Tungsten alloy collimator is a device used to restrict the photon acceptance angle to provide positional information for detected photons. SPECT tungsten alloy collimator is a thick sheet of tungsten alloy of high atomic number, pierced by an array of holes.
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In theory, only photons that traverse these holes without striking the septa between them can be detected. In practice, some photons penetrate the septa and are detected, degrading the resolution and contrast of the reconstructed image. This is more likely for isotopes that emit higher-energy photons, such as 111In, frequently used for tumor imaging, with primary gamma-ray emissions at 171 and 245 keV.
In parallel-hole tungsten alloy collimators, all holes are parallel to one another and perpendicular to the detector face. The fundamental design parameter for such a SPECT collimator is the hole size. Smaller holes provide better information on the location of the emission and, consequently, better spatial resolution, but accept fewer photons, thereby increasing noise.
