Collimators

Collimators play a very important role in nuclear medicine. Their presence is necessary for forming an image in every type of equipment bar PET. Their purpose is to ensure that a given pixel of the detector will only be stricken by photons arriving perpendicularly to the detector pixel (in case of parallel hole collimators) or reaching it from a well defined direction (in case of diverging or converging collimators). Of course these constructions cannot be perfect because the probability of a photon arriving at exactly 90° is zero, this way no pixel would be stricken. Furthermore, it would be impossible to create collimators that are ‘perfect’. In general we can say that photons can enter the collimator only within a very narrow spatial angle region.
Fundamentally we can say that collimators can belong to one of four types:

• parallel hole collimators
• converging collimators
• diverging collimators (a great surface can be imaged using a small detector)
• pinhole collimators (camera obscura – they can provide a magnified image)