to

1
to Delivering High Quality Images in Medical
Monitors
Welcome!
2
Your Instructor Sencore contact
Scott Reardon
1-800-SENCORE www.sencore.com
3
About Sencore

• Founded 1951
• Located in Sioux Falls, SD
• Leading manufacturer of innovative electronic
  test
  measurement equipment
• Computer Monitor - high resolution computer
  monitor repair and calibration
• Digital Video - MPEG-2/ATSC, MPEG-2/DVB, CCIR601
  Serial/Parallel, QAM, and HDTV
• High Definition TV - demonstration, testing and
  calibration applications
• Professional A/V - high-end video display
  calibration and sound system analysis, Home
  Theater
• Medical - Safety Testers, Compliance Testers, and
  Analyzers, including SPO2 and Patient Simulators
• Cable - RF distribution including CATV, MATV,
  satellite antenna systems QAM applications
• Broadcast - broadcast and studio settings,
  especially digital video and MPEG-2 applications

4
Introduction
Medical Monitors Need Calibration!

• Critical diagnoses depends on correct image
  display.
• Incorrect white balance causes false color
  imaging.
• Room lighting affects perceived color,
  brightness, and contrast.
• Overdriven CRTs cause picture distortion.
• Misadjusted User controls distort contrast and
  brightness ratios.
• Small picture flaws are magnified on high
  resolution displays.
• CRTs age - recheck calibration every 6 months

5
Introduction
Why Perform Video Calibration?

• Displays are aligned on location after
  installation and moving is completed
• Picture is optimized for room lighting conditions
  
• Reduce Doctors complaints about the display
• That it doesn't look right.
• The red of the tissue doesn't look right.
• The display is too dark or the display is too
  light.
• The picture is blurry.
• Assure the doctor that the image quality is
  correct
• Produces full range of accurate colors
  grayscale
• Telemedicine - displays need to be calibrated to
  match what is happening on site

6
Video Review - Producing an Image
CRT Operation

• CRT gun (F1/F2, K, G1, G2) produces an electron
  beam
• Electron beam is accelerated by high voltage
• Phosphor screen gives off light when struck by
  electron beam

7
Video Review - Producing an Image
How do video displays make BW?

• Changing bias (G1/K voltage) changes electron
  beam intensity
• Higher bias less beam current less bias more
  beam current
• Less beam current less light output more beam
  current more light output

No light output
Medium light output
Maximum light output
High bias
Medium bias
Low bias
8
Video Review - Producing an Image

• White - all phosphor producing light
• Black - no phosphor producing light
• Contrast ratio - difference between black level
  and white level
• Ambient light affects contrast ratio

9
Video Review - Producing an Image

• Level of video signal corresponds to CRT gun
  conduction brightness
• Brightness (luminance) average DC level
• Contrast (difference between bright/dark
  portions) peak-to-peak amplitude
• Video signal levels specified in IRE

10
Video Review - Producing Color
How do video displays make color?

• Color CRTs use three electron guns
• Each gun is fed a separate video signal (RGB)
• Red Green Blue white

11
Video Review - Producing Color
No light output

• Changing bias of all three guns equally produces
  shades of gray
• Neutral gray is call White Balance
• Good color tracking maintains neutral gray at
  all luminance levels

High bias
Medium light output
Maximum light output
Medium bias
Low bias
12
Video Review - Video Signals
13
Video Review - Signal Types
Composite Video vs.
S-Video (Y/C)

• Composite video contains luminance, chrominance,
  and sync on the same signal line.
• S-Video (Y/C) contains luminance and sync on one
  signal line, with chrominance on a second signal
  line.

14
Video Review - Signal Types
Advantages of S-video connection over composite
(RCA) connector

• Separates luminance information from color
  information to increase BW resolution
• Fewer artifacts with modern video sources like
  DSS and DVD

15
Video Review - Signal Types
Advantage of Component vs. S-video

• Increased color resolution

16
Video Review - Signal Types
Advantage of RGB over component

• RGB signals correspond directly to the CRT guns /
  light sources
• Provides best possible picture quality for analog
  CRT displays

HDTV uses component rather than RGB

• Requires less bandwidth
• More content can be stored on a DVD or broadcast.
  

17
Video Review - Signal Types
Advantage of DVI vs. Component

• DVI is digital equivalent of analog RGB
• DVI better for fixed pixel (non-CRT) displays
  connected to digital sources (DVD, digital cable
  set top converter, video games)
• Removes unnecessary analog-digital-analog
  conversion - enables pure digital signal to
  display
• DVI has bandwidth to address each pixel in a
  digital TV display individually - enabling the
  highest possible picture quality
• Component better for analog (CRT) displays -
  analog has theoretically better color and
  luminance range

18
Video Review - Formats
19
Video Review - Resolution

• Measure of maximum number of light to dark
  transitions produced horizontally or vertically
• Horizontal resolution usually expressed in pixels
• Vertical resolution expressed in pixels or lines

20
Video Review - Scanning
Interlace Scanning

• Requires 2 vertical deflection cycles to display
  a complete frame
• Vertical Jitter
• Achieves Higher resolution with lower bandwidth
• Not used in high resolution monitors

2 Fields (interlaced)
1 Frame


21
Video Review - Scanning
Progressive Scanning

• Scans every vertical line every frame.
• Helps prevents fast motion artifacts.
• No vertical jitter

1 Frame
22
Light Color Theory

• Light is electromagnetic energy within a narrow
  range of frequencies
• Each different wavelength of light energy (if
  seen alone) is perceived by the human eye/brain
  as a different, fully saturated, color

23
Light Color Theory

• If light from the sun is equally reflected from a
  surface, the eye/brain sees the surface as white.
  
• If a surface absorbs all light energy and
  reflects none, the surface appears black.

24
Light Color Theory

• If more than one wavelength is reflected, the
  eye/brain performs a mixing of all light energy
  present and perceives a single color which is a
  result of the mixture.

25
Human Vision

• Eye is tri-stimulus - sees light using red,
  green, blue receptors
• Each receptor has a different response to the
  light spectrum
• CIE Standard Observer Response Curve shows
  average response of each receptor across light
  spectrum

26
Human Vision
Three characteristics define how our eye/brain
sees light
27
Human Vision

• Hue Dominant wavelength, color or tint of the
  color.

28
Human Vision

• Saturation Degree of purity from light of other
  wavelengths
• (zero saturation white equal energy of all
  wavelengths)

29
Human Vision

• Brightness Perceived light energy level

30
Light Color Measurement

• Light Measurement Units
• Luminance (brightness)
• Footlambert U.S. unit of luminance (radiated
  light), 1 lumen per square foot
• Nit (cd/m2) S.I. unit of luminance (radiated
  light), 1 candela per square meter

Illuminance ambient light that illuminates the
display contributes to luminance that is
observed from the image display reduces the
contrast in the image need to block from
measurement
31
Light Color Measurement

• Light Measurement Units
• Color (hue and saturation)
• CIE chromaticity coordinates (x,y) From 1931 CIE
  Chromaticity Diagram (Kelly Chart)

What color?
32
Color Measurement
CIE Chromaticity Diagram

• Graphically depicts the relationship between hue
  and saturation.
• Diagram shows the pure spectral colors around the
  curved border
• The results of mixing any of these spectral
  colors are shown at the base and center of the
  diagram.

33
Color Measurement
How do video displays make white?

• By combining the proper mix of RGB
• All three guns are turned on to make white

34
Color Measurement

• C.I.E. coordinate x 0.313 y 0.329 (D) is the
  white color which was chosen as the standard
  white reference for all video display systems.
  (D6500)

35
Color Measurement
How do video displays make black?

• All three guns are turned off to make black.
• The same point on the chart represents white and
  black

36
Color Measurement
How do video displays make color?

• Using RGB light sources
• Any three colors not lying on a straight line
  with one another are color primaries.
• The points shown are the NTSC specified CRT
  phosphor color primaries for US televisions.
• The connecting triangle encloses the range of
  colors able to be produced by a CRT using these
  color phosphors.

37
Color Measurement
Color Temperature

• Color Temperature is sometimes also used to
  specify different near-white colors.
• Color Temperature is referenced to color of
  carbon when heated to different temperatures
  (measured in Kelvin -- Celsius plus 273).

38
Color Measurement

• Different whitescorrespond to different color
  temperatures
• White sheet of paper looks different under
  different light
• CRT radiates light so color temperature not
  affected by lighting conditions (but brightness
  and contrast are)

39
Color Measurement

• There is an approximate correlation between color
  temperature and CIE chromaticity
• Accurate calibration is done using CIE coordinates

40
Color Measurement

• Colors which are created by heating carbon form a
  continuous line across the CIE Chromaticity
  Diagram - known as the black body curve.
• Only colors exactly on the curve are specified by
  original absolute color temperature.

41
Calibration Overview
1. Calibrate Display Geometry

• Size
• Centering
• Pincushion
• Trapezoid
• Rotation
• Linearity

42
Calibration Overview
2. Calibrate Display Convergence

• Precisely overlay red, green, and blue images at
  all points on display.
• Eliminate all color fringing effects.

43
Calibration Overview
3. Calibrate Display White Balance

• Use color analyzer to measure color of white in
  center white window at high and low luminance
  levels

44
Calibration Overview
4. Calibrate Display User Controls

• Brightness (Black Level)
• Contrast (White Level)
• Color (Saturation)
• Tint (Hue)
• Sharpness

45
Video Generator
Introducing the VP400 401 VideoPro
The VP400 401 VideoPro Multimedia video
generators deliver the HDTV, NTSC, RGB, DVI RF
Video Signals You Need for Accurate Service
Alignments of Monitors Direct View Displays.
46
Video Generator
DVI (VP401)
NTSC RF (VP401)
Battery operated (8 hours)
Component/RGB VESA/HDTV
Composite
S-Video
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SMPTE Bar
Video Generator
Check and adjust color level and hue controls.
Check color demodulator accuracy.
48
Video Generator
Color Bar
Check for primary (red, green, blue) and
secondary (cyan, magenta, yellow) colors to
confirm proper chroma processing.
49
Decoder Check
Video Generator
75 white reference
75 saturated color level
0
5
10
20
15
25
-25
20
-10
-15
-5
Check color decoder/matrix circuit to determine
if any red or green color emphasis (push) or
de-emphasis. The 0 center bar of each color as
viewed through its respective color filter should
be the same intensity as the 75 reference white.
50
Decoder Adjust
Video Generator
75 white reference
Primary color sections
Secondary color bars
Set color decoder/matrix circuits for accurate
color. View color primary through color filter
adjust chroma gain to match intensity of white
bar and surrounding color. Adjust chroma hue to
match intensity of secondary color bars.Blue
section set User Menu Color Hue
controls. Red green sections set Service
controls
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Video Generator
Focus
Check for proper static dynamic focus operation
at screen center edges.
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Converge
Video Generator
Converge display over entire picture area. The
red, green and blue segments should form straight
lines of equal thickness. The line intersections
show steps from the straight ideal converged
lines.
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Crosshatch (43)
Video Generator
Check and adjust convergence and linearity.
54
Linearity
Video Generator
Check and adjust deflection linearity, set
geometry controls, color convergence adjustments.
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Anamorphic
Video Generator
Check accuracy of widescreen stretch feature
(from 43 to 169).
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Overscan
Video Generator
5 overscan (green line)
10 overscan (red line)
electrical center
Check and adjust display geometry, including
picture centering, size, trapezoid (keystone)
correction, pincushion (bow) correction, and
linearity.
57
Video Generator
Staircase
Check video amplifier linearity and gray scale
tracking. Adjust G2 (screen) control to proper
setting.
58
Horizontal Staircase
Video Generator
Check gray scale tracking of CRT projectors with
red and blue overdrive on sides of screen.
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Pluge (Picture Line-Up Generator Equipment)
Video Generator
Black
10 IRE
Alternating
Check for proper black level (brightness) and
white level (contrast) setup. Includes
alternating black/blacker-than-black section.
60
Video Generator
Needle
Check for Sweep Velocity Modulation (SVM)
operation, high voltage regulation, and setting
Contrast Brightness on CRT displays.
61
Video Generator
Ramp
Visual check of grayscale tracking. Poor
performance causes visual color tint at one or
more light levels. Checks digitizing linearity of
video signal processors. Poor performance causes
vertical bands.
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Window
Video Generator
Check chromaticity with color analyzer.
Adjustable IRE level window with pluge bars to
monitor black level.
63
Video Generator
Regulate
Black / White alternating
1 pixel wide
Check high voltage and scan current regulation.
64
Video Generator
Raster
Check purity white balance. Includes adjustable
0-100 IRE levels. Color can be gated on/off to
produce red, green, blue, cyan, magenta yellow
raster.
65
Checker
Video Generator
Check for regulation of low voltage supply to
video amps and video ringing. Should see sharp
vertical lines with no video ringing.
66
Lo-Hi Tracking
Video Generator
0 (black)
2.5
- 4IRE (blacker than black)
5
97.5
100 (white)
95
Set Contrast Brightness on fixed-pixel
(non-CRT) displays. Increase brightness to see
black (-4 IRE) vertical bar, then decrease until
bar just disappears. Increase brightness control
if needed to make 2.5 and 5 boxes both visible.
Set contrast so 95 and 97.5 boxes are visible.
67
Video Generator
Multiburst
Check for ringing, smearing, and sharp
resolution. Increase Sharpness control until all
bursts are equal brightness without ghosting.
68
Video Generator
Sharpness
Clear, same brightness
Straight, same width
No blooming
Align controls that artificially enhance picture
transitions (contrast, picture, aperture,
scan-velocity modulation). SVM - bottom center
vertical lines straight same width, set for
least impact on width of black lines compared to
white lines. Contrast/Picture/Aperture - sharp
B/W transitions, white edges not blurred or
blooming, no ghosting, clear, bright multiburst.
69
Color Calibration

• Different display types produce different light
  spectrums
• Our eyes still see the same result
• To provide accurate measurements Colorimeter must
  use CIE standard observer filters that replicate
  eye response
• What worked for CRTs doesnt work for other
  display types

70
Color Calibration

• Colorimeter is a tri-stimulus measurement device
• Filters and light sensors to separate light into
  R,G B
• Output voltages proportional to applied light

71
Color Calibration
CP5000 Color Analyzer
CP5001 ColorPro Color Analyzer

• Accurate method to set white balance (set to
  industry specs for best video display
  performance)
• Objective test (measure light produced by
  display)
• Easy to use and interpret (x-y and RGB bar
  graphs)
• CP500x accurate for all display types
• NIST traceable

CP5000 ColorPro Color Analyzer
72
Color Calibration
ColorPro Analyzer overview

• Probe connects to PCs serial or USB port or
  Pocket PC
• Select standard preset white references, or input
  custom setting
• Flexible refresh rate synchronization for testing
  at a fixed display format or to follow changing
  formats.
• Luminance measured in units of nits or
  footlamberts.

73
Color Calibration
ColorPro Analyzer overview

• Delta RGB analog bar graph - quick adjustment of
  cutoff and drive controls

• Selectable color reference

• Change resolution for fine touchup adjustments

74
Color Calibration
ColorPro Analyzer overview

• Chromaticity diagram display provides quick
  visualization of control interaction

• Center target defines selected white reference
  color coordinates

• Measurement cursor color steering lines track
  control adjustments

75
Color Calibration
ColorPro Analyzer overview

• Calibration Report printout provides
  documentation re-calibration reference
• Pre-and post-calibration performance easily
  documented
• Standard or extended form tracks display at high
  low luminance, or at multiple luminance points
  (ISF)

76
Color Calibration
ColorPro Analyzer overview
Identifies when next calibration is due
Documentation that display is optimized
Shows before and after color temperatures
77
Color Calibration
Calibration Library- On-line library of digital
service menu adjustment codes available to
ColorPro owners to simplify white balance
calibration without service literature.
78
White Balance/Gray-Scale Tracking Overview
White Balance Calibration

• Accurate image colors are only possible when
  white balance is correct tracks accurately from
  black to white.
• White balance is produced at all luminance levels
  when
• 1) RGB sources/CRT guns balanced/biased to
  produce desired white reference at darkest gray
• 2) RGB video amplifiers have correct gain
  linearity to produce desired white reference at
  all luminance levels from black to gray to white

• Brightness control adjusts light level of the
  darkest parts of picture (should be called black
  level control)
• Contrast control adjusts light level of the
  brightest picture parts (should be called
  brightness control).

79
White Balance/Gray-Scale Tracking Overview -
continued
White Balance Calibration

• Brightness Contrast controls affect RGB output
  equally and do not affect the white balance of
  dark or bright picture portions
• If the displays gray scale doesnt track
  accurately adjusting brightness or contrast
  controls produces a noticeable color shift due to
  the luminance level change

Normal
Lower Luminance Poor tracking
Lower Luminance Good tracking
80
White Balance - preliminary setup
White Balance Calibration

• 1. Display the Pluge, Window, or Staircase
  pattern.

81
Pluge Pattern
Black
10 IRE
alternating 0/7.5 IRE
82
White Balance - preliminary setup
White Balance Calibration

• 2. With the contrast control set to approximately
  3/4 of full scale, adjust the brightness control
  for proper display of black in the Pluge or
  Window pattern (for the Staircase pattern, make
  the first bar black and the second bar just
  barely visible).

83
White Balance - preliminary setup - CRT displays
White Balance Calibration
3. Display the Needle pattern. 4. Adjust the
contrast control for maximum brightness with no
signs of blooming in the maximum white bar of the
gray scale or raster distortion of the needle
pulses.
84
White Balance - preliminary setup - Fixed pixel
displays
White Balance Calibration
3. Display the Lo-Hi Tracking pattern. 4. Adjust
Contrast control as high as possible without
white clipping or compression.
97.5
100 (white)
95
85
White Balance Calibration
White Balance

• Color Tracking Adjustment Controls
• Cutoff/Bias controls adjust color balance at low
  luminance levels.
• Gain/Drive controls adjust color balance at high
  luminance levels.

Gain/Drive Controls
Cutoff/Bias Controls
86
(No Transcript)
87
White Balance Calibration

• 1. Determine the white reference/CIE chromaticity
  coordinates to which the display is to be
  adjusted.
• Note Most displays should be adjusted to the CIE
  D65 daylight standard (x 0.313, y 0.329).
  This is the NTSC and ATSC white reference
  standard.

88
White Balance Calibration
2. Display a 20 or 30 IRE white window pattern on
the monitor (1 lumen - just enough light to get a
reading). Note If starting from scratch after a
CRT or other video component replacement, preset
the displays drive (gain) controls for maximum
brightness and the cutoff (bias) controls for
minimum brightness.
89
White Balance Calibration
3. Leaving the cutoff control for the initially
strongest color (as viewed on the dim CRT) at its
original or preset level, adjust the other two
cutoff controls to obtain color balance at the
desired white reference.
90
White Balance Calibration
4. Select a 80-100 IRE level window pattern. 5.
Leave the drive control for the initially weakest
color (as viewed on the bright CRT) at its
original or preset level, and adjust the other
two drive controls to again obtain color balance
at the desired white reference.
91
White Balance Calibration
6. Repeat steps 2-5 until the displays color
temperature remains relatively constant (tracks)
over the full range of IRE levels or contrast
control adjustment.
92
Calibrating User Controls
When the color temperature is calibrated perform
final adjustment of the User controls at typical
ambient lighting.
CRT Displays 1. Brightness (black Level) 2.
Sharpness/Enhancement 3. Contrast (white Level
) 4. Color 5. Hue/Tint
Fixed Pixel Displays 1. Brightness Contrast 2.
Sharpness/Enhancement 3. Color 4. Hue/Tint
93
Brightness Control

• Brightness control adjusts light level of the
  back or darkest luminance portions of the
  picture.
• If set too dark
• subtle dark gray details of the scene are lost to
  black
• If set too bright
• darkest grays and deep blacks are all a lighter
  gray, which effectively lowers the contrast ratio
  of the display and reduces picture quality

94
Contrast Control

• Contrast control adjusts light level of the white
  or high luminance portions of a scene.
• Sometimes labeled picture control - should be
  labeled white level control
• If set too low
• picture image is dim, whites become dull image
  loses its luster because the contrast ratio is
  too low.
• If set too high (this is often the factory
  setting)
• power supply may be overdriven causing raster
  distortion
• may cause pixel blooming (occurs when the screen
  pixels are struck so hard that light spreads to
  the adjacent pixels, defocusing the white image)

95
Enhancement Controls - (Sharpness, Picture,
Aperture, Scan Velocity Modulation)

• Artificially enhance picture transitions between
  black white
• Recommend turning SVM off

96
Calibrating User Controls
Brightness Control - CRT displays 1. Select
VP300 Pluge pattern. 2. Watch second box from the
inside (alternating between 7.5 IRE/black 0
IRE/blacker-than-black). 3. Adjust the control
until the 7.5 IRE 0 IRE levels appear the same
and no flashing can be seen. The outer 10 IRE box
should still be slightly visible.
97
Calibrating User Controls
Brightness Control - Fixed Pixel displays 1.
Select VideoPros Lo-Hi Tracking pattern. 2.
Increase brightness to see black (-4 IRE)
vertical bar, then decrease until bar just
disappears.
0 (black)
- 4IRE (blacker than black)
97.5
100 (white)
95
98
Calibrating User Controls
Brightness Control - Fixed Pixel displays 3.
Increase the Brightness control if needed to make
2.5 and 5 boxes both visible. 4. Set contrast
so 95 and 97.5 boxes are visible.
2.5
0 (black)
5
- 4IRE (blacker than black)
97.5
100 (white)
95
99
Calibrating User Controls
Sharpness/Enhancement Controls - Fixed Pixel
CRT displays

• 1. Select the VideoPros Sharpness pattern.
• 2. Adjust sharpness control while watching the
  bottom vertical lines. Set for best balance
  between ghosting and poor focus.
• too high - faint ghosting beside line
• too low - appears out of focus and lost
  resolution

Straight, same width, no ghosting
100
Calibrating User Controls
Sharpness/Enhancement Controls - Fixed Pixel
CRT displays
3. If the top multiburst section becomes too dull
or unclear increase the Sharpness control for the
best compromise
Clear, same brightness
101
Calibrating User Controls
Contrast Control - CRT Displays

• 1. Select VideoPros Needle Pulse pattern.
• 2. Watch the vertical lines as you adjust the
  contrast
• top white bar is same width as 4 gray bars below
• vertical lines are straight.
• If contrast is too high, needle pulses will bend
  boxes bloom
• 3. Recheck Brightness control setting and
  readjust if needed.

same width (no blooming)
Straight (no raster distortion)
102
Calibrating User Controls
Color Level Control 1. Select SMPTE Color Bars
pattern. 2. View pattern with blue filter, paying
attention to large top outside blue white bars,
and small bars below them. 3. Adjust Color
control to make these bars appear to have the
same intensity (brightness) level. 4. Remove blue
filter and view yellow red bars. They should be
very colorful without blooming into adjacent bars.
103
Calibrating User Controls
Tint Control 1. Select SMPTE Color Bars
pattern. 2. View pattern with blue filter, paying
attention to the Cyan/Magenta bars that are
located above and below one another. 3. Adjust
Tint/Hue control to make these bars appear to
have the same intensity (brightness) level.
104
Color, Hue and Decoder/matrix

• Use if display has 2 User Controls and color
  Service Adjustments (RYR/red gain, RYB/red
  hue, GYR/green gain, GYB/green hue)
• Set color decoder/matrix circuits for accurate
  color.

Decoder Adjust
105
75 white reference
Primary color sections
Secondary color bars
1. View color primary through color filter
adjust chroma gain to match intensity of white
bar and surrounding color. Blue section set
User Menu Color Hue controls. Red green
sections set Service controls (RYR/red gain,
RYB/red hue, GYR/green
gain, GYB/green hue) 2. Adjust chroma hue to
match intensity of secondary color bars in each
color block.
106

• Benefits of a Properly Calibrated Video Display
• Displays a sharper-focused, full resolution
  image.
• Shows full detail in both the darkest brightest
  parts of all scenes.
• Is properly matched to the viewing environment.
• Produces a full range of accurate colors,
  including flesh tones.
• Minimizes picture artifacts (distortions)
• Satisfied user

107
DICOM Overview
Imagine the following
A trauma patient is admitted at an outpatient
clinic or rural clinic at a remote location.
Several X-rays are taken and sent to a hospital
for consultation.
108
DICOM Overview
How can we make sure that what the physician sees
at the clinic matches what is being displayed on
the radiologist softcopy workstation at the
hospital?
?
?
?
?
109
DICOM - Overview
The image is also sent to another radiologist for
a second opinion. This radiologist has a
diagnostic viewing station from another vendor -
a different monitor and video display card. As a
matter of fact, he has a new flat panel display
instead of a conventional CRT. How do we make
sure that the image again looks identical?
?
?
?
?
?
110
DICOM - Overview

• Everyone misses something among the myriads of
  pixels that are tape archived for colleagues and
  lawyers to examine with 20/20 hindsight vision.
• Perhaps the missed findings were apparent in the
  image data, but a once-new PACS monitor had lost
  its ability to display those subtle differences.
• How can you know? What will you say when they ask
  what steps you took to assure that the display
  equipment you used was up to par?

111
DICOM - Overview
A digital signal from an image can be measured,
characterized, transmitted, and reproduced
objectively and accurately. However, the visual
interpretation of that signal is dependent on the
varied characteristics of the systems displaying
that image. Without a standard images produced
by the same signal may have completely different
visual appearance, information, and
characteristics on different display devices. In
medical imaging it is important that there be a
visual consistency in how a given digital image
appears, whether viewed, for example, on the
display monitor of a workstation or as a film on
a light-box.
excerpts taken from the Forward to Digital
Imaging and Communications in Medicine
(DICOM) Part 14 Grayscale Standard Display
Function
112
DICOM - Overview

• These issues are addressed the Digital Imaging
  and Communications in Medicine (DICOM) standard
  (PS 3.14-2003)
• Part 14 of this standard deals with grayscale
  consistency.
• Jointly developed by American College of
  Radiology (ACR) National Electrical
  Manufacturers Association (NEMA)
• Standard method for transferring images and
  associated information
• Allows images to be displayed consistently on
  different softcopy and hardcopy devices
• devices manufactured by various vendors
• devices having different technologies (CRT, LCD)
• devices with different characteristics (max.
  Brightness and/or Density)
• different ambient light environments.

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DICOM - Display Gamma
CRT and LCD monitors have a fixed power-law
relationship between output luminance and input
drive signal (gamma) of about 2.5 output
luminance input drive2.5 Signals applied to
CRTs LCD require an inverse gamma offset to
counteract the displays gamma This
correction produces a linear relationship between
the driving signal and resulting light output
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DICOM - Display Gamma

• Our eye-brain detection system is not linear
• much less sensitive in the blacks than in the
  whites
• why map all black values into luminance changes
  to display on the monitor when we cant notice
  the differences between small black value
  changes?
• DDL values of darker areas mapped into large
  luminance increments
• DDL values of whiter areas mapped to smaller
  luminance increments
• Resulting curve is standardized as the DICOM
  Grayscale Standard Display Function (GSDF)

Digital Driving Level (DDL) digital value
applied to a Display System to produce a
luminance output
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DICOM - Perceptual Linearization

• GSDF produces a display function that matches the
  perceptual characteristics of our eyes
• Just-Noticeable Difference (JND) - smallest
  luminance change that an average person can just
  perceive (approximately 256 shades of gray)
• Perceptually linearized display - equal changes
  in driving levels produce perceptually equivalent
  changes in luminance across the entire luminance
  range.
• Most digital radiologic images display 256 shades
  of gray - the approximate maximum number of Just
  Noticeable Differences that we can perceive

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DICOM - Perceptual Linearization
Equal steps in perceived brightness represent
equal steps in the acquired image data
Different change in absolute luminance
Same number of Just Noticeable Difference same
perceived contrast
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DICOM - Perceptual Linearization
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DICOM - Perceptual Linearization

• GSDF provides a way to standardize image
  displayed on monitors having different inherent
  brightness and response curves
• Used to recalibrate monitors whose luminance and
  response curves have changed over time
• Simply calibrate the monitor printers according
  to this curve
• Calibration requires luminance meter that can be
  placed on the monitor sothe light output
  (luminance) can be measured at multiple digital
  driving levels (DDLs) / luminance levels
• Luminance meter output is compared with the
  actual Grayscale Standard Display Function (GSDF)
  and differences can be calculated and saved
• A corrective transfer function is then loaded
  directly to the video display controller

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DICOM - Color Lookup Table

• Video Card Look Up Table
• Digitized images are made up of a matrix of
  pixels, each possessing at least three
  dimensions two (or more) spatial and one
  intensity value. The quantized dimensions are
  stored on the computer as a file of binary
  numbers.
• In order to see the image on the computer
  monitor, the image pixel values must be mapped,
  one-to-one, to screen pixel values, via a Look Up
  Table (LUT). The LUT transfer function determines
  what screen values correspond to image pixel
  intensity values at all coordinates in the image.

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DICOM - Color Lookup Table

• If each image intensity value is mapped to its
  corresponding screen value the LUT is a linear
  function and can be graphed as a 45 straight
  line.
• By modifying the slope of the transfer function
  and screen mapping, LUT image intensity values
  may be selectively increased or decreased to
  perceptually equalize luminance steps in the image

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DICOM - Calibrating Displays

• Procedure for Calibrating Gray scale (BW)
  Monitors
• Calibrate each monitor regularly
• Calibrate under normal viewing lighting
  conditions
• Adjust monitor contrast and brightness
  before/during calibration and never again
• Calibrate with always the same background
  intensity (about 20, or a value comparable to
  the average intensity emitted from the monitor)
• Calibrate to match the standard display function
  (DICOM)

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Questions?
Remember
Your Sencore contact
Scott Reardon
1-800-SENCORE www.sencore.com