Assessment of effective doses and organ doses to patients from computed tomography

Since medical radiation is the greatest man -made source of exposure, although not subjected to individual dose limits, concerns on the risk of medical radiation are increasing. Especially, for CT examinations incurring relatively large doses per single procedure it is important to under st and the effective dose, an index quantity for stochastic risk of radiation, to the patients. Using MIRD- type mathematical anthropomorphic phantoms and the Monte Carlo code MCNP4B, the tissue equivalent doses and the effective doses were calculated for typical CT procedures. T he illuminating X- ray spectrum was select ed from SPECT 78 data library and the circulating exposure mode was approximated by averaging the doses from four directions : AP (Antero- Posterior ), PA(Postero-Ant erior ), LLAT (Left lateral) and RLAT (Right lateral). Protocols applied to the HiSpeed Advantage 2.X model was used as an example. The resulting effective doses to adult patients were in the range of 2∼5 mSv depending on the body part diagnosed. The size of effective doses are in the order of thoracic- spine, lumber - spine, abdomen, Cervical- spine, pelvis, chest , and head examinations. Considering that the effective dose due to single examination is at the level of annual doses of typical radiation workers, it is evident that further development of dosimetric methodology is of worth for CT examinations