1. A system for detecting fraud from medical imaging data including image data and associated metadata, the system comprising: input means arranged to receive the medical imaging data from at least one source; and processing means arranged to analyze the medical imaging data received by the input means to determine whether it meets a fraud criterion, and if the medical imaging data does meet the fraud criterion to generate a fraud indicator output, the image data including values for each of a plurality of pixels or voxels that make up the image represented by the image data; wherein the processing means is arranged to compare at least one parameter of the image data with a corresponding reference parameter, and the fraud criterion relates to the similarity of the parameters; the processing means is arranged to compare one image with another using image subtraction; and the fraud criterion relates to residuals of the image subtraction.