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We agree with Cakir and colleagues [1Cakir H. Yurekli I. Iner H. Yazman S. Kestelli M. The probability of incomplete or hypoplastic circle of Willis (letter).Ann Thorac Surg. 2019; 107: 1583Abstract Full Text Full Text PDF Scopus (2) Google Scholar] that an incomplete or hypoplastic circle of Willis can increase the risk of inadequate left cerebral perfusion during aortic arch repair using regional cerebral perfusion on cardiopulmonary bypass. In addition, even during extended resection and repair of aortic coarctation by lateral thoracotomy, a communicating circle of Willis is mandatory for adequate bilateral cerebral perfusion, when left-sided supraaortal vessels are clamped. In our study [2Rüffer A. Tischer P. Münch F. et al.Comparable cerebral blood flow in both hemispheres during regional cerebral perfusion in infant aortic arch surgery.Ann Thorac Surg. 2017; 103: 178-185Scopus (20) Google Scholar], an open circle of Willis was verified in all patients by combined transfontanellar and transtemporal ultrasound examination. An incomplete or hypoplastic circle of Willis would have been present if blood flow velocities measured in the majority of left-sided cerebral arteries during regional cerebral perfusion had been significantly lower when compared with blood flow velocities in the contralateral vessels. We have tried to discuss the phenomenon of significantly lower blood flow velocities measured exclusively in the left internal carotid artery (ICA) when compared with the contralateral side. On the one hand, “full-flow” cardiopulmonary bypass was maintained by cannulation of the innominate artery, thereby leading to increased flow in the right ICA. On the other hand, diminished antegrade blood flow in the left ICA can result from structural aortic arch hypoplasia or concomitant vascular malformations. As a combination of both increased perfusion of the right ICA and reduced antegrade perfusion of the left ICA, blood flow direction can even be inverted by “retrograde” perfusion of the left ICA over a communicating circle of Willis (see Fig 3D in our article [2Rüffer A. Tischer P. Münch F. et al.Comparable cerebral blood flow in both hemispheres during regional cerebral perfusion in infant aortic arch surgery.Ann Thorac Surg. 2017; 103: 178-185Scopus (20) Google Scholar]). As a consequence, concurring blood flow directions can lead to flow annihilation in that vessel. We thank Dr Cakir and colleagues [1Cakir H. Yurekli I. Iner H. Yazman S. Kestelli M. The probability of incomplete or hypoplastic circle of Willis (letter).Ann Thorac Surg. 2019; 107: 1583Abstract Full Text Full Text PDF Scopus (2) Google Scholar] for their important comment and want to emphasize that combined transfontanellar and transtemporal ultrasound is a well-performable, noninvasive tool to verify anatomic consistence of the circle of Willis, and it can be easily implemented before aortic arch operations in children with an open fontanelle. The Probability of Incomplete or Hypoplastic Circle of WillisThe Annals of Thoracic SurgeryVol. 107Issue 5PreviewWe congratulate Rüffer and colleagues [1] for their study. We want to share our opinion about this paper. Rüffer and colleagues performed regional cerebral perfusion through the innominate artery and compared the rate of blood flow in the two hemispheres of patients who underwent aortic arch surgical procedures [1]. Figure 4 in their paper reveals that the blood flow rate was higher in the right internal carotid artery than in the left internal carotid artery during total body perfusion. An incomplete or hypoplastic circle of Willis is a common phenomenon and is a recognized anatomic variant. Full-Text PDF