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BackgroundMagnetic resonance arterial spin labeling (ASL) imaging with multiple post-labeling delays (PLDs) provides the bolus arrival time (BAT) as well as cerebral blood flow (CBF) to characterize the cerebral hemodynamics. However, the complexity of data acquisition and processing inhibits the calculation of BAT. We developed a simple method for approximating BAT using single-PLD ASL imaging and single-photon emission computed tomography (SPECT). We conducted a proof-of-concept study in patients with carotid artery stenosis.MethodsWe introduced the ASL/SPECT ratio, calculated by dividing the tissue magnetization in pulsed continuous ASL by the CBF measured using SPECT. In theory, the ASL/SPECT ratio has a positive relationship with BAT. Our proof-of-concept study included 63 patients who underwent carotid endarterectomy (CEA) in our hospital from 2014 to 2019. After preprocessing the ASL and SPECT datasets using three-dimensional stereotactic surface projection, we calculated the ASL/SPECT ratio at each voxel. We investigated the correlation between the preoperative BAT and the postoperative CBF.ResultsWe found a positive correlation between the delay of BAT and the increase rate of CBF in the ipsilateral middle cerebral artery territory (Pearson’s correlation coefficient, 0.444; 95% confidence interval, 0.220–0.623; p=0.000269). Four patients (6.3%) presented with hyper-perfusion phenomenon. Visualization of the BAT revealed that the area prone to postoperative hyper-perfusion presented with a delayed preoperative BAT.ConclusionsOur findings suggest the feasibility of the BAT approximated using ASL and SPECT in patients with chronic steno-occlusive cerebrovascular diseases. The proposed concept is also applicable to ASL and any modalities that measure CBF.