Reproducibility of SPECT measurement of benzodiazepine receptors in human brain with iodine-123-iomazenil

Iodine-123-iomazenil is a SPECT radiotracer used to image and quantity benzodiazepine receptors. The reproducibility of the measurement of benzodiazepine receptors in human brain with [¹²³]iomazenil and SPECT was investigated with a test/retest paradigm. Methods: Six subjects underwent two experiments during a 1-wk interval. Iodine-123-iomazenil was injected as a single bolus (12 mCi). The time-activity curves of the tracer in the arterial plasma were measured and corrected for metabolites. Regional time-activity curves of five brain regions were measured with the CERASPECT camera for 145 min postinjection with serial 2-min acquisitions. Data were analyzed using three kinetic models that included a two-compartment model, an unconstrained three-compartment model and a three-compartment model with a constraint on the nondisplaceable compartment. Results: The results from the various analyses and fitting strategies were compared. The variability (average absolute difference between test and retest, expressed as a percentage of the mean of both measurements) was 10% to 17%, depending on the outcome measure and the fitting procedure. The most reproducible outcome measure was the regional tracer distribution volume relative to the total arterial concentration (V(T)'). V(T)' showed an average regional variability of 10% ± 2%, with an intraclass correlation coefficient of 0.81. Conclusion: SPECT measurement of regional [¹²³I]iomazenil V(T)' is reproducible and reliable. The use of regional ratios results in a significant loss of information.