1. First FRCR Examination in Clinical Radiology Diagnostic Radiology & Radionuclide Radiology (4b) Patient Dosimetry John Saunderson Radiation Protection Adviser
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2. RCR Syllabus
Methods
Diagnostic reference levels (including high dose techniques)
Magnitude and measurements
Radiation detectors and dose meters
Measurement of absorbed dose and dose rate in air .
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3. Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air
Ionisation of air detectors
Ionisation chambers
Geiger-Muller tubes
Proportional counters
Others
Scintillation detectors
Solid state (e.g. diodes)
Stimulated luminescence (TLD / OSLD)
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4. Ionisation Chambers Conducting anode (+) and Cathode (-)
Typically between 100 and 400 V between
When irradiated, some air atoms are ionised
Positive ions attracted to cathode, negative to anode
A current flows and can be measured
Current  absorbed dose to air rate
1 coulomb of charge in dry air is released by joules of absorbed energy (no need to learn this!)
joules ÷ kg of air in chamber = grays absorbed dose to air

5. But we are interested in dose to tissue not to air?

6. Dose to air vs dose to tissue?
i.e. from 10 keV to 100 MeV ratio of absorbed dose to air to absorbed dose to tissue within around +/- 3%

7. Examples of current measured by an ionisation chamber
1 coulomb of charge in dry air is released by joules of absorbed energy
Fluoro patient entrance dose of 10 mGy/min
0.4 nanoamps through a 60 cc ionisation chamber
0.04 nA through a 6 cc ionisation chamber
i.e. very small currents (hence very delicate instruments, cable and connectors)
The bigger the chamber, the more sensitive
Detailed working out (not needed for FRCR exam!!)
Fluoro patient entrance dose of 10 mGy/min measured by a 60 cc ion chamber
60 c.c. air = 78 microgrammes of air
10 mGy per minute = 10 mJ/kg per min = 0.78 J per 78 g per min
0.78 J/min ÷ J/C = 23 nC per minute = 0.4 nA

8. Ionisation Chambers
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9. Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air
Ionisation of air detectors 
Ionisation chambers 
Geiger-Muller tubes
Proportional counters
Others
Scintillation detectors
Solid state (e.g. diodes)
Stimulated luminescence (TLD / OSLD)
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10. Geiger-Muller (GM Tubes)
Low pressure gas, high voltage (1000 V)
When irradiated, some gas atoms are ionised
High voltage accelerates electrons andions towards electrodes, giving them more energy
These ionise more gas atoms causing a cascade
A pulse is produced, which is not dependant on the energy of the radiation in

11. GM Tube Energy response i.e. “grays per click”

12. GM Tube uses
Very sensitive to small amounts of radioactive material, so good contamination monitor
Thin window can detect alpha particles
If energy compensated, sensitive dose rate meter, for a particular range.
Can be very small and still sensitive, so good for pocket dosemeters.

13. Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air
Ionisation of air detectors 
Ionisation chambers 
Geiger-Muller tubes 
Proportional counters
Others
Scintillation detectors
Solid state (e.g. diodes)
Stimulated luminescence (TLD / OSLD)
21/09/2018

14. Proportional counter
Half way between an ionisation chamber and a GM-tube
The size of the “click” is proportional to the energy of the photon

15. Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air
Ionisation of air detectors 
Ionisation chambers 
Geiger-Muller tubes 
Proportional counters 
Others
Scintillation detectors
Solid state (e.g. diodes)
Stimulated luminescence (TLD / OSLD)
21/09/2018

16. Scintillation Detector
Good contamination monitor
Pulse height gives energy of X-ray photon
Can also be used for dose rate meter

17. Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air
Ionisation of air detectors 
Ionisation chambers 
Geiger-Muller tubes 
Proportional counters 
Others
Scintillation detectors 
Solid state (e.g. diodes)
Stimulated luminescence (TLD / OSLD)
21/09/2018

18. Silicon diode detector
Act like ionisation chamber
Silicon much more dense than air, so can be a lot smaller than air ionisation chamber
Used for testing X-ray sets
Need to correct for energy to get tissue dose.

19. Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air
Ionisation of air detectors 
Ionisation chambers 
Geiger-Muller tubes 
Proportional counters 
Others
Scintillation detectors 
Solid state (e.g. diodes) 
Stimulated luminescence (TLD / OSLD)
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20. Methods General radiology Fluoroscopy Computed Tomography
Nuclear Medicine .
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21. General radiology ESD Dose-Area Product (DAP) Effective dose .
Thermoluminescent dosemeter (TLD)
exposure factors
Dose-Area Product (DAP)
Effective dose .
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22. T.L.D. Crystals, e.g. lithium fluoride
Radiation causes electrons to be caught in “traps”
At lab. TLDs heated to 240oC
Electrons released, light emitted
Amount of light emitted proportional to dose .
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23. T.L.D.s +/- Small Tissue equivalent Read 1 Gy Easy for radiographer
No direct readout
Sensitive to heat, UV, dirt
Tricky to calibrate
Easy to loose
Special ones needed for low dose (e.g. chest) .
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24. Dose Area Product
Because dose falls with 1/d2 and area increases with d2, DAP is independent of distance.
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25. DAP +/- No fiddly TLD for radiographers! Only one number to record
Instant answer
Doesn’t take into account backscatter
Initial cost (several thousand £)
Units sometimes cause confusion (cGy.cm2, or Gy.cm2, Gy.m2) .
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26. Effective dose Complicated to calculate from ESD or DAP
Can use computer models which make assumptions on
field size
patient size
field position
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32. Fluoroscopy Dose-Area Product (DAP) Exposure factors Effective dose .
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33. DAP +/- for fluoroscopy
Instant answer, etc.
DAP moves with the tube
Gives good indication of relative risks of inducing cancer
Not directly linked to erythema risk.
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34. Exposure factors
Based on assumed FSDs, etc.
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35. Effective dose
Can be “fudged” using radiograph software.
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36. GAFCHROMIC film
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39. GAFCHROMIC film optical density is proportional to the absorbed dose
0.01 Gy to 50 Gy
Energy independent from 30 keV to 30 MeV
£20 per 14” x 17” sheet

40. Computed Tomography CT Dose Index (CTDI) Dose-length Product (DLP)
Effective dose
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41. CT Dose Index (CTDI) Applies to a single slice Can be used to compare
different slice widths
different physical filter
different scanners
etc.
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42. Dose-Length Product (DLP)
DLP = CTDI x n x T
Gives an idea of relative dose for a whole scan
Can be used to compare effect of pitch, etc.
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43. Effective dose
NRPB program similar to radiography one
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45. CT dose/risk calculation
e.g. wrong patient referred for CT scan (laryngectomy)
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49. Diagnostic Reference Levels
Early 80’s survey
DRLs today
IRMER
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50. Mid-80’s survey Method Survey of twenty UK hospitals
13 most common views
For each patients (60-80kg) at
DAP or ESD by TLD measured.
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51. Mid-80’s survey Results E.g. Chest PA Median ESD = 0.18 mGy
Minimum ESD = 0.03 mGy
Maximum ESD = 1.43 mGy
Max / min = 48 !!.
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53. Mid-80’s survey Recommendation
Use 75th percentile as reference value
i.e. carry out local surveys and take action if average dose is greater than ¾ of national survey doses
e.g. for chest PA reference = 0.3mGy ESD
Send results to NRPB to review national reference doses every 5 years.
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54. DRLs today
A DRL is essentially a guide to the rather indistinct border between “good and normal practice” and “bad and abnormal practice”.
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55. IRMER National DRLs Local DRLs set as 3rd quartile
average from survey of “standard patients”
should be below DRL
Local DRLs
Usually use average of room averages for Trust
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57. National DRLs
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60. Staff and Environmental Monitoring
Devices
TLD
OSLD
Electronic
Body
Extremity
Environment
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61. Relevant measurement techniques
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62. Measuring Dose Luxel dose badges TLD finger rings
Can be cold sterilized
Heat sensitive
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63. Luxel badges Wear underneath lead rubber apron
Assume dose to badge = effective dose
Can be worn for 2 weeks to 3 months (usually 1 month)
Must be returned promptly.
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64. Electronic Dosemeters
Used by Radiation Physics staff to test x-ray unit and measure environmental doses
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65. f i n
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