3D SAP-EPI motion-corrected fast susceptibility weighted imaging

Introduction: Susceptibility-weighted imaging (SWI) has been utilized as a useful contrast mechanism in MRI that accentuates the paramagnetic properties of blood products (1,2). With the use of both magnitude and phase images, SWI can provide improved conspicuity of venous blood vessels and other sources of susceptibility effects (2). Typically, the SWI acquisition uses a high-resolution, three-dimensional gradient echo (GRE) sequence. However, the GRE acquisition used for SWI suffers from a long scan time (~5 mins at 3T), which decreases patient through-put and increases the chances of motion artifacts. A 3D GRE-EPI trajectory has been proposed as a faster alternative. However, unless the data are acquired with several interleaves, the images may suffer from considerable blurring and geometric distortion artifacts. The problem with using multiple interleaves is that, like standard GRE, it makes 3D GRE-EPI vulnerable to motion. Here, a 3D short-axis readout propeller (SAP)-EPI trajectory (3) is suggested as an alternative approach to 3D GRE and 3D GRE-EPI. SAP-EPI can achieve higher resolution than EPI with significantly reduced distortions (4). As a result, fewer interleaves can be used (GRAPPA-acceleration factor R ≤ 4), making the use of parallel imaging (PI) applicable. With PI, each interleave can be acquired with the full image FOV, making one ‘blade’ consistent (that is, inter-blade motion is negligible), while the acquisition of one ‘brick’ can be performed with full brain coverage in a few seconds. The speed of 3D SAP-EPI makes the risk of intra-brick motion (ghosting) small – leaving only the inter-brick 3D motion to be corrected (3). Here we show human 3D SAP-EPI SWI images, as well as demonstrate initial motion-corrected SWI SAP-EPI human data from controlled motion experiments. 3D SAP-EPI is also compared with interleaved 3D EPI as well as with standard 3D GRE.