Parallel excitation has been introduced as a means of accelerating multidimensional, spatially-selective excitation using multiple transmit coils, each driven by a unique RF pulse. Previous approaches to RF pulse design in parallel excitation were either formulated in the frequency domain or restricted to echo-planar trajectories, or both. This paper presents an approach that is formulated as a quadratic optimization problem in the spatial domain and allows the use of arbitrary k-space trajectories. Compared to frequency domain approaches, the new design method has some important advantages. It allows for the specification of a region of interest (ROI), which improves excitation accuracy at high speedup factors. It allows for magnetic field inhomogeneity compensation during excitation. Regularization may be used to control integrated and peak pulse power Parallel excitation was recently proposed (1,2) as a means of accelerating multidimensional selective excitation using multiple coils driven with independent waveforms. In a manner analogous to parallel imaging methods, such as sensitivity encoding (SENSE) (4) and generalized autocalibrating partially parallel acquisition (GRAPPA) (5), a reduced excitation k-space trajectory (6) can be used to achieve a desired excitation pattern by exploiting the blurring behavior of coil sensitivity patterns in the excitation k-space domain to deposit RF energy in regions that are not traversed by the trajectory. Accelerated selective excitation is useful for reducing specific absorption rate (SAR) (2), and shortening multidimensional RF pulses in such applications as compensation for B 1 and B 0 inhomogeneity (7-10). The feasibility of parallel excitation was also recently verified experimentally (11,12).Several methods currently exist for designing small-tipangle RF pulses in parallel excitation (1-3). The pioneering pulse design methods were introduced by Katscher et al. (1) and Zhu (2). The method introduced by Katscher et al. (1), dubbed transmit SENSE, is characterized by the explicit use of transmit sensitivity patterns in the pulse design process, and its formulation is based on a convolution in excitation k-space. It allows usage of arbitrary kspace trajectories. Zhu's (2) method makes explicit use of transmit sensitivity patterns, but is formulated as an optimization problem in the spatial domain, and, as described, is restricted to echo-planar k-space trajectories. Griswold et al. (3) proposed a k-space domain method that is analogous to GRAPPA imaging. It is unique in that it does not require prior determination of sensitivity patterns. Instead, it involves an extra calibration step in the pulse design process. It also appears to be restricted to echo-planar k-space trajectories.In this paper we propose an alternative RF pulse design method that is closely related to transmit SENSE (1), but is formulated in the spatial domain. It is a multicoil generalization of the iterative pulse design method proposed by Yip et al. (13), and is based on the minimization of a qua...

Parallel transmitter techniques are a promising approach for reducing transmitter B 1 inhomogeneity due to the potential for adjusting the spatial excitation profile with independent RF pulses. These techniques may be further improved with transmit sensitivity encoding (SENSE) methods because the sensitivity information in pulse design provides an excitation that is inherently compensated for transmitter B 1 inhomogeneity. This paper presents a proof of this concept using transmit SENSE 3D tailored RF pulses designed for small flip angles. An eightchannel receiver coil was used to mimic parallel transmission for brain imaging at 3T. The transmit SENSE pulses were based on the fast-k z design and produced 5-mm-thick slices at a flip angle of 30°with only a 4.3-ms pulse length. It was found that the transmit SENSE pulses produced more homogeneous images than those obtained from the complex sum of images from all receivers excited with a standard RF pulse. Magn Reson Med 57:842-847, 2007.

Proton-exchange membranes (PEM) display unique ion-selective transport that has enabled a breakthrough in high-performance proton-exchange membrane fuel cells (PEMFCs). Elemental understanding of the morphology and proton transport mechanisms of the commercially available Nafion® has promoted a majority of researchers to tune proton conductive channels (PCCs). Specifically, knowledge of the morphology-property relationship gained from statistical and segmented copolymer PEMs has highlighted the importance of the alignment of PCCs. Furthermore, increasing efforts in fabricating and aligning artificial PCCs in field-aligned copolymer PEMs, nanofiber composite PEMs and mesoporous PEMs have set new paradigms for improvement of membrane performances. This perspective profiles the recent development of the channels, from the self-assembled to the artificial, with a particular emphasis on their formation and alignment. It concludes with an outlook on benefits of highly aligned PCCs for fuel cell operation, and gives further direction to develop new PEMs from a practical point of view.

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To overcome alkali-resistant and synthetic hurdles to alkaline anion-exchange membranes (AAEMs) for alkaline fuel cells, the polyacylation of pre-quaternized monomers as a straightforward and versatile approach has been proposed for the first time. Via this approach, novel aromatic anion-exchange polyelectrolytes featuring a long pendent spacer (i.e.,-O-(CH 2) 4-) instead of a conventional benzyl-type spacer (i.e.,-CH 2-) are successfully synthesized, and exhibit not only high OH À and CO 3 2À conductivity (91 mS cm À1 and 51 mS cm À1 at 60 C, respectively) but also outstanding alkaline stability (e.g., no degradation of ammonium groups after aging in 6 mol dm À3 NaOH at 60 C for 40 days). Using this kind of AAEM, a promising peak power density of 120 mW cm À2 is obtained on a preliminary H 2 /O 2 single cell at 50 C. This powerful synthetic approach together with exceptional membrane properties should pave the way to the practical application of this kind of AAEMs in alkaline fuel cells.