Flying focal spot (FFS) in cone-beam CT

In the beginning of 2004 medical spiral-CT scanners that acquire up to 64 slices simultaneously became available. Most manufacturers use a straightforward acquisition principle, namely an x-ray focus rotating on a circular path and an opposing cylindrical detector whose rotational center coincides with the x-ray focus. The 64-slice scanner available to us, a Somatom Sensation 64 spiral cone-beam CT scanner (Siemens, Medical Solutions, Forchheim, Germany), makes use of a flying focal spot (FFS) that allows for view-by-view deflections of the focal spot in the rotation direction (αFFS) and in the z-direction (zFFS) with the goal of reducing aliasing artifacts. The FFS feature doubles the sampling density in the radial direction (channel direction, αFFS) and in the longitudinal direction (detector row direction or z-direction, zFFS). The cost of increased radial and azimuthal sampling is a two- or four-fold reduction of azimuthal sampling (angular sampling). To compensate for the potential reduction of azimuthal sampling the scanner simply increases the number of detector read-outs (readings) per rotation by a factor two or four. Then, up to four detector readings contribute to what we define as one view or one projection. A significant reduction of in-plane aliasing and of aliasing in the z-direction can be expected. Especially the latter is of importance to spiral CT scans where aliasing is known to produce so-called windmill artifacts. We have derived and analyzed the optimal focal spot deflection values ∂α and ∂z as they would ideally occur in our scanner. Based upon these we show how image reconstruction can be performed in general. A simulation study showing reconstructions of mathematical phantoms further provides evidence that image quality can be significantly improved with the FFS. Aliasing artifacts, that manifest as streaks emerging from high-contrast objects, and windmill artifacts are reduced by almost an order of magnitude with the FFS compared to a simulation without FFS. Patient images acquired with our 64-slice cone-beam CT scanner support these results.