MRI Technique Detects Blood Flow Velocity and Stroke Risk in 4-D

This article was originally published in the Northwestern University McCormick School of Engineering News Center. 

Atrial fibrillation is the most common form of cardiac arrhythmia, affecting 33.5 million patients worldwide. As if having an irregular heart beat wasn’t troubling enough, patients with atrial fibrillation are also much more likely to have a stroke. However, a new imaging technique developed by a multidisciplinary Northwestern University team may predict who, among that group, is most at risk. It’s a breakthrough that could lead to better treatment and outcomes for all patients with atrial fibrillation.

“Atrial fibrillation is thought to be responsible for 20 to 30 percent of all strokes in the United States,” said Michael Markl, PhD, the Lester B. and Frances T. Knight Professor of Cardiac Imaging at Northwestern University Feinberg School of Medicine. “While atrial fibrillation is easy to detect and diagnose, it’s not easy to predict who will suffer a stroke because of it.”

Atrial fibrillation is linked to stroke because it slows the patient’s blood flow. The slow blood flow can lead to clots, which can then travel to the brain and initiate stroke. Markl developed a cardiac magnetic resonance (CMR) imaging test that can detect the blood’s velocity through the heart and body. Called atrial 4-D flow CMR, the technique is non-invasive and does not require contrast agents. The imaging program, which images blood flow dynamically and in the three spatial dimensions, comes in the form of software that can also be integrated into current MRI equipment without the need of special hardware and scanners or equipment upgrades.

4-D flow CMR can be employed to measure 3-D blood flow dynamics in the living heart and atria. Derived flow stasis maps in the left atrium and left atrial appendage are a novel concept to visualize and quantify regions with low flow, which are known to cause clot formation and risk for stroke.

“We simply programmed the scanner to generate information differently — in a way that wasn’t previously available,” Markl, who is also a professor of radiology at Feinberg and biomedical engineering in the McCormick School of Engineering, said. “It allows you to measure flow, diffusion of molecules, and tissue elasticity. You can study the human body in a very detailed manner.”

Better Treatment, Reduced Risks

Historically, physicians have attempted to assess stroke risk in atrial fibrillation patients by using a risk scoring system, which takes risk factors, such as age, general health, and gender, into account. Higher risk patients are then given medicine to prevent blood clots that lead to stroke.

“It’s very well accepted that these therapies significantly reduce the risk of stroke,” Markl said. “But they also increase risk of bleeding complications. It’s a dilemma that physicians face. They want to reduce one risk without introducing another risk. It’s particularly difficult for younger patients who might be on these medications for a long period of time. Maybe the risk of bleeding is initially small. But after taking medication for 20 or 30 years, it’s more and more likely that they’ll experience complications.”

Markl’s 4-D flow imaging technique can give a more precise assessment of who needs the medication, preventing physicians from over treating their patients. In a pilot study with 60 patients and a control group, Markl found that atrial fibrillation patients who would have been considered high risk for stroke by the traditional scoring system in fact had normal blood flow, while patients who were considered lower risk sometimes had the slow blood flow indicative of potential clotting.

“About 50 or 60 percent of patients who you would consider high risk actually had normal flows,” Markl said. “You could then hypothesize that those 50 percent don’t really need the treatment.”

“The challenge lies in the complexity of the technology,” Markl added. “We want to integrate tools that help deliver test results within a matter of minutes.”

Markl plans to continue following atrial fibrillation patients as a part of a long-term study to better understand the predictive power and diagnostic value of his new imaging technique. His team is also developing algorithms and tools to make it easier to analyze the data.