Understanding Your Cardiac PET Scan Results

Introduction

A cardiac PET report contains information that can seem overwhelming to patients encountering these concepts for the first time. The report describes perfusion patterns, quantifies blood flow in unfamiliar units, and categorizes findings using specialized terminology. Understanding what these findings mean helps patients engage productively with their physicians and participate in treatment decisions.

The interpretation of cardiac PET extends beyond simply labeling results as normal or abnormal. The location, extent, and severity of perfusion abnormalities all matter. Quantitative measurements of myocardial blood flow and coronary flow reserve provide prognostic information independent of whether the scan appears visually abnormal. The relationship between PET findings and coronary anatomy is complex and sometimes counterintuitive.

This article explains what cardiac PET reports contain and what the findings mean clinically. Patients can use this information to formulate questions for their physicians and understand how results influence treatment planning. Related articles address fundamental concepts, technical factors affecting results, and how findings translate into clinical decisions.

What does a cardiac PET report include and what are the key findings to understand?

A standard cardiac PET report includes patient identification, clinical indication, imaging protocol, radiotracer used, and technical quality assessment. The interpretive portion describes perfusion images qualitatively, reports quantitative flow measurements when available, and provides conclusions regarding the presence, severity, and extent of coronary artery disease.

The qualitative description addresses whether tracer uptake appears uniform throughout the left ventricle or shows regional differences between rest and stress. Abnormalities are localized to coronary artery territories (left anterior descending, left circumflex, and right coronary artery distributions) and characterized by size and severity. Reports typically include summed scores that quantify total stress abnormality, rest abnormality, and the difference between them (Schelbert et al., 2003).

Quantitative flow data appears in most contemporary reports. This includes myocardial blood flow at rest and stress for each vascular territory and globally, along with calculated coronary flow reserve. These numerical values carry prognostic significance beyond what visual interpretation provides. The conclusion synthesizes all findings into a clinical impression regarding probability and severity of coronary disease.

How are perfusion defects described and categorized on cardiac PET?

Perfusion defects are regions where tracer uptake is reduced compared to normally perfused myocardium. Reports describe defects by location (which coronary territory), extent (small, medium, or large), severity (mild, moderate, or severe), and reversibility (fixed versus reversible). Each characteristic provides distinct clinical information.

Location correlates with the coronary artery supplying that region. Anterior and septal defects suggest left anterior descending artery disease. Lateral defects indicate left circumflex territory. Inferior and inferoseptal abnormalities point to right coronary or circumflex disease depending on coronary dominance. Accurate localization guides decisions about which vessels to evaluate or treat (Schindler et al., 2010).

Extent refers to what percentage of the left ventricle shows abnormal perfusion. Small defects involve less than 10% of the myocardium, medium defects 10-20%, and large defects more than 20%. The total extent of ischemia predicts clinical outcomes and influences treatment recommendations. Guidelines use ischemic extent thresholds to guide revascularization decisions.

What is the difference between a fixed defect and a reversible defect?

A fixed defect shows reduced tracer uptake at both rest and stress imaging, with no significant change between the two conditions. Fixed defects typically represent scar from prior myocardial infarction where muscle cells have died and been replaced by fibrous tissue. Scar cannot be restored to normal function by revascularization.

A reversible defect shows reduced uptake during stress but improved or normalized uptake at rest. This pattern indicates ischemia, where viable myocardium receives inadequate blood flow only when demand increases during stress. Reversible defects identify territories that might benefit from revascularization if the supplying coronary artery is opened.

Partially reversible defects show some improvement from stress to rest but do not fully normalize. This pattern suggests a mixture of scar and viable ischemic tissue within the same territory. The distinction between fixed and reversible defects is fundamental to treatment planning (Guduguntla and Weinberg, 2025).

What do the terms ischemia and infarct mean on a cardiac PET report?

Ischemia refers to inadequate blood supply to meet myocardial oxygen demand. On PET imaging, ischemia manifests as stress-induced perfusion defects that improve at rest. The affected muscle remains alive and can contract normally when blood supply is adequate. Ischemia indicates flow-limiting coronary artery disease that might benefit from treatment.

Infarct means irreversible death of heart muscle, typically from coronary artery occlusion during a heart attack. Infarcted tissue appears as a fixed perfusion defect because dead muscle cannot take up tracer regardless of blood flow conditions. The location and extent of infarct affect cardiac function and provide information about prior cardiovascular events.

Reports sometimes describe “peri-infarct ischemia” where reversible defects surround fixed defects. This pattern indicates viable tissue adjacent to prior infarct that experiences inadequate blood supply during stress. Identifying this pattern matters clinically because the viable tissue may be salvageable through revascularization (Chen et al., 2019).

How is the severity of a perfusion abnormality graded?

Severity grading reflects how much tracer uptake is reduced compared to normally perfused regions. Most systems use a scale from 0 (normal) to 4 (absent uptake), applied to each myocardial segment. Mild defects show slightly reduced uptake but remain clearly visible. Moderate defects show substantially reduced but detectable uptake. Severe defects show near-absent tracer activity.

Interpreting physicians evaluate severity both visually and using quantitative software that calculates uptake values for each segment. The combination of extent and severity determines overall risk. A large mild defect may carry similar prognostic significance to a small severe defect. Multiple studies have established the relationship between perfusion abnormality severity and clinical outcomes (Nayfeh et al., 2023).

Reporting systems vary in their specific grading criteria, which can complicate comparison of results between facilities. The general principles remain consistent: more severe and more extensive abnormalities indicate greater disease burden and higher risk. Trends over time at a single facility using consistent methods are more reliable than cross-facility comparisons.

What is summed stress score and summed difference score?

The summed stress score (SSS) quantifies total perfusion abnormality during stress imaging. The left ventricle is divided into 17 segments, each scored from 0 to 4 based on tracer uptake. The SSS equals the sum of all segment scores. Higher SSS indicates more extensive and severe perfusion abnormality.

The summed rest score (SRS) applies the same methodology to rest images. The summed difference score (SDS) equals SSS minus SRS and represents the amount of reversible ischemia. SDS specifically quantifies the extent of ischemia that might benefit from revascularization (Schelbert et al., 2003).

These semi-quantitative scores have been validated extensively for predicting clinical outcomes. An SSS greater than 4 is generally considered abnormal. SDS greater than 2 indicates significant ischemia. Different guidelines use various thresholds, but the principle remains consistent: higher scores indicate greater abnormality and generally higher risk.

What are normal values for myocardial blood flow at rest and during stress?

Normal resting myocardial blood flow (MBF) ranges from approximately 0.6 to 1.0 mL/min/g in most healthy individuals. Resting flow varies relatively little between people and remains stable across different coronary territories. Values significantly below 0.6 mL/min/g may indicate severely reduced rest perfusion from tight stenosis or prior infarction.

Normal stress MBF depends on the effectiveness of coronary vasodilation and ranges from approximately 2.5 to 4.0 mL/min/g or higher in healthy young individuals. Stress flow above 2.0 mL/min/g is generally considered normal, while values below 1.5-1.8 mL/min/g suggest significant flow limitation (Valenta and Schindler, 2024).

These normal ranges derive from populations studied with specific tracers and protocols. Different laboratories may establish local normal values based on their equipment and methods. Interpreting physicians account for technical factors when applying flow thresholds to individual patients.

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What is a normal coronary flow reserve and what values indicate impairment?

Coronary flow reserve (CFR) is the ratio of stress to rest myocardial blood flow. Normal CFR exceeds 2.5-3.0 in healthy individuals with normal coronary arteries and microvasculature. This reserve capacity allows the heart to increase blood flow substantially during exercise or other stress conditions.

Values between 2.0 and 2.5 are borderline and may indicate mild impairment. CFR below 2.0 is generally considered abnormal and indicates compromised vasodilator capacity. CFR below 1.5 indicates severely impaired coronary circulation and is associated with high event rates (Schindler et al., 2010).

The specific threshold for abnormality varies by laboratory and population studied. Some experts use 2.0 as the cutoff, others use 2.5. Rather than focusing on a single threshold, the clinical interpretation considers the overall clinical context, including the degree of impairment, regional versus global patterns, and concordance with other findings.

How do age, sex, and other demographics affect normal reference ranges for CFR?

Coronary flow reserve declines with age even in the absence of obstructive coronary disease. Older adults have lower CFR than younger individuals with similar coronary anatomy. This age-related decline reflects changes in both large arteries and microvasculature. Reference ranges should be age-adjusted when interpreting CFR values (Nayfeh et al., 2023).

Sex differences in CFR exist but are less pronounced than age effects. Women tend to have slightly higher resting blood flow than men after adjusting for body size. Some studies report higher CFR in women, while others find no significant difference. The clinical significance of sex differences remains under investigation.

Cardiovascular risk factors including hypertension, diabetes, obesity, and dyslipidemia all reduce CFR independent of obstructive coronary disease. These conditions impair microvascular function, which limits flow reserve even when large coronary arteries appear normal. Interpreting CFR in patients with multiple risk factors requires recognition that impairment may reflect microvascular disease rather than epicardial stenosis.

What does globally reduced coronary flow reserve indicate versus a regional abnormality?

Globally reduced CFR, where flow reserve is impaired throughout all coronary territories, suggests either balanced three-vessel coronary artery disease or diffuse microvascular dysfunction. Balanced three-vessel disease produces uniform flow limitation that may not create regional differences visible on relative perfusion imaging. Quantitative flow measurement detects this pattern that qualitative imaging misses (Chen et al., 2019).

Regional CFR reduction affecting one or two territories suggests focal coronary artery stenosis in the supplying vessels. The territory with lowest CFR typically corresponds to the most severely diseased coronary artery. Regional patterns help identify culprit vessels for potential intervention.

Distinguishing globally reduced CFR from balanced epicardial disease versus microvascular dysfunction often requires additional testing. Coronary CT angiography or invasive angiography can assess anatomic stenosis. Purely microvascular dysfunction shows open epicardial arteries despite reduced CFR. The treatment implications differ substantially between these conditions.

Can cardiac PET detect microvascular disease even when large coronary arteries are open?

Cardiac PET can detect coronary microvascular dysfunction (CMD) by identifying reduced CFR in patients with angiographically normal or minimally diseased epicardial coronary arteries. This capability represents a unique strength of quantitative PET imaging. Patients with CMD experience angina and face increased cardiovascular risk despite open large arteries.

The diagnosis of CMD requires demonstrating impaired vasodilator response when anatomic obstruction has been excluded. PET showing reduced CFR provides objective evidence supporting CMD diagnosis (Valenta and Schindler, 2024). Many patients with CMD have normal stress tests and normal coronary angiograms, making PET particularly valuable for this population.

CMD affects women more frequently than men and often occurs in patients with traditional cardiovascular risk factors. Recognition has increased substantially over the past decade. Cardiac PET has become an important diagnostic tool for identifying CMD and guiding treatment with medications targeting microvascular function.

What is the difference between epicardial coronary artery disease and microvascular dysfunction?

Epicardial coronary artery disease involves atherosclerotic plaque buildup in the large coronary arteries visible on angiography. Significant stenoses restrict blood flow, particularly during stress when demand increases. Treatment options include medications, stenting, and bypass surgery depending on disease severity and extent.

Microvascular dysfunction affects the small vessels within the heart muscle that angiography cannot visualize. These vessels normally dilate to increase flow during stress. When microvascular function is impaired, flow cannot augment adequately despite open large arteries. Treatment focuses on medications that improve endothelial function and vasodilator capacity.

Patients may have either condition alone or both simultaneously. Cardiac PET can identify both patterns and help clinicians determine which mechanism predominates in an individual patient (Guduguntla and Weinberg, 2025). The distinction matters because treatment strategies differ significantly between epicardial and microvascular disease.

How does cardiac PET quantify the extent and severity of disease?

Extent quantification involves determining what percentage of left ventricular myocardium shows abnormal perfusion or flow. Software programs calculate the total hypoperfused area as a fraction of total myocardial volume. Larger extent indicates more widespread disease and higher risk.

Severity quantification assesses how severely flow is reduced in abnormal regions. This combines visual scoring (the 0-4 scale applied to each segment) with quantitative flow measurements in mL/min/g. More severe reductions indicate tighter stenoses or more advanced disease.

The combination of extent and severity provides comprehensive disease characterization not available from either measure alone (Schelbert et al., 2003). A patient with large but mild abnormality may have similar risk to one with small but severe abnormality. Both parameters should be considered in risk assessment and treatment planning.

What does left ventricular ejection fraction on PET indicate and how is it measured?

Left ventricular ejection fraction (LVEF) quantifies what percentage of blood in the left ventricle is pumped out with each heartbeat. Normal LVEF is typically 55-70%. Values below 40-50% indicate impaired systolic function. Severely reduced LVEF below 35% significantly affects prognosis and treatment eligibility.

Gated PET imaging acquires data synchronized to the cardiac cycle, allowing reconstruction of images at end-diastole (when the ventricle is full) and end-systole (when the ventricle has contracted). Software calculates LVEF from the difference in ventricular volumes between these phases.

LVEF measured during stress may differ from rest LVEF. Normal stress LVEF increases or remains stable compared to rest. A drop in LVEF during stress suggests extensive ischemia or stunned myocardium. Post-stress stunning with reduced LVEF after pharmacologic stress indicates severe ischemia even if perfusion abnormalities appear modest (Nayfeh et al., 2023).

What findings suggest balanced ischemia?

Balanced ischemia occurs when all three major coronary artery territories have similar degrees of flow limitation, preventing the relative differences that perfusion imaging typically detects. Because perfusion images compare regions to each other, uniformly reduced flow may appear normal when no territory is clearly worse than others.

Clues suggesting balanced ischemia include globally reduced CFR on quantitative assessment, transient ischemic dilation of the left ventricle during stress, and stress-induced reduction in LVEF. These secondary markers help identify severe three-vessel disease that might otherwise be missed (Schindler et al., 2010).

Quantitative flow measurement is particularly valuable for detecting balanced ischemia. A patient with globally reduced stress MBF and CFR below 2.0 throughout all territories likely has significant three-vessel disease even if visual perfusion images appear relatively normal. This represents one of the key advantages of quantitative PET over purely qualitative imaging.

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How do interpreting physicians vary in their readings?

Reader variability exists in cardiac PET interpretation as in all imaging modalities. Studies comparing independent readers show agreement rates of 80-90% for major diagnostic categories (normal versus abnormal, presence of significant CAD). Disagreement increases for borderline cases and subtle abnormalities.

Variability arises from differences in training, experience, threshold for calling findings abnormal, and attention to specific image features. Readers with higher volume and more experience generally show better agreement with expert consensus. Quantitative measurements reduce but do not eliminate reader variability compared to purely visual interpretation (Schelbert et al., 2003).

For patients, reader variability means that interpretations of borderline studies may differ between physicians. When results significantly affect treatment decisions, obtaining a second read from an experienced nuclear cardiologist may be valuable. High-volume centers with experienced readers generally provide more reliable interpretations.

What does cardiac PET not show that other tests might reveal?

Cardiac PET provides excellent functional assessment of blood flow but does not directly image coronary artery anatomy. The test cannot determine the exact location of a stenosis within a coronary artery, cannot distinguish 50% from 80% blockage, and cannot assess plaque morphology. Anatomic imaging with CT angiography or invasive angiography complements PET for comprehensive evaluation.

PET does not assess coronary artery calcification. Calcium scoring requires a separate CT acquisition. While many PET/CT protocols include low-dose CT for attenuation correction, this typically does not provide diagnostic calcium score assessment unless specifically requested.

Structural heart abnormalities, valve disease, and pericardial conditions are not evaluated by perfusion PET. Echocardiography remains the primary modality for structural cardiac assessment (Pelletier-Galarneau et al., 2024). Patients with suspected structural heart disease in addition to coronary concerns may need multiple imaging studies.

Can cardiac PET determine the exact location or percentage of a coronary artery blockage?

Cardiac PET identifies which coronary artery territories have inadequate blood flow but cannot precisely localize stenoses within those arteries. A perfusion defect in the left anterior descending territory indicates disease somewhere in that vessel but does not distinguish proximal from mid from distal stenosis.

The test cannot measure stenosis percentage. A 50% stenosis and a 70% stenosis may produce similar perfusion abnormalities depending on lesion characteristics and collateral blood supply. Functional significance matters more than anatomic severity for clinical outcomes in many cases (Chen et al., 2019).

When anatomic detail is required for treatment planning, invasive coronary angiography or CT angiography provides the necessary information. PET and anatomic imaging are complementary rather than competing modalities. The combination of functional and anatomic assessment provides the most complete picture of coronary artery disease.

What is the relationship between PET perfusion findings and actual anatomic stenosis severity?

The correlation between perfusion abnormality and stenosis severity is imperfect. Lesions with similar anatomic appearance can have very different functional significance depending on lesion length, eccentricity, entrance effects, and collateral supply. Studies show that stenoses between 50-70% severity have highly variable effects on blood flow.

Flow-limiting stenoses typically require at least 50-70% diameter narrowing to reduce perfusion under stress conditions. Tighter stenoses more consistently limit flow, but even severe stenoses may have minimal perfusion impact if robust collaterals supply the downstream territory. Conversely, moderate stenoses may cause significant ischemia if hemodynamically unfavorable (Valenta and Schindler, 2024).

This anatomic-functional mismatch explains why functional testing adds value beyond anatomic imaging alone. A 60% stenosis that limits flow warrants different consideration than a 60% stenosis that does not. PET identifies which stenoses matter functionally, which helps guide treatment decisions.

How should patients interpret findings of mildly reduced versus moderately reduced flow reserve?

Mildly reduced CFR (approximately 2.0-2.5) indicates some impairment of coronary vasodilator reserve. This may reflect early atherosclerosis, cardiovascular risk factors affecting microvascular function, or subclinical disease. Mildly abnormal CFR warrants attention to risk factor modification and may influence decisions about further testing.

Moderately reduced CFR (approximately 1.5-2.0) represents more significant impairment with higher event risk. Patients in this range often benefit from aggressive medical therapy and may warrant additional evaluation to determine the underlying cause. Moderately reduced CFR carries independent prognostic significance regardless of perfusion image appearance (Nayfeh et al., 2023).

Severely reduced CFR (below 1.5) indicates substantially compromised coronary circulation. Patients with values this low face significantly elevated cardiovascular risk and typically require comprehensive evaluation and treatment. The distinction between moderate and severe impairment affects the urgency and intensity of clinical response.

Conclusion

Cardiac PET reports contain multiple types of information that together characterize coronary artery disease and cardiovascular risk. Visual perfusion findings identify regions with inadequate blood supply. Quantitative flow measurements provide prognostic information and detect disease patterns that visual interpretation misses. Understanding what these findings mean helps patients engage in treatment discussions.

Key concepts include the distinction between fixed and reversible defects, the prognostic significance of extent and severity measures, and the meaning of myocardial blood flow and coronary flow reserve values. Normal ranges depend on demographics and technical factors. Borderline findings warrant thoughtful interpretation rather than automatic categorization as normal or abnormal.

Related articles explain basic cardiac PET concepts, technical factors affecting results, and how findings translate into treatment decisions. For patients preparing to discuss results with their physicians, understanding these concepts enables more productive conversations about what findings mean and what to do next.