Evaluation of Estimated Exposure of the Lens Due to Cone-beam CT and Area Detector CT

Objective: Recently, the exposure threshold for cataract has been reduced, and interest in lens exposure is deepening. The importance of cone-beam CT (CBCT) is increasing in the field of intracranial endovascular treatment, and opportunities of substituting CT after endovascular treatment with CBCT has also increased. While the exposure by angiography and CT must be considered comprehensively for the management of lens exposure, total dose management and direct comparison are difficult due to the difference in the evaluation methods. Therefore, we compared the direct doses measured at the lens between the modalities.Methods: We measured the doses at five points (center, upper, lower, left, and right) in an acrylic phantom 16 cm in diameter and simulating the head for the determination of the CT dose index (CTDI) under various conditions of non-helical scanning of area detector CT (ADCT) and CBCT and compared the mean of the five points, which was assumed to be the dose of the whole head, and the upper point, which was assumed to correspond to the lens.Results: The ratio of the dose by ADCT relative to that by 15-second MIDDLE mode CBCT was 1.1 as the mean of the five sites but was 2.3 at the site corresponding to the lens.Conclusion: Compared with CT, by which the object is scanned over 360°, the exposure of the lens can be reduced by CBCT because it collects data from an arc of dorsal 200° and does not directly apply X-ray to the lens.