Transcranial optical monitoring of cerebrovascular hemodynamics in acute stroke patients

Correlations Between Quantitative EEG Parameters and Cortical Blood Flow in Patients Undergoing Extracorporeal Membrane Oxygenation With and Without Encephalopathy

PURPOSE

The neurologic examination of patients undergoing extracorporeal membrane oxygenation (ECMO) is crucial for evaluating irreversible encephalopathy but is often obscured by sedation or neuromuscular blockade. Noninvasive neuromonitoring modalities including diffuse correlation spectroscopy and EEG measure cerebral perfusion and neuronal function, respectively. We hypothesized that encephalopathic ECMO patients with greater degree of irreversible cerebral injury demonstrate less correlation between electrographic activity and cerebral perfusion than those whose encephalopathy is attributable to medications.

METHODS

We performed a prospective observational study of adults undergoing ECMO who underwent simultaneous continuous EEG and diffuse correlation spectroscopy monitoring. (Alpha + beta)/delta ratio and alpha/delta Rartio derived from quantitative EEG analysis were correlated with frontal cortical blood flow index. Patients who awakened and followed commands during sedation pauses were included in group 1, whereas patients who could not follow commands for most neuromonitoring were placed in group 2. (Alpha + beta)/delta ratio-blood flow index and ADR-BFI correlations were compared between the groups.

RESULTS

Ten patients (five in each group) underwent 39 concomitant continuous EEG and diffuse correlation spectroscopy monitoring sessions. Four patients (80%) in each group received some form of analgosedation during neuromonitoring. (Alpha + beta)/delta ratio-blood flow index correlation was significantly lower in group 2 than group 1 (left: 0.05 vs. 0.52, P = 0.03; right: -0.12 vs. 0.39, P = 0.04). Group 2 ADR-BFI correlation was lower only over the right hemisphere (-0.06 vs. 0.47, P = 0.04).

CONCLUSIONS

Correlation between (alpha + beta)/delta ratio and blood flow index were decreased in encephalopathic ECMO patients compared with awake ones, regardless of the analgosedation use. The combined use of EEG and diffuse correlation spectroscopy may have utility in monitoring cerebral function in ECMO patients.

Wavelet and time-based cerebral autoregulation analysis using diffuse correlation spectroscopy on adults undergoing extracorporeal membrane oxygenation therapy

INTRODUCTION

Adult patients who have suffered acute cardiac or pulmonary failure are increasingly being treated using extracorporeal membrane oxygenation (ECMO), a cardiopulmonary bypass technique. While ECMO has improved the long-term outcomes of these patients, neurological injuries can occur from underlying illness or ECMO itself. Cerebral autoregulation (CA) allows the brain to maintain steady perfusion during changes in systemic blood pressure. Dysfunctional CA is a marker of acute brain injury and can worsen neurologic damage. Monitoring CA using invasive modalities can be risky in ECMO patients due to the necessity of anticoagulation therapy. Diffuse correlation spectroscopy (DCS) measures cerebral blood flow continuously, noninvasively, at the bedside, and can monitor CA. In this study, we compare DCS-based markers of CA in veno-arterial ECMO patients with and without acute brain injury.

METHODS

Adults undergoing ECMO were prospectively enrolled at a single tertiary hospital and underwent DCS and arterial blood pressure monitoring during ECMO. Neurologic injuries were identified using brain computerized tomography (CT) scans obtained in all patients. CA was calculated over a twenty-minute window via wavelet coherence analysis (WCA) over 0.05 Hz to 0.1 Hz and a Pearson correlation (DCSx) between cerebral blood flow measured by DCS and mean arterial pressure.

RESULTS

Eleven ECMO patients who received CT neuroimaging were recruited. 5 (45%) patients were found to have neurologic injury. CA indices WCOH, the area under the curve of the WCA, were significantly higher for patients with neurological injuries compared to those without neurological injuries (right hemisphere p = 0.041, left hemisphere p = 0.041). %DCSx, percentage of time DCSx was above a threshold 0.4, were not significantly higher (right hemisphere p = 0.268, left hemisphere p = 0.073).

CONCLUSION

DCS can be used to detect differences in CA for ECMO patients with neurological injuries compared to uninjured patients using WCA.

A comprehensive overview of diffuse correlation spectroscopy: Theoretical framework, recent advances in hardware, analysis, and applications

Diffuse correlation spectroscopy (DCS) is a powerful tool for assessing microvascular hemodynamic in deep tissues. Recent advances in sensors, lasers, and deep learning have further boosted the development of new DCS methods. However, newcomers might feel overwhelmed, not only by the already-complex DCS theoretical framework but also by the broad range of component options and system architectures. To facilitate new entry to this exciting field, we present a comprehensive review of DCS hardware architectures (continuous-wave, frequency-domain, and time-domain) and summarize corresponding theoretical models. Further, we discuss new applications of highly integrated silicon single-photon avalanche diode (SPAD) sensors in DCS, compare SPADs with existing sensors, and review other components (lasers, sensors, and correlators), as well as data analysis tools, including deep learning. Potential applications in medical diagnosis are discussed and an outlook for the future directions is provided, to offer effective guidance to embark on DCS research.

Interferometric near-infrared spectroscopy (iNIRS) reveals that blood flow index depends on wavelength

Blood flow index (BFI) is an optically accessible parameter, with unit distance-squared-over-time, that is widely used as a proxy for tissue perfusion. BFI is defined as the dynamic scattering probability (i.e. the ratio of dynamic to overall reduced scattering coefficients) times an effective Brownian diffusion coefficient that describes red blood cell (RBC) motion. Here, using a wavelength division multiplexed, time-of-flight- (TOF) - resolved iNIRS system, we obtain TOF-resolved field autocorrelations at 773 nm and 855 nm via the same source and collector. We measure the human forearm, comprising biological tissues with mixed static and dynamic scattering, as well as a purely dynamic scattering phantom. Our primary finding is that forearm BFI increases from 773 nm to 855 nm, though the magnitude of this increase varies across subjects (23% ± 19% for N = 3). However, BFI is wavelength-independent in the purely dynamic scattering phantom. From these data, we infer that the wavelength-dependence of BFI arises from the wavelength-dependence of the dynamic scattering probability. This inference is further supported by RBC scattering literature. Our secondary finding is that the higher-order cumulant terms of the mean squared displacement (MSD) of RBCs are significant, but decrease with wavelength. Thus, laser speckle and related modalities should exercise caution when interpreting field autocorrelations.

Diffuse optical detection of global cerebral ischemia in an adult porcine model

Rapid screening for ischemic strokes in prehospital settings may improve patient outcomes by allowing early deployment of vascular recanalization therapies. However, there are no low-cost and convenient methods that can assess ischemic strokes in such a setting. Diffuse correlation spectroscopy (DCS) is a promising method for continuous, noninvasive transcranial monitoring of cerebral blood flow. In this study, we used a DCS system to detect cerebral hemodynamics before and after acute ischemic stroke in pigs. Seven adult porcines were chosen to establish ischemic stroke models via bilateral common carotid artery ligation (n = 5) or air emboli (n = 2). The results showed a significant difference in blood flow index (BFI) between the normal and ischemic groups. Relative blood flow index (rBFI) exhibited excellent results. Therefore, the diffuse optical method can assess the hemodynamic changes in acute cerebral ischemic stroke onset in pigs, and rBFI may be a promising biomarker for identifying cerebral ischemic stroke.

Dynamic cerebral autoregulation measured by diffuse correlation spectroscopy

Dynamic cerebral autoregulation (dCA) can be derived from spontaneous oscillations in arterial blood pressure (ABP) and cerebral blood flow (CBF). Transcranial Doppler (TCD) measures CBF-velocity and is commonly used to assess dCA. Diffuse correlation spectroscopy (DCS) is a promising optical technique for non-invasive CBF monitoring, so here we aimed to validate DCS as a tool for quantifying dCA. In 33 healthy adults and 17 acute ischemic stroke patients, resting-state hemodynamic were monitored simultaneously with high-speed (20 Hz) DCS and TCD. dCA parameters were calcaulated by a transfer function analysis using a Fourier decomposition of ABP and CBF (or CBF-velocity). Strong correlation was found between DCS and TCD measured gain (magnitude of regulation) in healthy volunteers (r = 0.73, p < 0.001) and stroke patients (r = 0.76, p = 0.003). DCS-gain retained strong test-retest reliability in both groups (ICC 0.87 and 0.82, respectively). DCS and TCD-derived phase (latency of regulation) did not significantly correlate in healthy volunteers (r = 0.12, p = 0.50) but moderately correlated in stroke patients (r = 0.65, p = 0.006). DCS-derived phase was reproducible in both groups (ICC 0.88 and 0.90, respectively). High-frequency DCS is a promising non-invasive bedside technique that can be leveraged to quantify dCA from resting-state data, but the discrepancy between TCD and DCS-derived phase requires further investigation.

Assessment of dynamic cerebral autoregulation in near-infrared spectroscopy using short channels: A feasibility study in acute ischemic stroke patients

Introduction

In acute ischemic stroke, progressive impairment of cerebral autoregulation (CA) is frequent and associated with unfavorable outcomes. Easy assessment of cerebral blood flow and CA in stroke units bedside tools like near-infrared spectroscopy (NIRS) might improve early detection of CA deterioration. This study aimed to assess dynamic CA with multichannel CW-NIRS in acute ischemic stroke (AIS) patients compared to agematched healthy controls.

Methods

CA reaction was amplified by changes in head of bed position. Long- and short channels were used to monitor systemic artery pressure- and intracranial oscillations simultaneously. Gain and phase shift in spontaneous low- and very low-frequency oscillations (LFO, VLFO) of blood pressure were assessed.

Results

A total of 54 participants, 27 with AIS and 27 age-matched controls were included. Gain was significantly lower in the AIS group in the LFO range (i) when the upper body was steadily elevated to 30. and (ii) after its abrupt elevation to 30°. No other differences were found between groups.

Discussion

This study demonstrates the feasibility of NIRS short channels to measure CA in AIS patients in one single instrument. A lower gain in AIS might indicate decreased CA activity in this pilot study, but further studies investigating the role of NIRS short channels in AIS are needed.

Intraoperative Cerebral Hemodynamic Monitoring during Carotid Endarterectomy via Diffuse Correlation Spectroscopy and Near-Infrared Spectroscopy

Objective: This pilot study aims to show the feasibility of noninvasive and real-time cerebral hemodynamic monitoring during carotid endarterectomy (CEA) via diffuse correlation spectroscopy (DCS) and near-infrared spectroscopy (NIRS). Methods: Cerebral blood flow index (CBFi) was measured unilaterally in seven patients and bilaterally in seventeen patients via DCS. In fourteen patients, hemoglobin oxygenation changes were measured bilaterally and simultaneously via NIRS. Cerebral autoregulation (CAR) and cerebrovascular resistance (CVR) were estimated using CBFi and arterial blood pressure data. Further, compensatory responses to the ipsilateral hemisphere were investigated at different contralateral stenosis levels. Results: Clamping of carotid arteries caused a sharp increase of CVR (~70%) and a marked decrease of ipsilateral CBFi (57%). From the initial drop, we observed partial recovery in CBFi, an increase of blood volume, and a reduction in CVR in the ipsilateral hemisphere. There were no significant changes in compensatory responses between different contralateral stenosis levels as CAR was intact in both hemispheres throughout the CEA phase. A comparison between hemispheric CBFi showed lower ipsilateral levels during the CEA and post-CEA phases (p < 0.001, 0.03). Conclusion: DCS alone or combined with NIRS is a useful monitoring technique for real-time assessment of cerebral hemodynamic changes and allows individualized strategies to improve cerebral perfusion during CEA by identifying different hemodynamic metrics.

Lawrence K, Tachtsidis I, Tong Y, Torricelli A, Urner T, Wabnitz H, Wolf M, Wolf U, Xu S, Yang C, Yodh AG, Yücel MA, Zhou W. Optical imaging and spectroscopy for the study of the human brain: status report

This report is the second part of a comprehensive two-part series aimed at reviewing an extensive and diverse toolkit of novel methods to explore brain health and function. While the first report focused on neurophotonic tools mostly applicable to animal studies, here, we highlight optical spectroscopy and imaging methods relevant to noninvasive human brain studies. We outline current state-of-the-art technologies and software advances, explore the most recent impact of these technologies on neuroscience and clinical applications, identify the areas where innovation is needed, and provide an outlook for the future directions.

Cerebral Blood Flow Hemispheric Asymmetry in Comatose Adults Receiving Extracorporeal Membrane Oxygenation

Peripheral veno-arterial extracorporeal membrane oxygenation (ECMO) artificially oxygenates and circulates blood retrograde from the femoral artery, potentially exposing the brain to asymmetric perfusion. Though ECMO patients frequently experience brain injury, neurologic exams and imaging are difficult to obtain. Diffuse correlation spectroscopy (DCS) non-invasively measures relative cerebral blood flow (rBF) at the bedside using an optical probe on each side of the forehead. In this study we observed interhemispheric rBF differences in response to mean arterial pressure (MAP) changes in adult ECMO recipients. We recruited 13 subjects aged 21–78 years (7 with cardiac arrest, 4 with acute heart failure, and 2 with acute respiratory distress syndrome). They were dichotomized via Glasgow Coma Scale Motor score (GCS-M) into comatose (GCS-M ≤ 4; n = 4) and non-comatose (GCS-M > 4; n = 9) groups. Comatose patients had greater interhemispheric rBF asymmetry (ASYM_rBF) vs. non-comatose patients over a range of MAP values (29 vs. 11%, p = 0.009). ASYM_rBF in comatose patients resolved near a MAP range of 70–80 mmHg, while rBF remained symmetric through a wider MAP range in non-comatose patients. Correlations between post-oxygenator pCO₂ or pH vs. ASYM_rBF were significantly different between comatose and non-comatose groups. Our findings indicate that comatose patients are more likely to have asymmetric cerebral perfusion.

Complete head cerebral sensitivity mapping for diffuse correlation spectroscopy using subject-specific magnetic resonance imaging models

We characterize cerebral sensitivity across the entire adult human head for diffuse correlation spectroscopy, an optical technique increasingly used for bedside cerebral perfusion monitoring. Sixteen subject-specific magnetic resonance imaging-derived head models were used to identify high sensitivity regions by running Monte Carlo light propagation simulations at over eight hundred uniformly distributed locations on the head. Significant spatial variations in cerebral sensitivity, consistent across subjects, were found. We also identified correlates of such differences suitable for real-time assessment. These variations can be largely attributed to changes in extracerebral thickness and should be taken into account to optimize probe placement in experimental settings.

Fast diffuse correlation spectroscopy with a low-cost, fiber-less embedded diode laser

Diffuse correlation spectroscopy (DCS), a popular optical technique for fast noninvasive measurement of blood flow, is commonly implemented using expensive fiber-coupled long coherence length laser systems. Here, we report the development of a portable and fiber-less approach that can be used as a low-cost alternative to illuminate tissue in DCS instruments. We validate the accuracy and noise characteristics of the fiber-less DCS laser source, by comparisons against traditional DCS light sources, with experiments on controlled tissue-simulating phantoms and in humans.

Accuracy of diffuse correlation spectroscopy measurements of cerebral blood flow when using a three-layer analytical model

Diffuse correlation spectroscopy (DCS) is a non-invasive optical technology for the assessment of an index of cerebral blood flow (CBFi). Analytical methods that model the head as a three-layered medium (i.e., scalp, skull, brain) are becoming more commonly used to minimize the contribution of extracerebral layers to the measured DCS signal in adult cerebral blood flow studies. However, these models rely on a priori knowledge of layer optical properties and thicknesses. Errors in these values can lead to errors in the estimation of CBFi, although the magnitude of this influence has not been rigorously characterized. Herein, we investigate the accuracy of measuring cerebral blood flow with a three-layer model when errors in layer optical properties or thicknesses are present. Through a series of in silico experiments, we demonstrate that CBFi is highly sensitive to errors in brain optical properties and skull and scalp thicknesses. Relative changes in CBFi are less sensitive to optical properties but are influenced by errors in layer thickness. Thus, when using the three-layer model, accurate estimation of scalp and skull thickness are required for reliable results.

Performance assessment of laser sources for time-domain diffuse correlation spectroscopy

Time-domain diffuse correlation spectroscopy (TD-DCS) is an emerging optical technique that enables noninvasive measurement of microvascular blood flow with photon path-length resolution. In TD-DCS, a picosecond pulsed laser with a long coherence length, adequate illumination power, and narrow instrument response function (IRF) is required, and satisfying all these features is challenging. To this purpose, in this study we characterized the performance of three different laser sources for TD-DCS. First, the sources were evaluated based on their emission spectrum and IRF. Then, we compared the signal-to-noise ratio and the sensitivity to velocity changes of scattering particles in a series of phantom measurements. We also compared the results for in vivo measurements, performing an arterial occlusion protocol on the forearm of three adult subjects. Overall, each laser has the potential to be successfully used both for laboratory and clinical applications. However, we found that the effects caused by the IRF are more significant than the effect of a limited temporal coherence.

The slope of cerebral oxyhemoglobin oscillation is associated with vascular reserve capacity in large artery steno-occlusion

Inadequate cerebral perfusion is a risk factor for cerebral ischemia in patients with large artery steno-occlusion. We investigated whether prefrontal oxyhemoglobin oscillation (ΔHbO₂, 0.6-2 Hz) was associated with decreased vascular reserve in patients with steno-occlusion in the large anterior circulation arteries. Thirty-six patients with steno-occlusion in the anterior circulation arteries (anterior cerebral artery, middle cerebral artery, and internal carotid artery) were included and compared to thirty-six control subjects. Patients were categorized into two groups (deteriorated vascular reserve vs. preserved vascular reserve) based on the results of Diamox single- photon emission computed tomography imaging. HbO₂ data were collected using functional near-infrared spectroscopy. The slope of ΔHbO₂ and the ipsilateral/contralateral slope ratio of ΔHbO₂ were analyzed. Among the included patients (n = 36), 25 (69.4%) had deteriorated vascular reserve. Patients with deteriorated vascular reserve had a significantly higher average slope of ΔHbO₂ on the ipsilateral side (5.01 ± 2.14) and a higher ipsilateral/contralateral ratio (1.44 ± 0.62) compared to those with preserved vascular reserve (3.17 ± 1.36, P = 0.014; 0.93 ± 0.33, P = 0.016, respectively) or the controls (3.82 ± 1.69, P = 0.019; 0.94 ± 0.29, P = 0.001). The ipsilateral/contralateral ΔHbO₂ ratio could be used as a surrogate for vascular reserve in patients with severe steno-occlusion in the anterior circulation arteries.

Blood flow response to orthostatic challenge identifies signatures of the failure of static cerebral autoregulation in patients with cerebrovascular disease

Background

The cortical microvascular cerebral blood flow response (CBF) to different changes in head-of-bed (HOB) position has been shown to be altered in acute ischemic stroke (AIS) by diffuse correlation spectroscopy (DCS) technique. However, the relationship between these relative ΔCBF changes and associated systemic blood pressure changes has not been studied, even though blood pressure is a major driver of cerebral blood flow.

Methods

Transcranial DCS data from four studies measuring bilateral frontal microvascular cerebral blood flow in healthy controls (n = 15), patients with asymptomatic severe internal carotid artery stenosis (ICA, n = 27), and patients with acute ischemic stroke (AIS, n = 72) were aggregated. DCS-measured CBF was measured in response to a short head-of-bed (HOB) position manipulation protocol (supine/elevated/supine, 5 min at each position). In a sub-group (AIS, n = 26; ICA, n = 14; control, n = 15), mean arterial pressure (MAP) was measured dynamically during the protocol.

Results

After elevated positioning, DCS CBF returned to baseline supine values in controls (p = 0.890) but not in patients with AIS (9.6% [6.0,13.3], mean 95% CI, p < 0.001) or ICA stenosis (8.6% [3.1,14.0], p = 0.003)). MAP in AIS patients did not return to baseline values (2.6 mmHg [0.5, 4.7], p = 0.018), but in ICA stenosis patients and controls did. Instead ipsilesional but not contralesional CBF was correlated with MAP (AIS 6.0%/mmHg [− 2.4,14.3], p = 0.038; ICA stenosis 11.0%/mmHg [2.4,19.5], p < 0.001).

Conclusions

The observed associations between ipsilateral CBF and MAP suggest that short HOB position changes may elicit deficits in cerebral autoregulation in cerebrovascular disorders. Additional research is required to further characterize this phenomenon.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12883-021-02179-8.

Dissecting the microvascular contributions to diffuse correlation spectroscopy measurements of cerebral hemodynamics using optical coherence tomography angiography

Significance: Diffuse correlation spectroscopy (DCS) is an emerging noninvasive, diffuse optical modality that purportedly enables direct measurements of microvasculature blood flow. Functional optical coherence tomography angiography (OCT-A) can resolve blood flow in vessels as fine as capillaries and thus has the capability to validate key attributes of the DCS signal. Aim: To characterize activity in cortical vasculature within the spatial volume that is probed by DCS and to identify populations of blood vessels that are most representative of the DCS signals. Approach: We performed simultaneous measurements of somatosensory-evoked cerebral blood flow in mice in vivo using both DCS and OCT-A. Results: We resolved sensory-evoked blood flow in the somatosensory cortex with both modalities. Vessels with diameters smaller than 10    μ m featured higher peak flow rates during the initial poststimulus positive increase in flow, whereas larger vessels exhibited considerably larger magnitude of the subsequent undershoot. The simultaneously recorded DCS waveforms correlated most highly with flow in the smallest vessels, yet featured a more prominent undershoot. Conclusions: Our direct, multiscale, multimodal cross-validation measurements of functional blood flow support the assertion that the DCS signal preferentially represents flow in microvasculature. The significantly greater undershoot in DCS, however, suggests a more spatially complex relationship to flow in cortical vasculature during functional activation.

Towards detection of brain injury using multimodal non-invasive neuromonitoring in adults undergoing extracorporeal membrane oxygenation

Extracorporeal membrane oxygenation (ECMO) is a form of cardiopulmonary bypass that provides life-saving support to critically ill patients whose illness is progressing despite maximal conventional support. Use in adults is expanding, however neurological injuries are common. Currently, the existing brain imaging tools are a snapshot in time and require high-risk patient transport. Here we assess the feasibility of measuring diffuse correlation spectroscopy, transcranial Doppler ultrasound, electroencephalography, and auditory brainstem responses at the bedside, and developing a cerebral autoregulation metric. We report preliminary results from two patients, demonstrating feasibility and laying the foundation for future studies monitoring neurological health during ECMO.

The effect of head positioning on cerebral hemodynamics: Experiences in mild ischemic stroke

BACKGROUND AND PURPOSE

It is generally agreed that optimal head positioning is an important consideration in acute stroke management regime. However, there is limited literature investigating the effect of head positioning changes on cerebrovascular physiology in acute ischemic stroke (AIS). We aim to assess cerebral autoregulation (CA) and associated hemodynamic responses during gradual head positioning (GHP) changes, between AIS and controls.

METHODS

Cerebral blood flow velocity (CBFV, transcranial Doppler), blood pressure (BP, Finometer) and end-tidal CO₂ (capnography) were recorded between lying flat (0°) and sitting up (30°) head position, in 16 controls (8 women, mean age 57 ± 16 yrs) and 15 AIS patients (7 women, 69 ± 8 yrs). AIS patients carried out three visits at 13.3 ± 6.9 h, 4.8 ± 3.2 days and 93.9 ± 11.5 days from symptom onset, respectively.

RESULTS

AIS patients were significantly hypertensive (p = 0.005), hypocapnic (p < 0.001), and had lower CBFV (p = 0.02) compared to controls, in both head positions. When comparing 5-min FLAT to SIT head position, reductions in BP (both AIS and controls, p < 0.001) and CBFV (controls only: dominant hemisphere p = 0.001 and non-dominant hemisphere p = 0.05) were demonstrated. Of note, a reduction in autoregulation index was observed in AIS, after 5-min SIT head positioning, at all 3 visits (p = 0.018).

CONCLUSION

Key hemodynamic changes were demonstrated when the head position changes from 5-min FLAT to SIT head position (GHP) in mildly affected stroke patients. Importantly, these were associated with non-significant changes in CBFV but reduced measures of CA following AIS, which may be relevant in determining the optimal head position and the ideal timing of mobilisation. Clinical Trial Registration - URL: http://www.clinicaltrials.gov. Unique Identifier: NCT02932540.

Hematocrit significantly confounds diffuse correlation spectroscopy measurements of blood flow

Diffuse correlation spectroscopy (DCS) is an optical modality used to measure an index of blood flow in biological tissue. This blood flow index depends on both the red blood cell flow rate and density (i.e., hematocrit), although the functional form of hematocrit dependence is not well delineated. Herein, we develop and validate a novel tissue-simulating phantom containing hundreds of microchannels to investigate the influence of hematocrit on blood flow index. For a fixed flow rate, we demonstrate a significant inverse relationship between hematocrit and blood flow index that must be accounted for to accurately estimate blood flow under anemic conditions.

Cerebral haemodynamics with head position changes post-ischaemic stroke: A systematic review and meta-analysis

The effects of upright postures on the cerebral circulation early post-ischaemic stroke are not fully understood. We conducted a systematic review and meta-analysis to investigate the effects of head positioning on cerebral haemodynamics assessed by imaging methods post-ischaemic stroke. Of the 21 studies included (n = 529), 15 used transcranial Doppler. Others used near-infrared, diffuse correlation spectroscopy and nuclear medicine modalities. Most tested head positions between 0° and 45°. Seventeen studies reported changes in CBF parameters (increase at lying-flat or decrease at more upright) in the ischaemic hemisphere with position change. However, great variability was found and risk of bias was high in many studies. Pooled data of two studies ≤24 h (n = 28) showed a mean increase in cerebral blood flow (CBF) velocity of 8.5 cm/s in the ischaemic middle cerebral artery (95%CI,-2.2-19.3) from 30° to 0°. The increase found ≤48 h (n = 50) was of 2.3 cm/s (95%CI,-4.6-9.2), while ≤7 days (n = 38) was of 8.4 cm/s (95%CI, 1.8-15). Few very early studies (≤2 days) tested head positions greater than 30° and were unable to provide information about the response of acute stroke patients to upright postures (sitting, standing). These postures are part of current clinical practice and knowledge on their effects on cerebral haemodynamics is required.

Beyond diffuse correlations: deciphering random flow in time-of-flight resolved light dynamics

Diffusing wave spectroscopy (DWS) and diffuse correlation spectroscopy (DCS) can assess blood flow index (BFI) of biological tissue with multiply scattered light. Though the main biological function of red blood cells (RBCs) is advection, in DWS/DCS, RBCs are assumed to undergo Brownian motion. To explain this discrepancy, we critically examine the cumulant approximation, a major assumption in DWS/DCS. We present a precise criterion for validity of the cumulant approximation, and in realistic tissue models, identify conditions that invalidate it. We show that, in physiologically relevant scenarios, the first cumulant term for random flow and second cumulant term for Brownian motion alone can cancel each other. In such circumstances, assuming pure Brownian motion of RBCs and the first cumulant approximation, a routine practice in DWS/DCS of BFI, can yield good agreement with data, but only because errors due to two incorrect assumptions cancel out. We conclude that correctly assessing random flow from scattered light dynamics requires going beyond the cumulant approximation and propose a more accurate model to do so.

Assessment of the brain ischemia during orthostatic stress and lower body negative pressure in air force pilots by near-infrared spectroscopy

A methodology for the assessment of the cerebral hemodynamic reaction to normotensive hypovolemia, reduction in cerebral perfusion and orthostatic stress leading to ischemic hypoxia and reduced muscular tension is presented. Most frequently, the pilots of highly maneuverable aircraft are exposed to these phenomena. Studies were carried out using the system consisting of a chamber that generates low pressure around the lower part of the body - LBNP (lower body negative pressure) placed on the tilt table. An in-house developed 6-channel NIRS system operating at 735 and 850 nm was used in order to assess the oxygenation of the cerebral cortex, based on measurements of diffusely reflected light in reflectance geometry. The measurements were carried out on a group of 12 active pilots and cadets of the Polish Air Force Academy and 12 healthy volunteers. The dynamics of changes in cerebral oxygenation was evaluated as a response to LBNP stimuli with a simultaneous rapid change of the tilt table angle. Parameters based on calculated changes of total hemoglobin concentration were proposed allowing to evaluate differences in reactions observed in control subjects and pilots/cadets. The results of orthogonal partial least squares-discriminant analysis based on these parameters show that the subjects can be classified into their groups with 100% accuracy.

Enhancement of diffuse correlation spectroscopy tissue blood flow measurement by acoustic radiation force

The current research on acousto-optic effects focuses on the interactions of acoustic waves with static optical properties rather than dynamic features such as tissue blood flow. Diffuse correlation spectroscopy (DCS) is an emerging technology capable of direct measurements of tissue blood flow by probing the movements of red blood cells (RBCs). In this article, we investigated the relations between the acoustic radiation force (ARF) and ultrasonic patterns by the finite element simulations. Based on the outcomes, we experimentally explored how the ultrasound-generated ARF enhance the DCS data as well as the blood flow measurements. The results yield the optimal pattern to generate ARF and elucidate the relations between the ultrasonic emission and flow elevations. The flow modality combing the DCS with ARF modulations, which was proposed in this study for the first time, would promote disease diagnosis and therapeutic assessment in the situation wherein the blood flow contrast between healthy and pathological tissues is insufficient.

Transient Laterality of Cerebral Oxygenation Changes in Response to Head-of-Bed Manipulation in Acute Ischemic Stroke

Background: Cerebral oxygenation monitoring provides important information for optimizing individualized management in patients with acute ischemic stroke (AIS). Although changes in cerebral oxygenation are known to occur in response to head-of-bed (HOB) elevation within 72 h after onset, changes in cerebral oxygenation during stroke recovery are unclear. We compared changes in total- (tHb), oxygenated- (HbO2), and deoxygenated-hemoglobin (deoxyHb) concentrations in response to HOB manipulation between the timeframes within 72 h and 7–10 days after AIS onset. Methods: We measured forehead ΔtHb, ΔHbO2, and ΔdeoxyHb in response to HOB elevation (30°) within 72 h (first measurement) and 7–10 days (second measurement) after AIS onset using time-resolved near-infrared spectroscopy. Results: We enrolled 30 participants (mean age 72.8 ± 11.3 years; 13 women) with a first AIS. There were no significant differences in ΔtHb, ΔHbO2, or ΔdeoxyHb measurements on the infarct or contra-infarct side. At the first measurement, ΔtHb, ΔHbO2, and ΔdeoxyHb measured on the contra-infarct side did not correlate with those measured on the infarct side: ΔtHb (r = 0.114, p = 0.539); ΔHbO2 (r = 0.143, p = 0.440); ΔdeoxyHb (r = 0.227, p = 0.221). Notably, at the second measurement, correlation coefficients of ΔtHb and ΔHbO2 between the contra-infarct and infarct sides were statistically significant: ΔtHb (r = 0.491, p = 0.008); ΔHbO2 (r = 0.479, p = 0.010); ΔdeoxyHb (r = 0.358, p = 0.054). Conclusion: Although changes in cerebral oxygenation in response to HOB elevation had a laterality difference between hemispheres within 72 h of AIS onset, the difference had decreased, at least partially, 7–10 days after AIS onset.

Perfusion Enhancement with Respiratory Impedance After Stroke (PERI-Stroke)

Intrathoracic pressure influences cardiac output and may affect cerebral blood flow (CBF). We aimed to quantify the cerebral hemodynamic response to intrathoracic pressure reduction in patients with acute ischemic stroke using a noninvasive respiratory impedance (RI) device. We assessed low-level (6 cm H2O) and high-level (12 cm H2O) RI in 17 spontaneously breathing patients within 72 h of anterior circulation acute ischemic stroke. Average age was 65 years, and 35% were female. Frontal lobe tissue perfusion and middle cerebral artery velocity (MCAv) were continuously monitored with optical diffuse correlation spectroscopy (DCS) and transcranial Doppler ultrasound, respectively. High-level RI resulted in a 7% increase in MCAv (p = 0.004). MCAv varied across all studied levels (baseline vs low-level vs high-level, p = 0.006), with a significant test of trend (p = 0.002). Changes were not seen in DCS measured tissue perfusion by nonparametric pairwise comparison. Mixed effects regression analysis identified a small increase in both MCAv (low-level RI: β 2.1, p < 0.001; high-level RI: β 5.0, p < 0.001) and tissue-level flow (low-level RI: β 5.4, p < 0.001; high-level RI: β 5.9, p < 0.001). There was a small increase in mean arterial pressure during low-level and high-level RI, 4% (p = 0.013) and 4% (p = 0.017), respectively. End-tidal CO2 remained stable throughout the protocol. RI was well tolerated. Manipulating intrathoracic pressure via noninvasive RI was safe and produced a small but measurable increase in cerebral perfusion in acute ischemic stroke patients. Future studies are warranted to assess whether RI is feasible and tolerable for prolonged use in hyperacute stroke management.

Diffuse Correlation Spectroscopy Analysis Implemented on a Field Programmable Gate Array

Diffusive correlation spectroscopy (DCS) is an emerging optical technique that measures blood perfusion in deep tissue. In a DCS measurement, temporal changes in the interference pattern of light, which has passed through tissue, are quantified by an autocorrelation function. This autocorrelation function is further parameterized through a non-linear curve fit to a solution to the diffusion equation for coherence transport. The computational load for this non-linear curve fitting is a barrier for deployment of DCS for clinical use, where real-time results, as well as instrument size and simplicity, are important considerations. We have mitigated this computational bottleneck through development of a hardware analyzer for DCS. This analyzer implements the DCS curving fitting algorithm on digital logic circuit using Field Programmable Gate Array (FPGA) technology. The FPGA analyzer is more efficient than a typical software analysis solution. The analyzer module can be easily duplicated for processing multiple channels of DCS data in real-time. We have demonstrated the utility of this analyzer in pre-clinical large animal studies of spinal cord ischemia. In combination with previously described FPGA implementations of auto-correlators, this hardware analyzer can provide a complete device-on-a-chip solution for DCS signal processing. Such a component will enable new DCS applications demanding mobility and real-time processing.

Cerebral Blood Flow Response During Bolus Normal Saline Infusion After Ischemic Stroke

GOALS

We quantified cerebral blood flow response to a 500 cc bolus of 0.9%% normal saline (NS) within 96 hours of acute ischemic stroke (AIS) using diffuse correlation spectroscopy (DCS).

MATERIALS AND METHODS

Subjects with AIS in the anterior, middle, or posterior cerebral artery territory were enrolled within 96 hours of symptom onset. DCS measured relative cerebral blood flow (rCBF) in the bilateral frontal lobes for 15 minutes at rest (baseline), during a 30-minute infusion of 500 cc NS (bolus), and for 15 minutes after completion (post-bolus). Mean rCBF for each time period was calculated for individual subjects and median rCBF for the population was compared between time periods. Linear regression was used to evaluate for associations between rCBF and clinical features.

RESULTS

Among 57 subjects, median rCBF (IQR) increased relative to baseline in the ipsilesional hemisphere by 17% (-2.0%, 43.1%), P< 0.001, and in the contralesional hemisphere by 13.3% (-4.3%, 36.0%), P < .004. No significant associations were found between ipsilesional changes in rCBF and age, race, infarct size, infarct location, presence of large vessel stenosis, NIH stroke scale, or symptom duration.

CONCLUSION

A 500 cc bolus of .9% NS produced a measurable increase in rCBF in both the affected and nonaffected hemispheres. Clinical features did not predict rCBF response.

Clinical Brain Monitoring with Time Domain NIRS: A Review and Future Perspectives

Near-infrared spectroscopy (NIRS) is an optical technique that can measure brain tissue oxygenation and haemodynamics in real-time and at the patient bedside allowing medical doctors to access important physiological information. However, despite this, the use of NIRS in a clinical environment is hindered due to limitations, such as poor reproducibility, lack of depth sensitivity and poor brain-specificity. Time domain NIRS (or TD-NIRS) can resolve these issues and offer detailed information of the optical properties of the tissue, allowing better physiological information to be retrieved. This is achieved at the cost of increased instrument complexity, operation complexity and price. In this review, we focus on brain monitoring clinical applications of TD-NIRS. A total of 52 publications were identified, spanning the fields of neonatal imaging, stroke assessment, traumatic brain injury (TBI) assessment, brain death assessment, psychiatry, peroperative care, neuronal disorders assessment and communication with patient with locked-in syndrome. In all the publications, the advantages of the TD-NIRS measurement to (1) extract absolute values of haemoglobin concentration and tissue oxygen saturation, (2) assess the reduced scattering coefficient, and (3) separate between extra-cerebral and cerebral tissues, are highlighted; and emphasize the utility of TD-NIRS in a clinical context. In the last sections of this review, we explore the recent developments of TD-NIRS, in terms of instrumentation and methodologies that might impact and broaden its use in the hospital.

Microvascular cerebral blood flow fluctuations in association with apneas and hypopneas in acute ischemic stroke

In a pilot study on acute ischemic stroke (AIS) patients, unexpected periodic fluctuations in microvascular cerebral blood flow (CBF) had been observed. Motivated by the relative lack of information about the impact of the emergence of breathing disorders in association with stroke on cerebral hemodynamics, we hypothesized that these fluctuations are due to apneic and hypopneic events. A total of 28 patients were screened within the first week after stroke with a pulse oximeter. Five (18%) showed fluctuations of arterial blood oxygen saturation ( ≥ 3 % ) and were included in the study. Near-infrared diffuse correlation spectroscopy (DCS) was utilized bilaterally to measure the frontal lobe CBF alongside respiratory polygraphy. Biphasic CBF fluctuations were observed with a bilateral increase of 27.1 % ± 17.7 % and 29.0 % ± 17.4 % for the ipsilesional and contralesional hemispheres, respectively, and a decrease of - 19.3 % ± 9.1 % and - 21.0 % ± 8.9 % for the ipsilesional and contralesional hemispheres, respectively. The polygraph revealed that, in general, the fluctuations were associated with apneic and hypopneic events. This study motivates us to investigate whether the impact of altered respiratory patterns on cerebral hemodynamics can be detrimental in AIS patients.

Portable Near-Infrared Technologies and Devices for Noninvasive Assessment of Tissue Hemodynamics

Tissue hemodynamics, including the blood flow, oxygenation, and oxygen metabolism, are closely associated with many diseases. As one of the portable optical technologies to explore human physiology and assist in healthcare, near-infrared diffuse optical spectroscopy (NIRS) for tissue oxygenation measurement has been developed for four decades. In recent years, a dynamic NIRS technology, namely, diffuse correlation spectroscopy (DCS), has been emerging as a portable tool for tissue blood flow measurement. In this article, we briefly describe the basic principle and algorithms for static NIRS and dynamic NIRS (i.e., DCS). Then, we elaborate on the NIRS instrumentation, either commercially available or custom-made, as well as their applications to physiological studies and clinic. The extension of NIRS/DCS from spectroscopy to imaging was depicted, followed by introductions of advanced algorithms that were recently proposed. The future prospective of the NIRS/DCS and their feasibilities for routine utilization in hospital is finally discussed.

Early microvascular cerebral blood flow response to head-of-bed elevation is related to outcome in acute ischemic stroke

BACKGROUND AND AIMS

Previously, microvascular cerebral blood flow (CBF) response to a mild head-of-bed (HOB) elevation has been shown to be altered in acute ischemic stroke (AIS) by diffuse correlation spectroscopy (DCS). We have hypothesized that early CBF response is related to the functional outcome.

METHODS

Patients with a non-lacunar AIS in the anterior circulation were monitored by DCS to measure relative CBF (ΔrCBF) on the frontal lobes bilaterally during a 0°-30° HOB elevation at early (≤ 12) or late (> 12) hours from symptom onset. National Institutes of Health Stroke Scale (NIHSS) scores were recorded at baseline at 24 and at 48 h. Functional outcome was measured by the modified Rankin Scale (mRS) at 3 months.

RESULTS

Thirty-eight (n = 38) AIS patients [baseline NIHSS = 19 (interquartile range: 16, 21)] were studied. ΔrCBF decreased similarly in both hemispheres (p = 0.4) when HOB was elevated and was not associated with baseline and follow-up NIHSS scores or patient demographics. At the early phase (n = 17), a lower or paradoxical ΔrCBF response to HOB elevation was associated with an unfavorable functional outcome (mRS > 2) in the ipsilesional (but not in the contralesional) hemisphere (p = 0.010). ΔrCBF response in the late acute phase was not related to mRS.

CONCLUSIONS

Early CBF response to mild HOB elevation in the ipsilesional hemisphere is related to functional outcome. Further studies may enable optical monitoring at the bedside to individualize management strategies in the early phase of AIS.

A Systemic Review of Functional Near-Infrared Spectroscopy for Stroke: Current Application and Future Directions

Background: Survivors of stroke often experience significant disability and impaired quality of life. The recovery of motor or cognitive function requires long periods. Neuroimaging could measure changes in the brain and monitor recovery process in order to offer timely treatment and assess the effects of therapy. A non-invasive neuroimaging technique near-infrared spectroscopy (NIRS) with its ambulatory, portable, low-cost nature without fixation of subjects has attracted extensive attention. Methods: We conducted a comprehensive literature review in order to review the use of NIRS in stroke or post-stroke patients in July 2018. NCBI Pubmed database, EMBASE database, Cochrane Library and ScienceDirect database were searched. Results: Overall, we reviewed 66 papers. NIRS has a wide range of application, including in monitoring upper limb, lower limb recovery, motor learning, cortical function recovery, cerebral hemodynamic changes, cerebral oxygenation, as well as in therapeutic method, clinical researches, and evaluation of the risk for stroke. Conclusions: This study provides a preliminary evidence of the application of NIRS in stroke patients as a monitoring, therapeutic, and research tool. Further studies could give more emphasize on the combination of NIRS with other techniques and its utility in the prevention of stroke.

Time-domain near-infrared spectroscopy in acute ischemic stroke patients

Large vessel occlusion (LVO) stroke might cause different degrees of hemodynamic impairment that affects microcirculation and contributes to metabolic derangement. Time-domain near-infrared spectroscopy (TD-NIRS) estimates the oxygenation of microcirculation of cerebral outer layers. We measure hemoglobin species and tissue oxygen saturation ( StO 2 ) of anterior circulation stroke patients, classified as LVO or lacunar, and assess the differences compared with controls and according to LVO recanalization status. Fiducial markers categorize the brain region below each TD-NIRS probe as ischemic or nonstroke areas. The study includes 47 consecutive acute ischemic stroke patients and 35 controls. The ischemic area has significantly higher deoxy-hemoglobin (HbR) and total hemoglobin (HbT) compared with controls in both recanalized and nonrecanalized patients but lower StO 2 only in recanalized patients. Recanalized patients have significantly lower mean StO 2 in the ipsilateral hemisphere compared with nonrecanalized patients. This is the first study to report TD-NIRS measurements in acute ischemic stroke patients. TD-NIRS is able to detect significant differences in hemoglobin species in LVO stroke compared with controls and according to recanalization status. This preliminary data might suggest that StO 2 can serve as a surrogate functional marker of the metabolic activity of rescued brain tissue.

Approaches to denoise the diffuse optical signals for tissue blood flow measurement

Various diseases are relevant to the abnormal blood flow in tissue. Diffuse correlation spectroscopy (DCS) is an emerging technology to extract the blood flow index (BFI) from light electric field temporal autocorrelation data. To account for tissue heterogeneity and irregular geometry, we developed an innovative DCS algorithm (i.e., the Nth order linear algorithm, or simply the NL algorithm) previously, in which the DCS signals are fully utilized through iterative linear regressions. Under the framework of NL algorithm, the BFI to be extracted is significantly influenced by the linear regression approach adopted. In this study, three approaches were proposed and evaluated for performing the iterative linear regressions, in order to understand what are the appropriate regression methods for BFI estimation. The three methods are least-squared minimization (L₂ norm), least-absolute minimization (L₁ norm) and support vector regression (SVR), where L₂ norm is a conventional approach to perform linear regression. L₁ norm and SVR are the approaches newly introduced here to process the DCS data. Computer simulations and the autocorrelation data collected from liquid phantom and human tissues are utilized to evaluate the three approaches. The results show that the best performance is achieved by the SVR approach in extracting the BFI values, with an error rate of 2.23% at 3.0 cm source-detector separation. The L₁ norm method gives a medium error of 2.81%. In contrast, the L₂ norm method leads to the largest error (3.93%) in extracting the BFI values. The outcomes derived from this study will be very helpful for the tissue blood flow measurements, which is critical for translating the DCS technology to the clinic.

Small separation diffuse correlation spectroscopy for measurement of cerebral blood flow in rodents

Diffuse correlation spectroscopy (DCS) has shown promise as a means to non-invasively measure cerebral blood flow in small animal models. Here, we characterize the validity of DCS at small source-detector reflectance separations needed for small animal measurements. Through Monte Carlo simulations and liquid phantom experiments, we show that DCS error increases as separation decreases, although error remains below 12% for separations > 0.2 cm. In mice, DCS measures of cerebral blood flow have excellent intra-user repeatability and strongly correlate with MRI measures of blood flow (R = 0.74, p<0.01). These results are generalizable to other DCS applications wherein short-separation reflectance geometries are desired.

Prolonged monitoring of cerebral blood flow and autoregulation with diffuse correlation spectroscopy in neurocritical care patients

Monitoring of cerebral blood flow (CBF) and autoregulation are essential components of neurocritical care, but continuous noninvasive methods for CBF monitoring are lacking. Diffuse correlation spectroscopy (DCS) is a noninvasive diffuse optical modality that measures a CBF index ( CBF i ) in the cortex microvasculature by monitoring the rapid fluctuations of near-infrared light diffusing through moving red blood cells. We tested the feasibility of monitoring CBF i with DCS in at-risk patients in the Neurosciences Intensive Care Unit. DCS data were acquired continuously for up to 20 h in six patients with aneurysmal subarachnoid hemorrhage, as permitted by clinical care. Mean arterial blood pressure was recorded synchronously, allowing us to derive autoregulation curves and to compute an autoregulation index. The autoregulation curves suggest disrupted cerebral autoregulation in most patients, with the severity of disruption and the limits of preserved autoregulation varying between subjects. Our findings suggest the potential of the DCS modality for noninvasive, long-term monitoring of cerebral perfusion, and autoregulation.

Cerebral autoregulatory performance and the cerebrovascular response to head-of-bed positioning in acute ischaemic stroke

Background and purpose

Cerebrovascular responses to head‐of‐bed positioning in patients with acute ischaemic stroke are heterogeneous, questioning the applicability of general recommendations on head positioning. Cerebral autoregulation is impaired to various extents after acute stroke, although it is unknown whether this affects cerebral perfusion during posture change. We aimed to elucidate whether the cerebrovascular response to head position manipulation depends on autoregulatory performance in patients with ischaemic stroke.

Methods

The responses of bilateral transcranial Doppler ultrasound‐determined cerebral blood flow velocity (CBFV) and local cerebral blood volume (CBV), assessed by near‐infrared spectroscopy of total hemoglobin tissue concentration ([total Hb]), to head‐of‐bed lowering from 30° to 0° were determined in 39 patients with acute ischaemic stroke and 17 reference subjects from two centers. Cerebrovascular autoregulatory performance was expressed as the phase difference of the arterial pressure‐to‐CBFV transfer function.

Results

Following head‐of‐bed lowering, CBV increased in the reference subjects only ([total Hb]: + 2.1 ± 2.0 vs. + 0.4 ± 2.6 μM; P < 0.05), whereas CBFV did not change in either group. CBV increased upon head‐of‐bed lowering in the hemispheres of patients with autoregulatory performance <50th percentile compared with a decrease in the hemispheres of patients with better autoregulatory performance ([total Hb]: +1.0 ± 1.3 vs. −0.5 ± 1.0 μM; P < 0.05). The CBV response was inversely related to autoregulatory performance (r = −0.68; P < 0.001) in the patients, whereas no such relation was observed for CBFV.

Conclusion

This study is the first to provide evidence that cerebral autoregulatory performance in patients with acute ischaemic stroke affects the cerebrovascular response to changes in the position of the head.

Cerebral vasoreactivity in response to a head-of-bed position change is altered in patients with moderate and severe obstructive sleep apnea

Motivation

Obstructive sleep apnea (OSA) can impair cerebral vasoreactivity and is associated with an increased risk of cerebrovascular disease. Unfortunately, an easy-to-use, non-invasive, portable monitor of cerebral vasoreactivity does not exist. Therefore, we have evaluated the use of near-infrared diffuse correlation spectroscopy to measure the microvascular cerebral blood flow (CBF) response to a mild head-of-bed position change as a biomarker for the evaluation of cerebral vasoreactivity alteration due to chronic OSA. Furthermore, we have monitored the effect of two years of continuous positive airway pressure (CPAP) treatment on the cerebral vasoreactivity.

Methodology

CBF was measured at different head-of-bed position changes (supine to 30° to supine) in sixty-eight patients with OSA grouped according to severity (forty moderate to severe, twenty-eight mild) and in fourteen control subjects without OSA. A subgroup (n = 13) with severe OSA was measured again after two years of CPAP treatment.

Results

All patients and controls showed a similar CBF response after changing position from supine to 30° (p = 0.819), with a median (confidence interval) change of -17.5 (-10.3, -22.9)%. However, when being tilted back to the supine position, while the control group (p = 0.091) and the mild patients with OSA (p = 0.227) recovered to the initial baseline, patients with moderate and severe OSA did not recover to the baseline (9.8 (0.8, 12.9)%, p < 0.001) suggesting altered cerebral vasoreactivity. This alteration was correlated with OSA severity defined by the apnea-hypopnea index, and with mean nocturnal arterial oxygen saturation. The CBF response was normalized after two years of CPAP treatment upon follow-up measurements.

Conclusion

In conclusion, microvascular CBF response to a head-of-bed challenge measured by diffuse correlation spectroscopy suggests that moderate and severe patients with OSA have altered cerebral vasoreactivity related to OSA severity. This may normalize after two years of CPAP treatment.

Dynamic autoregulation of cerebral blood flow measured non-invasively with fast diffuse correlation spectroscopy

Cerebral autoregulation (CA) maintains cerebral blood flow (CBF) in the presence of systemic blood pressure changes. Brain injury can cause loss of CA and resulting dysregulation of CBF, and the degree of CA impairment is a functional indicator of cerebral tissue health. Here, we demonstrate a new approach to noninvasively estimate cerebral autoregulation in healthy adult volunteers. The approach employs pulsatile CBF measurements obtained using high-speed diffuse correlation spectroscopy (DCS). Rapid thigh-cuff deflation initiates a chain of responses that permits estimation of rates of dynamic autoregulation in the cerebral microvasculature. The regulation rate estimated with DCS in the microvasculature (median: 0.26 s-1, inter quartile range: 0.19 s-1) agrees well (R = 0.81, slope = 0.9) with regulation rates measured by transcranial Doppler ultrasound (TCD) in the proximal vasculature (median: 0.28 s-1, inter quartile range: 0.10 s-1). We also obtained an index of systemic autoregulation in concurrently measured scalp microvasculature. Systemic autoregulation begins later than cerebral autoregulation and exhibited a different rate (0.55 s-1, inter quartile range: 0.72 s-1). Our work demonstrates the potential of diffuse correlation spectroscopy for bedside monitoring of cerebral autoregulation in the microvasculature of patients with brain injury.

Compact dual-mode diffuse optical system for blood perfusion monitoring in a porcine model of microvascular tissue flaps

In reconstructive surgery, the ability to detect blood flow interruptions to grafted tissue represents a critical step in preventing postsurgical complications. We have developed and pilot tested a compact, fiber-based device that combines two complimentary modalities-diffuse correlation spectroscopy (DCS) and diffuse reflectance spectroscopy-to quantitatively monitor blood perfusion. We present a proof-of-concept study on an in vivo porcine model (n=8). With a controllable arterial blood flow supply, occlusion studies (n=4) were performed on surgically isolated free flaps while the device simultaneously monitored blood flow through the supplying artery as well as flap perfusion from three orientations: the distal side of the flap and two transdermal channels. Further studies featuring long-term monitoring, arterial failure simulations, and venous failure simulations were performed on flaps that had undergone an anastomosis procedure (n=4). Additionally, benchtop verification of the DCS system was performed on liquid flow phantoms. Data revealed relationships between diffuse optical measures and state of occlusion as well as the ability to detect arterial and venous compromise. The compact construction of the device, along with its noninvasive and quantitative nature, would make this technology suitable for clinical translation.

Clinical applications of near-infrared diffuse correlation spectroscopy and tomography for tissue blood flow monitoring and imaging

OBJECTIVE

Blood flow is one such available observable promoting a wealth of physiological insight both individually and in combination with other metrics.

APPROACH

Near-infrared diffuse correlation spectroscopy (DCS) and, to a lesser extent, diffuse correlation tomography (DCT), have increasingly received interest over the past decade as noninvasive methods for tissue blood flow measurements and imaging. DCS/DCT offers several attractive features for tissue blood flow measurements/imaging such as noninvasiveness, portability, high temporal resolution, and relatively large penetration depth (up to several centimeters).

MAIN RESULTS

This review first introduces the basic principle and instrumentation of DCS/DCT, followed by presenting clinical application examples of DCS/DCT for the diagnosis and therapeutic monitoring of diseases in a variety of organs/tissues including brain, skeletal muscle, and tumor.

SIGNIFICANCE

Clinical study results demonstrate technical versatility of DCS/DCT in providing important information for disease diagnosis and intervention monitoring.

Non-Invasive Respiratory Impedance Enhances Cerebral Perfusion in Healthy Adults

Optimization of cerebral blood flow (CBF) is the cornerstone of clinical management in a number of neurologic diseases, most notably ischemic stroke. Intrathoracic pressure influences cardiac output and has the potential to impact CBF. Here, we aim to quantify cerebral hemodynamic changes in response to increased respiratory impedance (RI) using a non-invasive respiratory device. We measured cerebral perfusion under varying levels of RI (6 cm H₂O, 9 cm H₂O, and 12 cm H₂O) in 20 healthy volunteers. Simultaneous measurements of microvascular CBF and middle cerebral artery mean flow velocity (MFV), respectively, were performed with optical diffuse correlation spectroscopy and transcranial Doppler ultrasound. At a high level of RI, MFV increased by 6.4% compared to baseline (p = 0.004), but changes in cortical CBF were non-significant. In a multivariable linear regression model accounting for end-tidal CO₂, RI was associated with increases in both MFV (coefficient: 0.49, p < 0.001) and cortical CBF (coefficient: 0.13, p < 0.001), although the magnitude of the effect was small. Manipulating intrathoracic pressure via non-invasive RI was well tolerated and produced a small but measurable increase in cerebral perfusion in healthy individuals. Future studies in acute ischemic stroke patients with impaired cerebral autoregulation are warranted in order to assess whether RI is feasible as a novel non-invasive therapy for stroke.

Application of optical methods in the monitoring of traumatic brain injury: A review

We present an overview of the wide range of potential applications of optical methods for monitoring traumatic brain injury. The MEDLINE database was electronically searched with the following search terms: "traumatic brain injury," "head injury," or "head trauma," and "optical methods," "NIRS," "near-infrared spectroscopy," "cerebral oxygenation," or "cerebral oximetry." Original reports concerning human subjects published from January 1980 to June 2015 in English were analyzed. Fifty-four studies met our inclusion criteria. Optical methods have been tested for detection of intracranial lesions, monitoring brain oxygenation, assessment of brain perfusion, and evaluation of cerebral autoregulation or intracellular metabolic processes in the brain. Some studies have also examined the applicability of optical methods during the recovery phase of traumatic brain injury . The limitations of currently available optical methods and promising directions of future development are described in this review. Considering the outstanding technical challenges, the limited number of patients studied, and the mixed results and opinions gathered from other reviews on this subject, we believe that optical methods must remain primarily research tools for the present. More studies are needed to gain confidence in the use of these techniques for neuromonitoring of traumatic brain injury patients.

Continuous cerebral hemodynamic measurement during deep hypothermic circulatory arrest

While survival of children with complex congenital heart defects has improved in recent years, roughly half suffer neurological deficits suspected to be related to cerebral ischemia. Here we report the first demonstration of optical diffuse correlation spectroscopy (DCS) for continuous and non-invasive monitoring of cerebral microvascular blood flow during complex human neonatal or cardiac surgery. Comparison between DCS and Doppler ultrasound flow measurements during deep hypothermia, circulatory arrest, and rewarming were in good agreement. Looking forward, DCS instrumentation, alone and with NIRS, could provide access to flow and metabolic biomarkers needed by clinicians to adjust neuroprotective therapy during surgery.

A compact instrument to measure perfusion of vasculature in transplanted maxillofacial free flaps

The vascularization and resulting perfusion of transferred tissues are critical to the success of grafts in buried free flap transplantations. To enable long-term clinical monitoring of grafted tissue perfusion during neovascularization and endothelialization, we are developing an implantable instrument for the continuous monitoring of perfusion using diffuse correlation spectroscopy (DCS), and augmented with diffuse reflectance spectroscopy (DRS). This work discusses instrument construction, integration, and preliminary results using a porcine graft model.

Fast blood flow monitoring in deep tissues with real-time software correlators

We introduce, validate and demonstrate a new software correlator for high-speed measurement of blood flow in deep tissues based on diffuse correlation spectroscopy (DCS). The software correlator scheme employs standard PC-based data acquisition boards to measure temporal intensity autocorrelation functions continuously at 50 - 100 Hz, the fastest blood flow measurements reported with DCS to date. The data streams, obtained in vivo for typical source-detector separations of 2.5 cm, easily resolve pulsatile heart-beat fluctuations in blood flow which were previously considered to be noise. We employ the device to separate tissue blood flow from tissue absorption/scattering dynamics and thereby show that the origin of the pulsatile DCS signal is primarily flow, and we monitor cerebral autoregulation dynamics in healthy volunteers more accurately than with traditional instrumentation as a result of increased data acquisition rates. Finally, we characterize measurement signal-to-noise ratio and identify count rate and averaging parameters needed for optimal performance.

Risk of pneumonia associated with zero-degree head positioning in acute ischemic stroke patients treated with intravenous tissue plasminogen activator

INTRODUCTION

In the hyperacute phase of ischemic stroke, a 0° position is recommended to increase cerebral perfusion in nonhypoxic patients able to tolerate lying flat. However, use of 0° positioning is not uniformly applied in clinical practice, most likely due to concerns of aspiration pneumonia. We aimed to determine the risk of pneumonia associated with 0° head of bed positioning in acute stroke patients treated with thrombolytic therapy.

METHODS

A retrospective descriptive study was conducted using prospectively collected, consecutive acute ischemic stroke patients treated with IVtPA whose head of bed was positioned at 0° for the first 24 h. Rates of hospital-acquired pneumonia were determined using a strict adjudication process to insure accuracy of pneumonia diagnoses. Quantitative characteristics were analyzed in SPSS to compare differences between "true" pneumonia cases and nonpneumonia cases.

RESULTS

Twenty-four of 333 (7.2%) patients had mention the diagnosis of pneumonia in the registry and/or medical record. Of these cases, only 15 (4.5%) met evidence-based diagnostic criteria for hospital-acquired pneumonia. The 15 adjudicated cases had similar median admission NIHSS scores to nonpneumonia cases (10 vs. 9, respectively; P = ns), but were older (74 ± 15 vs. 64 ± 17 years; mean difference 9.889, 95 CI = 1.2-18.6; P = 0.026). A total of eight patients with pneumonia were intubated and mechanically ventilated, and one patient received bilevel positive airway pressure ventilation during the 0° positioning period. Pneumonia cases had significantly longer hospitalizations (14.5 ± 12 vs. 6.6 ± 9 days; mean difference 7.97, 95% CI = 1.1-14.8; P = 0.026) and higher median discharge mRS score (4 vs. 3: P = 0.003).

CONCLUSIONS

Zero-degree head of bed positioning in the first 24 h following an acute ischemic stroke treated with IV-tPA was associated with acceptable rates of pneumonia. Rates for pneumonia may be further reduced by eliminating use of a 0° protocol in intubated/mechanically ventilated patients.

Simultaneous blood flow and blood oxygenation measurements using a combination of diffuse speckle contrast analysis and near-infrared spectroscopy

A combined diffuse speckle contrast analysis (DSCA)-near-infrared spectroscopy (NIRS) system is proposed to simultaneously measure qualitative blood flow and blood oxygenation changes in human tissue. The system employs an optical switch to alternate two laser sources at two different wavelengths and a CCD camera to capture the speckle image. Therefore, an optical density can be measured from two wavelengths for NIRS measurements and a speckle contrast can be calculated for DSCA measurements. In order to validate the system, a flow phantom test and an arm occlusion protocol for arterial and venous occlusion were performed. Shorter exposure times (<1 ms ) show a higher drop (between 50% and 66%) and recovery of 1/K²S values after occlusion (approximately 150%), but longer exposure time (3 ms) shows more consistent hemodynamic changes. For four subjects, the 1/K²S values dropped to an average of 82.1±4.0% during the occlusion period and the average recovery of 1/K²S values after occlusion was 109.1±0.8% . There was also an approximately equivalent amplitude change in oxyhemoglobin (OHb) and deoxyhemoglobin (RHb) during arterial occlusion (max RHb=0.0085±0.0024 mM/DPF, min OHb=-0.0057±0.0044 mM/DPF). The sensitivity of the system makes it a suitable modality to observe qualitative hemodynamic trends during induced physiological changes.