# Intraoperative Imaging of Cortical Blood Flow by Camera-Based Photoplethysmography at Green Light
## 4 Discussion
Regardless of the theoretical model of light modulation in blood vessels, the APC parameter in cbPPG systems describes the relative change of blood flow arteries of medium and small caliber, as it is seen comparing APC maps (Panels B and E in Figures 1 and 3) before and after surgical intervention. The feature of cbPPG to visualize blood flow simultaneously in the whole cortex favorably distinguishes this system from other intraoperative imaging techniques in open neurovascular surgery. It should be noted that, in our pilot experiments, we did not intend to reach the highest possible resolution to visualize the smallest pulsatile arteries. Nevertheless, it can be readily achieved by zooming in the camera lens and/or by using a camera matrix with higher pixel numbers. Recent video-capillaroscopy experiments showed feasibility of resolving PPG waveforms in the smallest skin capillaries [18; 30]. In addition, a relatively simple algorithm for processing PPG data allows software to be developed to obtain results in real time, with a delay of about 10 seconds, necessary to accumulate information about several cardiac cycles. In our experiments, the variations of the light intensity reemitted from the cortical microvessels were recorded synchronously with ECG, thus allowing for accurate measurements of the time needed for the blood-pressure wave to travel from the heart to the cortex (PAT) [24]. This is valuable feature for brain-pathology assessment and prognosis because pulsatile, heart-related variations in blood pressure and flow play an important role in homeostasis [31]. It should be noted that there are only two noninvasive methods that allow for full-field imaging of microcirculation pulsatility: (i) LSCI [32], and (ii) cbPPG presented in this study. Despite the fact that both systems are based on processing of data obtained by a digital camera, technically they are fundamentally different. While LSCI fundamentally requires the use of coherent light [33], cbPPG operates at incoherent illumination [24]. This difference makes the latter method more advantageous. (i) LEDs used in cbPPG are more stable and reliable light sources providing more uniform illumination than can be done by a laser-diode needed for LSCI. (ii) LSCI uses high-speed cameras (114 frames per second, [32]) in contrast to the conventional camera (30-40 frames per second) in cbPPG. (iii) Intensity of the brain illumination in the LSCI system was several times higher than (20 mW/cm\({}^{2}\) in versus 3 mW/cm\({}^{2}\)). An advanced feature of the cbPPG system is simultaneous, spatially resolved (in every pixel) assessment of PAT in the full field of view of the cortex. Since the heart is the only generator of a blood-pressure wave in the cardiovascular system, we suppose that the relative difference in PAT observed in adjacent areas of the cortex indicates a difference in the method of blood supply. It should be underlined that surgical intervention has changed significantly the spatial distribution of the PAT parameter in all our experiments. In addition, the shape of PPG pulses and its variability in space and time (see Figures 2 and 4) bears important information about the processes of blood supply to cerebral microvessels. Intraoperative video recording of the cerebral cortex illuminated with green light and subsequent processing of recorded images synchronously with ECG made it possible to assess the parameters of blood pulsations in cortical microvessels in most of the regions accessible for observation with high spatial and temporal resolution. In the case of the patient without a significant lesion of the cerebral arteries, the APC and PAT parameters were rather evenly distributed over the cortex in the preoperative period (see Figure 1B,C). However, postoperative maps (Figure 1E,F) clearly revealed an area of pia mater damage associated with transcritical access to the underlying structures of the brain.