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Two better than one? Not always so when treating brain aneurysms

In this video, a catheter (green tube) placed inside a blood vessel delivers a flow diverter (blue mesh) across the opening of an aneurysm (light gray area near the top right). The device slows blood flow into the aneurysm, minimizing the risk that it will rupture.

By GROVE POTTER

Published February 15, 2017

“We used equations from fluid mechanics to model the blood flow, and we used structural mechanics to model the devices.”
Robert Damiano, PhD student
Department of Mechanical Engineering

The old adage about two being better than one doesn’t necessarily apply to brain surgery.

That’s according to a study by UB engineers that used high-performance computing to examine how to best treat aneurysms.

To reduce blood flow into aneurysms, surgeons often insert a flow diverter — tiny tubes made of weaved metal, like stents — across the opening of an aneurysm. With the blood flow into the aneurysm reduced, the risk of rupture is minimized.

If the opening, or neck, of an aneurysm is large, surgeons will sometimes overlap two diverters to increase the density of the mesh over the opening. Another technique is to compress the diverter to increase the mesh density and block more blood flow.

But which technique is better?

Research published recently in the American Journal of Neuroradiology points to the single, compressed diverter, provided that it produces a mesh denser than the two overlapped diverters and that it covers at least half of the aneurysm opening.

“We’re able to quantify that,” says Hui Meng, professor of mechanical engineering and lead author of the study. “When doctors see the simulated blood flow in our models, they’re able to visualize it. They see that they need to put more of the dense mesh here or there to diffuse the jets (of blood) because the jets are dangerous.”

Meng holds appointments in UB’s School of Engineering and Applied Sciences and the Jacobs School of Medicine and Biomedical Sciences. She is also co-director of the Toshiba Stroke Research Center at UB.

Using UB’s supercomputer

Working with UB’s supercomputing facility, the Center for Computational Research, Robert Damiano and Nikhil Paliwal, both PhD candidates in Meng’s lab, used virtual models of three types of aneurysms — fusiform, large saccular and medium saccular — and applied engineering principles to model the pressure and speed of blood flowing through the vessels.

The engineers modeled three different diverter treatment methods — single non-compacted, two overlapped and single compacted — and ran tests to determine how they would affect blood flow in and out of the aneurysm using computational fluid dynamics.

“We used equations from fluid mechanics to model the blood flow, and we used structural mechanics to model the devices,” Damiano says. “We’re working with partial differential equations that are complex and typically unsolvable by hand.”

These equations are converted to millions of algebraic equations and are solved using the supercomputer. The very small size of the mesh added to the need for massive computing power.

“The diverter mesh wires are 30 microns in diameter,” Paliwal says. “To accurately capture the physics, we needed to have a maximum of 10 to 15 micron grid sizes. That’s why it is computationally very expensive.”

Compressed versus overlapped

The models showed that compressing a diverter produced a dense mesh that covered 57 percent of an elongated, or fusiform, type of aneurysm. That proved more effective than overlapping two diverters.

The compacted diverter was less effective in saccular aneurysms. As diverters are compressed, they become wider and bump into the sides of the vessel, so they could not be compressed enough to cover a small opening of an aneurysm. Compression was more effective in a large-necked aneurysm, producing a dense mesh that covered 47 percent of the opening.

Complete coverage of an aneurysm using a solid diverter is not favorable because a porous scaffold is needed to allow cell and tissue growth around the neck of the aneurysm, Paliwal says. In addition, the danger of blocking off smaller arteries prevents the use of solid diverters.

Next, as part of a National Institutes of Health-funded project, the team wants to look back over hundreds of previous cases to determine how blood flow was affected by the use of diverters. The idea is to build a database so that more definitive conclusions can be drawn.

“We’re going to look at and model previous cases, and hopefully we’ll have a way to determine the best treatment to cause the best outcome for new aneurysm cases,” Damiano says.