Reducing Dose in Cardiac CT: Mature Methods are Making a Difference

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CVB030401Recently, medical professionals and CT system manufacturers have sought ways to reduce radiation exposure from coronary CT angiography (CCTA) exams. Today, it is possible to perform a CCTA study that delivers a lower radiation dose than the commonly used nuclear stress test. However, education and training are integral to establishing uniform coronary CCTA scanning standards and less dose to patients.

For years, not much attention was paid to the radiation dose associated with CT scanning. Several seminal studies in the early part of the 21st Century linked medical radiation exposure with the possible development of cancer later in life. Since then, there has been a concerted effort to reduce dose as much as diagnostically possible.

Several insurance companies now flag patients who either exceed a radiation dose threshold or have had more than five CT exams to the most vulnerable parts of their anatomy. The FDA suggests that people carry a “radiation dose card” that details all their medical radiation exposure. And CT equipment manufacturers have developed technology that, when used properly, can significantly minimize dose.

Radiation dose is a concern for chest pain patients undergoing CCTA to assess the presence of coronary artery disease. Therefore, it is incumbent upon physicians and technologists that they perform CCTA in accordance with the ALARA principal (As Low As Reasonably Achievable).

Regardless of where the CCTA exam takes place—the emergency department or a physician practice—one of the biggest problems associated with excessive radiation exposure is operator unfamiliarity with dose-reduction protocols. “Whether a patient is imaged electively or in the emergency department, there shouldn’t be a lot of variation in radiation exposure,” says John Lesser, MD, from the Minneapolis Heart Institute. “With CCTA, you have to manipulate parameters to fit the patient. Differences arise because of the patient’s heart rate or size, or operator knowledge.”

Ideally, the CCTA dose should be the same or less than the more commonly used stress nuclear scan. “With earlier models of dual-isotope SPECT scanners, the radiation dose of a cardiac test can be as high as 24 mSv, while a SPECT sestamibi scan can be as high as 12 mSv. We should expect most cardiac CT scans to range between 3 and 12 mSv,” Lesser says.

“The diagnostic results obtained through a CCTA study far outweigh the slight risk of cancer development in later years,” according to Jörg Hausleiter, MD, of the University of Munich, Germany. “However, there are practical means of reducing radiation dose. With modern CT technologies, we will be able to reduce the dose into a range of approximately 5 mSv, or even lower.”

Importance of PROTECTION I
The PROTECTION I trial, of which Hausleiter was lead investigator, was an international, prospective, multicenter study that included 1,965 patients undergoing CCTA between February and December 2007 (JAMA 2009;301:500-507). Researchers found that the median dose-length product (DLP) of patients was 885 mGy cm, which corresponds to an estimated radiation dose of 600 chest x-rays. They also observed a high variability in DLP between study sites (range of median DLPs per site, 331 to 2,146 mGy cm).

The authors wrote that the study “demonstrates that radiation exposure can be reduced substantially by uniformly applying the currently available strategies for dose reduction, but these strategies are used infrequently. An improved education of physicians and technologists performing CCTA on these dose-saving strategies might be considered to keep the radiation dose ‘as low as reasonably achievable’ in every patient undergoing CCTA.”

Dose-reduction techniques
Some available methods for reducing CCTA radiation dose include:
  • Tube Current Modulation, a helical scan: “You administer a full program dose to certain parts of the cycle, and then you administer a dose reduction of 20 percent for the rest of the cycle,” Lesser says.
  • Step-and-Shoot Method, an axial scan. “It exposes without overlapping radiation from adjacent scanner detectors, and exposes a small segment of the cycle. This method also requires a very regular heartbeat and reduces radiation dose the most,” Lesser says.
  • Tube Voltage Reduction from the conventional 120 kV to 100 kV, which “can be done very safely without losing diagnostic quality,” Hausleiter says.
  • With the new Siemens Definition Flash CT scanner, the “table feed goes very fast, which makes the dose miniscule,” Lesser says, but it can only be employed when the heart rate is less than 60 bpm. Hausleiter’s University of Munich is a test site for the Flash CT and he says dose is as low as 1 mSv.
In addition to newer methods, there also are larger detectors, beyond 64-slice, such as Toshiba America Medical Systems’ Aquilion 320-slice CT and Philips Healthcare’s Brilliance 256-slice CT, which perform the exam much quicker, resulting in less radiation and do not require the physician to lower the heart rate, according to Kevin M. Takakuwa, MD, from Thomas Jefferson University Hospital in Philadelphia. However, Takakuwa adds that physicians need to be selective about which patients receive which technique, especially in regard to heart rate.

Triple rule-out controversy
The CT triple rule-out exam—testing for aortic dissection, pulmonary embolism and coronary artery disease in one scan—has received a great deal of criticism due to the potentially high radiation exposure to the patient. In fact, “one study estimated that the radiation emitted from a CT triple rule-out study is between 35 mSv and 50 mSv, which is an extraordinary amount of radiation,” Takakuwa explains.

“We [at Thomas Jefferson] knew that estimation was too high, so we set out to quantify its actual radiation dose, and also to see if we could lower it to a number that could be safely and routinely used in patients,” he says. He and his colleagues discovered that employing the tube current modulation method halves the radiation dose exposure.

According to their results, among the 172 patients evaluated without tube current modulation, effective dose averaged 18 mSv. Of the 95 patients who underwent the exam with tube current modulation, effective dose was significantly lower at 8.75 mSv, and image quality was better as compared with exams without tube current modulation (Am J Roent 2009;192:866-872). “There were no significant radiation differences by patient age, but tube current modulation decreased radiation exposure by at least half,” they wrote.

“Based on our findings, we believe the triple rule-out exam is an excellent test in an emergency department setting,” Takakuwa says.

Addressing discrepancies
PROTECTION I highlights the vast discrepancies of radiation dose with CCTA between sites. Like many aspects of medicine, however, greater familiarity with the procedure leads to better, more consistent outcomes. Conversely, technologists and physicians who are unfamiliar with dose-reduction techniques could potentially compromise image quality and over-expose their patients. The most important components of reducing radiation dose in the CCTA setting are to reduce kV levels and manage the scan field to minimize overlap. 

“When PROTECTION was published, we examined our methods and dose rates, and re-evaluated our outcomes with each change,” Lesser says. At first, Lesser and colleagues had a mean dose of 19 mSv. They adopted the MinDose method (reduced systolic tube current) and dropped the kV level. They then dropped kV further based on patient weight, and finally added step-and-shoot axial scanning. “We now are achieving a mean dose of 5 mSv for our coronary scans,” he says.

Hausleiter also encourages “feedback mechanisms for labs, so personnel can compare their performance to global benchmarks.” Participating sites in PROTECTION I immediately changed protocols to reduce dose when they were informed that their exposure rates were above the median.

As more operators begin employing techniques such as step-and-shoot and tube current modulation, and as more facilities adopt newer technologies, the concern about radiation dose may begin to dissipate. “The discussion about radiation dose with CCTA, which is important right now, will disappear in a few years, because everyone will be employing these new effective methods,” Hausleiter concludes.

New SCCT Guidelines Provide Foundation for Reducing CT Dose
In June, the Society of Cardiovascular Computed Tomography (SCCT) issued guidelines to establish a consensus of the minimally required standards for appropriate coronary CT angiography (CCTA) acquisition, as well as to provide recommendations for methods to avoid unnecessarily high radiation exposure. They were published in the May/June issue of the Journal of Cardiovascular Computed Tomography.

Wm. Guy Weigold, MD, co-chair of the guidelines committee and director of cardiac CT at Washington Hospital Center in Washington, D.C., says that the society thought the timing was appropriate, because people were “clamoring” for such standards due to the expanding usage of CCTA over the past few years. “Both recent adopters and long-term users are seeking to learn how to perfect their technique with CCTA, in acquiring the highest image quality with the lowest radiation dose,” he says. The committee, however, was hesitant to designate a numeric dose range for fear that certain institutions would find it insurmountable, according to Weigold.

Yet, the guidelines outline specific techniques, which have been proven effective in reducing radiation, such as tube voltage and tube current reduction; automatic exposure control; ECG-based tube current modulation (or step and shoot); reduced scan range; cardiac gating; and shielding. Weigold says that the step-and-shoot method is the most effective, with its ability to reduce dose up to 80 percent.

For administrators, the guidelines recommend that “radiation doses be stored in a format that allows for retrieval and periodic review of representative samples of the data” for reasons of transparency, evaluation and reporting.

Weigold acknowledges that CT technologies are rapidly evolving and that these guidelines will evolve as well. But he says the committee wanted to “to step into the moving stream, to provide the best criteria for its current operators.” He adds that the guidelines will serve as a “strong foundation, and will make it easier to build upon in the future.”   

The writing group was comprised of seven cardiologists and radiologists.