# 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 | Perfusion Pattern | Stress | Rest | Interpretation | |-------------------|--------|------|----------------| | Normal | Normal uptake | Normal uptake | No ischemia or scar | | Reversible defect | Decreased | Normal | Ischemia | | Fixed defect | Decreased | Decreased | Scar/infarction | | Partially reversible | Decreased | Partial improvement | Scar + ischemia (peri-infarct viability) | | Reverse redistribution | Normal | Decreased | Uncommon; post-revascularization | 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.