Analysis of optimal delayed scanning times with Florbetapir F-18 (Amyvid) in patients with cognitive impairment

Objectives When imaging targeted tracers it may be beneficial to perform delayed imaging to allow for clearance of nonspecific binding. Using the newly approved Amyvid, we imaged patients at different time-points to assess scan quality in patients with suspected Alzheimer's (AD) using both qualitative and quantitative analysis. Quantitatively, they were analyzed by comparing frontal, temporal, and posterior gray-to-white (GTW) matter ratio near the centrum semiovale.

Methods We analyzed a limited number of patients (n=5, average age 59 yrs) at approximately 30, 60, and 90 minutes post injection of 10 mCi Amyvid. All images were acquired on a GE Discovery VCT PET-CT scanner using a dynamic brain acquisition protocol. Images were acquired dynamically, 4 frames for 5 minutes each. Studies were then summed and transferred to a work station for qualitative and quantitative analysis using MIMneuro™ software.

Results Qualitatively, images were excellent (4) at 30 minutes after tracer injection. When compared to this score, images at 60 minutes were slightly less than excellent (3.5) and at 90 minutes were judged as good (3). The early images demonstrated the best GTW distinction relative to adjacent cortical regions. Quantitatively, there was no appreciable change in the GTW matter ratio over time, and in patients suspected of having AD the ratio of average GTW uptake at 90 minutes compared to the average uptake at 30 minutes was 1.05. In subjects with low suspicion of AD the ratio of average GTW uptake at 90 minutes compared to the average GTW uptake at 30 minutes was 0.99 and not significantly changed compared to patients with suspected AD.

Conclusions There is no advantage to Amyvid delayed imaging. Since, there is an overall decrease in brain counts due to tracer decay, there is image reconstruction resolution loss, accounting for the reduced distinction of gray-to-white matter uptake in cortical mantle, the area most often used to assess for abnormal uptake.