A new non-invasive method for measuring brain blood flow with light performs similarly to functional magnetic resonance imaging but at less cost.
This is the claim of biomedical engineers and neurologists at the University of California, Davis, whose advance has been used to detect brain activation.
Dubbed functional interferometric diffusing wave spectroscopy (fiDWS) the new method could be used for assessing brain injuries, or in neuroscience research. The work is published in Science Advances.
“Now we can assess how well the brain regulates blood flow, and even detect brain activation noninvasively in adult humans, using principles similar to functional magnetic resonance imaging (fMRI), but at a fraction of the cost,” said Vivek Srinivasan, adjunct associate professor of biomedical engineering at UC Davis and senior author on the study.
The human brain makes up two per cent of human body weight but takes 15 per cent to 20 per cent of blood flow from the heart. Measuring cerebral blood flow is important for diagnosing strokes, and for predicting secondary damage in subarachnoid hemorrhages or traumatic brain injuries.
Doctors who provide neurological intensive care would also like to monitor a patient’s recovery by imaging brain blood flow and oxygenation. This can be done with existing technology such as such as MRI or CT scanners, but it is expensive and cannot be applied continuously or at the bedside.
The new method is said to take advantage of the fact that near-infrared light can penetrate through body tissues. If a near-infrared laser is shone on someone’s forehead, the light will be scattered many times by tissue, including blood cells. By picking up the fluctuation signal of the light that finds its way back out of the skull and scalp, information can be gained about blood flow inside the brain. That signal is tangible but extremely weak.
According to UC Davis, Srinivasan and postdoctoral researcher Wenjun Zhou overcame that problem by making use of interferometry, which exploits the ability of light waves to superimpose, reinforce or cancel one another. In particular, through interferometry, a strong light wave can boost a weak light wave by increasing its detected energy.
They first split the laser beam into ‘sample’ and ‘reference’ paths. The sample beam goes into the patient’s head and the reference beam is routed so that it reconnects with the sample beam before going to the detector. Through interferometry, the stronger reference beam boosts the weak sample signal. This allowed the team to measure the output with the type of light-detecting chip found in digital cameras, instead of photon counting detectors. They then use software to calculate a blood flow index for different locations in the brain.
Srinivasan and Zhou worked with Dr. Lara Zimmerman, Dr. Ryan Martin and Dr. Bruce Lyeth at the UC Davis Department of Neurological Surgery to test the technology. They found that with this new technology, they could measure blood flow more rapidly and deeper below the surface than with current light-based technology. They could measure pulsating cerebral blood flow and could also detect changes when volunteers were given a mild increase in carbon dioxide.
When volunteers were given a simple mathematics problem, the researchers were able to measure activation of the prefrontal cortex through the forehead.