A method, including: determining a line of response for an imaging apparatus, the line of response being defined by respective locations of a pair of detector crystals of the imaging apparatus; defining an array of emission points corresponding to the determined line of response; determining, for each point in the array of emission points corresponding to the line of response, a solid angle subtended by surfaces of the pair of detector crystals that define the line of response; averaging the determined solid angles to generate an average solid angle; determining a depth of interaction factor dependent upon penetration of a gamma ray in the pair of detector crystals of the imaging apparatus; and calculating a geometric corrective factor for the determined line of response by multiplying a reciprocal of the average solid angle by the determined depth of interaction factor.