Transcript 19. Optimization of protection in mammography: Part 4

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN
DIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
Part 19.04: Optimization of protection in
Mammography
Practical exercise
IAEA
International Atomic Energy Agency
Overview
• To be able to apply quality control protocol
to mammography equipment
• To measure the tube voltage accuracy and
reproducibility
• To measure the radiation output and linearity
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 19.04: Optimization of protection in
Mammography
Topic 1: kV accuracy and reproducibility
IAEA
International Atomic Energy Agency
Accuracy of tube voltage
• Mammographic image quality and patient
dose are dependent on the kilovoltage (kV).
• A non-invasive tube voltage measurement
over the clinical kV-range at 1 kV intervals is
necessary
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Reproducibility of tube voltage
• The reproducibility is measured by repeated
exposures at one fixed clinical tube voltage,
e.g., 28 kV
• A digital kVp meter (designed for
mammography) is required
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Accuracy and reproducibility of tube
voltage
Limiting value : Accuracy for 25-31 kV: < ± 1 kV
Reproducibility < ± 0.5 kV
Frequency :
Annually
Equipment :
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Digital kVp meter
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 19.04: Optimization of protection in
Mammography
Topic 2: Measurement of radiation output linearity
IAEA
International Atomic Energy Agency
Measurements of Radiation Output (I)
• The tube output is determined by the ratio of
entrance surface dose (mGy) and mAs
• A high radiation output is desirable to ensure
that exposure times are sufficiently short to
minimize patient movement and discomfort
• The measurement can be performed either
in air or with a phantom
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Measurements of Radiation Output (II)
• The output is generally measured as a function kV
and mAs
• There are two experimental arrangements used for
the determination of the radiation output under
clinical conditions.
• An ionization chamber is positioned at a fixed distance
from the focal spot with the beam collimated to the
chamber
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Measurements of Radiation Output
(III)
X-Ray tube
Filter
FFD
Phantom
Ion chamber
Table top
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Lead sheet
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Measurements of Radiation Output
(IV)
• In both geometries, a sheet of lead is placed
on the top of the table to obtain standard
backscatter conditions.
• The radiation output can be determined
under the following operating conditions:
Consistency checks for output are made by
repeating measurements at constant
exposure factors, e.g., 28 kV, 50 mAs
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Measurements of Radiation Output (V)
• The output is measured at various kVs , keeping
the mAs constant.
• The mAs required for the reference exposure
should be used
• Correct for the distance from the focal spot to the
detector. Calculate the output at 1 metre and the
output rate at a distance equal to the focus-to-film
distance (FFD).
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Measurements of Radiation Output linearity
• The linearity of the output with the tube
current can be determined by varying the
mAs, keeping the kilovoltage constant
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Tube output
Limiting value : acceptable: >30 mGy/mAs at 1 m
desirable: 40-75 mGy/mAs at 1 m
acceptable: > 7.5 mGy/s at a
distance equal to the FFD
desirable: 10-30 mGy/s at a
distance equal to the FFD
Frequency :
Equipment :
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Anually and when problems occur
Dosimeter, exposure timer
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Digital systems
For systems with digital detectors additional
issues need to be considered in order to
optimize age quality.
Please see the training material on Digital
Radiology:
https://rpop.iaea.org/RPOP/RPoP/Content/Ad
ditionalResources/Training/1_TrainingMaterial/
DigitalRadiology.htm.
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Where to Get More Information
European protocol for the quality control of the
physical and technical aspects of mammography
screening.
http://euref.org/index.php?option=com_phocado
wnload&view=category&id=1&Itemid=8
American College of Radiology Mammography
Quality Control Manual, Reston VA, 1999.
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