FDG PET-CT
Fluorodeoxyglucose (FDG) positron emission tomography with CT shows the glucose use of tissues. FDG is a glucose analogue labelled with fluorine-18.
It enters cells through glucose transporters and is trapped as FDG-6-phosphate, so cells with a high glucose demand, such as most cancers, activated inflammatory cells and active brain and heart muscle, show high uptake.
The dose is 3 to 4 MBq for each kg (usually 250 to 400 MBq) intravenously after fasting for 4 to 6 hours, with a blood glucose under about 11 mmol/L (200 mg/dL).
The patient rests quietly in a warm room for about 60 minutes while the tracer is taken up.
Imaging from the skull base to the mid-thigh takes 15 to 30 minutes, and a whole body study, including the legs and brain, is used for melanoma and some other indications.
The effective dose is about 7 mSv for the tracer and 3 to 10 mSv for the CT, depending on the technique. Cancer is the leading cause of death in many countries, and FDG PET-CT is the most widely used molecular imaging method.
It changes the stage in 20 to 30% of patients in many cancers, and its sensitivity and specificity are around 85 to 95% in lymphoma, lung cancer, oesophageal cancer, head and neck cancer, melanoma and colorectal cancer.
Indications.
- Staging and restaging of lymphoma, lung cancer, oesophageal, head and neck, cervical, colorectal and other cancers.
- Assessment of response to treatment, early and at the end of treatment (lymphoma, lung, oesophageal and others).
- Solitary pulmonary nodule of intermediate risk and larger than about 8 mm.
- Cancer of unknown primary.
- Detection of recurrence when tumour markers rise or imaging is equivocal.
- Radiotherapy planning (delineation of metabolically active tumour).
- Fever of unknown origin, suspected infection (spinal, prosthetic, vascular graft, endocarditis, bone), and the extent of inflammation.
- Large vessel vasculitis, sarcoidosis, and other inflammatory conditions.
- Myocardial viability when other tests are inconclusive, and cardiac sarcoidosis.
- Brain: epilepsy focus before surgery, and dementia (patterns of reduced metabolism in neurodegenerative disease).
Contraindications and precautions.
- Pregnancy: avoid unless essential. Breast feeding: interrupt for a short time as the local rule requires, and limit close contact with infants for several hours.
- Fasting for 4 to 6 hours. Water is allowed and encouraged. Intravenous glucose and parenteral nutrition are stopped.
- Blood glucose: scan if under about 11 mmol/L. With higher values, uptake in tumours falls and muscle uptake rises, so the scan is postponed or insulin is given as the local rule requires. Insulin injected shortly before the tracer causes muscle uptake.
- Diabetes: oral drugs are continued, but metformin increases bowel uptake, and it is stopped for 48 hours when bowel disease is the question.
- Strenuous exercise for 24 hours before the scan raises muscle uptake.
- Brown fat: keep the patient warm. Beta blocker or a benzodiazepine can lower uptake in selected cases.
- Claustrophobia is uncommon, but the patient must lie still for the scan.
- Iodinated contrast is used only when a diagnostic CT is planned.
Preparation and technique.
The patient is weighed, and the glucose is checked. FDG is injected through a vein away from the area of interest. The patient rests in a quiet, warm room for 60 minutes, avoids talking and chewing, and empties the bladder before the scan.
The CT is taken first, followed by the PET acquisition in 6 to 8 bed positions at 1 to 3 minutes each. Time-of-flight reconstruction and respiratory gating improve image quality. Delayed images at 2 to 3 hours may help in selected cases.
The report describes the site, size and avidity of lesions. The uptake value (SUV) is measured but depends on technique, weight, glucose and time, so comparisons use one scanner and one method.
