# PET-CT Indications

> PET-CT with FDG for NEET-PG Radiology: glucose analogue principle, oncology indications, infection and dementia uses, and pitfalls.

- Canonical URL: https://prepelephant.com/topics/neet-pg/radiology/pet-ct-indications
- Exam / course: NEET-PG · Subject: Radiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "PET-CT Indications", PrepElephant, https://prepelephant.com/topics/neet-pg/radiology/pet-ct-indications

## Direct answer

Positron emission tomography combined with CT (PET-CT) uses fluorine-18 fluorodeoxyglucose (FDG), a glucose analogue trapped in metabolically active cells, to image glucose metabolism fused with CT anatomy in a single examination. Its principal indications are oncological — characterising indeterminate solitary pulmonary nodules, staging and response assessment of lymphoma, melanoma and many solid tumours, detecting recurrence with rising tumour markers, and differentiating radiation necrosis from recurrent brain tumour — plus selected infection and inflammation imaging and, with FDG or amyloid tracers, dementia and epilepsy evaluation. False positives from physiological uptake, infection and inflammation, and false negatives in hyperglycaemia, slow-growing or small tumours are the key pitfalls NEET-PG expects.

## What you must remember

- Principle: F-18 FDG is transported like glucose and phosphorylated by hexokinase but not further metabolised, so it accumulates intracellularly in proportion to glucose use; F-18 half-life is about 110 minutes.
- PET detects the two 511 keV photons from positron annihilation; CT fusion provides anatomical correlation and attenuation correction in one visit.
- Patient preparation: fast typically four to six hours, blood glucose controlled (elevated glucose reduces tumour uptake), physical inactivity before injection to minimise muscle uptake; diabetic protocols adjust insulin timing.
- High-yield oncology uses: solitary pulmonary nodule characterisation; staging and restaging of Hodgkin and non-Hodgkin lymphoma (replaced gallium-67); melanoma and head-neck cancer staging; unknown primary with cervical nodal metastasis; rising tumour markers with negative conventional imaging; glioma recurrence versus radiation necrosis.
- Lower or variable utility: prostate carcinoma (FDG-avidity poor; PSMA PET is preferred in current practice), mucinous and low-grade gastrointestinal tumours, and renal cell carcinoma detection within the urinary tracer background.
- Non-oncological uses: fever of unknown origin and prosthetic or vascular graft infection evaluation, sarcoidosis activity, epilepsy focus localisation (FDG interictal hypometabolism) and amyloid PET in dementia assessment.
- Pitfalls: physiological uptake in brain, heart, bowel, urinary tract and brown fat; false positives from tuberculosis, other granulomatous disease and post-chemotherapy thymic or marrow rebound; false negatives from hyperglycaemia, tiny lesions below resolution and indolent histology.
- Reporting pairs uptake intensity (standardised uptake value) with CT morphology — the SUV is semiquantitative, not absolute proof of malignancy.

## Common confusion

The predictable mistake is treating FDG uptake as synonymous with cancer — tuberculosis, sarcoidosis and healing fractures are FDG-avid, a serious issue in India where TB mimics lymphoma on PET. The second error is forgetting hyperglycaemia: competing glucose drives FDG out, causing false negatives, hence the fasting and glucose-control protocol. Finally, remember PET-CT is a problem-solving and staging tool; it does not replace biopsy for histology, and its resolution limits miss subcentimetre disease.

## Exam-focused takeaway

Expect the FDG principle (glucose analogue, trapped after phosphorylation), the F-18 half-life (about 110 minutes), and indication matching — lymphoma staging, unknown primary, radiation necrosis versus recurrence. Pitfall questions contrast TB granulomas and malignancy uptake, and glucose loading effects. Distinguish PET-CT from bone scan (metabolic versus osteoblastic) and know PSMA has superseded FDG for prostate cancer.

## Frequently asked questions

### What is FDG and how does it work?

Fluorine-18 fluorodeoxyglucose, a glucose analogue transported into cells and phosphorylated by hexokinase without further metabolism, so it remains trapped in proportion to glucose utilisation, making hypermetabolic tumours bright on PET.

### Which tumours are best evaluated by FDG PET-CT?

Lymphoma, melanoma, lung cancer, head and neck cancers and most aggressive solid tumours for staging, response and recurrence. FDG is unreliable for prostate and low-grade mucinous tumours, where PSMA or conventional imaging is preferred.

### Why must patients fast before FDG PET?

Fasting lowers insulin and blood glucose so tumours take up FDG competitively; hyperglycaemia and recent insulin drive tracer into muscle and reduce lesion visibility, producing false-negative studies.

### Can PET-CT distinguish tumour recurrence from radiation necrosis?

In glioma, recurrent tumour shows FDG hypermetabolism whereas necrosis is hypometabolic; amino-acid tracers such as methionine and FET perform even better in current practice.

### What are the common false-positive causes of FDG uptake?

Physiological brain, myocardial, bowel, urinary and brown-fat activity; infection and inflammation including tuberculosis and sarcoidosis; and post-therapy changes such as thymic rebound and marrow stimulation — clinical correlation and CT morphology resolve most.