Estimation of 6-Degree-of-Freedom (6-DOF) Rigid-Body Patient Motion from Projection Data by the Principal-Axes Method in Iterative Reconstruction

We developed a method for estimating and compensating rigid-body translation and rotation from scatter-and-attenuation-compensated projection data in iterative reconstruction when multiple projection angles are acquired at the same time. During reconstruction, both the non-attenuated and attenuated line-integrals are calculated. Their ratios are then multiplied times the scatter-corrected projection data to obtain estimated scatter-and-attenuation-compensated projection data. At the end of each iteration, the sets of compensated projection data for the angles acquired at the same time are then employed to calculate the center-of-mass and the inertia tensor. These are used to estimate the location and orientation of the patient by the principle-axes method. The estimated motion is applied in the next iteration to reposition the estimated slices and attenuation map in the projector and back-projector to match the pose of the patient at time the projections were acquired. To evaluate our method, we simulated an acquisition of the MCAT phantom with a 3-head SPECT system and imaged the Data Spectrum anthropomorphic phantom on a 3-head IRIX SPECT system. In simulations the phantom translated and rotated by the same amount 9 times. A numerical projector modeling the motion, attenuation, and distance-dependent blurring was used to generate the projection data. Poisson noise was added and 30 noise-realizations were generated. In the experiment with the anthropomorphic phantom, 4 360-degree acquisitions were performed with the phantom translated or rotated beforehand. A motion-present dataset was made by mixing the 4 acquisitions.