An apparatus including processing circuitry configured to: acquire a plurality of sets of medical imaging data of a region of a subject, each set of data corresponding to a respective different measurement period; apply a filter to the plurality of medical imaging data sets to produce a plurality of filtered medical imaging data sets corresponding to the different measurement periods, wherein the applying of the filter is such that, for each of the medical imaging data sets, the filtering uses at least some information from the other medical imaging data sets acquired at the different time periods and wherein the applying of the filter comprises applying at least one constraint or condition and the constraint or condition comprises preserving at least one measure of intensity for each medical imaging data set.