1. A method, comprising: acquiring a set of all rays which represent a tomographic image; selecting a subset of rays from the set of all rays to use in an error calculation for a constrained conjugate gradient minimization problem; calculating an approximate error using the subset of rays; calculating a minimum in a conjugate gradient direction based on the approximate error; and using the minimum in the conjugate gradient direction in order to complete the conjugate gradient minimization problem related to the tomographic image.