Optimisation in digital radiography9 October 2015

Radiation Dose Metrics

Dr Tim WoodHull and East Yorkshire Hospitals NHS Trust

tim.wood@hey.nhs.uk

• ‘X-ray tube’ metrics• ‘Patient dose’ metrics• ‘Detector dose’ metrics• How do these compare and what do they tell

us?• Summary

Overview

• An understanding of a range of dose metrics that may be used in planar X-ray imaging

• An appreciation of the advantages and limitations of different dose metrics, and how they may be used (or misused) in clinical practice

Objectives

• What this talk is:– A brief overview of common dose metrics that are

used in planar X-ray imaging, and how these may be used (or abused) during optimisation

• What this talk is not:– A debate about the merits or the underlying

science around effective dose and risk from ionising radiation

Pre-amble

• Accelerating potential (kVp)– Higher kVp = greater tube output (more X-ray photons)– Higher kVp = greater penetration (X-ray photons will pass through more

material before being attenuated)• Tube current time product (mAs)

– Proportional to patient dose – double mAs, double patient dose• Filtration

– Greater filtration hardens the beam and reduces skin dose – common on modern interventional X-ray equipment

– May reduce radiographic contrast, and increase tube wear• Not always obvious how these come together to determine patient

dose– However, these are required if want to calculate effective dose

X-ray tube metrics

• Incident Air KERMA (‘free-in-air’)– The sum of kinetic energy of all charged particles liberated per unit mass– Can make direct measurement of the output of the X-ray tube and

determine how much ‘dose’ goes into the patient– Does not include backscatter– Can be used to calculated effective dose with software such as PCXMC

2.0

X-ray tube metrics

• Entrance surface dose– Includes backscatter, which can be a significant

contribution to skin dose– Can be measured directly with ionisation chamber

on phantoms, TLD on patient, film, calculated from incident AK using backscatter factors

– Published factors may be used to estimate effective dose• Software such as PCXMC uses the incident Air KERMA

(scatter free)

– A number of national DRLs (and reference doses) are given in this quantity – may be useful for patient radiation dose audits

– Not the most ‘practical’ quantity for audit

Patient dose metrics

• Dose/KERMA Area Product (DAP/KAP)– Absorbed dose to air/Air KERMA averaged over the

area of the beam in a plane perpendicular to the beams axis, multiplied by the area of the beam in the same plane

– Wide range of units used, so compare numbers with extreme care!• Gycm2, cGycm2, mGym2, etc.

– Backscatter is excluded– Invariant with distance– Either measured with a large area ionisation chamber

on tube output, or determined via calculation on some modern systems • Relies on accurate & regular calibration by Engineer

Patient dose metrics

• Advantages– Widely available– Real time display– Relatively direct

measurement (calculation) of patient dose

– Easily converted to effective dose with published factors or software (e.g. PCXMC 2.0)

– National DRLs are widely expressed in DAP

– Useful ‘practical’ quantity for dose audit

Dose/KERMA Area Product

• Disadvantages– Wide range of units used

• Gycm2, cGycm2, mGym2, etc.• Beware of unit conversion

errors!

– This number in isolation does not tell the whole story • A higher DAP does not always

mean a higher dose to the patient!

– Calibration accuracy? Table transmission?

• Effective dose– Effective dose is the tissue-weighted sum of the equivalent doses in all

specified tissues and organs of the body – It represents the stochastic health risk – the probability of cancer

induction and genetic effects – It takes into account the type of radiation and the nature of each organ

or tissue being irradiated– It is NOT a practical quantity to use in the X-ray room

• There is no bolt on ‘effective dose meter’!

– It is an important consideration in any optimisation study• Remember, ALARP consistent with the intended purpose!

– Has to be calculated from other quantities taking into account beam spectrum, patient size, etc.• Need DAP, kVp, filtration, etc.

Patient dose metrics

• Detector air KERMA– Image noise is determined by the number of X-ray photons reaching the

detector, so detector dose is a prime target for optimisation– Often measured as part of routine QA– Important consideration for the configuration of the Automatic Exposure

Control (AEC) on any DR/CR system– BUT, consider what the number you measure actually means and how it

compares with others!– Lots of different techniques are used in the UK

• Solid state (no backscatter) versus ionisation chamber (with backscatter) measurement?

• Scatter-free (copper at the tube head) versus scatter (PMMA or water on table) geometry?

• Position of dose meter – corrections for grid transmission, inverse square law, etc.?

Detector dose metrics

Detector air KERMA

• All digital detectors will have a detector dose index to indicate the level of exposure

• All manufacturers use a different number to define this– E.g. EI, lgM, etc– There are moves to standardise this, but this has not yet happened

• The way this varies with dose is also different for each DDI– E.g. logarithmic, linear, exponential, …

• This makes the DDI very difficult to use for optimisation studies

• BUT DDI is still useful for monitoring system performance and checking for drift over time– i.e. Dose creep!

Detector dose indices

Optimisation in digital radiography9 October 2015

How do these dose metrics compare, and what do they tell us?

Lets look at a case study…

• Quite a complex task for optimisation due the wide range of anatomy– Large areas of low attenuation

– lungs– High attenuations regions –

spine, heart, diaphragm• Need to think very carefully

about how different parameters interact when seeking to optimise!– And now for some ‘back of an

envelope calculations’…

PA Chest X-ray

• Some common techniques for PA chest include;– 60 kVp, 10 mAs– 85 kVp, 4 mAs– 120 kVp, 2 mAs

• If we assume a typical general X-ray system, with 3.5 mm Al filtration…

• Air KERMA incident on the patient is;– 60/10 = 0.090 mGy– 85/4 = 0.076 mGy– 120/2 = 0.076 mGy

• Looks like 85 or 120 kVp will be best as less radiation into the patient…

X-ray tube metrics

• Entrance surface dose may be calculated from Air KERMA, but similar trend still apparent– Low kVp = higher skin dose– High kVp = lower skin dose– Is this actually a problem for a chest X-ray?

• Assume: same collimation and FDD of 180 cm;• Patient DAP is;

– 60/10 = 0.10 Gycm2

– 85/4 = 0.085 Gycm2

– 120/2 = 0.085 Gycm2

• Again, 85 or 120 kVp look like the best choice as lower DAP – This would look better in a patient dose audit, and when compared to

national DRLs!

Patient dose metrics

• Lets calculate effective dose… (using PCXMC 2.0)– 60/10 = 0.009 mSv– 85/4 = 0.012 mSv– 120/2 = 0.016 mSv

• So now it looks like actually low kVp should be the winner as it presents the lowest ‘risk’ to the patient (~25%)!

• What if this was an AP Chest x-ray?– 60/10 = 0.020 mSv– 85/4 = 0.022 mSv– 120/2 = 0.027 mSv

• Less significant effective dose saving (~10%) at low kVp as radiation sensitive organs are closer to the entrance surface!

Patient dose metrics

• BUT optimisation is not just about getting the lowest dose • What does the low kVp, low effective dose technique mean

for image quality?• Estimate detector dose (Air KERMA free in Air) with SRS78• With 110 mm ICRU Soft Tissue in primary beam (lung like)

– 60/10 = 5.7 µGy– 85/4 = 6.5 µGy– 120/2 = 7.9 µGy

• With 180 mm ICRU Soft Tissue (diaphragm like)– 60/10 = 0.7 µGy– 85/4 = 1.1 µGy– 120/2 = 1.6 µGy

Detector dose metrics

Approx. 14% higher for 85 kVp c.f. 60 kVp

Approx. 60% higher for 85 kVp c.f. 60 kVp

• So, low kVp gives lowest effective dose, despite the higher DAP and skin dose, but also yields the lowest detector dose

• Will this compromise image quality and give images that are too noisy?– If not, does this mean that the high

kVp techniques could just use lower mAs to give a more similar effective dose?

• There is another factor to consider…– The detector!

Detector dose metrics

Detector dose metrics

60 70 80 90 100 110 120 130 140 1500

5

10

15

20

25

30

35

CRDR (CsI)

kVp

Effici

ency

(%)

• CR plates are less efficient at absorbing X-rays than DR flat panels (CsI based)– DR can use lower mAs than CR at any

given kVp• Detector efficiency drops with kVp

for CR– 60 kVp ~18.0%– 85 kVp ~15.5%– 120 kVp ~13.5%

• So for CR, 60 kVp is ~15% more efficient than 85 kVp and ~33% more efficient than 120 kVp

Detector dose metrics

60 80 100 120 1400

5

10

15

20

25

30

35

CR

DR (CsI)

kVpEffi

cien

cy (%

)

• So for CR, reduced detector dose is offset by improved detector efficiency at 60 kVp (in the lung)

• 60 kVp will also benefit from improved radiographic contrast than high kV techniques!– Images may be noisier in the

diaphragm & spine – does this matter?• For DR, 60 kVp and 85 kVp yield

similar efficiency – Need to determine if lower noise at 85

kVp outweighs the improved contrast at 60 kVp

• In Hull, we have investigated this via computer simulation and found from observer studies (and now from clinical practice) that 60 kVp gives superior image quality for AGFA CR for matched effective doses

• This simple and common example of a PA Chest X-ray emphasises the point that no single dose metric can tell you all you need to know when optimising exposure protocols

PA Chest X-ray

• There are a range of dose metrics available that may aid the process of optimisation

• No single dose metric will tell you the full story!– Use each one in isolation with extreme caution e.g. during patient dose

surveys• All factors should be considered in the optimisation process,

and these should be combined with an appropriate assessment of image quality

• Remember, common patient dose metrics don’t always accurately reflect the ‘risk’ to the patient– e.g. a higher DAP reading from the introduction of a new technique

doses not necessarily mean the patient is getting a higher effective dose!

Summary

Optimisation in digital radiography9 October 2015

Thanks for listening

Any questions?