Reducing CT Coronary Angiography Radiation, Maintaining Imaging Quality

Through carefully calculated adjustments to coronary imaging using dose-reduction techniques, cardiologists have managed to reduce the radiation delivered to patients undergoing coronary angiograms considerably. Plus, it’s possible to maintain good image quality throughout, researchers found.

Lead investigator, Michelle Williams, MD, cardiology research fellow at the University of Edinburgh, UK, studied radiation dose, both before and after introducing dose-reduction techniques.

“We’ve optimized our scanning protocols to reduce the radiation dose for cardiac CT, without reducing image quality. So if we reduce heart rate, use the smallest detector size that can be fitted, if we optimize bolus tracking, narrow the phase window, and tailor tube current and voltage to our individual’s dimensions, we can reduce radiation dose dramatically,” said Williams, a presenter at ECR 2011 in Vienna, Austria.

In 2009, 70 million CT scans were conducted in the US and 4 million in the UK, and cardiac diagnostic imaging as a whole is responsible for up to 30 percent of radiation exposure.

According to Williams, there is wide scope to make alterations to method that can reduce the dose delivered to the patient. “Median radiation dose varies widely depending on the scanner, the protocol, and patient factors such as heart rate, patient size, and patient shape. The lifetime risk of cancer varies depending on who is scanned, so the risk for a young female is dramatically different to that of an old man,” she added.

For reference for Williams’ study, the Protection 1 study, published in the Journal of the American Medical Association (JAMA) in February 2009 sourced data from 50 study sites comprising nearly 2,000 CT scans. Median dose length product (DLP) was found to be 885, but the effective dose varied widely between sites from five to 30 millsieverts (mSv).

The research presented was a single center cohort study looking at patients referred for clinically indicated CT coronary angiography. Williams and colleagues used a 320-multidetector CT and did contrast enhanced, electrocardiogram-gated studies. Dose reduction techniques used in the study included, firstly, minimizing detector range as much as possible: “you can scan up to 16 cm, but most hearts fit into a much smaller range than that, so we worked with radiographers to ensure we got the smallest detector range physically possible,” said Williams.

Second, tube current and voltage were reduced and tailored to the patient’s BMI, which was based on the subjective image quality of the scans. Acquisition was limited to one heart-beat with a 70 percent to 80 percent phase window, and a 0.35s rotation time. Heart rate target was reduced from 65 beats per minute to 60 beats per minute because motion can decrease image quality and increase radiation dose.

“As overall radiation dose for CT coronary angiography for the main volume component of the scan reduces, then the proportional effect of the bolus tracking images become more important in the overall radiation dose that the patient is exposed to. We want to reduce this further as well, so by using a delayed, intermittent, very low dose for bolus tracking we were able to do that,” Williams said.

In total 63 patients were scanned, 31 before and 32 after the introduction of dose-reduction techniques. Average BMI in the two groups was 28 and 27 kg/m2. Heart rate was lower in the second group and the median DLP for the volume component of the scan fell from 309 to 171.The median DLP for bolus tracking fell from 31.7 to 20.0.

“Converting that to effective radiation dose, using a conversion factor of 0.014, the radiation dose from the CT scan fell from 4.3 to 2.1 mSv. However, scanner type and area of chest being imaged means that a more realistic conversion factor is 0.023. There’s still the same reduction in radiation dose but it is important to remember which conversion factor is being used when looking at effective doses.”

Importantly, she added, image quality was maintained throughout with these dose-reduction techniques.

In her conclusion, Williams said that consideration of K-factor when assessing radiation dose was important, and that radiation dose for bolus tracking and scout images should not be ignored when considering patient exposure as a whole.