# Intraoperative Imaging of Cortical Blood Flow by Camera-Based Photoplethysmography at Green Light
## Results
### Revascularization of Internal Carotid Artery in Patient with Arterial Atherosclerotic Occlusion  
Another pilot experiment was carried out during surgical intervention of a patient with atherosclerotic occlusion of the internal carotid artery after ischemic stroke. The patient underwent an operation of extra-intracranial bypass grafting between the temporal artery and the middle cerebral artery in connection with clinical signs of chronic cerebral hypoperfusion against the background of extracranial occlusion of the internal carotid artery. Images of the open brain and the parameters of the pulsatile blood flow measured before and after applying anastomosis are shown in the upper and lower rows of Figure 3, respectively. The anastomosis application resulted in significant diminishing of APC in the whole cortex as can be seen by comparing the maps of the parameter in Figure 3B,E (note the big difference in the color scale of these maps). The mean APCs in the whole cortex were \(2.13\pm 0.50\%\) and \(0.80\pm 0.07\%\) before and after the surgery, respectively. An increased amplitude of pulsations perhaps indicated a decreased vascular tone before surgery. Most likely, this was because blood supply to the brain was accomplished by the collateral artery while the functional vasodilator reserve was maximally involved. With impaired blood supply, the mechanism of cerebral autoregulation did not work due to exhaustion of the limits of vasodilation. We hypothesize that the observed decrease in APC after anastomosis is caused by an increase in the volumetric blood-flow velocity, which, through the autoregulation mechanism, increases the vascular tone, ensuring the constancy of cerebral perfusion. A similar  
Figure 2: Variations of the photoplethysmography (PPG)-pulse shape in time and space in the case of a tumor: Thin color lines in (**A–C**) show PPG pulses of 43 consecutive cardiac cycles calculated in the central pixel of a region of interest (ROI) marked by P1, P2, and P3, respectively. The positions of these ROIs in the cortex are shown in Figure 1. 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. decrease in APC in the cerebral cortex of animals was recently observed in response to an increase in systemic arterial pressure [20]. The assumption about a change in the pathways of the brain's blood supply was confirmed by comparing the spatial distributions of the PAT parameter before and after anastomosis (Figure 3C vs. Figure 3F). Prior to surgery, PAT had extremely heterogeneous distribution, and its magnitude varied from zero to values, exceeding the cardiac cycle duration (note the limits of the color scale in Figure 3C). After anastomosis, the travel time of the pulse wave turned out to be uniformly distributed (see Figure 3F) with more realistic value of about 140 ms. The prolonged arrival time of the pulse wave before anastomosis was caused by the brain's supply via the collateral artery. The mean PATs in the whole cortex were \(407\pm 137\) ms and \(147\pm 117\) ms before and after the surgery, respectively. The difference in hemodynamics before and after anastomosis is even more evident in the stability of the PPG-pulse shape. While the upper row in Figure 4 shows variability of the pulse shape in three representative ROIs before surgery, the graphs in the lower row demonstrate the shape variability of PPG pulses in the same ROIs both in time (A-C) and in space (D-F) after anastomosis. As can be seen from the graphs of the upper row in Figure 4, the PPG pulse shape is extremely unstable before surgery.