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Understanding Display Characteristics What You Should Know When Viewing Images from PACS www.sh.lsuhsc.edu/radiology Radiology Disclosures • Unfortunately no one is paying me to hawk their products • Gratefully acknowledge the use of slides from OTech and Barco www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology www.sh.lsuhsc.edu/radiology Radiology So Let’s Move on www.sh.lsuhsc.edu/radiology Radiology 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) www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology 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. www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology Luminance • Lightbox/Alternator • PC color monitor (CRT) • Medical imaging monitors – ACR recommended minimum – Low bright – LSU Clinical Barco CRT – NEC Color 15”, 18”, 20” LCD – LSU Diagnostic Barco CRT – High bright www.sh.lsuhsc.edu/radiology 200-600fL 20-40fL 47fL 50-60fL 65fL 71fL 85fL 100fL or higher Radiology LCD Display Examples Richardson Electronics Monitor Demonstration - May 28, 2003 ITEM DESCRIPTION Greyscale 3MP SIZE ORIENTATION TYPE INPUT RESOLUTION BRIGHTNESS CONTRAST FP2080HB-2IF MFG Image Systems 20.8" Landscape/ Protrait LCD Digital 2048x1536 600 cd/m(2) 176 fL 600 to 1 FP1900HB-1F Image Systems Greyscale 2MP 19" Landsacpe/ Portrait LCD Digital 1200x1600 800 cd/m(2) 235 fL 700 to 1 LCD-3000-BK NEC Color 30" Landscape/ Portrait LCD Analog DVI-D 1280x768 450 cd/m(2) 132 fL 350 to 1 LCD-2080UX NEC Color 20.1" Landscape/ Portrait w Matrox MED2MP-DVI/AGP LCD AnalogDigital 1200x1600 250 cd/m(2) 73 fL 350 to 1 LCD-1880SX NEC Color 18.1" Landscape LCD AnalogDigital 1280x1024 240 cd/m(2) 71fL 350 to 1 LCD-1560MBK NEC Color 15" Landscape LCD AnalogDigital 1024x768 250 cd/m(2) 73fL 400 to 1 Color Allows hardware Rotation for Portrait or Landscape Board DVI HD15 1920x1200 MED2MP-DVI Matrox Bus Type AGP 4X www.sh.lsuhsc.edu/radiology For NEC LCD's Radiology 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 www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology Glass Back-Light Fluorescent Lamp LCD material Pixel I-Guard www.sh.lsuhsc.edu/radiology Radiology Temperature Dependance www.sh.lsuhsc.edu/radiology Radiology Time Dependance www.sh.lsuhsc.edu/radiology Radiology Monitor Calibration • Brightness and contrast controls usually hidden, once set during calibration they should not be changed • Manual or automated monitoring • DICOM GSDF www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology GSDF Graph www.sh.lsuhsc.edu/radiology Radiology Calibrating to the GSDF Curve www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology SMPTE Test Pattern To properly display a test pattern on your monitor, go to 10.1.204.17/ami and click on ‘Test’ www.sh.lsuhsc.edu/radiology Radiology 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. www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology 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) www.sh.lsuhsc.edu/radiology Radiology 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 www.sh.lsuhsc.edu/radiology Radiology