Determination of CT scanner radiation output: correction factors for partial irradiation of thimble and pencil ionisation chambers by collimated fan beams.
CT radiation output (mGy mAs-1) measured free-in-air on axis is required for compliance testing protocols. It can also be used as an input parameter for computer programs for patient dose assessment for a range of CT examinations. Output correction factors have been investigated as a function of collimation width for two chambers; an in-beam, 6 cc general diagnostic chamber, and a 3 cc pencil CT chamber. For irradiation by a fan beam perpendicular to the chamber axis, a correction factor given by the ratio, chamber active length to beam collimation width, is applicable to within +/- 5% of a measured ratio for the pencil CT chamber. However, this is not the case for the short 6 cc thimble chamber where an exponential function was found to fit the data to within similar limits. It is recommended that a pencil chamber be used for determining radiation output in air for a CT scanner. For a given scan condition static measurements are only required, obviating a need for sequential movements of an occluded small volume chamber to sample a radiation profile.
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Radiological Physics Pty Ltd, St Vincent's Hospital, Sydney.
D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation; D015898:Tomography Scanners, X-Ray Computed