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Digital vs. Analog Mammography

Just as consumers today have to decide between analog and digital television sets, so, too, do physicians and radiology centers have to choose between analog and digital mammography units, with the latter rapidly gaining ground in imaging centers around the country.

As of late 2003, the U.S. Food and Drug Administration (FDA) had cleared four digital mammography units for clinical usethe Senographe 2000D Full-Field Digital Mammography systems, approved in January 2000 and manufactured and marketed by GE Health Systems; the SenoScan, approved in Sept 2001 and manufactured and marketed by Fischer Imaging; and the LORAD LDBI, approved in March 2002 and Hologic/Lorad Selenia Full Field Digital Mammography (FFDM) System, approved in October 2002, both manufactured and marketed by Lorad/Hologic. Fuji-Film Medical Systems is also pursuing a digital system.

Like analog mammography, digital mammography is basically an x-ray of the breast used for both screening and diagnosis. Patients notice no difference between the two forms, as the breast is squeezed between two plates and x-rays pass through the breast tissue.

Unlike traditional mammography, however, which uses film as both a receptor and a display for the image to produce static, fixed images, digital mammography uses detectors (similar to those found in digital cameras) that change the x-rays into electrical signals. These signals are then transferred to a digital receptor that converts the x-ray energy to numbers, processes the numbers, and produces an image that can be displayed on a monitor or printed on a high-resolution laser printer.

Digital detectors create an electronic image as pixels, either indirectly or directly. They use either charge-coupled devices (CCDs), flat-panel arrays or computed radiography cassettes. CCDs are self-scanning devices that provide an electronic readout of an image produced by a scintillator. Electrons, whose numbers are related to the image *produ ced by the scintillators, are then digitized to produce an image. One major problem with the images is that they can only be 5 x 5 cm, much smaller than the typical breast.

The flat-panel arrays, another indirect system, involves the detection of light by a series of smaller (100m) photo diodes arrayed over a flat panel measuring 19 x 23 cm. The special resolution is 100 m, with a similar resolution to CCD. It can be used to cover the entire breast.

The computed radiography cassette, utilizes laser beam scanning to de-energize the plate, resulting in light that is detected by a photomultiplier tube and then produces a digital image. It produces a special resolution of 50 m.

Digital mammography also has an intrinsically wider dynamic range, displaying all breast structures from dense parenchyma to skin. Because there is a lower level of intrinsic noise, a lower dose of x-ray may be used than is currently used in conventional mammography with no loss in accuracy. The imaging process can also be used to create display "windows" for masses and calcifications.

The soft copy display provides a direct-view display of the digital image on cathode ray tube (CRT) monitors or liquid crystal display (LCD), allowing for unlimited manipulation of the contrast and brightness of the image, including image reversal and electronic magnification.

Studies find several advantages to digital mammography systems, including:

  • A higher detected quantum efficiency (DQE), especially near the skin line and in dense breasts, providing better detection and diagnostic capabilities at a lower radiation level.2 One reason for the improved detection is that digital mammography can distinguish more shades of gray than film mammography.
  • The capability to acquire images in near-real time and to process them, i.e., to vary brightness and contrast levels, auto-windowing, image manipulation. Information can also be coupled with patient information such as risk probability analysis or other pertinent personal history data for improved decision-making.
  • The ability to electronically archive films. This not only reduces the risk of misplacing or damaging films, but provides for immediate electronic transmittal via the Internet for a second opinion, regardless of where the physician in located. Digital imaging is also ideal for telemedicine and mobile mammography, with units able to transfer digital images thousands of miles with no loss of viability.
  • Exams take less than half the time as with conventional, film-based mammograms, with a digitally displayed image available in about 10 seconds, compared to two minutes for a conventional film mammogram, which must then be developed.
  • Fewer patient callbacks because the ability to generate images within seconds and the flexibility to magnify and manipulate those images eliminate many call backs necessary with traditional film-based mammography systems.

    Several small trials suggest certain efficacies of full-field digital imaging over film-based screening. For instance, one study published in the journal Medical Physician, found that full-field digital mammography optimized low-contrast lesion detection by using a softer x-ray beam for thin breasts and a harder x-ray beam for thick breasts, resulting in superior detection compared to screen-film mammography for all but the thinnest breasts.

    Another analysis of the comparative performance of 18 digital mammography systems and 38 screen-film mammography systems showed that digital systems generally improve image quality for equal or lower breast doses, and provide tighter control on exposures and image quality.

    To determine the overall efficacy and accuracy of digital mammography compared to conventional mammography, the National Cancer Institute (NCI) and the American College of Radiology Imaging Network (ACRIN) launched in 2001 a large-scale, multicenter trial to compare t he two for breast cancer detection. The Digital Mammographic Imaging Screening Trial (DMIST) will involve 49,500 women in the United and Canada and cost $26.3 million. It is designed to test what smaller, earlier trials have suggested: that digital mammography may provide better detection of early breast cancer.

    Manufacturers are also pursuing numerous enhancements and improvements to existing digital mammography technology, including tomosynthesis, which reconstructs postacquisition two-dimensional data as three-dimensional representations, and contrast imaging using the digital scan.2 Additionally, several compute-assisted decision programs have been approved for use with digital mammography systems.

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