Download why dual energy

Dual energy radiology
• Introduction
– Conventional radiology
– Why digital?
– Why dual energy?
• Experimental setup
• Image acquisition
• Image processing and results
Introduction: what are X-rays
• X rays = electromagnetic
radiation (=photons) in the range
10-11
10-8
–
m<l<
m
– 31016 Hz < n < 3 1019 Hz
– 0.1 keV < E < 100 keV
Energy
10-9
10-6
10-3
eV
1
103
106
X-ray generation
ACTIVITY
1Bq = 1decay / second
• Sorgenti
• Radiazione di sincrotrone
• Tubi a raggi X
1Ci = 37GBq
X-ray tube
• Electrons emitted by cathod and accelerated towards the anode (W, Mo)
•Then in the anode do:
Breemstrahlung
Ionization
K L
Anode heating
At diagnostic energies
more than 99% of eenergy goes into
heating; less than 1% is
used for X-rays!
K-shell e- extraction
L K
Continuous spectrum Characteristic lines
Max. energy eV
X-ray interactions
Mass attenuation coefficient (cm2/g)
Silicon
Photoelectirc effect
Compton scattering
_
e+ e
production
X-ray absorption
• Intensity of a beam traversing a material  attenuation
I(x) = I0 e-mx
• Absorption coefficient: m(E) = N s = (srNA)/ A
• Radiographs are based on the different absorption
coefficient of different materials
Bone:
O
Ca
C
P
Other
43.5%
22.5%
15.5%
10.3%
8.2%
Soft Tissue:
O
70.8%
C
14.3%
H
10.2%
N
3.4%
Other
1.3%
Bones absorb
more X rays
than soft
tissue: appear
white on the
radiograph
(photons
darken the
film)
Conventional radiography: image receptors
• Direct-exposure X-ray film
– emulsion of grains of AgBr (  1 mm) suspended in gelatin
– X-rays interact mostly with Ag and Br
• Ag and Br have a larger s than the elements in gelatine
• A latent image is built up of sensitised BrAg grains
• The latent image is then developed (senitised grains converted to silver)
– Problem: very low efficiency
•  0.65% of incident X-rays are detected
• Screen-film combinations
– Phosphor screen to absorb X-ray photons and re-emit part of its
energy in the form of light fluorescent photons
– The light photons expose the film (emulsion of AgBr in gelatine)
• The interaction of light photons with the AgBr is a photochemical reaction
• The silver distribution forms the latent image
– Problem: compromise between detection efficiency and unsharpness
(=loss of edge details)
• The larger the screen thickness, the larger the efficiency, but also the unsharpness
Why digital radiology?
• Digital radiography has well known advantages over
conventional screen-film systems
– Enhance detecting efficiency w.r.t. screen-film
– Image analysis
– Easy data transfer
Why silicon detectors?
Main characteristics of silicon detectors:
• Small band gap (Eg = 1.12 V)
 good resolution in the deposited energy
3.6 eV of deposited energy needed to create a pair of
charges, vs. 30 eV in a gas detector
•Excellent mechanical properties
•Detector production by means of microelectronic techniques
small dimensions
spatial resolution of the order of 10 mm
speed of the order of 10 ns
Eg=1.12 V
Introduction: why dual energy ?
• Dual energy techniques
Based on different energy
dependence of the
absorption coefficient of
different materials
• GOAL: improve image contrast
Enhance detail
visibility (SNR)
Decrease dose
to the patient
Decrease contrast
media concentration
Example 1: dual energy
mammography
Example 1: dual energy
mammography
E  15-20 keV:
Signal from cancer tissue
deteriorated by the
adipose tissue signal
E  30-40 keV
Cancer tissue not visible,
image allows to map glandular
and adipose tissues
Example 2: angiography
•Angiography = X-ray examination of blood vessels
 determine if the vessels are diseased, narrowed or blocked
 Injection of a contrast medium (Iodine) which absorbs X-ray
differently from surrounding tissues
•Coronary angiography
 Iodine must be injected into the heart or very close to it
 A catheter is inserted into the femoral artery and managed up to the
heart
→Long fluoroscopy exposure time to guide the catheter
→Invasive examination
•Why not to inject iodine in a peripheral vein?
 Because lower iodine concentration would be obtained, requiring
longer exposures and larger doses to obtain a good image
 But, if the image contrast could be enhanced in some way…
Example 2: angiography at the
iodine K-edge (II)
Iodine injected in
patient vessels acts as
radio-opaque contrast
medium
Dramatic change of
iodine absorption coeff. at
K-edge energy (33 keV)
Subtraction of 2 images taken with photons of
2 energies (below and above the K-edge)
→ in the resulting image only the iodine signal
remains and all other materials are canceled
Experimental setup
•
To implement dual energy imaging we need:
• a dichromatic beam
• a position- and energy-sensitive detector
Quasi-monochromatic beams
• ordinary X-ray tube + mosaic crystals
• instead of truly monochromatic
synchrotron radiation
Advantages: cost, dimensions, Linear array of silicon microstrips +
availability in hospitals
electonics for single photon counting
•Binary readout
• 1 or 2 discriminators (and
counters) per channel
• Integrated counts for each pixel
are readout
• Scanning required to build 2D image
Experimental setup: beam
Two spatially
separated
beams with
different
energies
 E-DE and
E+DE obtained
in 2 separate
beams
Bragg Diffraction on Highly
Oriented Pyrolitic Grafite Crystal
n.h.c
E 
B 2d sin 
B
Double slit
collimator
W anode tube
Experimental setup: Single Photon
Counting System
X-rays
current pulses
N. I. I/O cards PCI-DIO96
and DAQCard-DIO-24
100 mm
data,
control
Silicon strip detector
Integrated
circuit
PC
• Fully parallel signal processing for all channels
• Binary architecture for readout electronics
1 bit information (yes/no) is extracted from each strip
Threshold scans needed to extract analog information
• Counts integrated over the measurement period transmitted to DAQ
Detecting system
Silicon microstrip detector
each strip is an independent detector
which gives an electric signal when an Xray photon crosses it and interacts with
a silicon atom
4 cm
Chip RX64 → counts incident photons on
each strip of the detector
6.4 mm
10 strip = 1 mm
micro-bondings
Knowing from which strip the electric
signal comes from,the position of the
incoming X-ray phonton is reconstructed.
Experimental setup: RX64 chip
Cracow U.M.M. design - (28006500 mm2) - CMOS 0.8 µm process
(1) 64 front-end channels
a) preamplifier
b) shaper
c) 1 or 2 discriminators
(2) (1 or 2)x64 pseudo-random counters (20-bit)
(3) internal DACs: 8-bit threshold setting and 5-bit for bias settings
(4) internal calibration circuit (square wave 1mV-30 mV)
(5) control logic and I/O circuit (interface to external bus)
2
Detector
1
5
4
3
System calibration setup in Alessandria
Detector in Front config.
Fluorescence target
(Cu, Ge, Mo, Nb, Zr, Ag, Sn)
Cu anode X-ray tube
→ X-ray energies = characteristic lines of target material
System calibration
Mo K
Counts
150
Sn K
Ge K
100
Ag K
Cu K
Mo K
Ag K
Rb K
50
0
100
200
Source Am+Rb target
Source Am+Mo target
Source Am+Ag target
Tube+Cu target
Tube+Ge target
Tube+Mo target
Tube+Ag target
Tube+Sn target
Sn K
300
400
500
Threshold (mV)
241Am
source with rotary target holder (targets: Cu, Rb, Mo, Ag, Ba)
Cu-anode X-ray tube with fluorescence targets (Cu, Ge, Mo, Ag, Sn)
System
Tp
GAIN
mV/el.
ENC
Energy
resolution
6 x RX64
0.7 ms
64
≈170 el.
≈0.61 keV
6 x RX64DTH
0.8 ms
47
≈ 200 el.
≈0.72 keV
Imaging test
1-dimensional array of strips → 2D image obtained by scanning
Test Object
5 mm
Collimator (0.5 mm)
Detector
Cd-109 source (22.24 keV)
Imaging test
Pasos
Scanning
1-dimensional array of strips → 2D image obtained by scanning
210
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
24,00
21,00
18,00
15,00
12,00
9,000
6,000
3,000
0
0
10
20
30
40
Canales
50
60
K-edge subtraction imaging
• Map the concentration of a particular element in a sample
 X-ray energies chosen so that the element under study has the Kedge discontinuity between them
 Cancel background structures by subtracting 2 images taken at the
2 energies
 For best background cancellation the 2 energies must be close
to each other
 Best choice: energies just above and below the K-edge of the
interesting material
• Art painting analysis
• Isolate one typical material (ec. Zn, Cd) to date a painting
• Medical imaging with contrast medium
 Suited for angiography at iodine K-edge
- Cancel background structures to enhance vessel visibility
 Possible application at the Gadolinium K-edge (50.2 keV)
 Possible application in mammography (study vascularization extent)
- Hypervascularity characterizes most malignant formations
Angiography setup
X-ray tube with dual energy
output
Phantom
Detector box with 2
collimators
1.
X-ray tube + mosaic crystal and 2 collimators to provide dual-energy output
- E1= 31.5 keV, E2 =35.5 keV (above and below iodine k-edge)
2.
Detector box with two detectors aligned with two collimators
3.
Step wedge phantom made of PMMA + Al with 4 iodine solution filled
cavities of 1 or 2 mm diameter
pixels
15
10
5
6
15
5
pixels
3
Conteggi (x10 )
Angiographic test results (I)
4
3
10
2
5
1
0
0
200
pixels
100
0
0
300
E = 31.5 keV
100
E = 35.5 keV
1,0
Counts / Max.Counts
0,8
0,6
logarithmic subtraction
0,4
C1  ln N35.5   C2  ln N31.5 
0,2
Measurement
Simulation
0,0
0
50
100
150
200
250
300
Counts / Max.Counts
Conc(I) = 370 mg/ml
E = 31.5 KeV
1,0
Conc(I) = 370 mg/ml
E = 35.5 KeV
0,6
0,4
0,2
Measurement
Simulation
0,0
0
50
100
350
-0.4
-0.6
150
200
250
Strip Number
15
10
5
-0.8
0
0
100
200
pixels
300
ln[count(E=35.5Kev)] - ln[count(E=31.5Kev)]
-0.2
pixels
log conteggi
0.0
Measurement
Simulation
Conc(I) = 370 mg/ml
0,2
0,0
-0,2
-0,4
-0,6
-0,8
-1,0
0
50
100
150
200
Strip Number
300
0,8
Strip Number
Phantom
structure not
visible in
final image
200
pixels
250
300
350
300
350
Angiographic test results (II)
5
-0.8
0.0
-0.2
10
5
-0.3
0
0.15
0.10
0.05
0.00
-0.05
-0.10
-0.15
0
0
100
200
300
100
200
300
5
0
pixels
Conc = 370 mg / ml
10
0
0
pixels
15
pixels
0.1
-0.1
log conteggi
-0.6
10
15
pixels
-0.4
15
pixels
-0.2
log conteggi
0.2
0.0
100
200
300
pixels
Conc = 92.5 mg / ml
Conc = 23.1 mg / ml
Signal contrast Sig .Counts  Bckgr.Counts
SNR 

Noise contrast
Bckgr. fluctuations
100
cavità 4 teor.
cavità 3 teor.
cavità 2 teor.
cavità 1 teor.
SNR
80
cavità 4
cavità 3
cavità 2
cavità 1
60
40
20
0
0
100
200
Concentrazione (mg/ml)
300
400
Possible decrease of iodine concentration keeping the same rad. dose
Results with a second phantom
Phantom
0
100
200
300
140
120
0
100
200
300
140
140
300 um pixel
100
120
300 um pixel
100
120
80
100
60
80
80
60
40 60
40
20 40
20
20
0
0
100
200
300
140
0
140
0
0
120
100
300 um pixel
Digital Subtraction
Angiography
Dual Energy
Angiography
Iodine conc. = 95 mg/ml
smaller cavity
(=0.4 mm)
visible in DEA
and not in DSA
100
200
100 um pixel
300
120
100
80
80
60
60
40
40
20
20
0
0
100
200
100 um pixel
300
Application to art painting analysis
 Detect the presence of cadmium in a painting
Cd red
Test object
Cd K-edge = 26.7 keV
Cu red
60
50
40
30
60
E = 24.2 keV
20
10
0
0
100
200
300
logarithmic
subtraction
50
40
30
20
60
10
50
0
40
0
30
20
E = 27.5 keV
10
0
0
100
200
300
100
200
300
After subtraction:
• Cd grains contrast enhanced
• Cu wires contrast decreased