RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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RADIATION PROTECTION INDIAGNOSTIC
ANDINTERVENTIONAL RADIOLOGY
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Part 15.3 Optimization of protection in
  radiography
• Practical exercise - HVL measurement

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Overview / Objectives

• Subject matter HVL measurement
• Step by step procedure to be followed to
  implement the considered QC test
• Interpretation of results

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Part 15.3 Optimization of protection in
radiography
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• HVL (Filtration) Measurement

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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

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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

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Tape Measure, Spirit Level
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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

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HVL Measurement Geometry
X-Ray Tube Collimator Aluminium Sheets X-Ray
Beam Detector Table top
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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

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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

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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
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Minimum HVL Values (IEC)
Acceptable
Not acceptable
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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/QC20Book.html