# Intraoperative Imaging of Cortical Blood Flow by Camera-Based Photoplethysmography at Green Light
## 4 Discussion  
Good quality and highly resolved spatial distributions of hemodynamic parameters APC and PAT were obtained in all studied cases. As evident from Figures 1 and 3, a surgical intervention leads to significant changes in both parameters differently in different areas. As we discuss below, these parameters reveal important information about the spatiotemporal dynamics of cerebral blood flow. We used an advanced cbpPG system [24] to acquire two-dimensional distribution of the pulsatile blood-flow parameters in the open cortex. All measurements were carried out using incoherent polarized green light (530 \(\pm\) 30 nm). Since RBCs strongly absorb green light, its penetration depth into the skin tissue does not exceed 0.6 mm [29]. Considering a higher density of blood vessels in the cortex, we suppose that green light penetrates at even smaller depths into the cortex, thus interacting mainly with upper microvessels. Beside this, we have to consider the difference in the anatomical and physiological structures of blood vessels in the cortex and skin [19]. Cortical vessels are organized in the principle of reverse radial branching: from the cerebral cortex to the subcortical structures. Arteries and arterioles are situated at the upper surface of the cortex, whereas capillaries are located inside and have an additional layer of glial cells that also absorb green light. Since the green light first interacts with pulsatile arteries, its heart-related modulation in the case of interaction with the cortex stems from periodical modulation of the blood volume in these vessels. However, the detailed mechanism of light modulation has not yet been established. Obviously, the source of the variable component is not only medium-sized arteries but also smaller vessels, which branch to a minimum size before penetrating through the cortex to the underlying sections (cortical arteries), thus becoming unresolved  
Figure 4: Variations of PPG-pulse shape in time and space in the case of carotid artery revascularization: The thin color lines in (**A–C**) show PPG pulses of 35 consecutive cardiac cycles calculated in the central pixel of ROIs marked by P1, P2, and P3, respectively. The positions of these ROIs in the cortex are shown in Figure 3. The thick green lines in these graphs are PPG pulses averaged over the cardiac cycles. The thin color lines in graphs (**D–F**) show the mean PPG pulses calculated in each of 49 neighboring pixels within respective ROIs marked by P1, P2, or P3. The thick pink lines in these graphs are the waveforms averaged over all pixels within the ROI. The upper and lower rows show the graphs before and after surgery, respectively. in the current camera resolution. This is indicated by the continuous distribution of pulsating areas outside the regions with distinct vascular structure. Nevertheless, we cannot completely exclude the presence of light modulation due to periodical mechanical compression of brain tissue by large pulsating arteries [14]. Signs of the latter modulation mechanism can be seen while comparing the APC maps before and after revascularization (Figure 3B,E). If small arteries are well resolved in Figure 3B, being highlighted by red because of the higher amplitude of the blood volume pulsations, the APC is more uniformly distributed within areas bounded by blue-highlighted, non-pulsating veins in Figure 3E. The relationship between both modulation mechanisms has not yet been established. It is possible that it has a complex, nonlinear character. The contribution to the pulsatile component of wide pulsating veins is less probable, since the APC of these vessels is several times lower than the surroundings in all maps. At the same time, venous blood flow can determine the DC component of the PPG waveform due to the significant volume of blood in these vessels.