What is PET (Positron Emission Tomography)?

Positron Emission Tomography (PET): Principles and Applications

PET is a non-invasive diagnostic imaging technique that provides tomographic images of metabolic activity in tissues by detecting positron-emitting radiotracers, most commonly 18F-fluorodeoxyglucose (FDG), which measures glucose metabolism in cells. 1

Basic Principles of PET

• PET measures the three-dimensional distribution of positron-emitting labeled radiotracers in the body 1
• The most commonly used tracer is 18F-FDG, a glucose analog that:
  • Is taken up by cells via glucose transporters
  • Becomes metabolically trapped after phosphorylation by hexokinase
  • Accumulates in cells with high metabolic activity 1
• 18F is a cyclotron-produced radioisotope with a half-life of 109.7 minutes, allowing imaging within a few hours after injection 1
• Increased FDG uptake in tissues reflects increased glucose utilization, which is characteristic of most cancers due to:
  • Overexpression of glucose transporters (particularly GLUT1 and GLUT3)
  • Increased hexokinase activity 1

PET/CT Hybrid Imaging

• Modern PET scanners are typically combined with CT (PET/CT) or MRI (PET/MRI) to provide both functional and anatomical information 1
• PET/CT fusion improves:
  • Anatomical localization of metabolic abnormalities
  • Tumor staging accuracy
  • Delineation of tumors associated with surrounding structures 1
• The integrated system provides co-registered morphologic and functional datasets as part of a single examination 1
• PET/CT is superior to PET alone, CT alone, or visual correlation of both techniques separately 1

Clinical Applications

Oncology

• Diagnosis of indeterminate pulmonary nodules with sensitivity of 96% and specificity of 79% 1
• Staging of malignancies, particularly for lymph node involvement 1, 2
• Detection of distant metastases and unexpected additional primary malignancies 1
• Evaluation of treatment response and early detection of recurrence 1
• Radiation therapy planning 1

Infection and Inflammation

• Diagnosis and management of infectious and inflammatory disorders 1
• Detection of sites of infection with unknown origin 1
• Monitoring treatment response in inflammatory conditions 1

Neurology

• Evaluation of neurodegenerative disorders, particularly dementias
• Assessment of seizure foci in epilepsy
• Differentiation of tumor recurrence from radiation necrosis in brain tumors 3

Technical Considerations

• Patient preparation requires:
  • Fasting for 4-6 hours before the scan
  • Blood glucose control (ideally <150-180 mg/dl) 2
• After FDG administration, a 60-90 minute uptake period is needed before imaging 2
• Interpretation involves both visual assessment and semi-quantitative evaluation using standardized uptake values (SUVs) 2

Limitations and Pitfalls

• False-negative results can occur in:
  • Small lesions (<1 cm) due to partial volume effects
  • Low-grade or well-differentiated tumors with low metabolic activity
  • Certain cancer types like bronchoalveolar cell carcinoma 1
• False-positive results can occur in:
  • Inflammatory or infectious processes
  • Granulomatous diseases
  • Post-surgical or post-radiation changes 1

Emerging Applications

• Development of new radiotracers beyond FDG:
  • Amino acid analogs for protein synthesis imaging
  • Choline for membrane lipid synthesis imaging
  • Acetate for cytoplasmic lipid synthesis imaging 4, 5
• These newer tracers may be more specific for certain tumor types than FDG 5

PET imaging, particularly with hybrid systems like PET/CT, has revolutionized diagnostic imaging by providing crucial metabolic information that complements anatomical imaging, leading to improved diagnosis, staging, and treatment monitoring across multiple medical disciplines.