Detailed Assessment of 18F-THK5351 Distribution Pattern in the Midbrain: Comparison With Progressive Supranuclear Palsy and Corticobasal Syndrome

Kenji Ishibashi, Masanori Kurihara, Ryoko Ihara, Mana Higashihara, Atsushi Iwata, Kenji Ishii

18F-THK5351 PET is used to image ongoing astrogliosis by estimating monoamine oxidase B levels. 18F-THK5351 preferentially accumulates around the substantia nigra (SN) and periaqueductal gray (PG) in the midbrain under healthy conditions and exhibits a "trimodal pattern." In progressive supranuclear palsy (PSP) and corticobasal syndrome (CBS), the midbrain 18F-THK5351 uptake can be increased by astrogliosis, collapsing the "trimodal pattern." We aimed to elucidate cases in which the "trimodal pattern" collapses in PSP and CBS. Participants in the PSP (n = 11), CBS (n = 17), Alzheimer disease (n = 11), and healthy control (n = 8) groups underwent 18F-THK5351 PET. Volumes of interest (VOIs) were placed on the SN, PG, and their midpoints. The midbrain uptake ratio (MUR) was calculated to assess the trimodal pattern as follows: MUR = (VOI value on the midpoint)/(VOI value on the SN and PG). Approximately, the trimodal pattern can be identified at MUR <1 but not at MUR >1. Compared with the healthy control group, MUR significantly increased in the PSP (P < 0.01) and CBS (P < 0.01) groups, but was unchanged in the Alzheimer disease group (P = 0.10). In the PSP group, all patients, including 2 with mild symptoms and a short disease duration, showed MUR >1. In the CBS group, MUR varied widely. In PSP, the trimodal pattern can collapse even in the early phase when symptoms are mild. In CBS, the trimodal pattern may or may not collapse depending on the underlying pathology.