draft bir shielding book june99t

Shielding Design for Diagnostic X-ray rooms Draft of June 1999 1

The Shielding of Radiographic Facilities at DiagnosticEnergies

1. Preface

2. Introduction

2.1. Scope of this report.

2.2. Definition of Dose

2.3. Design criteria.

2.4. Sources of radiation

2.5. Occupancy Factors

2.6. Use of reference doses.

2.7. Room Layouts

3. Building Materials

4. Dealing with secondary radiation

4.1. Leakage Radiation

4.2. Scattered Radiation

5. Transmission through building materials

6. Assessment of shielding

7. Methodologies and worked examples

7.1. Radiographic facilities

7.2. Mammographic facilities

7.3. R&F

7.4. C Arm

7.5. CT

7.6. Dental

7.7. Mobile

7.8. Bone Densitometry

8. Appendix


2 INTRODUCTION

2.1 Scope Of This Report

This will be a small section outlining what the aims of the report are and how thingsare structured.

2.2 Definition of Dose

When using dose limitation and calculating shielding requirements, it is important tobe clear about what is meant by the term ‘dose’. The constraint refers to the effectivedose to the person who may be irradiated. However, this report considers the need toshield the individual from radiations which are generally measured in terms of airkerma. It should also be recognised that individual doses, as recorded on a personaldose monitor, are assessed in terms of the operational quantity, personal doseequivalent, Hp(d). Hp(d). is assessed at depths (d) of 0.07 and 10 mm.

The relationship between the derived quantities and air kerma is complex, dependingon radiation spectrum, and, in the case of effective dose, distribution of photon fluenceand the posture of the exposed individual. In fact, whilst air kerma represents asignificant overestimate of effective dose due to self shielding within the body it isactually less than Hp(10) by 20 to 70% in the energy range used for diagnosticradiology,

It is neither practical nor realistic to use effective dose when calculating shieldingrequirements. All calculations in the report are therefore based on air kerma which,to retain clarity, is expressed in units of mGy and µGy.

2.3 Design Criteria

The Ionising Radiations Regulations 1999 require that work involving exposure toexternal radiations should be performed in rooms which are provided with adequateshielding. If there is public access to the surrounding area or access is permitted toemployees who are not directly involved in the work, the shielding should be designedto reduce dose rates to the lowest level that is reasonably practicable. It is thereforenecessary to formulate design criteria to ensure that this requirement is met.

A design limit based on the annual dose limit for members of the public (1 mSv) doesnot comply with the needs of the legislation and certainly does not represent an ‘aslow as reasonably practicable’ (ALARP) solution.

There is considerable weight of advice suggesting that dose optimisation procedurescan be performed by the adoption of constraints. However, the legislative frameworkis not prescriptive as to the actual value of the constraint to be applied.

In their advice following the publication of ICRP Report 60, the NRPB recommendedthat the constraint on optimisation for a single source of radiation should not exceed30% of the dose limit, in this case 0.3 mSv per annum (NRPB 1993 a, b). They alsoemphasise that the introduction of dose constraints does not replace the requirement


on operators to optimise their use of sources or their management of practices toensure that exposures to members of the public are kept as low as reasonablyachievable.

Dose constraints, therefore, represent an upper bound on the outcome of anyoptimisation procedure. However, there is also an acceptance that on-site exposuresto members of the public should not be assessed in terms of either continuousoccupancy or exposure. In terms of shielding, therefore, the application of the doseconstraint must be made using realistic assumptions.

The Working Party considered the application of alternatives to the 0.3 mSv perannum dose constraint as recommended by NRPB. One such alternative was to basethe criterion on the minimum amount of radiation detectable by a personal radiationmonitor. It can be argued that such an approach may represent an ALARP solution.However, this concept was rejected because of its arbitrary nature, given for examplethe potential for changes in the detection limits of radiation monitors.

The conclusion of the Working Party was that any design would have to be based onthe dose limit for members of the public of 1mSv per annum and that the principle ofoptimisation left no alternative but to utilise the concept of a dose constraint of 30%.The ALARP design criterion for all examples was therefore chosen as 0.3 mSv perannum. However, it should be stressed that in order to use this criterion effectively, itis essential that realistic allowances be made for mitigating factors such asoccupancy.

In designing shielding one further dose constraint may need to be considered. Filmmay be stored in a room adjacent to the x-ray room and some attention may need tobe given to inadvertent blackening of unexposed film. Film itself is relativelyinsensitive to radiation requiring doses in excess of about 10 µGy to cause anysignificant increase in fog. The period of storage in the department may be severalmonths so that it is prudent to aim to restrict doses further in those areas where longterm film store is planned and an annual design limit of 30 µGy (?100µGy) isrecommended.

Film in cassette is much more sensitive. By definition, exposure of 400 speed film to2.5 µGy will produce a density of 1.0 above base plus fog and perceptible fog will beproduced at a dose which is one-tenth of this. In a busy department it is unlikely thata film cassette will remain unused for more than one day which means that shieldingshould be designed to reduce the dose to 0.25 µGy per day or 60 µGy per yearbased on a five day week. This only applies to those areas where film cassettes areto be kept and is of most significance when considering the specification of the screenin radiography or R&F rooms.

2.4 Sources of Radiation.

The three sources of radiation which need to be considered in any examination of theshielding problem can be grouped into two distinct types, primary and secondaryradiation. Secondary radiation has two components, scatter and leakage. The threesources are briefly discussed below.

Primary Radiation.

The primary beam consists of the spectrum of radiation emitted by the x-ray tube prior


to any interaction with the patient, grid, table, image intensifier etc. The fluence of theprimary beam will be several orders of magnitude greater than that of either of thesecondary radiations discussed below.

In the majority of all radiography, the primary beam will be collimated so that the entirebeam interacts with the patient. Exceptions include extremity radiography and areconfined to low kVp, low mAs exposures..

Interactions within the patient results in considerable attenuation of the primaryradiation. Whilst typical entrance doses are of the order of mGy, exit doses aremeasured in µGy. The exit beam is however considerably more penetrating than theentrance beam. Take the simple example of an 85 kVp constant potential primarybeam with a total filtration of 3.5 mm Al. The half value layer of this radiation is 3.66mm Al and it has an effective energy of 47 keV. After passing through 16 cm of tissueequivalent material and 2 cm of bone, its fluence is reduced by a factor ofapproximately 330, but the exit beam has an effective energy of 62 keV and an HVLof 8.93 mm Al.

Scattered Radiation.

Scattered radiation is inevitable in diagnostic radiology and is a direct result of theCompton effect. The fluence of scattered radiation depends on the volume of thepatient irradiated, the spectrum of the primary beam and the field size employed.Both the fluence and quality of the scattered radiation are dependent on angle atwhich they are measured. A simple generalisation is that the scattered kerma isbetween 10-5 and 10-6 of the incident kerma per cm2 of the incident beam.

Figure 2.1 shows data measured by the working party indicating how the scatterkerma varies with angle for three different accelerating potentials. This data isconsistent with other published data used in section 4.2 (Williams 1996) Figure 2.2shows how the HVL, (i.e. beam quality) varies with angle of scatter for an 85kVpprimary beam incident on the pelvis of a rando phantom. The primary beam has aconstant wave form and a half value layer of 3.7 mm Al. It will be seen from the figurethat for angles below approximately 120 degrees, the scattered radiation will beharder than the primary radiation.

Leakage Radiation.

Leakage radiation arises because x-rays are emiited in all directions by the target.The outer shell of the tube housing is generally constructed of a light alloy withadequate mechanical properties. Evidently, such an alloy will not absorb enough ofthe radiation to reduce the kerma of the unwanted radiation to the legal maximum.The housing is therefore lined with lead where appropriate. Any radiation transmittedthrough this protective shield is termed leakage radiation. Manufacturers often protecttubes well beyond the legal minimum with the possible exception of those used inmobile radiography, where weight is especially important. Because it generallypasses through two or more mm of lead, leakage radiation will be considerablyharder than radiation in the primary beam.

Leakage is usually defined at the maximum operating potential of an x-ray tube /generator combination and is specified at the maximum continuous tube currentpossible at that potential (the leakage technique factors). Consider the example of aconstant potential x-ray tube with an inherent filtration of 1.2 mm Al and leakagetechnique factors of 3mA at 150 kVp. To reduce the leakage radiation to the legal


maximum of 1mGy at 1 metre in 1 hour requires the addition of between 2.1 and 2.5mm of lead. As a result of the hardening effect of the lead the HVL of the 150 kVpleakage radiation will be of the order of 14 mm Al. However, with the exception ofhigh kVp chest techniques, where mAs values are in any event low, the majority ofconventional radiography is carried out at less than 100 kVp. The measureableleakage will therefore be a lot lower than the legally defined maximum. For example,assume that the tube housing has indeed been shielded as required by the aboveexample and that 1mGy is measurable at 1 metre at 150 kVp. At 80 kVp the leakageradiation will have an HVL of about 12 mm Al. However, the kerma of the leakageradiation will be of the order of 14 µGy, i.e. it will be reduced by a factor of about 70.

Variation of scatter factor with scattering angle

0

1

2

3

4

5

6

7

8

9

0 20 40 60 80 100 120 140 160

scattering angle

scat

ter

fact

or

(uG

y/(G

y.cm

^2))

Figure 2.1 Variation of scatter factor with scattering angle. _____ 105 kVp ------- 85 kVp ......... 50 kVp.

HVL variation with angle of scatter

2.5

33.5

44.5

5

5.56

6.5

0 50 100 150

angle of scatter

HV

Lin

mm

of

Al

Figure 2.2 Variation of HVL with angle of scatter.


2.5 Occupancy factors

As stated in 2.2 above, the application of the dose constraint must be made usingrealistic assumptions regarding the occupancy of areas which are relevant in terms ofthe shielding problem. To be realistic, the occupancy factor for an area should not beconsidered as being an indication of the time during which it is occuppied by ageneric group of people (such as patients in a waiting room). Instead, it is thefraction of time spent by the single person who is there the longest. In this context, itis most likely that the critical groups for shielding purposes will not be patients orpatients' visitors but non radiation workers employed by the hospital. Given thisassumption, the occupancy factor is best defined as being the fraction of an 8 -hourday or 2000 hour year for which a particular area is occupied by a single person.

NCRP 49 contains examples of suggested occupancy factors and Dixon (1997) hasmade more recent suggestions. For ease of reference, extracts from both sets of dataare reproduced in table 2.1. The data in the table do not represent hard and fastrequirements for occupancy factors; rather they are for indicative purposes only andare intended for use only when no realistic data is available. It will be noted that theminimum occupancy factor in the table is 1/40. This is a result of a US requirementthat persons who are not occupationally exposed should not, on average, receivemore than 2mR (20 µGy) from any one source in any one week. It is notrecommended that either set of occupancy factors in table 2.1 be used without seriousconsideration being given to the specific task in hand. Rather, it is considered that amore appropriate course of action is to make an informed assessment of the particularinstallation being considered. The assessment should be made in terms of the 0.3mSv constraint and should involve detailed appraisal of available architecturaldrawings. The drawings consulted should not only be of the room to be shielded itself,but should also encompass the surrounding area.

Whilst making such an assessment, it is important to bear in mind the fact that the useto which a particular room is put may change with time. It is also important toconsider all of the surrounding rooms, not just those adjacent to the area beingshielded. For example, although a corridor may have low occupancy, an officeaccross the corridor may be occupied on a full time basis and this must be taken intoaccount when specifying the construction of the corridor wall.

Above : The Americans have a max of 2mR per week hence 1/40. Do we have abasis for a minimum occupancy factor ?

2. 6 Estimating workload : use of reference doses and otherdata.

A prerequisite to designing shielding for any x-ray facility is a knowledge of the use towhich the room is going to be put and of the number of patients that are expected tobe imaged in a year. This information will allow estimates of workload to be made.Without doubt, the best estimates of workload are those which take into accouint localpractice, rather than generic figures which represent 'busy departments', 'DGHdepartments' and so on. Accordingly, whichever technique is adopted for estimatingshielding requirements, the recommendation made in this publication is that thedesign be based on workload data extracted from audit of present practice.

For example, in much of this report it is recommended that DAP is used as the


measure of workload. In the UK most fluoroscopy sets have dose-area product (DAP)meters fitted . They are also becoming increasingly common on fixed radiographicunits. In addition, most physicists who are involved in shielding calculations will havebeen involved in patient dose audits and will have their own data. It is thereforerelatively simple to estimate total DAP from the projected clinical workload of theproposed x-ray facility. Similarly the number of CT slices performed in any one yearshould be easily accessible in any site where there is a existing CT facility

If workload cannot be extracted from locally available data, for example in the case ofthe implementation of a new technique, then there are several alternatives open to theperson making the calculation. For example, DAP values have been published forsome common examinations(ref) and for high dose interventional procedures (ref)and are reproduced at appropriate points in this publication. Alternatively, andprobably more conservatively, there are the reference doses produced by the NRPBfollowing their 1984 survey (NRPB 1984) and adopted by the CEC. More up to dateinformation is available such as the 1995 review of doses to patients undergoingmedical examinations in the UK (NRPB 1995). . Paediatric data can be found in theCEC guidelines on quality criteria in paediatric radiography (CEC 1996) CT referencelevels ?? CEC CT quality criteria ??

New NRPB report on trends in radiography etc. One assumes we can include datafrom this publication when it comes out

2.7 Room Layout

The official UK guidance on the design of radiological facilities ({Scottish} HealthBuilding Note 6) suggests that general, specialised (including angiographic) and CT x-ray rooms should be designed to a minimum dimension of 38 m2 . No specificrecommendation is given for mammography facilities. It is accepted that theserecommended dimensions do not necessarily reflect the situation encountered inpractice, where rooms may be considerably smaller. Nevertheless given that there isno other available recommendation, the majority of examples in this document will bebased on x-ray rooms having a floor area of 38m2.

References

CEC 1996 European guidelines guidelines on quality criteria for diagnosticradiographic images in paediatrics. Euopean Commission , Luxembourg.

Dixon RL 1997. Application of new concepts for radaiation shielding of medicaldiagnostic x-ray facilities. Presented at RSNA Chicago, November, 1997.

National Council on Radiation Protection and Measurements (1976) Structuralshielding design and evaluation for medical use of x rays and gamma rays of energiesup to 10 MeV. NCRP Report No 49

NRPB 1984 A National survey of doses to patients undergoing a selection of routinex-ray examinations in UK hospitals.

NRPB 1993 a

NRPB 1993 b


NRPB (1995) Doses to patients undergoing medical examinations in the UK - 1995review. NRPB report R-289 , NRPB Chilton, Oxfordshire

Scottish Health Building Note 6.Radiology Department. The Scottish Office 1994

Williams JR (1996) Scatter dose estimation based on dose-area product and thespecification of radiation barriers. Br J Radiol, 69, 1032-7.


Table 2.1 Some suggestions for occupancy factors when no other data is available.

LOCATION T Dixon T NCRP49

Adjacent x-ray room 1 1

Attended waiting roomc 1 -

Employee Rest Area 1 1

Film Reading Area 1 -

Laundry 1 -

Nuclear Medicine Scanning Room 1 1

Offices, shops, living quarters, children’s indoor play areas,occupied space in nearby buildings

1 1

Ultrasound Scanning Room 1 -

X-ray Control Room 1 1

Barium Kitchen 1/2 -

Cafeterias 1/2 -

Kitchens 1/2 -

Nurses stations 1/2 1

Patient Dressing Rooms 1/2 -

Patient Examination and Treatment rooms 1/2 -

Corridors 1/8 1/4

Employee lounge 1/8 -

Patient roomsb 1/8 -

Toilets or bathrooms 1/8 1/16

Outdoor areas with seating 1/20 -

Storage rooms 1/20 -

Unattended vending areas 1/20 -

Attics 1/40 -

Outdoor areas with only transient pedestrian or vehicular traffic 1/40 1/16

Patient Dressing room 1/40 -

Stairways 1/40 1/16

Unattended elevators 1/40 -

Unattended parking lots 1/40 1/4

Unattended waiting rooms 1/40 1/16

Vehicular drop off areas (unattended) 1/40 1/16


3 Building Materials


4 Dealing With Secondary Radiation

Secondary radiation comprises of a scatter component and a leakage component.Both components must be taken into account when considering the transmission ofsecondary radiation. It is often assumed that the scattered radiation will have thesame transmission properties as the primary beam whilst the leakage component willbe harder. As exemplified in section 2.4, the first assumption will only be true atspecific scattering angles.

One approach which has been used to allow for the two types of secondary radiationis that of NCRP 49 and is based on work done in the 1950s. The foundation of themethod is that the contribution of leakage and scattered radiation can be assessedseparately and barrier requirements for each determined independently. The larger ofthe two is taken to be the final result, unless they have the same magnitude, in whichcase the ‘add one half value layer’ approach is adopted. This concept is significantlyflawed as has been pointed out by several authors (Archer 1983, Simpkin 1987,Simpkin 1998).

4.1 Leakage Radiation

In the traditional treatment of leakage radiation, it is usual to assume that it is allgenerated at the maximum potential of the generator / tube combination. This canlead to extremely conservative design parameters given that much radiography isperformed at potentials below 100 kVp whilst leakage parameters are frequentlyspecified at 150 kVp. Simpkin and Dixon (1998) have reworked the issue of thetransmission of secondary radiation to take this fact into account. In doing so, theyhave demonstrated that that the NCRP49 approach to leakage radiation can result insolutions which are up to 8300 times too conservative.

Table 2.1 shows data extracted from the work of Simpkin and Dixon anddemonstrates the ratio of leakage to scatter at 90 degrees for a range of acceleratingpotentials. Figure 2.1 is a graph showing this ratio plotted against the kVp. The dataare for a field size of 1000 cm2 and are specified at 1 metre from the sources of thescatter and leakage. The assumption is also made that protection against leakageradiation is only sufficient to ensure an air kerma of 1 mGy/hour at 1 metre withleakage factors of 150 kVp and 3.3 mA. As has been pointed out previously themajority of x-ray tubes have more protection than this in place.

Even making this conservative assumption it is evident that there will be considerablyless leakage radiation than scatter at commonly used energies. Consequently, in thispublication the ‘add one half value layer’ approach is rejected. Instead, secondaryradiation transmission curves which take into account the variation of leakageradiation with kilovoltage are provided. See section 5.


4.2 ASSESSMENT OF SCATTER DOSE

Scatter dose is a function of kVp, scattering angle, entrance surface dose, and thearea of the x-ray beam. This principle was the basis of the standard equation used forscatter dose calculation given in NCRP Report 49 (NCRP 1976). This equation canbe written as:

K KF

400s ua= ⋅ . (4.2.1)

in which Ks is the scatter air kerma at 1 m, Ku is the kerma free in air at the beam entrypoint, F cm2 is the x-ray beam area at the image receptor and a is an experimentallydetermined constant which is a function of kV and scattering angle. Values for agiven in NCRP 49 were based on data from Trout and Kelley (1972).

More recently, Williams (1996) proposed that DAP be used for scatter dose estimationDAP meters are calibrated in terms of area-kerma product which is essentially thesame as Ku.F in equation 4.2.1. The only difference is that F is defined at the imagereceptor and not at the position at which air kerma is specified. Williams measuredscatter dose as a function of angle and kV. He defined the scatter factor, S, as:

SK

DAPs= (4.2.2)

Experimental values of S are plotted as a function of angle for a range of kVps inFigure 4.2.1. It can be approximated by the equation:

S a b c d e kV f= ⋅ + ⋅ + ⋅ + ⋅ + ∗ − ∗ +( ) (( ) )θ θ θ θ4 3 2 85 1 (4.2.3)

for which the fitting constants are given in Table 1.

It is recommended that where possible, scatter dose is calculated from DAP using thescatter factor given by equation 3. The examples in this report follow this suggestion.Where it is not possible to do so, for example, in mammography, other methods ofderiving scatter dose are presented.

It is important to note that for a surface parallel to the central axis of the x-ray beam, itcan be shown that the maximum scatter dose is at an angle of 117° . This is the mostcommon geometry for shielding calculations on room walls. It corresponds to verticallydirected x-ray beams and to non-vertical beams when the axis of rotation from thevertical is perpendicular to the wall. This generally applies to tilting tables and tochest radiography, for example. With this geometry it can be shown that themaximum wall scatter (Smax) at 1 m is given by:

Smax = (0.031 kV + 2.5) µGy (Gy.cm2)-1 (4.2.4)

For distances greater than 1 m, the inverse square law can be applied.

It should be noted that scatter factors were also assessed by Simpkin and Dixon(1998). They used Trout and Kelley’s original data with certain modifications. In


particular they changed the low kV (50 and 70 kV) data which were originallymeasured in x-ray beams with very low filtration. The data were modified on the basisof measurements by Dixon (1994) for one scattering angle (90° ). In their re-analysisof Trout and Kelley’s data, Simpkin and Dixon adopted a more conservative methodfor dealing with the variations in scatter due to variations in scatter path length in thephantom arising from differences in field area and the position of the beam centre.The revised scatter factors (a’) were normalised to 1 cm2 rather than to a 400 cm2

beam, i.e. a a'= 400 .

Their data may be compared with that used here. Figure 4.2. 2 shows the scatterfactors at 85 kV. It can be seen that, although there is good agreement between 90°and 120° , the shape of the curves is very different. The ratio of maximum to minimumscatter factor is 3.7 and 1.5 as assessed by Williams and Simpkin & Dixonrespectively

References

Archer BR , Thorny JI, Bushong SC. (1983) Diagnostic x-ray shielding design basedon an empirical model of photon attenuation. Health Physics 44 , 507-517.

Dixon RL (1994) On the primary barrier in diagnostic x-ray shielding. Med Phys, 21,1785-93.

Hart D, Jones DG and Wall BF (1994) Estimation of effective dose in diagnosticradiology from entrance surface dose and dose-area product measurements. NRPBReport R262.

Hart D, Hillier MC, Wall BF, Shrimpton PC and Bungay D (1996) Dose to patientsfrom medical x-ray examinations in the UK - 1995 review. NRPB Report R289.

Legare JM, Carrieres, PE, Manseau A et al. (1977) Blindage contre les grandschamps de rayons x primaires et diffuse des appareils triphase au moyen depanneaux de verre de gypse et de plomb acoustique. Radioprotection 13 79-95

National Council on Radiation Protection and Measurements (1976) Structuralshielding design and evaluation for medical use of x rays and gamma rays of energiesup to 10 MeV. NCRP Report No 49.

Simpkin DJ and Dixon RL (1998) Secondary radiation shielding barriers for diagnosticx-ray facilities: scatter and leakage revisited. Health Physics, 74, 350-65.

Trout ED and Kelley JP (1972) Scattered radiation from a tissue-equivalent phantomfor x-rays from 50 to 300 kVp. Radiology, 104, 161-9.

Williams JR (1996) Scatter dose estimation based on dose-area product and thespecification of radiation barriers. Br J Radiol, 69, 1032-7.


kVp Ratio of scatter to leakage50 3.45 x108

70 2 x 104

100 22.6125 14.6150 13.5

Table 4.1 Ratio of scattered to leakage radiation at a 90 degree scatteringangle.

a -1.042 x 10-7

b 3.265 x 10-5

c -2.751 x 10-3

d 8.371 x 10-2

e 1.578f 5.987 x 10-3

Table 4.2 Factors required for the calculation of the scatter factor, S, from kV andscattering angle using equation 3. These data were derived frommeasured values over a kV range of 50 to 125 kV and scatteringangles between 30° and 150° .


Figure 4.1 The ratio of scatterd to leakage radiation at 90 degree scattering angle.

0

2

4

6

8

10

12

0 30 60 90 120 150 180

Angle of scatter

Sca

tter

fact

or,

SµG

y.(G

y.cm

2 )-1

125 kV100 kV85 kV70 kV50 kV

Figure 4.2 Variation of scatter with angle from Williams (1996)

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

40 60 80 100 120 140 160

kVp

Rat

io


0

1

2

3

4

5

6

7

8

9

10

0 30 60 90 120 150 180Scatter angle

Sca

tter

fact

or

µGy/

(cG

y.cm

2 )

Williams

Simpkin & Dixon

Figure 4.3 Comparison of scatter factors at 85 kV from Williams (1996) andSimpkin and Dixon (1998)


5. Transmission of X-rays through Shielding Materials

5.1 Unattenuated Primary Radiation

The most commonly available publications in the UK containing information whichMedical Physicists can use to derive the characteristics of shielding materials areHPA Report 41 (HPA 1984) and the Radiological Protection Handbook (HMSO 1971).The first was published in 1984 and the second in 1971; both are now out of print.Another potential reference source is the publication NCRP 49 (NCRP 1976) Much ofthe transmission data in all three sources is based on single phase measurements orwas derived under narrow beam conditions and can therefore be discounted for use ina pragmatic situation..

In 1983, Archer et al (1983) developed an empirical model (equation 1) to describethe broad beam attenuation of x-rays through a material provided that the parametersα, β, λ could be determined for the particular medium.

( )B x= +

−

−

1

1

βα

αλ βα

λexp. 5.1)

.In the above equation, B is the broad beam transmission, x is the thickness of materialand α, β, λ are the fitting parameters.

The equation can be inverted to enable the calculation of the amount materialrequired to provide the desired transmission thus :

x =+

+

−1

1αλ

βα

βα

λ

lnB

5.2)

Work by Archer (1994), Rossi et al (1991) , Simpkin (1995), Christensen andSayeg(1979) amongst others has resulted in a body of data for three phase andconstant potential transmission through a variety of materials. based on empiricalmeasurement and theoretical modeling. As a result the parameters α, β, λ have alsobeen determined for these materials. Simpkin (1995) has published a compendium ofavailable data and has also demonstrated how relatively simple techniques can beused to fit α, β and λ to experimentally obtained transmission measurements.Tsalaffoutas et al (1998) have recently applied this procedure to an aerated concretebuilding material. The working party have done the same for barium plaster usingdata supplied by the manufacturer, British Gypsum (1991). Table 5.1 gives fittingparameters for a selection of materials at representative constant potential energies.A more comprehensive collation of fitting parameters can be found in Simpkin (1995).

Equation 5.2 has been used to predict transmission values for a selection ofcommonly used shielding materials at differing kVps using the available data. Table5.2 shows the density assumed for each material. Figures 5.1 and 5.2 show derivedtransmission curves through lead and concrete, for radiation arising from a Tungsten


target at 50, 60, 75, 90 and 120 kVp. Also shown in these figures are transmissioncurves for 30 kVp Mo/Mo radiation. Figure 5.3 shows transmission curves for Baplaster at 50, 75 100 and 125 kVp . Figure 5.4 shows transmission through woodand gypsum for 30 kVp Mo/Mo radiation. All data are plotted for B=1 to B=10-6.

Figure 5.5 demonstrates a plot of thickness vs transmission for lead, concrete,barium plaster , plate glass, steel wood and gypsum at 75kVp. Such curves areinstructive and can be generated at other potentials using the coefficients provided intable 5.2

Inspection of curves like those in figure 5.5 shows that there is an obvious correlationbetween the transmission provided by differing materials at any accelerating potential.It is possible to fit polynomial equations to the data and thus relate the quantities ofthe different materials required to provide the same transmission. Table 5.3 goves thecoefficients of cubic polynomial fits using thickness of lead as the dependent variableand concrete, glass, gypsum, steel, wood and barium plaster as the independentvariables. The polynomial fit was applied over transmission values 1 to 10-8. Alsoshown (Table 5.4) are the coefficients where concrete thickness is the independentvariable and equivalent thickness of lead is the dependent variable. The cubicpolynomials fitted have no intercept and are of the form y = b1x + b2x

2 +b3x3 . A cubic

fit was performed in all cases for consistency even though in some cases a linear fitprovided almost as good a result. The tendency towards linearity is increasinglypronounced as the kVp increases In all cases r2>0.999. A typical example is given infigure 5.6 which shows the equivalence between concrete and lead at 60 kVp.

The coefficients in tables 5.1, 5.3 and 5.4 can easily be incorporated into spreadsheets and used to evaluate either the required thicness or the equivalent thickness'of material. Two example are given below :

1) Suppose that calculation had shown that the requisite shielding could be providedby 2.2 mm lead at 75 kVp. Use of Table 5.3 shows that the same degree ofprotection can be obtained using

252002222221787222215091226387 .......... =×××+××+× mm concrete

2) Suppose that calculation showed that a transmission factor of 0.002 was requiredto achieve the desired specification and that the majority of radiography in the roomwas carried out at 75 kVp or lower. Equation 5.2 shows that the required shieldingcan be provided with 6 mm Ba plaster.

When considering equivalent thickness of concrete, it is worth recalling that HPAreport 41(1984) points out that in the UK ordinary concrete has a density of 2200 kg m-3 whilst in the US the standard specification has a density of 2350 kg m -3 . The datapresented here is based on the US specification but, provided that the material is notloaded with granite or other aggregates, the data can just be scaled appropriately. Sofor example, if the transmission curves and / or equations presented here are used toderive a desired thickness of concrete, and the actual material to be used has adensity of 2200 kg m-3, then the answer should be scaled by a factor of 2.35/2.2.

5.2 Unattenuated primary radiation

As outlined in section 2 of this report, primary radiation is significantly attenuated andhardened following transmisssion through a patient. It is not unreasonable and alsoconservative to treat this radiation as being of constant HVL. It is easy to use equation


5.1 to show that

αβ+

λ−α−= 1ln

1)Bln( x 5.3)

As x becomes large the second term in this equation becomes increasinglyinsignificant and the transmission equation tends to a simple exponential with aconstant equal to α Thus, one can treat the primary, attenuated radiation as having aconstant (or assymptotic) HVL of (ln 2)/α. The final column of table 5.1 shows typicalhigh attenuation HVLs calculated in this manner. The validity of this approach can beseen by inspecting the transmission curves (figs 5.1 - 5.4). It can be seen that fortransmission less than 10-3 the form of the logarithmic plot is effectively linear. Incases such a chest radiography, where the transmiussion through the patient is such(< 10-3) that the 'high attenuation' assumption is probably not valid, the result ofadopting it will be a conservative design since the true transmitted beam will not be asprenetrating as that modelled.

5.3 Secondary Radiation

5.3.1 Radiography.

Simpkin and Dixon (1998) have used equation 5.1 above and applied it to thesecondary radiation problem. They have made the simplifying assumption that thescattered and primary beams have the same attenuation. Values of α, β, and λ havebeen derived for secondary radiation transmitted through various media at a 90degree angle and for a field size of 1000 cm2 at an FFD of 1m. The leakagetechnique factors assumed, and then modified to reflect the operating potential beingconsidered, were 5 mA at 50 kVp for radiography below 50 kVp and 3.3 mA at 150kVp for radiography above 50 kVp.

The data of Simpkin and Dixon have been used here to produce secondarytransmission curves. Although they have not considered the variation of scatter qualitywith angle experimental measurements performed by the working party suggest thatat 117 degrees, which is the angle for maximum scatter dose (see section 4.2) , theprimary and scattered radiation have very similar HVLs (figure 2.2). The use of thedata is therefore considered to be justifiable. As in the case of primary radiation,equation 5.2 has been used to predict transmission values for a selection ofcommonly used shielding materials at differing kVps using the available data..Figures 5.7 and 5.8 show derived transmission curves through lead and concrete, forsecondary radiation arising from a primary beam (Tungsten target) at 50, 70, 100 and125 kVp. Also shown in these figures are transmission curves for seconary radiationarising from a 30 kVp Mo/Mo primary beam. Figure 5.9 shows transmission ofsecondary through wood and gypsum for a 30 kVp Mo/Mo primary beam. All data areplotted for B=1 to B=10-6. Selected fitting factors at representative energies are givenin table 5.5 as is the high attenuation Half Value Layer. A more comprehensivecollation of fitting parameters can be found in Simpkin and Dixon (1998).

The equivalence of materials has also been determined, using the cubic polynomialapproach outlined above. The equations of each fitted curve are given in tables 5.6and 5.7. It will be seen from the figures and equations that, as expected, thesecondary radiation requires more shielding than the corresponding primary radiation.This is especially the case especially at higher values of kVp.


Hopefully can put some Monte Carlo results in here - i.e. what the effect ofscattering angle really is and when Simpkin & Dixon's work can be used. Needto determine what the effect of the HVL variation is.

The penetrating ability of the scattered radiation decreases as the scatterfraction increases - (I don't think this is at all remarkable) - . In the experimentperformed as part of this work, it was lower than the primary beam at anglesgreater than about 120 degrees. The question is : is it in fact all stopped byabout the same amount of shielding since as the fluence goes down, thepenetrating ability goes up and one balances the other out.

5.3.2 Computed Tomography

The spectrum of scattered radiation which is produced as a result of CT scanning willbe considerably harder than that observed in general radiogrpahy. Allowance mustbe made for this when designing computed tomography suites. There is very littledata available on the transmission characteristics of such radiation. Simpkin (1990)has used Monte Carlo tehniques to simulate CT scatter spectra and has modelled thetransmission characteristics of a variety of commonly used materilals but has madeno allowance for leakage. radiation His results are given in table 5.6 in the form ofcoefficients for equations 5.1 and 5.2. Figure 5.10 is a demonstration of the use of thistable, and shows predicted transmission through concrete at 120 and 140 kVp.

5.4 References.

Archer BR , Thorny JI, Bushong SC. (1983) Diagnostic x-ray shielding design basedon an empirical model of photon attenuation. Health Physics 44 , 507-517.

Archer BR, Fewell TR, Conway BJ & Quinn PW. (1994). Attenuation properties ofdiagnostic x-ray shielding materials. Medical Physics 21 1499-1507

British Gypsum (1991) Thistle x-ray. Technical Information leaflet.

Chritensen Rc & Sayeg JA. 1979 Attenuation characteristics of gypsum wallboard.Health Physics 36 595-600.

HMSO 1971 Handbook of Radiological Protection. HMSO London.

HPA 1984 Report no 41 :Notes on Building Materials and Reference Data onShielding Data for use below 300 kVp. HPA (now IPEM) , York

Legare JM, Carrieres, PE, Manseau A et al. (1977) Blindage contre les grandschamps de rayons x primaires et diffuse des appareils triphase au moyen depanneaux de verre de gypse et de plomb acoustique. Radioprotection 13 79-95

National Council on Radiation Protection and Measurements (1976) Structuralshielding design and evaluation for medical use of x rays and gamma rays of energiesup to 10 MeV. NCRP Report No 49.

RossiRP, Ritenour R, Christodoulou E. 1991 Broad beam transmission properties ofsome common shieldin materials for use in diagnostic radiology. Health Physics 61601-608

Simpkin DJ 1989 Shielding requirements for constant potential diagnostic x-raybeams determined by a monte carlo calculation. Health Physics 56 151-164


Simpkin DJ 1990 Transmission of scatter raduiation from computed tomography (CT)scanners determined by a monte carlo calculation. Health Physics 58 363-367

Simpkin DJ 1995 Transmission data for shielding diagnostic x-ray facilities. HealthPhysics 68 704-709

Simpkin DJ and Dixon RL (1998) Secondary radiation shielding barriers for diagnosticx-ray facilities: scatter and leakage revisited. Health Physics, 74, 350-65.

Tsalafoutas A, Yakoumakis A, Manetou A & Flioni-Vyza A. 1998 The diagnostic x-rayprotection characteristics of Ytong, an aerated concrete based building material. BJR71 944-949


1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0.0 1.0 2.0 3.0 4.0 5.0

mm

Tra

nsm

issi

on

Tx 30

Tx 50

Tx 60

Tx 75

Tx 90

Tx 120

Figure 5.1 Transmission of primary radiation through Lead

Figure 5.2 Transmission of primary radiation through concrete.

.

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0 400.0

mm

Tra

nsm

issi

on

Tx 30

Tx 50

Tx 60

Tx 75

Tx 90

Tx 120


.

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0

mm

Tra

nsm

issi

on Tx 50

Tx 75

Tx 100

Tx 125

Figure 5.3 Transmission of primary radiation Through Barium Plaster

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0.0 100.0 200.0 300.0 400.0 500.0 600.0

mm

Tra

nsm

issi

on

Wood

Gypsum

Figure 5.4 Transmission of primary radiation Through Wood & Gypsum 30 kVp


Figure 5.5 Transmission of primary radiation at 75 kVp.

CONCRETE

LEAD

1.61.41.21.0.8.6.4.20.0

200

100

0

Cubic fit

Observed Data

Figure 5.6 Equivalence between concrete and lead at 60 kVp.

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01

Transmission

Th

ickn

ess

Lead

Steel

Gypsum

Plate Glass

Concrete

Ba Plaster


1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0.0 1.0 2.0 3.0 4.0 5.0 6.0

mm

Tra

nsm

issi

on Tx 30

Tx 50

Tx 70

Tx 100

Tx 125

Figure 5. 7 Transmission of secondary radiation through lead.

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0 400.0

mm

Tra

nsm

issi

on Tx 30

Tx 50

Tx 70

Tx 100

Tx 125

Figure 5.8 Transmission of secondary radiation through concrete.


1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0.0 50.0 100.0 150.0 200.0 250.0 300.0

mm

Tra

nsm

issi

on

Gypsum

Wood

Figure 5.9 Transmission of secondary radiation for 30 kVp primary

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 400.00 450.00 500.00

Barrier Thickness (mm)

Tra

nsm

issi

on

120 kVp

140 kVp

Figure 5.10 Transmission of scattred CT radiation through concrete.


30 kVp αααα ββββ λλλλ ∆ = β/ α∆ = β/ α∆ = β/ α∆ = β/ α AssymptoticHVL =( ln 2)/αααα

Lead 38.8 178 0.3473 4.58763 0.017865Steel 5.716 43.41 0.3959 7.59447 0.121264Gypsum 0.1208 0.7043 0.3613 5.8303 5.737973Plate glass 0.3061 1.599 0.3693 5.22378 2.264447Concrete 0.3173 1.698 0.3593 5.3514 2.184517wood 0.02166 0.03966 0.3732 1.831025 32.0012550 kVpLead 8.8014 27.2737 0.2956 3.09879 0.078754Steel 1.817 4.845 0.4024 2.66648 0.381479Gypsum 0.03883 0.08732 0.05106 2.24878 17.85082Plate glass 0.09721 0.1798 0.4912 1.8496 7.13041Concrete 0.09032 0.1712 0.2324 1.89548 7.674349Ba Plaster 0.447049 1.222173 0.263357 2.73387 1.55049660 kVpLead 6.951 24.89 0.4198 3.58078 0.099719Steel 1.183 4.219 0.4571 3.56636 0.585923Gypsum 0.02985 0.07961 0.6169 2.667 23.22101Plate glass 0.07452 0.1539 0.5304 2.06522 9.301492Concrete 0.06251 0.1692 0.2733 2.70677 11.08858Ba Plaster75 kVpLead 4.666 22.69 0.6618 4.86284 0.148553Steel 0.5793 3.629 0.5908 6.26446 1.196525Gypsum 0.2066 0.6649 0.775 3.2183 3.35502Plate glass 0.5291 1.28 0.6478 2.4192 1.310049Concrete 0.04797 0.1663 0.4492 3.46675 14.4496Ba Plaster 0.609028 2.357352 0.582836 3.870677 1.1381290 kVpLead 3.067 18.83 0.7726 6.13955 0.226002Steel 0.3971 2.913 0.7204 7.33568 1.745523Gypsum 0.01633 0.05039 0.8585 3.08573 42.44624Plate glass 0.0455 0.1077 0.8522 2.36703 15.234Concrete 0.04228 0.1137 0.469 2.68921 16.39421120 kVpLead 2.246 8.95 0.5873 3.98486 0.308614Steel 0.2336 1.797 0.8116 7.69264 2.96724Gypsum 0.01235 0.03047 0.9566 2.46721 56.12528Plate glass 0.03758 0.06808 1.031 1.8116 18.44458Concrete 0.03566 0.07109 0.6073 1.99355 19.43767

Table 5.1 Selected coefficients to generate primary transmission curves(equations 5.1, 5.2 & 5.3)


Material Density

Lead 11350 kgm-3

Concrete 2350 kg m-3

Steel 7400 kgm-3

Gypsum 705 kgm-3

Plate Glass 2560 kgm-3

Wood 550 kgm-3

Table 5.2 Density of Materials


30 kVp b1 b2 b3CONCRETE 112.096 51.0661 -78.89GLASS - 120.829 42.2184 -74.817GYPSUM 277.034 207.506 -309.99STEEL 5.1375 8.7427 -13.859WOOD 3103.48 -5936.3 8776.53Ba Plaster - - -

50 kVpCONCRETE 120.679 -30.717 11.2057GLASS - 150.514 -47.992 13.2935GYPSUM 203.376 -113.94 42.8306STEEL 6.2303 -0.7349 .1146BA PLASTER 20.3472 -1.6765 .7105

60 kVpCONCRETE 105.721 -9.9460 4.2142GLASS - 138.504 -33.431 7.9825GYPSUM 337.873 -72.798 16.5428STEEL 6.2363 -0.1632 0.0228

Ba Plaster - - -75 kVp

CONCRETE 87.6303 1.1509 .1787GLASS - 116.682 -15.916 2.6675GYPSUM 296.416 -37.643 6.134STEEL 6.2954 0.837 -0.1226BA PLASTER 7.7994 -0.1512 0.0322

90 kVpCONCRETE 76.0696 -3.0833 .4431GLASS - 99.6588 -11.927 1.3144GYPSUM 258.95 -26.028 2.8502STEEL 6.6.17 .3716 -0.0386BA PLASTER

125 kVpCONCRETE 91.3923 -6.9006 .5278GLASS - 103.422 -10.843 .8456GYPSUM 290.882 -25.812 1.9735STEEL 9.9312 .2880 -.0303BA PLASTER 12.8985 .0098 .0003

Table 5. 3Cubic coefficients relating lead thickness with that of other material


30 kVp b1 b2 b3LEAD .0089 -3e-05 3.9e-0750 kVp b1 b2 b3LEAD .0080 2.9E-05 -1.E-0760 kVp b1 b2 b3LEAD 0.0095 9.0E-06 -4.E-0870 kVp b1 b2 b3LEAD 0.0109 -2E-06 -3.E-0980 kVp b1 b2 b3LEAD 0.0119 9.7E-07 -5.E-0990 kVp b1 b2 b3LEAD 0.013 8.9E-06 -2.E-08100 kVp b1 b2 b3LEAD 0.0136 1.6E-05 -3E-08110 kVp b1 b2 b3LEAD 0.0127 1.8E-05 -3.E-08125 kVp b1 b2 b3LEAD 0.0101 1.8E-05 -2E-08

Table 5. 4Cubic coefficients relating concrete thickness with that of lead


αααα ββββ λλλλ ∆ = β/ α∆ = β/ α∆ = β/ α∆ = β/ α HVL (=ln(2)/αααα)30 kVpLead 38.79 180 0.356 4.64037 0.017869Steel 7.408 42.49 0.4021 5.73569 0.093567Gypsum 0.1198 0.7137 0.3703 5.95743 5.78587Plate glass 0.306 1.62 0.3793 5.29412 2.265187Concrete 0.3174 1.725 0.3705 5.43478 2.183829wood 0.02159 0.03971 0.03971 1.83927 32.1050150 kVpLead 8.801 27.28 0.2957 3.09965 0.078758

Steel 1.817 4.84 0.4021 2.66373 0.381479Gypsum 0.0388 0.0873 0.5105 2.25 17.86462Plate glass 0.09721 0.1799 0.4912 1.85063 7.13041Concrete 0.0903 0.1712 0.2324 1.8959 7.67604970 kVpLead 5.369 23.49 0.5883 4.37512 0.129102

Steel 0.7149 3.798 0.5381 5.31263 0.969572Gypsum 0.023 0.0716 0.73 3.11304 30.13683Plate glass 0.05791 0.1357 0.5968 2.34329 11.96939Concrete 0.0509 0.1697 0.3849 3.33399 13.61782100 kVpLead 2.507 15.33 0.9124 6.11488 0.276485Steel 0.3424 2.456 0.9388 7.1729 2.024378Gypsum 0.0147 0.04 0.9752 2.72109 47.15287Plate glass 0.04279 0.08948 1.029 2.09114 16.19881Concrete 0.0395 0.0844 0.5191 2.13671 17.54803125 kVpLead 2.233 7.888 0.7295 3.53247 0.310411Steel 0.2138 1.69 1.086 7.90458 3.242035Gypsum 0.012 0.0267 1.079 2.225 57.76227Plate glass 0.03654 0.0579 1.093 1.58456 18.96955Concrete 0.0351 0.066 0.7832 1.88034 19.74778

Table 5.5 Selected coefficients to generate secondary transmission curves at90 degrees(equations 5..1 & 5.2).


30 kVp b1 b2 b3CONCRETE 112.351 51.7292 -81.807GLASS - 121.035 41.8356 -74.904GYPSUM 277.503 217.189 -324.34STEEL 4.9526 2.5928 -4.9301WOOD 2028.45 -6270.3 10390.8

50 kVp b1 b2 b3CONCRETE 120.675 -30.657 11.2004GLASS - 150.465 -47.944 13.2771GYPSUM 355.969 -93.493 23.3248STEEL 6.2303 -0.7366 0.1155

70 kVp b1 b2 b3CONCRETE 91.6283 1.3991 0.4601GLASS - 123.435 -19.366 3.7135GYPSUM 310.695 -45.936 8.4881STEEL 6.2219 0.6853 -0.1175

100 kVp b1 b2 b3CONCRETE 69.1344 -2.6302 0.2774GLASS - 85.3562 -8.0393 0.7137GYPSUM 229.377 -17.588 1.5571STEEL 7.0278 0.0908 -0.0082

125 kVp b1 b2 b3CONCRETE 81.9378 -4.6953 0.3615GLASS - 90.6395 -7.5868 0.5849GYPSUM 257.496 -18.028 1.3748STEEL 10.1464 0.1501 -0.0150

Table 5.6Cubic coefficients relating lead thickness with that of other material for

secondary radiation


30 kVp b1 b2 b3LEAD .0089 -3e-05 4e-0750 kVp b1 b2 b3LEAD .0080 2.9E-05 -1.E-0770 kVp b1 b2 b3LEAD 0.0109 -2E-06 -3.E-09100 kVp b1 b2 b3LEAD 0.0143 1.1E-05 -2.E-08125 kVp b1 b2 b3LEAD 0.0117 1.5E-05 -2E-08

Table 5.7Cubic coefficients relating concrete thickness with that of lead for secondaryradiation


αααα ββββ γγγγ ∆=∆=∆=∆=ββββ//// αααα

Pb120-kVp 2.70214 6.2227 0.7721 2.303140-kVp 2.86862 4.6590 0.7921 1.624

Concrete120-kVp 0.03829 0.0142 0.6582 0.371140-kVp 0.03359 0.0122 0.5185 0.363

Gypsum wallboard120-kVp 0.00100 0.0268 0.4125 26.800140-kVp 0.01177 0.0167 1.3910 1.419

Steel120-kVp 0.27957 1.5191 1.2357 5.434140-kVp 0.19215 0.9519 0.9649 4.954

Plate glass120-kVp 0.03213 0.0146 0.2280 0.454140-kVp 0.03544 .00975 0.9450 0.275

Table 5.8 Coefficients which can be used to generate transmission of scatteredradiation from CT installations.


6 PRACTICAL ASSESSMENT OF SHIELDING

6.1 INTRODUCTION

When an X-ray facility is complete, it is necessary to check the integrity of theshielding provided and ensure that it fulfils the design criteria. This is one element ofthe critical examination required by the Ionising Radiation Regulations. Such checkscan be carried out through discussions with the builder and visual inspection of allparts during the construction phase. However, a complete inspection of all parts of thefacility at this stage may not be practicable, in which case it is necessary for checks ofthe shielding to be made when the construction is complete. Use of a radiationsource and detector during the critical examination allows the integrity of all parts ofthe shielding in a completed installation to be checked. The most suitable type ofsource is a sealed source of 241Am as the main γ-ray has an energy of 60 keV which iswithin the upper part of the photon energy range of an X-ray beam (Hewitt 1982). Avial of 99mTc may be used as an alternative to determine the thickness of lead used toprovide protection if an 241Am source is not available. However, a knowledge ofshielding materials employed is essential for interpretation of results, because theenergy of the main 99mTc γ-ray (141 keV) is significantly higher than the normal X-rayrange and relative attenuation properties of shielding materials at this energy differfrom those at X-ray energies. An alternative method is to make the assessment usingradiographic exposures. However, this method is more time consuming and lessflexible and so will only be considered briefly in this report.

6.2 METHODOLOGY

6.2.1 Radioactive source method

There are two main aims of the practical assessment of shielding:

1) to detect any places where there are gaps in the shielding provided2) to check that the protection is of the level specified.

The apparatus consists of a source, a detector and positioning rod, a tapemeasure and calibration charts. Two persons are required to carry out the tests, bothfor practicality of making the measurements and for purposes of ensuring security ofthe radioactive source and minimising exposure of other staff. Procedures must be inplace to comply with current Ionising Radiation Regulations and ensure effectivesupervision of the source. One person is required to position and control the sourceand the second to determine the location of the beam on the far side of the barrierusing a suitable detector. A tape measure is useful for defining the approximatelocation of the source, in order to assist the second person in locating the radiationbeam. Persons carrying out the assessment may choose to use the source anddetector on whichever side of the boundary is better both for access and for safetypurposes. It is easier to locate the source at fixed points on the side of a wall to whichaccess is limited and leave areas of wall on the other side where more space may beavailable to allow the person using the detector more space to search for the radiationbeam. Other things being equal, positioning of the source on the inside of the X-rayroom will simulate more closely the situation in practice. It will be necessary to movethe detector back and forth over the wall surface to locate the position of maximumresponse.

Detector


The detector used should have a rapid response to facilitate location of theradiation beam on the far side of the barrier from the source. A scintillation detectorsuch as a Mini Instruments 900 Monitor with a Type 44A probe, which has a 32 mmdiameter x 2.5 mm thick sodium iodide scintillation crystal, is suitable. <For situationsin which the expected degree of protection is less than 1mm Pb equivalent, an endwindow geiger tube such as a Mini Instruments Type E probe can be used.>> A rigidmeasuring bar with 1 cm graduations attached to the probe housing is useful toenable the probe face to be positioned at a fixed distance of 1 to 15 cm from the wallsurface (figure 6.1).

SourcesAs already mentioned, the source may be 241Am or 99mTc. The source must be

housed in a shielded container having a window or cap which can be readilyremoved, so that the aperture can be placed against the barrier to be tested. A designfor a lockable container with a safety shutter for an 241Am source (Hewitt 1982) isshown in figure 6.2. A spring loaded rotating lead shutter may be held open when thecontainer is positioned against a barrier and closes automatically when the containeris moved away from the barrier. This minimises the exposure to scattered radiation ofthe person handling the source.

241-Americium241Am emits a gamma ray of energy 60 keV which is close to the peak of the

photon energy spectrum for a diagnostic X-ray unit. Plots of count rate against sourcedetector separation are given in figure 6.3 for barriers made from different materials.Results are normalised with respect to the count rate obtained with a source detectorseparation of 30 cm with 1 mm of lead in the beam.

In order to obtain a high enough radiation level for the check to be made, a source of15-40 GBq must be used. This requires the use of a type A transport container andassociated procedures under the Radioactive Material (Road Transport) (GreatBritain) Regulations 1996 including consignment note, labelled vehicles and carryingof a fire extinguisher. In addition the Radiation (Emergency Preparedness and PublicInformation) Regulations require the operator holding and using an 241Am source ofactivity more than 300 MBq to carry out a hazard assessment and risk evaluation forthe Health and Safety Executive. Sources with a valid special form certificate areexempt from this requirement.

99m- TechnetiumFor hospitals which do not possess an 241Am source, a vial of 99mTc provides

an alternative. 99mTc is readily available in Nuclear Medicine departments, isinexpensive and the potential hazard is low because of the short half-life. As a result,a vial of 99mTc can be carried as an excepted package. Activities required are typically50 MBq or 100 MBq. It is recommended that standard activities are used for whichcalibration charts can be prepared. If the amount of liquid is much smaller than thevolume of the vial, the position of the activity will depend on the inclination of the vial.It is therefore recommended that a volume of liquid is used which is sufficient to fill themajority of the vial.

The disadvantage of 99mTc is that the γ-ray has an energy of 141 keV, which issignificantly higher than the photon energies in most diagnostic X-ray beams. As aresult the relative attenuation by different shielding materials such as lead, concreteand X-ray plaster is not the same for 99mTc as for an X-ray beam. The attenuation bylead is high because the K absorption edge for lead is at 88 keV, but attenuation byother materials used for shielding is much lower than that for diagnostic X-ray beams.


Thus the attenuation for a beam of 99mTc γ-rays can be used to determine the amountof lead protection, but is seldom satisfactory for assessing barriers of other materials.

Plots of normalised count rates versus source detector separation for differentthicknesses of lead and other materials are given in figure 6.4.

Checks for gaps in shieldingChecks for gaps in protection should be made with the detector in contact with

the wall. It will be necessary to move the detector back and forth over the wall surfaceto locate the position of maximum response. This will normally be the position wheresource and detector are directly opposite each other on either side of the wall, but inplaces where there is a gap in the protection in a cavity wall, this may not necessarilybe the case. Places where gaps are more likely to occur are where different forms ofshielding meet. These include:

Frames of shielded windows and doorsJoins between two parts of a shieldWhere sockets, etc, breach the integrity of the wall.

Practical assessment of level of protectionThe source container with the window or lid open is held against the wall for

the measurements (figure 6.1). A tripod and baseplate are useful for holding thesource in a fixed position. An option to include an additional 1 mm of lead in the beamis useful to increase the range of attenuation that can be assessed. In order to make ashielding assessment it is necessary to know the thickness of the wall, so that theseparation of the source and the detector can be set. If there is a door in the wall, thiscan be determined with relative ease, but if not, it may be necessary to measuredistances along a number of walls to determine the wall thickness, as illustrated infigure 6.5. Where several measurements are required the accuracy of the distancemay be limited. Values for the thickness may also be available from plans of thefacility or from measurements made during the construction phase. Where the wall issome distance from any door to the room, these may be the only thicknessesavailable.

The results of the measurement depend upon the separation of the source anddetector. A separation between 20 cm and 50 cm is recommended. If a shorterdistance is employed, error due to inaccuracy in the distance becomes large, while ifthe distance is too large count rates may be too low to obtain an accuratemeasurement. As already discussed, it is necessary to know what shielding materialswere used if a realistic result is to be obtained. The data in figures 6.3 and 6.4 allowresults to be converted to thicknesses of different shielding materials

Normalised count rates are plotted as a function of separation of source and detectorto allow flexibility in the configuration used. It is suggested that the user make ameasurement of the count rate obtained at 30 cm through 1 mm lead using theirchoice of activity This measurement could be used to calibrate the user’s monitor anda set of curves of count rate versus distance determined from the figures. Variations inthe measurements relating to the relative position of the source, detector and barrierwere ±15%.Experimental results indicate that use of a type E geiger as opposed to atype 44 monitor will not alter the magnitude of this error See Daves figure at the end

6.2.2 Measurements using X-ray equipment

Assessments of shielding may be made using X-ray equipment, although this methodtends to be more time consuming and requires a radiation detector with good dynamic


range. The measurements may be performed with the unit installed in the room, ifthere is sufficient flexibility in positioning of the X-ray tube, or a mobile radiographicunit may be used. The energy of the X-ray beam can be chosen to match that ofexaminations performed in the room and so used to obtain the wall attenuationdirectly. An ionisation chamber of appropriate sensitivity should be used to measureair kerma for an exposure using standard factors on either side of the barrier to betested. The attenuation can then be determined directly from the ratio of the twomeasurements with an inverse square law correction applied to allow for the differentpositions. Alternatively the air kerma at the distance to be used for the assessmentcould be determined from measurements made in the room. The use of X-rayequipment and film can be useful for direct demonstration of gaps in shieldingdetected using the source method.

ReferencesHewitt JM (1982) A self-contained method for assessing the lead equivalenceof protective barriers in diagnostic X-ray departments. J. Soc. RadiologicalProtection 2, 22-26.


Figure 1 Arrangement for assessment of shielding using a source andscintillation detector. Thickness of the wall = A + B.

Figure 2 Shielded container for 241Am source. (N.B. Awaiting informationfrom Graham Ramsden, NRPB, Leeds on design)

Figure 3 Graphs of normalised count rate from a 241Am source versusdistance between probe and source for different thicknesses of a)lead and b) barytes plaster and brick. Data are normalised withrespect to the count rate with 1 mm lead at a source probedistance of 30 cm.

Figure 4 Graphs of normalised count rate from a 99mTc source versusdistance between probe and source for different thicknesses of a)lead and b) barytes plaster. Data are normalised with respect tothe count rate with 1 mm lead at a source probe distance of 30cm.

Figure 5 Determination of wall thickness A from distances along interveningwalls.

LastFigure

Dave Sutton results


Wall

A BScintillation Contamination Monitor

Source

Measuring Bar

Figure 6.1


Figure 63a

Americium-241 Lead Attenuator

0.001

0.01

0.1

1

10

100

0 10 20 30 40 50 60 70

Source Detectro Distance (cm)

No

rmal

ised

Co

un

tR

ate

(c/s

)

0.5 mm

1.0 mm

1.5 mm

2.0 mm

3.0 mm


Americium-241 - Barium Plaster Attenuator

0.001

0.01

0.1

1

10

0 10 20 30 40 50 60 70

Source Detector Distance (cm)

No

rmal

ised

Co

un

tR

ate

10 mm

20 mm

30 mm

Figure 6.3 b


Technetium-99m - Lead Attenuator

0.01

0.1

1

10

100

0 10 20 30 40 50 60 70

Source Detector Distance (cm)

No

rmal

ised

Co

un

tR

ate

0.5 mm

1.0 mm

1.5 mm

2.0 mm

2.5 mm

3.0 mm

Figure 6.4a


Technetium 99m Ba Plaster Attenuator

0.01

0.1

1

10

0 10 20 30 40 50 60 70

10 mm Barytes

20 mm Barytes

30 mm Barytes

10 cm Brick

21.5 cm Brick

Figure 6.4 b


A

B C

D

Wall thickness A = D - B - C

Figure 5


scint & geiger comparison

1

10

100

10 00

100 00

10 00 00

0 .0 0 0 .50 1.00 1.5 0 2 .0 0 2.50 3.0 0 3 .5 0 4 .00 4.50

sc in t i l la t ion p r ob e

g eig er t u be

Daves Figure to show the similarity - counts are normalised.


7 Methodologies and Worked Examples

7.1 RADIOGRAPHIC FACILITIES

Radiographic rooms are used in a more versatile manner than most other x-ray installations.Generally there are two main imaging stations – a table and a vertical bucky stand usedprincipally for chest radiography. Examinations on the table most commonly involve theradiation beam firing downwards with the cassette either in the bucky tray or on the table top(generally for extremities). For these locations protection is required for the secondaryradiation and for the transmitted primary radiation. The area of the wall or floor exposed to thelatter component being both limited and predictable. However, other parts of the floor or wallsmay be exposed to transmitted primary due, for example, to lateral views taken with the patienton the table and examination of patients on trolleys.

7.1.2 Methods

1. NCRP

The conventional method of calculating barrier thickness is to use therecommendations in NCRP Report 49 (NCRP 1976). The NCRP method considersthe three components of radiation (primary, leakage and scatter) separately and hasthe following features:

• Workload is expressed in mA.min• A single, high kVp is assumed for the total workload• Primary barrier thickness is designed on the assumption that there is no

attenuation in the beam directed towards the wall or floor• Leakage radiation is assumed to be at the maximum specified rate, i.e. 1 mGy h-1

• Scatter radiation is computed on the basis of equation 4.1.1 using scatter factorsbased on single phase, low filtration sets.

Changes in the methodology for primary shielding were proposed by Dixon and Simpkin(1998) and for secondary barriers by Simpkin and Dixon (1998) which retained the use ofmA.min as the unit of workload.

For the specification of primary barriers, the biggest changes made by Dixon and Simpkin(1998) are concerned with more realistic assumptions regarding primary attenuation andworkload spectra. They provide the following data:

• transmission through the patient, image receptor, and supporting structures based on workby Dixon (1994);

• tube output at 1 m per mA.min;• workload spectrum based on a survey by Simpkin (1996);• use factors, i.e. proportion of the workload for which the beam is directed at the floor or

specific walls.

The data can be used to specify shielding for any workload spectrum and pattern of usage.However, they give a general equation for calculating transmission B through a barrier ofthickness, x , in association with pre-shielding in the patient, image receptor, etc. which isassumed to have an equivalent thickness, xpre:

( ) N0Dd

TP

)xx(B1

2

pre

=+ (1)


in which P is the dose constraint, T is the occupancy factor, d is the focus to barrier distance,D1(0) is the unshielded primary dose per patient at 1 m for the standard workload spectrum,and N is the number of patients per week.

Values of these parameters are given by Dixon and Simpkin (1998) and reproduced in Table1. For cross-table radiography, they recommend the use of xpre = 0.3 mm lead or 30 mmconcrete.

For secondary radiations, Simpkin and Dixon (1998) recommended that scatter and leakageradiations are combined and used with modified transmission data to account for thehardening of the leakage spectrum. This approach was discussed in Section 4. They provide asimplified equation for the calculation of the secondary barrier based on the same workloaddata as given above combined with the assumption that all vertical bucky work including chestradiography is with a 35 x 43 cm2 cassette and that the average field area for tableradiography is 1000 cm2. The total secondary dose per patient at 90° and distance of 1 m

( 1secD ) is given in Table 1 for the standardised workload.

The unshielded secondary dose is given by:

2sec

1sec

secd

ND)0(D = (2)

in which dsec is the distance to the area to be shielded and N is the number of patients.

2. Working party

Alternative methods are proposed here for shielding calculations. For primaryradiation it is proposed that film dose is used and secondary radiation shielding shouldbe assessed from workload in terms of DAP as outlined in Section 4 of this report.This approach is described in more detail.

400 speed radiographic film requires, by definition, a dose of 2.5 µGy to produce adensity of 1.0 plus base plus fog. Some areas of the film will have higher densitiesbecause a) radiologists generally prefer somewhat darker films, b) the beam may belarger than the body part (for example for extremities), and c) there are densityvariations across the film. In addition slower films may be used for extremities(although for protection purposes, these are associated with low kVps and aretherefore of much less relevance). It is proposed to use a value of 10 µGy as theaverage for the maximum dose on any part of the film. This represents significantattenuation of the primary which may vary between about 10-3, e.g. for a lateral viewof the lumbar spine with an entrance skin dose of 10 mGy to 0.1 for a chestradiograph with an ESD of 0.1 mGy.

Making the assumption that the beam is fully collimated to the area of the cassette,there is then further attenuation produced in the cassette itself and in the structure ofthe cassette holder and the table base or vertical bucky stand.

Attenuation needs to be estimated for three geometries:� Table radiography with attenuation in the cassette plus table assembly� Cross table radiography with attenuation in the cassette alone� Vertical bucky radiography with attenuation in the cassette plus bucky assembly

Dixon (1994) measured primary attenuation in 8 types of film cassette. Average values areshown in Table 2. However, these data cannot be applied to the primary transmitted throughthe patient because of beam hardening. To estimate this effect, the primary transmission


factors can be used to estimate lead and concrete equivalence of the cassette. These valuesare shown in the Table. The equivalent thicknesses can then be used to calculate thetransmission of the filtered spectrum in lead (A) and concrete (B) using the limiting values ofHVL for the primary beam derived as for the data in Table 5.1. It can be seen that thetransmission of the filtered beam is much greater and it is recommended that 50%transmission is assumed at all energies.

Transmission through a table assembly with a cassette in place was given by Dixon(1994). This included transmission through the tabletop and grid. Primarytransmission through these components are given separately. The data are shown inTable 3. Using primary transmission data the lead equivalence of the completeassembly and of the tabletop plus grid can be calculated. From these values thetransmission of the filtered beam through the cassette plus the table base can becalculated using the limiting values of HVL.

Dixon (1994) measured primary transmission through 3 models of vertical bucky standat 125 kVp. The same type of analysis as for the table has been made for the modelwith maximum transmission. The result shown in Table 3 indicates that thetransmission of the filtered beam is 0.25.

For secondary radiation, the working party recommend that the methods described inSection 4 are used with workload based on DAP.

7.1.3 Workload

Primary radiation

For primary radiation it is necessary to know which examinations may involve the useof a horizontal beam. Most commonly horizontal beams are used with the verticalbucky with the patient standing or lying on a trolley. The vertical bucky may be usedfor the following examinations:

� Chests – kVp depends on local practice and may vary over virtually the fulldiagnostic range (60 to 150 kVp). It is advisable to design shielding on the basisthat practice may change to the upper end of this range since this has beenrecommended in a number of publications both for patient dose reduction andimage quality.

� Shoulders, cervical spines (at between 60 and 75 kVp).� Abdomens (70 to 90 kVp) –required for erect patients in rare circumstances.� Standing knees, femur (60 to 70 kVp) and erect spines (70 to 90 kVp) generally in

rooms serving orthopaedics.� Skulls, sinuses, facial bones (60 to 75 kVp) – may preferably be done in rooms

with isocentrically mounted skull units.

The examinations of the skull, etc. may also be done on the table with the filmpropped up. There are other cross-table radiographic views but these are relativelyrare except with seriously injured patients. It is also possible that the radiographicworkload will include ‘sky-line’ views of the knee. However, this is with the tubepointing towards the patient who will be protected by a lead coat and will effectivelyprovide local shielding.

In A&E horizontal beams are more common particularly for seriously injured patients.Examinations include: skull; spine; hips; and extremities. The film may be taken usinga vertical bucky, if that is available, or with the film propped up or held.


Scatter

For scatter calculations workload is required in terms of DAP. There are fewpublications of DAP for radiographic doses. However, it is possible to infer typicalvalues using average entrance skin doses (ESD) from NRPB (1996) and the ratio ofESD and DAP to effective dose conversion factors from NRPB (1994). These valuesare given in Table 4 with upper limit values calculated from the reference doses. Alarge proportion of the workload in terms of the numbers of films will be forextremities. However, the DAPs for these examinations will be low (in the range 0.01to 0.1 Gy.cm2) so that they make a relatively small contribution to scatter and thekVps are also relatively low (50 to 70 kV) requiring less shielding.

For a busy radiographic room, the workload will be in the region of 500 Gy.cm2 butthis will depend on case mix. The average DAP weighted kVp will be in the range 80to 90 kVp. This will be determined by the relative proportion of examinations in thepelvic and abdominal regions which are associated with the highest DAPs and tubepotentials.

7.1.4 Example

Room diagram to calculate at wall situated at 1.5 m from Table (primaryand secondary), cubicle at 3 m (secondary), door (3.5 m – secondary),wall 1 m behind chest stand (primary), floor below table.

For this example the total weekly workload is assumed to be 500 Gy.cm2. Thisincludes:

• 400 films on the table at a DAP weighted average of 90 kVp• 10 cross-table spine films at 100 kV directed towards wall A.• 100 chest films at 125 kVp• An average DAP per chest film equal to 0.15 cGy cm2

Any other examination using the vertical bucky is ignored in this calculation. It hasbeen assumed that the occupancy factor of adjacent rooms and of the room below is100%.

Wall APrimary

10 films at 1 m FFD giving 10 µGy per film with 50% transmission throughthe film plus cassette.

Transmission =

( )72.0

5.001.010

115.1

523.0

2

=××

+×

At 100 kV the HVLs of lead and concrete are 0.276 mm and 17.5 mm(Table 5.5) so that 0.13 mm of lead or 8 mm of concrete is required.

SecondaryMaximum scatter dose on the wall is given by equation 4.4 at 90 kVp:

[ ] weekperGy11755005.11)5.290031.0(S

2max µ=××+×=


The required transmission is therefore given by 0049.0175.1

523.0

B ==

This attenuation would be provided by 0.9 mm of lead or 70 mm ofconcrete

The total thickness for this wall is based on the higher of these two values, i.e.0.9 mm of lead or 70 mm of concrete.

Wall B

Protection is required for primary transmission through the wall behind thebucky stand resulting form 100 films per week taken at 3.5 m FFD, with a film-to-barrier distance of 1 m.

Transmission =

( )038.0

25.001.0100

5.35.31

523.0

2

=××

+×

This requires 4.7 HVLs which equals 1.5 mm of lead or 93 mm of concreteusing the data from Table 5.5.

DoorFor a barrier at 3.5 m from the table, the scatter dose is given by:

[ ] weekperGy2165005.31)5.290031.0(S

2max µ=××+×=

Giving a required transmission of 026.0216.0

523.0

B == which can be achieved

with 0.5 mm of lead.

Protective screen

The scatter dose to the protective screen at a distance of 3 m iscalculated in the same way as for the door and is equal to 294 µGy.However, with a dose constraint of 60 µGy per year, the transmissionmust be less than 0.0039. This requires 1 mm of lead.

Floor

Primary transmission has to be calculated for 400 films taken at 100 cm FFDwith a distance to the occupants in the room below equal to 3.5 m. Thetransmission through the Table assembly of a 100 kVp beam from Table **(0.09) has been used in these calculations.

Transmission =

( )32.0

09.001.0400

115.3

523.0

2

=××

+×

This requires 1.6 HVLs of concrete which is equal to 27 mm at 90 kVp.


NCRP method

The NCRP method will be applied to the shielding requirements for Walls A and B and thefloor. It is assumed that the workload spectrum in Dixon and Simpkin (1998) applies and thatfor there are 1.3 films per patient examination on the Table and 1.0 films per patient on thevertical bucky. The total number of patients examined on the table is then 310 per week.

Wall APrimary

The primary transmission should be less than 00025.009.031015.5

)5.11(523.0 2

=××

+×.

Assuming an average kVp of 80 kVp (the peak of the workload spectrum in Dixon andSimpkin 1998), this requires 1.4 mm of lead. Subtracting xpre = 0.3 mm (the leadequivalence of the shielding provided by the patient and cassette) leads to a barrierrequirement of 1.1 mm lead.

Secondary

The secondary transmission should be less than 0018.03101031.2

5.1523.0

2

2

=××

×−

which can be provided by 0.9 mm lead or 79 mm concrete at 80 kVp.

The total barrier thickness needs to be

Wall BPrimary radiation only will be included. Application of the Dixon and Simpkin (1998)method requires that the primary transmission is less than

00016.031025.2

)15.3(523.0 2

=×

+×.

This transmission is achieved at 125 kVp with 2.7 mm of lead or 204 mm of concrete.The equivalent shielding provided by the imaging assembly (xpre) is 0.85 mm and 72mm of the two materials respectively (Table **) implying a wall thickness of 1.8 mmlead or 132 mm of concrete.

FloorThe required primary transmission through the floor is less than

00020.031085.1

)15.3(523.0 2

=×

+×

At 80 kVp this is achieved with 123 mm of concrete. For the floor, xpre = 74 mm so thatthe required thickness of concrete is 49 mm.

7.1.4 Other points to consider

A&ESkull tubesChest rooms


Table 7.1.1 Normalised workload and unshielded primary and secondary doses andequivalent thicknesses of shielding in the image receptor and support from Dixon and Simpkin(1998)

General radiographic room

All barriers Wall withchest bucky

Floor andother walls

Chest room

Workload per patient (mA.min) 2.45 0.601 1.85 0.216Unshielded primary dose perpatient D1(0) mGy

7.41 2.25 5.15 1.21

Equivalent thickness xpre mmLead 0.87 0.85 0.94 0.91

Concrete 73 72 74 72Unshielded secondary doseper patient Dsec

1 mGy3.42 x 10-2 5.30 x 10-3 2.31 x 10-2 2.69 x 10-3

Table 7.1. 2

60 kVp 80 kVp 100 kVp 125 kVpPrimary attenuation 0.0553 0.107 0.156 0.208Lead equivalence 0.14 0.17 0.19 0.21Concrete equivalence 16.5 16.7 19.2 20.7Filtered beam transmission – A 0.37 0.51 0.62 0.62Filtered beam transmission – B 0.36 0.47 0.47 0.48


Table 7.1. 3

Table assembly

80 kVp 100 kVp 125 kVp

Chestbucky

(125 kVp)Tabletop primary transmission 0.769 0.801 0.820 -Grid primary transmission 0.406 0.450 0.488 0.488Combined transmission 0.312 0.360 0.400 0.488Table assembly primary transmission 0.0017 0.0062 0.016 0.027Pb equivalence of cassette holder +table base

0.89 0.98 0.76 0.62

Transmission through cassette holder +table base

0.03 0.09 0.19 0.25

Table 7.1.4

DAP Gy.cm2

Examination Average Upper limitLumbar spine AP 2.4 3.8Lumber spine Lat 2.6 5.2Lumbar spine LSJ 2.6 3.5Chest AP 0.2Chest PA 0.1 0.2Chest Lat 0.4 0.8Abdomen AP 2.8 4.8Pelvis AP 2.5 5.3Skull AP/PA 0.6 1.1Skull Lat 0.3 0.6Thoracic spine AP 1.3 2.2Thoracic spine Lat 2.5 4.2IVU 16 40


7.2 MAMMOGRAPHY FACILITIES

X-ray mammography possesses a number of unique properties when compared toother x-ray modalities:

• the maximum energy of the primary x-ray beam is in the range 25 - 35 kV, and willtypically be <30kV;

• the typical unit, with a molybdenum anode and molybdenum filter is lightly filtered(0.3 - 0.4 mm Al), giving a very low average x-ray energy;

• equipment is designed so that the primary beam is constrained to fall within thearea of the image receptor, and therefore in practice only scattered radiationneeds to be considered.

Although this simplifies shielding assessments, there may be other mitigating factorsto be considered:

• newer mammography units have multiple target/filter combinations, typically usingmolybdenum/rhodium and tungsten/rhodium. These are specifically designed tocreate a slightly ‘harder’ x-ray beam, with HVLs of 0.4 - 0.5 mm Al;

• Health Building Notes do not give minimum sizes for mammography rooms, andthe equipment is often placed in rooms where the distance from the imagereceptor to the nearest barrier will be within the range 1 - 2 metres. These roomswill frequently be situated in or near out-patient clinics and office accommodationwhere a high occupancy factor must be assumed;

• mammography equipment is frequently used in mobile units as part of the BreastScreening Programme, and thus consideration must be given to appropriatestandards of protection that will withstand the rigours of regular transportation.

IPEM Report 59 (2nd edition) (IPEM 1994) on >the commissioning and routine testingof mammographic x-ray systems’ does not specifically deal with issues surroundingroom design, although it does refer to earlier work in the UK (Walker and Hounsell1989). More recent information is available from the USA (Simpkin 1996).

7.2.1 Workload / Calculations

The 1986 Forrest Report on Breast Cancer Screening suggested a maximumworkload of 80 women per day, having two single-view mammograms each (or amaximum of 160 mammograms per day). Data from a busy screening centresuggests that 60 women per day, having an average of three films each, is a moretypical. For the purpose of this report, we will assume that the maximum is likely to be200 films per day.

A pessimistic assumption of 30 kVp and 100 mAs per exposure has been used tocreate a safety margin for actual usage (I don’t think that there is any reference forthis W&H use 35 kVp). At 30 kVp, the tube output is approximately 50 µGy mAs-1 at1m or 140 µGy mAs-1 at 60cm, the typical distance to the image receptor.

Data from both the Walker and Hounsell (1989) and Simpkin (1996) give similarvalues for the scatter ratios, although they are assessed somewhat differently.Simpkin’s data cover a wider range of scatter angles and energies. He showed thatfor an 18 x 24 cm2 film size, the maximum scatter ratio is 5.4 x 10-4 at 1630 and it is0.9 x 10-4 at 900. The scatter ratio is defined as the ratio of scatter air kerma at 1m


from the patient to the primary dose at the image receptor.

From these data the maximum scatter air kerma (Ks) at 1 m per typical exposure at100 mAs can be calculated:

Ks = 5.4 x 10-4 x 140 x 100 = 7.6 µGy

This is the maximum scatter intensity at 163° . For cranio-caudal views (i.e. with thebeam directed towards the floor, the scatter dose to a wall at a distance of 1 m will besignificantly less than this. However, for lateral and oblique views the dose to the wallbehind the tube will receive this level of dose. On the principle of specifying shieldingfor the worst possible case, this dose will be assumed to be incident on a wall at 1 mdistance.

Multiplying by the workload data (200 films per day) and assuming 250 working daysper year, the annual incident scatter dose is shown to be 380 mGy.

7.2.2 Practical Solutions

The transmission data of section 5 can be used to determine the thicknesses ofmaterial required to reduce the dose to the design criterion (0.3 mGy per year). Therequired transmission is 0.3/380 = 7.9x10-4. Use of equation 5.1 and the data in table5.5 results in the solutions shown in table 7.2.1. The table shows that normal buildingmaterials and thicknesses are likely to be more than adequate to provide a sufficientlevel of shielding.

No data have been presented to calculate the doses to rooms above or below themammography unit. Given the requirement for structural integrity for the floor orceiling suggests adequate protection at normal diagnostic x-ray energies, they willcertainly provide adequate protection.

The wall behind the patient will receive virtually no scattered radiation, since it will beabsorbed by the patient.

Protection of doors is a difficult issue since the amount of lead required is trivial. It isworth noting however, that a 1 cm thick glass door will provide adequate protection

In the case of mobile units the data in Table 7.2.1 show that typical constructionmaterials would provide adequate shielding to fulfil the design criteria. Given thenature of these units, the annual occupancy factor at any given location givesadditional margins for safety.

Data have not been presented to calculate the doses to the space below the mobilemammography unit. Given the requirement for structural integrity for the floor and thelikely zero occupancy factor, adequate protection will almost certainly be guaranteed.


Table 7. 2.1 Required shielding thicknesses for a wall at 1 m.

Material Thickness (mm)Lead 0.0814Steel 0.4255Plasterboard 24.3Wood 128Concrete 9.6Glass 10.2

References(1) The commissioning and routine testing of mammographic x-ray systems. J

Law, DR Dance et al. IPEM Report 59, 2nd edition, 1994.(2) X-ray protection considerations for mammography screening centres. A

Walker and AR Hounsell. BJR, 1989, 62, 554-557.(3) Scatter radiation intensities about mammography units. DJ Simpkin.

Health.Phys., 1996, 70(2), 238-245.(4) Review of mammographic equipment and its performance. NHS Breast

Screening Programme Publication 24, 1992.

7.3 R&F Rooms

Radiography and fluoroscopy rooms (R&F) are mainly used for barium contraststudies. A typical room has an undercouch screening system with two x-ray tubes.One is mounted in the tilting table for undercouch fluoroscopy and the second tube isceiling mounted and is used for radiography with a table bucky and possibly a verticalbucky stand as well. An alternative is a system with a single column mountedovercouch tube. For the radiation protection of the staff, this system would be usedremotely, i.e. with the Radiologist operating the set from behind the protective screen.Other examinations which may be done in this room include ERCPs. The room mayalso act as a back up for radiography, in particular it is not uncommon to use R&Frooms for IVUs when barium contrast studies are not scheduled.

Barium contrast examinations have two parts, fluoroscopy and radiography. Forfluoroscopy the only significant radiation to be considered for shielding is scatterbecause the transmitted primary will be heavily attenuated in the image intensifiershield. Radiography contributes to as much as 90% of the total DAP (Hart et al.1994). Images may be captured in a number of ways including directly from theimage intensifier in digital format, using a cut film camera linked to the imageintensifier output screen, or using a conventional film cassette positioned in front ofthe image intensifier. In each case, the image intensifier mount provides localshielding from the transmitted primary and shielding only needs to consider scatteredradiation.

For barium enemas large format film (35 x 43 cm2) may be needed for certainprojections. These have to be taken using a separate table bucky and x-ray tube.However, the bucky assembly and table structure should be sufficient to attenuate thetransmitted primary radiation.

For undercouch screening tables the direction of maximum scatter is below the tabletop and the table structure provides significant shielding. In addition, for scattering


angles less than 90° , there is considerable local shielding provided by the lead apronattached to the image intensifier, the structure of the image intensifier mount, and itsshield. Therefore, scatter doses are likely to be significantly overestimated.Overcouch systems have little local shielding so that the doses to the walls will bemuch greater.

7.3.1 Workload

Average DAPs for barium contrast studies have been published by Hart et al. (1996).These data are reproduced in Table 1. In addition data are included for ERCPs. Abusy department might perform 25 barium enemas and 25 other contrast studies perweek, i.e. 10 patients per day. It can be seen that with these patient numbers, thetotal weekly DAP would be in the region of 1000 Gy.cm2. This figure will be used fordesign purposes.

7.3.2 Practical solutions

Shielding for an R&F room which is 6.5x5.9 m2 will be calculated. This is slightlylarger than Health Building Notes recommendations It will be assumed that the tableis positioned as close as possible to one of the walls at a distance of 1.6 m. Theprotective screen and room doors are each 2.5 m from the centre of the table. Bariumexaminations are generally carried out at high kV and an average value of 100 kV isassumed. The maximum scatter factor (Smax) at a distance of 1 m is given byequation (4) in Chapter 4.1. At 100 kV, Smax = 5.6 µGy (Gy.cm2)-1. This is for a wallparallel to the central axis of the beam which will be assumed to correspond to thegeometry for each If the barriers to be considered. The maximum weekly scatter doseat 1 m is therefore 5.6 mGy for the assumed workload (1000 Gy.cm2 per week).

Wall

Required transmission %26.016.1

526.53.0

2

=

×

×<

At 100 kV this requires 1.4 mm lead or 120 mm of concrete (1800 kg m-3).This is the specification for the wall closest to the examination table but itwould probably be applied to all walls.

Door and protective screen

Required transmission %64.015.2

526.53.0

2

=

×

×<

At 100 kV this requires 1.0 mm lead. A 1 mm lead door is therefore sufficientparticularly as the occupancy directly behind the door will be significantly lessthan 100%. However, the screen will also be used to shield films in cassettesand a dose constraint equivalent to 60 µGy per year has been recommended.The shielding requirement is then to provide less than 0.16% transmissionwhich requires 1.6 mm lead. This demonstrates that the standard 2 mm leadprotective screen should be provided in this room.

Floor/ceilingFloor shielding should be considered for undercouch systems and ceilingprotection for overcouch. The maximum scatter is back towards the tube andis approximately 10 µGy (Gy.cm2)-1 at a distance of 1 m. It may be assumed


that a person standing in the room below or above is at a distance equal to thefloor-to-ceiling height. This is generally not less than 3.5 m so that therequired transmission is given by:

%70.015.3

520.103.0

2

=

×

×<

This can be achieved using 1 mm lead or 93 mm concrete (1800 kg m3). Mostmodern structures will provide this level of shielding.

Other considerations

For part of the examination, particularly for barium swallows and meals, the table willbe tilted to the vertical position. This will increase the amount of scatter to the wallbehind the x-ray tube. This may need to be considered if the wall is particularly closeto the set and it may influence the shielding specified for the door if it is in thisposition.

A vertical bucky may be used in the room. Shielding behind the bucky may berequired in accordance with section 7.1.

Table 7.3.1 Typical DAP values for examinations performed in an R&F roomBarium contrast study data from Hart et al. (1996). ERCPs fromWilliams (personal communication).

Examination Ave DAP(Gy.cm2)

Barium enema 27.0Barium follow-through 12.0Barium meal 13.0Barium swallow 9.8ERCP 11.0

7.4 C-ARM EQUIPMENT

The general dose constraint of 0.3 mGy per year. Since C-arms are not generallyused with film-screen systems, there should be no need for more stringent dosetargets. The dose constraint may be relaxed when occupancy is considered.However, this type of room is likely to be surrounded by working areas such asrecovery rooms, nursing stations, etc. so that occupancy is unlikely to be less than25%.

Calculation of structural shielding for C-arm x-ray sets is relatively simple becausethe only significant source of radiation is scatter from the patient. There should beno possibility of primary exposure since it is a requirement that under all operatingconditions the x-ray beam falls entirely within the area of the image intensifier faceand its surround, and that the housing has a lead equivalence of at least 2 mm.

In Chapter 4.1 it was shown that the intensity of scatter is directly related to DAP


rate and is a function of scattering angle and kV. For the situation in which the x-ray beam is parallel to a wall, equation (4) from Chapter 4.1 can be used to showthat the maximum dose due to scatter at a wall 1 m from the beam centre variesfrom 4.7 to 5.6 µGy (Gy cm2)-1 for 70 to 100 kV x-rays respectively. This is theapproximate kV range used for the more common procedures comprising themajority of workload for these sets. The dose is greater if the tube is angledtowards the wall and may increase to approximately 10 µGy (Gy cm2)-1. Thenarrow range of values relative to the inherent uncertainties in calculation ofshielding permits the use of a single factor which for the purpose of this report istaken as 6 µGy (Gy cm2)-1. Inverse square law (ISL) can be applied to calculatemaximum scatter doses at greater distances.

C-arms are used for a variety of procedures which include angiography,interventional radiology, in orthopaedic surgery, and endoscopy. In thesesituations, the procedure is carried out in a relatively large room to allowunimpeded access around the patient. Health building notes recommend roomsizes of 40 m2 for an operating theatre and 38 m2 for a specialised radiology room.It is unlikely, therefore, that any side of the room will be much less than 6 m andsince the bed or patient support is generally positioned centrally in the room, theminimum practical distance between the centre of the beam and any wall is 2.5 m.Although individual rooms may need special calculations, for the purpose ofgeneral guidance it can therefore be assumed that the maximum scatter dose onany wall is unlikely to exceed 1 µGy (Gy cm2)-1.

7.4.1 Workload

The workload is expressed in terms of total DAP. There have been a number ofpublished surveys of DAP arising from high dose x-ray procedures. Some of theseare summarised in Table 1. It is difficult to use these data to estimate the dailyworkload for the planned new room because of the uncertainty in the mix ofprocedures, local clinical practice and in the projected case numbers. Although thedose per case is relatively high, the daily patient throughput is likely to be lowcompared with other rooms because of the extended time needed for each case.For example a busy Cardiac Lab is unlikely to exceed 10 cases in a standard (8hour) working day. Table 2 summarises audit data collected for 5 rooms with C-arm x-ray sets. These values illustrate the range which may be anticipated. Themaximum weekly DAP for these five rooms was 1750 Gy cm2.

The most common use of C-arms in theatres is in orthopaedics and the area ofmost frequent use is in trauma. With modern sets fitted with last image hold, pulsefluoroscopy, thermal printers, etc., DAP values should be low for most cases (ofthe order of a few cGy cm2. However, certain procedures may require significantlyhigher doses. An audit of cases in a busy trauma theatre showed that the averageweekly DAP was 22 Gy.cm2 for a total of 28 patients for whom the C-arm wasused.

7.4.2 Practical solutions

A. Cardiac Lab.

Figure 7.4.1 shows the layout drawing for a cardiac lab formed out of an existingfacility. The layout was dictated by the need to have patient access from the leftside of the room as shown in the drawing. The x-ray set was angled in order toallow trolleys to be taken into both the preparation and recovery areas. This also


served to maximise the working space for the cardiologists. The protected area isused during the procedure principally by the physiological measurementtechnician. It is situated at the foot end of the table allowing the technician to havea clear view of the patient and cardiologist. It was the local preference to haveopen access between the x-ray room and the protected area so that there wouldbe direct voice communication between the technician and cardiologist without theneed for an intercom system. Access to the protected area does not need to bethrough the x-ray room which is important both for radiation protection and forcontrol of infection. It allows ready access for other staff to observe proceduresand to communicate with those in the room.

The total area of the room is 36.6 m2 which is marginally less than isrecommended in Building Note 6. Under normal operating conditions, the nearestdistance between the area under examination and any wall is 2.6 m. It has beenassumed that at this distance the scatter dose is 1 µGy (Gy cm2)-1.

For the shielding specification, the maximum workload has been taken as 50%greater than the maximum for a cardiac lab in Table 2, that is 2600 Gy.cm2 perweek. The resultant maximum annual scatter dose to the nearest wall isapproximately 140 mGy. With 100% occupancy, the 0.3 mGy annual doseconstraint leads to a requirement for less than 0.21% transmission. Typicallycardiac procedures are carried out at 80 kV. This shielding would be achievedwith 0.9 mm lead or 100 mm concrete (1800 kg m-3). However, it would beprudent to assume a higher kV. At 100 kV 1.5 mm lead or 130 mm concretewould be needed.

This example is one in which conservative assumptions on workload, occupancy,and kV move the shielding specification towards the 2 mm of lead solution. Morerealistic assumptions would however suggest that 1 mm of lead is sufficient andthis would be the preferred option for doors to the recovery and preparation areasin which occupancy will be less than 100% and the people in the room wouldnormally work at least 1 m further away than the door itself.

Protection to the floor also needs to be considered. The maximum scatter dose islikely to be 10 µGy (Gy cm2)-1 at a distance of 1 m. The midline of the patient isapproximately 1 m above the floor and with a standard floor-to-floor height of 4 m,the distance from mid-trunk of a person standing in the room below will be about3.5 m from the source of scatter. This annual scatter dose is then given by:

mGy1105.3

15226001010

23 ≈

×××× −

For 100% occupancy this leads to a transmission specification of less than 0.27%.This can be achieved with 90 mm concrete (1800 kg m-3). This corresponds to theaverage thickness of a typical concrete slab floor.

B. Orthopaedic theatre

The following data have been assumed:• Distance to nearest wall - 3 m• Workload - 25 Gy.cm2 per week• Maximum scatter dose at 1 m - 6 µGy (Gy.cm2)-1

• Dose constraint - 0.3 mGy/year• Occupancy of adjacent areas - 33%


• Average operating potential - 90 kV

Occupancy is assumed to be less than 100% because the theatre is in 24 houruse and no individual member of staff is liable to be in the vicinity for more thanone-third of the time that the set is in use.

The required maximum transmission level is given by:

%10413

)1065233.025(

3.02

3=

×

×××× −

This implies that no protection is required on the walls of this theatre in order toachieve a dose constraint of 0.3 mGy per year to adjacent areas. Even at threetimes the workload, the required shielding would be achieved with 22 mm ofplasterboard showing that theatres do not normally need additional shielding evenif lightweight construction walls are used. However, theatres used forintraoperative interventional procedures could require significant shielding. Theseprocedures are likely to become more common in the future.

C. Coronary care unit

C-arms are used in coronary care units. Typically the DAP for the insertion of atemporary pacemaker is about 5 Gy cm2 (I have made up this figure, I am gettingsome better data). These are relatively infrequent procedures with normally fewerthan 2 patients per week. The average weekly DAP is therefore unlikely to exceed10 Gy cm2. The procedures rooms in which pacing is done tend to be relativelysmall so that less attenuation is provided due to ISL.

The following data have been assumed:• Distance to nearest wall - 1.5 m• Workload - 10 Gy.cm2 per week• Maximum scatter dose at 1 m - 6 µGy (Gy.cm2)-1

• Dose constraint - 0.3 mGy/year• Occupancy of adjacent areas - 25%• Average operating potential - 80 kV

Transmission calculation:

0 3

10 52 6 10 1

15

122%

3

2. .

× × × ××

=−

This can be achieved with 0.1 mm lead or 30 mm plasterboard. Using doublethickness plasterboard would therefore provide sufficient protection on a lightweight wall. In practice it is likely that only one area of the wall would require thislevel of protection. It might also be legitimate to assume that the person on theother side of the wall will be at a significantly greater distance than 1.5 m from thecentre of the x-ray beam.


Table 7.4.1. Typical doses from angiography and interventional procedures. Thesedata are averages from the following sources: (1) Vano et al, BJR, 68, 1215 (1995),(2) Marshall et al, BJR, 68, 495, (1995), (3) Steele and Temperton, BJR, 68, 452,(1993), (4) Williams, BJR, 70, 498, (1997), (5) Broadhead et al, BJR, 70, 492, (1997),(6) Zweers et al, BJR, 71, 672 (1997), (7) Betsou et al, BJR, 71, 634 (1997), (8)McParland, BJR, 71, 175 (1997), (9) Ruiz Cruces et al, BJR, 71, 42 (1997) and (10)Williams (personal communication).

Procedure No ofstudies

DAPGy cm2

DAP rangeGy cm2

Refs

Cerebral angiography 5 53 24 - 74 1,2,3,8,10Coronary angiography 5 40 23 - 67 1,5,7,10Abdominal angiography 4 120 61 - 180 4,8,9,10Femoral angiography 6 60 30 - 88 1,3,4,8,9,10Cerebral embolisation 3 110 105 - 122 2,8,10PTCA 5 55 13 - 88 1,5,7,10TIPSS 6 260 77 - 524 1,4,6,8,10

Table 7.4.2 Results of dose audit in 5 x-ray rooms used for high dose procedures.

Weekly workloadNo of cases DAP Gy cm2

Cardiac Lab – 1 44 1150Cardiac Lab – 2 35 1750Vascular studies 23 1700Hepato-biliary interventions 12 850Neuroradiology 11 450


7.5 CT INSTALLATIONS

7.5.1 INTRODUCTIONWhen a patient undergoes an examination in a CT scanner, the X-ray

tube is rotated in a 360o arc around the body. Tube potentials of 120-140 kVpare employed which give high levels of Compton scattered radiation. Thedose distribution is well defined and reproducible, because the position of thegantry is fixed and the X-ray tube follows the same path in space for eachrotation. The gantry provides shielding against the primary beam and thescatter pattern is determined by the volume of tissue irradiated and interveningabsorber in the form of the gantry and the patient’s body. Examinations of thetrunk tend to produce more scatter than head scans because a larger volumeof tissue is irradiated, but the level of scatter behind the gantry for head scansmay be greater because there is less tissue beyond the irradiated volume toprovide attenuation. The resulting pattern of scattered radiation has theappearance of an hour glass, which has 360o symmetry about the axis ofrotation ( figure 7.5.1). The dose distribution which depends on exposureparameters, beam collimation, filtration and gantry shielding is characteristicfor each type of scanner. Dose distributions provided by manufacturers shouldbe employed to determine shielding requirements. Because the magnitude ofthe dose distribution varies dramatically with position, the orientation of ascanner within a room must be decided before shielding requirements arefinalised.

7.5.2 METHODOLOGYInformation on scatter levels provided by manufacturers usually takes

form of isodose curves for a single slice using particular scan parameters andphantoms. Two drawings are required, one in the horizontal plane (floor plan)and one in the vertical plane (elevation). Sometimes only a single contour at aparticular dose level is provided or a sequence of doses measured at a rangeof positions at the same distance from the isocentre. In such cases it should beassumed that the dose declines with distance from the isocentre according toan inverse square law at distances beyond the limit of dose contour plots. Thedecline in scatter with distance from the isocentre may be plotted for criticaldirections to determine the dose level at relevant boundaries (figure 7.5. 2).Scatter diagrams provided by X-ray companies would normally have beenproduced using standard PMMA phantoms representing the head (16 cmdiameter) and body (32 cm diameter). Scatter from a body phantom may be20-100% higher than that from a head phantom when the same exposurefactors are used, as a result of the greater volume of tissue irradiated and theuse of different filter options. Self-shielding by the body is generally notincluded and may reduce the dose by 50% in certain directions. However, It isnot recommended that any adjustments are made to allow for this, since it willdepend on body size and the scanner will be used to scan phantoms duringQA tests.

In order to estimate dose levels from scatter plots, the workload in thedepartment must be predicted in terms of the scan parameters used.


Information on the likely breakdown of cases between head and trunk togetherwith typical numbers of slices for standard examinations is required from thedepartment. Data on recommended scan protocols in terms of mAs and kVshould be obtained from the X-ray company. There is little difference in scatterlevels for standard and helical scans if a similar pitch is used for each (Langerand Gray 1998), but since the time taken for helical scans may be 50% less,there is likely to be a larger patient throughput. Current helical scannerscannot achieve double the workload without reaching heat limits, so anassumption of a potential 50% increase in workload when a standard scanneris replaced by a helical one may be reasonable. Patient throughput mayincrease as tube technology improves.

Scatter plots are usually given for a single slice with a typical mAs orother specified mAs value. These should be used to determine doses at criticalpositions. These will be not only at nearest distances to individual barrierssuch as walls, doors and control cubicles, as in other X-ray rooms, but, moreimportantly, in directions of greatest scatter. It is helpful to sketch isodosecontours onto a scaled plan of the X-ray room in order to identify parts of thewalls exposed to the highest dose levels and so the directions and angleswhich are the most critical (figure 7.5. 2). These directions can then beidentified on the manufacturer’s isodose plot using a protractor and the scaleddistances read off for each dose contour. Results should be plotted in alogarithm-linear format (figure 7.5. 3) to determine the dose at the distance ofthe barrier. Isodose scatter curves are usually also provided in a vertical planethrough the scanner. It is important that consideration is given to shielding inthe floor and ceiling since the level of protection required for modern helicalscanners may be greater than that provided by a standard concrete floor(Langer and Gray 1998).

The scatter plots provide data on doses for a standard slice and thismust be multiplied by the number of slices in a given period and the ratio of theactual mAs to that used in deriving the scatter plot. The scatter dose is usuallydirectly proportional to the slice width, so a simple ratio adjustment can beused to relate results to a standard thickness (e.g. 10 mm). The same principlemay not be true for thin slices on some scanners (e.g. 1 mm), but thecontribution made to the total workload is usually small. Relative contributionsto scatter dose from different slice widths can be assessed from values of thecomputed tomography dose index (CTDI) which provides a measure of theradiation dose per mAs from one slice. If CTDI values are similar, then dosewill be proportional to slice width. As reference patient doses are set in termsof the weighted computed tomography dose index (CTDIW) using CTDI valuesmeasured at the periphery and centre of perspex whole body or headphantoms (CEC 1997) or dose length product (DLP), it may be possible tobase exposure parameters for standard examinations upon these values.

There are differences in scatter for head and body scans. These can beequated in terms of the ratio of scatter doses from one slice in each phantomfor a given mAs. This can be determined from the decline in scatter dose inseveral directions for the two configurations. It is simplest to relate all to a setof factors for which a scatter plot is available. The one representing the bulk of


examinations to be performed on the scanner should normally be used. Insome cases an isodose plot may only be available for either a head or bodyphantom. If this is the case and no other data is available on relative scatterlevels, conservative assumptions should be made. When only data for a headscan is available, conversion to the scatter dose for a body with similarexposure factors should be represented by multiplication by a factor of two,while when only data for a body scan is available, the same scatter dose permAs should be used for the head.

The dose permitted in each adjacent area, including occupancy factors,should be divided by the unshielded dose from the scanner at that position todetermine the transmission required for each barrier. Once the requiredtransmission has been established the thickness of a barrier with theappropriate transmission for 120 kVp or 140 kVp X-rays can be determinedfrom figure 5.10 or Table 5.7 These data on X-ray transmission were derivedusing the formula of Archer et al (1983) fitted to results derived by Simpkin(1990) using X-ray spectra typical of the primary beams of CT scanners at 120kVp (filtration 5.8 mm Al, half-value layer 6.9 mm Al) and 140 kVp (filtration 6.8mm Al, half-value layer 7.3 mm Al).

EXAMPLEA CT scanner in a busy city hospital is to be located in a room

measuring 6.5 m x 5.8 m (area 38 m2, Building Note 6) with the operator’scubicle in an adjacent room. The separation of floor and ceiling slabs is 4.0 m.Each is constructed from 140 mm thick lightweight concrete (density 1840 kgm-3). The scanner isocentre is located 0.9 m above floor level. Isodose curveshave been provided for a 120 kVp, 250 mAs, 10 mm slice on a 320 mmdiameter PMMA body phantom and a 350 mAs, 10 mm slice on a 160 mmdiameter head phantom.

The scanner is to be located towards the right hand side of the room asshown in the plan (figure 7.5. 2). Patients will enter through the double door onthe left hand wall. The scanner is positioned at an angle of 30o so that theoperator can obtain a good view of the lower half of the scanner. The doorfrom the cubicle into the room is located in the corner to minimise spacerequirements and the operator cubicle window extends for most of theremaining length of the wall to provide a complete view of the scanner andother door.

The projected workload for the CT scanner comprises 120 body and 80head examinations per week. An average body examination in the departmentcomprises 22 slices of 10 mm width at a pitch of 14 mm, with exposure factorsof 120 kVp and 250 mAs per slice.

Number of 250 mAs body slices per week = 120 x 22 = 2640 slices

An average head examination consists of ten slices of 10 mm width and fiveslices of 5 mm width, with exposure factors of 120 kVp and 350 mAs per slice.The isodose plots indicate that the scatter dose per mAs from a 10 mm slicethrough the head is half that from a slice through the trunk.


Number of 350 mAs, 10 mm head slices per week = [10 +(5 x 5/10)] x 80 = 1000

Equivalent no. of 250 mAs, 10 mm body slices = 1000 x 350 = 700 slices perweekfor head examinations performed each week 250 x 2

The total workload is equivalent to 2640 + 700 = 3340 body slices of 250 mAsand 10 mm width per week. Therefore a workload of 3340 slices per week withparameters similar to those used to obtain the isodose plots (figure 7.5. 2) hasbeen assumed. Calculation of the shielding requirement for one wall (A) (figure7.5. 2) is given below.

Wall AThe maximum dose contour extends at an angle of 150o directly

towards the wall which is at a distance of 2 m from the isocentre. The dose perslice at the wall, determined from the plot in figure 7.5. 3, is 2.0 µGy.

The dose per week from 3340 slices = 3340 x 2.0 µGy = 6680 µGy

The dose to other persons from the scanner is to be kept below 300 µSv peryear, which is equal to 6 µSv per week. The area beyond the wall is a viewingarea, where the occupancy is estimated to be 0.5.

The required transmission for the barrier = 6 = 0.00186680 x 0.5

This transmission is provided by 1.8 mm of lead or 160 mm of concrete(Chapter ?, figure 7.5. ?, Table ?).

RoofThe dose contours provided only extend to 1.5 m above the isocentre.

The highest dose contour along the length of the scanner corresponds to 2.1µGy from a single slice at a distance of 1.7 m from the isocentre in a direction36o from the vertical. The sensitive organs of a person on the floor above willbe 0.6 m above the ceiling if seated and so will be a height of 3.7 m above theisocentre. The distance along the line of greatest scatter will be (3.7/cos 36o)m = 4.6 m. Applying an inverse square law correction along the line of greatestscatter will give:

Dose from slice at distance 4.6 m from isocentre = 2.1 x 1.72/ 4.62 µGy = 0.28 µGy

The dose per week from 3340 slices = 3340 x 0.28 µGy = 935 µGy

The space above the CT room is a ward, where patients and staff could bepresent throughout the day, so an occupancy of 1.0 is assumed. If a dosecriterion of 6 µSv per week is applied.

The required transmission for the barrier = 6/935 = 0.0064This transmission would be provided by 120 mm of standard concrete(density 2350 kg m-3) (figure 7.5. 4) or 120 x 1840/2350 mm = 153 mm of


lightweight concrete.

The protection already present in the floor is 140 mm of lightweightconcrete. The thickness along the line of scatter is 140/cos 36o = 173 mm.Because radiation is incident obliquely, an equivalent barrier thickness equal tothe mean of the actual thickness and that in the direction of scatter isassumed, equal to 156 mm. Thus the protection provided by the structuralconcrete in the floor is sufficient. The thickness of barrier (B) required toattenuate radiation incident at angle θ can be calculated from the equation B =T ( 1 + cos θ ) / 2, where T is the thickness of material to provide the requiredattenuation. Thus for this example B = 153 x ( 1 + cos 36o) / 2 mm = 138 mmof lightweight concrete.

7.5.3 THINGS TO LOOK OUT FORRooms smaller than the 38 m2 recommended are frequently used and

here walls intercepting the direction of high scatter may need considerablygreater protection. It may in some cases be appropriate to include additionallead sheets localised to the area of high scatter (Harpen 1998), but care mustbe taken to ensure that additional protection of this type is located in thecorrect position.

Floor and ceiling slabs in new buildings are often thinner as well asbeing made from lightweight concrete and may require additional protection.Concrete floors and ceilings are often poured on a metal base with a ridgedcross-section, giving a trapezoidal variation in thickness, for which theminimum thickness, which should be used in calculating the protection, mayonly be 800 mm.

The height of shielding in walls must be given careful consideration asshielding to a height of 2 m is unlikely to be sufficient for a CT installation.Shielding would normally be to the full height of a room in directions where thescatter is high, since the level of radiation scattered from the ceiling slab intoan adjacent room may exceed the dose criterion set. This becomes of greaterimportance in rooms significantly smaller than the size recommended. Inrooms with false ceilings it may be necessary to extend the wall shielding tothe roof slab. In these configurations the level of protection required in theupper part of the room is unlikely to be as high as that in the low walls, since itis required primarily to offer protection against secondary scattered radiationand the process of scattering from a concrete barrier will reduce the dose-rateto about 1% of that incident on the ceiling slab (McRobbie 1997). If thedistance between the false ceiling and the ceiling slab is large it may be morecost effective to include the lead in the false ceiling, as this will provideadditional protection to both the floor above and adjacent rooms.

The control cubicle for a CT scanner would usually have a continuousshielded boundary, including a protected door. If a department does not wishto include a door, calculations must be performed to ensure that levels ofradiation scattered from adjacent walls will not result in the required dose-ratebeing exceeded.


REFERENCESArcher BR, Thornby JI and Bushong SC (1983). Diagnostic X-ray shieldingdesign based on an empirical model of photon attenuation. Health Physics 44,507-517.

Commission of the European Community (CEC) (1997) Quality criteria forcomputed tomography. CEC Working Document EUR 16262.

Harpen MD (1998) An analysis of the assumptions and their significance in thedetermination of required shielding of CT installations. Medical Physics, 25,194-198.

Langer SG and Gray JE (1998). Radiation shielding implications of computedtomography scatter of exposure to the floor. Health Physics 75, 193-196.

McRobbie DW (1997). Radiation shielding for spiral CT scanners. Brit. J.Radiol. 70, 226.

Simpkin DJ (1990). Transmission of scatter radiation from computedtomography (CT) scanners determined by a Monte Carlo Calculation. HealthPhysics 58, 363-367.

Figure CaptionsFigure7.5. 1

Isodose contour map showing the form of the scatter dose distribution in thehorizontal plane for a 10 mm slice through a head phantom using 120 kVpand 350 mAs.

Figure7.5. 2

Plan (a) and elevation (b) of CT room with isodose contours sketched in for a10 mm slice using 120 kVp and 250 mAs for a phantom. Walls for whichshielding calculations have been performed are labelled A-H.

Figure7.5. 3

Plot of scatter dose verses distance in the directions A - H in the plan shownin figure 7.5. 2.


Table 1 Data from calculation of shielding for each wall in example

Description Wall Wall Window Door Wall Door Roof Floor

Code A B C D E F G H

Use of adjoining room ViewingRoom

Office Operator Operator Corridor Corridor Ward PlantRoom

Distance to barrier(m)

2.3 2.2 3.5 4.6 4.2 4.3 3.1 0.9

Distance to nearestperson (m)

4.6 4.2

Dose per 250 mAsbody slice

2.0 2.1 0.43 0.20 0.43 0.056 0.30 0.36

Dose / wk (µGy) 6546 7014 1450 655 1450 187 1005 1188

Occupancy 0.5 1 1 0.5 0.2 0.5 1 0.2

Transmission 0.0018 0.0009 0.0042 0.018 0.02 0.064 0.0064 0.02

Lead Thickness (mm) 1.8 2.1 1.5 1.0 1.0 0.6 1.4 1.0

Concrete thickness(mm)

160 170 90 120 95

Lightweight concretethickness (mm)

138 110

Lightweight concretebarrier thickness(mm)

153 121


Figure 7.5.1


Figure 7.5.2 a


Figure 7.5.2b


0.01

0.1

1

10

100

0.01 0.1 1 10

1/(distance)2

Do

sep

ersl

ice

(mic

roG

ray)

A B

C

D

E

F

G H

Figure 7.5.3

7.6 DENTAL X-RAYSShielding design for dental radiology may be considered a trivial exercise incomparison to medical radiology because of the relatively low radiation doses.However, the problem should not be completely dismissed for a number of reasons.

• Dental surgeries are rarely purpose built. Surgery layout may be compromisedwith, for example, the x-ray set being used close to a wall.

• It is not uncommon for a dental practice to have a single set in a separate room toserve two or three surgeries. In these circumstances the designated room may bevery small.

• General Dental Practitioners own there own premises and additional expenditureon shielding may represent a significant increase over the cost of the x-ray set.


Intra-oral radiography

For intra-oral radiography, the following dose data may be used as guidance:

For a 60 kV set used with E-speed film, ESD should be approximately 2 mGy. Withcircular collimation, the maximum scatter dose at a distance of 1 m would be:

Scatter dose = 2 x 10-3 x 28 x 8 = 0.45 µGy per film

The primary beam in intra-oral radiography should always be intercepted by thepatient. Transmission of the primary depends on many factors. For the purpose ofshielding calculations it can be assumed that primary transmission is no greater than2 µGy per film.

Assuming that intra-oral films are taken with the beam pointing in one of threedirections (corresponding to the patient’s left, right and straight on), and that each ofthese directions is equally probable, then the weighted average primary plus scatterdose at a distance of 1 m is 1 µGy per film. It is recommended that this single value isused for shielding specifications unless there is good reason to suggest that the dosemight be significantly different.

Table 7.6.1 shows the required attenuation for a range of barrier distances and weeklyworkloads based on an annual dose constraint of 0.3 mGy.

An indication of the number of films taken in dental practice can be inferred fromNRPB (1994). Using data from the Dental Practice Board, it was reported that theaverage number of radiographs taken in the General Dental Service in the period1990 to 1993 was 16.47 million per year using 17,100 x-ray sets. This is equivalent to960 films per year or just under 20 films per week on each set.

The following conclusions may be made from the data in the table:

• No shielding is required if the workload is no more than 20 films per week and thedistance between the patient and the wall is at least 2 m.

• Surgery walls using brick or blockwork should provide sufficient protection in anycircumstance. At 70 kV (generally the upper kV limit for intra-oral radiography),100 mm concrete block with a relatively low density (1500 kg m-3) transmits lessthan 0.2%.

• Partition walls with 10 mm plasterboard on both sides will provide sufficientprotection in most circumstances. 20 mm of plasterboard has approximately 25%transmission at 70 kV. The Table indicates those situations in which this amountof shielding may be inadequate.

Entrance surface dose (ESD): 1 to 10 mGyDepends on projection, patient size, film speed, kV, focus-skin distance,

etc.

Field area at tip of spacer: 28 cm2 (6 cm diameter collimator)12 cm2 (rectangular collimator)

Dose area product (DAP): 1 to 30 cGy cm2

Based on ESD and area

Applied voltage: 50 to 70 kV

Scatter factor (S): 3 to 8 µGy (Gy cm2)-1


Panoramic radiology

For panoramic dental x-ray sets, shielding calculations need only be made for scatterradiation since the cassette and cassette holder provide sufficient shielding for theradiation transmitted through the patient.

DAP can be used for scatter dose calculation. For equipment working with rare earthscreens, DAP should be in the range 5 to 15 cGy cm2. The maximum scatter dosefactor for a fixed point can be calculated by integration of the scatter dose curve.Between 30° and 150° , i.e. through a 120° rotation representing half of the fullmovement, the average scatter factor is equal to 4.7 µGy (Gy cm2)-1 at 85 kV. Themaximum scatter dose at 1 m will therefore be 0.7 µGy per examination. However, itis likely that the unit will be mounted closer to the wall than this and incident doses atthe wall between 1 and 2 µGy per examination might be expected.

Example

Distance to wall = 70 cm

Scatter dose = 0.7 / (0.7)2≈ 1.4 µGy

Transmission through 20 mm plasterboard = 35% (at 85 kV)

Max. no of examinations per week without additional shielding

= 300/(1.4 * 0.35 * 52) = 12

In practice this is a conservative estimate since it is unrealistic to assumecontinuous occupancy directly behind the wall.

General

It may be necessary to consider shielding for windows and doors. A window with 5mm glass provides only 50% attenuation at 70 kV. However, in practice it is unusualto have high occupancy immediately outside a window and inverse square law isnormally a sufficient attenuator.

Internal doors could be a problem but these are also unlikely to be close to the patientunless the x-ray room is very small. In that situation, the operator may need to standoutside the door to be 2 m from the patient and he/she would control access to thearea outside the door.

For most intra-oral projections, the x-ray beam lies within 15° of the horizontal.Shielding to floors and ceilings is therefore only needed for scatter radiation. Evenwith a low floor-to-ceiling height (3 m) it is very unlikely that the workload would besufficient to require any additional shielding.

The most cost effective dose for additional shielding is generally the use of an extrasheet of plasterboard. This would only be needed over restricted areas of the walltowards which the beam might be directed and generally within a distance of 1.5 or 2m from the patient.

No account of occupancy has been used in these calculation. However, within thedental premises it is unlikely that there would be many adjacent rooms for which lessthan 100% occupancy can be assumed.

Reference

NRPB (1994) Guidelines on radiological standards for primary dental care. Docs ofthe NRPB, 5(3).


Table 7.6.1 Maximum transmission permitted for a dose constraint of 0.3 mSv peryear as a function of workload and barrier distance. Data based on average scatterplus primary dose equal to 1 µGy per film at a distance of 1 m from the patient.

Barrier distance (m)Films/week 1 1.5 2 2.5 3

10 0.58 - - - -20 0.29 0.65 - - -50 0.12 0.26 0.46 0.72 -

100 0.06 0.13 0.23 0.36 0.71200 0.03 0.06 0.12 0.18 0.35

7.7 MOBILE RADIOGRAPHY

Mobile X-ray Equipment – DRAFT 2

When it is not possible or advisable to move patients, such as those in NNU, PICU,ITU and resuscitation rooms, mobile radiographic equipment provides the onlyfeasible radiographic option. In most cases, the need for primary and secondary X-rayshielding will be obviated by the relatively low kV used, the relatively low levels ofscattered radiation produced and the infrequency of examinations. However, practicallimitations e.g. the unpredictable nature and urgency of procedures in recusitationrooms, should be taken into account. In many ways, as far as radiation shielding isconcerned, the use of mobile or ceiling suspended X-ray equipment (as in someresuscitation rooms) is analogous to the use of static X-ray equipment in X-raydepartments. Hence, shielding for surrounding walls, floors and ceiling can beassessed in a similar way to that used for radiography carried in a purpose designedX-ray room.

There are three potential sources of exposure of staff to ionising radiation: leakageradiation; primary radiation; scattered or secondary radiation.

Leakage RadiationWeight is an important consideration in the design of mobile X-ray equipment. Thespecified upper limit of 1mGy at 1m in one hour for radiation leakage is used as afixed constraint in the design of X-ray tube primary shielding. Hence, at comparable X-ray beam mean energies, the leakage radiation from mobile X-ray equipment will behigher than from most static X-ray equipment. Simpkin (Simpkin, 1998) has calculatedthat a primary shield thickness of 2.32mm, 2.20mm and 1.93mm of lead placedaround a diagnostic X-ray tube is sufficient to reduce the leakage air kerma to0.87mGy at 1m for a X-ray tube in continuous operation at respectively: 150kV,3.3mA; 125, 4mA; 100kV, 5mA. In table 1, figures adapted from Simkin (Simkin,1998) show the leakage radiation’s air kerma values at 1m (obtained with the threepreceding shielding thickness’) for a range of kVs and for an exposure of 10mAs.


kVp 2.32mm of Pb and10mAs (nGy)

2.20mm of Pb and10mAs (nGy)

1.93 mm of Pb and10mAs (nGy)

150 737 NA NA125 426 608 NA100 165 240 48780 3.0 5.2 15.960 <<1 <<1 <<1

Table 1. Unshielded leakage radiation air kerma values at 1m with the minimum X-raytube shielding in place for an exposure of 10mAs (adapted from Simpkin, 1998).

In most cases, apart from when equipment is used at the highest kV, leakageradiation may be neglected.

Primary and Secondary RadiationShielding for primary and secondary radiation is best considered in relation to thesituation in which the X-ray equipment is being used.

Mobile Radiographic Equipment used in NNU and PICUWorkload distributionsFigures 1 and figure 2 show workload distribution from one hospital’s NNU and PICU.The distributions have been compiled from over 700 records. Although these workloaddistributions should be treated with caution, it is not unreasonable to postulate that thedistributions are reasonably representative of the situation in other hospitals in the UK.

The mean kVs from the histograms are 65kV and 75kV (rough estimates). MeanDAP values have been estimated assuming a DAP value of 0.5 cGy cm2 for each X-ray.

Primary radiationThe patient, grid, cassette and bed will attenuate the primary beam significantly. Inaddition, there should be, with proper collimation, no possibility of a primary X-raybeam impinging directly surrounding structures.

Often a grid will not be used with paediatrics. However, exposure factors will bereduced to maintain the air kerma at the required level at the film surface.

(I suggest I obtain some typical cassette doses and do somemeasurements on the air kerma values on the exit side of cassettes. Ican then reinforce this with some reference to the lead equivalence ofcassettes etc. I can then make a statement similar to Jerry’s)


Secondary RadiationFigures 3 and 4 show plots of the scattered radiation produced from a chest X-ray onsubstitute anthropomorphic phantoms for a neonate and 5 year old child.

In general, compared to adult radiography, low kV and small scattering volumes leadto low levels of scattered radiation being produced. At the side of an incubator and atthe edge of a bed in PICU, referring to figure 3 and 4, scatter radiation levels for chestradiography are unlikely to substantially exceed a few hundred nGy per exposure. Forthe incubator, levels of scattered radiation at a distance of 1m from the centre of thephantom are insignificant (some value needed). Similarly, scattered radiation levelsat a distance of 1m from the edge of the PICU bed are likely to be less than 100nGyper exposure. On most units, X-rays will only be taken on a relatively infrequentbasis. Hence, ordinarily, there would seem to be little need for secondary radiationshielding. However, in NNU (although unlikely to receive significant radiation doses) itwould seem prudent, following the application of the ALARP principle, for staff whoare required to hold patients to wear protective lead coats.

Adult radiography on ITU, Wards and in resuscitation roomsWorkload figuresIn figure 5 a histogram of the types and frequency of exposures carried in onehospital’s ITU department has been included (Study underway). The mean kVfactors are ??. Mean DAP readings are also included in the figure (I can do this fromthe above histogram).

Primary RadiationThe calculations relating to primary radiation should be carried out in a similar way tothat in section ? (general X-ray)

Secondary RadiationFigure 6 shows data produced by Herman (Herman, 1980) for scattered radiation airkerma levels around an ITU bed from a chest X-ray taken at 80kVp. The air kermalevels at 1m from the centre of the phantom and at the side of the bed are of theorder of 400nGy per exposure. North (North, 1984) suggests that the most intensescattered radiation is back towards the source of radiation. He suggests that thescattered radiation intensities at 90 degrees to the beam direction are less than halfthose at 165 degrees. This statement broadly agrees with Williams (Williams, 1996). Acalculation following the method given in section ?? can be made:

Secondary radiation from mobile X-rays on ITUsRepresentative DAP: 0.1 Gy cm2

Work load: 10 chest X-rays a day. Factors: 80kV and 2mAsDistance: 2mMaximum scatter factor: 1.24 µGy ((Gy cm2)-1) (Williams, 96)

The maximum scattered radiation at 2m from the patient is of the order of 100 to 200nGy per exposure.

The need to consider the use of secondary shielding most often arises in the case ofresuscitation rooms. This is due to the proximity of walls and adjoining beds, types ofX-ray views taken, the number of staff involved and the urgency of procedures.

Domiciliary X-raysAlthough very unusual, radiography using low powered transportable X-ray equipmentis known to be carried out in patient’s homes. The requirement for radiation shielding,


outside the use of lead coats, is obviated by the one-off natures of these types of X-rays. However, the positioning of the patient and X-ray equipment should be carefullyconsidered and procedures should be fully referenced in local rules.

Secondary shielding for mobile X-raysIf an RPA considers that radiation shielding is required then the following approachesare recommended.

Inter-Cubicle (or bed bay) ShieldingIt is unlikely that it would be possible to designate one cubicle specifically where X-rays may be used. The question then becomes how to provide shielding economicallyfor a number of cubicles? Also, any shielding between cubicles must take account ofclinical staff’s need for easy access and for shelf space and visibility. The followingare offered as possible solutions:

• Fixed ScreensThese offer the most elegant solution. Windows and shelving can beinstalled as required. The main disadvantages are: the cost of providingscreens for a number of cubicles; clinical staff’s objections to the rigiddemarcation between cubicles.

• Mobile ScreensMobile screens offer a degree of flexibility and potential cost savings. A purpose-designed screen is shown in figure. Disadvantages include difficulties inpositioning and inconvenience.

< Figure 7. Photograph of premise’s screen>

• Lead CurtainsLead curtains have the advantage of ease of use, but the disadvantages of limitedlead equivalence and a relatively poor record for structural integrity.


Mobile Trailers

During the past few years, mobile trailers, with CT scanners and a limited number ofC-arm fluoroscopic units installed in them, have been increasingly used, usually on ahire basis, by Hospitals. Initially, the trailers were primarily used to provide a first lineservice. However, especially in the case of CT scanners, trailers are increasinglybeing used to add additional capacity and to provide continuity in service. As far asthe design of radiation shielding for mobile trailers is concerned the situation iscomplicated by restrictions in space and weight.

Shielding CalculationsThe appropriate methods to be used for shielding calculations have been covered in othersection (??? and ???) of this publication. In reality, it is very unlikely that an RPA will beasked to advise a coach building company on the requirements for radiation shielding duringthe design and building of a trailer. Usually an RPA will be faced with a fait accompli and mustfocus on the rules for the positioning and use of the trailer. Having said this, if shielding isbeing designed the following design consideration should be borne in mind.

Figures 1 and 2 show schematics of a typical CT and cardiac catheter laboratorytrailer.

< Schematics - already obtained >

Figure 1. Schematic of a CT mobile trailer(courtesy of Calumet Coach Company)

Figure 2. Schematic of a cardiac catheter laboratory mobile trailer(courtesy of Calumet Coach Company)

In relation to figure 1 and 2 the following points should be noted:

• when designing any shielding, reference should be made to national roadregulations governing weight per axial/tandem/spread and overall weightincluding the tractor unit;

• the floor of the trailer is 1.5m above the ground;

• available space is severely limited. One or two extendible sides are usually usedto increase available space;

• access to the underside of the mobile trailer is restricted by the trailer’sundercarriage;

• there is no easy access to the roof;

• the patient entry door is usually only accessible via a lift;

• there is usually a substantial amount of plant at the back end of the trailer;

RoofUnder normal circumstances and based upon the restriction of access and nobuildings directly overlooking the trailer, shielding should not be required in thetrailer’s roof.


FloorEssentially, the trailer’s floor can be divided into two areas:

• the area below which access is restricted by the trailer’s undercarriage. Shieldingshould not be required in this area;

• the areas, mainly below the extendible sides, to which access is possible. Theneed for shielding, to a certain degree, is obviated by the use of portable barriers.However, this approach is generally discouraged due to a lack of directsupervision. It would be prudent in this case to consider the installation ofshielding.

WallsRadiation shielding should be installed in the side walls that enclose the X-ray room.Shielding on the back wall is unlikely to be required due to the shielding effect of plantthat is usually attached in this area.

Control Room ScreenThe close proximity of the control room to the X-ray source should be consideredwhen deciding on thickness of shielding.

Operational ConsiderationsTrailers will need to be parked in an area with easy access to the hospital and apower supply. In the sighting of mobile trailers, the following additional point shouldbe considered:

• whenever possible, the trailer should not be sighted adjacent to buildings;

• the trailer should not be parked directly beneath buildings.


ReferencesWilliams J. R., 1996. ‘Scatter dose estimation based on dose-area product and thespecification of radiation barriers’. The British Journal of radiology, 69, 1032-1037.

Simpkin D. J.; Dixon R. L.; 1998. ‘Secondary Shielding barriers for diagnostic X-ray facilities:scatter and leakage revisited’. Health Physics, March 1998: Vol. 74, No. 3.

Herman M. W., Patrick J., Tabrisky J, 1980.; ‘A comparative study of scatteredradiation levels from 80-kVp and 240-kVp X-rays in the surgical intensive care unit’.Radiology 137:552-553, Nov 1980.

North D., 1985. ‘Patterns of Scattered Exposure from portable radiographs’. HealthPhysics, Vol. 49, No. 1 (July), 92-93

Other Thoughts

A checklist of questions to ask architects etc. would be a good thing.

Notes on Building Materials

Walls - these may be constructed in or lined with:

Brick: External quality, preferably without a “frog” or mortarholes.

Concrete Block: 100 or 150 mm.

Lead Sheet: Existing low density walls may be lined with sheet lead securedat regular intervals to reduce creeping. This is not the bestmethod of lead-lining and is not recommended generally.

Lead-backedBoards: These may be used for lining or structurally and are available in

plasterboard or plywood and different thicknesses of lead.They may be obtained ready-finished in melamine or preparedfor skimming with plaster or for decorating. Boards must bescrew fixed. Joints must be made over lead-lined battenshaving the same thickness of lead. Lead and lead-lined boardsare available in standard commercial thicknesses. Sometimes,thinner lead shielding is adequate, but special thicknesses,although available, are more expensive.

Mammography: A double layer of plasterboard, with staggered joints, issufficient. Alternatively, aluminium sheet may be used.

Barium Plaster: Brick or block walls may be coated in barium (X-ray) plaster.The plaster is heavy to apply.

For both bricks and concrete blocks, the density should be > 1850 kgm-3 and themortar beds and perpends must be well filled to avoid cavities. In general, 100 mmthick brick or block of the above density is equivalent to ~ 1mm lead.


7.8 BONE DENSITOMETRY

Bone Densitometry units have not been considered in the past a source of particularradiation protection problems. However, in the light of units with increasing patient andoperator doses e.g. fan beam units, this assumption must be reconsidered. Nospecific layout will be given and references only made to distances from protectedpersons.

Occupancy of surrounding areas with requisite shielding will be the overall planningconsideration. The specific requirements will be strongly dependent on the type ofDXA scanner used.

There should in general be no primary barrier requirements. The protection requiredwill be provided by secondary barriers or distance as required.

7.8.1 Methodology

Full occupancy should be assumed in the first instance. Only if the resulting barrierrequirements appear grossly excessive or excessively costly should the designassumptions need to be examined again.

Patel, Blake, Batchelor and Fogelman[1] have investigated the dosimetry of variousDXA scanner using a mixture of real patients and phantoms. Table 7.8.1 presentstheir data for spine and hip examinations.

In the case of the first two units A and B shown in Table 7.8.1 the dose limit would beachieved under the given workloads, in the latter case by increasing the distance tothe operator or the person to be protected. In the cases C and D additional shieldingwould be necessary

7.8.2 Example

A QDR 4500 is planned in a room where there is a waiting room for ante natalpatients the other side of a thin plasterboard partition barrier. Workloads of 100patients per week are expected. The nearest member of the public will be 2 metresaway from the scanner.

From Table 7.8.1 the annual unshielded dose at 1 metre would be 3.02 mSv perannum. At 2 metres this reduces to 0.76 mSv per annum. An attenuation factor of 0.4is therefore required for the barrier. This would be met by 1.32 HVLs of lead. Becauseof the heavily filtered nature of DXA radiation we will use data highly attenuatedbeams. From table 5.5 one can conservatively assume the HVL to be 0.28 mm lead.The required degree of shielding is therefore 0.37 mm lead, 23 mm concrete or 62mm gypsum wall board..

For operator protection, as recommended in the study by Patel et al [1] the distancefrom the unit must be increased to a least 2 metres to achieve a dose level of lessthan 1 mSv per annum. The dose constraint of 0.3 mSv per year could only beachieved by using a operator distance slightly in excess of 3 metres. Alternatively, theuse of a 0.5 mm lead protective shield would enable this requirement to be met.


The screens and doors may require protection, which should present no difficulties.The floor concrete density and thickness will need to be considered to verify that thespecified thickness will be adequate - in most cases no additional protection will berequired. Section 5 provides appropriate information.

References[1] Patel,R, Blake,G.M.,Batchelor,S,Fogelman,I - ref to follow.

[2] National Radiological Protection Board, Occupational Public and MedicalExposure, Guidance on the 1990recommendations of ICRP, Doc. NRPB, 4. No.2, 32-41, 1993, London, HMSO.

[3] International Commission on Radiological Protection (ICRP), Radiologicalprotection of the worker in medicine and dentistry, ICRP Publication 57, Annals of theICRP 20 No.3, 1989, Pergamon Press, Oxford.

Scanner (A) Lunar DPX (B) Hologic QDR1000

(C) HologicQDR 2000 plus

(D) HologicQDR 4500*

Dose per Scan(µSv)

0.01 0.08 0.42 0.58

No. patients perweek

100 100 100 100

Annual Dose at1 metre (mSv)

0.05 0.42 2.18 3.02

Attenuationfactor for 0.3mSv/year

not required 0.72 0.14 0.10

Table 7.7.1

draft bir shielding book june99t

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