Test-retest reproducibility of extrastriatal binding with 123I-FP-CIT SPECT in healthy male subjects

I-123-labeled 2 beta-carbomethoxy-3 beta-(4-iodophenyl)-N-(3-fluoropropyl) nortropane (I-123-FP-CIT) is used to assess striatal dopamine transporter (DAT) expression, but it can also quantify extrastriatal serotonin transporter (SERT) expressions. While FP-CIT uptake in extrastriatal regions has been quantified, no information exists on the reproducibility of the I-123-FP-CIT specific uptake ratio (SUR) in extrastriatal regions. We investigated test-retest reproducibility of I-123-FP-CIT binding in the striatum, the midbrain, and cortical regions in eight healthy male subjects. All subjects underwent two I-123-FP-CIT SPECT scans, and SUR was calculated using the cerebellum as the reference. We found good test-retest reproducibility of I-123-FP-CIT SUR in the midbrain, and in the lateral frontal/temporal cortex and combined cortical regions. The overall variability and intraclass correlation of SUR were, respectively, 4.9-7.8% and 0.90-0.96 in striatal regions, 8.6% and 0.79 in the midbrain, and 3.6-9.1% and 0.84-0.95 in the lateral frontal/temporal cortex and combined cortical regions. Our results provide evidence that I-123-FP-CIT SPECT is a valid technique for analyzing striatal DAT, as well as extrastriatal SERT in areas such as the SERT-enriched midbrain. In addition, our data suggest that I-123-FP-CIT could be used for analyzing SERT in regions with relatively low SERT expression (e.g., temporal or frontal cortices).