Abstract
Synthetic computed tomography (sCT) from cone-beam CT (CBCT) is a major objective in online adaptive radiotherapy. Planning magnetic resonance image (MRI) can supply strong soft-tissue priors to improve sCT synthesis in combination with CBCT inputs. Prior multimodal approaches, however, combine daily CBCT and planning MRI through simple channel-wise concatenation, and it remains unclear whether this fusion mechanism itself limits the benefit obtained from the MRI prior. We systematically compare three fusion strategies within a common Residual-in-Residual Dense Block (RRDB) conditional GAN backbone: input concatenation with channel attention, a large separable kernel attention (LSKA) spatial gate, and a proposed cross-modal spatial gating (CMSG) that couples modality-specific encoders with joint channel and spatial recalibration. On a cohort of 190 brain-metastasis patients (152 train, 38 hold-out), with three sequential CBCTs and a planning CT reference, we evaluate every fusion strategy across four input configurations and assess significance with patient-level paired Wilcoxon signed-rank tests in native Hounsfield-Unit (HU) space. The proposed CMSG achieves the best fidelity (PSNR 30.98 dB, MAE 55.87 HU) and reveals a fusion × modality interaction: the planning MRI yields a significant MAE reduction only when fused with modality-specific encoders (median -6.61 HU within CMSG, p<10^-5; a net -3.06 HU against the best CBCT-only baseline, p=2.2 × 10^-3), while under input concatenation with a spatial gate its benefit is not significant (p=0.20). These results demonstrate that the design of the fusion mechanism is key for a better exploitation of the planning MRI prior. Code is available at github.com/vhxjaouen/CrossModalSpatialGating.
Links to Paper and Supplementary Materials
Main Paper (Open Access Version): https://papers.miccai.org/miccai-2026-sat/paper/MIART_021.pdf
SharedIt Link: Not yet available
SpringerLink (DOI): Not yet available
Supplementary Material: Not Submitted
Link to Open Review
Open Review Page: Not Available
BibTex
@InProceedings{JaoVin_CrossModal_MICCAISAT2026,
author = { Jaouen, Vincent AND Bourbonne, Vincent AND Bert, Julien AND Schick, Ulrike AND Conze, Pierre-Henri AND Visvikis, Dimitris},
title = { { Cross-Modal Spatial Gating of Planning-MRI Priors for Longitudinal CBCT-to-CT Synthesis } },
booktitle = {Medical Image Computing and Computer Assisted Intervention -- MICCAI 2026 Workshops and Challenges},
year = {2026},
publisher = {Springer Nature Switzerland},
volume = {LNCS 17274},
month = {pending},
page = {pending}
}