Apparatus for detecting radiation emitted from a number of volume elements of a body. The apparatus includes a first plurality of detector elements, each detector element being configured to output signals indicative of an intensity of radiation that is incident thereon. The apparatus also includes a first plurality of adjustable collimator channels, each adjustable collimator channel being associated with and being positioned between a respective detector element and the body, each adjustable collimator channel having a second plurality of dimensional configurations defining respective different sets of the volume elements from which emitted radiation impinges on the respective detector element. A processor computes a radiation intensity from at least a portion of the volume elements in response to the signals output by the detector elements in at least two of the dimensional configurations of the adjustable collimator channels.