Book Description

Magnetic resonance imaging (MRI) guided focused ultrasound (MRgFUS) is a promising non-invasive and non-ionizing therapy in which the acoustic energy penetrates to the target through the intact surrounding tissue without causing any significant bioeffects and without any incisions. At the target, ultrasound energy is converted to heat, causing tissue coagulation and necrosis. MRI guidance is used with FUS to provide high quality tumor margin definition as well as the ability to monitor the temperature of the tissue and assess the treatment. Among the MRgFUS applications are treatments of tumors in the prostate, breast, uterine fibroids, liver and brain. To make these applications widely acceptable and available to patients, improvement of image guidance is essential. The goal of this work was to develop novel MRI tools for visualization of the focal spot and for adaptive focusing of ultrasound. Visualization of the ultrasound focus is performed to con rm that the beam's focus is placed on the target anatomy. Currently, it is achieved by producing a small temperature rise in a test spot, which is detected using MRI temperature monitoring techniques. Focus localization often relies on multiple applications of ultrasound, the cumulative effects of which can lead to potentially irreversible changes in healthy tissue. In addition, visualization of the temperature rise with MRI is problematic in tissue with high fat content such as breast. The thesis addresses the challenges of focus visualization by introducing novel imaging methods, that image displacement of tissue due to acoustic radiation force: 2D Fourier Transform (2DFT) spin-echo and single-shot echo planar imaging (EPI) MR guided acoustic radiation force imaging (MR-ARFI). The new pulse sequences are demonstrated ex vivo and optimized for in vivo brain applications. One of the challenges of using FUS to treat brain pathology is overcoming the phase aberrations of ultrasound caused by heterogeneous acoustic properties of the skull bone. These aberrations result in partial or complete destruction of the focal spot, and therefore prevent the delivery of sufficient energy to the targeted volume, and cause heating where it was not intended. The current correction method estimates the aberrations from the thickness and density of the skull bone obtained from pre-operative computerized tomography (CT) images of the patient's head. Recently introduced alternative methods use adaptive focusing approach combined with MR-ARFI. The transducer elements' emissions are manipulated until the acoustic intensity, which is proportional to tissue displacement, is maximized. Promising, but very time consuming, these methods do not o er a practical solution. In this work, it is shown how using Zernike polynomials, actively utilized in optics, can increase the efficiency of MR-ARFI-based adaptive focusing, making it a more suitable technique for clinical applications.