Cerebral blood flow is essential for normal brain function and often perturbed in neurological disease. If one shines a source of coherent light on perfused tissue, the detected speckles, or "grains" of light fluctuate, or "dance", at a rate proportional to blood flow in the volume sampled by the light. In brain tissue, this concept can be harnessed to measure the cerebral blood flow index (CBFi).
However, to date, implementations of this principle for noninvasive adult human brain monitoring—collectively known as diffuse correlation spectroscopy (DCS)—have achieved limited brain sensitivity. This is because the brain is 1–2 centimeters deep beneath the scalp and skull, meaning that the light must sample the superficial tissue before reaching the brain. While the collection points can be moved further from the source to address this issue by improving sampling of the brain, this strategy requires many photon counting channels to detect highly attenuated light far from the source. DCS becomes prohibitively expensive as the number of channels increases.
In a breakthrough study, Dr. Mingjun Zhao from New York University Langone Health, United States, and her team, has optimized a novel approach called interferometric diffusing wave spectroscopy (iDWS). Their findings were made available online on 13 February 2025 and was published in Volume 31, Issue 4 of the IEEE Journal of Selected Topics in Quantum Electronics in July-August 2025.
"Our novel iDWS approach involves boosting of the weak optical field returning from the brain by coherent amplification with a stronger reference field. In this way, interferometry enables a non-scientific complementary-metal-oxide-semiconductor sensor to parallelize measurements of weak coherent light fluctuations," explains Dr. Zhao.
In this work, the researchers further optimized iDWS at 852 nm with respect to the number of independent channels, camera duty cycle and full well capacity, incident laser power, noise and artifact mitigation, and data processing, achieving an over 20x overall improvement in signal to noise ratio. In addition, they demonstrated pulsatile CBFi monitoring at 4–4.5 cm source-collector separation in adults with moderate pigmentation (Fitzpatrick 4).
More recently, the team optimized a 1,064 nm iDWS system using the concepts presented in this work and achieved pulsatile CBFi measurement at over 5 cm source-collector separation.
This novel iDWS approach provides several advantages over promising non-interferometric extensions of conventional DCS. Compared to DCS with a 512×512 single-photon avalanche diode array, the proposed complementary-metal-oxide-semiconductor sensor is currently approximately two orders of magnitude less expensive. In addition, iDWS achieves higher brain sensitivity compared to speckle contrast optical spectroscopy, a DCS-inspired approach, due to the short exposures that capture the rapid speckle fluctuations originating from the high brain CBFi.
Besides technical advancements, the researchers are also actively working on clinical translation. "Stabilizing an interferometer without pneumatically isolated optical tables in space-starved clinical settings is challenging. In this work, we have successfully built the system on a cart and have defined conditions for stable operation. The engineering of a demonstrably stable cart-based iDWS system represents a milestone in clinical translation of interferometric diffuse optical methods," remarks Dr. Zhao.
With the mobile iDWS system, the team also reports preliminary clinical measurements of CBFi in a patient in the Neuro Intensive Care Unit (Neuro ICU) in this work. These results push the boundaries of iDWS CBFi monitoring performance beyond previous reports.
In the future, the researchers will test and optimize the long-term operability of the 852 nm iDWS system presented here in the Neuro ICU environment, implement time-of-flight filtering on the new iDWS, and finally, further validate iDWS in diagnosis and treatment monitoring of neurological disorders, including ischemic stroke and traumatic brain injury.
