Room: Room 207
Purpose: To evaluate the feasibility of correcting system-dependent (B0-inhomogeneity and gradient-nonlinearity) residual geometric distortion of a 700mm-wide bore MR-simulator (Magnetom-Aera, Siemens Healthcare, Erlangen, Germany) by image unwarping according to distortion map obtained up to five months prior to acquisitions, for MR-based radiotherapy applications.
Methods: 3D-gradient-echo (VIBE) and spin-echo (SPACE) with coronal-acquisition, 400Hz/Pixel bandwidth were investigated with console integrated-correction enabled. A bore-sized (55x55x32.5cm3) geometric accuracy phantom, filled with 3892 markers (CT/MR visible) arranged in a 25mm-cubic-grid (FIGURE-1) was constructed and marker displacements from reference CT to MR images.The marker displacement of the first MR-image of each sequence was employed to compute a three-dimensional b-spline image geometric-transformation which aligned markers in MR against their counterparts in CT. Additional acquisitions were obtained within a week (Apr2nd, Apr3rd) and were repeated in the next five consecutive months (May, Jun, Jul, Aug, Sep), yielding 7 testing images for each sequence.The maker displacements within various diameter-sphere-volumes (DSVs), before and after unwarping were measured. The maximum DSVs that offered <1mm displacement of each testing image were recorded as a comparison against CT-simulators (AAPM-TG66 report).
Results: Unwarping further reduced marker displacements by at least 50% and enlarged 1mm-displacement DSVs by more than 50mm in all cases (FIGURE-2) compared to integrated-correction. Both sequences demonstrated similar displacements and >500mm of 1mm-displacement DSVs after unwarping in Apr2nd and Apr3d. The performance of unwarping fluctuated and slowly declined in the subsequent months, suggesting less effective correction beyond a prolonged period of time after obtaining reference distortion.
Conclusion: Image unwarping is recommended to suppress system-dependent geometric distortion. Our results hinted a monthly update of the unwarping function for the best performance. After unwarping, the over 500mm 1mm-displacement DSVs allows a large usable field-of-view to suit a wide-range of radiotherapy applications. Further study is needed to quantify the subject-dependent distortion.