TiO2 nanotube cold cathode X-ray source

In a conventional medical X-ray tube, a tungsten filament is heated to high temperatures in order to extract electrons, which bombarding a metal target in order to generate X-ray photons via bremstrulung radiation mechanisms. The basic principle of the X-ray source has not undergone fundamental changes since Coolidge’s proposal in 1913. However, “hot” thermo-emission cathodes have several important limitations, such as slow response time, high power consumption, high operating temperatures, and random distribution of electron velocities. A potential solution to these limitations is to employ an electron source that can operate in field emission or “cold” cathode mode. In this case, electrons are emitted at room temperatures and have a much shorter response time and an output current that is voltage-controllable. In addition, use of field emission electron sources allows device miniaturization that could lead to more portable and miniature X-ray sources. Although field emission cathodes have been widely used in a variety of vacuum electronic devices, for use in medical X-ray tubes cold emission cathodes have not been successful due to several reasons. One of the biggest limitations is fast degradation due to oxidation of the nanotubes by residual oxygen in a vacuum chamber and poor adhesion to conductive substrates that increases the interface layer’s electrical resistance, leading to heating effects.

 

Titanium dioxide nanotubes (TNTs) is considered a potential field emitter for such x-ray sources for a number of properties: 1) TNT is a natural oxide, so exposure to O2 will not adversely affect its properties; and 2) TNT grows on titanium (Ti) sheet as the latter oxidizes, so a good electrical contact between the TNT film and conductive Ti sheet is intrinsically guaranteed. Another advantage TNT is its fabrication simplicity: thesetype of nanotubes can be grown by electrochemical oxidation (anodization), which requires simple experimental setup, making this method very cost-effective. The equipment needed for TNT growth consists of a container, electrolyte, platinum electrode, and DC supply. In addition, TNT has a lower work function range 3.9-4.5 eV compared to traditional carbon nanotubes, which similar work function has been report range in 5.0-5.3 eV. Also, TNTs, have a higher degree of nanotube array uniformity to ensure narrower electron kinetic energy distribution which can produce a smaller focal spot. Additionally, oxide materials have been proven to be very radiation-tolerant.

A new TiO2 nanotube X-ray tube design could be a significant advance not only in single source X-ray technology, but also in development a distributed source which does not require any moving components. In single source CT X-ray tube motion causes blurring of projection motion, resulting in reconstruction artifacts, and limits the CT system’s detectability.

 

It is important to enhance the cardiac CT system’s temporal and spatial resolutions for accurate diagnosis. Use a distributed multi-beam X-ray source with stationary X-ray tubes offers a solution to this problem. This design places multiple emitters statically around a CT gantry, and when electronically switched, each emitter fires independently, eliminating X-ray source motion and waiting time. Electronic X-ray source switching can occur on the order of single microseconds instead of the tens of milliseconds that are required for mechanical motion in conventional CT.  This reduces the total rotational scan time, which in turn has a large impact on final scan time, as many more usable slices can be acquired per cardiac cycle. Gating image acquisition, either prospective or retrospective, can be used to capture conditions in the heart using limited projections from each cardiac cycle when the heart is moving most slowly, during the end of the diastole.  This requires small, high-intensity X-ray tubes that can be placed close to each other in a CT gantry. Thus, the development of distributed X-ray sources shows great potential for significantly increasing cardiac-CT image quality, and decreasing scan time and patient dose.