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Keywords:

  • B1 mapping;
  • RF mapping;
  • flip angle;
  • Bloch-Siegert shift;
  • off resonance;
  • parallel transmit

Abstract

A novel method for amplitude of radiofrequency field (Bmath image) mapping based on the Bloch-Siegert shift is presented. Unlike conventionally applied double-angle or other signal magnitude–based methods, it encodes the B1 information into signal phase, resulting in important advantages in terms of acquisition speed, accuracy, and robustness. The Bloch-Siegert frequency shift is caused by irradiating with an off-resonance radiofrequency pulse following conventional spin excitation. When applying the off-resonance radiofrequency in the kilohertz range, spin nutation can be neglected and the primarily observed effect is a spin precession frequency shift. This shift is proportional to the square of the magnitude of Bmath image. Adding gradient image encoding following the off-resonance pulse allows one to acquire spatially resolved B1 maps. The frequency shift from the Bloch-Siegert effect gives a phase shift in the image that is proportional to Bmath image. The phase difference of two acquisitions, with the radiofrequency pulse applied at two frequencies symmetrically around the water resonance, is used to eliminate undesired off-resonance effects due to amplitude of static field inhomogeneity and chemical shift. In vivo Bloch-Siegert B1 mapping with 25 sec/slice is demonstrated to be quantitatively comparable to a 21-min double-angle map. As such, this method enables robust, high-resolution Bmath image mapping in a clinically acceptable time frame. Magn Reson Med 63:1315–1322, 2010. © 2010 Wiley-Liss, Inc.