A practical method for measuring rotation angles in partial-angle CT scan by using CTDI phantoms, metal rods, and a real-time dosimeter

Partial-angle computed tomography (CT), including CT fluoroscopy and C-arm cone-beam CT systems, plays an important role in image-guided interventions. In these systems, the X-ray tube rotation angle is a critical factor affecting the dose evaluation. However, scans are often performed with partial rotations of approximately 180°–240°, and reliable methods for measuring the actual rotation angle have not been established. In this study, we propose a method for measuring the rotation angle in partial-angle CT scans and improving dose evaluation accuracy by incorporating the measured angles into Monte Carlo simulations. Measurements were performed using CT fluoroscopy and angiography. A CT dose index phantom was placed on the patient table, and metal rods and a real-time dosimeter were inserted into the peripheral holes and center hole, respectively. The rotation angles were identified from the X-ray attenuation caused by the metal rods, and the dose profiles for a single scan were determined by combining measurements acquired with and without the rods. The measured rotation angles and dose profiles were incorporated into Monte Carlo simulations and then validated against measured values. The differences between the displayed and measured rotation angles were 3.4% for CT fluoroscopy and 8.1% for angiography. Compared with the conventional method, the maximum discrepancy in dose evaluation was reduced by 9.0% for CT fluoroscopy and 4.8% for angiography. The proposed method facilitates rotation angle quality assurance in partial-angle CT scans and contributes to appropriate dose management.