Viscoelastic modulus reconstruction using time harmonic vibrations

This paper presents a new iterative reconstruction method to provide high-resolution images of shear modulus and viscosity via the internal measurement of displacement fields in tissues. To solve the inverse problem, we compute the Fr\'echet derivatives of the least-squares discrepancy functional with respect to the shear modulus and shear viscosity. The proposed iterative reconstruction method using this Fr\'echet derivative does not require any differentiation of the displacement data for the full isotropic linearly viscoelastic model, whereas the standard reconstruction methods require at least double differentiation. Because the minimization problem is ill-posed and highly nonlinear, this adjoint-based optimization method needs a very well-matched initial guess. We find a good initial guess. For a well-matched initial guess, numerical experiments show that the proposed method considerably improves the quality of the reconstructed viscoelastic images.Comment: 15 page

Contactless Remote Induction of Shear Waves in Soft Tissues Using a Transcranial Magnetic Stimulation Device

This study presents the first observation of shear wave induced remotely within soft tissues. It was performed through the combination of a transcranial magnetic stimulation device and a permanent magnet. A physical model based on Maxwell and Navier equations was developed. Experiments were performed on a cryogel phantom and a chicken breast sample. Using an ultrafast ultrasound scanner, shear waves of respective amplitude of 5 and 0.5 micrometers were observed. Experimental and numerical results were in good agreement. This study constitutes the framework of an alternative shear wave elastography method

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

Pilot study on the feasibility of replacing invasive heart pressure measurements with non-invasivemagnetic resonance elastography as a way to reduce rodent numbers in pre-clinical research

Background: Imagem e Espectroscopia por Ressonância Magnética são técnicas fenotípicas que permitem a caracterização não-invasiva da função cardíaca em roedores. No entanto, ao avaliar a função miocárdica, as fases do ciclo cardíaco onde ocorrem as taxas máximas de geração de pressão e relaxamento não são directamente acessíveis com técnicas de imagem. Esta informação está apenas disponível recorrendo a cateterismo, sendo que se trata de um procedimento invasivo e terminal em roedores. A Elastografia por Ressonância Magnética mensura a deformação de um tecido após a estimulação mecânica e tem o potencial de aceder de forma não-invasiva à pressão ventricular in vivo, como foi demonstrado com sucesso em porcos e corações humanos. Este projecto tem como objectivo a construção de um sistema que permite a excitação mecânica no interior do magnete com a adequada amplitude e gama de frequência, necessárias para experiências murinas. Metodologia: Foi necessário o desenvolvimento do hardware de Elastografia necessário à estimulação mecânica de tecidos, compatível com o sistema existente de manipulação dos animais. Sequências de Ressonância Magnética foram ainda implementadas e equipadas com um gradiente de sensibilização de movimento por forma a codificar a deformação das ondas de cisalhamento. O sistema de Elastografia bem como as sequências formam testados em géis de agarose com uma gama de frequências até aos 1,5 kHz de vibração, essenciais para dimensões do coração de ratos. Posteriormente, um gel de dois compartimentos com diferentes concentrações de agarose foi excitado utilizando a configuração de Elastografia. Finalmente, um rato morto foi submetido a exames de Elastografia por Ressonância Magnética. Resultados: Houve uma clara e uniforme penetração das ondas de cisalhamento nas experiências com géis de agarose. Todos os espectros mostraram um padrão de deslocamento de fase em toda a amostra, o que correspondeu à propagação das vibrações mecânicas. O gel de 0,5% de agarose deformou mais do que o gel de 1%, resultando num comprimento de onda mais curto que o comprimento de onda encontrado no gel mais rígido. Os resultados também mostram maior atenuação das ondas no gel de 1%, quer a 500 Hz quer a 1000 Hz de frequência de vibração, quando comparado com a propagação de ondas no gel de 0,5%. Na amostra de dois compartimentos, observou-se que as ondas penetraram não-uniformemente os dois compartimentos. Houve uma melhor propagação das ondas no gel de 0,5% de agarose do que no gel de 2%, resultando num comprimento de onda mais curto no gel menos rígido. No gel com maior rigidez não foi observável uma onda inteira numa única imagem, enquanto que no gel mais soft são observados seis ciclos completos de vibração. Nas experiências com o rato, verificou-se uma forte distinção dos tecidos aquando da passagem das vibrações. Os tecidos deformaram-se em função da sua rigidez que se traduziu em diferentes variações de fase no fígado e no coração. Conclusões: Neste projecto foi concebível um sistema bem-sucedido de estimulação mecânica dos tecidos. Foi qualitativamente demonstrado que a aplicação de excitação mecânica por ondas de cisalhamento pode ser um método para codificar propriedades mecânicas de um objecto heterogéneo. Assim, este projecto representou um enorme passo no desenvolvimento de uma técnica capaz de fazer medições de pressão ventricular em ratos.Background: Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) are phenotyping techniques that allow for a non-invasive characterization of cardiac function in rodents. However, when assessing myocardial function, the isovolumic phases of the cardiac cycle are not directly accessible to imaging techniques, yet this is where maximal rates of pressure generation and relaxation occur. This type of information is only available using LV catheterization, which in rodents is an invasive and terminal procedure, requiring new experimental groups every time-point. Magnetic Resonance Elastography (MRE) measures shear deformation following mechanical tissue stimulation and has the potential to non-invasively access ventricular pressure in vivo, as successfully demonstrated in pig and human hearts. Therefore, this project aims to build a setup, which allows for mechanical stimulations inside the magnet with the appropriate amplitude and frequency range required for murine experiments. Methodology: Core to this project was the development of the MRE hardware required for the mechanical tissue stimulation and compatible with the existing animal handling system. MRI sequences needed to be implemented to equip them with motion-sensitizing gradient to encode shear deformation. In order to validate the MRE setup and the MRI sequences, phantoms of different concentrations of agarose were excited over a wide range of frequencies. The frequencies ranged from hundreds of Hz up to 1.5 kHz. MRE experiments were subsequently performed on a two-compartment phantom with different concentrations of agarose. Finally, a dead mouse was subjected to MRE examinations. Results: There was a clear uniform penetration of shear waves in phantom experiments. All phantoms demonstrated a pattern of phase shifts across the sample, which corresponded to propagation of mechanical vibrations. The gel of 0.5% agarose deformed more than the 1% agarose gel, resulting in a wavelength shorter than the wavelength found in stiffer gel. The results also show higher attenuation of shear waves in 1% agarose gel, either at 500 Hz or 1000 Hz of vibration frequency, when compared with the propagation of waves in 0.5% agarose gel. In the two-compartment phantom, shear waves penetrated non-uniformly the phantom. There was a better propagation of waves in the 0.5% gel than in the 2% one, resulting in a shorter wavelength in the softer gel than the rigid one. In the gel with higher level of stiffness the wavelength exceeded the dimensions of the gel while in the softer gel were observed six complete vibration cycles. In the mouse experiments, there was a strong tissue distinction when vibrations passed through them. The tissues deformed depending on their rigidity which translated into different spins phase variations either in the liver or in the heart. Conclusions: A setup to provide tissue mechanical stimulation in a small bore pre-clinical MR system was successfully developed. It was qualitatively demonstrated that the application of shear wave mechanical excitation may be a method to encode mechanical properties of a heterogeneous object. Thus, this project represented a major step in the ability to develop a technique for indirect LV pressure measurements in mice

K-space data processing for Magnetic Resonance Elastography (MRE)

International audienceObject: Magnetic Resonance Elastography (MRE) requires substantial data processing based on phase image reconstruction, wave enhancement and inverse problem solving. The objective of this study is to propose a new, fast MRE method based on MR raw data processing, particularly adapted to applications requiring fast MRE measurement or high elastogram update rate.Material and Methods: The proposed method allows measuring tissue elasticity directly from raw data without prior phase image reconstruction and without phase unwrapping. Experimental feasibility is assessed both in a gelatin phantom and in the liver of a porcine model in vivo. Elastograms are reconstructed with the raw MRE method and compared to those obtained using conventional MRE. In a third experiment, changes in elasticity are monitored in real-time in a gelatin phantom during its solidification by using both conventional MRE and raw MRE.Results: The raw MRE method shows promising results by providing similar elasticity values to the ones obtained with conventional MRE methods while decreasing the number of processing steps and circumventing the delicate step of phase unwrapping. Limitations of the proposed method are the influence of the magnitude on the elastogram and the requirement for a minimum number of phase offsets.Conclusion: This study demonstrates the feasibility of directly reconstructing elastograms from raw data