Computer simulation of functional materials for therapeutic ultrasound applications
Ahmed, Sheikh Jamil
SubjectMagnetic resonance guided high intensity focused ultrasound (MRgFUS)
Dielectric dissipation by hysteresis loss
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Magnetic resonance imaging guided high intensity focused ultrasound is a potential non-invasive treatment which uses constructive interference patterns to concentrate ultrasound energy generated by a piezoelectric (ferroelectric) transducer to thermally ablate affected tumor and cancerous tissues. However, currently used actuators (ultrasound generators) suffer from heating of the ferroelectric materials during operation which causes the dampening of ultrasound by changing the effective thickness frequency relationship and/or depolarization of the material. The excess thermal energy also contributes to the shorter heating and longer cooling cycle of operation which in turn results in higher treatment cost because of the long operating time. Such heating is caused by an energy loss (dielectric dissipation) that takes place when an alternating electric field is applied to the ferroelectric material to generate the ultrasound waves. The loss is related to the area of the hysteresis loop of the material. The project aims at establishing a framework to reduce the dielectric dissipation in ferroelectric materials during their operation as ultrasound transducers. At the initial stage, to study the associated material characteristics, first principle approaches have been adapted as a method in our research rather than experimental methods which would consume more efforts in terms of equipment, money and time. For the purpose of this study, an all electron density functional package WIEN2k is being used along with the advantage of high performance computing. In order to determine the ferroelectric parameters which are related to the polarization based property of materials, an additional software package, BerryPI has been developed in the framework of our research. The switching of ferroelectric materials which is a macroscopic effect has been studied at the atomistic level. A microscopic interpretation has been made on the growth of domains which is an essential contributor to the ferroelectric hysteresis loss. The findings of the study can be used as a model to assist in the screening of potential ferroelectric materials for ultrasound transducers. In addition, an energy efficient method to apply the electric field has been proposed that will drive the ferroelectric crystal with optimum power and thus, with reduced dielectric dissipation.