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.