Effects of size and shape (aspect ratio) on the hemodynamics of saccular aneurysms: A possible index for surgical treatment of intracranial aneurysms

OBJECTIVE: The present study was undertaken to explore the relationship bet ween the characteristic geometry of aneurysms prone to rupture and the bloo d flow patterns therein, using microsurgically produced aneurysms that simu lated human middle cerebral artery aneurysms in scale and shape. METHODS: We measured in vivo velocity profiles using our 20-MHz, 80-channel , Doppler ultrasound velocimeter. We produced small (less than or equal to 5 mm, 5 cases) and large (16-13 mm, 12 cases) aneurysms with round, dumbbel l, or multilobular shapes. RESULTS: The fundamental patterns of intra-aneurysmal flow were composed of inflow, circulating flow, and outflow. The inflow, which entered the aneur ysm only during the systolic phase, was strongly influenced by the position and size of the neck and the flow ratio into the distal branches. The outf low was usually nonpulsatile and of low velocity. The circulating flow depe nded on the aspect ratio (depth/neck width). A single recirculation zone wa s observed in aneurysms with aspect ratios of less than 1.6. This circulati on did not seem to extend to areas with aspect ratios greater than this val ue; in aneurysms with aspect ratios of more than 1.6, a much slower circula tion was observed near the dome. Furthermore, in the dome of dumbbell-shape d aneurysms and daughter aneurysms, no flow was detected. Intra-aneurysmal flow was determined by the aspect ratio, rather than the aneurysm size. CONCLUSION: The localized, extremely low-flow condition that was observed i n the dome of aneurysms with aspect ratios of more than 1.6 is a common flo w characteristic in the geometry of ruptured aneurysms, so great care shoul d be taken for patients with unruptured intracranial aneurysms with aspect ratios of more than 1.6.