Transcript 08. Optimising DR Displays - RPOP

Radiation Protection in Digital Radiology
Optimising DR Displays
L08
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International Atomic Energy Agency
Educational Objectives
• List three major differences between DR displays
and transilluminated films
• Explain how CRTs and LCDs differ with respect to
the display of medical images
• Appreciate the differences between medical and
commercial grade flat panels
• Give an example of how differences between a
technologist’s display and a radiologist’s display
can contribute to unnecessary radiation exposure.
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The Cathode Ray Tube (CRT) is a fifty-year
old device for displaying electronic images
• Electrons produced in a
vacuum tube strike a
luminescent screen
• The path of the electron
beam is deflected by a coil
• The amount of light
produced in any position is
related to the intensity of
the electron beam at that
time
• Color can be produced by
means of a shadow mask
or aperture grill
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Bushberg et al. 2002 The Essential Physics of Medical Imaging 2nd Ed
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The CRT provides a dynamic display of
even stationary images
• The electron beam scans
across the face of the display in
a raster fashion
• The standard video frame rate
is 30 fps (SMPTE)
• Historical lowest rate to avoid
perception of flicker
• Convenient: ½ of 60 Hz
• Alternate frame rates, such as
24 fps for motion pictures
• Interleaved display would use 2
frames for one image – higher
spatial resolution
• A picture element (pixel) is an
arbitrary segment of a scan line
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Bushberg et al. 2002 The Essential Physics of Medical Imaging 2nd Ed
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Active Matrix Liquid Crystal Displays (AMLCD)
present electronic images by a different method
• The LCD controls the transmission
of a uniform backlight
• The transmission of light through a
given LC cell can be considered
binary (on/off)
• Actually much more complex
• A pixel is composed of six
components arranged in a chevron
pattern
• Two domains
• Three colors
• “Active” refers to control of each
pixel independently via TFT array
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“Flat Panel”
Bushberg et al. 2002 The Essential Physics of Medical Imaging 2nd Ed
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Light transmitted by a flat panel is a
composite of pixel component states
• Un-calibrated response is irregular
• Display controller needs 10-12 bits for medical
applications
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Ability to produce color, limits
performance of commercial flat panels
• Color filter allows only 3-5% transmittance of
backlight vs. 8-15% for monochrome
• Combination of sub-pixel intensities to yield true
white is additional complication
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Off-axis viewing is problematic with flat
panels
• CRT emissive luminance is Lambertian, the
intensity appears the same from all viewing angles
• Flat panel transmissive luminance is nonLambertian, the intensity appears different from
any viewing angle other than normal (rounds?)
• This is not a problem for single viewer, unless the
viewer must move (interventional?, surgery?)
• Even if radiographer has same display as
radiologist, off-axis viewing differences can cause
discrepancy in rendering the image
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Radiation Protection in Digital Radiology
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“How do you know that it’s okay for the
physician to view images on that display?”
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Capabilities of the display technology
Characteristics of the images to be viewed
Idiosyncrasies of human visual system
Configuration of the display device
Calibration of the display device
Local viewing environment
Workstation software and controls
Viewing task to be performed
Active maintenance of display quality
Ambient lighting condition
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Radiation Protection in Digital Radiology
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Digital imaging is often“display-limited”
• The information contained in the image cannot be
presented by the display in a single rendering
• Spatial resolution
• Contrast resolution
• Dynamic range
• Workstations address this problem by software
tools to display a portion of the image at full
resolution
• Zoom and Pan
• Window-width and window-level
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The smallest feature that can be displayed is
limited by the number of pixels
CRT Beam spot size for 300 X 400 mm Field
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Pixels
Array size
Spot size
(mm)
1MP
900 X 1100
"1K X 1K”
0.35
2 MP
1200 X 1600
“1K X 1.5K”
0.25
3 MP
1500 X 2000
“1.5K X 2K”
0.20?
5 MP
2000 X 2500
“2K X 2K”
0.15
Flynn MJ 2004
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L08 Optimising DR Displays
“Corduroy” artefact
• Interference
pattern between
fixed grid lines
and downsampling rate for
display
• Disappeared on
zoom
• Bad choices
• Display default
magnification
factor
• Line rate of grid
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Radiation Protection in Digital Radiology
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This is one place where size doesn’t matter
• The physical size of the display affects only viewing
distance
• The pixel matrix is what matters
• Large displays are useful for interventional, surgical, and
multiple simultaneous observers
• Projection Displays
• Plasma Displays
• DLP displays
• Small displays may be useful for
reference or
image navigation
• PDA
• Cell Phone
• LEP (light emitting polymers)
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Spatial resolution of the display is limited
by blur
• Blur is a major factor in CRT displays because of
the dynamic way the pixel is produced
• Blur is much improved in flat panels because of the
stationary structure of the pixel
• a 3MP flat panel performs as well as a 5MP CRT display
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Electronic displays are limited with
respect to maximum luminance
• Typical light box luminance is >500 fL (1700 cd/m2)
• Typical medical CRT is 70-90 fL (240-300 cd/m2 )
• minimum ACR is 50 fL (171 cd/m2 ) for primary
interpretation
• Typical general purpose CRT is 30 fL (100 cd/m2 )
• Medical monochrome LCD is 200 fL (700 cd/m2 )
• Typical consumer electronics color LCD display is
60 fL (200 cd/m2 )
• Paper SSA20-06 Visser M et al. RSNA 2005
describes prototype backlight up to 2000 cd/m2
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Because of their limited luminance, the viewing
environment for electronic displays is critical
• Ambient illumination limits the contrast that can be
appreciated from an electronic display
• The higher the ambient illumination, the higher the
maximum luminance the display will need.
• The more reflective the display, the lower the allowable
level of ambient illumination.
• Big problem with CRT
• Changes in ambient illumination strongly affect contrast in
the dark areas of the display, so one strategy is to raise the
minimum luminance.
• Convenient for flat panels with poor black levels
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The luminance function of electronic displays
is not appropriate for viewing digital images
• The luminance function is modified by a software look-uptable (LUT) in an attempt to elicit equal human visual
response for equal changes in grayscale value.
• The mathematics of this transformation are defined in
DICOM Part 14 Grayscale Standard Display Function
(GSDF).
• The result is that a graph of luminance expressed in units
of “just noticeable differences” (JND) is linear with respect
to grayscale value.
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Luminance (Cd/m^2) versus PV
(serial number A2I-05975)
Luminance of
properly calibrated
display is curved
function of
greyscale value
Luminance (Cd/m^2)
1000
100
Measured (Nominal Lmax=600)
10
GSDF
1
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
Pixel Value
JND Index versus Input Pixel Value
(serial number A2I-05975)
600
500
JND Index
Luminance
translated into
JND is linear
function of
greyscale value
400
300
R^2 = 0.99984
200
100
Series1
Least Squares Fit
0
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
Pixel Value
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Viewer software can affect display function
Dell Model E771P Color Monitor
600
y = 4.3844x + 60.704
R2 = 0.9706
JND Index
500
400
Stentor
Webb1000
300
Linear (Stentor)
Linear (Webb1000)
200
y = 4.888x + 36.444
R2 = 0.9856
100
0
0
20
40
60
80
100
SMPTE %
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“The best electronic image, improperly
displayed is terrible.”
• CRT monitors degrade over time. LCDs last longer.
• The wrong display Look-up-table (LUT) can spoil a
great electronic image (DICOM Part 14 GSDF)
• Test patterns, notably the SMPTE, can make display
problems obvious.
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Just because you ran the calibration routine does
not mean the display is DICOM Part 14 compliant
JND index
Dome Flatpanel post cal
900
800
700
600
500
400
300
200
100
0
y = 5.8183x + 202.51
R2 = 0.9721
0
20
40
60
80
100
120
SMPTE%
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For GSDF conformance, consumer color flat panels
require control of downloadable color ramps
HPDome
Color Flatpanel post cal
700
JND index
600
y = 4.286x + 150.2
R2 = 0.9717
500
400
300
200
100
0
0
20
40
60
80
100
120
SMPTE%
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Display problems affect the radiologist’s ability
to practice digital radiology
• Potential errors (hard or soft copy)
• Incorrect GSDF calibration
• Inadequate matrix
• Moire’ (interference) patterns
• Inadequate spatial resolution
• Incorrect or missing demographics or annotations
• Inadequate viewing conditions
• QC => Radiologist “Film” critique
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Quantifiable Consequences of Degraded
Performance
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Loss of Contrast Sensitivity
Loss of Sharpness/Spatial Resolution
Loss of Dynamic Range
Increase in Noise
Decrease in System Speed
Geometric Distortion
Artefacts
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AAPM Task Group 18 has developed
procedures for assessing display quality
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GSDF
Luminance Uniformity
Noise (Low contrast performance)
Resolution and resolution uniformity (CRT only)
Veiling Glare (CRT only)
Geometric Distortion (CRT only)
Bandwidth Artifacts (CRT only)
Dead Pixel Count (LCD only)
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AAPM Task Group 18 report on assessment of display
performance for medical imaging systems
• Recommended tests and
frequency
• Useful test patterns
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http://aapm.org/pubs/reports/OR_03.pdf
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Some aspects of display performance
can change over time
 GSDF
 Luminance Uniformity
 Noise (Low contrast performance)
 Resolution and resolution uniformity (CRT only)
X Veiling Glare (CRT only)
 Geometric Distortion (CRT only)
 Bandwidth Artifacts (CRT only)
 Dead Pixel Count (LCD only)
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Radiation Protection in Digital Radiology
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To assure display quality, you are
going to have to measure it
• Will need photometer
• May need a chromaticity attachment
• Will need test patterns
• MANY available from TG18
• Will need to measure more stuff, more
frequently (monthly) with CRT
• Useful lifetime of CRTs is limited compared to
flat panels
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Good news is that remote QC
technology is available
• Automatic GSDF calibration
• Automatic monitoring and compensation for
changes in ambient lighting and maximum
luminance
• Remote monitoring, reporting, and adjustment via
SNMP client.
• Luminance level, drive level, system temperature,
etc
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Ref: Raimond Pohlman and Jeff Shepard, UT MDACC
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Difference in appearance on two GSDF
calibrated displays
Acquisition Station
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PACS Display
Even with proper calibration, viewer interpretation of
greyscales can differ …
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Discrepancy between the DX VOI LUT
and the PACS Linear LUT
• PACS viewer applied
linear LUT to
greyscales intended to
have sigmoidal LUT
• Consequence: clipped
light and dark regions
WW=2747, WL=4897
16384
Output Grayscale
12288
DX VOI LUT
8192
PACS Linear LUT
4096
0
0
4096
8192
12288
16384
Input Grayscale
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Radiation Protection in Digital Radiology
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Teleradiology – the forgotten display
• With remote viewing, one can no longer control
what display is going to be used to display the
image.
• ACR Standard calls for transmission and
assessment of SMPTE test pattern weekly.
• Only workable approach is to provide the physician
with an assessment tool at session log-in where he
must affirm that he can see features.
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Field Effect Display (FED) may challenge
AMLCD
• Can be built as thin as LCD
• Emissive display: no backlight
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Conclusions:
• Active Matrix Liquid Crystal Displays will
continue to displace Cathode Ray Tube Displays
for medical imaging
• Displays for medical imaging require special
calibration according to DICOM GSDF
• Increasing use of pseudo-color in digital imaging
imposes special demands on displays
• Novel display technologies are likely to find use
in specific limited applications, except possibly
FEDS
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Answer True or False
•
•
•
The spatial resolution in flat panel monitors
are better than CRT
There can be artefacts arising from display
screens
The display systems can be used in any
kind of environment
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Answer True or False
• True. Blur in flat panel monitors is less than
the CRT monitors because of the stationary
structure of the pixel.
• True. Corduroy artefact. It is the interference
pattern between fixed grid lines and downsampling rate for display.
• False. Ambient illumination limits the
contrast that can be appreciated from an
electronic display
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References:
• Bushberg JT, Seibert JA, Leidholdt EM Jr, and Boone JM. The Essential
Physics of Medical Imaging 2nd Ed. Lippincott Williams and Wilkins.
Philadelphia. (2002) 933).
• Flynn MJ. Softcopy Display: Technology, Performance, and Quality. In
Specifications, Performance Evaluations and Quality Assurance of
Radiographic and Fluoroscopic Systems in the Digital Era. Goldman LW
and Yester MV eds. AAPM Monograph No. 30.Medical Physics
Publishing. Madison. (2004) 335-351.
• Baldano A. Principles of Cathode-Ray Tube and Liquid Crystal Display
Devices. In Advances in Digital Radiography: RSNA Categorical Course
in Diagnostic Radiology Physics. (2003) 91-102.
• Samei E, Badano A, Chakraborty D et al. Assessment of display
performance for medical imaging systems: Executive summary of
AAPM TG18 report. Medical Physics 32(4)(2005)1205-1225.
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