Our study showed a mean individual carotid flow difference percentage of 42.9% in the AA1 group. The result was consistent with several previous studies showing that the carotid flow ipsilateral to A1 aplasia is significantly lower than that of the contralateral side, with the flow difference percentage ranging from 34 to 50% [15–18]. Moreover, we also demonstrated that there was a significantly higher individual carotid flow difference percentage (30%) when A1 asymmetry was present. Since each A1 segment carries about 10% of the total brain flow to the ipsilateral hemisphere [17, 24], A1 segment asymmetry indicates redistribution of bilateral A1 flow and is crucial for computational hemodynamic studies for 2 reasons. First, A1 hypoplasia is associated with AcomA aneurysms [1, 13, 23, 27]. This relation can be explained by hemodynamic studies using either an experimental design with an A1 diameter ratio of ≥50% [25], or patient-specific 3D geometry with an unequal A1 inflow [28]. Both types of studies [25, 28] showed elevated intra-aneurysmal wall shear stress in the setting of A1 hypoplasia or unequal flow that would trigger AcomA aneurysm formation. Our results further emphasized that A1 asymmetry, even with a diameter difference of < 50%, could also cause unequal inflow and might be associated with Acom aneurysm formation. Second, several studies have indicated the importance of using patient-specific inflow boundary conditions to obtain reliable computational fluid dynamic results, especially for aneurysms with more than 2 inflow avenues, such as AcomA aneurysms [3, 19, 25, 28, 29]. Venugopal et al. [19] showed that the wall shear stress distribution on an aneurysm surface is sensitive to the bilateral A1 flow ratio and flow rate by using different inflow boundary conditions for a patient-specific AcomA aneurysm geometry with an original flow ratio of 1.87. Similarly, Karmonik et al. [29] reported that changes in the flow distribution of bilateral A1 segments could cause variations of the average wall shear stress as high as 43%, again using a patient-specific AcomA aneurysm model with an original flow ratio of 1.72. On the other hand, blood flow changes of the parent artery would not change the characterization of the intra-aneurysmal flow pattern substantially in the setting of a side-wall aneurysm/terminal aneurysm or AcomA aneurysm with relatively symmetric A1 segments [30]. In our study, we proposed a linear equation between the ipsilateral ICA diameter and ICA flow volume to provide a representative inflow boundary condition for the numerical simulation while A1 asymmetry is present.