In Vivo Mapping of Brain Elasticity in Small Animals Using Shear Wave Imaging (original) (raw)
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2010 IEEE International Symposium on Biomedical Imaging: From Nano to Macro, 2010
Shear Wave Imaging (SWI) is an ultrasound based technique for elasticity imaging that has been successfully tested on several organs in the framework of cancer diagnosis. In this work, the potential of this technique to map brain elasticity in vivo on trepanned small animals is investigated. From a SWI scan of the rat brain, 3D elasticity maps are reconstructed reaching a spatial resolution of 800 m. The dynamic modulus of the brain tissues exhibits values in the 1 to 16 kPa range and is quantified for different anatomical regions. The propagation of shear waves is found to be anisotropic, which could be a consequence of fiber orientation. Finally, the interest of brain elasticity mapping for the monitoring of brain ischemia is investigated on a rat model. Focal cerebral ischemia is shown to induce a dramatic decrease of elasticity in the lesion.
We use supersonic shear wave imaging (SSI) technique to measure not only the linear but also the nonlinear elastic properties of brain matter. Here, we tested six porcine brains ex vivo and measured the velocities of the plane shear waves induced by acoustic radiation force at different states of pre-deformation when the ultrasonic probe is pushed into the soft tissue. We relied on an inverse method based on the theory governing the propagation of small-amplitude acoustic waves in deformed solids to interpret the experimental data. We found that, depending on the subjects, the resulting initial shear modulus µ 0 varies from 1.8 to 3.2 kPa, the stiffening parameter b of the hyperelastic Demiray–Fung model from 0.13 to 0.73, and the third-(A) and fourth-order (D) constants of weakly nonlinear elasticity from −1.3 to −20.6 kPa and from 3.1 to 8.7 kPa, respectively. Paired t test performed on the experimental results of the left and right Yi Jiang and Guoyang Li have contributed equally to this study. lobes of the brain shows no significant difference. These values are in line with those reported in the literature on brain tis-sue, indicating that the SSI method, combined to the inverse analysis, is an efficient and powerful tool for the mechanical characterization of brain tissue, which is of great importance for computer simulation of traumatic brain injury and virtual neurosurgery.
Ex Vivo Evaluation of Mouse Brain Elasticity Using High-Frequency Ultrasound Elastography
IEEE Transactions on Biomedical Engineering, 2019
Most neurodegenerative diseases are highly linked with aging. The mechanical properties of the brain should be determined for predicting and diagnosing age-related brain diseases. A preclinical animal study is crucial for neurological disease research. However, estimation of the elasticity properties of different regions of mouse brains remains difficult because of the size of the brain. In this study, highfrequency ultrasound elastography (HFUSE) based on shear wave imaging was proposed for mapping the stiffness of the mouse brain at different ages ex vivo. Methods: For HFUSE, a 40-MHz ultrasound array transducer with an ultrafast ultrasound imaging system were used in the present study. The accuracy and resolution during HFUSE were determined through a mechanical testing system and by conducting phantom experiments. Results: In the experiments, the error in the elastic modulus measurement was approximately 10% on average, and the axial resolution was 248 µm. Animal testing was conducted using mice that were 4 (young aged) and 11 (middle aged) months old. The elasticity distributions of the cortex and hippocampus in the mouse brains were obtained through HFUSE. Conclusion: The average shear moduli of the cortex and hippocampus were 3.84 and 2.33 kPa for the 4-month-old mice and 3.77 and 1.94 kPa for the 11-month-old mice, respectively. No statistical difference was observed in the cortex stiffness of mice of different ages. However, the hippocampus significantly softened with aging.
Journal of Biomechanical Engineering, 2008
In this study, the magnetic resonance (MR) elastography technique was used to estimate the dynamic shear modulus of mouse brain tissue in vivo. The technique allows visualization and measurement of mechanical shear waves excited by lateral vibration of the skull. Quantitative measurements of displacement in three dimensions during vibration at 1200Hz were obtained by applying oscillatory magnetic field gradients at the same frequency during a MR imaging sequence. Contrast in the resulting phase images of the mouse brain is proportional to displacement. To obtain estimates of shear modulus, measured displacement fields were fitted to the shear wave equation. Validation of the procedure was performed on gel characterized by independent rheometry tests and on data from finite element simulations. Brain tissue is, in reality, viscoelastic and nonlinear. The current estimates of dynamic shear modulus are strictly relevant only to small oscillations at a specific frequency, but these esti...
Frequency-dependent viscoelastic parameters of mouse brain tissue estimated by MR elastography
Physics in Medicine and Biology, 2011
Viscoelastic properties of mouse brain tissue were estimated non-invasively, in vivo, using magnetic resonance elastography (MRE) at 4.7 T to measure the dispersive properties of induced shear waves. Key features of this study include (i) the development and application of a novel MR-compatible actuation system which transmits vibratory motion into the brain through an incisor bar, and (ii) the investigation of the mechanical properties of brain tissue over a 1200 Hz bandwidth from 600-1800 Hz. Displacement fields due to propagating shear waves were measured during continuous, harmonic excitation of the skull. This protocol enabled characterization of the true steady-state patterns of shear wave propagation. Analysis of displacement fields obtained at different frequencies indicates that the viscoelastic properties of mouse brain tissue depend strongly on frequency. The average storage modulus (G ) increased from approximately 1.6 to 8 kPa over this range; average loss modulus (G ) increased from approximately 1 to 3 kPa. Both moduli were well approximated by a power-law relationship over this frequency range. MRE may be a valuable addition to studies of disease in murine models, and to pre-clinical evaluations of therapies. Quantitative measurements of the viscoelastic parameters of brain tissue at high frequencies are also valuable for modeling and simulation of traumatic brain injury.
Quantitative Evaluation of Neonatal Brain Elasticity Using Shear Wave Elastography
Journal of Ultrasound in Medicine, 2020
Objectives-To demonstrate the feasibility of 2-dimensional brain ultrasound shear wave elastography (SWE) and to define the average elasticity values of the gray and white matter in term neonates. Methods-This work was a prospective observational single-center study including 55 healthy term neonates consecutively recruited in the maternity ward between the second and third postnatal days. All were successfully evaluated with a cerebral SWE examination performed with a multifrequency 4-9-MHz transducer. Bilateral sagittal planes of the thalamus and corona radiata were used to measure stiffness using a quantitative SWE method. Several elastograms with 5 to 15 nonoverlapping areas were obtained from the 2 different anatomic locations. The 5 most central measurements were averaged as representative values. Results-The 55 neonates ranged from 37 to 40 weeks' gestation. The estimated mean velocity values of the thalamus (1.17 m/s; 95% confidence interval, 1.13, 1.22 m/s) and corona radiata (1.60 m/s; 95% confidence interval, 1.57, 1.64 m/ s) were statistically different (P < .001). There was no significant influence of laterality, gestational age, cephalic perimeter, sex, length, or type of delivery on the stiffness measurements. Conclusions-Brain ultrasound SWE is feasible and allows measurements of neonatal brain elasticity. The elasticity of the thalamus and corona radiata at the frontal white matter in healthy term neonates is different. The knowledge of normal SWE ranges in term neonates allows comparative studies under pathologic conditions. Key Words-acoustic radiation force impulse imaging; brain; elastography; neonate; ultrasound E lasticity is defined as the ability of an object or material to resume its normal shape after being stretched or compressed. Soft tissue elasticity depends both on the molecular composition and structural organization of tissue components. 1 Under pathologic conditions, both compositional and organizational properties undergo substantial changes that might alter the tissue elastic properties. Tissue elasticity changes allow depiction of the existence of tissue abnormalities, grading a lesion and defining the extension of the damage. Ultrasound elastographic techniques are able to characterize biomechanical properties. 2,3 Many manufacturers are implementing quantitative 2-dimensional shear wave elastography (SWE) on conventional and commercially available ultrasound equipment 4,5 using
Journal of Neurotrauma, 2013
Traumatic brain injury (TBI) presents a variety of causes and symptoms, thus making the development of reliable diagnostic methods and therapeutic treatments challenging. Magnetic resonance elastography (MRE) is a technique that allows for a noninvasive assessment of the mechanical properties of soft biological tissue, such as tissue stiffness, storage modulus, and loss modulus. Importantly, by quantifying the changes in the stiffness of tissue that is often associated with disease, MRE is able to detect tissue pathologies at early stages. Recent improvements in instrumentation have allowed for the investigation of small samples with microscopic resolution (lMRE). We hypothesize that lMRE can sensitively detect variations in micromechanical properties in the brain caused by the compressive and shearing forces sustained during TBI. To test this hypothesis, we randomized 13 C57BL mice to receive a controlled cortical impact at a 0.5 mm or 0.75 mm depth, with both sham and naïve mice as controls. Our objective was to propagate mechanical shear waves throughout the brain for in vivo TBI lMRE imaging. The mechanical properties of the injured brain tissue were determined at days 0, 1, 7, and 28 post-injury. For both groups, we observed a significant drop in the stiffness of the impacted region immediately following the injury; the 0.75 mm animals experienced increased tissue softness that lasted longer than that for the 0.5 mm group. Although the shear stiffness, storage modulus, and loss modulus parameters all followed the same trend, the tissue stiffness yielded the most statistically significant results. Overall, this article introduces a transformative technique for mechanically mapping the brain and detecting brain diseases and injury.
Viscoelasticity Imaging of Biological Tissues and Single Cells Using Shear Wave Propagation
Frontiers in Physics, 2021
Changes in biomechanical properties of biological soft tissues are often associated with physiological dysfunctions. Since biological soft tissues are hydrated, viscoelasticity is likely suitable to represent its solid-like behavior using elasticity and fluid-like behavior using viscosity. Shear wave elastography is a non-invasive imaging technology invented for clinical applications that has shown promise to characterize various tissue viscoelasticity. It is based on measuring and analyzing velocities and attenuations of propagated shear waves. In this review, principles and technical developments of shear wave elastography for viscoelasticity characterization from organ to cellular levels are presented, and different imaging modalities used to track shear wave propagation are described. At a macroscopic scale, techniques for inducing shear waves using an external mechanical vibration, an acoustic radiation pressure or a Lorentz force are reviewed along with imaging approaches proposed to track shear wave propagation, namely ultrasound, magnetic resonance, optical, and photoacoustic means. Then, approaches for theoretical modeling and tracking of shear waves are detailed. Following it, some examples of applications to characterize the viscoelasticity of various organs are given. At a microscopic scale, a novel cellular shear wave elastography method using an external vibration and optical microscopy is illustrated. Finally, current limitations and future directions in shear wave elastography are presented.
Shear wave elasticity imaging: a new ultrasonic technology of medical diagnostics
Ultrasound in Medicine & Biology, 1998
Shear wave elasticity imaging (SWEI) is a new approach to imaging and characterizing tissue structures based on the use of shear acoustic waves remotely induced by the radiation force of a focused ultrasonic beam. SWEI provides the physician with a virtual "finger" to probe the elasticity of the internal regions of the body. In SWEI, compared to other approaches in elasticity imaging, the induced strain in the tissue can be highly localized, because the remotely induced shear waves are attenuated fully within a very limited area of tissue in the vicinity of the focal point of a focused ultrasound beam. SWEI may add a new quality to conventional ultrasonic imaging or magnetic resonance imaging. Adding shear elasticity data ("palpation information") by superimposing color-coded elasticity data over ultrasonic or magnetic resonance images may enable better differentiation of tissues and further enhance diagnosis. This article presents a physical and mathematical basis of SWEI with some experimental results of pilot studies proving feasibility of this new ultrasonic technology. A theoretical model of shear oscillations in soft biological tissue remotely induced by the radiation force of focused ultrasound is described. Experimental studies based on optical and magnetic resonance imaging detection of these shear waves are presented. Recorded spatial and temporal profiles of propagating shear waves fully confirm the results of mathematical modeling. Finally, the safety of the SWEI method is discussed, and it is shown that typical ultrasonic exposure of SWEI is significantly below the threshold of damaging effects of focused ultrasound. © 1998 World Federation for Ultrasound in Medicine & Biology.
Journal of Biomechanics, 2013
Characterization of the dynamic mechanical behavior of brain tissue is essential for understanding and simulating the mechanisms of traumatic brain injury (TBI). Changes in mechanical properties may also reflect changes in the brain due to aging or disease. In this study, we used magnetic resonance elastography (MRE) to measure the viscoelastic properties of ferret brain tissue in vivo. Threedimensional (3D) displacement fields were acquired during wave propagation in the brain induced by harmonic excitation of the skull at 400 Hz, 600 Hz and 800 Hz. Shear waves with wavelengths in the order of millimeters were clearly visible in the displacement field, in strain fields, and in the curl of displacement field (which contains no contributions from longitudinal waves). Viscoelastic parameters (storage and loss moduli) governing dynamic shear deformation were estimated in gray and white matter for these excitation frequencies. To characterize the reproducibility of measurements, two ferrets were studied on three different dates each. Estimated viscoelastic properties of white matter in the ferret brain were generally similar to those of gray matter and consistent between animals and scan dates. In both tissue types G 0 increased from approximately 3 kPa at 400 Hz to 7 kPa at 800 Hz and G 00 increased from approximately 1 kPa at 400 Hz to 2 kPa at 800 Hz. These measurements of shear wave propagation in the ferret brain can be used to both parameterize and validate finite element models of brain biomechanics. assessment of the rheological behavior of human organs using multifrequency MR elastography: a study of brain and liver viscoelasticity. Physics in Medicine and Biology 52, 7281-7294. Kleiven, S., 2002. Finite element modeling of the human head. Ph.D. Dissertation. A new method to measure cortical growth in the developing brain.