Laser Doppler, speckle and related techniques for blood perfusion mapping and imaging

Evaluation of laser speckle flowmetry for imaging cortical perfusion in experimental stroke studies: quantitation of perfusion and detection of peri-infarct depolarisations

Laser speckle imaging of the exposed cerebral cortex allows detailed examination of the time course and topography of perfusion under different experimental conditions. Here we examine the quantitative capacity of the method and its sensitivity for the detection of peri-infarct depolarisations (PIDs). In four cats anaesthetised with chloralose, the right hemisphere was exposed and the right middle cerebral artery was occluded. The brain was illuminated with a laser diode, the speckle pattern was imaged, and images of inverse speckle correlation time (ICT) were derived from the calculated speckle contrast images. We examined the relationship of ICT with perfusion, as imaged quantitatively using umbelliferone clearance (CBF(umb)). Values of ICT and CBF(umb) were compared and regression parameters were calculated for each experiment. In eight cats, cortical surface direct current (DC) potential was monitored at two locations and detection of PIDs by DC potential and ICT change was compared. ICT- and CBF(umb)-derived values of perfusion were closely correlated, with a high degree of significance (P<0.0001). Overall, monitoring of DC potential detected 90% of PIDs, whereas ICT detected 56%. We conclude that (1) laser speckle imaging provides an index of perfusion that has a linear relationship with the clearance rate of umbelliferone within the range of levels of perfusion examined; (2) this relationship is relatively stable between experiments; and (3) the method's ability to detect blood flow changes associated with PIDs likely depends on the noise level of the speckle measurements.

Paradoxical response to water immersion in replanted fingers

When the fingers are immersed in water digital blood flow is reduced and the skin wrinkles, suggesting that vasoconstriction plays a role. However, is wrinkling due to sympathetic activation or some intrinsic property of the skin? To define the role of autonomic innervation in skin wrinkling we measured changes in blood perfusion (Laser Doppler Perfusion Imager) in 20 successfully replanted fingers (mean time after complete amputation 17.5 +/- 5.2 months). We compared responses in replanted fingers with those in corresponding contralateral normal fingers before and immediately after immersion in saline (0.5 mol/l NaCl at 40 degrees C). In the normal fingers, immersion caused a decrease in blood flow by 27.6 +/- 11.5% and skin wrinkling. However, in the replanted fingers blood flow increased by 22.8 +/- 19.6% and the skin did not wrinkle. The paradoxical cutaneous vasodilatation and absence of wrinkling in the denervated fingers suggests that sympathetic innervation is important in water-immersion skin wrinkling.

High-resolution angle-resolved measurements of light scattered at small angles by red blood cells in suspension

Red blood cells (RBCs) scatter light mainly in the forward direction, where the scattering phase function has a narrow peak. We performed an experimental investigation into the angular distribution of light scattered by blood in the small-angle domain. A highly diluted suspension of RBCs (hematocrits in the range 5 x 10(-5)-10(-2)) was illuminated with a He-Ne laser with 633 nm wavelength. We focused our research on two main topics: the scattering efficiency of the RBCs given by the mean scattering cross section and the scattering anisotropy obtained from the angular distribution of the scattered photons. The collimated beam transmission and the angular distribution of scattered light were measured and compared with the predictions of the effective phase function model. The RBCs' mean scattering cross section and scattering anisotropy were obtained by fitting of the experimental data.

Absorption and scattering coefficient dependence of laser-Doppler flowmetry models for large tissue volumes

Based on quasi-elastic scattering theory (and random walk on a lattice approach), a model of laser-Doppler flowmetry (LDF) has been derived which can be applied to measurements in large tissue volumes (e.g. when the interoptode distance is >30 mm). The model holds for a semi-infinite medium and takes into account the transport-corrected scattering coefficient and the absorption coefficient of the tissue, and the scattering coefficient of the red blood cells. The model holds for anisotropic scattering and for multiple scattering of the photons by the moving scatterers of finite size. In particular, it has also been possible to take into account the simultaneous presence of both Brownian and pure translational movements. An analytical and simplified version of the model has also been derived and its validity investigated, for the case of measurements in human skeletal muscle tissue. It is shown that at large optode spacing it is possible to use the simplified model, taking into account only a 'mean' light pathlength, to predict the blood flow related parameters. It is also demonstrated that the 'classical' blood volume parameter, derived from LDF instruments, may not represent the actual blood volume variations when the investigated tissue volume is large. The simplified model does not need knowledge of the tissue optical parameters and thus should allow the development of very simple and cost-effective LDF hardware.

Spatial extent of oxygen metabolism and hemodynamic changes during functional activation of the rat somatosensory cortex

The spatial extent of the changes in oxy-hemoglobin (HbO), deoxy-hemoglobin (HbR), total hemoglobin concentration (HbT), cerebral blood flow (CBF), and the cerebral metabolic rate of oxygen (CMRO(2)) in response to forepaw and whisker stimulation were compared in the rat somatosensory cortex using a combination of multi-wavelength reflectance imaging and laser speckle contrast imaging of cerebral blood flow. The spatial extents of the response of each hemodynamic parameter and CMRO(2) were found to be comparable at the time of peak response, and at early times following stimulation onset, the spatial extent of the change in HbR was smaller than that of HbO, HbT, CBF, and CMRO(2). In addition, a slight spatial dependence was found in the power law coefficient relating changes in CBF and HbT. Although the CMRO(2) response is a metabolic measure and thus expected to have a more localized response than the hemodynamic parameters, the results presented here suggest that this may not be the case in general, possibly due to the increased sensitivity of optical imaging techniques to superficial cortical layers where the lateral extent of the metabolic and neuronal activation is larger compared to that in layer IV. In addition, we found that the measured spatial extent of the CMRO(2) changes was insensitive to assumptions made in the calculation of the CMRO(2) changes such as baseline hemoglobin concentrations, vascular weighting constants, and wavelength dependence of tissue scattering. Multi-parameter full field imaging of the functional response provides a more complete picture of the hemodynamic response to functional activation including the spatial and temporal estimation of CMRO(2) changes.

Imaging the development of an ischemic core following photochemically induced cortical infarction in rats using Laser Speckle Contrast Analysis (LASCA)

Laser Speckle Contrast Analysis (LASCA) has been used to image the development of an ischemic core following photochemically induced infarction in rats up to 1 h post-lesion. Using LASCA, we have been able to image a central ischemic core which had little or no perfusion surrounded by a penumbral region with reduced perfusion. In addition, we have shown the existence of a surrounding region of hyperemic tissue. A potential feature of this imaging approach is its capability to track cerebral blood flow (CBF) changes in the region within and outside the ischemic core besides demonstrating the real-time progression of the ischemic core into the penumbral region. We have demonstrated the continuous disruption of CBF to the ischemic core that eventually affected the blood supply to the surrounding regions. The penumbral flow is shown to exhibit a sudden increase post-ischemic induction followed by a slow decline to the final baseline level. Interestingly, we observed an interaction (P < 0.03) between penumbral flow peak and the time effects of increase in pixel area from the infarct region to the surrounding penumbral region. Using a paired-sample t test, we observed that the mean pixel area was larger for the infarct region than for the penumbral region (P < 0.004) during the time interval between the induction of ischemia and the time point of peak flow in the penumbral region.

Spatiotemporal characteristics of low-frequency functional activation measured by laser speckle imaging

Changes in neuronal activity have been shown to be accompanied by alteration in regional cerebral blood flow . In the present study, laser speckle imaging (LSI) was employed to measure stimulus-evoked neuronal activities in rat barrel cortex. The spatiotemporal characteristics of hemodynamic response to mechanical stimuli from 1 to 3 Hz were examined. Time to peak amplitude reduced from 4.5 to 3.5 s with increasing frequencies. Spatially, the response was confined to a small circular region at the beginning and then spread out asymmetrically to the surrounding regions. The maximal area of activation ranged from 2.2 to 3.5 mm2, while the time to reach maximal area occurred between 5.5 and 6 s. Moreover, there was a high correlation between LSI and laser-Doppler flowmetry in terms of peak response magnitude and the time to reach peak. These two values were linearly dependent on stimulus frequency whereas area of activation and time to maximal area appeared to be independent of this parameter. LSI's high sensitivity, low cost of the equipment, and size and complexity make this a suitable technique for fundamental neurophysiological investigations.

Determination of optimal exposure time for imaging of blood flow changes with laser speckle contrast imaging

Laser speckle contrast imaging is becoming an established method for full-field imaging of cerebral blood flow dynamics in animal models. The sensitivity and noise in the measurement of blood flow changes depend on the camera exposure time. The relation among sensitivity, noise, and camera exposure time was investigated experimentally by imaging the speckle contrast changes in the brain after electrical forepaw stimulation in rats. The sensitivity to relative changes in speckle contrast was found to increase at longer exposure times and to reach a plateau for exposure times greater than approximately 2 ms. However, the speckle contrast noise also increases with exposure time and thus the contrast-to-noise ratio was found to peak at an exposure time of approximately 5 ms. Our results suggests that approximately 5 ms is an optimal exposure time for imaging of stimulus-induced changes in cerebral blood flow in rodents.

Temporal clustering analysis of cerebral blood flow activation maps measured by laser speckle contrast imaging

Temporal and spatial orchestration of neurovascular coupling in brain neuronal activity is crucial for comprehending the mechanism of functional cerebral metabolism and pathophysiology. Laser speckle contrast imaging (LSCI) through a thinned skull over the somatosensory cortex is utilized to map the spatiotemporal characteristics of local cerebral blood flow (CBF) in anesthetized rats during sciatic nerve stimulation. The time course of signals from all spatial loci among the massive dataset is hard to analyze, especially for the thousands of images, each of which composes millions of pixels. We introduce a temporal clustering analysis (TCA) method, which is proven as an efficient method to analyze functional magnetic resonance imaging (fMRI) data in the temporal domain. The timing and location of CBF activation shows that contralateral hindlimb sensory cortical microflow is activated to increase promptly in less than 1 s after the onset of 2-s electrical stimulation and is evolved in different discrete regions. This pattern is similar but slightly elaborated from the results obtained from laser Doppler flowmetry (LDF) and fMRI. We present this combination to investigate interacting brain regions, which might lead to a better understanding of the nature of brain parcellation and effective connectivity.

Methods of monitoring shock

Intensive monitoring is a crucial component of the management of shock. However, there is little consensus about optimal strategies for monitoring. Although the pulmonary artery catheter has been widely used, conflicting data exist about the utility of this device. A variety of other techniques have been developed in hopes of providing clinically useful information about myocardial function, intravascular volume, and indices of organ function. In addition, there is evolving evidence that targeting and monitoring certain physiological goals may be most important early in the course of shock. In this chapter, we examine many of the available monitoring techniques and the evidence supporting their use.

Translational and Brownian motion in laser-Doppler flowmetry of large tissue volumes

This study reports the derivation of a precise mathematical relationship existing between the different p-moments of the power spectrum of the photoelectric current, obtained from a laser-Doppler flowmeter (LDF), and the red blood cell speed. The main purpose is that both the Brownian (defining the 'biological zero') and the translational movements are taken into account, clarifying in this way what the exact contribution of each parameter is to the LDF derived signals. The derivation of the equations is based on the quasi-elastic scattering theory and holds for multiple scattering (i.e. measurements in large tissue volumes and/or very high red blood cell concentration). The paper also discusses why experimentally there exists a range in which the relationship between the first moment of the power spectrum and the average red blood cells speed may be considered as 'linear' and what are the physiological determinants that can result in nonlinearity. A correct way to subtract the biological zero from the LDF data is also proposed. The findings should help in the design of improved LDF instruments and in the interpretation of experimental data.

Optical imaging of the spatiotemporal dynamics of cerebral blood flow and oxidative metabolism in the rat barrel cortex

Oxidative metabolism and cerebral blood flow (CBF) are two of the most important measures in neuroimaging. However, results from concurrent imaging of the two with high spatial and temporal resolution have never been published. We used flavoprotein autofluorescence (AF) and laser speckle imaging (LSI) in the anaesthetized rat to map oxidative metabolism and CBF in response to single vibrissa stimulation. Autofluorescence responses reflecting oxidative metabolism demonstrated a fast increase with a delay of 0.1 s. The sign-reversed speckle contrast reflecting CBF started to rise with a delay of 0.6 s and reached its maximum 1.4 s after the stimulation offset. The fractional signal changes were 2.0% in AF and 9.7% in LSI. Pixelwise modelling revealed that CBF maps spread over an area up to 2.5-times larger than metabolic maps. The results provide evidence that the increase in cerebral oxidative metabolism in response to sensory stimulation is considerably faster and more localized than the CBF response. This suggests that future developments in functional imaging concentrating on the metabolic response promise an increased spatial resolution.

Pronounced hypoperfusion during spreading depression in mouse cortex

We studied unique cerebral blood flow (CBF) responses to cortical spreading depression in mice using a novel two-dimensional CBF imaging technique, laser speckle flowmetry. Cortical spreading depression caused a triphasic CBF response in both rat and mouse cortex. In rats, mild initial hypoperfusion (approximately 75% of baseline) was followed by a transient hyperemia reaching approximately 220% of baseline. In mice, the initial hypoperfusion was pronounced (40-50% of baseline), and the anticipated hyperemic phase barely reached baseline. The duration of hypoperfusion significantly correlated with the duration of the DC shift. As a possible explanation for the pronounced hypoperfusion, mouse cerebral vessels showed enhanced resistance to relaxation by acetylcholine (3 microM) after K+ -induced preconstriction (20, 40, and 80 mM) but dilated normally in response to acetylcholine after preconstriction with U46619, a synthetic thromboxane A2 analog. By contrast, rat vessels dilated readily to acetylcholine after preconstriction by K+. The transient normalization of CBF after hypoperfusion in the mouse was abolished by L-NA but not 7-NI. In summary, the CBF response to cortical spreading depression in mice contrasts with the rat in that the initial hypoperfusion is pronounced, and the hyperemic phase is markedly diminished. The differences in CBF response between species may be in part caused by an increased sensitivity of mouse cerebral vessels to elevated extracellular K+.

Toxicities of hemoglobin solutions: in search of in-vitro and in-vivo model systems

Several hemoglobin-based oxygen carriers (HBOCs) have been developed with a rationale focused on exploiting one or more physicochemical properties (e.g., oxygen affinity, molecular weight, viscosity, and colloid osmotic pressure) resulting from the chemical or recombinant modification of hemoglobin (Hb). Several chemically modified Hbs have reached late stages of clinical evaluation in the United States and Canada. These Hbs, in general, demonstrated mixed preclinical safety and efficacy, and reasonable safety in Phase I trials. However, as clinical development shifted into later stages, an undesirable safety and efficacy profile became clear in patient populations studied, and as a result some products were withdrawn from further clinical pursuit. Several questions still remain unanswered regarding the safety of Hb products for their proposed clinical indication(s). For example, 1) were preclinical studies predictive of clinical outcome? And, 2) were the most appropriate preclinical studies performed to predict clinical outcome? The primary objectives of this analysis are to explore prelinical safety issues associated with HBOCs and provide an overview of the in-vitro and in-vivo models employed. The methods for obtaining data to serve as a basis for discussion are compiled from a literature-based survey of safety and efficacy derived from biochemical, cellular, and whole animal assessment of HBOCs. Results from this overview of a vast body of published data may provide a means for identifying critical preclinical safety issues, which may ultimately lead to identification of potential limitations in the effective clinical use of certain HBOCs.

Laser speckle flowmetry for the study of cerebrovascular physiology in normal and ischemic mouse cortex

Laser speckle flowmetry (LSF) is useful to assess noninvasively two-dimensional cerebral blood flow (CBF) with high temporal and spatial resolution. The authors show that LSF can image the spatiotemporal dynamics of CBF changes in mice through an intact skull. When measured by LSF, peak CBF increases during whisker stimulation closely correlated with simultaneous laser-Doppler flowmetry (LDF) measurements, and were greater within the branches of the middle cerebral artery supplying barrel cortex than within barrel cortex capillary bed itself. When LSF was used to study the response to inhaled CO2 (5%), the flow increase was similar to the response reported using LDF. For the upper and lower limits of autoregulation, mean arterial pressure values were 110 and 40 mm Hg, respectively. They also show a linear relationship between absolute resting CBF, as determined by [C]iodoamphetamine technique, and 1/tau(c) values obtained using LSF, and used 1/tau(c) values to compare resting CBF between different animals. Finally, the authors studied CBF changes after distal middle cerebral artery ligation, and developed a model to investigate the spatial distribution and hemodynamics of moderate to severely ischemic cortex. In summary, LSF has distinct advantages over LDF for CBF monitoring because of high spatial resolution.

Spatiotemporal quantification of cerebral blood flow during functional activation in rat somatosensory cortex using laser-speckle flowmetry

Laser-speckle flowmetry was used to characterize activation flow coupling after electrical somatosensory stimulation of forepaw and hindpaw in the rat. Quantification of functional activation was made with high transverse spatial (microm) and temporal (msec) resolution. Different activation levels and duration of stimulation were quantitatively investigated, and were in good agreement with previous laser-Doppler measurements. Interestingly, the magnitude but not the overall shape of the response was found to scale with stimulus amplitude and the distance from the activation centroid. The results provide new insights about the spatial characteristics of cerebral blood flow response to functional activation, and the method should lead to improved understanding of the coupling of neuronal activity and hemodynamics under normal and pathologic conditions.

Simultaneous imaging of total cerebral hemoglobin concentration, oxygenation, and blood flow during functional activation

A simple instrument is demonstrated for high-resolution simultaneous imaging of total hemoglobin concentration and oxygenation and blood flow in the brain by combining rapid multiwavelength imaging with laser speckle contrast imaging. The instrument was used to image changes in oxyhemoglobin and deoxyhemoglobin and blood flow during cortical spreading depression and single whisker stimulation in rats through a thinned skull. The ability to image blood flow and hemoglobin concentration changes simultaneously with high resolution will permit detailed quantitative analysis of the spatiotemporal hemodynamics of functional brain activation, including imaging of oxygen metabolism. This is of significance to the neuroscience community and will lead to a better understanding of the interrelationship of neural, metabolic, and hemodynamic processes in normal and diseased brains.

The impact of ocular blood flow in glaucoma

Two principal theories for the pathogenesis of glaucomatous optic neuropathy (GON) have been described--a mechanical and a vascular theory. Both have been defended by various research groups over the past 150 years. According to the mechanical theory, increased intraocular pressure (IOP) causes stretching of the laminar beams and damage to retinal ganglion cell axons. The vascular theory of glaucoma considers GON as a consequence of insufficient blood supply due to either increased IOP or other risk factors reducing ocular blood flow (OBF). A number of conditions such as congenital glaucoma, angle-closure glaucoma or secondary glaucomas clearly show that increased IOP is sufficient to lead to GON. However, a number of observations such as the existence of normal-tension glaucoma cannot be satisfactorily explained by a pressure theory alone. Indeed, the vast majority of published studies dealing with blood flow report a reduced ocular perfusion in glaucoma patients compared with normal subjects. The fact that the reduction of OBF often precedes the damage and blood flow can also be reduced in other parts of the body of glaucoma patients, indicate that the hemodynamic alterations may at least partially be primary. The major cause of this reduction is not atherosclerosis, but rather a vascular dysregulation, leading to both low perfusion pressure and insufficient autoregulation. This in turn may lead to unstable ocular perfusion and thereby to ischemia and reperfusion damage. This review discusses the potential role of OBF in glaucoma and how a disturbance of OBF could increase the optic nerve's sensitivity to IOP.