Use of the specific binding ratio distribution to characterise multiple system atrophy in advanced iodine-123-labelled N-(3-fluoropropyl)-2β-carbomethoxy-3β-(4-iodophenyl) nortropane serotonin transporter imaging

Objective(s): Sudden death in multiple system atrophy (MSA) is caused by decreased serotonergic innervation, but there is no routine test method for this decrease. In addition to dopamine transporters, iodine-123-labelled N-(3-fluoropropyl)-2β-carbomethoxy-3β-(4-iodophenyl) nortropane (¹²³I-FP-CIT) binds serotonin transporters (SERTs). We noted a binding potential to quantify the total quantity of¹²³I-FP-CIT binding to its receptors. Following Mintun’s binding-potential concept, this study aimed to evaluate the relationship between the specific binding ratio (SBR) and total SERT tissue amount, but not SERT binding, and to develop an SBR imaging method to measure brain-stem SERT. We sought to establish a binding-potential imaging procedure using SBR images to examine differences in the brain-stem SERT distribution between healthy subjects and MSA patients. Methods: Single-photon emission computed tomography (SPECT) and T1-weighted magnetic resonance (MR) images were aligned. The MR (T1) images were used to set a reference site for the occipital-lobe SBR in each subject, and measurements were made from the SPECT image at the same position. The pixel values and accumulation ratios compared with the occipital lobe were calculated, and a regional SBR distribution image was created. We identified areas with SERT accumulation above a certain level. Results: The SERT accumulation site was visualised as an SBR value on MR images. The accumulation distribution (SERT distribution) on the SBR images significantly differed between the healthy subjects and patients with MSA. Conclusion: SERT accumulation was noted in the brain-stem region, indicating that SBR imaging was useful for viewing and quantifying SERT accumulation.