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  • A small and speedy x-ray source

Oct 2009
Hank Hogan,

CHAPEL HILL, N.C. – Smaller x-ray machines that offer better resolution and faster imaging than current technology can produce are on the horizon if research at the University of North Carolina pans out.

According to physics professor Otto Zhou, the key to devising this next-generation x-ray technology is the use of carbon nanotubes for an electron source. The new equipment could find applications in a number of areas, he said. “In a few years we could have nanotube-based x-ray imaging devices in hospitals for clinical use and at airports for security inspections.”

Otto Zhou and his colleagues at the University of North Carolina have generated x-rays using carbon nanotubes as the electron source. Images courtesy of University of North Carolina.

Conventional x-ray sources are based on a century-old technique in which a tungsten filament sitting in a vacuum tube is heated to 1000 °C. As a result, the filament releases electrons that subsequently are accelerated under an applied voltage and then slammed into a metallic anode, creating x-rays.

The North Carolina group, which includes professors Jianping Lu and Sha X. Chang, M.D., has replaced this hot source with a cold one constructed out of carbon nanotubes. The structures have sharp tips that make it possible to extract electrons from them using a relatively low gate electrode voltage. These electrons can then be accelerated to create x-rays in much the same manner as is done in conventional systems.

Zhou noted that demonstrating x-ray emission using a carbon nanotube source is not that difficult. What is challenging – and what the group spent a great deal of effort on – is creating carbon nanotube sources with a high emission current and high voltage stability. Both attributes are required in a commercial system because devices need a high flux and a long lifetime to be a viable alternative to the traditional approach.

Zhou reported that the researchers have succeeded in developing a suitable fabrication process and have applied this to prototype commercial devices. “Our team invented the spatially distributed multibeam field emission x-ray, which is the basis for the current development on stationary imaging systems for medical and security applications.”

Using its new x-ray generation technique, Zhou's group imaged a mouse, revealing calcified plaque along the aorta.

Nanotubes allow smaller x-ray devices to be built, which can be important for some applications. Another advantage of the new approach is that it enables the electron source, and the x-rays, to be turned on and off very quickly. A traditional system does this using mechanical shutters or a grid, but the cycling rate is slower than what can be done using nanotubes.

This speed allows for multiple beams in a system to be switched on and off in a rapid sequence, making it possible to image without the blurring caused by breathing or heartbeat and, thus, boosting the quality. The group reported on some of these imaging results in Physics in Medicine and Biology, published in April 2009.

A clinical test of a first-generation nanotube-based imaging system for high-speed image-guided radiotherapy is planned for the end of the year. The system was developed by Siemens and XinRay Systems LLC, a joint venture between Siemens and a University of North Carolina startup company, Xintech Inc., based in Research Triangle Park.

Technique that defocuses activity from surrounding planes by means of the relative motions at the point of interest.
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