Advanced 123I-FP-CIT Serotonin Transporter Imaging: Specific Binding Ratio(SBR) Distribution Helps to Characterize MSA

Background: Sudden death in MSA is caused by a decrease in serotonergic innervation, but there is currently no method to test for this at the level of routine medical care. Therefore, we decided to investigate whether serotonergic nerve can be evaluated using dopamine transporter SPECT. 123 I-FP-CIT can bind and accumulate not only dopamine transporters but also serotonin transporters (SERT). While the common method of SERT analysis by SPECT is considered unreliable in many respects, in this study we developed a more reliable SBR imaging technique to measure SERT in the brainstem. Objective: Using a simple model, we derived a binding potential [1] to quantify the total quantity of 123 I-FP-CIT specifically binding to its receptors in brain tissue. We then tried to establish a procedure for imaging that binding potential. Moreover, we investigated whether it was possible to observe differences in SERT distribution in the brainstem region using SBR images from healthy individuals and patients with MSA. Subjects　A total of 17 healthy individuals and 11 patients with MSA were included in this study. Methods: SPECT with TEW （Triple Energy Window） scatter correction and CT-AC and MR (T1) images were acquired. The MR images were aligned with the SPECT images. First, a reference site for the SBR calculation was set in the occipital lobe of each subject using the MR (T1) image and measurements were made from the SPECT image at the same position. For each pixel of the SPECT image, a pixel value ( 123 I-FP-CIT accumulation) and accumulation ratio in comparison to the occipital lobe (reference site) were calculated, and a regional SBR distribution image (SBR image) was created. An image showing SERT accumulation was created from the SBR image using a constant threshold method, and the MR image of the same subject was aligned and fused with this image. Results: The site of serotonin transporter accumulation was visualized as an SBR value on MRI images. Furthermore, SBR images revealed that the distribution of accumulation (distribution of SERT) was different between controls and MSA patients. Conclusion: An apparent accumulation of SERT was noted in the brainstem region, suggesting that SBR imaging can be useful in determining and quantifying SERT accumulation.