SU‐E‐J‐107: SEMAC MRI Metal Artifact Reduction Technique for Radiotherapy Planning Using CT/MRI Fusion

Abstract

Purpose: To assess the utility of SEMAC MRI metal artifact reduction technique for radiotherapy (RT) planning using CT/MRI fusion in patients with spine stabilisation devices. Methods: Conventional MRI spin‐echo (SE) sequences used for spinal cord delineation and radiotherapy planning in the spine were compared with a prototype implementation of Slice Encoding for Metal Artifact Correction (SEMAC) (WARP works‐in‐progress software package, Siemens Healthcare, Erlangen, Germany). Sequences were first optimised at 1.5T (Siemens MAGNETOM Aera) using a phantom constructed of a spine stabilisation device suspended in gelatin and then applied in patient studies (n=2). Both patient and phantom MRI series were co‐registered with spine CT data to assess artifact extent and geometrical image distortion. Registered data sets were assessed in terms of artifact size, distortions, image quality, visualisation of the spinal cord, spinal fluid, disease site and soft tissues adjacent to metal implants. Results: The optimal parameters were found as a compromise between artifact reduction and MRI acquisition time, while maintaining image quality comparable to conventional sequences: TR/TE 560/15ms, res: 0.8×0.8×4mm, GRAPPA=3, BW=587Hz/Px, 30 slices, TA=3:57min, SEMAC parameters: Slice encoding steps=4, Excitation & Refocusing RF duration=1500us, slice correction factor=1.2. The metal artifact volumes (signal voids and pile‐ups) were reduced using 2D SEMAC sequence on average by 43±16 % from 15.6±2.5 cm3 to 8.9±1.4 cm3. The difference in spinal cord, fluid and lesion volumes outlined using conventional and SEMAC sequences were 7±0.15, 2±0.42 and 35±3.11% respectively in the 6cm long spine sections affected by metallic artifacts. Conclusion: The work shows the importance of MRI metal artifact reduction techniques for reliable RT planning in the vicinity of metallic implants. The use of SEMAC sequence improves geometrical accuracy of outlined volumes in the presence of spine stabilization devices which degrade geometrical information from both CT and conventional SE MRI.