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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology. RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY. Part 15.3: Optimization of protection in radiography Practical exercise - HVL measurement. Overview / Objectives.

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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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  1. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY Part 15.3: Optimization of protection in radiography Practical exercise - HVL measurement

  2. Overview / Objectives • Subject matter : HVL measurement • Step by step procedure to be followed to implement the considered QC test • Interpretation of results 15.3: Optimization of protection in radiography

  3. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 15.3: Optimization of protection in radiography HVL (Filtration) Measurement

  4. Half Value Layer (HVL) (I) • Possibly the most important test • Checks whether there is sufficient filtration in the x-ray beam to remove damaging low energy radiation 15.3: Optimization of protection in radiography

  5. What Minimum Equipment is Needed? • Electronic device (multi-function meter) to measure dose • High purity aluminium layers. (Most aluminium has high levels impurities, e.g., Cu) • Stand to hold the dosimeter, e.g., laboratory stand • Tape measure, spirit level • Roll of removable tape 15.3: Optimization of protection in radiography

  6. Tape Measure, Spirit Level 15.3: Optimization of protection in radiography

  7. Half Value Layer (II) • Method (simplest - using a multifunction test meter) : • Place the detector half way between the front of the collimator and the table top. • Set 80 kVp, fixed mAs (e.g., 50 mAs) • Collimate x-ray beam to size of detector • Measure exposure three times 15.3: Optimization of protection in radiography

  8. HVL Measurement Geometry X-Ray Tube Collimator Aluminium Sheets X-Ray Beam Detector Table top 15.3: Optimization of protection in radiography

  9. Half Value Layer (III) • Tape a 1 mm sheet of aluminium to the front of the collimator and measure the dose • Add another 1 mm sheet of aluminium, measure again, and repeat until the dose has fallen to below 50% of the initial, unattenuated, value • Remove all aluminium, and make 3 more exposures 15.3: Optimization of protection in radiography

  10. Half Value Layer (IV) Analysis : • Average all the “no Al” measurements • Plot all results on semi-log graph paper • From the graph, find the thickness of Al required to reduce the unattenuated dose by 50% - this is the HVL • The HVL must be at least that specified in local regulations, e.g., 2.3 mm Al at 80 kVp • Ideally, it should be at least 3.0 mm Al or higher to assist in minimizing patient dose 15.3: Optimization of protection in radiography

  11. HVL Measurement Dose • Be careful of beam hardening (semi-log plot is not a straight line) • Use points either side of half initial value • Calculate HVL : (initial value = 9 50% of this = 4.5, thus HVL = 2.6 mm Al) mm Al 15.3: Optimization of protection in radiography

  12. Minimum HVL Values (IEC) Acceptable Not acceptable 15.3: Optimization of protection in radiography

  13. Where to Get More Information Quality Control in Diagnostic Imaging, Gray JE, Winkler NT, Stears J, Frank ED. Available at no cost. http://www.diquad.com/QC%20Book.html 15.3: Optimization of protection in radiography

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