Representative cases of aneurysms with flow complexity. Top row (aneurysm A and B), unruptured aneurysms with simple flow; bottom row (aneurysm C and D), ruptured aneurysms with complex flow; recirculation zone marked with number and arrow. Two recirculation areas were involved in aneurysm C, and three recirculation zones were observed in aneurysm D (see the text for detailed description).