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Understanding Display Characteristics

Understanding Display Characteristics. What You Should Know When Viewing Images from PACS. Disclosures. Unfortunately no one is paying me to hawk their products Gratefully acknowledge the use of slides from OTech and Barco. Terms.

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Understanding Display Characteristics

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  1. Understanding Display Characteristics What You Should Know When Viewing Images from PACS

  2. Disclosures • Unfortunately no one is paying me to hawk their products • Gratefully acknowledge the use of slides from OTech and Barco

  3. Terms • AAPM TG-18 American Association of Physicists in Medicine Task Group 18 • COTS Common Off the Shelf • CRT Cathode Ray Tube • LCD Liquid Crystal Display • DVI Digital Video Interface • DICOM Digital Imaging and Communications in Medicine • GSDF Gray Scale Display Function

  4. Preface • We can see radiological images on any pc • So why don’t we just use our desktop pcs (is a display just a display?) • We need to understand the characteristics that differentiate displays • We need to use that understanding to properly deploy image viewing workstations

  5. Image Display • Images currently can be viewed on a CRT (cathode ray tube) or LCD (liquid crystal display) monitors, or film can be viewed on a lightbox • The various modalities impose specific requirements on workstations • Nuclear Medicine and Ultrasound require color displays, but lower resolution • CT/MRI depend more on contrast (bit depth) • CR/DR require high spatial resolution

  6. CRT Display • CRT • Can display a true black • Industry moving to LCD because this is what customers want (not necessarily need) • Sexy • Take up less room • Produce less heat • Ability to remote monitor • GE already sent out ‘end of service’ on CRTs

  7. CRT Display • Our current CRTs require specialized video cards and cables which are different from standard digital pc video displays today • These cards are becoming very difficult to find

  8. So Let’s Move on

  9. LCD Display • LCD • High even luminance and no positional drift • Long lifetime • Small pixel sizes • Reduced reflection and sensitivity to ambient light • Off-angle viewing is an issue • No true black (look at LCD in dark room)

  10. LCD Monitors • Color monitors 1024x768 (15”), 1280x1024 (17,18,19”) or 1600x1200 (20,21”) • Gray scale monitors typically 1280x1024 (1Megapixal), 1600x1200 (2Mp clinical) or 2k x 1.5k (3Mp diagnostic) [Mammo 4kx4k CRT] • When use portrait (up and down) display, can represent typical CR/DR at almost full resolution on 3 Megapixels • On 2Mp monitors, only portions of the image can be displayed at maximal resolution, and have to ‘pan’ to view areas of the image

  11. Video Cards • Medical grade cards that can be properly calibrated and monitored • COTS cards that for the most part do not provide an ability to calibrate them • Always use a DVI card when using digital (LCD) monitors. Analog signals do not translate accurately to a digital format.

  12. Major Concerns • Given that we will be moving to LCD displays, there are two major concerns • Luminance and the ability to calibrate it • Stability of the display • Consistent voltage to lamp • No changes over time as monitor warms up • Ability to monitor changes and calibrate as needed

  13. Luminance • Luminance is the quantity describing the brightness of a monitor (from black to white) • ACR standards specify a luminance of at least 160 candela per square meter (cd/m2), or 47 fL (foot Lamberts), for diagnostic reading • Gray scale monitors provide higher luminance than color monitors • Luminance affects both contrast resolution and spatial resolution (very important) • High-performance monitors are still less bright than a view box by a factor of 5 to 10, and windowing and leveling are needed to compensate

  14. Luminance • Lightbox/Alternator 200-600fL • PC color monitor (CRT) 20-40fL • Medical imaging monitors • ACR recommended minimum 47fL • Low bright 50-60fL • LSU Clinical Barco CRT 65fL • NEC Color 15”, 18”, 20” LCD 71fL • LSU Diagnostic Barco CRT 85fL • High bright 100fL or higher

  15. LCD Display Examples

  16. Color vs Monochrome • Color monitors have decreased luminance, hence decreased contrast and spatial resolution • Color becoming more useful (US, NucMed, 3D reconstructs etc.) • Human eye has greater dynamic range (JND) color (500) vs grayscale (150-200) • But has decreased spatial resolution in the color spectrum • Most displays will be in the grayscale spectrum of the color video/monitor system (bit depth) • Grayscale monitors do provide higher contrast capability

  17. Stability • The backlight is an important source of instabilities • Temperature dependent • Time dependent • COTS monitor luminance • drops drastically over the first year of use (and contrast and spatial resolution drops with it) • Doesn’t stabilize for up to an hour or more when first turned on

  18. Temperature Dependance

  19. Time Dependance

  20. Monitor Calibration • Brightness and contrast controls usually hidden, once set during calibration they should not be changed • Manual or automated monitoring • DICOM GSDF

  21. Monitor Calibration • DICOM Part 14 Grayscale Display Function (GSDF) • Use of LUT’s (look up tables) for monitors allows calibration to provide consistent image display on different monitors • Enables images to look the same regardless of the display or printer by calibrating JND points to specific luminance output

  22. GSDF Graph

  23. Calibrating to the GSDF Curve

  24. Window Width and Level • This is not the same as contrast and brightness (monitor controls for this) • Actually displaying a subset of the available dynamic range of data • Take a CT image represented in 12 bits – this provides 4,096 different values • We can only discern around 200 values, or just noticable differences

  25. AAPM TG-18 Tests • Document ‘Assessment of Display Performance for Medical Imaging Systems’ • Not a standard but a documented system for monitoring and tracking displays • Defined test pattern images to enable assessment of display abnormalities • SMPTE test pattern (next slide) • The 95/100% box is often not discernible on flat panel monitors due to lack of true black

  26. SMPTE Test Pattern To properly display a test pattern on your monitor, go to 10.1.204.17/ami and click on ‘Test’

  27. AAPM • The test patterns used by the methodology in the TG-18 document are stored under the user ‘AAPM, TEST PATTERNS’ • Document can be found at the AAPM web site, www.aapm.org.

  28. Perfect World • If you have a need to view an image • You have a need to view it on a workstation that is subject to quality control

  29. Realities • Need to calibrate and monitor the display systems that allow us to do that (diagnostic and clinical) • Impossible to provide this for every pc that someone may view an image on • COTS pcs can be valid for viewing as long as there is a diagnostic report available • If there is no report, contact a Radiologist before making a clinical decision (a good thing to do in any case)

  30. Future Path • Move to LCD displays • Move manual ‘at the display’ calibration every 1-2 months to remote monitoring • Provide quality control at least at our diagnostic and clinical display stations • Understand the limitations of COTS display stations we use at the institution

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