Image-guided high intensity focused ultrasound (HIFU) is becoming increasingly accepted as a form of noninvasive ablative therapy for the treatment of prostate cancer, uterine fibroids and other tissue abnormalities. In principle, HIFU beams can be focused within small volumes which results in forming precise lesions within the target volume (e.g. tumor, atherosclerotic plaque) while sparing the intervening tissue. With this precision, HIFU offers the promise of noninvasive tumor therapy. The goal of this thesis is to develop an image-guidance mode with an interactive image-based computational modeling of tissue response to HIFU. This model could be used in treatment planning and post-treatment retrospective evaluation of treatment outcome(s).Within the context of treatment planning, the challenge of using HIFU to target tumors in organs partially obscured by the rib cage are addressed. Ribs distort HIFU beams in a manner that reduces the focusing gain at the target (tumor) and could cause a treatment-limiting collateral damage. We present a refocusing algorithms to efficiently steer higher power towards the target while limiting power deposition on the ribs, improving the safety and efficacy of tumor ablation. Our approach is based on an approximation of a non-convex to a convex optimization known as the semidefinite relaxation (SDR) technique. An important advantage of the SDR method over previously proposed optimization methods is the explicit control of the sidelobes in the focal plane. A finite-difference time domain (FDTD) heterogeneous propagation model of a 1-MHz concave phased array was used to model the acoustic propagation and temperature simulations in different tissues including ribs.The numerical methods developed for the refocusing problem are also used for retrospective analysis of targeting of atherosclerotic plaques using HIFU. Cases were simulated where seven adjacent HIFU shots ($~5000 W/cm^2, 2 $sec exposure time) were focused at the plaque tissue within the posterior wall of external femoral artery. After segmentation of the ultrasound image obtained for the treatment region in-vivo, we integrated this anatomical information into our simulation to account for different parameters that may be caused by these multi-region anatomical complexities. An FDTD heterogeneous model was used for both acoustic field and temperature computations. The acoustic field simulation considered a concave (40-mm radius of curvature) 32-element array operating at 3.5 MHz. To account for the blood flow in the vicinity of the target (plaque), we have used a modified transient bioheat transfer equation (tBHTE) with a convective term. The results from the numerical simulation were in good agreement with the thermal lesions identified by histological examination of the treated tissues.Within the context of accounting for the blood flow in tBHTE, the estimation of the displacement of tissue and blood motion are addressed. A new multi-dimensional speckle tracking method (MDST) utilizing the Riesz transform with subsample accuracy in all dimensions is described. Field II simulation of flow data in a channel is generated to provide a validation of the accuracy of the method. In addition, the new MDST method is applied to imaging data from the carotid artery of a healthy human volunteers. The results obtained show that using Riesz transform produces more robust estimation of the true displacement compared to previously published results.
University of Minnesota Ph.D. dissertation. May 2014. Major: Electrical/Computer Engineering. Advisor: Emad S. Ebbini. 1 computer file (PDF); ix, 126 pages, appendix A.
Almekkawy, Mohamed Khaled Ibrahim.
Optimization of focused ultrasound and image based modeling in image guided interventions.
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