Technology for
X-Ray Imaging
Volker Rasche
0731 – 500 45014
Volker.Rasche@uniklinik-ulm.de
Experimental Cardio
Experimental Vascular Imaging
Fundamental X-Ray Experiment
X-Ray Source
XR Generation
XR Interaction
with matter
Detector
XR Detection
XR Image Processing
Experimental Cardio
Experimental Vascular Imaging
Generation of X-Rays
EM-waves ~ 10^-10 m
(Order of Angström)
Experimental Cardio
Experimental Vascular Imaging
X-Ray Tube
U
I
- - Kathode
Anode
Experimental Cardio
Experimental Vascular Imaging
Maximal Photonenenergy
E
c
hv and E
ch
eU
min
U [kV]
eU
min
•h
•e
•U
• min
h
e
Plank‘s constant
Charge of an Electron
tube voltage (anode)
minimal wavelenght
=
=
6.626 10 -34 Js
1.60217733 e-19 C
[nm] Application area
10
0,1242
„medium“, X-Ray diagnosis
(Mammography)
100
0,012
„hard“, X-Ray diagnosis
Experimental Cardio
Experimental Vascular Imaging
Bremsstrahlung (BS)
– Fast deceleration of the electrons by the positive field of the nuclei
(Protons)
– Continuous spectrum (limited by the maximal electron energy)
Experimental Cardio
Experimental Vascular Imaging
Spectral Intensity of BS
• Assumptions
– Maximal energy of the
electrons given by vmax
– Any possible loss of
energy has equal
probability
– Interaction processes are
statistical independent
Experimental Cardio
Experimental Vascular Imaging
Spatial Intensity Distribution of BS
• Multiple interaction during traveling thru matter
– Change of direction with each interaction
– Almost isotropic radiation from focal spot
Focus
Experimental Cardio
Experimental Vascular Imaging
Characteristic Radiation (CS)
Photoeffect
•
•
•
Release of electron out of an inner shell of the atom
Filling of the free space of inner shell by electron from higher shell under radiation
of an em-wave with frequency defined by the energy difference between the two
shells involved
Discrete spectrum (defined by the energy difference between the two shells)
Experimental Cardio
Experimental Vascular Imaging
K & L Lines (CS)
The K Lines
2) An electron from the L or M shell "jumps in" to
fill the vacancy. In the process, it emits a
characteristic x-ray unique to this element and in
turn, produces a vacancy in the L or M shell.
The L Lines
3) When a vacancy is created in the L shell
by either the primary excitation x-ray or by
the previous event, an electron from the M
or N shell "jumps in" to occupy the vacancy.
In this process, it emits a characteristic xray unique to this element and in turn,
produces a vacancy in the M or N shell.
Experimental Cardio
Experimental Vascular Imaging
CS – Energy Level
Min energy 70KeV
Experimental Cardio
Experimental Vascular Imaging
X-Ray Spectrum in Focal Spot
Experimental Cardio
Experimental Vascular Imaging
X-Ray Spectrum in Tube
Bremsstrahlung
+
Characteristic Radiation
Filter to reduce patient dose
Low energy X-ray phantoms will
more likely be absorbed in the body
99 % of energy absorbed within the anode/tube
Heating up of the anode is the
Ex
Ca
major issue
Ex
V
I
perimental
perimental
rdio
ascular maging
X-Ray Tube - Efficiency
• Defined as quotient of resulting X-Ray power and
electrical input power
vmax
J ges
IU
J v dv
0
IU
• Experiments yields
k U Z , k 10
9
Wolfram Z
74,U
100kV ,
0.7%
• I = 1A, kV = 100kV -> 99,3kW in heat, 0.7 kW in XRays
Experimental Cardio
Experimental Vascular Imaging
Interaction with Matter
Experimental Cardio
Experimental Vascular Imaging
Interaction of X-Rays with Matter
• Each of this interactions
occurs with a certain
likelihood
Photoeffect
R
classical scatter
C
Compton scatter
x
pair generation
k
nuclear reaction
Minimally required energy
not reached in medical
applications
Ex
perimental Cardio
Experimental Vascular Imaging
Interaction of X-Rays with Matter
Photoeffect
•
•
•
Release of electron out of an inner shell of the atom
Filling of the free space of inner shell by electron from higher shell under radiation
of an em-wave with frequency defined by the energy difference between the two
shells involved
Discrete spectrum (defined by the energy difference between the two shells)
Experimental Cardio
Experimental Vascular Imaging
Interaction of X-Rays with Matter
2
+
Classical
scatter
1
=
2
1
5% of interactions
in diagnostic range
(25-150keV)
Interaction with strongly bound electrons
Experimental Cardio
Experimental Vascular Imaging
Interaction of X-Rays with Matter
Compton
electron
Compton
scatter
+
1
1
<
2
2
Interaction with loosely bound electron
Experimental Cardio
Experimental Vascular Imaging
Interaction of X-Rays with Matter
Energy
transfer
Classical scatter
Compton scatter
Photoeffect
Experimental Cardio
Experimental Vascular Imaging
Interaction of X-Rays with Matter
• Interaction with matter attenuates the X-Ray beam
– Transition of X-ray beam thru dx yields attenuation of the Xray beam by dN
N
s
x dx
dN
Ndx
N (d )
N 0e
d
N ( s)
N 0e
0
N-dN
– N0
– d
Number of X-Ray photons
Thickness of the material
– Definition: mass attenuation coefficient
• Attenuation != absorption
– Scatter
– Absorption
M
Experimental Cardio
Experimental Vascular Imaging
Interaction of X-Rays with Matter
Experimental Cardio
Experimental Vascular Imaging
Interaction of X-Rays with Matter
Compton scatter
Main interactions in
diagnostic range (25150keV)
Energy range, relevant for medical diagnostics
Experimental Cardio
Experimental Vascular Imaging
The Patient as X-Ray Source
• Main attenuation component
at medical imaging energies
– Compton scatter
• Compton scatter
– Change of the direction of
the X-Ray photon
• Patients acts as source
Experimental Cardio
Experimental Vascular Imaging
Attenuation Summary I
3
dN
3
3
dN
Z3
dN
dN
edx
Experimental Cardio
Experimental Vascular Imaging
Attenuation Summary II
• Tissue specific
– Differences in the attenuation coefficient define the contrast in X-ray
images
• Includes scatter and absorption
– For measuring µ, scatter radiation must be suppressed
• Contribution of the different effects strongly dependent on energy
– Soft-tissue 10-100kV predominantly Compton
– Strong increase with decreasing energy
• Complete absorption of X-rays of low energy. No contribution to image
formation from low-energy X-rays. But contribution to the patient X-ray
dose.
– Strong decrease with increasing energy
• Little differences in the absorption coefficients for high X-energies yield
poor contrast
Experimental Cardio
Experimental Vascular Imaging
Attenuation Summary III
• Energies utilized in medical imaging
– Strongly depend on the application
•
•
•
•
Mammography
Extremities
Angiography
CT
<10-30kV
30-50kV
40-100kV
120-140kV
– Maximal energy limited by the reduced absorption
– Minimal energy limited by complete absorption
• Filtering of X-Ray beam to minimize low-energy contributions
• Aluminum or copper filters
Experimental Cardio
Experimental Vascular Imaging
Some Tube Issues
Experimental Cardio
Experimental Vascular Imaging
Point-Spread-Function Tube
• Point source is blurred
focal spot
• Blur depends on
S_ISO
– System geometry
• Amplification factor
point source
ISO _ I
S _ ISO
ISO_I
– Focal spot size d
x
d
• Focal spot size as small as
possible !!!
Projection of
The point source
SID = SISO + ISOI
Experimental Cardio
Experimental Vascular Imaging
Requirements X-Ray Tube
•
Requirements
– High power
• Short exposure times
• Good SNR
– Small focus
• Sharp images
– Variable X-ray energies
1600°C
• Contrast optimization
– Cost effective production
– Long lasting with minimal
maintenance
•
Main problems
– Heat, heat, heat
– 99% of the power results in heat
Experimental Cardio
Experimental Vascular Imaging
At low temperatures, the heat
transfer is dominating
100%
Strahlung
Anteil
Leitung
50%
0%
1000
800
hoch
600
400
Anodentemperatur [°C]
200
0
niedrig
Abkühlzeit
Experimental Cardio
Experimental Vascular Imaging
Schematics Modern X-Ray Tube
• Size of the focal spot and material of the focal spot define the
maximal applicable power.
• Increase of the size of the focal spot by
– Tilted anodes
• Increase of the physical size of the focal spot, but
• Dependency of the focal spot size of the “viewing” direction
– Rotation of the anode
• Physical focal spot results to a ring
Rotor with bearing
Rotating anode
Oil
Vacuum
Tube housing
Radiation
protection
shielding
+
-
Cathode
X-Rays
Electron beam
Experimental Cardio
Experimental Vascular Imaging
Focus Geometry
vertical
horizontal
Experimental Cardio
Experimental Vascular Imaging
Anode material
• High atomic numbers
– Efficiency (stopping power)
• High melting point
– Power
• Excellent heat transport
– Cooling
• Quality measures
– Static anode tube:
– Rotating anode tube:

heat transport

density
c
specific heat
Q = Z Tmax
Q = Z Tmax (
c
Experimental Cardio
Experimental Vascular Imaging
Anode material
Experimental Cardio
Experimental Vascular Imaging
Schematics Anode
WO-RH
M0
C
Melting point – Stopping Power
Melting point + heat transfer
Specific Heat
Experimental Cardio
Experimental Vascular Imaging
X-Ray Detection
Experimental Cardio
Experimental Vascular Imaging
The Basic Principle
X-Ray Photons
Light Photons
Ions / Electrons
Detection
Experimental Cardio
Experimental Vascular Imaging
The Basic Principle
X-Ray Photons
Detector Quantum Efficiency
Light Photons
Ions
(DQE)
System Noise (less than Quantum Noise)
Detection
Dynamic
Range
Experimental Cardio
Experimental Vascular Imaging
X-Ray Detection
• X-Ray detectors
–
–
–
–
–
–
X-Ray films
Intensifier screens
Image plate
Xeroradiography
Image intensifier
Flat panel detectors
static
dynamic
• CT detectors
– Gas chamber
– Semi-conducter
dynamic
Experimental Cardio
Experimental Vascular Imaging
X-Ray Film
• Standard black & white film
– Silver bromide and silver iodide (Ag+Br-, Ag+I-)
• Illumination produces Silver seeds
– Br- + hv -> Br + e– Ag+ + e -> Ag
• Development increases the aggregation of Ag at
seeds
• Fixation removes remaining AgBr, AgI
• Negative: light is absorbed at Ag seeds
– Exposed areas (small µ) appear dark
– Non-exposed areas (huge µ) appear bright
Experimental Cardio
Experimental Vascular Imaging
X-Ray Film
Logarithmic sensitivity of
the human eye
• Optical density defined according to
S
J L0
log
JL
JL transmitted light intensity
JL0 in-coming light intensity
• Absorption is logarithmic proportional to the in-coming
X-ray intensity
D0
ln
D
J R 0T
ln
J RT
J R0
ln
JR
d
JR transmitted X-ray intensity
JR0 in-coming X-ray intensity
Efficiency of X-Ray photon capture
Experimental Cardio
Experimental Vascular Imaging
X-Ray Film
• Optical density curve
Optical
density
X-Ray dose
• Image should be concentrated in the linear part of the curve
– Steepness of the curve defined as -value ( = atan( ))
• large : good contrast, small dynamic range
• small : poor contrast, large dynamic range
Experimental Cardio
Experimental Vascular Imaging
X-Ray Film
• Excellent spatial resolution
– 0.025mm
– Defined by the size of the silver seeds
• Poor quantum efficiency
– AgBr-layer < 0.1mm
– about 1% of X-Ray photons are absorbed
• Doubling of DQE by double-coated films
Experimental Cardio
Experimental Vascular Imaging
Intensifier Screens
• Conversion of XR – Photon into Light Photon(s)
– 100KeV photon can theoretically produce about 44.000 light photons
at 550nm = 2.26eV
• Reduced spatial resolution
– Distance event – film
– Crossover-Effect in case of double-coated screens
• Visible light opaque coating of backside of film
Experimental Cardio
Experimental Vascular Imaging
Dynamic X-Ray Detectors
X-Ray
Scintillator
Transform X-Ray
photons into light
photons
Scintillator
Means for dynamic
light detection
Light
Experimental Cardio
Experimental Vascular Imaging
X-Ray Image Intensifier
• Good sensitivity in the 50-60
KeV range
• Decreasing with increasing
photon energy
• Less suited below 40 KeV
CsI - Structure
Caesium-Jodid CsI
Experimental Cardio
Experimental Vascular Imaging
X-Ray Image Intensifier
• Scintillator converts X-ray photons in light photons
• Light photons are converted into electrons
(Photocathode)
• Electrons are accelerated (25-30kV) and focussed on
output screen (intensification)
• Output screen signal is recorded with either a film, or a
CCD camera
Experimental Cardio
Experimental Vascular Imaging
X-Ray Image Intensifier
• Materials
– Input screen
Al
• 90% transparent for X-Ray photons in the relevant energy
window
– Scintillator
Caesium-Jodid CsI
• High X-ray absorpotion
• High conversion rate into visible light photons
• Pole structure
– Thick layers (<0.5mm) without significant loss of spatial resolution
• DQE = 70%
– Photocathode
Antimon-Cäsium SbCs3
• Low exit energy
• Long life time
– Exit screen
ZnCdS:Ag (BW TV)
Experimental Cardio
Experimental Vascular Imaging
X-Ray Image Intensifier
• Electron optic
– Projection of each point of the photocathode onto a specific
point of the output screen
– Compensation for different exit angle of the electrons
– Static E-field, realized by various metal rings (coils) at
different potentials
Experimental Cardio
Experimental Vascular Imaging
X-Ray Image Intensifier
Distortion
• Geometry (bended input screen)
• External magnetic fields (earth
magnetic field)
• Depends on the orientation of the
system to earth magnetic fiels
• Can cause several mm of distortion
Relative brightness
Vignetting
Distance to center
Experimental Cardio
Experimental Vascular Imaging
X-Ray Image Intensifier
CCD Camera
Vacuum Tube
Input screen
Experimental Cardio
Experimental Vascular Imaging
Solid-State Flat Panel Detetcor
PD bias line
X-ray converter
gate line
ADC
readout
addressing
readout line
photodiode
switching TFT
Amorphous Silicon
Experimental Cardio
Experimental Vascular Imaging
Linearity of X-Ray Detection
Experimental Cardio
Experimental Vascular Imaging
Quality Measures
• Noise
– Electronic noise (components)
– Quantum noise (Physic)
• Detector Quantum Efficiency (DQE)
– Measure for X-Ray photons used for image formation
• Modulation Transfer Function (MTF)
– System Transfer Function
– Transfer properties of the detector
• Scatter
• Image Contrast
Experimental Cardio
Experimental Vascular Imaging
Noise
• Noise in X-Ray is Poisson distributed
– is a discrete probability distribution that expresses the probability of a
number of events occurring in a fixed period of time
– events occur with a known average rate and independently of the
time since the last event.
– Distribution
– Deviation
px
µxe x
x!
x
– The measurement of n X-Ray photons results in an image noise of
n
Experimental Cardio
Experimental Vascular Imaging
Detector Quantum Effeciency DQE
• Quantum noise is a physical property
– Not suited for characterization of an imaging system
• Important question
– How much noise does a system add …
DQE
2
SNRoutput
SNRX
with
2
input
SNR
DQE X
Ray
Signal
Noise
n
n
noutput
Ray
ninput
Experimental Cardio
Experimental Vascular Imaging
Modulation Transfer Function
• Erinnerung
g x, y
f x, y
h x, y
f x,y h x x,y
y dx dy
– „Übertragung einer Frequenz v durch das
abbildende System“
– Beschreibt die gesamte Systemcharakteristik
Experimental Cardio
Experimental Vascular Imaging
Modulation Transfer Function
• Line-pairs per mm
– Number of bright/dark stripes per mm
– “Line-pairs equivalent to frequencies 1lp/mm = 1000Hz”
Experimental Cardio
Experimental Vascular Imaging
Modulation Transfer Function
MTF v
Contrast of image at frequency v
Contrast of input at frequency v
Experimental Cardio
Experimental Vascular Imaging
X-Ray Image Intensifier
 low dynamic range of the CCD (10Bits)
 distortion by earth magnetic field
 DQE
Vacuum Tube
CCD Camera
Experimental Cardio
Experimental Vascular Imaging
Flat Dynamic X-Ray Det.
 high dynamic range of the
ADC (14Bits)
 No distortion by earth
magnetic field
 DQE
CsI-Scintillator
+
amorphous Si
Photodiode
Experimental Cardio
Experimental Vascular Imaging
Scatter
Scatterratio P := nsc/(np + nsc)
Experimental Cardio
Experimental Vascular Imaging
Scatter
Scatterratio P := nsc/(np + nsc)
Anatomical region
KV
P
%scatter
Head
70
0.45
90
Lung
120
0.55
120
Pelvis
80
0.90
900
Experimental Cardio
Experimental Vascular Imaging
Streustrahlgitter
Schachtverhältnis: h/D
Experimental Cardio
Experimental Vascular Imaging
Focussed ASG
In XR always a compromise, since SID is adjustable!!!!
Experimental Cardio
Experimental Vascular Imaging
Anti-Scatter Grid
• Definitions
Acquired on film, so the lamellas of
the ASG are visible
– T : Transparency of the grid
• T = Traster/Tno raster
– TP: Transparency for
primary photons
• TP close to 1
– TS: Transparency for
secondary photons
• Ts close to 0 0
– S: Selectivity
• S=TP/TS
P ~ 0.7; TS = 0.1; TP = 0.78
Experimental Cardio
Experimental Vascular Imaging
Anti-Scatter Grid
Experimental Cardio
Experimental Vascular Imaging
Image contrast
• Image contrast
k
I1
I1
I2
I2
I
2I
• Main contributions
– Absorption coefficients
• In case of projections difference of the line integrals
– Scatter
– Noise
Experimental Cardio
Experimental Vascular Imaging
Image Contrast
I
I 0e
µ s ds
I
ln
I0
µ s ds
• µ describes the attenuation of an X-ray beam per unit
length considering all x-ray - tissue interactions
• µ significantly depends on the X-ray energy
• differences between tissues larger at low energies
• only single Parameter: kV of the X-ray photons
Experimental Cardio
Experimental Vascular Imaging
Tissue Attenuation Coefficients
Experimental Cardio
Experimental Vascular Imaging
Tissue Attenuation Coefficients
Experimental Cardio
Experimental Vascular Imaging
Hounsfield Values
HU
1000
material
water
water
CT values characterize the linear attenuation coefficient of the tissue in
each volume element relative to the µ-value of water. The CT values of
different tissues are therefore defined to be relatively stable and to a high
degree independent of the x-ray spectrum.
Experimental Cardio
Experimental Vascular Imaging
Hounsfield Units
Experimental Cardio
Experimental Vascular Imaging
Image contrast
• Scatter
– Contrast, no scatter
– Contrast, scatter
k0
kS
k
– Contrast, scatter, ASG
S
S
Ip
2IP
Ip
2 IP
IS
1
k0
TS I S
1
TP I P
1
k0
I
1 S
IP
1
k0
1 IS
1
S IP
Experimental Cardio
Experimental Vascular Imaging
Image contrast
• Example
– Pelvis
IP = 20%, IS = 80%
kS
0.2k0
– ASG sensitivity S=12
kS
0.75k0
Experimental Cardio
Experimental Vascular Imaging
Image contrast
No noise
N=256
N=16
Experimental Cardio
Experimental Vascular Imaging
Scatter and noise
• Signal to noise
n
n
– Poisson distribution
SNR
– Scatter
SNRscatter
– Example nS = nP
SNRscatter
n
np
np
np
nS
np
1
np
np
1
SNR 0.71SNR
2
• Doubled dose for compensation
SNR(2n)
2n
2 n
SNRscatter (2n)
SNR(n)
Experimental Cardio
Experimental Vascular Imaging
Systemarchitektur
Experimental Cardio
Experimental Vascular Imaging
Dose Adaptation
• Emitteted XR-dose
D
k Z I A U A2 T
• Considering stronger
attenuation of XR photons in
the object yields imaging
dose
D
k * Z I A U An T
n 3
• Dose is free parameter,
realized thru
– kV, mA, T
– kV, mA
– kV
Experimental Cardio
Experimental Vascular Imaging
Dose Adaptation
• Dose and kV settings specific
for application
• Automatic dose adaptation
– Changing anatomy during
data acquisition
– XR – Fluoroscopy, CINE
– Avoids over/under-exposure
– Small soft-tissue objects
(Mammography)
• Low kV
– Huge bony structures
• High kV
detector
– Small bony structures
(Extremities)
• Medium kV
I
I x, y
x
y
Di
Di
1
I
Experimental Cardio
Experimental Vascular Imaging
Image Filtering
• Image
enhancement
– edge enhancement
– noise reduction
– segmentation
–…
Experimental Cardio
Experimental Vascular Imaging
Image Filtering
I x, y
I x, y
I x, y
Sy
I x, y
Sx
I x, y
I x, y
I x x, y
I x x, y
I y x, y
Experimental Cardio
Experimental Vascular Imaging
Filter: Unsharp Mask
-
I x, y
I x, y
Aw
*
-
Experimental Cardio
Experimental Vascular Imaging
Image Filtering
I x, y
I x, y
I x, y
Aw
Level & Window
w
w
weighting
width of kernel
w=3
l
Experimental Cardio
Experimental Vascular Imaging
Further Imaging Techniques
•
•
•
•
•
•
•
Contrast-Enhanced XR
X-Ray Fluoroscopy
X-Ray CINE
Angiography
Digital Subtraction Angiography
Quntitative Angiography
Rotational X-Ray Imaging
Experimental Cardio
Experimental Vascular Imaging
Kontrastmittel in XR
• Röntgenpositiv
• Röntgennegativ
– Verstärkung der XR
Abschwächung
– Reduktion der XR
Abschwächung
– Bariumhaltige KM
(Magen-Darm-Trakt)
– Wasserunlöslicher
jodhaltige KM (MagenDarm-Trakt)
– Wasserlösliche jodhaltige
KM (Gefäße)
– Luft (Kolon)
– Methyzellulose
(Dünndarm)
– CO2 alternative zu
jodhaltigen KM bei
Unverträglichkeit
• Reduzierter Kontarst
Experimental Cardio
Experimental Vascular Imaging
Abominal Enhanced XR
• Stomach
– Filled with air
• Intestines
– Filled with BaSo4
Experimental Cardio
Experimental Vascular Imaging
Intestines BaSo4 Enhanced
Experimental Cardio
Experimental Vascular Imaging
Intestines BaSo4 Enhanced
Experimental Cardio
Experimental Vascular Imaging
Angiography of the Foot
Experimental Cardio
Experimental Vascular Imaging
XR-CINE - Angiography
RCA
LCA
Experimental Cardio
Experimental Vascular Imaging
X-Ray Fluoroscopy
Experimental Cardio
Experimental Vascular Imaging
Digital Subtraction Angiography (DSA)
Mask image
No-CA
AIM: To reduce background signal
CA enhanced
image
Displayed
Image
Mask
CA
Mask - CA
Experimental Cardio
Experimental Vascular Imaging
Digital Subtraction Angiography (DSA)
1) Extract edges for the identification of
2) Identify shift of segments
-) field of view
-) location with high information content
3) Estimate transformation
-) rigid-body
-) affine
-) polynoms
e.g. Prewitt
4) Perform MC subtraction
Experimental Cardio
Experimental Vascular Imaging
Digital Subtraction Angiography (DSA)
DSA
MC-DSA
Experimental Cardio
Experimental Vascular Imaging
Digital Subtraction Angiography (DSA)
• Selective injection of dye
into the
– Aorta
– Left renal artery
Experimental Cardio
Experimental Vascular Imaging
XR – Fluoroscopy – Device
Boosting
Experimental Cardio
Experimental Vascular Imaging
XR – Fluoroscopy – Device
Boosting
• Clinical Research
– Evaluation of the
technique for stents
– Evaluation of the
technique for other
devices
• Closure
• Umbrella
• Pacemarker lead
Experimental Cardio
Experimental Vascular Imaging
XR – Fluoroscopy – Stenoses Boosting
Experimental Cardio
Experimental Vascular Imaging
Lung Nodule Detection
Edge Filter
Blob Detection
???
Experimental Cardio
Experimental Vascular Imaging
Vitual Roadmaps
Experimental Cardio
Experimental Vascular Imaging