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SPECT Reconstruction Artifact from Center of Rotation Error

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SPECT Reconstruction Artifact from Center of Rotation Error

A SPECT bone scan of the lumbar spine shows a subtle ring artifact and blurring that was not present on prior studies. The technologist reviews the QC log and notices that the center of rotation (COR) check was last performed 3 weeks ago and showed a COR offset of 2.5 mm; the department's tolerance is 1.5 mm. The camera has been used daily since then without a repeat COR check. The patient's study is being read as possible metastasis.

1. Explain how a COR offset produces the observed artifacts and whether the study should be interpreted as-is.

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A COR offset means the projection data are misregistered relative to the axis of rotation. During backprojection, this misregistration causes point sources to appear as rings or arcs rather than points, and edges blur. The effect worsens with distance from the COR and with higher resolution collimators.

A 2.5 mm offset exceeds the 1.5 mm tolerance and is consistent with the ring artifact and blurring. The study should not be interpreted as-is for a possible metastasis, because the artifact could mimic or obscure a lesion. The study should be repeated after COR correction, or at minimum the finding should be flagged and correlated with other imaging. The QC log shows the camera was out of tolerance for 3 weeks, so other studies in that period may also be affected and should be reviewed.

2. What corrective action is required, and what documentation should accompany it?

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Corrective action:

  1. Perform the COR calibration per manufacturer instructions and confirm offset ≤ 1.5 mm.
  2. Re-run a uniformity flood and verify it passes.
  3. Repeat the patient study if clinically feasible.
  4. Review studies acquired during the 3-week out-of-tolerance period for artifacts.

Documentation:

  • Record the COR offset before and after correction.
  • Note the date and time of correction and the person performing it.
  • Document the repeat study and any clinical communication.
  • File the QC record per regulatory requirements.