NEITHER cause nor cure is known for breast cancer, a serious disease that may affect one out of every nine women in the United States. Early detection is the only known means for increasing a victim's chances for survival; mammography is currently the best means of cancer detection in women showing no symptoms.|
The power of mammography is proven. Yet, some breast cancers are missed, usually because the cancer is not imaged or because its indications in the image are too subtle to be recognized. The difficulty of visually detecting the cancer's subtle warning signs (in particular, sorting out significant microcalcifications--the calcium-rich deposits that are clues to malignant breast cancers) point to the need to improve image quality and the means of interpreting mammograms.
In 1991, help for improved breast cancer detection came from an unexpected source--Lawrence Livermore scientists and engineers working on national defense projects. They began to recognize that their technologies had important medical applications. Clint Logan, an engineer with expertise in materials imaging, an important aspect of nondestructive evaluations (see article in this issue), proposed using digital computer analysis on film mammograms. His proposal was carried out in a three-part project, first described in Energy and Technology Review, Nov.-Dec. 1992, pp. 27-36. The first part was to digitize mammograms, that is, to convert the data on the film record into numbers, applying a high spatial and contrast resolution to the entire mammogram. When digitized, data could be displayed with a variety of contrast settings, which allow clearer viewing than film studied over a light box.
The second part of this work was to develop computer algorithms to automatically detect microcalcifications in the digitized mammograms. The objective was to provide a "mammographer's assistant" that would quickly and objectively detect and flag microcalcifications for radiologists and doctors. The algorithm, developed by biomedical image processing specialist Laura Mascio, first performs two types of high-frequency analysis on a digitized image. One procedure extracts contrast (intensity difference) information, saving structures that have abrupt changes in brightness (from edges, for example) and are larger than several pixels in size. The other procedure extracts spatial, or size, information and thus saves small, textured structures.
Adding together what has been preserved by the two high-frequency analyses produces an image that is brightest where it contains detail common to both. When a selective erosion or enhancement (SEE) filter is applied over this image, it further reinforces image pixels that show strong evidence of belonging to a microcalcification and erodes pixels that show otherwise. The method developed by Mascio forms the basis of a computer algorithm that distinguishes between microcalcifications and mimicking spots, such as specks and flecks on the film. It was the first microcalcification-detection algorithm to use a gray-scale morphology for extracting frequency and texture information. It served as a model for further development of mammography screening algorithms.
The third part of the project was the design of a filmless, directly digital mammography system. Such a system would provide information and detection superior to the conventional film-based system, yet it would require a lesser x-ray dose to the patient. In collaboration with Fischer Imaging Corp., Logan and Jose M. Hernandez, another Livermore engineer, developed a digital screening unit with a novel x-ray source that can be adjusted for each patient's body size and an image detector that uses a charge-coupled device camera. Early trials indicate that this system yields images with better signal-to-noise ratios than conventional x rays. And because the images are digital, they can be manipulated in terms of contrast, magnification, and area of interest for the best view.
Improving Detection Algorithms
For the Next-Generation System|
As a result of this collaboration, four direct-digital screening systems produced by Fischer Imaging Corp. have been installed at sites around the U.S. Even as they are being introduced to the general population, Jeff Kallman, a Lawrence Livermore engineer, is starting research on the sensors for a new generation of mammography screening. He proposes to generate three-dimensional images of soft breast tissue speedily and painlessly with linear ultrasonic diffraction tomography. Because breast tissue has neither large sonic variations nor appreciable multiple scattering, linear imaging techniques can be used. There is some evidence that cancerous tissue has sound speed and attenuation properties different from normal tissue; the hope is that such an imaging system will be able to distinguish between them.
Data collection would be done while the breast is immersed in water or gel, bypassing the breast compression that makes conventional mammography uncomfortable and even painful for some women. Furthermore, it would involve no ionizing radiation, thus eliminating concerns about x-ray exposure. With appropriate data-acquisition technology, which Kallman is investigating, breast cancer screening in the future would be done quickly as well as safely.
-- Gloria Wilt
Key Words: breast cancer, data compression, detection algorithms, digital mammography, linear ultrasonic diffraction tomography, mammogram library, microcalcifications.
For further information contact J. Patrick Fitch (510) 422-3276 (firstname.lastname@example.org).