I'm not completely certain I have the timeline right on this. But this late article from Ha'aretz says that Ariel Sharon was given a CT scan after the surgery to relieve bleeding in his brain was completed and that on the basis of that test he was taken back for further surgery. The latest from the Jerusalem Post was simply that surgery had been completed after six hours.

Late Update: Now the Jerusalem Post confirms and elaborates. The CT scan revealed additional areas of bleeding in the brain. Thus the return to the operating room.