Uniformity Drift on a Gamma Camera After PM
A dual-head gamma camera was just serviced (photomultiplier tube replaced in head 2). A technologist runs a daily intrinsic flood with Co-57. Head 1 shows an integral uniformity of 3.1% and a differential uniformity of 2.4%. Head 2 shows an integral uniformity of 5.8% and a differential uniformity of 4.9%. The department's acceptance threshold is integral uniformity ≤ 4.5% and differential ≤ 3.5% (per NEMA). The camera is scheduled for a full clinical schedule in 30 minutes. The service engineer says the PMT needs to 'settle' and suggests running the flood again after the schedule.
1. Interpret the uniformity results against the threshold and determine whether the camera should be used clinically.
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Head 1 passes both criteria (3.1% ≤ 4.5%; 2.4% ≤ 3.5%). Head 2 fails both: integral 5.8% > 4.5% and differential 4.9% > 3.5%.
A failing uniformity flood means the image is non-uniform, which can produce ring or blob artifacts, false-positive or false-negative interpretations, and inaccurate quantification. The camera should not be used for clinical imaging on head 2 until uniformity is corrected. The 'settling' argument is not a substitute for passing QC; the PMT gain must be tuned and the flood re-run. If head 1 passes and the system can operate in single-head mode, head 1 could be used, but the protocol and physician must agree, and the failing head must be locked out. Otherwise, reschedule or divert patients.
2. What is the most likely cause of the head 2 uniformity failure, and what corrective steps should be taken before re-running the flood?
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The most likely cause is that the replacement PMT's high voltage or gain was not properly tuned, or the energy correction/linearity map was not updated after the PMT change. A single PMT with incorrect gain produces a localized hot or cold spot that degrades both integral and differential uniformity.
Corrective steps:
- Verify PMT high-voltage settings against the service manual and tune the new PMT gain.
- Re-run the energy peaking and uniformity correction (flood source) calibration.
- Confirm the energy window is centered on the Co-57 122 keV photopeak.
- Re-run the intrinsic flood and confirm integral ≤ 4.5% and differential ≤ 3.5% on both heads.
- Document the correction and the passing flood before returning the camera to clinical use.