1. A method of reconstructing projection data indicative of a plurality of positron annihilation events, the method comprising: using a first processor to perform a first portion of a reconstruction of a subset of the projection data; using a second processor to perform a second portion of the reconstruction of the subset of the projection data; using the results of the first and second portions of the reconstruction to update an object estimate; repeating the steps of using a first processor, using a second processor, and using the results for each of a plurality of subsets; repeating the step of repeating a plurality of times; generating a human readable image indicative of the object estimate.