A novel exact fan-beam image reconstruction formula is presented and validated using both mathematical phantom data and clinical data. This algorithm takes the form of the standard ramp filtered backprojection (FBP) algorithm plus local compensation terms. An equal weighting scheme is utilized in this algorithm in order to properly account for redundantly measured projection data. The algorithm has the desirable property of maintaining a mathematically exact result for: the full scan mode (2π), the short scan mode (π+ full fan angle), and the super-short scan mode (less than (π + full fan angle)). Another desirable feature of this algorithm is that it is derivative-free. The derivative-free nature of this algorithm distinguishes it from other exact fan-beam FBP algorithms.
Metal artifacts in many cases significantly limit non-invasive imaging in evaluation of cerebrovascular patients who have undergone prior aneurysm clipping or coiling. The data inconsistency due to the presence of metal produces streaks in the reconstructed CT slices, which then manifest themselves in the coronal and sagittal reprojections most often used to display CT angiographic data. In DSA, no CT reconstructions are performed and the presence of metal only produces a reduction in SNR behind the metal unless misregistration produces artifacts. In this paper, we have begun to investigate a new method to obtain DSA-like images by using a CT scanner. In this approach, sinogram data is obtained from the multi-slice scanner using the same scan parameters before and after contrast injection. These sets of data are registered, subtracted and rebinned to generate radiography-like images. This new method to form DSA-like images from a CT scanner is called Digital Subtraction Topography (DST). Importantly, CT image reconstruction procedure is not performed to obtain DST images. In principle, the disturbing metal artifacts in the CT images do not appear in the DST images. A number of topographic images representing each of the gantry angles are obtained. These images give clinical information at all angles with AP and RL resolution equivalent to that in the CT slices. Resolution in the SI direction is determined by the CT slice thickness, which can be sub millimeter. The conventional CT image reconstruction can also be applied to DST datasets to generate CT DSA images. In the absence of misregistration, the metal artifacts in the reconstructed CT DSA images could be reduced.