Fast high resolution whole brain T2* weighted imaging using echo planar imaging at 7 T

Abstract

Magnetic susceptibility based (T2* weighted) contrast in MRI at high magnetic field strength is of great value in research on brain structure and cortical architecture, but its use is hampered by the low signal-to-noise ratio (SNR) efficiency of the conventional spoiled gradient echo sequence (GRE) leading to long scan times even for a limited number of slices. In this work, we show that high resolution (0.5mm isotropic) T2* weighted images of the whole brain can be obtained in 6min by utilizing the high SNR efficiency of echo-planar imaging (EPI). A volumetric (3D) EPI protocol is presented and compared to conventional 3D GRE images acquired with the same resolution, amount of T2* weighting, and imaging duration. Spatial coverage in 3D EPI was increased by a factor of 4.5 compared to 3D GRE, while also the SNR was increased by a factor of 2. Image contrast for both magnitude and phase between gray and white matter was similar for both sequences, with enhanced conspicuity of anatomic details in the 3D EPI images due to the increased SNR. Even at 7T, image blurring and distortion is limited if the EPI train length remains short (not longer than the T2* of the imaged tissue). 3D EPI provides steps (speed, whole brain coverage, and high isotropic resolution) that are necessary to utilize the benefits of high field MRI in research that employs T2* weighted imaging.