CT: Slice War Slows Down, Dose War Heats Up

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Volume-rendered chest and abdomen image produced by Philips iCT 256-slice CT scanner demonstrating the vascular structures and boney landmarks of a chest and upper abdomen, highlighting the pulmonary tree and arterial vascular structures.

The past five years has seen enormous strides made in the development of multidetector CT (MDCT) technology. Systems leapt quickly from 16-slice configurations to 32- and 40-slice offerings. The 64-slice design took the medical imaging world by storm just a few years later and boosted vendor sales across the board. And when that wasn’t enough, 256-slice and 320-slice systems dawned—and now they are delivering significant patient care benefits. Today, purchasing decisions are driven by volumes—both patient volumes and data-rich image volumes, rather than slices, and increasingly are made with radiation dose conservation in mind. What do volume CT scanners offer that lower slice count can’t match?

The Somatom Definition dual-source system from Siemens Healthcare made its debut in 2005, which was followed a short 18 months later with the unveiling of Philips Healthcare’s Brilliance iCT 256-slice solution and Toshiba Medical Systems’ Aquilion ONE 320-slice scanner.

GE Healthcare was not sitting idly by watching its competition; the firm launched its LightSpeed CT750 HD CT scanner with the Gemstone detector, a garnet-based substance with substantially high optical properties in a system that allows helical dual-energy data acquisition with a single source and detector.

The concept of dual energy is far from new. In the early 1980s, researchers experimented with performing two separate scans at different energy levels on a single tube system. But technical problems precluded development. With newer CT technology, scan times are shorter and it is possible to scan multiple locations with one detector, as well as switch kV energies in two back-to-back rotations very quickly on a single-tube system.

Philips also has been developing the capability of scanning tissues at different energies. The company has developed a prototype system with a “multi-energy detector” that scans tissues at different energies simultaneously.

“Now that MDCT scanners have the capability to scan the entire heart or brain in 3 to 5 gantry rotations [with most 64-row detector scanners] or in a single rotation [as in 256-row detector or 320-row detector scanners], it is reasonable to expect the end of the slice wars,” writes Mahadevappa Mahesh, PhD, in a recent issue of the Journal of the American College of Radiology (March 2009, Vol.6:3, pp. 201-202).

As the slice war winds down, lowering radiation dose without compromising image quality has moved to the forefront. A concurrent rise in lay media attention to radiation dose from CT exams has both users and vendors striving to deliver dose rates adhering to the as low as reasonably achievable (ALARA) principle.

“The efforts made by CT manufacturers that are contributing to the dose wars may seem altruistic in nature, but to a large extent, they are among the salient objectives in selling scanners,” writes Mahesh. “Because of the demand for lower radiation doses, CT scanner manufacturers are making efforts to develop various strategies to reduce radiation dose by improving dose modulation techniques, making hardware improvements such as adding additional beam filters, improving detector dose efficiency, developing protocols that do not overlap excessively [prospective triggering], and developing reconstruction algorithms that can accommodate lower tube current, thereby creating the possibility to lower radiation doses.”

Driving down dose
Steven Braff, chairman of the department of radiology and chief radiologist at Fletcher Allen Health Care, University of Vermont College of Medicine in Burlington, Vt., is getting hands-on experience with a super premium CT system. The facility recently acquired a Philips Brilliance iCT 256-slice scanner.

Fletcher Allen is a Level One trauma center that has multiple CT scanners on site. The department also has 16-, 40-, 64- and 128-slice CT systems.
“We added the 256-slice iCT for cardiac imaging—we’re currently using it to do a study on triple rule out with our emergency department,” Braff says. “The 8-cm detector on the system allows us to capture the entire heart, using the step-and-shoot protocol, in just two heart beats. This allows us to image patients with heart rates into the 80s [beats per minute] without any medication.”