3D Wavelet Latent Diffusion Model for Whole-Body MR-to-CT Modality Translation

--Magnetic Resonance (MR) imaging plays an essential role in contemporary clinical diagnostics. It is increasingly integrated into advanced therapeutic workflows, such as hybrid Positron Emission T omography/Magnetic Resonance (PET/MR) imaging and MR-only radiation therapy. These integrated approaches are critically dependent on accurate estimation of radiation attenuation, which is typically facilitated by synthesizing Computed T omography (CT) images from MR scans to generate attenuation maps. However, existing MR-to-CT synthesis methods for whole-body imaging often suffer from poor spatial alignment between the generated CT and input MR images, and insufficient image quality for reliable use in downstream clinical tasks. In this paper, we present a novel 3D Wavelet Latent Diffusion Model (3D-WLDM) that addresses these limitations by performing modality translation in a learned latent space. By incorporating a Wavelet Residual Module into the encoder-decoder architecture, we enhance the capture and reconstruction of fine-scale features across image and latent spaces. T o preserve anatomical integrity during the diffusion process, we disentangle structural and modality-specific characteristics and anchor the structural component to prevent warping. We also introduce a Dual Skip Connection Attention mechanism within the diffusion model, enabling the generation of high-resolution CT images with improved representation of bony structures and soft-tissue contrast. Quantitative assessments demonstrate that our method, 3D-WLDM, achieves superior results, with PSNR improvements of up to 3.98 dB (1.04 dB over the best baseline), SSIM improvements of up to 0.36 (0.02 over the best baseline), and an MAE reduction of up to 53.76 (7.76 lower than the best baseline). Qualitative evaluations and clinical utility assessments using an open-source organ segmentation model further reveal substantial gains in segmentation accuracy, highlighting the translational potential of our method for radiation planning, hybrid imaging, and broader biomedical applications requiring high-fidelity MR-to-CT synthesis.