Experimental Evaluation of CT Number Changes in 320-Row CBCT Volume Scan for Proton Range Calculation

We investigated the longitudinal positional dependence of CT number in 320-row Cone Beam Computed Tomography (CBCT) volume scan (320-row volume scan) using a simple geometric phantom (SGP) and a chest simulation phantom (CSP) in order to evaluate its effect on proton range calculation. The SGP consisted of lung substitute material (LSM) and a cylindrical phantom (CP) made of high-density polyethylene. The CSP was an anthropomorphic phantom similar to the human chest. The two phantoms were scanned using 320-row volume scan in various longitudinal positions from the central beam axis. In experiments using the SGP, an image blur at the boundary of the two materials became gradually evident when the LSM was placed far away from the beam central axis. The image blur of the phantom was consistent with the gradation in CT number. The maximum difference in CT numbers between the 64-row helical scan and 320-row volume scan at the boundary of the two materials was consistent with approximately 50% of the relative proton stopping power. In contrast, the CT number profile in each longitudinal position was fairly consistent and longitudinal positional dependence rarely occurred in the CSP experiments. Pass lengths of CT beams through areas with widely different electron densities were shorter, and thus did not significantly impact CT numbers. Based on findings from the CSP experiments, we considered 320-row volume scan to be feasible for proton range calculation in clinical settings, although the relatively large longitudinal positional dependence of CT number should be accounted for when doing so.

Discussion about Improvement of Stability of the Scan Timing by Placing Small ROI in Cerebral 3D-CTA

In three-dimensional computed tomography angiography (3D-CTA) in our facility, we usually scan the volume of the brain according to the bolus tracking method. Fluoroscopic slice is placed at the Willis’s ring and the timing of scan is determined subjectively by a radiological technologist after strong enhancement of the basal cerebral artery is confirmed. In these procedures, however, variation of scan timing is often problematic. Therefore, we design the surpassing method to place the small region-of-interest (ROI) at the basal cerebral arteries and to start CT scan automatically. In this protocol, the fluoroscopic slices of the distal internal carotid arteries are selected referring to the precontrast volume data, small ROIs are set in bilateral internal carotid arteries, and scan trigger of CT is started automatically at the threshold of 170 HU. The maximum 80 mL of iodine contrast agent 300 mgI/mL is injected intravenously at the rate of 4.0 mL/sec, and the volume of the arterial phase is scanned automatically. We measure ROIs at the internal carotid arteries based on the obtained volume data of arterial phase and estimate the optimal scan timings from the fluoroscopic CT images reformatted at the intervals of 0.1 sec. In 38 of 53 patients, placement of the small ROIs is succeeded and automatic or manual CT scan is performed. In the patients who succeed in placement of the small ROIs, optimal scan timing of the arterial phase is obtained, while in the patients who fail placement of the small ROIs, a large variation is observed in their scan timings. Their results suggest that more stable scanning of the arterial phase is available by means of small ROI placement and automatic scanning. The clinical significance is large because the stability and reproducibility of the examination provide a quantitative analysis and more accurate diagnosis.