Non-Rigid Registration
Why Non-Rigid Registration In many applications a rigid transformation is sufficient. (Brain) Other applications: Intra-subject: tissue deformation Inter-subject: anatomical variability across individuals Fast-Moving area: Non-rigid
Registration Framework In terms of L.Brown.(1992) –Feature Space –Transformation –Similarity Measure –Search Strategy (Optimization) Rigid vs. Non-rigid in the framework
Feature Space Geometric landmarks: Points Edges Contours Surfaces, etc. Intensities: Raw pixel values
Transformation
Rigid transformation: 3DOF (2D) 6 DOF (3D) Affine transformation: 12 DOF
Transformation Additional DOF. Second order polynomial-30 DOF Higher order: third-60, fourth-105,fifth-168 Model only global shape changes
Transformation For each pixel (voxel), one 2d(3d) vector to describe local deformation. Parameters of non-rigid >> that of rigid
Similarity Measure Point based ---The distance between features, such as points,curves,or surfaces of corresponding anatomical structure. --- Feature extraction. Voxel based ---Absolute Difference, Sum of squared differences, Cross correlation, or Mutual information
Search Strategy Registration can be formulated as an optimization problem whose goal is to minimize an associated energy or cost function. General form of cost function: C = -C similarity +C deformation
Search Strategy Powell’s direction set method Downhill simplex method Dynamic programming Relaxation matching Combined with Multi-resolution techniques
Registration Scheme
Non-rigid Registration Feature-based –Control Points: TPS –Curve/Edge/Contour –Surface Intensity-based –Elastic model –Viscous fluid model –Others
Thin-plate splines (TPS) Come from Physics: TPS has the property of minimizing the bending energy.
TPS (cont.) Splines based on radial basis functions Surface interpolation of scattered data
Description of the Approach Select the control points in the images. Calculate the coefficients for the TPS. Apply the TPS transformation on the whole image.
Synthetic Images T1 T2
TPS-Results(1)
TPS-Results(2)
Rigid and non-rigid registration Rigid Registration as pre-processing (global alignment) Non-rigid registration for local alignment
Next time Affine-mapping technique