Media stories about patients being overexposed during CT scans, estimated risks of cancer incidence from CT scans and the increased use of CT imaging has generated considerable attention on the topic of medical radiation exposure. New techniques and technology, however, have proven successful in reducing the standard patient dose by 50 to 90 percent. Now, the only impediment to lower dose is for these largely software-based techniques to become a routine part of clinical practice.
Smith-Bindman et al in the December 2009 Archives of Internal Medicine concluded that the increasing number of CT scans in the U.S. (70 million in 2007) could potentially lead to “15,000 excess deaths” as a result of cancer. They found that the average radiation dose emitted to patients undergoing coronary CT angiography (CCTA) was 22 mSv. However, new dose-lowering techniques can reverse this trend, as they result in exposures of 1 to 5 mSv.
Prospective EKG gating, tube current modulation and scanners with faster temporal resolution deliver much lower radiation levels without suffering degradation in image quality. Prospective gating, also known as step-and-shoot, involves the x-ray tube turning off for a brief part of the cardiac cycle, resulting in no overlapping images. This technique can reduce dose by 70 to 80 percent, says James P. Earls, MD, director of cardiovascular CT and MRI at Fairfax Radiological Consultants in Virginia. Conversely, retrospective gating exposes the heart to four to five overlapping regions of x-rays, which considerably increases a patient’s radiation exposure.
“Prospective gating is a big deal,” says Tony DeFrance, MD, clinical associate professor at Stanford University Medical School in Stanford, Calif., and director of CVCTA Education, a cardiac CT training center, in San Francisco. His facility utilizes the Aquilion One (Toshiba America Medical Systems) 320-slice CT scanner, which acquires images in roughly 300 msec in a single rotation, offering 40 to 50 percent less dose than a 64-slice CT exam at about 3 to 5 mSv. At Stanford, prospective gating is used in 85 percent of CT exams.
At the Medical University of South Carolina (MUSC) in Charleston, S.C., imagers carefully adjust tube settings depending on a patient’s body type, says U. Joseph Schoepf, MD, director of cardiovascular imaging. Patients with a body mass index (BMI) of 25 or less with a moderate heart rate receive 20 percent less kilovolts (100 kV) than the normal tube output of 120 kV. For pediatric patients and smaller adults, tube currents can be reduced to 80 kV, resulting in a “substantial radiation dose savings.”
In an effort to reduce dose at the Lenox Hill Heart and Vascular Institute in New York City, technologists utilize prospective gating on its 256-slice scanners (Brilliance iCT, Philips Healthcare). Exams take three to five seconds, emit a radiation dose of 2.5 to 5 mSv and yield a 75 percent reduction in dose, says Harvey Hecht, MD, director of cardiovascular CT.
|FDA Steps Up Its Scrutiny of CT Radiation Dose|
In March, the FDA invited stakeholders to educate the agency regarding CT imaging radiation dose. While speakers expressed support for the FDA’s focus on radiation exposure, they raised a range of concerns, including the timing and coordination of implementing changes that the FDA might impose and who would be accountable to whom.
Sean M. Boyd, from the FDA’s Center for Devices and Radiological Health, who moderated the meeting, contends that a national consolidation of information is appropriate, but it is “not clearly decided who should maintain and manage it.” Also, ensuring that facilities use such data to improve internal processes lies outside FDA’s regulatory scope, he says. As a result, figuring out accountability for new stipulations remains a work in progress.
Cardiovascular Business asked Boyd to discuss the recent meeting and the FDA’s increased interest in medical radiation exposure.
Cardiovascular Business: What is the FDA’s overall impression of the meeting?
CVB: What has the agency learned from the meeting?