A novel radio-frequency (RF) pulse design algorithm is presented that generates fast slice-selective excitation pulses that mitigate

We introduce a novel algorithm for the design of fast slice-selective spatially-tailored magnetic resonance imaging (MRI) excitation pulses. This method, based on sparse approximation theory, uses a second-order cone optimization to place and modulate a small number of slice-selective sinc-like radio-frequency (RF) pulse segments (“spokes”) in excitation k-space, enforcing sparsity on the number of spokes allowed while simultaneously encouraging those that remain to be placed and modulated in a way that best forms a user-defined in-plane target magnetization. Pulses are designed to mitigate B1 inhomogeneity in a water phantom at 7 T and to produce highly-structured excitations in an oil phantom on an eight-channel parallel excitation system at 3 T. In each experiment, pulses generated by the sparsity-enforced method outperform those created via conventional Fourier-based techniques, e.g., when attempting to produce a uniform magnetization in the presence of severe B1 inhomogeneity, a 5.7-ms 15-spoke pulse generated by the sparsity-enforced method produces an excitation with 1.28 times lower root mean square error than conventionally-designed 15-spoke pulses. To achieve this same level of uniformity, the conventional methods need to use 29-spoke pulses that are 7.8 ms long.

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 B 1 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 B 1 transmit inhomogeneities using parallel excitation. The many positive benefits of high-field MRI are accompanied by destructive interference of the transmit RF fields within a typical volume excitation coil (1,2). This effect arises when the wavelength of the electromagnetic fields in the body approaches the dimension of the human head or body. In this case the RF fields generated by different parts of the coil can destructively interfere at some locations. For cylindrically symmetric coils, such as conventional birdcage designs, the center of the object is equidistant from all the rungs in the coil, which ensures an equal phase shift for the fields generated from each rung. For the phase relationship of the standard uniform mode of the birdcage, this leads to constructive interference at this location. In the periphery of the object, fields produced from different rungs travel unequal distances and can destructively interfere. The net effect is the center-brightening phenomena that is common in uniform mode birdcage coils at 3T and 7T. Although the high dielectric constant of water is, in practice, critical for shortening the wavelength, Collins et al. (2) have pointed out that the phenomenon does not require a dielectric media, and the phenomenon is not a dielectric resonance effect.The B 1 excitation field inhomogeneity in the transmit coil leads to unwanted spatial variations in the tissue contrast and image intensity for most pulse sequences. The severity of the effect depends on the flip-angle dependence of the sequence, and since the problem arises during excitation, it is not easily dealt with in postprocessing. Where the intrinsic contrast information is not present locally, ima...

Purpose:To investigate the behavior of whole-head and local specific absorption rate (SAR) as a function of trajectory acceleration factor and target excitation pattern due to the parallel transmission (pTX) of spatially tailored excitations at 7T. Materials and Methods:Finite-difference time domain (FDTD) simulations in a multitissue head model were used to obtain B 1 ϩ and electric field maps of an eight-channel transmit head array. Local and average SAR produced by 2D-spiral-trajectory excitations were examined as a function of trajectory acceleration factor, R, and a variety of target excitation parameters when pTX pulses are designed for constant root-mean-square excitation pattern error.Results: Mean and local SAR grow quadratically with flip angle and more than quadratically with R, but the ratio of local to mean SAR is not monotonic with R. SAR varies greatly with target position, exhibiting different behaviors as a function of target shape and size for small and large R. For example, exciting large regions produces less SAR than exciting small ones for R Ն4, but the opposite trend occurs when R Ͻ4. Furthermore, smoother and symmetric patterns produce lower SAR. Conclusion:Mean and local SAR vary by orders of magnitude depending on acceleration factor and excitation pattern, often exhibiting complex, nonintuitive behavior. To ensure safety compliance, it seems that model-based validation of individual target patterns and corresponding pTX pulses is necessary.

At high magnetic field, B1+ 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° and 180° spin-echo slice-selective excitations that mitigate severe B1+ inhomogeneity. The method was validated on an 8-channel transmit array at 7T using a water phantom with B1+ 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 B0 field strengths where large B1+ inhomogeneity is present.