Parallel-transmission-enabled three-dimensional T2 -weighted imaging of the human brain at 7 Tesla.

Abstract

A promise of ultra high field MRI is to produce images of the human brain with higher spatial resolution due to an increased signal to noise ratio. Yet, the shorter radiofrequency wavelength induces an inhomogeneous distribution of the transmit magnetic field and thus challenges the applicability of MRI sequences which rely on the spin excitation homogeneity. In this work, the ability of parallel-transmission to obtain high-quality T2 -weighted images of the human brain at 7 Tesla, using an original pulse design method is evaluated. Excitation and refocusing square pulses of a SPACE sequence were replaced with short nonselective transmit-SENSE pulses individually tailored with the gradient ascent pulse engineering algorithm, adopting a kT -point trajectory to simultaneously mitigate B1 (+) and ΔB0 nonuniformities. In vivo experiments showed that exploiting parallel-transmission at 7T with the proposed methodology produces high quality T2 -weighted whole brain images with uniform signal and contrast. Subsequent white and gray matter segmentation confirmed the expected improvements in image quality. This work demonstrates that the adopted formalism based on optimal control, combined with the kT -point method, successfully enables three-dimensional T2 -weighted brain imaging at 7T devoid of artifacts resulting from B1 (+) inhomogeneity.