What is centre-of-rotation (COR) and why is it important in SPECT?

The centre of rotation (COR) in SPECT refers to the precise geometric axis around which the gamma camera rotates during image acquisition. Accurate alignment of this rotational axis with the reconstruction matrix is essential for correct tomographic reconstruction.

If the COR is misaligned, projection data will not be correctly mapped during reconstruction, leading to image distortion or ring artefacts. COR calibration is therefore a critical component of SPECT quality control.

The centre of rotation defines the camera’s rotational axis in SPECT; accurate alignment is essential to prevent reconstruction artefacts and preserve spatial accuracy.

Understanding the physics

SPECT reconstruction assumes that the camera rotates around a fixed, well-defined axis. Each projection is mathematically back projected or iteratively reconstructed under the assumption that the detector position relative to the reconstruction matrix is known precisely.

If the actual mechanical centre of rotation differs from the assumed centre used in reconstruction, a small positional offset is introduced at every projection angle. Because reconstruction integrates information from all angles, even small systematic offsets can accumulate into visible artefacts.

A common manifestation of COR error is a ring artefact in reconstructed slices. This occurs because activity that should be spatially consistent across projections is slightly displaced in a circular pattern.

COR errors may arise from:

Mechanical misalignment
Detector sag
Electronic drift

Regular quality control testing measures the apparent centre of rotation using a point source. If deviation exceeds acceptable limits, correction factors are applied within the reconstruction software.

Where this matters clinically

Uncorrected COR errors degrade spatial resolution and introduce reconstruction artefacts, particularly in high-resolution SPECT studies. Routine COR calibration ensures accurate tomographic reconstruction and reliable clinical interpretation.