15.3 Acquisition, Positioning, and Image Quality
Key Takeaways
- Correct positioning, anatomical markers, and immobilization reduce motion artifact and laterality errors; label right/left and injection sites when protocol requires.
- Acquisition parameters—matrix, time/counts, energy window, collimator, orbit, and zoom—must match the clinical question and photon energy.
- Technologists evaluate images at a technical level for completeness, count density, motion, and obvious artifacts before releasing the patient; know when to repeat or add views.
- SPECT/CT and PET/CT require accurate co-registration; CT misalignment, truncation, and metal can create attenuation-correction artifacts that mimic disease.
- Before administration: reconcile patient identity with RP label, calculate volume for the prescribed activity from assayed concentration, and use the ordered route/technique (IV bolus vs infusion, oral, inhalation).
15.3 Acquisition, Positioning, and Image Quality
Quick Answer: Reconcile patient ↔ RP ↔ order, calculate the correct volume, administer by the ordered route, then acquire with correct collimator, energy window, matrix, time/counts, and positioning. Spot artifacts early, fix SPECT/CT–PET/CT registration, and repeat or add views when the study is incomplete or non-diagnostic.
Image quality is a technologist product. Physicians interpret disease; technologists deliver technically adequate, correctly labeled, artifact-controlled datasets. Domain V acquisition items often present a flawed image description and ask what went wrong—or what to do next before releasing the patient.
Patient / RP Reconciliation and Volume Calculation
Before administration, perform a final three-way check:
- Patient identity (two identifiers)
- Radiopharmaceutical label (agent, activity, calibration time, expiration, route)
- Order / protocol (study type, prescribed dosage range)
Assay the dose in the dose calibrator (or verify unit-dose assay) and apply decay correction to administration time. If concentration is known:
[ \text{Volume (mL)} = \frac{\text{Prescribed activity}}{\text{Concentration (activity/mL)}} ]
Account for residual in the syringe after injection when residual activity matters for quantification or therapy records. Never administer outside the prescribed range without authorized adjustment. If the assayed activity is outside the ordered window after decay correction, stop—redraw, reorder, or obtain authorized user direction rather than “close enough” injection.
Administration Route and Technique
| Route | Technique keys |
|---|---|
| IV | Patent access; avoid infiltrate; flush per protocol; note site for markers |
| Bolus | Tight bolus for first-pass/flow studies; arm position consistent |
| Infusion | Pump rates for pharmacologic stress or certain therapies as ordered |
| Oral | Capsule integrity; NPO rules; document time of ingestion |
| Inhalation | Seal, breathing pattern, xenon trap / exhaust systems |
| Intrathecal / other special | Physician-performed; tech supports sterile field and imaging timing |
Infiltrate destroys quantification (renogram, GFR, gastric emptying curves) and may require re-injection decisions by the authorized user. Document and image the site when helpful.
Positioning, Markers, and Immobilization
- Align anatomy to protocol: supine vs upright (gastric emptying), arms up for torso SPECT/PET when tolerated, head immobilized for brain SPECT/PET.
- Place anatomical markers (cod liver oil capsules, point sources, electronic markers) for laterality, landmarks, or injection sites per lab SOP.
- Use straps, head holders, and knee supports to reduce motion—never force a painful position that guarantees motion or injury.
- Remove attenuating objects: jewelry, ECG leads when possible, thick clothing zippers, external metal.
Imaging Parameters
| Parameter | Effect on image |
|---|---|
| Collimator | LEHR/LEAP for Tc-99m; medium/high energy for Ga-67, I-131, In-111 as appropriate |
| Energy window | Centered on photopeak(s); wrong window loses counts or admits scatter |
| Matrix | Finer matrix → better potential detail, needs more counts; coarser for low-count studies |
| Time / counts | Information density; premature stop → noisy non-diagnostic images |
| Zoom / orbit | SPECT radius as close as safe; 360° vs 180° cardiac orbits per protocol |
| Gating | ECG quality for MUGA/MPI; bad gating → wall-motion errors |
Match parameters to photon energy and clinical question. Using a low-energy collimator for I-131 is a classic artifact factory (septal penetration). Multi-peak radionuclides (e.g., Ga-67, In-111) need all protocol windows enabled—missing a peak silently starves counts.
Normal vs Abnormal Patterns at the Technologist Level
Technologists are not the final interpreters, but they must recognize technical completeness and gross unexpected findings that require extra views or urgent physician notification (e.g., obvious injection failure, missing organ in field of view, severe motion, empty syringe residual suggesting total infiltrate).
| Check | Action if failed |
|---|---|
| Correct body part / FOV | Reposition; add views |
| Count density adequate | Extend time or repeat |
| Motion | Repeat sequence |
| Laterality labeled | Correct labels before archive |
| Hybrid alignment | Re-register or repeat CT segment per protocol |
Artifacts and Causes (High-Yield)
| Artifact | Common cause |
|---|---|
| Motion blur / streaks | Patient movement, free-breathing mismatch |
| Hot spots on skin | Contamination (urine, saliva, injection drip) |
| Cold defects from metal | Pacemaker, jewelry, orthopedic hardware on attenuation map |
| Truncation artifact | Arms-down CT FOV too small on SPECT/CT or PET/CT |
| Misregistration | Patient shifted between emission and CT |
| Photomultiplier / crystal issues | Camera QC failures—do not image clinically |
| Wrong collimator / window | Septal penetration, low counts, scatter |
| Attenuation without correction | Soft-tissue (breast, diaphragm) on SPECT MPI |
Contamination hunt: if an unexpected focus appears, survey gown/table/skin and re-image after cleaning when appropriate.
Co-registration: SPECT/CT and PET/CT
Hybrid systems use CT for anatomy and attenuation correction. Co-registration errors create false hot/cold regions—especially near diaphragm, head/neck, and extremities.
Tech responsibilities: instruct breath-hold or free-breathing protocols consistently; minimize table shifts; verify overlay before finalizing reconstruction; flag metal and contrast that distort attenuation maps. When AC images look non-physiologic, review non-AC images—an exam favorite troubleshooting step. Arm position must match between emission and CT whenever the protocol allows; arms-down CT with arms-up PET/SPECT is a common self-inflicted misregistration.
When to Repeat or Add Views
Repeat or add views when:
- Motion, low counts, or wrong FOV make the study non-diagnostic
- Suspected contamination vs true lesion needs confirmation
- Protocol requires post-void, delayed, oblique, or spot views not yet obtained
- SPECT shows edge-of-FOV truncation of the organ of interest
Do not repeat unlimited times for minor cosmetic issues after a diagnostic dataset is secured—ALARA and schedule reality matter. Escalate ambiguous cases to the supervising physician.
Bottom line: quality chain = right patient/dose/volume/route → right geometry/parameters → artifact control → hybrid registration → know when to fix before the patient leaves.
A PET/CT attenuation-corrected image shows a sharp photopenic defect overlying a hip prosthesis that is less severe on the non-attenuation-corrected images. What is the most likely explanation?
Prescribed activity is 20 mCi and the assayed concentration of the Tc-99m agent is 10 mCi/mL at administration time. What volume should be drawn (ignoring residual), and what must still be verified?
Which acquisition choice is most appropriate for routine Tc-99m imaging versus I-131 imaging?