Learn · Part 2. The tests · 5 min read
Stress SPECT myocardial perfusion imaging
How SPECT shows relative blood flow at stress and rest, how to read reversible and fixed defects, and why balanced ischemia and attenuation matter.
60-second take
- SPECT images the distribution of a radiotracer at stress and rest. A reversible defect suggests ischemia; a fixed defect suggests scar. Gated images add ventricular function.
- It shows relative perfusion, so balanced ischemia across territories can be missed.
- Attenuation (dense breasts, obesity) and motion can create or hide defects. Attenuation correction or prone imaging may reduce false positives.
- Exercise is preferred when adequate. Vasodilator stress is used when the patient cannot exercise, and also for LBBB or paced rhythm, to avoid an artifactual septal defect.
- A technetium SPECT study averages about 10 mSv; where PET is available, guidelines consider it reasonable in preference to SPECT.
What it is and how it works
Single-photon emission computed tomography (SPECT) myocardial perfusion imaging shows how blood reaches the heart muscle. A radioactive tracer is injected into a vein, the heart muscle takes it up in proportion to its blood flow, and a gamma camera records where the tracer went. Images are taken after stress and again at rest, then compared.
The tracers named in the ASNC guideline are the technetium-99m agents (sestamibi and tetrofosmin) and thallium-201 . The 2021 guideline advises against dual-isotope studies that use thallium . Labs may do rest and stress imaging, or a stress-first protocol, so the length of the visit varies .
Reading the pictures
- Reversible defect: less tracer uptake at stress than at rest. This is consistent with ischemia .
- Fixed defect: low uptake at both stress and rest. This indicates scar .
The images show the relative distribution of flow: each area is judged against the best-perfused area. That is the main strength and the main weakness of the method, as the next sections explain.
What you get
The test shows perfusion abnormalities, left ventricular function and high-risk findings such as transient ischemic dilation of the ventricle . Gating, which times the pictures to the ECG, is what allows ejection fraction and wall motion to be measured with the perfusion images.
The criteria define the size of the abnormality in simple terms. On stress nuclear imaging, mild ischemia is ischemia in under 10% of the myocardium, and moderate to severe ischemia starts at 10% ischemic myocardium, which is linked to an annual risk of cardiac death or nonfatal MI of 5% or more .
Strengths and limitations
Strengths
- A large body of evidence for diagnosis and prognosis , and it can be paired with exercise or pharmacologic stress .
- Perfusion and function in one study, with the size and location of the abnormal region .
- Exercise can be combined with imaging, which adds to the diagnostic accuracy of the exercise test alone and keeps capacity and hemodynamic data .
Limitations
- Attenuation, motion and soft-tissue artifacts can create or hide defects, and the extent of disease can be underestimated . Dense breast tissue is a recognized cause , and image quality in obese patients is a concern for all imaging tests .
- Motion artifact, attenuation defects, arrhythmia or apical thinning can make a study inconclusive . Where PET is available, the 2021 and 2023 guidelines consider it reasonable in preference to SPECT to improve diagnostic accuracy and decrease nondiagnostic results .
- Ionizing radiation. A rest-stress technetium study averages about 10 mSv ; the ASNC guideline gives a 3 to 20 mSv range for a technetium SPECT or fluorine-18 flurpiridaz PET study .
Attenuation correction or prone imaging may reduce false-positive results in intermediate-to-high-risk patients, and the guideline gives it a weaker, class 2b recommendation .
Practical points
| Radiation | Yes. If pregnancy is possible, a pregnancy test is mandatory before any radiotracer injection, and breastfeeding guidance applies (24 hours of interruption is recommended for technetium agents) . |
|---|---|
| Tracer, contrast | Technetium-99m tracer through an IV; no iodinated contrast. Proton-pump inhibitors can add gastric uptake that confuses inferior wall images, and stopping them for 3 to 5 days may help . |
| Stressor | Exercise, or a vasodilator (regadenoson, adenosine or dipyridamole), with dobutamine as the alternative when a vasodilator is contraindicated. |
| Suits | Patients who need an ischemia test but cannot use the exercise ECG alone, because the resting ECG is uninterpretable or exercise is limited; a vasodilator can replace exercise . |
| Struggles | Dense breasts and other sources of attenuation, patient motion and arrhythmia , and balanced multivessel disease. |
Exercise or a drug?
Exercise is the preferred stress when the patient can reach an adequate workload. Vasodilator stress is appropriate instead when the patient cannot exercise adequately, and also for some people who can exercise: those having blood flow quantification, those with LBBB or ventricular pacing, and those with acute left ventricular thrombus, a significant aortic aneurysm or acute deep vein thrombosis . Regadenoson is a single fixed-dose bolus, whereas adenosine needs a continuous infusion .
Many patients booked for exercise end up converting to regadenoson if they cannot work hard enough. Typical triggers are under 5 METs or under 4 to 6 minutes of exercise without ischemic symptoms or ECG changes, but no single number should decide it . For that reason, anyone booked for exercise SPECT should follow the caffeine rules: abstain for at least 12 hours, and ideally 24 hours. Dipyridamole must also be held for 48 hours .
With exercise SPECT, the tracer is injected as close to peak exercise as possible, and the patient keeps exercising for at least a minute afterward .
In LBBB or ventricular pacing, the septum can show a mild resting defect that becomes more prominent as the heart rate rises. With exercise or dobutamine, this can produce an artifactual reversible defect in the septum, in the territory of the left anterior descending artery. Vasodilator agents raise the heart rate only modestly, so they are recommended instead, and the ASNC guideline adds that the defects are more common on SPECT than on PET and that PET is preferred over SPECT in this situation. A mild fixed septal defect may be a normal finding in these patients . The ASE guideline reaches a related conclusion from the echo side .
The ASNC guideline stresses that exercise adequacy is more than a heart rate target: reaching 85% of the maximum predicted rate should not be the only reason to stop, and a fit patient with above-average capacity and no troubling symptoms is unlikely to need conversion even if the heart rate response is submaximal. Typical angina or diagnostic ST depression can serve as an adequate endpoint, so the tracer can be given without reaching a target rate . The EXERRT trial showed that regadenoson can safely be given during recovery from an inadequate exercise test when there is no ischemia .
Example from the tool
CCTA with moderate stenosis 50%-69% (CAD-RADS 3)
A moderate stenosis on CCTA raises the question of whether it actually limits blood flow. See how the criteria rate each test for that question.
- Exercise ECGMay Be Appropriate
- Exercise SPECTAppropriate
- Pharmacologic SPECTAppropriate
- Pharmacologic PETAppropriate
- Exercise echoAppropriate
- Dobutamine echoAppropriate
- Stress CMRAppropriate
- CAC scoreRarely Appropriate
- CCTANot applicable
- Invasive angiographyAppropriate
- No testMay Be Appropriate
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