Minisymposia Abstracts

Speaker: Armando Manduca and Richard Ehman
Mayo Clinic
Title: Magnetic Resonance Elastography: Challenges and Opportunities
Abstract: Magnetic resonance elastography (MRE) is a phase contrast based MRI imaging technique that can directly visualize and quantitatively measure propagating acoustic strain waves in tissue-like materials subjected to harmonic mechanical excitation. The data acquired allows the calculation of local quantitative values of shear modulus and viscosity and the generation of images that depict viscoelastic properties of tissue. We will discuss current approaches for the inversion of MRE data and some of the challenges faced in such inversions. We will also briefly highlight current and potential future applications.

Speaker: Mark Palmeri
Duke University Medical School
Title: Quantitative Shear Wave Elasticity Imaging Techniques to Noninvasively Characterize Soft Tissue Stiffness
Abstract: Diseased tissue, such as malignant tumors and fibrotic scarring in response to organ insult, can exhibit changes in stiffness as healthy organ parenchyma is replaced by cancerous cells or scar tissue. Over the past 10 years, there have been efforts to characterize tissue stiffness using non-invasive imaging techniques, such as acoustic radiation forcebased shear wave elasticity imaging, to provide the clinician with a bedside tool to evaluate soft tissue health in a clinical setting. Soft tissue stiffness can be indirectly characterized by estimating the speed of shear wave propagation in these tissues. Acoustic Radiation Force Impulse (ARFI) shear wave imaging is one such modality that has been developed and studied for such noninvasive stiffness measurements. Shear waves are generated using impulsive, focused acoustic radiation force excitations, and the speed of those propagating shear waves is estimated using a variety of time-of-flight (TOF) algorithms. On popular shear wave imaging configuration in the clinical setting has been for the characterization of liver fibrosis. ARFI shear wave imaging sequences have been implemented for liver stiffness reconstructions using a Siemens CH4-1 curvilinear transducer on a Siemens SONOLINE AntaresTMscanner. The radiation force excitations were focused at 49 mm with an F/2 focal configuration to deliver a 180 \u03bcs excitation at 2.0 MHz. Shear waves were tracked using 3.0 MHz tracking beams and 4:1 parallel receive spanning 20 mm laterally-offset from the radiation force excitation at a PRF of 4.8 kHz. We have explored several algorithms to robustly reconstruct shear wave speeds under assumptions of homogeneity of the liver and a priori knowledge of the direction of shear wave propagation. An overview of our Lateral Time-to-Peak (TTP), RANSAC, and Radon-sum transformation methods will be presented in the context of a 172 patient retrospective / prospective study looking at liver fibrosis in Non-alcoholic Fatty Liver Disease (NAFLD) patients. Significant fibrosis (F3-4) was able to be differentiated from insignificant-to-mild fibrosis (F0-2) with 90% sensitivity and specificity using a shear stiffness threshold of 4.24 kPa. Liver shear stiffness was not dependent on hepatocyte ballooning, inflammation, or imaging location.

Future directions for shear wave elasticity imaging include the visualization of structures that violate the homogeneity assumptions that were utilized in these liver fibrosis studies. The homogeneous TOF reconstruction algorithms require modification to accommodate shear wave reflections from stiffness interfaces. Shear wave leading-edge (TTP Slope) algorithms with smaller spatial reconstruction kernels can reduce the artifacts associated with shear wave reflections and generate accurate quantitative stiffness images of spherical inclusions in simulation and phantoms. Quantitative shear wave images suffer from decreased spatial resolution that is dependent on the kernel size with CNRs that are approximately half that of qualitative ARFI images using similar radiation force excitations (6.2 vs. 11.5). Combining the higher resolution of qualitative elasticity images with the quantitative shear wave images will allow for additional characterization of liver lesions to be performed in the clinical setting.


Speaker: Joyce McLaughlin
Rensselaer Polytechnic Institute
Title: Biomechanical Imaging in Tissue-Using Time dependent Data and Frequency Dependent Data
Abstract: Biomechanical imaging is inspired by the doctors' palpation exam where tissue is palpated to feel abnormalities beneath the tissue surface. To make images first displacement movies are created either by first making a sequence of interior radiation force pushes within the tissue or by making single or multiple frequency excitations on the tissue surface and then second acquiring a sequence of RF/IQ ultrasound data sets or a sequence of MR data sets. A mathematical model relates the tissue biomechanical parameters to the displacement. This talk focuses on the mathematical models, algorithms, and images that are created with arrival time of wave fronts or with frequency dependent multiple component displacement data.

Speaker: Gen Nakamura and Yu Jiang
Hokkaido University
Title: Data Analysis for Micro-MRE measured data
Abstract: To recover the viscoelasticity of soft tissues in living bodies or phantoms from MRE-measured data is an inverse problem. To solve this inverse problem, it requires a PDE model to describe the wave field inside soft tissues or phantoms. Data analysis for MRE is done based on this PDE model and it requires to have an appropriate inversion analysis to recover the viscoelasticity of soft tissues in living bodies or phantoms from MRE-measured data. In this talk, we will show our data analysis for the data obtained by Micro-MRE in Hokkaido University.


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