2010-04-16

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:

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