VBoussot/synthrad2023-impact-registration

🧭 SynthRAD2023 IMPACT Registrations (BSpline Transforms)

This repository provides Elastix B-spline transformation parameter files generated using the IMPACT method on the SynthRAD2023 dataset.

Each file corresponds to a non-rigid registration between a reference CT and another modality (MRI or CBCT), aligned into CT space using Elastix with the IMPACT similarity metric.

• Task 1: 315 transforms (45 excluded cases)
• Task 2: 290 transforms (70 excluded cases)

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🚀 Overview

High-quality multimodal registration is essential for supervised sCT generation.
Inaccurate alignment between MRI/CBCT and CT images can lead to blurred, anatomically inconsistent, or artifact-prone synthetic CTs.

By leveraging features from pretrained segmentation models, IMPACT improves the anatomical consistency of cross-modality alignments, ensuring that each voxel correspondence reflects a true anatomical match.

The B-spline transforms provided here can be directly applied to warp MRI or CBCT images into CT space for training or evaluation of sCT generation models.

🔎 B-spline Transform Details

All registrations were performed using a 3rd-order B-spline transform with a final grid spacing of 10 mm across 4 resolution levels. The IMPACT loss was configured as a multi-metric combination of MIND and M730 features extracted from the final network layers.

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🔧 Usage

To apply a transformation, use Transformix (from Elastix):

transformix -in Task1/brain/1BA001/mr.mha -tp Task_1/brain/1BA001.txt -out output/

Where:

• mr.mha → Input image (MRI or CBCT) from the SynthRAD2023 dataset (not included here)
• Task_1/brain/1BA001.txt → B-spline transformation file from this repository
• output/ → Directory where the warped image will be saved

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📂 Repository Structure

SynthRAD2023_IMPACT_Registrations/
├── Task_1/
│   ├── brain/
│   │   ├── 1BA001.txt
│   │   ├── 1BA005.txt
│   │   └── ...
│   └── pelvis/
│   │   ├── 1PA001.txt
│   │   └── ...
│   │── Exclude.txt
└── Task_2/
    ├── brain/
    ├── pelvis/
    │── Exclude.txt

• Task 1: MRI → CT registrations
• Task 2: CBCT → CT registrations
• All transforms are in standard Elastix parameter file format (.txt)

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⚠️ Restrictions

🗑️ Excluded Cases

45 cases were excluded from Task 1 due to poor image quality. The list of excluded cases is provided in Task_1/Exclude.txt.

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📚 References

If you use these transformations, please cite the following works:

1. IMPACT Method
Boussot V., Hémon C., Nunes J.-C., Dowling J., Rouzé S., Lafond C., Barateau A., Dillenseger J.-L.

IMPACT: A Generic Semantic Loss for Multimodal Medical Image Registration.
arXiv:2503.24121, 2025.
https://arxiv.org/abs/2503.24121

2. SynthRAD2023 Dataset
Thummerer A., van der Bijl E., Galapon A. Jr., Verhoeff J.J.C., Langendijk J.A., Both S., van den Berg C.A.T. (Nico), Maspero M.

SynthRAD2023 Grand Challenge Dataset: Generating Synthetic CT for Radiotherapy.
Medical Physics, 50(7):4664–4674, 2023. Wiley Online Library.
https://doi.org/10.1002/mp.16884

3. Registration for sCT Synthesis
Boussot V., Hémon C., Nunes J.-C., Dillenseger J.-L.

Why Registration Quality Matters: Enhancing sCT Synthesis with IMPACT-Based Registration.
arXiv:2510.21358, 2025.
https://arxiv.org/abs/2510.21358

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🧠 License

All transformation files are released under the Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).
You may reuse, modify, and redistribute them for non-commercial research purposes only, with appropriate attribution.

https://creativecommons.org/licenses/by-nc/4.0/