1. A method of compensating for truncated transmission projection data beyond a measured field of view of a nuclear medicine imaging detector using fixed transmission line sources for attenuation correction, comprising: (a) generating an initial estimate of a transmission reconstruction from truncated transmission projection data using a geometric model of transmission beams emanating from said sources as straight lines; (b) modifying the geometric model of the transmission line sources and transmission beams emanating from said sources based on calibration images; and (c) generating transmission projection data outside said measured field of view using the initial estimate together with the modified geometric model.