Equations for CT dose calculations on axial lines based on the principle of symmetry

Purpose: Radiation dose from CT examination depends on the position relative to the irradiated region in the longitudinal direction. The objective of this work is to present a mechanism for calculating dose at any point on a line parallel to the axis of rotation. Methods: In a medium that is uniform in the z direction, the cumulative dose at the midpoint D L (0) of the irradiated length ( L ) increases with L until the equilibrium dose ( D eq ) is reached. Using the approach to equilibrium function, H ( L ) = D L (0)/ D eq , the authors formulated equations for calculating dose at an arbitrary point on an axial line. Results: Dose calculation examples are presented for multidetector CT (MDCT) and micro‐CT. Conclusions: The new equations are suited for any functional form of H ( L ), and can be used for dose evaluations in MDCT, micro‐CT, cone‐beam CT, and other cases with media that are approximately uniform along the z direction.