What is intrinsic and extrinsic uniformity in gamma camera imaging?

Intrinsic uniformity refers to the uniformity of detector response across the scintillation crystal when measured without a collimator. It assesses how evenly the gamma camera detector records events over its field of view.

Extrinsic uniformity refers to overall system uniformity when the collimator is attached. It reflects the combined performance of the detector and collimator.

Intrinsic uniformity reflects detector response alone, while extrinsic uniformity reflects the combined performance of detector and collimator which determines clinical image consistency.

In clinical practice, extrinsic uniformity is more relevant because imaging is always performed with a collimator in place.

Understanding the physics

Uniformity describes how consistently the detector responds to a uniform radiation field. Ideally, if the camera is exposed to a uniform source of gamma photons, the resulting image should show equal counts across all regions.

Intrinsic uniformity

Intrinsic uniformity is measured by removing the collimator and placing a uniform radiation source close to the detector. This isolates the performance of:

The scintillation crystal
Photomultiplier tubes
Signal processing electronics

Non-uniformity may arise from:

Variations in crystal thickness
Differences in photomultiplier gain
Electronic drift

If intrinsic uniformity is poor, artefacts will appear across the image even if the collimator is functioning correctly.

Extrinsic uniformity

Extrinsic uniformity is measured with the collimator attached. It includes both intrinsic detector effects and any non-uniformity introduced by the collimator, such as:

Septal damage
Collimator contamination
Manufacturing defects

Because the collimator absorbs most photons, small structural imperfections can create visible artefacts.

Uniformity is typically evaluated using flood field images, where a large uniform source is imaged and analysed for count variation across the field.

Where this matters clinically

Poor uniformity produces artefacts that can mimic pathology or degrade image quality. Regular uniformity testing is essential for quality control, particularly before SPECT imaging, where non-uniformities can create ring artefacts in reconstructed images.