Technique as Well as Technology Can Help Lower CTA Dose
As coronary computed tomography angiography (CTA) becomes a more common, noninvasive diagnostic standard for evaluating cardiac patients, lowering the dose of ionizing X-ray radiation has became a priority as hospitals look to reduce patient safety risks.
While the easy answer to lowering radiation exposure is to purchase new scanners with high-tech, dose-lowering bells and whistles, the Michigan Advanced Cardiovascular Imaging Consortium is taking a more practical look at the issue. It is conducting an ongoing study of patient CTA doses at 40 hospitals in Michigan. The researchers monitored the doses of every patient receiving CTA scans over a year, and by providing oversight and instructing hospitals in best practices, they were able to reduce the median dose of radiation from 25 to 12 millisieverts (mSv), a 52 percent reduction. The details of the study, which included the review of more than 4,800 patients who underwent CTA, were explained in a session at the American Heart Association annual meeting this past fall.
The program director for the Michigan Advanced Cardiovascular Imaging Consortium is Gilbert Raff, M.D., director of the Ministrelli Center for Advanced Cardiovascular Imaging at William Beaumont Hospital in Royal Oak, MI, and chair of the Society of Cardiovascular Computed Tomography’s Guidelines Committee, which is working to publish definitive image acquisition guidelines for cardiac CT.
He said there was a definite need for reducing CTA doses after reviewing data from several hospitals. He assembled a team that included a lead CT technologist and a radiation physicist, and then set out to identify best practices for lowering radiation doses using the existing CT scanners at the participating hospitals. The recommendations for best practice so far include:
• Use of ECG X-ray tube current modulation (also called mA modulation, which dials down the X-ray intensity during systole)
• Reduced tube voltage in patients who weigh less than 185 pounds
• Limiting the field of view (limiting scans to the heart and preventing additional scanning of the abdomen)
• Lowering heart rate with beta-blockers to help shorten scan time. Dr. Raff said a slower heart rate helps allow for better images during diastole and cuts the number of cycles that need to be images to get usable images that are stitched together for a whole heart image.
Dr. Raff said the hospitals saw a constant lowering of the dose without impairing the quality of the final images.
“Cardiac CT has a lot of technical tricks for lowering radiation dosage, unlike many other exams where you just push the button,” said Dr. Raff. “The methods we used have been around for a couple years and have the ability to lower radiation to as little as 5 mSv, depending on the patient as some patients require higher doses for technical reasons. My hope is that physicians and radiologists who are not already using these techniques will utilize them in their practices in order to lower the radiation risk associated with CT to improve the quality of patient care.”
A starting point for most hospitals is to begin recording the radiation doses each patient receives during CTA scans. He said hospitals need to be proactive in this record keeping and build it into standard practice.
“Every hospital does not monitor their doses, so they don’t know what doses their patients are getting,” Dr. Raff said.
Due to the fact most hospitals don’t track patient doses, there are a variety of estimates as to the average CTA dose. Cynthia McCollough, Ph.D., department of radiology, professor of radiological physics and director of CT Clinical Innovation Center at the Mayo Clinic in Rochester, MN, says her research shows average CTA scans between 12-13 mSv, but 18 mSv is not uncommon. In comparison, she said typical angiogram yields doses of 5-10 mSv, and a typical nuclear perfusion imaging studies have an average dose of 13-16 mSv.
McCollough spoke on dose reduction strategies at the 2008 Radiological Society of North America (RSNA) meeting in Chicago. Her suggestions included maintaining proper equipment performance with regular QA testing, using patient-size-specific doses, and use of ECG gating combined with mA modulation with arrhythmia detection to cut the amount of unusable images. The diastole part of the cardiac cycle is the only portion used to reconstruct cardiac images, and this portion of the cycle only accounts for about 20 percent of the imaging time, experts say.
Dr. Raff said the best practices are designed to produce dose reductions with existing CT scanners, but said even greater reductions can be made using newly introduced technology. He said emerging technologies can help reduce a CTA dose down to 1-5 mSv. The most promising technologies are those that reduce scan time, including dual-source CT scanners, use of prospective ECG triggering and high-slice detectors that can image an entire heart in one rotation.
Siemens’ recently introduced Somatom Definition Flash dual-source CT scanner. The company says it can image the entire thorax in 0.6 seconds without a breath hold. A whole body scan can be done in 4.7 seconds using its flash mode spiral scanning technology. The company said a heart scan with the Definition Flash results in a dose of less than 1 mSv and is taken in less than a quarter of a heartbeat.
The scanner is also said to be the first with software providing organ-specific dose protection, which eliminates radiation sensitive organs, such as breasts, thyroids and ovaries. Siemens said the X-ray source is turned off as its field passes these organs, and then turns back on when they are clear.
Siemens said the Definition Flash opens the possibility for routine cardiac scans and helps coronary CTA become the gold standard scan for patients with low-and-intermediate chest pain.
“That is the goal,” said Bernd Montag, Ph.D., CEO of Siemens Healthcare imaging and IT division. “To become the gold standard, it needs to work always. It also needs to be a safe exam.”
He said it is easier to explain to the patient that natural background radiation is 4-6 mSv per year. Montag said patients’ concern over high CT doses can be alleviated when their scan is below that natural radiation exposure level.
GE Healthcare recently released the Lightspeed CT750, offering prospective ECG gating. The system uses the step-and-shoot technique that limits exposure only to the diastolic portion of the cardiac cycle and then turns the X-ray source off and moves the table during systole to be ready to image the next section of the heart.
The step-and-shoot has significantly lowered the dose, according to James Min, M.D., associate professor of medicine, Weill Cornell Medical College at Cornell University, assistant attending physician, New York-Presbyterian Hospital in Ithaca, NY.
“We probably use it on about 90 percent of our patients and they are getting radiation doses of about 3 to 4 mSv routinely,” said Dr. Min. “It has probably reduced [radiation] by about 75 percent.”
The only patient population that clinicians can’t use the step-and-shoot method on is patients with high heart rates, according to Dr. Min. In those instances, the image quality is poor.
The only 320-slice CT scanner currently on the market is the Toshiba Aquilion ONE. It was released in 2007 and the clinicians who have used it boast of its impressive image quality and speed. Toshiba said the Aquilion ONE can image the whole heart (a 16 cm area) in 350 milliseconds in one rotation with a dose of between 2-5 mSv. Since it images an entire heart in one pass, it does not require multiple images to be taken and stitched together as with scanners using fewer detector rows.
Sweden-based imaging software company Sapheneia manufactures the Clarity CT Solution that aims to lower radiation dose by utilizing proprietary post-processing algorithms, enabling the use of less radiation during the acquisition. The Cardiology Associates of Martin & SF in Larkspur, CA, recently began use the Clarity CT Solution during cardiac scans. According to James Adams, M.D., the radiation dose in coronary CTA scans and calcium scoring scans have decreased by more than 50 percent.