ADVANCES IN ULTRA-HIGH FIELD 7 TESLA (T) HUMAN MRI: DESIGN AND METHODOLOGY OF TRANSMIT (TX) AND RECEIVE (RX) RADIO FREQUENCY (RF) COILS

Magnetic Resonance Imaging (MRI) is known as non-invasive imaging modality that provides superb anatomical soft tissue details. Over a decade, studies in ultrahigh field (UHF) MRI have been widely carried out in order to further improve the technology as well as understand the diseases. RF coil design and performance studies have been widely conducted to fully utilize the advantages that are given by UHF MRI. The aim of the design is typically producing uniform electromagnetic (EM) field distribution within the volume of interest while lowering the specific absorption rate (SAR) for Tx array and preserving/enhancing high signal to noise ratio (SNR) with Rx array. Clinically available MRI such as 1.5 Tesla (T) and 3T, uses a whole body RF coil that is embedded in the scanner as the field exciter/transmitter, and separate receivers for different parts of the body. This body coil is not available in UHF MRI due to shorter wavelength resulting in inhomogeneous EM field distributions and the large size of the body coil challenges to tune and match the coil at resonant frequency of the UHF MRI, ~300MHz as well as the size of the UHF MRI market is too small that the vendors are not actively investing in the body coil development. Due to the inexistence of the body coil at UHF MRI, it is critical to develop a RF transmit coil systems that produce a uniform B1+ field (clockwise rotating circularly polarized transverse magnetic field that is responsible for excitation) and low specific absorption rate (SAR). Aim of this thesis is to develop and evaluate new design of transmit (Tx) arrays and receive (Rx) arrays for breast and extremity (knee and ankle/foot) MR imaging at 7T. This thesis consists of several articles associated with breast and extremity MR imaging, namely: 1) Experimental and numerical analysis of B1+ field and SAR with a new transmit array design for 7T breast MRI (published as a first author), 2) Development of a 7T RF coil system for breast imaging (published as a first author), 3) Two way - split RF array development for knee MRI at 7T (under preparation as a main author), 4) A new RF Transmit Coil for Foot and Ankle Imaging at 7T MRI (published as a second author), and Overall, the work of this thesis contributes toward the understanding RF coil design and evaluation for UHF MRI human imaging

Safety of Simultaneous Scalp and Intracranial Electroencephalography Functional Magnetic Resonance Imaging

Understanding the brain and its activity is one of the great challenges of modern science. Normal brain activity (cognitive processes, etc.) has been extensively studied using electroencephalography (EEG) since the 1930’s, in the form of spontaneous fluctuations in rhythms, and patterns, and in a more experimentally-driven approach in the form of event-related potentials allowing us to relate scalp voltage waveforms to brain states and behaviour. The use of EEG recorded during functional magnetic resonance imaging (EEG-fMRI) is a more recent development that has become an important tool in clinical neuroscience, for example, for the study of epileptic activity. The primary aim of this thesis is to devise a protocol in order to minimise the health risks that are associated with simultaneous scalp and intracranial EEG during fMRI (S- icEEG-fMRI). The advances in this technique will be helpful in presenting a new imaging method that will allow the measurement of brain activity with unprecedented sensitivity and coverage. However, this cannot be achieved without assessing the safety implications of such a technique. Therefore, five experiments were performed to fulfil the primary aim. First, the safety of icEEG- fMRI using body transmit RF coil was investigated to improve the results of previous attempts using a head transmit coil at 1.5T. The results of heating increases during a high-SAR sequence were in the range of 0.2-2.4 °C at the contacts with leads positioned along the central axis inside the MRI bore. These findings suggest the need for careful lead placement. Second, also for the body transmit coil we compared the heating in the vicinity of icEEG electrodes placed inside a realistically-shaped head phantom following the addition of scalp EEG electrodes. The peak temperature change was +2.7 °C at the most superior icEEG electrode contact without scalp electrodes, and +2.1 °C at the same contact and the peak increase in the vicinity of a scalp electrode contact was +0.6 °C (location FP2). These findings show that the S-icEEG-fMRI technique is feasible if our protocol is followed carefully. Third, the heating of a realistic 3D model of icEEG electrode during MRI using EM computational simulation was investigated. The resulting peak 10 g averaged SAR was 20% higher than without icEEG. Moreover, the superior icEEG placed perpendicular to B0 showed significant local SAR increase. These results were in line with previous studies. Fourth, the possibility of simplifying a complete 8-contact with 8 wires depth icEEG electrode model into an electrode with 1-contact and 1 wire using EM simulations was addressed. The results showed similar patterns of averaged SAR values around the electrode tip during phantom and electrode position along Z for the Complete and Simplified models, except an average maximum at Z = ~2.5 W/kg for the former. The SAR values during insertion depth for the Simplified model were double those for the Complete model. The effect of extension cable length is in agreement with previous experiments. Fifth, further simulations were implemented using two more simplified models: 8-contact with 1 wire shared with all contact and 8-contact 1 wire connected to each contact at a time as well as the previously modelled simplified 1-contact 1 wire. Two sets of simulations were performed: with a single electrode and with multiple electrodes. For the single electrode, three scenarios were tested: the first simplified model used only, the second simplified models used only and the third model positioned in different 13 locations. The results of these simulations showed about 11.4-20.5-fold lower SAR for the first model than the second and 0.29-5.82-fold lower SAR for the first model than the complete model. The results also showed increased SAR for the electrode close to the head coil than the ones away from it. For the multiple electrodes, three scenarios were tested: two 1-contact and wire electrodes in different separations, multiple electrodes with their wires separated and multiple electrodes with their wires shorted. The results showed interaction between the two tested electrodes. The results of the multiple electrodes presented 2 to ~10 times higher SAR for the separated setup than the shorted. The comparison between the 1-contact with 1 wire model and the complete model is still unknown and more tests are required to show it. From the findings of this PhD research, we conclude that a body RF coil can be utilized for icEEG-fMRI at 1.5 T; however, the safety protocol has to be implemented. In addition, scalp EEG can be used in conjunction with icEEG electrodes inside the body RF coil at 1.5 T and the safety protocol has to be followed. Finally, it is feasible to perform EM computational simulations using realistic icEEG electrodes on a human model. However, simplifying the realistic icEEG electrode model might result in overestimations of the heating, although it is possible that the simplification of the model can help to simulate more complex implantations such as the implantation of multiple electrodes with their leads open circuited or short circuited, which can provide more information about the safety of implanted patients inside the MRI