A method to image brain tissue frozen at autopsy.

Magnetic Resonance Imaging (MRI) can provide the location and signal characteristics of pathological regions within a postmortem tissue block, thereby improving the efficiency of histopathological studies. However, such postmortem-MRI guided histopathological studies have so far only been performed on fixed samples as imaging tissue frozen at the time of extraction, while preserving its integrity, is significantly more challenging. Here we describe the development of cold-postmortem-MRI, which can preserve tissue integrity and help target techniques such as transcriptomics. As a first step, RNA integrity number (RIN) was used to determine the rate of tissue biomolecular degradation in mouse brains placed at various temperatures between -20 °C and +20 °C for up to 24 h. Then, human tissue frozen at the time of autopsy was immersed in 2-methylbutane, sealed in a bio-safe tissue chamber, and cooled in the MRI using a recirculating chiller to determine MRI signal characteristics. The optimal imaging temperature, which did not show significant RIN deterioration for over 12 h, at the same time giving robust MRI signal and contrast between brain tissue types was deemed to be -7 °C. Finally, MRI was performed on human tissue blocks at this optimal imaging temperatures using a magnetization-prepared rapid gradient echo (MPRAGE, isotropic resolution between 0.3-0.4 mm) revealing good gray-white matter contrast and revealing subpial, subcortical, and deep white matter lesions. RINs measured before and after imaging revealed no significant changes (n = 3, p = 0.18, paired t-test). In addition to improving efficiency of downstream processes, imaging tissue at sub-zero temperatures may also improve our understanding of compartment specificity of MRI signal.

Epidemiology of protozoan and helminthic parasites in wild passerine birds of Britain and Ireland.

Avian endoparasites play important roles in conservation, biodiversity and host evolution. Currently, little is known about the epidemiology of intestinal helminths and protozoans infecting wild birds of Britain and Ireland. This study aimed to determine the rates of parasite prevalence, abundance and infection intensity in wild passerines. Fecal samples (n = 755) from 18 bird families were collected from 13 sites across England, Wales and Ireland from March 2020 to June 2021. A conventional sodium nitrate flotation method allowed morphological identification and abundance estimation of eggs/oocysts. Associations with host family and age were examined alongside spatiotemporal and ecological factors using Bayesian phylogenetically controlled models. Parasites were detected in 20.0% of samples, with corvids and finches having the highest prevalences and intensities, respectively. Syngamus (33%) and Isospora (32%) were the most prevalent genera observed. Parasite prevalence and abundance differed amongst avian families and seasons, while infection intensity varied between families and regions. Prevalence was affected by diet diversity, while abundance differed by host age and habitat diversity. Infection intensity was higher in birds using a wider range of habitats, and doubled in areas with feeders present. The elucidation of these patterns will increase the understanding of parasite fauna in British and Irish birds.

Multifield and inverse-contrast switching of magnetocaloric high contrast ratio MRI labels.

PURPOSE: Demonstrating multifield and inverse contrast switching of magnetocaloric high contrast ratio MRI labels that either have increasing or decreasing moment versus temperature slopes depending on the material at physiological temperatures and different MRI magnetic field strengths. METHODS: Two iron-rhodium samples of different purity (99% and 99.9%) and a lanthanum-iron-silicon sample were obtained from commercial vendors. Temperature and magnetic field-dependent magnetic moment measurements of the samples were performed on a vibrating sample magnetometer. Temperature-dependent MRI of different iron-rhodium and lanthanum-iron-silicon samples were performed on 3 different MRI scanners at 1 Tesla (T), 4.7T, and 7T. RESULTS: Sharp, first-order magnetic phase transition of each iron-rhodium sample at a physiologically relevant temperature (~37°C) but at different MRI magnetic fields (1T, 4.7T, and 7T, depending on the sample) showed clear image contrast changes in temperature-dependent MRI. Iron-rhodium and lanthanum-iron-silicon samples with sharp, first-order magnetic phase transitions at the same MRI field of 1T and physiological temperature of 37°C, but with positive and negative slope of magnetization versus temperature, respectively, showed clear inverse contrast image changes. Temperature-dependent MRI on individual microparticle samples of lanthanum-iron-silicon also showed sharp image contrast changes. CONCLUSION: Magnetocaloric materials of different purity and composition were demonstrated to act as diverse high contrast ratio switchable MRI contrast agents. Thus, we show that a range of magnetocaloric materials can be optimized for unique image contrast response under MRI-appropriate conditions at physiological temperatures and be controllably switched in situ.

Ex vivo MR microscopy of a human brain with multiple sclerosis: Visualizing individual cells in tissue using intrinsic iron.

PURPOSE: To perform magnetic resonance microscopy (MRM) on human cortex and a cortical lesion as well as the adjacent normal appearing white matter. To shed light on the origins of MRI contrast by comparison with histochemical and immunostaining. METHODS: 3D MRM at a nominal isotropic resolution of 15 and 18 µm was performed on 2 blocks of tissue from the brain of a 77-year-old man who had MS for 47 years. One block contained normal appearing cortical gray matter (CN block) and adjacent normal appearing white matter (NAWM), and the other also included a cortical lesion (CL block). Postmortem ex-vivo MRI was performed at 11.7T using a custom solenoid coil and T2*-weighted 3D GRE sequence. Histochemical and immunostaining were done after paraffin embedding for iron, myelin, oligodendrocytes, neurons, blood vessels, macrophages and microglia, and astrocytes. RESULTS: MRM could identify individual iron-laden oligodendrocytes with high sensitivity (70% decrease in signal compared to surrounding) in CN and CL blocks, as well as some iron-laden activated macrophages and microglia. Iron-deficient oligodendrocytes seemed to cause relative increase in MRI signal within the cortical lesion. High concentration of myelin in the white matter was primarily responsible for its hypointense appearance relative to the cortex, however, signal variations within NAWM could be attributed to changes in density of iron-laden oligodendrocytes. CONCLUSION: Changes in iron accumulation within cells gave rise to imaging contrast seen between cortical lesions and normal cortex, as well as the patchy signal in NAWM. Densely packed myelin and collagen deposition also contributed to MRM signal changes. Even though we studied only one block each from normal appearing and cortical lesions, such studies can help better understand the origins of histopathological and microstructural correlates of MRI signal changes in multiple sclerosis and contextualize the interpretation of lower-resolution in vivo MRI scans.

Sensory and optogenetically driven single-vessel fMRI.

Magnetic resonance imaging (MRI) sensitivity approaches vessel specificity. We developed a single-vessel functional MRI (fMRI) method to image the contribution of vascular components to blood oxygenation level-dependent (BOLD) and cerebral blood volume (CBV) fMRI signal. We mapped individual vessels penetrating the rat somatosensory cortex with 100-ms temporal resolution by MRI with sensory or optogenetic stimulation. The BOLD signal originated primarily from venules, and the CBV signal from arterioles. The single-vessel fMRI method and its combination with optogenetics provide a platform for mapping the hemodynamic signal through the neurovascular network with specificity at the level of individual arterioles and venules.

Magnetocaloric materials as switchable high contrast ratio MRI labels.

PURPOSE: To develop switchable and tunable labels with high contrast ratio for MRI using magnetocaloric materials that have sharp first-order magnetic phase transitions at physiological temperatures and typical MRI magnetic field strengths. METHODS: A prototypical magnetocaloric material iron-rhodium (FeRh) was prepared by melt mixing, high-temperature annealing, and ice-water quenching. Temperature- and magnetic field-dependent magnetization measurements of wire-cut FeRh samples were performed on a vibrating sample magnetometer. Temperature-dependent MRI of FeRh samples was performed on a 4.7T MRI. RESULTS: Temperature-dependent MRI clearly demonstrated image contrast changes due to the sharp magnetic state transition of the FeRh samples in the MRI magnetic field (4.7T) and at a physiologically relevant temperature (~37°C). CONCLUSION: A magnetocaloric material, FeRh, was demonstrated to act as a high contrast ratio switchable MRI contrast agent due to its sharp first-order magnetic phase transition in the DC magnetic field of MRI and at physiologically relevant temperatures. A wide range of magnetocaloric materials are available that can be tuned by materials science techniques to optimize their response under MRI-appropriate conditions and be controllably switched in situ with temperature, magnetic field, or a combination of both.

Optically controlled on-coil amplifier with RF monitoring feedback.

PURPOSE: To develop a new optically controlled on-coil amplifier that facilitates safe use of multi-channel radiofrequency (RF) transmission in MRI by real-time monitoring of signal phase and amplitude. METHODS: Monitoring was carried out with a 4-channel prototype system by sensing, down sampling, digitizing, and optically transmitting the RF transmit signal to a remote PC to control the amplifiers. Performance was evaluated with benchtop and 7 T MRI experiments. RESULTS: Monitored amplitude and phase were stable across repetitions and had standard deviations of 0.061 µT and 0.0073 rad, respectively. The feedback system allowed inter-channel phase and B1 amplitude to be adjusted within two iterations. MRI experiments demonstrated the feasibility of this approach to perform safe and accurate multi-channel RF transmission and monitoring at high field. CONCLUSION: We demonstrated a 4-channel transceiver system based on optically controlled on-coil amplifiers with RF signal monitoring and feedback control. The approach allows the safe and precise control of RF transmission fields, required to achieve uniform excitation at high field. Magn Reson Med 79:2833-2841, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Magnetic resonance imaging of odorant activity-dependent migration of neural precursor cells and olfactory bulb growth.

Neural progenitors or neuroblasts are produced by precursor cells in the subventricular zone (SVZ) and migrate along the rostral migratory stream (RMS) to the olfactory bulbs (OB) throughout life. In the OB, these adult born neurons either die or replace existing olfactory interneurons, playing a critical role in the stabilization of OB circuitry. Although several aspects of the addition of new neurons into the OB have been studied, it is unclear whether long-distance activity from the OB can regulate the influx of migrating neuroblasts along the RMS. In this study, iron oxide-assisted MRI was used to track the migration of neuroblasts in combination with reversible naris occlusion to manipulate odorant-induced activity. It was found that decreasing olfactory activity led to a decrease in the rate of neuroblast migration along the RMS. Removal of the naris occlusion led to an increase in migratory rate back to control levels, indicating that olfactory activity has regulatory function on neuroblast migration in the RMS. Blocking odorant activity also led to an arrest in OB growth and re-opening the block led to a rapid re-growth returning the bulb size to control levels. Furthermore, pharmacogenetic elimination of the neuroblasts demonstrated that they were required for re-growth of the bulb following sensory deprivation. Together, these results show that sensory activity, neural migration and OB growth are tightly coupled in an interdependent manner.

Deciphering laminar-specific neural inputs with line-scanning fMRI.

Using a line-scanning method during functional magnetic resonance imaging (fMRI), we obtained high temporal (50-ms) and spatial (50-µm) resolution information along the cortical thickness and showed that the laminar position of fMRI onset coincides with distinct neural inputs in rat somatosensory and motor cortices. This laminar-specific fMRI onset allowed us to identify the neural inputs underlying ipsilateral fMRI activation in the barrel cortex due to peripheral denervation-induced plasticity.

Breeding density, fine-scale tracking, and large-scale modeling reveal the regional distribution of four seabird species.

Population-level estimates of species' distributions can reveal fundamental ecological processes and facilitate conservation. However, these may be difficult to obtain for mobile species, especially colonial central-place foragers (CCPFs; e.g., bats, corvids, social insects), because it is often impractical to determine the provenance of individuals observed beyond breeding sites. Moreover, some CCPFs, especially in the marine realm (e.g., pinnipeds, turtles, and seabirds) are difficult to observe because they range tens to ten thousands of kilometers from their colonies. It is hypothesized that the distribution of CCPFs depends largely on habitat availability and intraspecific competition. Modeling these effects may therefore allow distributions to be estimated from samples of individual spatial usage. Such data can be obtained for an increasing number of species using tracking technology. However, techniques for estimating population-level distributions using the telemetry data are poorly developed. This is of concern because many marine CCPFs, such as seabirds, are threatened by anthropogenic activities. Here, we aim to estimate the distribution at sea of four seabird species, foraging from approximately 5,500 breeding sites in Britain and Ireland. To do so, we GPS-tracked a sample of 230 European Shags Phalacrocorax aristotelis, 464 Black-legged Kittiwakes Rissa tridactyla, 178 Common Murres Uria aalge, and 281 Razorbills Alca torda from 13, 20, 12, and 14 colonies, respectively. Using Poisson point process habitat use models, we show that distribution at sea is dependent on (1) density-dependent competition among sympatric conspecifics (all species) and parapatric conspecifics (Kittiwakes and Murres); (2) habitat accessibility and coastal geometry, such that birds travel further from colonies with limited access to the sea; and (3) regional habitat availability. Using these models, we predict space use by birds from unobserved colonies and thereby map the distribution at sea of each species at both the colony and regional level. Space use by all four species' British breeding populations is concentrated in the coastal waters of Scotland, highlighting the need for robust conservation measures in this area. The techniques we present are applicable to any CCPF.

Optically controlled switch-mode current-source amplifiers for on-coil implementation in high-field parallel transmission.

PURPOSE: We tested the feasibility of implementing parallel transmission (pTX) for high-field MRI using a radiofrequency (RF) amplifier design to be located on or in the immediate vicinity of an RF transmit coil. METHOD: We designed a current-source switch-mode amplifier based on miniaturized, nonmagnetic electronics. Optical RF carrier and envelope signals to control the amplifier were derived, through a custom-built interface, from the RF source accessible in the scanner control. Amplifier performance was tested by benchtop measurements as well as with imaging at 7T (300 MHz) and 11.7 T (500 MHz). The ability to perform pTX was evaluated by measuring interchannel coupling and phase adjustment in a two-channel setup. RESULTS: The amplifier delivered in excess of 44 W RF power and caused minimal interference with MRI. The interface derived accurate optical control signals with carrier frequencies ranging from 64 to 750 MHz. Decoupling better than 14 dB was obtained between two coil loops separated by only 1 cm. Application to MRI was demonstrated by acquiring artifact-free images at 7 T and 11.7 T. CONCLUSION: We propose an optically controlled miniaturized RF amplifier for on-coil implementation at high fields that should facilitate implementation of high-density pTX arrays. Magn Reson Med 76:340-349, 2016. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Live nephron imaging by MRI.

The local sensitivity of MRI can be improved with small MR detectors placed close to regions of interest. However, to maintain such sensitivity advantage, local detectors normally need to communicate with the external amplifier through cable connections, which prevent the use of local detectors as implantable devices. Recently, an integrated wireless amplifier was developed that can efficiently amplify and broadcast locally detected signals, so that the local sensitivity was enhanced without the need for cable connections. This integrated detector enabled the live imaging of individual glomeruli using negative contrast introduced by cationized ferritin, and the live imaging of renal tubules using positive contrast introduced by gadopentetate dimeglumine. Here, we utilized the high blood flow to image individual glomeruli as hyperintense regions without any contrast agent. These hyperintense regions were identified for pixels with signal intensities higher than the local average. Addition of Mn(2+) allowed the simultaneous detection of both glomeruli and renal tubules: Mn(2+) was primarily reabsorbed by renal tubules, which would be distinguished from glomeruli due to higher enhancement in T1-weighted MRI. Dynamic studies of Mn(2+) absorption confirmed the differential absorption affinity of glomeruli and renal tubules, potentially enabling the in vivo observation of nephron function.

Micro-engineered local field control for high-sensitivity multispectral MRI.

In recent years, biotechnology and biomedical research have benefited from the introduction of a variety of specialized nanoparticles whose well-defined, optically distinguishable signatures enable simultaneous tracking of numerous biological indicators. Unfortunately, equivalent multiplexing capabilities are largely absent in the field of magnetic resonance imaging (MRI). Comparable magnetic-resonance labels have generally been limited to relatively simple chemically synthesized superparamagnetic microparticles that are, to a large extent, indistinguishable from one another. Here we show how it is instead possible to use a top-down microfabrication approach to effectively encode distinguishable spectral signatures into the geometry of magnetic microstructures. Although based on different physical principles from those of optically probed nanoparticles, these geometrically defined magnetic microstructures permit a multiplexing functionality in the magnetic resonance radio-frequency spectrum that is in many ways analogous to that permitted by quantum dots in the optical spectrum. Additionally, in situ modification of particle geometries may facilitate radio-frequency probing of various local physiological variables.

Shaped Magnetogel Microparticles for Multispectral Magnetic Resonance Contrast and Sensing.

Multispectral magnetic resonance imaging (MRI) contrast agents are microfabricated three-dimensional magnetic structures that encode nearby water protons with discrete frequencies. The agents have a unique radiofrequency (RF) resonance that can be tuned by engineering the geometric parameters of these microstructures. Multispectral contrast agents can be used as sensors by incorporating a stimulus-driven shape-changing response into their structure. These geometrically encoded magnetic sensors (GEMS) enable MRI-based sensing via environmentally induced changes to their geometry and their corresponding RF resonance. Previously, GEMS have been made using thin-film lithography techniques in a cleanroom environment. While these approaches offer precise control of the microstructure, they can be a limitation for researchers who do not have cleanroom access or microfabrication expertise. Here, an alternative approach for GEMS fabrication based on soft lithography is introduced. The fabrication scheme uses cheap, accessible materials and simple chemistry to produce shaped magnetic hydrogel microparticles with multispectral MRI contrast properties. The microparticles can be used as sensors by fabricating them out of shape-reconfigurable, "smart" hydrogels. The change in shape causes a corresponding shift in the resonance of the GEMS, producing an MRI-addressable readout of the microenvironment. Proof-of-principle experiments showing a multispectral response to pH change with cylindrical shell-shaped magnetogel GEMS are presented.

Postmortem MRI of Tissue Frozen at Autopsy.

Introduction: Postmortem MRI provides insight into location of pathology within tissue blocks, enabling efficient targeting of histopathological studies. While postmortem imaging of fixed tissue is gaining popularity, imaging tissue frozen at the time of extraction is significantly more challenging. Methods: Tissue integrity was examined using RNA integrity number (RIN), in mouse brains placed between -20 °C and 20 °C for up to 24 hours, to determine the highest temperature that could potentially be used for imaging without tissue degeneration. Human tissue frozen at the time of autopsy was sealed in a tissue chamber filled with 2-methylbutane to prevent contamination of the MRI components. The tissue was cooled to a range of temperatures in a 9.4T MRI using a recirculating aqueous ethylene glycol solution. MRI was performed using a magnetization-prepared rapid gradient echo (MPRAGE) sequence with inversion time of 1400 ms to null the signal from 2-methylbutane bath, isotropic resolution between 0.3-0.4 mm, and scan time of about 4 hours was used to study the anatomical details of the tissue block. Results and Discussion: A temperature of -7 °C was chosen for imaging as it was below the highest temperature that did not show significant RIN deterioration for over 12 hours, at the same time gave robust imaging signal and contrast between brain tissue types. Imaging performed on various human tissue blocks revealed good gray-white matter contrast and revealing subpial, subcortical, and deep white matter lesions typical of multiple sclerosis enabling further spatially targeted studies. Conclusion: Here, we describe a new method to image cold tissue, while maintaining tissue integrity and biosafety during scanning. In addition to improving efficiency of downstream processes, imaging tissue at sub-zero temperatures may also improve our understanding of compartment specificity of MRI signal.

Wireless amplified nuclear MR detector (WAND) for high-spatial-resolution MR imaging of internal organs: preclinical demonstration in a rodent model.

PURPOSE: To assess the feasibility of imaging deep-lying internal organs at high spatial resolution by imaging kidney glomeruli in a rodent model with use of a newly developed, wireless amplified nuclear magnetic resonance (MR) detector. MATERIALS AND METHODS: This study was approved by the Animal Care and Use Committee at the National Institutes of Health/National Institute of Neurologic Disorder and Stroke. As a preclinical demonstration of this new detection technology, five different millimeter-scale wireless amplified nuclear MR detectors configured as double frequency resonators were chronically implanted on the medial surface of the kidney in five Sprague-Dawley rats for MR imaging at 11.7 T. Among these rats, two were administered gadopentetate dimeglumine to visualize renal tubules on T1-weighted gradient-refocused echo (GRE) images, two were administered cationized ferritin to visualize glomeruli on T2*-weighted GRE images, and the remaining rat was administered both gadopentetate dimeglumine and cationized ferritin to visualize the interleaved pattern of renal tubules and glomeruli. The image intensity in each pixel was compared with the local tissue signal intensity average to identify regions of hyper- or hypointensity. RESULTS: T1-weighted images with 70-µm in-plane resolution and 200-µm section thickness were obtained within 3.2 minutes to image renal tubules, and T2*-weighted images of the same resolution were obtained within 5.8 minutes to image the glomeruli. Hyperintensity from gadopentetate dimeglumine enabled visualization of renal tubules, and hypointensity from cationic ferritin enabled visualization of the glomeruli. CONCLUSION: High-spatial-resolution images have been obtained to observe kidney microstructures in vivo with a wireless amplified nuclear MR detector.

Progressive Spread of Beta-amyloid Pathology in an Olfactory-driven Amyloid Precursor Protein Mouse Model.

Years before Alzheimer's disease (AD) is diagnosed, patients experience an impaired sense of smell, and ß-amyloid plaques accumulate within the olfactory mucosa and olfactory bulb (OB). The olfactory vector hypothesis proposes that external agents cause ß-amyloid to aggregate and spread from the OB to connected downstream brain regions. To reproduce the slow accumulation of ß-amyloid that occurs in human AD, we investigated the progressive accumulation of ß-amyloid across the brain using a conditional mouse model that overexpresses a humanized mutant form of the amyloid precursor protein (hAPP) in olfactory sensory neurons. Using design-based stereology, we show the progressive accumulation of ß-amyloid plaques within the OB and cortical olfactory regions with age. We also observe reduced OB volumes in these mice when hAPP expression begins prior-to but not post-weaning which we tracked using manganese-enhanced MRI. We therefore conclude that the reduced OB volume does not represent progressive degeneration but rather disrupted OB development. Overall, our data demonstrate that hAPP expression in the olfactory epithelium can lead to the accumulation and spread of ß-amyloid through the olfactory system into the hippocampus, consistent with an olfactory system role in the early stages of ß-amyloid-related AD progression.

Direct imaging of macrovascular and microvascular contributions to BOLD fMRI in layers IV-V of the rat whisker-barrel cortex.

The spatiotemporal characteristics of the hemodynamic response to increased neural activity were investigated at the level of individual intracortical vessels using BOLD-fMRI in a well-established rodent model of somatosensory stimulation at 11.7 T. Functional maps of the rat barrel cortex were obtained at 150 × 150 × 500 µm spatial resolution every 200 ms. The high spatial resolution allowed separation of active voxels into those containing intracortical macro vessels, mainly vein/venules (referred to as macrovasculature), and those enriched with arteries/capillaries and small venules (referred to as microvasculature) since the macro vessel can be readily mapped due to the fast T2 decay of blood at 11.7 T. The earliest BOLD response was observed within layers IV-V by 0.8s following stimulation and encompassed mainly the voxels containing the microvasculature and some confined macrovasculature voxels. By 1.2s, the BOLD signal propagated to the macrovasculature voxels where the peak BOLD signal was 2-3 times higher than that of the microvasculature voxels. The BOLD response propagated in individual venules/veins far from neuronal sources at later times. This was also observed in layers IV-V of the barrel cortex after specific stimulation of separated whisker rows. These results directly visualized that the earliest hemodynamic changes to increased neural activity occur mainly in the microvasculature and spread toward the macrovasculature. However, at peak response, the BOLD signal is dominated by penetrating venules even at layers IV-V of the cortex.

Sensitivity enhancement of remotely coupled NMR detectors using wirelessly powered parametric amplification.

A completely wireless detection coil with an integrated parametric amplifier has been constructed to provide local amplification and transmission of MR signals. The sample coil is one element of a parametric amplifier using a zero-bias diode that mixes the weak MR signal with a strong pump signal that is obtained from an inductively coupled external loop. The NMR sample coil develops current gain via reduction in the effective coil resistance. Higher gain can be obtained by adjusting the level of the pumping power closer to the oscillation threshold, but the gain is ultimately constrained by the bandwidth requirement of MRI experiments. A feasibility study here shows that on a NaCl/D(2) O phantom, (23) Na signals with 20 dB of gain can be readily obtained with a concomitant bandwidth of 144 kHz. This gain is high enough that the integrated coil with parametric amplifier, which is coupled inductively to external loops, can provide sensitivity approaching that of direct wire connection.