Higher order geometrical distortion in serial MR brain imaging

distortion change as a function of Gr , the standard deviation of the (sx, sy, sz, sxy, sxz, syz) components was (0.27%, 0.06%, 0.03%, 0.11o, 0.37o, 0.05o) showing most change in the readout dependent directions (sx, sxy, sxz). The coefficient of linear correlation, r, for (sx, sxy, sxz) with decreasing, Gr , was (0.93, 0.87, 0.94) showing a high inverse linear correlation with Gr (see Fig 1). The dp4 residual was (32.94%, 39.61%, 5.60%, 1.88%) indicating that almost all the distortion change was either first or second order. For the various sequence parameters the largest components of linear distortion were also (sx, sxy, sxz). The mean (stdev) for (sx, sxy, sxz) for both sequences was - FFE2: sx -1.53 (0.28), sxy 0.60 (0.07), sxz 0.92 (0.21); SE: sx -1.35 (0.82), sxy 0.40 (0.21), sxz -2.41 (0.92). The sagittal shear, sxz , was significantly higher for SE than for FFE2 (p 0.9, n=33). The mean (n=33) dp4 measure was (44.83%, 30.16%, 9.85%, 0.9%) indicating that 75% of the distortion was first or second order.