GE combats dose issues with axial CT

In a move calculated to open doors for CT as a cardiac screening tool, GE Healthcare will market a software upgrade for its LightSpeed VCT that will dramatically reduce the radiation dose patients receive during cardiac CT.

The software upgrade, compatible with each of the 1200 VCTs installed globally, holds the patient table steady for a single axial turn of the 64-slice detector, then "steps" the patient ahead about the width of that detector. The next turn of the imaging chain is timed to coincide with the same point in the cardiac cycle as the first turn, a point determined by monitoring ECG signals. This process, called prospective triggered gating, continues until the heart is covered.

"Basically, we are triggering off the ECG, moving the area detector on the VCT across the heart generally at three or four table positions," said Dominic Smith, general manager of marketing and advanced applications for CT and molecular imaging at GE Healthcare. "This collapses the amount of x-ray 'on' time."

This intermittent x-ray exposure dramatically cuts x-ray dose so that CT becomes a viable way to screen patients and follow those treated interventionally or medically for heart disease.

Smith describes the software-based technology, which will be showcased at the upcoming RSNA meeting, as GE's nod to the past while stepping into the future. The CT method of step-and-shoot was common back when "axial" was CT's middle name. Recognizing the potential of this approach, GE engineered the patient table on its VCT system to perform the rapid, iterative steps needed by SnapShot Cine. This makes it possible for the thousand-plus VCT owners to upgrade to low-dose cardiac CT using only software.

"From the very beginning, we designed the table to move very quickly from position to position," Smith said. "By triggering the table movement off the ECG, we can integrate patient dynamics - the patient's individual heartbeats - into the table mechanics."

SnapShot Cine, cleared by the FDA several weeks ago, produces image quality comparable to that of conventional helical scanning, but at 30% or less of the patient radiation exposure. Experience at GE's initial eight clinical sites, six inside the U.S. and two outside, indicates that high-quality cardiac exams can be obtained at less than a millisievert, although exposures typically range from 2 mSv to 3 mSv.

By comparison, helical scans of the heart expose patients to a range between 6 mSv and 12 mSv. The difference is that the x-ray beam is on continually during conventional scans, but is on only periodically when using GE's SnapShot Cine.

Body habitus and heart rate dictate the amount of actual x-ray dose necessary. A thin patient with a slow heartbeat will require less radiation dose than a large patient with a quick heartbeat.

The return of axial scanning may take some in the imaging community by surprise. This approach became obsolete in the early 1980s with the introduction of spiral or helical scanning. Its return as the means to resolve the dose problem surrounding cardiac CT will come at a price, however.

Because step-and-shoot CT gathers data only at a specific point in the cardiac cycle, functional data are not gathered. This could be a problem for physicians who want all their answers from a single modality. Proponents of the technology argue, however, that functional data can be readily obtained from echocardiography and nuclear medicine studies.

By resolving the dose issue, SnapShot Cine clears the way for widespread use of cardiac CT to screen patients for cardiovascular diseases, as well as to monitor the health of patients after receiving bypass grafts or coronary stents. Doing so may usher in a new era of precision in cardiac assessment.