Reproducibility of CT Beam Width Measurements Using a Pencil Ion Chamber
Abstract
Purpose
Evaluate the precision of CT beam width measurements using a pencil ion-chamber.
Methods
For nominal beam widths of 10, 15, and 20mm, three repeated exposures per setting were acquired on a GE Discovery RT590 CT scanner at 80, 100, and 120kV and 50, 100, and 150mA. Beam profiles were sampled using the Radcal Accu-Gold electrometer pencil ion-chamber and radiopaque mask. For each exposure, a calibration factor was derived and used to compute the measured beam width. For 20mm collimation, the ion-chamber was placed at 0, 1, 3, and 5mm superior-inferior (SI) offsets from isocenter for each kV/mA combination. A linear model was used to test the sensitivity of beam width to set up errors across kV/mA combinations. Slopes of the overall deviance from the global mean vs offset were obtained for each combination with 95% confidence intervals, and pairwise slope differences were evaluated using Tukey-adjusted p-values.
Results
Across all protocols, overall beam width means were 13.0, 16.9, and 20.6mm for 10, 15, and 20mm settings, respectively; inter-protocol standard deviations were under 0.1mm. Within each kV/mA setting, repeated measurements showed standard deviations from 0.04–0.09mm, with no discernible trends in beam width versus kV/mA. Linear regression analysis of beam width deviation vs isocenter offset was performed, with no statistically significant differences in slope (p<0.05). Estimated slopes of beam width deviation vs offset ranged from -0.07 to -0.12/mm across kV/mA settings, with overlapping 95% confidence intervals and no statistically significant interaction for p<0.05.
Conclusion
The ion-chamber technique produced reproducible CT beam width estimates that were effectively independent of tube potential and current as well as SI isocenter shifts. These findings support the method’s utility for QA and longitudinal monitoring of scanner performance and suggest that any clinically reasonable kV/mA combination yields equivalent beam width measurements for this system.