Eigenmode Analysis of Transmit Coil Array for SAR-reduced B1 Mapping and RF shimming

نویسنده

  • K. Nehrke
چکیده

Introduction The B1 transmit field inhomogeneity represents a serious problem in whole-body high field MRI (≥3T). B1 shimming based on measured B1 maps is a promising approach to cope with this problem and represents the primary application for parallel transmission at this point in time [1,2]. However, B1 mapping is still an error-prone and time consuming process [3], potentially resulting in a SAR issue caused by the shimmed RF pulse and the mapping scan itself. In the present work, an eigenmode analysis of the transmit sensitivities is employed to accelerate the B1 mapping process and reduce the SAR of the shimmed RF pulses. Theory The coil sensitivities S of a transmit array can be given by its singular value decomposition (SVD) S=UΣV. The columns of S form vectors that contain the spatial sensitivities of the N individual Tx coils. The orthogonal matrix V represents a transformation from the N physical coil elements to the N normal (eigen) modes of the coil arrays, represented by their orthonormal transmit sensitivities stored in the columns of the matrix U. The N singular values σi, held by the diagonal matrix Σ, describe the conditioning of the coil array with respect to its normal modes, and hence, are characteristic properties of the array for both, the mapping and the shimming procedure. Small singular values result in a less precise mapping of the corresponding modes, and hence, in larger mapping errors. Apart from that, those contributions of the shimmed B1 originating from modes with smaller singular values need larger RF power (∼σi), and hence, will contribute disproportionately high to the SAR (Fig.1). On the other hand, RF shimming is often a rather benign problem, and the normal modes are, at least approximately, known in advance. This opens the potential to restrict the problem to a few significant modes, thus accelerating the B1 mapping and regularizing the RF shimming process. Introducing the diagonal matrix R, with zeros for the modes to be discarded and ones for the remaining, VRV represents a transformation matrix for the drive scales of the coil elements to filter out the discarded modes, and hence, to restrict both mapping and shimming to the desired modes.

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تاریخ انتشار 2008