Researchers develop superconducting nanowire single-photon detectors (SNSPDs) for cerebral blood flow measurement.
For a human brain to work properly, it needs consistent flow of blood through the cerebral arteries and veins, which deliver oxygen and nutrients and also remove metabolic byproducts. Cerebral blood flow is considered as a vital and sensitive marker of cerebrovascular function in the biomedical field. To monitor the blood flow, optical techniques are used like diffuse correlation spectroscopy (DCS), which involves the illumination of tissues with near-infrared laser rays. The focussed light is scattered by the movement of red blood cells and the resulting pattern formed is analyzed by a detector to determine blood flow.
For ideal operation of such optical techniques, large source–detector (SD) separation, high acquisition rates, and longer wavelengths is required. However, existing DCS devices use single-photon avalanche photodiode (SPAD) detectors, and cannot meet the above requirements. Due to high signal-to-noise ratio and low photon efficiency, they cannot allow an SD separation greater than 25 mm or wavelength greater than 900 nm.
Researchers from Massachusetts General Hospital, Harvard Medical School, and MIT Lincoln Laboratory have indicated the use of superconducting nanowire single-photon detectors (SNSPDs) in DCS devices.
SNSPDs consist of a thin film of superconducting material with excellent single-photon sensitivity and detection efficiency. They can outperform SPADs in multiple parameters, such as time resolution, photon efficiency, and range of wavelength sensitivity.
The researchers conducted cerebral blood flow measurements on 11 participants using both SNSPD-DCS and SPAD-DCS systems. 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 the conventional SPAD-based DCS.
According to the researchers, the improvement is due to the high wavelength of SNSPDs; they received seven to eight times more photons than SPAD detectors. Moreover, the SNSPDs has a higher photon detection efficiency (88 percent) compared to the SPAD’s photon detection efficiency of 58 percent. The SNSPD-DCS system allowed for the SD separation of 35mm., which SPAD based system could not be operated at due to their low SNR.
The developed SNSPD-DCS system facilitates higher photon collection, larger SD separations, and higher acquisition rates, leading to better accuracy. As a result, this system may allow for a noninvasive and more precise measurement of cerebral blood flow.
The work is described in the journal Neurophotonics.