Cerebral microvascular network geometry changes in response to functional stimulation.

The cortical microvessels are organized in an intricate, hierarchical, three-dimensional network. Superimposed on this anatomical complexity is the highly complicated signaling that drives the focal blood flow adjustments following a rise in the activity of surrounding neurons. The microvascular response to neuronal activation remains incompletely understood. We developed a custom two photon fluorescence microscopy acquisition and analysis to obtain 3D maps of neuronal activation-induced changes in the geometry of the microvascular network of the primary somatosensory cortex of anesthetized rats. An automated, model-based tracking algorithm was employed to reconstruct the 3D microvascular topology and represent it as a graph. The changes in the geometry of this network were then tracked, over time, in the course of electrical stimulation of the contralateral forepaw. Both dilatory and constrictory responses were observed across the network. Early dilatory and late constrictory responses propagated from deeper to more superficial cortical layers while the response of the vertices that showed initial constriction followed by later dilation spread from cortical surface toward increasing cortical depths. Overall, larger caliber adjustments were observed deeper inside the cortex. This work yields the first characterization of the spatiotemporal pattern of geometric changes on the level of the cortical microvascular network as a whole and provides the basis for bottom-up modeling of the hemodynamically-weighted neuroimaging signals.

Amyloid-ß-dependent compromise of microvascular structure and function in a model of Alzheimer's disease.

The majority of patients with Alzheimer's disease have cerebral amyloid angiopathy, thus showing deposition of amyloid-ß peptides in the walls of leptomeningeal and cortical arterioles. These deposits are believed to result from impaired clearance of parenchymal amyloid-ß peptides. In the current work, we examined the changes in cortical microvascular structure and function in situ in TgCRND8, a transgenic mouse model of Alzheimer's disease. In contrast to venules, cortical arterioles were shown to increase in tortuosity and decrease in calibre with amyloid-ß peptide accumulation. These structural changes were accompanied by progressive functional compromise, reflected in higher dispersion of microvascular network transit times, elongation of the transit times, and impaired microvascular reactivity to hypercapnia in the transgenic mice. Moreover, inhibition of amyloid-ß peptide oligomerization and fibrillization via post-weaning administration of scyllo-inositol, a naturally occurring stereoisomer of myo-inositol, rescued both structural and functional impairment of the cortical microvasculature in this Alzheimer's disease model. These results demonstrate that microvascular impairment is directly correlated with amyloid-ß accumulation and highlight the importance of targeting cerebrovascular amyloid angiopathy clearance for effective diagnosis, monitoring of disease progression and treatment of Alzheimer's disease.

Quantitative estimates of stimulation-induced perfusion response using two-photon fluorescence microscopy of cortical microvascular networks.

Functional hyperemia, or the increase in focal perfusion elicited by neuronal activation, is one of the primary functions of the neurovascular unit and a hallmark of healthy brain functioning. While much is known about the hemodynamics on the millimeter to tenths of millimeter-scale accessible by MRI, there is a paucity of quantitative data on the micrometer-scale changes in perfusion in response to functional stimulation. We present a novel methodology for quantification of perfusion and intravascular flow across the 3D microvascular network in the rat somatosensory cortex using two-photon fluorescence microscopy (2PFM). For approximately 96% of responding microvessels in the forelimb representation of the primary somatosensory cortex, brief (~2s) forepaw stimulation resulted in an increase of perfusion 20±4% (mean±sem). The perfusion levels associated with the remaining 4% of the responding microvessels decreased 10±9% upon stimulation. Vessels irrigating regions of lower vascular density were found to exhibit higher flow (p<0.02), supporting the notion that local vascular morphology and hemodynamics reflect the metabolic needs of the surrounding parenchyma. High dispersion (~77%) in perfusion levels suggests high spatial variation in tissue susceptibility to hypoxia. The current methodology enables quantification of absolute perfusion associated with individual vessels of the cortical microvascular bed and its changes in response to functional stimulation and thereby provides an important tool for studying the cellular mechanisms of functional hyperemia, the spatial specificity of perfusion response to functional stimulation, and, broadly, the micrometer-scale relationship between vascular morphology and function in health and disease.

Quantification of blood flow and volume in arterioles and venules of the rat cerebral cortex using functional micro-ultrasound.

Relative cerebral blood volume (rCBV), relative cerebral blood flow (rCBF), and blood flow speed are key parameters that characterize cerebral hemodynamics. We used contrast-enhanced functional micro-ultrasound (fMUS) imaging employing a disruption-replenishment imaging sequence to quantify these hemodynamic parameters in the anesthetized rat brain. The method has a spatial resolution of about 100 µm in-plane and around 600 µm through-plane, which is comparable to fMRI, and it has a superior temporal resolution of 40 ms per frame. We found no significant difference in rCBV of cortical and subcortical gray matter (0.89 ± 0.08 and 0.61 ± 0.09 times the brain-average value, respectively). The rCBV was significantly higher in the vascular regions on the pial surface (3.89 ± 0.71) and in the area of major vessels in the subcortical gray matter (2.02 ± 0.31). Parametric images of rCBV, rCBF, and blood flow speed demonstrate spatial heterogeneity of these parameters on the 100 µm scale. Segmentation of the cortex in arteriolar and venular-dominated regions identified through color Doppler imaging showed that rCBV is higher and flow speed is lower in venules than in arterioles. Finally, we show that the dependence of rCBV on rCBF was significantly different in cortical versus subcortical gray matter: the exponent α in the power law relation rCBV=s·rCBF(α) was 0.37 ± 0.13 in cortical and 0.75 ± 0.16 in subcortical gray matter. This work demonstrates that functional micro-ultrasound imaging affords quantification of hemodynamic parameters in the anesthetized rodent brain. This modality is a promising tool for neuroscientists studying these parameters in rodent models of diseases with a cerebrovascular component, such as stroke, neurodegeneration, and venous collagenosis. It is of particular import for studying conditions that selectively affect arteriolar versus venular compartments.

Functional micro-ultrasound imaging of rodent cerebral hemodynamics.

Healthy cerebral microcirculation is crucial to neuronal functioning. We present a new method to investigate microvascular hemodynamics in living rodent brain through a focal cranial window based on high-frequency ultrasound imaging. The method has a temporal resolution of 40ms, and a 100µm in-plane and 600µm through-plane spatial resolution. We use a commercially available high-frequency ultrasound imaging system to quantify changes in the relative cerebral blood volume (CBV) by measuring the scattered signal intensity from an ultrasound contrast agent circulating in the vasculature. Generalized linear model analysis is then used to produce effect size and significance maps of changes in cerebral blood volume upon electrical stimulation of the forepaw. We observe larger CBV increases in the forelimb representation of the primary somatosensory cortex than in the deep gray matter with stimuli as short as 2s (5.1 ± 1.3% vs. 3.3 ± 0.6%). We also investigate the temporal evolution of the blood volume changes in cortical and subcortical gray matter, pial vessels and subcortical major vessels, and show shorter response onset times in the parenchymal regions than in the neighboring large vessels (1.6 ± 1.0s vs. 2.6 ± 1.3s in the cortex for a 10 second stimulus protocol). This method, which we termed functional micro-ultrasound imaging or fMUS, is a novel, highly accessible, and cost-effective way of imaging rodent brain microvascular topology and hemodynamics in vivo at 100micron resolution over a 1-by-1cm field of view with 10s-100s frames per second that opens up a new set of questions regarding brain function in preclinical models of health and disease.

The effect of accelerometer location on the classification of single-site forearm mechanomyograms.

BACKGROUND: Recently, pattern recognition methods have been deployed in the classification of multiple activation states from mechanomyogram (MMG) signals for the purpose of controlling switching interfaces. Given the propagative properties of MMG signals, it has been suggested that MMG classification should be robust to changes in sensor placement. Nonetheless, this purported robustness remains speculative to date. This study sought to quantify the change in classification accuracy, if any, when a classifier trained with MMG signals from the muscle belly, is subsequently tested with MMG signals from a nearby location. METHODS: An arrangement of 5 accelerometers was attached to the flexor carpi radialis muscle of 12 able-bodied participants; a reference accelerometer was located over the muscle belly, two peripheral accelerometers were positioned along the muscle's transverse axis and two more were aligned to the muscle's longitudinal axis. Participants performed three classes of muscle activity: wrist flexion, wrist extension and semi-pronation. A collection of time, frequency and time-frequency features were considered and reduced by genetic feature selection. The classifier, trained using features from the reference accelerometer, was tested with signals from the longitudinally and transversally displaced accelerometers. RESULTS: Classification degradation due to accelerometer displacement was significant for all participants, and showed no consistent trend with the direction of displacement. Further, the displaced accelerometer signals showed task-dependent de-correlations with respect to the reference accelerometer. CONCLUSIONS: These results indicate that MMG signal features vary with spatial location and that accelerometer displacements of only 1-2 cm cause sufficient feature drift to significantly diminish classification accuracy. This finding emphasizes the importance of consistent sensor placement between MMG classifier training and deployment for accurate control of switching interfaces.

A Retrospective Study of the Association between Pain Intensity and Opioid Use with Length of Stay during Musculoskeletal Inpatient Rehabilitation after Primary Knee and Hip Arthroplasty.

BACKGROUND: The relationship between pain intensity, opioid consumption, and length of stay (LOS) has received little attention in primary, lower extremity joint arthroplasty patients admitted to inpatient musculoskeletal rehabilitation. OBJECTIVE: To assess how initial pain and other clinical factors are associated with rehabilitation LOS. DESIGN: Retrospective chart review. SETTING: Rehabilitation hospital. PARTICIPANTS: One hundred ninety nine patients admitted for inpatient rehabilitation. INTERVENTIONS: Not Applicable. MAIN OUTCOME MEASURES: Pain intensities on the Numeric Rating Scale (NRS) were completed 3 times daily and total daily opioid consumption recorded in terms of morphine equivalents (MEQ). Confounding variables included patient demographics, medical comorbidity burden using the Charlson Comorbidity Index (CCI), and early functional status as measured by the motor subscale from the Functional Independence Measure (FIM). RESULTS: Mean day 3 NRS values of ≥5.2 and total day 3 opioid consumption of >50 MEQ were associated with a prolonged LOS by nearly 3 and 2 days, respectively. Within a multivariate linear regression, age, mean day 3 pain, comorbidity burden, and early motor functional status accounted for 36% of the variability seen in joint replacement rehabilitation LOS. With all other variables remaining constant, for every unit increase in mean day 3 pain and CCI, this amounted to an additional 5% and 4% increase to LOS, whereas each unit increase in admission motor FIM decreased estimated LOS by 3%. CONCLUSION: Mean pain intensity and total opioid consumption on day 3 of inpatient rehabilitation is associated with LOS. For the rehabilitation physician, this is useful information as the earlier identification of patients with poorly controlled pain can lead to directed intervention, better patient care, and significant cost savings.