Shear stiffness estimation using intravoxel phase dispersion in magnetic resonance elastography.

Dynamic MR elastography (MRE) is a phase-contrast technique in which the periodic shear motion of an object is encoded as variations in the phase of the reconstructed images. An alternative MRE method is presented whereby waves are depicted as intensity variations in the magnitude images due to intravoxel phase dispersion (IVPD). A theoretical framework is developed to model how the IVPD magnitude data are related to the underlying shear wave motion, and how they can be used to estimate shear stiffness. The results are shown in a series of phantom experiments to demonstrate that IVPD MRE complements phase-contrast MRE.     

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. © 2009 Wiley‐Liss, Inc.     

Comparison of quantitative shear wave MR-elastography with mechanical compression tests.

The mechanical properties of in vivo soft tissue are generally determined by palpation, ultrasound measurements (US), and magnetic resonance elastography (MRE). While it has been shown that US and MRE are capable of quantitatively measuring soft tissue elasticity, there is still some uncertainty about the reliability of quantitative MRE measurements. For this reason it was decided to determine in vitro how MRE measurements correspond with other quantitative methods of measuring characteristic elasticity values. This article presents the results of experiments with tissue-like agar-agar gel phantoms in which the wavelength of strain waves was measured by shear wave MR elastography and the resultant shear modulus was compared with results from mechanical compression tests with small gel specimens. The shear moduli of nine homogeneous gels with various agar-agar concentrations were investigated. The elasticity range of the gels covered the elasticity range of typical soft tissues. The systematic comparison between shear wave MRE and compression tests showed good agreement between the two measurement techniques.     

Quantitative shear wave magnetic resonance elastography: comparison to a dynamic shear material test.

Magnetic resonance elastography (MRE), a phase contrast MRI technique, images the propagation of applied mechanical waves in tissue, allowing shear stiffness to be quantified in vivo. This MRE technique has been validated with static mechanical compression tests. Dynamic mechanical analysis (DMA) may be a more appropriate comparison to MRE because it directly measures the shear modulus dynamically, while compression tests convert the measured elastic modulus to shear modulus with an assumed Poisson ratio. This study compared the shear stiffness estimated with various MRE inversion algorithms (i.e., manual calculation, local frequency estimate, phase gradient, direct inversion, and matched filter) to the dynamic mechanical test. The shear stiffness of five agarose gels with concentrations ranging from 1.5 to 3.5% were measured using MRE and DMA. The phase gradient inversion algorithm overestimated the shear modulus at higher concentrations (i.e., 3-3.5% agar), while the results from the other techniques correlated well with the dynamic mechanical test.     

Determination of thigh muscle stiffness using magnetic resonance elastography

PURPOSE: To measure the elastic properties of the vastus lateralis (VL), vastus medialis (VM), and sartorius (Sr) muscles using magnetic resonance elastography (MRE). MATERIALS AND METHODS: To obtain a normative database of the aforementioned muscles, oblique scan directions were prescribed passing through each muscle. Shear waves were induced into the muscles using pneumatic and mechanical drivers at 90 and 120 Hz, respectively. These drivers were attached to the distal end of the right thigh with the knee flexed at 30 degrees . The foot was placed in a footplate containing MR-compatible load cells to record the force during a contraction (10% and 20% of the maximum voluntary contraction). RESULTS: The shear moduli measured at rest in the VL (N = 12), VM (N = 14), and Sr (N = 13) were 3.73 +/- 0.85 kPa, 3.91 +/- 1.15 kPa, and 7.53 +/- 1.63 kPa, respectively. The stiffness of both vasti increased with the level of contraction, while the stiffness of the Sr remained the same. CONCLUSION: The MRE technique was able to approximate the stiffness of different thigh muscles. Furthermore, the wave length was sensitive to the morphology (unipennate or longitudinal) and fiber composition (type I or II) in each muscle.     

Mechanical transient-based magnetic resonance elastography.

Magnetic resonance elastography (MRE) is a technique for quantifying material properties by measuring cyclic displacements of propagating shear waves. As an alternative to dynamic harmonic wave MRE or quasi-steady-state methods, the idea of using a transient impulse for mechanical excitation is introduced. Two processing methods to calculate shear stiffness from transient data were developed. The techniques were tested in phantom studies, and the transient results were found to be comparable to the harmonic wave results. Transient wave based analysis was applied to the brains of six healthy volunteers in order to assess the method in areas of complex wave patterns and geometry. The results demonstrated the feasibility of measuring brain stiffness in vivo using a transient mechanical excitation. Transient and harmonic methods both measure white matter (approximately 12 kPa) to be stiffer than gray matter ( approximately 8 kPa). There were some anatomic differences between harmonic and transient MRE, specifically where the transient results better depicted the deeper structures of the brain.     

Rapid magnetic resonance elastography of muscle using one-dimensional projection

PURPOSE: To demonstrate the feasibility of 1D MR elastography (MRE) to rapidly assess skeletal muscle stiffness in vivo. MATERIALS AND METHODS: Shear waves were induced in the vastus medialis muscle (VM) using a pneumatic driver at 90 Hz and 2D MRE data were collected. Spatially selective excitations were used to produce 1D projections of MRE data oriented along the direction of propagating waves in the muscle. Data were collected with the thigh muscles relaxed and contracted at 20% maximum voluntary contraction (MVC) and the knee flexed at 30 degrees . RESULTS: The muscle stiffness measured at rest and in contraction with 1D MRE was 3.69 +/- 0.80 kPa and 9.52 +/- 2.74 kPa, respectively, and 4.36 +/- 0.98 kPa and 9.22 +/- 1.29 kPa, respectively, with the 2D MRE technique. CONCLUSION: Muscle stiffness measured using 1D MRE was in agreement with 2D MRE while reducing the scan time by a factor of 4.     

Characterization of the dynamic shear properties of hyaline cartilage using high-frequency dynamic MR elastography.

This work evaluated the feasibility of dynamic MR Elastography (MRE) to quantify structural changes in bovine hyaline cartilage induced by selective enzymatic degradation. The ability of the technique to quantify the frequency-dependent response of normal cartilage to shear in the kilohertz range was also explored. Bovine cartilage plugs of 8 mm in diameter were used for this study. The shear stiffness (mu(s)) of each cartilage plug was measured before and after 16 hr of enzymatic treatments by dynamic MRE at 5000 Hz of shear excitation. Collagenase and trypsin were used to selectively affect the collagen and proteoglycans contents of the matrix. Additionally, normal cartilage plugs were tested by dynamic MRE at shear-excitations of 3000-7000 Hz. Measured micro(s) of cartilage plugs showed a significant decrease (-37%, P < 0.05) after collagenase treatment and a significant decrease (-28%, P < 0.05) after trypsin treatment. Furthermore, a near-linear increase (R(2) = 0.9141) in the speed of shear wave propagation with shear-excitation frequency was observed in cartilage, indicating that wave speed is dominated by viscoelastic effects. These experiments suggest that dynamic MRE can provide a sensitive quantitative tool to characterize the degradation process and viscoelastic behavior of cartilage.     

Magnetic resonance elastography of the lung: technical feasibility.

Magnetic resonance elastography (MRE) is a phase-contrast technique that can spatially map shear stiffness within tissue-like materials. To date, however, MRE of the lung has been too technically challenging-primarily because of signal-to-noise ratio (SNR) limitations and phase instability. We describe an approach in which shear wave propagation is not encoded into the phase of the MR signal of a material, but rather from the signal arising from a polarized noble gas encapsulated within. To determine the feasibility of the approach, three experiments were performed. First, to establish whether shear wave propagation within lung parenchyma can be visualized with phase-contrast MR techniques, MRE was performed on excised porcine lungs inflated with room air. Second, a phantom consisting of open-cell foam filled with thermally polarized (3)He gas was imaged with MRE to determine whether shear wave propagation can be encoded by the gas. Third, preliminary evidence of the feasibility of MRE in vivo was obtained by using a longitudinal driver on the chest of a normal volunteer to generate shear waves in the lung. The results suggest that MRE in combination with hyperpolarized noble gases is potentially useful for noninvasively assessing the regional elastic properties of lung parenchyma, and merits further investigation.     

Stiffness-weighted magnetic resonance imaging.

An imaging method is introduced in which the signal in MR images is affected by the stiffness distribution in the object being imaged. Intravoxel phase dispersion (IVPD) that occurs during MR elastography (MRE) acquisitions decreases the signal in soft regions more than in stiff regions due to changes in shear wave amplitude and wavelength. The IVPD effect is enhanced by lowpass filtering the MR k-space data with a circular Gaussian lowpass filter. A processing method is introduced to take the time series of MRE magnitude images with IVPD and produce a final stiffness-weighted image (SWI) by calculating the minimum signal at each pixel from a small number of temporal samples. The SWI technique is demonstrated in phantom studies as well as in the case of a preserved postmortem breast tissue specimen with a stiff lesion created by focused ultrasound ablation to mimic a breast cancer. When free of significant sources of depth-dependent wave attenuation, interference, and boundary effects, SWI is a simple, fast, qualitative technique that does not require the use of phase unwrapping or inversion algorithms for localizing stiff regions in an object.     

Rapid MR elastography using selective excitations.

Rapid MR elastography (MRE) techniques using spatially-selective excitations to reduce acquisition times to a few seconds or less were devised and tested. The techniques included reduced field of view (rFOV) MRE and 1D MRE (beam MRE) using 2D spatially selective RF excitations for gradient-echo (GRE) applications and intersecting 90 degrees and 180 degrees slice-selective excitations for spin-echo (SE) applications. It was shown that scan times could be reduced by a factor of 8 using rFOV MRE, and by 64 using beam MRE, while still obtaining stiffness estimates comparable to full-FOV MRE. Results were shown in gel phantom experiments as well as in the case of a preserved postmortem breast tissue specimen with a stiff lesion. These methods can be used to more rapidly interrogate regions of interest (ROIs) in tissue to quickly obtain information about the viscoelastic properties of that tissue.     

TREMR: Table‐resonance elastography with MR

Magnetic resonance elastography (MRE) is a noninvasive method of measuring tissue compliance. Current MRE methods rely on custom‐built hardware to elicit vibrations that are tracked by MR imaging. Knowledge of the wave propagation can be used to calculate the local shear stiffness of the tissue. We sought to determine whether the vibrations of the patient table that result from low‐frequency switching of the imaging gradients could be used as an alternative mechanical driving mechanism for MRE. We designed a pulse sequence that includes a gradient lobe specifically for the excitation of mechanical resonance, allowing control of the time between the onset of the vibrations and the velocity‐encoding of the readout. Data collected from a gelatin phantom with stiff cylindrical gelatin inserts demonstrated that wave propagation can be imaged with this method. Postprocessing to estimate the local spatial frequency of the waves also allows estimation of the local shear stiffness, where the stiff inserts are clearly identifiable. Data collected on the brain of a normal healthy volunteer showed clear rotational waves propagating from the skull inwards, also allowing generation of shear stiffness maps. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Assessment of liver viscoelasticity using multifrequency MR elastography

MR elastography (MRE) allows the noninvasive assessment of the viscoelastic properties of human organs based on the organ response to oscillatory shear stress. Shear waves of a given frequency are mechanically introduced and the propagation is imaged by applying motion‐sensitive gradients. An experiment was set up that introduces multifrequency shear waves combined with broadband motion sensitization to extend the dynamic range of MRE from one given frequency to, in this study, four different frequencies. With this approach, multiple wave images corresponding to the four driving frequencies are simultaneously acquired and can be evaluated with regard to the dispersion of the complex modulus over the respective frequency. A viscoelastic model based on two shear moduli and one viscosity parameter was used to reproduce the experimental wave speed and wave damping dispersion. The technique was applied in eight healthy volunteers and eight patients with biopsy‐proven high‐grade liver fibrosis (grade 3–4). Fibrotic liver had a significantly higher (P < 0.01) viscosity (14.4 ± 6.6 Pa · s) and elastic moduli (2.91 ± 0.84 kPa; 4.83 ± 1.77 kPa) than the viscosity (7.3 ± 2.3 Pa · s) and elastic moduli (1.16 ± 0.28 kPa; 1.97 ± 0.30 kPa) of normal volunteers. Multifrequency MRE is well suited for the noninvasive differentiation of normal and fibrotic liver as it allows the measurement of rheologic material properties. Magn Reson Med 60:373–379, 2008. © 2008 Wiley‐Liss, Inc.     

Magnetic resonance elastography of the brain using multishot spiral readouts with self‐navigated motion correction

Magnetic resonance elastography (MRE) has been introduced in clinical practice as a possible surrogate for mechanical palpation, but its application to study the human brain in vivo has been limited by low spatial resolution and the complexity of the inverse problem associated with biomechanical property estimation. Here, we report significant improvements in brain MRE data acquisition by reporting images with high spatial resolution and signal‐to‐noise ratio as quantified by octahedral shear strain metrics. Specifically, we have developed a sequence for brain MRE based on multishot, variable‐density spiral imaging, and three‐dimensional displacement acquisition and implemented a correction scheme for any resulting phase errors. A Rayleigh damped model of brain tissue mechanics was adopted to represent the parenchyma and was integrated via a finite element‐based iterative inversion algorithm. A multiresolution phantom study demonstrates the need for obtaining high‐resolution MRE data when estimating focal mechanical properties. Measurements on three healthy volunteers demonstrate satisfactory resolution of gray and white matter, and mechanical heterogeneities correspond well with white matter histoarchitecture. Together, these advances enable MRE scans that result in high‐fidelity, spatially resolved estimates of in vivo brain tissue mechanical properties, improving upon lower resolution MRE brain studies that only report volume averaged stiffness values. Magn Reson Med 70:404–412, 2013. © 2012 Wiley Periodicals, Inc.     

MR elastography of the lung with hyperpolarized 3He.

MR elastography (MRE) is a phase contrast-based technique for spatially mapping the mechanical properties of tissue-like materials. While hyperpolarized noble gases such as helium-3 ((3)He) have proven to be an ideal contrast mechanism for imaging of the lung using conventional MR techniques, their applicability for lung MRE is unknown, due to the fact that gases do not support shear. In this study, we report on the application of MRE to an ex vivo porcine lung specimen inflated with a hyperpolarized noble gas. Unlike proton MRE, shear wave propagation is encoded into the gas entrapped within the alveolar spaces rather than the parenchyma itself. These data provide first evidence of the technical feasibility of MRE of the lung using hyperpolarized noble gases.     

Measurement of liver stiffness with two imaging techniques: magnetic resonance elastography and ultrasound elastometry

Purpose

To cross‐validate the magnetic resonance elastography (MRE) technique with a clinical device, based on an ultrasound elastometry system called Fibroscan.

Materials and Methods

Ten healthy subjects underwent an MRE and a Fibroscan test. The MRE technique used a round pneumatic driver at 60 Hz to generate shear waves inside the liver. An elastogram representing a map of the liver stiffness was generated allowing for the measurement of the average liver stiffness inside a region of interest. The Fibroscan technique used an ultrasound probe (3.5 MHz) composed of a vibrator that sent low‐frequency (50 Hz) shear waves inside the right liver lobe. The probe acts as an emitter‐receptor that measures the velocity of the waves propagated inside the liver tissue.

Results

The mean shear stiffness measured with the MRE and Fibroscan techniques were 1.95 ± 0.06 kPa and 1.79 ± 0.30 kPa, respectively. A higher standard deviation was found for the same subject with Fibroscan.

Conclusion

This study shows why MRE should be investigated beyond the Fibroscan. The MRE technique provided elasticity of the entire liver, meanwhile the Fibroscan provided values of elasticity locally. J. Magn. Reson. Imaging 2008;28:1287–1292. © 2008 Wiley‐Liss, Inc.     

MR elastography of human lung parenchyma: technical development, theoretical modeling and in vivo validation

Purpose:

To develop a novel MR‐based method for visualizing the elastic properties of human lung parenchyma in vivo and to evaluate the ability of this method to resolve differences in parenchymal stiffness at different respiration states in healthy volunteers.

Materials and Methods:

A spin‐echo MR Elastography (MRE) pulse sequence was developed to provide both high shear wave motion sensitivity and short TE for improved visualization of lung parenchyma. The improved motion sensitivity of this approach was modeled and tested with phantom experiments. In vivo testing was then performed on 10 healthy volunteers at the respiratory states of residual volume (RV) and total lung capacity (TLC).

Results:

Shear wave propagation was visualized within the lungs of all volunteers and was processed to provide parenchymal shear stiffness maps for all 10 subjects. Density corrected stiffness values at TLC (1.83 ± 0.22 kPa) were higher than those at the RV (1.14 ± 0.14 kPa) with the difference being statistically significant (P < 0.0001).

Conclusion:

¹H‐based MR elastography can noninvasively measure the shear stiffness of human lung parenchyma in vivo and can quantitate the change in shear stiffness due to respiration. The values obtained were consistent with previously reported in vitro assessments of cadaver lungs. Further work is required to increase the flexibility of the current acquisition and to investigate the clinical potential of lung MRE. J. Magn. Reson. Imaging 2011;33:1351–1361. © 2011 Wiley‐Liss, Inc.     

Magnetic resonance elastography in the liver at 3 Tesla using a second harmonic approach

Magnetic resonance elastography (MRE) using mechanical stimulation has demonstrated diagnostic value and clinical promise in breast, liver, and kidney at 1.5 Tesla (T). However, MRE at 1.5T suffers from long imaging times and would benefit from greater signal‐to‐noise for more robust postprocessing. We present an MRE sequence modified for liver imaging at 3.0T. To avoid artifacts in the phase images, the sequence maintains a short TE by using a second harmonic approach, including stronger motion encoding gradients, shorter radio frequency pulses and an echo‐planar readout. Scan time was decreased by a factor of ～2 relative to 1.5T by using an EPI readout and a higher density sampling of the phase waveform was used to calculate shear stiffness and viscosity. Localized (small region of interest) and global (whole‐liver region of interest) measurements in normal healthy subjects compared very favorably with previously published results at 1.5T. There was no significant difference between global and localized measures. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Magnetic resonance elastography of the kidneys: Feasibility and reproducibility in young healthy adults

Purpose:

To evaluate the feasibility and reproducibility of renal magnetic resonance elastography (MRE) in young healthy volunteers.

Materials and Methods:

Ten volunteers underwent renal MRE twice at a 4–5 week interval. The vibrations (45 and 76 Hz) were generated by a speaker positioned beneath the volunteers' back and centered on their left kidney. For each frequency, three sagittal slices were acquired (eight phase offsets per cycle, motion‐encoding gradients successively positioned along the three directions of space). Shear velocity images were reconstructed using the curl operator combined with the local frequency estimation (LFE) algorithm.

Results:

The mean shear velocities measured in the renal parenchyma during the two examinations were not significantly different and exhibited a mean variation of 6% at 45 Hz and 76 Hz. The mean shear velocities in renal parenchyma were 2.21 ± 0.14 m/s at 45 Hz (shear modulus of 4.9 ± 0.5 kPa) and 3.07 ± 0.17 m/s at 76 Hz (9.4 ± 0.8 kPa, P < 0.01). The mean shear velocities in the renal cortex and medulla were respectively 2.19 ± 0.13 m/s and 2.32 ± 0.16 m/s at 45 Hz (P = 0.002) and 3.06 ± 0.16 m/s and 3.10 ± 0.22 m/s at 76 Hz (P = 0.13).

Conclusion:

Renal MRE was feasible and reproducible. Two independent measurements of shear velocities in the renal parenchyma of the same subjects showed an average variability of 6%. J. Magn. Reson. Imaging 2011;. ©2011 Wiley‐Liss, Inc.