Background: CT is a diagnostic imaging modality giving higher patient dose in comparison with other radiological procedures, so the calculation of organ dose in CT exams is very important. While methods to calculate the effective dose have been established (ICRP 26 and ICRP 60), they depend heavily on the ability to estimate the dose to radiosensitive organs from the CT procedure. However, determining the radiation dose to these organs is problematic, direct measurement is not possible and comparing the dose as functions of scan protocol such as mA is very difficult. One of the most powerful tools for measuring the organ dose is Monte Carlo simulation.

Materials and Methods: Today the predominant method for assessment of organ absorbed dose is the application of conversion coefficients established by the use of Monte Carlo simulations. One of the most famous dose calculation software is CTDOSE, which we have used it for calculation of organ dose. In this work we measured the relationship between the mA, KV and scanner type with the equivalent organ dose and effective dose in mathematically standard phantom (Hermaphrodite 170cm/70Kg) in an abdomen-pelvis CT exam by Monte Carlo method. For this measurement we increased the mA in steps of 10 mA and plot curves for organ dose as a function of mA for different KV setting.

Results: As expected, with increasing mA, patient organ dose increased, but the simulation results showed that the slope of organ dose as a function of mA increased with KV increasing. By increasing KV from 120 to 140 the increase in slope of curves representing patient organ dose versus mA for different scanner types show almost similar behavior whereas the slope of the corresponding curves in scanners which equipped xenon detectors was almost 22% more than the slope of scanners equipped with scintillation detectors.

Conclusion: Our research showed that regarding equivalent dose the system incorporating scintillation detector has a superior performance. Incorporating such software in various CT scanners, marketed by different vendors, will offer the ability to get a print out of patient organ dose in any examination according to the imaging parameters used for imaging any part of the body. Iran . J. Radiat. Res., 2004 1(4): 187-194