Down, Down, Down: Methods to Reduce Dose in CCTA

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HIIT070603
A heart, scanned with the FlashSpiral Cardio of the Somatom Definition Flash from Siemens Healthcare.
Recently, medical professionals and 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 gold-standard nuclear stress test. However, education and training are integral to establishing uniform coronary CT angiography scanning standards.

For years, not much attention was paid to the radiation dose associated with CT scanning. Several seminal studies in the early 21st Century linked medical radiation exposure with the 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 manufacturers have developed technology that, when used properly, can significantly minimize dose.

Radiation dose is a particular 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 an operator’s 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 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,” says Jörg Hausleiter, MD, from the department of cardiology at Heart Center Munich, Clinic at the Technical University in 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, for which Hausleiter was lead investigator, was an international, prospective, multicenter study that included 1,965 patients undergoing CCTA between February and December 2007. It was published in a February issue of the Journal of American Medical Association. 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 current methods for reducing CCTA radiation dose included
  • Current Tube Modulation, a helical scan: “You administer a full program dose to certain parts of the cycle, and then you administer a dose reduction to approximately 20 percent for the rest of the cycle,” Lesser says. This method also requires a very regular heartbeat.
  • Step-and-Shoot Method, an axial scan. “It exposes without overlapping certain parts of the body, and exposes a small segment of the cycle. This method 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 rates become miniscule,” Lesser says, but it can only be employed when the heart rate is less than 60 bpm. The 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 are also 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 more quickely. 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, he adds that physicians need to be selective about which patients receive which technique, especially in regards to heart rate. 

Addressing discrepancies
PROTECTION I highlights the vast discrepancies of radiation dose among CCTA exams between sites. However, like many aspects of medicine, 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 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 these techniques and more facilities adopt newer technologies, the concern about radiation dose may begin to dissipate, Hausleiter concludes.

Diffusing Dose Across A Variety of Conditions
Stroke: St. Elizabeth Medical Center’s emergency department has changed its stroke patient treatment strategy based on CT. Previously, in the ER, stroke patients would undergo a CT exam as well additional tests such as an MR exam. St. Elizabeth now has Toshiba America Medical Systems’ 320-slice Aquilion One CT to capture up to 16 cm of coverage, allowing the physicians to create a perfusion map of the brain in less than one minute with less than 5 mSv of radiation dose. By diagnosing stroke within the hour, physicians can plan stent treatment to open the artery passage, and potentially prevent a stroke.

Cardiac: German Heart Centre Munich was a beta site for Siemens Healthcare’s Somatom Definition Flash CT, which combines dual-source and the Flash Spiral concept—the latter of which can increase scan speed up to 430 mm/s. Physicians here say the scanner’s two x-ray tubes enable acquisition of the entire heart in 250 ms within a single diastolic phase, with a reduced dose down to an 1 mSv or less. For coronary CT angiography exams, this combines flexible X-ray pulsing with 75 ms of temporal resolution, allowing for a combination of low dose coronary imaging and functional information.

Oncology: The University of Chicago Medical Center installed Philips Healthcare’s 256-slice Brilliance iCT system, which will be used for thoracic CT, among other procedures. Thoracic CT, the most common MDCT exam, holds particular importance for the lung cancer care cycle. A patient at risk for or diagnosed with lung cancer can receive several CT scans in the course of his or her treatment; therefore, minimizing cumulative radiation dose for thoracic exams was of particular importance. Since the thorax has inherently low x-ray attenuation and high contrast due to the presence of air in the lungs, X-axis dose modulation can be used to achieve dose reductions up to 50 percent. Low scan dose protocols of approximately 1 mSv can be achieved for pulmonary lung nodule imaging studies.

Trauma: Royal Berkshire Hospital in England installed GE Healthcare’s Discovery CT750 HD, which provides large coverage functional information through its Volume Helical Shuttle, allowing clinicians to scan volumes by 32 cm coverage using up to 50% less dose. These whole organ perfusion studies are important for emergency procedures. The Discovery CT750 HD has a Gemstone detector, which enables spatial resolution improvements of up to 33 percent for routine body imaging. During the daytime, the hospital uses it for routine work, and during the night for emergency care.