By Katherine Harmon
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The mammograms most women receive are decidedly two-dimensional. An x-ray machine takes images of the breast from the sides, and radiologists examine the resulting image to see if it offers up any hits of potentially cancerous irregularities. These tests, however, are far from perfect. Normal calcium deposits and fibrous tissue can align to create a composite image that resembles a suspect mass. And small lesions can go undetected if they are hidden among normal tissue.
A team of researchers has added a new dimension to screening for better accuracy. This new technique allows traditional x-ray machines to capture images of the breast in three dimensions and provides a new display to let radiologists examine width, length and depth. The multidimensional snapshot may help to avoid false-positives of tissue that is not really cancerous as well as well as missed lesions that can go on to develop into tougher-to-treat cancer.
Stereographic digital mammography produces an inner view of the breast in a similar manner to the way our eyes and brain create the three dimensional representation of the world around us.
“Our eyes see the world from two slightly different perspectives,” Carl D’Orsi of Emory University School of Medicine, co-author of a paper in the November 13 issue of Radiology, noted in a prepared statement. “In this technique, the x-ray tube functions as the eyeball, with two different images providing slightly different views of the internal structure of the breast.” To create this stereoscopic view, the x-ray technologist shifts the x-ray tube 10 degrees (standard, straight-on x-rays would be at 0 degrees, so the two images are +5 and -5 degrees off center, respectively) to collect two different angles that can be reassembled into a more three-dimensional image of the breast.
For the study, 779 women identified to have a high risk of breast cancer (related to family history or other factors, such as a previous history of the disease) volunteered to have two mammograms completed during a single visit—one with traditional 2-D x-rays and another with the 3-D, stereoscopic technique.
For the stereoscopic view, the system fuses both images and conveys the depth perspective. The research team created a display system that fed the two different perspectives onto respective display monitors that were connected top-to-bottom, with the top monitor angled in toward the viewer, creating a 110-degree angle. The two displays were cross-polarized. A glass plate covered with reflective coating on the top separated the two screens and reflected the top image to a viewer while letting the bottom image through. The technician wore cross-polarizing glasses. “The radiologist’s visual system then fuses the two images into a single, directly visible in-depth image of the breast,” the researchers described in their paper.
Some 12.9 percent of scans done by traditional x-ray indicated that a woman needed a follow-up visit for further testing, but only 9.6 percent of the scans required this with 3-D imaging. The percentage of women who were called for subsequent examinations based on the screening and found to actually have cancer was double that with the 3-D data than with the 2-D approach (although the overall numbers are small, four of 83 and three of 126, respectively), suggesting that the false-positive rate was lower with the new technique. The researchers are not sure if these findings will carry over to the general population, as the women in the study were already at a substantially increased risk for breast cancer.
The additional x-ray snapshot required only an extra 90 seconds for scanning. But it is unclear whether the new technique would increase costs (due to additional equipment and training) or reduce them (because of reduction in false positives and unnecessary follow-up procedures).
Concern has also been mounting that the radiation exposure during mammograms could slightly increase the risk for breast cancer. And the stereoscopic x-ray technique exposed subjects to twice as much radiation as the traditional, single-perspective scan because “we acquired each of the images comprising the stereo pairs with a full standard x-ray dose,” D’Orsi said. But they are working to reduce these levels. “Now that we know the technique is worthwhile, we’re repeating the study in the general population with a dose comparable to routine screening mammography.”