Elastografia rezonansu magnetycznego: przegląd piśmiennictwa

Magnetic Resonance Elastography (MRE) is a rapidly developing, non-invasive, precise and reproducible imaging technique used for imaging the mechanical properties of tissues and for quantitative evaluation of shear wave propagation in the examined tissues.Magnetic resonance elastography based on three general steps, which can be described as induction of shear wave with a frequency of 50 - 5000 Hz in the tissue, then imaging of propagation of the waves inside the body (organ) using a special phase-contrast MRI technique and after all processing the acquired data in order to generate images which reveal tissue stiffness.MRE enables detection and grading of chronic hepatic fibrosis. It can be used to monitor the response to treatment or to evaluate the progress of the disease. Attempts are made to use elastography in the assessment of different organs such as liver, heart, breast, lungs, kidneys, prostate, brain tissue and spinal cord, cartilages, muscles and bones.Elastografia rezonansu magnetycznego (MRE) jest szybko rozwijającą się, nieinwazyjną, dokładną i odtwarzalną metoda diagnostyczną, wykorzystywaną dla obrazowania własności mechanicznych tkanek oraz dla ilościowej oceny propagacji fal sprężystych w  badanych tkankach. Zasadniczo technika ta składa się z trzech podstawowych kroków. Po pierwsze generowanie w obszarze zainteresowania fal sprężystych o częstotliwości w zakresie 50-5000 Hz. Następnie pozyskiwanie obrazów rezonansu magnetycznego, które przedstawiają rozchodzenie się wyindukowanych fal sprężystych. Ostatecznie przetwarzanie obrazów szerzenia się fal sprężystych na ilościowe mapy sztywności tkanek zwane elastogramami.MRE umożliwia wykrywanie i stopniowanie przewle-kłego włóknienia wątroby. Sekwencja może być wyko-rzystywana dla monitorowani odpowiedzi na leczenie lub dla oceny progresji choroby.Trwają badania naukowe z użyciem elastografii rezo-nansu magnetycznego w ocenie innych organów takich jak serce, piersi, płuca, nerki, prostata, tkanka mózgowa i rdzeń kręgowy, układ mięśniowo-szkieletowy

Magnetic resonance elastography: a review

Magnetic Resonance Elastography (MRE) is a rapidly developing, non-invasive, precise and reproducible imaging technique used for imaging the mechanical properties of tissues and for quantitative evaluation of shear wave propagation in the examined tissues. Magnetic resonance elastography based on three general steps, which can be described as induction of shear wave with a frequency of 50 - 5000 Hz in the tissue, then imaging of propagation of the waves inside the body (organ) using a special phase-contrast MRI technique and after all processing the acquired data in order to generate images which reveal tissue stiffness. MRE enables detection and grading of chronic hepatic fibrosis. It can be used to monitor the response to treatment or to evaluate the progress of the disease. Attempts are made to use elastography in the assessment of different organs such as liver, heart, breast, lungs, kidneys, prostate, brain tissue and spinal cord, cartilages, muscles and bones.Elastografia rezonansu magnetycznego (MRE) jest szybko rozwijającą się, nieinwazyjną, dokładną i odtwarzalną metoda diagnostyczną, wykorzystywaną dla obrazowania własności mechanicznych tkanek oraz dla ilościowej oceny propagacji fal sprężystych w  badanych tkankach. Zasadniczo technika ta składa się z trzech podstawowych kroków. Po pierwsze generowanie w obszarze zainteresowania fal sprężystych o częstotliwości w zakresie 50-5000 Hz. Następnie pozyskiwanie obrazów rezonansu magnetycznego, które przedstawiają rozchodzenie się wyindukowanych fal sprężystych. Ostatecznie przetwarzanie obrazów szerzenia się fal sprężystych na ilościowe mapy sztywności tkanek zwane elastogramami. MRE umożliwia wykrywanie i stopniowanie przewle-kłego włóknienia wątroby. Sekwencja może być wyko-rzystywana dla monitorowani odpowiedzi na leczenie lub dla oceny progresji choroby. Trwają badania naukowe z użyciem elastografii rezo-nansu magnetycznego w ocenie innych organów takich jak serce, piersi, płuca, nerki, prostata, tkanka mózgowa i rdzeń kręgowy, układ mięśniowo-szkieletowy

Characterizing anisotropy in fibrous soft materials by MR elastography of slow and fast shear waves

The general objective of this work was to develop experimental methods based on magnetic resonance elastography (MRE) to characterize fibrous soft materials. Mathematical models of tissue biomechanics capable of predicting injury, such as traumatic brain injury (TBI), are of great interest and potential. However, the accuracy of predictions from such models depends on accuracy of the underlying material parameters. This dissertation describes work toward three aims. First, experimental methods were designed to characterize fibrous materials based on a transversely isotropic material model. Second, these methods are applied to characterize the anisotropic properties of white matter brain tissue ex vivo. Third, a theoretical investigation of the potential application of MRE to probe nonlinear mechanical behavior of soft tissue was performed. These studies provide new methods to characterize anisotropic and nonlinear soft materials as well as contributing significantly to our understanding of the behavior of specific biological soft tissues

MAGNETIC RESONANCE ELASTOGRAPHY FOR APPLICATIONS IN RADIATION THERAPY

Magnetic resonance elastography (MRE) is an imaging technique that combines mechanical waves and magnetic resonance imaging (MRI) to determine the elastic properties of tissue. Because MRE is non-invasive, there is great potential and interest for its use in the detection of cancer. The first part of this thesis concentrates on parameter optimization and imaging quality of an MRE system. To do this, we developed a customized quality assurance phantom, and a series of quality control tests to characterize the MRE system. Our results demonstrated that through optimizing scan parameters, such as frequency and amplitude, MRE could provide a good qualitative elastogram for targets with different elasticity values and dimensions. The second part investigated the feasibility of integrating MRE into radiation therapy (RT) workflow. With the aid of a tissue-equivalent prostate phantom (embedded with three dominant intraprostatic lesions (DILs)), an MRE-integrated RT framework was developed. This framework contains a comprehensive scan protocol including Computed Tomography (CT) scan, combined MRI/MRE scans and a Volumetric Modulated Arc Therapy (VMAT) technique for treatment delivery. The results showed that using the comprehensive information could boost the MRE defined DILs to 84 Gy while keeping the remainder of the prostate to 78 Gy. Using a VMAT based technique allowed us to achieve a highly conformal plan (conformity index for the prostate and combined DILs was 0.98 and 0.91). Based on our feasibility study, we concluded that MRE data can be used for targeted radiation dose escalation. In summary, this thesis demonstrates that MRE is feasible for applications in radiation oncology

Viscoelasticity Imaging of Biological Tissues and Single Cells Using Shear Wave Propagation

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

Dynamic Magnetic Resonance Elastography

Magnetic Resonance Elastography (MRE) is a medical imaging technique used to generate a map of tissue elasticity. The resulting image is known as an elastogram, and gives a quantitative measure of stiffness in the examined tissue. The method is indirect; the elasticity, itself, is not measured. Instead, the physical response to a known stress is captured using magnetic resonance imaging, and is related to an elasticity parameter through a mathematical model of the tissue. In dynamic elastography, a harmonic stress is externally applied by a mechanical actuator, which is oriented to induce shear waves through the tissue. Once the system reaches a quasi-steady state, the displacement field is measured at a sequence of points in time. This data is the input to elasticity reconstruction algorithms. In this dissertation, the tissue is modelled as a linearly viscoelastic, isotropic continuum, undergoing harmonic motion with a known fundamental frequency. With this model, viscoelasticity is described by the complex versions of Lamé's first and second parameters. The second parameter, known as the complex shear modulus, is the one of interest. The term involving the first parameter is usually deemed negligible, so is ignored. The task is to invert the tissue model, a system of linear differential equations, to find the desired parameter. Direct inversion methods use the measured data directly in the model. Most current direct methods assume the shear modulus can be approximated locally by a constant, so ignore all derivative terms. This is known as the local homogeneity assumption, and allows for a simple, algebraic solution. The accuracy, however, is limited by the validity of the assumption. One of the purposes of MRE is to find pathological tissue marked by a higher than normal stiffness. At the boundaries of such diseased tissue, the stiffness is expected to change, invalidating the local homogeneity assumption, and hence, the shear modulus estimate. In order to capture the true shape of any stiff regions, a method must allow for local variations. Two new inversion methods are derived. In the first, a Green's function is introduced in an attempt to solve the differential equations. To simplify the system, the tissue is taken to be incompressible, another common assumption in direct inversion methods. Unfortunately, without designing an iterative procedure, the method still requires a homogeneity assumption, limiting potential accuracy. However, it is very fast and robust. In the second new inversion method, neither of the local homogeneity or incompressibility assumptions are made. Instead, the problem is re-posed in a quadratic optimization form. The system of linear differential equations is set as a constraint, and any free parameters are steered through quadratic programming techniques. It is found that, in most cases, there are no degrees of freedom in the optimization problem. This suggests that the system of differential equations has a fully determined solution, even without initial, boundary, or regularization conditions. The result is that estimates of the shear modulus and its derivatives can be obtained, locally, without requiring any assumptions that might invalidate the solution. The new inversion algorithms are compared to a few prominent, existing ones, testing accuracy and robustness. The Green's function method is found to have a comparable accuracy and noise performance to existing techniques. The second inversion method, employing quadratic optimization, is shown to be significantly more accurate, but not as robust. It seems the two goals of increasing accuracy and robustness are somewhat conflicting. One possible way to improve performance is to gather and use more data. If a second displacement field is generated using a different actuator location, further differential equations are obtained, resulting in a larger system. This enlarged system is better determined, and has improved signal-to-noise properties. It is shown that using data from a second field can increase accuracy for all methods