Slice‐selective RF pulses for in vivo B inhomogeneity mitigation at 7 tesla using parallel RF excitation with a 16‐element coil

Abstract

Slice-selective RF waveforms that mitigate severe B 1 ؉ inhomogeneity at 7 Tesla using parallel excitation were designed and validated in a water phantom and human studies on six subjects using a 16-element degenerate stripline array coil driven with a butler matrix to utilize the eight most favorable birdcage modes. The parallel RF waveform design applied magnitude least-squares (MLS) criteria with an optimized k-space excitation trajectory to significantly improve profile uniformity compared to conventional least-squares (LS) designs. Parallel excitation RF pulses designed to excite a uniform in-plane flip angle (FA) with slice selection in the z-direction were demonstrated and compared with conventional sinc-pulse excitation and RF shimming. In all cases, the parallel RF excitation significantly mitigated the effects of inhomogeneous B 1 ؉ on the excitation FA. The optimized parallel RF pulses for human B 1 ؉ mitigation were only 67% longer than a conventional sinc-based excitation, but significantly outperformed RF shimming. Key words: parallel excitation; slice-selective excitation; RF inhomogeneity mitigation; multidimensional RF pulse; RF coil array Slice-selective excitation plays a crucial role in MRI. With the push toward higher magnetic field strength, dramatic B 1 ϩ inhomogeneity for human imaging has become a serious issue, causing inhomogeneous flip-angle (FA) distribution in-plane for slice-selective excitations and detrimental nonuniformity for both signal-to-noise ratio (SNR) and image contrast. Several RF design approaches have been suggested to compensate for this inhomogeneity, including adiabatic pulses (1,2), RF-shimming (3-6), and spatially tailored excitation designs (7-11).For relatively mild B 1 ϩ inhomogeneity, using the low-FA approximation (12) with appropriate echo-volumnar k-space trajectories (9 -11), termed either "fast-k z " or "spokes" excitation trajectories, the within-slice FA inhomogeneity can be corrected. With these pulses, slice selection is achieved with a conventional sinc-like RF pulse during each k z traversal (a spoke), and in-plane FA inhomogeneity is mitigated by the appropriate choice of the complex-valued amplitude that modulates the RF waveform of each spoke. Nonetheless, if the transmit (Tx) B 1 ϩ field is rapidly varying with position, a large number of spokes will be required at correspondingly high k x and k y locations, rendering the RF pulse too lengthy for practical use.With the introduction of parallel excitation systems (13-16), the k-space trajectory can be undersampled significantly to accelerate the RF pulse and reduce its duration. A number of successful demonstrations of this concept have been reported (e.g., . For example, it has been demonstrated at 3T (18,21) and 4.7T (17) that a parallel RF design method using low-FA approximation with spokebased excitation trajectories can produce highly uniform slice-selective excitation with reasonable excitation durations.In this work we use spoke-based excitation in combination with magnitude least-squar...

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