Modeling clustered activity increase in amyloid-beta positron emission tomographic images with statistical descriptors

BACKGROUND: Amyloid-beta (Aβ) imaging with positron emission tomography (PET) holds promise for detecting the presence of Aβ plaques in the cortical gray matter. Many image analyses focus on regional average measurements of tracer activity distribution; however, considerable additional information is available in the images. Metrics that describe the statistical properties of images, such as the two-point correlation function (S(2)), have found wide applications in astronomy and materials science. S(2) provides a detailed characterization of spatial patterns in images typically referred to as clustering or flocculence. The objective of this study was to translate the two-point correlation method into Aβ-PET of the human brain using 11C-Pittsburgh compound B (11C-PiB) to characterize longitudinal changes in the tracer distribution that may reflect changes in Aβ plaque accumulation. METHODS: We modified the conventional S(2) metric, which is primarily used for binary images and formulated a weighted two-point correlation function (wS(2)) to describe nonbinary, real-valued PET images with a single statistical function. Using serial 11C-PiB scans, we calculated wS(2) functions from two-dimensional PET images of different cortical regions as well as three-dimensional data from the whole brain. The area under the wS(2) functions was calculated and compared with the mean/median of the standardized uptake value ratio (SUVR). For three-dimensional data, we compared the area under the wS(2) curves with the subjects’ cerebrospinal fluid measures. RESULTS: Overall, the longitudinal changes in wS(2) correlated with the increase in mean SUVR but showed lower variance. The whole brain results showed a higher inverse correlation between the cerebrospinal Aβ and wS(2) than between the cerebrospinal Aβ and SUVR mean/median. We did not observe any confounding of wS(2) by region size or injected dose. CONCLUSION: The wS(2) detects subtle changes and provides additional information about the binding characteristics of radiotracers and Aβ accumulation that are difficult to verify with mean SUVR alone.