Good point, Steve. At first look, it seemed to me that the staircase artifacts are due to large slice spacing, but maybe you are right that it’s due to motion. Either way, resampling to isotropic voxel size will not hurt.
You may reduce staircase artifacts by using Smoothing effect. Try different smoothing methods and parameters and browse through slices to check how well the smoothed segment matches the organ’s shape. If they are not good (not smooth enough or oversmoothed), then you can just undo the smoothing and try with different method or parameter setting.