Helical X-ray tube trajectory estimation via image noise analysis for enhanced CT dosimetry

Information on the helical trajectory of the X-ray tube is necessary for accurate dose evaluation during computed tomography (CT). We aimed to propose a methodology for analyzing the trajectory of the X-ray tube. The novelty of this paper is that the incident direction of X-rays is estimated from the standard deviation (SD) distribution. The X-ray incident direction for each slice was analyzed using a distribution function of SD values, in which the analysis regions were placed in the air region. Then, the helical trajectory of the CT scan was estimated by fitting a three-dimensional helical function to the analyzed data. The robustness of our algorithm was verified through phantom studies: the analyzed X-ray incident directions were compared with instrumental log data, in which cylindrical polyoxymethylene resin phantoms and a whole-body phantom were scanned. Chest CT scanning was mimicked, in which the field of view (FOV) was set at the lung region. The procedure for analyzing the X-ray incident direction was applicable to cylindrical phantoms regardless of the phantom size. In contrast, in the case of the whole-body phantom, although it was possible to apply our procedure to the chest and abdomen regions, the shoulder slices were inappropriate to analyze. Therefore, the helical trajectory was determined based on chest and abdominal CT images. The accuracy in X-ray incident direction analysis was evaluated to be 7.5°. In conclusion, we have developed an algorithm to estimate a three-dimensional helical trajectory that can be used for dose measurements and simulations.