Electronic network monitors multicenter image quality

Researchers at the Mallinckrodt Institute of Radiology at Washington University in St. Louis may have overcome a problem not uncommon in digital imaging: CT and chest x-ray image acquisition devices at trial screening centers vary with respect to vendor and model, complicating image quality assurance.

The Mallinckrodt Institute, enlisted as the quality assurance coordinating center for the National Lung Screening Trial, devised an electronic system to help manage the QA process (J Digit Imaging 2005 Sep;18(3):242 50, e-Pub ahead of print).

"We established an electronic network incorporating the various devices to monitor image quality and provide quality control throughout the lung study," said Stephen Moore, a research assistant professor of radiology at Mallinckrodt.

The network includes a workstation at each screening center that de-identifies the data, a DICOM storage service at the QA coordinating center, and Web-based systems for presenting images and QA evaluation forms to the QA radiologists. QA data are collated and analyzed by an independent organization.

The network provides the means to collect randomly selected digital imaging studies from screening centers in electronic format, distribute the studies to QA radiologists, and collect image quality review data, according to Moore.

One problem was obtaining the correct values for certain technical parameters from equipment manufacturers.

"This was a time-consuming process because of variation in the location of these parameters in the DICOM headers," Moore said.

Multislice CT scanners, for example, have two measures of exposure: mAs and effective mAs. Effective mAs is calculated as the ratio of mAs/pitch, where pitch is a measure of how far the table moves during one cycle of the instrument. Greater table movement per cycle (higher pitch) implies lower effective mAs and, therefore, lower exposure.

Because pitch and mAs affect image quality and radiation exposure, their monitoring is essential to quality control, but some equipment makers report mAs in the DICOM attribute "0018 1152: Exposure," while other manufacturers use that attribute to report effective mAs, Moore said.

DICOM conformance statements were helpful to Moore in deciphering this information, but for some scanners the QA team had to contact the corporations' engineering divisions.

"In the end, we had to write custom software for each of the eight different models of CT scanners from four different makers," he said.