They typically consist of a lead disc drilled with tens of thousands of closely packed holes, separated from each other by septa.
Each hole only accepts Gamma rays to travel through a narrow channel. The path of these rays, and therefore their origin of location, can be accurately mapped. All other rays traveling in various other directions are absorbed by the septa, and do not contribute to the image.
Hence, the primary function of the collimator in Gamma cameras is for accurate spatial localization, and not scatter reduction (as is the case in general radiography).
The collimator is placed over the scintillator crystal of the Gamma camera, and positioned as close as possible to the patient, to maximize spatial resolution.
Collimator types vary based on the specific photopeaks of the radionuclides being used.
- low energy - used for nuclides emitting photons up to 160 keV
- medium energy - used for nuclides emitting photons up to 250 keV
- high energy - used for nuclides emitting photons > 250 keV
Note that the septal thickness will primarily determine the amount of photon energies being accepted.
Hole shape and diameter will primarily determine the sensitivity and spatial resolution of the Gamma camera.
Collimators can also vary based on their design:
- parallel hole - multiple holes which run parallel to each other (most common design)
- pinhole - single hole with a single aperture, providing a magnified and inverted image with superior spatial resolution. Used in imaging small structures.
- converging - multiple holes which converge onto a central point, providing a magnified image with improved spatial resolution. Used in imaging small structures.
- diverging - multiple holes which fan away from the center, providing a minified image. Used in whole body imaging where a larger field of view is required.