In this article the present knowledge of the collimators used for single photon imaging in Nuclear Medicine is recapped. In the introduction, the classic formulas to describe the performances of multihole collimator are presented with an overview of the basic concepts. In the subsequent sections, the tools available for the characterization of multihole collimators are described. The characterization is divided into aspects related to the geometrical response and aspects related to effects due to interaction of photons with the collimator. Nowadays a rather complete theoretical description of the collimator geometrical response is available. In section Collimator characterization: theory a general formulation that is valid for a wide range of collimators is outlined. In section Collimator characterization: numerical methods the kind of knowledge that can be derived today from numerical computing methods about the penetration and scatter effects is presented. The problem of performances optimization in collimator design is described in section Collimator design. Difficulties and some new perspectives on avoiding both penetration and hole pattern artifacts while designing high-energy collimators are mentioned.
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