Temporal resolution and the evaluation of candidate algorithms for four-dimensional CT.

The four-dimensional computed tomography ("4D-CT") with area detector has been developed for dynamic volumetric imaging with large longitudinal coverage. In this paper one of the key technologies for 4D-CT development is discussed: Image reconstruction algorithm with high temporal resolution. All of the cone-beam algorithms investigated previously assume that the object is stationary. In this paper a new class of cone-beam problem is addressed: a dynamic volumetric (4-D) imaging. A continuously rotating circular (stationary couch) scanning is employed, and then, a generalized version of the well-known Feldkamp algorithm with the following three steps is performed: (1) applying a weighting function (along the time axis) to projection data, (2) filtering the weighted data along the detector row direction, (3) cone-beam backprojecting of the filtered data along the corresponding x-ray path. The weighting function controls the time center, the temporal resolution, and the image quality. Four weighting functions developed for fan-beam reconstruction were applied to the first step: (a) a constant weight fixed at 0.5 (FS-FDK), (b) feathering both edges of the (time) window (OS-FDK), (c) Parker's weight for a half-scan (HF-FDK), and (d) an extended Parker's weight, which allows us to use a larger range of projection data up to one rotation (NHS-FDK). We evaluated them in terms of temporal resolution, image noise, and image quality. Also, the cause of the artifact has been investigated. The temporal resolution of NHF-FDK equals that of HS-FDK, which is half of the one rotation period. For the moving object, NHS-FDK offers the best image quality. The images with FS-FDK are degraded by streak artifacts; HS-FDK provides poor image quality with good temporal resolution; and images by OS-FDK are blurred due to insufficient temporal resolution. The cause of the artifact was found as an inconsistency of projection data due to object motion (in FS-FDK) and lost 3-D-Radon data caused by applying Parker's weight (in HS-FDK). A hand toy was employed for the preliminary evaluation of dynamic volumetric imaging with the real 256-slice scanner. In an overall evaluation, NHS-FDK provides the stable and the sufficient image quality both with moving and stationary objects.