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

Radiologywww.sh.lsuhsc.edu/radiology

Understanding Display Understanding Display CharacteristicsCharacteristics

What You Should Know When What You Should Know When Viewing Images from PACSViewing Images from PACS


DisclosuresDisclosures

• Unfortunately no one is paying me to Unfortunately no one is paying me to hawk their productshawk their products

• Gratefully acknowledge the use of Gratefully acknowledge the use of slides from OTech and Barcoslides from OTech and Barco


TermsTerms

• AAPM TG-18 American Association of AAPM TG-18 American Association of Physicists in Medicine Task Group 18Physicists in Medicine Task Group 18

• COTS Common Off the ShelfCOTS Common Off the Shelf• CRT Cathode Ray TubeCRT Cathode Ray Tube• LCD Liquid Crystal Display LCD Liquid Crystal Display • DVI Digital Video InterfaceDVI Digital Video Interface• DICOM Digital Imaging and DICOM Digital Imaging and

Communications in MedicineCommunications in Medicine• GSDF Gray Scale Display FunctionGSDF Gray Scale Display Function


PrefacePreface

• We can see radiological images on any We can see radiological images on any pc pc

• So why don’t we just use our desktop pcs So why don’t we just use our desktop pcs (is a display just a display?)(is a display just a display?)

• We need to understand the We need to understand the characteristics that differentiate displayscharacteristics that differentiate displays

• We need to use that understanding to We need to use that understanding to properly deploy image viewing properly deploy image viewing workstationsworkstations


Image DisplayImage Display

• Images currently can be viewed on a Images currently can be viewed on a CRT (cathode ray tube) or LCD (liquid CRT (cathode ray tube) or LCD (liquid crystal display) monitors, or film can crystal display) monitors, or film can be viewed on a lightboxbe viewed on a lightbox

• The various modalities impose specific The various modalities impose specific requirements on workstationsrequirements on workstations– Nuclear Medicine and Ultrasound require Nuclear Medicine and Ultrasound require

color displays, but lower resolutioncolor displays, but lower resolution– CT/MRI depend more on contrast (bit CT/MRI depend more on contrast (bit

depth)depth)– CR/DR require high spatial resolutionCR/DR require high spatial resolution


CRT DisplayCRT Display

• CRTCRT– Can display a true blackCan display a true black– Industry moving to LCD because this is Industry moving to LCD because this is

what customers want (not necessarily what customers want (not necessarily need)need)•SexySexy

•Take up less roomTake up less room

•Produce less heatProduce less heat

•Ability to remote monitorAbility to remote monitor

•GE already sent out ‘end of service’ on CRTsGE already sent out ‘end of service’ on CRTs


CRT DisplayCRT Display

• Our current CRTs require specialized Our current CRTs require specialized video cards and cables which are video cards and cables which are different from standard digital pc different from standard digital pc video displays todayvideo displays today

• These cards are becoming very These cards are becoming very difficult to finddifficult to find



So Let’s Move onSo Let’s Move on


LCD DisplayLCD Display

• LCDLCD– High even luminance and no positional driftHigh even luminance and no positional drift– Long lifetimeLong lifetime– Small pixel sizesSmall pixel sizes– Reduced reflection and sensitivity to Reduced reflection and sensitivity to

ambient lightambient light– Off-angle viewing is an issueOff-angle viewing is an issue– No true black (look at LCD in dark room)No true black (look at LCD in dark room)


LCD MonitorsLCD Monitors• Color monitors 1024x768 (15”), 1280x1024 Color monitors 1024x768 (15”), 1280x1024

(17,18,19”) or 1600x1200 (20,21”)(17,18,19”) or 1600x1200 (20,21”)

• Gray scale monitors typically 1280x1024 Gray scale monitors typically 1280x1024 (1Megapixal), 1600x1200 (2Mp clinical) or 2k x (1Megapixal), 1600x1200 (2Mp clinical) or 2k x 1.5k (3Mp diagnostic) [Mammo 4kx4k CRT]1.5k (3Mp diagnostic) [Mammo 4kx4k CRT]

• When use portrait (up and down) display, can When use portrait (up and down) display, can represent typical CR/DR at almost full resolution on represent typical CR/DR at almost full resolution on 3 Megapixels3 Megapixels

• On 2Mp monitors, only portions of the image can On 2Mp monitors, only portions of the image can be displayed at maximal resolution, and have to be displayed at maximal resolution, and have to ‘pan’ to view areas of the image‘pan’ to view areas of the image


Video CardsVideo Cards

• Medical grade cards that can be properly Medical grade cards that can be properly calibrated and monitoredcalibrated and monitored

• COTS cards that for the most part do not COTS cards that for the most part do not provide an ability to calibrate themprovide an ability to calibrate them

• Always use a DVI card when using digital Always use a DVI card when using digital (LCD) monitors. Analog signals do not (LCD) monitors. Analog signals do not translate accurately to a digital format.translate accurately to a digital format.


Major ConcernsMajor Concerns

• Given that we will be moving to LCD Given that we will be moving to LCD displays, there are two major displays, there are two major concernsconcerns– Luminance and the ability to calibrate itLuminance and the ability to calibrate it– Stability of the displayStability of the display

•Consistent voltage to lampConsistent voltage to lamp

•No changes over time as monitor warms upNo changes over time as monitor warms up

•Ability to monitor changes and calibrate as Ability to monitor changes and calibrate as neededneeded


LuminanceLuminance

• Luminance is the quantity describing the Luminance is the quantity describing the brightness of a monitor (from black to white)brightness of a monitor (from black to white)

• ACR standards specify a luminance of at ACR standards specify a luminance of at least 160 candela per square meter (cd/mleast 160 candela per square meter (cd/m22), ), or 47 fL (foot Lamberts), for diagnostic or 47 fL (foot Lamberts), for diagnostic readingreading

• Gray scale monitors provide higher Gray scale monitors provide higher luminance than color monitorsluminance than color monitors

• Luminance affects both contrast resolution Luminance affects both contrast resolution and spatial resolution (very important)and spatial resolution (very important)

• High-performance monitors are still less High-performance monitors are still less bright than a view box by a factor of 5 to 10, bright than a view box by a factor of 5 to 10, and windowing and leveling are needed to and windowing and leveling are needed to compensatecompensate


LuminanceLuminance

• Lightbox/AlternatorLightbox/Alternator 200-600fL200-600fL• PC color monitor (CRT)PC color monitor (CRT) 20-40fL20-40fL• Medical imaging monitorsMedical imaging monitors

– ACR recommended minimumACR recommended minimum 47fL47fL– Low brightLow bright 50-60fL50-60fL– LSU Clinical Barco CRTLSU Clinical Barco CRT 65fL65fL– NEC Color 15”, 18”, 20” LCDNEC Color 15”, 18”, 20” LCD 71fL71fL– LSU Diagnostic Barco CRTLSU Diagnostic Barco CRT 85fL85fL– High brightHigh bright 100fL or 100fL or

higherhigher


LCD Display ExamplesLCD Display Examples

Richardson Electronics Monitor Demonstration - May 28, 2003

ITEM MFG DESCRIPTION SIZE ORIENTATION TYPE INPUT RESOLUTION BRIGHTNESS CONTRAST

FP2080HB-2IF Image

Systems Greyscale

3MP 20.8" Landscape/ LCD Digital 2048x1536 600 cd/m(2) 600 to 1 Protrait 176 fL

FP1900HB-1F Image

Systems Greyscale

2MP 19" Landsacpe/ LCD Digital 1200x1600 800 cd/m(2) 700 to 1 Portrait 235 fL

LCD-3000-BK NEC Color 30" Landscape/ LCD Analog 1280x768 450 cd/m(2) 350 to 1 Portrait DVI-D 132 fL

LCD-2080UX NEC Color 20.1" Landscape/ LCD Analog- 1200x1600 250 cd/m(2) 350 to 1 Portrait w Matrox Digital 73 fL MED2MP-DVI/AGP

LCD-1880SX NEC Color 18.1" Landscape LCD Analog- 1280x1024 240 cd/m(2) 350 to 1 Digital 71fL

LCD-1560M-BK NEC Color 15" Landscape LCD Analog- 1024x768 250 cd/m(2) 400 to 1

Digital 73fL

MED2MP-DVI Matrox Bus Type Color Allows hardware Board DVI 1920x1200 For NEC

LCD's AGP 4X Rotation for Portrait HD15 or Landscape


Color vs MonochromeColor vs Monochrome

• Color monitors have decreased luminance, hence Color monitors have decreased luminance, hence decreased contrast and spatial resolutiondecreased contrast and spatial resolution

• Color becoming more useful (US, NucMed, 3D Color becoming more useful (US, NucMed, 3D reconstructs etc.)reconstructs etc.)

• Human eye has greater dynamic range (JND) color Human eye has greater dynamic range (JND) color (500) vs grayscale (150-200)(500) vs grayscale (150-200)

• But has decreased spatial resolution in the color But has decreased spatial resolution in the color spectrumspectrum

• Most displays will be in the grayscale spectrum of Most displays will be in the grayscale spectrum of the color video/monitor system (bit depth)the color video/monitor system (bit depth)

• Grayscale monitors do provide higher contrast Grayscale monitors do provide higher contrast capabilitycapability


StabilityStability

• The backlight is an important source of The backlight is an important source of instabilitiesinstabilities– Temperature dependentTemperature dependent– Time dependentTime dependent

• COTS monitor luminance COTS monitor luminance – drops drastically over the first year of use (and drops drastically over the first year of use (and

contrast and spatial resolution drops with it)contrast and spatial resolution drops with it)– Doesn’t stabilize for up to an hour or more Doesn’t stabilize for up to an hour or more

when first turned onwhen first turned on



Temperature DependanceTemperature Dependance


Time DependanceTime Dependance


Monitor CalibrationMonitor Calibration

• Brightness and contrast controls Brightness and contrast controls usually hidden, once set during usually hidden, once set during calibration they should not be calibration they should not be changedchanged

• Manual or automated monitoringManual or automated monitoring

• DICOM GSDFDICOM GSDF


Monitor CalibrationMonitor Calibration

• DICOM Part 14 Grayscale Display DICOM Part 14 Grayscale Display Function (GSDF)Function (GSDF)– Use of LUT’s (look up tables) for monitors Use of LUT’s (look up tables) for monitors

allows calibration to provide consistent allows calibration to provide consistent image display on different monitorsimage display on different monitors

– Enables images to look the same Enables images to look the same regardless of the display or printer by regardless of the display or printer by calibrating JND points to specific calibrating JND points to specific luminance outputluminance output


GSDF GraphGSDF Graph


Calibrating to the GSDF Calibrating to the GSDF CurveCurve


Window Width and LevelWindow Width and Level

• This is not the same as contrast and This is not the same as contrast and brightness (monitor controls for this)brightness (monitor controls for this)

• Actually displaying a subset of the Actually displaying a subset of the available dynamic range of dataavailable dynamic range of data

• Take a CT image represented in 12 Take a CT image represented in 12 bits – this provides 4,096 different bits – this provides 4,096 different valuesvalues

• We can only discern around 200 We can only discern around 200 values, or just noticable differencesvalues, or just noticable differences



AAPM TG-18 TestsAAPM TG-18 Tests

• Document ‘Assessment of Display Document ‘Assessment of Display Performance for Medical Imaging Systems’Performance for Medical Imaging Systems’

• Not a standard but a documented system Not a standard but a documented system for monitoring and tracking displaysfor monitoring and tracking displays

• Defined test pattern images to enable Defined test pattern images to enable assessment of display abnormalitiesassessment of display abnormalities

• SMPTE test pattern (next slide)SMPTE test pattern (next slide)• The 95/100% box is often not discernible The 95/100% box is often not discernible

on flat panel monitors due to lack of true on flat panel monitors due to lack of true blackblack


SMPTE Test PatternSMPTE Test PatternTo properly display a test pattern on your monitor, go to 10.1.204.17/ami and click on ‘Test’


AAPMAAPM

• The test patterns used by the The test patterns used by the methodology in the TG-18 document methodology in the TG-18 document are stored under the user ‘AAPM, are stored under the user ‘AAPM, TEST PATTERNS’TEST PATTERNS’

• Document can be found at the AAPM Document can be found at the AAPM web site, www.aapm.org.web site, www.aapm.org.


Perfect WorldPerfect World

• If you have a need to view an imageIf you have a need to view an image

• You have a need to view it on a You have a need to view it on a workstation that is subject to quality workstation that is subject to quality controlcontrol


RealitiesRealities

• Need to calibrate and monitor the display Need to calibrate and monitor the display systems that allow us to do that systems that allow us to do that (diagnostic and clinical)(diagnostic and clinical)

• Impossible to provide this for every pc that Impossible to provide this for every pc that someone may view an image onsomeone may view an image on

• COTS pcs can be valid for viewing as long COTS pcs can be valid for viewing as long as there is a diagnostic report availableas there is a diagnostic report available

• If there is no report, contact a Radiologist If there is no report, contact a Radiologist before making a clinical decision (a good before making a clinical decision (a good thing to do in any case)thing to do in any case)


Future PathFuture Path

• Move to LCD displaysMove to LCD displays

• Move manual ‘at the display’ calibration Move manual ‘at the display’ calibration every 1-2 months to remote monitoringevery 1-2 months to remote monitoring

• Provide quality control at least at our Provide quality control at least at our diagnostic and clinical display stationsdiagnostic and clinical display stations

• Understand the limitations of COTS Understand the limitations of COTS display stations we use at the institutiondisplay stations we use at the institution

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

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