Could brain vessels’ blood flow predict approach to aneurysms?

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Could brain vessels’ blood flow predict approach to aneurysms?

UW specialists in neurosurgery, math and physics team up, use 3-D-printed models to study fluid forces on vessel walls
Brian Donohue

On a computer monitor at Harborview Medical Center, Dr. Michael Levitt rotates a 3-D angiogram that maps the blood flow to a patient’s brain. As a collective, the vessels evoke a tree canopy; then Levitt zooms in, revealing a single branch in spellbinding detail. 

Levitt in office
Brian Donohue
Dr. Michael Levitt is studying the whether blood flow in and around brain aneurysms might inform a patient's course of care.
Dr. Michael Levitt is studying the whether blood flow in and around brain aneurysms might inform a patient's course of care.

Such sophisticated imagery makes it easy for neurosurgeons like Levitt to spy unruptured aneurysms – pouches of weak tissue often no bigger than a pencil eraser, which emerge from vessels like cul de sacs from streets. The high-res scans give real-time, sub-millimeter guidance as doctors take steps to halt blood flow that might burst a pouch and cause a stroke. 

For all they illuminate, though, the scans offer no data about which treatment has the best chance of success with any aneurysm. 

“You place a stent or coil, but for some patients, the aneurysm either sticks around or grows larger. Those are treatment failures,” said Levitt, an assistant professor of neurological surgery and mechanical engineering at the University of Washington. 

“We want to figure out why procedures are helpful for some but not others, and eventually to figure out why some aneurysms grow and others stay the same.”

It’s a novel, imaginative approach he’s taking with a small UW team of engineering and mathematics problem-solvers. 

The digital angiogram is first turned into high-fidelity, 3-D printed model of the aneurysms and its vessel. Then fluid can be sent through the resin model at the same rate as the patient’s blood flow, and the team can monitor the fluid dynamics around and inside the aneurysm. 

aneurysm 3 views
(Click to enlarge.) The computerized model (top) gives rise to a 3-D-printed, actual-size model (middle). At bottom, blood-flow forces on the aneurysm at time of surgery can be visualized.
The computerized model (top) gives rise to a 3-D-printed, actual-size model. At bottom, blood-flow forces at time of surgery can be visualized.

“Part of it is the size and the angle at which blood strikes the inside of the aneurysm. We’re looking at forces on the walls or in the main blood vessel that can either keep the aneurysm open or let it clot off, depending on how you treat it,” Levitt explained. “We’re trying to figure out if we can correlate changes in those forces, when we do the treatment, with patients who end up needing second or third treatments – or with patients who get better.”

To measure the fluid flow rate, the team deploys a Doppler-tipped “microwire” typically used in vasculature around patients’ hearts.

“We’re the only group I know of using this wire in the brain,” Levitt said. 

By accumulating data with the 3-D models, the investigators hope to define characteristics that will predict success when a patient presents with an unruptured aneurysm. 

“That’s really the translational aspect of this research, Levitt said. “We’d like to obviate the need for 3-D printing and use only a digital model. The idea would be to take measurements before treatment, then while you’re placing the coils or stent, check to see whether there’s enough of a change in the forces inside the aneurysm to say, ‘We’re done, let’s go home’ or ‘We need to do something more.’ ”

Aneurysms develop for reasons unknown, in 1-2 percent of the population – usually people 40 or older. Unruptured aneurysms often are asymptomatic, revealed by happenstance in scans for unrelated concerns. They can remain unruptured for decades, but physicians typically address them upon discovery. 

Conventionally, aneurysms have been treated surgically; part of the skull is removed and a clip is placed across the outside of the pouch. The endovascular approach is more recent; a catheter is inserted in the patient’s femoral artery and directed up to the brain using X-ray guidance. Using the catheter, a neurovascular specialist fills the aneurysm with coils of metal or places a stent across the pouch to block blood flow. Without blood, the aneurysm eventually clots off.

UW collaborators in the research include Alberto Aliseda, associate professor of mechanical engineering, and Pierre Mourad, associate professor of bioengineering, mathematics and neurological surgery. The principle investigator on the grant with Levitt is Louis Kim, associate professor of neurological surgery and chief of neurosurgery at Harborview. They are supported by a $2 million grant from the National Institutes of Health.