Radiation Dose Reduced During Triple Rule-Out Coronary Computed Tomography Angiography
PHILADELPHIA, April 9, 2009 — The amount of radiation used during coronary computed tomography angiography (CTA) scans to rule out coronary artery disease, aortic dissection, pulmonary embolism, and other chest diseases can be significantly reduced without compromising image quality, a new study has shown. Investigators showed that the use of tube-current modulation cuts the amount of radiation in half, compared with triple rule-out coronary CTA without tube-current modulation.
"People have criticized the triple rule-out study by saying there is too much radiation to the patient," lead investigator Dr Kevin Takakuwa from Jefferson University in Philadelphia, Pennsylvania told Heartwire. "There is definitely more radiation used than in cardiac CT scans because it's a larger area . . . This study shows that you can reduce the amount of radiation exposure and, with better technology and faster scanners, we're going to be able to get the radiation doses down even lower."
The results of the study are published in the April 2009 issue of the American Journal of Roentgenology.
Speaking with Heartwire, Takakuwa explained that a triple rule-out study is a cardiac CT scan that includes a scan for pulmonary embolism and thoracic dissection. He said the study is a new approach for evaluating emergency department (ED) patients presenting with symptoms suggestive of acute coronary syndrome. It is a larger scan that includes the lungs, and is a promising technique in the ED when clinicians are trying to determine who has a disease, such as a difficult-to-diagnose thoracic dissection, and who doesn't, said Takakuwa.
Although the study is able to rule out different diseases, triple rule-out CT requires specialized techniques, particularly the timing of intravenous contrast material, so that clinicians are able to image the lungs and coronary arteries as they pass through the CT scanner. Some studies have suggested that the scan requires a radiation dose as high as 40 mSv, and this can pose a cancer risk, particularly among women who have a higher lifetime risk of developing cancer as a result of radiation.
In this study, researchers evaluated patients with a modified radiation protocol known as tube-current modulation. Takakuwa said the protocol reduces the amount of radiation used by not collecting CT images in phases of the cardiac cycle in which it is not needed. In total, 267 low- to moderate-risk ED patients were included in this prospective analysis. Of these, 172 were evaluated without the modified protocol and 95 were scanned using the tube-current modulation.
Among patients who underwent the triple rule-out CTA scans with the modified protocol, the amount of radiation used was 8.75 mSv, which is significantly lower than the 18.0 mSv used with the conventional approach. The reduced radiation did not compromise image quality. In fact, the modified dose-sparing approach yielded more scans classified as "excellent image quality" than the unmodified approach.
"One of the nice findings is that when you use the dose modulation, you get the radiation dose down significantly, but it's also lower than the average for the most common alternative test, which is the nuclear stress test," said Takakuwa.
Not every patient is a candidate for the dose-modulation radiation technique, though. Patients with elevated heart rates or abnormal rhythms are excluded because the CT scan might not capture these irregular beats. However, roughly 80% to 90% of patients who present to the ED are typical candidates for the modified radiation approach. In this study, there were no differences in radiation doses among men and women who underwent tube-current modulation, although obese patients received more radiation than overweight and normal weight patients in both the modified and conventional protocols.
In another study appearing in the April 2009 issue of the American Journal of Roentgenology, Dr Tobias Pflederer from the University of Erlangen in Germany, et al showed that the use of lower tube voltage also results in reductions in radiation exposure during coronary CTA.
In total, 100 patients underwent cardiac CTA using a tube voltage of either 120 kV or 100 kV. The radiation dose used for patients scanned with 120 kV ranged from 8.8 to 16.9 mSv, whereas the dose for patients scanned with 100 kV ranged from 4.9 to 11.9 mSv. Although radiation was reduced, image quality was retained. Patient-based and vessel-based image quality scores were not significantly different between the 2 radiation protocols, reported investigators.
Source: Heartwire
