C-arm systems may be used as front ends for cone-beam CT. The resulting image quality is affected by several factors, including the source trajectory, the reconstruction algorithm, and the accuracy of the data. The standard source trajectory is a circular arc spanning a little more than 180 degrees. However, since a planar source trajectory satisfies Tuy's completeness condition only within a subset of the source plane, the resulting images are bound to exhibit "cone-beam artifacts" off the source plane. The cure consists in using a source trajectory that satisfies Tuy's completeness condition everywhere within the volume of interest. Such a source trajectory must be non-planar. To keep the scan time short, the source
trajectory should also consist of a single, smooth segment. A favorable source trajectory of this kind is a curve known as spherical spiral. We implemented a spherical spiral on a laboratory
C-arm system, along with a standard circular arc.
An anthropomorphic head phantom was scanned using both source trajectories and otherwise identical scan parameters.
Images were reconstructed using a short scan version of the FDK algorithm (circular arc) and the cone-beam Fourierfiltered
backprojection (CBFFBP) algorithm presented earlier. Images obtained with the circular arc showed cone-beam artifacts. Images obtained with the spherical spiral did not. The results also demonstrate the good performance of the CBFFBP algorithm.
The tomographic reconstruction of the beating heart requires dedicated methods. One possibility is gated
reconstruction, where only data corresponding to a certain motion state are incorporated. Another one is motioncompensated
reconstruction with a pre-computed motion vector field, which requires a preceding estimation of
the motion. Here, results of a new approach are presented: simultaneous reconstruction of a three-dimensional
object and its motion over time, yielding a fully four-dimensional representation. The object motion is modeled
by a time-dependent elastic transformation. The reconstruction is carried out with an iterative gradient-descent
algorithm which simultaneously optimizes the three-dimensional image and the motion parameters. The method
was tested on a simulated rotational X-ray acquisition of a dynamic coronary artery phantom, acquired on a
C-arm system with a slowly rotating C-arm. Accurate reconstruction of both absorption coefficient and motion
could be achieved. First results from experiments on clinical rotational X-ray coronary angiography data are
shown. The resulting reconstructions enable the analysis of both static properties, such as vessel geometry and
cross-sectional areas, and dynamic properties, like magnitude, speed, and synchrony of motion during the cardiac
cycle.
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