Although reflective-detected laser speckle contrast imaging (LSCI) is a powerful tool for monitoring blood flow circulation at the surface layers of tissue, it is inadequate for detecting signals from thick tissue layers. A scientific team from Huazhong University of Science and Technology explored the capability of transmissive-detected LSCI (TR-LSCI), an alternative approach to monitoring blood flow in thick tissue. Using TR-LSCI, the researchers were able to obtain a strong signal-to-background ratio (SBR) for deep signals that were barely detectable using windowless LSCI in the reflective-detected mode. With the help of tissue windows, reflective-detected LSCI can be used to observe cortical blood flow distribution and map cutaneous blood flow at single-blood-vessel resolution. However, when the reflective-detected mode is used without windows, the strength of the static speckle in the upper tissue layer becomes much greater than the dynamic speckle signal in the deep targeted layer. The superficial static speckle limits contrast and resolution, potentially making blood flow undetectable. Even with the assistance of optical windows, reflective-detected LSCI can provide acceptable resolution in the superficial layers of tissue only. The Huazhong team compared the performance of transmissive and reflective detection LSCI. The reflective-detected mode performed better than TR-LSCI when the target layer was at the surface, while the transmissive-detected mode obtained a stronger SBR for thick tissue. When the transmission detection method was used, the dynamic speckle signals from the vascular layer became stronger than the static speckle information from the upper tissue layer, improving the SBR needed for thick-tissue blood flow detection. (a) A diagram of the TR-LSCI imaging system. (b) A diagram of the conventional reflective-detected LSCI system. (c) A comparison between TR-LSCI and conventional LSCI for cutaneous/subcutaneous blood flow mapping on different parts of the mouse body. In (c), it is obvious that TR-LSCI provides better imaging quality for each body part, especially for the thick parts. (d) TR-LSCI for dynamic blood flow monitoring in the mouse hindlimb after an acetyl choline (a drug that dilates blood vessels) injection. TR-LSCI was capable of distinguishing individual cutaneous blood vessels. The plots in (d) represent the dynamics in the femoral vein and in the branch of femoral vein, showing that, compared to the femoral vein, the blood flow velocity in the branch of the femoral vein responded more strongly but recovered more quickly. (e) A comparison between TR-LSCI and conventional LSCI for subcutaneous blood flow mapping on different parts of human hands. As shown in (e), the blood flow in the individual blood vessels was resolvable in TR-LSCI, while conventional LSCI was not able to distinguish it. (f) Conventional LSCI and TR-LSCI for reactive hyperemia experiments on human hands. Conventional LSCI was used to monitor superficial perfusion, while TR-LSCI was used to monitor blood flow in the deep vessels. The histograms in (f) represent the dynamics in the superficial perfusion and deep blood flow. The results suggest that blood flow in the deep larger vessels is affected by pressure more and recovers more slowly after pressure is released. Courtesy of Dong-Yu Li, Qing Xia, Ting-Ting Yu, Jing-Tan Zhu, and Dan Zhu. In experiments, the researchers demonstrated TR-LSCI’s ability to achieve superior results for thick-tissue imaging and showed that the imaging quality could be further improved by using longer wavelengths of near-infrared light. The researchers used the Monte Carlo method to simulate the SBR of LSCI in reflective- and transmissive-detected modes, with varying signal depth and tissue thickness. To evaluate the validity of the Monte Carlo simulation results, they performed experiments on tissue phantoms, animal tissue, and human tissue. They used TR-LSCI to map and track blood flow in mouse body parts with various thicknesses, including the ear, hindlimb, back, and paw. They evaluated the validity of the simulation and the capability of TR-LSCI to provide the structural and functional information about blood vessels required for microcirculation research. The researchers also used conventional LSCI and TR-LSCI to image different positions of the human hand. They showed that TR-LSCI was capable of monitoring adult human blood flow changes in fingers or palms with individual-vessel resolution. They needed only 6 mJ/cm2 laser irradiation for the imaging procedure — far lower than that used in low-level laser therapy, the researchers said. “Our systematic in vivo results also suggested that TR-LSCI could perform blood flow mapping with promising resolution in thick tissue without the assistance of tissue windows,” the researchers said. They were able to acquire blood flow signals in the human hand with TR-LSCI, suggesting that LSCI in the transmissive mode could be successfully used on other areas of the human body. “The successful acquisition of blood flow signal in the human hand implied that TR-LSCI might be further applied in other human body parts whose thickness is feasible for light to penetrate, such as ear, lip, toe, and instep,” the researchers said. “Thus, the development of TR-LSCI might accelerate the clinical research of microcirculation and related diseases, such as diabetic foot ulcer, rheumatoid arthritis, and dermatitis.” Both theoretical and experimental results demonstrate that TR-LSCI is capable of obtaining thick-tissue blood flow information. In the future, the Huazhong researchers will further optimize TR-LSCI by using a longer wavelength laser to obtain greater imaging depth and quality, and by using algorithms to improve the quality of the final image. Thanks to its noninvasiveness, low cost, and high spatiotemporal resolution, TR-LSCI could become a valuable tool for microcirculation research and clinical applications. Scientists could select the most suitable imaging scheme — conventional LSCI or TR-LSCI — depending on whether the application was for monitoring blood flow in surface tissue or thick tissue. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-021-00682-8).