TH‐C‐18A‐04: Validation of Dosimetric Measurement of CT Radiation Profile Width

Purpose: The ACR now requires that the CT radiation profile width be measured at all clinically used collimations. We developed a method for measuring the profile width using dosimetry alone to allow a faster and simpler measurement of beam widths. Methods: A pencil ionization chamber is used to take two dose‐length product measurements in air for a wide collimation. One of these is taken with a 1cm tungsten mask on the pencil chamber. The difference between these measurements is the calibration factor, or the DLP in air per unit length. By dividing the doselength product for any given collimation by this factor, we can rapidly determine the beam profile width.We measured the beam width for all available detector configurations and focal spot sizes on three different CT scanners from two different manufacturers. The measurements were done using film, CR cassette, and the present dosimetric method. Results: The beam widths measured dosimetrically are approximately 2% wider than those measured using film or computed radiography; this difference is believed due to off‐focus or scattered radiation. After correcting for this, the dosimetric beam widths match the film and CR widths with an RMS difference of approximately 0.2mm. The measured beam widths are largely insensitive to errors in positioning of the mask, or to tilt errors in the pencil chamber. Conclusion: Using the present method, radiation profile widths can be measured quickly, with an accuracy better than 1mm.