SLC6A4 and Parkinson disease: Serotonin (5-HT) positron emission tomography (PET) utilizes tracers (e.g., [11C] DASB, [18F]altanserin) to visualize serotonin transporter (SERT) or receptor distribution, revealing abnormalities in the raphe 5-HT system and its association with attention and emotional regulation deficits in PD-MCI.