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New views of cerebral blood flow from near-IR spectroscopy

11 Jan 2023

Poland's ICTER uses parallel interferometry to monitor prefrontal cortex.

Optical monitoring of cerebral blood flow is a potentially life-saving technology in clinical emergencies, and a number of spectroscopic techniques have been applied to the task.

Near-IR spectroscopy (NIRS) is the attractive approach to monitoring the optical and dynamic properties of the human brain, able to measure blood oxygen saturation and the concentrations of oxy- and deoxy-hemoglobin thanks to their selective absorption in the 660 to 940 nanometer range.

It has also been combined with other modalities, as in the platform developed by ICFO applying both NIRS and diffuse correlation spectroscopy (DCS) together, taking advantage of the latter's ability to detect blood flow using the speckled fluctuations generated by the flow itself when the vessels are illuminated.

A project at Poland's ICTER research center, part of the Polish Academy of Sciences, has now developed a variant approach intended to improve the light throughput detected by an NIRS technique, potentially improving its ability to monitor cerebral blood flow in vivo.

As reported in Biomedical Optics Express, the new platform address the limitation of single-mode fibers being the usual means of light collection in interferometric near-IR spectroscopy, which inherently reduces the detected light throughput. This has previously been compensated for with longer measurement or integration times, but these in turn restrict the monitoring of rapid blood flow changes.

ICTER's solution employs parallel interferometric near-infrared spectroscopy, christened πNIRS ("pi-NIRS"), using multi-mode fibers for light collection and a high-speed two-dimensional camera for light detection. This architecture allows each camera pixel to essentially act as a single iNIRS detection channel, while the processed signals from each pixel are spatially averaged to reduce the overall integration time.

Towards a brain-computer interface

This improvement translates into greater sensitivity of the system and accuracy of detection itself, according to the ICTER project team, making it possible to detect rapid changes in blood flow related to the activation of neurons in response to an external stimulus or administered drug.

The modified approach could be helpful for diagnosing cerebral blood flow-related neuronal disorders, and evaluating the effectiveness of therapeutic approaches for neurodegenerative diseases.

In trials the πNIRS platform was used to measure blood flow in the forearm and forehead of human subjects, producing data in agreement with baseline data from diffuse correlation spectroscopy. In the latter case, the device registered the increase in blood flow associated with the subject reading from text.

The platform's main limitation at present is the cost of the CMOS camera, but the project anticipates that this cost will be reduced as other applications in lidar and ophthalmology make increasing use of such fast camera sensors.

"Continuous and non-invasive monitoring of blood flow could help treat significant brain diseases," said Dawid Borycki of ICTER. "In addition, quick detection of cerebral blood flow will bring us closer to developing a non-invasive brain-computer interface. And our project will strengthen the tradition of Polish development in diffusion optics."

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