Dual-source CT promises faster scans and lower dose, more data

The release of Siemens' Somatom Definition may forever change CT. The scanner, unveiled just days before the RSNA meeting and featured at the Siemens booth on the exhibit floor, combines two 64-slice scanners in one, incorporating two x-ray tubes and two detectors. Together, they generate 128 slices every 330 msec. The new product is distinguished less by number of slices or even the novelty of two imaging chains, however, that by what the technology can do: double temporal resolution and cut acquisition time in half.

Mounted at right angles to one another, the tubes and detectors generate a slice in 83 msec, half the time needed by a conventional 64-slice scanner. This speed frees the system from motion artifact when imaging the heart. The new scanner may also offer advantages in peripheral CT angiography as well as imaging of obese, pediatric, and trauma patients.

"In the beginning, probably, a lot of people will say, 'Oh, come on! Not another gadget,'" said Dr. Axel Kuettner, a specialist in radiology and cardiovascular imaging at the Friedrich-Alexander University of Erlangen Nuremberg in Germany. "But as soon as they discover its potential, dual-source CT is going to catch fire."

The reason is its ability to cut not only acquisition time but dose, he said. This seems counterintuitive, as the use of two x-ray tubes would presumably cause dose to go up, not down. But because scan time is minimized, the Somatom Definition actually cuts dose.

"We are estimating a reduction in x-ray exposure of about 40% compared with 64-slice CT," he said.

Kuettner was the first physician in the world to operate a Somatom Definition. He will be joined by physicians at 10 more sites by spring 2006. Siemens expects the number of installations to reach as many as 100 by year's end.

Users will pay a hefty price of between $2.3 million and $2.5 million. The price could be worth it.

"We can get incredible scan speed, because the Somatom Definition can use power reserves up to 160 kW, driven by two 80-kW tubes and generators," said Bernd Ohnesorge, Siemens' vice president of CT. "We can use these power reserves to scan always at top speed and get high image quality independent of patient size and weight."

This capability could be especially important for the growing obese population in the U.S., which presents special challenges for CT evaluation. Dual-source CT offers a way to crank up the power and improve image quality without overexposing the patient.

"With the use of this second tube, you could dramatically reduce the noise in the images without exceeding the safe level of dose application," Kuettner said.

The Somatom Definition may also be valuable in the ER. It could serve as a one-stop shop, handling a broad range of applications from trauma to the assessment of patients with abdominal or chest pain, including patients suspected of myocardial infarction. Kuettner notes that 64-slice CT is more than fast enough to handle whatever trauma scan is needed, but the flexibility in power provided by dual-source imaging would offer an advantage.

"You think of patients coming in with braces or medical gadgets attached, and they present a difficult scan situation," he said. "Definitely, the power reserve (on the Somatom Definition) would be a plus."

The decreased dose delivered by the Somatom Definition makes it a natural for pediatrics. Radiologists typically turn down the power when imaging children, and some diagnostic information can be lost in the process. Dual-energy CT offers the chance to keep the dose low and still get high-quality images.

In cardiological applications, patients can be scanned without the need for beta blockers. These drugs are typically used to slow the heart rate during CT scans to reduce motion artifact. This practice, however, can lead to artifacts of its own, potentially masking the very abnormalities physicians are trying to find.

Rather than decreasing heart rate, physicians would like to increase it, as they do during stress echocardiography. In the case of a midgrade stenosis of a coronary artery, impaired cardiac function may appear only under stress. The most advanced single-source scanner cannot visualize such impairments, but the Somatom Definition is fast enough to stop the heart in midbeat, according to Kuettner.

"I see cardiac stress CT as a future potential of this system," he said.

Such rapid acquisition is possible because of the Somatom Definition's unique design. Whereas a single-source scanner acquires a slice after a 180 degrees rotation, the new scanner, with two imaging chains mounted at right angles, acquires the same information with a 90 degrees turn.

The short scan time is achieved without the use of segmentation algorithms that compile data from sequential rotations. If Siemens were to use such algorithms, temporal resolution would double again to about 42 msec. Ohnesorge does not consider such mathematical manipulations necessary.

"It has been shown in EBT (electron-beam tomography) that if you are able to go below 100 msec, there is no motion artifact in coronary artery imaging," he said.

Somatom Definition software developments include syngo Circulation, which automatically tracks coronary vessels and performs cardiac functional analyses. Automatic vessel tracking is also integrated into syngo InSpace, which supports other CTA exams.

These capabilities barely hint at the underlying power of the system. With two separate imaging chains, Somatom Definition has the potential to conduct simultaneous exams at different x-ray energies, a possibility Kuettner expects to begin exploring in the next few months. In these studies, each tube would be charged at a different energy; for example, 80 kV and 120 kV.

The technique might be used in cardiovascular imaging to differentiate the vessel wall from surrounding tissue. It could also be used to isolate bone and blood vessels, as in the case of complex vasculature near the skull or across the knee.

Kuettner calls this approach CT spectroscopy, because data obtained at different energies will be used to characterize different chemicals. Chemicals absorb x-rays differently, which raises the prospect of distinguishing among them on the basis of their absorption patterns. This capability might be tuned to work with virtual colonography; fecal matter, tagged with barium, could be selectively subtracted from CT reconstructions.

"A lot more people might undergo this procedure if they didn't have to go through the bowel cleansing part of it," Kuettner said.

Bones could be subtracted in much the same way. The means for doing so is relatively straightforward and could be helpful in visualizing complex vasculature, especially at the base of the skull where CTA signals are most difficult to interpret, he said. This approach might be used in the lung to characterize pulmonary nodules, distinguishing benign nodules, which are calcified, from malignant ones, which are not.

CT spectroscopy might even be used to exactly characterize blood vessels and plaques, according to Kuettner. Differential absorption patterns could be translated into colorized CT images.

"Calcified content might be colored in red, whereas the lumen would appear in white," he said.

This kind of application is theoretical, at least for the moment. Studies aimed at evaluating this use are scheduled to begin at the University of Erlangen by spring.