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)1 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. (c) 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4742070]