Parallel magnetic resonance imaging: Encoding theory and antenna design

The conventional two-dimensional Fourier transform (2DFT) magnetic resonance imaging (MRI) encodes one dimension of spatial information into frequency component of the time-domain signals using a constant magnetic field gradient applied during data acquisition. The second dimension of the spatial information is encoded into the phase component by repeating the data acquisitions with different phase encoding gradient strengths to create another pseudo-time dimension. Because the number of repetitions of the data acquisitions is equivalent to the time resolution, a high resolution image requires a longer scan time. Recently two practical methods, Simultaneous Acquisition of Spatial Harmonics (SMASH) method in the time domain and Sensitivity-Encoded (SENSE) imaging method in the frequency domain, changed such sequential data acquisition into a partially parallel process by using MRI phased array coils, reducing the scan time several fold.; Unlike the numerical approach developed for SMASH, the author developed an analytical transform to perform the parallel spatial encoding, and a Fourier-Hilbert transform method to synchronize the phases of the signals from the individual phased array coils so that the analytical approach can be implemented in a real MRI environment. A parallel image reconstruction algorithm incorporating the transform and phase synchronization was implemented on real MRI data acquired with a four-channel MRI phased-array to achieve a two-fold scan-time reduction.; The conventional MRI phased-array coil is described in details with both the design and the analysis of a broadband 31P and 23Na phased-array system developed with author's contribution that made the system functional. However, for the purpose of parallel MRI, current MRI phased array configurations are unable to deploy a large number of coil elements due to the limitations of the loop structure and the decoupling method. The author proposed, designed, and fabricated a new planar strip array that essentially eliminates the coupling problem. For a four-channel version fabricated for a 1.5T clinical MRI scanner, the coupling between any two coils was less than −60dB. The decoupling is accomplished by both the geometry of the strips and dielectric constant of the substrate and the overlay. Such a structure allows a very large number of the element detectors to simultaneously acquire the data.; Combining the new parallel encoding theory and new antenna array design together provides a way to achieve true parallel spatial encoded MRI.