Cortical bone characterization using guided wave ultrasonography

• Author / Creator

  Tran, Tho Nguyen Hoang Thi

• Osteoporosis is currently the most common metabolic musculoskeletal disease which causes brittle bones with a consequent increase in bone fragility and susceptibility to fracture. This “silent epidemic” is mainly characterized by the loss of bone mass, micro-structural deterioration, and cortical thinning, resulting in changes to the mechanical properties of bone. The disease has significant morbidity and mortality affecting over 200 million people throughout the world, especially the elderly and post-menopausal women. The prevalence of osteoporosis and osteoporotic fractures rises up exponentially with the rapid growth of the aged population, which significantly increases the associated health-care costs. Due to the serious impact of osteoporosis and its related fractures on the quality of life, there is a huge call for reliable diagnostic approaches to assess osteoporosis and the fracture risk.

  Dual-energy X-ray absorptiometry (DXA) is today the gold standard to measure bone mineral density (BMD) for osteoporosis assessment. The technique is based on photon absorptiometry to measure the attenuation of photon energy by different tissues using X-ray source. DXA exposes patients to ionizing radiation and only measures bone mass thus is incapable of providing the bone mechanical properties, most notably elastic parameters, which are important determinants of bone quality especially in the early stage of osteoporosis.

  Ultrasound is an indispensable imaging modality to study soft tissues in diagnostic radiology. The use of ultrasound to study hard tissues is not yet common. But over the last two decades, the idea of using quantitative ultrasound to characterize bone properties has been evolving. Ultrasound has the merits over the other medical imaging modalities because it is portable, cost-effective, lack of ionizing radiation, and sensitive to the mechanical stiffness of cortical bones. Particularly, axial transmission ultrasonography (ATU) has shown great potential to be a non-invasive diagnostic tool to evaluate cortical thinning at multiple peripheral skeletal sites, e.g. radius and tibia. ATU excites ultrasonic guided waves (UGW) propagating in bone. Guided wave techniques have been successfully used in non-destructive testing (NDT) to study waveguide structures such as plates, cylinders, and pipes. Quantitative guided wave ultrasonography (GWU) is attractive because of the sensitivity of guided waves to the geometric, architectural, and material properties of the cortex. The cortex of long bones is a hard tissue layer bounded above and below by soft tissue and marrow, resulting in high impedance contrast interfaces, and therefore is a natural waveguide for ultrasonic energy to propagate.

  The aim of this thesis is to develop a model-based inversion scheme for parametric characterization of cortical bone tissues. The dispersion inversion problem is formulated in the frequency-phase velocity domain. The developed algorithm extracts cortical thickness and elastic velocities from the multi-frequency UGW signals and is applied to numerical simulation data, ex-vivo bone phantom data, and in-vivo human data.

  To image the dispersive energy of UGW, a sparsity-promoting Radon transform method is implemented. This signal processing technique not only provides high-resolution dispersion map but also can be used for signal-to-noise ratio (SNR) enhancement, wave field filtering, and guided mode extraction.

  In order to simulate the velocity dispersion of UGW, semi-analytical finite element (SAFE) method is used. The formulation works fine for both free and immersed solid media. The dispersion curves of the UGW propagating in cortical bone coupled with soft tissues can be accurately computed. This work is the first in the bone ultrasound research community to consider a multi-layered long bone model with the cortex surrounded by soft tissues.

  To solve the nonlinear inverse problem, the grid search technique is chosen. It simultaneously reconstructs the cortical thickness and compression and shear wave speeds in the bone from UGW data. The inversion scheme is validated based on the combination of numerical and experimental benchmark tests and is subsequently applied to in-vivo bone data.

  In conclusion, this research demonstrates that using guided wave ultrasonography to characterize osteoporotic cortical thinning is feasible. The cortical properties can be inverted reliably from UGW signals to assess the bone health status and osteoporotic fracture risk prediction. This study increases our fundamental understanding of ultrasound interaction of bone tissue under the impact of the overlying soft tissue and the governing physical principles involved.

• Subjects / Keywords

  • dispersion
  • cortical bone
  • ultrasonic guided waves
  • axial transmission
  • ultrasound
  • inversion

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-631f-9817

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.