Helvie, M.D., Mark
Article by Mark Helvie, M.D.
FULL FIELD DIGITAL MAMMOGRAPHY
A new type of mammography equipment called full-field digital mammography has generated a good deal of press during its first year of availability in the United States. Mammography remains the best method of early breast cancer detection. However, traditional film-screen mammography is limited in its ability to detect some cancers, especially those occurring in women with radiographically "dense" breasts. For this reason, extensive research efforts to improve mammography have occurred. Digital mammography offers theoretical advantages compared to film-screen mammography for cancer detection. I will discuss this new technology, its current use, and future applications.
What is digital mammography?
Conventional film screen mammography uses low energy x-rays that pass through a compressed breast during a mammographic examination. The exiting x-rays are absorbed by film (x-ray film) which is then developed into a mammographic image that can be held and looked at by the radiologist. This traditional process is analogous to personal photographic cameras and photographic film where light is focused on the film and developed to produce a negative which can be printed as a picture. With digital mammography, low energy x-rays pass through the breast exactly like conventional mammograms but are recorded by means of an electronic digital detector instead of the film. This electronic image can be displayed on a video monitor like a TV or printed onto film. Again, this is similar to digital cameras that produce a digital picture that can be displayed on a computer screen or printed on paper. The radiologist can manipulate the digital mammogram electronically to magnify an area, change contrast, or alter the brightness.
Most radiographic imaging is currently done by digital means including CT scans, ultrasound, MRI, nuclear scans, and the some standard radiographs such as bone or chest examinations. Mammography was the last area of radiologic imaging to use digital technology due to the unique needs of mammography. The high resolution and contrast needed to produce excellent quality mammographic imaging exceeds all other areas of digital imaging. For this reason, the technology to produce digital mammography is very complex, expensive, and time consuming to develop reliable systems. Patients undergoing digital mammographic examinations will see few differences during the examination itself. There is more equipment in the room which includes a small TV monitor which allows the mammographic technologist to view the mammogram in several seconds instead of developing films and waiting ten minutes to see an image. Breast compression, unfortunately, is still required to produce optimal images at the lowest possible radiation dose.
Digital mammography offers potential and practical advantages over film screen mammography by improving contrast and signal to noise ratios. These may allow better cancer detection. Near instantaneous image acquisition and the ability to transmit images electronically around the world are also significant advantages. One can have a mammogram at a remote site with the images sent, displayed, and interpreted at a medical center. However, the most important scientific application is the potential to use advanced computer and electronic technologies to manipulate the image in order to better "see" certain breast tumors that are currently difficult to visualize on film screen mammography systems. These applications include direct computer-aided diagnosis (computer reading of mammograms), 3-D imaging, dual energy mammography, and the contrast-enhanced mammography. Most of these processes remain in the development stage.
Current Digital Mammography
The Food and Drug Administration regulates all mammography equipment. The FDA approval process for digital mammography equipment in the USA requires manufacturers to show that their equipment performs at least at the level of film screen mammography. There was not a requirement that the digital systems needed to be superior. Several manufacturers have now met the FDA requirements and offer full field digital systems, which should perform at least as well as film screen mammography.
Although there have been scientific reasons to expect superior performance of digital systems, the largest clinical trials involving several thousand women undergoing screening mammograms with both film-screen and digital systems have shown no significant difference in cancer detection between film screen mammography and digital mammography. While some critics have considered these similar results to be a "negative" for digital mammography, one must remember that to achieve similar cancer detection performance with first generation equipment is a major accomplishment. One study showed the call-back rate from screening (needing to return for additional diagnostic mammographs after a screening mammogram) was better with full field digital mammography than screen-film systems. That is, fewer women needed to return for extra views with digital imaging. This is probably due to the ability of the radiologist to magnify an image and change contrast with digital systems. Since digital images are available within seconds of exposure, interventional procedures such as needle localizations requiring x-ray guidance performed on digital machines are much faster for the patient. The time the breast is in compression is shorter.
Equipment costs for digital mammography systems is 3-5 times the cost of film screen mammography. Additional on-going costs of maintenance and image storage compound the price differential. Some insurers, such as the federal government, have recognized these cost differences and reimburse digital mammography at a slightly higher rate than film screen mammography.
Full field digital image (Fig. 1) shows a small mass with calcifications (arrow). Notice that mass is better demonstrated on the digital image than on the standard film screen mammogram (Fig. 2). This mass turned out to be an invasive cancer.
Much digital research is directed at current weak areas of film screen mammography, particularly the detection of cancerous masses in women with dense breast tissue. Dense breast tissue can mask or camouflage some cancers from mammographic detection. Using 3D imaging, contrast, and dual energy methods holds promise that currently invisible cancers will be accurately shown and detected. Directly applied computer-aided diagnostic systems may also improve detection. Scientific proof for these applications will require time.
Digital mammography is in its infancy similar to CT and MRI scanning of 20 years ago. Current technology offers similar imaging quality to film screen mammography at a much faster speed benefiting women undergoing biopsies and reducing the number of call-backs. On-going advances in electronic and computer technology are expected to produce improvements in digital mammography that would allow better breast cancer detection especially in those women with radiographically dense breasts.
Mark Helvie, M.D.
Professor of Radiology, Director of Breast Imaging Division
University of Michigan Medical Center, Ann Arbor, MI.