FDG-PET Preparation & Physiologic Uptake

Quick Answer

Good FDG-PET starts with preparation: fast ~4–6 h, control glucose (typically < ~150–200 mg/dL), avoid strenuous exercise, keep the patient warm and quiet, and use a consistent uptake time (~60 min). Interpretation then depends on knowing the normal biodistribution (brain, diet-dependent myocardium, liver/spleen reference, bowel, urinary tract) and the physiologic mimics — brown fat, muscle, thymic rebound, G-CSF marrow, cyclical ovarian/endometrial, and tonsillar/lymphoid uptake — that cause most false positives.

Why Preparation Matters

FDG competes with glucose for cellular uptake, so hyperglycemia and recent insulin shift tracer away from tumor (and into muscle), degrading tumor conspicuity. Physical and physiologic activity redistributes FDG (muscle, brown fat, myocardium). Consistent prep and uptake time are also essential for valid SUV comparison across serial scans.

Patient Preparation

• Fast ~4–6 hours (water permitted); glucose checked and controlled (reschedule if markedly high).
• No strenuous exercise for ~24 h (reduces muscle uptake).
• Warm environment (~before and during uptake) to reduce brown-fat activation; quiet, no talking/chewing (laryngeal/muscle uptake).
• Consistent uptake time (~60 min); hydration and voiding before imaging (urinary activity).
• Manage diabetes carefully (timing of insulin/oral agents; note metformin effect below).
• For cardiac sarcoid (suppress myocardium) or viability (promote myocardium), preparation is disease-specific and opposite (see those pages).

Normal Biodistribution

Physiologic Mimics & How to Recognize Them

• Brown adipose tissue (BAT): symmetric supraclavicular/paraspinal/mediastinal uptake; more in cold, lean, young patients — not nodal disease. Warming (and, per protocol, benzodiazepine or beta-blocker) reduces it.
• Skeletal muscle: exertion, tension, insulin — diffuse/symmetric along muscle groups.
• Thymic rebound: post-chemotherapy anterior-mediastinal uptake (young patients).
• G-CSF / anemia marrow: diffuse marrow uptake after growth factor/chemotherapy — mimics/masks disease.
• Cyclical ovarian/endometrial: menstrual-phase/ovulation-dependent pelvic uptake.
• Tonsillar/Waldeyer, salivary: often symmetric; asymmetry raises concern.
• Bowel / diverticulitis / metformin: metformin causes intense bowel uptake (can obscure abdominal disease).

The Hyperglycemia & Metformin Caveats

• Hyperglycemia/recent insulin: competes with FDG → reduced tumor uptake and increased muscle uptake; reschedule if very high.
• Metformin: markedly increases bowel FDG uptake; holding it ~24–48 h (per protocol) improves abdominal reads.

Reporting Checklist

• Document glucose, fasting status, uptake time, and any BAT-reduction measures.
• Attribute physiologic patterns explicitly (BAT, muscle, thymic rebound, G-CSF marrow, cyclical pelvis) rather than over-calling disease.
• Use liver/blood-pool reference for response (Deauville/PERCIST) and keep prep consistent across serial scans.

Board Pearls

Preparation is half the study: fast ~4–6 h, control glucose, avoid exercise, keep the patient warm and quiet, and use a consistent ~60-min uptake time. Hyperglycemia competes with FDG — reducing tumor uptake and increasing muscle uptake — so reschedule if very high.

Know the physiologic mimics cold: brown fat (symmetric supraclavicular/paraspinal, cold/lean/young — reduce with warming), muscle (exertion/tension/talking), thymic rebound and G-CSF marrow (post-therapy), and cyclical ovarian/endometrial uptake — these cause most false positives.

Metformin markedly increases bowel uptake (hold ~24–48 h per protocol for abdominal reads); liver and blood pool are the reference organs for Deauville/PERCIST; and consistent preparation/uptake time is essential for valid SUV comparison. Disease-specific cardiac prep is the exception — suppress myocardium for sarcoid, promote it for viability.

Related Pages

• Tracer: FDG; response: Oncology response criteria (RECIST/PERCIST), Lymphoma staging (Deauville).
• Pitfalls: Incidental findings on FDG-PET, Pearls, pitfalls & normal variants.

Figure / Diagram Suggestions

• A normal FDG biodistribution whole-body plate (brain, myocardium, liver, bowel, urinary).
• A physiologic-mimic panel (brown fat, muscle, thymic rebound, G-CSF marrow).
• A hyperglycemia/metformin effect illustration (tumor-uptake shift; bowel uptake).

Self-Check (Board-Style)