Understanding Display Characteristics

1
Understanding Display Characteristics

• What You Should Know When Viewing Images from PACS

2
Disclosures

• Unfortunately no one is paying me to hawk their
  products
• Gratefully acknowledge the use of slides from
  OTech and Barco

3
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

4
Preface

• We can see radiological images on any pc
• So why dont 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

5
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

6
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

7
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

8
(No Transcript)
9
So Lets Move on
10
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)

11
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

12
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.

13
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

14
Luminance

• Luminance is the quantity describing the
  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

15
Luminance

• Lightbox/Alternator 200-600fL
• PC color monitor (CRT) 20-40fL
• Medical imaging monitors
• ACR recommended minimum 47fL
• Low bright 50-60fL
• LSU Clinical Barco CRT 65fL
• NEC Color 15, 18, 20 LCD 71fL
• LSU Diagnostic Barco CRT 85fL
• High bright 100fL or higher

16
LCD Display Examples
17
Color vs Monochrome

• Color monitors have decreased luminance, hence
  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

18
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)
• Doesnt stabilize for up to an hour or more when
  first turned on

19
(No Transcript)
20
Temperature Dependance
21
Time Dependance
22
Monitor Calibration

• Brightness and contrast controls usually hidden,
  once set during calibration they should not be
  changed
• Manual or automated monitoring
• DICOM GSDF

23
Monitor Calibration

• DICOM Part 14 Grayscale Display Function (GSDF)
• Use of LUTs (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

24
GSDF Graph
25
Calibrating to the GSDF Curve
26
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

27
(No Transcript)
28
AAPM TG-18 Tests

• Document Assessment of Display Performance 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

29
SMPTE Test Pattern
To properly display a test pattern on your
monitor, go to 10.1.204.17/ami and click on Test
30
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.

31
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

32
Realities

• Need to calibrate and monitor the display 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)

33
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