PACS Monitor

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PACS Monitor
Nel Leung
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• Grayscale Standard Display Function (GSDF)
• Look-up Table (LUT)
• LCD vs CRT
• Requirement of a PACS Monitor

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Digital Imaging and Communications in Medicine
(DICOM) Part 14 Grayscale Standard Display
Function (GSDF)
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GSDF

• provide a standard for equipment manufacturers
  such that images look similar on different
  display devices
• enhance image interpretation

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Multiple Factors Affecting Display Quality

• luminance response
• reflection
• glare
• spatial resolution
• noise
• geometrical distortions
• temporal response (e.g. temporal noise-and
  flicker)

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DICOM standard currently covers only the
luminance response aspect of display quality
through the DICOM GSDF
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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

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• 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

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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

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L1 is perceived as the same contrast difference
as L2
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Look-up Table (LUT)
8 to 8 bit LUT
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This gives not only a high color precision, but
also a high color variety to exactly reproduce
the image.
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More of the initial image information is
maintained.
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LCD CRT
Much 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 lt 1)
Uniform Sharpness Less Uniform Sharpness
10001 Contrast Ratio 30001 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
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LCD CRT
Low 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
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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

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(Ambient Light Compensation)
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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
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• b) DICOM GSDF Support
• With a non-linearized display, the observers took
  longer to reach a decision and ended up being
  mistaken more often.

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• 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.

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• d) Color
• Some applications (CT/MR/US) require Color
  display
• Performance of Monochrome is 2530 better than
  Color displays
• Color displays calibrated to the DICOM GSDF are
  preferred.

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END