Recent Developments in Medical Imaging
Characteristics and Performance
Evaluation of Digital Image Displays
Hans Roehrig
University of Arizona
The Totally Digital and Film-less Radiology Department is
rapidly replacing the conventional film-screen based radiology
department.
Digital imaging sensors and digital displays are used instead of
the traditional sensor and display, namely the film-screen
combination and the associated film-light box. CRTs and LCDs
are the most common digital displays
In digital systems the functions of detection and
display are independent.
As a result, the detector and the
display can be optimized
separately by image processing.
Medical Imaging is a visual science. It is the task of the radiologist to
diagnose and quantify disease processes in patients with the aid of mainly
anatomical images that are created by a variety of physical and physiological
processes. Some of these processes are:
•In Projection Radiography: The absorption of x-rays according to the
attenuation coefficients µ/ρ and thickness t with different energy in the diseased
organs, tissues and bones of the patients.
I = I0 × e− ( µ / ρ )× ρ × t
Radiologist “reading” radiographs. Notice the environment, the
ambient light is moderate in order to enhance the perception
1
Computed Tomography: Imaging of slices. Acquisition using
projections and reconstruction by (usually filtered) back-projection.
•In Diagnostic Ultrasound: The reflection and transmission of the
mechanical energy of inaudible sound waves according to differences
of the acoustic impedance at both sides of tissue interfaces, using
pulse-echo techniques.
Snell’s law of
Refraction
•In Magnetic Resonance Imaging: The magnetic resonance of mainly
Hydrogen nuclei (Protons), generated using radio-frequency pulses at
different pulse repetition times TR and different pulse echo times TE.
T1 Weighting
T2 Weighting
The correct diagnosis depends critically
on several physical and psycho-physical
factors, all of which are related to the
quality of the displayed image and the
observer’s perception capabilities:
M or Mz
represents
longitudinal
magnetization
Mxy represents
transverse
magnetization
2
In particular:
•The accuracy and precision with which the clinical images represent
the patient’s anatomy and pathology.
•The specifics of digital image processing and image manipulation, i.e.,
contrast enhancement and spatial resolution compensation.
•The accuracy and precision with which the human observer, the
radiologist, can perceive the patient’s pathology.
•A particular influence on the perception by the radiologist has the
widely applied DICOM “Greyscale Standard Display Function”
(GSDF). This display function perceptually linearizes the display.
•Perceptual linearization means equal numbers of
digital steps in the image at the display input lead
to equal number of perceptual luminance steps at
the display output
It is clear that amongst all components of the imaging chain, the
display has a prominent importance: It is the component, where the
bits meet the brain. That’s where the psycho-physical characteristics
of the human observer come in. The display certainly must meet the
requirements for spatial resolution, display contrast and SNR (see
Rose-Model below).
Digital X-Detector (DR)
Quantum Efficiency ηx
Integration Time tint,detector
Image on the
display
Image in the
radiologist’s
brain
X-ray photon
fluence rate Φx
Background area ab
Object area ao
Display signal-to-noise in
image detector, on the display and hopefully in the brain:
(S/N)Display = (Φ
Φ x ao tint, detector ηx)1/2 (C/(2-C)1/2)
Display of large amounts of data on film,
CT slices of one patient study on the
traditional alternator.
Above: Characteristic curves (H&D) of
film and film-screen combinations : DLog E response
Below: MTFs of film and film-screen
combinations; superb resolution
The problem of film: Film serves as
detector and as display. Image
processing is very hard to do with
data displayed on film. One needs a
film digitizer at least.
3
Schematic of CR, Computed
Radiography, the first
practical digital imaging
techniques for Chest
Radiography
Electronic Display Devices
CRT
LCD
In digital systems the functions
of detection and display are
independent.
As a result, the detector and the
display can be optimized
separately by image processing.
In
The most common
displays for filmless radiology
Single LCD Pixel: Plane of transmitted light
CRT
Display of chest images
on two CRTs (above
right) and on two
LCDs (below right).
Both CRTs and both
LCDs are standing side
by side, and each
one having the
capability to display
images with a pixel
matrix of 2048 x 2560
pixels.
LCD
Plane of back light
4
Psychophysical Evaluation of Displays
Human observers are the detectors of objects and backgrounds.
•
Experimental Setup: Human
observer views objects
displayed on a background
and determines presence or
absence of object. The
human observer serves as
the detector.
Clinical Object
Simulated Object
Signal = ∆L = LB – LO
SNR =
Noise = σ LB , LO ,Tot = (1 / 2) × σ
Background
LB , B
Background
LB , B
Object
LO, O
Object
LO, O
Schematic illustrating the simple Rose Model of Vision. It
relates the Probability of Detection to signal-to-noise ratio
and contrast threshold. Presence of spatial noise on the
display may reduce the signal-to-noise ratio and consequently
lead to the wrong diagnosis.
The Rose Model
OBJECT
AREA
Detection of simulated objects on simulated backgrounds, all of
which can be described analytically: Psychometric curves,
Threshold Contrasts, Detection Probability (PD), Just Noticeable
Differences (JND), Rose-Model, signal detection theory.
BACKGROUND
AREA
2
LB
+σ
2
LO
∆L
σ O ,B , L
Contrast = ∆L/Lmax
Threshold − Contrast = CT =
Background
LB
Object
LO
∆L JND
=
LB
LB
Detection of a
uniform aperiodic
object on a uniform
aperiodic background
Psychometric
Curve
(2 – C)
k2
ao = ---------- * ---------ΦO η tint
C2
aO = Area of object to be detected
ΦO = Photon fluence rate incident on object
Observer η = Efficiency of photon utilization
C
tint
k
RANGE OF VIEWING ANGLES:
0.1 Degrees to 0.53 Degrees
Observer
(detector or human observer)
= Contrast of object relative to background, defined as (Φ O - ΦB)/ΦO
= Integration time or exposure time
= Signal-to-noise ratio
SNR =
∆L
σ O,B , L
JND
5
There are different detection strategies and configurations with
different objects, all of which result in different threshold values
and different JNDs
Uniform
aperiodic
object
(square or
disc) on
uniform
background
Uniform
periodic object
(sine-wave or
square-wave
patterns) on
uniform
background
Non-uniform
aperiodic
object
(Gaussian
intensity
distribution)
on uniform
background
Series of
uniform
aperiodic
objects (disks
of different
diameter and
different
intensity) on
uniform
background
DICOM Calibration, Barten Model
∆L JND
Threshold − Contrast = CT =
=
LB
LB
Perceptual Linearization: Equal steps in driving levels DDL
result in equal steps in perception or JNDs
Threshold Contrasts and JNDs, the Basis for the DICOM Standard
Sinewave
Pattern
Frequency:
4 lp/deg
Background, Luminance Lb
Object, Luminance Lo > Lb
Typical Viewing Conditions:
Viewing Distance dv = 0.5 m
Viewing Angle αv = 2 deg
Contrast C = (|Lb - Lo|) / Lb
At Threshold (50 % Probability
of Detection): Lb - Lo = JND
Lb, n+1 = Lb, n + JNDn
Standard Display Function:
Mathematical Interpolation of 1023 Luminance Levels from L = 0.050 (cd/m2)
L = 4000 (cd/m2)
Log10 L I =
a + c • ( Ln I) + e • ( Ln I )2 + g • ( Ln I )3 + j • ( Ln I )4
2
3
4
5
1 + b • ( Ln I) + d • ( Ln I ) + f • ( Ln I ) + h • ( Ln I ) + k • ( Ln I )
I = index (1 to 1023) of the Luminance Levels LI of the JNDs
a = -1.30119
b = -0.025840
C = 0.080243
d = - 0.10320
f = 0.028746
g = - 0.025468
h = - 0.0031979
j = 0.0013635
JND
1
2
3
4
.
.
20
21
22
to
L (cd/m2)
0.0500
0.0546
0.0594
0.0644
JNDI (cd/m2)
0.0046
0.0048
0.0050
0.0052
0.1750
0.1839
0.1930
0.0089
0.0091
0.0094
JND
993
994
995
996
.
.
1021
1022
1023
e = 0.13647
k = 0.00012993
L (cd/m2)
3283.6680
3304.9660
3326.4010
3347.9740
JNDI (cd/m2)
21.42
21.55
21.7
21.83
3934.7960
3960.2810
3985.9310
25.4850
25.6500
25.8150
In General: JNDn+1 > JNDn
6
One Single Display Function is better as long
as it can be repeated and serve as a standard
Additional advantage: Perceptual
Linearization: The display is matched to the
contrast sensitivity of the human eye!
LUMINANCE [CD/M**2]
1000.0
Equal steps in driving levels DDL result in equal
steps in Perception or JNDs
100.0
1000
10.0
600
Display Function
Before Calibration
Luminance(Cd/m2)
0.1
0
256
512
JND INDEX
768
1024
JND Distance
100
1.0
400
10
Display Function
After Calibration
200
1
DICOM Standard Display Function. The function is defined for the
luminance range from 0.05 to 4000 cd/m2 . The just-noticeable
difference applies to 2-degree targets with sinusoidal modulation of
4 c/deg calculated with the Barten model of human contrast
sensitivity
0
590 JNDs
Lmax,2
Lmax,1
80:1
200
100.0
200:1
Lmin,2
Monitor 1
1000.0
10.0
Lmin,1
610 JNDs
1.0
Right: Multitude of display
functions of CRT for different
contrast and brightness setting.
Select one as standard.
0.1
0
256
512
JND INDEX
768
1024
Luminance [cd/m^2]
LUMINANCE [CD/M**2]
100
Digital Driving Levels(DDLs)
Left: Characteristic curve of
Film-Screen Combination:
Only one fixed curve; no contrast
change possible
1000.0
Monitor 2
0
0.1
Gamma
1.97
100.0
Brightness
189
100
Gamma
1.93
10.0
Contrast
255
200
Gamma
2.77
Gamma
2.97
1.0
0.1
1
10
100
Command Level [ADU]
1000
7
What can go wrong in the Reading Room?
What can go wrong with a display ?
The luminance of the digital displays can change, the spatial
resolution can change, the display can be set-up
incorrectly…. a myriad of problems. This needs to be
evaluated in the Reading Room.
There is a need for Image Quality
Control in the Reading Room
The image quality of the digital displays also needs to be
tested and certified when the displays are acquired from the
display companies. This testing is called “Acceptance
Testing”.
Acceptance Testing is
done in the Laboratory.
Original Image
Good MTF
1.00
0.80
MTF
0.60
0.40
0.20
0.00
0.00
0.50
1.00
1.50
2.00
Spatial Frequency (lp/mm)
8
Good Display
Function
Bad MTF
1.00
Luminance
0.80
MTF
0.60
0.40
0.20
Command Level
0.00
0.00
0.50
1.00
1.50
2.00
Spatial Frequency (lp/mm)
Bad Display
Function
Noise free
Luminance
Command Level
9
Construction and Operation of CRTs and LCDs
Noise present
Schematic of a Cathode Ray Tube Display, an analog Display
Cathode
Modulator
(G1)
D
First Anode
(G2)
D
f
b
B
t
Notice the “Triode-like” control grid G1 and the
brightness and contrast controls
Triode configuration in CRT electron gun
Serial input of information requires very high bandwidth
10
1000.0
ε
Generalized current vs. voltage equation for triode
Ic = Cathode Current
K = a constant of proportionality,
Vd = Drive voltage on the modulator (G1)
Vc = the cutoff voltage and
ψ, ε are exponents that must satisfy the equation ψ+ε=3/2.
ψ is commonly called gamma γ
Typical values for γ are between 1.5 and 3.5
Gamma
1.97
100.0
Brightness
189
100
Gamma
1.93
Contrast
255
200
10.0
Gamma
2.77
Characteristic curves of a CRT monitor
for contrast settings 200 through 255 at
brightness settings100 (dashed curves)
and 189 (solid curves). Addressable
matrix: 2048 x 2560 pixels
Gamma
2.97
1.0
1000
0.1
1
10
100
Command Level [ADU]
Characteristic curves of a CRT monitor for
brightness settings 200 through 255 at
contrast settings 150 (dashed curves) and
222 (solid curves). Addressable matrix:
2048 x 2560 pixel
1000
Luminance [cd/m^2]
I c = KVd Vc
Luminance [cd/m^2]
ψ
Gamma
1.66
Gamma
1.94
100
Brightness
255
200
10
Contrast
150
Gamma
1.88
Gamma
2.32
1
1
Video Signal and Scanning
Contrast
222
10
100
Command Level (ADU)
1000
Ideal line raster with no overlap and no residual raster
modulation, and real CRT line raster with overlap of lines
and finite raster modulation.
11
Rise- and Falltime of the Amplifier and the Effect on Bandwidth
Pixel on
White Level
Black Level
Pixel off
Pixel on
90 %
10 %
tpix
trise
fall
tpix >> tfall + trise
Assuming tfall = trise
Bandwidth f = 1/ (4 trise )
For 2048 x 2560 pixel matrix, 71 Hz refresh rate, blanking time 26 % of frame time:
Ideally:
Typically:
tpix = 2 E-9 sec
trise = 1.0 E-10 sec: Df = 2.5 GHz
trise = 6.25 E-10 sec: Df = 400 MHz
Resolution-Addressability Ratio: RAR
Scanned Raster,
(horizontal and
vertical) defining
nominal pixel size
ah,nx av,n = Anominal
Beam Spot Shape,
defining actual pixel
size aFWHM in terms
of FWHM
FWHM
50%
Amplitude
CRT-Vertical Raster
LCD –Pixel Structure
The CRT is to a certain degree an analog-discrete system,
analog and continuous in the horizontal direction,
discrete in the vertical direction, while the LCD is
discrete in both directions, a totally digital system
RAR =
((1 / 2) × FWHM ) 2 × π
AFWHM
=
Ano min al
ah , n × av , n
12
AMPLITUDE [RELATIVE UNITS]
100
SPOT FIXED
80
60
SPOT BROADENED
DUE TO MOTION
40
SPOT BROADENED
DUE TO LIMITED
ELECTRONICS
BANDWIDTH
20
0
2
4
6
8
TIME [NSEC]
10
12
Schematic illustrating the width of a single pixel as given by a Gaussian
spot, which moves during the “video-on time” for a single pixel (approx
2 nsec) and which is convolved with the time response function of the
electronics (rise time and fall time about 1.4 nsec each.
Attenuation of ambient light by adding a neutral-density filter
(“panel”) to the faceplate and by making the faceplate itself
a neutral-density glass. 33 % of the phosphor light is
transmitted, but only 9.3 % of the incident ambient light
is returned
13
Spatial Noise of CRTs: Phosphor Granularity
CCD Camera images of two types of CRT phosphor
layers taken with high optical magnification
P45
Binned 8 x 8 CCD Pixels
P104
P45
P104
P 45
P 104
In this case the CRT line structure was removed by defocussing
CRT electron optics, which is why the SNR is high.
Schematic of LCD Display
Single LCD Pixel: Plane of transmitted light
Plane of back light
Single pixel
consisting of
3 sub-pixels
14
15
Intrinsic display function of LCD and the use of reference
Potentials to achieve a gamma of about 2.2
Display Functions of an LCD (LCD2) and their differential
before and after calibration at a precision of 12.58 bits.
1000
20
1.20
Schematic Illustrating Cause of Non-Monotonic
Display Functions of LCD Display.
Before Calibration
16
Intrinsic display
function of liquid
crystal with steep
slope, not useful
for display of
gray scale
0.40
Intrinsic display function of liquid
crystal, made useful by applying
8 reference voltages to achieve
gradually changing grayscale.
Frequently the effect of the
reference voltages is a nonsmooth display function with
edges and kinks.
L/ DDL (Cd/m2)
0.80
Luminance(Cd/m2)
Relative Transmission of LCD
100
12
After Calibration
10
8
Derivative before
calibration
1
Discontinuity due to one of
the 8 reference potentials
4
Derivative after
calibration
0.00
0.1
0.00
0.04
0.08
1000
600
Display Function
Before Calibration
JND Distance
100
400
Display Function
After Calibration
200
Digital Driving Levels(DDLs)
300
590 JNDs
Lmax,2
1000.0
LUMINANCE [CD/M**2]
Equal steps in driving levels DDL result in equal
steps in Perception or JNDs
Luminance(Cd/m2)
100
0
Additional advantage: Perceptual
Linearization: The display is matched to the
contrast sensitivity of the human eye!
10
0
0.12
Relative Driving Level
Monitor 2
Lmax,1
80:1
100.0
200:1
Lmin,2
Monitor 1
10.0
Lmin,1
610 JNDs
1.0
200
1
0.1
0
0.1
0
100
200
Digital Driving Levels(DDLs)
0
256
512
JND INDEX
768
1024
16
Example of Spatial Noise for LCDs:
Temporal Noise: LCDs exhibit also temporal noise
Spatial Noise are the Local and Stationary Luminance Variations from
Pixel to Pixel and from Sub-Pixel to Sub-Pixel
In general, subtraction of two CCD images generated shortly
one after another in time and at the same exposure time results
in the disappearance of the spatial features. The RMS in the
subtracted images is substantially smaller than that in the
original images.
In addition, subtraction of two subtracted images leads to an
increase in the RMS by a factor of square-root of about 2,
indicating that the noise remaining in the subtracted images is
practically temporal noise which increases when the respective
images are subtracted.
Temporal noise represents a small fraction of an
LCD’s total noise (see next slide).
CRT-Vertical Raster
LCD –Pixel Structure
The CRT is to a certain degree an analog-discrete system,
analog and continuous in the horizontal direction,
discrete in the vertical direction, while the LCD is
discrete in both directions, a totally digital system
17
(a)
(a)
(b)
(b)
Figure 13 Display contrast expressed as
dL/L as a function of the viewing
direction for a medical imaging CRT (a)
and for an AMLCD [(b) is in the
horizontal direction and © is in the
diagonal direction]
Figure 4.12 Display luminance
curves as a function of the
viewing direction for a medical
imaging CRT (a), and for an
AMLCD [(b) is in the horizontal
direction, and (c) is in the
diagonal direction]
(c)
(c)
Physical Evaluation of Displays
Use of CCD Camera for evaluation of an LCD
18
Quantitative evaluation of Veiling Glare
Comparision of Veiling Glare in CRTs and LCDs
0.0500
Measurement of Veiling Glare
0.0400
(Image of a Circle)
White
Surround
Veiling Glare
CRT
Black
Center
0.0300
0.0200
Disk Diameter D
VG =
0.0100
L(Disk, Center) - L(Monitor, Dark)
L(White Surround)
LCD
0.0000
0
50
100
150
200
250
300
350
400
Diameter of the disk (mm)
19
Uniformity of Totoku LCD 1 at 38.78 cd/m2 (View 1)
Maximum Luminance Deviation: 27.79%
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Use of CCD Camera for evaluation of an LCD
Measurement of
Luminance at 40 positions
throughout the display
Optical magnification:
Select 8 x 8 CCD pixels to represent 1 CRT pixel
in order to achieve “Oversampling”
CRT, 1 Pixel,
width wCRT
CCD, 8 Pixels,
total width (8 * wCCD)
Squarewave Pattern for MTF
Single Line for MTF
Examples of actual CCD Images
Optical Magnification:
mo = (8 * wCCD) /wCRT
Result of “Oversampling”:
Oversampling will permit to consider the actually discrete LCD
and CRT (at least in the vertical direction) a continuous system,
so we can do FFTs as in continuous systems.
“Pixel Emitted Intensity”
Horizontal pixel strings of
different length: 1 pixel, 2 pixels
3 pixels, 4 pixels …. 10 pixel
20
Evaluation of Spatial Resolution:
The Modulation Transfer Function (MTF)
Spatial Resolution:
MTF: Involve 1-D or 2-D Fourier Transform of
image with stimulus
The MTF is defined as the Fourier Transform of the imaging system’s
Point Spread Function (PSF) or Impulse Response, normalized to unity
at zero spatial frequency:
MTF = |P(f)| / P(0)
Requirement:
The imaging system must be a linear system
Noise:
Noise Power Spectrum (NPS): Involve 2_D
Fourier Transform of uniform image
Unfortunately, a CRT is not a linear system:
The luminance as a function of the command level ADU is given by
L = a (ADU)
with between 1.5 and 3.5
However, with a small signal ∆ADU as input superimposed on a mean
value ADUmean,, a non-linear system can be considered a linear system:
ADU = ADUmean + ∆ ADU
So we display a small signal ∆ ADU on a uniform background ADUmean,
The general input-output relation for a linear shift invariant system may be
written as a convolution or folding integral:
wout(u) = p(u - u') win(u') du'
The general shorthand notation is wout(u) = p(u) * win(u)
If the delta function is the input (“impulse”), win(u) = δ(u),
the output is then woutδ(u) = p(u-u')δ(u')du' = p(u) = impulse response
Fourier transform:
1{w in(u)*p(u)}
Examples of input stimuli for the measurement of MTF:
Squarewaves (superpositions of sinewaves), lines (rectangles) and white noise
Vertical
SquareWave
ADUmean
ADUmean + ∆ADU
= Win(f) P(f)
In Fourier Space: convolution turns in to multiplication:
Wout(f) = Win(f) P(f)
Vertical
Line
ADUmean
ADUmean + ∆ADU
Also: p(u), which was considered the system Point Spread Function,
turns in Fourier Space into
MTF = |P(f)| / P(0)
If the input cannot be a perfect “impulse”, i.e. a delta
function, corrections
for the actual input have to be made
P(f) = Wout(f) / Win(f)
White
Noise
ADUmean
Standard
Deviation
∆(ADU)
21
1.20
Comparison of the MTFs of an aperture modulated
monochrome LCD (Nyquist frequency: 1.46 lp/mm)
and a 5M-pixel CRT with a P104 phosphor (Nyquist
frequency: 3.47 lp/mm)
Horizontal MTF obtained from the Line Spread Function
(Totoku LCD, 8 bits mode, Nyq. Freq: 2.42 lp/mm)
1.2
32.75 cd/m2
0.8
181.38 cd/m2
LCD: Horizontal MTF
0.80
6.21 cd/m2
MTF
LCD: Vertical MTF
Pixel Size
0.206 mm
0.4
Vertical MTF obtained from the Line Spread Function
(SIEMENS 5M LCD, Nyq. Freq: 3.03 lp/mm)
1.2
MTF
43.82 cd/m2
CRT: Vertical MTF
0
0.8
0
0.40
0.5
1
1.5
2
2.5
Spatial Freq (lp/mm)
Pixel Size
0.165 mm
10.26 cd/m2
MTF
CRT: Horizontal MTF
203.02 cd/m2
0.4
0.00
0.00
0.40
0.80
1.20
0
Relative Spatial Frequency (Nyquist)
0
1
2
3
4
Spatial Frequency (lp/mm)
10
CCD-Camera Image of Uniform LCD display.
Notice the LCD structure, leading to the spikes
in the NPS
NPS of a P104 CRT
1
The pixel dimension of the CRT is 2048 * 2560.
It is measured from a 127 ADU uniform image.
The mean value of the CCD image has been
normalized to 1. The Mag. is 8.
0.1
NPS (m m 2 )
0.01
2D NPS of Totoku LCD at 32.75 cd/m2 (8 bits mode)
0.001
0.0001
Y direction
Spatial Noise of CRTs: Phosphor Granularity
X direction
1E-005
CCD Camera images of two types of CRT phosphor
1E-006
layers taken with high optical magnification
Nyquist
Frequency (N f):
1E-007
3.4722 lp/mm
4*Nf : 13.8889 lp/mm
1E-008
0
10
20
Frequency (lp/mm)
30
Spatial Noise for CRT
2-D Noise Power Spectrum
P 45
P 104
22
Pixel Structure of LCDs used for Frame-Rate Modulation
or for Sub-Pixel Modulation or for both. Two Pixels are
shown. Each Pixel has 3 Sub-Pixels
Pixel 1 with
3 sub-pixels
Pixel 2 with
3 sub-pixels
Comparison of Horizontal and Vertical NPS
( DDL 55)
1
0.1
0.01
10
NPS of a LCD
The pixel dimension is 2048 * 1536.
It is measured from a 137 ADU uniform image.The mean value
of the CCD image has been normalized to 1. The Mag. is 15.
1
0.001
0.1
X direction
LCD spatial
noise alone
0.01
NPS (mm 2 )
LCD Structure
Noise and LCD
spatial noise
0.0001
Y direction
1E-005
0.001
0.0001
1E-006
1E-005
Nyquist
Frequency (Nf):
1E-006
1E-007
2.4369 lp/mm
0
1E-007
0
10
The pixel dimension of the CRT is 2048 * 2560.
It is measured from a 127 ADU uniform image.
The mean value of the CCD image has been
normalized to 1. The Mag. is 8.
0.1
10
Y direction
NPS of a LCD
0.1
1E-006
X direction
Nyquist
Frequency (Nf):
3.4722 lp/mm
0.01
4*Nf : 13.8889 lp/mm
1E-008
0
10
20
Frequency (lp/mm)
NPS (mm2)
1E-007
20
o
o
Display a uniform field on display
Take an image with the CCD
camera
o
Perform 2-D Fourier Transform (FT)
o
Integrate NPS and reduce to 1 display
pixel (“down-sampling”)
The pixel dimension is 2048 * 1536.
It is measured from a 137 ADU uniform image.The mean value
of the CCD image has been normalized to 1. The Mag. is 15.
1
X direction
1E-005
16
Procedure for finding the SNR per display pixel of a display
from the response to a uniform input to the display under test:
0.001
0.0001
12
40
LCD Noise seemes to be
similar or even larger than
CRT noise
0.01
NPS (m m 2 )
20
30
Frequency (lp/mm)
Comparison of LCD noise
with CRT Noise:
NPS of a P104 CRT
1
8
Structure permitting Aperture Modulation with almost 1800 levels
1E-008
10
4
4*Nf : 9.7276 lp/mm
Y direction
0.001
0.0001
30
1E-005
Nyquist
Frequency (Nf):
1E-006
2.4369 lp/mm
1E-007
Example of a uniform display field
4*Nf : 9.7276 lp/mm
1E-008
0
10
20
30
Frequency (lp/mm)
40
23
Illustration of derivation of the 5 x 5 pixel image of a 5 x
5 pixel section of an LCD from a 15 x 15 CCD pixel
CCD image. “Convolution and Down-Sampling”.
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.
1.00E+0
Normalized Spatial Noise Power Spectra
for Sub-Pixel Modulated LCD
1.00E-1
NPS in Horizontal Direction
NPS in Vertical
Direction
1.00E-2
NPS (mm^2)
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Integration of the Noise-Power Spectrum to find the
Variance σ2
1.00E-3
1.00E-4
CCD image of 5 x 5
pixel section of an LCD,
taken at a 3:1 pixel-ratio
(3 x 3 CCD pixels per
LCD pixel). The CCD
image is a 15 x 15 array
of CCD pixels
Ideal 5 x 5 pixel image of
a 5 x 5 pixel section of an
LCD. For simplicity we
assume an LCD pixel is
a square
1.00E-5
LCD-Nyquist
Frequency
2.42 lp/mm
1.00E-6
0.00
4.00
8.00
12.00
16.00
20.00
Frequency (lp/mm)
σ2
LCD, pix
=
NPSonedf ≈
f ≠0
NPSupsamp*sin c2df
f ≠0
Plotting the Resulting 9 SNRs
What tests should be done in the
Reading Room and in the Laboratory?
Ask the AAPM Task Group 18!
24
American Association of Physicists in
Medicine (AAPM) Task Group 18
Assessment of Display Performance for
Medical Imaging Systems
Types of TG 18 tests:
• There are visual tests, often called
qualitative or basic tests, where a human observer
views the display and makes a decision on the
presence or absence of a test object.
AAPM On-Line Report No. 03, 2005
Can be downloaded from website: www.duke.edu/~samei
• There are quantitative or advanced tests, where an
instrument like a photometer or a CCD camera is
used to make a
measurement and provide
quantitative data.
25
and therefore
26
27
28
29
We developed a CCD Camera-lens combination.
It can be used hand-held in the Reading Room
It can be used for both types of TG18 tests.
The Optical Layout for the
It can be used for both CRTs and LCDs.
04
Case of an LCD
Focus Adjustment
Image Quality Control of Display Systems in the
PACS Environment.
Hans Roehrig1, Jerry Gaskill2,
Jiahua Fan1
1University
2Image
Total camera length < 110mm
Working F/#: 9.46
The Optical Layout for the
Case of a CRT
of Arizona, Tucson AZ
Smith’s Inc., Germantown, MD
This work was supported by a grant from NIH and by the EIZO
NANAO Corporation.
CCD
Camera
Focus Adjustment
Display Glass
Thickness
CCD
Camera
Pixel matrix:
640 x 480
8 Bits
Contrast
Total camera length < 90mm
Working F/#: 9.46
30
The Software: VeriLUM from Image-Smiths
VeriLUM
provides
• Tracking
• Calibration
• MTF measurement (click
MTF button)
• SNR
measurement
(click SNR
button)
• Results
Verifying Focusing of the CCD Camera
Note the status bar at
the bottom of the
image. It displays the
gain, max pixel value
and the standard
deviation of the image
that help in focusing
the camera.
The Gain is
automatically adjusted
so that the image is
not saturated.
Check of Focusing and Alignment of the Camera
and Determining the Magnification Ratio MR
VeriLUM displays this
test pattern to check for
alignment, to verify focusing
of the camera and to determine the pixel ratio MR,
(the ratio of the number of CCD
pixels per display pixel)
while the camera is held firmly
against the display or held on a
firm stand on a table.
The MTF of the display under test
is found from the line response
Here we input all the
parameters required for
the calculation of an
MTF:
• Horizontal or Vertical
Line
• Line and Background
Values
Gain Max Pixel Value Focus Index
31
The input stimulus for the MTF from the
line response: A line displayed on the display,
and the line image taken with the CCD
The Profile of the Line
Image: The LSF
60
Pixel Intensity
40
20
0
Single line test pattern
the stimulus or input to
the display system
CCD camera image of the
single line from the display
which is the output of the
display system i.e. lineresponse of interest
Plotting the Resulting MTF
-20
0
100
200
300
400
500
Pixel Position
This line profile from the subtracted image is subjected to
a Fourier Transform to get the MTF.
Procedure for finding the SNR per display pixel of a display
from the response to several (perhaps even 9) uniform inputs
to the display under test:
o
o
Display a uniform field on display
Take an image with the CCD
camera
o
Perform 2-D Fourier Transform (FT)
o
Integrate NPS and reduce to 1 display
pixel
Example of one of
several perhaps even 9
uniform fields
32
Illustration of derivation of the 5 x 5 pixel image of a 5 x
5 pixel section of an LCD from a 15 x 15 CCD pixel
CCD image. “Convolution and Down-Sampling”.
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Plotting the SNRs for the case of 9
different luminance values
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.
CCD image of 5 x 5
pixel section of an LCD,
taken at a 3:1 pixel-ratio
(3 x 3 CCD pixels per
LCD pixel). The CCD
image is a 15 x 15 array
of CCD pixels
Ideal 5 x 5 pixel image of
a 5 x 5 pixel section of an
LCD. For simplicity we
assume an LCD pixel is
a square
33