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SPECT MPI

SPECT MPI compares relative myocardial tracer uptake during stress and rest to detect ischemia, infarction, and high-risk physiology.

Core Principle

Coronary stenosis → impaired stress hyperemia → relatively reduced tracer delivery.

Because uptake is normalized to the region with the highest activity, SPECT demonstrates relative, not absolute, perfusion.

Common Protocols

Protocol Use
1-day rest–stress Common; higher stress dose limits shine-through
1-day stress–rest Allows stress-first evaluation
2-day stress–rest Improved count quality in larger patients
Stress-only Rest imaging omitted when stress study is unequivocally normal

Tc-99m sestamibi and tetrofosmin are the principal contemporary tracers. See Radiopharmaceuticals.

Systematic Interpretation

  1. Review the indication, symptoms, stress response, and ECG.
  2. Inspect raw projection images.
  3. Confirm adequate acquisition, reconstruction, and gating.
  4. Compare stress and rest perfusion.
  5. Define defect location, size, severity, and reversibility.
  6. Assess wall motion, thickening, LV volumes, and LVEF.
  7. Look for high-risk and extracardiac findings.
  8. Integrate everything into a final risk assessment.

Perfusion Patterns

Stress Rest Interpretation
Normal Normal Normal perfusion
Abnormal Normal or improved Ischemia
Abnormal Abnormal without change Scar or artifact
Abnormal Partially improved Mixed scar and ischemia

A fixed defect with normal regional wall motion and thickening favors attenuation artifact. A fixed defect with matching dysfunction favors infarction.

Semiquantitative Scoring

Each of the 17 LV segments is scored:

  • 0 = normal
  • 1 = mildly reduced
  • 2 = moderately reduced
  • 3 = severely reduced
  • 4 = absent uptake
\[ \text{SDS} = \text{SSS} - \text{SRS} \]
  • SSS: summed stress score
  • SRS: summed rest score
  • SDS: reversible component

Approximate ischemic burden:

\[ \%\text{ ischemic myocardium} = \frac{\text{SDS}}{68}\times100 \]
Defect size Approximate LV involvement
Small <10%
Moderate 10%–20%
Large >20%

Gated SPECT

ECG gating provides:

  • LVEF
  • LV volumes
  • Regional wall motion
  • Regional wall thickening
  • Evidence of post-stress stunning

Wall thickening is reflected by increasing regional brightness during systole. Gating may be unreliable with significant arrhythmia, poor counts, or incorrect R-wave detection.

High-Risk Findings

  • Large or multiple reversible defects
  • Ischemia involving multiple vascular territories
  • Transient ischemic dilation
  • Stress-induced fall in LVEF
  • Post-stress regional stunning
  • Increased lung uptake
  • Increased RV uptake or RV dilation
  • Severe LV dysfunction
  • High-risk exercise or ECG findings

Balanced ischemia

Left main or multivessel CAD may produce uniformly reduced perfusion and an apparently normal relative study. Suspect it when perfusion appears normal but TID, LV dysfunction, severe coronary calcium, ischemic ECG changes, or other high-risk findings are present.

Reporting Essentials

Include:

  • Stress method and adequacy
  • Study quality
  • Defect location, size, severity, and reversibility
  • LV function and volumes
  • TID and other high-risk findings
  • Comparison with prior imaging
  • Overall interpretation and risk

Board Pearls

  • Reversible defect = ischemia.
  • Fixed defect + abnormal motion = infarction.
  • Fixed defect + normal motion = consider attenuation.
  • TID with abnormal perfusion suggests extensive CAD.
  • A normal relative study does not exclude balanced ischemia.
  • Review raw data before interpreting reconstructed images.

References