Contents I. Introduction
MRI-guided focused ultrasound (MRgFUS) is a noninvasive biomedical system used to treat medical disorders such as essential tremor. The system comprises a focused ultrasound system to treat tissues and MRI to improve the accuracy of localizing the target. MRgFUS is composed of an array of ultrasound transducers with a common conductive ground and a large bath of fluid (transfer medium) used to transmit the ultrasonic energy to the patient’s head. These structures affect the distribution of the radiofrequency magnetic field B1 of the MRI system, which is used to excite the nuclei and to detect signal. In fact, propagation of radiofrequency fields is affected by local variations of the dielectric properties of the materials. For example, High-Permittivity Materials (HPM) pads and helmets have been used to impact the B1 field distribution, improving the SNR and the transmit efficiency, and ultimately the quality of the images [1] – [4]. In a recent study, it was shown that variation of the permittivity of the materials is beneficial also for MRgFUS systems: specifically, the B1 field was improved by using a slot ground and a different permittivity of the transfer medium [5]. In this work, through numerical simulations, we optimize the value of the permittivity of the transfer medium to maximize the SNR and the transmit efficiency of an MRgFUS system.
