Abstract
In tomography, the element r sub ij in the R matrix represents the transit length of the ith arc over the jth cell; its determination is often the most time-consuming step in image reconstruction. While in straight-ray tomography R is calculated only once, most algorithms that take ray-bending into account require the frequent reevaluation of R, and the time required to calculate it has a major effect on the algorithm's afficiency. Attention is presently given to two methods of determining the coefficients in R, assuming straight rays in order to simplify this presentation. (O.C.)