kT -points: short three-dimensional tailored RF pulses for flip-angle homogenization over an extended volume

Magn Reson Med. 2012 Jan;67(1):72-80. doi: 10.1002/mrm.22978. Epub 2011 May 16.

Abstract

With Transmit SENSE, we demonstrate the feasibility of uniformly exciting a volume such as the human brain at 7T through the use of an original minimalist transmit k-space coverage, referred to as "k(T) -points." Radio-frequency energy is deposited only at a limited number of k-space locations in the vicinity of the center to counteract transmit sensitivity inhomogeneities. The resulting nonselective pulses are short and need little energy compared to adiabatic or other B 1+-robust pulses available in the literature, making them good candidates for short-repetition time 3D sequences at high field. Experimental verification was performed on three human volunteers at 7T by means of an 8-channel transmit array system. On average, whereas the standard circularly polarized excitation resulted in a 33%-flip angle spread (standard deviation over mean) throughout the brain, and a static radio-frequency shim showed flip angle variations of 17% and up, application of k(T) -point-based excitations demonstrated excellent flip angle uniformity (8%) for a small target flip angle and with sub-millisecond durations.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Algorithms*
  • Brain / anatomy & histology*
  • Humans
  • Image Enhancement / methods
  • Image Interpretation, Computer-Assisted / methods*
  • Imaging, Three-Dimensional / methods*
  • Magnetic Resonance Imaging / methods*
  • Organ Size
  • Pattern Recognition, Automated / methods*
  • Reproducibility of Results
  • Sensitivity and Specificity