Image domain Propeller FSE (iProp-FSE)

Introduction: Fast-spin echo (FSE) imaging is one of the most frequently used pulse sequences for brain imaging due to its scan time efficiency, good tissue contrast, and high effective resolution. Practical problems that occur are mostly due to patient motion, which leads to blurring and imaging ghosting (Fig. 1b). To make the acquisition robust to motion, the T2-w PROPELLER pulse sequence can be used, where each excitation is followed by an RF-refocused blade readout in k-space that rotates between acquisitions (Fig. 2a). In our experience however, the contrast of FSE is still preferred over PROPELLER, as our neuroradiologists find that the latter has less GM/WM conspicuity. Hence, in practice, one faces the trade off between optimal contrast and the risk of motion artifacts. In this work, we present a new pulse sequence, image domain Propeller FSE (iProp-FSE), whereby propeller blades are acquired in the image domain instead of in k-space (PROPELLER), with the phase encoding direction aligned with the short-axis of the blade as in PROPELLER. Similar to PROPELLER, iPropFSE also allows one to perform motion correction, but there are considerable differences between both sequences. Specifically, iProp-FSE has three useful properties: (1) the overlap of blades occurs in the image domain, whereby Nblades averages are obtained in the center of the brain, which is the most SNR-starved region for ‘multi’-channel coils. Unlike PROPELLER, this approach comes without the penalty of colored noise; (2) Even if each blade covers only a strip of the final FOV, there is plenty of overlap between adjacent blades, making motion correction straightforward; (3) Since the image phase of the blade does not need to be preserved when combining the blades in the image domain, the gridded image is immune to potential spatially-varying non-linear phase changes, such as seen in DWI.