RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

1
RADIATION PROTECTION INDIAGNOSTIC
ANDINTERVENTIONAL RADIOLOGY
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Part 19.4 Optimization of protection in
  Mammography
• Practical exercise

2
Overview / objective

• 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

3
Part 19.4 Optimization of protection in
Mammography
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 1 kV accuracy and reproducibility

4
Accuracy of tube voltage

• Mammographic image quality and patient dose are
  dependent on any variation in the generator
  kilovoltage (kV) of the x-ray set. Therefore an
  accurate kV calibration is required.
• A non-invasive tube voltage check over the whole
  used kV-range at 1 kV intervals should be
  performed

5
Reproducibility of tube voltage

• The reproducibility is measured by repeated
  exposures at one fixed tube voltage that is
  normally used clinically (e.g. 28 kV)
• A digital kVp-meter (specially designed for
  mammography) is presently the most suitable for
  this purpose

6
Accuracy and reproducibility of tube voltage

• Limiting value Accuracy for 25-31 kV lt 1 kV
• Reproducibility lt 0.5 kV
• Frequency Every six months
• Equipment Digital kVp-meter

7
Part 19.4 Optimization of protection in
Mammography
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 2 Measurement of radiation output -
  linearity

8
Measurements of Radiation Output (I)

• The tube output is determined by the ratio of
  entrance surface dose (mGy) and focal spot charge
  (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
  under phantom conditions

9
Measurements of Radiation Output (II)

• The output is generally measured as a function of
  tube and generator factors (e.g. kV, mA, s).
• 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 focus of the X-ray tube in well
  collimated beam conditions.
• The ionization chamber should be equipped with a
  suitable electrometer, preferably a direct
  readout device

10
Measurements of Radiation Output (III)
X-Ray tube
Filter
FFD
Phantom
Ion. chamber
Lead slab
Table top
11
Measurements of Radiation Output (IV)

• In both geometry, 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 on the tube output are made by
  repeating measurements at constant exposure
  factors (e.g. 28 kV, 50 mAs)

12
Measurements of Radiation Output (V)

• The output is measured at different kV by varying
  the kilovoltage in fixed steps while keeping the
  mAs product constant
• Similar mAs product to that required for the
  reference exposure should be used for the
  measurement.
• Correct for the distance from the focal spot to
  the detector and calculate the specific output at
  1 metre and the output rate at a distance equal
  to the focus-to-film distance (FFD).

13
Measurements of Radiation Output - linearity

• The linearity of the output with the tube current
  can be examined by changing the mAs product while
  keeping the kilovoltage constant

14
Tube output

• Limiting value acceptable gt30 mGy/mAs at 1 m
• desirable 40-75 mGy/mAs at 1 m
• acceptable gt 7.5 mGy/s at a distance equal
  to the FFD
• desirable 10-30 mGy/s at a distance equal to
  the FFD
• Frequency Every six months and when problems
  occur
• Equipment Dosimeter, exposure timer