PET Scan — Procedure Guide, Recovery & Risks | MyMedicPlus

A positron emission tomography (PET) scan is a nuclear medicine imaging technique that maps metabolic activity in tissues throughout the body by detecting gamma rays emitted by a positron-emitting radiotracer. In clinical practice, PET is almost always combined with computed tomography (PET-CT) to simultaneously provide metabolic and anatomical information. The most widely used tracer is fluorodeoxyglucose (FDG, 18F-FDG), a radiolabelled glucose analogue that accumulates in metabolically active cells — principally cancer cells, which consume glucose at higher rates than normal tissue. PET-CT is a cornerstone of oncological imaging, used for cancer staging, treatment response assessment, detection of recurrence, and planning for radiotherapy. Specialised tracers beyond FDG — including 68Ga-DOTATATE for neuroendocrine tumours, PSMA for prostate cancer, and Florbetapir for brain amyloid imaging — have expanded PET applications significantly. PET scanning provides functional metabolic information that complements the anatomical detail of CT and MRI. The most widely used tracer is 18F-fluorodeoxyglucose (FDG), a radiolabelled glucose analogue that accumulates in metabolically active tissues — tumours, sites of infection, and normal structures with high glucose uptake (brain, heart, bladder). FDG PET-CT is the standard of care for staging and response assessment in lymphoma, lung cancer, colorectal cancer, head and neck cancer, melanoma, thyroid cancer, and oesophageal cancer. Non-FDG tracers include Ga-68 DOTATATE (neuroendocrine tumours), F-18 PSMA (prostate cancer), F-18 NaF (bone metastases), and F-18 florbetapir (amyloid imaging in dementia). Standard uptake values (SUV) quantify tracer uptake; SUVmax above 2.5 is typically considered abnormal for FDG.

FDG PET-CT is indicated for staging of lung cancer, lymphoma, colorectal cancer, oesophageal cancer, head and neck cancers, breast cancer, melanoma, and other solid tumours. It detects distant metastases that alter clinical staging and management in 20–30% of patients compared to CT alone. PET-CT is used to assess treatment response in lymphoma (Deauville criteria after chemotherapy), to guide biopsy to the most metabolically active disease site, and to detect recurrence when tumour markers rise without a CT-identifiable lesion. Non-oncological indications include cardiac viability assessment (FDG), evaluation of fever of unknown origin and large-vessel vasculitis (FDG), and brain amyloid imaging for Alzheimer's disease diagnosis. Contraindications include pregnancy (relative) and severe uncontrolled hyperglycaemia, which reduces FDG tumour uptake (blood glucose must be under 8–10 mmol/L at time of injection).

The patient fasts for 4–6 hours before the scan to minimise background glucose and maximise FDG tumour-to-background contrast. Blood glucose is checked on arrival; if elevated, the scan may be rescheduled. FDG is injected intravenously and the patient rests quietly in a warm, dimly lit room for 60–90 minutes to allow tracer distribution and cellular uptake. Physical activity is avoided during uptake to prevent skeletal muscle FDG uptake that could obscure findings. The patient then lies on the scanner couch for simultaneous CT (3–5 minutes) and PET acquisition (20–30 minutes scanning head to thigh). The CT component provides attenuation correction and anatomical localisation of PET signal. Images are reviewed by a nuclear medicine physician or radiologist; the standardised uptake value (SUV) quantifies tracer uptake at each site. Scans for brain amyloid, cardiac viability, and PSMA follow adapted protocols with different tracers and uptake periods. A period of quiet rest in a warm, low-stimulus environment for 30–45 minutes minimises FDG uptake in muscle and brown fat (which can mimic or obscure lesions). Low-dose CT is used for attenuation correction and anatomical localisation. Diagnostic quality CT with contrast can be performed simultaneously (PET-CT with diagnostic CT). Total scan time is 15–25 minutes for whole-body imaging. Images are reviewed by a nuclear medicine physician or radiologist with PET reporting expertise. Quantitative assessment using SUVmax and total lesion glycolysis (TLG) supports response assessment to chemotherapy or radiotherapy (Deauville score for lymphoma).

FDG PET-CT detects distant metastases and upstages or downstages cancer in 20–30% of patients, directly altering the treatment plan and avoiding futile surgery in metastatic disease. In lymphoma, PET-adapted therapy using the Deauville 5-point scale after 2 cycles of chemotherapy allows treatment intensification or de-escalation based on metabolic response, improving outcomes. PET-CT has higher sensitivity for small mediastinal lymph node involvement in lung cancer than CT alone (80–90% vs 60–70%). PSMA PET for prostate cancer biochemical recurrence detects disease at PSA levels as low as 0.2 ng/mL, far exceeding conventional CT or bone scan sensitivity. PET is significantly more sensitive than CT for detecting FDG-avid recurrence at previously treated sites, guiding salvage therapy decisions. PET-CT detects disease at an earlier stage than conventional imaging — identifying lesions missed on CT alone — and alters management in 20–30% of cancer patients by upstaging or downstaging disease, avoiding futile surgery, or identifying occult primary sites. FDG PET-CT reduces the need for diagnostic surgery in patients with mediastinal lymphadenopathy or solitary pulmonary nodules by characterising metabolic activity non-invasively.

PET-CT carries a small radiation dose from both the radiotracer (FDG effective dose approximately 5–7 mSv) and the CT component (additional 3–10 mSv), for a combined effective dose of approximately 10–15 mSv — equivalent to 3–5 years of background radiation. The associated lifetime additional cancer risk from a single PET-CT is estimated at less than 1 in 1,000. FDG injection carries negligible pharmacological risk at the diagnostic dose used. False-positive results occur at sites of inflammatory activity (granulomas, surgical wounds, arthritic joints) since FDG accumulates in activated macrophages as well as tumour cells. False-negative results occur in low-grade tumours (prostate adenocarcinoma, low-grade lymphoma, mucinous tumours) with low FDG avidity. Claustrophobia and inability to remain still during the 20–30-minute scan are practical limitations, managed with mild anxiolytics when needed.

No recovery is required after a PET scan. Patients may eat, drink, and resume all normal activities immediately after the scan. To reduce radiation exposure to others, patients are advised to avoid prolonged close contact (within 1 metre for more than 30 minutes) with pregnant women, infants, and young children for approximately 6 hours after the scan, as the radiotracer continues to decay and emit radiation during this period. Generous fluid intake is encouraged for the remainder of the day to flush the tracer through the urinary tract. Diabetic patients resuming insulin or oral hypoglycaemic agents should do so promptly after the scan. Results are reported by the nuclear medicine physician, typically within 24–48 hours, and are discussed with the referring oncologist or specialist at the next clinical appointment.