In a method and apparatus for analyzing medical imaging data of a subject from an imaging modality using a tracer, data from detection of tracer emission events at least one region of a scanned imaging volume are obtained, and the data processed to establish sub-periods or time points within a scan period for the emission events. A rate of change of emission events per unit time for a given region is then calculated, for example by obtaining a number of emission events in each sub-period in the given region, and fitting a line through the values for the sub-periods.