GE engineers integrate future technologies with LightSpeed VCT

It used to be that engineers did what they could and hospitals bought what they made. Those "old days" of radiology are long past. Clinical feedback now drives the process, marking a change in approach that presents extraordinary challenges to modern-day engineers. This is especially true for the CT design teams at GE Healthcare.

"We used to write documents telling them that we wanted spatial resolution of a certain number of line pairs," said Gene Saragnese, GE's global vice president of molecular imaging and CT. "Now we spec things in clinical goals."

In an exclusive discussion with DI SCAN at GE headquarters in Waukesha, WI, Saragnese and other top brass in the CT business unit discussed the thinking behind the company's latest and coming developments in CT. They explained how unique clinical problems posed by the physiological nature of different tissues are leading their engineers to approaches that could radically change the way exams are conducted, yielding information far beyond the reach of currently installed scanners. And yet, this information will be obtained using the same basic scanner platform, the LightSpeed VCT. Making this happen is a challenge that GE has no choice but to accept.

By late spring, GE had about 800 VCTs installed worldwide with a 200-unit backlog of orders. The 64-slice CT scanner, the design process for which began years ago, will be required in the years ahead to distinguish materials that have the same Hounsfield units, which for decades have served as the means for distinguishing soft tissue from bone. If the VCT is to differentiate the densities of plaques and tissues, and go beyond that to identify subtle changes in plaque density, it will have to do so in ways other than the shades of gray currently used, Saragnese said.

"Our mission is to assess patients more closely, to move from visualizing the lumen to understanding wall morphology and the physiological meaning of plaques," Saragnese said.

A big step in that direction is an experimental technique called spectral CT. Presented June 14 at the International Symposium on Multidetector-Row CT in San Francisco, spectral CT has the potential to distinguish contrast media from calcifications and bone from soft tissue, a particularly helpful clinical tool when blood vessels and bone intermingle (DI SCAN 6/14/06, Spectral CT technique mines signal for additional information). This technique, which translates photon strikes into anatomic position and material characteristics, calls for a low dose, a win-win for those who want more information and less patient exposure to radiation.

Also at the MDCT conference, GE described efforts to develop a "helical shuttle scan" and dual-energy CT with a single x-ray source (DI SCAN 6/12/06, GE equipment performs CT helical shuttle, dual-energy scans). The dual-energy approach produces images tuned to different x-ray energies suited to imaging different types of tissues, making it a single-beam answer to Siemens' dual-source Somatom Definition. The single beam, according to Brian J. Duchinsky, global general manager for GE Healthcare's CT business, contains the two energies in an overlapping spectrum.

"We want to use the same beam to allow a simultaneous acquisition that doesn't expose the patient to two beams," he said.

The helical shuttle scan has no competitor, at least none known to DI SCAN. It is a type of dynamic CT angiogram that affords whole-organ coverage by "shuttling" the scanner back and forth over the patient. With this capability, the 40-mm coverage of the VCT detector is transformed into 80 mm, according to Saragnese. It may have immediate clinical impact in the assessment of stroke patients.

"It can be used to look with extreme accuracy at temporal events, determining how much tissue is being perfused," he said.

GE engineers are now looking at applying this approach to whole-body scans, which will mean adapting to the different kinds of information being gathered and body areas being covered. The helical scan might be set to shuttle back and forth over the heart, then speed up in one direction to acquire data along the aorta, before sweeping back and forth again, this time to capture perfusion through an organ, such as the liver.

"It would be the first dynamic helical technique," said Duchinsky, who noted that a product with these capabilities might be available in a year or two.

Targeted for release later this year is a feature now awaiting FDA clearance called Cardiac Freeze Frame. This will literally turn the x-ray beam on and off, using a technique called prospective gating. Bad beats, the kind of out-of-synch, abnormal contractions that generate unusable data, will be skipped. Also different from today's helical scan will be a resurrection of the "step-and-shoot" method. In this, one section of the heart is captured at a specific point in time. The patient is then moved incrementally in the gantry to allow the next section to be acquired at the same point in the cardiac cycle. The process continues until the entire heart is scanned with each sweep slightly overlapping the last.

Step-and-shoot exams will take seven beats rather than the current five, but the added few seconds will be worth the wait. Patients will be exposed to much less radiation - between 70% and 75% less - than with current helical scans.

The GE executives liken the process of developing new CT techniques to making Russian dolls with one fitting inside the other. The helical shuttle scan, dual-energy CT, spectral CT, and such additional features as Cardiac Freeze Frame, all depend on clearly imaging the coronaries. But these techniques will have a clinical impact only if they can be adapted to run on the existing CT scanner platform. Saragnese says they will.

"The VCT platform is designed around the future needs of healthcare that we envisioned two and three years ago," he said. "Our installed base is very important to us."