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, 6cc general diagnostic chamber, and a 3cc 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 6cc 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. Copyright (1998) Australasian Physical and Engineering Sciences in Medicine