Table 2:

Variations in the circle of Willis and volume flow rates

Variations in the Circle of Willis	“Textbook” Type (n = 105)	A1 Hypoplasia (n = 6)	P1 Hypoplasia (n = 6)
Total volume flow (mL/min) (mean ± SD)	781 ± 151	744 ± 119	763 ± 129
Right internal carotid artery or internal carotid artery ipsilateral to hypoplastic A1 or P1	311 ± 73	236 ± 48	355 ± 66
Left internal carotid artery or internal carotid artery contralateral to hypoplastic A1 to P1	304 ± 74	367 ± 67	317 ± 60
Basilar artery	165 ± 43	141 ± 39	90 ± 24

Note:— A1 indicates precommunicating segment of the anterior cerebral artery; P1, precommunicating segment of the posterior cerebral artery.