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CT Scans of the Heart Can Be Done with Low Radiation Dose

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Coronary artery, as imaged
by a 64-slice CT scanner
    February 4, 2009 (updated) -- Physicians are able to perform high-quality CT angiograms of the heart with minimal radiation exposure, according to a study published in today's Journal of the American Medical Association (JAMA). Using dose-reduction strategies, some centers included in this study, dubbed PROTECTION I, were able to perform a 64-slice CT angiogram with a measured radiation exposure of 2.1 mSv (millisieverts), equivalent to the level of normal annual background radiation encountered by a resident of New York City -- and they were able to do this without degradation of the image.

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However, some of the 50 international centers in this study performed similar CT angiograms at ten times that level of radiation, 21 mSv, prompting the authors to conclude:

"Median doses of CCTA (Cardiac Computed Tomography Angiography) differ significantly between study sites and CT systems. Effective strategies to reduce radiation dose are available but some strategies are not frequently used. The comparable diagnostic image quality may support an increased use of dose-saving strategies in adequately selected patients."

Interestingly enough, the overall median value of radiation exposure in this study was 12 mSv which, accordingly to an editorial in the same issue by Andrew J. Einstein, MD, PhD, was "somewhat less than the value reported in several earlier studies using 64-slice scanners". 12 mSv is, in fact, at the lower end of radiation exposure for the widely-used nuclear stress test -- which can go to 22 mSv for a thallium 201 test.

PROTECTION I was a cross-sectional, international, multicenter, observational study of 50 sites, including 21 university hospitals and 29 community hospitals. The team, headed by Dr. Jörg Hausleiter of Munich, looked at the estimated radiation dose in 1,965 patients undergoing CCTA between February and December 2007. PROTECTION I is one of the first studies to look at and measure actual radiation dose from CT heart scans. The important finding is that there is very wide variation among centers; the important message is that there are strategies for reduction of radiation levels for cardiac CT angiograms, and that centers performing these tests need to be aware of and trained in these techniques.

According to Dr. Tony DeFrance, Medical Director of CVCTA Education in San Francisco:

Tony DeFrance MD
Tony DeFrance, MD
Medical Director
CVCTA Education
San Francisco, CA

"We need to get the word out to the centers and I think education, ongoing dialogue, even site visits for people are important to optimize their radiation reduction strategies. Step one is measuring their doses. What we’ve done, and what's really changed our practice, is measuring the dose on every patient and recording it. That's the first step to awareness: knowing how much dose you're delivering. Then getting educated about the different dose reduction strategies, from dose modulation to prospective-gated imaging.

About a year-and-a-half ago, we were getting an average of 16 mSv. Since we've instituted our dose reduction strategy our average dose now is 6.8. It's just a matter of doing prospective imaging as much as possible, using dose modulation and really optimizing the scanner technology and parameters to do it. I think a lot of centers aren't using them -- they're just using retrospective on everyone. So the key is teaching these people that you can halve the dose or even decrease it by 80%.

Dr. DeFrance and CVCTA Education use the acronym LOWDOSE to summarize what CT centers need to think about:

Length of scan minimized
Optimized X-ray parameters and gating technique to the individual
Working with vendors to optimize the system
Dose Modulation
Operator Training (CT Technologist and physician)
Scrutinize the effective dose on each case
Evaluate dose at regular intervals and implement changes to minimize it

Implications for Patients
The PROTECTION I study also has significance for patients who are scheduled for a CT angiogram. Dr. Michael Poon, past president of the Society of Cardiovascular Computed Tomography (SCCT) and Director of the Center of Advanced Cardiac Imaging at Stony Brook University Medical Center, told Angioplasty.Org:

"The patient has to request this. Patients may read about this, may see the headlines, and then they need to go back and ask the imaging center, 'What method are you using to lower the dose?' And if they don't know what you're talking about, I would say, 'See you later!'

"I'm very serious. I'm doing that at Stony Brook. I've told the hospital that when the patients come, I say, 'Look, I am doing this with the lowest dose possible. If you can find a lower dose then go there.' But they can't, because I know the method, I know what it takes, I know what to do to get the patient into the range so that you can take advantage of this latest technology to get the lowest dose."

    Michael Poon, MD, FACC
Michael Poon, MD, FACC
Stony Brook University
Medical Center

One of the other caveats in interpreting the results of the PROTECTION I study is that it used data from the year 2007, which does not take into account the extensive dose-reduction protocols and equipment advances from all the major manufacturers of CT scanning equipment that occurred in the past year, from GE's prospective gating to Toshiba's 320-slice scanner and similar programs for Philips and Siemens, so the radiation exposure is most likely lower than what is reported. Dr. Poon stated, "The article is not representative of the latest technology that we are using."

Aquilion™ ONE 320 detector row
Aquilion™ ONE 320 detector row
CT scanner

Dr. DeFrance notes that much incorrect information is being distributed. For example, many people might think that Toshiba's 320-slice scanner produces excessive radiation, compared to a 64-slice machine. But, in fact, the opposite is true because it is able to scan an entire heart in one gantry rotation of 300 milliseconds. Dr. DeFrance currently does CT angiograms, achieving radiation doses of only 1 or 2 mSv.

Similarly GE's new high-definition CT system is able to image a very high quality at low exposures -- neither of these technologies were in wide use during the time frame of the PROTECTION I study.

The conclusion and rationale for this study is that, with training and education, CTA radiation levels can be significantly reduced. Yet many news reports of the study only emphasize the highest radiation levels measured. Dr. DeFrance opines:

"There's so much misinformation out there. And CTA is such a good modality. I think we're losing the media war. There are a lot of people with vested interests that don't want this to succeed. And new technology adoption is center-focus for CMS, for the new Administration and, in how to bring a new technology with comparative effectiveness, CTA is the frontline battleground of this. What always bothers me is that the media throw these numbers around, but what they don’t realize is the risk-benefit ratios and the risk of coronary disease vs. the theoretical risk of cancer, they don’t even compare it, and the articles don’t even address that."

An important point, emphasized by all cardiac CT practitioners, is that CTA is not a screening tool. It is optimally used for patients who are experiencing symptoms of angina, but in whom other tests have proved inconclusive. Even at low dose, CTA is able to accurately rule out coronary artery disease (CAD). The alternative is to send a patient for an invasive cardiac catheterization, which is significantly more expensive, which exposes the patient to the risks of an invasive procedure and which 37% of the time results in a negative finding for CAD, one that could have been detected much more simply by using CTA.

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Reported by Burt Cohen, February 4, 2009