A positron emission tomography (PET) scan is an imaging test that reveals how tissues and organs function by tracking the movement of a small amount of radioactive material through the body. Unlike imaging tests that primarily show anatomy, such as computed tomography (CT) scans, PET scans can highlight changes in cellular activity that may indicate cancer, neurological disease, heart problems, or other conditions.
Doctors often use PET scans to investigate suspicious findings, determine how far a disease has spread, assess whether treatment is working, and identify changes in the body before they become obvious on conventional imaging. However, a PET scan does not automatically identify a specific disease. Its findings must be interpreted alongside symptoms, medical history, laboratory results, and other tests.
Understanding what a PET scan measures, what it can reveal, and where its limitations lie helps explain why it has become an important tool in modern medicine.
How a PET scan works
A PET scan detects changes in the distribution of a radioactive tracer, a substance introduced into the body that emits tiny particles detectable by specialized equipment. The tracer is designed to participate in or reflect a particular biological process, allowing doctors to observe activity inside tissues rather than simply their physical structure.
One of the most commonly used PET tracers is fluorodeoxyglucose, or FDG. This compound resembles glucose, the sugar that cells use for energy. FDG contains a radioactive form of fluorine that allows the scanner to track where the compound accumulates.
After the tracer is injected into a vein, it travels through the bloodstream and enters tissues. Cells that take up glucose tend to take up FDG as well, although the amount varies according to the tissue’s metabolic activity and other biological factors.
As the radioactive material decays, it releases positrons, which are positively charged particles. When a positron encounters an electron, the two particles annihilate each other, producing two gamma rays that travel in nearly opposite directions. The PET scanner detects these paired signals and uses them to calculate where the tracer is concentrated.
A computer then reconstructs the detected signals into images showing the distribution of tracer throughout the body. Areas with relatively high tracer uptake may appear as bright or intense regions, depending on the display settings.
These patterns provide clues about how tissues are behaving. For example, many cancer cells use glucose at an unusually high rate, so they may accumulate more FDG than surrounding tissue. However, normal tissues such as the brain and heart can also show substantial uptake, and inflammation can produce intense signals. A bright region therefore indicates increased tracer activity, not necessarily cancer.
Other PET tracers are designed to reveal different biological processes, including blood flow, the availability of certain receptors, and the presence of particular proteins associated with disease. The tracer selected determines, in large part, what the scan can reveal.
What a PET scan can reveal about cancer
Cancer is one of the most important uses of PET imaging. Many tumors contain cells with altered metabolism, increased glucose consumption, or other biological characteristics that make them detectable with an appropriate tracer.
A PET scan can help doctors identify metabolically active tumors and assess whether cancer has spread beyond its original location. It can also help distinguish between different possible explanations for an abnormality found on another imaging test, although it cannot always resolve the uncertainty.
Detecting cancer and evaluating suspicious findings
When a CT scan or another test identifies an unexplained mass, PET imaging may help determine whether the area has metabolic characteristics associated with malignancy. A suspicious lymph node, for example, may accumulate more FDG than expected if it contains active cancer cells.
However, increased uptake is not proof of cancer. Infections, autoimmune disorders, healing tissues, and other inflammatory processes can produce similar patterns. Conversely, some cancers have low FDG uptake and may be difficult to detect with this tracer.
The usefulness of PET therefore depends on the type of cancer, its biological characteristics, the size and location of the abnormality, and the clinical question being investigated.
Determining whether cancer has spread
Cancer staging describes the extent of disease, including the size and location of the original tumor, involvement of nearby lymph nodes, and spread to distant organs. This information helps doctors choose an appropriate treatment strategy.
PET imaging can reveal suspicious activity in lymph nodes, bones, or other organs that may not be clearly abnormal on structural imaging alone. When combined with CT, it can show both the anatomical location of a finding and its metabolic activity.
A PET scan can also help identify unexpected sites of disease. However, very small tumors may escape detection, and some cancers do not accumulate enough of the selected tracer to produce a conspicuous signal. PET findings must therefore be interpreted in the context of the cancer type and other clinical evidence.
Monitoring treatment response and detecting recurrence
Cancer treatments can change the activity of tumor cells before they produce a substantial change in tumor size. A tumor may become less metabolically active after effective treatment even if a mass remains visible on CT.
Comparing PET scans obtained before and after treatment can help doctors assess whether a tumor’s activity has decreased, remained stable, or increased. These changes may provide useful evidence about treatment response, although their significance depends on the treatment, timing of the scan, and type of cancer.
PET imaging can also help investigate suspected recurrence after treatment. For example, it may help distinguish active disease from certain forms of residual tissue change. Nevertheless, inflammation caused by recent surgery, radiation therapy, or other treatment can complicate interpretation.
A PET scan does not replace every other method of evaluating cancer. Some findings require biopsy, in which a small tissue sample is examined under a microscope, to establish a diagnosis or confirm whether cancer remains active.
How PET scans help evaluate heart disease
PET imaging can reveal important information about the heart’s blood supply and the condition of its muscle. Depending on the tracer and scanning protocol, it can help evaluate coronary artery disease, assess whether damaged heart muscle remains viable, and investigate certain inflammatory conditions.
Measuring blood flow to the heart
The heart muscle requires a continuous supply of oxygen and nutrients through the coronary arteries. When these arteries become narrowed or blocked, blood flow may become insufficient, particularly during physical activity or stress.
Certain PET scans measure blood flow through the heart muscle. Doctors may use these studies to investigate whether parts of the heart receive less blood than expected and to evaluate the functional significance of suspected coronary artery disease.
Some protocols also measure myocardial blood flow quantitatively, providing information about the circulation across the heart muscle rather than relying solely on visual comparisons between regions.
These measurements can help guide decisions about additional testing and treatment. They do not, by themselves, replace the need to consider symptoms, cardiovascular risk factors, and other findings.
Determining whether damaged heart muscle may recover
After a heart attack or prolonged reduction in blood flow, some heart muscle may become permanently scarred, while other areas may remain alive but function poorly because they receive inadequate blood.
This second condition is sometimes called myocardial hibernation. Certain PET studies can help distinguish potentially viable muscle from tissue that has been irreversibly damaged.
That distinction can matter when doctors consider treatments intended to improve blood supply, such as coronary revascularization procedures. The value of testing depends on the patient’s overall condition and whether the results are likely to change management.
Detecting inflammation in the heart
Specialized PET protocols can help identify abnormal inflammatory activity in cardiac tissue. One example is cardiac sarcoidosis, a condition in which clusters of inflammatory cells can affect the heart’s electrical system and muscle.
Because normal heart muscle also uses glucose, specialized preparation is often necessary to reduce normal myocardial FDG uptake before an inflammation-focused scan. Otherwise, normal metabolic activity may obscure the findings.
Even with careful preparation, PET results must be interpreted alongside other evidence, such as symptoms, electrocardiograms, echocardiography, cardiac magnetic resonance imaging, and sometimes tissue sampling.
What PET scans can reveal about the brain
The brain consumes substantial energy and depends on carefully regulated blood flow and chemical signaling. PET imaging can examine these processes, making it useful in selected neurological and psychiatric evaluations.
Different tracers allow researchers and clinicians to investigate glucose metabolism, blood flow, neurotransmitter systems, and the accumulation of certain proteins. The information obtained depends on the tracer used and the condition being investigated.
Evaluating dementia and other cognitive disorders
Some forms of dementia produce characteristic patterns of reduced glucose metabolism in particular brain regions. FDG PET can help identify these patterns when a person’s symptoms and other test results leave uncertainty about the underlying cause of cognitive decline.
For example, Alzheimer’s disease can produce reduced metabolic activity in specific areas of the cerebral cortex, while other neurodegenerative conditions may produce different patterns. These patterns can support a diagnosis, but they are not always unique to one disease.
Other PET tracers are designed to detect amyloid or tau proteins, which are associated with Alzheimer’s disease. Amyloid PET can help determine whether substantial amyloid deposits are present in the brain. Tau PET can reveal the distribution of certain tau deposits, although its clinical role varies by setting.
An abnormal amyloid scan does not necessarily mean that a person’s symptoms are caused by Alzheimer’s disease. Amyloid can be present in people who have no noticeable cognitive impairment, and dementia can arise from several different conditions. Results must be interpreted in the context of the person’s symptoms, examination, and other findings.
PET imaging is not routinely needed for every person with memory problems. Doctors generally begin with a clinical assessment and use specialized imaging when it is likely to clarify the diagnosis or affect care.
Investigating seizures and brain tumors
PET can help evaluate selected cases of epilepsy, particularly when seizures continue despite treatment and doctors are considering surgery. Areas of reduced glucose metabolism may help identify brain tissue associated with seizure activity, although these findings are not sufficient on their own to determine where seizures begin.
PET also has a role in evaluating certain brain tumors. Because some tumors have metabolic characteristics that differ from normal brain tissue, specialized tracers can help characterize lesions, assess tumor activity, or investigate possible recurrence.
In neuro-oncology, amino acid PET tracers may be useful for distinguishing active tumor tissue from some treatment-related changes. This can be important because radiation and other therapies can alter brain tissue in ways that resemble tumor progression on conventional imaging.
The choice of tracer and the interpretation of the results depend on the tumor type and the specific clinical question.
PET scans for infection and inflammation
Inflammation is the body’s response to injury, infection, or other threats. Immune cells become active, change their metabolism, and often consume more glucose than they do under resting conditions. Because FDG follows glucose-related activity, PET can reveal areas where inflammation is occurring.
This ability can make PET useful when doctors suspect a deep-seated infection, an inflammatory disorder, or an unexplained process affecting several parts of the body.
For example, PET imaging may help investigate certain bone infections, infections involving implanted medical devices, or inflammatory diseases that affect multiple organs. It may also help assess conditions such as large-vessel vasculitis, in which inflammation affects major arteries.
However, the scan generally shows the location and distribution of abnormal activity rather than identifying the specific microorganism or immune process responsible. Increased uptake cannot reliably distinguish infection from cancer or other inflammation in every case.
Additional testing may be needed to identify the cause. Depending on the suspected condition, this may include blood tests, cultures, targeted imaging, or biopsy.
Preparation is particularly important when PET is used to assess inflammation. Recent infections, surgery, exercise, and certain medications can affect tracer distribution or complicate interpretation. Doctors must consider these factors when deciding whether an abnormal signal is meaningful.
Why PET scans are often combined with CT or MRI
PET imaging is especially valuable for showing biological activity, but it does not always provide enough anatomical detail to identify the exact structure responsible for a signal. This is why PET is frequently combined with CT in a single examination.
A PET/CT scan acquires functional and anatomical information in one session. The CT images help locate areas of increased tracer uptake and show their relationship to nearby organs, bones, lymph nodes, and other structures.
This combination is particularly useful in cancer care, where doctors need to know both whether a suspicious area is metabolically active and where it is located.
PET can also be combined with magnetic resonance imaging in selected settings. PET/MRI offers metabolic information alongside MRI’s detailed views of soft tissues, which can be valuable in certain neurological, pelvic, and other examinations.
CT and MRI remain essential in their own right. CT is useful for assessing anatomy, detecting many types of structural abnormalities, and evaluating emergencies. MRI provides excellent soft-tissue contrast and can reveal detailed structural changes without using ionizing radiation. Neither test is universally superior to PET; each answers different questions.
A physician chooses among these methods according to the suspected disease, the information already available, and the type of information needed to guide care.
What happens during a PET scan
Most PET examinations begin with instructions about preparation. The details depend on the tracer and the purpose of the study, but patients commonly receive guidance about eating, drinking, physical activity, and medications.
For an FDG PET scan, patients are often asked to avoid food for several hours before the appointment. Blood glucose levels may be checked because elevated glucose and other metabolic factors can affect tracer uptake and image quality. People with diabetes may need individualized instructions about food and medication.
After arriving, the patient receives the tracer, usually through an intravenous injection. The tracer then needs time to distribute through the body and accumulate in the tissues being examined. During this waiting period, the patient typically rests quietly and avoids unnecessary movement or strenuous activity.
The patient then lies on a scanning table that moves through the scanner. The machine detects signals from the tracer and reconstructs them into images. Remaining still is important because movement can blur the results.
The imaging portion commonly takes tens of minutes, although the total appointment may be considerably longer because of preparation and the tracer uptake period. The duration varies with the examination and equipment used.
Most people can resume their usual activities afterward unless they have been given other instructions. Drinking fluids and urinating regularly can help the body eliminate the tracer, although the exact guidance depends on the examination and the patient’s medical circumstances.
How safe is a PET scan?
PET scans are generally well tolerated. The tracer is administered in a small quantity, and its radioactivity decreases through radioactive decay while the body eliminates much of the substance.
Nevertheless, PET involves exposure to ionizing radiation, which can damage cells at sufficiently high levels. The potential long-term risk from a medical imaging examination is generally considered small, but it is not zero. The amount of radiation depends on the tracer, the administered activity, and whether CT is also performed.
In PET/CT, the CT component contributes additional radiation exposure. The medical team should use an examination protocol appropriate to the clinical question and avoid unnecessary repeat scans.
Allergic reactions to PET tracers are uncommon, but patients should tell their medical team about previous reactions to injected substances and relevant medical conditions. The radioactive tracer used for PET is not the same as the iodine-based contrast material sometimes injected during CT, and a reaction to one does not automatically imply a reaction to the other.
Pregnancy requires special consideration because radiation can affect a developing fetus. Anyone who is pregnant or might be pregnant should inform the medical team before the examination. If a patient is breastfeeding, the team should also be told, because instructions may vary according to the tracer used.
The scan itself is painless apart from the needle used for the injection. Some people find it uncomfortable to remain still or lie on the scanning table, but PET generally does not require surgery or recovery time.
What a PET scan cannot tell you
PET imaging provides powerful clues about biological activity, but its results have important limitations.
First, increased tracer uptake is not a diagnosis. Cancer, infection, inflammation, and normal physiological processes can produce overlapping patterns. A scan may identify an abnormal area without establishing why it is abnormal.
Second, a normal or negative scan does not rule out every disease. Very small lesions may be difficult to detect because their signals blend with surrounding tissue. Some tumors and other conditions show little uptake of the tracer used. A disease may also be present at a stage when it does not produce a detectable imaging abnormality.
Third, PET findings can be affected by preparation and normal body function. The brain normally uses large amounts of glucose, the heart’s uptake depends on its metabolic state, and muscles may accumulate FDG after strenuous activity. Brown fat, which helps regulate body temperature, can also show uptake under certain conditions. These normal patterns can complicate the interpretation of an examination.
Finally, the meaning of a PET result depends on the clinical context. Doctors consider the distribution and intensity of tracer uptake, the patient’s symptoms, the suspected disease, and findings from other tests. A suspicious result may lead to additional imaging, laboratory testing, follow-up scans, or biopsy. In some cases, the most appropriate next step is observation rather than immediate intervention.
How doctors interpret PET scan results
PET reports describe the location and pattern of tracer uptake and explain how those findings relate to the suspected condition. In cancer imaging, reports may also include measurements of uptake intensity, such as the standardized uptake value, or SUV. This is a measure used to help characterize tracer accumulation, adjusted for factors such as the administered activity and the patient’s body size.
SUV can help compare areas within a scan or assess changes across examinations performed under comparable conditions. However, it is not a direct measurement of how aggressive a cancer is, and there is no single SUV threshold that reliably separates cancer from benign conditions in every situation.
A radiologist or nuclear medicine physician interprets the images and prepares a report for the treating clinician. The report may describe findings as suspicious, indeterminate, or consistent with a particular process, depending on the evidence. Indeterminate means that the scan does not provide enough information to reach a confident conclusion.
The clinician then combines the imaging results with the rest of the medical evaluation. If a PET scan suggests a tumor, for example, tissue examination may still be necessary to determine its exact type. If the scan shows inflammation, additional testing may be needed to identify the cause.
The most useful PET scan is not necessarily the one that produces the most striking image. It is the one that answers a clearly defined medical question and provides information that can improve diagnosis, treatment selection, or follow-up.
When a PET scan is useful
PET scans are most valuable when information about cellular activity can answer a question that conventional examination, laboratory testing, or structural imaging cannot adequately resolve. Their established applications include evaluating many cancers, assessing selected heart conditions, investigating certain neurological disorders, and locating some infections or inflammatory processes.
They are not necessary for every suspected disease, and they are not a universal screening test for healthy people. Their usefulness depends on whether the suspected condition is detectable with the available tracer and whether the result is likely to influence medical decisions.
By revealing patterns of biological activity throughout the body, PET imaging helps doctors look beyond what tissues look like and examine how they behave. That distinction can provide crucial information, but the scan is most reliable when its findings are interpreted alongside the broader clinical picture.