CT vendors compete to cut patient cardiac scan doses

September 1, 2007

Radiation dose from coronary CT angiography can be dramatically reduced by technologies either currently or soon to be on the U.S. market.

Radiation dose from coronary CT angiography can be dramatically reduced by technologies either currently or soon to be on the U.S. market.

Two vendors are putting a new spin on the step-and-shoot technique that preceded spiral scanning. Two others are leveraging next-generation technologies to increase acquisition speed. Each is answering concerns raised over the summer by research published in the July 18 Journal of the American Medical Association indicating that routine screening with coronary CTA could pose an increased lifetime risk of cancer.

Using a computational model based on health effects of radiation, Dr. Andrew J. Einstein and colleagues from the Columbia University College of Physicians and Surgeons in New York City found that the risk of cancer from routine coronary CTA ranged from one in 143 for a 20-year-old woman to one in 3261 for an 80-year-old man.

The weaknesses of the study include the unlikely prospect that many 20-year-old women would be routinely screened for heart disease and the dependence on data taken from survivors of the atomic bombs dropped on Japan in 1945 rather than actual CT exposure. No one in the imaging community, however, debates the need to minimize patient exposure to radiation.

"We are aware of the need to keep track of radiation dose, and, increasingly, our patients are aware of that, too," said Dr. Samuel Wann, medical director for the diagnostic laboratories at Wisconsin Heart Hospital in Milwaukee.

To get a handle on the radiation issue, Wann began using this spring an experimental version of Philips' step-and-shoot product. As the name implies, this technology is a throwback to prespiral days of CT. Patients advance through the gantry in stutter steps, and each turn of the gantry produces a single slice. Modern step-and-shoot protocols turn the x-ray beam off between axial acquisitions, reducing patient exposure to radiation by 70% or more.

Preliminary tests have produced doses averaging between 3 mSv and 4 mSv, compared with 6 to 12 mSv during a helical study of the heart.

The step-and-shoot approach harnessed by Philips Medical Solutions, which is scheduled for commercial release in a few weeks, is similar to the SnapShot Pulse product that GE Healthcare launched late last year for its LightSpeed VCT.

"You get a radical dose reduction because you don't have the overlap associated with helical imaging," said Gene Saragnese, GE vice president and general manager of CT and molecular imaging.

Under ideal conditions, Fairfax Radiological Consultants in northern Virginia used SnapShot Pulse to get the dose below 1 mSv. These ideal conditions are created by a slow, regular heartbeat. But this is usually not the case, especially when examining patients believed to be at risk of developing coronary artery disease.

There are methods, however, of handling problem cases. Philips' Rate Responsive technologies are designed to handle variable heart rates. These technologies continuously track the ECG signal and adapt the x-ray acquisition accordingly, such as during a preventricular contraction, according to Phillip J. Prather, director of Global Cardiology CT for Philips Medical.

"If an arrhythmia occurs during the scan, the scanner will stop the x-ray beam, wait for the next sinus rhythm, and then continue," Prather said.

The end result is repeatable high-quality cardiac imaging at dose levels approximately 80% lower than through traditional spiral techniques, Prather said. Wann prefers, however, to use Philips' step-and-shoot selectively.

"We don't use step-and-shoot in older patients with rapid or irregular heart rates," he said. "If we are going to take pictures, we want to be sure to use enough radiation dose to get the answer."

Radiation dose can be constrained by another technique called ECG-triggered tube current modulation. This method decreases the computer-modeled risks cited in the JAMA study to one in 219 for the 20-year-old woman and one in 5017 for the 80-year-old man. Siemens Medical Solutions uses this method on all its multislice CTs, including its 64-slice scanner, according to clinical business manager Praveen Nadkarni.

Siemens is also promoting what it says is a much more effective way to cut dose than ECG-triggered exposures and step-and-shoot combined: the dual-source Somatom Definition. The Definition reduces overall patient exposure, despite the simultaneous use of two beams of x-rays, primarily by drastically reducing exposure time, according to Nadkarni. Technologies include an adaptive reconstruction algorithm that makes the use of dose reduction methods possible.

"This filtration algorithm can improve the pixel quality in the images and reduce the confounding elements that you can have from reduced radiation exposure of the patient," he said.

The ultimate expression of dose cutting could come in less than a year, when Toshiba expects to begin selling its 256-slice scanner in the U.S. Single-rotation capture of the heart will bring an immediate reduction in dose. The reason, according to Rich Mather, Ph.D., Toshiba senior manager clinical sciences, is in the basic design of the scanner.

"There is no overlap, which you get with helical scanning and even with step-and-shoot," Mather said. "There is none with the 256, so you are lowering the dose."

Image quality is improved as well, he said, because there is no need to "piece" the heart together from a series of slices, as must be done with step-and-shoot.

"You can get the entire heart in a single rotation with no chance of misregistration and no chance of contrast differences," Mather said.

No data have yet been published to substantiate this claim, but they will soon, he said. Luminaries at clinical sites, including Johns Hopkins University, in midsummer were compiling data for submission to peer-reviewed journals.

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