Fig. 5.
a: Prospective correction with the rejection module enabled. A sample EPI-slice from a volunteer before (column 1) and after (column 2) a large, abrupt through-plane head-nod, with difference images shown (column 3). The example demonstrates a large rotational displacement θp and minimal intra-volume motion Δθp (the opposite of Fig. 2, row 2); matching slices with prospective correction ON (row 1: Iafter at θp = 6.11°, Δθp ~ 0°) and OFF (row 2: Iafter at θp = 6.33°, Δθp ~ 0°) are compared. The uncorrected slice (row 2) moving through the frontal sinus illustrates the extreme nature of the motion, while its position is maintained with correction (row 1). Residual error in the difference image is emphasized by white and black arrows at the corners of the brain that denote areas of positive and negative difference, respectively – this indicates an uncorrected non-rigid-body shear in the left-right phase-encode direction. b: Field maps illustrate the effective shim-change within the brain due to motion. Pixel-shift maps r of prospectively corrected images are shown after in-plane (column 1: Iafter at θs = 4.30°, Δθs = 1.11°) and through-plane (column 2: Iafter at θp = 5.38°, Δθp = 1.76°) rotations. Maps r were windowed so that the background gray-level = 0 mm pixel-shift, while white/black map to +3/−3 mm pixel-shifts. For the in-plane case, pixel-shifts in r did not exceed 0.5 mm. For the through-plane case, pixel-shifts of up to +3 mm and −1.5 mm were observed in the anterior and posterior regions, respectively. The difference image with prospective correction only (column 3) contains the same shear-like distortion in the anterior/posterior regions as present in Fig. 5a. When additional retrospective distortion-correction is applied (column 4), the shear-like artifact is significantly reduced.