An x-ray collimator can be constructed from multiple subassemblies, which at least includes a first subassembly that reduces the leakage of x-ray radiation between adjacent apertures and a second subassembly that reduces the spill of x-ray radiation around the detector face. Each of these subassemblies has numerous apertures. In the first subassembly these apertures correspond to focal spots on an x-ray source, and in the second subassembly, these apertures are shaped such that the dimensions increase from smaller entrances to larger exits.