Add ImpactSynth extension

[vboussot]

New extension

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This PR adds ImpactSynth, a new extension enabling synthetic CT (sCT) generation from MRI or CBCT directly inside 3D Slicer, specifically designed for radiotherapy workflows.

Slicer IMPACT-Synth provides a dedicated GUI for the IMPACT-Synth framework and leverages KonfAI Apps for fast and fully configurable inference pipelines.
It integrates state-of-the-art models from the SynthRAD2025 Challenge (ranked 3rd in both MRI→CT and CBCT→CT tasks) and includes embedded evaluation and uncertainty workflows to support clinical Quality Assurance.

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Included capabilities

Image Synthesis

• MRI ➜ CT generation
• CBCT ➜ CT generation

Segmentation & QA

• Automatic anatomical segmentation of input and generated volumes
• Quantitative metrics (e.g., Dice, MAE, PSNR, MS-SSIM)
• Synchronized 2D/3D inspection of predictions
• Exportable uncertainty maps for dose propagation

Integration-focused design

• GPU acceleration when available
• One-click execution of the full workflow
• Based on KonfAI Apps, enabling transparent workflows and versioning
• Future compatibility with IMPACT-Reg for dose accumulation

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We welcome feedback and suggestions from the Slicer team!