CT doses in cylindrical phantoms.

A single CT scan of thickness T in a cylindrical phantom produces a three-dimensional dose distribution, which depends primarily on the photon energy spectrum, the x-ray beam shaping filter and the size and composition of the irradiated phantom. Monte Carlo simulations employing monoenergetic photons were employed to investigate the effect of each of these factors on phantom dose distributions. The fractional energies scattered, imparted and transmitted through the CT phantom were calculated. A dose index (D(r)), which is a function of phantom radius r, was computed. Phantom materials investigated included lung, fat, water, soft tissue, acrylic and bone with calculations performed for head (160 mm diameter) and body (320 mm diameter) phantoms. All dose and energy imparted data generated for CT phantoms were normalized using an 'in air' dose (Dair), which is defined as the axial dose (in acrylic) at the isocentre in the absence of any phantom. Results obtained show how CT parameters impact on doses in cylindrical phantoms. These dosimetry data are likely to be useful to estimate energy imparted to phantoms (and patients) undergoing CT examinations.