Interpretation of myocardial perfusion images (SPECT, PET)

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Interpreting myocardial perfusion SPECT images involves a systematic comparison of rest and stress images to assess myocardial ischemia and infarction. The key steps in this process are outlined below.

Image acquisition and evaluation

Stress Imaging: Initially performed using exercise or pharmacologic agents to elevate myocardial oxygen demand (or simulate increased demand using vasodilatory agents). A radiotracer is administered at peak stress to capture blood flow distribution under these conditions. If stress imaging results are entirely normal, further imaging may not be necessary.
Rest Imaging: Conducted after a predefined interval to allow for radiotracer clearance or with a different radiotracer to capture myocardial blood flow under baseline conditions.
Raw Image Analysis: Raw images are reviewed to identify potential artifacts, extracardiac tracer activity, and assess image quality and patient motion.
Alignment: Ensure proper alignment of rest and stress images to accurately compare corresponding myocardial regions, as misalignment can lead to incorrect interpretation of perfusion differences.

Perfusion assessment

The assessment of myocardial regions in myocardial perfusion imaging (MPI) is based on principles similar to those used in echocardiography. The 17-segment model of the left ventricle serves as a standardized framework for identifying and localizing areas of ischemia or infarction. This segmentation facilitates precise reporting and ensures consistent communication of findings across clinicians. Perfusion defects are categorized by their size, encompassing:

Small defects: <10% of the left ventricular myocardium.
Medium defects: 10–20% of the myocardium.
Large defects: >20% of the myocardium.

Perfusion defects are further classified based on the intensity of tracer uptake and associated myocardial changes:

Mild defects: Characterized by decreased tracer counts while preserving normal wall thickness, often indicative of early or mild ischemia.
Moderate defects: Associated with wall thinning, suggesting a more advanced level of ischemia or partial scarring.
Severe defects: Defined by absent tracer uptake, with signal intensity comparable to background activity, typically representing infarcted tissue or non-viable myocardium.

Perfusion patterns and reversibility

Perfusion patterns are systematically analyzed by comparing tracer uptake across all myocardial regions. Areas of reduced perfusion, which manifest as less intense or absent uptake, are identified and categorized. A critical aspect of this evaluation is the determination of reversibility, which involves comparing stress and rest images to distinguish between:

Reversible defects: Indicative of ischemia, where reduced perfusion during stress normalizes at rest.
Irreversible defects: Suggestive of infarction or scar tissue, where perfusion remains reduced under both conditions.
Reverse redistribution: A less common pattern where defects appear worse at rest than during stress, potentially indicating microvascular dysfunction or other pathologies.

The software allows for detailed regional analysis of myocardial perfusion, highlighting specific segments of the left ventricle that demonstrate abnormalities. These tools compare perfusion data to population-based reference databases, often adjusted for sex and age, to identify deviations from normal values. This enhances the ability to detect subtle perfusion defects and differentiate between normal and abnormal patterns.

Reverse redistribution (RR)

Reverse redistribution (RR) in SPECT myocardial perfusion imaging is an unusual phenomenon in which a perfusion defect becomes evident or worsens in rest images compared to stress images. This pattern contrasts with the typical expectation in myocardial perfusion imaging, where defects are more pronounced during stress.

RR is characterized by normal or near-normal perfusion observed during stress, followed by the appearance of a perfusion defect in rest images. It has been documented with thallium-201 and technetium-99m tracers (sestamibi and tetrofosmin). Approximately 5% of all SPECT studies exhibit RR, with a higher prevalence when using thallium tracers (Kashefi et al.).

The phenomenon is most commonly observed in regions supplied by the right coronary artery (RCA). Notably, RR often occurs in patients with normal epicardial coronary arteries, suggesting a possible association with microvascular dysfunction. The prognostic significance of reverse redistribution warrants further investigation, although it is considered an unfavorable prognostic marker (Swinkels et al).

Functional imaging of left ventricular function

ECG-gated myocardial perfusion imaging adds an important layer of information by allowing simultaneous evaluation of left ventricular function. This includes assessing wall motion and thickening in addition to perfusion, which can help differentiate between viable and non-viable myocardial tissue. Parameters such as ejection fraction, end-diastolic volume, and end-systolic volume can be quantified. SPECT measurements of LV volumes and ejection fraction (EF) show a high correlation with cardiac MRI measurements. There is, however, a tendency for SPECT to underestimate LV volumes compared to MRI, particularly in patients with dilated ventricles (van Derwall et al.).

Normal variants and artifacts

It is important to distinguish true perfusion abnormalities from normal variants or artifacts. Common normal variants include:

Apical thinning: Seen as a fixed perfusion defect in the apical inferior wall or septum with normal wall motion, often observed in both SPECT and PET imaging.
Basal lateral perfusion defect: Seen as a fixed defect in the basal lateral wall, particularly on ¹³N-ammonia PET/CT, with normal wall motion. This can be a normal finding.
Breast and diaphragmatic attenuation: Breast tissue or diaphragmatic attenuation can result in fixed perfusion defects in the anterior or inferior walls, respectively. These artifacts can often be differentiated by repeating the imaging in a prone position or using attenuation correction methods.

High-risk features in perfusion images

MPI provides significant prognostic information based on the presence of high-risk features. According to Table 18.4, high-risk features in MPI include:

Large perfusion defects: Large single or multi-territorial fixed and/or reversible perfusion defects involving more than 15% of the left ventricular (LV) mass suggest extensive coronary artery disease.
Transient ischemic dilation (TID): An apparent increase in the size of the LV cavity during stress compared to rest indicates extensive subendocardial ischemia and is a marker of multivessel or left main disease.
Stress-induced myocardial stunning: A drop in left ventricular ejection fraction (LVEF) post-stress suggests significant ischemia and is considered a high-risk finding.
Increased pulmonary or right ventricular tracer uptake: This indicates elevated left ventricular filling pressures during stress, reflecting increased risk.

Transient ischemic dilatation

Transient Ischemic Dilation (TID) is an apparent increase in the size of the LV cavity during stress compared to rest. TID is considered a marker of severe, often multivessel CAD, and is associated with a high risk of adverse cardiac events. TID can also occur as an artifact in patients with significant attenuation, such as those with obesity.

Increased lung uptake

Increased lung uptake of radiotracer during stress is another high-risk marker indicative of elevated left ventricular filling pressures and severe ischemia. It reflects impaired LV function and correlates with extensive CAD, including left main or multivessel disease. Increased lung uptake is often associated with transient ischemic dilation and other markers of poor prognosis.