Stroke is a leading cause of disability, death and health care expenditure. It is the second most common cause of death worldwide, exceeded only by heart disease. Ischemic stroke constitutes 83 to 90% of stroke cases in western countries. Occlusion of the Middle Cerebral Artery (MCA) in the anterior circulation, or its branches is the most common site, accounting for approximately 90% of infarcts and two thirds of all first strokes. Available therapies include intravenous recombinant tissue plasminogen activator (IV rt-PA) and mechanical recanalization using thrombectomy (MT) devices. Both treatment approaches are primarily focused on recanalization, i.e. aiming to restore artery potency and brain perfusion to reduce the volume of cerebral infarction which thereby reduces functional deficits. Recent meta-analysis of the results demonstrated that although MT is an effective therapy, still over 50% of the treated patient do not recover. IV rt-PA treatment suffers from similar, more significant, lack of effect in majority of patients. This lack of treatment success is probably a result of inadequate micro circulatory collateral blood flow in the involved brain tissue. Treatment targeting collateral cerebral blood flow augmentation is a promising therapy, either as a stand-alone treatment, or as a complementary treatment post recanalization. BrainsGate's device, the ISS, augments brain perfusion through the collateral circulation. The augmentation of collateral blood flow is a result of stimulation of the Spheno-Palatine Ganglion (SPG). The SPG is the source of parasympathetic innervations to the anterior cerebral circulation, which comprises the middle cerebral artery, the anterior cerebral artery, and their tributaries. Studies in rodents, dogs and monkeys have demonstrated that stimulation of SPG neurons leads to a profound ipsilateral increase in the CBF because of arterial vasodilatation, and that this leads in turn to augmentation of tissue perfusion (see Figure 1). Moreover, previous studies, including histopathologic studies in humans have revealed that the main neurotransmitters of this neuronal pathway are acetylcholine (ACh), vasoactive intestinal peptide (VIP), and nitric oxide (NO). It is interesting to note that NO is best known for its potent vasodilatory activity and its capacity for neuroprotection and neurogenesis. Several studies have demonstrated that the cerebral vasodilatation induced by SPG stimulation is indeed mediated through NO mechanisms and that ablation of the SPG efferents to the cerebral vasculature leads to an increased infract size in MCA-occluded rats. So far, imaging studies aiming to demonstrate collateral blood flow augmentation due to SPG stimulation were not yet performed in human stroke patients. It is highly important to study the effect of SPG stimulation on the augmentation of collateral blood flow and to relate it to the subject's cerebral blood flow status, the extent of the collateral vessel potency prior to the stimulation and the relation of the vessel occlusion site to the vasodilatory effect. The purpose of the ImpACT-24col sub-study is to explore these important questions by using digital subtraction angiography (DSA), the gold standard imaging technique to demonstrate collateral blood flow dynamics. The results of this study will further promote the knowledge towards optimization of SPG stimulation to treat acute ischemic stroke patients. The ImpACT-24col study will be a sub-study of the ImpACT-24B trial and eligible patients will be enrolled to both trials.