Figure 1.
Gradient calibration protocol flowchart.
Flowchart detailing the processes involved in the protocol and the order in which they should be implemented. The two major subdivisions of the technique are the system calibration (a) and the post-processing correction (b). Expected image deformations are illustrated using schematics.
Figure 2.
CAD drawing of phantom sections and photograph of the assembled phantom, which was created using 3D printing. To assemble, the grid section (a) was inserted into the outer chamber (b) which was sealed by the chamber cap (c). It can then be filled through the s-bend (d) and sealed using a cap (e). An irregular prism (f) in the centre of the grid structure aids in the orientation of images.
Figure 3.
Axial (a) and coronal (b) slices from CT data and corresponding axial (c) and coronal (d) slices from 3D gradient echo MRI data. Landmarks for distance measurements are shown (b) for Z axis (red) and Y axis (blue) (landmarks in X axis are orthogonal to those in the Y axis at same Z coordinates).
Figure 4.
Gradient scaling values before and after system calibration.
Sagittal slices of CT phantom images (white) overlaid on MRI images (filler inside phantom shown in green, phantom structure shown in black, 26×60 mm FOV ) showing alignment before (a, b) and after (c, d) system calibration. The errors in the scaling factors (e) prior to calibration (time point −1) are reduced after system adjustment (TP 0) and the factors calculated using 100 µm data (X, Y, Z) are in good agreement with the 40 µm data (x40, y40, z40).
Figure 5.
Displacement fields generated from post-processing correction.
Projection along the X axis from CT phantom data (white) overlaid on MRI images (filler inside phantom shown in green, phantom structure shown in black) show good alignment after post-processing correction (a). Displacement fields generated from the non-rigid registration show displacements increase along the Z axis (b) as distance from the centre increases. This is also observed in the X (c) and Y (d) axes for central slices. Slices taken further from the centre show the displacements in the X (e) and Y (f) directions increase more rapidly with distance from the slice centre.