BioPhotonics spoke with Euiheon Chung, a biomedical science and engineering professor at Gwangju Institute of Science and Technology in South Korea. Chung and his colleagues recently published a paper (www.doi.org/10.1364/ optica.422871) about the use of a technique called optical speckle image velocimetry to quantitatively measure blood flow in the cortical brain as an indicator of health or disease.

You have indicated that traditional laser speckle methods are not quantitative. Why is quantitative measurement so vital in terms of analysis or diagnosis?

Traditional laser speckle methods, such as laser speckle contrast imaging (LSCI), are semiquantitative and only provide relative measures in the region of interest, and thus are not suitable in long-term studies or comparisons among different experimental groups. So far, only a generic semiquantitative measure such as blood flow index (in arbitrary units) has been used, rather than an absolute unit of blood flow velocity. Thus, quantitative measurement is vital for rigorous testing of a new drug or a novel therapy that affects blood flow rate. Our optical speckle image velocimetry (OSIV) technique enables the absolute blood flow vector map.

Why are a wide field of view and sequential images important in this study?

There have been techniques for quantitative red blood cell (RBC) speed measurement via galvanometric scanning across blood vessels in confocal or two-photon laser scanning microscopes. However, these can only measure one or several blood vessels and have shortcomings for a comprehensive analysis of interconnected blood flows. Thus, wide field of view image velocimetry is crucial for hemodynamic network analysis. For example, to understand the dynamic nature of ischemic stroke before, during, and after a cerebrovascular accident, the OSIV methodology provides sequential images with quantitative velocity maps in real time. Furthermore, OSIV could help the development of novel therapies by monitoring disease progression longitudinally. Currently, the maximum OSIV velocity is limited by the camera frame rate.

What does this technique reveal about strokes that we didn’t already know, and what is the next step in your research?

After we confirmed the parabolic velocity profile in a blood vessel with the OSIV technique, we applied the approach to a photothrombosis-based ischemic stroke model, revealing the intricate spatiotemporal dynamics of local velocity distributions. However, this was a proof-of-principle study at this stage. Before OSIV can be commercialized, we would need to improve the speed of calculations and further develop computational algorithms to monitor flow changes in various types of blood vessels. In terms of instrumentation, compact, portable, and head-mounted OSIV hardware could be created to allow functional brain imaging in awake animals.

In our research, we used a 532-nm green laser because of its availability in our lab and since its power was high enough to conduct the investigation. However, this wavelength was not optimal due to high hemoglobin absorption in the blue-green range, leading to a lower signal-to-noise within the vessels, making some results inconclusive. Currently, we are testing a longer-wavelength laser to reduce such absorption by the RBCs, as our speckle imaging relies more on their scattering rather than their absorption for now. Thus, there exists more room for tweaking and improving our setup.