Assessment of stiffness changes in the ex vivo porcine aortic wall using magnetic resonance elastography

Magnetic resonance elastography (MRE) is a noninvasive phase-contrast technique for estimating the mechanical properties of tissues by imaging propagating mechanical waves within the tissue. In this study, we hypothesize that changes in arterial wall stiffness, experimentally induced by formalin fixation, can be measured using MRE in ex vivo porcine aortas. In agreement with our hypothesis, the significant stiffness increase after sample fixation was clearly demonstrated by MRE and confirmed by mechanical testing. The results indicate that MRE can be used to examine the stiffness changes of the aorta. This study has provided evidence of the effectiveness of using MRE to directly assess the stiffness change in aortic wall. The results offer motivation to pursue MRE as a noninvasive method for the evaluation of arterial wall mechanical properties.     

Effect of off-frequency sampling in magnetic resonance elastography

In magnetic resonance elastography (MRE), shear waves at a certain frequency are encoded through bipolar gradients that switch polarity at a controlled encoding frequency and are offset in time to capture wave propagation using a controlled sampling frequency. In brain MRE, there is a possibility that the mechanical actuation frequency is different from the vibration frequency, leading to a mismatch with encoding and sampling frequencies. This mismatch can occur in brain MRE from causes both extrinsic and intrinsic to the brain, such as scanner bed vibrations or active damping in the head. The purpose of this work was to investigate how frequency mismatch can affect MRE shear stiffness measurements. Experiments were performed on a dual-medium agarose gel phantom, and the results were compared with numerical simulations to quantify these effects. It is known that off-frequency encoding alone results in a scaling of wave amplitude, and it is shown here that off-frequency sampling can result in two main effects: (1) errors in the overall shear stiffness estimate of the material on the global scale and (2) local variations appearing as stiffer and softer structures in the material. For small differences in frequency, it was found that measured global stiffness of the brain could theoretically vary by up to 12.5% relative to actual stiffness with local variations of up to 3.7% of the mean stiffness. It was demonstrated that performing MRE experiments at a frequency other than that of tissue vibration can lead to artifacts in the MRE stiffness images, and this mismatch could explain some of the large-scale scatter of stiffness data or lack of repeatability reported in the brain MRE literature.     

Magnetic resonance elastography using an air ball-actuator

The purpose of this study was to develop a new technique for a powerful compact MR elastography (MRE) actuator based on a pneumatic ball-vibrator. This is a compact actuator that generates powerful centrifugal force vibrations via high speed revolutions of an internal ball using compressed air. This equipment is easy to handle due to its simple principles and structure. Vibration frequency and centrifugal force are freely adjustable via air pressure changes (air flow volume), and replacement of the internal ball. In order to achieve MRI compatibility, all parts were constructed from non-ferromagnetic materials. Vibration amplitudes (displacements) were measured optically by a laser displacement sensor. From a bench test of displacement, even though the vibration frequency increased, the amount of displacement did not decrease. An essential step in MRE is the generation of mechanical waves within tissue via an actuator, and MRE sequences are synchronized to several phase offsets of vibration. In this system, the phase offset was detected by a four-channel optical-fiber sensor, and it was used as an MRI trigger signal. In an agarose gel phantom experiment, this actuator was used to make an MR elastogram. This study shows that the use of a ball actuator for MRE is feasible.     

Diffraction-biased shear wave fields generated with longitudinal magnetic resonance elastography drivers

Magnetic resonance elastography (MRE) is a technique for quantifying the acoustic response of biological tissues to propagating waves applied at low frequencies in order to evaluate mechanical properties. Application-specific MRE drivers are typically required to effectively deliver shear waves within the tissue of interest. Surface MRE drivers with transversely oriented vibrations have often been used to directly generate shear waves. These drivers may have disadvantages in certain applications, such as poor penetration depth and inflexible orientation. Therefore, surface MRE drivers with longitudinally oriented vibrations are used in some situations. The purpose of this work was to investigate and optimize a longitudinal driver system for MRE applications. A cone-like hemispherical distribution of shear waves being generated by these drivers and the wave propagation being governed by diffraction in the near field are shown. Using MRE visualization of the vector displacement field, we studied the properties of the shear wave field created by longitudinal MRE drivers of various sizes to identify optimum shear wave imaging planes. The results offer insights and improvements in both experimental design and imaging plane selection for 2-D MRE data acquisition.     

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

The increasing prevalence of Alzheimer's 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 Alzheimer's disease.     

Modeling shear modulus distribution in magnetic resonance elastography with piecewise constant level sets

Magnetic resonance elastography (MRE) is designed for imaging the mechanical properties of soft tissues. However, the interpretation of shear modulus distribution is often confusing and cumbersome. For reliable evaluation, a common practice is to specify the regions of interest and consider regional elasticity. Such an experience-dependent protocol is susceptible to intrapersonal and interpersonal variability. In this study we propose to remodel shear modulus distribution with piecewise constant level sets by referring to the corresponding magnitude image. Optimal segmentation and registration are achieved by a new hybrid level set model comprised of alternating global and local region competitions. Experimental results on the simulated MRE data sets show that the mean error of elasticity reconstruction is 11.33% for local frequency estimation and 18.87% for algebraic inversion of differential equation. Piecewise constant level set modeling is effective to improve the quality of shear modulus distribution, and facilitates MRE analysis and interpretation.     

Analysis of wave patterns in MR elastography of skeletal muscle using coupled harmonic oscillator simulations

The ability to study muscle elasticity in vivo would be of great clinical interest. Magnetic resonance elastography (MRE) has the potential to quantify noninvasively the distribution of the shear modulus in muscle tissue. Elasticity information may be derived by extracting frequencies from the wave patterns of phase-contrast MRE images. In a new approach, MRE wave patterns were reconstructed using 3D coupled harmonic oscillator calculations (CHO). To analyze in vivo MRE measurements of the biceps brachii of healthy volunteers, different anisotropic fibrous structures for the couplings between the muscle elements have to be assumed. V-shaped wave patterns as observed when excitation was applied on the tendon were reproduced by a model, where in a central band of stiff fascicles wave propagation was about twice as fast as that in surrounding tissue. Planar waves were observed for excitation near the muscle surface. They could be reconstructed by assuming a simultaneous wave excitation of all muscle fibers, where fibers along the main muscle axis were coupled more strongly than those perpendicular to the axis. The results show that CHO calculations provide a fast and reliable method for incorporating anatomical information of the investigated tissue in the reconstruction of complex wave patterns.     

Feasibility of MR elastography of the intervertebral disc

Low back pain (LBP) is a costly and widely prevalent health disorder in the U.S. One of the most common causes of LBP is degenerative disc disease (DDD). There are many imaging techniques to characterize disc degeneration; however, there is no way to directly assess the material properties of the intervertebral disc (IVD) within the intact spine. Magnetic resonance elastography (MRE) is an MRI-based technique for non-invasively mapping the mechanical properties of tissues in vivo. The purpose of this study was to investigate the feasibility of using MRE to detect shear wave propagation in and determine the shear stiffness of an axial cross-section of an ex vivo baboon IVD, and compare with shear displacements from a finite element model of an IVD motion segment in response to harmonic shear vibration. MRE was performed on two baboon lumbar spine motion segments (L3–L4) with the posterior elements removed at a range of frequencies (1000–1500 Hz) using a standard clinical 1.5 T MR scanner. Propagating waves were visualized in an axial cross-section of the baboon IVDs in all three motion-encoding directions, which resembled wave patterns predicted using finite element modeling. The baboon nucleus pulposus showed an average shear stiffness of 79 ± 15 kPa at 1000 Hz. These results suggest that MRE is capable of visualizing shear wave propagation in the IVD, assessing the stiffness of the nucleus of the IVD, and can differentiate the nucleus and annulus regions.     

磁共振弹性成像技术在肝纤维化检测中的研究

对肝脏组织的弹性应力分析在肝纤维化程度的精确定量评价方面是非常关键的,因此从弹性力学角度结合磁共振成像技术的磁共振弹性成像研究在肝脏疾病的早期诊断和治疗方面将具有重要意义和广泛应用. 该文针对弹性磁共振成像技术的理论与技术进行了全面的研究,开发了磁共振弹性图研究平台,主要包括：组织生物力学模型分析与建模,剪切波激励装置的开发,弹性成像序列的设计,弹性拟合算法研究与实现4个方面的工作. 基于上述工作,进行了相关体模和离体猪肝以及志愿者在体肝脏实验,实验结果表明,研究平台基本可以满足肝脏弹性成像研究的需要.     

Magnetic resonance elastography and diffusion-weighted imaging of the sol/gel phase transition in agarose

The dynamics of the sol/gel phase transition in agarose was analyzed with magnetic resonance elastography (MRE) and diffusion-weighted imaging, providing complementary information on a microstructural as well as on a macroscopic spatial scale. In thermal equilibrium, the diffusion coefficient of agarose is linearly correlated with temperature, independent of the sol/gel phase transition. In larger agarose samples, the transition from the sol to the gel state was characterized by a complex position and temperature dependency of both MRE shear wave patterns and apparent diffusion coefficients (ADC). The position dependency of the temperature was experimentally found to be qualitatively similar to the behavior of the ADC maps. The dynamics of the temperature could be described with a simplified model that described the heat exchange between sol and gel compartments. The experiments supported the approach to derive temperature maps from the ADC maps by a linear relationship. The spatially resolved dynamics of the temperature maps were therefore employed to determine the elasticities. For this reason, experimental MRE data were simulated using a model of coupled harmonic oscillators. The calculated images agreed well with the experimentally observed MRE wave patterns.     

Rapid acquisition technique for MR elastography of the liver

Magnetic resonance elastography (MRE) of the liver is a novel noninvasive clinical diagnostic tool to stage fibrosis based on measured stiffness. The purpose of this study is to design, evaluate and validate a rapid MRE acquisition technique for noninvasively quantitating liver stiffness which reduces by half the scan time, thereby decreasing image registration errors between four MRE phase offsets. In vivo liver MRE was performed on 16 healthy volunteers and 14 patients with biopsy-proven liver fibrosis using the standard clinical gradient recalled echo (GRE) MRE sequence (MREs) and a developed rapid GRE MRE sequence (MREr) to obtain the mean stiffness in an axial slice. The mean stiffness values obtained from the entire group using MREs and MREr were 2.72 ± 0.85 kPa and 2.7 ± 0.85 kPa, respectively, representing an insignificant difference. A linear correlation of R² = 0.99 was determined between stiffness values obtained using MREs and MREr. Therefore, we can conclude that MREr can replace MREs, which reduces the scan time to half of that of the current standard acquisition (MREs), which will facilitate MRE imaging in patients with inability to hold their breath for long periods.     

On the feasibility of elastic wave visualization within polymeric solids using magnetic resonance elastography

In this paper, the feasibility of extending previously described magnetic resonance elastography (MRE) dynamic displacement (and associated elasticity) measurement techniques, currently used successfully in tissue, to solid materials which have much higher shear rigidity and much lower nuclear spin densities, is considered. Based on these considerations, the MRE technique is modified in a straightforward manner and used to directly visualize shear wave displacements within two polymeric materials, one of which is relatively stiff.     

Static and dynamic liver stiffness: An ex vivo porcine liver study using MR elastography

Magnetic resonance elastography (MRE) is an MRI-based noninvasive technique for quantitatively assessing tissue stiffness. The hypothesis of this study is that stiffness increases with portal pressure. We further hypothesized that the rate of stiffness change with pressure would be larger in liver tissue treated to simulate the stiffening effects of fibrosis. In agreement with our hypothesis, the formalin-treated livers were stiffer than the untreated livers, and in both groups the liver stiffness increased with portal venous pressure. The rate of stiffness change with portal pressure was significantly greater after formalin treatment. In this study, we have developed an ex vivo liver model incorporating portal venous pressure variations and observed significant changes in liver stiffness due to portal pressure. This model could be useful for understanding and investigating the changes in the static and dynamic components of liver stiffness.     

Stiffness change of the supraspinatus muscle can be detected by magnetic resonance elastography

Magnetic resonance elastography (MRE) and ultrasound shear wave elastography (SWE) are imaging techniques to measure stiffness of the soft tissue using magnetic resonance imaging (MRI) and ultrasound images, respectively. The purpose of this study was to explore the feasibility of the MRE measurement to evaluate the change in supraspinatus (SSP) muscle stiffness before and after rotator cuff tear, and to compare the result with those of SWE. Six swine shoulders were used. The skin and subcutaneous fat were removed, and the stiffness value of the SSP muscle was measured by MRE and SWE. The MRE measurement was performed with 0.3 T open MRI and the vibration from a pneumatic driver system with active driver to a passive driver to create the shear wave in the tissue. The passive driver was placed on the center of the SSP muscle. The stiffness was estimated from the wave images using local frequency estimation methods. In the SWE measurement, the probe of the ultrasound was placed on the center of the SSP muscle. The shear wave propagation speed was measured at a depth of 1 cm from the surface, and the stiffness was calculated. After those measurements, the rotator cuff tendon was detached from the greater tuberosity, and MRE and SWE measurements were then performed in the same manner again. The differences in the stiffness values were compared between before and after the rotator cuff tendon tear on both the MRE and SWE measurements. The results indicated that stiffness values on MRE and SWE were 9.3 ± 1.8 and 10.0 ± 1.2 kPa respectively before the rotator cuff tear, and 7.3 ± 1.3 and 8.0 ± 0.8 kPa respectively after the tendon detachment. Stiffness values were significantly lower after the tendon detachment on both the MRE and SWE measurements (p < 0.05). Our results demonstrated that stiffness values of the SSP muscle on MRE and SWE were lower after rotator cuff detachment. From this result, MRE may be a feasible method for quantification of the change in rotator cuff muscle stiffness.     

MR elastography of the head and neck: Driver design and initial results

Purpose

The purpose was to describe the design and fabrication of a driver suitable for magnetic resonance elastography (MRE) of the head and neck and to assess its performance in evaluating human parotid gland, lymph nodes and thyroid at 3.0 T.

Materials and methods

A head and neck driver was fabricated using a commercial transducer, headrest mould and piston extension. Driver performance was tested using a motion-sensitized spin-echo MRE pulse sequence. Six healthy volunteers and three patients (two metastatic nodes and one papillary carcinoma) were evaluated using MRE. Viscoelastic maps were computed to obtain storage modulus (G’) and loss modulus (G”) of the normal parotid and thyroid, metastatic node and thyroid cancer. Reproducibility was assessed by coefficient of variation.

Results

All subjects completed MRE examination without discomfort. Initial G’ and G” values were as follows: normal parotid gland, 1.12 kPa and 0.48 kPa; thyroid, 0.58 kPa and 0.42 kPa; metastatic node, 0.66 kPa and 0.58 kPa; and thyroid cancer, 0.17 kPa and 0.28 kPa. Based on parotid data, the coefficient of variation for G’ and G” was 4.7% and 9.8%.

Conclusion

A new MRE driver for head and neck was successfully implemented, and our initial results suggested the device was suitable for the mechanical assessment of tissues in the head and neck.     

Modelling and characterization of a piezoceramic inertial actuator

An inertial actuator (also known as a proof mass actuator) applies forces to a structure by reacting them against an “external” mass. This approach to actuation may provide some practical benefits in the active control of vibration and structure-borne noise: system reliability may be improved by removing the actuator from a structural load path; effective discrete point-force actuation permits ready attachment to curved surfaces, and an inherent passive vibration absorber effect can reduce power requirements.This paper describes a class of recently developed inertial actuators that is based on mechanical amplification of displacements of an active piezoceramic element. Important actuator characteristics include resonance frequencies, clamped force, and the drive voltage to output the force frequency response function.The paper addresses one particular approach to motion amplification, the “dual unimorph,” in detail. A model of actuator dynamic behavior is developed using an assumed-modes method, treating the piezoelectrically induced stresses as external forces. Predicted actuator characteristics agree well with experimental data obtained for a prototype actuator. The validated actuator dynamic model provides a tool for design improvement.     

Effects of gadoxetic acid on liver elasticity measurement by using magnetic resonance elastography

The purpose of this study was to evaluate the effect of gadoxetic acid (Gd-EOB-DTPA) on measurements of liver stiffness by using magnetic resonance elastography (MRE). In this study, 104 consecutive patients (mean age, 67.7±9.4 years) underwent MRE using a 1.5-T MR scanner equipped with a cylindrical passive driver that was placed across the right chest wall for delivering vibrations. Axial gradient-echo images, which were automatically converted to elastograms that represented stiffness (kPa), were acquired using a continuous sinusoidal vibration of 60 Hz. Two raters independently placed a region of interest on the right lobe of the liver on the elastograms obtained before and after Gd-EOB-DTPA was administered. Liver stiffness was measured using these two elastograms and compared using a paired t test and correlation analysis. No significant difference was observed in liver stiffness before and after Gd-EOB-DTPA was administered (Rater 1, P=.1200; Rater 2, P=.3585). The correlation coefficients were 0.986 (Rater 1) and 0.984 (Rater 2), indicating excellent correlation between the stiffness values before and after Gd-EOB-DTPA was administered. Liver stiffness measured by MRE did not differ before and after Gd-EOB-DTPA was administered.     

Thin-disk piezoceramic ultrasonic motor. Part I: design and performance evaluation

The purpose of this study is to gain the knowledge and experience in the design of thin-disk piezoceramic-driving ultrasonic actuator dedicated. In this paper, the design and construction of an innovative ultrasonic actuator is developed as a stator, which is a composite structure consisting of piezoceramic (PZT) membrane bonded on a metal sheet. Such a concentric PZT structure possesses the electrical and mechanical coupling characteristics in flexural wave. The driving ability of the actuator comes from the mechanical vibration of extension and shrinkage of a metal sheet due to the converse piezoelectric effect, corresponding to the frequency of a single-phase AC power. By applying the constraints on the specific geometry positions on the metal sheet, the various behaviors of flexural waves have been at the different directions. The rotor is impelled by the actuator with rotational speeds of 600 rpm in maximum using a friction-contact mechanism. Very high actuating and braking abilities are obtained. This simple and inexpensive structure of actuator demonstrates that the mechanical design of actuator and rotor could be done separately and flexibly according to the requirements for various applications. And, its running accuracy and positioning precision are described in Part II.A closed loop servo positioning control i.e. sliding mode control (SMC) is used to compensate automatically for nonlinearly mechanical behaviors such as dry friction, ultrasonic vibrating, slip-stick phenomena. Additionally, SMC scheme has been successfully applied to position tracking to prove the excellent robust performance in noise rejection.     

悬线式物镜力矩器动态特性的仿真实验研究

悬线式物镜力矩器是目前光学头系统中使用最为广泛的力矩器,其动态特性直接决定了光学头的读写性能。基于ANSYS参数化设计语言(APDL)建立了悬线式光学头物镜力矩器的仿真设计平台。通过仿真平台进行了力矩器一阶共振频率和峰值、二阶共振频率和峰值、扭转共振频率、静态灵敏度、动态灵敏度和相位等动态特性参数的仿真实验,仿真计算的结果与实际测量的结果相比,最大相对误差为4.2%。该仿真平台对于二维乃至三维悬线式物镜力矩器的设计和评价都具有重要的指导意义。     

Active damping control unit using a small scale proof mass electrodynamic actuator

This paper presents a study on the design and use of a small scale proof mass electrodynamic actuator, with a low mounting resonance frequency, for velocity feedback control on a thin rectangular panel. A stability-performance formula is derived, which can be effectively used to assess the down scaling effects on the stability and control performance of the feedback loop. The design and tests of a velocity feedback loop with a prototype small scale proof mass actuator are also presented. When a feedback control having a gain margin of about 6 dB is implemented, so that there is little control spillover effect around the fundamental resonance of the actuator, reductions of vibration between 5 dB and 10 dB in the frequency band between 80 Hz and 250 Hz have been measured at the control position.