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2013 version

2013 version. First FRCR Examination in Clinical Radiology General Radiation Protection (3h) John Saunderson Radiation Protection Adviser. General Radiation Protection.

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2013 version

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  1. 2013 version

  2. First FRCR Examination in Clinical RadiologyGeneral Radiation Protection(3h)John SaundersonRadiation Protection Adviser

  3. General Radiation Protection

  4. Medical and Dental Guidance Notes A good practice guide on all aspects of ionisingradiation protection in the clinical environment • 240 pages, £20 (discount for bulk purchase!) • Buy from http://www.ipem.ac.uk “an essential reference book for all those working with ionising radiation in medical or dental practice, including medical and dental staff, radiographers, scientific and technical staff, and their employers.”

  5. Medical and Dental Guidance Notes 1. General measures for radiation protection 2. Radiation protection of persons undergoing medical exposures 3 - 4. Diagnostic & interventional radiology 5 - 6. Dental radiology 7- 9. Radiotherapy 10-18. Nuclear medicine and other uses of radioactive materials (+ Appendices 1 - 21)

  6. Radiation protection of the patient • Justification -  • Optimisation -  • Limitation - X

  7. Ways to Optimising Patient Doses in Radiology • minimise exposure time • minimise radiation field size • maximise tube voltage (kV) • maximise beam filtration • maximise tube to patient distance (FSD) • minimise pulses per second • minimise acquisition images • minimise use of CT • have a good quality assurance programme

  8. OPTIMISATIONField Size • Don’t irradiate more tissue than necessary • Larger fields = • larger doses to patient • larger doses to staff (from scattered radiation) • more scatter, resulting in more blurred images Some examples of poorly collimated paediatric chest radiographs

  9. OPTIMISATIONExposure Time • Don’t use X-rays at all if other non-radiation techniques can reasonably be used • Don’t irradiate for longer than necessary • If you are not looking at the monitor stop screening • Use the freeze frame facility whenever appropriate

  10. OPTIMISATIONX-Ray Tube Voltage (kV) (more detail in lecture 4) On some sets kV can be selected by the operator. On fluoroscopy sets it is normally selected automatically, but different user selectable settings will affect how the automatic kV works. Higher kV radiation is more penetrating, so lower intensity beam into patient required to give same intensity out of patient to imager

  11. From NIST Physical Reference Data (http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html)

  12. Photoelectric Absorption •   m x Z3 / E3 •  = linear attenuation coefficient for PE effect • m = mass density (kg/m3) • Z = atomic number • E = photon energy

  13. Compton Scattering •   m x e / E •  = linear attenuation coefficient for PE effect • m = mass density (kg/m3) • e = electron density (e- per kg) [Z/A] • E = photon energy

  14. From NIST Physical Reference Data (http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html)

  15. OPTIMISATIONX-Ray Tube Voltage (kV) (continued) Higher kV radiation is more penetrating, so lower intensity beam into patient required to give same intensity out of patient to imager Therefore, higher kV = lower patient dose e.g. changing up from 90 to 110 kV may lead to a 26% reduction in skin dose changing down from 90 to 70 kV may lead to a 62 % increase in skin dose So at lower kV values the threshold for erythema can be reached in a shorter screening time On some sets “low dose mode” forces the X-ray set to use higher kV settings, and “high contrast mode” forces it to use lower kV settings.

  16. OPTIMISATIONX-Ray Tube Voltage (kV) (continued) • Example – a 1 mm wire in the trunk, comparing 90kV to 110kV beam • Skin dose 26% lower for 110 kV • Effective dose 10% lower for 110 kV • Contrast 15% higher for 90 kV But, because a higher kV beam is more penetrating then it will also less contrast between different materials Therefore, higher kV = less contrast You must have sufficient contrast to see what you need to in order to perform the clinical procedure, but a higher than necessary contrast can result in a higher risk of patient skin reactions, a higher risk of cancer induction in patients and staff, and a higher risk of exceeding your eye or other dose limit which could prevent you working with X-rays until the following year.

  17. OPTIMISATIONBeam filtration (more detail in lecture 4) All X-ray sets must have a metal filter (typically around 2 mm of aluminium) to remove very low energy X-rays which would never penetrate the patient and reach the imager, but would significantly increase the dose to the patient. Most modern fluoroscopy sets also have removable copper filters of a few tenths of a millimetre built into the X-ray tube housing. On some fluoroscopy sets the added copper filtration can be selected by the operator. On others it is normally selected automatically, but different user selectable settings will affect how much filtration is automatically driven into the beam.

  18. OPTIMISATIONBeam filtration (continued) On some sets “low dose mode” forces the X-ray set to use extra copper filters, and “high contrast mode” forces it to use no copper filter. • Example – a 1 mm wire in the trunk in a 90kV beam, • comparing no copper filter with using a 0.1 mm copper filter • Skin dose 29% lower with 0.1 mm copper filter • Effective dose 4.3% lower with 0.1 mm copper filter • Contrast is 4.5% higher for no copper More filtration effectively means the average energy of the X-rays is higher, and so with more filtration the beam is more penetrating. In exactly the same way as higher kV beams give lower patient skin dose but also lower contrast, more filtration will do exactly the same. Use the maximum filtration you can while still maintaining adequate image quality for the task you are undertaking.

  19. Transmission through 10 cm tissue • 80 keV  16 % • 60 keV  13 % • 50 keV  10 % • 40 keV  7 % • 30 keV  2 % • 20 keV  0.04 % • 15 keV  0.000008 % • 10 keV  10-21 %

  20. Minimum Filtration • General tube  2.5 mm aluminium • Mammography  0.03 mm molybdenum or 0.5 mm Al • Dental ( 70kVp)  1.5 mm Al • Dental (> 70kVp)  2.5 mm Al

  21. Tube to Patient Distance

  22. Tube to Patient Distance Greater FSD = lower patient dose e.g.  from 50 to 70 cm  49% skin dose Greater FSD = less magnification (so fewer distortions) Tube to patient distance never < 30cm, preferably > 45cm for chests > 60 cm.

  23. Fluoroscopy • Only expose when looking at monitor • Keep patient close to image intensifier and far from tube (at least 30 cm from tube for mobile, 45 cm for static) • Use low dose setting, unless image unacceptable (i.e. high kV, high filtration) • Magnification increases dose rate to skin (although a smaller area irradiated) • Cone down where practicable • Special care if skin dose likely to exceed 1 Gy.

  24. Entrance Dose Rates for Standard Phantom

  25. Time to Reach 2 Gy for Standard Phantom

  26. Skin dose rate (mGy/min) Remedial level: 50 mGy/min for largest field (standard patient) Suspension level: 100 mGy/min (standard patient)

  27. Fluoroscopy Dose Modes • “Low dose” • higher kV (lower mA) • more copper filtration • therefore, lower contrast • “High contrast” • lower kV (higher mA), • less copper filtration • therefore higher dose

  28. CT Philips Brilliance, CHH (2011) In body CTDI/mAs vs Slice Thickness 24 mm  40 uGy/mAs 18 mm  40 uGy/mAs 12.0 mm  42 uGy/mAs 6.0 mm  52 uGy/mAs 3.0 mm  74 uGy/mAs 1.2 mm  72 uGy/mAs • High dose, so justification important • Lowest mA practicable • Minimum number of slices necessary • Angulation of gantry can substantially reduce eye dose • CT: 10 x 1mm slices may give higher dose than 1 x 10mm slice . .

  29. Dose quantities in CT • Computed Tomography Dose Index (CTDI, in Gy) • Average dose inside the beam

  30. Weighted CTDI100 • CTDIw = 1/3CTDI100(centre) + 2/3CTDI100(peripheral)

  31. Dose quantities in CT • Dose length Product (DLP, in Gy.mm)

  32. NRPB - W67 Doses from Computed Tomography (CT) Examinations in the UK - 2003 Review

  33. NRPB - W67 Doses from Computed Tomography (CT) Examinations in the UK - 2003 Review

  34. NRPB - W4 Radiation Exposure of the UK Population from Medical and Dental X-Ray Examinations, 2002

  35. CT Dose Optimisation • Don’t CT unless sufficient benefit • Don’t use very thin inefficient slices if unnecessary • Lowest mAs/slice for acceptable noise level • Understand Dose Modulation systems and use where appropriate • Largest pitch • Shortest scan length

  36. Pregnancy -can be downloaded from http://www.hpa.org.uk/webw/HPAweb&HPAwebStandard/HPAweb_C/1238230848780?p=1199451989432 (link from http://www.hullrad.org.uk)

  37. Pregnancy - Radiation Risks

  38. Fetal Doses from Medical Exposure (mGy)

  39. Fetal Doses from Medical Exposure (mGy)

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