Objective   To investigate the normal anatomy and variation of Willis circle by 64-slice volume CT digital subtraction angiography (VCTDSA).    Methods   VCTDSA data of 174 patients with cephalocervical and cephalic scanning in our hospital from February 2006 to September 2008 were analyzed. Developmental condition

shape

size

integrity

and perforating branches of Willis circle were observed.    Results   (1) Hypoplasia was displayed in any component vessels of Willis circle. Incidence rate of hypoplasia was 20.1% in left posterior communicating artery

16.1% in right posterior communicating artery

13.8% in right anterior cerebral artery A1

9.2% in right posterior cerebral artery P1

7.3% in anterior communicating artery

6.3% in left posterior cerebral artery P1

and 3.4% in left anterior cerebral artery A1. Congenital arterial absence occurred more frequently in right posterior communicating artery (54.6%)

left posterior communicating artery (52.3%)

and anterior communicating artery (15.8%). (2) Hypoplasia or congenital absence of posterior circulation and anterior-posterior circulation occurred with a rate of 55.8% and 29.8% respectively. Integrity of Willis circle was only 9.2%. There were 17 perforating branches and 26 heubner inversion arteries observed in 174 Willis circle

with an average diameter of 0.82 mm and 0.94 mm respectively.    Conclusion   VCTDSA can display Willis circle completely and clearly. Using three-dimensional rotation may detect all kinds of variation and perforating branches of Willis circle.