Scanners in magnetic resonance imaging (MRI) are characterized by their main magnetic field strength. Commercially available clinical MR scanners commonly have main field strengths of 1.5 and 3 Tesla. Researchers increasingly explore clinical benefits of higher field strength scanners as they provide higher signal to noise ratio or higher resolution images. On the other hand, high field strength comes with image non-uniformity as well as increased tissue heating due to radiofrequency (RF) energy deposition (also called specific absorption rate or SAR). Parallel RF transmission was proposed to address both of these challenges by optimization of RF pulses transmitted from multiple channels simultaneously. However, both the RF pulse design and RF safety management becomes more complicated with parallel RF transmission.
This talk will describe a parallel transmission RF pulse design approach to mitigate image non-uniformity that is slice selective, works for arbitrary flip angle RF excitations and incorporates explicit SAR constraints to effectively reduce RF heating at high field MRI. Excitation and refocusing RF pulse pairs are designed with this approach and are fed into a RARE (rapid acquisition with relaxation enhancement) imaging sequence and experimental validation of image non-uniformity correction is demonstrated with a 7T scanner on a head phantom. Next, a comprehensive RF safety management workflow is described that enables local SAR monitoring with a 7T parallel transmit system for head imaging. In terms of the level of image non-uniformity and limitation by RF heating, 7T head imaging and 3T fetal imaging are two very similar problems. Hence the techniques developed above are also applied to 3T fetal imaging. Furthermore, using special constraints on the fetus SAR, we ensure the RF heating for the fetus does not increase using parallel transmission compared to the standard mode of operation, a serious concern associated with parallel transmission fetal imaging.
Thesis Committee: Elfar Adalsteinsson (supervisor), Larry Wald, Ellen Grant, Jacob White