A gamma camera detects photons using a large scintillation crystal coupled to an array of photomultiplier tubes (PMTs). When a gamma photon interacts with the crystal, light is produced and detected by multiple PMTs. The relative signals from these PMTs determine the calculated position of the interaction.

For accurate imaging, each part of the detector must respond equally to identical photon flux.

Uniformity testing is performed by placing a uniform flood source in front of the detector. This source may be:

• An intrinsic source (no collimator, using a point source at a distance)

• An extrinsic flood source (collimator in place, using a sheet source)

If the detector system is functioning correctly, the resulting image should show evenly distributed counts across the entire field.

Non-uniformities may arise from:

• PMT gain drift

• Crystal defects

• Electronic noise

• Collimator damage (in extrinsic testing)

Even small variations can produce ring artefacts in SPECT reconstruction, because reconstruction algorithms amplify systematic detector non-uniformities during rotational acquisition.

Uniformity is often quantified using parameters such as integral uniformity and differential uniformity, which measure count variation across the field of view.

Because the gamma camera assumes uniform detector response during reconstruction, daily uniformity testing is critical.