GEOMETRIC CALIBRATION METHOD FOR MULTIPLE-HEAD CONE-BEAM SPECT SYSTEM

A method is presented for estimating the geometrical parameters of con e beam systems with multiple heads, each head having its own orientati on, In tomography, for each head, the relative position of the rotatio n axis and of the collimator do not change during the data acquisition . We thus can separate the parameters into intrinsic parameters and ex trinsic parameters. The intrinsic parameters describe the detection sy stem geometry and the extrinsic parameters the position of the detecti on system with respect to the rotation axis. Intrinsic parameters are measured directly, once for each collimator. Extrinsic parameters must be estimated each time the acquisition geometry is modified. Extrinsi c parameters are estimated by minimizing the distances between the mea sured position of a point source projection and the computed position obtained using the estimated extrinsic parameters. The main advantage of this method is that the extrinsic parameters are only weakly correl ated when the intrinsic parameters are known. Thus we can use any simp le least square error minimization method to perform the estimation of the extrinsic parameters. Giving a fixed value to the distance betwee n the point source and the rotation axis in the estimation process, en sures the coherence of the extrinsic parameters between each head. We show that with this calibration method, the full width at half maximum measured with point sources is very close to the theoretical one, and remains almost unchanged when more than one head is used. Simulation results and reconstructions on a Jaszczak phantom are presented that s how the capabilities of this method.