A California supercomputer is advising heart specialists in Minneapolis on when they should — and more importantly when they shouldn't — thread instruments inside patients' blood vessels to examine blockages. The so-called HeartFlow system uses images from a patient's CT scan and analyzes them against volumes of data on the human vascular system and the science of fluid dynamics. As a result, it can diagnose patients' needs — and help doctors avert heart attacks and stroke — without the costly and invasive procedure of inserting a catheter. "It should decrease the number of people who end up in the cath lab who don't have lesions that matter," said Dr. John Lesser, who directs cardiovascular imaging at Allina Health's Minneapolis Heart Institute.