April 18, 2008

April 18, 2008 (Chicago, IL) – A new study shows that the radiation emitted by cardiac computed tomography angiography (CCTA) varies widely, depending on a number of variables, and that education of CT personnel is required to help minimize radiation. Dr Jorg Hausleiter (German Heart Centre, Munich, Germany) reported the findings of the Prospective Multicentre Study on Radiation Dose Estimates of Cardiac CT Angiography in Daily Practice (PROTECTION 1) during a late-breaking abstracts session at the recent American College of Cardiology 57th Annual Scientific Session.

"For the first time, we have demonstrated that radiation varies significantly--both by site and by the machine employed. There is great potential to reduce the dose," Hausleiter told heartwire . "There are already strategies to reduce the dose, but these are used very rarely in daily practice, and we need better education of CT personnel, such as radiologists and cardiologists. Also, they need to understand that there is no standard, one-size-fits-all approach--they need to adjust the procedure to the individual."

However Hausleiter stressed to heartwire that the benefits of CCTA are still great in terms of the impact that the information obtained may have on the diagnosis and treatment of cardiac disease. "And most of the patients we are scanning are 60 years old, and it will take 10 to 20 years before any cancer might develop from radiation, so the risk is minimal to them. But obviously, the lower the dose, the better," although he added this is of most importance in a younger patient.

Radiation doses "all over the place"

As background, Hausleiter explained that use of CCTA is increasing, and it is now used widely to rule out coronary artery disease, "but the downside is that there is radiation associated with the procedure. However, a lot of people--both physicians and patients--are not aware of the dose of radiation emitted."

In the study, which was observational and industry-independent, the researchers examined 50 sites worldwide and analyzed 1965 CT angiographies performed during one month. The majority (94%) of machines were 64-slice; the remainder 16-slice. Most were a single-source configuration, with just one being dual source. They determined the magnitude of CCTA radiation dose estimates and the use and efficacy of radiation dose-saving algorithms.

Unexpectedly, the radiation dose estimates differed significantly between the study sites, ranging from 5.7 mSv to 36.5 mSv, with a median of 15.4 mSv. "We were very surprised by this. The doses were all over the place at different sites. Some were 10 mSv and below, while others were 25 or 30 mSv or higher," Hausleiter said.

Use of dose-saving strategies erratic

Next, they examined a number of dose-saving algorithms. First, they looked at automatic exposure control, which Hausleiter explains adjusts the tube current to the anatomy of the patient. They found that this did not reduce the dose of radiation, something Hausleiter said he was not surprised by, "because I've done previous work on this that has shown it does not reduce radiation dose in cardiac studies."

Second, they looked at so-called "ECG pulsing," whereby radiation is used at 100% only during diastole and then reduced to 20% to 25% for the remaining phases of cardiac contraction. Almost 80% of centers did use this method, and this reduced radiation by 20%, he noted.

Third, the "100-kV" strategy, whereby the tube voltage can be reduced from 120 kV to 100 kV in nonobese patients, "is actually very effective, reducing the radiation dose by 50%," Hausleiter said. But unfortunately, this was used in only 6% of the population who were scanned. "There were only a few centers aware of it;the majority were not using it," he noted. He added that only three of the four manufacturers whose machines were studied actually have this capability, with Philips being the one that doesn't. They also studied image quality at 100 kV and found that it was as good as the image seen at 120 kV.

Finally, they evaluated the sequential scan-dosing algorithm, also known as "step-and-shoot." Hausleiter explained that this is "an old methodology that is being reintroduced, and you can get dose reductions of about 68% compared with conventional spiral imaging." With spiral imaging, the table is moving all the time at the same speed while the radiation tube rotates around the body, he noted, but with the sequential method, the table stops at one position to acquire images rather than being in continuous motion, so that radiation is administered only in predefined snapshots during the cardiac cycle, rather than during the whole cycle.

"We found it reduced the dose very effectively, and the image quality was as good as in spiral data sets in this observational study," he noted. But again, this strategy was employed in only 6% of the population scanned. However Hausleiter points out that "most of the scanners don't have 'step-and-shoot' capability. It is just beginning to be installed or upgraded in machines, so it's not surprising it's been used so rarely."

Other factors play a role, including, surprisingly, the make of machine

Hausleiter explained that his team then performed a linear regression analysis to look at individual predictors of radiation dose and found some other surprises. The length of the scanned arteries plays a big role, they discovered. "If you look carefully and try to minimize the scan length rather than scanning the entire thorax, you can reduce the radiation dose." Also, the length of time that a center had been performing CCTA had some effect on radiation dose, too--"it was a tiny effect, but it was significant," he noted.

"But most surprising of all was the tremendous impact of the different CT systems: four vendors producing five 64-slice systems," he noted. After correcting for all other variables in a multivariate adjustment, they found that some systems have a significantly higher radiation dose than the others. The Siemens single-source 64 was the best, emitting the lowest doses of radiation, followed by the Phillips 64 and the Siemens dual-source 64. The worst two studied were the Toshiba 64 and the GE 64.

But Hausleiter declined to say which system he would recommend buying; rather, he commented: "You need to be aware of the capabilities of the machine and keep looking at the updated systems, as all the manufacturers have something in the pipeline. This is a field that is rapidly evolving, and there are always additional possibilities to reduce the radiation dose."

In conclusion, he noted: "This study will increase the awareness of the radiation exposure of cardiac CTs. In addition, we think this study will result in an increased use of already available dose-saving algorithms, which will substantially lower the radiation exposure of patients. Finally, it strengthens the need for further education of cardiologists and radiologists about CT technology to ensure low radiation exposure to the patient."

The complete contents of Heartwire , a professional news service of WebMD, can be found at www.theheart.org, a Web site for cardiovascular healthcare professionals.



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