Investigation of RF Front-Ends For Simultaneous Multinuclear MR Imaging and Spectroscopy

Abstract

Simultaneous multinuclear acquisition has long been of interest due to the fact that it can provide from multiple nuclear information in a single scan. This directly reduces acquisition time and provides a potential application on quantitative analysis at multiple nuclei, e.g. hyperpolarization and MR Fingerprinting. A direct sampling is preferred for the simultaneous acquisition since the structure can simplify phase synchronization relative to mix the frequency down to baseband and sample. But this places a great demand on data throughput while digitizing a whole spectrum. Undersampling technique was introduced and applied in this dissertation but it requires a passband filtering around the resonant frequency. Another issue is that there can be a great difference in signal sensitivity between multiple nuclei. In this dissertation, a reconfigurable triplexer front-end is proposed which can provide separate filtering at different nuclei and gain controls on each signal path temporarily to improve SNR performance in a single channel digitization. Simultaneous MR images (1H/23Na/2H) were acquired by comparing the proposed narrow band front-end filtering and a wideband, low pass filtering within a single channel. The front-end approach demonstrates that a higher SNR was achieved because the front-end can provide a narrow band filtering and separate gain controls on the resonant frequencies. Both simultaneous MR images and MR spectroscopies (1H/23Na/2H and 1H/13C/15N) are further compared with single nuclear acquisition from the Varian system, showing that the front-end performance is comparable to a commercial system but using a less complex structure. Additionally, for simultaneous MR spectroscopy, single channel digitization performs as well as three channel digitization separately. This validates that an improvement on data throughput by generating only one third data size when comparing to digitizing each of the nuclei separately and allows other digitizer channels to be available for array application. Large signal dynamic range is also conducted in this dissertation to verify the benefit of the triplexer front-end can achieve at a higher SNR for all nuclei. An array can offer multiple advantages over single element and is desirable, but coupling between elements has always been a major issue. Currently, conventional approaches to decouple are limited either by coil geometry or to a single frequency and a complex matching and tuning circuit may be required to decouple multiple nuclei with existing approaches. Therefore, a preamp decoupling approach was proposed with a fixed series resonant circuit and was evaluated based on a high input impedance Op Amp across multiple bands. Noise models of an Op Amp were developed to find an acceptable NF region condition. Impedance transformation and decoupling were also evaluated at 1H/13C/15N. The overall evaluation of the approach provides a possibility on the multiband array decoupling with an ease and fixed structure.