Technical Development of Accelerated and Multi-contrast Echo-planar Imaging

Abstract

Echo-planar imaging (EPI) is a popular magnetic resonance imaging (MRI) technique used in functional MRI (fMRI) and other studies that require high temporal resolution. However, the single-shot EPI technique has limited spatial resolution due to signal decay and contains geometric distortion caused by a relatively long echo spacing. To overcome these limitations, multi-shot and parallel MRI schemes can be employed, and multi-band/simultaneous multi-slice techniques can be used to further improve temporal resolution. However, all these EPI schemes are vulnerable to Nyquist artifact, which reduces signal-to-noise ratio (SNR) and image quality due to inconsistencies between positive and negative-readout ky lines. We propose a novel inherent coil-signature-based 2D phase correction method for removing Nyquist artifact in EPI data obtained with different schemes. The method is based on inherent coil sensitivity profile and does not require time-consuming 2D phase calibration scans. Our proposed method effectively removes 2D Nyquist artifact and aliasing artifacts caused by acceleration schemes (multi-band or parallel imaging) and multi-shot technique, and the resulting default mode network activation mapping are more accurate than conventional 1D phase correction method. In the field of quantitative MRI, multi-contrast imaging can be utilized to generate multi-parametric mapping of different parameters such as T1, T2 and T2* and magnetic susceptibility, capable of detecting microstructural processes related to aging and neurological diseases. The spoiled gradient recalled acquisition in the steady state (SPGR) sequence is commonly used for multi-contrast imaging, which uses a spoiling gradient to eliminate the residual transverse magnetization that is present at the end of each repetition time. As a result, the SPGR sequence is not capable of generating T2-weighted (T2-w) images. Additionally, the requirement of a relatively long TE for T2*-weighted (T2*-w) images results in dephasing. Double-echo steady state (DESS) imaging technique has been proved to provide multi-contrast images with both T2 and T2* contrast, based on the balanced steady-state free precession (bSSFP) pulse sequence that involves a balanced gradient echo sequence with a TR. DESS imaging offers high SNR and excellent tissue contrast, especially for midbrain region, due to its sensitivity to T2/ T2* contrasts. However, DESS imaging has a relatively long scan time, which limits its application in certain clinical settings. To improve time efficiency, we utilize the PulSeq framework to generate a customized MRI pulse sequence called double-echo steady state EPI (DESS-EPI), which acquires two echoes in each TR interval and facilitate the acquisition of both T2 and T2* contrast. Our preliminary results are promising in reducing acquisition time (three times less compared to DESS imaging) and achieving contrast in the midbrain region.