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 a single 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
]. 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
]. 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 μm) 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
]. The key feature of PPG is synchronization with heartbeat modulation of the light interacting with live biological tissue. 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
] 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 µm 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
]. 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.
Good quality and highly resolved spatial distributions of hemodynamic parameters APC and PAT were obtained in all studied cases. As evident from Figure 1
and Figure 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 ± 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 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 3
B,E). If small arteries are well resolved in Figure 3
B, 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 3
E. 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.
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 Figure 1
and Figure 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
]. 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/cm2
in versus 3 mW/cm2
). 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 Figure 2
and Figure 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 1
B,C). However, postoperative maps (Figure 1
E,F) clearly revealed an area of pia mater damage associated with transcortical access to the underlying structures of the brain. In this area, the PPG waveforms are distorted as seen in panels C and F of the lower row in Figure 2
that probably corresponded to temporal violation of microvessel blood flow in this region of the cortex. It is worth noting that our experiments revealed high sensitivity of PPG-pulse-shape variability to changes in hemodynamics. Recently reported experimental observation of significant increases in variability of PPG-pulse shape measured at the facial area of patients with systemic sclerosis [34
] may have similar physiological reasons.
In the case of vascular lesion, APC in the preoperative period was also distributed rather evenly (Figure 3
B). In contrast, the PAT map (Figure 3
C) was highly heterogeneous, which was the result of high instability and asynchronicity of PPG waveforms shown in the upper row in Figure 4
. Revascularization of the middle cerebral artery led to restoration of blood pulsation synchronism in cortical microvessels (the lower row in Figure 4
). After surgical intervention, the spatial distribution of both APC and PAT became uniform (Figure 3
E,F). The observed postoperative decrease in the APC parameter is mainly due to a decrease in the amplitude of the heart-related alternating component. We hypothesize that more efficient blood supply to previously hypoperfused areas caused by anastomosis triggers the compensatory mechanism that increases the tone of arterial vessels, thus increasing regional vascular resistance. Accordingly, the amplitude of arterial lumen oscillations decreases, leading to a paradoxical drop in APC. A deeper study of basic reasons for light interaction with blood vessels, including animal experiments, will certainly offer a reliable interpretation of the observations.
It is worth noting that the PPG technique cannot provide accurate quantitative evaluation of the volumetric velocity of cerebral blood flow. However, the proposed system can obviously be useful for a semiquantitative analysis of the dynamics of such indicators such as vascular tone and functional reserve of cerebral blood flow. These indicators are very important for both assessing the effectiveness of surgery and revealing the regions of critical circulatory disorders by using vasodilators (e.g., carbonic anhydrase inhibitors) or by analyzing the correlation between APC and systemic arterial pressure. The parameters PAT, pulse shape, and their variability can indicate areas of the cerebral cortex with the most significant change in cerebral blood supply.
In conclusion, we showed that the green-light cbPPG system offers a new approach to objective assessment of the blood-flow changes in cerebral microvessels during surgical intervention. Synchronous recording of video and ECG allowed us to obtain highly resolved spatial distributions of three parameters of the blood flow in cerebral microvessels (APC, PAT, and shape of PPG pulses) that provides useful information for maximizing positive surgical outcome and minimizing risk. The low cost of the components of the developed cbPPG system, the high information content and noninvasive nature of the method, as well as its ease of use determine great prospects for its wide application in clinical practice.