Measurement of cerebral vascular reserves with I-123 IMP SPECT without an arterial input function using the microsphere model and radiopharmaceutical dose calibration Abstract Objectives Cerebral vascular reserve (CVR) is an important indicator for the management of and therapy for cerebral arterial occlusive disease (CAOD). Vasodilatory function is measured using the standard IMP-ARG method. The IMP autoradiography (IMP-ARG) method employed here uses a standardized input function, which was derived from 12 patients between 31 and 71 years of age. Because the population of elderly patients continues to increase in Japan, additional therapies are required to assess CVR in elderly patients with chronic cardiopulmonary disease or a history of smoking, in particular. Despite its popularity, alternatives to the IMP-ARG method are necessary. Here, we proposed the microsphere (MS) method without an input function. Method Using this method and the IMP-ARG method, we measured the CVRs of 18 CAOD patients. Results The CVRs derived with these two methods were significantly and linearly correlated (r = 0.89, p < 0.01). CVRs categorized by severity were also found to correspond between the two methods (κ = 0.87). Conclusions Thus, the method proposed here may serve as a supplemental to and be compatible with the IMP-ARG method for the assessment of CVR. Furthermore, the two methods, when used in conjunction, may result in less error than either would alone.