In order to function properly, the brain requires a gentle flow of blood through the cerebral arteries & veins, which deliver oxygen and nutrients and also remove metabolic byproducts. Therefore, cerebral blood flow is taken into account an important and sensitive marker of cerebrovascular function. Optical methods offer a noninvasive approach for measuring cerebral blood flow. Diffuse correlation spectroscopy (DCS), a way gaining popularity, involves the illumination of tissues with near-infrared laser rays. the light is scattered by the movement of red blood cells & therefore the resulting pattern formed is analyzed by a detector to know blood flow.
The ideal operating conditions for accurate measurement are:
- Large source–detector (SD) separation (>30 mm),
- High acquisition rates, and
- Longer wavelengths (>1000 nm). However, current DCS devices—which use single-photon avalanche photodiode (SPAD) detectors—cannot attain that ideal. thanks to high signal-to-noise & low photon efficiency, they can’t allow an SD separation greater than 25 mm or wavelength greater than 900 nm.
To enable the operation of DCS devices under ideal conditions, researchers from Massachusetts General Hospital, Harvard Medical School , & MIT Lincoln Laboratory recently proposed the utilization of superconducting nanowire single-photon detectors (SNSPDs) in DCS devices.
SNSPDs, first demonstrated 20 years ago, contains a thin-film of superconducting material with excellent single-photon sensitivity & detection efficiency. Commonly utilized in telecommunications, optical quantum information, & space communications, SNSPDs are seldom utilized in biomedicine. SNSPDs outperform SPADs in multiple parameters, like time resolution, photon efficiency, and range of wavelength sensitivity.
To demonstrate the operational superiority of the new SNSPD-DCS system, the researchers conducted cerebral blood flow measurements on 11 participants using both SNSPD-DCS & SPAD-DCS systems provided by Quantum Opus. The SNSPD-DCS system operated at a wavelength of 1064 nm with two SNSPD detectors, whereas the SPAD-DCS system operated at 850 nm.
The SNSPD-based DCS system showed significant improvement in SNR compared to conventional SPAD-based DCS. This improvement was due to two factors. First, with illumination at 1064 nm, the -SNSPD detectors received 7-8 times more photons than SPAD detectors at 850 nm did. Second, SNSPD features a higher photon detection efficiency (88%) than SPAD’s photon detection efficiency of 58%. While the SPAD-DCS could only allow signal acquisition at 1 Hz at 25 mm SD separation due to low SNR, the 16 times increase in SNR for the SNSPD-DCS system allowed signal acquisition at 20 Hz at an equivalent SD separation allowing clear detection of arterial pulses.
As cerebral blood flow sensitivity increases substantially for measurements taken at larger SD separation, the researchers also performed measurements at 35 mm SD separation. The SNSPD-DCS system recorded a 31.6% relative increase in blood flow sensitivity. In contrast, the SPAD-DCS system couldn’t be operated at 35 mm SD separation due to its low SNR.
Finally, the performance of SNSPD-DCS system was validated by measurements taken during breath-holding & hyperventilation exercises. Theoretically, blood flow increases during 1st 30 seconds of breath-holding & slowly returns to normal thereafter. During hyperventilation, blood flow to scalp increases and blood-flow to brain decreases. SNSPD-DCS measurements showed a rise of 69% and a decrease of 18.5% in relative cerebral blood flow for breath-holding & hyperventilation, respectively. These measurements are in agreement with those obtained from PET & MRI studies.
The SNSPD-DCS system facilitates higher photon collection, larger SD separations, & higher acquisition rates, resulting in better accuracy. Given these advantages, this novel system may allow for noninvasive & more precise measurement of cerebral blood flow—an important marker of cerebrovascular function—for adult clinical applications.
The findings were published on SPIE.