PET Scan: What It Is, Procedure & What to Expect

What Is a PET Scan?

A PET scan (positron emission tomography) is a nuclear medicine imaging technique that creates detailed 3D images showing how your organs and tissues are functioning. By detecting the metabolic activity of cells, PET scans can identify diseases at the cellular level, often before structural changes become visible on other imaging tests like CT or MRI.

Positron emission tomography represents one of the most significant advances in diagnostic imaging of the past few decades. Unlike traditional X-rays, CT scans, or MRI, which primarily show the anatomical structure of organs and tissues, PET scans reveal how well these structures are functioning at the molecular and cellular level. This functional imaging capability makes PET scans particularly valuable for detecting and monitoring diseases that affect cellular metabolism.

The technology works by detecting gamma rays emitted from a radioactive tracer that has been injected into your bloodstream. The most commonly used tracer is fluorodeoxyglucose (FDG), a modified form of glucose that contains a radioactive fluorine atom. Since cancer cells typically consume glucose at much higher rates than normal cells, areas of increased tracer uptake often indicate the presence of malignant tissue. However, PET imaging extends far beyond cancer detection—it's also used to evaluate heart function, brain disorders, and inflammatory conditions.

Today, PET scans are almost always performed in combination with CT scans, a technique called PET-CT. This combination provides the best of both worlds: the CT component shows the precise anatomical location and structure of organs, while the PET component reveals how those structures are functioning. This fusion of functional and anatomical imaging has revolutionized the diagnosis and management of many diseases, particularly cancer.

How Does a PET Scan Work?

The science behind PET imaging is elegantly simple yet technologically sophisticated. When the radioactive tracer (most commonly FDG) is injected into your bloodstream, it travels throughout your body and is taken up by cells according to their metabolic activity. Cells that are more metabolically active—such as cancer cells, heart muscle during exercise, or brain tissue during thought—absorb more of the tracer.

As the radioactive atoms in the tracer decay, they emit positrons (positively charged particles). When a positron collides with an electron in nearby tissue, both particles are annihilated and two gamma rays are produced, traveling in exactly opposite directions. The PET scanner contains a ring of detectors that capture these gamma ray pairs simultaneously. By analyzing which detectors are hit and when, sophisticated computer algorithms can determine exactly where in your body the annihilation occurred, creating a precise map of tracer distribution.

This process generates thousands of data points that are compiled into detailed three-dimensional images. Areas with high tracer uptake appear as "hot spots" on the scan, while areas with low metabolic activity appear darker. Radiologists and nuclear medicine physicians then interpret these patterns to identify abnormalities and make diagnoses.

What Is a PET Scan Used For?

PET scans are primarily used for cancer diagnosis, staging, and treatment monitoring. They're also valuable for evaluating heart conditions (cardiac viability), brain disorders (Alzheimer's disease, epilepsy), and detecting hidden infections or inflammatory diseases. The ability to detect metabolic changes before structural changes makes PET uniquely powerful for early disease detection.

The clinical applications of PET imaging have expanded dramatically since the technology became widely available. While oncology remains the dominant use case, accounting for approximately 85% of all PET scans performed, the technology has proven invaluable across multiple medical specialties. Understanding when and why a doctor might order a PET scan can help patients better prepare for and understand their diagnostic journey.

Cancer Diagnosis and Staging

Cancer cells typically exhibit significantly higher metabolic rates than normal cells, consuming glucose at rates 10 to 50 times higher than surrounding healthy tissue. This metabolic hyperactivity makes tumors appear as bright "hot spots" on PET scans, allowing detection of cancerous lesions as small as 7-8 millimeters in diameter—often before they're visible on CT or other imaging modalities.

PET scans are particularly valuable for staging cancer, which means determining how far the disease has spread. Accurate staging is crucial because treatment decisions often depend on whether cancer remains localized or has metastasized to lymph nodes or distant organs. A whole-body PET scan can identify metastatic disease throughout the body in a single examination, providing information that might otherwise require multiple separate tests.

The types of cancer most commonly evaluated with PET scans include lung cancer, lymphoma, melanoma, colorectal cancer, breast cancer, esophageal cancer, head and neck cancers, and thyroid cancer. For lymphoma in particular, PET scanning has become central to diagnosis, staging, and treatment response assessment, often serving as the primary imaging modality throughout the patient's care journey.

Treatment Response Monitoring

One of the most valuable applications of PET imaging is assessing how well cancer treatment is working. Because PET scans detect metabolic activity rather than just tumor size, they can reveal treatment response much earlier than anatomical imaging. A tumor may remain the same size initially even while therapy is killing the cancer cells inside—the metabolic activity drops before the tumor shrinks.

This early response assessment allows oncologists to determine within weeks whether a treatment regimen is effective. If PET shows persistent or increased metabolic activity, the treatment plan can be modified without waiting months for the tumor to grow on CT. This adaptability can be life-saving, particularly for aggressive cancers where time is critical.

Heart Conditions

In cardiology, PET scans are used to assess myocardial viability—determining whether heart muscle that isn't contracting normally is alive and potentially recoverable, or is scar tissue that cannot recover. This distinction is crucial when deciding whether a patient will benefit from revascularization procedures like bypass surgery or stenting.

PET can also measure myocardial blood flow with high accuracy, helping identify coronary artery disease and assess its severity. The combination of flow and viability information helps cardiologists make optimal treatment decisions for patients with heart failure or coronary artery disease.

Brain Disorders

The brain's high metabolic demands make it an excellent target for PET imaging. In neurology, PET scans are used to evaluate dementia, helping distinguish Alzheimer's disease from other forms of cognitive decline. Specific tracers have been developed that bind to the amyloid plaques characteristic of Alzheimer's, providing definitive diagnosis even in early stages.

For patients with epilepsy who don't respond to medication, PET scans can identify the brain region responsible for seizures by showing areas of decreased metabolism between seizures. This localization is essential for surgical planning when seizure surgery is being considered.

How Do I Prepare for a PET Scan?

Preparation for a PET scan involves fasting for 4-6 hours before the exam, avoiding strenuous exercise for 24-48 hours, following a low-carbohydrate diet the day before, staying well hydrated, and wearing comfortable clothing. Diabetic patients receive special instructions. Proper preparation is essential for accurate results.

Proper preparation is crucial for obtaining high-quality PET images. The goal of preparation protocols is to minimize background glucose uptake by normal tissues while maximizing the contrast between abnormal and normal areas. Following your preparation instructions carefully will help ensure your scan provides the most accurate and useful information possible.

Dietary Preparation

The day before your PET scan, you should follow a low-carbohydrate diet. This means avoiding sugary foods and drinks, bread, pasta, rice, potatoes, fruit, and other high-carbohydrate items. Instead, focus on protein-rich foods like meat, fish, eggs, cheese, and non-starchy vegetables. This dietary modification helps lower your baseline insulin levels and reduces glucose uptake by normal tissues, improving the visibility of abnormal areas.

You must fast for 4-6 hours before your appointment. During this fasting period, you may drink plain water—in fact, staying well hydrated is encouraged as it helps the tracer circulate and is eventually eliminated through urine. However, you should avoid all other beverages including coffee, tea, juice, and anything containing sugar or artificial sweeteners. Even sugar-free gum or mints should be avoided as they can affect blood glucose levels.

Activity Restrictions

Physical activity increases glucose uptake in muscles, which can interfere with scan interpretation. You should avoid strenuous exercise for 24-48 hours before your scan. This includes running, gym workouts, heavy lifting, and intense sports. Light walking and normal daily activities are generally acceptable, but check with your imaging facility for specific guidance.

On the day of your scan, try to stay calm and relaxed. Stress and anxiety can affect your body's glucose distribution. If you're nervous about the procedure, let the staff know—they can help you relax and may have strategies to reduce anxiety.

Special Considerations for Diabetic Patients

If you have diabetes, you'll receive specific instructions tailored to your medication regimen and blood glucose control. Generally, your blood glucose level should be below 200 mg/dL (11 mmol/L) at the time of your scan. If it's higher, the scan may need to be rescheduled as elevated blood glucose can significantly degrade image quality.

Insulin and diabetes medications may need to be adjusted before your scan. Typically, you should take your medications as usual up until the fasting period begins, but specific instructions vary based on your medication type and the facility's protocols. Never adjust your diabetes medications without explicit guidance from your healthcare provider or the imaging center.

What to Bring and Wear

Wear comfortable, loose-fitting clothing without metal components such as zippers, buttons, or underwire. You may be asked to change into a hospital gown, so avoid wearing jewelry, watches, or accessories that you'll need to remove. Bring your identification, insurance information, and any relevant medical records or previous imaging studies.

If you're claustrophobic or anxious about enclosed spaces, inform the facility when scheduling. The PET scanner is more open than an MRI machine, but some patients still experience discomfort. Arrangements can often be made to help you feel more comfortable.

What Happens During a PET Scan?

During a PET scan, you receive a radioactive tracer injection and rest quietly for 45-60 minutes while it distributes throughout your body. You then lie on a table that moves through a ring-shaped scanner for 20-45 minutes while images are captured. The procedure is painless, though you must remain still during scanning.

Understanding what happens during your PET scan appointment can help reduce anxiety and ensure you're fully prepared. The entire process typically takes 2-3 hours, with most of that time spent in preparation and waiting rather than the actual imaging procedure.

Arrival and Registration

Plan to arrive 15-30 minutes before your scheduled appointment time. After checking in, you'll complete any necessary paperwork and change into a hospital gown if required. A technologist will review your medical history, confirm your preparation, and check your blood glucose level with a finger-stick test. If your glucose is too high, the scan may need to be rescheduled.

You'll have an opportunity to use the restroom before the procedure begins, as a full bladder can interfere with pelvic imaging and you won't be able to use the restroom during the uptake period. The technologist will also insert a small intravenous (IV) catheter, usually in your arm or hand, which will be used to administer the radioactive tracer.

Tracer Injection and Uptake Period

Once the IV is in place, the radioactive tracer—typically FDG—is injected through the catheter. The injection itself is painless and feels no different from a routine blood draw or IV medication administration. After the injection, you'll rest quietly in a comfortable reclining chair for 45-60 minutes while the tracer circulates through your bloodstream and is absorbed by your cells.

During this uptake period, it's important to remain as still and quiet as possible. Physical activity or talking can increase muscle uptake of the tracer, potentially creating artifacts on your images. You should avoid using your phone, reading, or engaging in any mentally stimulating activities. Some facilities provide a quiet room with dimmed lights to help you relax. You may bring headphones to listen to calm music or an audiobook, but check with your facility first.

The Scanning Process

After the uptake period, you'll be taken to the scanner room. The PET-CT scanner looks like a large ring or donut with a table that slides through the center. The opening is quite spacious—typically about 70 centimeters (28 inches) in diameter—which is larger and less confining than an MRI machine.

You'll lie on your back on the scanning table with your arms positioned above your head (for body scans) or at your sides (for head scans). The technologist will ensure you're comfortable and may use straps, pillows, or foam supports to help you maintain your position. Remaining still during the scan is crucial for image quality—even small movements can blur the images.

During the scan, the table moves slowly through the scanner ring while the detectors capture the gamma rays emitted by the tracer. You may hear humming or clicking sounds from the machine, which is normal. For a whole-body PET-CT scan, the imaging typically takes 20-45 minutes. Brain scans may be shorter (15-20 minutes), while some specialized protocols take longer.

Throughout the scan, a technologist monitors you from an adjacent control room and can communicate with you through an intercom. If you feel uncomfortable or need assistance at any time, you can alert them immediately.

After the Scan

Once imaging is complete, the IV catheter is removed, and you can change back into your clothes. There are no restrictions on eating, drinking, or activities after the scan—in fact, drinking plenty of fluids and urinating frequently will help flush the remaining tracer from your system more quickly.

The radioactive tracer has a very short half-life (about 110 minutes for FDG), meaning it decays rapidly and will be essentially eliminated from your body within a few hours. The small amount of radiation you receive during the scan poses minimal risk and is comparable to or less than the radiation from a CT scan.

Is a PET Scan Safe?

PET scans are generally very safe. The radiation exposure is low—comparable to or less than a standard CT scan—and the radioactive tracer is eliminated from your body within hours. Side effects are rare and usually limited to minor discomfort from the injection. PET scans are not recommended during pregnancy unless medically necessary.

Safety concerns are natural when undergoing any medical procedure involving radiation. Understanding the actual risks and benefits of PET imaging can help you make informed decisions about your healthcare and reduce any anxiety about the procedure.

Radiation Exposure

PET scans do involve exposure to ionizing radiation, but the amount is relatively small. A typical PET-CT scan delivers a radiation dose of approximately 10-25 millisieverts (mSv), which is similar to what you would receive from several years of natural background radiation exposure. For comparison, the average person receives about 3 mSv per year from natural background sources including cosmic rays and radon gas.

The radioactive tracer used in PET scans has a very short half-life. FDG, the most common tracer, has a half-life of only 110 minutes, meaning half of the radioactivity is gone within about two hours. After 24 hours, virtually no detectable radioactivity remains in your body. This rapid decay is a significant safety advantage compared to some other medical procedures involving radioactive materials.

While any radiation exposure carries some theoretical risk, the diagnostic benefits of PET scans generally far outweigh this small risk. The information obtained from a PET scan often guides critical treatment decisions that can significantly impact health outcomes. Medical imaging guidelines recommend that scans be performed when the clinical benefit justifies the small radiation risk.

Allergic Reactions and Side Effects

Allergic reactions to PET tracers are extremely rare. Unlike iodinated contrast agents used in CT scans, radioactive tracers like FDG very rarely cause allergic responses. If you have a history of severe allergies or have had reactions to previous nuclear medicine procedures, inform your healthcare team, but true allergic reactions to PET tracers occur in less than 0.1% of patients.

The most common side effect of a PET scan is minor discomfort or bruising at the injection site, similar to what you might experience after a routine blood draw. Some patients report feeling slightly cold during the tracer injection, and patients with diabetes may experience changes in blood glucose levels. Serious complications are exceptionally rare.

Special Populations

PET scans are generally not performed during pregnancy unless absolutely medically necessary, as the developing fetus is more sensitive to radiation. If you are pregnant or think you might be pregnant, inform your healthcare team immediately. In urgent situations where a PET scan is crucial for diagnosis, the procedure may still be performed after careful consideration of risks and benefits.

If you are breastfeeding, you should discuss this with your healthcare provider. Depending on the tracer used, you may need to temporarily suspend breastfeeding for several hours after the scan. Pumping and discarding breast milk during this period ensures that your baby is not exposed to radioactivity.

Children can safely undergo PET scans when medically indicated. Pediatric protocols use weight-based tracer dosing to minimize radiation exposure while maintaining image quality. The benefits of accurate diagnosis typically outweigh the small radiation risks, but PET scans in children are ordered judiciously and only when other imaging options are insufficient.

How Are PET Scan Results Interpreted?

PET scan results are interpreted by specialized physicians (nuclear medicine doctors or radiologists) who analyze tracer uptake patterns and compare them to normal distributions. Results typically include SUV values (standardized uptake value) that quantify metabolic activity. Reports are usually available within 1-3 days and are sent to your ordering physician.

Interpreting PET scan images requires specialized training and expertise. Understanding how results are analyzed and reported can help you have more informed conversations with your healthcare team about your diagnosis and treatment options.

What Doctors Look For

When reviewing your PET scan, physicians examine the distribution of the radioactive tracer throughout your body. Areas with high tracer uptake appear as bright "hot spots" on the images, while areas with low uptake appear darker. The pattern of uptake, its location, intensity, and how it changes over time provide valuable diagnostic information.

Normal PET scans show tracer uptake in predictable locations. The brain, heart, and kidneys normally show high FDG uptake due to their high metabolic activity. The bladder appears bright because the tracer is excreted in urine. Some uptake in muscles, especially if the patient was physically active before the scan, is also normal. Physicians are trained to distinguish these normal patterns from abnormal findings that may indicate disease.

Understanding SUV Values

PET reports often include SUV (standardized uptake value) measurements. SUV is a semi-quantitative measure of tracer uptake that allows comparison between different areas of the body and between scans performed at different times. Higher SUV values indicate greater metabolic activity and, in the context of cancer imaging, often suggest more aggressive disease.

Typical SUV values for normal tissue range from 1-3, while cancerous tumors often show SUVs of 5-20 or higher, depending on the tumor type and aggressiveness. However, SUV interpretation must be done in clinical context—some benign conditions like infection or inflammation can also cause elevated SUV values, and some slow-growing cancers may have relatively modest uptake.

Getting Your Results

After your scan, a specialist physician will analyze your images and prepare a detailed report. This process typically takes 1-3 days, though urgent cases may be reviewed more quickly. The report is sent to the healthcare provider who ordered your scan, who will then discuss the results with you and explain what they mean for your care.

PET scan results should always be interpreted in the context of your complete clinical picture, including your symptoms, other test results, and medical history. A single PET finding is rarely diagnostic on its own—rather, it's one piece of information that contributes to your overall care plan. Your healthcare team will help you understand what your results mean and what steps, if any, should be taken next.

What Is the Difference Between PET and PET-CT?

PET-CT combines positron emission tomography (PET) with computed tomography (CT) in a single examination. While PET shows metabolic function, CT provides detailed anatomical structure. Together, they allow physicians to see both what tissues look like and how they're functioning, providing the most comprehensive diagnostic information available.

The development of combined PET-CT scanners has transformed medical imaging and is now the standard approach for most PET examinations. Understanding why these technologies are combined helps explain the superior diagnostic capability of modern PET imaging.

Why Combine PET and CT?

PET imaging excels at detecting functional abnormalities—it shows which cells are metabolically active—but it provides limited anatomical detail. A hot spot on a PET scan tells you something abnormal is happening, but it may not clearly show exactly where in the body that abnormality is located or what anatomical structure is involved. The images can appear somewhat blurry compared to CT or MRI.

CT scanning provides excellent anatomical detail, showing the precise size, shape, and location of organs and abnormalities, but it doesn't provide information about cellular function. A lymph node that appears slightly enlarged on CT could be reactive (benign) or malignant—CT alone often cannot distinguish between these possibilities.

By combining both modalities in a single examination, PET-CT provides the best of both worlds. The CT component precisely localizes abnormalities detected on PET, while the PET component reveals whether anatomically visible abnormalities are metabolically active. This fusion dramatically improves diagnostic accuracy compared to either test alone.

How PET-CT Works

In a PET-CT scanner, both imaging systems are housed in the same machine, typically with the CT scanner on one end and the PET scanner on the other. During your examination, you first pass through the CT scanner, which takes just a few minutes, then continue through the PET scanner for the longer acquisition.

Computer software then fuses the two image sets, perfectly aligning the anatomical CT images with the functional PET images. The result can be viewed in multiple ways: as separate PET and CT images, as combined fusion images, or with the PET signal overlaid on the CT anatomy in color. This flexibility allows physicians to examine your scan from multiple perspectives.

Benefits of PET-CT Over PET Alone

Studies have consistently shown that PET-CT is more accurate than PET or CT alone for cancer staging and many other applications. The combined examination can detect smaller abnormalities, more precisely localize disease, and provide more confident diagnoses. In some studies, PET-CT changed the treatment plan in up to 30% of patients compared to what would have been planned based on CT alone.

From a practical standpoint, PET-CT is also more efficient. Rather than undergoing separate PET and CT examinations on different days, you receive both in a single visit. The CT component also provides information needed for accurate PET image reconstruction (attenuation correction), improving the quantitative accuracy of PET measurements.

What Are the Limitations of PET Scans?

PET scan limitations include false positives from inflammation or infection, false negatives from slow-growing tumors, limited spatial resolution compared to CT/MRI, difficulty imaging small lesions (under 7-8mm), and interference from high blood glucose levels. Cost and availability can also be limiting factors.

While PET scanning is a powerful diagnostic tool, understanding its limitations helps set appropriate expectations and ensures the technology is used optimally. No single imaging test is perfect for all situations, and PET is most valuable when used in appropriate clinical contexts.

False Positives

Not all metabolically active tissue is cancerous. Inflammation, infection, and healing tissues can all show increased FDG uptake, potentially being mistaken for malignancy. For example, a healing surgical site, an area of pneumonia, or arthritic joints may all appear as hot spots on PET. Physicians must consider these possibilities and may recommend biopsy or follow-up imaging to distinguish benign from malignant processes.

False Negatives

Some cancers don't show high FDG uptake and may be missed or underestimated on PET scans. These include certain types of prostate cancer, well-differentiated neuroendocrine tumors, mucinous cancers, and some low-grade lymphomas. For these tumor types, other imaging modalities or specialized PET tracers may be more appropriate.

Spatial Resolution

PET imaging has lower spatial resolution than CT or MRI, meaning very small abnormalities may not be reliably detected. Lesions smaller than 7-8 millimeters are often at or below the detection threshold. This limitation is particularly relevant when looking for small lymph node metastases or tiny tumor deposits.

Cost and Availability

PET scans are expensive compared to many other imaging tests, and not all healthcare facilities have PET scanners. Insurance coverage varies, and prior authorization may be required. While these factors shouldn't prevent you from getting a medically necessary scan, they may influence your healthcare team's recommendations about when PET imaging is most appropriate.