Vijayanand Alagappan

Parallel RF transmission with eight channels at 3 tesla

Spatially selective RF waveforms were designed and demonstrated for parallel excitation with a dedicated eight‐coil transmit array on a modified 3T human MRI scanner. Measured excitation profiles of individual coils in the array were used in a low‐flip‐angle pulse design to achieve desired spatial target profiles with two‐ (2D) and three‐dimensional (3D) k‐space excitation with simultaneous transmission of RF on eight channels. The 2D pulse excited a high‐resolution spatial pattern in‐plane, while the 3D trajectory produced high‐quality slice selection with a uniform in‐plane excitation despite the highly nonuniform individual spatial profiles of the coil array. The multichannel parallel RF excitation was used to accelerate the 2D excitation by factors of 2–8, and experimental results were in excellent agreement with simulations based on the measured coil maps. Parallel RF transmission may become critical for robust and routine human studies at very high field strengths where B1 inhomogeneity is commonly severe. Magn Reson Med, 2006.

Magnitude least squares optimization for parallel radio frequency excitation design demonstrated at 7 Tesla with eight channels

Spatially tailored radio frequency (RF) excitations accelerated with parallel transmit systems provide the opportunity to create shaped volume excitations or mitigate inhomogeneous B1 excitation profiles with clinically relevant pulse lengths. While such excitations are often designed as a least‐squares optimized approximation to a target magnitude and phase profile, adherence to the target phase profile is usually not important as long as the excitation phase is slowly varying compared with the voxel dimension. In this work, we demonstrate a method for a magnitude least squares optimization of the target magnetization profile for multichannel parallel excitation to improve the magnitude profile and reduce the RF power at the cost of a less uniform phase profile. The method enables the designer to trade off the allowed spatial phase variation for the improvement in magnitude profile and reduction in RF power. We validate the method with simulation studies and demonstrate its performance in fourfold accelerated two‐dimensional spiral excitations, as well as for uniform in‐plane slice selective parallel excitations using an eight‐channel transmit array on a 7T human MRI scanner. The experimental results are in good agreement with the simulations, which show significant improvement in the magnitude profile and reductions in the required RF power while still maintaining negligible intravoxel phase variation. Magn Reson Med 59:908–915, 2008.

Slice-Selective RF pulses for In-vivo B1+ Inhomogeneity Mitigation at 7 Tesla using Parallel RF Excitation with a 16-Element Coil

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. For example the standard deviations (SDs) of the in‐plane FA (averaged over six human studies) were 16.7% for conventional sinc excitation, 13.3% for RF shimming, and 7.6% for parallel excitation. This work demonstrates that excitations with parallel RF systems can provide slice selection with spatially uniform FAs at high field strengths with only a small pulse‐duration penalty. Magn Reson Med 60:1422–1432, 2008.

Fast slice‐selective radio‐frequency excitation pulses for mitigating B +1 inhomogeneity in the human brain at 7 Tesla

A novel radio‐frequency (RF) pulse design algorithm is presented that generates fast slice‐selective excitation pulses that mitigate B  +1 inhomogeneity present in the human brain at high field. The method is provided an estimate of the B  +1 field in an axial slice of the brain and then optimizes the placement of sinc‐like “spokes” in kz via an L1‐norm penalty on candidate (kx, ky) locations; an RF pulse and gradients are then designed based on these weighted points. Mitigation pulses are designed and demonstrated at 7T in a head‐shaped water phantom and the brain; in each case, the pulses mitigate a significantly nonuniform transmit profile and produce nearly uniform flip angles across the field of excitation (FOX). The main contribution of this work, the sparsity‐enforced spoke placement and pulse design algorithm, is derived for conventional single‐channel excitation systems and applied in the brain at 7T, but readily extends to lower field systems, nonbrain applications, and multichannel parallel excitation arrays. Magn Reson Med 59:1355–1364, 2008.

Degenerate mode band-pass birdcage coil for accelerated parallel excitation.

An eight‐rung, 3T degenerate birdcage coil (DBC) was constructed and evaluated for accelerated parallel excitation of the head with eight independent excitation channels. Two mode configurations were tested. In the first, each of the eight loops formed by the birdcage was individually excited, producing an excitation pattern similar to a loop coil array. In the second configuration a Butler matrix transformed this “loop coil” basis set into a basis set representing the orthogonal modes of the birdcage coil. In this case the rung currents vary sinusoidally around the coil and only four of the eight modes have significant excitation capability (the other four produce anticircularly polarized (ACP) fields). The lowest useful mode produces the familiar uniform B1 field pattern, and the higher‐order modes produce center magnitude nulls and azimuthal phase variations. The measured magnitude and phase excitation profiles of the individual modes were used to generate one‐, four‐, six‐, and eightfold‐accelerated spatially tailored RF excitations with 2D and 3D k‐space excitation trajectories. Transmit accelerations of up to six‐fold were possible with acceptable levels of spatial artifact. The orthogonal basis set provided by the Butler matrix was found to be advantageous when an orthogonal subset of these modes was used to mitigate B1 transmit inhomogeneities using parallel excitation. Magn Reson Med 57:1148–1158, 2007.

Broadband slab selection with B1+ mitigation at 7T via parallel spectral-spatial excitation.

Chemical shift imaging benefits from signal‐to‐noise ratio (SNR) and chemical shift dispersion increases at stronger main field such as 7 Tesla, but the associated shorter radiofrequency (RF) wavelengths encountered require B  1+ mitigation over both the spatial field of view (FOV) and a specified spectral bandwidth. The bandwidth constraint presents a challenge for previously proposed spatially tailored B  1+ mitigation methods, which are based on a type of echovolumnar trajectory referred to as “spokes” or “fast‐kz”. Although such pulses, in conjunction with parallel excitation methodology, can efficiently mitigate large B  1+ inhomogeneities and achieve relatively short pulse durations with slice‐selective excitations, they exhibit a narrow‐band off‐resonance response and may not be suitable for applications that require B  1+ mitigation over a large spectral bandwidth. This work outlines a design method for a general parallel spectral‐spatial excitation that achieves a target‐error minimization simultaneously over a bandwidth of frequencies and a specified spatial‐domain. The technique is demonstrated for slab‐selective excitation with in‐plane B  1+ mitigation over a 600‐Hz bandwidth. The pulse design method is validated in a water phantom at 7T using an eight‐channel transmit array system. The results show significant increases in the pulses spectral bandwidth, with no additional pulse duration penalty and only a minor tradeoff in spatial B  1+ mitigation compared to the standard spoke‐based parallel RF design. Magn Reson Med 61:493–500, 2009.

High-flip-angle slice-selective parallel RF transmission with 8 channels at 7 T.

At high magnetic field, B(1)(+) non-uniformity causes undesired inhomogeneity in SNR and image contrast. Parallel RF transmission using tailored 3D k-space trajectory design has been shown to correct for this problem and produce highly uniform in-plane magnetization with good slice selection profile within a relatively short excitation duration. However, at large flip angles the excitation k-space based design method fails. Consequently, several large-flip-angle parallel transmission designs have recently been suggested. In this work, we propose and demonstrate a large-flip-angle parallel excitation design for 90 degrees and 180 degrees spin-echo slice-selective excitations that mitigate severe B(1)(+) inhomogeneity. The method was validated on an 8-channel transmit array at 7T using a water phantom with B(1)(+) inhomogeneity similar to that seen in human brain in vivo. Slice-selective excitations with parallel RF systems offer means to implement conventional high-flip excitation sequences without a severe pulse-duration penalty, even at very high B(0) field strengths where large B(1)(+) inhomogeneity is present.

Simultaneous z-shim method for reducing susceptibility artifacts with multiple transmitters.

The signal loss susceptibility artifact is a major limitation in gradient‐echo MRI applications. Various methods, including z‐shim techniques and multidimensional tailored radio frequency (RF) pulses, have been proposed to mitigate the through‐plane signal loss artifact, which is dominant in axial slices above the sinus region. Unfortunately, z‐shim techniques require multiple steps and multidimensional RF methods are complex, with long pulse lengths. Parallel transmission methods were recently shown to be promising for improving B1 inhomogeneity and reducing the specific absorption rate. In this work, a novel method using time‐shifted slice‐select RF pulses is presented for reducing the through‐plane signal loss artifact in parallel transmission applications. A simultaneous z‐shim is obtained by concurrently applying unique time‐shifted pulses on each transmitter. The method is shown to reduce the signal loss susceptibility artifact in gradient‐echo images using a four‐channel parallel transmission system at 3T. Magn Reson Med 61:255–259, 2009.

Four-dimensional spectral-spatial RF pulses for simultaneous correction of B1+ inhomogeneity and susceptibility artifacts in T2*-weighted MRI.

Susceptibility artifacts and excitation radiofrequency field B1+ inhomogeneity are major limitations in high‐field MRI. Parallel transmission methods are promising for reducing artifacts in high‐field applications. In particular, three‐dimensional RF pulses have been shown to be useful for reducing B1+ inhomogeneity using multiple transmitters due to their ability to spatially shape the slice profile. Recently, two‐dimensional spectral‐spatial pulses have been demonstrated to be effective for reducing the signal loss susceptibility artifact by incorporating a frequency‐dependent through‐plane phase correction. We present the use of four‐dimensional spectral‐spatial RF pulses for simultaneous B1+ and through‐plane signal loss susceptibility artifact compensation. The method is demonstrated with simulations and in T2*‐weighted human brain images at 3 T, using a four‐channel parallel transmission system. Parallel transmission was used to reduce the in‐plane excitation resolution to improve the slice‐selection resolution between two different pulse designs. Both pulses were observed to improve B1+ homogeneity and reduce the signal loss artifact in multiple slice locations and several human volunteers. Magn Reson Med 64:1–8, 2010.