The input power density of eroding rotating anode X-ray sources restricts the achievable spatial image resolution in X-ray systems, especially for medical computed tomography (CT). The development of anodes that sustain higher input power density has stalled in recent decades, despite substantial investment and sophisticated material analysis. The grain structure of the conversion layer, typically sintered and forged tungsten/rhenium, erodes during tens of millions of thermal cycles. Anodes of high-performance tubes are under extreme thermomechanical stress and rotate near angular burst velocities. To overcome this challenge, we propose a paradigm shift using a stream of very fast moving tungsten microparticles. Volume heating, twice the mass heat capacity and much shorter residence times under electron impact may render an order of magnitude improvement of the focal spot input power density. This corresponds to a threefold improvement of the source MTF in each orthogonal direction for a standard focal spot. We made sure by Monte-Carlo simulation, that the new microparticle target would not charge negatively upon electron impact in the tube voltage range of medical imaging. Hence, it would be electrically compatible with the spectral requirements. We propose technical implementations. We further suggest a source of high intensity and highly monochromatic bremsstrahlung based on microparticle technology that may replace synchrotrons for a variety of experiments. After thorough simulations we believe that the remaining engineering problems, such as separating the microparticle space from the cathode region, storage, acceleration, capturing, cooling, and recycling, can be solved in the near future.

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.

Background

The permitted input power density of rotating anode x-ray sources is limited by the performance of available target materials. The commonly used simplified formulas for the focal spot surface temperature ignore the tube voltage despite its variation in clinical practice. Improved modeling of electron transport and target erosion, as proposed in this work, improves the prediction of x-ray output degradation by target erosion, the absolute x-ray dose output and the quality of diagnostic imaging and orthovolt cancer therapy for a wide range of technique factors.

Purpose

Improved modeling of electronic power absorption to include volume effects and surface erosion, to improve the understanding of x-ray output degradation, enhance the reliability of x-ray tubes, and safely widen their fields of use.

Methods

We combine Monte Carlo electron transport simulations, coupled thermoelasticity finite element modelling, erosion-induced surface granularity, and the temperature dependence of thermophysical and thermomechanical target properties. A semi-empirical thermomechanical criterion is proposed to predict the target erosion. We simulate the absorbed electronic power of an eroded tungsten-rhenium target, mimicked by a flat target topped with a monolayer of spheres, and compare with a pristine target.

Results

The absorbed electronic power and with it the conversion efficiency varies with tube voltage and the state of erosion. With reference to 80 kV (100%), the absorption of a severely eroded relative to a pristine target is 105% (30 kV), 99% (100 kV), 97% (120 kV), 96% (150 kV), 93% (200 kV), 87% (250 kV), and 79% (300 kV). We show that, although the simplistic Müller–Oosterkamp model of surface heating underestimates the benefit of higher tube voltages relative to operation at 80 kV, the error is limited to below −6% for 30 kV (reduction advised) and +13% for 300 kV (input power increase permitted).

Conclusions

Correcting the x-ray conversion efficiency of eroded target material, that is typically not accessible by measuring the tube current, may imply corrections to existing x-ray dose calculations. The relative increase of the allowable anode input power of rotating anode x-ray tubes with increasing tube voltage is substantially smaller than predicted by volume heating models that only rely on the focal spot surface temperature. The widely used voltage agnostic Müller–Oosterkamp formalism fails to predict the tube voltage dependency of the surface temperature of rotating anode targets, ignores the temperature dependency of the thermal diffusivity of tungsten-rhenium, and the granularity of the material. Nevertheless, we show theoretically why, backed by experience, the practical use of the Müller–Oosterkamp formalism is justified in medical imaging and provides a basis for comparison with new microparticle based targets. The reason for this surprising finding is that voltage dependent material erosion must be primarily considered as a precursor of thermal runaway effects.

The spatiotemporal resolution of diagnostic X-ray images is limited by the erosion and rupture of conventional stationary and rotating anodes of X-ray tubes from extreme density of input power and thermal cycling of the anode material. Conversely, detector technology has developed rapidly. Finer detector pixels demand improved output from brilliant keV-type X-ray sources with smaller X-ray focal spots than today and would be available to improve the efficacy of medical imaging. In addition, novel cancer therapy demands for greatly improved output from X-ray sources. However, since its advent in 1929, the technology of high-output compact X-ray tubes has relied upon focused electrons hitting a spinning rigid rotating anode; a technology that, despite of substantial investment in material technology, has become the primary bottleneck of further improvement. In the current study, an alternative target concept employing a stream of fast discrete metallic microparticles that intersect with the electron beam is explored by simulations that cover the most critical uncertainties. The concept is expected to have far-reaching impact in diagnostic imaging, radiation cancer therapy and non-destructive testing. We outline technical implementations that may become the basis of future X-ray source developments based on the suggested paradigm shift.

As an alternative to rigid anodes, a novel concept of X-ray targets consisting of a stream or a multitude of streams of fast tungsten microparticles has recently been proposed. Low-density microparticle streams resemble thin targets with nearly constant intensity distribution over a wide range of photon energies, abruptly terminating at the Duane–Hunt limit of maximum photon energy instead of falling off smoothly. According to our simulations, fast microparticles outperform classical rigid targets and enable extremely high electronic input power density and X-ray output. This opens new possibilities for generating high-intensity, nearly monochromatic X-rays. Such keV-type X-ray sources could replace expensive electron synchrotrons in appropriate applications. Furthermore, for sufficiently thin microparticle streams, the output X-ray spectra are functions of particle size, allowing modulation of the mean photon energy. We simulated the spectral response of tungsten microparticles using Monte Carlo methods and confirmed the validity of our new concept to generate near-monochrome spectra and high intensity with microparticle-based X-ray sources, outperforming classical X-ray tubes. Furthermore, we confirm a weak size dependence of the mean energies of filtered X-rays. We complement previous results highlighting the advantages of microparticle-based X-ray targets and aim at the implementation of the new concept in the future.