Iterative image reconstruction allows coronary CT angiography (CTA) to be administered at lower radiation exposure and with less iodine than conventional filtered-back projection reconstruction, and with no loss of image quality, according to a prospective study of more than 200 patients at several imaging centers.
Presented by Bin Lu, M.D., the RSNA 2013 research, “Effect of Reduced X-ray Tube Voltage, Low Iodine Concentration Contrast Medium and Iterative Reconstruction on Image Quality and Radiation Dose at Coronary CT Angiography: A Prospective Multicenter Study,” was conducted at nine hospitals in China and spearheaded by researchers at Fuwai Hospital, Beijing, and the Chinese Academy of Medical Sciences.
Dr. Lu and colleagues compared image quality for two CTA protocols. The first protocol used a tube voltage of 120kVp, a contrast agent of 370mgI/ml iopromide and filtered back projection reconstruction; the second used 100kVp, a contrast agent of 270mgI/ml iodixanol and sinogram affirmed iterative reconstruction (SAFIRE). The two groups, 115 in the reduced dose group and 116 in the control group, were comparable in size, age, body mass index and contrast volume.
Images were read by radiologists who had attended training sessions to enable them to assign image quality scores consistently. Image quality scores in the two groups were comparable, and there were no statistically significant differences in mean attenuation, image noise, or contrast-to-noise ratio. The mean iodine dose was 27 percent lower with the 100kVp protocol, and the mean effective radiation dose was 35 percent lower.
Dr. Lu cited two limitations for the study. The first was that the body mass index (BMI) of the Chinese study subjects was, on average, significantly lower than that of the general population in both Europe and North America, and further study is needed to determine whether higher BMI might affect the results. The second was that the protocol wasn’t designed to measure whether the lower iodine load would translate to a reduced risk of contrast-induced nephropathy.
Knowledge of the intrinsic variability in radiation dose delivered to patients undergoing diagnostic CT imaging would aid efforts to evaluate and lower the radiation patients receive, said an RSNA 2013 presenter.
Radiation dose can be almost two-fold higher for a patient receiving an identical repeated CT in identical conditions, said Douglas G. Larson, M.D., during his presentation, “CT Dose Variability for Patients Undergoing Repeat Identical CT Scans: A Retrospective Analysis of 2,606 Patients Undergoing 12,632 CT Scans.”
“All we know is the average dose number is being brought lower,” said Dr. Larson, an abdominal imaging fellow in the radiation department at Duke University School of Medicine. “Almost nobody is talking about the standard deviation. It turns out there is quite a bit of unpredictability in the CT system.”
Dr. Larson discovered the minimum to maximum variation ratio of radiation dose ranged from 1.56 times to 2.02 times in patients. “We pushed all the same buttons, we did everything the same and one of the scans had double the dose of one of the others without necessarily imparting increased clinical value,” Dr. Larson said.
Patient size, table height, scanner manufacturer and scanner model can all affect dose variability. No variation was seen with patient age or patient gender. “An important step in understanding CT variability is identifying the controllable – the things we can manipulate about the scan and the patient – as well as the intrinsic components of dose variability,” Dr. Larson said.
Dr. Larson retrospectively examined the records of more than 2,000 patients who had undergone more than 12,000 combined repeat CT scans at his institution. He identified colon cancer, rectal cancer, lung cancer and renal stone patients who underwent the same CT protocol at least twice between January 2007 and February 2013.
Data collected included study protocol; CT dose index (CTDI) and dose-length product (DLP); table height; pitch; noise index and effective mA; peak kilovolt (kVp); date; patient age and gender; and patient size.
The researchers also wanted to know the role played by the scanners themselves - the manufacturer, the model and the unique ID of each. They found a significant statistical difference does exist.
“Even if the scanners look identical they don’t actually perform the same,” Dr. Larson said. “Each piece of equipment has its own dose distribution characteristics. If you look at different manufacturers or generations of scanners, everything behaves a little differently.”
Other contributing factors likely exist, including particular CT technologists, which were not taken into account during his research, he said.
As the push for lowering radiation dose moves forward, Dr. Larson said the question involves more than simply reducing the average dose received by patients as a whole.
“I feel it’s a real blind spot in all of the work that’s happened to date,” he said. “I fully support everything we’ve done to lower CT dose. But as we try to stretch the limits, it’s time to start looking at some other factors as well.”
Understanding the controversies surrounding the relationship between CT radiation dose and potential cancer risks better equips radiologists to find ways to reduce the dose, according to an RSNA 2013 presenter.
The National Research Council’s Biological Effects of Ionizing Radiation VII (BEIR VII) report, which estimated the associated cancer risk for patients undergoing CT exams, is mathematically complex and not very digestible for radiologists, said Justin E. Costello, D.O., a third-year radiology resident at the San Antonio Military Medical Center.
“Even for most general radiologists, it’s difficult to have a really good understanding of BEIR VII,” said Dr. Costello, who presented “CT Radiation Dose: A Review of the Current Controversies and Dose Reduction Strategies through Clinical Scenarios and Patient/Clinician Questions.” “Our goal was to present the information in a neutral way so that the general radiologist could at least make some sense of it, and may be able to convey that to an ordering provider,” Dr. Costello said.
Among the controversies regarding BEIR VII are the report’s reliance on atomic bomb survivor data (ABSD), the difficulty in calculating exact CT dose, and use of the linear no threshold (LNT) model, which makes a linear extrapolation of cancer risk at higher radiation doses to include individuals exposed at lower doses, Dr. Costello said.
“It’s an assumption that these lower levels of radiation are actually going to cause cancer,” he said. “The Health Physics Society has looked at that and said there isn’t concrete data to support it. We’re causing fear in patients, and we’re not even sure if this causes risks.”
Among the issues with using ABSD is that the radiation exposure they experienced differs from that given patients undergoing medical imaging, which is a fractionated dose compared to the one-time dose survivors were exposed to. The researchers took the ABSD and extrapolated down to levels considered medical radiation, between 5-10 or 5-100 millisieverts.
“When you compared that to people who weren’t exposed to radiation, there’s no statistical difference between increased risk of incidence of cancer at those levels,” Dr. Costello said.
Despite the controversies, radiologists should still be seeking ways to reduce radiation dose to patients. He suggested several strategies and techniques to reduce radiation dose, including angular current modulation and iterative reconstruction. “The dose reductions we can get from current modulation are pretty dramatic,” Dr. Costello said. “If you looked at five years ago, some of the factors we’re doing to current modulation have saved at least half a dose from a CT scan.”
Current modulation results in the same savings in breast dose found using bismuth breast shields. However, the shields are no longer used at most institutions due to their considerable drawbacks, which include substantial increases in CT numbers, streaking and beam hardening artifacts, and increased image noise.
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