Leptomeningeal collaterals regulate reperfusion in ischemic stroke and rescue the brain from futile recanalization.

Recanalization is the mainstay of ischemic stroke treatment. However, even with timely clot removal, many stroke patients recover poorly. Leptomeningeal collaterals (LMCs) are pial anastomotic vessels with yet-unknown functions. We applied laser speckle imaging, ultrafast ultrasound, and two-photon microscopy in a thrombin-based mouse model of stroke and fibrinolytic treatment to show that LMCs maintain cerebral autoregulation and allow for gradual reperfusion, resulting in small infarcts. In mice with poor LMCs, distal arterial segments collapse, and deleterious hyperemia causes hemorrhage and mortality after recanalization. In silico analyses confirm the relevance of LMCs for preserving perfusion in the ischemic region. Accordingly, in stroke patients with poor collaterals undergoing thrombectomy, rapid reperfusion resulted in hemorrhagic transformation and unfavorable recovery. Thus, we identify LMCs as key components regulating reperfusion and preventing futile recanalization after stroke. Future therapeutic interventions should aim to enhance collateral function, allowing for beneficial reperfusion after stroke.

Specific and Nonuniform Brain States during Cold Perception in Mice.

The quest to decode the complex supraspinal mechanisms that integrate cutaneous thermal information in the central system is still ongoing. The dorsal horn of the spinal cord is the first hub that encodes thermal input which is then transmitted to brain regions via the spinothalamic and thalamocortical pathways. So far, our knowledge about the strength of the interplay between the brain regions during thermal processing is limited. To address this question, we imaged the brains of adult awake male mice in resting state using functional ultrasound imaging during plantar exposure to constant and varying temperatures. Our study reveals for the first time the following: (1) a dichotomy in the response of the somatomotor-cingulate cortices and the hypothalamus, which was never described before, due to the lack of appropriate tools to study such regions with both good spatial and temporal resolutions. (2) We infer that cingulate areas may be involved in the affective responses to temperature changes. (3) Colder temperatures (ramped down) reinforce the disconnection between the somatomotor-cingulate and hypothalamus networks. (4) Finally, we also confirm the existence in the mouse brain of a brain mode characterized by low cognitive strength present more frequently at resting neutral temperature. The present study points toward the existence of a common hub between somatomotor and cingulate regions, whereas hypothalamus functions are related to a secondary network.

In vivo whole brain microvascular imaging in mice using transcranial 3D Ultrasound Localization Microscopy.

BACKGROUND: Non-invasive high-resolution imaging of the cerebral vascular anatomy and function is key for the study of intracranial aneurysms, stenosis, arteriovenous malformations, and stroke, but also neurological pathologies, such as degenerative diseases. Direct visualization of the microvascular networks in the whole brain remains however challenging in vivo. METHODS: In this work, we performed 3D ultrafast ultrasound localization microscopy (ULM) using a 2D ultrasound matrix array and mapped the whole-brain microvasculature and flow at microscopic resolution in C57Bl6 mice in vivo. FINDINGS: We demonstrated that the mouse brain vasculature can be imaged directly through the intact skull at a spatial resolution of 20 µm and over the whole brain depth and at high temporal resolution (750 volumes.s-1). Individual microbubbles were tracked to estimate the flow velocities that ranged from 2 mm.s-1 in arterioles and venules up to 100 mm.s-1 in large vessels. The vascular maps were registered automatically with the Allen atlas in order to extract quantitative vascular parameters such as local flow rates and velocities in regions of interest. INTERPRETATION: We show the potential of 3D ULM to provide new insights into whole-brain vascular flow in mice models at unprecedented vascular scale for an in vivo technique. This technology is highly translational and has the potential to become a major tool for the clinical investigation of the cerebral microcirculation. FUNDING: This study was supported by the European Research Council under the European Union's Seventh Framework Program (FP/2007-2013) / ERC Grant Agreement n° 311025 and by the Fondation Bettencourt-Schueller under the program "Physics for Medicine". We acknowledge the ART (Technological Research Accelerator) biomedical ultrasound program of INSERM.

XDoppler: Cross-Correlation of Orthogonal Apertures for 3D Blood Flow Imaging.

Row column addressing (RCA) transducers have the potential to provide volumetric imaging at ultrafast frame rate using a low channel count over a large field of view. In previous works we have shown that vascular imaging of large arteries as well as functional neuroimaging of the rat brain were feasible using RCA orthogonal plane wave imaging (OPW), but these applications required to transmit many plane waves, significantly reducing the frame rate. In this study, we introduce XDoppler imaging, a novel method to increase the performances of RCA flow imaging by taking advantage of the blood spatial decorrelation statistics combined with the limited spatial overlap of the point spread functions (PSF) of the two orthogonal apertures of the RCA transducer. We provide at first a theoretical basis to understand how the correlation operation reduces the sidelobe level. Then, we demonstrate both in vitro and in vivo in the human carotid artery and in the rat brain that XDoppler provides a significant gain in contrast-to-noise ratio (CNR) (between 3 and 6 dB depending on the application) compared to OPW. This improvement also leads to a sensitivity increase in the rat brain as more blood vessels are detected by XDoppler imaging.

Whole-Brain 3D Activation and Functional Connectivity Mapping in Mice using Transcranial Functional Ultrasound Imaging.

Functional ultrasound (fUS) imaging is a novel brain imaging modality that relies on the high-sensitivity measure of the cerebral blood volume achieved by ultrafast doppler angiography. As brain perfusion is strongly linked to local neuronal activity, this technique allows the whole-brain 3D mapping of task-induced regional activation as well as resting-state functional connectivity, non-invasively, with unmatched spatio-temporal resolution and operational simplicity. In comparison with fMRI (functional magnetic resonance imaging), a main advantage of fUS imaging consists in enabling a complete compatibility with awake and behaving animal experiments. Moreover, fMRI brain mapping in mice, the most used preclinical model in Neuroscience, remains technically challenging due to the small size of the brain and the difficulty to maintain stable physiological conditions. Here we present a simple, reliable and robust protocol for whole-brain fUS imaging in anesthetized and awake mice using an off-the-shelf commercial fUS system with a motorized linear transducer, yielding significant cortical activation following sensory stimulation as well as reproducible 3D functional connectivity pattern for network identification.

Pharmaco-fUS: Quantification of pharmacologically-induced dynamic changes in brain perfusion and connectivity by functional ultrasound imaging in awake mice.

There is a critical need for reliable quantitative biomarkers to assess functional brain alterations in mouse models of neuropsychiatric diseases, but current imaging methods measuring drug effects through the neurovascular coupling, face issues including poor sensitivity, drug-induced changes in global brain perfusion and the effects of anesthesia. Here we demonstrate the proof-of-concept of a minimally-invasive fUS imaging approach to detect the acute cholinergic modulatory effects of Scopolamine (ScoP) on functional brain connectivity in awake and behaving mice, through the intact skull. A machine-learning algorithm constructed an ad-hoc pharmacological score from the ScoP-induced changes in connectivity patterns of five mice. The discrimination model shows important ScoP-induced increase of the hippocampo-cortical connectivity. The pharmacological score led to robust discrimination of ScoP treatment from baseline in an independent dataset and showed, in another independent group, dose-dependent specific effects of central cholinergic modulation of functional connectivity, independent from global brain perfusion changes. In conclusion, we introduce pharmaco-fUS as a simple, robust, specific and sensitive modality to monitor drug effects on perfusion and functional connectivity in the awake mouse brain.