A method, apparatus, medium and signal for producing a representation of a measurable property which varies in time and space are disclosed. The apparatus includes a receiver for receiving a plurality of sets of values representing measurements of the property across the object at respective measurement times, each set being associated with a respective measurement time. The apparatus further includes a processor circuit in communication with the receiver. The processor circuit is programmed to produce a plurality of sets of values representing the property at a plurality of locations throughout the object at the respective measurement times, by minimizing a figure of merit function relating the values representing the measurements with the values representing the property at the plurality of locations, with a shape constraint imposed on the values representing the property at the plurality of locations.