Phillip J. Rossman

Magnetic resonance elastography of skeletal muscle.

While the contractile properties of skeletal muscle have been studied extensively, relatively little is known about the elastic properties of muscle in vivo. Magnetic resonance elastography (MRE) is a phase contrast‐based method for observing shear waves propagating in a material to determine its stiffness. In this work, MRE is applied to skeletal muscle under load to quantify the change in stiffness with loading. A mathematical model of muscle is developed that predicts a linear relationship between shear stiffness and muscle load. The MRE technique was applied to bovine muscle specimens (N = 10) and human biceps brachii in vivo (N = 5). Muscle stiffness increased linearly for both passive tension (14.5 ± 1.77 kPa/kg) and active tension, in which the increase in stiffness was dependent upon muscle size, as predicted by the model. A means of noninvasively assessing the viscoelastic pro‐perties of skeletal muscle in vivo may provide a useful method for studying muscle biomechanics in health and disease. J. Magn. Reson. Imaging 2001;13:269–276.

Continuously moving table data acquisition method for long FOV contrast‐enhanced MRA and whole‐body MRI

A method is presented in which an extended longitudinal field of view (FOV), as required for whole‐body MRI or MRA peripheral runoff studies, is acquired in one seamless image. Previous methods typically either acquired 3D data at multiple static “stations” which covered the extended FOV or as a series of 2D axial sections. The method presented here maintains the benefits of 3D acquisition while removing the discrete nature of the multistation method by continuous acquisition of MR data as the patient table moves through the desired FOV. Although the technique acquires data only from a homogeneous central volume of the magnet at any point in time, by spatially registering all data it is possible to extend the FOV well beyond this volume. The method is demonstrated experimentally with phantoms, in vivo angiographic animal studies, and in vivo human studies. Magn Reson Med 47:224–231, 2002.

Feasibility of In Vivo MR Elastographic Splenic Stiffness Measurements in the Assessment of Portal Hypertension

OBJECTIVE Liver stiffness is associated with portal hypertension in patients with chronic liver disease. However, the relation between spleen stiffness and clinically significant portal hypertension remains unknown. The purposes of this study were to determine the feasibility of measuring spleen stiffness with MR elastography and to prospectively test the technique in healthy volunteers and in patients with compensated liver disease. MATERIALS AND METHODS Spleen stiffness was measured with MR elastography in 12 healthy volunteers (mean age, 37 years; range, 25-82 years) and 38 patients (mean age, 56 years; range, 36-60 years) with chronic liver disease of various causes. For patients with liver disease, laboratory findings, spleen size, presence and size of esophageal varices, and liver histologic results were recorded. Statistical analyses were performed to assess all measurements. RESULTS MR elastography of the spleen was successfully performed on all volunteers and patients. The mean spleen stiffness was significantly lower in the volunteers (mean, 3.6 +/- 0.3 kPa) than in the patients with liver fibrosis (mean, 5.6 +/- 5.0 kPa; range, 2.7-19.2 kPa; p < 0.001). In addition, a significant correlation was observed between liver stiffness and spleen stiffness for the entire cohort (r(2) = 0.75; p < 0.001). Predictors of spleen stiffness were splenomegaly, spleen volume, and platelet count. A mean spleen stiffness of 10.5 kPa or greater was identified in all patients with esophageal varices. CONCLUSION MR elastography of the spleen is feasible and shows promise as a quantitative method for predicting the presence of esophageal varices in patients with advanced hepatic fibrosis.

Vascular wall elasticity measurement by magnetic resonance imaging.

The goal of this current study was to determine whether an MRI‐based elastography (MRE) method can visualize and assess propagating mechanical waves within fluid‐filled vessels and to investigate the feasibility of measuring the elastic properties of vessel walls and quantitatively assessing stenotic lesions by using MRE. The ability to measure the Youngs modulus‐wall thickness product was tested using a thin‐walled latex vessel model. Also tested in vessel models was the ability to quantitate the degree of stenosis by measuring transmitted and reflected mechanical waves. This method was then applied to ex vivo porcine models and in vivo human arteries to further test its feasibility. The results provide preliminary evidence that MRE can be used to quantitatively assess the stiffness of blood vessels, and provide a non‐morphologic method to measure stenosis. With further development, it is possible that the method can be implemented in vivo. Magn Reson Med, 2006. Published 2006 Wiley‐Liss, Inc.

Vibration safety limits for magnetic resonance elastography.

Magnetic resonance elastography (MRE) has been demonstrated to have potential as a clinical tool for assessing the stiffness of tissue in vivo. An essential step in MRE is the generation of acoustic mechanical waves within a tissue via a coupled mechanical driver. Motivated by an increasing volume of human imaging trials using MRE, the objectives of this study were to audit the vibration amplitude of exposure for our IRB-approved human MRE studies, to compare these values to a conservative regulatory standard for vibrational exposure and to evaluate the applicability and implications of this standard for MRE. MRE displacement data were examined from 29 MRE exams, including the liver, brain, kidney, breast and skeletal muscle. Vibrational acceleration limits from a European Union directive limiting occupational exposure to whole-body and extremity vibrations (EU 2002/44/EC) were adjusted for time and frequency of exposure, converted to maximum displacement values and compared to the measured in vivo displacements. The results indicate that the vibrational amplitudes used in MRE studies are below the EU whole-body vibration limit, and the EU guidelines represent a useful standard that could be readily accepted by Institutional Review Boards to define standards for vibrational exposures for MRE studies in humans.

Developments in dynamic MR elastography for in vitro biomechanical assessment of hyaline cartilage under high-frequency cyclical shear.

The design, construction, and evaluation of a customized dynamic magnetic resonance elastography (MRE) technique for biomechanical assessment of hyaline cartilage in vitro are described. For quantification of the dynamic shear properties of hyaline cartilage by dynamic MRE, mechanical excitation and motion sensitization were performed at frequencies in the kilohertz range. A custom electromechanical actuator and a z‐axis gradient coil were used to generate and image shear waves throughout cartilage at 1000–10,000 Hz. A radiofrequency (RF) coil was also constructed for high‐resolution imaging. The technique was validated at 4000 and 6000 Hz by quantifying differences in shear stiffness between soft (∼200 kPa) and stiff (∼300 kPa) layers of 5‐mm‐thick bilayered phantoms. The technique was then used to quantify the dynamic shear properties of bovine and shark hyaline cartilage samples at frequencies up to 9000 Hz. The results demonstrate that one can obtain high‐resolution shear stiffness measurements of hyaline cartilage and small, stiff, multilayered phantoms at high frequencies by generating robust mechanical excitations and using large magnetic field gradients. Dynamic MRE can potentially be used to directly quantify the dynamic shear properties of hyaline and articular cartilage, as well as other cartilaginous materials and engineered constructs. J. Magn. Reson. Imaging 2007.

Magnetic resonance elastography of the brain in a mouse model of Alzheimer’s disease: initial results

The increasing prevalence of Alzheimers disease (AD) has provided motivation for developing novel methods for assessing the disease and the effects of potential treatments. Magnetic resonance elastography (MRE) is an MRI-based method for quantitatively imaging the shear tissue stiffness in vivo. The objective of this research was to determine whether this new imaging biomarker has potential for characterizing neurodegenerative disease. Methods were developed and tested for applying MRE to evaluate the mouse brain, using a conventional large bore 3.0T MRI system. The technique was then applied to study APP-PS1 mice, a well-characterized model of AD. Five APP-PS1 mice and 8 age-matched wild-type mice were imaged immediately following sacrifice. Brain shear stiffness measurements in APP-PS1 mice averaged 22.5% lower than those for wild-type mice (P = .0031). The results indicate that mouse brain MRE is feasible at 3.0T, and brain shear stiffness has merit for further investigation as a potential new biomarker for Alzheimers disease.

Determination and analysis of guided wave propagation using magnetic resonance elastography

We present a novel extension of standard magnetic resonance elastography (MRE) measurement and analysis methods, which is applicable in cases where the medium is characterized by waveguides or fiber bundles (i.e., muscle) leading to constrained propagation of elastic waves. As a demonstration of this new method, MRI is utilized to identify the pathways of the individual fibers of a stalk of celery, and 3D MRE is then performed throughout the volume containing the celery fibers for a measurement of the displacements. A Helmholtz decomposition is performed permitting a separation of the displacements into longitudinal and transverse components, and an application of a hybrid Radon transform permits a spectral decomposition of wave propagation along the fibers. Dot product projections between these elastic displacements measured in the global coordinate system and three Frenet vectors representing the tangent and two corresponding orthogonal vectors along any particular fiber orientation yield the displacement contributions to wave propagation along the fiber as if it were a waveguide. A sliding window spatial Fourier transform is then performed along the length of each fiber to obtain dispersion images that portray space–wavenumber profiles. Therefore, this method can permit localized tracking and characterization of wave types, velocities, and coupling along arbitrarily oriented fibers. Magn Reson Med, 2005. Published 2005 Wiley‐Liss, Inc.

Controlled experimental study depicting moving objects in view-shared time-resolved 3D MRA

Various methods have been used for time‐resolved contrast‐enhanced magnetic resonance angiography (CE‐MRA), many involving view sharing. However, the extent to which the resultant image time series represents the actual dynamic behavior of the contrast bolus is not always clear. Although numerical simulations can be used to estimate performance, an experimental study can allow more realistic characterization. The purpose of this work was to use a computer‐controlled motion phantom for study of the temporal fidelity of three‐dimensional (3D) time‐resolved sequences in depicting a contrast bolus. It is hypothesized that the view order of the acquisition and the selection of views in the reconstruction can affect the positional accuracy and sharpness of the leading edge of the bolus and artifactual signal preceding the edge. Phantom studies were performed using dilute gadolinium‐filled vials that were moved along tabletop tracks by a computer‐controlled motor. Several view orders were tested using view‐sharing and Cartesian sampling. Compactness of measuring the k‐space center, consistency of view ordering within each reconstruction frame, and sampling the k‐space center near the end of the temporal footprint were shown to be important in accurate portrayal of the leading edge of the bolus. A number of findings were confirmed in an in vivo CE‐MRA study. Magn Reson Med, 2009.

Magnetic Resonance Elastography with a Phased-Array Acoustic Driver System

Dynamic MR elastography (MRE) quantitatively maps the stiffness of tissues by imaging propagating shear waves in the tissue. These waves can be produced from intrinsic motion sources (e.g., due to cardiac motion), from external motion sources that produce motion directly at depth in tissue (e.g., amplitude‐modulated focused ultrasound), and from external actuators that produce motion at the tissue surface that propagates into the tissue. With external actuator setups, typically only a single transducer is used to create the shear waves, which in some applications might have limitations due to shadowing and attenuation of the waves. To address these limitations, a phased‐array acoustic driver system capable of applying independently controlled waveforms to each channel was developed and tested. It was found that the system produced much more uniform illumination of the object, improving the quality of the elastogram. It was also found that the accuracy of the stiffness value of any arbitrary region of interest could be improved by obtaining maximal shear wave illumination with the phased array capability of the system. Magn Reson Med, 2009.