Fludeoxyglucose F-18 (brain metabolism)

Fludeoxyglucose F-18 (FDG) is the general-purpose PET metabolic tracer used across oncology, cardiology and neurology; it is not brain-specific as a product, but brain FDG PET is a distinct, well-established clinical application. Reduced glucose metabolism follows a regional pattern that differs by disease: temporoparietal and posterior cingulate hypometabolism is typical of Alzheimer's disease, while frontal and/or anterior temporal hypometabolism points to frontotemporal dementia, and this pattern difference is the main clinical use of FDG-PET in cognitive impairment.

In the United States, Medicare's national coverage determination for FDG-PET in dementia (NCD 220.6.13) covers a scan specifically to help differentiate Alzheimer's disease from frontotemporal dementia, in patients with a recent dementia diagnosis and at least 6 months of documented cognitive decline whose presentation leaves this distinction clinically uncertain after a comprehensive specialist evaluation; a prior brain SPECT or FDG-PET scan for the same question generally precludes a repeat scan within a year. All other dementia uses of FDG-PET (for example distinguishing other neurodegenerative diseases, or use outside a CMS-approved clinical trial in mild cognitive impairment) remain nationally non-covered under this determination, a materially narrower coverage pathway than the one now in force for amyloid PET.

FDG-PET brain imaging does not require thyroid blockade or specialised handling beyond standard F-18 PET radiopharmacy practice, and dosing follows general oncologic/neurologic FDG protocols (typically around 185-370 MBq IV, imaging beginning about 30-45 minutes later, with the patient resting quietly in a darkened, low-stimulation room during uptake to standardise regional brain activity). Because FDG uptake reflects neuronal activity rather than a specific pathological protein, it complements rather than replaces amyloid and tau PET: FDG shows the functional consequence of neurodegeneration, while amyloid and tau tracers show the underlying molecular pathology.