Abstract： Objective:To explore the feasibility of CT angiography (CTA) in investigating vascular dilatation of anterior choroidal artery (AChA) and posterior communicating artery (PComA) in patients with Moyamoya syndrome (MMS).Methods:From July 2017 to July 2018, the clinical and imaging data of MMS patients with brain CTA and DSA performed were analyzed retrospectively. According to DSA results, 71 MMS patients were divided into unilateral MMS group (20 cases, 20 hemispheres) and bilateral MMS group (51 cases, 102 hemispheres). There were 20 cases in unilateral MMS group, 10 males and 10 females, with an average age of (45±9) years; 51 cases in bilateral MMS group, 24 males and 27 females, with an average age of (44±12) years. The hemispheres were divided into dilated group and non-dilated group according to the dilatation of AChA or PComA. Kappa analysis was used to evaluate the consistency of two inspection methods to judge the expansion of AChA. The lumen diameters of PComA, P1 and P2 segments of posterior cerebral artery were measured on CTA images, and the ratio of PComA/P1 and PComA/P2 were calculated. The repeatability of CTA measures was evaluated by intra-group correlation coefficient. Independent sample t-test was used to compare CTA measurement results between PComA dilated group and non-dilated group, and ROC curve was drawn to calculate the best threshold for diagnosis of PComA expansion. The sensitivity, specificity, accuracy, positive predictive value and negative predictive value of CTA measures were calculated. Results:The sensitivity, specificity, accuracy, positive predictive value and negative predictive value of CTA diagnosis of AChA expansion inunilateral MMS were all 100.00%. The sensitivity, specificity, accuracy, positive predictive value and negative predictive value of CTA diagnosis of AChA expansionin bilateral MMS were 90.00%, 93.90%, 93.14%, 78.26% and 97.47%. Compared with DSA, there was no significant difference in the diagnostic performance of AChA expansion between single and bilateral MMS diagnosed by CTA ( P>0.05). The two methods had strong consistency (Kappa value was 1.00 and 0.79 respectively, P<0.01). A total of 46 patients (69 cerebral hemispheres) were included in the evaluation of PComA. PComA/P1 (1.09±0.41) and PComA/P2 (0.86±0.13) in the dilated group were significantly higher than those in the non-dilated group (0.71±0.21 for PComA/P1 and 0.75±0.23 for PComA/P2). The differences were statistically significant ( t=-4.59, -2.50, P<0.05). The best threshold in diagnosing PComA expansion was 0.87 (PComA/P1) and 0.76 (PComA/P2), and the sensitivity, specificity, accuracy, positive predictive value and negative predictive value were 84.62%, 83.33%, 84.06%, 86.84%, 80.65% and 79.49%, 60.00%, 71.01%, 72.09% and 69.23%, respectively. Compared with DSA, the Kappa value of CTA measures in diagnosis of PComA expansion was 0.68 (PComA/P1) and 0.40 (PComA/P2), respectively, and the difference was statistically significant ( P<0.05). Conclusions:CTA has a strong consistency with DSA in evaluating the AChA expansion in MMS. When the PComA/P1 ratio on CTA is greater than 0.87, it can be used as the diagnosis criterion for PComA expansion.