Echo Planar Diffusion-weighted Mri of the Abdomen and Pelvis: Comparison of Free- Breathing Monopolar and Bipolar Spin-echo Sequences in Image Quality and Geometric Distortions

Introduction: Diffusion-weighted magnetic resonance imaging (DWI) exploits the random, thermally induced displacement of water molecules in biological tissue. Most DWI applications in the abdomen and pelvis employ single-shot spin-echo echo-planar imaging (ssEPI) due to its fast acquisition times and resultant robust performance against motion artifacts. Nevertheless EPI-based DWI suffers from increased sensitivity to magnetic field inhomogeneities, which ensue from susceptibility differences at soft tissue interfaces and eddy currents generated by the high-amplitude diffusion-weighting gradients [1-3]. Geometric distortion results when image readout takes place in the presence of eddy currents, whereby the current-associated residual magnetic field is misread as a spatial encoding gradient field. A bipolar double spin-echo sequence with alternating polarity diffusion sensitising gradients has been employed to counteract the effect of eddy currents [4]. The purpose of this study was to compare artifacts, signal-to-noise ratios (SNR), fat suppression and apparent diffusion coefficient values (ADCs) using monopolar (conventional Stejkal and Tanner) and bipolar ssEPI-DWI on abdominal and pelvic images.