Continuing its leadership in the development and use of cardiac CT, Massachusetts General Hospital Imaging has recently achieved dramatic reductions in radiation exposure for patients undergoing the test.
A cardiac CT angiography scan examines the heart for evidence of coronary artery disease using 3D images of the heart obtained in a non-invasive, 10-minute exam. Although the test has proven extremely useful, physicians have employed it cautiously because of the initial perception that it required a relatively high amount of radiation. The Cardiovascular Imaging group within the Department of Radiology has now shown that a combination of new technology and attention to individual patient factors can make a big difference.
"The combination of the new scanner technology, and significantly improved tailoring of dose to the patient led to markedly decreased doses, yet improved image quality" said Brian Ghoshhajra, MD, MBA, the Mass General Imaging radiologist who spearheaded the work.
"Just a few years back, cardiac CT was regarded as the highest dose CT examination," added Mannudeep Kalra, MD, another Mass General radiologist. "Flash forward to present day, and cardiac CT is now in most cases the lowest dose CT examination."
Each cardiac CT patient receives thorough care at Mass General Imaging. Two physicians—an attending physician and a cardiovascular radiology fellow, both with board certification and subspecialty training in cardiac imaging—jointly supervise each exam along with the highly trained technologist who performs the exam.
Brian Ghoshhajra, MD, MBA
After analyzing more than 6,500 past exams to identify the factors that led to high or low doses of radiation, the team developed a procedure in which the CT technologist calculates the patient’s BMI (body mass index) while the physicians analyze the patient’s heart rate, rhythm, and the clinical indication for the CT scan. Gathering this information allows the team to adjust the amount of radiation in two ways:
- In general, the larger the patient, the more radiation is required to image the heart. Knowing the patient’s BMI allows the team to select a lower-power beam of radiation when possible.
- Because the heart is always moving, it is difficult to capture sharp images of it. In fact, until very recent technology was developed, this examination was not even possible. A technique called prospective ECG (electrocardiogram) gating turns the CT radiation on and off repeatedly during scanning, based on signals from heart-activity monitors attached to the patient’s skin. This means that image capture occurs at the same point in each heartbeat cycle, resulting in clear pictures. It also uses dramatically less radiation than the alternative method, which involves leaving the radiation beam on continuously and later using only selected exposures to build the final CT image.
At the same time this work was underway, Mass General Imaging installed an advanced 128-slice, "dual-source" CT scanner that features more detectors and higher speed than previous scanners, as well as a mode designed specifically for cardiac CT. During the study, the median radiation dose for a cardiac CT at Mass General decreased by 74.8 percent, from 13.1 milliSieverts (mSv) to 3.3 mSv. (The milliSievert is a measure of radiation based on biological effects; 1 mSv is equivalent to the amount of normal background radiation a person receives in three to four months living at sea level, or about 10 chest X-rays.) In fact, cardiac CT radiation doses are routinely less than 1 mSv, and as low as 0.6 mSv in some patients. The alternative non-invasive test is a nuclear perfusion imaging exam, which requires 13 mSv in all patients.
Radiation exposure from cardiac CT angiography exams at Mass
General Imaging has fallen dramatically with the addition of new
technology and the consistent application of dose-reduction techniques.
"By tracking our performance, and then instituting a relatively simple method of technologist-driven, size-based default scan protocols, patient radiation exposures were reduced by approximately 75 percent while maintaining scan quality," Ghoshhajra said.
Importantly, he adds, this new strategy did not compromise the most important aspect of Mass General's cardiac imaging program: individualized care. "We strongly feel that each patient's exam must be directly supervised," Ghoshhajra said. For example, only physicians can decide whether it is appropriate to administer a small amount of medication to slow the heart down, which makes it easier to use the technology that cycles the radiation on and off during scanning. "This can reduce the radiation dose dramatically," he said. "If this method is applied in combination with the technologists' use of lower doses in smaller patients, you can see how these joint efforts can be very powerful."
The team has presented its results at several national meetings and received an Honorable Mention award at a recent scientific meeting of the North American Society of Cardiac Imaging. This success story is also a great example of the way Mass General Imaging employees across the board continuously work to reduce radiation dose.