Investigation of ¹¹C-PiB equivocal PET findings

We have encountered occasional equivocal findings when assessing cerebral cortical amyloid retention with ¹¹C-Pittsburgh compound B (PiB) PET. We investigated the diagnostic significance of equivocal PiB PET findings.

This retrospective study included 101 consecutive patients complaining of cognitive disorders (30 Alzheimer’s disease, 25 mild cognitive impairment, 8 Lewy body disease, 7 frontotemporal lobar degeneration, 31 others) who underwent both ¹¹C-PiB PET and ¹⁸F-fluorodeoxy-d-glucose (FDG) PET. We visually classified PiB-positive, PiB-equivocal or PiB-negative ratings according to cortical uptake. For quantitative assessments of PiB PET, standard uptake values referred to cerebellar cortex (SUVR) were calculated in regional template volume of interests (frontal, temporoparietal, precuneus/posterior cingulate cortex, cerebral white matter and cerebellar cortex). The results of visual assessment were compared with the regional and mean cortical SUVRs and cortical-to-white matter ratio of PiB uptake, as well as clinical and FDG PET findings.

Among the 101 scans, 41 were PiB negative, 11 were PiB equivocal, and 49 were rated PiB positive in the visual assessments. The mean cortical SUVR and cortical-to-white matter ratio were 0.97 ± 0.07 and 0.57 ± 0.21 in PiB-negative, 1.51 ± 0.17 and 0.75 ± 0.06 in PiB equivocal and 2.10 ± 0.33 and 0.97 ± 0.11 in PiB-positive group, respectively. Nine of 11 subjects with PiB-equivocal findings had cognitive impairments and FDG distribution compatible with Alzheimer’s disease or dementia with Lewy bodies.

We considered equivocal visual findings on PiB PET equivalent to PiB-positive with slight cortical uptake. In addition, slight cortical amyloid deposits were considered to cause cerebral metabolic abnormality and cognitive impairment. Although mean cortical SUVR was more sensitive than visual assessment because of low cortical-to-white matter contrast due to non-specific accumulation in white matter, it is important not to overlook small amounts of cortical uptake of PiB in visual inspection for exact diagnosis.