Daily QC: Water CT Number Drift and a New Ring Artifact
On Monday morning, before the first patient, a technologist runs the daily QC scan on an 8-year-old 64-slice scanner. The water phantom is scanned with the routine head protocol. Results: the mean CT number of water is +9 HU (historical baseline 0 +/- 2 HU), image noise has risen from 0.34% to 0.52%, and a faint but complete ring is visible, centered near the phantom edge. The tube was replaced two years ago; the last air calibration was six months ago. Seven patients are already checked in, including one scheduled for a coronary calcium score and one for CT urography. The service engineer is four hours away. The physicist is reachable by phone.
1. What do you check before you call the engineer, and what are the plausible causes of these three findings together?
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Rule out operator and phantom error first - it is far more common than a hardware failure.
Check locally
- Phantom position: is it centered on the isocenter using the alignment lasers? Off-center positioning changes both the measured CT number and the noise, and it can smear a single bad channel into a partial or complete ring.
- Air bubbles, debris, or a scratched phantom: a bubble in the water produces a focal artifact; a dirty phantom housing changes attenuation.
- Correct protocol and reconstruction kernel selected for the QC scan.
- Repeat the scan after correction - a single run is not a trend.
If reproducible, the differential is
- Detector/channel drift: one bad or drifting channel produces a ring and shifts the mean CT number and noise.
- Tube aging or filtration change: the beam hardening characteristics shift, moving water off zero and increasing noise.
- Calibration interval: six months since air calibration; a calibration is due or overdue depending on the manufacturer's specification.
- Reconstruction/software change pushed since the last QC run.
Tolerances to reference: water CT number typically within +/-5 HU and noise within roughly +/-10-15% of baseline - this run fails both.
2. Can the scanner be used for patients today? How does the day's schedule change your answer?
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The answer is not a blanket yes or no - it depends on what each study depends on.
Studies that are sensitive to absolute CT numbers
- Coronary calcium scoring applies a fixed 130 HU threshold to every calcific voxel. A mean shift of +9 HU, plus increased noise, changes which voxels cross the threshold and can move the Agatston score, changing risk category. Hold or reschedule this patient; do not scan until QC is back within tolerance.
- Any study where a Hounsfield measurement drives a decision - fat quantification, liver iron or steatosis assessment, nodule enhancement thresholds, dual-energy material decomposition.
Studies that are relatively tolerant
- Contrast-enhanced studies where the task is morphological (CT urography for filling defects, obstruction, or mass). A ring artifact and modest noise increase degrade image quality but usually do not invalidate them - but the radiologist should know, and the images should be reviewed for the ring's position relative to the anatomy of interest.
Practical plan: call the physicist, restrict the scanner to studies that do not depend on quantitative CT numbers, notify the radiologist of the artifact, and document the decision. If the ring cannot be corrected, escalate to taking the scanner off-line for anything but urgent work.
3. What corrective actions and records are required before the scanner returns to unrestricted clinical use?
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- Repeat QC after repositioning; if the values return to baseline, record both runs - the failed run and the corrected run - because the failed run documents that the technologist caught and fixed a positioning error.
- If it is not positioning: perform the manufacturer's air (and water) calibration sequence if your department is authorized to do so, then re-run QC. Do not return the scanner to service on the basis of a calibration alone - verify with the phantom.
- Escalate to service for detector or tube evaluation if the ring persists. A single-channel failure will not be fixed by calibration.
- Document everything: date and time, phantom used, protocol, measured CT number and noise, the visible artifact, actions taken, who was notified (physicist, engineer, supervising radiologist), and the disposition of any patients scanned during the interval. This log is what an accrediting body inspects.
- Re-baseline only with physicist approval. Do not simply edit the baseline to match the new +9 HU reading - that converts an unresolved fault into a permanent, invisible one.
- Follow up: re-run QC daily until the values are stable for several consecutive days, and review any patient studies performed during the drift window.