Siemens seeks to expand CT uses with new cardiac imaging package

Company pushes performance of spiral technology

Siemens Medical Systems is pushing the limits of spiral CT technology in its quest to develop cardiac imaging applications for the modality. The Iselin, NJ, vendor has begun shipping SubSecond Cardio CT, a new upgrade package that enables users of the company's premium Somatom Plus 4 system to conduct cardiac scans. Siemens hopes the package will spur the clinical investigation of techniques that eventually could lead to the broad-based use of CT for cardiac imaging.

Like MRI, CT has been restricted from cardiac applications due to the rapid motion of the heart, which creates artifacts, and for years the only CT developer targeting cardiac imaging was Imatron of South San Francisco, CA. Imatron's electron beam tomography (EBT) technology uses electron beams to create x-rays rather than a mechanical CT x-ray tube. The firm's systems are capable of 50- to 100-millisecond scans, far faster than the 1-second scan times standard on most CT systems. Imatron has focused its technology on the detection of calcifications in the coronary arteries, which can be a precursor to heart disease in asymptomatic patients.

Companies with mechanical CT scanners are beginning to target cardiac applications as well, however. At last year's Radiological Society of North America meeting, Elscint released its own cardiac calcification scoring package, for its premium CT-Twin scanner (SCAN Special Report 12/97). The technique is based on the twin-slice architecture of CT-Twin, which enables the system to collect twice as much data in the same amount of time as conventional CT scanners.

Like Imatron and Elscint, Siemens sees cardiac CT as a promising new market. SubSecond Cardio CT is based on the short-geometry architecture of Somatom Plus 4. Although Plus 4 is a mechanical scanner, its short geometry enables the system's x-ray tube to complete a 360 revolution of the patient in 750 milliseconds, 25% faster than the one-second CT scanners that predominate in the CT market.

To enable radiologists to work with even shorter scan times, Siemens added a cardiac triggering function that uses EKG signals to tell the CT scanner to begin its acquisition when the heart isn't moving. EKG triggering enables SubSecond Cardio CT users to time the acquisition of a 500-millisecond partial scan to the resting period of the heart, thus reducing heart motion and better visualizing heart structures, according to John Sandstrom, division manager of CT. Sandstrom assumed the top spot at the company's U.S.-based CT division in February, after predecessor Diane Cerny left Siemens.

Somatom Plus 4 scanners with SubSecond Cardio CT are capable of initiating scans at any point in the x-ray tube's revolution, rather than at the same point in each cycle. The 500-millisecond scan times are a big improvement over conventional one-second CT scanners for heart imaging, Sandstrom said.

"Not only do we see vessels that are well delineated, but the myocardial wall is well delineated. We can see the capillary muscles, and the diagnostic value of that is much higher," Sandstrom said. "What would have appeared smeared (due to motion artifact) is no longer smeared."

Siemens is investigating SubSecond Cardio CT for many of the same applications that MRI proponents are investigating: anatomy, function, and perfusion. CT has an additional application over MRI, however: the ability to visualize coronary calcifications, which can be a precursor to heart disease. Siemens plans to file a 510(k) application for calcium scoring uses, which would be packaged with SubSecond Cardio CT. The company hopes to have a product available by the end of this year. Like calcium scoring, many of the applications possible with SubSecond Cardio CT are investigational, and must be proven in clinical practice before they can be marketed.

Siemens began shipping SubSecond Cardio CT in March, with a list price of $100,000. The package can be sold to any Somatom Plus 4 user, and includes cardiac monitoring equipment and cardiac analysis software. The company's goal is to get the package into the hands of physicians, who can then generate the epidemiological studies that will validate the technology.

Relationship with Imatron. The release of SubSecond Cardio CT marks a new turn in Siemens' investigation of cardiac CT technology. The company for several years had rights to distribute Imatron's ultrafast CT scanners under a relationship that expired on April 1. Although the distribution agreement was the more well known aspect of the relationship, the partnership also had a technology development side. Siemens and Imatron were working together to see if they could develop an electron beam tomography scanner at a price point that could win mass-market acceptance, Sandstrom said.

A prototype unit was developed, but the system still would have sold for a premium over conventional spiral CT scanners. Siemens decided that the higher cost was not worth the incremental improvement in performance that the prototype system would offer, especially given recent improvements in spiral technology that bring it closer to EBT. Siemens ultimately decided to end the collaboration with Imatron.

"Although you may be able to do new applications a little bit easier with that technology, will you be able to survive in the long haul if those same applications could be addressed using existing spiral CT technology?" Sandstrom said. "We think we can drive spiral CT further and closer to what EBT can do. High performance is nothing if you are three times more expensive than everything else on the market."

Siemens will continue to support those EBT scanners sold through its channels, Sandstrom said, and the company intends to make sure that these customers have access to future upgrades developed by Imatron.

Bringing to market cardiac applications for CT might seem to be a quixotic mission, given the rapid developments going on in cardiac MRI (SCAN 4/15/98 and 4/1/98). MRI proponents hope that their technology will one day be able to provide a one-stop shop for noninvasively assessing all aspects of the heart. If successful, MRI could effectively take procedures away from other modalities.

Sandstrom, however, believes that there will be room for cardiac CT, even if MRI lives up to its potential. He cites recent history to prove his point. The arrival of MRI in the late 1980s was believed to be the death knell for less advanced technologies like CT. Instead, CT has been able to grow and thrive, partly through new technological innovations like spiral CT and partly by carving out new applications for which it is uniquely suited. Cardiac CT will probably develop in a similar fashion, especially as many facilities with an installed base of CT scanners pursue new applications for their systems. CT's lower cost structure compared with MR could be another advantage, Sandstrom said.

"If you can look at the cardiac anatomy with CT, which I think we demonstrated we can; if we can get a perfusion study, which the early results indicate has definite potential; if we are able to get some functional information, then we are in the same situation as MR," Sandstrom said. "Then you can add one piece to it. With MR, you can't see calcification, with CT you can. So you have a different question that can be answered."