When doctors want to know more about a patient's risk of cardiovascular disease, they may order a cardiac stress test. But when it comes to stroke risk, there is no equivalent, scalable, cost-effective test of brain function to help doctors counsel patients about their potential risk. A questionnaire that asks patients about contributing risk factors is currently the best tool to estimate such risk.
Now, a team of engineers and scientists from Caltech and the Keck School of Medicine of USC has developed a device with headphones that can be used to non-invasively assess a patient's stroke risk by tracking changes in blood flow and volume while the participant holds their breath. The device incorporates a laser-based system and has shown promising results in terms of differentiating between individuals at low and high risk for stroke.
Stroke affects nearly 800,000 Americans each year and is the leading cause of serious long-term disability in the United States. It is caused by the blockage or rupture of an artery in the brain, resulting in reduced blood flow. Without oxygen, brain cells die rapidly: about 2 million every minute during a stroke.
“With this device, for the first time, we will have a way to know whether someone's risk of having a stroke in the future is significant or not based on a physiological measurement,” says Simon Mahler, co-lead author of a paper describing the new technique and device in the magazine Express Biomedical Optics and postdoctoral fellow in the laboratory of Changhuei YangThomas G. Myers Professor of Electrical Engineering, Bioengineering, and Medical Engineering at Caltech and a researcher at the Heritage Medical Research Institute. “We think this can really revolutionize the way stroke risk is assessed and will eventually help doctors determine whether a patient's risk is stable or worsening.”
“Our optical technology for non-invasively measuring blood flow is expected to be useful for a number of brain disease applications,” says Yang, who is also executive director of electrical engineering at Caltech. He noted that this project is part of a broader collaborative effort with Dr. Charles Liu, professor of clinical neurological surgery, surgery, psychiatry and behavioral sciences and biomedical engineering at the Keck School of Medicine of USC, and his team.
Speckle Contrast Optical Spectroscopy for Stroke Risk Assessment
In general, blood vessels become stiffer as a person ages, meaning it is more difficult for them to dilate to allow blood to pass through. This, in turn, means that the person is more likely to suffer a stroke.
The Caltech team developed a compact device that projects infrared laser light through the skull to the brain in one spot and then uses a special camera nearby to collect the light that bounces back after it is scattered by blood flowing inside blood vessels. . The method, called speckle contrast optical spectroscopy (SCOS), measures the decrease in light intensity from the point where it enters the skull to the place where the bounced light is collected to determine the volume of blood in the brain's blood vessels. ; It also analyzes the way light scatters and creates specks in the camera's field of view. The specks fluctuate in the images depending on the speed of blood flow in the blood vessels. The faster the blood flows, the more rapidly the speckle field changes.
Researchers can use those measurements to calculate a relationship between flow and the volume of blood flowing through the vessel to get an idea of that patient's stroke risk.
The team conducted a study of 50 participants. They used the currently used stroke risk questionnaire, The Cleveland Stroke Risk Calculatorto divide participants into two groups: one low risk and one high risk. They then measured blood flow in each volunteer for three minutes, quantifying the rate of flow and the volume of blood reaching the brain. After a minute, they asked the participants to hold their breath.
Holding your breath stresses the brain when it begins to notice that it is taking in too much carbon dioxide and not enough oxygen. It goes into what Mahler calls “panic mode” and begins pumping oxygen from the rest of the body into itself. This greatly increases blood flow in the brain. Once you stop holding your breath, oxygen levels return to their baseline value. While this happens in both people at low and high risk for stroke, the researchers found that there were differences between the groups in terms of how blood moved through the vessels.
The SCOS technique allows researchers to measure how much blood vessels expand while the subject holds their breath and how fast blood flows through the vessels in response. “These reactive measurements are indicative of the stiffness of the vessels,” says Yang. “Our technology makes it possible to perform these types of measurements non-invasively for the first time.”
The findings
“What we found is clear and striking evidence of a different reaction of blood flow and blood volume between the two groups,” says Yu Xi Huang, co-senior author of the new paper and a graduate student in Yang's lab.
In the low stroke risk group, the researchers observed a smaller increase in blood flow during the breath-hold exercise compared to the high stroke risk group, but a greater increase in blood volume, an indication that more blood is able to flow through the widened blood vessels.
“We can clearly see that the highest risk group has a higher flow-volume ratio, where they have a faster flow but a lower blood volume when holding their breath,” Mahler says. This is due to the stiffness of the blood vessels and indicates a higher chance of rupture. “If someone comes in with an extremely high flow-volume ratio value, we might suspect that person will have a stroke in the near future.”
A promising future
The team is conducting additional research using the current prototype of the imaging device on patients at a hospital in Visalia, California, to collect additional data from a larger, more diverse population. The researchers also plan to incorporate machine learning into the device's data collection process and conduct a clinical trial that would involve following patients for more than two years to improve the technology. They hope that the device could eventually be widely used not only for pre-screening for stroke risk, but also to help detect exactly where in the brain a stroke might have already occurred.
Additional authors of the paper, “Correlation of stroke risk with noninvasive cerebrovascular perfusion dynamics using a portable speckle-contrast optical spectroscopy laser device,” are Julian Michael Tyszka, associate director of the Caltech Brain Imaging Center; and Dr. Aidin Abedi, Yu Tung Lo, Patrick D. Lyden and Dr. Jonathan Russin of the Keck School of Medicine of USC. The work was supported by the National Institutes of Health and the USC Neurorestoration Center.
Leave feedback about this