PET in Cardiology: Heart Blood Flow, Viability and Inflammation

A cardiologist who wants to know whether narrowed coronary arteries are limiting blood flow to the heart muscle has several imaging choices. PET is a less common alternative to the more familiar SPECT stress test, with distinct strengths and practical limits. This page covers perfusion imaging with rubidium-82 or ammonia N-13, blood flow and flow reserve, viability imaging with FDG, and FDG for cardiac inflammation. Which test is right for any person is a decision for the cardiologist and the imaging team, and protocols differ between centres.

Perfusion PET: rubidium-82 and ammonia N-13

Myocardial perfusion imaging compares blood flow to the heart muscle at rest and under stress, created either by exercise or by a medicine that widens the coronary arteries. A review in Clinical Physiology and Functional Imaging names the most common perfusion tracers in clinical cardiac PET as ammonia labelled with nitrogen-13, rubidium-82 and oxygen-15 water. Tracers in general are discussed in PET radiotracers explained.

Rubidium-82 is produced by a generator, a small device holding its parent isotope strontium-82, which has a half-life of 26 days; the review says the generator needs replacing only every 4 to 6 weeks. Rubidium-82 itself has a half-life of about 75 seconds, which allows fast back-to-back imaging and high throughput without an on-site cyclotron. Ammonia N-13 needs a cyclotron to make it, which the same review says limits its availability to larger centres that have one. The joint position statement of the American Society of Nuclear Cardiology and the Society of Nuclear Medicine and Molecular Imaging adds that a complete rest and stress study can be acquired in less than one hour when rubidium-82 is used, with image acquisition times of about five minutes.

Myocardial blood flow and flow reserve

Ordinary perfusion images are relative: one part of the heart is compared with another. PET can also be done dynamically, recording the tracer over time, so that blood flow can be calculated in absolute units of millilitres per minute per gram of tissue, at rest and at stress. The ratio of stress to rest flow is called myocardial or coronary flow reserve. It describes how much extra flow the heart muscle can call on when needed.

The position statement explains why this matters. Regional flow reserve shows the physiological significance of narrowing in the main coronary arteries, analogous to the fractional flow reserve measured by a catheter inside the artery. Where the main arteries are not narrowed, a reduced flow reserve can point to problems in the tiniest vessels, the microcirculation. The statement says routine quantification of flow in this way is unique to PET among noninvasive options and can improve accuracy and risk stratification. Quantification with rubidium-82 is more challenging because its short half-life makes the signal noisy, although its values agree closely with ammonia N-13, which is better validated.

When PET is chosen over SPECT

The two societies state that rest-stress PET is a first-line preferred test for people with known or suspected coronary artery disease who qualify for stress imaging but cannot complete a diagnostic-level exercise test, and that they know of no scenario in which PET should not be considered a preferred test for people who need a medicine-induced stress. They also recommend it when a previous stress scan was poor quality, equivocal or affected by attenuation artefact (tissue absorbing the signal and creating false defects); when body characteristics commonly affect image quality, including obesity with a body mass index above 30, large breasts or breast implants; in high-risk patients, such as those with chronic kidney disease stage 3 to 5, diabetes or known multivessel disease; in younger patients who may need repeated scans; and when blood flow quantification is needed.

The evidence base is described in the Clinical Physiology review. It reports a sensitivity and specificity for cardiac SPECT in detecting a significant narrowing of 87 percent and 73 percent, and for cardiac PET perfusion of 91 percent and 89 percent, from the studies it cites (with wide confidence intervals), and says PET offers better spatial resolution and robust attenuation correction. It also notes that SPECT is well validated and cost effective. These figures come from different studies and patient groups, so they show a general direction only.

Radiation is part of the comparison. The position statement says a complete rest-stress PET study routinely exposes patients to less than 5 millisieverts and as little as 1 millisievert with three-dimensional protocols; the Clinical Physiology review gives about 2.8 millisieverts for a typical rubidium-82 study, or 3.2 to 5.2 millisieverts with the added CT scans. For context on how such numbers compare with everyday exposure, see radiation dose in context, and for how the CT component works, PET-CT and PET-MRI.

Viability imaging with FDG

After a heart attack, or in heart failure, part of the muscle may move poorly. The question is whether that muscle is stunned or hibernating, meaning alive and potentially able to recover if blood flow is restored, or replaced by scar. A practice document from the American Society of Nuclear Cardiology states that this distinction is critical for treatment and prognosis, and that FDG-PET was the only technique approved by the US Food and Drug Administration for assessing myocardial viability at the time of writing.

The method compares a rest perfusion image with an FDG image showing whether muscle still takes up glucose. The practice document explains that reduced blood flow with preserved or enhanced FDG uptake indicates viable muscle, called a perfusion-metabolism mismatch, whereas proportionally reduced flow and FDG uptake indicates scar. Preparation is deliberate: fasting for roughly 6 to 12 hours followed by a glucose load, with insulin used as needed, to push the heart toward using glucose so that the images are clear. Your imaging centre gives instructions suited to your health, especially if you have diabetes; the general routine is in what to expect on scan day.

Cardiac sarcoidosis and inflammation

A joint consensus document of the Society of Nuclear Medicine and Molecular Imaging and the American Society of Nuclear Cardiology, published in the Journal of Nuclear Medicine, explains that glucose metabolism is increased in inflammatory cells, so FDG-PET can detect cellular inflammation. The current imaging approach, it says, pairs FDG with a perfusion study to capture both the inflammatory and the scarring components. It adds that cardiac involvement may be clinically evident in less than 10 percent of people with sarcoidosis who have no cardiac symptoms, though autopsy and imaging series report much higher rates, more than 20 percent in the United States and more than 50 percent in Japan.

Healthy heart muscle also takes up FDG, which could mask inflammation, so preparation differs from viability scans. The consensus describes prolonged fasting (12 to 18 hours), a high-fat low-carbohydrate diet and intravenous heparin, often combined, to shift the heart away from glucose. It advises excluding significant coronary artery disease before the scan, because that can also alter perfusion and FDG uptake. The document says serial FDG scans may help assess response to treatment, but that more data are needed, and that evidence for the benefit of immunosuppressive treatment is itself limited.

Availability and honest limits

The position statement describes myocardial perfusion PET as underused relative to its advantages and widely available in the United States, which may not be true elsewhere. Where there is no PET perfusion service, SPECT remains the established alternative, and the sarcoidosis consensus accepts SPECT perfusion as a substitute. Rubidium-82 has a longer positron range than fluorine-18, which slightly reduces the sharpness gain over SPECT, as the Clinical Physiology review notes. Scans show patterns of flow, metabolism or inflammation; the diagnosis and plan come from the cardiologist, who also weighs symptoms and other tests. Reading a PET report can help you prepare questions.

The short version

Cardiac PET with rubidium-82 or ammonia N-13 images heart blood flow and can measure it in absolute terms, which is the basis of flow reserve. FDG-PET addresses viability and, with special preparation, inflammation. Guidelines prefer PET in some situations, and SPECT remains a valid choice elsewhere, with availability and local policy shaping the options.