Quality Control Procedures
Gamma camera, SPECT, PET/CT, dose calibrator, and generator QC, cadence and pass criteria
Quick Answer
A defined quality-control program keeps imaging quantitative and detects failures before they mimic disease. The essentials: gamma camera — daily uniformity and energy peak, periodic spatial resolution/linearity (bar phantom) and SPECT center-of-rotation (COR); PET/CT — daily normalization/blank and cross-calibration; dose calibrator — daily constancy, periodic linearity, accuracy, geometry; generator — Mo-99 breakthrough (≤ 0.15 µCi/mCi) and aluminum (≤ 10 µg/mL). Each test has a cadence and an acceptance criterion; failing one has a characteristic artifact.
Gamma Camera (SPECT) QC
| Test | Cadence | What it detects / criterion |
|---|---|---|
| Energy peak (photopeak) | Daily | Correct window centering; drift causes count loss/artifact |
| Uniformity (flood) | Daily | Non-uniform detector response → ring/patchy SPECT artifacts; integral/differential uniformity (commonly ≤ ~5% for CFOV) |
| Spatial resolution & linearity (bar phantom) | Weekly | Four-quadrant bar phantom; smallest resolved bars = resolution; bar straightness = linearity |
| Center of rotation (COR) | Weekly–monthly | SPECT reconstruction alignment; COR error → "tuning-fork"/doughnut ring artifacts and blurring |
| Tomographic uniformity / sensitivity | Periodic | System sensitivity (cpm/µCi) and SPECT uniformity |
Extrinsic floods include the collimator (test the clinical system); intrinsic floods are without the collimator (test the detector). A non-uniformity projects as a ring artifact at the corresponding radius on SPECT — the single most important reason daily uniformity is mandatory.
PET/CT QC
| Test | Cadence | Purpose |
|---|---|---|
| Daily normalization / blank scan | Daily | Detector-block stability; drift degrades uniformity/quantitation |
| Calibration / well-counter cross-calibration | Periodic | Keeps SUV quantitative (scanner ↔ dose calibrator ↔ clock) |
| CT QC | Per CT program | Attenuation-map accuracy, CT number/water |
| NEMA NU-2 performance | Acceptance/annual | Resolution, sensitivity, scatter fraction, NECR, image quality |
A miscalibrated cross-calibration (scanner, dose calibrator, and clock must agree) silently biases SUV — a quantitation error, not a visual one.
Dose Calibrator QC
The instrument behind every administered activity — its QC is non-negotiable:
| Test | Cadence | Method / criterion |
|---|---|---|
| Constancy | Daily | Long-lived reference source (Cs-137, Co-57); reading stable within ~±5–10% |
| Accuracy | Annual (and at install) | NIST-traceable source(s); within ~±5% (±10% action) |
| Linearity | Quarterly | Decay method or shield (Calicheck) method across the activity range |
| Geometry | At installation | Volume/container-dependence correction factors |
Well Counter & Survey Instruments
- Well counter: daily constancy and energy calibration; efficiency for quantitative counting (blood dosimetry, wipe tests).
- Survey / GM meters: annual calibration, daily battery/constancy check; used for contamination and exposure surveys and package/area monitoring.
Generator & Radiopharmaceutical QC
| Test | Limit | Note |
|---|---|---|
| Mo-99 breakthrough | ≤ 0.15 µCi Mo-99 / mCi Tc-99m | Radionuclidic purity, at administration |
| Aluminum (Al³⁺) | ≤ 10 µg/mL | Column breakthrough (colorimetric strip); excess disrupts labeling/colloids |
| Radiochemical purity | typically ≥ 90–95% | Chromatography; free-pertechnetate / hydrolyzed-reduced species |
| Sterility/endotoxin | Per USP | Compounded and PET products |
Failure → Artifact Map
| QC failure | Characteristic artifact |
|---|---|
| Non-uniformity | Ring artifact on SPECT (at the defect radius) |
| COR error | Doughnut / tuning-fork blurring, point-source ring |
| Wrong photopeak/energy | Count loss, degraded contrast |
| Collimator–isotope mismatch | Septal-penetration star (e.g. low-energy collimator with I-131) |
| Cross-calibration error | Biased SUV (silent quantitative error) |
| Mo-99 breakthrough | Added dose, degraded images, high-energy contamination |
Board-Level Synthesis
The daily anchors are gamma-camera uniformity + energy peak, PET normalization/blank, and dose-calibrator constancy. The periodic anchors are bar-phantom resolution/linearity and SPECT COR (weekly–monthly), and dose-calibrator linearity (quarterly)/accuracy (annual)/geometry (install).
A non-uniformity projects as a ring artifact at the corresponding SPECT radius, and a COR error causes doughnut/tuning-fork blurring — the two failures most likely to mimic or obscure disease, and the reason their QC is frequent.
Two limits anchor generator radionuclidic/chemical purity: Mo-99 breakthrough ≤ 0.15 µCi/mCi and aluminum ≤ 10 µg/mL. The dose calibrator underlies every administered activity — daily constancy, quarterly linearity, annual accuracy, and geometry at install — and PET cross-calibration (scanner ↔ dose calibrator ↔ clock) keeps SUV trustworthy, a quantitation error that is invisible on the image.
Related Pages
- Physics: Radiation detection & instrumentation, counting statistics & ROC (chi-square/constancy), radiopharmaceuticals & production (generator QC).
- Pitfalls: Pearls, pitfalls & normal variants (artifacts).
Figure / Diagram Suggestions
- A QC cadence calendar (daily/weekly/quarterly/annual by instrument).
- A failure → artifact plate (ring, doughnut, septal-penetration star).
- A dose-calibrator four-test schematic (constancy/accuracy/linearity/geometry).
Self-Check
Q1. Which gamma-camera QC tests are performed daily, and what artifact does a uniformity failure cause?
Answer: Daily uniformity (flood) and energy-peak checks; a non-uniformity projects as a ring artifact at the corresponding radius on SPECT.
Q2. What are the four dose-calibrator QC tests and their cadences?
Answer: Constancy (daily), linearity (quarterly), accuracy (annual), and geometry (at installation).
Q3. State the Mo-99 breakthrough and aluminum limits for a Tc-99m generator eluate.
Answer: Mo-99 ≤ 0.15 µCi per mCi Tc-99m (at administration) and aluminum ≤ 10 µg/mL.
Q4. A SPECT study shows a doughnut/tuning-fork blurring artifact. Which QC parameter is likely off?
Answer: The center of rotation (COR) — misalignment causes point-source ring/doughnut and tomographic blurring.
Evidence & sources
https://nuc.med/qc-procedures/ Report an issue