Laser speckle contrast imaging of cerebral blood flow

Laser speckle contrast imaging (LSCI) has emerged over the past decade as a powerful, yet simple, method for imaging of blood flow dynamics in real time. The rapid adoption of LSCI for physiological studies is due to the relative ease and low cost of building an instrument as well as the ability to quantify blood flow changes with excellent spatial and temporal resolution. Although measurements are limited to superficial tissues with no depth resolution, LSCI has been instrumental in pre-clinical studies of neurological disorders as well as clinical applications including dermatological, neurosurgical and endoscopic studies. Recently a number of technical advances have been developed to improve the quantitative accuracy and temporal resolution of speckle imaging. This article reviews some of these recent advances and describes several applications of speckle imaging.     

Adrenalectomy increases local cerebral blood flow in the rat hippocampus

The present study examined the effect of glucocorticoid manipulations on local cerebral blood flow in the hippocampus. We measured local cerebral blood flow in the hippocampus at 1-h intervals over a 1-day period in freely moving rats, by means of the H₂ clearance method, before and after sham adrenalectomy, adrenalectomy or adrenalectomy with corticosterone replacement. We also measured local cerebral blood flow in the prefrontal cortex before and after adrenalectomy. Four weeks after the adrenalectomy, hippocampal blood flow at each time of day was an average of 47% greater than before the operation, showing diurnal variation as before. After the sham adrenalectomy or adrenalectomy with corticosterone replacement, hippocampal blood flow did not change significantly with respect to either its level or its diurnal variation. Local cerebral blood flow in the prefrontal cortex increased by only 19% after adrenalectomy. The present study demonstrates that adrenalectomy causes a remarkable increase in hippocampal blood flow, probably due to a lack of corticosterone.     

Capillary cerebral amyloid angiopathy is associated with vessel occlusion and cerebral blood flow disturbances

The role of cerebral amyloid angiopathy (CAA) in the pathogenesis of Alzheimer's disease (AD) is not fully understood. Here, we studied whether CAA is associated with alterations in microvascularisation in transgenic mouse models and in the human brain. APP23 mice at 25–26 months of age exhibited severe CAA in thalamic vessels whereas APP51/16 mice did not. Wild-type littermates were free of CAA. We found CAA-related capillary occlusion within the thalamus of APP23 mice but not in APP51/16 and wild-type mice. Magnetic resonance angiography (MRA) showed blood flow alterations in the thalamic vessels of APP23 mice. CAA-related capillary occlusion in the branches of the thalamoperforating arteries of APP23 mice, thereby, corresponded to the occurrence of blood flow disturbances. Similarly, CAA-related capillary occlusion was observed in the human occipital cortex of AD cases but less frequently in controls. These results indicate that capillary CAA can result in capillary occlusion and is associated with cerebral blood flow disturbances providing an additional mechanism for toxic effects of the amyloid β-protein in AD.     

Local cerebral blood flow in the brain during bicuculline-induced seizures and the modulating influence of inhibition of prostaglandin synthesis

The purpose of this study was to measure changes in local cerebral blood flow (1-CBF) during generalized seizures, and to study whether or not formation of prostaglandins or related substances contributes to the increased flow rates. Seizures were induced in ventilated rats maintained on 70% N₂O and 30% O₂ by the i. v. injection of the GABA receptor blocker bicuculline (1.2 mg-kg^-1). Formation of prostaglandins was inhibited by the administration of the fatty acid cyclo-oxygenase inhibitor indomethacin (10 mg · kg^-1)-Local CBF in 21 defined brain structures was measured autoradiographically with ^HC-iodoantipyrine as the diffusible tracer. After 20 min of continuous seizure activity 1-CBF increased 1.5-5-fold, the smallest increases (<200% of control) being observed in frontal and auditory cortex and in the caudoputamen, and the largest (>400% of control) in substantia nigra, thalamus, visual cortex, lateral geniculate and hypothalamus. In general, the largest increases in 1-CBF occurred in sensory and limbic systems (and hypothalamus) while motor systems showed a pronounced variability. In the majority of structures examined indomethacin failed to modify the CBF response during seizures. Although this result suggests that seizures, in contrast to hypercapnia, lead to an increased CBF by other mechanisms than those related to prostaglandin formation, some structures (nucleus ruber, cerebellum, and superior colliculus) showed a clearly reduced 1-CBF in indomethacin-treated animals.     

Regional brain activation by bicuculline visualized by functional magnetic resonance imaging. Time-resolved assessment of bicuculline-induced changes in local cerebral blood volume using an intravascular contrast agent

Functional magnetic resonance imaging (fMRI) has been applied to study rat focal brain activation induced by intravenous administration of the GABA(A) antagonist bicuculline. Using magnetite nanoparticles as a blood pool contrast agent, local changes in cerebral blood volume (CBV) were assessed with high temporal (10 s) and spatial (0.35 x 0.6 mm(2)) resolutions. Upon infusion of the bicuculline region-specific increases in CBV have been observed, suggesting CBV to reflect brain activity. During the first 2 min, the signal increases were predominant in the cortex, followed by increases in other brain areas, such as the caudate putamen, thalamus and cerebellum. Ten minutes after the start of infusion, a dominant response was observed in the thalamus, while in the caudate putamen a biphasic response pattern was seen. The magnitude of the signal responses in all brain regions was dependent on the dose of bicuculline and, in general, matched the known distribution of GABA(A) binding sites. This study suggests that pharmacological fMRI, displaying brain function at the highly specific level of drug-receptor interaction, should foster our understanding of normal and pathological brain function.     

The influence of nicotine on local cerebral blood flow in rats.

Local cerebral blood flow (LCBF) was measured in conscious rats during an acute nicotine infusion. LCBF was measured using the autoradiographic iodoantipyrine method. LCBF was unchanged in most brain structures during nicotine infusion compared to controls. Significant (P less than 0.05) increases were found in 3 structures (lateral geniculate body, superior colliculus, anteroventral nucleus of the thalamus). These structures have already shown increases in local glucose utilization in a previous study [2]. The observed increases in LCBF are interpreted to be secondary to metabolic activation by nicotine indicating the lack of a direct action of nicotine on cerebral blood vessels.     

Functional laser speckle imaging of cerebral blood flow under hypothermia

Hypothermia can unintentionally occur in daily life, e.g., in cardiovascular surgery or applied as therapeutics in the neurosciences critical care unit. So far, the temperature-induced spatiotemporal responses of the neural function have not been fully understood. In this study, we investigated the functional change in cerebral blood flow (CBF), accompanied with neuronal activation, by laser speckle imaging (LSI) during hypothermia. Laser speckle images from Sprague-Dawley rats (n = 8, male) were acquired under normothermia (37°C) and moderate hypothermia (32°C). For each animal, 10 trials of electrical hindpaw stimulation were delivered under both temperatures. Using registered laser speckle contrast analysis and temporal clustering analysis (TCA), we found a delayed response peak and a prolonged response window under hypothermia. Hypothermia also decreased the activation area and the amplitude of the peak CBF. The combination of LSI and TCA is a high-resolution functional imaging method to investigate the spatiotemporal neurovascular coupling in both normal and pathological brain functions.     

Fractal features for localization of temporal lobe epileptic foci using SPECT imaging

Single photon emission computed tomography (SPECT) is an accurate imaging method for the diagnosis of refractory partial epilepsy. Two scans are carried out: interictal and ictal. The interest of this method is to provide an image in the ictal period, which allows hyperperfused areas linked to the seizure to be localized. The epileptic foci localization is improved by subtracting the two acquisitions (subtracted ictal SPECT: SIS). In some cases, the SIS method is not effective and does not isolate the seizure foci. In this article, we investigate a new method based on texture analysis using fractal geometry features. Fractal geometry features were extracted from each scan in order to quantify the heterogeneity change resulting from the hyperperfusion. A support vector machine (SVM) classification algorithm was used to classify the voxels into two classes: focal and healthy. Quantitative evaluation was performed on simulated images and clinical images from 22 patients with temporal lobe epilepsy. Results on both experiments showed that the proposed method is more specific and more sensitive than the SIS method.     

Simulation of determinant and dependent foci of epileptic activity in the rat cerebral cortex

Foci of increased activity with independent firing patterns were created by means of weak solutions of strychnine in acute experiments on rats. A hyperactive focus of excitation created by means of concentrated strychnine solutions played the role of determinant structure. Its role was to determine the character of activity of the other epileptogenic foci, to enhance their paroxysmal activity, to combine them into a single functional complex, and to determine the behavior of the whole complex. This complex could be destroyed by suppressing the activity of the determinant focus. Elimination of any of the dependent foci forming the complex did not disturb the complex itself. These investigations confirm, on a new model, the general concept of the role of determinant structure in the activity of the CNS.Department of Pathological Physiology, N. I. Pirogov Odessa Medical Institute. (Presented by Academician of the Academy of Medical Sciences of the USSR A. M. Chernukh.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 87, No. 3, pp. 217–220, March, 1979.     

Laser speckle contrast imaging of cerebral blood flow in freely moving animals

We designed a miniature laser speckle imager that weighs ～20 g and is 3.1-cm high for full-field high-resolution imaging of cerebral blood flow (CBF) in freely moving animals. Coherent laser light illuminates the cortex through a multimode optical fiber bundle fixed onto the supporting frame of the imager. The reflected lights are then collected by a miniature macrolens system and imaged by a high-resolution CMOS camera at a high frame rate (50 fps). Using this miniature imager, we achieve high spatiotemporal resolution laser speckle contrast imaging of CBF in freely moving animals in real time.     

Chronic stress exposure influences local cerebral blood flow in the rat hippocampus.

To examine the influence of chronic stress on the brain, we measured local cerebral blood flow in the hippocampus of rats which had been exposed to chronic stress by the hydrogen clearance method in the freely moving status. Rats were exposed, once a day for 12 weeks, to stress of a 15-min immersion in cold water at 4 degrees C (the stress group) or slightly handled for about 1 min (the control group). Local cerebral blood flow values in the hippocampus, which were measured after a 12-week recovery period, were lower in rats in the stress group than those of rats in the control group only in the dark cycle, but not in the light cycle. Accordingly, local cerebral blood flow in the hippocampus of rats in the stress group did not have a daily fluctuation, i.e. lower in the light cycle and higher in the dark cycle, as was shown in rats in the control group. There were no significant changes in motor activity in rats in the stress group as compared to those in the control group. Severe structural damages were observed in the CA2 and CA3 cell fields of the hippocampus of rats in the stress group. We found that an increase in local cerebral blood flow in the hippocampus in the dark cycle was blunted following chronic stress exposure, suggesting that chronic stress exposure caused hippocampal neurons to be less responsive to environmental stimuli derived from motor activity during the dark cycle.     

Evoked local cerebral blood flow induced by somatosensory stimulation is proportional to the baseline flow

The purpose of this study was to determine the relationship between the increase in local cerebral blood flow during neuronal activation (evoked LCBF) and the baseline flow level. We measured the hemodynamics in alpha-chloralose-anesthetized rats using laser-Doppler flowmetry during somatosensory stimulation under the hypocapnic, normocapnic and hypercapnic conditions. The baseline levels of LCBF and red blood cell (RBC) velocity under hypocapnia (PaCO(2)=26.4+/-1.1 mmHg) were, respectively, 10 and 11% lower than those under normocapnia (PaCO(2)=34.2+/-1.4 mmHg) (P<0.01). The evoked response magnitude of LCBF and RBC velocity under hypocapnia were, respectively, 22 and 18% lower than those under normocapnia. There was no significant difference in the baseline level and evoked response magnitude of RBC concentration. On the other hand, the baseline levels of LCBF, RBC velocity and RBC concentration under hypercapnia (PaCO(2)=73.4+/-13.3 mmHg) were, respectively, 47, 24 and 14% higher than those under normocapnia (PaCO(2)=34.7+/-2.5 mmHg) (P<0.01). The evoked response magnitude of LCBF, RBC velocity and RBC concentration under hypercapnia were, respectively, 96, 82 and 62% greater than those under normocapnia. After normalization with respect to each baseline level, there was no significant difference in normalized evoked response magnitude of LCBF, RBC velocity and RBC concentration, either between hypocapnic and normocapnic conditions or between hypercapnic and normocapnic conditions, indicating that evoked LCBF is proportional to the baseline flow. These results suggest that the amount of evoked LCBF is not determined by the demand for metabolic substrates.     

Measurement of local cerebral blood flow with iodo [14C] antipyrine

The autoradiographic diffusible tracer technique for the measurement of local cerebral blood flow was originally designed for use with the radioactive, inert gas 131I-labeled trifluoroiodomethane and is applicable only with tracers that exhibit unrestricted diffusion through the blood-brain barrier. Because of the technical problems associated with the use of gaseous tracers, a suitable nonvolatile tracer has been sought. [14C] Antipyrine has been used previously and found to be unsuitable because of limitations in its diffusion through the blood-brain barrier. An analogue of [14C]antipyrine, iodo [14C]antipyrine, exhibits higher partition coefficients than [14C]antipyrine between nonpolar solvents and water and might, therefore, be expected to diffuse more freely through the barrier. Its use as the tracer in the local blood flow technique leads to values considerably above those obtained with [14C]antipyrine in the rat and cat and essentially the same as those obtained with the gas trifluoro[131I]iodomethane in the cat. Iodo[14C]antipyrine appears, therefore, to be a satisfactory nonvolatile tracer for the measurement of local cerebral blood flow.     

Patch-based local learning method for cerebral blood flow quantification with arterial spin-labeling MRI

Arterial spin-labeling (ASL) perfusion MRI is a non-invasive method for quantifying cerebral blood flow (CBF). Standard ASL CBF calibration mainly relies on pair-wise subtraction of the spin-labeled images and controls images at each voxel separately, ignoring the abundant spatial correlations in ASL data. To address this issue, we previously proposed a multivariate support vector machine (SVM) learning-based algorithm for ASL CBF quantification (SVMASLQ). But the original SVMASLQ was designed to do CBF quantification for all image voxels simultaneously, which is not ideal for considering local signal and noise variations. To fix this problem, we here in this paper extended SVMASLQ into a patch-wise method by using a patch-wise classification kernel. At each voxel, an image patch centered at that voxel was extracted from both the control images and labeled images, which was then input into SVMASLQ to find the corresponding patch of the surrogate perfusion map using a non-linear SVM classifier. Those patches were eventually combined into the final perfusion map. Method evaluations were performed using ASL data from 30 young healthy subjects. The results showed that the patch-wise SVMASLQ increased perfusion map SNR by 6.6% compared to the non-patch-wise SVMASLQ.     

Effect of cerebrocrast on local cerebral blood flow and EGG in cats after brain hemorrhage

Laboratory of Pharmacology of the Cerebral Circulation, Research Institute, of Pharmacology, Tomsk Scienctific Center, Academy of Medicial Sciences of Russia. Institute of Organic Synthesis, Academy of Sciences of Latvia, Riga. (Presented by Academician of the Russian Academy of Medical Sciences E. D. Gol'dberg.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 114, No. 7, pp 49–52, July, 1992.     

Stress and local cerebral blood flow: studies on restrained and unrestrained rats

Summary The degree of stress has been compared between two protocols used for the measurement of local cerebral blood flow (LCBF) in conscious rats. The first method involved acute surgical procedures (cannulation of both femoral veins and arteries) under halothane anesthesia. It was followed by a recovery period (2–3 h) during which the rat was before LCBF measurement. The second method employed chronic cannulation of the abdominal aorta and vena cava, allowing the LCBF assays to be performed on freely moving rats. Plasma corticosterone and a glucose tolerance tests showed that the freely moving rats were less stressed than the gently restrained ones. The LCBF of the two groups were not significantly different except in the frontal and parietal cortex, where it was more elevated in the freely moving rats. LCBF may be sensitive to the environmental conditions in freely moving rats whereas these vascular effects may be reduced after 2–3 h of gentle restraint. The two protocols tested in this study could be considered as good methods for studying LCBF in conscious rats, although some stress remained in gently restrained rats. Freely moving rats can be used for behavioural studies providing that the time lag of the arterial samples is taken into account. Since the basal LCBF values of gently restrained rats are minimally affected by the stress inherent in the preparation, this convenient protocol could be considered as useful for numerous investigations.     

Changes in the local cerebral blood flow accompanying elevation of the systemic arterial pressure in animals

The systemic arterial pressure was raised in normal rabbits and in rabbits with experimental renal hypertension by intravenous injection of noradrenalin and the local cerebral blood flow was recorded at two points of the cortex or white matter of the cerebral hemispheres by the hydrogen-clearance method, and the EEG also was recorded. A series of successive changes in the local cerebral blood flow was observed and these changes could be similar or different in duration and character at different electrodes. Pathological forms of activity were revealed on the EEG.