Rotating anode x-ray tube technology at the limit

The input power density of rotating anode x-ray sources and hence the spatial image resolution of the x-ray system must be fundamentally restricted due to the erosion of anode material. The efficacy of computed tomography would benefit from much smaller x-ray focal spots with equal or increased photon output. A switch to carbon fiber reinforced rotor members that may enable higher rotor velocity has been suggested, or increasing the tube voltage for deeper implantation of electronic input power. Alternatively, we are proposing a new fast moving tungsten microparticle target that avoids focal track erosion, offers high mass heat capacity from increased temperature swing and reduced material residence time in the electron beam. This novel technology concept promises to eliminate the bottleneck and allow for an order of magnitude improvement of the focal spot input power density. However, before investing in implementation the ultimate limitations of current technology should be better known than currently. To gain knowledge, we improved the modeling of electron transport and target erosion of rotary anodes. We infer a criticality parameter that enables predicting the risk of anode erosion for a wide range of technique factors and focal spot sizes based on a few reference life cycle tests. In conclusion, despite the deficits of assumptions made in the classic Müller-Oosterkamp theory that ignores tube voltage, the derived specifications of commercial x-ray tubes are justified. Limited by anode erosion, the gain of permitted power density with increasing tube voltage is smaller than predicted by some alternative volume heating models. We further discovered the necessity to introduce a correction for calculations of the applied patient x-ray dose and pointed to the related error in the standards for radiation safety.