Tips and tools optimize soft-copy displays

Monitors meet digital mammography's demanding standards

By: Deborah R. Dakins

Due to its high spatial requirements, mammography is one of the most challenging applications for soft-copy display. It's not surprising that it is among the last digital modalities to migrate from cathode-ray tube to flat-panel liquid crystal displays. In fact, it is only in recent months that FDA-cleared LCDs for digital mammography have become available.

Achieving and maintaining high-quality soft-copy display is critical in all imaging modalities but is particularly important in digital mammography. Whether the display is a CRT or LCD, multiple factors come to the fore in assuring optimal soft-copy displays: luminance, contrast, resolution, sharpness, and uniformity across the display.

"On mammography images, you are looking at very subtle gradations in shading," said Lynda Domogalla, marketing manager for Barco, a display manufacturer headquartered in Belgium. "And within those gradations, you are looking for very small details. As a result, both contrast and resolution are very important."

Another important but behind-the-scenes player in soft-copy optimization is the display's graphics controller. It networks with the workstation computer to load digital images onto the monitor and must adeptly manage large digital mammography data sets that can range from 120 to 200 MB per study.

"The graphics controller powers the display, and it must have enough memory so that the entire data set of current and prior images can be manipulated," said Albert Xthona, product manager for digital mammography at Barco. "If a radiologist wants to switch between the craniocaudal view and the mediolateral view, for example, it needs to happen instantly."

Researchers at the University of Arizona have spent years defining which parameters in soft-copy display lead to best performance and image quality. Published research has focused on CRTs but holds implications for LCDs as well.

Studies based on UA research have documented the importance of luminance and have determined that higher is better. Moreover, published findings illustrate the myriad soft-copy display factors that influence radiologist performance, such as the type of phosphor used in manufacturing monitor faceplates. The UA team was the first to establish that radiologists perform better when displays are calibrated to the DICOM standard. Ongoing work focuses on the impact of monitor veiling glare on user workflow.

"We've systematically addressed all the physical param-eters of (CRT) monitors that could potentially influence the performance," said Elizabeth Krupinski, Ph.D., an assistant professor of radiology research

at UA.

At the 2004 Society for Computer Applications in Radiology meeting, Krupinski and colleagues presented a study of the impact of modulation transfer function (MTF) on digital mammography soft-copy interpretation.

CRTs are inherently nonisotropic, with MTF that is different in the horizontal and vertical directions. As a result, images displayed on a CRT monitor are degraded in both spatial and contrast resolution, she said.

"You lose contrast modulation in the higher spatial frequencies. The potential exists in digital mammography, especially with microcalcifications, to not be able to see them or characterize them as benign or malignant. Since seeing sharp detail is important, we wanted to find if image processing could compensate for what the monitor is degrading," Krupinski said.

The UA team devised an algorithm that compensates for limitations in MTF and found that it improved radiologists' performance when viewing soft-copy mammograms. Moreover, radiologists did better during the study at identifying microcalcifications in images processed to compensate for MTF versus those that were unprocessed.

ISSUES FOR LCDs

With the emergence of flat-panel LCDs, new display issues have emerged that need to be investigated and resolved. One of these is angle of regard.

When LCDs are viewed off-axis as compared with straight on, the gray levels change. Krupinski and colleagues found that the effect, most prominent at 45¼ off center, can have a substantial effect on radiologist performance.

Because it's common practice, particularly at teaching sites, for radiologists to share findings with residents or clinician colleagues who may be sitting side by side, the study carries a practical message.

"The person who is providing image interpretation really needs to be looking at the monitor straight on," Krupinski said. "If you are sitting on the side, you have to be aware that LCDs are not perfect yet."

The situation has improved greatly as vendors have introduced new generations of LCDs, according to Krupinski. In the past, looking just a bit off-axis would cause everything on the monitor to disappear, but newer versions are much better.

These and other issues, such as maintaining pixel uniformity in LCDs, highlight the importance of quality control for digital mammography displays.

"All displays are going to introduce some amount of noise, which means that some parts of the screen are inherently a little brighter or a little darker," Xthona said. "This unevenness is added to the image."

To counteract that effect, Barco's newly introduced Coronis 5MP Mammo LCD display system includes proprietary algorithms. Per-pixel uniformity technology measures and adjusts luminance output at the pixel level and eliminates screen noise, ensuring that uniformity is corrected to the DICOM curve independently. Defect pixel compensation technology identifies defective pixels in each individual display.

Barco's LCD digital mammography displays benefit from intervention-free quality assurance software that performs ongoing QA and DICOM calibration in the background.

"Quality assurance requirements are much higher for digital mammography than for other imaging applications," Xthona said. "It's very important that a display system have a well-designed QA program that doesn't add extra effort to the medical practice."

HIGHER STANDARD

The high standard for mammography displays is illustrated by the keen attention the FDA pays to new products. While existing PACS workstations that have already cleared the agency's 510(k) process can be used to review mammography images, the monitors used as part of the workstation must be cleared for mammographic image display. At a minimum, the displays must be able to capture 5-megapixel full-breast images and express sufficient diagnostic detail.

Newly introduced workstations and monitors developed specifically for digital mammography must meet a higher threshold of performance to obtain 510(k) approval. In recent months, a handful of products tailored for digital mammography soft copy have met the FDA standard, including Agfa's IMPAX PACS, a multimodality dedicated mammography workstation that uses CRT displays, Barco's CRT and LCD dedicated digital mammography displays, and Planar's LCD monitor.

As a mature display technology, CRTs have already met and managed most optimization issues for soft copy. But the march is on toward LCDs. Spurred by the promise of improved brightness, life span, image uniformity, resolution, life-cycle costs, and performance enhancements, many sites now routinely read images on flat-panel monitors. Now that LCDs are available for digital mammography, the transition to that modality is not far behind.

FUTURE VIEWS

Achieving the highest resolution possible continues to be the defining factor in optimal digital mammography soft-copy display. Solutions could arise from a $1.2 million grant project sponsored by the National Cancer Institute. The project will integrate and test use of an advanced 9-megapixel display in Fischer Imaging's SenoScan review workstation.

The display incorporates IBM's T221 flat-panel unit with a viewing area that is 25% larger than current CRT monitors. The workstation was shown as a work-in-progress at last year's RSNA meeting, and clinical evaluation will begin at the University of North Carolina by 2005.

But Brian Hemminger, an adjunct assistant professor of radiology at the University of North Carolina, Chapel Hill, cautions that such a display, should it reach commercial introduction, is likely to be too pricey for use beyond the research level.

"First, we have to determine if the increased resolution is better, and then whether it is worth many thousands of dollars more to achieve," he said. "Clinical trials will tell us how much better 9 megapixels is in terms of quality of care, and then users can decide if it is worth the extra investment."

"There are potentially unlimited applications for stem cells," said Dr. Jeff W.M. Bulte, an associate professor of radiology at Johns Hopkins School of Medicine. "People have shown that in many different animal models."