Transmit SENSE

Abstract

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...

High‐resolution gradient‐recalled echo imaging at 9.4T using 16‐channel parallel transmit simultaneous multislice spokes excitations with slice‐by‐slice flip angle homogenization

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Optimizing signal‐to‐noise ratio of high‐resolution parallel single‐shot diffusion‐weighted echo‐planar imaging at ultrahigh field strengths

“…8). This drawback might be addressed, for instance, by RF shimming (37,41) or multidimensional RF pulses (42,43). Moreover, the theoretically achievable SNR gain is opposed by enhanced through-plane B 0 dephasing at ultrahigh field strengths.…”

Hadamard‐encoding combined with two‐dimensional‐selective radiofrequency excitations for flexible and efficient acquisitions of multiple voxels in MR spectroscopy

“…1a) could be skipped, which would allow for echo times as short as 14 ms. Parallel excitation techniques (e.g., Katscher et al) (18) can help to reduce the 2DRF pulse duration which may be beneficial regarding relaxation during the excitation, however, will not shorten the echo time further.…”