Influence of gantry overrun angle on CTDI variability: a Monte Carlo study

This study used Monte Carlo simulation to evaluate the influence of the X-ray tube start-angle uncertainty and gantry rotation overrun on computed tomography dose index (CTDI) measurements during axial scanning. Polymethyl methacrylate phantoms with diameters of 16 and 32 cm were modeled, and CTDI_100,c, CTDI_100,p, and CTDI_w were calculated while varying the overrun angle from 0° to 120°. For each condition, the tube start angle varied from 0° to 359°, and the relative standard error was used to quantify the CTDI variability. When the overrun angle was 0°, the CTDI variability was negligible. However, the relative standard error increased with an increasing overrun angle, particularly at the peripheral positions and in the 32-cm phantom. These findings indicate that gantry overrun and uncertainty in the tube start angle can contribute to CTDI measurement variability and that repeated measurements may be required under conditions with large overrun angles.