SLC6A4 and Parkinson disease: Furthermore, three studies previously used Statistical Parametric Mapping and the PETPVE12 toolbox to analyse the specific binding ratio (SBR) to investigate the binding of 123I-FP-CIT single-photon emission computed tomography (SPECT) to SERT outside the striatum in PD (8-10), and all three studies generated SBR from the SPECT image and used it as an index of the amount of binding between 123I-FP-CIT and SERT.