10.3 PET and PET/CT Systems and QC
Key Takeaways
- PET detects near-simultaneous opposite annihilation photons (coincidence) to define lines of response (LORs); time-of-flight (TOF) uses arrival-time differences to localize events along the LOR
- Daily blank scans and periodic normalization keep detector efficiencies matched; failed blank/normalization QC blocks accurate imaging
- Well-counter (or equivalent dose calibrator/cross-calibration) pathways support quantitative SUV—activity concentration relative to injected dose and body size
- CT provides attenuation correction and localization on PET/CT; metal, motion, and truncation distort AC maps
- SUV pitfalls include wrong clock/dose times, residual syringe activity, hyperglycemia for FDG, and partial-volume underestimation of small lesions
10.3 PET and PET/CT Systems and QC
Quick Answer: PET = coincidence along LORs (± TOF). Daily blank scan, normalization, and solid SUV cross-calibration. CT AC fails with metal, motion, and truncation; SUV fails with timing, glucose, and partial volume.
Positron emission tomography (PET) images positron emitters (e.g., F-18, Ga-68, Rb-82). A positron annihilates with an electron, producing two 511 keV photons emitted roughly 180° apart. The scanner registers a true event when opposing detectors detect both photons within a short coincidence timing window.
Coincidence, LOR, and Time-of-Flight
| Concept | Meaning |
|---|---|
| Coincidence detection | Two 511 keV interactions accepted as a pair if within the timing window and energy windows |
| Line of response (LOR) | The line connecting the two detection points—annihilation is assumed somewhere along that line |
| True / scatter / random coincidences | Trues = one annihilation; scatter = deflected photon; randoms = unrelated photons within the window |
| Time-of-flight (TOF) | Measures small arrival-time difference to localize the event along the LOR, improving SNR especially in large patients |
PET does not use lead parallel-hole collimators for primary spatial encoding; electronic collimation via coincidence defines geometry. Physical septa appear in some historical 2-D systems (below).
2-D vs 3-D Acquisition Concepts
| Mode | Idea | Trade-off |
|---|---|---|
| 2-D | Interplane septa reduce out-of-plane scatter and randoms | Lower sensitivity |
| 3-D | Septa retracted/absent; accept more oblique LORs | Higher sensitivity; more scatter/randoms—needs good corrections |
Most modern whole-body PET/CT systems operate in 3-D with sophisticated scatter and randoms correction. Exam items may still contrast the concepts.
Core PET QC
Daily Blank Scan
A blank scan (or equivalent daily detector check using a transmission rod/source or manufacturer daily protocol) verifies that detectors respond uniformly enough for clinical use. It is analogous in spirit to a gamma-camera flood: catch dead blocks, unstable electronics, or temperature issues before patients.
| If blank fails | Action |
|---|---|
| Localized cold block | Service; do not image through a dead detector region |
| Global count loss | Check sources, HV, cooling, coincidence timing |
| Pass | Document; proceed with remaining daily checks |
Normalization
Normalization measures relative efficiency of each LOR or detector crystal pair so reconstruction can compensate for efficiency differences. It is performed on a schedule (and after major service) using manufacturer protocols with a known source geometry. Skipping normalization after crystal or module replacement yields patterned artifacts and bad quantitation.
Cross-Calibration / Well Counter for SUV
Standardized uptake value (SUV) compares lesion activity concentration to injected activity normalized by body weight (or lean body mass/BSA variants):
SUV = (activity concentration in ROI) / (injected activity / body mass)
(with consistent units and decay correction)
The PET scanner’s quantitative scale must match the dose calibrator used to assay the syringe. Departments perform cross-calibration (sometimes involving a well counter or phantom with known activity) so that 1 Bq/mL in the phantom reads correctly on PET. Without it, SUVs drift between cameras or after software updates.
CT Attenuation Correction on PET/CT
CT images are converted to a 511 keV attenuation map for PET reconstruction. Benefits: fast AC, anatomic fusion, and localization. Requirements: same patient position and good registration between CT and PET bed positions.
PET Artifacts and Pitfalls
| Problem | Effect on images / SUV |
|---|---|
| Metal implants | CT streak artifacts → incorrect μ → over/under-corrected emission |
| Patient motion | Misregistration; false defects or hot rims; wrong SUV |
| Truncation | Body outside CT FOV → incomplete μ-map |
| Residual activity / wrong injection time | Wrong decay correction → SUV error |
| Extravasation | Dose not fully in patient; SUV underestimated |
| High plasma glucose (FDG) | Competes with FDG; tumor SUV may fall |
| Partial-volume effect | Small lesions look colder than true concentration |
Timing discipline: record assay time, injection time, and scan start; assay residual syringe activity when protocol requires. Clock mismatch between hot lab and scanner is a classic quantitative error.
SUV Basics for the Exam
| Factor | Direction of effect (typical) |
|---|---|
| Longer uptake time (FDG) | Tumor SUV often rises over early intervals |
| Larger patient / wrong weight entry | SUV changes if weight wrong |
| Hotter reconstruction / sharper filter | Can raise max SUV |
| Larger ROI / mean vs max | Different numeric SUV |
Report which SUV metric (SUVmax, SUVmean) and reconstruction the site uses. Compare serial studies only when protocols match.
Randoms, Dead Time, and High-Activity Studies
At high count rates (e.g., Rb-82 MPI, high-activity dynamic studies), random coincidences and dead time rise. Systems estimate randoms (delayed window or singles-based methods) and apply dead-time correction, but extreme rates can still degrade quantitation. Follow manufacturer activity guidelines for injected dose and start times.
Daily / periodic PET QC summary (teaching):
| Test | Typical role |
|---|---|
| Blank / daily detector check | Catch failed blocks before patients |
| Normalization | Relative LOR/crystal efficiencies |
| Well-counter / dose-calibrator cross-cal | Absolute scale for SUV |
| Phantom (uniform / ACR-type) | Image quality, SUV recovery, artifact survey |
Worked SUV pitfall: Injected assay 10.0 mCi at 08:00; residual syringe 1.0 mCi at 08:05 not subtracted; scan at 09:00 with decay correction assuming 10.0 mCi in the patient. True net injected activity is lower → SUV systematically high. Always apply residual and correct clocks.
TOF note: TOF does not remove the need for AC or scatter correction; it improves effective sensitivity/SNR by constraining where along the LOR the event likely occurred.
Memory aid: PET QC Blank, Normalization, Cross-cal → “BNC” before trusting SUV.
In PET imaging, a line of response (LOR) is best described as:
A daily PET blank scan shows a persistent cold detector block. The most appropriate next step is:
Which factor most directly causes underestimation of FDG SUVmax in a small lung nodule even when calibration is correct?