Myocardial Perfusion Imaging (SPECT)⁹⁹ᵐTc-sestamibi / tetrofosmin · ²⁰¹Tl

Quick Answer

SPECT MPI compares myocardial tracer uptake at stress versus rest to detect ischemia (reversible defect) and infarct (fixed defect). Tc-99m-sestamibi or tetrofosmin are the usual agents; ECG-gating simultaneously yields LVEF, volumes, and wall motion. Stress is by exercise or a pharmacologic vasodilator/agent. Attenuation correction (CT or prone imaging) reduces artifacts.

Clinical Importance

SPECT MPI is a workhorse for evaluating stable chest pain, risk-stratifying known/suspected CAD, and assessing myocardium after revascularization. Normal MPI carries a low annual event rate; increasing defect burden predicts higher risk.

Patient Preparation

• Hold caffeine and methylxanthines ~12–24 h before vasodilator stress (they block adenosine receptors and blunt hyperemia).
• Hold nitrates and, per protocol, some anti-ischemics for diagnostic studies.
• NPO per local policy; diabetic-medication and glucose management for gated accuracy.
• Confirm no contraindication to the chosen stress agent.

Protocols (common variants)

• Same-day rest/stress or stress/rest (Tc-99m agents).
• Two-day stress/rest for larger patients (higher activity, better counts).
• Stress-first / stress-only — if stress is normal with good-quality gating and attenuation correction, rest imaging may be omitted (lower dose).
• Dual-isotope (Tl-201 rest + Tc-99m stress) — less used now due to dose.

Acquisition

Gated SPECT ~15–60 min after stress and rest injections (timing agent-dependent). Supine ± prone or upright imaging; CT attenuation correction where available. Ensure adequate counts and correct gating.

Processing

Reconstruct short-axis, vertical and horizontal long-axis slices; generate polar (bull's-eye) maps; quantify perfusion (summed scores) and function (LVEF, EDV/ESV, TID). Compare stress and rest datasets.

Interpretation

Normal Findings

Homogeneous tracer uptake throughout the left ventricle at stress and rest, normal wall motion/thickening, normal LVEF, no TID.

Abnormal Findings

• Reversible defect (present at stress, resolves at rest) → ischemia.
• Fixed defect (present at stress and rest) → infarct/scar.
• Partially reversible → mixed scar and peri-infarct ischemia.
• High-risk markers: large/multiterritory ischemia, TID, post-stress LV dysfunction, increased lung uptake (Tl-201).

Attenuation correction & artifact mitigation

Soft-tissue attenuation is the commonest source of false-positive defects, so most labs use one or more corrections:

• CT-based attenuation correction (SPECT/CT): the reference method; watch for CT–emission misregistration.
• Prone imaging: shifts the diaphragm and reduces inferior-wall attenuation (breast attenuation may improve or worsen).
• Upright / two-position imaging and dedicated cardiac cameras (CZT) improve counts and reduce artifact.
• ECG gating is the great disambiguator: preserved wall motion/thickening in an apparent fixed defect points to attenuation artifact rather than scar.

Stress-only / stress-first imaging

A stress-first protocol images stress before rest; if the stress study is unequivocally normal with good-quality gating and attenuation correction, the rest acquisition can be omitted (stress-only) — cutting radiation dose and scanner time roughly in half. It requires confidence in image quality and is best in lower-pretest-probability patients.

Quality checks that change interpretation

Confirm adequate counts, absence of significant patient motion, correct ECG gating (arrhythmia degrades gated EF), and no subdiaphragmatic (bowel/liver) activity overlapping the inferior wall. Increased lung uptake (thallium) and a dilated post-stress cavity (TID) are high-risk signs read off the same images.

Stress vs rest, and the two questions gating rescues

SPECT MPI compares tracer uptake at stress vs rest: a reversible defect (present at stress, resolves at rest) means ischemia; a fixed defect (present at both) means infarct/scar. ECG gating is the great disambiguator — preserved wall motion/thickening in an apparent fixed defect points to attenuation artifact, not scar — and it simultaneously yields LVEF, volumes, and TID. Grade extent and severity; don't stop at "abnormal."

Dose-sparing and the artifacts that fool you

A stress-first / stress-only protocol images stress before rest and, if the stress study is unequivocally normal with good gating and attenuation correction, omits the rest acquisition — roughly halving dose and scanner time, best in lower-pretest-probability patients. The commonest false-positive source is soft-tissue attenuation (breast anteriorly, diaphragm inferiorly), mitigated by CT attenuation correction (watch CT–emission misregistration), prone/upright imaging, and gating. And always screen for caffeine/methylxanthines (~12–24 h hold) before vasodilator stress — they blunt hyperemia and cause false negatives — plus balanced multivessel ischemia that can look falsely uniform (watch TID, post-stress LV dysfunction, thallium lung uptake).

Attenuation artifact vs true defect

The single most-tested SPECT skill — separating a soft-tissue artifact from real scar:

High-Yield Pearls

• Grade the study — extent and severity (summed scores) matter more than a binary "abnormal."
• Gating rescues interpretation: preserved wall motion in a fixed apparent defect suggests attenuation artifact, not scar.

Common Pitfalls

• Attenuation artifacts: breast (anterior) and diaphragm (inferior) mimic defects — use attenuation correction, prone imaging, and gating to distinguish.
• Balanced ischemia in multivessel disease can look falsely uniform on relative SPECT — watch for TID, LV dysfunction, and consider PET flow.
• Caffeine before vasodilator stress → false-negative for ischemia.
• Motion and subdiaphragmatic (bowel/liver) activity artifacts.

Reporting

State protocol/agent/stress type, image quality and attenuation correction, perfusion (location, size, severity, reversibility with summed scores), gated LVEF/volumes, TID, and an integrated risk impression. See the cardiac reporting page.

Self-Check

Key References

• ASNC imaging guidelines for SPECT MPI acquisition, processing, and interpretation.
• SNMMI procedure standard for myocardial perfusion imaging.