PET scan

**Principle:** A radiotracer labeled with a positron-emitting isotope (18F, 11C, 13N, 15O, 68Ga, 82Rb) is injected. The positron travels a short distance, then annihilates with an electron, producing two 511-keV gamma photons emitted ~180° apart. Detectors register coincident photons, and the scanner reconstructs a 3D map of tracer distribution. Modern scanners are almost always PET/CT (or PET/MRI), combining metabolic with anatomic data and allowing attenuation correction.

**FDG-PET:** 18F-FDG is a glucose analog. It is taken up by GLUT transporters and phosphorylated by hexokinase to FDG-6-phosphate, which cannot proceed through glycolysis (and is poorly dephosphorylated in most tumor cells), so it is trapped intracellularly. Cancer cells show the Warburg effect (increased aerobic glycolysis, increased GLUT1 and hexokinase), so they accumulate FDG. The signal is semiquantified by the standardized uptake value (SUV).

**Clinical uses (high-yield):** - **Oncology:** staging and restaging (lymphoma, lung cancer, esophageal, head and neck, melanoma, colorectal), evaluating a solitary pulmonary nodule, detecting occult metastases, assessing response to therapy (e.g., Deauville score in lymphoma), and distinguishing residual tumor from post-treatment fibrosis. - **Neurology:** Alzheimer disease (bilateral temporoparietal hypometabolism), frontotemporal dementia (frontal/anterior temporal hypometabolism), and localizing seizure foci (interictal hypometabolism). Amyloid PET and tau PET are also used. - **Cardiology:** myocardial viability (FDG uptake in hypoperfused myocardium = hibernating, viable tissue; "perfusion-metabolism mismatch"), and perfusion imaging with 82Rb or 13N-ammonia. PET is more accurate than SPECT for coronary perfusion. - **Other:** large-vessel vasculitis, sarcoidosis, fever/inflammation of unknown origin, and infection. Prostate cancer is imaged with PSMA-PET (68Ga- or 18F-labeled) and neuroendocrine tumors with 68Ga-DOTATATE.

**Limitations and pitfalls:** - **False positives:** infection, inflammation, granulomatous disease (TB, sarcoid), recent surgery or radiation, brown fat, and physiologic uptake in the brain, heart, kidneys/bladder, and muscle. - **False negatives:** low-FDG-avid tumors (well-differentiated adenocarcinoma/bronchioloalveolar carcinoma, carcinoid, mucinous tumors, hepatocellular carcinoma, renal cell carcinoma, prostate cancer) and small lesions (<~5–8 mm). - **Hyperglycemia** and **high insulin** levels competitively inhibit FDG uptake or shunt it to muscle. Patients fast about 4–6 hours, and diabetics need glucose control. - Brain, heart, and bladder normally show high uptake, which limits evaluation of nearby lesions. - Radiation exposure; relatively contraindicated in pregnancy. Breastfeeding precautions may apply.

**PET vs. other modalities:** CT/MRI show anatomy (structure). PET shows function (metabolism). SPECT uses single-photon emitters (99mTc, 201Tl, 123I) and has lower resolution and sensitivity than PET. Isotope half-lives are short (18F ≈ 110 min; 11C ≈ 20 min; 82Rb ≈ 75 sec), and many require an on-site cyclotron (82Rb comes from a generator).

**Exam pearls:** "Hot spot" on PET in a patient with a lung nodule suggests malignancy. Alzheimer disease shows temporoparietal hypometabolism. A viable but hibernating myocardium shows reduced perfusion with preserved FDG uptake, and it will benefit from revascularization. Hyperglycemia can cause a false-negative study.