Insights and improvements in correspondence between axonal volume fraction measured with diffusion-weighted MRI and electron microscopy.

Biophysical diffusion-weighted imaging (DWI) models are increasingly used in neuroscience to estimate the axonal water fraction ( ), which in turn is key for noninvasive estimation of the axonal volume fraction ( ). These models require thorough validation by comparison with a reference method, for example, electron microscopy (EM). While EM studies often neglect the unmyelinated axons and solely report the fraction of myelinated axons, in DWI both myelinated and unmyelinated axons contribute to the DWI signal.

Towards a representative reference for MRI-based human axon radius assessment using light microscopy.

Non-invasive assessment of axon radii via MRI bears great potential for clinical and neuroscience research as it is a main determinant of the neuronal conduction velocity. However, there is a lack of representative histological reference data at the scale of the cross-section of MRI voxels for validating the MRI-visible, effective radius (r). Because the current gold standard stems from neuroanatomical studies designed to estimate the bulk-determined arithmetic mean radius (r) on small ensembles of axons, it is unsuited to estimate the tail-weighted r.

Biophysically motivated efficient estimation of the spatially isotropic component from a single gradient-recalled echo measurement.

Purpose: To propose and validate an efficient method, based on a biophysically motivated signal model, for removing the orientation-dependent part of using a single gradient-recalled echo (GRE) measurement.

Methods: The proposed method utilized a temporal second-order approximation of the hollow-cylinder-fiber model, in which the parameter describing the linear signal decay corresponded to the orientation-independent part of . The estimated parameters were compared to the classical, mono-exponential decay model for in a sample of an ex vivo human optic chiasm (OC).

Single-subject independent component analysis-based intensity normalization in non-quantitative multi-modal structural MRI.

Non-quantitative MRI is prone to intersubject intensity variation rendering signal intensity level based analyses limited. Here, we propose a method that fuses non-quantitative routine T1-weighted (T1w), T2w, and T2w fluid-saturated inversion recovery sequences using independent component analysis and validate it on age and sex matched healthy controls. The proposed method leads to consistent and independent components with a significantly reduced coefficient-of-variation across subjects, suggesting potential to serve as automatic intensity normalization and thus to enhance the power of intensity based statistical analyses.

Afferent Visual Pathway Affection in Patients with PMP22 Deletion-Related Hereditary Neuropathy with Liability to Pressure Palsies.

Background: The PMP22 gene encodes a protein integral to peripheral myelin. Its deletion leads to hereditary neuropathy with liability to pressure palsies (HNPP). PMP22 is not expressed in the adult central nervous system, but previous studies suggest a role in CNS myelin development.

Multifrequency magnetic resonance elastography of the brain reveals tissue degeneration in neuromyelitis optica spectrum disorder.

Objectives: Application of multifrequency magnetic resonance elastography (MMRE) of the brain parenchyma in patients with neuromyelitis optica spectrum disorder (NMOSD) compared to age matched healthy controls (HC).

Methods: 15 NMOSD patients and 17 age- and gender-matched HC were examined using MMRE. Two three-dimensional viscoelastic parameter maps, the magnitude |G*| and phase angle φ of the complex shear modulus were reconstructed by simultaneous inversion of full wave-field data in 1.

Higher-resolution MR elastography reveals early mechanical signatures of neuroinflammation in patients with clinically isolated syndrome.

Purpose: To assess if higher-resolution magnetic resonance elastography (MRE) is a technique that can measure the in vivo mechanical properties of brain tissue and is sensitive to early signatures of brain tissue degradation in patients with clinically isolated syndrome (CIS).

Materials And Methods: Seventeen patients with CIS and 33 controls were investigated by MRE with a 3T MRI scanner. Full-wave field data were acquired at seven drive frequencies from 30 to 60 Hz.

Cerebral multifrequency MR elastography by remote excitation of intracranial shear waves.

The aim of this study was to introduce remote wave excitation for high-resolution cerebral multifrequency MR elastography (mMRE). mMRE of 25-45-Hz drive frequencies by head rocker stimulation was compared with mMRE by remote wave excitation based on a thorax mat in 12 healthy volunteers. Maps of the magnitude |G*| and phase φ of the complex shear modulus were reconstructed using multifrequency dual elasto-visco (MDEV) inversion.

Cerebellar neurochemical alterations in spinocerebellar ataxia type 14 appear to include glutathione deficiency.

Autosomal dominant ataxia type 14 (SCA14) is a rare usually adult-onset progressive disorder with cerebellar neurodegeneration caused by mutations in protein kinase C gamma. We set out to examine cerebellar and extracerebellar neurochemical changes in SCA14 by MR spectroscopy. In 13 SCA14 patients and 13 healthy sex- and age-matched controls, 3-T single-voxel brain proton MR spectroscopy was performed in a cerebellar voxel of interest (VOI) at TE = 30 ms to obtain a neurochemical profile of metabolites with short relaxation times.

High-resolution mechanical imaging of the human brain by three-dimensional multifrequency magnetic resonance elastography at 7T.

Magnetic resonance elastography (MRE) is capable of measuring the viscoelastic properties of brain tissue in vivo. However, MRE is still limited in providing high-resolution maps of mechanical constants. We therefore introduce 3D multifrequency MRE (3DMMRE) at 7T magnetic field strength combined with enhanced multifrequency dual elasto-visco (MDEV) inversion in order to achieve high-resolution elastographic maps of in vivo brain tissue with 1mm(3) resolution.

Towards an elastographic atlas of brain anatomy.

Cerebral viscoelastic constants can be measured in a noninvasive, image-based way by magnetic resonance elastography (MRE) for the detection of neurological disorders. However, MRE brain maps of viscoelastic constants are still limited by low spatial resolution. Here we introduce three-dimensional multifrequency MRE of the brain combined with a novel reconstruction algorithm based on a model-free multifrequency inversion for calculating spatially resolved viscoelastic parameter maps of the human brain corresponding to the dynamic range of shear oscillations between 30 and 60 Hz.

Compression-sensitive magnetic resonance elastography.

Magnetic resonance elastography (MRE) quantifies the shear modulus of biological tissue to detect disease. Complementary to the shear elastic properties of tissue, the compression modulus may be a clinically useful biomarker because it is sensitive to tissue pressure and poromechanical interactions. In this work, we analyze the capability of MRE to measure volumetric strain and the dynamic bulk modulus (P-wave modulus) at a harmonic drive frequency commonly used in shear-wave-based MRE.

Imaging of magnetic microfield distortions allows sensitive single-cell detection.

Cell tracking with magnetic resonance imaging (MRI) is mostly performed using superparamagnetic iron oxide (SPIO) nanoparticle-labeled cells. However, negative contrast in T2*-weighted imaging is inherently problematic as a homogeneous background signal is required to visualize the negative signal. In a magnetic field, SPIO-labeled cells develop their own magnetization, distorting the main field.

MR elastography of the liver and the spleen using a piezoelectric driver, single-shot wave-field acquisition, and multifrequency dual parameter reconstruction.

Purpose: Viscoelastic properties of the liver are sensitive to fibrosis. This study proposes several modifications to existing magnetic resonance elastography (MRE) techniques to improve the accuracy of abdominal MRE.

Methods: The proposed method comprises the following steps: (i) wave generation by a nonmagnetic, piezoelectric driver suitable for integration into the patient table, (ii) fast single-shot 3D wave-field acquisition at four drive frequencies between 30 and 60 Hz, and (iii) single-step postprocessing by a novel multifrequency dual parameter inversion of the wave equation.

Fractal network dimension and viscoelastic powerlaw behavior: I. A modeling approach based on a coarse-graining procedure combined with shear oscillatory rheometry.

Recent advances in dynamic elastography and biorheology have revealed that the complex shear modulus, G*, of various biological soft tissues obeys a frequency-dependent powerlaw. This viscoelastic powerlaw behavior implies that mechanical properties are communicated in tissue across the continuum of scales from microscopic to macroscopic. For deriving constitutive constants from the dispersion of G* in a biological tissue, a hierarchical fractal model is introduced that accounts for multiscale networks.

Measurement of vibration-induced volumetric strain in the human lung.

Noninvasive image-based measurement of intrinsic tissue pressure is of great interest in the diagnosis and characterization of diseases. Therefore, we propose to exploit the capability of phase-contrast MRI to measure three-dimensional vector fields of tissue motion for deriving volumetric strain induced by external vibration. Volumetric strain as given by the divergence of mechanical displacement fields is related to tissue compressibility and is thus sensitive to the state of tissue pressure.

Multifrequency inversion in magnetic resonance elastography.

Time-harmonic shear wave elastography is capable of measuring viscoelastic parameters in living tissue. However, finite tissue boundaries and waveguide effects give rise to wave interferences which are not accounted for by standard elasticity reconstruction methods. Furthermore, the viscoelasticity of tissue causes dispersion of the complex shear modulus, rendering the recovered moduli frequency dependent.

Viscoelasticity-based MR elastography of skeletal muscle.

An in vivo multifrequency magnetic resonance elastography (MRE) protocol was developed for studying the viscoelastic properties of human skeletal muscle in different states of contraction. Low-frequency shear vibrations in the range of 25-62.5 Hz were synchronously induced into the femoral muscles of seven volunteers and measured in a cross-sectional view by encoding the fast-transverse shear wave component parallel to the muscle fibers.

In vivo magnetic resonance elastography of human brain at 7 T and 1.5 T.

Purpose: To investigate the feasibility of quantitative in vivo ultrahigh field magnetic resonance elastography (MRE) of the human brain in a broad range of low-frequency mechanical vibrations.

Materials And Methods: Mechanical vibrations were coupled into the brain of a healthy volunteer using a coil-driven actuator that either oscillated harmonically at single frequencies between 25 and 62.5 Hz or performed a superimposed motion consisting of multiple harmonics.

MR-elastography reveals degradation of tissue integrity in multiple sclerosis.

In multiple sclerosis (MS), diffuse brain parenchymal damage exceeding focal inflammation is increasingly recognized to be present from the very onset of the disease, and, although occult to conventional imaging techniques, may present a major cause of permanent neurological disability. Subtle tissue alterations significantly influence biomechanical properties given by stiffness and internal friction, that--in more accessible organs than the brain--are traditionally assessed by manual palpation during the clinical exam. The brain, however, is protected from our sense of touch, and thus our current knowledge on cerebral viscoelasticity is very limited.

Scatter-based magnetic resonance elastography.

Elasticity is a sensitive measure of the microstructural constitution of soft biological tissues and increasingly used in diagnostic imaging. Magnetic resonance elastography (MRE) uniquely allows in vivo measurement of the shear elasticity of brain tissue. However, the spatial resolution of MRE is inherently limited as the transformation of shear wave patterns into elasticity maps requires the solution of inverse problems.

The impact of aging and gender on brain viscoelasticity.

Viscoelasticity is a sensitive measure of the microstructural constitution of soft biological tissue and is increasingly used as a diagnostic marker, e.g. in staging liver fibrosis or characterizing breast tumors.

Cardiac magnetic resonance elastography. Initial results.

Objectives: To develop cardiac magnetic resonance elastography (MRE) for noninvasively measuring left ventricular (LV) pressure-volume (P-V) work.

Material And Methods: The anterior chest wall of 8 healthy volunteers was vibrated by 24.3-Hz acoustic waves for stimulating oscillating shear deformation in myocardium and adjacent blood.

In vivo determination of hepatic stiffness using steady-state free precession magnetic resonance elastography.

Objective: The objective of this study was to introduce an magnetic resonance elastography (MRE) protocol based on fractional motion encoding and planar wave acquisition for rapid measurements of in vivo human liver stiffness.

Materials And Methods: Vibrations of a remote actuator membrane were fed by a rigid rod to the patient's surface beneath the right costal arch resulting in axial shear deflections of the liver. Data acquisition was performed using a balanced steady-state free precession (bSSFP) sequence incorporating oscillating gradients for motion sensitization.

Shear wave group velocity inversion in MR elastography of human skeletal muscle.

In vivo quantification of the anisotropic shear elasticity of soft tissue is an appealing objective of elastography techniques because elastic anisotropy can potentially provide specific information about structural alterations in diseased tissue. Here a method is introduced and applied to MR elastography (MRE) of skeletal muscle. With this method one can elucidate anisotropy by means of two shear moduli (one parallel and one perpendicular to the muscle fiber direction).