Collimators for two-dimensional scans of a radiation sources and methods of scanning are provided. One system includes a scan unit for scanning and collecting ionizing radiation emitted from a radiation emitting object is provided. The scan unit includes an array of at least one pixelated radiation detector having an imaging surface including a two-dimensional (2D) array of pixels. The scan unit also includes a collimator positioned between the radiation detector and the radiation emitting object, with the collimator including a 2D array of columns having openings and septa forming bores, wherein the columns are arranged in groups along rows of the 2D array of columns and the bores within one of the groups have a different aspect ratios than the bores in another one of the groups.