Concern about the cancer risk from low medical level radiation, particularly low-dose radiation delivered from CT scans, has been growing in the healthcare community. One controversial study suggests that about 1.5 to 2 percent of all cancers in the U.S. might be caused by the clinical use of CT alone. And the National Council on Radiation Protection and Measurements says radiation exposure has increased seven-fold since 1980. The drive is on to drive down CT dose—and new cardiovascular scanning techniques are making that possible, some with 80 percent less dose, without any negative effects on image quality.
A controversial study published in December 2009 in the 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. Rebecca
Smith-Bindman, MD, of the University of California, San Francisco, and colleagues found that the radiation dose emitted to patients undergoing coronary CT angiography (CCTA) was 22 mSv. Comparatively, doses as low as 10 mSv have been shown to raise the risk of cancer in Hiroshima and Nagasaki atomic bomb blast survivors.
Specifically, the amount of dose delivered to the breast during CCTA was equated to 15 mammography exams, while that emitted to the lungs was equivalent to 711 chest radiography series, Smith-Bindman et al found. In addition, the study projected that more women than men are vulnerable to dose: one in 270 women who underwent cardiac CT at age 40 would develop cancer as a result, compared to one in 595 men.
With CT exams still on the rise, radiation exposure to patients is a growing concern. The study also documented that the number of CT exams performed annually jumped from 3 million per year in 1980 to 70 million in 2007.
Once a scan is deemed necessary to proper patient care, physicians and healthcare professionals seek to minimize the effects of dose on the patient—often utilizing new vendor-specific dose lowering techniques during CCTA exams. Cardiovascular CT exams that once carried a radiation dose of up to 29 mSv, through new dose-reducing techniques now carry doses into the range of 1 to 5 mSv and often less—with no sacrifices in image quality.
Prior to dose-reduction techniques, a 64-slice CT scan exposed the patient to an average dose of 10 to 23 mSv. Today, 256- and 320-slice scanners perform these same exams in less than second and can reduce dose by up to 80 percent. “This is less than one-third of the natural background radiation that a person receives while living in the U.S. for one year,” says U. Joseph Schoepf, MD, director of cardiovascular imaging at the Medical University of South Carolina (MUSC) in Charleston, S.C.
Better techniques, less dose
New dose-reduction techniques such as prospective electrocardiographic gating, tube current modulation and the use of faster 256- and 320-slice scanners, deliver lower radiation levels concurrent with higher quality image. The new techniques, however, also add considerable expense to the purchase of a CT scanner.
Prospective gating, or triggering (step-and-shoot method), is gaining widespread acceptance. Prospective gating “is basically turning the tube on and off for a brief part of the cardiac cycle to minimize the radiation dose,” says Tony DeFrance, MD, clinical associate professor at Stanford University Medical School and director of CVCTA in San Francisco.
Previously, retrospective gating was the principal technique to minimize dose. Yet, retrospective gating exposes the heart to four to five overlapping regions of x-rays which considerably increases a patient’s radiation exposure, according to James P. Earls, MD, director of cardiovascular CT and MRI at Fairfax Radiological Consultants in Northern Virginia. Because prospective triggering acquires non-overlapping images during a limited portion of the cardiac cycle, the radiation dose is reduced substantially compared with retrospective gating. Thus, prospective gating can reduce radiation dose by 70 to 80 percent, Earls says.
“Prospective gating is the big deal,” says DeFrance, whose facility utilizes the Toshiba America Medical Systems’ Aquilion One 320-slice CT scanner. The scanner acquires images in roughly 300 msec in a single rotation, and with 40 to 50 percent less dose than a 64-slice CT exam.
At Stanford, prospective gating is used in 85 percent of CT exams. The 320-slice scanner captures a full image of the heart