The tools for mammography and its interpretation have undergone a Renaissance over the past couple decades. Film-screen mammography and the classic light-box alternator diagnostic paradigm are rapidly giving way to full-field digital mammography (FFDM) acquisition with DR and CR technology and soft-copy interpretation, either on cathode ray tube or flat-panel liquid crystal displays (LCDs).
The introduction of LCD panel displays in the commercial off-the-shelf computer space has thoroughly supplanted CRT-based monitors for the consumer market. The filmless reading room in radiology also has seen a surge in deployment, in part due to the reduced footprint, easier quality assurance, and longer lifetime of LCD monitor technology.
Flat-panel displays have made great strides in the medical marketplace since their introduction. A combination of liquid-crystal processes tuned for medical applications, greater noise reduction as well as more uniform, stable and powerful backlit illumination have eliminated many of the previous drawbacks of LCDs.
In 2005, Elizabeth Krupinski, PhD, from University Medical Center in Tucson, Ariz., noted that care should be taken when viewing radiographic images off axis on LCD displays.
“Residents and other clinicians who tend to find themselves seated next to or looking over the shoulder of the radiologist seated in front of the display need to be aware of the fact that the LCD display distorts luminance, gray levels and contrast and that these distortions can lead to degradations in performance especially for low-contrast lesions,” she wrote in the Proceedings of SPIE (Vol. 5749).
In the intervening years, LCD panel manufacturers have addressed this issue through the utilization of in-plane switching, which maintains high contrast over a much larger viewing angle—of particular importance in dual- and quad-monitor configurations commonly deployed for radiological reading environments.
The physical characteristics and quality of display are of great import for any diagnostic imaging interpretation, and particularly so in digital mammography. Luminance, contrast, resolution, sharpness, and uniformity across the display are critical for diagnostic-quality digital mammography display. High spatial requirements necessary to display subtle gradations in shading, and the very small details within that shading, have made this sub-specialty among the last to migrate from CRT to LCD monitors.
However, a cornucopia of 5 megapixel (MP) and higher offerings from display developers that meet the specific needs of the sub-specialty, and approved by the FDA for primary interpretation of digital mammograms, is rapidly elbowing out conventional CRT monitors in many mammography reading rooms. In addition, multi-site, scientific studies have confirmed the efficacy of LCD technology for digital mammography interpretation.
Research published in the American Journal of Roentgenology (Dec. 2006) found that 5MP flat-panel monitors are at least equivalent to, and in some aspects superior to, 5MP CRTs in the display of FFDM images.
“As we move out of the research arena with digital mammography and into the high-volume use of this technology, it is imperative that we work to make this technique fit into our preexisting PACS systems,” the authors note. “In doing so, we will help control the overall costs of the systems both in dollars spent and in interpretation time. Because LCD monitors are now widely used for the primary interpretation of other techniques, it seemed logical to evaluate their use for mammography.”
The study determined that the LCDs were rated better for the sharpness of mass margins and mass conspicuity. For calcium features, the LCDs were rated better than the CRTs for lesion conspicuity and number of calcifications
For architectural distortions, there was no statistically significant difference between the monitors in any of the features evaluated. For display characteristics, the LCDs were better for luminance, while the CRTs were significantly better for image noise. In the overall ratings, there was no statistically significant difference between the two displays.
Optimizing flat-panel deployment
When it comes to soft-copy interpretation of digital mammography images in the reading room trenches, LCD monitors trump CRTs for a host of economic and ergonomic reasons.
Radiology is moving away from CRT monitors for a variety of reasons. CRTs typically have a low luminance with 300 candelas per square meter (cd/m2), requiring low ambient light in reading rooms that makes comparisons with light-box based prior images difficult.
In addition, CRTs have a life expectancy of approximately 30,000 hours (about three years of daily clinical use); and they degrade over time, requiring their replacement rather than repair. Because CRTs require the screen to be constantly refreshed due to phosphor stimulation decay, eye fatigue also is a factor for radiologists interpreting digital mammograms on this technology.
From a deployment perspective, CRTs for digital mammography image display are heavy (at approximately 40 lbs.), have a large footprint (decreasing the amount of available desk space), and produce a significant heat output—sometimes resulting in the requirement of additional air conditioning for a reading room. As such, LCDs are quickly becoming the display of choice for radiologists interpreting mammography images.
“We first had CRTs when we started our digital interpretation,” says Stamatia Destounis, MD, with Elizabeth Wende Breast Care in Rochester, N.Y. “When we moved to flat-panel (LCD) monitors, the image quality improved. The black was black; the differentiation in the grays and the whites for the dense tissue was much easier for our eyes and also for interpretation.”
For facilities considering the switch to a flat-panel reading environment, Destounis offers some ergonomic pearls garnered from experience. Physicians should have a comfortable, adjustable chair from which to interpret images and control over the ambient lighting for their reading area, she says.
“You should position yourself no further than an arm’s length away from the monitors, and we’ve found a slight angle between displays in a two-panel setup reduces incidental lighting from one monitor to the other,” Destounis says. “Also, a 15- to 20-degree angle below eye level for the viewing area [where the top of the monitor is at eye level] really helps improve visualization and reduce strain.”
Michael A. Trambert, MD, has been using exclusively LCD-flat panel displays for digital mammography interpretation since the modality was introduced to the Santa Barbara, Calif.-based Cottage Health System.
In the multi-site arena in which Trambert and his group operate, workflow has been an important key to the success of digital mammography interpretation in the flat-panel environment.
“You should digitize your prior exams so that they can be presented as a comparison study as if they are a prior digital mammogram,” he says. “Do so with a digitizer that allows you to break it up into right and left breast and bi-view position so you can take advantage of the user interface when you go to read the current digital exam in comparison with that prior study.”
In addition, a single data repository for all images, mammography as well as other radiological exams, is critical for being able to access all relevant information and studies needed to provide a diagnosis, Trambert says.
“You really want to read all your digital mammography from PACS,” he says. “When you’re reading a breast MR, or a follow-up mammogram in correlation with a breast MR, if it’s on a completely separate system, you might not know a prior exists. You really want it all on the same database; it’s just good patient care.”
From the light-box alternator, to CRT monitors, to LCD panels—the display of images for mammography interpretation has made giant strides in the past two decades; so what’s coming up on the interpretative horizon? Some developers are looking to the 3D projection of mammography images as the next leap forward in diagnostic capabilities.
For example, stereoscopic 3D full-field digital mammography (FFDM) can provide greater positive lesion detection and reduce false-negative lesion detection, according to results of three-year clinical trial conducted at the Emory University Breast Imaging Center in Atlanta.
“The results suggest that stereo mammography could bring a substantial improvement over standard mammography in the accuracy of lesion detection and, with that, substantial gains in the cost-effectiveness of breast cancer screening,” according to David J. Getty, PhD, who shared the trial outcomes at the International Workshop on Digital Mammography (IWDM) in Tucson, Ariz., late this summer.
Stereoscopic 3D FFDM utilizes a combination of image acquisition, software and display technology to achieve volumetric 3D image viewing capabilities for an interpreting physician. Getty, of Cambridge, Mass.-based BBN Technologies, is a developer of the system.
“Stereo mammography exams acquire a pair of images captured with the x-ray tube rotated by 10-degrees between the two acquisitions while the breast remains uncompressed and unmoved,” Getty says.
The stereo mammograms were viewed on a prototype medical stereo display from the firm, the StereoMirror SD2250.
The prototype consists of two 5MP, grayscale monitors, mounted one above the other, with an angular separation of 110 degrees between the faces. The images, each displayed on one of the two monitors, are cross-polarized and are transmitted and reflected onto a coated glass plate that bisects the angle between the two monitor faces.
“The radiologist wears lightweight passive cross-polarized glasses such that the left eye sees only the reflected image from the upper monitor, while the right eye sees only the transmitted image from the lower monitor,” Getty said. “The radiologist’s visual system fuses the two images into a single in-depth image of the internal structure of the breast.”
The Emory study enrolled 1,458 women at an elevated risk for breast cancer who agreed to undergo a standard FFDM study on a GE Healthcare Senographe 2000D as well as a stereoscopic FFDM exam utilizing the same modality. Screening exams consisted of cranio-caudal (CC) and medio-lateral-oblique (MLO) views.
The standard and stereo digital mammograms for each patient were interpreted independently by two radiologists, with prior mammograms available for comparison in 99 percent of the cases.
Standard mammography demonstrated a true-lesion detection rate of 61.4 percent, while stereoscopic 3D provided a true-lesion detection rate of 75.7 percent; a 23.7 percent detection rate.
Standard mammography was responsible for 130 of 192 false-positive lesion detections (67.7 percent), while stereoscopic 3D was responsible for 70 (36.5 percent).
“This 46 percent reduction in false-positive reports with stereo mammography is highly statistically significant,” Getty noted.
He believes that this can be attributed to the volumetric structure of a lesion being visualized on the prototype.
“We believe the large reduction in false positives is due to the fact that normal tissue or unrelated calcifications at different depths, that would be superimposed in a 2D projection and resemble a focal lesion, are seen in the stereo mammogram as layers or normal tissue or unrelated calcifications lying at different depths through the breast,” Getty said.
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