A passageway called the circle of Willis opens up to several major arteries at the base of the brain, ensuring a steady flow of oxygen. The sheer volume of blood moving through makes this region susceptible to aneurysms, weaknesses in arterial walls that can lead to stroke and debilitating brain damage. Leopold Grinberg, an applied mathematician at Brown University, developed a fluid dynamics model and ran it at the Pittsburgh Supercomputing Center to learn more about blood flow at the circle of Willis.

He and his team use CT and MRI scans of aneurysm-afflicted arteries to create patient-specific simulations. Then they release virtual blood into the model and examine how differences in circulation affect aneurysms. They can even digitally remove the aneurysm to see how the flow changes. "We're working backward to see the flow that led to the problem in the first place," he says. In this image, Grinberg visualizes the large-scale circulation through the entire region, but he can also zoom in to view the journey of individual blood cells.