Cavitation methods in therapeutic ultrasound : techniques, mechanisms, and system design

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, February 2004.Includes bibliographical references (leaves 134-151).Focused ultrasound is currently being developed as a non-invasive thermal ablation technique for benign and cancerous tumors in several organ systems. Although these therapies are designed to ablate tissue purely by thermal means, cavitation, the formation and collapse of gas bubbles, can occur. These bubbles can be unpredictable in their timing and location and often interfere with thermal therapies. Therefore, focused ultrasound techniques have tried to avoid bubbles and their effects. However, gas bubbles in vivo have some potential useful features for therapy. They greatly enhance local ultrasound absorption, and can on their own induce mechanical damage to the tissue. In addition, bubble clouds can block ultrasound wave propagation, providing a means to protect vital tissues during ablation of nearby pathology. If induced and controlled properly, cavitation in focused ultrasound therapy could potentially be very beneficial. The first aim of this research is to design and test in vivo ultrasound exposures that induce cavitation at appropriate times and take advantage of their absorption enhancing properties. In addition, methods to monitor and control cavitation induction and the associated therapy will be investigated. Second, a theoretical bubble model and acoustic field simulations will be used to design optimal pressure fields which very tightly control the cavitation location. These models will also be used to investigate methods for reducing the acoustic powers needed to induce cavitation while preventing off focus cavitation. For the final phase of the research a multi-channel, multi-frequency ultrasound amplifier system capable of delivering optimal exposures via large scale phased array systems will be developed and tested. In(cont.) total, the thesis research will justify applications for cavitation in ultrasound therapy, and develop the technology and methodology to optimally use cavitation and monitor its effects in vivo.by Shunmugavelu D. Sokka.Ph.D

Design and evaluation of linear intracavitary ultrasound phased array for MRI-guided prostate ablative therapies

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1999.Includes bibliographical references (p. 76-82).by Shunmugavelu D. Sokka.S.M

A Stress Ratio Parameter for Studying the Workability of Metals-Extrusion

The extrusion process has been simulated using the rigid-viscoplastic finite element analysis code ALPID 2.1. Various die geometries, billet materials, and ram speeds were included in the intrinsic workability studies. Conical dies, constant strain rate, and streamlined dies of different length with 9:1 as well as 4.4:1 reduction ratios were studied. Hypothetical materials with various strain rate sensitivities as well as aluminum 2024 and aluminum 1100 were used in the simulations. The rate of change of the square of the stress ratio (mean stress/effective stress) with respect to the log of strain rate as well as several other parameters were plotted for materials in the deformation region of the die. These plots are used to illustrate the stability of materials during the extrusion deformation process. Four different experimental process conditions were used with a streamline die to help confirm the results from the simulations

A Stress Ratio Parameter for Studying the Workability of Metals-Tension and Compression

Axisymmetric tension and compression tests were simulated using the rigid-viscoplastic finite element analysis code ALPID 2.1. The tension test simulations were run for strain rate sensitivity parameters in the range of 0.2-–1.1. The compression test simulations were run for 0.2, 0.4, and 0.8 strain rate sensitivity parameters and for 0.0, 0.001, 0.150, and 0.800 friction factors. Variations of the stress ratiog (mean stress/effective stress) in combination with the strain rate and with respect to time have been studied. The rate of change of the square of theg parameter with respect to the log of the strain rate allows a useful description of the material workability within the simulated test specimens during the deformation processes

Extrusion Through Controlled Strain Rate Dies

The workability of a material during deformation processing is determined by (a) the die geometry which, in turn, determines the flow field during deformation, and, (b) the inherent workability of the material under the imposed processing conditions of strain rate and temperature. Most common alloys have good inherent workability and can be successfully formed over wide ranges of temperature and strain rate. Products can be successfully formed from these alloys even with dies which impose large variations in strain rate during deformation. However, many of the new alloys and composites can be deformed only in very narrow processing regimes, and control of the strain rate during deformation of such materials becomes important. For example, extrusion of a whisker-reinforced aluminum alloy composite is possible only when the strain rate is controlled to within one order of magnitude. This paper describes the development of a method for obtaining preliminary shapes of controlled strain rate extrusion dies, a special case being the constant strain rate die. The theoretical basis for such die design processes is presented, followed by some examples of die geometries. Since this design procedure ignores the material flow properties, the designed die shapes must be verified using the finite element method or physical modeling. Results of simulations with the program ALPID are also presented

Deformation processing of an aluminum alloy containing particles: Studies on AI-5 pct Si alloy 4043

Al-5 wt pct Si alloy is processed by upset forging in the temperature range 300 K to 800 K and in the strain rate range 0.02 to 200 s−1. The hardness and tensile properties of the product have been studied. A “safe” window in the strain rate-temperature field has been identified for processing of this alloy to obtain maximum tensile ductility in the product. For the above strain rate range, the temperature range of processing is 550 K to 700 K for obtaining high ductility in the product. On the basis of microstructure and the ductility of the product, the temperature-strain rate regimes of damage due to cavity formation at particles and wedge cracking have been isolated for this alloy. The tensile fracture features recorded on the product specimens are in conformity with the above damage mechanisms. A high temperature treatment above ≈600 K followed by fairly fast cooling gives solid solution strengthening in the alloy at room temperature