Comparison of gated and dynamic cone-beam CT reconstruction methods

In the past few years, there has been an increasing interest of the radiation oncology community in cone-beam computed tomography (CT) scanners integrated into the gantry of medical linear accelerators. This imaging device allows to acquire CT images of the patient in treatment position. Unfortunately, like for other CT scanners, the respiratory motion of the patient during acquisition causes artifacts such as blur, streaks and bands. The current methods available to correct these artifacts can be categorized in 3 classes, depending if they use (1) no motion information, (2) only simplified motion information like a respiratory signal, in which case the method is called respiratory-correlated or gated imaging [2, 5, 7, 10], or (3) precise motion information like a dense vector field or a B-spline model, in which case the method is called dynamic reconstruction [1, 4, 6]. The respiratory-correlated or gated technique consists in selecting a subset of the cone-beam projections, on the basis of their position in the respiratory cycle, and to use these projections in a conventional static reconstruction algorithm to obtain an image of a respiratory instant. On the contrary, dynamic reconstruction uses all the projections, but the reconstruction algorithm is modified to compensate for the motion of the thorax. Both methods have been studied in the past but, to our knowledge, they have not been directly compared. In this study, we compared the gated and dynamic CT reconstructions of a moving mechanical phantom from cone-beam projections acquired using a linear accelerator with a kV cone-beam scanner. The motion was sinusoidal in the cranio-caudal direction only. This made it easy to extract both the respiratory signal and the 4D motion, which would not have been the case for real patient acquisitions. The resulting CT images were compared quantitatively, by reference to the static CT reconstruction from the set of cone-beam projections obtained with the fixed platform.