Two tubes are better than one, if you ask Siemens Medical Solutions. Simultaneous x-ray beams rotating every 330 msec on the Somatom Definition CT gantry stop any motion in its tracks, even that of the heart. Now Siemens is taking the concept a step further.

Early results, presented at the European Congress of Radiology, indicate what can be accomplished when the two beams are pumped at two different energy levels: 80 kVp and 140 kVp.

Bone removal algorithms, employed since earlier this year at the Mayo Clinic in Rochester, MN, automatically remove bone in images of the head and elsewhere to selectively reveal blood vessels, said Cynthia H. McCollough, Ph.D., an associate professor of radiologic physics at Mayo.

A plaque removal algorithm draws to crystal clarity the lumen of severely calcified peripheral vasculature, and a perfused blood volume algorithm can demonstrate perfusion deficits in the lung secondary to a pulmonary embolus.

The trick is found in the absorption characteristics of different materials in the body, as these algorithms mathematically decompose materials, McCollough said. Fat, for example, produces a relatively low CT number because of its lesser attenuation of x-rays. Calcium absorbs more energy, producing a higher CT number. Iodine absorbs the most energy and is associated with the highest CT number. These materials can be color-coded or subtracted using these numbers.

"We are particularly enthusiastic about bone removal in peripheral runoffs, where the bone is removed so we can focus on the vasculature without operator intervention," she said. "By taking it a step further, we can remove even small, discrete plaques."

X-ray dose and overall exam time might be reduced with a tool that produces a "virtual noncontrast" image from data obtained after the administration of a contrast medium. The image is created by processing two data sets, each at the different energy levels. X-rays from one are passed through the iodinated medium, and from the other are absorbed.

Subtracting data associated with iodine produces the equivalent of an image that would have otherwise been possible only in the absence of the contrast medium. This allows the comparison of contrast and virtual noncontrast images. Alternatively, an image might even be produced from a mixture of data to highlight the iodinated regions, she said.

"With dual-energy CT, we have this additional data in our back pocket," McCollough said.

The possibilities of how such data might be used are wide open. Material decomposition might allow novel analyses, she said. Preliminary studies at Mayo have determined that kidney stones composed of uric acid might be distinguished from others, just as tendons and cartilage might be selectively visualized.

Willi Kalendar, Ph.D., the wunderherr behind much of Siemens' CT development from his position as director of the Institute of Medical Physics at the University of Erlangen in Germany, explained how such things are possible in the context of nuances surrounding dual-source CT, particularly the need to make the second detector substantially smaller than the first. Although vendors have upped the ante in a decade-long slice war, Siemens is not likely to add any more x-ray beams to the Definition. There simply is not enough room.

"I see many problems there," he said. "I don't want to motivate them to go further. I think it would be in vain."