Early-phase Amyloid PET as a Perfusion-weighted Surrogate of Neurodegeneration

Purpose of the Report: Amyloid PET is acquired after an uptake interval during which regional tracer distribution is governed largely by cerebral perfusion. This review examines whether images obtained during it provide clinically usable information about neurodegeneration, and defines the limits of that claim, for nuclear medicine physicians and neuroradiologists. Materials and Methods: Published work on early-phase amyloid PET was reviewed across tracer kinetics, concordance with F-18 fluorodeoxyglucose (FDG) PET, classification at the individual level, differential diagnosis, prognosis, vascular confounding, acquisition, and interpretation. The strength of evidence in each domain was graded. Results: Early-phase images provide a perfusion-weighted delivery image rather than a quantitative blood flow map. Group-level concordance with FDG PET is consistent across tracers, and single-subject studies report comparable pattern recognition. In 379 clinical scans, visual reading identified neurodegeneration status in 78.8% of amyloid-negative patients and assigned 75.3% of amyloid-negative, neurodegeneration-positive patients to the correct diagnostic category. Baseline perfusion predicted subsequent cognitive decline in 2 cohorts. Where discriminative power was compared directly in the same patients, FDG performed better than early-phase measures despite strong correlation between them. No universal acquisition window or reference region has been established. Conclusions: Early-phase amyloid PET is a promising perfusion-weighted surrogate that provides complementary information related to neurodegeneration and differential diagnosis. It is not interchangeable with FDG PET and is not a validated neurodegeneration biomarker. Standardized acquisition, structured reporting that identifies the early phase as investigational, and prospective validation in mild cognitive impairment are the appropriate next steps.