# Intraoperative Imaging of Cortical Blood Flow by Camera-Based Photoplethysmography at Green Light
## 1 Introduction  
Evaluation of blood perfusion in the brain cortex during surgical intervention is an important but hitherto unresolved problem. Several methods were tested for intraoperative imaging of cortical blood flow. Perfusion computed tomography (PCT) requires sophisticated equipment that can be used only in specially equipped hybrid operational theaters. Moreover, the interpretation of PCT data is not always unambiguous and does not reveal the dynamic component of perfusion. In addition, both the PCT method and magnetic resonance imaging disrupt the surgical procedure and require significant extra time for preparation. Laser Doppler flowmetry measures blood velocity in only asingle point, whereas laser Doppler imaging is a rather expensive system and has poor temporal resolution [1]. Moreover, blood flow in Doppler methods is characterized by means of relative perfusion units, the physical significance of which has a contentious interpretation [2]. The method of video angiography that uses intravenous administration of indocyanine green (ICG) with subsequent video recording of the cortex was also proposed for intraoperative assessment of vessel patency during neurovascular surgery [3]. The video-angiography system is rather simple and provides views of the full field of cerebral blood flow. However, the need to administer the intravenous dye ICG to increase the contrast of functioning vessels does not allow them to be visualized continuously and in real time. Another camera-based method, laser speckle contrast imaging (LSCI), is also capable of full-field visualization of cortical blood flow [4]. In contrast to video angiography, LSCI does not require an exogenous contrast agent, thus providing continuous monitoring of cerebral blood flow in two dimensions with high resolution [5; 6]. More advanced modifications of the LSCI technique, dynamic and multi-exposure, allow visualization of blood flow in the full field of view, pixel by pixel, but require the use of high-speed video cameras [7; 8]. Although speckle contrast values are indicative of the level of motion in the sample, they are not directly proportional to red blood cell (RBC) velocity or blood flow [4]. Moreover, the results of the LSCI technique are ambiguous in interpretation due to multiple light scattering in biological tissue [9]. None of the above methods can provide a quantitative assessment of blood perfusion and dynamic parameters of the cerebral cortex supply in real time and with high spatial resolution allowing for estimation of the state of microcirculation simultaneously over the full field of view. However, it is the assessment of blood flow in the microvessels (arteries of medium and small caliber down to 30 \(\upmu\)m) that provides the most important information about the problems of brain function. Superficial microvessels are situated in the pia mater, constituting about 80% of the total blood flow of the brain. Continuity of blood circulation in microvessels of the convexital brain surface prognosticates restoration of the cortex functions as it was shown in experiments with animals [10]. Optical methods are widely considered as very promising for contactless measurements of blood-flow parameters in skin. Nowadays, remote assessment of cutaneous blood perfusion is usually provided by two commercially available optical techniques, both using coherent-light illumination: laser Doppler perfusion imaging and LSCI [11]. However, another optical method proposed 80 years ago to evaluate blood supply to the skin and referred to as photoplethysmography (PPG) [12] until now has not been used to assess cutaneous microcirculation. This may be caused by still insufficient knowledge of the mechanism of light interaction with pulsatile blood vessels. Notwithstanding the long history of research in this field, the origin of the PPG signal remains the subject of continuing debates [11; 13; 14; 15]. The key feature of PPG is synchronization with heartbeat modulation of the light interacting with live biological tissue.