SDSU Department of Physics Master’s Thesis Defense: Vadim Malis

Speaker: Vadim Malis

Title: “Advanced Magnetic Resonance Imaging Techniques to Probe Muscle Structure and Function”

Time: 12:45 pm, Tuesday, April 29, 2014

Place: PA-250

 

Abstract:

Structural and functional Magnetic Resonance Imaging (MRI) studies of skeletal muscle allow the elucidation of muscle physiology under normal and pathological conditions. Continuing on the efforts of the Muscle Imaging and Modeling laboratory, the focus of the thesis is to (i) extend and refine two challenging imaging modalities: structural imaging using Diffusion tensor imaging and functional imaging based on Velocity Encoded Phase Contrast Imaging and (ii) apply these methods to explore age related structure and functional differences of the gastrocnemius muscle.

Diffusion tensor imaging allows the study of tissue microstructure as well as muscle fiber architecture. The images, based on an ultrafast single shot EPI sequence, suffer from geometric distortions and low signal to noise ratio. A processing pipeline was developed to correct for distortions and to improve image SNR. DTI acquired on a senior and young cohort of subjects were processed through the pipeline and differences in DTI derived indices and fiber architecture between the two cohorts were explored. The DTI indices indicated that at the microstructural level, fiber atrophy was accompanied with a reduction in fiber volume fraction. At the fiber architecture level, fiber length and pennation angles decreased with age that potentially contribute to the loss of muscle force with age.

Velocity encoded Phase contrast imaging provides tissue (e.g. ,muscle) velocity at each voxel which allows the study of strain and strain rate (SR) under dynamic conditions. The focus of the thesis was to extract 2D strain rate tensor maps from the velocity images and apply the method to study age related differences. The tensor mapping can potentially provide unique information on the extracellular matrix and lateral transmission, the role of these two elements has recently emerged as important determinants of force loss with age. In the cross sectional study on aging, strain rate during isometric contraction was significantly reduced in the seniors; presumably from decrease in muscle slack and increase in stiffness with age. Other parameters of interest from this study that allow inferences on the ECM and lateral transmission are the asymmetry of deformation in the fiber cross section as well as the angle between the SR and muscle fiber.

The last part of thesis, which is a ‘work-in-progress’ is the extension to 3D SR tensor mapping using a 3D spatial, 3D velocity encoded imaging sequence. This is combined with Diffusion tensor imaging to obtain the lead eigenvector (muscle fiber direction) at each voxel. The 3D SR is then rotated to the basis of the DTI to obtain a ‘Fiber Aligned Strain rate: FASR’. The off diagonal elements of FASR are shear strain terms. Detailed analysis of the shear strain will provide a unique non-invasive method to probe lateral transmission.

 

Committee Members:

Dr. Usha Sinha (Chair, Dept. Of Physics)

Dr. Valafar (depart. of computer science)

Dr. Shantanu Sinha (depart. of Radiology, UCSD)

 
 
 

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