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Fig 3. Pictorial view of collimation (drawings are not to scale). A, Parallel-hole collimator. The object is projected onto the detector without magnification. The projection does not use the whole detector. Ideally, photons reach the detector only traveling along lines (LORs, dashed lines) parallel to the collimator channels. B, The finite width of the collimator channels allows lines nearly parallel to the collimator also to reach the detector. C, Longer channels restrict the acceptance angle and resolution improves (see how a more restricted region of space than that in B is seen from each channel). Resolution worsens with distance. D, A fan-beam collimator magnifies the object, projecting it on the whole detector. LORs converge to the focal line (a point in the transverse section shown).
