A study of CT dose distribution in an elliptical phantom and the influence of automatic tube current modulation in the x-y plane.

Computed tomography (CT) performance assessments relating to patient dose to the body are made conventionally in 320 mm diameter cylindrical acrylic phantoms. The cross section of the human trunk is closer to an ellipse and automatic tube current modulation (ATCM) systems adjust the exposure level with orientation in the x-y plane, changing the dose distribution within the body. This study has investigated differences in the distributions of dose within a standard cylindrical body phantom and an elliptical dosimetry phantom for Toshiba, General Electric and Philips CT scanners, and recorded changes with the application of the ATCM. Single slice dose profiles have been recorded within the phantoms using Gafchromic film. CT dose indices along 100 mm lengths have been calculated and data sets combined to simulate helical scans, from which values for cumulative doses have been derived. The doses in the centre of the elliptical phantom are 70-100% larger than for the cylindrical one and in the anterior are around 20-40% larger, while the doses in the lateral positions are similar for the two phantom shapes. The differences between the anterior and lateral doses were larger for the Toshiba scanner and this is thought to be linked to the narrower profile of the beam produced by the bow-tie filter. When the ATCM mode for the Toshiba scanner is implemented, the doses in the anterior and posterior positions are reduced preferentially, bringing them closer to the doses in the lateral positions.