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.     

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.     

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.     

Fractional encoding of harmonic motions in MR elastography.

In MR elastography (MRE) shear waves are magnetically encoded by bipolar gradients that usually oscillate with the same frequency fv as the mechanical vibration. As a result, both the repetition time (TR) and echo time (TE) of such an MRE sequence are greater than the vibration period 1/fv. This causes long acquisition times and considerable signal dephasing in tissue with short transverse relaxation times. Here we propose a reverse concept with TR相似文献   

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.     

In vivo MR elastography of the prostate gland using a transurethral actuator

Conventional approaches for MR elastography (MRE) using surface drivers have difficulty achieving sufficient shear wave propagation in the prostate gland due to attenuation. In this study we evaluate the feasibility of generating shear wave propagation in the prostate gland using a transurethral device. A novel transurethral actuator design is proposed, and the performance of this device was evaluated in gelatin phantoms and in a canine prostate gland. All MRI was performed on a 1.5T MR imager using a conventional gradient‐echo MRE sequence. A piezoceramic actuator was used to vibrate the transurethral device along its length. Shear wave propagation was measured transverse and parallel to the rod at frequencies between 100 and 250 Hz in phantoms and in the prostate gland. The shear wave propagation was cylindrical, and uniform along the entire length of the rod in the gel experiments. The feasibility of transurethral MRE was demonstrated in vivo in a canine model, and shear wave propagation was observed in the prostate gland as well as along the rod. These experiments demonstrate the technical feasibility of transurethral MRE in vivo. Further development of this technique is warranted. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

基于肺部血流MRI信号在心动周期内变化的灌注成像技术研究

目的探讨血流变化对肺部MRI信号的影响，并研究1种新的MR肺血流灌注成像方法。方法对健康志愿者15例，采用相位对比电影MRI技术测量大肺动脉血流速度和流量在心动周期内的变化；并选用单次激发半傅立叶变换超快速自旋回波序列观察肺实质MR信号的相应改变，评价其相关性；根据不同心动期相肺实质MR信号的差异进行图像减影。结果肺实质．MRI信号表现为心脏收缩期降低，舒张期升高。大肺动脉的瞬时速度、瞬时流量与其呈负相关(r=-0．878、-0．770，P=0，002、0．015)。经肺部MRI信号差异最大的舒张末期和收缩中期的MRI减影可获得肺灌注像。结论肺实质MRI信号的改变与肺血流模式和速度有关。该技术是1种简便易行的非对比剂性的MR肺灌注评价新方法。     

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.     

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.     

MR elastography of breast lesions: understanding the solid/liquid duality can improve the specificity of contrast-enhanced MR mammography.

The purpose of this analysis is to explore the potential diagnostic gain provided by the viscoelastic shear properties of breast lesions for the improvement of the specificity of contrast enhanced dynamic MR mammography (MRM). The assessment of viscoelastic properties is done via dynamic MR elastography (MRE) and it is demonstrated that the complex shear modulus of in vivo breast tissue follows within the frequency range of clinical MRE a power law behavior. Taking benefit of this frequency behavior, data are interpreted in the framework of the exact model for wave propagation satisfying the causality principle. This allows to obtain the exponent of the frequency power law from the complex shear modulus at one single frequency which is validated experimentally. Thereby, scan time is drastically reduced. It is observed that malignant tumors obtain larger exponents of the power law than benign tumors indicating a more liquid-like behavior. The combination of the Breast Imaging Reporting and Data System (BIRADS) categorization obtained via MRM with viscoelastic information leads to a substantial rise in specificity. Analysis of 39 malignant and 29 benign lesions shows a significant diagnostic gain with an increase of about 20% in specificity at 100% sensitivity.     

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.     

Accelerating MR elastography: a multiecho phase-contrast gradient-echo sequence

PURPOSE: To demonstrate the feasibility of using a multiecho phase-contrast (PC) gradient-echo sequence with motion-sensitizing gradient (MSG) to accelerate MR elastography (MRE) acquisitions in comparison to single-echo PC sequences. MATERIALS AND METHODS: The sequence was implemented and compared with a conventional single-echo sequence as the standard of reference in both agarose phantoms and in vivo in the biceps of three healthy volunteers. For reconstruction of the elasticity modulus, a local frequency estimation (LFE) algorithm was used. ETL factors of 1-16 were evaluated. RESULTS: Phantom experiments demonstrated excellent consistency between single-echo and multiecho measurements in terms of wave equivalency, SNR, and reconstructed shear modulus. Additionally, the in vivo MRE examinations showed an excellent correspondence to the single-echo results. Minor loss of wave amplitude was observed at higher ETL factors. CONCLUSION: The results demonstrate that a multiecho sequence is suitable for accelerating MRE in nearly homogeneous tissue, such as muscle. It provides equivalent elasticity values in a significantly reduced scan time compared to a single-echo sequence. The maximum achievable ETL factor must be individually determined for the target tissue.     

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.     

MR measurement of pulsatile pressure gradients

A magnetic resonance (MR) imaging method for evaluating pulsatile pressure gradients in laminar blood flow is presented. The technique is based on an evaluation of fluid shear and inertial forces from cardiac-gated phase-contrast velocity measurements. The technique was experimentally validated by comparing MR and manometer pressure gradient measurements performed in a pulsatile flow phantom. Analyses of random noise propagation and sampling error were performed to determine the precision and accuracy of the method. The results indicate that a precision of 0.01–0.03 mmHg/cm and an accuracy of better than 8% can be achieved by using standard clinical pulse sequences in tubes exceeding 6 mm in diameter. The authors conclude that MR measurement of pressure gradients is feasible and that additional hemodynamic information may be derived from conventional phase-contrast imaging studies.     

Magnetic resonance elastography of the human brain: a preliminary study

Purpose: To study the application of magnetic resonance elastography (MRE) in the human brain.

Material and Methods: An external force actuator was developed, which produced propagating shear waves in brain tissue. A modified phase-contrast gradient-echo sequence was developed. The propagating shear waves within the brain were directly imaged. The wave images were processed to obtain the elasticity image. Shear waves at 100 Hz, 150 Hz, and 200 Hz were applied.

Results: The propagating shear waves in the brain were visualized on wave images. The elasticity image revealed the difference in tissue elasticity between gray and white matter of the brain.

Conclusion: MRE could be an imaging technique for assessing the elasticity of brain tissue.     

Application of DENSE‐MR‐elastography to the human heart

Typically, MR‐elastography (MRE) encodes the propagation of monochromatic acoustic waves in the MR‐phase images via sinusoidal gradients characterized by a detection frequency equal to the frequency of the mechanical vibration. Therefore, the echo time of a conventional MRE sequence is typically longer than the vibration period which is critical for heart tissue exhibiting a short T₂. Thus, fast acquisition techniques like the so‐called fractional encoding of harmonic motions were developed for cardiac applications. However, fractional encoding of harmonic motions is limited since it is two orders of magnitude less sensitive to motion than conventional MRE sequences for low‐frequency vibrations. Here, a new sequence is derived from the so‐called displacement encoding with stimulated echoes (DENSE) sequence. This sequence is more sensitive to displacement than fractional encoding of harmonic motions, and its spectral specificity is equivalent to conventional MRE sequences. The theoretical spectral properties of this new motion‐encoding technique are validated in a phantom and excised pork heart specimen. An excellent agreement is found for the measured displacement fields using classic MRE and displacement encoding with stimulated echoes MRE (8% maximum difference). In addition, initial in vivo results on a healthy volunteer clearly show propagating shear waves at 50 Hz. Thus, displacement encoding with stimulated echoes MRE is a promising technique for motion encoding within short T₂* materials. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Functional MR imaging of pulmonary ventilation using hyperpolarized noble gases

The current status of experimental and clinical applications for functional MR imaging of pulmonary ventilation using hyperpolarized noble gases are reviewed. 3-helium (3He) and 129-xenon (129Xe) can be hyperpolarized by optical pumping techniques such as spin exchange or metastability exchange in sufficient amounts. This process leads to an artificial, non-equilibrium increase of the density of excited nuclei which represents the source of the MR signal. Those hyperpolarized gases are administered mostly via inhalation, and will fill airways and airspaces allowing for ventilation imaging. Recent human studies concentrate on imaging the airways and airspaces with high spatial resolution. Normal ventilation is reflected by an almost complete and homogeneous distribution of the hyperpolarized gas represented by the signal detected. Loss of signal or inhomogeneous signal distribution represent mass effects and ventilatory abnormalities. Even healthy subjects with seasonal allergies without pulmonary symptoms have been observed to exhibit transient ventilation defects. Real-time imaging of ventilation has become feasible for 3He MR imaging and allows for assessment of ventilation-distribution. Furthermore, functional oxygen-sensitive 3He MR imaging opens the field of non-invasive assessment of regional intrapulmonary oxygen concentrations in vivo. Knowing that the diffusion of gas is affected by the geometry and nature of its environment, diffusion measurements are under investigation as a sensitive marker of diseases that involve structural changes of lung parenchyma, such as emphysema and fibrosis. Whereas 3He is not absorbed and is restricted to the airspaces, 129Xe is soluble in blood and lipid-rich tissue. This presents the opportunity for additional dissolved-phase imaging, providing a step towards simultaneous ventilation-perfusion studies.     

MR Imaging with Hyperpolarized 3He Gas

Magnetic resonance images of the lungs of a guinea pig have been produced using hyperpolarized helium as the source of the MR signal. The resulting images are not yet sufficiently optimized to reveal fine structural detail within the lung, but the spectacular signal from this normally signal-deficient organ system offers great promise for eventual in vivo imaging experiments. Fast 2D and 3D GRASS sequences with very small flip angles were employed to conserve the nonrenewable longitudinal magnetization. We discuss various unique features associated with performing MRI with hyperpolarized gases, such as the selection of the noble gas species, polarization technique, and constraints on the MR pulse sequence.     

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.