Transcript RADIATION PROTECTION IN DIAGNOSTIC 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
IAEA
International Atomic Energy Agency
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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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 15.3: Optimization of protection in
radiography
HVL (Filtration) Measurement
IAEA
International Atomic Energy Agency
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
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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
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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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X-Ray Tube
HVL Measurement Geometry
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
• Be careful of beam
hardening (semi-log plot is
not a straight line)
• Use points either side of
half initial value
• Calculate HVL :
Dose
10
(initial value = 9
50% of this = 4.5,
thus HVL = 2.6 mm Al)
1
0
1
2
3
4
mm Al
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Minimum HVL Values (IEC)
4.5
4
HVL (mm Al)
3.5
Acceptable
3
2.5
2
1.5
Not acceptable
1
0.5
0
50 60 70 80 90 100 110 120 130 140 150
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kVp
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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/QC%20Book.html
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