Phase sensitive imaging with multi-channel radio-frequency arrays requires sophisticated channel combination. Combining signal from multiple channels without considering the spatial sensitivity profile of those channels can lead to destructive interference and poor quality phase images. This work outlines a phase combination method which interpolates SVD derived relative sensitivity estimates from a prescan using a solid harmonic basis to allow for phase alignment that is extensible to the remainder of the imaging session. Furthermore, this phase alignment method is computationally efficient and applicable to any coil configuration.
The prescan used to derive relative sensitivity estimates via SVD was a 10 image 8mm isotropic prescan and is routinely collected as part of the $$$B_1^+$$$ shimming protocol. Once calculated, these relative sensitivity estimates were globally corrected for shared signal. This was done to ensure that the sensitivities did not contain extraneous phase from other sources, e.g. $$$B_0$$$ and $$$B_1^+$$$, prior to solid harmonic interpolation. Relative sensitivities were fit to the solid harmonics using an iterative least squares fitting algorithm which also removes a common complex scaling term every iteration in order to move the sensitivities closer to the solid harmonic solution and improve interpolation. This fitting process consistently takes under ten seconds. These fits can be stored as coefficients of solid harmonics and permit interpolation of relative sensitivities to any image size. This interpolation can be used to align the phases of the receiver channels in subsequent acquisitions for improved phase sensitive combination. This method is more computationally efficient than performing SVD combination on every image set collected.
Three image sets were examined to determine the effectiveness of the proposed phase combination algorithm, two sets in human subjects and one set in an oil phantom. All images were collected on the Siemens Magnetom 7T head-only MRI system located at the Centre for Functional and Metabolic Mapping . Human data was collected with the approval of the University of Western Ontario Research Ethics Board. The oil phantom data was collected using a multi-echo gradient echo sequence (3mm isotropic voxels, 12 echos, TE=3.11 ms-53.38 ms, FA=10 , TR=100 ms) and a symmetric whole head coil with 32 receive channels5. The human data was collected using a multiband EPI sequence4 (2 mm isotropic voxels, TE=20 ms, FA=45 , TR=1.25 s, multiband factor 3 and GRAPPA factor 3) and two coils, the same symmetric whole head coil and an asymmetric coil with 32 receive channels targeting the occipital parietal region6. The efficacy of the method was evaluated using a quality ratio, a modification of the quality factor7, where the denominator is the SVD combined image as opposed to the summed magnitudes. This removes the effect of noise bias from the quality factor.
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