Geometric calibration method for multiple heads cone-beam SPECT system

The authors present a method for performing geometric calibration of cone beam systems with multiple heads, each having its own orientation. In tomography, for each head, the relative position of the rotation axis and of the collimator does not change during the acquisition. The authors thus separate the parameters into intrinsic parameters and extrinsic parameters. The intrinsic parameters describe the detection system geometry and the extrinsic parameters the position of the detection system with respect to the rotation axis. Intrinsic parameters are measured only on the collimator. Extrinsic parameters must be estimated each time the acquisition geometry is modified. Extrinsic parameters are estimated by minimizing distances between measured point source projection positions and computed positions with the estimated parameters. Extrinsic parameters are almost not correlated because intrinsic parameters are precisely known. Thus one can use any simple least square error minimization method to perform this estimation. Giving a fix value to the distance between the point source and the rotation axis in the estimation process, one ensures the coherence of the extrinsic parameters between each head. The authors show with this calibration method that the Full Width at Half Maximum measured on point sources is very close to the theoretical one and remains unchanged when one uses more than one head. The authors present reconstructions on point sources and on Jaszczak phantom that show the capabilities of this method