Measurement of organ dose in chest CT examination using Monte Carlo simulation

Background: Exposure conditions in CT examinations are quite different from conventional Xray. In CT examination higher dose is given to patient in comparison with the dose in other diagnostic examinations. In order to calculate organ effective dose in chest CT, Monte Carlo simulation has been used in this study. Materials and Methods: The Impact survey data were used to determine the parameters related to patient dose. This was done by correlating the measurements from the NRPB scanners with the effective dose calculated, using the CTDOSE software. Patient dose index in air (CTDIair) was measured as function of tube exposure ranged from 90 to 225mAs at constant kVp and slice thickness, using a stack of TLD chips which was long enough to fully encompass the dose profile that could have been used. Results: Dose profile of each exposure was measured with approximately Gaussian distribution shape. The full width at half maximum (FWHM) of these profiles was nearly equal, and on average it was equal to 8 cm. Also the maximum CTDIair for these profiles, as expected increased with mAs ranging from 29.2 to 50.606 mGy. CTDIair was measured by two methods using conversion coefficient established by using software, based on Monte Carlo simulations (CTDOSE) and the other was measured in the area under the dose profile distribution. Conclusion: The slice thickness measured from FWHM and those thicknesses set by the operator were nearly equal proving that the measurements using TLD were accurate. The effective dose for chest increased with increasing mAs. By these measurements, it was also noted that the maximum equivalent dose and sharpest slope variation were for lungs, heart and breast respectively, whereas the minimum equivalent dose with lowest slope variation was related to thyroid, liver, spleen, stomach wall and kidneys respectively. Iran. J. Radiat. Res., 2007 4 (4): 205-209