Introduction –Conventional radiology –Why digital? –Why dual energy? Experimental setup Image...
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Transcript of Introduction –Conventional radiology –Why digital? –Why dual energy? Experimental setup Image...
• Introduction
– Conventional radiology– Why digital?– Why dual energy?
• Experimental setup• Image acquisition• Image processing and results
Dual energy radiologyDual energy radiology
Introduction: what are X-raysIntroduction: what are X-rays
Energy
10-9
10-6
10-3
1
103
106
eV
• X rays = electromagnetic radiation (=photons) in the range
– 10-11 m < < 10-8 m– 31016 Hz < < 3 1019
Hz– 0.1 keV < E < 100 keV
X-ray generationX-ray generation
• Sorgenti• Radiazione di
sincrotrone• Tubi a raggi X
37GBq1Ci
second/1decay1Bq
=
=ACTIVITY
X-ray tubeX-ray tube
At diagnostic energies more than 99% of e- energy goes into heating; less than 1% is used for X-rays!
Anode heating
K L
Ionization
Characteristic lines
KL
K-shell e- extraction
• Electrons emitted by cathod and accelerated towards the anode (W, Mo)•Then in the anode do:
Breemstrahlung
Continuous spectrumMax. energy eV
X-ray interactionsX-ray interactions
Photoelectirc effect
Compton scattering
e+e _
production
Mass attenuation coefficient (cm2/g)
Silicon
X-ray absorptionX-ray absorption• Intensity of a beam traversing a material attenuation
I(x) = I0 e-x
• Absorption coefficient: (E) = N = (NA)/ A• Radiographs are based on the different absorption
coefficient of different materials
Bones absorb more X rays than soft tissue: appear white on the radiograph (photons darken the film)
Bone:O 43.5%Ca 22.5%C 15.5%P 10.3%Other 8.2%
Soft Tissue:O 70.8%C 14.3%H 10.2%N 3.4%Other 1.3%
Conventional radiography: image receptorsConventional radiography: image receptors• Direct-exposure X-ray film
– emulsion of grains of AgBr ( 1 m) suspended in gelatin– X-rays interact mostly with Ag and Br
• Ag and Br have a larger 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?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?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 m
speed of the order of 10 ns
Eg=1.12 V
• Dual energy techniques
• GOAL: improve image contrast
Based on different energy dependence of
the absorption coefficient of different
materials
Enhance detail visibility (SNR)
Decrease dose to the patient
Decrease contrast media concentration
Introduction: why dual energy ?Introduction: why dual energy ?
Example 1: dual energy Example 1: dual energy mammographymammography
Example 1: dual energy Example 1: dual energy mammographymammography
E 15-20 keV:Signal from cancer tissue deteriorated by the adipose tissue signal
E 30-40 keVCancer tissue not visible, image allows to map glandular and adipose tissues
Example 2: angiographyExample 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 angiographyIodine must be injected into the heart or very close to itA 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 Example 2: angiography at the iodine K-edge (II)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 setupExperimental 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 radiationAdvantages: cost, dimensions, availability in hospitals
Linear array of silicon microstrips + 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: beamExperimental setup: beam
Bd
chnB
Esin2
..
Bragg Diffraction on Highly Oriented Pyrolitic Grafite Crystal
W anode tube
Double slit collimator
Two spatially separated beams with different energies E-E and E+E obtained in 2 separate beams
• Fully parallel signal processing for all channels• Binary architecture for readout electronics
1 bit information (yes/no) is extracted from each stripThreshold scans needed to extract analog information
• Counts integrated over the measurement period transmitted to DAQ
• Fully parallel signal processing for all channels• Binary architecture for readout electronics
1 bit information (yes/no) is extracted from each stripThreshold scans needed to extract analog information
• Counts integrated over the measurement period transmitted to DAQ
data, control
Silicon strip detector Integrated circuit
100 m
current pulses
X-rays
PC
N. I. I/O cards PCI-DIO-N. I. I/O cards PCI-DIO-96 96
and DAQCard-DIO-24and DAQCard-DIO-24
Experimental setup: Single Photon Experimental setup: Single Photon Counting SystemCounting System
Detecting systemDetecting system
Chip RX64 → counts incident photons on each strip of the detector
4 cm
6.4 mm10 strip = 1 mm
micro-bondings
Silicon microstrip detectoreach strip is an independent detector which gives an electric signal when an X-ray photon crosses it and interacts with a silicon atom
Knowing from which strip the electric signal comes from,the position of the incoming X-ray phonton is reconstructed.
Experimental setup: RX64 chipExperimental setup: RX64 chipCracow U.M.M. design - (28006500 m2) - CMOS 0.8 µm process
(1) (1) 64 front-end channels a) preamplifierb) shaperc) 1 or 2 discriminators
(2)(2) (1 or 2)x64 pseudo-random counters (20-bit)
(3)(3) internal DACs: 8-bit threshold setting and 5-bit for bias settings
(4)(4) internal calibration circuit (square wave 1mV-30 mV)
(5)(5) control logic and I/O circuit (interface to external bus)
Cracow U.M.M. design - (28006500 m2) - CMOS 0.8 µm process
(1) (1) 64 front-end channels a) preamplifierb) shaperc) 1 or 2 discriminators
(2)(2) (1 or 2)x64 pseudo-random counters (20-bit)
(3)(3) internal DACs: 8-bit threshold setting and 5-bit for bias settings
(4)(4) internal calibration circuit (square wave 1mV-30 mV)
(5)(5) control logic and I/O circuit (interface to external bus)
11 22
3344
55
Det
ecto
rD
etec
tor
System calibration setup in AlessandriaSystem 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
150
100
50
0
Co
un
ts
500400300200100
Threshold (mV)
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
Cu K
Mo K
Ge K
Rb K
Ag K
Sn K
Ag K
Mo K
Sn K
System Tp
GAINV/el.
ENC Energy resolution
6 x RX64 0.7 s 64 ≈170 el. ≈0.61 keV
6 x RX64DTH 0.8 s 47 ≈ 200 el. ≈0.72 keV
241241Am source with rotary target holder (targets: Cu, Rb, Mo, Ag, Ba)Am source with rotary target holder (targets: Cu, Rb, Mo, Ag, Ba)Cu-anode X-ray tube with fluorescence targets (Cu, Ge, Mo, Ag, Sn)Cu-anode X-ray tube with fluorescence targets (Cu, Ge, Mo, Ag, Sn)
System calibrationSystem calibration
Imaging testImaging test1-dimensional array of strips → 2D image obtained by scanning
Cd-109 source (22.24 keV)
Detector
Collimator (0.5 mm)
Tes
t O
bje
ct
5 mm
Imaging testImaging test1-dimensional array of strips → 2D image obtained by scanning
0 1 0 2 0 3 0 4 0 5 0 6 0
5 0
6 0
7 0
8 0
9 0
1 0 0
1 1 0
1 2 0
1 3 0
1 4 0
1 5 0
1 6 0
1 7 0
1 8 0
1 9 0
2 0 0
2 1 0
C a n a le s
Pas
os
0
3 , 0 0 0
6 , 0 0 0
9 , 0 0 0
1 2 , 0 0
1 5 , 0 0
1 8 , 0 0
2 1 , 0 0
2 4 , 0 0
Sca
nn
ing
• Map the concentration of a particular element in a sample X-ray energies chosen so that the element under study has
the K-edge discontinuity between them Cancel background structures by subtracting 2 images taken
at the 2 energiesFor 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
K-edge subtraction imagingK-edge subtraction imaging
X-ray tube with dual energy output
Phantom
Detector box with 2 collimators
1.1. X-ray tube + mosaic crystal and 2 collimators to provide dual-energy output 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)- E1= 31.5 keV, E2 =35.5 keV (above and below iodine k-edge)
2.2. Detector box with two detectors aligned with two collimatorsDetector box with two detectors aligned with two collimators
3.3. Step wedge phantom made of PMMA + Al Step wedge phantom made of PMMA + Al with 4 iodine solution filled with 4 iodine solution filled cavities of 1 or 2 mm diametercavities of 1 or 2 mm diameter
Angiography setupAngiography setup
15
10
5
0
pixe
ls
3002001000pixels
-0.8
-0.6
-0.4
-0.2
0.0
log
con
tegg
i
0 50 100 150 200 250 300 350
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2 Conc(I) = 370 mg/ml Measurement Simulation
ln[c
ou
nt(
E=
35.5
Kev)]
- ln
[co
un
t(E
=31.5
Kev)]
Strip Number
15
10
5
0
pix
els
3002001000pixels
161412108642
Co
nte
gg
i (
x1
03 )
0 50 100 150 200 250 300 350
0,0
0,2
0,4
0,6
0,8
1,0 Conc(I) = 370 mg/ml E = 31.5 KeV
Measurement Simulation
Co
un
ts / M
ax.C
ou
nts
Strip Number
E = 31.5 keVE = 31.5 keV
15
10
5
0
pix
els
3002001000pixels
6
5
4
3
2
1
Co
nte
gg
i (x
103 )
0 50 100 150 200 250 300 350
0,0
0,2
0,4
0,6
0,8
1,0
Strip Number
Measurement Simulation
Conc(I) = 370 mg/ml E = 35.5 KeV
Co
un
ts / M
ax.C
ou
nts
E = 35.5 keV
5.3125.351 lnln NCNC logarithmic subtraction
Phantom structure not
visible in final image
Angiographic test results (I)Angiographic test results (I)
15
10
5
0
pix
els
3002001000pixels
-0.8
-0.6
-0.4
-0.2
0.0
log
co
nte
gg
i
Conc = 370 mg / mlConc = 370 mg / ml
15
10
5
0
pix
els
3002001000pixels
-0.3
-0.2
-0.1
0.0
0.1
0.2
log
co
nte
gg
iConc = 92.5 mg / mlConc = 92.5 mg / ml
15
10
5
0
pix
els
3002001000pixels
-0.15
-0.10
-0.05
0.00
0.05
0.100.15
log
co
nte
gg
i
Conc = 23.1 mg / mlConc = 23.1 mg / ml
100
80
60
40
20
0
SN
R
4003002001000Concentrazione (mg/ml)
cavità 4 teor. cavità 4 cavità 3 teor. cavità 3 cavità 2 teor. cavità 2 cavità 1 teor. cavità 1
Possible decrease of iodine concentration keeping the same rad. dose
Angiographic test results (II)Angiographic test results (II)
nsfluctuatioBckgr
CountsBckgrCountsSig
contrastNoise
contrastSignalSNR
.
..
Results with a second phantomResults with a second phantom
140 140
120 120
100 100
80 80
60 60
40 40
20 20
0 0
300
um p
ixel
300
300
200
200
100
100
0
0
100 um pixel
140 140
120 120
100 100
80 80
60 60
40 40
20 20
0 0
300
um p
ixel
300
300
200
200
100
100
0
0
100 um pixel
140 140
120 120
100 100
80 80
60 60
40 40
20 20
300
um p
ixel
300
300
200
200
100
100
0
0
100 um pixel
Phantom
Digital SubtractionAngiography
Dual Energy Angiography
smaller cavity (=0.4 mm) visible in DEA and not in DSA
Iodine conc. = 95 mg/ml
Application to art painting analysisApplication to art painting analysis Detect the presence of cadmium in a painting
60
50
40
30
20
10
0
3002001000
E = 24.2 keV
60
50
40
30
20
10
0
3002001000
E = 27.5 keV
Cd K-edge = 26.7 keV
Cd red
Cu red
Test object
60
50
40
30
20
10
0
3002001000
logarithmic subtraction
After subtraction:• Cd grains contrast enhanced• Cu wires contrast decreased