Improving T2-weighted imaging at high field through the use of kT-points

Abstract

Purpose At high magnetic field strengths (B0 ≥ 3 T), the shorter radiofrequency wavelength produces an inhomogeneous distribution of the transmit magnetic field. This can lead to variable contrast across the brain which is particularly pronounced in T2-weighted imaging that requires multiple radiofrequency pulses. To obtain T2-weighted images with uniform contrast throughout the whole brain at 7 T, short (2-3 ms) 3D tailored radiofrequency pulses (kT-points) were integrated into a 3D variable flip angle turbo spin echo sequence. Methods The excitation and refocusing "hard" pulses of a variable flip angle turbo spin echo sequence were replaced with kT-point pulses. Spatially resolved extended phase graph simulations and in vivo acquisitions at 7 T, utilizing both single channel and parallel-transmit systems, were used to test different kT-point configurations. Results Simulations indicated that an extended optimized k-space trajectory ensured a more homogeneous signal throughout images. In vivo experiments showed that high quality T2-weighted brain images with uniform signal and contrast were obtained at 7 T by using the proposed methodology. Conclusion This work demonstrates that T2-weighted images devoid of artifacts resulting from B1+ inhomogeneity can be obtained at high field through the optimization of extended kT-point pulses. © 2013 Wiley Periodicals, Inc.