Room: Karl Dean Ballroom C
Purpose: CT-MRI registration is commonly used in MR based treatment planning and delivery. However, the multimodality registration often results in distortion of the bony anatomies which may have undesired dosimetric consequences. This study aims to improve multimodality registration accuracy by including a bone rigidity term in the deformable registration cost function.
Methods: The bones were first segmented on the CT images using thresholding. A b-spline deformable registration was performed from CT to MRI using mutual information to measure the similarity. A rigidity term was imposed on the segmented bone volumes to penalize changes in the shape and volume. The bone rigidity regularized registrations using the proposed method are compared to rigid registration, and a b-spline deformable image registration without the rigidity penalty for three sites including the brain, abdomen and pelvis. In addition to visual inspection of the registration quality, the determinant of the spatial Jacobian was used to evaluate the preservation of the bone volumes.
Results: Visually, rigidity regularized deformable registration matches the exterior soft tissue better than the rigid registration, while maintaining better bone spatial integrity compared to the b-spline deformable registration. Using the rigidity penalty in deformable registration significantly reduced the range of the Jacobian determinant from 23.0 to 1.88 across the entire imaging volume, indicating an ability to control large, unfeasible deformations. Within the bone volumes, the average Jacobian determinant was improved from 0.844Â±0.543 to 0.974Â±0.0574. The improved bone volume preservation was not site dependent.
Conclusion: Including a bone rigidity term in the deformable registration significantly improves the MR-CT registration and reduces the confusion of bone-air volumes, which is particularly important for the MR based dose calculation accuracy. Compared to rigid registration, the new framework allows greater freedom of matching the soft tissues.
Funding Support, Disclosures, and Conflict of Interest: NIH U19AI067769 DE-SC0017687 NIH R21CA228160 DE-SC0017057 NIH R44CA183390 NIH R43CA183390 NIH R01CA188300