A single photon emission computed tomography system includes a detector assembly adjacent a field of view and a collimating assembly disposed between the detector assembly and the field of view. The collimating assembly includes at least two spaced-apart collimating vanes of photon-attenuating material. The system further includes a photon-blocking member disposed between the field of view and the detector. The blocking member has an aperture defined therethrough. The system further includes a mask disposed adjacent the detector assembly having at least one aperture defined therethrough. A displacement actuator moves the photon-blocking member relative to the detector assembly.