DICOM Conformance: The Differentiator Between Medical- & Consumer-Grade Displays
“For a reliable and correct diagnosis, the image quality of the monitors is indispensable,” says Jochen Hansmann, MD, assistant medical director for diagnostic radiology at the University of Heidelberg Clinical Center in Heidelberg, Germany. “The best radiograph may become useless, if the image is not displayed on the monitor correctly and faithfully in detail.”
Heidelberg’s radiology department consists of 25 radiologists and 50 medical technologists. Earlier in the decade, Hansmann recognized the advantages and potential of making diagnoses with digital data. He was significantly involved in the decision in 2003 to install a RIS/PACS in the department. With the further installation of a hospital information system (HIS), the entire facility became filmless by 2005. Today, every RIS/PACS station consists of three monitors: two for diagnosis (a 2 megapixel [MP] monochrome and a 2 MP color) and one for showing RIS data (1 MP color).
Hansmann and colleagues chose the RadiForce monitors from Eizo. “The best feature was the grayscale reproduction of these color monitors. Up to now, the quality with color monitors was lower than with grayscale monitors. But with these monitors, there is no difference,” Hansmann says.
Choosing a medical displayStandard computer displays offer limited resolution that is not optimized for diagnostic imaging. Medical-grade displays, on the other hand, offer resolutions up to 2048 x 2560 (5 MP) in portrait or landscape that correspond better to the image format of DICOM medical images.
Consumer-grade displays typically offer a maximum luminance of 250 to 300 candela per square meter (cd/m2). State-of-the-art medical displays, by contrast, achieve luminance levels of more than 1,000 cd/m2, which is much closer to conventional film. According to DICOM 3.14, “a larger luminance range results in a broader spectrum of grayscales that can be discerned by the human eye [also known as Just Noticeable Differences or JNDs].” As a result, subtle lesions will be easier to detect on a medical display and radiologists can reach a diagnosis faster.
Luminance is not the only important parameter for diagnostic reading. For many applications, contrast is even more important than luminance. Medical displays offer a contrast (up to 1,000:1) that is substantially better than most consumer displays, which have on average contrast ratio of only 300:1. Displays with better contrast ratios are capable of rendering more DICOM JNDs than their low-contrast counterparts.
The number of available shades of gray on most consumer displays is limited to 256 (8 bit). Medical displays have a much wider grayscale range, enabling them to render every grayscale as defined by DICOM. The Coronis grayscale display family from Barco, for instance, offers up to 4,096 shades of gray (12 bit), while the Dome family of displays, part of the NDS Surgical Imaging portfolio of systems, can be optimized for up to 3,061 shades of grays.
Such an extensive range is necessary to comply with the guidelines set forth by American Association of Physicists in Medicine (AAPM) and the European Reference Organization for Quality Assured Breast Screening and Diagnostic Services (EUREF).
All LCD displays, however, suffer from luminance non-uniformity. This means that images will appear slightly differently in the corner of the display than in the center. This luminance non-uniformity can be as much as 25 to 30 percent. But the bar for diagnostic image quality is being continuously raised. AAPM and EUREF already have proposed to limit this non-uniformity to 10 percent. Medical display vendors such as NDS Surgical Imaging, Barco and Eizo typically offer a luminance uniformity correction function that provides homogeneous brightness from one corner of the screen to the other.
Right display/right userMedical images are typically viewed by many physicians outside of radiology. But not every physician needs the same high-quality display used in radiology. A dual-head 5-MP grayscale monitor, for example, would be complete overkill and much too expensive for use in emergency rooms, operating rooms, doctors’ offices and nursing stations.
To distribute digital imaging throughout the enterprise, hospitals need displays whose performance, specifications and costs are tailored to the needs of each user. They need a suite of displays such as:
- Diagnostic displays, which are used for radiology and mammography where high resolution and grayscale rendering are absolutely critical. These displays are DICOM-calibrated 2-, 3- or 5-MP grayscale monitors and are at the high end of the cost spectrum.
- Referral displays, which are designed for use in ORs, ERs, intensive care units and other care-giving areas where excellent contrast is important, but not at the level required for radiology. These displays are typically 2- or 3-MP monitors and have DICOM calibration as well.
- Clinical displays, which are used in physician offices, nursing stations and for patient-bedside education. These 1-MP desktop displays can be used for multiple purposes, but support DICOM calibration for consistency of image viewing.
- Specialized displays, which are designed for specific medical uses, such as mobile viewing station solutions for use in the OR.
The DICOM curve ballThe way the human eye responds to contrasts in light levels is not linear. At low levels, humans can notice small changes in luminance. At higher luminance, the change needs to be much greater before the human eye can perceive the difference from one level to the next. A curve can be plotted that shows measured luminance levels versus increments of perceived difference. This sensitivity to changes in contrast is known as the DICOM curve and was developed by the AAPM.
On the DICOM curve, an image is represented on a scale in incremental values from zero to 1,023, with zero being the darkest and 1,023 the lightest. Each grayscale increment corresponds to an increment in luminance, between 0.05 cd/m2 and 4,000 cd/m2, which can just be noticed by the eye. The result is a mapping of levels of luminance versus steps in visual perception. As a result, this DICOM curve describes the specific grayscale output of a display in a defined range of luminance values that are nearly linear in perception.
By allowing the standardization and mapping of the contrast sensitivity to that of the human eye, DICOM calibration assures that a given gray level will appear the same from one display to the next. It’s the standard for viewing of grayscale medical images and is designed to reproduce approximately the same experience as one would get when viewing film.
Quality controlAlthough flat-panel displays generally don’t require the constant maintenance that cathode ray tube displays did, they do require a rigorous quality assurance (QC) program to ensure DICOM calibration of their display systems. QC in the radiology reading room is pretty straightforward, but becomes more challenging as images migrate throughout the hospital enterprise. It would be inefficient to perform QC checks on each individual display. Vendors today offer dashboard-type tools that allow PACS administrators and IT managers to view and correct calibration problems from a central location.
The Dome Dashboard, for example, offers central management of Dome displays across the enterprise. These dashboard solutions typically collect and summarize data into organized reports, which also serve to help facilities comply with local and national accreditation standards. Eizo offers RadiNET Pro QC software, which the Worcestershire Royal NHS Trust in the U.K. uses.
“Our administrators are now able to track all networked monitors both onsite and remotely, observe usage hours, plan quality assurance, implement plans remotely and have all the benefits of being able to manage their assets from a single admin control point,” says Richard Szabranski, PACS manager at Worcestershire Royal NHS Trust.