What is a PET Scan?
A PET (Positron Emission Tomography) scan is a noninvasive nuclear medicine imaging technique that creates three-dimensional images showing the metabolic and functional activity of tissues by detecting pairs of gamma rays emitted from positron-emitting radioactive tracers injected into the body. 1
Basic Physics and Mechanism
PET imaging works through a unique detection system:
Positron emission and annihilation: When a positron emitted from the radioactive tracer collides with a nearby electron, they mutually annihilate and produce two gamma photons that travel in opposite directions (180 degrees apart). 1
Coincidence detection: The PET camera uses detectors positioned opposite each other to simultaneously detect these paired photons, creating a "line of response" that indicates the tracer was somewhere along that line. 1, 2
Electronic collimation: Unlike conventional nuclear medicine imaging, PET uses coincidence electronics rather than physical collimators, resulting in higher image resolution and sensitivity. 3
Quantitative capability: PET uniquely provides quantitative measurements of tracer concentrations through attenuation correction using transmission scans, allowing precise assessment of metabolic activity. 4, 3
Most Common Tracer: FDG
The predominant radiotracer used clinically is 18F-fluorodeoxyglucose (FDG):
FDG is a glucose analogue that enters cells via glucose transporters (particularly GLUT1) and becomes phosphorylated by hexokinase, but unlike glucose, it becomes metabolically trapped inside the cell. 1
Cancer cells demonstrate markedly increased FDG uptake due to overexpression of glucose transporters and elevated hexokinase activity, reflecting the Warburg effect of increased glycolysis in malignant cells. 1
The 18F isotope has a half-life of 109.7 minutes, allowing imaging within 2-3 hours after injection while requiring cyclotron production. 1, 5
Clinical Applications
Oncology (Primary Use)
PET scanning is predominantly used in cancer management:
Diagnosis of indeterminate pulmonary nodules: FDG-PET achieves 96% sensitivity and 79% specificity in distinguishing benign from malignant lung lesions ≥1 cm, significantly superior to CT alone. 1
Cancer staging: PET detects lymph node metastases and distant metastases with higher accuracy than anatomic imaging, with negative predictive values equal or superior to invasive procedures like mediastinoscopy. 1
Treatment response assessment: PET evaluates metabolic changes following therapy, often detecting response earlier than anatomic imaging. 1, 6
Recurrence detection: Whole-body PET identifies cancer recurrence and unexpected additional primary malignancies. 1
Neurological Disorders
PET provides functional brain imaging for:
Dementia evaluation: Different patterns of glucose metabolism help distinguish Alzheimer's disease from other dementias. 2
Epilepsy: Identifies seizure foci showing decreased metabolism between seizures. 2
Movement disorders and brain tumors: Assesses dopaminergic function and tumor metabolism. 2
Cardiology
PET measures myocardial perfusion and viability:
Quantitative myocardial blood flow measurement in ml/g/min to diagnose coronary artery disease. 3
Viability assessment: Identifies hibernating myocardium with preserved glucose metabolism despite reduced function, predicting recovery after revascularization. 3
Infectious and Inflammatory Diseases
Pyrexia of unknown origin (PUO): FDG-PET/CT identifies the fever source in 48% of cases with 80-100% sensitivity, leading to treatment modifications in 53% of patients. 7
Bacteremia of unknown origin: PET/CT detects infection sites in 56.4% of cases and has high clinical impact in 47.3% of patients. 7
PET/CT: The Current Standard
Modern PET is typically combined with CT in integrated scanners:
PET/CT provides simultaneous metabolic and anatomic information, overcoming PET's limitation of poor anatomic localization and CT's inability to distinguish metabolic activity. 1
CT serves dual purposes: provides attenuation correction for quantitative PET measurements and anatomic detail for precise lesion localization. 1
PET/CT is superior to PET alone, CT alone, or visual correlation of separate studies, particularly for T3/T4 tumor staging and delineating tumors associated with atelectasis. 1
Important Limitations and Pitfalls
False Positives
Inflammatory conditions (sarcoidosis, granulomatous diseases, infections) show increased FDG uptake and can mimic malignancy. 1
Recent procedures or implants cause inflammatory uptake. 7
Physiologic uptake in brain, heart, urinary tract, and bowel must be distinguished from pathologic uptake. 7
False Negatives
Subcentimetric lesions may not accumulate sufficient tracer for detection. 1
Bronchoalveolar cell carcinomas often show minimal FDG uptake despite being malignant. 1
Prior antibiotic therapy reduces inflammatory activity and can cause false negatives in infection imaging. 7
Small mobile vegetations in endocarditis may be missed. 7
Technical Requirements
PET imaging requires specialized infrastructure:
On-site cyclotron or nearby production facility is necessary due to short half-lives of positron emitters (particularly for C-11, N-13, O-15 tracers with half-lives of minutes). 5, 4
Radiochemistry unit for tracer synthesis, though automated synthesis units have simplified this process. 4
Patient preparation: 4-6 hours fasting, adequate hydration, blood glucose <150-180 mg/dL, with special myocardial suppression protocols when cardiac imaging is needed. 7
Image acquisition: Typically begins 60 minutes post-injection, with whole-body imaging from skull base to mid-thigh as standard. 7