LightSpeed VCT upgrade cuts cardiac dose by 70%

GE Healthcare introduced at the 2006 RSNA meeting an upgraded version of its flagship CT scanner with software that cuts patient x-ray dose for cardiac scans by 70% or more. The 64-slice LightSpeed VCT XT uses a technique called SnapShot Pulse to change the scanner mode from conventional helical acquisition to step-and-shoot.

Each gantry rotation acquires data on a swath of the heart the width of the detector, then steps the patient ahead that distance - about 3.5 cm - in preparation for the next shot. This step-and-shoot method repeats until the heart is covered, reducing overall dosage by turning the x-ray beam off while the table steps forward. Prospective ECG gating ensures that data are obtained at the same phase in the cardiac cycle.

GE also introduced Volume Shuttle, which uses a similar approach to perform dynamic angiography and perfusion elsewhere in the body. But Volume Shuttle steps the patient forward, then back to the beginning position to cover old ground, before retracing its step forward again. The table shuttles back and forth, doubling the coverage width of the anatomy while minimizing dose, as the x-ray beam is o only during the axial acquisitions.

The company emphasized SnapShot Pulse because of its potential for making CT a viable means to screen patients and follow those treated interventionally or medically. Early clinical studies conducted globally have produced coronary CT angiography images as good or better than ones obtained using helical scanning, according to luminaries who presented their findings this week at the RSNA meeting.

Dr. Jean-Louis Sablayrolles, head of CT Cardiac Imaging Radiology at the Centre Cardiologique du Nord in Saint-Denis, France, reported that all of the 340 exams he conducted with SnapShot Pulse produced excellent image quality yet exposed the patient to less than 8.5 mSv, the threshold in European guidelines for chest exams.

In one case, Dr. James Earls, vice president and medical director of Fairfax Radiological Consultants in Virginia, visualized a stenosis not apparent in the helical scan of a five-foot, 200-pound woman. The SnapShot Pulse scan was performed with an exposure of 3.9 mSv versus 23 mSv for the nondiagnostic scan.

Use of SnapShot Pulse does have a downside, however. 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. But it doesn't bother Earls, who sees SnapShot Pulse as the way to screen patients for cardiac disease and follow heart patients being treated medically.

LightSpeed VCT XT will begin shipping the end of this year. Both SnapShot Pulse and Volume Shuttle will be available as upgrades for the installed base by mid-2007.