The interaction of collimator lattice periodicity and detector pixelation

Collimators, which are just periodic arrays of holes in lead, are used on gamma cameras in nuclear medicine to create images. Generally, the collimator hole pattern is not visible in clinical images because the holes are separated by distances smaller than the intrinsic resolution of the gamma camera. However, new (CZT) solid-state detectors are characterized by a regular grid of detector pixels. The interaction between the collimator-hole lattice and the detector grid almost assures that moire patterns will appear in the images unless the two grids are commensurate. Unfortunately, collimators with hole spacings commensurate with the pixel size are far from optimal in nuclear medicine. A mathematical analysis of the imaging process in such systems is provided and used to demonstrate the effects. Two strategies for minimizing the effects are examined: (1) introducing a gap between the collimator and the detector and (2) constructing the collimator with numerous small segments with displaced grid structures. Both strategies have serious drawbacks