PiB & florbetapir provide consistent results for Alzheimer’s eval

Beta-amyloid status was highly consistent regardless of the radiotracer used, according to results of a small study of individuals who underwent PET imaging exams with Pittsburgh compound B (PiB) and florbetapir. The findings were published in the January issue of The Journal of Nuclear Medicine.

Although PiB has demonstrated its utility in Alzheimer’s disease research, its short half-life translates into limited use. F-18 labeled radioligands such as florbetapir, provide a longer half-life and overcome this challenge. Both tracers have been validated, but direct comparisons of PiB and florbetapir in the same individual are limited.

Susan M. Landau, PhD, of the Helen Wills Neuroscience Institute at the University of California, Berkeley, and colleagues undertook a retrospective study designed to compare measurements of cortical retention ratios of PiB and florbetapir.

The researchers focused on a subset of 32 individuals, eight cognitively normal and 24 diagnosed with mild cognitive impairment, enrolled in the Alzheimer’s Disease Neuroimaging Initiative who underwent two PiB studies and one florbetapir scan at one- to two-year intervals.

Cortical retention ratios were comparable between the two PiB scans and the second PiB scan and florbetapir study. The researchers studied several image analysis methods, including smoothing, spatial normalization to a PET template and use of a reference region. The effect of the data preprocessing method was minimal, according to Landau et al. The differences and percentage changes between scans also were “reasonably consistent.”

Beta-amyloid positive and negative status was concordant as well. The researchers used a florbetapir cutoff of 1.10, which resulted in 18 of 32 participants categorized as positive. They applied a PiB cutoff of 1.47 to the second PiB scan, and found the positive-negative categorization was consistent for 31 patients, indicating 97 percent agreement.

Landau and colleagues noted the limitations of the study: its small sample size and time interval between the PET exams of approximately one to two years. “These analyses are an initial step in addressing the need for standardization of AB methodologies…Here, we demonstrate that despite these acquisition differences, reliable numeric conversions can be made,” concluded the researchers.