Download Display Characteristics

Understanding Display
Characteristics
What You Should Know When
Viewing Images from PACS
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Disclosures
• Unfortunately no one is paying me to
hawk their products
• Gratefully acknowledge the use of slides
from OTech and Barco
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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
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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
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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
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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
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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
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So Let’s Move on
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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)
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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
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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.
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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
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Luminance
• Luminance is the quantity describing the
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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
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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
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200-600fL
20-40fL
47fL
50-60fL
65fL
71fL
85fL
100fL or higher
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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
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For NEC
LCD's
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Color vs Monochrome
• Color monitors have decreased luminance, hence
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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
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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
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Glass
Back-Light
Fluorescent
Lamp
LCD material
Pixel
I-Guard
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Temperature Dependance
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Time Dependance
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Monitor Calibration
• Brightness and contrast controls usually
hidden, once set during calibration they
should not be changed
• Manual or automated monitoring
• DICOM GSDF
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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
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GSDF Graph
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Calibrating to the GSDF Curve
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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
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AAPM TG-18 Tests
• Document ‘Assessment of Display Performance
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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
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SMPTE Test Pattern
To properly display a test pattern on your monitor, go to
10.1.204.17/ami and click on ‘Test’
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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.
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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
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Realities
• Need to calibrate and monitor the display
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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)
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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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