What Is a PET Scan and When Is It Used?

PET scanning, formally known as positron emission tomography, occupies a unique place among imaging technologies. Where most imaging modalities show what the body's structures look like, PET shows what they are doing. The technology can identify regions of increased or decreased metabolic activity, areas where cells are dividing rapidly, and various other functional characteristics that anatomic imaging alone cannot reveal. This functional information complements the anatomic information from CT and MRI, with the combination often providing more comprehensive evaluation than either approach alone.

The clinical applications of PET have expanded substantially over recent decades. The technology was once a research tool with limited clinical use. It is now central to oncology, where PET supports diagnosis, staging, treatment monitoring, and detection of recurrence for many cancers. Beyond oncology, PET has expanding roles in neurology for evaluation of dementia and various other brain conditions, in cardiology for evaluation of heart function and viability, and in various other clinical situations where functional information complements anatomic findings.

The technology behind PET reflects a remarkable combination of physics, chemistry, and engineering. Radioactive tracers, manufactured specifically for medical use, are administered to the patient. The tracers travel to specific tissues or accumulate based on metabolic activity. The scanner detects the radiation emitted by these tracers and constructs images showing where in the body the tracer has accumulated. The combination of PET with CT (PET-CT) or MRI (PET-MRI) provides anatomic correlation that supports interpretation of the functional findings.

This article explains how PET scans work, what specific tracers are used, how PET differs from other imaging modalities, what conditions PET is commonly used to evaluate, and what patients can expect when undergoing PET imaging.

For more on imaging broadly, see Advanced Imaging Technologies in Medicine.

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How PET Scans Work

PET imaging operates on principles fundamentally different from those of CT, MRI, or ultrasound.

Tracer Injection and Distribution

The patient receives a small amount of radioactive tracer, typically given by intravenous injection. The tracer is a molecule labeled with a radioactive isotope that emits positrons (positively charged particles) as it decays.

After injection, the tracer distributes throughout the body:

• Different tracers have different patterns of distribution
• FDG (fluorodeoxyglucose) is a glucose analog that is taken up by tissues based on their metabolic activity
• Tissues with high metabolic activity, including many cancers, take up more FDG than less active tissues

Radiation Detection

The scanner detects the radiation produced when positrons emitted by the tracer interact with electrons in surrounding tissue:

• The interaction produces gamma rays that travel in opposite directions
• The scanner uses detectors arranged around the patient to identify these gamma rays
• The pattern reveals where in the body the tracer has accumulated

Combined Imaging

Modern PET scanners often combine PET with CT (PET-CT) or MRI (PET-MRI):

• Combined system acquires PET and the corresponding anatomic imaging during the same examination
• Supports more accurate interpretation by correlating functional findings (where tracer has accumulated) with anatomic structures

Timing

The whole process, from injection of tracer to completion of imaging, typically takes 1 to 2 hours. The actual scan time is often 20 to 40 minutes, with additional time for tracer uptake before scanning begins.

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What PET Tracers Show

Different PET tracers reveal different aspects of biology. Several specific tracers have established clinical applications.

FDG (Fluorodeoxyglucose)

FDG is the most widely used PET tracer:

• The molecule is similar to glucose but is labeled with the radioactive isotope fluorine-18
• When taken up by cells, FDG cannot be metabolized as glucose normally would
• Accumulates in cells based on their metabolic activity
• Cancer patterns: Many cancers have higher metabolic activity than surrounding normal tissues, leading to increased FDG uptake
• Pattern support: Identification of cancers, evaluation of their extent, and assessment of treatment response
• Beyond cancer: Also used for evaluation of inflammation, infection, and various non-cancer conditions where altered metabolism provides diagnostic information

Other Tracers for Cancer

Several other tracers target specific molecular features of cancers:

• PSMA-targeted tracers (such as gallium-68 PSMA) target prostate-specific membrane antigen, which is expressed by prostate cancer cells
  • Transformation: These tracers have transformed prostate cancer imaging
  • Detection: Allowing detection of metastases that conventional imaging often misses
• DOTATATE and similar tracers target somatostatin receptors expressed by neuroendocrine tumors
  • Support: evaluation of neuroendocrine cancers including carcinoid tumors and various other types
• Various other: targeted tracers exist for specific cancer types and continue to be developed

Tracers for Neurology

Several tracers support evaluation of brain conditions:

• FDG-PET of the brain shows patterns of glucose metabolism
  • Alzheimer disease: specific patterns of decreased metabolism in particular brain regions support diagnosis
  • Other dementias: different patterns may be seen
  • Support: distinction among different types of dementia
• Amyloid PET tracers (such as florbetapir, florbetaben, and flutemetamol) bind to amyloid plaques in the brain
  • Presence or absence: of amyloid supports evaluation of Alzheimer disease
  • Particularly: useful in distinguishing it from other conditions producing similar symptoms
• Tau PET tracers bind to tau protein tangles, another feature of Alzheimer disease
  • Support: evaluation of tau pathology in dementia
  • Research: various research applications

Tracers for Cardiology

Several tracers support evaluation of cardiac conditions:

• Myocardial perfusion tracers (such as rubidium-82 or various others) support evaluation of blood flow to the heart muscle
  • Patterns: of reduced perfusion can identify coronary artery disease
• FDG-PET of the heart can evaluate viability of heart muscle
  • Hibernating muscle: alive but not functioning normally because of reduced blood flow
  • Support: decisions about whether revascularization (improving blood flow) might restore function

Other Specialized Tracers

Various other tracers support specific clinical applications:

• Bone-specific tracers evaluate bone metabolism
• Specific tracers target molecular features of various conditions
• Development: of new tracers continues, expanding the clinical applications of PET

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How PET Differs From Other Imaging

Understanding how PET differs from other imaging modalities clarifies its specific role.

Functional Versus Anatomic Imaging

PET shows function rather than just structure:

• Where CT and MRI show what tissues look like
• PET shows what they are doing metabolically
• Functional information complements the anatomic information from other modalities
• For some clinical: questions, functional information is critical
  • Identification: of small cancer metastases
  • Distinction: between active and inactive disease
  • Various other: questions benefit from functional imaging

Whole-Body Imaging

PET typically images the entire body or substantial portions of it:

• Whole-body capability supports identification of disease at multiple sites
• Comprehensive staging: and various other applications where comprehensive evaluation matters
• CT and MRI: typically image specific body regions rather than the entire body
• The targeted: approach is often appropriate
• For situations: requiring whole-body assessment, PET provides this capability

Combination With Other Modalities

PET is typically combined with CT (PET-CT) or sometimes MRI (PET-MRI):

• Combined approach: provides both functional and anatomic information from a single examination
• Integration: supports more accurate interpretation of findings

Different Types of Information

The combination of PET with anatomic imaging provides three types of information:

• Anatomic information: from the CT or MRI shows what structures are present
• Functional information: from the PET shows metabolic activity
• Combined interpretation: correlates these aspects, supporting more comprehensive evaluation than either alone

For more on this comparison, see MRI vs CT Scan: Differences, Uses, and Risks.

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When PET Is Used in Cancer Care

Cancer is the most common application of PET imaging. Several specific roles support cancer care.

Cancer Staging

For many cancers, PET supports staging by identifying disease at sites where conventional imaging may miss it:

• Whole-body capability: and the ability to identify metabolically active disease support comprehensive staging
• Particularly useful: for cancers that often metastasize to distant sites
  • Lung cancer, lymphoma, melanoma, and various other cancers commonly involve PET imaging

Treatment Monitoring

For cancers being treated, PET can evaluate whether treatment is working:

• Decreased FDG uptake suggests reduced cancer activity
• Persistent or increased: uptake suggests treatment is not effectively reducing the disease
• Information support: decisions about whether to continue current treatment, change to a different approach, or pursue other strategies

Detection of Recurrence

For patients in remission after cancer treatment, PET can identify recurrence often before symptoms develop or anatomic imaging shows changes:

• Functional information: may identify metabolically active cancer cells that have not yet produced visible masses

Characterization of Findings

For findings identified on other imaging that are uncertain in nature, PET can help distinguish cancer from benign processes:

• Increased FDG uptake suggests cancer
• Though some: benign conditions also show increased uptake
• Information from PET, combined with other findings, supports diagnostic decisions

Specific Cancer Applications

Different cancers have specific patterns of PET use:

• Lung cancer: PET-CT is routinely used for initial staging, evaluation of treatment response, and detection of recurrence
• Lymphoma: PET-CT is central to staging and treatment monitoring
  • Deauville criteria: used for evaluation of treatment response in lymphoma, are based on PET findings
• Colorectal cancer: PET supports evaluation of suspected recurrence and characterization of liver metastases
• Melanoma: PET supports staging in higher-stage disease and detection of recurrence
• Breast cancer: PET has more limited routine use, with applications in specific situations including detection of distant disease in advanced cases
• Prostate cancer: PSMA PET has transformed imaging, identifying metastases that conventional imaging often misses

For more on cancer detection, see How Medical Imaging Helps Detect Cancer Early.

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When PET Is Used in Neurology

Beyond cancer, PET has expanding roles in neurology.

Evaluation of Dementia

For evaluation of dementia, PET can support specific diagnoses:

• FDG-PET: of the brain shows patterns of glucose metabolism that vary among different types of dementia
  • Support distinction: between Alzheimer disease, frontotemporal dementia, dementia with Lewy bodies, and various other conditions
• Amyloid PET: shows whether amyloid plaques are present in the brain
  • Support Alzheimer: disease diagnosis, particularly when the clinical picture is unclear
  • Recent introduction: of amyloid-targeting therapies for Alzheimer disease has gained additional clinical importance
• Tau PET: shows whether tau tangles are present, providing additional information about Alzheimer pathology

Evaluation of Movement Disorders

For movement disorders, specific PET tracers can evaluate the dopamine system:

• Information support: diagnosis of Parkinson disease
• Distinction: from related conditions

Evaluation of Epilepsy

For epilepsy, particularly when surgery is being considered, PET can identify regions of reduced metabolism that may be the source of seizures:

• Information support: surgical planning

Brain Tumors

For brain tumors, PET can complement MRI in:

• Evaluating: tumor activity
• Distinguishing: tumor recurrence from treatment effects
• Various other: applications

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When PET Is Used in Cardiology

Cardiac PET applications have expanded substantially.

Myocardial Perfusion

For evaluation of coronary artery disease, PET myocardial perfusion imaging shows patterns of blood flow to the heart muscle:

• Information identifies: areas of reduced perfusion that may indicate significant coronary disease
• PET myocardial: perfusion typically uses tracers such as rubidium-82 or N-13 ammonia
• Technology provide: excellent quantification of blood flow that can support specific clinical decisions

Myocardial Viability

For patients with reduced heart function, PET can evaluate whether the affected heart muscle is alive but not functioning (hibernating) or has been replaced by scar tissue:

• Distinction matter: for treatment decisions
• Hibernating muscle: potentially recovering function with revascularization
• FDG-PET: of the heart, often combined with perfusion imaging, supports viability assessment

Sarcoidosis and Other Conditions

For specific cardiac conditions including cardiac sarcoidosis, PET can support diagnosis and treatment monitoring:

• Functional information: complements other cardiac imaging

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What to Expect During a PET Scan

The patient experience during PET imaging follows reasonably consistent patterns.

Preparation

Specific preparation depends on the type of PET being performed:

• For FDG-PET: fasting for 4 to 6 hours before the examination is typically required
  • Diabetic patients: may need specific instructions about medications and blood sugar management
  • Avoidance of: strenuous exercise for 24 hours before the scan is often recommended because exercise can affect tracer distribution
• For other tracers: specific preparation instructions vary

Tracer Injection

The tracer is administered through an IV:

• Injection itself: is brief and typically involves only the discomfort of an IV insertion
• Uptake period: of 30 to 90 minutes follows, during which the tracer distributes throughout the body
• Patient typically: rests during this period, often in a quiet room with minimal stimulation
• Movement and: conversation are typically limited because muscle activity can affect tracer distribution

Scanning

The actual scanning occurs after the uptake period:

• Patient lies: on a scanner table that moves through the scanner
• Must remain: still throughout the scan, which typically takes 20 to 40 minutes
• For PET-CT: the CT portion is acquired during the same session, often at the beginning or end of the PET scan
• For PET-MRI: the combined acquisition is more complex and typically takes longer

After the Scan

After the scan, the patient can typically resume normal activities:

• Radioactive tracer: continues to decay over hours
• Remaining radioactivity: is below levels of concern by the time the patient leaves the facility
• Advised to: drink fluids to support clearance of the tracer
• Special precautions: about contact with others are typically not needed
• Though some: institutions provide specific guidance for limited contact with young children or pregnant women for the rest of the day

Scheduling Considerations

The total time at the facility for a PET examination is typically 2 to 4 hours:

• Includes preparation, uptake period, scanning, and any post-scan instructions
• Patients should: plan for this time when scheduling
• For some specific: PET protocols, the uptake period may be longer, extending the total time required

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Risks and Safety

PET imaging involves specific safety considerations.

Radiation Exposure

PET involves radiation exposure from the radioactive tracer:

• Typical dose: approximately 8 to 14 millisieverts depending on the specific protocol
• PET-CT involve: the combined dose from both components
• Comparable: to that of a typical CT scan of the abdomen and pelvis
• Cancer risk: from a single PET-CT is small but not negligible
• Benefits: of PET imaging for clinical situations where it has been recommended typically substantially outweigh the small radiation risk

For more on radiation, see Are Medical Imaging Tests Safe? Understanding Radiation Risks.

Allergic Reactions

Allergic reactions to PET tracers are uncommon but possible:

• Most reactions: are mild
• Severe reactions: are rare

Pregnancy Considerations

PET is generally avoided during pregnancy:

• Because of: radiation considerations
• Alternative imaging: without radiation is preferred when possible
• When PET: is genuinely needed during pregnancy, careful consideration of risks and benefits is appropriate

Breastfeeding Considerations

For breastfeeding patients, specific recommendations about temporary cessation of breastfeeding may apply for some tracers:

• Duration of: cessation depends on the specific tracer and varies among institutions

Diabetes Considerations

For patients with diabetes, blood sugar management before PET-FDG is important:

• Elevated blood: sugar can affect tracer distribution and reduce image quality
• Specific protocols: for diabetic patients support optimal imaging

Contraindications

Few absolute contraindications exist for PET, though specific situations may warrant alternative imaging.

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How PET Results Are Interpreted

The interpretation of PET findings involves several specific considerations.

Visual Interpretation

The radiologist or nuclear medicine physician evaluates the PET images visually:

• Identifies areas: of abnormal tracer uptake
• Increased uptake: in patterns suggesting cancer, decreased uptake in regions where reduced metabolism is concerning, and various other patterns support specific diagnoses

Quantitative Measurement

Beyond visual assessment, quantitative measurement of tracer uptake supports more precise evaluation:

• Standardized uptake: value (SUV) provides a measurement of tracer concentration
• Can be compared: between different patients and over time
• For oncology: SUV measurements support evaluation of cancer activity, treatment response, and various other considerations
• Specific thresholds: and patterns guide interpretation in different clinical situations

Correlation With Anatomic Imaging

The integration of PET with CT or MRI supports correlation of functional findings with anatomic structures:

• Areas of abnormal: uptake are localized to specific anatomic structures
• Supporting interpretation: of what the findings represent

Comparison With Prior Studies

For patients with prior PET studies, comparison over time supports evaluation of changes:

• Decreasing uptake: suggests improvement
• Increasing uptake: suggests progression
• New areas: of uptake may suggest new disease

Limitations

PET interpretation has specific limitations:

• False positive: findings can occur from inflammation, infection, post-treatment changes, and various other benign conditions that can produce increased uptake
• False negative: findings can occur from cancers with low metabolic activity, small lesions below the resolution of PET, and various other situations
• Integration: of PET findings with clinical context, other imaging, and various other information supports accurate diagnosis

For more on imaging interpretation, see Can Medical Imaging Be Misinterpreted?.

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False Positives and False Negatives

Understanding the limitations of PET supports realistic expectations.

Common False Positive Sources

Several conditions can produce increased FDG uptake that mimics cancer:

• Inflammation: produces increased glucose metabolism
  • Active inflammation: in any tissue can produce FDG uptake that resembles cancer
  • Recent surgery, infections, and various inflammatory conditions can all produce this pattern
• Granulomatous diseases: including sarcoidosis and tuberculosis can produce intense FDG uptake
• Brown fat, a type of fat tissue that can be metabolically active, can produce FDG uptake
  • More common: in younger patients and in cold conditions
• Recent radiation: therapy can produce increased FDG uptake in treated areas, sometimes lasting weeks to months
• Various benign: tumors can show increased FDG uptake, requiring additional evaluation to distinguish from cancer

Common False Negative Sources

Some cancers have low metabolic activity and may not show increased FDG uptake:

• Slow-growing cancers: including some thyroid cancers, low-grade neuroendocrine tumors, and various others may not have substantially increased uptake
• Very small lesions: below the resolution of PET may not be detected
  • Resolution of PET: is typically 4 to 8 millimeters
  • Lesions smaller: than this may be missed
• Cancers with: specific metabolic characteristics that do not preferentially use glucose may have limited FDG uptake
• For these situations: alternative imaging or different tracers may be needed

Managing Uncertainty

When PET findings are uncertain, several approaches help:

• Comparison: with prior imaging provides context
• Correlation: with clinical findings supports interpretation
• Additional imaging: with different modalities or different tracers may resolve uncertainty
• Biopsy: of accessible findings provides definitive information

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Cost and Insurance

The financial aspects of PET affect practical access.

Cost

PET scans are typically expensive:

• Costs varying: among institutions and the specific protocol
• PET-CT: is often more expensive than CT alone but provides additional information

Insurance Coverage

Insurance coverage for PET varies:

• Most plans: cover PET for established clinical indications including specific cancer applications
• For some: applications, pre-authorization is required to confirm clinical appropriateness
• For some applications: including specific neurological evaluations, coverage may be more limited
• Specific situation: determines whether coverage applies

Access

PET scanners are less widely available than CT or MRI scanners:

• Patients in: some areas may need to travel for PET imaging
• For specialized: PET protocols including some neurology applications, availability may be more limited
• Specific institutions: providing certain capabilities

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Recent Developments and Future Directions

The field of PET imaging continues to evolve.

New Tracers

Development of new PET tracers continues:

• Tracers targeting: specific molecular features of various cancers and other conditions
• PSMA tracers: for prostate cancer
• FAPI tracers: targeting fibroblasts in tumors
• Various others: have entered clinical use or are in development

Total-Body PET

Newer scanner technology allowing imaging of the entire body simultaneously has emerged:

• Technology offer: potential advantages including faster scanning, lower tracer doses, and improved image quality

Combination With MRI

PET-MRI combines PET with the soft tissue detail of MRI:

• Technology has: specific advantages for certain applications including evaluation of brain tumors, some oncology applications, and various others

Theranostics

The combination of imaging and therapy using related compounds (theranostics) has emerged as an important approach:

• Tracers used: for imaging can identify patients who may benefit from therapy with similar compounds
• PSMA-based: imaging and therapy for prostate cancer is one example of this approach

Artificial Intelligence

Artificial intelligence applications in PET image analysis continue to develop:

• Specific applications: support image reconstruction, automatic identification of findings, quantitative analysis, and various other aspects of PET imaging

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Common Questions About PET Scans

Several questions arise commonly about PET imaging.

How Does PET Compare to Other Imaging?

PET provides functional information that complements anatomic information from CT and MRI:

• Combination: of PET with CT or MRI provides comprehensive evaluation
• For specific: clinical questions, PET may be the most appropriate imaging
• For others: anatomic imaging alone is adequate

Will the Radiation Hurt Me?

The radiation from PET is at levels comparable to other imaging studies using radiation:

• Benefits: for clinical situations where PET has been recommended typically substantially outweigh the small radiation risk

Why Do I Need to Fast?

For FDG-PET, fasting reduces blood sugar levels, which improves the contrast between cancer and normal tissues:

• Elevated blood: sugar can reduce image quality

How Long Will the Scan Take?

The actual scan typically takes 20 to 40 minutes:

• Additional time: for tracer uptake before scanning
• Total time: at the facility is typically 2 to 4 hours

Can I Drive Home Afterward?

For most PET scans, driving afterward is not problematic:

• For scans: involving sedation, alternative transportation arrangements are needed

Is the Tracer Permanent in My Body?

The radioactive tracer decays over hours:

• Most eliminated: through urine within hours after the scan
• By the next: day, residual radioactivity is essentially gone

What Does the Cost Cover?

PET scan costs typically include:

• The tracer, the scanner time, professional interpretation, and various other components
• Specific billing: varies among institutions

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When to See a Specialist

Several specialists may be involved in PET imaging.

Ordering Physician

The ordering physician, typically the specialist managing the relevant condition:

• Often an: oncologist for cancer or a neurologist for brain conditions
• Determines: whether PET is appropriate and explains the reasoning

Imaging Specialists

Nuclear medicine physicians or radiologists with specific training in PET interpret the studies:

• Their expertise: supports accurate interpretation of complex findings

Multidisciplinary Review

For complex situations, multidisciplinary review may include input from multiple specialists:

• Integrated interpretation: of PET and other findings supports comprehensive care
• For situations: where PET findings affect significant clinical decisions, second opinions on imaging interpretation can sometimes provide value

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Conclusion

PET imaging provides functional information that complements the anatomic information from CT and MRI. The technology has applications across oncology, neurology, cardiology, and various other specialties, with the specific tracer used determining what biological process is being evaluated.

For patients undergoing PET imaging, understanding what to expect helps frame the experience. The preparation, tracer injection, uptake period, and scanning together typically take 2 to 4 hours. The radiation dose is comparable to other imaging studies using radiation, with benefits substantially outweighing risks for appropriate clinical applications.

The interpretation of PET findings involves visual assessment, quantitative measurement, correlation with anatomic imaging, and integration with clinical context. False positives and false negatives can occur, with awareness of common patterns supporting accurate interpretation.

The field of PET imaging continues to evolve, with new tracers, improved technology, and expanding applications supporting better diagnostic care. The trajectory of improvement suggests further developments in coming years.

For most patients, the practical message is that PET imaging is a powerful diagnostic tool that, when appropriately applied, provides information that supports better clinical decisions. The specific applications depend on the clinical situation, with the imaging recommended typically reflecting careful consideration of how the information will affect care.

The encouraging element of modern PET imaging is that the available capabilities allow visualization of biological processes that would have been invisible to clinicians a generation ago. The combination of PET with other imaging modalities provides comprehensive evaluation that supports accurate diagnosis and effective treatment.

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Medical Disclaimer

This content is for informational purposes only and does not constitute medical advice. Individuals should consult a qualified healthcare provider for diagnosis and treatment.