Caltech and USC device improves assessment of stroke risk for patients

01 Oct 2024

Head-mounted system monitors blood flow while a patient holds their breath.

A project from Caltech and USC's Keck School of Medicine has developed a method of monitoring blood flow intended to improve assessment of stroke risk in patients.

At present there is no scalable and cost-effective test of the brain's function targeted specifically at helping physicians counsel patients on their potential risk of suffering from stokes, according to the researchers.

Often clinicians are obliged to fall back on assessing a patient's lifestyle and contributory risk factors via interview or questionnaire, so an improved and quantitative method would be valuable in tackling a global health threat.

The project has designed an optics-based headset device to non-invasively assess a patient's stroke risk by monitoring changes in blood flow and volume while a participant holds their breath.

Oxygen deprivation during breath holding initiates a number of physiological changes in the cerebrovascular system, including increase in both cerebral blood volume and cerebral blood flow.

This is turn triggers autoregulation and blood vessel dilation. But this response is impaired in diseased atherosclerotic vessels, providing an indication of where a potential risk for stroke might be present.

Infra-red optical methods are increasingly effective at monitoring blood flow and mapping blood vessels, and using them to observe hemodynamics has been a focus of research for some time. Examples have included the 2018 ICFO study combining diffuse correlation spectroscopy (DCS) and near-infrared diffuse optical spectroscopy (NIRS-DOS) for bedside monitoring of patients who have suffered a stroke.

As described in Biomedical Optics Express the new device uses speckle contrast optical spectroscopy (SCOS), known to be an effective method for observing blood cells moving through capillaries and blood vessels. Back in 2014 another ICFO study determined that SCOS could combine relatively inexpensive detectors with high imaging frame rates, both advantageous for clinicians.

“With this device, for the first time, we are going to have a way of knowing if the risk of someone having a stroke in the future is significant or not based on a physiological measurement,” said Simon Mahler from Caltech. “We think this can really revolutionize the way stroke risk is assessed and will eventually help doctors determine if a patient's risk is stable or worsening.”

Stiffness of blood vessels means higher risk of rupture

The project's SCOS device compares the intensity of 785-nanometer light delivered by the headset to that of the returned light which bounces back, and also records how the light scatters and creates speckles in a CMOS sensor's field of view.

The variation in intensity provides a measure of the volume of blood present at the time of imaging, while the speckle field allows the rate at which that blood is flowing to be assessed simultaneously.

A study of 50 participants divided into low- and high-risk categories based on current standard stroke risk assessments was undertaken. Blood flow was measured in each volunteer for three minutes, quantifying the flow rate and volume of blood reaching the brain. After one minute, the participants held their breath. SCOS then measured how much the blood vessels expanded and how much faster blood flowed through the vessels in response.

"We can clearly see that the higher risk group has a higher flow-to-volume ratio, where they have faster flow but a lower volume of blood during breath holding," said Simon Mahler. "This is caused by the stiffness of the blood vessels and indicates a higher chance of rupture. If someone came in with an extremely high flow-to-volume ratio value, we might suspect that this person will have a stroke in the near future."

The team's next steps will be to incorporate machine learning into the device's data collection process, and to conduct a clinical trial that would involve patient tracking over more than two years to enhance the technology. They hope the device might eventually be used not only for stroke risk prescreening but also to help detect where exactly in the brain a stroke might have already occurred.

“These reactive measurements are indicative of vessel stiffness,” commented Caltech's Changhuei Yang. “Our technology makes it possible to make these type of measurements non-invasively for the first time.”

• Yu Xi Huang of Caltech will deliver a related presentation Correlating stroke risk with non-invasive tracing of brain blood dynamic via a portable speckle contrast optical spectroscopy laser device at the forthcoming Photonics West conference in San Francisco, on January 27th, 2025.