FDG-PET imaging

What FDG-PET is

FDG-PET is a nuclear medicine scan that measures the brain's use of glucose. The brain is the body's most glucose-hungry organ, and different dementias produce characteristic patterns of reduced metabolism in specific brain regions. The scan involves an intravenous injection of a small dose of radiolabelled glucose analogue, followed by a 30-minute uptake period, then a scan lasting around 20 minutes.

NICE NG97 recommends FDG-PET when Alzheimer's Disease is suspected but the diagnosis remains uncertain after standard work-up (recommendation 1.2.15), and when Frontotemporal Dementia is suspected and structural imaging is inconclusive (recommendation 1.2.23).

What FDG-PET shows in each dementia

Alzheimer's Disease

Reduced glucose metabolism in the parietal and posterior temporal cortices, often with the posterior cingulate cortex affected early. The pattern is usually bilateral but can be asymmetric in atypical Alzheimer's.

Frontotemporal Dementia

Reduced metabolism in the frontal and anterior temporal cortices. Often asymmetric, with the left hemisphere more affected in language-led presentations.

Dementia with Lewy Bodies

Reduced metabolism in the occipital cortex (a specific feature) alongside more general reductions. The pattern overlaps with Alzheimer's but the occipital involvement is a clue.

Vascular Dementia

Patchy reductions corresponding to areas of infarction or chronic ischaemia, often in subcortical and watershed regions.

Posterior Cortical Atrophy

Reduced metabolism in the parietal and occipital cortices.

Primary Progressive Aphasia

Asymmetric reduced metabolism in the language network, with patterns differing by variant.

When FDG-PET is requested

• The clinical picture is atypical and the structural MRI is inconclusive;
• Distinguishing Alzheimer's Disease from Frontotemporal Dementia would change management (for example, considering Cholinesterase Inhibitors);
• Young-onset dementia or rapidly progressive cognitive decline;
• Where a clear diagnosis is needed for clinical trial enrolment or anti-amyloid antibody therapy consideration.

What to expect on the day

You will be asked to fast for 4 to 6 hours beforehand (water is allowed). A cannula is placed in a vein and the radiolabelled glucose is injected. You then rest quietly for 30 minutes. The scan itself lasts around 20 minutes, lying still in a doughnut-shaped scanner (less enclosed than MRI). Total time at the centre is around 90 minutes.

The radiation dose is small, comparable to about three years of natural background radiation. The dose is justified by the diagnostic benefit. Pregnancy is a contraindication; breastfeeding requires a short interruption.

UK availability

NHS FDG-PET access is variable by region; some areas have prompt access, others require referral to a regional centre with waits of several months. Private FDG-PET is available in many UK cities, typically costing £1,200 to £2,000, with two- to four-week appointments. The Dementia Service can arrange private FDG-PET as part of structured assessment where clinically indicated.

What FDG-PET cannot do

FDG-PET shows brain metabolism but does not directly visualise the underlying pathological proteins. For specific amyloid or tau measurement, amyloid PET or tau PET are used. These are largely research tools in the UK currently, though amyloid PET use will likely grow with anti-amyloid antibody therapy access.