Parallel excitation with an array of transmit coils

TLDR

Traditional multidimensional excitation pulses create complex magnetization patterns but require prolonged RF and gradient pulses. The authors present a closed‑form design for accelerated small‑tip‑angle multidimensional excitations that suppresses aliasing lobes via a SENSE analogy, and they design and experimentally evaluate SAR‑reduced excitation pulses. Theoretical and experimental results show that parallel excitation with a transmit coil array accelerates excitation, reduces RF power deposition, and faithfully produces desired excitation profiles without increasing gradient strain. Published in Magn Reson Med 51:775–784 (2004); © 2004 Wiley‑Liss, Inc.

Abstract

Abstract Theoretical and experimental results are presented that establish the value of parallel excitation with a transmit coil array in accelerating excitation and managing RF power deposition. While a 2D or 3D excitation pulse can be used to induce a multidimensional transverse magnetization pattern for a variety of applications (e.g., a 2D localized pattern for accelerating spatial encoding during signal acquisition), it often involves the use of prolonged RF and gradient pulses. Given a parallel system that is composed of multiple transmit coils with corresponding RF pulse synthesizers and amplifiers, the results suggest that by exploiting the localization characteristics of the coils, an orchestrated play of shorter RF pulses can achieve desired excitation profiles faster without adding strains to gradients. A closed‐form design for accelerated multidimensional excitations is described for the small‐tip‐angle regime, and its suppression of interfering aliasing lobes from coarse excitation k ‐space sampling is interpreted based on an analogy to sensitivity encoding (SENSE). With or without acceleration, the results also suggest that by taking advantage of the extra degrees of freedom inherent in a parallel system, parallel excitation provides better management of RF power deposition while facilitating the faithful production of desired excitation profiles. Sample accelerated and specific absorption rate (SAR)‐reduced excitation pulses were designed in this study, and evaluated in experiments. Magn Reson Med 51:775–784, 2004. © 2004 Wiley‐Liss, Inc.