Experimental approach of internal dose map visualization during helical CT examinations: importance of X-ray incident direction analysis and central internal dose estimation

During computed tomography (CT) examination, radiation exposures should be appropriately managed taking into considering the effects of bowtie filter, the heel effect and over-beaming effect. Furthermore, the analysis of an X-ray incident direction is important. The purpose of this study is to develop a procedure to obtain two-dimensional (2D) internal dose distributions based on actual measurements of surface dose distribution and central internal dose data. Experiments were conducted using a clinical CT scanner and four cylindrical polyacetal resin (POM) phantoms having diameters of 15–30 cm. The entrance surface doses and the central internal dose were measured by placing the optically stimulated luminescence (OSL) dosimeters on the surface and inner part of the phantom, respectively, during helical CT scans. The X-ray incident direction at the slice containing the dosimeter was estimated based on the noise distribution analysis of the CT image. Then, circumferential surface dose distributions were determined as a function of the X-ray incident direction. Based on these experimental data, we succeeded in visualizing the 2D dose distributions. The obtained dose distribution was inhomogeneous, clearly reflecting the influence of factors such as the heel effect. The uncertainty due to our methodology was estimated to be from 4.3 % to 7.4 %. Our methodology needs central internal dose data, and the absence of this data introduced additional systematic uncertainties of +6.9 % to −11.4 %. In conclusion, correcting for the effect of the X-ray incident directions for entrance surface dose and adding the central inner dose data can improve the reliability of the internal dose distribution.