pacs monitors
Post on 12-Jul-2015
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PACS Monitor
Nel Leung
• Grayscale Standard Display Function (GSDF)
• Look-up Table (LUT)
• LCD vs CRT
• Requirement of a PACS Monitor
Digital Imaging and Communications in Medicine
(DICOM)
Part 14: Grayscale Standard Display Function (GSDF)
GSDF
• provide a standard for equipment manufacturers such that images look similar on different display devices
• enhance image interpretation
Multiple Factors Affecting Display Quality
• luminance response • reflection • glare • spatial resolution • noise • geometrical distortions • temporal response (e.g. temporal noise-
and flicker)
DICOM standard currently covers only the luminance response
aspect of display quality through the DICOM GSDF
Grayscale Standard Display Function
• based on human Contrast Sensitivity
• non-linear : human eye is relatively less sensitive in the dark areas of an image than it is in the bright areas of an image
• easier to see small relative changes in luminance in the bright areas of the image than in the dark areas of the image
• GSDF adjusts the pixel brightness resulting in the same level of perceptibility at all luminance levels → “perceptually linearized”
• with a non-linearized display, the observers took longer to reach a decision and ended up being mistaken more often
Just-Noticeable Difference (JND)
• the luminance difference of a given target under given viewing conditions that the average human observer can just perceive
• JND = kL0LB where k is constant, L0 is the object luminance, and LB is the background luminance
L1 is perceived as the same contrast difference as L2
Look-up Table (LUT)
8 to 8 bit LUT
This gives not only a high color precision, but also a high color variety to exactly reproduce the image.
More of the initial image information is maintained.
LCD CRTMuch Lighter and requires much less Space
Much Heavy and Bulky
Perfect Geometry Geometric Corrections needed
Perfect Modulation Transfer Function (MTF = 1)
Imperfect Modulation Transfer Function (MTF < 1)
Uniform Sharpness Less Uniform Sharpness
1000:1 Contrast Ratio 3000:1 Contrast Ratio
Contrast Ratio dependent on Viewing Angle
Contrast Ratio independent of Viewing Angle
Low Reflection of Ambient Light High Reflection of Ambient Light
Lower Response Speed Instantaneous Response Speed
Backlight Aging Phosphor Aging
LCD CRTLow Power Consumption High Power Consumption
No Image Flicker Image Flicker Present
Narrow Viewing Angle Wider Viewing Angle
Usually produce only crisp images in their "native resolution"
Can switch between Multiple Resolution Settings without a significant loss in Sharpness
More Expensive at initial purchase Less Expensive at initial purchase
Image is always Perfectly “Focused” over the entire screen
Has Clarity or Focus issues at the screen edge
Emission-Free, causes no Electromagnetic Interference
Can generate Electric, Magnetic and even X-Ray Emissions
Less Stable More Stable
Luminance Degradation of LCD
• Temperature changes, particular at startup (may take more than half hour to stabilize)
• The luminance slowly degrades because the phosphors used in the lamps wear out
(Ambient Light Compensation)
Requirement of a PACS Monitor
a) Minimal Resolution Requirement
CT/MR 1248 x 1024
Angio/DSA 1248 x 1024
DF 1248 x 1024
US 1248 x 1024
Thorax 2048 x 1536
Skeletal 2048 x 1536
Mammo 2560 x 2048
b) DICOM GSDF Support
• With a non-linearized display, the observers took longer to reach a decision and ended up being mistaken more often.
c) Brightness
• Diagnostic-review monitors have been defined by the American College of Radiology as requiring a minimum brightness of 170 Cd/m2.
• 300 and 500 Cd/m2 is recommended for use
in a properly darkened diagnostic reading room.
d) Color
• Some applications (CT/MR/US) require Color display
• Performance of Monochrome is 25%–30% better than Color displays
• Color displays calibrated to the DICOM GSDF are preferred.
END
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