IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN
DIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
Part 15.5: Optimization of protection in
radiography
Practical exercise - focal spot
IAEA
International Atomic Energy Agency
Contents
• Subject matter : quality control of
general radiography system
• 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 15.5: Optimization of protection in
radiography
Focal spot size measurement
(star pattern method)
IAEA
International Atomic Energy Agency
What Minimum Equipment is Needed?
•
•
•
•
Focal spot tool : star pattern
Radiographic film
Non-screen cassette
Roll of removable tape
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Focal spot size (star pattern) (1)
• The focal spot size measurement is intended to determine
its effective dimensions at installation or when resolution
has markedly decreased. For routine quality control the
evaluation of spatial resolution is considered adequate
• A magnified X-ray image of the test device is produced
using a non-screen cassette
• This can be achieved by placing the film to be exposed
between two sheets of opaque film (optical density greater
than 3.0) inside of the cassette.
• Alternatively, a mammography screen-film system can be
used.
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Focal spot size (star pattern) (2)
1. Select the focal spot size, 70 kV and an mAs to
obtain an optical density between 0.80 and 1.40
above base-plus-fog (measured in the central
area of the image)
2. Tape the star pattern to the bottom of the
collimator with it centered using the collimator
cross hairs and the quadrants aligned parallel
and perpendicular to the anode-cathode axis.
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Focal spot size (star pattern) (3)
• Place the cassette on the table top
• Make an exposure and process the film
• For digital systems the procedure is the
same except that measurements are made
on a digital image
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Focal spot size (star pattern) (4)
•
The magnification is determined as the ratio of the
diameter of the test pattern image to the diameter
of the physical test pattern.
•
The focal spot dimensions can be estimated from
the 'blurring diameter' on the of the star pattern
•
The distance between the outermost blurred
regions is measured in two directions:
perpendicular and parallel to the tube axis
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Focal spot size (star pattern) (5)
•
The focal spot size is calculated using:
pxq
Dblur
f =
x
180 ( M star - 1)
where :
q
: the angle of the radiopaque spokes (1° or
0.5°),
Dblur : the diameter of the blur.
Mstar : the magnification factor determined by
measuring the diameter of the star pattern on
the acquired image (Dimage) and the diameter of
the device itself (Dstar), directly on the star
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Focal spot size (star pattern) (6)
According to the IEC Standard:
• a nominal 0.8 focal spot should not exceed
1.2 mm in width and 1.6 mm in length.
• A nominal 1.2 focal spot should not exceed
1.7 mm in width and 2.4 mm in length.
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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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