A method of calibrating a PAI system (401-413) is described. The method includes a signal processing unit (404) determining optical fluence at voxels within arteries of a human or animal, and interpolating from these measurements to provide a fluence map with a fluence value for all voxels of interest. The system (404) stores the fluence map for subsequent use in making PAI measurements. The arterial optical fluence may be determined on the basis that the arterial oxygen saturation (SaO2) is the same throughout the arterial part of the circulation system.
