Calibration for C-arm Reconstruction

Due to their open design and mechanical instability, C-arm imaging systems acquire projections along non-ideal scan trajectories. Volume reconstruction from C-arm projections requires a very precise knowledge of the imaging geometry. These knowledge can be gained by calibration.

"Fortunately, the C-arm motion is highly reproducible over a period of six months (Fahrig and Holdsworth 2000), and so the calibration can be done off-line with updating only needed about twice per year in clinical environments."

Calibration is a process to determine the transformation between 3D world coordinate and 2D image coordinate.

xw = P * xi

Here xw and xi homogenous coordincate of world system and image system. P is the projection matrix that need to be computed by calibration.

The calibration method can be off-line or on-line:

• On-line calibration can real-time compute the projection matrix, but it requires more complicated image processing algorithm. Till now, online calibration is still under research in the academic community.
• Off-line calibration is an standard approach in industry and clinic practices. A calibration phantoms of known geometry is used. 

"The measured 2D location of the spheres in the cone-beam projection together with their known 3D positions then allows us to obtain expressions for all calibration parameters."

Reference:

• http://www5.informatik.uni-erlangen.de/Forschung/Publikationen/2007/Hoppe07-GCO.pdf
• http://deposit.d-nb.de/cgi-bin/dokserv?idn=994417551&dok_var=d1&dok_ext=pdf&filename=994417551.pdf
• http://www.cs.huji.ac.il/~josko/papers/cas-02-miccai-calibration.pdf
• http://spie.org/x648.html?product_id=479945