scatter xray radiation in concrete

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A study of scatter in diagnostic X-ray rooms

G MCVEY, D Phil and H WEATHERBURN, PhD

Medical Physics Department, The Churchill, Oxford Radcliffe Hospital NHS Trust, Headington, Oxford OX3 7LJ, UK

Abstract. An important part of determining the radiation protection requirements during X-ray room design isthe calculation of the amount of scatter inside and outside the planned locations of the shielding barriers. In thiswork, a Monte Carlo code has been developed to calculate the percentage scatter so that the current data can beconsolidated and new data can be provided as required. Calculations have been compared with measurementsto show that they are representative of scatter found in X-ray rooms. Scatter from the dose–area product meter

and the collimator system were found to provide large contributions to the measured scatter. A fluoroscopyroom containing a C-arm X-ray set was modelled with the Monte Carlo code. The scatter dose was calculatedat an X-ray room entrance behind the protective screen, acting as a secondary barrier, at the radiographer’sconsole. The variation of scatter with the position of the protective screen was studied. An empirical calculation,

which provided reasonable agreement with the Monte Carlo calculations, was found by using new data for thevariation of scatter from concrete with field size. If a door was placed in the X-ray room entrance behind theprotective screen, the results showed that it would not need to be lead lined.

The doses at points outside the planned locations for

shielding barriers, such as walls and doors, have to be

calculated as part of the radiation protection design

process for an X-ray room. This design has to take into

account primary, scatter and leakage radiation.

Appropriate data are obtained from various reports [1–4].

The older reports [1–3] provide data on the amount of

scatter and the transmission of primary radiation which

were measured using X-ray units with single phase

generators. These data need to be updated as the use of

modern X-ray units with medium frequency generators isbecoming increasingly common. The differences in the

penetrating power of the X-rays produced by the different

generators prompted Archer et al [5] and Simpkin [6, 7] to

produce new transmission data for primary and leakage

radiation. These new transmission data are mutually

consistent. A recent BIR report [4] presents new scatter

data which were measured by Williams [8]. However, this

work does not appear to be consistent with the earlier

reports. There is considerable variation of Williams’

scatter values in the forward and backward directions

compared with the older measured values [9, 10].

Therefore, scatter distributions in X-ray rooms are

investigated in this paper.A Monte Carlo code XYZSCAT, based on the EGS4

code system [11], has been developed for this purpose. The

scatter from blocks of solid water and concrete have been

calculated and compared with measurements carried out in

a fluoroscopy room at the Churchill Hospital, Oxford to

show that they are representative of scatter in a clinical

X-ray room. The comparison was carried out for tubevoltages between 49 kV and 121 kV, for scattering angles

between 45˚ and 150˚ and for field areas between 100 cm2

and 900 cm2. The XYZSCAT code calculates the scattercontributions from different components modelled in theX-ray room, for example, the dose–area product (DAP)meter. This work highlights the significant contributions tothe total scatter from materials other than the patient.

Sutton and Williams [4] provide worked examples for awide variety of shielding problems which may beencountered. However, they only give limited advice onthe common shielding problem of how to estimate the dosebehind the protective screen at the radiographer’s consoleproduced by scatter from the ceiling. Instantaneous dose isdifficult to measure at this position. Thus, a practical

benefit of Monte Carlo calculations is that they canprovide data for these situations. A fluoroscopy room atthe Royal Marsden Hospital, London was modelled withthe XYZSCAT code and the dose calculated at a roomentrance behind a protective screen at the radiographer’sconsole. In the past, an empirical calculation would havebeen used to estimate the scatter behind the protectivescreen. This method estimates the scatter from the concreteceiling as 5% at 1 m [4] using a 10 000 cm2 field area [1].

The XYZSCAT code has been used to provide new databy calculating the scatter from concrete for varying fieldarea. The methods described in this work are generallyapplicable and may be used for radiographic rooms aswell.

Methods and materials

Output and scatter measurements

Measurements were carried out in a fluoroscopy room at theChurchill Hospital, Oxford. The X-ray generator used was aPhilips Medio 65 CP-H (Philips Medical Systems, Eindhoven,

The Netherlands)with a Philips overcouch X-ray tube SRO 33100 with a 13˚target angle, 3.4 mmAl total filtration and a 3%peak voltage ripple. All measurements were performed in

Received 27 November 2002 and in final form 4 June 2003, accepted 7 August 2003.

Current address for G McVey, North Wales Medical Physics, GlanClwyd Hospital, Bodelwyddan, Denbighshire LL18 5UJ, UK.

Current address for H Weatherburn, Princess Noorah OncologyCenter, King Abdulaziz Medical City, PO Box 9515, Jeddah 21423,

Kingdom of Saudi Arabia.This work has been supported by a grant from Anglia and OxfordHealth Authority.

The British Journal of Radiology, 77 (2004), 28–38 E 2004 The British Institute of Radiology

DOI: 10.1259/bjr/93969091

28 The British Journal of Radiology, January 2004



radiographic mode. The incident air kerma (without back

scatter) and half value layers were measured using a Keithley

15 cm3 ionization chamber (Keithley Instruments, OH)

connected to a 37D electrometer (Pitman Instruments Ltd,

Surrey) for tube voltages between 49 kV and 121 kV. The

chamber had a calibration traceable to PTB (Physikalisch-

Technische Bundesanstalt), Germany. A DIGI-X instru-

ment using the Keithley chamber, remote controlled with‘‘oRTIgo’’ software (RTI Electronics AB, Molndal,

Sweden), was used to measure the X-ray tube filtration

and voltage waveform ripple.Figure 1 shows the experimental set up used to measure

the scatter dose. A solid water block (WT1 material) or an

aerated concrete block was positioned against one of walls

in the X-ray room, at a distance of 100 cm from the X-ray

tube focus. The WT1 material was employed as a scatterer

to simulate the patient as it is commonly used as a tissue

substitute. The aerated concrete block was used as its

composition is similar to the materials used in the con-

struction of X-ray rooms although it had a lower density

(0.48 g cm23) than standard concrete. Barytes plaster is

required to provide adequate shielding if aerated concrete

blocks are used in the construction of an X-ray room.

The centre of the field was aligned with the centre of the

phantom’s incident surface area using the light field. The

dimensions of the WT1 block were 30.5 cm long, 30.5 cm

wide and 10 cm thick and the dimensions of the concrete

block were 44.3 cm long, 21.3 cm wide and 10 cm thick.

Measurements of scattered air kerma were carried out with

a Pitman 35 cm3 ionization chamber connected to the

37D electrometer. This chamber also had a calibration

traceable to PTB. The centre of the chamber volume was

positioned 100 cm from the centre of the field size on the

incident surface of the phantom. Measurements were

obtained at a scattering angle of 135˚ with tube voltagesbetween 49 kV and 121 kV for an incident field area of

400 cm2 and with a tube voltage of 121 kV for field areasof 100 cm2 and 900 cm2.

Figure 2 shows the experimental set up used toinvestigate the scatter contributions from the X-ray tubehead. The solid water block was positioned in the centre of the X-ray room. Measurements of scattered air kermawere undertaken at scattering angles of 45 , 87 , 120 , 135˚

and 150˚ with a tube voltage of 69 kV for an incident fieldarea of 400 cm2. These measurements were undertakenwith and without a mobile shield placed between the X-ray

tube head and the detector. The mobile shield was placedas close as possible to the X-ray tube head to ensure that itdid not block any scatter from the phantom reaching thedetector. Thus for the measurements at the 120 , 135˚ and150˚ scattering angles, the shield is placed away from thephantom and for the measurements at the 45˚ and 87˚scattering angles, the shield is placed closer to thephantom. Transmission measurements were carried outwith the mobile shield to ensure that the scatter from theX-ray tube head was reduced to a negligible value.

The leakage from the X-ray tube head was also

measured and the readings of scatter dose were correctedif it was non-negligible. The percentage scatter was found

as the ratio of the scattered air kerma to the incident airkerma without backscatter. The overall uncertainty in thismeasured quantity was ¡11%. The large uncertainty was

due to the difficulty in reproducing the position of thechamber in a large dose gradient as the scatter variesrapidly with both scattering angle and distance from thephantom.

The XYZSCAT Monte Carlo code

The XYZSCAT code was developed for this study bymodifying the XYZDOS code [12]. Figure 3 shows an

Figure 1. The experimental set up used to measure and calcu-

late the scatter from a phantom supported against one of thewalls with the X-ray tube focus and detector 1 m from the

incident surface of the phantom. DAP, dose–area product.

Figure 2. The experimental set up used to measure and calcu-

late the scatter from a phantom in the centre of the room and

to study the effect of scatter from the X-ray tube head. The

positions of the detector and mobile shield are shown for themeasurements carried out at scattering angles of 45 , 87˚ and

135 . DAP, dose–area product.


29The British Journal of Radiology, January 2004



example of the model that can be used with the

XYZSCAT code where voxels define a slab phantom

and the X-ray room walls. The XYZSCAT code transports

photons from the X-ray tube focus to the phantom from

where the photons are scattered throughout the X-ray

room. Scattering will also occur from other materials in

the X-ray room. The program calculates the air kerma in

all voxels specified as air. The scattered air kerma is found

by combining fluence with the scattered photon energy and

the mass energy transfer coefficient [13] for the scattered

photon energy. The scattered fluence is calculated by using

the tracklength of the photons traversing the voxel and

dividing by the voxel volume [14, 15]. A voxel size of

10 cm610 cm610 cm was used to score the fluence. The

radial distribution of scatter is found by using voxels with

their centre at 1 m from the phantom surface at scattering

angles between 30˚ and 150 . The percentage scatter is

found by dividing the scattered air kerma with the incident

air kerma without backscatter.The XYZSCAT code was used to simulate four models

in this paper which are described below. The XYZSCAT

calculations were carried out with between 506106 and4006106 photons divided into 10 batches in order to

achieve a statistical uncertainty of ¡1% for the simulation

of scatter distributions surrounding slab phantoms

(models 1 to 3). The statistical uncertainty achieved forthe simulation of scatter behind the protective screen(model 4) was ¡3%. These calculations were carried outwith 2406106 photons divided into 10 batches. TheXYZSCAT code samples a lagged-Fibonacci randomnumber generator [16] to provide the large number of photon histories required for all of these simulations. The

photon histories were followed down to an energy of 1 keV. The interactions simulated included photoelectricabsorption with the production of K fluoroescent photons

for lead, Compton scattering without electron bindingcorrections and coherent scattering. Electrons were nottransported using the EGS4 code. The photon interactioncoefficients, in the energy range between 1 keV and200 keV, were prepared for use with the XYZSCATcode by the PEGS4 program [11].

Models used with the XYZSCAT code

The first model was used to compare the calculated

and measured scatter from blocks of WT1 material andaerated concrete as shown in Figure 1. The Philipsfluoroscopy room was modelled with dimensions of 6 mlong, 6 m wide and 3 m high. The simulated blocks of

scattering material have the same dimensions as those usedin the measurements. Table 1 shows the compositions anddensities of all materials used in the calculations. Thecomposition and density of the WT1 material wasobtained from White et al [17]. The walls, ceiling andfloor were simulated as 20 cm thick concrete with astandard density of 2.35 g cm23. The composition of concrete was obtained from Simpkin [6] with either themeasured density of the aerated concrete blocks or the

standard concrete densityused. TheDAP meter (Physikalisch-Technische Werkstatten, PTW type B 727085, Freiberg,Germany), used to monitor DAP, was included in thecalculation model. The dimensions of the meter are

16.2 cm long, 18.1 cm wide and 1.7 cm thick. It isconstructed from layers of Perspex 0.2 cm thick with anair gap between them. The DAP meter was modelled as asolid block of Perspex with a composition given by theInternational Commission on Radiation Units andMeasurements (ICRU) [18], with an average density of 0.319 g cm23 found by the mass of Perspex divided by thevolume of the DAP meter. The air in the X-ray room wasincluded in the simulation. Its composition and densitywere given by the National Physics Laboratory (NPL) [19].

The calculations were carried out for tube voltagesbetween 49 kV and 121 kV with a field area of 400 cm2

and for field areas between 100 cm2 and 900 cm2 at a tube

voltage of 121 kV.

Figure 3. An example of the voxel geometry that can be used

to define the patient (WT1 material), the surrounding air and

the X-ray room walls (concrete) with the XYZSCAT Monte

Carlo code.

Table 1. The compositions and densities of materials used in the Monte Carlo calculations

Material Density

(g cm23)

Elemental composition (percentage by mass)

H C N O Na Mg Al Si S Cl Ar K Ca Fe Sb

Air 0.1205 0 0 78.1 21.0 0 0 0 0 0 0 0.9 0 0 0 0

WT1 1.015 8.1 67.3 2.4 19.8 0 0 0 0 0 0.1 0 0 2.3 0 0

Concrete 0.48 or 2.35 0.6 0 0 49.8 1.7 0.2 4.6 31.6 0.1 0 0 1.9 8.3 1.2 0

Soft tissue 1.00 10.1 11.1 2.6 76.2 0 0 0 0 0 0 0 0 0 0 0

Perspex 0.319 8.0 60.0 0 32.0 0 0 0 0 0 0 0 0 0 0 0Carbon fibre or wood 0.50 or 0.30 6.2 44.4 0 49.4 0 0 0 0 0 0 0 0 0 0 0

G McVey and H Weatherburn




The second model was used to investigate the scatterfrom the X-ray tube head as shown in Figure 2. The

calculations were carried out for tube voltages between

49 kV and 121 kV with a field area of 400 cm2.The third model simulates the scatter from a concrete

barrier (with a standard density of 2.35 g cm23) with

varying field area as shown in Figure 4. The calculations

were carried out for 140 kV X-rays incident on a 20 cm

thick concrete barrier with square field areas increased

from 4 cm2 to 40 000 cm2. The calculations were under-

taken with a plane parallel beam and with a divergent

beam with a focus–surface distance (FSD) of 100 cm.

The fourth model simulates a fluoroscopy room

containing a C-arm X-ray set at the Royal MarsdenHospital, London as shown in Figure 5. The patient’s

torso was modelled as a slab of average soft tissue (90 cm

long, 22 cm wide and 18 cm thick) supported by a carbon

fibre couch top (220 cm long, 56 cm wide and 1 cm thick).

The calculation model includes the DAP meter (PTW type

B), concrete ceiling, floor and walls (all 20 cm thick,2.35 g cm23 density), a false plywood ceiling (1.3 cm thick,

0.3 g cm23 density) and a protective screen (2 mm lead

equivalence). The data for average soft tissue were

obtained from the ICRU [20] and the data for woodand carbon fibre were obtained from the NPL [19].

Undercouch, overcouch and lateral orientations of theX-ray tube were simulated for the barium enema, barium

swallow and other examinations including barium meals

and nephrostomy. The X-ray set would not normally beused in an overcouch orientation but it has been included

for completeness. Table 2 shows the imaging parameters

for the different examinations and tube orientations. Thenumber of examinations per week represents the workload

in a busy department [4]. The fluoroscopy time was

obtained from the recommended national reference doses

for complete examinations [21]. The incident dose rates at

the patient entry point, shown in Table 2, are typical for apatient thickness of 20 cm [22, 23]. Large field sizes were

employed in the calculations to produce a large amount of

scatter and hence provide a conservative estimate of thedose at the X-ray room entrance. For a C-arm X-ray set,there is an image intensifier present behind the phantom

which has 2 mm lead equivalence in order to absorb

primary radiation. The intensifier has not been modelled

so that the results of the calculations are also applicable toradiographic rooms.

The amount of scatter reaching the X-ray room entrance

is dependent on the position and size of the protective

screen at the radiographer’s console. Thus, the effect on

the scatter of varying the distance of the screen from theX-ray room walls and ceiling and the height of the screen

has been studied. The scatter dose was calculated in order

to decide whether a door at the X-ray entrance situatedbehind a protective screen is required to be lead lined. Theamount of shielding x in the door required to provide a

dose less than a dose constraint Dc of 0.3 mSv per year is

found by using Equation 1 and increasing x until the dose

(a) (b)

Figure 5. The calculation model of the Philips fluoroscopy room at the Royal Marsden Hospital: (a) the overhead view with theX-ray tube in a lateral orientation and (b) the lateral view with the X-ray tube in the overcouch orientation. The patient access to

the X-ray room is not shown in the figure. DAP, dose–area product.

Figure 4. The geometry for the calculation of scatter at a 150˚scattering angle for 140 kV X-rays incident on a 20 cm thick

concrete barrier.





behind the door D(x) is less than Dc.

D(x)~DeT e(x)zDsT s(x)zDoT o(x) ð1Þ

Equation 1 takes into account the leakage and scatter doseat the X-ray room entrance from the barium enema,barium swallow and the other examinations (De, Ds andDo, respectively) and the corresponding transmissionsT e(x), T s(x) and T o(x) because the different examinationshave different tube voltages. The transmission data forlead and wood (0.55 g cm23 density) were obtained fromSimpkin [7]. The dose behind a typical door of 4.4 cmthick wood [5] was also calculated using Equation 1.

Empirical calculation of scatter behind the protective

screen

The XYZSCAT calculations of scatter at the X-rayroom entrance behind the protective screen were compared

with an empirical calculation described by Equation 2 [24].

S ~100Apatient

100

0:0005

d 2ceiling

!Aceiling

100

0:0005

d 2entrance

!ð2Þ

where: Apatient is the field area incident on the patient;

Aceiling is the nominal irradiated area of the ceiling; d ceilingis the distance from the patient to a position on the ceilingabove the protective screen; d entrance is the distance from

the ceiling to a point at the X-ray room entrance.Equation 2 calculates the percentage scatter, S , in two

parts. In the first bracket, the amount of scatter from thepatient incident on the ceiling above the protective screenis calculated. The scatter factor is 0.0005 at 1.0 m from apatient for a field area of 100 cm2. Table 2 shows thevalues Apatient for each examination and tube orientation.d ceiling is 2.96 m. In the second bracket, the scatter fromthe ceiling reaching the X-ray room entrance is calculated

for two positions. The first position is at a gonad height of 0.7 m above the floor (d entrance53.66 m). The secondposition is at an eye height of 1.7 m above the floor(d entrance53.05 m). For simplicity, the same scatter factorof 0.0005 at 1 m from the ceiling is used for a field area of 100 cm2 [1]. Aceiling is chosen from measured or calculated

data for the variation of scatter with field size and will bediscussed later.

X-ray spectra

The X-ray spectra used in the XYZSCAT code wascalculated using the Birch and Marshall Model [25]. Thespectra for the Philips X-ray tube were calculated using itsX-ray tube characteristics mentioned above. A detaileddescription of the X-ray spectra calculations is given byMcVey [26].

Results and discussionScatter from tissue equivalent phantoms

Table 3 shows the comparison of the measured scatterwith the values calculated using the first model (Figure 1)for X-rays incident on the solid water phantom. Thescatter is compared at a 135˚ scattering angle for tubevoltages between 49 kV and 121 kV and for field areasbetween 100 cm2 and 900 cm2. Table 4 compares the

measured scatter with the values calculated using thesecond model (Figure 2). The first set of results in Table 4compares the measured and calculated scatter for a400 cm2 field area for different tube voltages andscattering angles. There is reasonable agreement betweenthe measured and calculated scatter due to the largemeasurement uncertainty. However, the calculated scatterconsistently underestimates the measured value. This isprobably due to scatter from the X-ray collimators not

Table 2. Imaging parameters for the examinations carried out with a Philips C-arm system

Parameter type Image system parameters

Barium enema Barium swallow Other examinations

Tube voltage (kV) 100 85 90

FSD (cm) 82 87 82

Field size (Apatient) (cm6cm) 25625 (Frontal) 1566 25625 (Frontal)

25618 (Lateral) (Frontal and lateral) 25618 (Lateral)Workload (exams week21) 25 12 13

Time/examination (min) 2.7 2.3 3.8

Dose rate (mGy min21) 11.4 9.8 10.1

Annual dose (mGy year21) 38360 13560 25310

FSD, focus–surface distance.

Table 3. The comparison of the measured and calculated scatter from the WT1 material phantom for field areas between 100 cm2

and 900 cm2 and for tube voltages between 49 kV and 121 kV

Tube voltage (kV) Percentage scatter for different field sizes

10 cm610 cm 20 cm620 cm 30 cm630 cm

Measured values Calculated values Measured values Calculated values Measured values Calculated values

49 — — 0.176 0.144 — —

69 — — 0.229 0.181 — — 121 0.0742 0.0597 0.288 0.235 0.593 0.517





accounted for in the XYZSCAT model and is discussed inmore detail in the next section. A rigorous validation of

the XYZSCAT code is described by McVey [26].The measured and calculated scatter is briefly compared

with the values given in the literature. For scatteringangles less than 90 , the measured and calculated scatter(Table 4) tends to be larger than that reported by Troutand Kelley [9] and Williams [8]. This is due to the solidwater block used in our work being thinner than thephantoms used in the published work. For scatteringangles greater than 90 , the measured and calculatedscatter (Tables 3 and 4) is substantially lower than thevalues of Williams [8] and greater than those of Trout andKelley [9] as their experimental arrangement did notinclude a DAP meter. Our values are lower than those of

Williams as in his experimental arrangement the X-raytube head was closer to the phantom which resulted in agreater scatter contribution from the DAP meter.

Both the measured and the calculated scatter have asimilar variation with tube voltage and field area. Thescatter at the 135˚ scattering angle increases by 64% forincreasing the tube voltage from 49 kV to 121 kV. There isalso a linear relationship between the scatter and field areafor slab phantoms. This agrees with the work of Bomfordand Burlin [10] for scatter in the backward direction.Trout and Kelley [9] did not find a linear relationship,which was possibly due to their use of a human torsoshaped phantom. This implies that the variation of scatter

from patients is not linear with field area and needs furtherinvestigation [26].

The effect of scatter from the surrounding materials in

an X-ray room

The second set of results in Table 4 shows the measured

scatter with the mobile shield in place (Figure 2) andcalculations carried out without the DAP meter includedin the simulation. There is reasonable agreement betweenthe measured and calculated percentage scatter due to thelarge measurement uncertainty. In Table 4, the differencesbetween the measured and calculated scatter are decreasedby shielding out the scatter from the X-ray tube head.

The scatter from the X-ray collimators could not readilybe modelled with the XYZSCAT code. Therefore, the

scatter produced by the collimator system was estimated asthe difference between the measured scatter without theshield and the scatter calculated with the DAP meter

included in the model. From Table 4, the scatter producedby the collimator system and its associated uncertainty are

estimated to be between 0.026¡0.008% to 0.077¡0.033%.Therefore, it is a significant proportion of the total

measured scatter. The estimated collimator scatter issimilar to the value of 0.04% measured by Trout andKelley [27].

Figure 6 shows the calculated scatter from the DAPmeter for tube voltages of 49 kV and 121 kV. It variesbetween 0.025% and 0.085% for scattering angles between30˚ and 150 . This variation appears to be independent of tube voltage. The scatter from the DAP meter can also beestimated from Trout and Kelley [27] data. They measuredthe variation of scatter from different thicknesses of Perspex placed in front of the collimator system. Thus for

4 mm thick Perspex simulating the DAP meter, theestimated scatter is 0.09%, which agrees well with thecalculated values.

Scatter from concrete barriers and blocks

Table 5 compares the scatter from a concrete barriercalculated using the third model (Figure 4) with the valuesgiven by the HMSO Handbook of Radiological Protection[1] for scattering angles between 120˚ and 150 . The

Table 4. The first comparison (columns 3 and 4) is between the measured and calculated scatter with varying tube voltage and scat-

tering angle. The second comparison (columns 5 and 6) is between the scatter measured with the mobile shield in position and the

values calculated with the dose–area product (DAP) meter not included in the computation

Tube voltage

(kV)

Scattering

angle ( ˚ )

Percentage scatter

Measured

values

Calculated

values

Measured

values with

mobile shield

Calculated

values without

DAP meter

49 135 0.181 0.140 0.136 0.126

69 45 0.137 0.090 0.0916 0.0731

69 87 0.0783 0.0524 0.0267 0.0304

69 120 0.195 0.143 0.149 0.125

69 135 0.237 0.181 0.183 0.167

69 150 0.252 0.269 0.229 0.207

121 135 0.312 0.234 0.242 0.221

Figure 6. The variation of scatter from the dose–area productmeter at the scattering angle positions 1 m from the phantom

surface for tube voltages of 49 kV and 121 kV.





average of the HMSO scatter factors corresponds to thevalue of 0.0005 used in Equation 2. The table also shows

the comparison of the measured scatter from an aeratedconcrete block with the values calculated using the firstmodel (Figure 1). All values are for a field area of

100 cm2. The HMSO measured values are considerablylarger than the other values as they will include scatter

from the surroundings and they apply to a range of tubevoltages between 100 kV and 300 kV. The calculated andmeasured scatter from the aerated concrete block is largerthan the calculated values from the concrete barrier asthey include scatter from the DAP meter. There is verygood agreement between the measured and calculatedvalues for scatter from the aerated concrete block.

Variation of scatter from a concrete barrier with field

size

Figure 7 compares the variation of the HMSO [1]percentage scatter and the XYZSCAT calculated valueswith field area for a plane parallel beam and a divergent

beam with a FSD of 100 cm. The HMSO data weremeasured for tube voltages between 100 kV to 300 kV at a160˚ scattering angle. Figure 7 shows that the shapes of themeasured and calculated curves are very different as

the field area increases from 4 cm2 to 40 000 cm2. Thecalculated scatter values are 6.3% and 4.3% for the plane

parallel and divergent beams, respectively, for a field areaof 40 000 cm2. The calculated scatter has a linear

relationship with field size for areas less than 2500 cm2.The HMSO values show a linear relationship with fieldsize for areas below 4000 cm2 and reach a maximum value

of 3.6% for a field area of 18 000 cm2. It is difficult toknow the reason for the large difference in the shape of the

calculated and published curves as the HMSO report givesvery little information on the experimental arrangementused to measure the scatter.

The variation of scatter at a room entrance behind the

protective screen at the radiographer’s console

For all studies in this section, a barium enemaexamination (Table 2) was simulated with the X-raytube in a lateral orientation to the patient. The scatterwas calculated at eye and gonad height for an individualstanding in the centre of the X-ray room entrance. Theradiosensitive organs in the human body all lie within this

range of heights. Figure 8a shows the variation of scatterat the X-ray room entrance with the depth of theradiographer’s console, i.e. the distance between thescreen and the wall with the room entrance (Figure 5).At the minimum depth investigated of 78 cm, the scatter at

eye height is 10% lower and the scatter at gonad height is40% lower than the scatter values for the screen at a depthof 178 cm. At the maximum depth investigated of 268 cm,the scatter at eye height is 14% higher and the scatter atgonad height is 29% higher than the scatter values for thescreen at a depth of 178 cm. There are some oscillations inthe scatter for the screen at distances of between 229 cmand 245 cm from the wall. This is possibly due to the largegradient in the scatter distribution surrounding the DAP

meter (Figure 6). At these positions, the screen is close tothe DAP meter, which is at a distance of 236 cm from thewall. The decrease in the scatter reaching the roomentrance, shown in Figure 8a, may result from the scatter

from the DAP meter being locally absorbed by the screen.The increase in scatter at the room entrance for increasingdepth is probably due to the position of the screen beingcloser to the patient—the primary source of scatter. Forthe X-ray tube in the overcouch and undercouchorientations, the calculations show the expected contin-uous increase in scatter for increasing depth of the screen.The overall variation in scatter is smaller than might havebeen anticipated considering the large variation in the area

behind the protective screen.Figure 8b shows the variation of scatter at the X-ray

room entrance with the width of the radiographer’s

Figure 7. The variation of scatter from a concrete barrier cal-

culated with the XYZSCAT code for a parallel beam and adivergent beam (focus–surface distance (FSD)5100 cm) and the

values given by the HMSO [1] for 100 kV to 300 kV X-rays.

Table 5. Comparison of the calculated and measured scatter at 1 m from concrete barriers or blocks for a 100 cm2 field area

Source kV or kV range Percentage scatter from concrete barriers

120˚ scattering angle 135˚ scattering angle 150˚ scattering angle

HMSO [1] 100 to 300 kV 0.0440 0.0520 0.0660

3rd XYZSCAT Model (Figure 4) 140 kV 0.0144 0.0200 0.0248

Percentage scatter from concrete blocksSource kV or kV range

120˚ scattering angle 135˚ scattering angle 150˚ scattering angle

Measurements 121 kV — 0.0318 —

1st XYZSCAT Model (Figure 1) 121 kV 0.0243 0.0284 0.0463





console, i.e. the distance of the screen from the right handwall. At the minimum width investigated of 100 cm, the

scatter at eye height is 14.3 times higher and the scatter at

gonad height is 22.6 times higher than the scatter values

for the screen with a width of 270 cm. At the maximum

width investigated of 350 cm, the scatter at eye height is

0.92 times lower and the scatter at gonad height is 0.95

times lower than the scatter values for the screen with a

width of 270 cm. Thus, screen widths greater than 200 cm

provide adequate protection of the X-ray room entrance

and as the width decreases then it will no longer be

adequate. There is considerably greater variation in scatter

with changing the width of the protective screen than the

depth of the radiographer’s console due to the X-ray

entrance becoming increasingly exposed to patient scatter

as the width is reduced.

Figure 8c shows the variation of scatter at the X-rayroom entrance as the height of the X-ray room is varied

with the height of the protective screen being kept constant

at 200 cm, i.e. the gap between the screen and ceiling is

varied. At the minimum gap distance of 53 cm with the

suspended ceiling touching the screen, the scatter at eye

height is 51% lower and the scatter at gonad height is 42%

lower than the scatter values for a gap distance of 141 cm.

The scatter at eye height reaches a maximum for a gap of

100 cm which is 4% higher than the scatter value for a gap

of 141 cm. The scatter at gonad height is similar for gaps

between 90 cm and 220 cm. At the maximum gap distance

investigated of 300 cm, the scatter at eye height is 29%

lower and the scatter at gonad height is 18% lower than

the scatter values for a gap of 141 cm. For gaps between

53 cm and 100 cm, the scatter at eye height increases as

Figure 8. The variation of scatter at the X-ray room entrance behind a protective screen for (a) increasing the depth of the radiogra-

pher’s console area, i.e. the distance between the screen and the wall containing the room entrance; for (b) increasing the width of

the radiographer’s console area; for (c) increasing the distance between the protective screen and the ceiling; and for (d) increasing

the height of the protective screen.





the area on the ceiling from which the scatter can reachthis position increases more rapidly than the square of thedistance from the ceiling. For gaps between 100 cm and300 cm, the scatter decreases as the area on the ceilingincreases less rapidly than the square of the distance fromthe ceiling.

Figure 8d shows the variation of scatter at the X-ray

room entrance when the height of the protective screen isincreased from a minimum height of 200 cm [4] until ittouches the suspended ceiling at a height of 288 cm. With

the protective screen touching the suspended ceiling, thescatter at eye height is 74% lower and the scatter at gonadheight 61% lower than the scatter values for a screenheight of 200 cm. The scatter values at eye and gonadheight tend to converge for increasing screen height. Thisimplies that the solid angle into which the scatter from theceiling can reach both the eyes and the gonads is similarfor a screen height of 288 cm.

Scatter dose at a room entrance behind the protective

screen at the radiographer’s console

Table 6 shows the total scatter dose in mGy per year atthe X-ray room entrance calculated by the XYZSCATcode. The dose for each examination is calculated by thepercentage scatter times the incident dose given in Table 2and then summed to give the total for each projection. TheXYZSCAT calculated dose is compared with the scattereddose calculated empirically by Equation 2. The problemwith using Equation 2 is that the area on the concreteceiling Aceiling has to be chosen. If the whole area of

the ceiling was used then the percentage scatter reachingthe entrance may be unrealistically large. Therefore in thepast, a field area of 10 000 cm2 [1] was chosen which

corresponds to a scatter value of 5% at 1 m [4]. This fieldarea was chosen as the scatter values given by the HMSO[1] did not increase for areas above 10 000 cm2 (Figure 7).However, the XYZSCAT calculated values increase forfield areas above 10 000 cm2 (Figure 7) and therefore, afield area of 35 000 cm2 was also chosen for use inEquation 2.

Table 6 shows the Monte Carlo calculated values arelarger than both the empirically calculated values in mostcases except for the undercouch orientation for the35 000 cm2 field area. Empirical calculations have pre-viously been the only method of estimating the dose at theX-ray room entrance and it is of concern that the older

data (10 000 cm2 field area) give dose values which aresignificantly smaller than the Monte Carlo values. The

empirical calculation using the larger field area of 35 000 cm2 provides an estimate of the dose at theentrance similar to that calculated by the Monte Carlomethod. However, this comparison is only valid for thespecific situation shown in Figure 5 as for example,increasing the screen height would significantly reducethe scatter (Figure 8d).

Table 6 shows that the Monte Carlo calculated dosesare larger at eye height than at gonad height which impliesthat the X-rays are being scattered from the ceiling. The

doses are larger for the overcouch orientation of the X-raytube than for other projections as most X-rays arebackscattered from the patient to the ceiling. The scatterfrom the undercouch orientation is similar to the scatterfrom the lateral orientation. There was no rapid increasein dose for the undercouch orientation where the primarybeam is pointed at the ceiling. This shows that the primarybeam is attenuated greatly by the patient. The annual dosedue to leakage radiation at the X-ray room entrancebehind the protective screen was conservatively estimatedto be 0.18 mGy at gonad height. This was empirically

calculated using the maximum leakage dose rate of 1 mGy h21 at 1 m [28] from the X-ray tube assembly

and the field area on the ceiling of 35 000 cm2. It can beseen that the dose due to scatter is slightly larger than thatdue to leakage radiation.

Table 7 shows the thicknesses of lead and wood thatwould be required to reduce the scatter and leakage dosesto the dose constraint of 0.3 mSv per year. The table alsoshows the annual transmitted doses through a typical44 mm thick wood door [5]. A significant amount of shielding is required at eye height and when the X-ray tubeis in the overcouch position. However, this is only atheoretical study and the door would not be shielded forthese situations. It can be seen that the average dosesbehind a typical wooden door are close to the dose con-straint. Therefore, it would be possible to have a doorwithout lead lining at the room entrance providing that theprotective screen is used as a secondary barrier.

Conclusions

Data for estimating the amount of scattered radiationare essential when designing a diagnostic X-ray facility. Ithas often been observed that there are insufficient data todo this properly [4]. Monte Carlo models provide amethod for determining appropriate data [26].

The scatter values calculated by a Monte Carlo codehave been compared with the scatter measured in a clinical

Table 6. The total scatter doses calculated by the XYZSCAT code and the empirical calculations using field areas on the concrete

ceiling of 10 000 cm2 and 35 000 cm2. The annual doses have been calculated for lateral, undercouch and overcouch orientations of

the X-ray tube using the total incident dose for each orientation

Tube orientation Height Total annual dose (mGy year21)

XYZSCAT

calculation

Empirical calculation

with 10000 cm2 area

Empirical calculation

with 35000 cm2 area

Lateral Gonad 0.23 0.064 0.22

Eye 0.41 0.092 0.32

Undercouch Gonad 0.24 0.087 0.31

Eye 0.41 0.13 0.44

Overcouch Gonad 0.32 0.087 0.31Eye 0.63 0.13 0.44





X-ray room. This comparison was more difficult than

expected owing to the scatter from surrounding materials

in the X-ray room. The DAP meter and collimator system

were shown to give large contributions to the measured

scatter in the X-ray room. Further experimental work is

necessary to find the source of the small remainingsystematic differences between the calculated and mea-

sured scatter.Monte Carlo models are an accurate method for

calculating the dose where instantaneous dose measure-

ments are difficult to undertake. The Monte Carlo model

has been employed to calculate the dose at the X-ray room

entrance behind a protective screen. The current method of

estimating this dose is to use empirical calculations.

However, this method using HMSO data [1] has been

shown to underestimate significantly the Monte Carlo

calculated dose. A reasonable empirical calculation may

be used by increasing the field area to give results

consistent with the Monte Carlo calculated data forthe specific situation simulated. However, this may not

be the case for other situations, for example a different

screen height, as the calculated dose decreases with

increasing screen height. Finally, Monte Carlo calculations

of dose at the X-ray room entrance show that if a door

was placed in this position then it would not need to be

lead lined.

References

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Table 7. The thicknesses of lead and wood required to reduce the total of the scatter (Table 6) and leakage doses at the X-ray

room entrance to less than a dose constraint of 0.3 mSv per year for the lateral, undercouch and overcouch orientations of the

X-ray tube. The table also shows the total dose behind a typical door constructed of 44 mm thick wood placed in the X-ray room

entrance and the average dose behind the door for each orientation of the X-ray tube

Tube orientation Height Door thickness to achieve the dose constraint Dose behind a typical wooden door

Lead thickness (mm) Wood thickness (mm) Dose (mGy) Average dose per tube

direction (mGy)

Lateral Gonad 0.019 39 0.29 0.38

Eye 0.056 100 0.47

Undercouch Gonad 0.021 42 0.30 0.38

Eye 0.056 99 0.46

Overcouch Gonad 0.032 62 0.35 0.48

Eye 0.084 137 0.62





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for the protection of persons against ionising radiations

arising from medical and dental use. London: HMSO, 1988.



scatter xray radiation in concrete

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