Analysis of size-specific dose profiles beyond the scan range in water phantoms undergoing computed tomography examinations

Background: The purpose of this study is to analyze the cumulative dose profile tail beyond the scan lengths of 10-90 cm in CT examinations on central and peripheral axes of water phantoms (10-50 cm diameter). The analysis includes extension, trend, and dependency of the tails on the scan length and phantom diameter. Materials and Methods: A validated Monte Carlo model of a GE LightSpeed 16-slice CT scanner was used to obtain a dose profile from narrow beam collimation (1 mu m). The cumulative dose profile tail was developed by applying the well-known "convolution method", i.e. convolving a dose profile with a RECT function. The tails were analyzed according to the phantom diameter and scan length statistically. Results: For all phantoms, the statistical analysis shows no significant differences between tails of scan lengths of 10-90 cm for peripheral axis and those of scan lengths > 30 cm for the central axis. The tails have an exponential falloff from the scan edge modeled by D(z)=Ae(-Bz). The A and B are related to the falloff speed and amplitude of the tails. By increasing the phantom diameter, the amplitude and falloff speed decrease 82% and 59% on the central axis and 65% and 31% on the peripheral axis respectively. Conclusion: A simple equation was suggested for dose calculation at any point beyond the scan range on the central and peripheral axes of water phantoms. The equation is independent of scan length and is useful for evaluating the dose of organs located at the edge or outside of the scan region.