Layer-specific connectivity revealed by diffusion-weighted functional MRI in the rat thalamocortical pathway

Investigating neural activity from a global brain perspective in-vivo has been in the domain of functional Magnetic Resonance Imaging (fMRI) over the past few decades. The intricate neurovascular couplings that govern fMRI's blood-oxygenation-level-dependent (BOLD) functional contrast are invaluable in mapping active brain regions, but they also entail significant limitations, such as non-specificity of the signal to active foci. Diffusion-weighted functional MRI (dfMRI) with relatively high diffusion-weighting strives to ameliorate this shortcoming as it offers functional contrasts more intimately linked with the underlying activity. Insofar, apart from somewhat smaller activation foci, dfMRI's contrasts have not been convincingly shown to offer significant advantages over BOLD-driven fMRI, and its activation maps relied on significant modelling. Here, we study whether dfMRI could offer a better representation of neural activity in the thalamocortical pathway compared to its (spin-echo (SE)) BOLD counterpart. Using high-end forepaw stimulation experiments in the rat at 9.4 T, and with significant sensitivity enhancements due to the use of cryocoils, we show for the first time that dfMRI signals exhibit layer specificity, and, additionally, display signals in areas devoid of SE-BOLD responses. We find that dfMRI signals in the thalamocortical pathway cohere with each other, namely, dfMRI signals in the ventral posterolateral (VPL) thalamic nucleus cohere specifically with layers IV and V in the somatosensory cortex. These activity patterns are much better correlated (compared with SE-BOLD signals) with literature-based electrophysiological recordings in the cortex as well as thalamus. All these findings suggest that dfMRI signals better represent the underlying neural activity in the pathway. In turn, these advanatages may have significant implications towards a much more specific and accurate mapping of neural activity in the global brain in-vivo

The (un)conscious mouse as a model for human brain functions: key principles of anesthesia and their impact on translational neuroimaging

In recent years, technical and procedural advances have brought functional magnetic resonance imaging (fMRI) to the field of murine neuroscience. Due to its unique capacity to measure functional activity non-invasively, across the entire brain, fMRI allows for the direct comparison of large-scale murine and human brain functions. This opens an avenue for bidirectional translational strategies to address fundamental questions ranging from neurological disorders to the nature of consciousness. The key challenges of murine fMRI are: (1) to generate and maintain functional brain states that approximate those of calm and relaxed human volunteers, while (2) preserving neurovascular coupling and physiological baseline conditions. Low-dose anesthetic protocols are commonly applied in murine functional brain studies to prevent stress and facilitate a calm and relaxed condition among animals. Yet, current mono-anesthesia has been shown to impair neural transmission and hemodynamic integrity. By linking the current state of murine electrophysiology, Ca(2+) imaging and fMRI of anesthetic effects to findings from human studies, this systematic review proposes general principles to design, apply and monitor anesthetic protocols in a more sophisticated way. The further development of balanced multimodal anesthesia, combining two or more drugs with complementary modes of action helps to shape and maintain specific brain states and relevant aspects of murine physiology. Functional connectivity and its dynamic repertoire as assessed by fMRI can be used to make inferences about cortical states and provide additional information about whole-brain functional dynamics. Based on this, a simple and comprehensive functional neurosignature pattern can be determined for use in defining brain states and anesthetic depth in rest and in response to stimuli. Such a signature can be evaluated and shared between labs to indicate the brain state of a mouse during experiments, an important step toward translating findings across species

Étude de la réponse hémodynamique dans un modèle réussi de vieillissement chez le rat par imagerie optique intrinsèque

RESUME L'enjeu de cette these de doctorat est de mesurer les changements de parametres neurovasculaires et les modications de la reponse hemodynamique au cours du vieillissement chez le rat. La reponse hemodynamique est le processus par lequel, suite a une augmentation de l'activite neuronale, il se produit une augmentation locale du debit sanguin pour suivre les changements de l'activite metabolique venant de l'activite neuronale. La mesure de la reponse hemodynamique est a la base de l'imagerie fonctionnelle et permet de suivre de facon indirecte les changements d'activite neuronale a travers le couplage neurovasculaire. Toutefois, de nombreuses proprietes physiologiques du cerveau (debit, volume sanguin, compliance des vaisseaux, densite vasculaire, etc.) peuvent ^etre modiees au cours du vieillissement et aecter le couplage neurovasculaire. Cette these vise donc a mieux comprendre les changements de couplage neurovasculaire au cours du vieillissement et leur eet sur les mesures obtenues en imagerie fonctionnelle. Dans le cadre de cette these, les changements de reponse hemodynamique ont ete mesur es a l'aide d'un systeme d'imagerie optique intrinseque(IOI) developpe au laboratoire. Cette technique recente d'imagerie cerebrale se base sur les proprietes d'absorption de la lumiere visible dans le cortex. La technique a une tres bonne resolution spatiale et une faible profondeur de penetration ce qui en fait une technique tres bien adaptee a l'etude chez le rat. En IOI, la lumiere visible illumine le cortex et voyage dans la couche supercielle du cerveau avant d'^etre re echie a la surface du cortex et mesuree a l'aide d'une camera CCD. Lors de sa propagation a travers le cortex, une partie de la lumiere est absorbee par les deux principaux chromophores presents (l'hemoglobine et la deoxyhemoglobine). Ainsi, les changements de concentration d'un chromophore peuvent ^etre determines a travers des changements d'intensit e lumineuse. Ce qui permet ensuite d'etudier les concentrations de sang oxygene et desoxygene. En plus des mesures d'IOI, le systeme mesure simultanement les changements de debit sanguin a travers une mesure par granularite laser. La premiere partie des resultats mesure les changements de la reponse hemodynamique au cours du vieillissement. Les principales observations consistent en une diminution de l'amplitude de la reponse hemodynamique et une augmentation du temps d'activation de la reponse hemodynamique en fonction de l'^age. On observa aussi des changements spatiaux de la reponse hemodynamique. Ainsi, l'amplitude de la reponse hemodynamique diminue plus lentement en fonction----------ABSTRACT The aim of this thesis is to measure changes in neurovascular parameters and in hemodynamic response in aging rats. The hemodynamic response is the process by which, following an increase of neuronal activity, the blood ow increase locally to follow the changes in metabolic activity from neural activity. Measuring the hemodynamic response is the key process of functional imaging and allows to monitor indirectly changes of neuronal activity through the neurovascular coupling. However, many physiological properties in the brain ( ow, blood volume, vessel compliance, vascular density, etc.) may be modied during aging and aect neurovascular coupling. The thesis aims to better understand the changes in neurovascular coupling during aging and their eect measurements obtained in functional imaging. In this work, changes in hemodynamic response were measured using an intrinsic optical imaging system (IOI) developed in the laboratory. This recent imaging technique is based on the absorption properties of the visible light in the cortex. The technique has a very good spatial resolution and low depth of penetration which makes it well suited to study brain activity in rats. In IOI, visible light illuminates the cortex and travels in the surface layer of the brain before being re ected at the surface of the cortex and measured using a CCD camera. In the journey through the cortex, a part of the light is absorbed by the two main chromophores present (hemoglobin and deoxyhemoglobin). Thus, changes in chromophore concentration can be determined through changes in light intensity. This allows to study the concentrations of oxygenated blood and deoxygenated. In addition to measures of IOI, the system simultaneously measures changes in blood ow through a laser speckle measurement technique. The rst part of the results then shows the changes in the hemodynamic response during aging. The main observations is that during aging we observe a decrease in the amplitude of the hemodynamic response and an increase in the activation time of the hemodynamic response. Age also produce changes of the hemodynamic response. Thus, the amplitude of the hemodynamic response decreases more slowly as a function of aging on the side ipsilateral to stimulation from the contralateral side. The second part of the work studies neurovascular coupling using three dierent biophysical models. The three biophysical models can reproduce the dierent types of hemodynamic response found in our rats population. A comparison of models by log evidence did not foound signicative dierence between the performance of the three models. However, the model-Boas Huppert that we developed has an advantage of nding neurovascular restin

Magnetic resonance imaging of appetite-induced hypothalamic activity

Tesis doctoral inédita leida en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 26-10-2013La obesidad es un síndrome pandémico que subyace a las enfermedades más prevalentes y mórbidas en los países desarrollados. Es resultado de un desequilibrio en la regulación del apetito y del gasto energético; dos mecanismos que están fundamentalmente controlados por el hipotálamo. Las técnicas de imagen por resonancia magnética constituyen una herramienta excelente de evaluación anatómica y funcional del cerebro, en condiciones fisiológicas y patológicas, proporcionando un constante creciente tipo de aplicaciones. La tesis que aquí presento se ha centrado en el desarrollo de nuevas estrategias para evaluar de forma no invasiva los procesos de regulación cerebral del apetito en ratones y seres humanos, utilizando técnicas de resonancia magnética pesada en difusión. El Capítulo 1 proporciona una introducción a los principios básicos de la imagen por resonancia magnética, así como alguna de sus aplicaciones, recopilando las principales nociones fisiológicas de los mecanismos hipotalámicos de la regulación del apetito. También incluye s una breve compilación de las técnicas de neuroimagen más utilizadas en la evaluación de procesos relacionados con el apetito. En el Capítulo 2 describo el desarrollo y la implementación de una nueva técnica de imagen funcional basada en difusión para la detección de la actividad hipotalámica por apetito, en ratones y en seres humanos. El Capítulo 3 está dedicado a la aplicación de esta técnica a cuatro situaciones experimentales distintas, diseñadas para evaluar la respuesta específica de los núcleos hipotalámicos en procesos en los que la regulación del apetito y el balance energético están alterados. Finalmente, en el Capítulo 4 muestro la comparación del uso de la metodología analizada con distintos modelos de difusión- con los resultados obtenidos mediante la aplicación de una técnica funcional más convencional, ambas aplicadas al estudio de los efectos hipotalámicos de la administración de glucosa a ratones ayunados. En conclusión, mi Tesis doctoral demuestra que la técnica de imagen de resonancia magnética pesada en difusión proporciona un instrumento nuevo y robusto para el estudio de la regulación del apetito de forma no invasiva.Obesity is a pandemic syndrome that underlies the most morbid and prevalent diseases in developed countries. It results from an imbalance in appetite regulation and energy expenditure, two processes that are fundamentally controlled by the hypothalamus. Magnetic Resonance Imaging methods are excellently endowed to assess brain anatomy and function, under physiological and pathological conditions, providing an always increasing array of approaches. In this dissertation, I will introduce a collection of new strategies to evaluate non-invasively appetite regulation in the brain of mice and humans, based in the use of diffusion weighted magnetic resonance imaging methods. Chapter 1 introduces general concepts on the magnetic resonance imaging phenomenon and its applications, reviewing the key physiological mechanisms supporting hypothalamic appetite regulation. A short compilation of the most common neuroimaging techniques used to evaluate appetiterelated processes is also included. Chapter 2 describes the development and implementation of a new functional diffusion weighted imaging method applied to the detection of hypothalamic activity by fasting in mice and humans. Chapter 3 covers four different experimental manipulations designed to probe the role of specific hypothalamic nuclei in the regulation of appetite control and energy balance, under conditions where these are intentionally altered. Finally, Chapter 4 compares the use of the methodology analysed with different models of diffusion- with the results obtained with a more conventional functional imaging technique, both applied to the paradigm of glucose administration to fasting mice. In conclusion, this dissertation demonstrates that diffusion weighted magnetic resonance imaging methods provide a novel and useful approach to investigate appetite regulation non-invasively

QUANTATITIVE STUDY OF WATER DYNAMICS IN BIOMIMETIC MODELS AND LIVING TISSUE BY NMR AND MRI: PERSPECTIVES ON DIRECT DETECTION NEURONAL ACTIVITY

Detection of neuronal activity noninvasively and in vivo is a desideratum in medicine and in neuroscience. Unfortunately, the widely used method of functional magnetic resonance imaging (fMRI) only indirectly assesses neuronal activity via its hemodynamic response; limiting its temporal and spatial accuracy. Recently, several new fMRI methods have been proposed to measure neuronal activity claiming to be more direct and accurate. However, these approaches have proved difficult to reproduce and are not widely applied mainly because of a dearth of “ground truth” experiments that convincingly establish the correlation between the magnetic resonance (MR) signals and the underlying neuronal activity. In addition, limited knowledge of water dynamics in living tissue restricts our understanding of the underlying biophysical sources of these candidate fMRI signals. To address the first problem, we developed a novel test system to assess and validate fMRI methods, in which real-time fluorescent intracellular calcium images and MR recording were simultaneously acquired on organotypic rat-cortex cultures without hemodynamic confounds. This experimental design enables direct correlation of the candidate functional MR signals with optical indicia of the underlying neuronal activity. Within this test bed, MR signals with contrasts from water relaxation times, diffusion, and proton density were tested. Diffusion MR was the only one shown to be sensitive to the pathological condition of hyperexcitability, e.g., such as those seen in epilepsy. However, these MR signals do not appear to be sensitive or specific enough to detect and follow normal neuronal activity. Efforts were made toward improving our understanding of the water dynamics in living tissue. First, water diffusivities and relaxation times in a biomimetic model were measured and quantitatively studied using different biophysical-based mathematical models. Second, we developed and applied a rapid 2D diffusion/relaxation spectral MR method, to better characterize the heterogeneous nature of tissue water. While the present study is still far from providing a complete picture of water dynamics in living tissues, it provides novel tools for advancing our understanding of the possibilities and limits of detecting neuronal activity via MR in the future, as well as providing a reproducible and reliable way to assess and validate fMRI methods

Functional Electrical Impedance Tomography of adult and neonatal brain function.

Electrical Impedance Tomography (EIT) is a fast, portable imaging technique that produces tomographic images of the internal impedance of an object from surface electrode measurements. This thesis reports the first use of EIT to image evoked brain activity in adults and neonates and determines whether accurate EIT images could be obtained from the adult and neonatal brain. In addition, a realistic head-tank phantom was developed to test the performance of EIT with known impedance changes placed within a real human skull. Two EIT systems were used. Images were obtained using 31 or 21 Ag/AgCl EEG scalp electrodes in adults and neonates, respectively, with either 256 or 187 individual impedance measurements from different electrode combinations: 2 applied a safe, alternating current and 2 measured the resultant scalp voltage. Imaging was performed using a block design with 6-15 stimulation periods of between 10-75s during either: 1) Visual, 2) Somatosensory or 3) Motor stimuli. Impedance changes were detected in 38/39 adults and 9/9 neonates within 0.6-5.8s after stimulus onset, and returned to baseline 7.6-36s after stimulus cessation. Reconstructed images were noisy: -20-70% images showed correct localisation to the expected area of cortex stimulated by the visual, motor or somatosensory paradigms. As EIT images from the head-tank localised changes within 10% of the impedance perturbation, this indicated that poor localisation in humans was not due to the head-shape or the skull, but may be related to unknown physiological factors. An improved EIT reconstruction algorithm, using a computerised finite-element model of the head, showed improved localisation for the adult images. This is the first demonstration that EIT can detect and image impedance changes in the head, probably due to increased regional cerebral blood volume in the activated cortex. Improvements may enable more accurate neuroimaging of the adult and neonatal brain for use in clinical practice

Techniques for imaging small impedance changes in the human head due to neuronal depolarisation

A new imaging modality is being developed, which may be capable of imaging small impedance changes in the human head due to neuronal depolarization. One way to do this would be by imaging the impedance changes associated with ion channels opening in neuronal membranes in the brain during activity. The results of previous modelling and experimental studies indicated that impedance changes between 0.6%and 1.7% locally in brain grey matter when recorded at DC. This reduces by a further of 10% if measured at the surface of the head, due to distance and the effect of the resistive skull. In principle, this could be measured using Electrical Impedance Tomography (ElT) but it is close to its threshold of detectability. With the inherent limitation in the use of electrodes, this work proposed two new schemes. The first is a magnetic measurement scheme based on recording the magnetic field with Superconducting Quantum Interference Devices (SQUIDs), used in Magnetoencephalography (MEG) as a result of a non-invasive injection of current into the head. This scheme assumes that the skull does not attenuate the magnetic field. The second scheme takes into consideration that the human skull is irregular in shape, with less and varying conductivity as compared to other head tissues. Therefore, a key issue is to know through which electrodes current can be injected in order to obtain high percentage changes in surface potential when there is local conductivity change in the head. This model will enable the prediction of the current density distribution at specific regions in the brain with respect to the varying skull and local conductivities. In the magnetic study, the head was modelled as concentric spheres, and realistic head shapes to mimic the scalp, skull, Cerebrospinal Auid (CSF) and brain using the Finite Element Method (FEM). An impedance change of 1 % in a 2cm-radius spherical volume depicting the physiological change in the brain was modelled as the region of depolarisation. The magnetic field, 1 cm away from the scalp, was estimated on injecting a constant current of 100 µA into the head from diametrically opposed electrodes. However, in the second scheme, only the realistic FEM of the head was used, which included a specific region of interest; the primary visual cortex (V1). The simulated physiological change was the variation in conductivity of V1 when neurons were assumed to be firing during a visual evoked response. A near DC current of 100 µA was driven through possible pairs of 31 electrodes using ElT techniques. For a fixed skull conductivity, the resulting surface potentials were calculated when the whole head remained unperturbed, or when the conductivity of V1 changed by 0.6%, 1 %, and 1.6%. The results of the magnetic measurement predicted that standing magnetic field was about 10pT and the field changed by about 3fT (0.03%) on depolarization. For the second scheme, the greatest mean current density through V1 was 0.020 ± 0.005 µAmm-2, and occurred with injection through two electrodes positioned near the occipital cortex. The corresponding maximum change in potential from baseline was 0.02%. Saline tank experiments confirmed the accuracy of the estimated standing potentials. As the noise density in a typical MEG system in the frequency band is about 7fT/√Hz, it places the change at the limit of detectability due to low signal to noise ratio. This is therefore similar to electrical recording, as in conventional ElT systems, but there may be advantages to MEG in that the magnetic field direcdy traverses the skull and instrumentation errors from the electrode-skin interface will be obviated. This has enabled the estimation of electrode positions most likely to permit recording of changes in human experiments and suggests that the changes, although tiny, may just be discernible from noise

Electrical impedance tomography of human brain function.

Electroencephalography (EEG) has been used for over 70 years to record the electrical signals of the brain. Electrical impedance tomography (EIT) is a more recent imaging technique which when applied to brain function and structure has the potential to provide a rapid portable bedside neuroimaging device. The purpose of this work has been to investigate several applications of EIT and EEG in the imaging of brain function. EEG does not always give the required spatial information, especially if the current generator is in the deep brain structures such as the hypothalamus. Dipole source localisation has become a common research tool that can be used estimate the current sources that are responsible for the EEG signals recorded on the scalp. Using this method, the accuracy and ease of use for four commercially available headnets was assessed. No headnet performed better at localisation, with all localising one dipole well, and two or three dipoles poorly. EIT has the potential to image the impedance changes that occur during neuronal depolarisation. Modelling work has been carried out to predict the size of these impedance changes and this thesis presents some work carried out in an attempt to record these changes in human subjects. The levels of noise at present are too great to record the impedance changes, but suggestion for improving the signal to noise ratio are given. Previous work on EIT and fMRI studies has shown that there are changes in blood volume (and as a result changes in impedance) after interictal spike activity. The impedance changes relating to the blood flow response to interictal epileptiform activity were recorded using EEG-correlated continuous EIT acquisition from scalp electrodes from patients on telemetry. Despite averaging up to 900 spikes, there was no recordable change in impedance after the interictal activity. Bioimpedance changes also occur due to pathological conditions. Multifrequcncy HIT makes use of the differences in impedance properties between healthy and ischaemic or tumour tissue, in an attempt to image these conditions. Data were collected from patients with tumours and other conditions and healthy volunteers, and the raw data and images compared. No differences were seen in the raw data between the different patients groups thought changes were seen in individual patients. . These results will inform the design of an EIT system which operates at a lower frequency band where the largest changes in impedance are seen