13.1 Myocardial Perfusion Imaging
Key Takeaways
MPI maps relative myocardial blood flow with Tc-99m sestamibi/tetrofosmin SPECT (± ECG gating, prone, SPECT/CT attenuation) or PET agents Rb-82 and N-13 ammonia
One-day Tc protocols use low rest then high stress (≈3× activity); two-day uses roughly equal high doses; inject at peak exercise or peak pharmacologic hyperemia
Patient prep: NPO as ordered, hold caffeine 12–24 h for vasodilator stress, hold beta blockers for exercise stress when the order requires them
Common artifacts—breast attenuation (anterior/anteroseptal), diaphragmatic (inferior), motion, and LBBB-related septal defects—must be recognized before calling ischemia or scar
Gated SPECT adds wall motion and EF; fixed defect with normal thickening often favors attenuation over true infarct
13.1 Myocardial Perfusion Imaging
Quick Answer: MPI images relative coronary flow at stress and rest. Tc-99m sestamibi/tetrofosmin SPECT (± gating, prone, SPECT/CT AC) is standard; PET uses Rb-82 or N-13 ammonia. Sequence and dose ratios (one-day low→high vs two-day equal) protect image quality. Prep = NPO, no caffeine 12–24 h for vasodilators, hold β-blockers for exercise when ordered. Flag breast, diaphragm, motion, LBBB artifacts.
Domain V cardiac items test protocol choice, timing, prep, and tech-level artifact recognition—not full cardiologist scoring of every coronary territory.
Clinical Role of MPI
| Indication | Teaching point |
|---|---|
| Suspected CAD / chest pain | Stress–rest patterns support ischemia vs scar |
| Known CAD risk stratification | Extent/severity of ischemia guides management |
| Preoperative cardiac risk | Functional capacity plus perfusion when indicated |
| Viability adjunct (selected) | Rest uptake / PET pathways (see §13.2) |
The technologist owns dose, injection timing, positioning, gating, and raw-data QC.
Correlate symptoms, ECG changes, and achieved heart rate or pharmacologic adequacy with the images. A technically perfect scan after a failed stress still underestimates ischemia—document peak HR, symptoms, ST changes, and whether caffeine or β-blockers were held.
Planar vs SPECT MPI
Early MPI used planar multi-view imaging (anterior, LAO, left lateral). Planar is historical foundation and rare fallback; modern practice is SPECT (and hybrid SPECT/CT or PET/CT). Tomography separates overlapping walls that planar cannot.
SPECT MPI: Gated and Non-Gated
After IV injection, wait for blood-pool and hepatobiliary clearance (often ~30–60 min rest, ~15–45 min post-stress; tetrofosmin often earlier than sestamibi). Acquire SPECT with arms up when tolerated.
| Mode | What you get | Why it matters |
|---|---|---|
| Non-gated SPECT | Perfusion only | Use if irregular rhythm prevents gating |
| ECG-gated SPECT | Perfusion + wall motion, thickening, LVEF | Normal motion favors attenuation over infarct |
| Supine SPECT | Default geometry | Breast and diaphragm attenuation common |
| Prone SPECT | Belly down | Often reduces inferior diaphragmatic attenuation |
| SPECT/CT | SPECT + low-dose CT attenuation correction (AC) | Still review non-AC and raw projections |
Prone + supine (or AC + non-AC) comparison helps when fixed inferior or anterior defects look artifactual. CT-AC caveats: misregistration from breathing or position change creates false defects—check fused alignment before trusting AC-only slices.
PET Myocardial Perfusion
| Agent | Half-life / source | Teaching notes |
|---|---|---|
| Rb-82 chloride | ≈ 75 s; Sr-82 generator | Rest and stress same session; absolute flow possible |
| N-13 ammonia | ≈ 10 min; cyclotron | High extraction; needs nearby cyclotron |
| O-15 water (selected) | Very short; cyclotron | Quantitation/research |
PET offers higher resolution and routine CT attenuation correction. Stress is usually pharmacologic because of short half-lives. Inject and image on a tight clock—especially Rb-82.
Tc-99m Protocols: One-Day vs Two-Day
Tc-99m agents show minimal clinical redistribution; separate stress and rest injections are required.
| Protocol | Dose pattern (adult order-of-magnitude) | Sequencing logic |
|---|---|---|
| One-day rest → stress | Rest ~8–12 mCi, stress ~24–36 mCi (≈3:1) | High second dose dominates residual |
| One-day stress → rest (selected) | Stress first then higher rest, or stress-only if normal | Lab-specific triage pathways |
| Two-day | ~20–30 mCi each day (roughly equal) | Large patients; avoid same-day residual |
| Stress-only (selected low-risk) | Single stress ~25–40 mCi class | Rest canceled if stress clearly normal |
Injection timing: at peak treadmill exercise (continue ~1 min when safe) or at peak pharmacologic hyperemia (during adenosine, after regadenoson bolus, or timed with dipyridamole). Secure IV—extravasation ruins relative counts.
Same-day dual-isotope protocols (historical Tl-201 rest + Tc-99m stress) appear on older exams but are less common now; know that different energies and redistribution properties change processing. For pure Tc one-day work, never invert the high/low ratio without a written protocol reason.
Patient Preparation (High-Yield)
| Prep item | Why |
|---|---|
| NPO / light meal as ordered | Lower GI activity; safer stress |
| Hold caffeine 12–24 h before adenosine, dipyridamole, regadenoson | Caffeine antagonizes A2A hyperemia → false-negative MPI |
| Hold β-blockers for exercise when ordered | Protects diagnostic heart-rate response |
| Other meds per order | Document what the patient took |
| History: LBBB, paced rhythm, COPD/asthma, caffeine today | Chooses modality; predicts artifacts |
| Pregnancy screen; IV; coach stillness | Safety and motion control |
Common Artifacts and Pitfalls
| Artifact | Typical appearance | Tech response |
|---|---|---|
| Breast attenuation | Fixed anterior/anterolateral | Rotating cine; prone or AC; normal gated motion |
| Diaphragmatic attenuation | Fixed inferior wall | Prone, AC, gated thickening |
| Motion | Cine discontinuities, false defects | Repeat; validated motion correction |
| LBBB / RV paced rhythm | Septal changes ± “reversible” look | Prefer pharmacologic stress when ordered; document rhythm |
| Subdiaphragmatic liver/bowel | Inferior scatter / normalization error | Delay, hydrate, fatty snack, re-image |
Review the rotating projection cine on every study before reconstruction sign-off. Truncation, upward creep, and arm-down positioning create defects that look like CAD until the raw data are inspected.
Bottom line: match agent and camera mode to the question, protect dose ratio and injection timing, enforce caffeine and med holds, and review raw projections + gating before calling ischemia or scar.
A one-day rest–stress Tc-99m sestamibi protocol is ordered. Which dose relationship best matches standard teaching practice?
Rest ~8–12 mCi followed by stress ~24–36 mCi so the second injection dominates residual activity
Equal 5 mCi doses at rest and stress on the same day to minimize total exposure
Rest 30 mCi and stress 10 mCi so residual rest activity is maximized during stress imaging
A single 5 mCi stress dose only, with rest always omitted regardless of findings
Why must patients avoid caffeine for 12–24 hours before pharmacologic vasodilator MPI with adenosine, dipyridamole, or regadenoson?
Caffeine increases Tc-99m sestamibi hepatobiliary clearance and always voids the rest dose
Caffeine permanently labels red cells and blocks ECG gating triggers
Caffeine antagonizes adenosine-receptor–mediated coronary hyperemia and can cause false-negative perfusion results
Caffeine converts Rb-82 into N-13 ammonia inside the generator
Which statement best describes a common inferior-wall fixed defect on supine SPECT MPI in a male patient with normal inferior wall motion on gated images?
It must represent a completed inferior infarct and needs no technical review
It is pathognomonic for LBBB and requires conversion to exercise stress only
It proves Rb-82 PET was performed instead of Tc-99m SPECT
It often reflects diaphragmatic attenuation; prone imaging, attenuation correction, and normal thickening support an artifactual cause
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