Patient dose assessment in computed tomography examination using physical measurement and computational algorithms

The first stage in computed tomography (CT) dose optimization is the knowledge of the amount of radiation a patient receives during CT examination. The main objective of this study was to evaluate and determine radiation doses received by selected radiosensitive organs of patients during routine head, chest and abdominopelvic CT examinations and validating the techniques employed. The methodology of the study was in four different phases. Phase 1 benchmarked the doses estimated for 150 patients at Oslo University Hospital (OUH) using DoseWatch against doses from CT-Expo and conversion coefficient to estimate the effective dose. Effective doses estimated by DoseWatch in this phase were significantly lower than that of CT-Expo and conversion coefficient (p #<0.001) up to about 50%. For organ doses, DoseWatch recorded lower doses than CT-Expo for all the organs except for testis (p #<001) and eye lens (p #<0.026). Organ dose measurements on anthropomorphic phantom were performed at the Greater Accra Regional Hospital (GARH) using thermoluminescent dosimeters (TLDs) and the measured doses verified with CT-Expo software in phase 2 of the study. The TLD measured organ doses varied between 3.97 mGy for oesophagus and 56.22 mGy for brain. High doses were recorded in the brain (37.80 - 56.22 mGy) and eye lens (29.94 - 36.16 mGy). Comparing organ dose measurements between TLD and CT-Expo, the maximum organ dose difference was observed in the eye lens (31.60%). The effective doses from TLD measurements were 2.78, 6.67 and 17.39 mSv for head, chest and abdominopelvic CT respectively, while effective doses from CT-Expo were 2.20, 10.30 and 16.70 mSv for the head, chest and abdominopelvic CT respectively.Phase 3 utilised 150 patients' data retrospectively collected from the database of GARH and effective doses estimated using recorded dose-length product (DLP) and conversion coefficients III(k) method. The average patient absorbed dose to the brain and eye lens at GARH were 30.59 ± 3.92 and 38.81 ± 5.01 mGy respectively. In the chest region, the highest dose was obtained in the oesophagus (8.37 mGy). For the abdominopelvic CT, the highest mean dose was obtained in the stomach (15.79 ± 6.69 mGy). Consequently, the mean effective dose ranged from 1.73 ± 0.32 to 9.13 ± 5.19 mSv. In the final phase of the study, radiation dose to the patients undergoing the CT examinations at GARH were simulated with Monte Carlo N-Particle 6.2 software. The simulated effective dosesfrom MCNP were 1.80, 5.40 and 7.80 mSv for head, chest and abdominopelvic CT examinations respectively. The highest MCNP simulated dose was obtained in the eye lens, with a dose 40.56 mGy and the lowest dose was obtained in heart, with a dose of 10.62 mGy.The study has validated the techniques employed in organ and effective dose estimations in CT procedures. The highest mean organ doses from OUH and GARH were observed in the eye lens. Irrespective of the slight variations in the doses from the two facilities, they were all lower than values reported by ICRP publication 103. Variation in estimated doses was attributed to differences in CT scanner specifications, examination protocols and the expertise of the imagingtechnologists. Importance of the dose monitoring tool (such as DoseWatch) in patient dose optimization has been highlighted in the study. All techniques used to estimate organ and effective doses in this study have been proven to be suitable, reliable and complimentary. (Author)