Residency · Residency · Diagnostic Radiology

Myocardial Perfusion Imaging: SPECT and PET

Introduction

Myocardial perfusion imaging (MPI) is a well-established nuclear medicine technique for the non-invasive evaluation of coronary artery disease (CAD). Both SPECT and PET provide information on myocardial blood flow, viability, and left ventricular function.

SPECT Myocardial Perfusion Imaging

Radiopharmaceuticals

Technetium-99m sestamibi (Cardiolite) is a lipophilic cation taken up by mitochondria in proportion to blood flow. It undergoes minimal redistribution and allows gated imaging. Technetium-99m tetrofosmin (Myoview) operates by a similar mechanism with faster hepatic clearance. Thallium-201 is a potassium analog actively taken up via the Na/K-ATPase pump. It undergoes redistribution over 3-4 hours, which is key for viability assessment. The Tc-99m agents provide better image quality due to their 140 keV photon energy and higher injectable doses.

Stress Protocols

Exercise stress (treadmill or bicycle) is preferred when the patient can achieve adequate workload. The target is at least 85% of the age-predicted maximum heart rate (calculated as 220 minus age), and the radiotracer is injected at peak stress with continued exercise for 1-2 minutes. Pharmacologic stress is used when exercise is contraindicated or inadequate. Vasodilators such as regadenoson (Lexiscan), adenosine, and dipyridamole cause coronary vasodilation and unmask flow-limiting stenoses by producing differential perfusion. Dobutamine, a beta-1 agonist, is used when vasodilators are contraindicated (such as in severe asthma or high-grade AV block) and works by increasing myocardial oxygen demand. Caffeine must be withheld for 12-24 hours before vasodilator stress because it is an adenosine receptor antagonist.

Image Acquisition and Display

SPECT images are acquired with a rotating gamma camera or dedicated cardiac cameras using CZT detectors. Gated SPECT provides simultaneous assessment of perfusion and LV function, including LVEF, wall motion, and wall thickening. Images are displayed in three standard planes: short axis, vertical long axis, and horizontal long axis. Stress images are compared directly with rest images using a 17-segment model.

PET Myocardial Perfusion Imaging

Radiopharmaceuticals

Rubidium-82 (Rb-82) is a potassium analog that is generator-produced with an ultra-short half-life of 76 seconds, requiring no cyclotron. Nitrogen-13 ammonia (N-13) is cyclotron-produced with a 10-minute half-life and provides excellent image quality. Oxygen-15 water is the gold standard for absolute myocardial blood flow quantification but has limited clinical use. Fluorine-18 flurpiridaz is an investigational agent with an improved extraction fraction.

Advantages of PET over SPECT

PET offers superior spatial resolution with lower attenuation artifacts and routine attenuation correction with CT. Its major advantage is the ability to quantify absolute myocardial blood flow (MBF) in mL/g/min and myocardial flow reserve (MFR). An MFR less than 2.0 is abnormal and associated with adverse prognosis. PET also provides better diagnostic accuracy for multivessel CAD, where balanced ischemia may be missed on SPECT.

Image Interpretation

Perfusion Patterns

A normal study shows homogeneous radiotracer uptake throughout the myocardium on both stress and rest images. A reversible defect shows decreased uptake on stress that normalizes at rest, indicating ischemia. A fixed defect shows decreased uptake on both stress and rest, indicating scar or infarction. A partially reversible defect shows some improvement at rest, suggesting a combination of scar and ischemia or viable myocardium within the infarct zone. Reverse redistribution, where the defect appears worse at rest than stress, is uncommon and often seen post-revascularization.

Territorial Distribution

The LAD territory supplies the anterior wall, septum, and apex. The LCx territory supplies the lateral wall and posterolateral segments. The RCA territory supplies the inferior wall and basal inferoseptum.

Semi-Quantitative Scoring

The Summed Stress Score (SSS) sums all segment scores on stress (0 equals normal, 4 equals absent uptake per segment). The Summed Rest Score (SRS) sums all segment scores at rest. The Summed Difference Score (SDS), which equals SSS minus SRS, represents the extent and severity of ischemia. An SSS greater than 13 or SDS greater than 7 indicates high-risk disease.

Artifacts and Pitfalls

Breast attenuation in females causes fixed anterior wall defects, and diaphragmatic attenuation in males causes fixed inferior wall defects, both mimicking scar. Patient motion causes misregistration between stress and rest or between emission and transmission scans. Hot spot artifacts result from bowel or liver activity adjacent to the myocardium. Balanced ischemia, where global reduction in flow occurs with multivessel disease, may cause perfusion to appear falsely normal on relative images. PET with absolute flow quantification overcomes this limitation.

Transient Ischemic Dilation (TID)

TID occurs when the LV cavity appears larger on stress than rest images, indicating severe and extensive ischemia or multivessel disease. A TID ratio greater than 1.2 is abnormal and portends a worse prognosis.

Viability Assessment

Thallium-201 redistribution detects viable but hibernating myocardium when uptake appears on delayed (4-hour or 24-hour) images in a previously fixed defect. FDG PET viability is performed in combination with a perfusion tracer. A mismatch pattern (decreased perfusion with preserved FDG uptake) indicates viable, hibernating myocardium likely to benefit from revascularization. A matched defect (decreased perfusion and FDG uptake) indicates scar that is unlikely to recover function.

Clinical Pearls

A reversible perfusion defect indicates ischemia and potentially flow-limiting coronary stenosis. PET MPI with absolute myocardial blood flow quantification overcomes the limitation of balanced ischemia in multivessel disease. Transient ischemic dilation on stress MPI is a high-risk marker for severe CAD. The perfusion-metabolism mismatch pattern on FDG PET identifies viable hibernating myocardium that may benefit from revascularization.

References

1. ASNC Imaging Guidelines for SPECT Nuclear Cardiology Procedures. J Nucl Cardiol. 2018;25(5):1784-1846.
2. PET Myocardial Perfusion Imaging: State of the Art. J Nucl Med. 2020;61(8):1161-1168.
3. Myocardial Blood Flow Quantification by PET: Technical Aspects and Clinical Applications. J Nucl Cardiol. 2019;26(3):973-985.
4. Multimodality Imaging Assessment of Myocardial Viability. JACC Cardiovasc Imaging. 2019;12(7 Pt 1):1199-1218.

For the intern

When you're called at 3am about myocardial perfusion imaging (MPI), your main job is recognizing high-risk patterns that need urgent cardiology consultation: extensive reversible defects (indicating severe ischemia), transient ischemic dilation (TID ratio >1.2 suggesting multivessel disease), or severely reduced left ventricular ejection fraction (LVEF) on gated images. Check if the patient is symptomatic—chest pain with a positive stress test warrants immediate evaluation. Most MPI interpretation can wait until morning when you have adequate time to review all three orthogonal planes (short axis, vertical long axis, horizontal long axis) and compare stress-to-rest images systematically. Remember that Single-Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) both assess myocardial blood flow, but PET offers absolute flow quantification that catches balanced ischemia SPECT might miss.

Common pitfalls

• Mistaking attenuation artifacts for true defects: Breast tissue causes anterior wall "defects" in women; diaphragm causes inferior wall "defects" in men. Both are typically fixed on stress AND rest. True ischemia is reversible.
• Missing balanced ischemia on SPECT: When all three coronary territories are equally underperfused, relative images may appear deceptively normal. Look for TID, reduced LVEF with stress, or request PET with flow quantification (myocardial flow reserve <2.0 is abnormal).
• Forgetting medication interactions: Caffeine blocks vasodilator stress agents (regadenoson, adenosine, dipyridamole). Always verify 12-24 hour abstinence before pharmacologic stress.
• Confusing fixed defects with viable myocardium: A fixed defect usually means scar, but check for viability using thallium-201 redistribution imaging or fluorodeoxyglucose (FDG) PET if revascularization is considered.

<image> Create a simple decision algorithm flowchart for interpreting myocardial perfusion imaging. Panel 1 (top): Start with "Compare stress and rest images in all three planes." Panel 2: Three branches showing perfusion patterns—left branch shows "Defect on STRESS only" leading to green box labeled "Reversible = ISCHEMIA"; middle branch shows "Defect on STRESS and REST" leading to yellow box labeled "Fixed = SCAR (consider viability assessment)"; right branch shows "Normal both" leading to blue box labeled "Normal perfusion." Panel 3 (bottom): Add a red warning box separate from the main flow stating "High-risk markers → urgent cardiology: TID ratio >1.2, extensive reversible defect (SDS >7), reduced LVEF with stress." Use simple arrows and minimal text in boxes. </image>

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