Characterization of spatial integrity with active and passive implants in a low-field magnetic resonance linear accelerator scanner

Background and purposeStandard imaging protocols can guarantee the spatial integrity of MR images utilized in radiotherapy (RT). However, the presence of metallic implants can significantly compromise this integrity. Our proposed method aims at characterizing the geometric distortions induced by both passive and active implants commonly encountered in planning images obtained from a low-field MR-Linac (0.35T, MRIdian, Viewray).Materials and methodsWe designed a spatial integrity phantom defining 1276 control points and covering a field of view (FOV) of 20x20x20cm3. This phantom was scanned in a water tank with and without different implants used in hip and shoulder arthroplasty procedures as well as with active cardiac stimulators. The images were acquired with the clinical planning sequence (balanced steady-state free-precession, resolution 1.5 mm3). Spatial integrity was assessed by the Euclidian distance between the control point detected on the image and their theoretical locations. A first plane free of artefact (FPFA) was defined to evaluate the spatial integrity beyond the last artefact fringe (or larger banding artefact).ResultsIn the region extending up to 20 mm from the largest banding artefacts, passive implants can cause distortions up to 2 mm while this value increases to 3 mm with active implants. Beyond this region the spatial integrity is recovered and the image can be considered as unaffected by the implants.ConclusionsWe characterized the impact of common implants on a low field MR-LINAC planning sequence. These measurements can support the creation of extra margin while contouring OARs and target volumes in the vicinity of implants.