Computer Simulation of Diagnostic and Therapeutic Applications of Ultrasound Propagation in Biological Tissues

• Author / Creator

  Vafaeian, Behzad

• Ultrasound has been widely utilized in diagnostic and therapeutic clinical practices. Further development of the application of ultrasound in the clinical practices requires comprehensive understanding of the physics of ultrasound propagation in biological tissues. In this research, computer simulation was used as a means for investigating the physics of ultrasound propagation in two particular diagnostic and therapeutic applications of ultrasound. In the field of diagnostic ultrasound, quantitative ultrasound (QUS) for bone assessment was considered. Accurate diagnosis and monitoring osteoporosis based on measuring velocity and attenuation of ultrasonic waves transmitted into cancellous bone are the eventual aims of QUS. In this regards, the interaction between the micro-structure of cancellous bone and ultrasonic waves should be fully understood. Although many researches have been dedicated towards this goal, the physics of ultrasound propagation in cancellous bone has not yet been fully revealed. One of the approaches towards investigating the physics of the propagation is using computer simulation. QUS has been conventionally simulated using the finite difference time-domain (FDTD) method. However, large discrepancy (up to 75%) has been reported in the results of simulated QUS experiments when the FDTD method was employed. Some part of the discrepancy is logically believed to originate from the FDTD scheme for resolving the micro-structure of cancellous bone, and implicit formulations of the material discontinuity at the solid-fluid (bone-fluid) interface in the heterogeneous medium of cancellous bone. To mitigate the effect of these discrepancy sources, the standard Galerkin finite element method (FEM) in time domain was used as an alternative to the FDTD method. To demonstrate the capability of the FEM for simulating ultrasound propagation in cancellous bone, three-dimensional finite element models of six water-saturated cancellous bone samples were created and analyzed. The obtained relations between the simulated ultrasonic parameters (velocity and attenuation of the ultrasound) and the bone density of the samples showed that different degree of osteoporosis presented by the sample could be clearly distinguished by the simulations. Moreover, comparing the results with other experimental and simulation studies indicated that the finite element simulations were in agreement with them; therefore, the FEM was demonstrated to be capable of simulating ultrasound propagation in water-saturated cancellous bone. To further investigate the accuracy of the method, finite element simulations and QUS experiments of ultrasound propagation in cancellous bone-mimicking phantoms, i.e. aluminum foams, were performed. The simulations and the experimental results had an average relative error of 10% when the simulated and experimental ultrasonic attenuation values were compared with each other. Assumed to be mainly caused by disregarding energy-absorbing mechanisms in the simulations, the observed discrepancy (10% on average) indicated that the FEM could effectively simulate QUS as along as the energy-absorbing mechanisms had relatively small contribution in total ultrasonic attenuation. Furthermore, the results strongly suggested that wave scattering and mode conversion might be the dominant attenuation mechanisms of ultrasound propagating in trabecular structures, particularly aluminum foams. The therapeutic effect of low intensity pulsed ultrasound (LIPUS) on orthodontically induced inflammatory root resorption (OIIRR) was the second application of ultrasound considered in this thesis. The ultrasound has been observed to have stimulatory effect on new cementum regeneration that may lead to the prevention or the treatment of OIIRR. It is hypothesized that the stimulatory effect of LIPUS on new cementum regeneration is through LIPUS-induced mechanical signals. However, the stimulatory mechanical mechanism triggering new cementum regeneration has not been clearly understood yet. The aim of this study was to evaluate the hypothesis on the stimulatory mechanical mechanism by seeking possible relations between the amounts of new cementum regeneration and ultrasonic parameters such as pressure amplitude and time-averaged energy density. To this end, the FEM was utilized to simulate the previously published experiment of ultrasonic wave propagation in the dentoalveolar structure of beagle dogs. The results demonstrated qualitative relations and quantitative positive correlations between the thickness of the regenerated cementum observed in the experiment and the magnitudes of the ultrasonic parameters.

• Subjects / Keywords

  • Finite-element-method
  • Ultrasound
  • Cancellous bone
  • Tooth root resorption

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3VH5CW1C

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.