Impact of High Pitch and Tube Starting Angle on Pediatric CT K-Factor Variability: A Monte Carlo Study
Abstract
Purpose
Monte Carlo simulations are widely used to derive effective dose conversion factors (k-factors) for CT; however, acquisition parameters such as helical pitch and tube starting angle are often randomly selected. The impact of these parameters on the variability of k-factor estimates, especially in high pitch scans, has not been systematically evaluated. This study investigated how tube starting angle and pitch influence k-factor variability in pediatric chest-abdomen-pelvis (CAP) CT across clinically relevant pitch settings.
Methods
Monte Carlo simulations of CAP CT examinations were performed using ICRP-reference pediatric extended cardiac-torso (XCAT) phantoms representing ages of newborn, 1, 5, 10, and 15 years. CT scanner (SOMATOM Force, Siemens Healthineers) simulated helical dual-source acquisitions using 120 kV across pitches of 0.5 to 3 in increments of 0.5. For each age and pitch, simulations were repeated for 12 tube starting angles uniformly distributed over 360°. Organ doses and their respective ICRP-103 tissue weighting factors were used to calculate effective dose. Pediatric k-factors were calculated by dividing the resulting effective dose by scanner-reported DLP. Angle-averaged mean k-factors and variability metrics, including coefficient of variation (CV) and peak-to-peak percent range, were evaluated.
Results
Across all age groups, the angle-averaged mean k-factor was nearly constant with pitch, indicating that pitch does not systematically bias mean k-factor estimates for CAP CT. As expected, k-factor variability across tube starting angles increased with pitch. For pitch ≤ 1, CV ranged from 0.2–1.2%, whereas for pitch values > 1, CV increased to as high as 6.5%, with peak-to-peak percent range reaching 17.3%.
Conclusion
While k-factors remain stable in their mean values across pitch, higher pitch increases sensitivity to tube starting angle. This work defines practical limits on k-factor precision and has implications for Monte Carlo dose modeling and interpretation of effective dose estimates in high-pitch pediatric CT.