Enhancing Transcranial Blood Flow Visualization with Dynamic Light Scattering Technologies: Advances in Quantitative Analysis

Monitoring of partial and full venous outflow obstruction in a porcine flap model using laser speckle contrast imaging

Chronic imaging of cerebral blood flow (CBF) is an important tool for investigating vascular remodeling after injury such as stroke. Although techniques such as Laser Speckle Contrast Imaging (LSCI) have emerged as valuable tools for imaging CBF in acute experiments, their utility for chronic measurements or cross-animal comparisons has been limited. Recently, an extension to LSCI called Multi-Exposure Speckle Imaging (MESI) was introduced that increases the quantitative accuracy of CBF images. In this paper, we show that estimates of chronic blood flow are better with MESI than with traditional LSCI. We evaluate the accuracy of the MESI flow estimates using red blood cell (RBC) photographic tracking as an absolute flow calibration in mice over several days. The flow measures computed using the MESI and LSCI techniques were found to be on average 10% and 24% deviant (n=9 mice), respectively, compared with RBC velocity changes. We also map CBF dynamics after photo-thrombosis of selected cortical microvasculature. Correlations of flow dynamics with RBC tracking were closer with MESI (r=0.88) than with LSCI (r=0.65) up to 2 weeks from baseline. With the increased quantitative accuracy, MESI can provide a platform for studying the efficacy of stroke therapies aimed at flow restoration.

Laser speckle contrast imaging is a robust and versatile blood flow imaging tool in basic and clinical research for its relatively simple construction and ease of customization. One of its key features is the scalability of the imaged field of view. With minimal changes to the system or analysis, laser speckle contrast imaging allows for high-resolution blood flow imaging through cranial windows or low-resolution perfusion visualization of perfusion over large areas, e.g. in human skin. We further utilize this feature and introduce a multi-scale laser speckle contrast imaging system, which we apply to study vasoreactivity in renal microcirculation. We combine high resolution (small field of view) to segment blood flow in individual vessels with low resolution (large field of view) to monitor global blood flow changes across the renal surface. Furthermore, we compare their performance when analyzing blood flow dynamics potentially associated with a single nephron and show that the previously published approaches, based on low-zoom imaging alone, provide inaccurate results in such applications.

Commentaries on Viewpoint: The human cutaneous circulation as a model of generalized microvascular function

Spinal cord ischemia leads to iatrogenic injury in multiple surgical fields, and the ability to immediately identify onset and anatomic origin of ischemia is critical to its management. Current clinical monitoring, however, does not directly measure spinal cord blood flow, resulting in poor sensitivity/specificity, delayed alerts, and delayed intervention. We have developed an epidural device employing diffuse correlation spectroscopy (DCS) to monitor spinal cord ischemia continuously at multiple positions. We investigate the ability of this device to localize spinal cord ischemia in a porcine model and validate DCS versus Laser Doppler Flowmetry (LDF). Specifically, we demonstrate continuous (>0.1Hz) spatially resolved (3 locations) monitoring of spinal cord blood flow in a purely ischemic model with an epidural DCS probe. Changes in blood flow measured by DCS and LDF were highly correlated (r = 0.83). Spinal cord blood flow measured by DCS caudal to aortic occlusion decreased 62%. This monitor demonstrated a sensitivity of 0.87 and specificity of 0.91 for detection of a 25% decrease in flow. This technology may enable early identification and critically important localization of spinal cord ischemia.

Laser-based medical techniques for evaluating blood flow (BF), such as laser Doppler flowmetry, laser speckle contrast imaging, etc., are known, but expensive and have some disadvantages. Recently, we have proposed a new technique—incoherent optical fluctuation flowmetry (IOFF), which is realized using a LED-based optical probe. This work aims to theoretically study the dependence of BF registered by IOFF on the source-detector distance (SDD) in the probe. For this purpose, we developed a three-layer optical model of skin and used Monte Carlo (MC) simulations of light propagation. All computations were performed for a wavelength of 810 nm and several SDDs from 1 to 14 mm. MC results showed that the BF depends nonlinearly on the SDD. Herewith, the BF is strongly correlated with the mean photon path length in a tissue (R = 0.92). Thus, flowmeters with different SDDs can give different BF values on the same patient. Based on the study results, to standardize BF measurements, it has been justified that BF magnitudes measured should be normalized to the exponential function of the SDD in the used optical probe in the form of [1 − exp(−b·SDD)], where b is a constant.

The integrity of endothelium-dependent vasodilation in the skin of patients with insulin-dependent diabetes mellitus (IDDM) is unclear, especially with respect to the role of nitric oxide. To examine this, forearm skin blood flow by laser Doppler flowmetry and total blood flow by venous occlusion plethysmography was measured in response to brachial artery infusions of an endothelium-dependent (methacholine) and -independent (sodium nitroprusside) vasodilator. Peak hyperemic forearm blood flow, following 5 min of arterial occlusion, was also determined. Responses were compared in 11 control subjects and 16 patients with insulin-dependent diabetes mellitus. In ten normal subjects, co-infusion of NG-monomethyl-L-arginine with methacholine produced a significant reduction in total forearm blood flow response to methacholine (p < 0.002), measured by venous occlusion plethysmography, as well as vascular conductance (p < 0.001), confirming that nitric oxide contributes to this response. In contrast, NG-monomethyl-L-arginine had no significant effect on the methacholine-induced increase in forearm skin blood flow measured by laser Doppler flowmetry indicating that factors other than nitric oxide may be involved. Increases in forearm skin blood flow in response to methacholine, sodium nitroprusside and to an ischemic stimulus were not significantly different between the normal subjects and patients with IDDM. Dose-related increases in total forearm blood flow and vascular conductance were not significantly different between control subjects and diabetic patients during infusions of methacholine. The increases in these parameters during infusions of sodium nitroprusside, however, were significantly less in the diabetic group than in the control group (p < 0.05) as was the peak reactive hyperemic blood flow (p < 0.05). Since skin blood flow was not affected, the reduced vasodilator responses to sodium nitroprusside and an ischemic stimulus in the diabetic group are in forearm skeletal muscle. The reduced muscle blood flow does not reflect a decreased vasodilatory capacity, but rather a functional impairment in response to nitric oxide and ischemia since the methacholine dilation was normal. The normal vasodilator responses in the forearm skin, which is predominantly capillary as opposed to arteriovenous anastomatic blood flow, indicate that the response to nitric oxide and an ischemic stimulus in this vascular bed is intact in patients with IDDM. This is, therefore, an unlikely cause of diabetic skin, complications in these areas.

Regulation of local tissue-type plasminogen activator release by endothelium-dependent and endothelium-independent agonists in human vasculature

A dual-imaging modality is demonstrated for high-resolution quantitative imaging of local cerebral blood flow in the rat cortex by combining simultaneous spectral-domain Doppler optical coherence tomography (SDOCT) and full-field laser speckle contrast imaging (LSCI). Preliminary studies in tissue flow phantom and cocaine-induced cerebral blood flow changes indicated that by correlating coregistered cortical arterial blood flow, the relative measurement of flow changes by LSCI could be accurately calibrated by the absolute flow imaging provided by SDOCT (least square fit, r(2) approximately 0.96). Quantitative LSCI of cerebral blood flow is crucial to the quantitative analyses of the spatiotemporal hemodynamics of functional brain activations and thus improved understanding of neural process.

Blood perfusion in human tissue can be measured in vivo by means of various optical methods, which seem to be very different from one another. The most prominent examples of them are laser Doppler flowmetry, laser speckle contrast imaging, diffuse correlation spectroscopy, and the most recently developed diffuse speckle contrast analysis. In this paper, we claim that these four seemingly different modalities are examining different aspects of the same entity-the temporal autocorrelation function of scattered photons. We will show how the observables in each modality can be theoretically derived from the temporal autocorrelation function, and will discuss the merits and drawbacks of each modality in its practical use.

Automated non-invasive laser speckle imaging of the chick heart rate and extraembryonic blood vessels and their response to Nifedipine and Amlodipine drugs.

.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 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.

Reactive hyperemia refers to an increase of blood flow in tissue post release of an occlusion in the local vasculature. Measuring the temporal response of reactive hyperemia, post-occlusion in patients has the potential to shed information about microvascular diseases such as systemic sclerosis and diabetes. Laser speckle contrast imaging (LSCI) is an imaging technique capable of sensing superficial blood flow in tissue which can be used to quantitatively assess reactive hyperemia. Here, we employ LSCI using coherent sources in the blue, green and red wavelengths to evaluate reactive hyperemia in healthy human volunteers. Blood flow in the forearms of subjects were measured using LSCI to assess the time-course of reactive hyperemia that was triggered by a pressure cuff applied to the biceps of the subjects. Raw speckle images were acquired and processed to yield blood-flow parameters from a region of interest before, during and after application of occlusion. Reactive hyperemia was quantified via two measures - (1) by calculating the difference between the peak LSCI flow during the hyperemia and baseline flow, and (2) by measuring the amount of time that elapsed between the release of the occlusion and peak flow. These measurements were acquired in three healthy human participants, under the three laser wavelengths employed. The studies shed light on the utility of in vivo LSCI-based flow sensing for non-invasive assessment of reactive hyperemia responses and how they varied with the choice source wavelength influences the measured parameters.

Diffuse Correlation Spectroscopy (DCS) is an increasingly popular non-invasive optical technique to clinically measure deep tissue blood flow, albeit at slow measurement rates of 0.5-1 Hz. We recently reported the development of a new ‘fast’ DCS instrument that continuously measures blood flow at 50-100 Hz (simultaneously from 8 channels), using conventional DCS sources/detectors, and optimized software computations. A particularly interesting result was our ability to optically record pulsatile micro-vascular blood flow waveforms, and therein readily identify high frequency features such as the dicrotic notch. Here, we showcase the utility and potential of high-speed measurements of blood flow (and arterial blood pressure) in a few clinical applications. First, we employ the fast-DCS instrumentation to measure cerebral autoregulation (CVAR) dynamics. Cerebral autoregulation refers to the mechanism by which cerebral blood flow (CBF) is maintained during fluctuations in blood pressure; CVAR is impaired in the injured brain. We derive an index of autoregulation by measuring the rates of decrease (and recovery) of blood flow and blood pressure following a sudden, induced change in systemic blood pressure (e.g., bilateral thigh cuff deflation). Our pilot experiments in healthy volunteers show that DCS measured rates of micro-vascular regulation are comparable to conventional large vessel regulatory metrics ( e.g., measured with transcranial Doppler ultrasound). Second, we utilized pulsatile blood flow oscillations in cerebral arteries to estimate the critical closing pressure (CrCP), i.e., the arterial blood pressure at which CBF approaches zero. Pilot experiments in healthy subjects show good agreement between CrCP measured with DCS and transcranial Doppler ultrasound.

The tissue optical clearing (TOC) technique could significantly improve the biomedical optical imaging depth, but most current investigations are limited to in vitro studies. For in vivo applications, the TOC method must provide a rapid treatment process, sufficient transparency, and safety for animals, which makes it more difficult. Recently developed innovative optical clearing methods for in vivo use show great potential for enhancing the contrast and resolution of laser speckle contrast imaging (LSCI) for blood flow monitoring. This paper gives an overview of recent progress in the use of TOC for vascular visualization with LSCI. First, the principle of TOC-induced improvement of LSCI and a quantitative analysis method for evaluating the improvement are described briefly. Second, the paper introduces transparent windows, including various skin windows and a cranial window, that permit LSCI to monitor dermal or cortical blood flow, respectively, with high resolution and contrast. Third, preliminary investigations of the safety of TOC demonstrate that the transparent skin window is switchable, which enables LSCI to repeatedly image blood flow. However, research on in vivo TOC is currently less advanced than that on in vitro TOC. Future work should focus on developing a highly effective, safe method and extending its applications.