Room: AAPM ePoster Library
Purpose: 4D-MRI is increasingly important for daily guidance of thoracic and abdominal radiotherapy. This study exploits the simultaneous multi-slice (SMS) technique to accelerate the acquisition of a balanced turbo-field echo (bTFE) coronal 4D-MRI sequence on the 1.5 T Unity MR-linac (Elekta AB, Stockholm, Sweden).
Methods: An SMS single-shot bTFE sequence was implemented to acquire a stack of 52 coronal 2D images over 30 dynamics (TR/TE=3.86/1.93ms, FOV=30(CC)x45(LR)x26(AP)cm³, voxel size=2x2x5mm³, SMS factor=2, SENSE=2.5). Simultaneously excited slices were separated by half the FOV. The middle half of the slice stack was used as navigator slices. Motion of the liver dome in the craniocaudal direction was used to extract an image-based self-sorting signal. Each dynamic was used as a reference, to which the others were rigidly registered. The end-exhale dynamic per slice location was identified and used for the self-sorting signal. A correction for amplitude variation was performed in anteroposterior direction. Slices were sorted into 10 amplitude bins, and the temporal relationship of simultaneously excited slices was used to generate sorted 4D-MRIs for 3 healthy volunteers. The self-sorting signal was validated using a 4D motion phantom (ModusQA, London, Canada).
Results: SMS-4D-MRI acquisition took 5 min on the MR-linac. The reconstruction of the sorted 4D-MRIs took 4 min using non-optimized MATLAB code on a local workstation. The peak-to-peak (SD) motion in the sorted 4D-MRIs of the volunteers was 9.4 (2.9), 10.3 (3.5), and 13.1 (3.5) mm. Motion extracted from the corresponding self-sorting signal agreed within 2.0 (0.5) mm. The average root-mean-square deviation (SD) of the liver dome smoothness was in the range of 2.1-2.2 (0.3-0.8) mm for the volunteers. In the phantom, the self-sorting signal showed a high correlation with the sinusoidal motion signals (=0.98).
Conclusion: The SMS-4D-MRI sequence yields anatomically plausible 4D representations and is thus a strong candidate for daily MR-guided radiotherapy.
Funding Support, Disclosures, and Conflict of Interest: MF Fast acknowledges funding by the Dutch Research Council (NWO) through project #17515 (BREATHE EASY).