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Photonics West 2021: New ways to study cerebral hemodynamics

16 Mar 2021

Turgut Durduran outlines how novel biomarkers and devices could assist stroke recovery.

New biophotonics techniques are revealing essential details of cerebral hemodynamics, details that can be of urgent clinical use in the treatment of injuries or strokes.

Speaking in the SPIE Photonics West Digital Forum, Turgut Durduran from Spain's ICFO research center described how continuing breakthroughs in the study of cerebral blood flow could assist both patient well-being and the treatment of disease.

"We use near-infrared spectroscopy (NIRS), generally time-resolved, to measure blood oxygen saturation and the concentrations of oxy- and deoxy-hemoglobin; and diffuse correlation spectroscopy (DCS) to measure the cerebral blood flow itself," said Durduran.

DCS aims to detect microvascular blood flow using the fluctuations or speckle patterns it generates under illumination with near-infrared light. ICFO has previously studied several applications of the technique, including its use to monitor blood flow in stroke victims.

"We can combine these two methods to measure parameters like the metabolic rate of oxygen extraction," noted Durduran. "But can we go further? Can we estimate intracranial pressure by these methods?"

Answering that question could assist clinicians to safely apply hyperventilation to traumatic brain injury patients, an approach which can directly cause vasoconstriction and reduce blood flow. The question is, does reducing the intracranial pressure help these patients, or hurt them?

One approach is to consider "cerebral misery perfusion," or CMP, the condition where cerebral autoregulatory capacity is exhausted and blood supply is insufficient to meet metabolic demand.

Simultaneously monitoring for both a decrease in cerebral flow and a concurrent increase in oxygen extraction fraction, or the ratio of blood oxygen that a tissue takes from the blood flow in order to keep functioning, could produce a potential new biomarker for CMP, and this can be done in real time with modern optical platforms.

"Technology has improved recently, and there are now fast DCS devices," noted Durduran. "There are DCS instruments that can measure at more than 10 hertz. And in fact, we have developed hardware in the European Horizon 2020 LUCA project which allows us to measure up to 32 channels of fast DCS signals. These are fast enough to even resolve the DCS cardiac pulsations of blood flow in the brain."

Wearable sensors

Another focus area involves the treatment of ischemic stroke patients. Long-standing research at ICFO and the University of Pennsylvania has shown that cerebral blood flow during mild orthostatic challenges, such as the gentle raising and lowering of the heads of patients in their clinical beds, can act as a biomarker for the likely return of normal autoregulation in patients.

Study of these effects could ultimately lead to the generation of a cerebral autoregulation index for a patient, calculated from the hemodynamic data and used to guide the patient's progress through standard or intensive physiotherapy, depending on the response of their particular brain.

"The relationship between cerebral blood flow (CBF) and intracranial pressure (ICP) is complicated," commented Durduran. "Our proposed method involves using DCS to measure the pulsatile microvascular blood flow, and then apply machine learning to map this microvascular CBF to an index of the resulting ICP. Here again, fast DCS enables this pulsatory behavior to become a new biomarker."

One goal is the development of a wearable battery-powered wireless CBF monitor able to be attached to a patient's head, and a prototype device employing an array of single-photon avalanche detectors around the head and a laser source in a separate unit on the patient's back has been trialed.

"We are working on turning it into a more convenient headband design, with a goal of achieving this in two years," said Durduran.

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