Speaker
Description
Objective: Upright radiotherapy requires CT images representative of the treatment position. To our knowledge, no paired upright and supine head and neck CT dataset is currently publicly available, limiting research into posture-specific treatment planning. Paired upright and supine low-field MRI can characterize postural changes without additional upright CT acquisitions. We developed a framework to synthesize upright CT by incorporating the extracted anatomic changes into supine CT through MRI-guided deformable registration.
Methods: We imaged 14 healthy volunteers using supine CT and paired supine-upright low-field T1-weighted MRI. Following rigid alignment, two-stage deformable registration addressed intermodality correspondence and postural deformation sequentially. The supine CT was registered to supine MRI using modality-independent neighbourhood descriptors, followed by supine-to-upright MRI registration using normalised cross-correlation. Each deformation field was represented by a coordinate-based neural network optimised individually for each volunteer and registration stage. Anatomical guidance comprised 591 landmark triplets across the cohort, supplemented by structure contours for the second stage. Regularisation constrained deformation magnitude and curvature, with an additional Jacobian penalty in the first stage. The resulting transformations mapped the original CT onto the upright MRI grid, retaining CT-derived intensities. Registration performance was assessed using cross-modality image agreement, structure overlap, target registration error (TRE) and Jacobian analysis of local folding.
Results: Upright synthetic CT (sCT) showed closer correspondence to upright than supine MRI in all 14 volunteers. With the original evaluation mask, mean mutual information was 0.246 for upright sCT-upright MRI versus 0.174 for upright sCT-supine MRI (mean per-volunteer increase: 45%). Using a common mask, upright sCT-upright MRI and registered supine sCT-supine MRI yielded mean mutual information of 0.194 and 0.226 and gradient agreement of 0.295 and 0.303, respectively. The similar gradient agreement supports preservation of anatomical correspondence after posture transfer. Across 11 volunteers with contours, mean Dice similarity coefficient (DSC) increased from 0.420 following rigid alignment to 0.799 with anatomical guidance, while mean 95th-percentile Hausdorff distance decreased from 7.28 to 2.02 mm. Overlap quantified agreement with contours also used for guidance. Guided registration reduced mean held-out TRE in both stages. Local folding occurred in eight volunteers, involving <0.09% of body voxels.
Outlook & Discussion: MRI-guided deformable registration enabled synthesis of upright CT anatomies with improved anatomical correspondence. The derived CT-MR pairs could support dosimetric investigation of posture-specific radiotherapy planning and AI model development and validation, including MRI-based upright CT synthesis. Synthetic CT provides a surrogate reference; independent validation against acquired upright CT remains necessary.