Amyloid PET imaging in Alzheimer’s disease: a comparison of three radiotracers

PurposeThe increasing use of amyloid PET in Alzheimer’s disease research and clinical trials has motivated efforts to standardize methodology. We compared retention of the 11C radiotracer Pittsburgh Compound B (PiB) and that of two 18F amyloid radiotracers (florbetapir and flutemetamol) using two study populations. We also examined the feasibility of converting between tracer-specific measures, using PiB as the common link between the two 18F tracers.MethodsOne group of 40 subjects underwent PiB and flutemetamol imaging sessions and a separate group of 32 subjects underwent PiB and florbetapir imaging sessions. We compared cortical and white matter retention for each 18F tracer relative to that of PiB, as well as retention in several reference regions and image analysis methods. Correlations between tracer pairs were used to convert tracer-specific threshold values for amyloid positivity between tracers.ResultsCortical retention for each pair of tracers was strongly correlated regardless of reference region (PiB–flutemetamol, ρ = 0.84–0.99; PiB–florbetapir, ρ = 0.83–0.97) and analysis method (ρ = 0.90–0.99). Compared to PiB, flutemetamol had higher white matter retention, while florbetapir had lower cortical retention. Two previously established independent thresholds for amyloid positivity were highly consistent when values were converted between tracer pairs.ConclusionDespite differing white and grey matter retention characteristics, cortical retention for each 18F tracer was highly correlated with that of PiB, enabling conversion of thresholds across tracer measurement scales with a high level of internal consistency. Standardization of analysis methods and measurement scales may facilitate the comparison of amyloid PET data obtained using different tracers.