Medical Electronics Manufacturing
Magazine
MEM Article Index
Medical Electronics Manufacturing Fall 1998
Display Technologies
Active-Matrix LCDs
Alan Lewis, director, display engineering and development, dpiX
Flat-panel displays are steadily replacing conventional cathode ray tubes (CRTs) in almost all applications requiring the electronic display of data and imagery, and medical systems are no exception. The potential benefits flat-panel displays offer for medical devices are significant for both patients and medical professionals. The performance and reliability requirements for medical displays, however, are particularly challenging and in some cases unique. Nevertheless, recent developments in active-matrix liquid crystal display (AMLCD) technology have provided substantial improvements in optical performance, and AMLCDs are now set to bring about a profound change in the technical infrastructure supporting the practice of medicine.
Flat-panel displays with 24-bit color are ideal for photorealistic and real-time video imaging.
The development of AMLCDs has traditionally been driven by the need for compact, low-power, lightweight displays for portable systems, particularly laptop computers. Such displays have also been used in portable patient monitoring systems. This has led to the development of portable diagnostic tools, such as handheld ultrasound imagers.
More important for the medical device industry is the trend in recent years toward larger-screen AMLCDs replacing CRTs in desktop and other static applications. These breakthroughs are also responsible for the development of AMLCDs that provide an alternative to film-based x-ray systems, enabling fully digital systems with on-demand remote access to images, telemedicine, and digital picture archiving communications systems (PACS).
One of the most significant advances in AMLCD technology for the medical electronics industry is improvement in viewing-angle technology. Displays based on in-plane switching (IPS), for example, achieve roughly symmetrical viewing characteristics with gray levels and colors that are relatively stable over a wide range of viewing angles. This technology overcomes the obstacle presented by the off-angle color shifts and image inversions that are seen in many conventional laptop computer displays.
IPS technology has drawbacks, however, notably in response speed and panel efficiency. In addition, other approaches achieve as good or better viewing characteristics. New optical compensation layers, for example, have enhanced the performance of conventional twisted nematic displays. Multidomain vertically aligned cells and other novel approaches provide extremely wide viewing angles without compromising performance, brightness, or color saturation.
Figure 1. Active-matrix liquid crystal display.
The physical structure of an AMLCD, with a backlight behind a plate of varying transmission representing an image, is similar to that of a conventional x-ray film viewer. The subjective appearance of the display—the look and feel presented to the viewer—is also very similar. This is important in making the device acceptable as a film replacement (see Figure 1).
Resolution and Gray Levels
Replacing conventional film and light box arrangements for displaying x-ray images—either at the point of image capture or remotely for diagnosis—requires much more than a similar appearance and a wide viewing angle. Color is not required, but resolution and gray levels are critical in allowing radiologists to make accurate diagnoses. For example, dpiX recently demonstrated a 19-in.-diagonal monochrome AMLCD with 2560 X 2048 pixels and 8-bit gray scale. Up to this point, true 8-bit gray scale AMLCDs have washed out at the lower tones.
Digital x-ray images, obtained either directly with an electronic image capture system or produced by precision scanning of an x-ray film, contain a great deal of important gray-scale data. Images are typically stored with 12–14 bits per pixel (4–16 kGy levels), although the human visual system is capable of resolving only about 1000 gray shades at best.
The most advanced AMLCDs can render 256 shades of gray using state-of-the art electronics. The response of the display can be adjusted dynamically to window in on a specific range of gray levels. That is, gray levels in the image can be distributed over the available levels on the display; this windowing can be achieved either under user control or automatically through image enhancement software. Thus, the subtlest intensity changes in the recorded image can be enhanced and magnified to emphasize a particular feature, offering a diagnostic advantage that is not available with conventional film.
The modulation transfer function (MTF) of an AMLCD is determined by the pixel geometry alone. This measure of the image sharpness is constant both over the surface of the display and over time. Interference between adjacent pixels is negligible, allowing the sharpest possible image.
By contrast, the spot size on a CRT is limited by the physics of the beam focus and deflection system. It is also typically larger than a single pixel, limiting the MTF. Because the MTF degrades away from the center of the screen, the focus and convergence tend to worsen over time so that the CRT needs periodic recalibration and servicing.
With AMLCDs, maintenance is limited to replacement of the backlight (typically after 20,000–30,000 hours of operation) to maintain brightness. Image sharpness and fidelity never degrade. Operating costs for an AMLCD can thus be much lower than for a comparable CRT.
AMLCDs are particularly useful for applications such as magnetic resonance imaging (MRI) because of their low emissions and low susceptibility to electromagnetic interference (EMI). AMLCD images are inherently immune to distortion by external magnetic fields, eliminating bulky shielding that is often needed with CRTs and therefore greatly simplifying the design of MRI systems.
AMLCDs are, of course, not the only flat-panel displays available. Large-screen plasma displays are being manufactured, but they do not match the pixel density and gray-scale precision of AMLCDs. At the other end of the spectrum, small screens based on field emission display (FED) technology are showing fast improvement but also do not yet match the image quality, size, and low-power requirements of AMLCDs. Electroluminescent displays are used now in some patient-monitoring applications.
If current AMLCDs are set to displace film-based x-ray systems, what about future applications? Predictions are always risky, but developments in reflective liquid crystal display technology promise to affect many industries, including medical electronics. Systems that are truly portable, in the sense of both requiring very low power and adapting naturally to ambient lighting conditions, will allow, for example, detailed diagnostic images to be downloaded and displayed on handheld viewers with image quality that matches film.
Although these devices are still some years off, the success of personal digital assistants and digital organizers despite the modest performance of their current-technology reflective LCD screens points to what will happen as the technology for reflective displays starts to reach its true potential.
