To test the feasibility of applying N-isopropyl-[123I]p-iodoamphetamine (IMP) for sequential measurements of regional cerebral blood flow (rCBF) with injection of two separate doses in a single procedure, kinetic analysis based on a two-compartment model was done using dynamic SPECT data. A microsphere model analysis without consideration of IMP washout from the brain was also tested for clinical application.
Methods: A dynamic SPECT scan consisting of fifty 1-min scans was obtained on 15 patients using a three-head rotating gamma camera with two separate doses of IMP (111 MBq each) at the beginning and 25 min after scan initiation. The reproducibility of two resting rCBF scans was tested in six patients and the cerebrovascular response shown by increased rCBF with acetazolamide (1 g) was assessed in nine patients.
Results: Two-compartment model analysis showed excellent reproducibility of resting rCBF scans and significantly different cerebrovascular reactivity to acetazolamide between areas with and without ischemia. Microsphere model analysis showed smaller values in the first rCBF image by 3% and in the second by 10%, resulting in lower values for cerebrovascular reactivity. The difference in cerebrovascular reactivity between ischemic and nonischemic areas, however, is highly significant.
Conclusion: The double-injection method for IMP is feasible for two sequential rCBF measurements in a single procedure and is applicable for acetazolamide challenge. Simple microsphere model analysis, as well as a two-compartment model analysis, provide reliable assessment for cerebrovascular reactivity despite the complex dynamics of IMP and are feasible for clinical application.