Impact of different X-ray tube positions on actual dose measurements during CT examinations -An effect of patient physique-

Dose management of patients is very important during X-ray Computed Tomography (CT) examinations, but because the patient's surface dose is inhomogeneous, it is difficult to measure the most probable value using a small passive-type dosimeter, lent to the patient. To solve this problem, our research group developed a precise dose analysis procedure in which a systematic uncertainty related to the X-ray incident direction (θin) is reduced. θin information was analyzed from CT images. However, the applicability of our procedure to actual patients with various physiques has not been examined. This study aims to propose a dose analysis procedure that can be applied to patients with various physiques, and to show its impact on dose measurement. Clinical data of 198 patients with Body Mass Index (BMI) values between 15 and 40 kg/m2 (mean value: 23.1 ± 3.8 kg/m2) who underwent chest CT scans were analyzed after dividing them into three groups based on BMI values. The absorbed dose was measured with a small-type Optically Stimulated Luminescence (OSL) dosimeter. To derive correction factors related to θin, the dependence of the actually-measured dose values of various patients on θin was analyzed. The correction coefficients were determined independently for the three groups classified by BMI values. By correcting the effect of θin, the systematic uncertainty element could be reduced, resulting in 30 % reduction of the uncertainty. Furthermore, it was found that our analysis procedure makes it possible to visualize outliers. In comparison with the expected dose values based on Computed Tomography Dose Index (CTDI) values, most of the data fell within the range of ±1.34 mGy (=1σ). However, 7 % of the data showed large deviations larger than 2σ. In conclusion, our research group has developed a procedure for measuring patient surface doses that can be applied to patients having various physiques, in which the effects of X-ray incident direction were accurately corrected. The procedure could be one solution to the problems with actual dose measurements during CT examinations, and will be useful for dose management based on the small-type dosimeter.