Calculating Organ Doses of Patients in Spiral CT Using EGS4 Monte-Carlo Simulation of Voxel Phantom

The doses of patients on X-ray CT imaging are higher than other general X-ray diagnoses. On the view point of radiation protection, the optimizations of X-ray CT imaging are very important. However, it is not easy to estimate patients\\u27 doses by direct measurements using dosemeters. We have calculated the patients\\u27 doses by the method of Monte-Carlo simulations. The mathematical phantom used in the simulations was a voxel phantom of a Japanese male, which have been developed in JAERI. It was based on the pictures of X-ray CT scanning for the whole body, and constructed of about 6.63 million voxels. The simulation code was EGS4, and the fortran compiler was g77 in a Pentium IV PC Linux machine. The geometry of the voxel phantom was included in the EGS4 user code. The material data of the voxels were soft tissue, lungs and bone data of ORNL. As the X-ray spectra with filters for each tube voltages, Birch\\u27s data were referred in the user code. For X-ray CT imaging conditions of calculations, some typical X-ray CT diagnoses were selected. As photon irradiation parameters, the exposure area, slice number, slice width and X-ray tube voltages were set. The center axis and X-ray tube focus positions were determined based on a real X-ray CT machine geometry. The energy depositions of organs were calculated, and patients\\u27 doses were estimated. The ratios of entrance surface dose and each organ doses can be obtained and used for the estimation of the doses. They are useful as indicators for the consideration of optimization on X-ray CT imaging in practice.11th International Congress of the International Radiation Protection Associatio