Projects:ConformalFlatteningRegistration

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Objective:

The goal of this project is for better visualizing and computation of neural activity from fMRI brain imagery. Also, with this technique, shapes can be mapped to shperes for shape analysis, registration or other purposes. Our technique is based on conformal mappings which map genus-zero surface: in fmri case cortical or other surfaces, onto a sphere in an angle preserving manner.

The explicit transform is obtained by solving a partial differential equation. Such transform will map the original surface to a plane(flattening) and then one can use classic stereographic transformation to map the plane to a sphere.

The process of the algorithm is brifly given below:

1. The conformal mapping f is defined on the originla surface Σ as \triangle f = (\frac{\partial}{\partial u} - i\frac{\partial}{\partial v})\delta_p. In that u and v are the conformal coordinates defined on the surface and the δ_p is a Dirac function whose value is non-zero only at point p. By solving this partial differential equation the mapping f can be obtained.
2. To solve that equation on the discrete mesh representation of the surface, finite element method(FEM) is used. The problem is turned to solving a linear system D'x = b. Since b is complex vector, the real and imaginary parts of the mapping f can be calculated separately by two linear system.
3. Having the mapping f, the original surface can be mapped to a plane.
4. Further, the plane can be mapped to a sphere by the stereographic projection.

Also, in the work of Multiscale Shape Segmentation, conformal flattening is used as the first step for remeshing the surface.

Progress:

This algorithm is now written into SandBox as itkConformalFlatteningFilter. More test is being made and then hopefully can be integrated into the ITK CVS repository.

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The flattening result for the real brain surface has some pseudo-coloring problem making it look not that nice. I'll improve the vtk display ASAP I come back from MICCAI.

References:

[1] S. Angenent, S. Haker, A. Tannenbaum, and R. Kikinis. On the Laplace-Beltrami operator and brain surface flattening. IEEE Trans. on Medical Imaging, Vol. 18, pp. 700-711, 1999
[2] Y. Gao, J. Melonakos, and A. Tannenbaum. Conformal Falttening ITK Filter. to appear in In 2006 MICCAI Open-source Workshop

Links

• Georgia Tech Algorithms
• NA-MIC Collaborations