Medical Displays: New Technologies and Strict Standards Drive a Need for Visual Quality Testing

Posted:  
Mon, November 4, 2019
Author: 
Anne Corning  | 

The development of medical imaging technologies has revolutionized healthcare, providing powerful diagnostic tools, supporting non-invasive assessment of injuries and internal issues, and enabling diseases to be detected far earlier than ever before.

Physicist Marie Curie helped develop X-rays—the first medical imaging technology—based on her Nobel-prize winning work with radioactive elements. Since then, MRI (magnetic resonance imaging), CT scans (computed tomography), ultrasound, nuclear medicine techniques such as positron emission tomography (PET), and other advanced imaging technologies have been added to the medical toolkit, offering different views into the human body. These non-invasive methods rely on different technologies to scan interior structures and produce images of bones, organs, and tissues to support diagnosis, medical monitoring, and treatment.

Radiologist reviewing brain-scan images on diagnostic-quality display screens.

When they initially emerged, imaging technologies produced two-dimensional images on films, which had to be held up to a light source for reading. Subsequently, techniques were developed to combine multiple scans together into 3D renderings and to generate and capture both 2D and 3D images digitally for viewing on a computer screen.

Today, all these imaging technologies depend on high-quality display screens that enable clinicians to review images in detail and discern fine nuances revealed in a patient’s scans. These scans typically contain a large amount of data, so high-resolution screens are needed to render the scan accurately. But screen resolution is just the first requirement for “medical-grade” or “hospital-grade” display monitors.

Medical-Grade Displays

Display monitors used in medical settings need to offer enhanced performance and greater longevity (due to their constant usage) compared to the typical commercial and consumer displays. Medical displays usually come with special image-enhancing technologies to ensure constant brightness and clear and consistent images over the lifetime of the display. Some of the key considerations for medical displays include:

  • Resolution – The higher the screen resolution, the greater the clarity and visible detail in an image—necessary for accurate diagnosis or research. The latest medical-grade display products offered by leading manufacturers include high-definition (HD), 4K, and up to 12-megapixel-resolution screens. The pixel pitch of typical devices is about 0.200 microns and the array sizes are 1536 x 2048 or larger.1
  • Luminance – Luminance (often referred to as “brightness”) is the amount of photon energy that reaches the eye, measured in units of candela (cd) per square meter (cd/m2), also called “nits”. The acceptable range for a radiology display, for example, is 350-420 nits.2
  • Contrast – Also important is contrast, measured as the range of luminance available between the maximum luminance of a display pixel and when that pixel is “off”.

Although in recent years manufacturers have been closing the gap with increased brightness, matrix sizes, decreased pixel pitch, and increased factory QA to ensure uniformity, consumer-grade displays haven’t produced the same luminance and contrast as medical displays. Even if an LCD consumer-grade display starts out at 400 nits, after 18 months, its backlight performance will already have decayed to about 350 nits, effectively ending its lifespan for radiological use. To counter this, medical displays are designed with “headroom”—a maximum luminance capacity above that of its normal operating level, which can be tapped over time to compensate for the anticipated performance decay. 

Many medical images (such as X-rays and mammograms) are monochromatic, so being able to discern fine luminance differences in grayscale is important. Medical imaging relies on the “just noticeable differences” (JND) standard first developed by NASA. JND is the smallest difference in luminance (e.g., between two gray levels) that the average observer can just perceive on the display system.

Monochromatic CT and MRI scans of the brain showing an intracerebral hemorrhage or mass.

Calibration – To maintain stable peak luminance from cold start to full warm-up, and throughout its lifetime, a medical display has a closed-loop control circuit tied to a built-in photometer. The photometer monitors gamma levels and measures peak luminance several times a second, ensuring a consistent level of luminance. Monitors can also be periodically calibrated to ensure accuracy. By contrast, a commercial-grade (consumer)display simply offers a manual brightness control where a user can set their own preference, without reference to absolute luminance levels. Monochromatic medical-grade monitors have calibration features that meet the DICOM part 14 Grayscale Standard

Color Gamut – You may think high-end home screens have a wide color gamut, but a modern medical-grade LCD monitor displays 1.074 billion colors, compared to the commercial version that displays a mere 16.7 million.3 Producing this range typically requires 10-bit-per-color output capability across the operating system, software, video card, and monitor; performance is measured as a percentage of “sRGB” (standard Red, Green, Blue). For example, a typical medical monitor might advertise “sRGB 99.9” to indicate its high color accuracy. Some surgical monitors offer enhanced red display capabilities (adding the deep red spectrum to provide sRGB 115%), which enables more precise visualization of internal tissues and organs.

Computed tomography imaging provides a 3D color rendering from an angiogram to assess the patient’s coronary arteries for disease.

Durability – In the dynamic environment of an emergency department, critical care unit, or operating room, medical display monitors may often be moved around, bumped, or have things taped to them. For a normal commercial LCD display, this treatment can cause damage to individual pixels, resulting in bad or dead pixels in the display. While commercial displays can be shipped to market with an “acceptable” number of bad/dead pixels, the minimum allowed defects for shipping medical displays is much lower, and pixels can’t be susceptible to easy damage during use.

To protect these displays, medical LCD panels have robust cover materials (glass or plastic) that will diffuse a blow and absorb shocks, protecting the pixels below. Cover materials can also be made with properties that enhance image visualization and reduce eyestrain.

Stability & Longevity – Medical screens need to maintain consistent luminance and remain free from artifacts through their lifetime. They are expected to last for 5+ years of “normal” use—which can mean 24 x 7 x 365—without degradation in image quality or performance. Images must be reproduced exactly from day-to-day and even year-to-year to allow clinicians to evaluate a patient’s condition over time.

Ambient Light Performance – How a display appears under bright light—such as in an emergency room or ICU—is another factor for medical screens. Reflection or glare off the screen surface can impede a user’s ability to see images clearly or with the proper contrast. Many medical monitors offer optical bonding or other anti-glare features, along with treatments to resist “fogging” from humidity.

View Angle – Medical displays—both clinical monitors (also called “review” monitors) that are often used in medical offices to review imaging results in meetings with staff or patients, and surgical monitors that are used in operating rooms and hands-on treatment settings—need to be able to provide clear and high-contrast images over a wide view angle to accommodate multiple users located at different places around the room.

Safety – Monitors for use in surgical settings often offer additional features such as an antimicrobial coating and fluid resistance bezel that enable them to be cleaned and sanitized regularly.

LCD Medical Displays

Because of these many performance demands, LCD screens with edge-lit LED backlights currently dominate the medical industry at 93% penetration.4 Even with the relative stability and dependable performance of LCD screens, medical LCD displays must incorporate supplemental features such as backlight stabilization (sensors that monitor backlight output) and a built-in photometer. Choice of underlying LCD panel technology is also an important performance factor to consider.

LCD Panel technologies include twisted nematic (TN), in-plane switching (IPS), vertical alignment (VA), multi-domain vertical alignment (MVA), and advanced fringe field switching (AAFS). Source: Cybernet5

One challenge developers have been working to overcome is the fact that pixels lose efficiency with use. This issue has been addressed with LCD screens thanks to approaches that incorporate calibration and headroom. While OLED and other newer technologies offer the potential of higher brightness and resolution than LCD, until recently they have not shown the stability and longevity required for medical applications. Because OLED pixels are driven independently (they are self-emitting), those pixels that are used more often may age more quickly than others, resulting in uneven color or luminance, image retention, and other issues over time. Recently, however, OLED performance has begun to show promise of reaching the level required for medical displays, and is expected to capture more than a $70 million share of what will be a $2.8 billion market by 2023.6

There can be variation in display performance requirements, depending on the healthcare environment in which a display will be used. For example, diagnostic radiology displays have higher demands, and clinical/review screens generally incorporate touchscreen functionality. Dentistry, mammography, pathology and other display types all have unique specifications. Display screens at a nurses’ station used simply for viewing patient chart data may even be conventional, commercial-grade monitors.

Dentistry display used for viewing patient X-ray images.

These days, tablet computers are also used by medical staff, with similar requirements beyond commercial-grade products. The level of acceptance testing, constancy checks, and calibration for the different screen types likewise varies accordingly.

Medical Display Standards & Regulatory Requirements

For medical display manufacturers and their customers, meeting the demands of medical-grade displays is not just a matter of performance but it’s also a matter of compliance. In addition to the DICOM grayscale standard mentioned above, medical displays are subject to an array of regulatory and industry standards.

The American College of Radiology (ACR), the American Association of Physicists in Medicine (AAPM), and the Society for Imaging Informatics in Medicine (SIIM) have all developed their own guidelines and standards. For example, the 2005 standards from AAPM Task Group 18 (TG-18) have been widely accepted and adopted across the country. The most recent update to the Technical Standard was produced in 2019, and includes performance parameters for flat-panel LCD and OLED displays.

Requirements for medical-grade monitors include, among others:

  • Certifications to meet DICOM (Digital Imaging and Communications in Medicine) international standards.
  • The 60601-1 medical electrical equipment standards from the AAMI (Association for the Advancement of Medical Instrumentation).
  • Compliance with the EN/IEC 60601-1 standard of the National Fire Protection Association (NFPA) Health Care Facilities Code (NFPA 99), under which monitors used within a patient care vicinity cannot exceed leakage current levels of 100 microamps. 
  • The Food & Drug Administration (FDA) 510(k) regulations that govern whether a specific monitor can be marketed as medical device.

Medical Display Trends

The medical display market is growing, with no slowdown in sight. General population growth and the increasing number of aging patients is driving growth in demand for healthcare services in the U.S. and around the globe. Medical display industry sales are projected to increase by 5% CAGR by 2022.7

Today, color displays account for a majority of medical screens sold, with the largest portion being purchased for various diagnostic imaging purposes.8 However, much of the imaging technology (such as X-ray) is monochromatic, thus multi-modal displays—displays that can be used effectively to display more than one image type—are a growing trend.

Multi-modal functions in displays allow operators to view both color and grayscale images in static and moving formats. They can have a color enhancement functionality that allows radiologists to get a detailed view of multiple images, such a both PET and ultrasound outputs. To learn about some of the latest advancements in medical display technology, check out what LG Display has to offer in the categories of review, x-ray, and surgical monitors:

Medical Display Quality Evaluation

With such stringent technical and performance standards, medical display manufacturers need reliable quality regimens and tools to test and measure products before they can be released on the market. Radiant Vision Systems offers a range of display metrology solutions for absolute measurement and qualification of displays for luminance, color, and uniformity down to the individual pixel level. Solutions include:

  • ProMetric® Imaging Photometers and Colorimeters, designed to replicate the human eye’s photopic response to brightness and color. These systems make precise spatial measurements of luminance and chromaticity in both the R&D lab and on the production line. Built around scientific-grade, thermoelectrically cooled sensors of up to 43-megapixel resolution, the systems ensure high measurement resolution with low noise for the optimal balance of clarity. Paried with our TrueTest™ automated visual inspection software, a Radiant solution provides a range of quality tests and defect detection tools including:
    • Uniformity
    • Gradient
    • Line Defects
    • Particle Defects
    • Pixel Defects
    • ANSI Brightness
    • ANSI Color Uniformity
    • Checkerboard Contrast
    • Chromaticity
    • Distortion
    • Focus Uniformity
    • Compare Points of Interest
    • Points of Interest
    • Color Edge Mura
    • Color Mura
    • Diagonal Pattern Mura
    • Polarizer Deformation
    • Spot Pattern Mura
  • Just Noticeable Differences (JND) – As an option, Radiant’s measurement systems may employ an image analysis algorithm specifically for computing JND in display uniformity, used to identify display mura. The JND detection algorithm is offered in our TrueMURA™ Software, and is based on U.S. patent 7,783,130, “Spatial Standard Observer” technology, licensed by Radiant from NASA.

Raw JND analysis of a screen image (top) captured by a ProMetric Imaging Colorimeter and TrueTest software. Image is lighter for higher values of JND and darker for lower values, showing mura at the center of the screen and light leakage and dark spot artifacts along the edge. False color representation of the JND map (bottom) showing areas with JND values greater than 1, which is the threshold for being "just noticeable". The spot in the bottom right has the largest computed value of JND, and the mottled area accross most of the display represents JND values of 0.7 or lower.

  • View Angle Performance measurement of flat panel displays using our FPD Conoscope Lens. This Lens enables high-resolution photopic measurement of the angular distribution of color, luminance, and contrast, using Fourier optics to qualify display output as seen from +/- 70 degrees in a single measurement.

A ProMetric Imaging Colorimeter with the FPD Conoscope Lens attached for measuring view angle performance.

  •  Pixel and subpixel-level mura detection and correction – Radiant’s photometric imaging systems have the capability to measure each and every pixel, identify non-uniformity in a pixel’s brightness or color, then calculate a correction coefficient using our patent-pending Demura process to adjust screen performance. As the medical display industry moves towards adoption of OLED and other emissive display technologies, the ability to measure and correct displays at the pixel (individual emitter) level will be increasingly important.

     

Analsys of corner mura on a display in the TrueMURA software.

As the display industry continues to develop screens with higher and higher resolution, and the medical industry moves towards the adoption of OLED and other newer technologies, using the most discerning display measurement systems from Radiant will help ensure performance and accuracy of current and future medical displays.

CITATIONS

  1. Flynn, M., “Displays Chapter 3: DICOM Basics Pertaining to Displays”, Society for Imaging Informatics in Medicine, retrieved 10/31/19.
  2. Hirschorn, D., “Displays Chapter 6: Alternatives to High-Performance Displays”, Society for Imaging Informatics in Medicine, retrieved 10/31/19.
  3. Lovinus, A., “What Does Medical Grade Mean for a Computer Monitor?”, SmartBuyer, January 30, 2018
  4. Waltz-Flannigan, A., “Introduction to Current Display Technologies for Medical Image Viewing”, Mayo Clinic, AAPM Annual Meeting, 2017.
  5. A Brief Introduction to Medical Grade Monitors”, Cybernet, June 20, 2017
  6. “$2.8 Billion Medical Display Monitor Market 2018 – Global Market Size, Share, Development, Growth, and Demand Forecast, 2013-2023”, Research and Markets, April 29, 2018.
  7. Medical Display Market by Technology (LED, OLED), Panel Size (≤22.9”, 27.0-41.9”, ≥42”), Resolution (≤2MP, 4.1-8MP, ≥MP), Application (Radiology, Mammography, Digital Pathology, Multi-Modality, Surgical) & Display color – Global Forecast to 2023" from Markets and Markets, March 2018
  8. “Global Medical Display Market 2018-2022 / Industry Analysis and Forecast / Technavio” Business Wire, December 28, 2018
     
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