# Intraoperative Imaging of Cortical Blood Flow by Camera-Based Photoplethysmography at Green Light
## 4 Discussion
In this area, the PPG waveforms are distorted as seen in panels C and F of the lower row in Figure 2 that probably corresponded to temporal violation of microvessel blood flow in this region of the cortex. It is worth noting that our experiments revealed high sensitivity of PPG-pulse-shape variability to changes in hemodynamics. Recently reported experimental observation of significant increases in variability of PPG-pulse shape measured at the facial area of patients with systemic sclerosis [34] may have similar physiological reasons. In the case of vascular lesion, APC in the preoperative period was also distributed rather evenly (Figure 3B). In contrast, the PAT map (Figure 3C) was highly heterogeneous, which was the result of high instability and asynchronicity of PPG waveforms shown in the upper row in Figure 4. Revascularization of the middle cerebral artery led to restoration of blood pulsation synchronism in cortical microvessels (the lower row in Figure 4). After surgical intervention, the spatial distribution of both APC and PAT became uniform (Figure 3E,F). The observed postoperative decrease in the APC parameter is mainly due to a decrease in the amplitude of the heart-related alternating component. We hypothesize that more efficient blood supply to previously hypoperfused areas caused by anastomosis triggers the compensatory mechanism that increases the tone of arterial vessels, thus increasing regional vascular resistance. Accordingly, the amplitude of arterial lumen oscillations decreases, leading to a paradoxical drop in APC. A deeper study of basic reasons for light interaction with blood vessels, including animal experiments, will certainly offer a reliable interpretation of the observations. It is worth noting that the PPG technique cannot provide accurate quantitative evaluation of the volumetric velocity of cerebral blood flow. However, the proposed system can obviously be useful for a semiquantitative analysis of the dynamics of such indicators such as vascular tone and functional reserve of cerebral blood flow. These indicators are very important for both assessing the effectiveness of surgery and revealing the regions of critical circulatory disorders by using vasodilators (e.g., carbonic anhydrase inhibitors) or by analyzing the correlation between APC and systemic arterial pressure. The parameters PAT, pulse shape, and their variability can indicate areas of the cerebral cortex with the most significant change in cerebral blood supply. In conclusion, we showed that the green-light cbPPG system offers a new approach to objective assessment of the blood-flow changes in cerebral microvessels during surgical intervention. Synchronous recording of video and ECG allowed us to obtain highly resolved spatial distributions of three parameters of the blood flow in cerebral microvessels (APC, PAT, and shape of PPG pulses) that provides useful information for maximizing positive surgical outcome and minimizing risk. The low cost of the components of the developed cbPPG system, the high information content and noninvasive nature of the method, as well as its ease of use determine great prospects for its wide application in clinical practice. Conceptualization, O.V.M., A.V.S., and A.A.K. ; data curation, A.A.K. ; formal analysis, O.V.M. ; funding acquisition, A.A.K. and R.V.R; investigation, O.V.M. and A.V.S. ; methodology, O.V.M. and A.V.S. ; project administration, O.V.M. and A.A.K. ; resources, A.V.S. ; software, A.A.K. and R.V.R. ; visualization, A.A.K. ; writing--original draft, A.A.K. ; writing--review and editing, O.V.M., A.V.S., A.A.K., and R.V.R. All authors have read and agreed to the published version of the manuscript. This study was funded by the Ministry of Science and Higher Education of the Russian Federation, grant No. 075-15-2020-800. The authors declare no conflict of interest.