A Dose of Good News for CCTA: New Techniques Offer Lower Dose, Higher Quality
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 in the time span of one heartbeat, says DeFrance. “We don’t worry about skipped beats or extra heart beats anymore,” he says.

With 64-slice CT scanners, the longer image acquisition time frame of 15 seconds—six to eight heart beats—catches images when the heart is moving and twisting, disrupting image quality and creating noise. With 320-slice CT, DeFrance says, image distortion is alleviated due to its rapid acquisition speed.

During the ERASIR I trial, of which Earls was co-principle investigator, he and colleagues examined the radiation doses of 1,150 patients who underwent a cardiac CT scan using GE Healthcare’s Adaptive Statistical Iterative Reconstruction (ASIR) technique.

The researchers separated study participants into three groups: a 64-slice CCTA with filtered back projection on 735 patients; a CCTA using prior techniques and ASIR on 247 patients; and finally, a CCTA using ASIR combined with other dose-reduction techniques, such as low kVp and body mass index (BMI)-adjusted tube current modulation, on 169 patients.

The team found that the effective radiation dose of cardiac CT scans that used the ASIR technique, combined with other dose-reduction techniques, was significantly lower than those studies using previous techniques, 1.3 mSv and 3.8 mSv, respectively.

At Fairfax Radiology Consultants, Earls and colleagues also use the ASIR technique, in combination with other dose-reduction methods, on its six outpatient CT scanners and during 92 percent of CT exams. With these techniques, Earls says that the facility tracks dose reductions of 25 to 40 percent during each patient CCTA exam. In addition, for patients with a BMI of 30 or less, tube voltage reduction techniques are utilized.

At MUSC, Schoepf and colleagues “meticulously adjust” tube settings depending on a patient’s body type. Patients with a BMI of 25 or less with a moderate rate receive 20 percent less than the normal tube output—100 kV rather than 120 kV. For pediatric patients and smaller adults, Schoepf says tube currents can be reduced even less to 80 kV, resulting in a “substantial radiation dose savings.”

At MUSC, the Iterative Reconstruction in Image Space (IRIS) technique reconstructs raw image data during a CT exam. The reconstruction decreases artifacts within patient images and lessens dose by 60 percent. In addition, the use of the Siemens Somatom Definition Flash CT scanner reduces patient radiation exposure to less than 1 mSv, says Schoepf. He says this new technique offers significant reductions compared to previous ones that exposed patients to 12 to 30 mSv of radiation per study. The IRIS technique also has specific dose reduction benefits for pediatrics and the obese.

In an effort to reduce dose at the Lenox Hill Heart and Vascular Institute in New York City, technologists utilize prospective gating on their 256-slice scanners (Philips Healthcare’s Brilliance).

Exams conducted via 256-slice CT 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 at Lenox Hill.

What are the tradeoffs?

While DeFrance, Earls, Hecht and Schoepf agree that these dose-reduction techniques and protocols enhance the quality of care during CCTA exams, they advise that there also can be tradeoffs.

Schoepf says that prospective gating can sometimes mislay information that would “otherwise be inherent in data sets acquired by retrospective EKG gating.” He says that use of the technique hinders the evaluation of cardiac function. However, he notes, “If I can get away with ruling out coronary artery disease with a radiation exposure of less than 1 mSv, then I can happily forgo this function of information.”

“Everyone wants to lower radiation dose,” says DeFrance, but “I think one of the main problems is getting nice, slow heart rates.” He preps his patients with beta blockers to achieve a heart rate less than 65 beats per minute before cardiac CT exams.

Some physicians, he says, don’t feel comfortable slowing down a patient’s heart rate with medication. Therefore, “they are not going to be as likely to use the prospective methods and subsequently, they are going to deliver higher radiation doses,” says DeFrance. 

To Hecht, prospective gating limits the data acquired during a cardiac CT exam compared to data obtained with the retrospective gating technique. Because prospective gating is done only during a quick, specific phase of the cardiac cycle, says Hecht, it limits the reconstruction of images because the image is obtained during a “narrow window.” During retrospective gating, an image may be reconstructed at any phase because the scan acquires data during the entire cardiac cycle. However, he says, “the tradeoff is well worth it.”

Another issue is using dose reduction techniqus properly. Clinical studies such as the Protection 1 trial, an international, prospective, multicenter study of 1,965 patients undergoing CCTA between February and December 2007 (JAMA 2009;301:500-507), show dose reduction techniques work when used uniformly—however, they learned that the techniques are used infrequently and many technologists are unfamiliar with using them. The research team recommended improved education of physicians and technologists performing CCTA on these dose-saving strategies. The study also pointed out vast discrepancies of CCTA radiation dose between facilities and the need to gauge facility performance vs. global benchmarks.

And what did they define as the most important factors to reducing CCTA radiation dose? Reducing kV levels and managing the scan field to minimize overlap.

Quality images without downsides

Traditionally, high doses of radiation produced a higher quality patient image during a cardiac CT, but today, the use of CCTA techniques lowers dose without deteriorating image quality.

Imaging coronary artery stents and dense coronary artery calcifications during a cardiac CT, explains Schoepf, has long been known to cause scatter and artifacts. “With the use of IRIS, you can reduce that scatter so that dense struts, coronary artery stents and dense calcifications actually look much more their real size.”

“We are addressing a traditional Achilles’ heel of cardiac CT, measuring stenosis severity in the presence of significant coronary artery calcifications,” Schoepf says.

Today, as widely available as dose lowering techniques are, efforts to reduce medical radiation in the U.S. have fallen behind those in the U.K., Europe and other areas of the world, and show dosages more than eight times higher, according to the Health Protection Agency in U.K. While these techniques slowly become adopted and the technology improves, the push to lower dose remains.

“It seems that manufacturers keep coming out with newer and better protocols, so it’s kind of a continuous learning and educational process,” says DeFrance. “We have to keep updating and tweaking protocols so we make sure we are really utilizing the tools.”