Fig. 2.

Difference images (I_after - I_before) obtained from one volunteer performing a continuous in-plane motion (row 1), and a second volunteer performing a continuous through-plane motion (row 2). A zero difference image reflects perfect correction. A reference EPI image (column 1) is shown alongside sample difference images generated from time-series acquired with prospective correction ON (column 2) and OFF (columns 3,4); column 3 images remained completely unprocessed, while column 4 images were retrospectively realigned using SPM. Row 1 was chosen to demonstrate an in-plane motion case with a large rotational displacement θ_s and moderate intra-volume motion Δθ_s; the closest matching slices with prospective correction ON (row 1, column 2: I_after acquired at θ_s = 3.52°, Δθ_s = 1.25°) and OFF (row 1, columns 3,4: I_after at θ_s = 4.38°, Δθ_s = 1.07°) are chosen for comparison. Row 2 was chosen to demonstrate through-plane motion with a small rotational displacement θ_p but a large intra-volume motion Δθ_p; matching slices with prospective correction ON (row 2, column 2: I_after at θ_p = 0.67°, Δθ_p = 2.37°) and OFF (row 2, columns 3,4: I_after at θ_p = 0.46°, Δθ_p = 2.53°) are compared. The difference image in row 1, column 3 is worse than row 2 column 3 since in this case θ_s >> θ_p. For all difference images, the corresponding slice in the first (reference) time-frame was chosen for I_before (θ_s,p, Δθ_s,p ~ 0°). All difference images are identically windowed.