Marlene Busko

July 16, 2013

MONTREAL, QC — Good-quality coronary CT angiography (CTA) scans can be obtained in clinical practice with a dose of radiation as low as 1 mSv--and possibly lower--by using new technology and applying careful strategies, Dr Jörg Hausleiter (German Heart Center, Munich) said in a presentation here at the Society of Cardiovascular Computed Tomography (SCCT) 2013 Scientific Meeting [1].

However, opinions are divided about whether clinicians still need to worry about current levels of radiation exposure from this procedure. Following a presentation by Dr Sandra S Halliburton (Cleveland Clinic, OH) about this, 59% of the audience, asked to vote using a touchpad device, agreed that there is cause for concern[2].

An "Artist" With Multiple Tools to Decrease Dose

"Doing a coronary CTA is like performing art," Hausleiter said. Getting the best image quality is critical, and now, clinicians can employ a combination of strategies to obtain "nice images" using a radiation dose as low as 1 mSv and possibly lower.

Beta-blockers can be used to slow a patient's heart rate, which produces images with enhanced sharpness, he stressed. Nitroglycerin can be used to induce coronary vasodilation, which improves the signal-to-noise ratio.

Hausleiter reported that he and colleagues showed that in the PROTECTION 2 trial, a coronary CTA protocol using a tube with a peak kilovoltage (kVp) of 100 maintained image quality and reduced radiation exposure by about 30% compared with a 120-kVp protocol[3]. Thus, "depending on [a patient's body-mass index] BMI, size of the chest, or heart rate, you should use a tube potential [that lowers] exposure," he said.

In PROTECTION 3 , among patients with stable, low heart rates, an axial-scan protocol maintained image quality but reduced radiation exposure by 69% compared with helical scanning[4]. Thus, an axial scan is recommended for patients with stable, low heart rates of less than 60 to 65 beats per minutes, he added.

"With these two important steps, a coronary CTA test went from 10 to 50 mSv to 3 to 4 mSV," Hausleiter noted.

Other investigators showed that by using a narrow acquisition window, radiation exposure was reduced from 3.4 mSv to 1.8 mSv without losing image quality.

Using a high-pitch spiral scan technique with a dual-source CT scan has made it possible to image the entire heart in 270 to 300 ms, Hausleiter noted.

When their group combined strategies such as reduced tube potential, a new high-pitch protocol, and iterative image reconstruction, they were able to achieve a radiation exposure of 0.56 mSv, he said.

"In clinical practice, we are already there--radiation doses of 1 mSv and below are possible," and it is feasible to lower radiation exposure to this level and still achieve diagnostic images, he concluded.

Reasons for and Against Worrying

There are several good reasons forworrying about radiation exposure from CT scans, Halliburton summarized. First, the number of CT procedures and doses per exam are both increasing. Currently, about 83 million CT scans are done in the US each year. Patients in the top 1% of radiation exposure receive an average of 120 mSv a year, while patients in the top 10% of radiation exposure receive 20 mSv of radiation a year.

"In 2009, the doses . . . were . . . 1 to 20 mSV for an average cardiac CT scan, with the higher numbers for older scanners and probably less experienced operators," she noted, adding that "we hope [the numbers] have gone down."

Three often-quoted nonmedical studies report that:

  • Atomic-bomb survivors who lived far from a bomb's epicenter and received on average 29 mSv of radiation had a 2% increased risk of cancer.

  • About 500 000 radiation workers in 15 countries who received on average 20 mSv radiation had a 2% increased risk of cancer.

  • Children in a UK study who had received radiation in utero had an even higher, 39% increased risk of childhood cancer.

On the other hand, humans have evolved to survive radiation from the earth and the atmosphere, Halliburton continued. Some highly controversial studies have even suggested that exposure to radiation may make a person "in a sense, stronger," she said. It is not really known what the effects of low-dose radiation are, since studies have analyzed data from people who had very high doses of radiation.

However, according to Halliburton, all data support risk from even a small dose of radiation, especially in vulnerable populations: "Studies have [consistently] shown that the risk [of cancer from radiation exposure] is greater for younger people and for women."

"We should be responsible," she advised. "We should expose only individuals with a favorable risk/benefit ratio to radiation from CT." Healthcare providers should apply as low as reasonably achievable dose strategies, follow the guidelines, and give a reasonable dose that does not lead to repeated scans due to poor image quality, she concluded.

Hausleiter has received grant/research support from Siemens Medical Solutions. Halliburton has received grant/research support from Bayer Radiology and research support form Phillips Medical Systems.

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