All the display vendors knew it. The 2005 meeting of the Radiological Society of North America was to be the first color RSNA. Grayscale is out and color is in. Why? Because who wants to be limited to shades of gray when there is so much more to see on a PACS workstation? Color overlays have been an integral part of Doppler flow imaging in ultrasound for decades, and most nuclear medicine images are better viewed using color scales. The need for color display in radiology images has only increased with the growth of PET-CT imaging, functional MRI, and computer-aided detection (CAD). The use of color in graphical user interfaces—things like buttons, scroll bars, and menus—makes them a lot easier to see and understand how to use. Color displays make so much sense that it makes one wonder why it took so long for them to be offered in the medical imaging market, and what makes them different from regular color LCD displays. The answers lie in the history of medical imaging monitors.
When the first picture archiving and communications systems (PACS) were deployed in the early 1990s, computer processors and networks were just barely capable of handling the data requirements of PACS. The standard computer monitor at the time was the color cathode ray tube (CRT), a technology still used in most televisions and many computer monitors today. These monitors were inadequate for diagnostic radiological image display for several reasons, but one of the main ones was that they were not bright enough. They could not display the minimum 256 shades of gray sufficiently different from each other to be discernable to the human eye. The step change from one shade of gray to the next was too small to make every step noticeably different from the next one.
BIRTH OF THE GRAYSCALE
 |
A new industry cropped up to meet the challenge of digital radiological image display. The grayscale, also called monochrome, CRT was introduced. Designed primarily to meet the needs of radiography, the monochrome phosphors they used could phosphoresce up to just over 50 foot-lamberts, the minimum brightness advocated by the American College of Radiology in its guidelines for digital image display. The earliest ones were not high resolution at under 1 megapixel (MP), but they were deemed adequate and put into use. Over time, these displays increased in brightness and resolution until they reached about 150 foot-lamberts and up to 5MP. Since grayscale display was at least a part of all radiology modalities, no distinction was made between radiography and CT, for example. Such a display was needed for both, and color simply was not an option.
In the late ’90s and early 2000s, liquid crystal display (LCD) technology spread through the consumer market. These displays were thinner, lighter, brighter, and sharper, and maintained their geometry longer than their CRT counterparts. Not missing a beat, the medical display manufacturers introduced their own version of LCD displays, which were grayscale and higher resolution: 3MP as opposed to the 1MP originally available for consumer LCDs. These displays were manufactured without the color filters generally used in the manufacturing process for one main reason: to make them brighter. Without the color filters, these LCD displays could reach 200, even 250 foot-lamberts, although the typical running brightness is set to about 120 to 150 foot-lamberts to prolong the life of the backlight and to not blind the radiologist.
It was well known that another way to make a brighter LCD display would be to simply boost the brightness of the fluorescent light bulbs in the back of the display. However, at that time, no such light bulb existed that could produce enough light without also producing too much heat, which would damage components of the display. This, in fact, is what changed in the past year. LCD backlight technology has advanced so as to produce light output levels in color LCDs fairly close to those of yesterday’s grayscale displays without producing too much heat.
Most consumer color LCD displays do not use these new light bulbs yet, but the Dell 2405 24-inch color flat panel display does. Designed for both computer display and DVD movie display, it runs at 2.3MP and the brightness can be set up to about 120 foot-lamberts. It also costs under $2,000 for a pair as opposed to the typical $13,000 for a pair of grayscale LCDs. This is very significant because the monitors are by far the bulk of the cost of a PACS workstation. Assuming a computer costs about $1,000 and adding a smaller color LCD display for such functions as worklists and web access at $500, these Dell monitors can bring the hardware price of a PACS workstation down from $14,500 to $3,100—almost five times cheaper.
DUE DILIGENCE
 David Hirschorn, MD |
In light of these changes in market offerings of displays, we have performed two studies of using consumer displays for primary diagnosis at Massachusetts General Hospital. The first involved typical 19-inch color LCD displays, which run at 1.3MP and about 70 foot-lamberts, commonly found in office desktops and homes. A pair of these displays was calibrated to the DICOM Part 14 Grayscale Display Function using VeriLUM from Image Smiths Inc (www.image-smiths.com). Two readers read 100 randomly selected CT scans on these test displays and recorded their findings for each case. Each finding was given a subjective measure of conspicuity on a scale of 1 to 5. After a time delay of at least 1 month, the readers were presented with the same cases in reverse order on a pair of 3MP grayscale LCDs running at 150 foot-lamberts, once again recording their findings and grading conspicuity. Not surprisingly, there was no significant difference in the findings between the two reading sessions. Most important, the 3MP grayscale displays did not show any new findings that were not seen before, and none of the findings were significantly more conspicuous.
Our second study examined the Dell 2405 (2.3MP, 120 foot-lamberts) against the 3MP grayscale (3MP, 150 foot-lamberts), this time for primary diagnosis of radiography (CR and DR). This study was similar in design to the previous one with one key difference. Most CT examinations have at least some positive findings, whereas most radiographs are negative. One radiologist observed that if he called all radiographs negative, he would be right 87% of the time! Therefore, 100 of the cases were selected specifically for containing subtle positive findings, such as small pneumothoraces or fractures. And 30 normal radiographs were added for balance. Again, all cases were read on the test monitors first by two readers, and then again at least 1 month later by the same readers in a different order. The results are only preliminary at this point, but they do seem to indicate no significant difference in the interpretations on both displays.
 Keith Dreyer, DO, PhD |
We at MGH believe that certain consumer color LCD displays, when properly deployed and calibrated, can be used for PACS workstations, even for radiography. As consumer displays improve in brightness and resolution, they will continue to drive down the cost of medical grade displays. What is more, the line between medical grade displays and consumer displays has become blurred and will probably only get blurrier over time.
The benefits of this trend are not just cost savings in PACS workstation hardware for hospitals and imaging centers. At under $4,000, health care institutions can afford to deploy many more workstations than previously thought possible and in more locations throughout the facility, including in doctors’ homes and offices. This increased access to diagnostic quality images should ultimately lead to improved patient care.
David Hirschorn, MD, is a research fellow in radiology informatics at Massachusetts General Hospital/Harvard Medical School and director of radiology informatics at Staten Island University Hospital. He is a member of the medical advisory board of a major monitor vendor.
Keith Dreyer, DO, PhD, is vice chairman for radiology informatics at Massachusetts General Hospital/Harvard Medical School. They have recently released the second edition of PACS: A Guide to the Digital Revolution.
