Radiation dose reductionthemes color RSNA 2008

Radiation dose reduction was a dominant theme at the 2008 RSNA scientific assembly.

If one point was clear to the nearly 60,000 people attending the meeting, it was that radiology has reasserted its authority over the use of ionizing radiation in diagnostic imaging.

Actions designed to shake radiologists from complacency about their application of x-rays in practice and new ideas for reducing dose appeared throughout the conference and show.

Reports from numerous hospitals suggest that radiologists are heeding calls to embrace radiation protection. Ultrasound and MRI utilizations have risen while the growth of multislice CT has slowed.

At the same time, some radiologists have expressed concern about the possibility of overstating the health risks of ionizing radiation and worry about a public overreaction against multislice CT and other lifesaving imaging modalities (see accompanying story).

Yet many radiologists remain in the dark about basic things they can do to reduce patient exposure. Dr. Ella A. Kazerooni, a professor of cardiothoracic radiology at the University of Michigan, described results from the CT Awareness of Radiation Exposure Study, a web-based survey of 150 radiologists that measured their knowledge about CT radiation exposure and strategies known to reduce it.

Though 89% of radiologists were aware that reducing mA would reduce radiation exposure, Kazerooni was surprised that 11% were not aware of that common precautionary practice.

The survey found that 87% of respondents were aware of the value of reducing the kV setting during routine CT scans and 82% knew about progressive gating triggering to reduce radiation exposure during cardiac CT procedures. The respondents believed, however, that they had limited access to most dose-reducing strategies. Only 61% said they could lower the mAs setting on their machines.

"I find it very sobering that 39% of radiologists didn’t know that they could go to the scanner to reduce the mA," Kazerooni said.

Several studies suggest physicians have altered their behavior, however.

Administrators at the Cincinnati Children’s Hospital and the Children’s Hospital of Philadelphia recently noticed a change in CT study volume. The emerging trend led them to investigate whether increased awareness of CT radiation safety issues had actually affected utilization, said principal investigator Kevin E. Gessner, an assistant vice president of radiology and medical imaging at Cincinnati Children’s Hospital.

Gessner and colleagues analyzed five years’ worth of procedural data for CT, ultrasound, and MRI at both institutions and confirmed that CT use at the two hospitals was slowing.

Overall imaging utilization increased almost 6% during the five-year period because of more orders for ultrasound and MR. Fiveyear growth rates for MRI, ultrasound, and CT were, respectively 9.8%, 8.6%, and 1.6%. CT volume has fallen since 2007 while ultrasound and MRI volumes have increased by healthy percentages.

"We speculate that these trends, in part, represent the impact of education about radiation risks and a resulting reduction in unnecessary CT examinations and shift to non-radiation- generating procedures," Gessner said.

A Massachusetts General Hospital study covering 10 years of CT experience seems to confirm this trend. Research fellow Dr. Pragya Dang released results from a review of 634,694 CT scans obtained between 1996 and 2005. While CT volume increased annually for all age groups, the estimated effective individual doses in children went down by 0.01%. The estimated effective dose for adults increased 3.61%. Both rate changes were statistically significant. The researchers also found that abdomen and chest scans produce the highest radiation doses in all age groups. In another MGH study, radiology resident Dr. Shlomit Goldberg-Stein evaluated a database of 46,100 patient records to identify 105 chest and 84 abdominopelvic CT exams performed on pregnant women from 1998 to 2007.

Clinicians increasingly relied on CT to evaluate known pregnant women during the period, according to Goldberg-Stein. Most scans were performed to diagnose suspected appendicitis or trauma. The average fetal radiation doses for a subset of 59 exams was 24.8 mGy, well below the 150 mGy threshold for malformation or miscarriage, she said. However, 5% exceeded the 50-mGy threshold for risk of childhood cancer.

3T MRI FOR APPENDICITIS

A University of Vermont study presented at the meeting suggested that 3T MRI could be a cost-effective alternative to CT for evaluating pediatric and young adult patients presenting to the emergency room with acute abdominal pain.

"Three-T MR is an excellent modality for evaluating acute abdominal pain for treatable conditions in children and adults," said coauthor Dr. Alisa Johnson. "It can diagnose multiple pathologies that cause right lower quadrant pain, including appendicitis."

CCTA DOSE REDUCTION

Scientific research presented in Chicago also reflected vendor and provider efforts to reduce radiation exposure during cardiac CT angiography exams. Scientific sessions featured innovative step-and-shoot tube current modulation techniques that emit radiation only during actual image acquisition. An improved CT gantry design was proposed to shift the x-ray beam away from vulnerable breast anatomy. Other studies demonstrated that the high acquisition speeds of 320-slice and dualdetector CT substantially reduce radiation dose.