New technology to help physicians and engineers pool resources to prevent stroke

May 8 2006

A professor at the University of Houston and his research students are working with physicians and scientists at the Methodist Neurological Institute on new technology to help identify which brain aneurysms are at highest risk of rupture and could cause a stroke.

Improving treatment of cerebral aneurysms, which are ballooning weak spots in the wall of a blood vessel in the brain, is at the center of this joint research. The goal of their study is to develop a fully-integrated computational medical tool that will be useful in helping to select patients for treatment whose aneurysms are most likely to rupture.

Ralph Metcalfe, a mechanical engineering professor at UH and deputy director of the UH biomedical engineering program and his graduate student, Aishwarya Mantha, work on this project with a Methodist team consisting of Drs. Charles Strother and Goetz Benndorf, interventional neuroradiologists, and Christof Karmonik, a researcher at the Methodist Hospital Research Institute.

Using computer simulations of blood flow in realistic geometric models of aneurysms, some blood flow characteristics have been identified that may contribute to aneurysm formation. These findings are described in a paper titled "Hemodynamics in a Cerebral Artery Before and After the Formation of an Aneurysm," appearing in the May issue of the American Journal of Neuroradiology, a scientific journal that publishes original articles dealing with the clinical imaging, endovascular therapy and basic science of the central and peripheral nervous system.

"According to the American Association of Neurological Surgeons, cerebral aneurysms affect up to six percent of the U.S. adult population," Metcalfe said. "Most aneurysms don't rupture, but if they do, the results are fatal in about 50 percent of the cases. The question is how to predict who is most at risk."

Since treatment of aneurysms is associated with some risk, Metcalfe's group and his Methodist colleagues are trying to develop a better method of identifying which aneurysms are most vulnerable for rupture. Once these patients are identified, physicians can then determine the best course of medical treatment, using existing technologies and best medical practices.

"One of the key points is that aneurysms don't seem to form randomly," Metcalfe said. "They do seem to form at locations that are associated with the fluctuations in the flow of blood, leading to the question of what it is about the flow of blood that tends to correlate with the formation of aneurysms."

The Methodist researchers acquire 3-D images of the intracranial vascular system by injecting dye into the vessels and rotating an X-ray tube around the patient's head, a technique that has become a standard for high-quality vascular imaging in this institution.

By using this geometric and blood flow data taken from a specific patient's clinical profile, Metcalfe's team can perform simulations in their computers of blood flow in that patient's arteries using existing computational fluid dynamics programs in novel applications. This is similar to the way that an aeronautical engineer would study the design of an airplane on a computer or in a wind tunnel. Strother and his colleagues at Methodist anticipate that this process will help researchers better understand how aneurysms form and ultimately discover ways to prevent strokes and death from this common disorder.

"We can't look at a person and tell the likelihood that an aneurysm will rupture," Strother said. "But we do know that force and stresses created by blood flow produces aneurysms. Our hope is that this study will help us learn enough to predict which ones are at high risk of rupture so that treatment can be offered before they become harmful."

This work has two potential applications. The first is as a research tool, with Metcalfe's team performing simulations of specific aneurysms. Using a technique employed by Karmonik to simulate removal of an aneurysm on the computer, they analyze how the blood behaves as it flows near the aneurysm site and determine if that can be correlated to a certain type of behavior of the blood at potential sites where aneurysms form. Very accurate simulations are done for a complete description of the flow fields, studying all the fluid dynamic variables in great detail, such as the wall shear stresses, the pressures and the velocity.

"The second application is as a potential clinical tool," Metcalfe said. "Once we have a reasonable idea of the fluid dynamic variables needed to study and identify a potential problem, we then use a program that provides a detailed, 3-D description of the aneurysms of the real patients."

Benndorf adds that the potential clinical importance of these computer simulations lies in the future possibility of directly predicting patient-specific blood flow so that patient-specific medical devices can be used in aneurysm treatment. He is studying how stents - small wire mesh tubes that are inserted into the artery to facilitate the occlusion of an aneurysm with small platinum coils - can be tailored to the patient's individual anatomy and blood flow in order to optimize their therapeutic effect and maximize the possibility of a successful outcome.

When Metcalfe's group imports a patient's images into a computer program, they remove some geometric glitches and generate a computational mesh that involves the mapping of hundreds of thousands of tiny elements that represent the area being studied. That mesh is then introduced into a program that actually solves the fluid dynamic equations of motion.

"It takes a lot of computer time to perform these simulations," Metcalfe said. "There are several hundred thousand elements that are discrete zones within a geometric mesh, and then there are 700 steps representing intervals of time over the cycle of each heart beat."

Requiring extremely fast computers, the group uses the Beowolf cluster at UH's Texas Learning and Computation Center (TLC2) to significantly improve the visualizations created by the simulations.

"The critical step here is to make these complicated flows much more accessible to people like medical researchers and physicians," Metcalfe said. "We're developing 3-D visualizations so doctors can go inside the virtual artery and actually see what's happening as the blood cells flow through."

Halliburton Company supports this joint project by funding the research analysis of the study's findings, which have the potential for substantial impact in neurology and medical science.