The idea of using parallel imaging to shorten the acquisition time by the simultaneous use of multiple receive coils can be adapted for the parallel transmission of a spatially-selective multidimensional RF pulse. As in data acquisition, a multidimensional RF pulse follows a certain k-space trajectory. Shortening this trajectory shortens the pulse duration. The use of multiple transmit coils, each with its own time-dependent waveform and spatial sensitivity, can compensate for the missing parts of the excitation k-space. This results in a maintained spatial definition of the pulse profile, while its duration is reduced. This work introduces the concept of parallel transmission with arbitrarily shaped transmit coils (termed "Transmit SENSE"). Results of numerical studies demonstrate the theoretical feasibility of the approach. The experimental proof of principle is provided on a commercial MR scanner. The lack of multiple independent transmit channels was addressed by combining the excitation patterns from two separate subexperiments with different transmit setups. Shortening multidimensional RF pulses could be an interesting means of making 3D RF pulses feasible even for fast T* 2 relaxing species or strong main field inhomogeneities. Other applications might benefit from the ability of Transmit SENSE to improve the spatial resolution of the pulse profile while maintaining the transmit duration. Key words: spatially-selective RF pulses; 2D RF pulses; SENSE; parallel imagingMultidimensional spatially-selective RF pulses (1-5) have found a number of useful applications in MRI. These RF pulses are able to generate or to refocus transverse magnetization within arbitrarily shaped, spatially restricted areas (2) in up to three spatial dimensions. They can be employed to perform volume selective excitation, outer volume suppression (2,6), and curved slice imaging (7), and they can serve as navigators for motion sensing (8).The basic principle of a spatially-selective RF pulse consists in a definite deposition of RF energy in the excitation k-space spanned by appropriate gradients applied simultaneously (1). Spatial definition using such multidimensional RF pulses is limited by the performance of the gradient system, as well as the finite lifetime of the transverse magnetization caused by T* 2 effects. Moreover, strong main-field inhomogeneities hinder clinical applications of 3D spatially-selective RF pulses, with typical durations of 20 -30 ms (5,9). The ability to shorten such RF pulses without losing spatial definition of the excited area is a prerequisite for exploring their use.Parallel imaging techniques, such as simultaneous acquisition of spatial harmonics (SMASH) (10) and sensitivity encoding (SENSE) (11), recently have been developed to accelerate MR image acquisition. These methodological breakthroughs triggered the question of whether it is possible to benefit from similar concepts in RF pulse design. In that respect, it is important to note that the physics of spatially-selective RF pulses show strong sim...

Magnetic resonance (MR) diffusion measurements of the abdomen were performed in 12 healthy volunteers by using a diffusion-weighted single-shot sequence both without and with pulse triggering for different trigger delays. Pulse triggering to the diastolic heart phase led to reduced motion artifacts on the diffusion-weighted MR images and to significantly improved accuracy and reproducibility of measurements of the apparent diffusion coefficients, or ADCs, of abdominal organs.

Selective 2H-, 13C-, and 17O-isotope labeling of the tyrosine amino acid has been used to map the unpaired π-electron spin-density distribution of the UV-generated neutral l-tyrosine phenoxy radical in alkaline frozen solution. The use of 13C and 17O labels allowed accurate determination of the full spin-density distribution and provided more insight in the geometrical structure of the neutral tyrosine radical in vitro. Simulations of the X-band (9.2 GHz) and Q-band (34.8 GHz) EPR powder spectra yielded the principal components of the 1H-, 13C-, and 17O-hyperfine tensors. For the two β-methylene hydrogens, a static conformational distribution of the dihedral angles (90° < θ1 < 60° and 60° < θ2 < 30°) was taken into account. The major proton hyperfine interactions and the principal g values for the neutral tyrosine radical, obtained from selectively deuterated samples, are consistent with literature values. The spin density at the specifically labeled postitions (C1‘, C2‘, C3‘, C4‘, C5‘, O4‘) was evaluated from the anisotropy of the 13C- and 17O-hyperfine tensors. A quantitative analysis of the positions C3‘ and C5‘ provided evidence for a planar distortion of the aromatic ring at these positions. 17O enrichment of the phenol oxygen O4‘ of the tyrosine radical unambiguously showed that the spin density at this oxygen is 0.26 ± 0.01. From the relatively large delocalization of the spin density over the carbonyl group of the tyrosine aromatic ring system, it is concluded that the C4‘−O4‘ bond has a double-bond character. The experimentally determined spin-density distribution is compared with several computational calculated spin-density distributions found in the literature. The isotropic 13C-hyperfine interactions are discussed in the framework of the Karplus−Fraenkel theory. This theory proved to be accurate for the determination of sign and magnitude of the isotropic 13C- and 17O-hyperfine interactions.

The reaction center (RC)-bound primary acceptor quinone QA of the photosynthetic bacterium Rhodobacter sphaeroides R26 functions as a one-electron gate. The radical anion Q~-is proposed to have an asymmetric electron distribution, induced by the protein environment. We replace the native ubiquinone-10 (UQ10) with specifically 13C-labelled UQ10, and use Q-band (35 GI-Iz) EPR spectroscopy to investigate this phenomenon in closer detail. The direct observation of the ~3C-hyperfine splitting of the gz-COmponent of UQ I 0~,-in the RC and in frozen isopropanol shows that the electron spin distribution is symmetric in the isopropanol glass, and asymmetric in the RC. Our results allow qualitative assessment of the spin and charge distribution for Q~-in the RC. The carbonyl oxygen of the semiquinone anion nearest to the S = 2 Fe2+-ion and QB is shown to acquire the highest (negative) charge density.

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