Understanding Aneurysms

Adel Malek, MD, PhD, is chief of neurovascular surgery at Tufts Medical Center, where he heads the Cerebrovascular and Endovascular Division in the Department of Neurosurgery. He is also an associate professor of neurosurgery at Tufts University School of Medicine. Malek’s research focuses on the mechanical forces on blood vessel walls and the relation between these forces and vascular disorders, primarily in the cerebral vasculature. Malek’s diverse background allows him to investigate mechanical shear stress (tangential frictional force resulting from blood flow) and tensile stress (force resulting from blood pressure) on many levels. He received a BS and an MS in electrical engineering from Massachusetts Institute of Technology, an MD from Harvard Medical School, and a PhD in medical engineering and medical physics from the Harvard-MIT Division of Health Sciences and Technology.

“Our lab is looking at the effects of hemodynamic forces on the development of brain aneurysms and cerebral vascular disorders in general,” says Malek. “An aneurysm is like an outpouching, a small balloon that forms in an area of focal weakness, usually around bifurcations where the vessel divides into two branches.” Brain aneurysms can have devastating consequences if they rupture. To study their development, the Malek lab has been interested in a phenomenon central to blood vessel biomechanics—the vascular optimality principle. In the branching arterial system of mammals, shear stress on the vessel wall is maintained at a fairly constant level—a level of optimal energy use; blood vessels remodel structurally by dilating or constricting to maintain this optimal level. “Our interest has been in trying to figure out whether diseases such as atherosclerosis or aneurysms occur because of a deviation from that optimality phenomenon, either because of geometric constraints or because of other modulation effects like smoking, diet, genetics, and so on,” says Malek.

Malek and his research group use computer-based simulations of fluid dynamics to analyze mechanical forces on the vessel wall. Their computational method is based on data from patient blood vessels that harbor aneurysms, and it enables the use of high-resolution 3D rotational angiography to visualize the inside of vessels. Malek worked with postdoctoral fellow Alexandra Lauric and Professor Eric Miller of the Department of Electrical and Computer Engineering at Tufts University to develop software that describes aneurysm curvature and shape based on parameters derived from the writhe number. (The writhe number measures coiling and is used often to describe the twists and turns of DNA.) Their hope is to predict where an aneurysm is likely to develop and to understand what size and shape characteristics make an aneurysm likely to rupture. “There are about 30,000 ruptures of brain aneurysms a year in the United States,” says Malek, “and it’s a devastating disease, leading to intracranial hemorrhage, which is fatal in about 10 to 20% of patients.”

Another aspect of Malek’s work is to understand how devices used to treat aneurysms—such as clips, coils, and stents—affect hemodynamics in an aneurysm-bearing vessel. The traditional technique involves brain microsurgery to place a clip on the outside of the neck of the aneurysm. The coiling technique involves sending a catheter through the femoral artery to the aneurysm and filling the lesion with tiny coils made of platinum wire, which stabilize the aneurysm. Stenting is sometimes used in conjunction with coils, especially for wide-necked aneurysms. Tiny stents are deployed (again via the femoral artery) to the neck of the aneurysm. The stent acts as a scaffold for the coils and prevents them from popping out of the aneurysm into the main flow of the blood vessel. “In 2001, coils were shown to be even better tolerated than clipping in ruptured aneurysms—patients do better clinically—but the limitation is that sometimes they require additional treatment down the line,” says Malek. “So we’re trying to understand why some of these treatments work and others don’t work so well. We have used numerical methods to simulate placing a coil in an aneurysm and looked at the implications on fluid mechanics inside the aneurysm. We found that, depending on the aneurysm, coil orientation can have significant effects on hemodynamics near the neck of the aneurysm, which can have significant effects on the potential recurrence of the aneurysm down the line.”

Traditionally, absolute size has been considered the single most important factor in whether an aneurysm is likely to burst or not. However, a recent study by Merih Baharoglu and colleagues in the Malek lab suggests that size is only significant for side-wall aneurysms, not for bifurcation aneurysms. “Autopsy studies show that about 2 to 4% of patients in general have aneurysms,” says Malek. “So one of the arguments is that physicians are in fact over-treating aneurysms, because we don’t know what the real rate of rupture is in incidentally discovered, unruptured aneurysms. Although treatment itself involves a risk of complications, medical observation also carries a risk of rupture, which can be devastating. So we’re hoping that these morphometric and fluid mechanical modeling tools can provide us with answers to guide the treatment and decision process involved in managing these potentially harmful lesions.”

For collaboration, Malek can offer his lab’s expertise in image processing tools, image-based computational fluid dynamics, numerical simulations for biological vessels, and shape analysis tools. Malek has current collaborations with the Department of Electrical and Computer Engineering and is looking into collaborating with the Department of Biomedical Engineering. He would like to include mechanical engineering in his collaborations if a researcher finds his work interesting. “We’d love to collaborate with anyone who’s interested; this is a very exciting field,” says Malek.

For more information, please contact Dr. Malek at amalek@tuftsmedicalcenter.org or at (617) 636-8200.