Invoking and Interaction with the Viewers
Figure 1. Invoking the Point-Based Registration Tool.
A Brief Description of the Robust Point Matching Framework
In Robust Point Matching (RPM), the correspondence and transformation are determined together iteratively in a robust manner accounting for outlier points which do not have a corresponding point in the other image. For more details see Duncan JS, Papademetris X, Yang J, Jackowski M, Zeng X, Staib LH. "Geometric strategies for neuroanatomic analysis from MRI." Neuroimage. 2004:23 Suppl 1:S34-45.The Main Point-Based Registration Control
Figure 2. The Point-Based Registration Tool.
The point based-registration control is divided into five parts:
- The "Common" controls frame (A in Figure 2) which defines the two surfaces to be registered, and some global parameters.
- The "parameters" controls frame (B1/B2 in Figure 2) which defines the specific parameters for the linear registration (B1) and the non-linear registration (B2).
- The "top-frame" controls frame (C in Figure 2) which has shortcuts to the two surface controls, and for closing the window.
- The "Viewer" frame containing a separate viewer for monitoring the progress of the registration (C in Figure 2).
- Finally, the update frame, at the bottom right (E in Figure 2) which contains settings for how often to update the display and how to display the surfaces.
An example: Computing a Linear (rigid registration)
The procedure to compute a linear registration between two surfaces is as follows:- Load the reference surface in the surface control of the Reference Viewer. Note the index of the surface it is loaded in – this is 1 by default.
- Similarly, load the target surface in the surface control of the Target Viewer, and note its index.
- Open the point-based registration controls.
- In the "Common Controls" (A), select the indices (most likely 1 and 1) for the reference and transform (target) surface.
- Press "Show Surfaces" in (C) to show the surfaces and adjust the viewer to your liking. If no surfaces are visible, press the "Va" button in frame (D) to reset the display.
- In Frame (B1), set the appropriate parameters. In particular, select the Linear tab and the "Rigid" transformation mode (other choices include Similarity and Affine). Set the desired number of points to use, in Max Landmarks. The only other parameters to touch are the "Initial Temperature" and "Final Temperature" which should be set to reflect (i) the maximum distance between the two surfaces prior to registration (initial temperature) and (ii) the point sampling distance of the surfaces, e.g. how closely sampled the points are (final temperature).
- Press the Start RPM button at the bottom of frame B1 to start the registration.
A second example: Computing a Non-Linear (nonrigid registration)
This is similar to the Linear case, with the following changes:- A non-linear registration is often initialized by a linear registration. Run the linear registration first and verify that it is successful.
- In step 6 above, there are a few more parameters to adjust. In
particular:
- First, select the Nonlinear tab. Enable Use Initial Transformation to use the last computed linear transformation (or the currently selected transformation in the transformation control of the Registration/Overlay tool.
- The Initial Temperature needs to be set to account for the distance at the end of the linear step.
- The Initial Control Spacing and Final Control Spacing reflect the values for the spacing of the tensor b-spline grid transformation that is to be computed. A rule of thumb is to select the final value to give the desired accuracy/computational cost and multiply this by 2 to set the initial value.
- The Initial Smoothness and Final Smoothness determine the value of the regularization weight at the start and end of the process. Most registrations will start with a relatively high smoothness to avoid local minima and progressively relax this to get improved accuracy.
- The computational cost is significantly higher!