A Generalized Phase Correction Technique for EPI-PROPELLER

INTRODUCTION: PROPELLER (Periodically rotated overlapping parallel lines with enhanced reconstruction [1]) based on echo planar imaging (EPI) [2,3] can acquire wider blades with increased time efficiency compared to fast-spin echo (FSE)-based PROPELLER [4]. However, EPI-based sequences suffer from Nyquist ghosts arising primarily from eddy currents and gradient anisotropy [5], thus requiring phase corrections. Although phase errors can be corrected using a reference scan for each blade, this approach can greatly compromise the data acquisition efficiency. A timeefficient phase correction technique based on only two reference scans was recently reported [6]. It has been shown that the technique can reduce the Nyquist ghosts as effectively as the method employing blade-specific reference scans. Additionally, the technique is also capable of reducing oblique Nyquist ghosts (ONG) [7,8] originating from gradient anisotropy. (We call the phase error arising from gradient anisotropy “oblique” phase error). While the concept of the phase correction technique has been illustrated in the specific case of axial scans, there is a need to generalize the technique for arbitrary imaging planes, including oblique planes. For oblique planes, the ONG can be accentuated, which imposes greater challenges on the phase correction technique. The goal of this study was to develop and demonstrate a generalized phase correction method addressing constant, linear and “oblique” phase errors all together, for any arbitrary imaging plane in EPI-PROPELLER.