Lossless and Lossy: Study Supports Compression of Mammograms

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Compression of images is not an alien concept in radiology. From radiography to CT, large image files transmitted between physicians’ offices or stored over the long term often undergo some kind of compression. Mammography—one of the last modalities to go digital—remains behind the curve, however. Can mammographic images be compressed while maintaining enough fidelity to be clinically useful?

Stamatia Destounis, MD

That’s the question that Stamatia Destounis, MD, a radiologist at Elizabeth Wende Breast Care LLC, Rochester, NY, hoped to answer with a recent study assessing the readability of mammograms compressed using an access-over-network (AON) wavelet compression engine developed by FUJIFILM Medical Systems USA Inc, Stamford, Conn. The algorithm is already used as part of the company’s Synapse PACS in order to compress other types of radiologic images, including musculoskeletal and CT images, for transmission between computers and for both short- and long-term storage.

“Compression has been slow to make it to mammography because of the resolution of the images and the fear that once you compress them, you’ll lose some vital information.”
—Stamatia Destounis, MD, Elizabeth Wende Breast Care

Destounis continues, “If you’re looking at a bone film, you’re looking specifically at a fracture or some other irregularity in the bone. You’re not looking for a small abnormality like cancer. Everything is so subtle with mammography. If you compress these subtle changes in glandular tissue, that could interfere with your reading.”

While lossless compression has become acceptable in mammography, lossy compression that in any way sacrifices data has remained the purview of other modalities. “Perfection is a big thing in breast imaging,” Destounis notes. “There’s a lot of governance that goes along with mammography.”

Destounis adds that because mammography was one of the last modalities to become digital, most facilities haven’t yet had to grapple with long-term storage of breast images. “We’ve been living in a screen/film world,” she says, “but in the past few years, as more and more facilities move into the digital sphere, we’re beginning to see these issues.” With file sizes routinely exceeding 60MB per study, the cost associated with storing mammographic images could be yet another barrier to widespread adoption of full-field digital mammography.

“These files can be quite large, and it’s impossible to store them forever in their original form,” Destounis says. “Some compression has to occur, or it slows down your PACS, and you have to invest so much money in your archiving and disaster recovery.” Large file sizes also lead to difficulties in image transmission that are not typically encountered when working with other modalities. “If you had a physician who wanted to look up a patient’s mammograms, but had only a laptop, he or she wouldn’t be able to view the images,” Destounis. “They’re so large that if you tried to transmit them, it could crash the system.”

A Blinded Study

To ascertain whether applying lossy compression to breast images compromised their diagnostic usefulness, Destounis and three colleagues at Elizabeth Wende—Posy Seifert, DO; Patricia Somerville, MD; and Philip Murphy, MD—set up a blinded study in which 16 experienced radiologists compared 100 digital mammograms in three compression modes. “We looked at the original images and then looked at images that were compressed at 20:1 and 40:1,” Destounis explains. “All images were in pairs, so you could be comparing lossless with 20:1, lossless with 40:1, or 20:1 with 40:1.”

The radiologists were instructed to compare each pair of images and decide which was more compressed, with the expectation that at a certain compression level, there would be a sufficient loss of fidelity for the image to be harder to read. “When you take a phantom in a lab, or any image that may have calcifications or masses, and you compress and compress, there’s a point where things start to lose their crispness,” Destounis says. “It was thought that when the images were irreversibly (lossily) compressed, you really couldn’t see as well.”

Readers recorded whether the quality of the images was sufficient for comparison with next-year mammograms and which image was more compressed. If the radiologist decided that both images were sufficient for comparison with next-year mammograms, he or she was asked to record that preference on a 7-point Likert scale ranging from +3 (meaning