Advances in Radiofrequency Coil Developments for 7 Tesla Human MRI

Abstract

Magnetic Resonance Imaging (MRI) is extensively used in clinics as a non-invasive medical diagnostic tool. MRI has excellent soft tissue contrast characteristics, utilizes non-ionizing radiation, and is considered safer than other imaging techniques, such as PET, CT, and X-Ray. Currently, clinics acquire MRI images using magnetic field intensities of 1.5 Tesla (T) and 3T. However, higher field strengths provide better resolution and improved tissue contrast. Two years following the FDA clearance of the first 7T scanner (Siemens, Terra), 7T related technologies are changing the standards of imaging and are expected to become the new clinical standard in the future. 7T MRI enables improved characterization of abnormalities in the human body and early identification of disease, especially in neuroimaging. Despite the promises of 7T MRI, several challenges still need to be addressed before it can be widely utilized in the clinic. The higher the magnetic field strength implicates a shorter wavelength of the electromagnetic waves inside the tissues. At 7T, the RF wavelength is approximately 12 cm inside the tissues, which is smaller than many parts of the body. This short wavelength causes strong RF interference, which produces inhomogeneous RF field distributions inside the regions of interest. These inhomogeneities cause voids or regions of low contrast in the images, thus limiting the diagnostic capabilities of 7T MRI. Additionally, the inhomogeneous electric field distribution at 7T might lead to higher local and global power deposition, which is a safety concern. In this dissertation work, optimized RF coil designs and methodologies are presented for head and extremity imaging at 7T. The simulated and experimentally-verified results show improved homogeneity of the RF magnetic field as well as reduction of power deposition in tissue during MRI scans