Respiratory motion estimation from slowly rotating xray projections:Theory and simulation. Med Phys

As radiotherapy has become increasingly conformal, ge-ometric uncertainties caused by breathing and organ motion have become an important issue. Accurate motion estimates may lead to improved treatment planning and dose calcula-tion in radiation therapy. However, respiratory motion is difcult to study by conventional X-ray CT imaging since object motion causes inconsistent projection views leading to artifacts in reconstructed images. We propose to estimate the parameters of a nonrigid mo-tion model from a set of projection views of the thorax that are acquired using a slowly rotating cone-beam CT scanner, such as a radiotherapy simulator. We use a conventionally reconstructed 3D thorax image, acquired by breath-hold CT, as a reference volume. We represent respiratory motion us-ing a exible parametric nonrigid motion model based on B-splines. The motion parameters are estimated by opti-mizing a regularized cost function that includes the squared error between the measured projection views and the re-projections of the deformed reference image. Preliminary 2D simulation results show that there is good agreement be-tween the estimated motion and the true motion. 1