CT Users Group

C J Martin, S Sookpeng and D J Gentle

Gartnavel Royal Hospital

Abstract

Phantoms used for CT dosimetry take the form of Perspex cylinders of standard dimensions. These provide a useful basis for comparisons of CT output, but such phantoms are not a good representation of the human trunk. An elliptical Perspex phantom has been constructed with axis dimensions of 330 mm and 220 mm similar to those for the trunk of an average patient. Holes for dosimetry measurements have been included at 10 mm from either end of the major and minor ellipse axes, and at the centre. Measurements have been made with Gafchromic film and ion chambers to compare distributions of dose within the elliptical phantom and a 320 mm diameter cylindrical phantom. Measurements of dose profiles for single rotations have been use to derive results for the CT dose index (CTDI). Results from multiple rotations have been combined to simulate helical scans and derive values for the cumulative doses near the mid-points of the phantoms. Measurements have been made on CT scanners manufactured by Toshiba, GE and Philips. The influence of the automatic tube current modulation (ATCM) systems in the x-y plane of the scan have been studied. For all scanners, measurements at the lateral peripheries of the elliptical phantom were similar to those in the cylindrical phantom. However, doses at the anterior (top) periphery and the centre of the elliptical phantom were substantially larger than the equivalent in the cylinder, with the peak dose at the centre of the ellipse being about double that at the centre of the cylinder with the Toshiba scanner. The higher dose values occur for two reasons. Firstly the anterior peripheral measurement points in the ellipse lie nearer to the isocentre where the attenuation of the fan beam by the central portion of the bow-tie filter is less. Secondly the attenuation of the thinner elliptical phantom in the anterior posterior direction is lower. The differences between the phantoms are dependent on the CT scanner type with results for the Toshiba Aquilion 64 being the largest. The reason for this appears to be because the central region of the fan beam profile for this scanner is narrower than for the other scanners. When the ATCMs are brought into play, these tend to equalise peripheral doses at the anterior and lateral positions. This has a greater influence on the distributions within the Toshiba scanner than those of the other manufacturers. Measurements of dose within an elliptical phantom show differences in dose distribution that are not apparent from measurements with a cylindrical phantom. They demonstrate how the shape of the bow tie filter affects dose distributions for different scanners and enable the effect of the ATCM on dose distribution to be assessed.

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