# Intraoperative Imaging of Cortical Blood Flow by Camera-Based Photoplethysmography at Green Light
## 1 Introduction
The consensual PPG model (originally developed for infrared light) suggests that this modulation originates from blood volume variations in major blood vessels (pulsatile arteries). This model provides rationale for a widely used clinical application of PPG referred to as pulse oximetry [16]. However, several experimental observations conflicting with the consensual PPG model were reported recently using the advanced camera-based PPG (cBPPG) technique. The first is paradoxical spectral dependence of the PPG signal that peaks at green [14; 17] despite the fact that this light cannot even reach pulsatile arterioles. Second, recent videocapillaroscopy study of RBC motion in nail-fold capillaries revealed that reemitted light is modulated at the heart rate not only in the capillaries but also in the space between them [18]. Nevertheless, we suggested that the cBPPG system operating at green illumination is capable of assessment microcirculation not only in the skin but also in the cortex. Grounds for this suggestion have arisen from anatomical-physiological structure of the brain vascular system in which small vessels and capillaries are supplied from arteries situated at the cortex surface [19]. The main blood supply of the brain is carried out by meningeal vessels penetrating brain tissue from the side of the convexital surface. These vessels depart from the Willis circle as paired anterior, middle, and posterior cerebral arteries and further ramify on gyri of cerebrum gradually decreasing in diameter from 700 to 40 \(\upmu\)m from conducting to precortical arteries. Next, from each precortical artery, 2-3 cortical arteries depart penetrating into the cortical space at right angles [19]. Unlike in the skin, all types of vessels are ideally accessible for observation in the cortex providing efficient interaction of green light with blood in microvessels. Effective use of the cbPPG technique for quantitative evaluation of changes in cortical blood perfusion of rats caused by painful stimuli was recently demonstrated by our group [20]. It was shown in these experiments that the amplitude of the PPG waveform is a measure of the cerebral arterial tone. A distinctive feature of the cbPPG system synchronized with the electrocardiogram (ECG) is not only the ability to quantify the time of the pulse-wave arrival to the measuring point (pulse arrival time, PAT) but also mapping a spatial distribution of PAT over large skin areas (e.g., face) [21]. Such maps allow one to obtain important additional information about the perfusion parameters, namely, to measure the spread in time of arrival of the blood-pressure pulse wave to different areas under study [22; 23]. Moreover, cbPPG is capable of estimating variations in the PPG-pulse shape both in time and space [24]. The wide range of hemodynamic parameters assessed by the advanced cbPPG system motivated us to carry out pilot experiments on measuring the parameters of pulsatile blood flow in open brain cortex during surgical interventions. The main objective of the research was estimation of eligibility to use the cbPPG system for remote visualization of cortical microcirculation in patients with various pathologies of the brain. We also aimed to show feasibility of monitoring the viability of brain structures with quantitative assessment of changes in parameters of blood supply (such as pulsation index, pulse arrival time, and pulse shape variability) to different areas of the brain before and after surgical intervention.