In this contribution, we present a new radiofrequency probe, a dual dielectric resonator used for transmission and reception of RF signals in Magnetic Resonance Microscopy. The probe is designed as a pair of magnetically coupled ceramic rings with the permittivity of 530 resonating at the Larmor frequency of protons at the magnetic field 17 T (730 MHz). The resonators are made of a unique low-loss ferroelectric composite ceramics, which allows doubling the scanned volume with increased signal-to-noise ratio in comparison to a metal solenoid probe. By adjusting the distance between the resonators and the position of the loop feed placed between them, the probe can be tuned and impedance-matched and does not require capacitors.
Magnetic resonance imaging (MRI) is an imaging technique exploiting the magnetic resonance (MR) of specific nuclear spins, like protons. In this article, MR probes based on dielectric ring resonators are investigated from a theoretical approach. We take advantage of the high-permittivity and low-loss properties of the ceramic material used for manufacturing these probes for microscopy applications. Magnetic resonance microscopy (MRM) aims at imaging tiny samples with a sufficient resolution to distinguish small details. In this framework, compact resonators, called volume probes, contain the investigated sample and are used for both signal transmission and reception. The newly developed semi-analytical model enables the estimation of the frequency of the first transverse electric mode of a cylindrical resonator. It also provides a method to compute the corresponding magnetic field distribution, the dielectric losses contributions from the probe and the sample, and the signal-to-noise ratio (SNR). The proposed approach aims at providing design guidelines for dielectric probes.
Genetic factors are known to contribute to Late Onset Alzheimer’s disease (LOAD) but their contribution to pathophysiology, specially to prodomic phases accessible to therapeutic approaches are far to be understood. To translate genetic risk of Alzheimer's disease (AD) into mechanistic insight, we generated transgenic mouse lines that express a ~195 kbp human BAC that includes only BIN1, a gene associated to LOAD. This model gives a modest BIN1 overexpression, dependent of the number of BAC copies. At 6 months of age, we detected impaired entorhinal cortex (EC)-hippocampal pathways with specific impairments in EC-dentate gyrus synaptic long-term potentiation, dendritic spines of granular cells and recognition episodic memory. Structural changes were quantified using MRI. Their whole-brain functional impact were analyzed using resting state fMRI with a hypoconnectivity centered on entorhinal cortex. These early phenotype defects independent of any changes in A-beta can be instrumental in the search for new AD drug targets.
In the past two decades, significant advances in magnetic resonance microscopy (MRM) have been made possible by a combination of higher magnetic fields and more robust data acquisition technologies. This technical progress has enabled a shift in MRM applications from basic anatomical investigations to dynamic and functional studies, boosting the use of MRM in biological and life sciences. This book provides a simple introduction to MRM emphasizing practical aspects relevant to high magnetic fields. It focuses on biological applications and presents a number of selected examples of neuroscience applications. The text is mainly intended for those who are beginning research in the field of MRM or are planning to incorporate high-resolution MRI in their neuroscience studies.
Background Sedation agents affect brain hemodynamic and metabolism leading to specific modifications of the cerebral blood oxygenation level. We previously demonstrated that ultra-high field (UHF) MRI detects changes in cortical blood oxygenation following the administration of sedation drugs commonly used in animal research. Here we applied the UHF-MRI method to study clinically relevant sedation drugs for their effects on cortical and subcortical (thalamus, striatum) oxygenation levels. Methods We acquired T2*-weighted images of Sprague-Dawley rat brains at 17.2T in vivo. During each MRI session, rats were first anesthetized with isoflurane, then with a second sedative agent (sevoflurane, propofol, midazolam, medetomidine or ketamine-xylazine) after stopping isoflurane. We computed a T2*-oxygenation-ratio that aimed at estimating cerebral blood oxygenation level for each sedative agent in each region of interest: cortex, hippocampus, thalamus and striatum. Results The T2*-oxygenation-ratio was consistent across scan sessions. This ratio was higher with inhalational agents than with intravenous agents. Under sevoflurane and medetomidine, T2*-oxygenation-ratio was homogenous across the brain regions. Intravenous agents (except medetomidine) induced a T2*-oxygenation-ratio imbalance between cortex and subcortical regions: T2*-oxygenation-ratio was higher in the cortex than the subcortical areas under ketamine-xylazine; T2*-oxygenation-ratio was higher in subcortical regions than in the cortex under propofol or midazolam. Conclusion Preclinical UHF MRI is a powerful method to monitor the changes in cerebral blood oxygenation level induced by sedative agents across brain structures. This approach also allows for a classification of sedative agents based on their differential effects on cerebral blood oxygenation level.
During general anesthesia it is crucial to control systemic hemodynamics and oxygenation levels. However, anesthetic agents can affect cerebral hemodynamics and metabolism in a drug-dependent manner, while systemic hemodynamics is stable. Brain-wide monitoring of this effect remains highly challenging. Because T2*-weighted imaging at ultra-high magnetic field strengths benefits from a dramatic increase in contrast to noise ratio, we hypothesized that it could monitor anesthesia effects on brain blood oxygenation. We scanned rat brains at 7T and 17.2T under general anesthesia using different anesthetics (isoflurane, ketamine-xylazine, medetomidine). We showed that the brain/vessels contrast in T2*-weighted images at 17.2T varied directly according to the applied pharmacological anesthetic agent, a phenomenon that was visible, but to a much smaller extent at 7T. This variation is in agreement with the mechanism of action of these agents. These data demonstrate that preclinical ultra-high field MRI can monitor the effects of a given drug on brain blood oxygenation level in the absence of systemic blood oxygenation changes and of any neural stimulation.
