On the application of phase correction and use of k-space entropy in partial Fourier diffusion-weighted EPI

Introduction: It is well-known that diffusion-weighted (DW) imaging is very sensitive to the effects of brain motion, even in single-shot (ss)-EPI [1-4]. While the extent of rigid body motion can be minimized through patient compliance and by securing the patient’s head, pulsatile brain motion is ubiquitous and can be significant. Pulsatile brain motion that occurs during the application of the DW gradients can result in the dispersion of k-space, corresponding to signal dropout and shading in the image domain. Severe brain motion may yield a k-space completely corrupted by brain motion [4]. Typically, partial Fourier (PF) encoding in the phase-encoding direction is used to reduce the echo time in DW-ssEPI. Here, the number of ‘overscans’ is used to denote how many extra lines of k-space are acquired past the k-space center. If k-space is dispersed in the case of pulsatile brain motion, the number of overscans acquired may not be enough to encode some of the dispersed signal and considerable information may be lost. In addition, the lack of phase information provided by the small central strip of k-space used for PF reconstruction may result in artifacts in the final image. This abstract shows that phase correction applied prior to partial Fourier reconstruction in ss-EPI is helpful for recovering signal lost in cases where k-space is corrupted by brain motion. Using the same k-space from several repetitions of a DW-ssEPI scheme, we explore the use of k-space entropy [5] as a metric to identify k-space corrupted by non-linear brain motion; the use of peripheral cardiac gating and non-gating; phase correction applied before both homodyne and POCS reconstruction; as well as the number of overscans that should be used to avoid significant artifacts due to pulsatile brain motion.