Noninvasive quantification of iodine-123-iomazenil SPECT

The feasibility of a noninvasive method for quantification of [123I]iomazenil binding using a standardized arterial input function and a single venous blood sample was assessed in normal volunteers.

Serial SPECT images and blood data from six healthy male volunteers after intravenous injection of [123I]iomazenil were used. The standardized input function was derived by averaging the six subjects' arterial curves. Individual input functions were estimated by calibrating the standardized input function with one-point venous blood radioactivity concentration. Ligand transport (K1) and receptor binding were computed from the estimated input functions and two separate SPECT scans using a table look-up procedure based on a three-compartment, two-parameter model. Reference values for K1 and receptor binding were determined from the serial SPECT data and individual arterial curves using a three-compartment, three-parameter model and curve fitting.

Analyses of the error caused by the calibration in relation to the time postinjection revealed that the optimal calibration time was 30 min postinjection. Receptor binding obtained by this simplified method correlated well with the reference values (r = 0.941) and was estimated within an error of 10% in the cerebral cortical regions. Although the estimated K1 showed relatively poor correlation (r = 0.699) with the reference value, it was an excellent relative measure in each subject.

Our method provided an absolute measure of the benzodiazepine receptor binding and a relative measure of ligand transport from two SPECT scans and a venous blood sample. This method would be useful for quantitative assessment of benzodiazepine receptors in clinical settings.