GE revisits step and shoot to reduce cardiac scan dose

GE Healthcare's latest invention, SnapShot Cine, is a multislice CT software enhancement that borrows its method from the days of axial scanning. The big difference is that it cuts the x-ray dose for cardiac scans by 70% or more compared with conventional CT.

SnapShot Cine was cleared just months ago by the FDA for use on the company's LightSpeed VCT. It holds the patient table steady for a single axial turn of the 64-slice detector, then bolts the patient ahead about the width of that detector for the next turn. This step-and-shoot process repeats until the heart is covered, dramatically reducing patient exposure to radiation. Just as important, it raises the prospect of turning CT into a viable means for screening patients and following those treated interventionally or medically.

Radiation dose is high for traditional helical scans, because the x-ray beam is on for the duration. Patients may be exposed to between 6 and 12 mSv, according to Dr. James Earls, vice president and medical director of Fairfax Radiological Consultants in Fairfax, VA. But SnapShot Cine exposes the patient to x-rays only intermittently. In one case, Earls squeaked by with less than 1 mSv of patient exposure, while producing high-quality images.

"Instead of four or five seconds of x-ray 'on' time, we are down in some cases to less than a second, which allows us to really collapse the millisieverts," he said.

Earls has been running SnapShot Cine for about a month at the nearby Prosperity Imaging Center, one of 14 freestanding imaging centers owned and operated by the Fairfax group, where he is director of cardiac CT. He has performed cardiovascular scanning with CT since 2002, using a 16-slice scanner initially before upgrading to a 64-slice device.

While patient exposures under a millisievert are possible, they are rare, he said. Body habitus and heart rate dictate the amount of x-ray dose necessary. One patient, whose scan required just 0.932 mSv, weighed 155 pounds and had a heart rate of between 34 and 38 beats per minute. Large patients with a quick pulse require a higher dose. But with SnapShot Cine they receive much less than they would otherwise. Patients typically are exposed to between 2 and 3 mSv.

SnapShot Cine works by making the best use of the dose applied. ECG leads pick up the heart rhythm that guides the software to turn the x-ray beam on and off at specific points in the cardiac cycle, a process called prospective triggered gating. The table remains motionless with the beam on, as the imaging chain turns, cutting an axial swath through the heart. With the beam off, the table steps the patient forward a measured distance into the gantry (about 3.5 cm). The process repeats until the heart is covered. This takes about five or six seconds, about the same time to complete a helical scan of the heart.

"When GE approached me with this, I thought we'd be doing scans at 15 to 20 seconds per acquisition, but that is not the case," Earls said.

This is possible because the table quickly thrusts the patient into position for the next axial scan. The rapid motion led Earls to initially worry that motion artifact would be a problem. This too was soon dismissed.

"We have compared our typical helical technique to this one and seen no statistical difference in image quality scores," he said.

Dominic Smith, general manager of marketing and advanced applications for CT and molecular imaging at GE Healthcare, describes the software-based technology as GE's "step back to the future." Its development was anticipated when the VCT was designed, he said, allowing the company to equip patient tables with the mechanical ability to handle the rapid, iterative steps that push a patient through the gantry. Some 1200 VCTs in GE's worldwide installed base can be upgraded to run SnapShot Cine with nothing more than a simple software upload. The product has been tested at eight clinical sites worldwide and was showcased for the first time at the 2006 RSNA meeting.

The concept behind GE's new software is rooted in the past. Axial scanning became obsolete with the introduction 15 years ago of spiral or helical scanning. But axial scanning is the most efficient way to reduce dose on a cardiac CT scan, and dose has been one of the main obstacles to widespread use of 64-slice CT as a cardiac screening tool.

With dose no longer a problem, CT is poised to have a major impact as a cardiac tool. There is, however, one downside. The step-and-shoot method gathers data only at a specific point in the cardiac cycle, which means functional data about the heart are not gathered. This could be a problem for physicians who want all their answers from a single modality. It doesn't bother Earls, who sees SnapShot Cine as the means to screen patients for cardiac disease and follow up heart patients being treated medically.

"This opens the door to a lot of things that we felt we couldn't do because of the radiation dose issue," he said. "In cases of bypass grafts or coronary stents, with this low dose, we are in a position to image patients routinely in a sequential manner over a period of years without worrying about cumulative radiation doses."