Magnetic resonance imaging of shear wave propagation in excised tissue

MR Elastography of the Breast: Evolution of Technique, Case Examples, and Future Directions

Recognizing that breast cancers present as firm, stiff lesions, the foundation of breast magnetic resonance elastography (MRE) is to combine tissue stiffness parameters with sensitive breast MR contrast-enhanced imaging. Breast MRE is a non-ionizing, cross-sectional MR imaging technique that provides for quantitative viscoelastic properties, including tissue stiffness, elasticity, and viscosity, of breast tissues. Currently, the technique continues to evolve as research surrounding the use of MRE in breast tissue is still developing. In the setting of a newly diagnosed cancer, associated desmoplasia, stiffening of the surrounding stroma, and necrosis are known to be prognostic factors that can add diagnostic information to patient treatment algorithms. In fact, mechanical properties of the tissue might also influence breast cancer risk. For these reasons, exploration of breast MRE has great clinical value. In this review, we will: (1) address the evolution of the various MRE techniques; (2) provide a brief overview of the current clinical studies in breast MRE with interspersed case examples; and (3) suggest directions for future research.

Changes of Material Elastic Properties during Healing of Ruptured Achilles Tendons Measured with Shear Wave Elastography: A Pilot Study

Therapy options for ruptured Achilles tendons need to take into account the right balance of timing, amount and intensity of loading to ensure a sufficient biomechanical resilience of the healing tendon on the one hand, and to enable an adequate tensile stimulus on the other hand. However, biomechanical data of human Achilles tendons after rupture during the separate healing stages are unknown. Shear wave elastography is an ultrasound technique that measures material elastic properties non-invasively, and was proven to have a very good correlation to biomechanical studies. Taking advantage of this technology, 12 patients who suffered from an acute Achilles tendon rupture were acquired and monitored through the course of one year after rupture. Nine of these patients were treated non-operatively and were included for the analysis of biomechanical behaviour. A significant increase of material elastic properties was observed within the first six weeks after trauma (up to 80% of baseline value), where it reached a plateau phase. A second significant increase occurred three to six months after injury. This pilot study suggests a time correlation of biomechanical properties with the biological healing phases of tendon tissue. In the reparative phase, a substantial amount of biomechanical resilience is restored already, but the final stage of biomechanical stability is reached in the maturation phase. These findings can potentially be implemented into treatment and aftercare protocols.

Acoustic radiation force optical coherence elastography for elasticity assessment of soft tissues

Biomechanical properties of soft tissues are important indicators of tissue functions which can be used for clinical diagnosis and disease monitoring. Elastography, incorporating the principles of elasticity measurements into imaging modalities, provides quantitative assessment of elastic properties of biological tissues. Benefiting from high-resolution, noninvasive and three-dimensional optical coherence tomography (OCT), optical coherence elastography (OCE) is an emerging optical imaging modality to characterize and map biomechanical properties of soft tissues. Recently, acoustic radiation force (ARF) OCE has been developed for elasticity measurements of ocular tissues, detection of vascular lesions and monitoring of blood coagulation based on remote and noninvasive ARF excitation to both internal and superficial tissues. Here, we describe the advantages of the ARF-OCE technique, the measurement methods in ARF-OCE, the applications in biomedical detection, current challenges and advances. ARF-OCE technology has the potential to become a powerful tool for in vivo elasticity assessment of biological samples in a non-contact, non-invasive and high-resolution nature.

Breast magnetic resonance elastography: a review of clinical work and future perspectives

This review on magnetic resonance elastography (MRE) of the breast provides an overview of available literature and describes current developments in the field of breast MRE, including new transducer technology for data acquisition and multi-frequency-derived power-law behaviour of tissue. Moreover, we discuss the future potential of breast MRE, which goes beyond its original application as an additional tool in differentiating benign from malignant breast lesions. These areas of ongoing and future research include MRE for pre-operative tumour delineation, staging, monitoring and predicting response to treatment, as well as prediction of the metastatic potential of primary tumours.

Minimally Invasive Micro-Indentation: mapping tissue mechanics at the tip of an 18G needle

Experiments regarding the mechanical properties of soft tissues mostly rely on data collected on specimens that are extracted from their native environment. During the extraction and in the time period between the extraction and the completion of the measurements, however, the specimen may undergo structural changes which could generate unwanted artifacts. To further investigate the role of mechanics in physiology and possibly use it in clinical practices, it is thus of paramount importance to develop instruments that could measure the viscoelastic response of a tissue without necessarily excising it. Tantalized by this opportunity, we have designed a minimally invasive micro-indenter that is able to probe the mechanical response of soft tissues, in situ, via an 18G needle. Here, we discuss its working principle and validate its usability by mapping the viscoelastic properties of a complex, confined sample, namely, the nucleus pulposus of the intervertebral disc. Our findings show that the mechanical properties of a biological tissue in its local environment may be indeed different than those that one would measure after excision, and thus confirm that, to better understand the role of mechanics in life sciences, one should always perform minimally invasive measurements like those that we have here introduced.

Longitudinal shear wave imaging for elasticity mapping using optical coherence elastography

Shear wave measurements for the determination of tissue elastic properties have been used in clinical diagnosis and soft tissue assessment. A shear wave propagates as a transverse wave where vibration is perpendicular to the wave propagation direction. Previous transverse shear wave measurements could detect the shear modulus in the lateral region of the force; however, they could not provide the elastic information in the axial region of the force. In this study, we report the imaging and quantification of longitudinal shear wave propagation using optical coherence tomography to measure the elastic properties along the force direction. The experimental validation and finite element simulations show that the longitudinal shear wave propagates along the vibration direction as a plane wave in the near field of a planar source. The wave velocity measurement can quantify the shear moduli in a homogeneous phantom and a side-by-side phantom. Combining the transverse shear wave and longitudinal shear wave measurements, this system has great potential to detect the directionally dependent elastic properties in tissues without a change in the force direction.

Shear Wave Imaging of Breast Tissue by Color Doppler Shear Wave Elastography

Shear wave elastography is a distinctive method to access the viscoelastic characteristic of the soft tissue that is difficult to obtain by other imaging modalities. This paper proposes a novel shear wave elastography [color Doppler shear wave imaging (CD SWI)] for breast tissue. Continuous shear wave is produced by a small lightweight actuator, which is attached to the tissue surface. Shear wave wavefront that propagates in tissue is reconstructed as a binary pattern that consists of zero and the maximum flow velocities on color flow image (CFI). Neither any modifications of the ultrasound color flow imaging instrument nor a high frame rate ultrasound imaging instrument is required to obtain the shear wave wavefront map. However, two conditions of shear wave displacement amplitude and shear wave frequency are needed to obtain the map. However, these conditions are not severe restrictions in breast imaging. This is because the minimum displacement amplitude is [Formula: see text] for an ultrasonic wave frequency of 12 MHz and the shear wave frequency is available from several frequencies suited for breast imaging. Fourier analysis along time axis suppresses clutter noise in CFI. A directional filter extracts shear wave, which propagates in the forward direction. Several maps, such as shear wave phase, velocity, and propagation maps, are reconstructed by CD SWI. The accuracy of shear wave velocity measurement is evaluated for homogeneous agar gel phantom by comparing with the acoustic radiation force impulse method. The experimental results for breast tissue are shown for a shear wave frequency of 296.6 Hz.

Magnetic resonance elastography (MRE) in cancer: Technique, analysis, and applications

Tissue mechanical properties are significantly altered with the development of cancer. Magnetic resonance elastography (MRE) is a noninvasive technique capable of quantifying tissue mechanical properties in vivo. This review describes the basic principles of MRE and introduces some of the many promising MRE methods that have been developed for the detection and characterization of cancer, evaluation of response to therapy, and investigation of the underlying mechanical mechanisms associated with malignancy.

Intraoperative magnetic resonance imaging–conditional robotic devices for therapy and diagnosis

Magnetic resonance imaging presents high-resolution preoperative scans of target tissue and allows for the availability of intraoperative real-time images without the exposure of patients to ionizing radiation. This has motivated scientists and engineers to integrate medical robotics with the magnetic resonance imaging modality to allow robot-assisted, image-guided diagnosis and therapy. This article provides a review of the state-of-the-art medical robotic systems available for use in conjunction with intraoperative magnetic resonance imaging. The robot functionalities and mechanical designs for a wide range of magnetic resonance imaging interventions are presented, including their magnetic resonance imaging compatibility, actuation, kinematics and the mechanical and electrical designs of the robots. Classification and comparative study of various intraoperative magnetic resonance image guided robotic systems are provided. The robotic systems reviewed are summarized in a table in detail. Current technologies for magnetic resonance imaging–conditional robotics are reviewed and their potential future directions are sketched.

Phase-sensitive imaging of tissue acoustic vibrations using spectrally encoded interferometry

Acoustic vibrations in tissue are often difficult to image, requiring high-speed scanning, high sensitivity and nanometer-scale axial resolution. Here we use spectrally encoded interferometry to measure the vibration pattern of two-dimensional surfaces, including the skin of a volunteer, at nanometric resolution, without the need for rapid lateral scanning and with no prior knowledge of the driving acoustic waveform. Our results demonstrate the feasibility of this technique for measuring tissue biomechanics using simple and compact imaging probes.

Incorporating endorectal MR elastography into multi-parametric MRI for prostate cancer imaging: Initial feasibility in volunteers

PURPOSE

To investigate the tolerability and technical feasibility of performing endorectal MR elastography (eMRE) in human volunteers within the representative age group commonly affected by prostate cancer.

MATERIALS AND METHODS

Endorectal MRE was conducted on seven volunteers in a 1.5 Tesla (T) MR imager using a rigid endorectal coil. Another five volunteers were imaged on a 3T MR imager using an inflatable balloon type endorectal coil. Tolerability was accessed for vibration amplitudes of ±1-50 μm and for frequencies of 100-300 Hz.

RESULTS

All 12 volunteers tolerated the displacements necessary to successfully perform eMRE. Shear waves with frequencies up to 300 Hz could propagate across the entire prostate using both coil designs.

CONCLUSION

The results of this study motivate further investigation of eMRE in prostate cancer patients to help determine if there is an added value of integrating eMRE into existing multi-parametric prostate MRI exams.

Monodomain shear wave propagation and bidomain shear wave dispersion in an elastic model of cardiac tissue

Cardiac tissue elastically deforms under an applied stress, permitting shear waves to propagate through the heart. Traditionally, this behavior has been modeled with a monodomain approach, in which the mechanical properties of the intracellular and extracellular spaces are averaged together. We consider a mechanical bidomain model of cardiac tissue in which the mechanics of the intracellular and extracellular spaces are considered individually with the two spaces coupled by a spring constant. We find two normal modes of oscillation: one in which the intracellular and extracellular spaces oscillate together (a monodomain mode) and the other in which they oscillate in opposition (a bidomain mode). These two modes have unique dispersion relationships. In the extreme approximation of equal shear moduli and mass densities of the intracellular and extracellular spaces, the dispersion in the bidomain mode depends on the spring constant, while it does not in the monodomain mode.

fMRI-Compatible Electromagnetic Haptic Interface

A new haptic interface device is suggested, which can be used for functional magnetic resonance imaging (fMRI) studies. The basic component of this 1 DOF haptic device are two coils that produce a Lorentz force induced by the large static magnetic field of the MR scanner. A MR-compatible optical angular encoder and a optical force sensor enable the implementation of different control architectures for haptic interactions. The challenge was to provide a large torque, and not to affect image quality by the currents applied in the device. The haptic device was tested in a 3T MR scanner. With a current of up to 1A and a distance of 1m to the focal point of the MR-scanner it was possible to generate torques of up to 4 Nm. Within these boundaries image quality was not affected.

Physics of MRI: a primer

This article is based on an introductory lecture given for the past many years during the "MR Physics and Techniques for Clinicians" course at the Annual Meeting of the ISMRM. This introduction is not intended to be a comprehensive overview of the field, as the subject of magnetic resonance imaging (MRI) physics is large and complex. Rather, it is intended to lay a conceptual foundation by which magnetic resonance image formation can be understood from an intuitive perspective. The presentation is nonmathematical, relying on simple models that take the reader progressively from the basic spin physics of nuclei, through descriptions of how the magnetic resonance signal is generated and detected in an MRI scanner, the foundations of nuclear magnetic resonance (NMR) relaxation, and a discussion of the Fourier transform and its relation to MR image formation. The article continues with a discussion of how magnetic field gradients are used to facilitate spatial encoding and concludes with a development of basic pulse sequences and the factors defining image contrast.

Transperineal prostate MR elastography: initial in vivo results

This article presents a new approach to magnetic resonance elastography of the prostate using transperineal mechanical excitation. This approach is validated using a prostate elasticity phantom and in vivo studies of healthy volunteers. It is demonstrated that the transperineal approach can generate shear wave amplitudes on the order of 6-30 μm in the mid-gland region. The driver was implemented using an electromagnetic actuator with a hydraulic transmission system. The magnetic resonance elastography acquisition time has been reduced significantly by using a "second harmonic" approach. Displacement fields are processed using the established three-dimensional local frequency estimation algorithm. The three-dimensional curl-based direct inversion was used to calculate the local wavelength. The traveling wave expansion algorithm was used to reconstruct the wave damping image for one case. Using the proposed method, it was possible to resolve lesions of 0.5 cc in the phantom study. Repeatability experiments were performed and analyzed. The results from this study indicate that transperineal magnetic resonance elastography--without an endorectal coil--is a suitable candidate for a patient study involving multiparametric magnetic resonance imaging of prostate cancer, where magnetic resonance elastography may provide additional information for improved diagnosis and image-based surveillance.

Acoustic radiation force-driven assessment of myocardial elasticity using the displacement ratio rate (DRR) method

A noninvasive method of characterizing myocardial stiffness could have significant implications in diagnosing cardiac disease. Acoustic radiation force (ARF)-driven techniques have demonstrated their ability to discern elastic properties of soft tissue. For the purpose of myocardial elasticity imaging, a novel ARF-based imaging technique, the displacement ratio rate (DRR) method, was developed to rank the relative stiffnesses of dynamically varying tissue. The basis and performance of this technique was demonstrated through numerical and phantom imaging results. This new method requires a relatively small temporal (<1 ms) and spatial (tenths of mm(2)) sampling window and appears to be independent of applied ARF magnitude. The DRR method was implemented in two in vivo canine studies, during which data were acquired through the full cardiac cycle by imaging directly on the exposed epicardium. These data were then compared with results obtained by acoustic radiation force impulse (ARFI) imaging and shear wave velocimetry, with the latter being used as the gold standard. Through the cardiac cycle, velocimetry results portray a range of shear wave velocities from 0.76-1.97 m/s, with the highest velocities observed during systole and the lowest observed during diastole. If a basic shear wave elasticity model is assumed, such a velocity result would suggest a period of increased stiffness during systole (when compared with diastole). Despite drawbacks of the DRR method (i.e., sensitivity to noise and limited stiffness range), its results predicted a similar cyclic stiffness variation to that offered by velocimetry while being insensitive to variations in applied radiation force.

The feasibility of endorectal MR elastography for prostate cancer localization

The objectives of this study were to evaluate the feasibility of using a rigid radio-frequency receiver endorectal coil for intracavitary prostate magnetic resonance elastography (MRE) and to demonstrate the capability of this technique for generating stiffness maps over a typical prostate volume. An endorectal coil is currently used to help improve the signal-to-noise ratio of images acquired with multiparametric magnetic resonance imaging. We propose that this same coil could also serve to generate shear waves in the prostate gland during imaging, opening up the possibility of incorporating prostate stiffness characterization into multiparametric magnetic resonance imaging. Prostate cancer has been shown to change the elasticity of tissue, suggesting that stiffness imaging (elastography) may provide supplementary diagnostic information. A rigid endorectal coil was mechanically coupled to a piezoceramic actuator and used to investigate full volume (27 slices, 2-mm thick) endorectal MRE in a prostate mimicking phantom. The low-amplitude vibrations (± 8-38 μm displacements) necessary to perform endorectal MRE did not affect the signal-to noise ratio of the coil and endorectal MRE was capable of resolving 0.1 cc (0.6 cm diameter) spherical inclusion volumes. Therefore, the results of this study, in combination with current clinical practice, motivate clinical evaluation of endorectal MRE in patients.

Imaging the elastic properties of tissue: the 20 year perspective

After 20 years of innovation in techniques that specifically image the biomechanical properties of tissue, the evolution of elastographic imaging can be viewed from its infancy, through a proliferation of approaches to the problem to incorporation on research and then clinical imaging platforms. Ultimately this activity has culminated in clinical trials and improved care for patients. This remarkable progression represents a leading example of translational research that begins with fundamentals of science and engineering and progresses to needed improvements in diagnostic and monitoring capabilities applied to major categories of disease, surgery and interventional procedures. This review summarizes the fundamental principles, the timeline of developments in major categories of elastographic imaging, and concludes with recent results from clinical trials and forward-looking issues.

Transurethral prostate magnetic resonance elastography: prospective imaging requirements

Tissue stiffness is known to undergo alterations when affected by prostate cancer and may serve as an indicator of the disease. Stiffness measurements can be made with magnetic resonance elastography performed using a transurethral actuator to generate shear waves in the prostate gland. The goal of this study was to help determine the imaging requirements of transurethral magnetic resonance elastography and to evaluate whether the spatial and stiffness resolution of this technique overlapped with the requirements for prostate cancer detection. Through the use of prostate-mimicking gelatin phantoms, frequencies of at least 400 Hz were necessary to obtain accurate stiffness measurements of 10 mm diameter inclusions, but the detection of inclusions with diameters as small as 4.75 mm was possible at 200 Hz. The shear wave attenuation coefficient was measured in vivo in the canine prostate gland, and was used to predict the detectable penetration depth of shear waves in prostate tissue. These results suggested that frequencies below 200 Hz could propagate to the prostate boundary with a signal to noise ratio (SNR) of 60 and an actuator capable of producing 60 μm displacements. These requirements are achievable with current imaging and actuator technologies, and motivate further investigation of magnetic resonance elastography for the targeting of prostate cancer.

A method for characterization of tissue elastic properties combining ultrasonic computed tomography with elastography

OBJECTIVE

The correlation between various diseases and the change in the local mechanical properties of soft tissues has been long known. Over the past 20 years, there have been increasing research efforts to characterize mechanical properties of biological tissues using ultrasonic elastography. However, most of these works were based on characterization of only 1 type of waves (longitudinal or shear). The goal of this work was to devise a comprehensive ultrasound-based imaging method capable of measuring elastic parameters by combining both backscattered elastography and through-transmitted ultrasonic computed tomography.

METHODS

Our suggested technique provides measurements of both longitudinal and shear wave velocities. This enables the noninvasive computation of several tissue elasticity parameters such as Young's and shear moduli, Poisson's ratio, and, more importantly, the bulk modulus, the determination of which requires both wave velocities. Four different phantom types were examined: agar-gelatin-based phantoms and porcine fat tissue, turkey breast tissue, and bovine liver tissue in vitro specimens. The values of Young's modulus, the shear modulus, and Poisson's ratio were estimated and were consistent with values published in the literature.

RESULTS

The average bulk modulus values of the phantoms +/- SD were 2.83 +/- 0.001, 2.25 +/- 0.01, 2.48 +/- 0.01, and 2.53 +/- 0.02 GPa, respectively. A statistically significant difference (P < .001) in the values of the bulk modulus of the different phantoms was found.

CONCLUSIONS

The bulk modulus is suitable for differentiation between different tissue types. The obtained results show the feasibility of using a comprehensive ultrasonic imaging technique for noninvasive quantitative tissue characterization.

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.

Optical tracking of acoustic radiation force impulse-induced dynamics in a tissue-mimicking phantom

Optical tracking was utilized to investigate the acoustic radiation force impulse (ARFI)-induced response, generated by a 5-MHz piston transducer, in a translucent tissue-mimicking phantom. Suspended 10-microm microspheres were tracked axially and laterally at multiple locations throughout the field of view of an optical microscope with 0.5-microm displacement resolution, in both dimensions, and at frame rates of up to 36 kHz. Induced dynamics were successfully captured before, during, and after the ARFI excitation at depths of up to 4.8 mm from the phantom's proximal boundary. Results are presented for tracked axial and lateral displacements resulting from on-axis and off-axis (i.e., shear wave) acquisitions; these results are compared to matched finite element method modeling and independent ultrasonically based empirical results and yielded reasonable agreement in most cases. A shear wave reflection, generated by the proximal boundary, consistently produced an artifact in tracked displacement data later in time (i.e., after the initial ARFI-induced displacement peak). This tracking method provides high-frame-rate, two-dimensional tracking data and thus could prove useful in the investigation of complex ARFI-induced dynamics in controlled experimental settings.

Shear modulus decomposition algorithm in magnetic resonance elastography

Magnetic resonance elastography (MRE) is an imaging modality capable of visualizing the elastic properties of an object using magnetic resonance imaging (MRI) measurements of transverse acoustic strain waves induced in the object by a harmonically oscillating mechanical vibration. Various algorithms have been designed to determine the mechanical properties of the object under the assumptions of linear elasticity, isotropic and local homogeneity. One of the challenging problems in MRE is to reduce the noise effects and to maintain contrast in the reconstructed shear modulus images. In this paper, we propose a new algorithm designed to reduce the degree of noise amplification in the reconstructed shear modulus images without the assumption of local homogeneity. Investigating the relation between the measured displacement data and the stress wave vector, the proposed algorithm uses an iterative reconstruction formula based on a decomposition of the stress wave vector. Numerical simulation experiments and real experiments with agarose gel phantoms and human liver data demonstrate that the proposed algorithm is more robust to noise compared to standard inversion algorithms and stably determines the shear modulus.

In vivo cardiac, acoustic-radiation-force-driven, shear wave velocimetry

Shear wave elasticity imaging (SWEI) was employed to track acoustic radiation force impulse (ARFI)-induced shear waves in the mid-myocardium of the left ventricular free wall (LVFW) of a beating canine heart. Shear waves were generated and tracked with a linear ultrasound transducer that was placed directly on the exposed epicardium. Acquisition was ECG-gated and coincided with the mid-diastolic portion of the cardiac cycle. Axial displacement profiles consistent with shear wave propagation were clearly evident in all SWEI acquisitions (i.e., those including an ARFI excitation); displacement data from control cases (i.e., sequences lacking an ARFI excitation) offered no evidence of shear wave propagation and yielded a peak absolute mean displacement below 0.31 microm after motion filtering. Shear wave velocity estimates ranged from 0.82 to 2.65 m/s and were stable across multiple heartbeats for the same interrogation region, with coefficients of variation less than 19% for all matched acquisitions. Variations in velocity estimates suggest a spatial dependence of shear wave velocity through the mid-myocardium of the LVFW, with velocity estimates changing, in limited cases, through depth and lateral position.

Magnetic resonance elastography hardware design: A survey

Magnetic resonance elastography (MRE) is an emerging technique capable of measuring the shear modulus of tissue. A suspected tumour can be identified by comparing its properties with those of tissues surrounding it; this can be achieved even in deep-lying areas as long as mechanical excitation is possible. This would allow non-invasive methods for cancer-related diagnosis in areas not accessible with conventional palpation. An actuating mechanism is required to generate the necessary tissue displacements directly on the patient in the scanner and three different approaches, in terms of actuator action and position, exist to derive stiffness measurements. However, the magnetic resonance (MR) environment places considerable constraints on the design of such devices, such as the possibility of mutual interference between electrical components, the scanner field, and radio frequency pulses, and the physical space restrictions of the scanner bore. This paper presents a review of the current solutions that have been developed for MRE devices giving particular consideration to the design criteria including the required vibration frequency and amplitude in different applications, the issue of MR compatibility, actuation principles, design complexity, and scanner synchronization issues. The future challenges in this field are also described.

Image quality, tissue heating, and frame rate trade-offs in acoustic radiation force impulse imaging

The real-time application of acoustic radiation force impulse (ARFI) imaging requires both short acquisition times for a single ARFI image and repeated acquisition of these frames. Due to the high energy of pulses required to generate appreciable radiation force, however, repeated acquisitions could result in substantial transducer face and tissue heating. We describe and evaluate several novel beam sequencing schemes which, along with parallel-receive acquisition, are designed to reduce acquisition time and heating. These techniques reduce the total number of radiation force impulses needed to generate an image and minimize the time between successive impulses. We present qualitative and quantitative analyses of the trade-offs in image quality resulting from the acquisition schemes. Results indicate that these techniques yield a significant improvement in frame rate with only moderate decreases in image quality. Tissue and transducer face heating resulting from these schemes is assessed through finite element method modeling and thermocouple measurements. Results indicate that heating issues can be mitigated by employing ARFI acquisition sequences that utilize the highest track-to-excitation ratio possible.

Real-time sonoelastography of hepatic thermal lesions in a swine model

Sonoelastography has been developed as an ultrasound-based elasticity imaging technique. In this technique, external vibration is induced into the target tissue. In general, tissue stiffness is inversely proportional to the amplitude of tissue vibration. Imaging tissue vibration will provide the elasticity distribution in the target region. This study investigated the feasibility of using real-time sonoelastography to detect and estimate the volume of thermal lesions in porcine livers in vivo. A total of 32 thermal lesions with volumes ranging from 0.2 to 5.3 cm3 were created using radiofrequency ablation (RFA) or high-intensity focused ultrasound (HIFU) technique. Lesions were imaged using sonoelastography and coregistered B-mode ultrasound. Volumes were reconstructed from a sequence of two-dimensional scans. The comparison of sonoelastographic measurements and pathology findings showed good correlation with respect to the area of the lesions (r2 = 0.8823 for RFA lesions, r2 = 0.9543 for HIFU lesions). In addition, good correspondence was found between three-dimensional sonoelastography and gross pathology (3.6% underestimate), demonstrating the feasibility of sonoelastography for volume estimation of thermal lesions. These results support that sonoelastography outperforms conventional B-mode ultrasound and could potentially be used for assessment of thermal therapies.

Magnetic resonance elastography of the brain

The purpose of this study was to obtain normative data using magnetic resonance elastography (MRE) (a) to obtain estimates of the shear modulus of human cerebral tissue in vivo and (b) to assess a possible age dependence of the shear modulus of cerebral tissue in healthy adult volunteers. MR elastography studies were performed on tissue-simulating gelatin phantoms and 25 healthy adult volunteers. The data were analyzed using spatiotemporal filters and a local frequency estimating algorithm. Statistical analysis was performed using a paired t-test. The mean shear stiffness of cerebral white matter was 13.6 kPa (95% CI 12.3 to 14.8 kPa); while that of gray matter was lower at 5.22 kPa (95% CI 4.76 to 5.66 kPa). The difference was statistically significant (p<0.0001).

Magnetic resonance elastography to observe deep areas: comparison of external vibration systems

MRE methods deform the sample using an external vibration system. We have been using a transverse driver, which generates shear waves at the object surface. One of the problems is that shear waves rapidly attenuate at the surface of tissue and do not propagate into the body. In this study, we compared the shear waves generated by transverse and longitudinal drivers. The longitudinal driver was found to induce shear waves deep inside a porcine liver phantom. These results suggest that the longitudinal driver will allow measurement of the shear modulus deep inside the body.

Congruence of imaging estimators and mechanical measurements of viscoelastic properties of soft tissues

Biomechanical properties of soft tissues are important for a wide range of medical applications, such as surgical simulation and planning and detection of lesions by elasticity imaging modalities. Currently, the data in the literature is limited and conflicting. Furthermore, to assess the biomechanical properties of living tissue in vivo, reliable imaging-based estimators must be developed and verified. For these reasons, we developed and compared two independent quantitative methods--crawling wave estimator (CRE) and mechanical measurement (MM) for soft tissue characterization. The CRE method images shear wave interference patterns from which the shear wave velocity can be determined and hence the Young's modulus can be obtained. The MM method provides the complex Young's modulus of the soft tissue from which both elastic and viscous behavior can be extracted. This article presents the systematic comparison between these two techniques on the measurement of gelatin phantom, veal liver, thermal-treated veal liver and human prostate. It was observed that the Young's moduli of liver and prostate tissues slightly increase with frequency. The experimental results of the two methods are highly congruent, suggesting CRE and MM methods can be reliably used to investigate viscoelastic properties of other soft tissues, with CRE having the advantages of operating in nearly real time and in situ.

Development of a magnetic resonance elastic microscope system

Magnetic resonance elastography (MRE) is a method that can visualize the propagating acoustic strain waves in elastic materials under mechanical excitation. The local quantitative values of shear modulus are derived from the acquired data. The MRE could improve early detection of pathology because it is known that malignant tumors tend to be much harder than normal tissues and most benign tumors. In order to observe tissues such as the early stage of tumors in mouse embryo, spatial resolution of the MRE image is not enough because of hardware limitation of the conventional MRI system. We developed the elasticity measurement system using the MR microscope which spatial resolution is about 200 mm. The external vibration system and MR pulse sequences are developed for an MR elastic microscope. Experiments were performed with homogeneous and heterogeneous agarose gel phantoms. These results suggest the developed MR elastic microscope system makes it possible to generate image that depict distribution of stiffness.

Performance analysis of steady-state harmonic elastography

Shear modulus estimation can be confounded by the ill-posed nature of the inverse elasticity problem. In this paper, we report the results of experiments conducted on simulated and gelatin phantoms to investigate the effect of various parameters (i.e., regularization, spatial filtering and the subzone generation process) associated with shear modulus reconstruction on the statistical accuracy (mean squared error), and image quality (i.e., contrast and spatial resolution) of the recovered mechanical properties. The results indicate several interesting observations. Firstly, the intrinsic spatial resolution of magnetic resonance elastography (MRE) is dependent on both regularization and spatial filtering. Secondly, the elastographic contrast-to-noise ratio (CNR(e)) increases with increasing regularization and spatial filtering, but it was not affected by the zoning parameters (i.e., the subzones and the extent of the overlap). Thirdly, the statistical accuracy (MSE) of the recovered property improved with increasing regularization, and spatial filtering weight, but the size of the subdomains and their overlap had no significant effect.

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 ( approximately 200 kPa) and stiff ( approximately 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.

Comparison of Motion Encoding Waveforms for Magnetic Resonance Elastography at 3T

Magnetic resonance (MR) elastography has the ability to combine the inherent soft-tissue contrast of traditional MR imaging with quantitative maps of tissue stiffness. Mechanical properties of tissues can vary greatly with disease and degeneration, and can illuminate structure-function understanding of tissues. Dynamic MR elastography is a phase contrast-based method for imaging the transmission of strain waves in an object. The present study evaluates the theoretical and empirical results of using trapezoidal and sinusoidal motion encoding gradients (MEGs) for the purposes of elastography at 3T. The study evaluated the phase-to-noise ratio for the methods, and found excellent agreement between the theoretical predictions and experimental results. The sinusoidal MEGs were predicted to have a relative PNR decrease of 21.5%, which compares very well with the experimental PNR decrease of 19.1% (95% CI = 17.1% to 21.0%). These results show the trapezoidal MEGs provide more sensitivity to strain wave transmission for the purposes of MR elastography.

Three-dimensional analysis of shear wave propagation observed by in vivo magnetic resonance elastography of the brain

Dynamic magnetic resonance elastography (MRE) is a non-invasive method for the quantitative determination of the mechanical properties of soft tissues in vivo. In MRE, shear waves are generated in the tissue and visualized using phase-sensitive MR imaging methods. The resulting two-dimensional (2-D) wave images can reveal in-plane elastic properties when possible geometrical biases of the wave patterns are taken into account. In this study, 3-D MRE experiments of in vivo human brain are analyzed to gain knowledge about the direction of wave propagation and to deduce in-plane elastic properties. The direction of wave propagation was determined using a new algorithm which identifies minimal wave velocities along rays from the surface into the brain. It was possible to quantify biases of the elastic parameters due to projections onto coronal, sagittal and transversal image planes in 2-D MRE. It was found that the in-plane shear modulus is increasingly overestimated when the image slice is displaced from narrow slabs of 2-5cm through the center of the brain. The mean shear modulus of the brain was deduced from 4-D wave data with about 3.5kPa. Using the proposed slice positions in 2-D MRE, this shear modulus can be reproduced with an acceptable error within a fraction of the full 3-D examination time.

Ultrasonic interrogation of tissue vibrations in arterial and organ injuries: preliminary in vivo results

Soft tissues surrounding vascular injuries are known to vibrate at audible and palpable frequencies, producing bruits and thrills. We report the results of a feasibility study where Doppler ultrasound (US) was used to quantitatively estimate the tissue vibrations after induced trauma in an animal model. A software-programmable US system was used to acquire quadrature-demodulated ensembles of received US echoes bypassing clutter filtering and other conventional Doppler processing stages. The waveforms of tissue velocity surrounding the injury site were then estimated from the clutter data using autocorrelation and analyzed to determine vibration characteristics. Six New Zealand white rabbits and two juvenile pigs were used for the study. The femoral artery of the anesthetized animal was punctured with an 18-gauge needle to model a peripheral arterial trauma, and the liver was surgically exposed and incised to model organ trauma. Two types of oscillatory tissue motion were observed: "vibrations" with high frequency (>50 Hz) and low peak-peak amplitude (<1 microm) and "flutter" with low frequency (<50 Hz) and high peak-peak amplitude (>1 microm). Active bleeding in femoral artery punctures produced tissue vibrations at the frequency of 323 +/- 214 Hz (mean +/- standard deviation, pooled for both rabbits and pigs) and the amplitude of 0.24 +/- 0.15 microm. Active bleeding in liver incisions produced vibrations at the frequency of 120 +/- 47 Hz and the amplitude of 0.33 +/- 0.25 microm. Flutter was observed in punctured arteries at the frequency of 28 +/- 13 Hz the amplitude of 2.92 +/- 1.75 microm, and in incised livers at the frequency of 26 +/- 6 Hz and the amplitude of 1.53 +/- 0.76 microm. In a punctured artery, the vibration frequency and phase of tissue surrounding the artery were highly correlated between neighboring locations in tissue (correlation coefficient = 0.98), and with the flow oscillations in the lumen (correlation coefficient = 0.96). This preliminary study indicates that tissue vibrations could provide additional physiologic information for detecting, localizing and monitoring internal bleeding using US.

Magnetic resonance elastography of the breast: correlation of signal intensity data with viscoelastic properties

OBJECTIVES

We sought to investigate the potential value of magnetic resonance (MR) elastography to improve the differentiation between benign and malignant tumors.

MATERIAL AND METHODS

Measurements of 5 patients with 6 malignant lesions, 11 patients with benign lesions, and 4 patients with no lesions at all were performed at 1.5 Tesla. After breast MR imaging, MR elastography was performed as a targeted measurement. Low-frequency mechanical waves (65 Hz) were transmitted into the breast tissue using an oscillator and were displayed by means of a MR sequence within the phase of the MR image. After reconstruction, the viscoelastic information was correlated with the signal intensity and morphology data.

RESULTS

All examinations were technically successful realized in approximately 25 minutes. Malignant tumors documented higher values of elasticity than benign corresponding with signal intensity and morphologic data.

CONCLUSION

A good separation exists between benign and malignant lesions in elasticity, corresponding with specific signal intensity and morphologic data. Further clinical studies with a larger number of patients are needed for extended validation.

Viscoelastic shear properties of in vivo breast lesions measured by MR elastography

Elastography is a technique to assess the viscoelastic properties of tissue by measuring an acoustic wave propagating though the object. Here, the technique is applied in the course of standard MR mammography to 15 patients with different pathologies (six breast cancer cases, six fibroadenoma cases and three mastopathy cases). Low-frequency mechanical waves are coupled longitudinally into the tissue in order to obtain sufficient wave amplitude throughout the entire breast. This leads to the presence of a substantial fraction of compressional waves, which contribute to the total displacement field. It is shown theoretically that the correct evaluation of these contributions from the compressional wave is rather difficult due to the almost incompressible nature of tissue. To overcome this problem, it is proposed to apply the curl-operator to the measured displacement field in order to completely remove contributions from the compressional wave. Results from simulations and a breast phantom demonstrate the feasibility of the technique. The in vivo results show a good separation between breast cancer and benign fibroadenoma utilizing the shear modulus. Breast cancer appears on average 2.2 (P<.001) times stiffer. All breast cancer cases showed a good delineation to the surrounding breast tissue with an average elevation of a factor of 3.3 (P< 1.4 x 10(-6)). The results as obtained for the shear viscosity do not indicate to be useful for separating benign from malignant lesions.

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.

Imaging anisotropic and viscous properties of breast tissue by magnetic resonance-elastography

MR-elastography is a new technique for assessing the viscoelastic properties of tissue. One current focus of elastography is the provision of new physical parameters for improving the specificity in breast cancer diagnosis. This analysis describes a technique to extend the reconstruction to anisotropic elastic properties in terms of a so-called transversely isotropic model. Viscosity is treated as being isotropic. The particular model chosen for the anisotropy is appealing because it is capable of describing elastic shear anisotropy of parallel fibers. The dependence of the reconstruction on the particular choice of Poisson's ratio is eliminated by extracting the compressional displacement contribution using the Helmholtz-Hodge decomposition. Results are presented for simulations, a polyvinyl alcohol breast phantom, excised beef muscle, and measurements in two patients with breast lesions (invasive ductal carcinoma and fibroadenoma). The results show enhanced anisotropic and viscous properties inside the lesions and an indication for preferred fiber orientation.

Imaging articular cartilage under compression?cartilage elastography

We constructed a device to compress small samples of articular cartilage while the samples were imaged in a 1.5 T imager. With the use of a piezoelectric piston, the device compressed 1-cm-diameter cylindrical samples of articular cartilage (200 microm) at a rate of 2 Hz. Simultaneously, we imaged the samples with a displacement-sensitive stimulated-echo acquisition mode (STEAM) sequence. We validated the technique using tissue that mimicked silicone samples. We compared the results from the same cartilage samples before and after they were degraded by digestion in trypsin. The extent of degradation was visualized from T(1)-weighted images of the samples after they were soaked in 0.5 mmolar of GdDTPA. The resulting elastographic images show compression and differential strain in directions both parallel and perpendicular to the surface of the cartilage. The static elastographic images that depict compression made before digestion and after 5 and 15 hr of trypsin digestion show that the elastic modulus of the samples decreased with a spatial variation consistent with the enzymatic digestion as revealed by the T(1) images. We believe this technique will be useful in studies of the mechanical properties of articular cartilage and other tissues, and may in the future be extended to the clinical setting.

Material Properties Estimation of Layered Soft Tissue Based on MR Observation and Iterative FE Simulation

In order to calculate deformation of soft tissue under arbitrary loading conditions, we have to take both non-linear material characteristics and subcutaneous structures into considerations. The estimation method of material properties presented in this paper accounts for these issues. It employs a compression test inside MRI in order to visualize deformation of hypodermic layered structure of living tissue, and an FE model of the compressed tissue in which non-linear material model is assigned. The FE analysis is iterated with updated material constant until the difference between the displacement field observed from MR images and calculated by FEM is minimized. The presented method has been applied to a 3-layered silicon rubber phantom. The results show the excellent performance of our method. The accuracy of the estimation is better than 15%, and the reproducibility of the deformation is better than 0.4 mm even for an FE analysis with different boundary condition.

Sonoelastographic imaging of interference patterns for estimation of the shear velocity of homogeneous biomaterials

The shear wave velocity is one of a few important parameters that characterize the mechanical properties of bio-materials. In this paper, two noninvasive methods are proposed to measure the shear velocity by inspecting the shear wave interference patterns. In one method, two shear wave sources are placed on the opposite two sides of a sample, driven by the identical sinusoidal signals. The shear waves from the two sources interact to create interference patterns, which are visualized by the vibration sonoelastography technique. The spacing between the pattern bands equals half of the shear wavelength. The shear velocity can be obtained by taking the product of the wavelength and the frequency. An alternative method is to drive the two vibration sources at slightly different frequencies. In this case, the interference patterns no longer remain stationary. It is proved that the apparent velocity of the moving patterns is proportional to the shear velocity in the medium. Since the apparent velocity of the patterns can be measured by analysing the video sequence, the shear velocity can be obtained thereafter. These approaches are validated by a conventional shear wave time-of-flight approach, and they are accurate within 4% on various homogeneous tissue-mimicking phantoms.

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.

Monitoring thermally-induced lesions with supersonic shear imaging

Thermally-induced lesions are generally stiffer than surrounding tissues. We propose here to use the supersonic shear imaging technique (SSI) for monitoring high-intensity focused ultrasound (HIFU) therapy. This new elasticity imaging technique is based on remotely creating shear sources using an acoustic radiation force at different locations in the medium. In these experiments, an HIFU probe is used to generate lesions in fresh tissue samples. A diagnostic transducer, controlled by our ultrafast scanner, is located in the therapeutic probe focal plane. It is used for both generating the shear waves and imaging the resulting propagation at frame rates reaching 5,000 images/s. Movies of the shear wave propagation can be computed off-line. The therapeutic and imaging sequences are interleaved and a set of wave propagation movies is performed during the heating process. From each movie, elasticity estimations have been performed using an inversion algorithm. It demonstrates the feasibility of detecting and quantifying the hardness of HIFU-induced lesions using SSI.

Evaluation of renal parenchymal disease in a rat model with magnetic resonance elastography

Alterations in the mechanical properties or "hardness" of tissues allow physicians to detect disease by palpation. Recently, attempts have been made to quantitate and image these tissue properties with the use of magnetic resonance elastography (MRE). This technique has been validated in ex vivo specimens, including kidney, breast, and prostate. In this study, in vivo MRE imaging of rat renal cortex is demonstrated and validated with a disease model that will facilitate further studies. Normal rats and rats with nephrocalcinosis induced with either 2 or 4 weeks of ethylene glycol exposure were studied with MRE. Histology in the diseased rats documented the presence of nephrocalcinosis. MRE measurements and images of shear stiffness were highly reproducible in individual rats. The shear stiffness of the renal cortex in normal rats was 3.87 kPa (95% CI 2.84-4.90 kPa). The shear stiffness increased to 5.02 kPa (95% CI 3.34-6.70 kPa) after 2 weeks of exposure, and to 6.49 kPa (95% CI 4.84-8.14 kPa) after 4 weeks of exposure (P = 0.0302, alpha < 0.05). MRE is capable of detecting alterations in the tissue mechanical properties of kidneys in vivo. It is a promising noninvasive technique that might have pathologic and prognostic significance.

Measurement of in vivo local shear modulus using MR elastography multiple-phase patchwork offsets

Magnetic resonance elastography (MRE) is a method that can visualize the propagating and standing shear waves in an object being measured. The quantitative value of a shear modulus can be calculated by estimating the local shear wavelength. Low-frequency mechanical motion must be used for soft, tissue-like objects because a propagating shear wave rapidly attenuates at a higher frequency. Moreover, a propagating shear wave is distorted by reflections from the boundaries of objects. However, the distortions are minimal around the wave front of the propagating shear wave. Therefore, we can avoid the effect of reflection on a region of interest (ROI) by adjusting the duration of mechanical vibrations. Thus, the ROI is often shorter than the propagating shear wavelength. In the MRE sequence, a motion-sensitizing gradient (MSG) is synchronized with mechanical cyclic motion. MRE images with multiple initial phase offsets can be generated with increasing delays between the MSG and mechanical vibrations. This paper proposes a method for measuring the local shear wavelength using MRE multiple initial phase patchwork offsets that can be used when the size of the object being measured is shorter than the local wavelength. To confirm the reliability of the proposed method, computer simulations, a simulated tissue study and in vitro and in vivo studies were performed.