SPECT imaging requires the gamma camera to rotate around the patient while acquiring projection images from multiple angles. Reconstruction algorithms assume that the camera rotates around a fixed and consistent centre.

If the true centre of rotation shifts slightly during acquisition, the projections will not align correctly in reconstruction. Even very small deviations (in the order of millimetres) can introduce significant artefacts.

These artefacts arise because reconstruction algorithms mathematically back-project counts assuming perfect geometry. If the geometric assumptions are violated, errors accumulate across projections, leading to:

• Blurring of structures

• Loss of spatial resolution

• Ring artefacts in transverse slices

• Distortion of quantitative values

Centre of rotation testing is typically performed using a point source placed at a known position. As the detector rotates, the position of the point source is tracked in projection images. If the centre of rotation is correct, the source should follow a predictable sinusoidal path.

Any deviation from this expected path indicates misalignment, which can then be corrected electronically.

COR testing is especially important for quantitative SPECT and for small lesion detection, where even subtle misalignment may significantly affect image quality.