For cutting CT dose, start with the scout scan

CT scout or scanogram images make up only about 4% of the typical chest/abdominal scan radiation dose, but are an easy target for dose reductions, according to a study presented Wednesday. Further, as technology changes and protocol updates reduce overall dose rates, scout images will make up a relatively larger part of the total and still represent a good target for cuts.

Researchers at the Baltimore VA surveyed radiologists on their use of scout scans and found they hold little value. They’re mainly used by technologists to assure the region of interest is covered in the diagnostic scan.

The survey of radiologists found that 63% almost never looked at the scout image, said Dr. Amee Patel, a resident, who presented the study. The survey also found a wide variation in actual dose levels used during the collection of scout images with one site 80% lower than others.

The most common setting has the scout image at 120 kVp/115 mAs, the equivalent of about 10 chest radiographs, and can represent up to 20% of the total CT scan dose, Patel said. Research has established that scout image dose reductions of 10-fold are possible while leaving overall landmarks still well visualized.

One problem encountered is that some scanners don’t permit lower settings for the scout images, Patel said.

Patel and Dr. Eliot Siegel, a coauthor of the study, both stressed the need to pay close attention to scout dose levels as lower dose imaging protocols emerge.

“In the National Lung Cancer Screening Trial, they are scanning at about 50 mAs. If the vendor would not allow them to go below 30 mAs, then the scanogram dose would be 50% of the entire scan, which is huge,” Siegel said. “We believe you go could all the way down to 3 mAs, which is similar to what we use for a typical test radiograph, and this would still allow the technologist to position the image with no problem. We believe you could reduce dose by 10- or 30-fold. The only reason we’re not is that vendors don’t allow us to go below 20 or 30 mAs.”