RF control for 7T body MRI: in search of the prostate

Abstract

MRI has gained a very prominent role in modern healthcare over the past decades. Not only the number of scanners has increased dramatically, but also the image quality, versatility and speed have reached unprecedented levels. Much of modern MRI research is aimed at the use of higher main magnetic field strengths, which allow larger signal-to-noise ratios and increase the sensitivity of various contrast mechanisms. However, the increase in main magnetic field strength up to 7 Tesla has introduced serious RF challenges for body MRI. One of the very important recent developments to cope with these challenges is the introduction of multi-transmit systems where the RF coil is no longer built as a large rigid transmit coil, but merely as an ensemble of smaller individual transmit antennas. This allows the electric and magnetic part of the RF field in the patient to be shaped by changing the phases and/or amplitudes of the individual antennas, a process generally referred to as RF shimming. The magnetic part can be altered to focus the magnetic excitation energy to the location of interest or for instance to make the magnetic excitation field more homogeneous. However, the electric part of the RF field will simultaneously change with the magnetic part and thereby the energy deposition in the patient will change. The development of RF transmit systems with suitable phase and amplitude settings therefore needs to consider the magnetic part together with the electric part of the RF field in the patient to obtain optimal excitation quality without exceeding RF heating guidelines. In this thesis different transmit systems are evaluated and new methods are developed that for each patient can find the optimal phase and/or amplitude settings that change the RF field in a controlled and effective manner. This allows the energy deposition in the patient to be controlled while the magnetic part of the RF field is optimized for imaging purposes. The aim of this thesis is to find the combination of an RF excitation system and shimming method that provides the best diagnostic images of the pelvic area without compromising patient safety and comfort.