MPI Interpretation & Grading

Quick Answer

Perfusion is graded on a 17-segment model, each segment scored 0–4 for tracer uptake at stress and at rest. Summing gives the Summed Stress Score (SSS), Summed Rest Score (SRS), and their difference the Summed Difference Score (SDS = SSS − SRS), which quantifies ischemia. Scores can be expressed as % of abnormal myocardium. Add transient ischemic dilation (TID) and gated LVEF/volumes for a complete high-risk assessment.

⚠️ Numeric cutoffs below are the widely-cited conventional ranges. Exact thresholds depend on the software package, protocol, and normal database used — confirm against your lab's reference values.

The 17-segment model

The left ventricle is divided into 17 segments across basal, mid, apical short-axis rings plus the apex, mapped to coronary territories (LAD, LCx, RCA). Each segment is visually or quantitatively scored:

Summed scores

• SSS = sum of all 17 stress segment scores → total burden of ischemia plus infarct.
• SRS = sum of all 17 rest segment scores → fixed defect (infarct/scar) burden.
• SDS = SSS − SRS → reversible defect burden = ischemia.

Conventional SSS severity ranges:

SDS (ischemia): roughly 0–1 none, 2–4 mild, 5–6 moderate, ≥7 severe (lab-dependent). Moderate-to-severe ischemia is a common threshold prompting consideration of revascularization.

Percent myocardium

Summed scores can be converted to a % of the myocardium involved:

% myocardium = (summed score ÷ maximum possible score) × 100

For the 17-segment model the maximum is 17 × 4 = 68 (older 20-segment models used a maximum of 80). Reporting "% ischemic" and "% scar" is often more intuitive than raw scores; a commonly cited actionable threshold is >10% ischemic myocardium.

Transient ischemic dilation (TID)

TID is the ratio of the LV cavity volume post-stress to at rest. An enlarged post-stress cavity (or apparent enlargement from diffuse subendocardial hypoperfusion) suggests severe and extensive CAD, including balanced multivessel disease.

• TID ratio upper-limit-of-normal is protocol- and software-dependent, commonly around ~1.2 for gated SPECT (published Rb-82 PET and specific protocols have their own normal values). Always use your protocol-specific cutoff.

Gated function

Gated acquisition adds LVEF, end-diastolic and end-systolic volumes, and wall motion/thickening. Post-stress LV dysfunction, a drop in EF from rest to post-stress, and elevated volumes are high-risk features that refine perfusion findings.

Coronary territories (17-segment map)

Perfusion defects are localized to vascular territories, which is what makes a report clinically actionable:

• LAD: anterior and anteroseptal walls, apex (segments 1, 2, 7, 8, 13, 14, 17).
• LCx: lateral wall (segments 5, 6, 11, 12, 16).
• RCA: inferior wall and basal/mid inferoseptum (segments 3, 4, 9, 10, 15).

Territory boundaries vary with coronary dominance and individual anatomy, so multivessel patterns and apical involvement should be described rather than force-fit to a single artery; the apex (segment 17) and apical segments in particular have variable supply.

High-risk features (beyond the summed scores)

A study is "high risk" when features cluster — any of these should be flagged prominently:

• Large, multiterritory, or ≥10% ischemic myocardium (high SDS / % ischemic).
• Transient ischemic dilation (TID) above the lab threshold.
• Post-stress LV dysfunction — a drop in EF from rest to post-stress, or a low post-stress EF.
• Increased lung uptake (thallium) — a marker of stress-induced LV dysfunction.
• Extensive fixed defect (large infarct burden) with low resting EF.

Worked example

SSS 12, SRS 4 → SDS 8. As % of the 68-point maximum: ~12% ischemic (SDS 8/68), ~6% scar (SRS 4/68). SSS 12 falls in the moderately abnormal band (9–13). Reported as: "moderately abnormal scan; moderate, predominantly reversible defect in the LAD territory; ~12% ischemic, ~6% scar; no TID; preserved gated LVEF" — a statement that conveys extent, territory, reversibility, and risk in one line. (Use the MPI Summed Score tool to compute this quickly.)

SDS is the ischemia number

On the 17-segment model (each segment 0–4), sum to SSS (stress = ischemia + infarct), SRS (rest = fixed scar), and SDS = SSS − SRS — the reversible / ischemia number that most directly informs revascularization discussions. Express as % myocardium (max 68) if clearer; >10% ischemic is a commonly cited actionable threshold. Then integrate cavity (TID) and function (gated LVEF/volumes) — high-risk features cluster.

High-Yield Pearls

• SDS is the ischemia number — it's what most directly informs revascularization discussions.
• Combine perfusion (SSS/SDS/% myocardium), cavity (TID), and function (LVEF/volumes) — high-risk features cluster.
• Report severity, not just presence: "mildly abnormal, 6% ischemic" communicates far more than "abnormal."

Common Pitfalls

• Quoting a fixed TID cutoff without accounting for the software/protocol normal database.
• Confusing SRS (fixed/scar) with SDS (reversible/ischemia).
• Attenuation artifact inflating SRS — correlate with gated wall motion before calling scar.

Self-Check

Key References

• Berman/Cedars-Sinai quantitative perfusion scoring and normal ranges.
• ASNC guidelines on quantification and interpretation of MPI (17-segment model, summed scores, TID).