DEPARTMENT
of
RADIOLOGY
Digital Image Displays
Resolution,
Brightness,
Grayscale
Michael Flynn
mikef@rad.hfh.edu
AAPM 2005
A.1 Fluoroscopic/Angiographic systems
A.2 QC image workstation
• Radiographers/technologists examine and approve the quality of
each radiographic image, adjusting display factors if necessary.
New Angio / Fluoro systems are now all
sold with LCD displays for in room use.
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• The QC station SHOULD be capable of displaying images with
comparable quality to that used for diagnostic interpretation.
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A.3 HFHS Tech stations
A.4 Radiologists Workspace
Delivery of images to the archive and RIS exam
completion is done at technologist’s workstations
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Two person workspace for
teaching and consultations
Full width keyboard bench provides
surface for both persons.
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A.6 Radiologhy Conference Room
A.5 HFHS ultrasound module
Ultrasound workstations utilizing
1600 x 1200 color monitors.
40” LCD Monitors with DICOM
calibrated grayscale and
matched luminance range
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A.7 Radiologhy Conference Room - Alternatives
A.8 HFHS Clinic stations
Clinical workstations
•
•
Enterprise desktop replacement
1550 single 1mp color lcd stations
Ceiling
mounted
DICOM
calibrated
LCD (DVI)
projectors
• Multi-input computer projection
• Computer, Film Camera, Slides, VHS, DVD
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A.9 HFHS Clinic stations
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A.10 HFHS Clinic stations
Clinical workstations
•
•
25 dual 1mp color lcd stations
Surgery and selected Clinics
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Specialized
wall mounts
with an
overhead
computer
shelf in areas
where space
was limited
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Main Campus ER
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B.1 Electro-optical Effect
B. LCD principles
Twisted Nematic (TN)
LC cell
Liquid Crystal Displays
Principles of Operation
When LC molecules
contact a grooved
surface, they align
parallel to the grooves.
• Liquid crystal display (LCD) devices have quickly
replaced CRT devices as wide viewing angle designs
have become available at modest cost.
• An understanding of the LCD principles of
operations is helpful when selecting devices for use
by different users in the medical center.
Adapted from Sharp Co. brochure
The director is altered by
external electric field. When
the director is twisted, light
polarization also twists.
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B.3 Elements of a TN LC Cell
B.2 Light Modulation With Polarizer
With polarizer filters, the LC electro-optical effect defines
light transmission as a function of applied cell voltage.
T
1
Top substrate
NB
Alignment
layer
90o twist
0
Polarizing Filter
Transparent
electrodes
NW
Vth
Bottom substrate
Applied voltage
Spacers
Backlight
For normally black (NB with aligned polarizers),
there is no transmission when voltage is applied.
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B.5 Brightness and Light Transmission
B.4 Brightness by Backlight
display
lamps
diffuser
Power efficiency and brightness are related
to optical performance of several layers.
display
100 %
reflector
40 %
•
•
•
•
•
Backlight:
Multiple-phosphor lamps, reflector, diffuser.
Behind panel (brighter) or on edge (uniformity).
HCFT (hot cathode fluorescent tube): bright, 10,000 h life.
CCFT (cold cathode fluorescent tube): 20,000 h life.
New tube designs provide even longer life and adjustment
of color temperature (multi-tube designs)
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backlight
20 %
polarizer
active matrix
electrode
3%
liquid crystal
electrode
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B.7 Off axis contrast ratio
B.6 Active Matrix Design
All pixels in a row are changed in sequence.
Contrast ratio at off-axis viewing directions for the
• The ratio of the
maximum luminance
to the minimum
luminance is known
as the contrast ratio
• Contrast ratio is
measured in the
absence of ambient
light.
• For LCD systems,
the contrast ratio
can be severely
degraded in relation
to viewing angle
• a-Si TFTs:
•
good switching performance.
low leakage in OFF state.
• Aperture ratio:
Typically 50%
• 80% (Sharp) high luminance
•
• Challenges:
scan lines resistance (lag).
• photo-conductivity.
•
C3 AMLCD (after Badano et al, RSNA'01)
900
horizontal
800
vertical
700
Contrast Ratio
No flicker even at modest refresh rates.
•
In color systems,
significant loss
comes from the
RGB color filters.
color filters
polarizer
diagonal 1
600
diagonal 2
500
400
300
200
100
0
-90
-45
0
45
90
Degrees off-axis
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B.8 The TN Viewing Angle Problem
B.9 In Plane Switching (IPS)
Due to the high anisotropy of light modulation:
For in-plane switching (IPS) designs, the rubbing directions are the same
on the top and bottom of the cell. When an electric field is applied, the
directors remain in plane producing improved viewing angle response.
The effective cell gap (ON/OFF state) changes.
• The effective LC orientation differs for
intermediate gray-level.
•
TN cell
–
Solutions:
–
–
–
OFF (W)
Compensation foils
Multiple sub-pixel domains (m)
In-plane switching (IPS, mIPS)
vertical alignment (VA, mVA)
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Advantage:
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Vertically Aligned LCD
ON (W)
High contrast (excellent black).
Disadvantages: low aperture ratio -> reduced brightness.
Relatively high power
slow response time.
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Emission angles can be distributed
by using multiple domains with
different orientations for each of
the sub-pixels structures.
• LC alignment is from a protrusion
producing directors that are
perpendicular to the display
surface. No rubbing processes
are employed.
• The sub pixel is divided into
several regions in which the
crystals are free to move,
independently of their neighbors,
in opposite directions (mVA).
Dual Domain RGB
• 2- and 4-domain designs demonstrated, 8-domain designs simulated.
• The domain alignment is fabricated using
• differential rubbing treatments and photolithographic steps.
• Patterned alignments with differential UV light exposure.
• Challenges:
• domain stability (polymer stabilization)
• more fabrication steps and cost
• Wide horizontal and vertical
viewing angle.
• Excellent low luminance response
(deep black).
• Switching times are ~1/2 that of
IPS designs producing improved
cine display.
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IPS cell
OFF (B)
B.11 Multi-domain Cells
B.10 Vertical Alignment (VA)
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ON (B)
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B.13 Dell 2000FP - 2MP
B.12 Macro Recording of LCD pixel structure
TN + Film
Macro photographs recorded with varying luminance.
Contrast/Brightness adjusted for similar appearance.
Leitz 24mm Summar
Ernst Leitz, Wetzlar (Germany)
Max aperture f2.0
Nikon PV4 bellows
Fuji S1 digital camera,ASA 1600
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B.14 NDS Nova (idTech panel) - 3MP
77 (7f)
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B.15 Totoku ME201L (Sharp panel) -2MP
IPS – dual domain
Macro photographs recorded with varying luminance.
Contrast/Brightness adjusted for similar appearance.
7 (0f)
f7
VA – dual domain
Macro photographs recorded with varying luminance.
Contrast/Brightness adjusted for similar appearance.
247 (f7)
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7 (0f)
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77 (7f)
247 (f7)
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B.17 Digital Display Controllers
B.16 SHARP T2020 - 2MP
VA – dual domain
Macro photographs recorded with varying luminance.
Contrast/Brightness adjusted for similar appearance.
•
165 Mpixels/sec
•
2Mp @ 82 Hz
Dual link mode:
•
7 (0f)
77 (7f)
247 (f7)
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control
control
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Luminance ratio:
LR = L’max/L’min should be the same
2. Luminance response:
Luminance vs P value should be the same.
1.
Equivalent contrast ?
Grayscale calibration
Maximum Luminance ?
Pixel Size ( Distance ) ?
Field of View?
8 bits?
Viewing angle?
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4Mp @ 82 Hz
Display
Controller
How can display devices be configured so
that the grayscale contrast of an image will
appear the same on all devices?
FAQ
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330 Mpixels/sec
•
Graphics
Controller
Pixel data
C.1 Equivalent Grayscale Appearance?
C Frequently Asked Questions
1.
2.
3.
4.
5.
6.
7.
•
Pixel data
TMDS RECEIVER
Single link mode:
•
TMDS TRANSMITTER
• In 1999, an industry Digital Display Working Group
defined specifications for digital display connection.
• The DVI specifications is now the most common
interface for graphic cards and monitors.
• Silicon Image’s PanelLink technology for Transition
Minimized Differential Signaling (TMDS) provides
the technical basis for the inteface.
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C. 1.b Luminance Range: Retinal response
C. 1.a Adapted Observer Performance
Luminance response of photo-receptors in the eye
1.0
Observer performance is best when visual system is
adapted to the average scene luminance.
A
B
P (relative photoreceptor response)
0.9
0.8
0.7
low average
luminance
0.6
medium average
luminance
high average
luminance
0.5
0.4
0.3
ADAPTATION
0.2
0.1
0.0
C
1
10
100
1000
10000
L (cd/m2)
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C.1.d Effect of Lmax/Lmin
C.1.c Fixed versus variable adaptation
Contrast threshold for varied visual adaptation (Flynn 1999b). The
contrast threshold, ∆L/L, for a just noticeable difference (JND)
depends on whether the observer has fixed (B) or varied (A)
adaptation to the light and dark regions of an overall scene.
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• Digital radiographs
should be displayed
using over a
luminance range of
250-350:1.
• Images prepared
for range of 250
that are display on
a monitor with large
range will have
poorly perceived
contrast in dark
regions.
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Flynn, Radiographics 1999
250:1
350:1
650:1
.1 to 2.50 FD
.1 to 2.65 FD
.1 to 2.90 FD
250:1
650:1
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C.1.e LR for LCD monitors
C.2 Grayscale calibration
How can the luminance response of
monitors with commercial graphic cards
be calibrated to the DICOM Gray Scale
Display Function (GSDF) ?
• For CRT monitors, LR is set by adjusting
brightness (Lmin) and contrast (Lmax).
• For LCD devices, only the backlight
intensity can be adjusted.
• For LCD devices
• Lmax is set by adjusting the backlight
brightness (current control).
• Lmin is set as a part of the grayscale
calibration (starting LUT value).
Measure the gray palette
Generate a look up table (LUT)
3. Install the LUT in the microsoft
graphic driver.
1.
2.
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C.2.a LCD uncalibrated luminance response
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C.2.b Luminance measurements
The uncalibrated luminance response of an
LCD panel typically has an irregular shape.
Calibration requires
precise measurement of
luminance for all gray
values in the palette.
1000
Luminance - 766 sequence
cd/cm2
100
10
4 min for 256
12 min for 766
30 min for 1786
1
0
100
200
300
400
500
600
700
800
900
Display Value
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An increasing palette of luminance generated by
sequentially incrementing R, G, and B values
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C.2.c dL/L for 766 palette sequence
C.2.d Calibration LUT
• Define the exact luminance for 256 luminance
values that follow the DICOM 3.14 standard
between a designated Lmin and Lmax.
• Adjust for ambient luminance by subtracting a
specified value for Lamb.
• From the measured palette, select those RGB
entries that most closely match the desired values.
The relative light change between sequential
palette entries varies discontinuously.
0.020
DL'/L'
0.018
0.016
DL'/L'
0.014
0.012
LutGenerate
0.010
0.008
Measured palette data is
converted to a calibration
LUT in a format that can be
automatically installed for
controllers with standard
microsoft drivers.
0.006
0.004
0.002
0.000
0
100
200
300
400
500
600
700
800
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C.2.e Grayscale calibration for enterprise systems
What should the maximum brightness be ?
This depends on the room lighting and
ambient luminance of the monitor.
2. Lmin should be 5 times Lamb.
3. Lmax is then Lmin time LR.
1.
• All ePACS stations have
program shortcuts to
display a test pattern and
to change LUTs.
• The DICOM LUT is
reinstalled at every login.
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C.3 Maximum brightness
• Calibration look-up tables were generated for a set
of Dell 1905FP monitors and found to be similar.
• A single generic LUT was identified for installation in
1000 workstations deployed for ePACS use.
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C.3.a Effect of Lamb
D: LCD reflection
• LCD monitors have a complex reflection distribution
Contrast reduction from diffuse added luminance
• Excellent performance can be obtained if lights sources are
located at more that 45 degrees from the surface normal
Luminance (cd/m2)
1000
DICOM
100
10 nit
10
1 nit
1
0.1
0
100
200
300
400
500
600
700
800
Display value
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C.4 Pixel Size
HFHS:
Radiology: Lmax = 500, LR = 350, Lmin = 1.4 cd/m2
Enterprise: Lmax = 250, LR = 350, Lmin = 0.7 cd/m2
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C.4.a HVS: Retinal anatomy
The retina of the human eye contains a network of
rods and cones interconnected by neural cells.
What should the pixel size of a medical
monitor be ?
This depends on the viewing distance,
so this should be specified first.
2. For the specified viewing distance
the pixel size should be just small
enough that the pixel structure is
not visible.
1.
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C.4.c. Visual Acuity
C.4.b HVS: Foveal response
A variety of test patterns are used to assess visual
acuity. Clinical measures are done typically with a
Snellen eye chart. Much psycho-visual research has
been done using sinusoidally modulated test targets.
Particularly thin cones (2 µm)
are densely packed in the
central 50 microns of the
fovea centralis. They provide
high detail color response.
At 60 cm, 1 degree
corresponds to a 1
cm field of view.
This area is
focused on a 288
micron region of
the retina, the
fovea centralis
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C.4.e Pixel Size at Maximum Spatial Acuity
C.4.d. Contrast Sensitivity & spatial acuity
Contrast sensitivity is the inverse of contrast threshold
Cs = 1/Ct
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• The pixel size of a display system that matches the resolving
power of the human eye depends on the observation distance.
• The visual spatial frequency limit and associated pixel size can
be defined as that for which Cs = 10% of maximum.
~0.5 c/mm
~2.5 c/mm
10% max
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Distance
frequency
pixel size
Close inspection
(0.33 m)
5 cycles/mm
0.100 mm/pixel
Normal viewing
(0.66 m)
2.5 cycles/mm
0.200 mm/pixel
Consultation view
(1.00 m)
1.7 cycles/mm
0.300 mm/pixel
Conference room
(3.00 m)
0.5 cycles/mm
1.000 mm/pixel
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Note: idTech 3MP panel, 0.207 mm/pixel
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C.5 Field of View
C.5.a Field of View
• 21 inch (diagonal) monitors with a field of 32 x 42
cm have provide an effective viewing field for
digital radiographs at a normal distance (2/3 m).
How large should the displayed area of a
monitor be?
This also depends on the viewing distance.
For the specified viewing distance the
diagonal dimension should be about 80%
of the viewing distance.
3. For this size, all regions of the image are
focused on regions of the retina with high
rod density.
1.
Eyeglass
lens
should be
optimized
for a
normal
viewing
distance
2.
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C.5.b Pixel array and Megapixels
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C.5.c Viewing distance and image zoom
• Use of image zoom features is ergonomically
better than leaning forward for close inspection.
• Split deck tables with a broad front deck usefully
prohibit close inspection with 3 MP monitors.
• The pixel size needed for visualizing full detail
and the field of view dictate the pixel array size
and the total number of pixels.
Distance
pixel size
32 x 42 cm
array size
MegaPixels
Close
(0.33 m)
0.100 mm
3200 x 4200
13.4
Normal
(0.66 m)
0.200 mm
1600 x 2100
3.4
• idtech 3 MP panel
.207 mm pixels / 20.8 inch (3.1 megapixels)
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C.5.d DR interpretation
C.5.e Regional zoom
4X
During interpretation, all regions of the recorded
radiograph should be viewed with a 1 -> 1 alignment of
image pixel values to display pixels.
1X
139 um DR
200 um CR
3 MP
5 MP
• Zoom levels of 1 -> 2 and 1-> 4
are needed to map detail at
the detector pixel level to
the region where visual
contrast sensitivity is
maximized.
• High zoom levels are of
particular importance for
direct DR detectors with
extended MTF performance.
3 MP
2 MP
1 MP
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C.5.f Minification
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C.6 256 gray levels
Is an 8 bit (256) grayscale sufficient ?
1.
2.
• Minification of 2 -> 1 can
also have value by increasing
the frequency of diffuse
structures and improving
their contrast sensitivity.
• Such minification is
commonly used for
mammography and chest
radiography.
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4.
5.
6.
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It better be.
The large majority of systems use standard
graphic calls with 256 level gray scales.
Specialized CRT graphic cards provide 10-12
bit gray levels.
However, for LCD systems the DVI interface
constrains the RGB graphic size.
No studies have indicated that 8 bit graphic
controll effects diagnostic performance.
However, artifacts can be illustrated using
test patterns with very gradual gray ramps.
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C.7 Viewing angle
Do the viewing angle characteristics of LCD monitors
cause problems ?
1.
2.
3.
4.
5.
Yes.
Manufacturers data on viewing angle can be
misleading.
Recents studies have suggested how viewing angle
can be specified for angles of acceptable contrast,
however this is not presently provided by medical
monitor suppliers.
Visual assessment using a contrast transfer test
pattern are very useful.
It is important to assess contrast and brightness
degradation in diagonal as well as hor/vert directions.
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?
Questions
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