February 25, 2008
Reducing Dose in CT Scans
By Annie Macios
Vol. 9 No. 4 P. 14
While the discussion continues about the overuse of CT scans, developers are working to reduce the radiation exposure per exam.
There is no doubt that CT scans offer an excellent view into the human body, and its use has grown dramatically. An estimated 68 million CT scans are performed in the United States every year, up from 3 million in 1980, according to a November 29, 2007, report in The New England Journal of Medicine (NEJM). These exams result in a significant amount of cumulative radiation for those who are scanned.
The report, authored by David Brenner, PhD, DSc, and Eric Hall, DPhil, DSc, of Columbia University, focuses on radiation exposure during CT and is causing physicians, patients, and manufacturers to take a closer look at what can be done to reduce exposure.
The overuse of diagnostic CT scans may cause as many as 3 million additional cancers in the United States during the next two to three decades, according to the report. However, researchers say they’re not trying to discourage all CT scan use. Rather, CT scanning is an invaluable tool in many cases, but doctors too often overlook its risks.
The NEJM report is a recent public example of what the radiology community already understands. CT manufacturers have incorporated various strategies and improved technology to lower dose exposure from their systems. From increasing the resolution to using different triggering techniques and real-time dose modulation to improving basic design elements, CT makers take radiation reduction seriously.
“CT is evolving in a way that improves dose savings to patients. Europeans have been conscious and sensitive to dose reduction for years, and now that the U.S. and Japan are competing in the low-dose market, it is definitely a welcome change,” says Peter Kingma, vice president of Siemens Medical Solutions’ CT division.
Gene Saragnese, vice president of global CT and molecular imaging at GE Healthcare, points to the medical value of CT, as well as the importance of reducing radiation exposure.
“We’ve been pushing this for years, even before the study came out. At RSNA 2006, we introduced the LightSpeed VCT XT for use in cardiac exam, which lowers the dose of cardiac scan radiation by up to 83%,” he says.
The images get better with each CT system evolution, which allows physicians to see more detail inside the volume. “One thing you hear less often nowadays is exploratory surgery,” says Saragnese. “That doesn’t happen as much today because of CT. So although there is the concern about radiation exposure during CT, you also have to recognize its medical benefits.”
SnapShot Pulse and VolumeShuttle are offered on the GE LightSpeed VCT XT configuration, capable of capturing images of the heart and coronary arteries in five heartbeats. With the SnapShot Pulse, the x-ray tube is only on when the heart is in the right position, resulting in a significantly lower dose compared with other techniques. “For the heart application, they’ve found that it provides an approximate 10% improvement in image quality and at a lower dose of radiation,” says Saragnese. There is also a 10-fold reduction in the amount of information sent to the PACS unit, so the physician gets better quality images and only what is truly needed. “The feedback has been very positive,” Saragnese adds.
James P. Earls, MD, of Fairfax (Va.) Radiology has seen great success since he began using the SnapShot Pulse, which he refers to as the prospective triggering CT, in October 2006 for cardiac CT. “We do 90% of our clinical patients using this technique and have done more than 1,500 scans on patients, and it performs great,” says Earls.
At Fairfax Radiology, they have reduced the dose given to patients by 83% and will publish these results in early spring. In addition to radiation reduction, they have also examined image quality using prospective vs. retrospective techniques and found higher quality images using prospective triggering.
However, there are additional differences in the overall technique. With prospective scanning—assuming the heart rate is the same—the technology monitors the heart rate and changes parameters on the fly to improve image quality. “With the helical scanner, the patient is moving and now, the patient is stationary for an axial acquisition. This improvement is a result of three or four components working together to produce a better image,” says Earls.
The only limitation Earls has found is with patients whose heart rates are higher than 70, but that only accounts for a small percentage of people.
Radiologists are pleased with the image quality produced with the SnapShot Pulse. Earls also sees that referring physicians are pleased with the radiation reduction for their patients. “More people, including patients, are more sensitive to the radiation issue in light of The Journal of the American Medical Association [July 19, 2007] and NEJM reports,” he says. The SnapShot Pulse has not only addressed this concern at his facility but also improved the process.
Toshiba recently introduced the AquilionONE, which can image 16 centimeters in a single rotation, enabling a scan of one organ in its entirety, including the heart and brain.
Today’s conventional cardiac scan is typically taken in 20 gantry rotations. “For a cardiac scan, the rotations are tight, sometimes with an 80% overlap. By eliminating the overlap in using one pass to image the heart, the radiation is reduced 80%,” says Robb Young, senior manager for CT with Toshiba Medical Systems.
Because it covers up to 16 centimeters of anatomy using 320 ultra–high-resolution 0.5 millimeter detector elements, time, radiation, and contrast dose are reduced. With the AquilionONE, the image is captured at one moment, eliminating the need to reconstruct slices from multiple points in time. “That’s why we call it a dynamic volume CT. The whole region is captured in the same moment of time, which is important because physicians can see all that is going on at once with no discontinuity,” says Young.
The AquilionONE also handles data differently than traditional CT scanners. Doctors would rather deal with volumes vs. slices, according to Young.
“Also, when we talk about dose, we feel contrast is another important issue for patients. Toshiba looked at all aspects of the patient’s scan. Our technology allows a reduction in contrast dose, generally from 120 cubic centimeters to 50 cubic centimeters per scan,” says Young. Toshiba leapfrogged from 64 slices to the AquilionONE’s 320 slices because of the contrast and radiation reduction benefits. “It’s as big of a breakthrough as helical was in the ‘90s,” he adds.
As of December 2007, there were five systems installed worldwide, and select installations will continue in early 2008 before the system is fully commercially launched.
Adapting on the Fly
Siemens has introduced the SOMATOM Definition AS scanner, available in 40-, 64-, and 128-slices that can be installed and upgraded on site with no gantry swap necessary. The Definition AS incorporates a real-time dosing modulation, CARE Dose 4D, which stands for combined applications to reduce exposure.
This technology actively measures dose during spiral acquisition to determine the minimum dose needed for optimal image quality. For example, it automatically decreases dose at the neck, and as it approaches the shoulders, it increases. At the lungs, it again decreases, and then in the pelvic area, it increases. It does this by measuring the optimal image quality and adjusts on the fly, according to Kingma.
Cardiac CT garners attention because potential recipients can be largely or slightly symptomatic. This exposes a reasonable amount of the population to diagnostic CT. For cardiac patients, this new method will image in the sequential mode and spiral helical modes.
In addition, electrocardiogram (ECG) pulsing allows heart imaging at full dose only when the heart is at rest. This enables the x-ray to be virtually off for the other parts of the cardiac cycle when the heart is in motion. The ability to use ECG pulsing helped Siemens to substantially reduce the dose to the patient.
Philips has taken a comprehensive approach from design to implementation that integrates aspects of radiation dose reduction into its CT and x-ray products. DoseWise is a unifying strategy that considers the shared responsibility between the manufacturers and users to reduce dose whenever possible, according to John Steidley, PhD, vice president of global CT marketing at Philips Healthcare.
“We try to reduce dose whenever possible, but sometimes it is necessary to use a normal dose depending on the patient’s needs. You have to weigh the risk vs. the reward. DoseWise gives our customers the tools they need to make the best diagnostic decisions and create treatment plans that will best suit their patient,” says Steidley.
One area that has recently raised concern is the dose during cardiac CT. “Our Step & Shoot for the Brilliance 64-slice CT is a cardiac imaging technique that has dramatic dose-reducing capabilities—up to 80%. And our Brilliance iCT can image the heart in just two beats. It decreases the dose but allows our users to get the image they need,” says Steidley.
“All aspects of our design concept take dose into consideration—from the x-ray tube and detector material to the design of the system. It’s not about one individual feature but incorporating dose consciousness into every feature,” says Scott Pohlman, director of Philips’ CT clinical science group.
In addition to cardiac CT, Pohlman highlights other areas in which Philips has made advances. Philips incorporates IntelliBeam filtration, an approach that aggressively filters the low-energy photons most likely absorbed by patients. “With this technique, the low energy, which doesn’t significantly contribute to the image quality, is filtered, and it minimizes the risk to the patient,” says Pullman. Steidley also notes that this technique is incorporated across the board in all of Philips’ x-ray technology.
Another feature to reduce radiation dose is the Eclipse collimator, which blocks radiation during the initial phase of the spiral acquisition where some of the radiation used doesn’t contribute to the image. “This is especially true for large coverage systems,” says Steidley, who adds that the technology is available on its new Brilliance iCT, a 256-slice scanner.
NanoPanel detectors, part of Philips’ Essence technology, is another concept that increases the detectors’ efficiency by decreasing the electronic noise associated with detection and improving the signal to noise ratio to make the detector more effective. “By increasing the detector’s efficiency, the image quality increases while dose decreases,” says Pohlman.
Combining all these techniques culminates in technology that benefits both physicians and patients. “Giving clinicians this wide range of tools is key to giving them the ability to tailor the scans to individual patients. To a clinician, it’s critical that they make the appropriate diagnoses, and being able to tailor their scans, in the long run, provides the best possible outcomes for patients and clinicians,” says Pohlman.
Siemens has engineered new dose-saving technology in its newest platform, the Definition AS. In addition to their current dose modulation technology, Siemens innovated an Adaptive Dose Shield, which protects the patient from unnecessary radiation. This unnecessary radiation is a function of CT scanners from all vendors; when starting a spiral or helical scan, CTs must rotate 180 degrees before and after the imaged areas.
“And as you get to bigger detectors, the problem gets worse and the area you radiate gets larger,” says Kingma. Siemens is rolling out a CT incorporating a collimator that blocks radiation at the beginning. It opens the window of the tube in the area to be imaged, then shuts the window to reduce exposure by approximately 25%. The first systems are currently installed in Europe, but the technology may be available in the United States this year.
— Annie Macios is a freelance medical writer based in Doylestown, Pa.