CT: Slice War Slows Down, Dose War Heats Up
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.”
Braff sees this capability to perform cardiac imaging with the use of beta blockers to be of patient benefit. “On the 40-slice and 64-slice CT systems, we have to beta block everybody—and that would take time,” he notes. “With the 256-slice system, we’re able to get emergency department cardiac patients in and out of the scanner in much less time.”
Perfusion imaging is another area that Braff cites as seeing advantages from the deployment of the new scanner. “Through the combination of propriety reformation software and thin-client technology, we’re able to see the results of a perfusion scan conducted by our stroke center from any location,” he says.
Dynamic volume for high volume
Time is critical in brain imaging, he notes, as the sooner a diagnosis is made, the sooner more brain can be saved.
The reduction in radiation dose that can be achieved by the scanner is another area that impresses Braff. “We’re able to do some scans with a dose as low as 2 mSv, which is equivalent to the same dose as background radiation,” he says.
“Total scan times [with 256-slice CT] for the most demanding examinations are only seconds long. We see image quality benefits in complex procedures like brain perfusion and neuro CT angiography as well as routine body imaging, bariatric and cardiovascular CT,” adds George Ebert, MD, PhD, director of CT at Fletcher Allen.
According to Ebert, “Through a combination of advancements in speed, power and coverage, the scanner is helping our clinicians gain valuable insights for a wide range of applications while incorporating the latest dose reduction.”
Huntington Memorial Hospital in Pasadena, Calif., recently acquired Toshiba’s Aquilion ONE dynamic volume 320-slice CT system to support a new emergency department as well as the hospital’s general radiology, cardiology, neurology and pediatric patients.
Designed to accommodate 30,000 patient visits per year, Huntington Hospital’s emergency department actually sees more than 60,000 patients—including 17,000 children. To address this need, Huntington is doubling the department’s size to serve 80,000 to 90,000 patients per year.
The new CT system can image the entire brain or heart in a single gantry rotation and in as few as 0.35 seconds. It also can show 4D dynamic movement, such as real-time brain function or a heart beating.
“The Aquilion ONE’s uniquely comprehensive exam will reduce the diagnosis time for patients suffering from life-threatening conditions, like chest pain and stroke,” says Christopher G. Hedley, MD, medical director of radiology at the facility. “Its ability to show real-time organ function, image patients quickly and perform multiple tests will improve patient care by empowering us to make a more accurate diagnosis faster than ever before.”
When it comes to super premium CT systems, Uwe J. Schoepf, MD, is arguably one of the more experienced users in the field. Schoepf, professor of radiology and cardiology and director, CT research and development at the Medical University of South Carolina (MUSC) in Charleston, was one of the first U.S. users of the Somatom Definition dual-source CT system, which incorporates two x-ray sources and two detectors in a single scanner.
In addition, he is working with the Somatom Definition AS 128-slice CT scanner at MUSC’s Ashley River Tower Hospital. The new scanner adapts to virtually any patient, offers advanced dose protection, allows for dynamic perfusion imaging, and features a wider gantry opening and higher maximal weight load to accommodate overly obese patients.
“Clinical needs have shifted and we have seen a significant upswing in the number of patients who undergo bariatric surgery,” says Schoepf. “In the past, we were limited by the bore size and the table weight, but the Somatom Definition AS allows us to adapt to the needs of this group of patients.”
The equipment also is being used for cardiac and oncology studies. The cardiac research is involved in stenosis detection in coronary arteries, while the oncologic research focuses on perfusion imaging in patients with head and neck cancer.
The system features an Adaptive Dose Shield that dynamically blocks unnecessary radiation dose before and after the spiral scan, ensuring that the only dose applied to the patient is dose that is clinically relevant. In addition, an Adaptive 4D Spiral mode of the system is able to address functional imaging (perfusion images of blood flow over time) of whole organs.
The super-premium systems at MUSC have allowed Schoepf to lower dose as well as amount of contrast material used in exams, he says. However, these systems pose an infrastructure and workflow challenge due to the amount of data they generate.
It’s not news that there is a pressing need to handle CT data overload. Too much information is a primary challenge cited by many in the industry, and it impacts both post-processing and storage.
Schoepf says that users of these systems should make sure that the physical infrastructure of their image data network is robust enough to handle the vast amounts of data these scanners generate and that their post-processing workflow is tuned to deliver reformations in a speedy and efficient manner.
Technology and technique
“Although it accounts for only 11 to 13 percent of radiologic examinations performed overall in the United States, CT is responsible for more than two thirds of the total radiation dose associated with medical imaging,” writes Amy K. Hara, MD, in a recent issue of the American Journal of Roentgenology (Sept. 2009, Vol. 193, pp. 764-761).
Hara, an associate professor of radiology at the Mayo Clinic in Scottsdale, Ariz., has been exploring the potential for dose reduction with GE’s LightSpeed CT750 HD CT dual-energy system and an adaptive statistical iterative reconstruction (ASIR) technique.
“In general, for body imaging, we’re able to reduce the dose anywhere from 30 to 60 percent with ASIR,” she says. The ASIR technique, according to Hara, is a unique CT reconstruction algorithm compared with the only one previously available, filtered backplane projection (FBP).
“Unlike with FBP, with adaptive statistical iterative reconstruction, it is not assumed that noise is evenly distributed across the entire image,” writes Hara and colleagues. “Instead, matrix algebra is used to selectively identify and then subtract a ability to selectively reduce image noise allows generation of a higher-quality image at a lower radiation dose.”
She and her colleagues at Mayo validated the ASIR technique with CT phantoms and then tested it in a group of 12 patients who had undergone routine-dose imaging of the abdomen or abdomen and pelvis.
“The results support the ability of ASIR to allow substantial reductions in radiation dose without the compromise in image quality due to noise that once was so troublesome,” the authors write. “Furthermore, reader assessments of overall image quality and low-contrast resolution were nearly identical for low-dose CT scans with ASIR and routine-dose images, further supporting the potential of this technique for routine imaging.”
Hara believes that the combination of new CT technology coupled with new reconstruction techniques may allow for even more aggressive reductions in radiation dose in the future.
“As hardware and software improve, more complex iterative reconstruction algorithms may be used clinically,” Hara writes, “resulting in even greater improvements in image quality.”