INTRODUCTION: In-room MRI is a promising image guidance strategy in external beam
radiotherapy to acquire volumetric information for moving targets. However,
limitations in spatio-temporal resolution led several authors to use 2D
orthogonal images for guidance. The aim of this work is to present a method to
concurrently compensate for non-rigid tumour motion and provide an approach for
3D reconstruction from 2D orthogonal cine-MRI slices for MRI-guided treatments.
METHODS: Free-breathing sagittal/coronal interleaved 2D cine-MRI were acquired in
addition to a pre-treatment 3D volume in two patients. We performed deformable
image registration (DIR) between cine-MRI slices and corresponding slices in the
pre-treatment 3D volume. Based on an extrapolation of the interleaved 2D motion
fields, the 3D motion field was estimated and used to warp the pre-treatment
volume. Due to the lack of a ground truth for patients, the method was validated
on a digital 4D lung phantom.
RESULTS: On the phantom, the 3D reconstruction method was able to compensate for
tumour motion and compared favourably to the results of previously adopted
strategies. The difference in the 3D motion fields between the phantom and the
extrapolated motion was 0.4 ± 0.3 mm for tumour and 0.8 ± 1.5 mm for whole
anatomy, demonstrating feasibility of performing a 3D volumetric reconstruction
directly from 2D orthogonal cine-MRI slices. Application of the method to patient
data confirmed the feasibility of utilizing this method in real world scenarios.
CONCLUSION: Preliminary results on phantom and patient cases confirm the
feasibility of the proposed approach in an MRI-guided scenario, especially for
non-rigid tumour motion compensation.