Motion Induced Signal Non-Uniformity Correction with Asymmetric bipolar MPG on Liver DWI

Body DWI has been proposed recently as a sensitive tumour detection method. One of the major issues with abdominal DWI is signal loss in soft tissues. Recent study (1) showed that this is mainly due to cardiac pulsation. Voxel deformation due to cardiac motion combined with strong diffusion gradient pulse induces intra-voxel spin phase dispersion that causes signal loss in both left and right lobes of the liver. Using a pair of bipolar gradient pulses, it is possible to compensate for this loss (2). We refer to this as motion compensated motion-probing gradient (MC MPG) approach. However, MC MPG is detrimental for diffusion weighting compared to conventional MPG. To achieve the same b-factor, higher gradient amplitude is required for MC MPG. This leads to significant prolongation of the minimum TR due to gradient hardware limitation. In this study, we propose a new type of MPG formed as a hybrid of conventional unipolar MPG and MC MPG(HybMC MPG) to shorten the minimum TE/TR. With this method, significant reduction of signal non-uniformity of DWI is possible while total scan time remains within clinically acceptable limits. Methods: MC MPG is a pair of bipolar gradient pulses (fig.1 middle) equivalent to standard 1-2-1 flow compensation gradient scheme. A unipolar gradient pulse is added in front of both bipolar pulses (fig.1 bottom). b-value is given by following formula: