Nanotube filaments offer advantages in x-ray tube technology

Product could be ready for market within two years

The core technology of radiography and CT, the carbon filament in the x-ray tube, is as old as Edison's light bulb. But that could soon change.

A new solution may be in the offing based on "nanotubes"--tiny hollow fibers--created from carbon. Just like solid filaments built into conventional x-ray tubes, these carbon fibers can be used to generate the electrons needed to create diagnostic x-rays, according to Otto Z. Zhou, Ph.D., developer of the technology. But they have several important advantages that might result in lower cost, lower operating temperatures, and more compact tubes.

The prototype is being built at Applied Nanotechnologies, a corporate spin-off founded on research done at the Center for Nanoscale Materials at the University of North Carolina at Chapel Hill. This

electron-emitting nanotechnology might be used in a range of different products. For the time being, x-ray applications are getting most of the attention.

The company has attracted the interest of two x-ray tube manufacturers that could shepherd this technology along, according to Zhou, an associate professor of physics and materials sciences at UNC and cofounder of the company charged with its development. Collaborations now under way could lead to a product within a year or two, he said. How and where such a product will be marketed, however, is not known. The companies expressing the most interest, companies whose names cannot be publicly disclosed, are not among the largest makers of medical x-ray tubes--and for good reason.

"Medium-size companies are more willing to take a risk," he said.

If the gamble succeeds, these companies could gain access to a technology that operates at room temperature yet generates enough electrons to produce x-rays in a volume great enough to create diagnostic radiographs or CT images. While a cool filament will reduce the heat dissipation problems faced by tube manufacturers, it will not eliminate the problem, Zhou said. Fewer than 1% of the electrons striking the anode target of the x-ray tube are converted into x-rays; the rest turn into heat. A lesser heat burden placed on the filament, however, could extend the overall life of the tube by allowing the filament to last longer. The use of carbon nanotubes may reduce the overall size of the tube or even the x-ray device itself, and the equipment might use less electricity. Possibly most important, the tube might be turned on and off very quickly.

"The fact that we can switch it on and off very easily means that the total exposure times will be much shorter than is possible with the x-ray tubes today," Zhou said.

The advantages provided by carbon nanotubes come primarily from their extraordinary structure. Each nanotube is made up of a single layer of carbon atoms that measures only a billionth of a meter in diameter. They were first created a decade ago and were soon shown capable of producing electrons. Until now, however, these nanotubes did not produce sufficient quantities of electrons to create radiographs. Nanotubes coming from Applied Nanotechnologies do. Zhou has produced radiographs of fish and even human hands, images that he considers comparable to ones taken using standard radiography equipment.

"We think our images eventually will be clearer than conventional ones since we have a more pointed, tunable source of electrons," he said.

Medical applications are the obvious uses but not the only ones. A lightweight, compact source of x-rays could serve industrial applications in machines built to assess aircraft wings, for example, or to scan baggage at airports. The technology might also be adapted for use in batteries to store electrical charges more efficiently. The carbon nanotubes made by Zhou and colleagues might also be turned into high-strength composite materials.

Zhou has been careful to apply for or receive patents on the techniques for making these materials or on the products themselves. Applied Nanotechnologies is charged with developing them into futuristic applications with some $4 million in seed money raised last year from just two benefactors.

"Not going to venture capitalists has given us more flexibility," said Zhou, who now serves as chairman of the board.

When it comes to developing new products, particularly in medical imaging, flexibility is a good thing to have.