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Local rigidity constraints for deformable image registration in CBCT-guided radiotherapy.
Tom J W Draper1, Cornel Zachiu1, Bas W Raaymakers1
1Department of Radiotherapy, UMC Utrecht, Heidelberglaan, CX, The Netherlands.
Medical Physics
|November 8, 2025
Summary
This study introduces a novel framework for deformable image registration (DIR) that uses local rigidity constraints to improve accuracy in adaptive radiotherapy. The method enhances anatomical consistency and motion estimation without significantly increasing computation time.
Area of Science:
- Medical Imaging
- Radiotherapy
- Computational Anatomy
Background:
- Deformable image registration (DIR) is crucial for radiotherapy planning and quality assurance.
- Conventional DIR methods lack tissue-specific biomechanical properties, leading to anatomical implausibility.
- Spatially varying models are needed to improve anatomical consistency in DIR.
Purpose of the Study:
- Introduce a lightweight framework for DIR incorporating local rigidity constraints.
- Improve anatomical consistency and motion estimation accuracy.
- Ensure computational efficiency and compatibility with existing DIR pipelines.
Main Methods:
- Jointly optimize rigid-body transformation and surrounding tissue deformations.
- Evaluate on 57 pelvic CT-CBCT image pairs with annotated landmarks.
- Impose local rigidity constraints on segmented bones (sacrum, hip, femur) and thoracic structures.
Main Results:
- Constrained methods improved landmark alignment, with more cases below 3 mm mean TRE.
- Significantly reduced unphysical deformations in rigid bones, validated by Jacobian analysis.
- Showed improved alignment of thoracic structures with marginal TRE increase in lungs; runtime increased by max 40s.
Conclusions:
- The proposed framework enhances DIR accuracy and anatomical reliability using local rigidity constraints.
- It effectively removes anatomically implausible deformations in rigid structures.
- The method is efficient and suitable for CBCT-guided adaptive radiotherapy.
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