A Comparison of the Distortion in the Same Field MRI and MR-Linac System With a 3D Printed Phantom

Abstract

PurposeA 3D printed geometric phantom was developed that can be scanned with computed tomography (CT) and magnetic resonance imaging (MRI) to measure the geometric distortion and determine the relevant dose changes.Materials and MethodsA self-designed 3D printed photosensitive resin phantom was used, which adopts grid-like structures and has 822 1 cm2 squares. The scanning plan was delivered by three MRI scanners: the Elekta Unity MR-Linac 1.5T, GE Signa HDe 1.5T, and GE Discovery-sim 750 3.0T. The geometric distortion comparison was concentrated on two 1.5T MRI systems, whereas the 3.0T MRI was used as a supplemental experiment. The most central transverse images in each dataset were selected to demonstrate the plane distortion. Some mark points were selected to analyze the distortion in the 3D direction based on the plane geometric distortion. A treatment plan was created with the off-line Monaco system.ResultsThe distortion increases gradually from the center to the outside. The distortion range is 0.79 ± 0.40 mm for the Unity, 1.31 ± 0.56 mm for the GE Signa HDe, and 2.82 ± 1.48 mm for the GE Discovery-sim 750. Additionally, the geometric distortion slightly affects the actual planning dose of the radiotherapy.ConclusionGeometric distortion increases gradually from the center to the outside. The distortion values of the Unity were smaller than those of the GE Signa HDe, and the Unity has the smallest geometric distortion. Finally, the Unity’s dose variation best matched with the standard treatment plan.

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“…These distortions become more pronounced further away from the magnet isocenter but can be largely mitigated through modern 3D distortion correction algorithms, increased gradient amplitude, increased bandwidth, and adequate shielding. For example, mean geometric distortions over a 40 cm diameter volume‐of‐interest have been reported at <1 mm and <1.2 mm for 1.5 T and 3 T diagnostic MRI scanners, respectively, 45,46 while maximum geometric displacements of <1–2 mm have been reported for both the Elekta Unity 47,48 and ViewRay MRIdian 49 systems. Patient‐level distortions are typically caused by phenomena such as material magnetic susceptibility differences and chemical shifts.…”