Abstract: A collimator for collimating high-energy photons, which may be used in medical imaging (e.g., nuclear medicine). The collimator has holes through the thickness (height) of the collimator, with the holes arranged in groups or clusters. The collimator may be used with a detector having an array of pixels, wherein each group of holes may be associated with a corresponding pixel, thereby providing multiple collimator holes per pixel. In one embodiment, each group of holes has septa of a given width separating the holes in that group, and each group of holes is separated from neighboring groups by septa of another, greater width. In another embodiment, the intra-group septa may be recessed from the top and/or bottom surface(s) of the collimator such that these septa have a smaller thickness (height) than the inter-group septa.