Embodiments relate to a slit collimator assembly including a first set of panels spaced at least partially around a longitudinal axis of the collimator assembly and extending generally parallel to the longitudinal axis. The slit collimator assembly further includes a second set of panels spaced at least partially around longitudinal axis of the collimator assembly and extending generally parallel to the longitudinal axis. The first set of panels and the second set of panels are arranged to define one or more slit apertures. The slit collimator assembly is configured so that movement of at least one of the first set of panels or the second set of panels adjusts an aperture size of at least one of the one or more slit apertures. The slit collimator assembly is configured so that gamma rays can pass through the one or more slit apertures, but the remainder of the collimator assembly is substantially gamma ray absorbent. Embodiments also relate to imaging systems and methods of changing collimator performance.