Optimization of Gradient Moment Nulling for Hybrid of Opposite-contrast Mra Sequence

Introduction To enhance slower arterial flow as well as faster flow without increasing imaging time, we have proposed a new MRA technique named hybrid of opposite-contrast MRA (HOP-MRA) that combined standard TOF white-blood (WB) MRA with the flow-sensitive black-blood (FSBB) sequence, resulting in excellent visualization of slow-flow vessels, as compared to the MTC-TOF MRA technique [1,2]. However, flow-void artifacts were notable especially in major brain arteries in the HOP-MRA technique. This was hypothesized to be due to the difference of gradient moment nulling (GMN) in the 1 echo (TOF) part of the sequence. “Phase-encode displacement (PED) artifacts” [3] are well known, in which blood vessels are shifted from the original position depending on velocity, angle between flow and read-out, and gradient shape in a spin warp GRE-based sequence. A full-velocity compensation (3-axis GMN) was employed in the TOF part of the HOP-MRA sequence to reduce the PED artifacts. A standard MTCTOF sequence employed the GMN in k-space center in the two phase-encode directions (2-axis GMN). The purpose of this study was to assess flowvoid artifacts and the degree of PED artifacts due to the GMN difference in TOF part in HOP-MRA. Methods Simulation for the PED artifacts was performed based on Nishimura’s method [3]. Simulation conditions were: time interval between center of phase-encode pulse and the echo peak (deltaT) was 2.85ms, radius (R) of blood-vessel was 3mm and the flowing direction was 45deg that provides a maximum displacement from the read-out direction (X) (Fig. 1a). Imaging was performed on a 1.5-T whole-body imager (EXCELART Vantage, Toshiba Medical Systems Corp.). Dual-echo 3D-gradient-echo sequence (FE3D) and parallel imaging with a reduction factor of 2 were employed. In the 1st echo for TOF, TE=6.4 ms and 1 order GMN was employed. In the 2nd echo (for FSBB), TE=24 ms and flow dephasing gradient of b=2 s/mm were employed. Volume images for HOP-MRA were obtained by simple-weighted subtraction using TOF signal, STOF and FSBB signal, SBB as SHOP=STOF-a*SBB and the scaling coefficient a=1.5 was used. Maximum intensity projection (MIP) was performed without volume selection. Two types of TOF with 3-axis GMN and 2-axis GMN for generating HOP-MRA were compared in simulation and in a volunteer study. Results and Discussion Simulation results showed that vessel walls remained within the ideal vessel positions at velocities below 50 cm/s, though the distortions of vessel profiles were pronounced in higher velocities (Fig. 1b). Flow void artifacts in the HOP-MRA using TOF with 3axis-GMN were decreased by using TOF with 2-axis GMN (Fig. 2). This was likely due to dephasing effects induced by higher order moments over 2 order. Vessel shift artifacts were pronounced in the TOF-MRA with 2-axis GMN but the vessel width remained within the original vessel walls similar to the results of the simulation. Those artifacts, however, were decreased in the HOP-MRA even with the 2-axis GMN due to the contribution of FSBB signal, and the width between vessel walls in the HOP-MRA was almost similar to that with the 3-axis GMN and FSBB where realistic profiles were probably provided. (Fig. 3). Considering less flow-void artifacts and minor vessel shift artifacts, 2-axis GMN is better for the TOF part on HOP-MRA sequence. The HOP technique is suitable for decreasing both flow-void and PED artifacts in MRA compared to TOF alone. References [1] Kimura T et al. MRM62: 450-458 (2009). [2] Kodama T. at al.: Proc of ISMRM, p3416, 2008. [3] Nishimura et al. MRM 22, 481-492, 1991.