In Reply: Impact of Cerebral Revascularization on Pial Collateral Flow in Patients with Unilateral Moyamoya Disease Using Quantitative Magnetic Resonance Angiography

To the Editor: We thank Drs Wang & Zhang for their letter regarding our recent publication1 and for sharing their institutional experience to enrich our discussion.2 We reiterate that our focus was to characterize shifts in total hemispheric and pial collateral flow after revascularization with the addition of the bypass graft. While moyamoya disease (MMD) can first present or even progress bilaterally, we only included patients with unilateral disease to create a construct to normalize flow ratios to an "unaffected side" that served as a control hemisphere. While we recognize that the smaller leptomeningeal and lenticulostriate MMD vessels were not quantified in our construct, we found flow through the major cerebral vessels—anterior cerebral artery, middle cerebral artery, and posterior cerebral artery—to be a reasonable stand-in measure. Typically, MMD collaterals in hypoperfused regions have been shown to arise from major vessels, such as anterior cerebral artery–posterior cerebral artery leptomeningeal collaterals supplying territories in the anterior circulation at greatest susceptibility to ischemia.3 While vessel wall imaging (VWI) is an emerging tool in distinguishing MMD from other etiologies of intracranial occlusive disease, such as atherosclerosis-associated moyamoya vasculopathy (AA–MMV) or dissection, digital subtraction cerebral angiography remains the gold standard in both MMD diagnosis and the exclusion of competing diagnoses, according to the most recent guidelines.4 In addition, VWI may be limited by visualization of submillimeter structures, coregistration between imaging sequences, and qualitative interpretation.5 Therefore, we determined angiography to be sufficient for confirmation of MMD to achieve our objective of globally characterizing hemodynamic changes following bypass. However, we do espouse the usage of VWI as an adjunctive modality to angiography to differentiate AA–MMV, as recent data have illuminated.6,7 As MMD may present higher risk of ischemic stroke than AA-MMV,8 leveraging VWI with quantitative flow metrics may provide a more robust model for ischemic stroke risk stratification in the future.