Signal-to-noise ratio improvement in NMR via receiver hardware optimization

The goal of this research was to increase available signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI) by applying specific knowledge of the imaging system to improve receiver probes (coils) and receiving hardware. A brief history of improvements in MRI receiver and coil design is presented, including the transition from large linear volume coils to local surface coils and quadrature volume coils. Then quadrature surface coils are introduced and finally multi-coil arrays with independent acquisition systems. The research covers improvements in these areas and begins with a surface coil which is adjustable in size to optimize performance given the region of interest. By careful design of trombone-like coil elements, physical adjustment can be made without electrical adjustment. Second, new understanding of noise correlation and crosstalk between coils is developed and applied to multi-coil arrays. This provides the ability to increase available SNR for such systems. Third, a method for optimally combining multiple coils in a transverse (extending perpendicular to the static magnetic field) array into a single channel by proper signal combination is presented. This method is termed generalized quadrature because of the similarity of the method to standard quadrature combination, but with freedom in weighting and phasing in the combination process. Fourth, several methods of manipulating the multiple signals from an array to allow separation after acquisition are presented. These methods require new hardware demands but allow significant improvements in SNR for either transverse or longitudinal arrays. Fifth, several novel design methods are demonstrated, including an algorithm for impedance matching, a generalized quadrature combination method, transmission synchronized rf shielding and a bird-cage surface coil. Finally, the potential future applications and benefits of this research are presented.