John Weiser, PhD

While the predominance of diagnostic medical imagery has been displayed on grayscale monitors, there has been a growing architectural shift in the construction of diagnostic workstations to include a color monitor as a third display device on a two-monitor workstation. The reason for this architectural shift is that more and more information associated with diagnostic imaging is in the form of text or color. Examples of the color monitor usage on a diagnostic workstation include:

  1. Display of text from the PACS database and/or voice dictation system, which is desired to be displayed in a larger size and lower brightness than could be displayed on a diagnostic grayscale monitor
  2. Display of color editing markers, color image overlays, color 3D renderings, and small format color images from modalities such as nuclear medicine and ultrasound.

The need for color information in association with images varies depending on the workflow of a given physician or technologist. Some physicians and technologists work primarily in the high-resolution grayscale modality environment, and therefore have little need of color information directly in association with images. The primary example of this is radiologists and radiology technologists who work in the projection radiography environment. With input modalities being primarily CR and DR, where the image resolution is 2K by 2.5K, and the modalities produce no color overlay information, the principal requirement of a PACS workstation is to have high-brightness, high-resolution, high-contrast grayscale display monitors. The use of color monitors in these workstations conflicts with the primary image display function and is best isolated onto a smaller, less bright, color-based monitor that is integrated into the overall workstation design.

While other modalities such as digital fluoroscopy, angiography, CT, MRI, nuclear medicine, and ultrasound also produce primarily grayscale images, there are some significant color components to either the primary images or postprocessed renderings of those images. Additionally, the image matrix size is small enough and the typical image contrast is high enough that color monitors currently on the market are able to adequately accommodate diagnostic display quality requirements.

Radiology reading environments where the modalities or their postprocessed studies typically contain color components include ultrasound, nuclear medicine, MRI, and CT. The use of high-resolution 2MP color monitors makes good sense on workstations dedicated to reading these modalities.

Outside of the radiology department, there is an even greater need for color display capabilities on PACS workstations. Clinicians who work in the emergency department wards, clinics, and operating rooms treat a wide variety of patients who have been imaged with a wide variety of modalities. To satisfy the image viewing requirements of these modalities, high-brightness, high-resolution color monitors are required. This requirement for color image viewing capabilities, as well as high-resolution grayscale image viewing capabilities, extends well beyond the walls of the radiology department into the wards, clinics, ICUs, operating rooms, examination rooms, treatment rooms, physician offices, and even physician homes. The general trend in medical imaging is for the spatial resolution to continue to increase and for the use of color overlays, such as orthopedic templates, textual information, and postprocessed imagery, such as 3D reconstructions, to also increase.

IMPROVED TECHNOLOGY

The situation for color LCD monitors has changed in recent years, thanks to the hundreds of thousands of consumers who sit in front of their computers, holding their game controllers, and demanding ever better graphics performance and visual detail for simulated reckless driving and other forms of violence. The LCDs and graphics cards being mass produced to satisfy the consumer gaming and video market have begun to approach the size, resolution, and performance that are needed in the medical market. Notice that we said “have begun to approach.” So, before we decide whether to replace those grayscale displays, or decide where to purchase a new color display, we may wish to consider a few points.

John Romlein

Contrast Ratio . The contrast ratio is the maximum white level divided by the minimum black level. It is the ratio, rather than the difference between white and black, that determines the amount of contrast that will be perceived by a human using the display. But this ratio does not relate exactly to the real world. In the real world, we have some amount of ambient light coming from our surroundings, reflecting back at us from the monitor, and adding to both the black level and the white level. When we account for this ambient light, we get a new term, called the “luminance ratio,” which gives us a better indication of the actual perceived contrast.

For comparison, we will look at three different LCD monitors that all have the same published contrast ratio, 600:1. They are all “2MP” displays (1600×1200 resolution), but each has a different maximum brightness. A typical 20.1-inch-diagonal, 2MP, consumer-grade LCD has brightness rated at 300 cd/m2. The typical ambient light reflected from the monitor in a dark radiology reading room is about 0.2 cd/m2. If we account for this ambient light, the luminance ratio is 430:1. So the effective contrast is not 600:1, but only 430:1. If we upgrade to a 500 cd/m2, high-brightness, medical-grade color LCD with the same contrast ratio, the luminance ratio is 485:1, and a 900 cd/m2 grayscale LCD has a luminance ratio of 530:1. Lots of numbers here, but the take-away point is that three monitors with the same published contrast ratio have very different perceived contrast when ambient light is taken into consideration. The brighter monitor will always yield better perceived contrast.

Published Brightness vs Operating Brightness . The brightness that is quoted in a product specification sheet is the maximum brightness, ie, with the backlight turned up to its maximum output level. This is not the desired condition in which to operate a medical display over an extended period of time. In order to extend the useful life of the backlight and be able to maintain constant brightness over time, medical-grade LCD monitors are typically operated at two thirds of their maximum output. The high-brightness monitor would run at 330 cd/m2, and the grayscale would be at 600 cd/m2. If the same criteria were applied to the consumer-grade color monitor, it would be operated at a mere 200 cd/m2. Along with operating brightness, we must consider brightness stability, which is important for a medical display, especially in the presentation of the darker areas of an image. This level of stability is not needed in the consumer market, so the consumer-grade display will most likely have less sophisticated circuitry and backlight hardware than the medical-grade displays.

EFFORT TO ACHIEVE QUALITY CONTROL

Monitors used for medical image display, whether inside or outside of the radiology department, and whether color or grayscale, should be calibrated to the DICOM Grayscale Standard Display Function (GSDF) and be included in an operational quality control program. At the high end of the medical-grade scale are the monitors that do not require manual calibration adjustment. These monitors maintain a constant brightness level and have individual firmware settings that adjust them to the GSDF standard. They are not quite “set it and forget it,” because their calibration should be verified periodically with an external photometer, but they do reduce greatly the amount of effort required to maintain an effective quality control program, and can also be teamed up with remote monitoring software that provides a console view of the status of the monitors on workstations throughout the network. These features are very useful, but they do come at an increased price. The consumer-grade and lower-end medical-grade monitors should also be calibrated. This can be done manually with a photometer and software that can be purchased from a third-party source. The medical monitor vendors also offer manual calibration software that is principally designed to work with their brand of monitors, and may also work with other products. This manual calibration method makes it necessary to physically visit the workstation location more frequently to verify the calibration and make adjustments if necessary.

CONCLUSION

It is clear at this point that there is an increasing use of color in medical imagery, not only in the radiology department, but also from all other image-dependent “ologies” that are now and will continue to be users of PACS technology. There is, therefore, a constant search for adequate but cost-effective monitors to satisfy the wide variety of medical image display requirements. In the medical environment, the total cost of ownership must include not only the bottom-line price of a monitor, but also the contributing factors of maintainability, reliability, and clinical sufficiency. The challenge for monitor designers, PACS workstation designers, and hospital IT managers is to match the available display technology with the image display requirements in a fiscally constrained environment.

John Weiser, PhD, is a senior partner and founder of Qualiteering Labs, [email protected].

John Romlein is a senior partner and founder of Qualiteering Labs, [email protected]