High-resolution diffusion-weighted 3D MRI, using diffusion-weighted driven-equilibrium (DW-DE) and multishot segmented 3D-SSFP without navigator echoes.

Abstract

In this work we report on the development of a novel technique for high-resolution diffusion-weighted (DW) MRI based upon 3D steady-state free precession (3D-SSFP). First the 3D-SSFP acquisition was segmented (each segment consisting of a series of RF pulses and gradient-recalled echoes), and then DW-driven equilibrium (DE) was inserted between each segment. The in-plane imaging matrix was typically 256 x 192 or 256 x 160, which resulted in high-resolution DW images. The DW-DE segmented SSFP signal was contaminated by the non-DW magnetization, which recovered and contributed signal during the readout train (T(1) contamination). Center-out slice encoding was used to place the greatest diffusion weighting at the center of k-space. A numerical simulation and supporting experiments were performed to evaluate the relationship of the transverse magnetization to imaging parameters, such as the b-value, echo-train length (ETL), echo-train (group) repetition time (TR(g)), and RF excitation TR (Delta t). Both the numerical simulation and the experiments suggested that the effect of T(1) contamination would be reduced with a longer TR(g), smaller b-value, shorter ETL, and center-out slice phase encoding. Phase errors caused by microscopic motions during the diffusion gradients were converted into amplitude errors by the tip-up pulse at the end of the diffusion-weighting segment. As a result, small bulk motions, such as CSF pulsation, did not cause motion-related ghosting artifacts, which would be typical in images from other multishot DWI techniques. This technique can be used for high-resolution DWI of nonbrain anatomies.