introduction –why digital? –why dual energy? experimental setup image acquisition image...
DESCRIPTION
Introduction: why digital ? Digital radiography has well known advantages over conventional screen-film systems –Enhance detecting efficiency w.r.t. screen-film –Image analysis –Easy data transferTRANSCRIPT
• Introduction– Why digital?– Why dual energy?
• Experimental setup• Image acquisition• Image processing and results
A silicon microstrip system A silicon microstrip system with the RX64DTH ASIC for with the RX64DTH ASIC for
dual energy radiologydual energy radiology
1) University of Eastern Piedmont and INFN, Alessandria, Italy L. Ramello;
2) University and INFN, Torino, Italy P. Giubellino, A. Marzari-Chiesa, F. Prino;
3) University and INFN, Ferrara, Italy; M. Gambaccini, A. Taibi, A. Tuffanelli, A. Sarnelli;
4) University and INFN, Bologna, Italy G. Baldazzi, D. Bollini;
5) AGH Univ. of Science and Technology, Cracow, Poland W. Dabrowski, P. Grybos, K. Swientek, P. Wiacek;
6) University of Antwerp, Antwerp, Belgium P. Van Espen;
7) Univ. de los Andes, Colombia C. Avila, J. Lopez Gaitan, J.C. Sanabria;
8) CEADEN, Havana, Cuba A.E. Cabal, C. Ceballos, A. Diaz Garcia, L. Bolaños;
9) CINVESTAV, Mexico City, Mexico L.M. Montano;
The CollaborationThe Collaboration
Introduction: why digital ?Introduction: why digital ?• Digital radiography has well known advantages over
conventional screen-film systems– Enhance detecting efficiency w.r.t. screen-film
– Image analysis– Easy data transfer
• Dual energy techniques
• GOAL: improve image contrast
Based on different energy dependence
of different materials
Enhance detail visibility (SNR)
Decrease dose to the patient
Decrease contrast media concentration
Introduction: why dual Introduction: why dual energy ?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 Example 2: angiography at the iodine K-edge (II)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 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 + electronics 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 (1)Experimental setup: beam (1)
Bdchn
BE sin2..
Bragg Diffraction on Highly Oriented Pyrolitic Grafite Crystal
W anode tube
1st and 2nd Bragg harmonics E and 2E are obtained in the same beam
Collimator
Experimental setup: beam (2)Experimental setup: beam (2)
Bdchn
BE sin2..
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
More on the dichromatic beamMore on the dichromatic beam
incidentspectraat 3 energysettings …
… spectra after 3 cm plexiglass
(measured with HPGe detector)
• 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
Experimental setup: PCBExperimental setup: PCB
detectorpitch adapter
ASICs
PCB:- One 400 strip detector- Pitch adapter- 6 RX64 chips
384 equipped channels- connector to DAQ card
2 protoype detectors:a) 6 x Single threshold RX64b) 6 x Dual threshold RX64
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.
Why silicon detectors?Why silicon detectors?Main characteristics of silicon detectors:• speed of the order of 10 ns• spatial resolution of the order of 10 m•small amount of material
0.003 X0 for a typical 300 m thickness
• excellent mechanical properties• 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
Silicon sensor diodeSilicon sensor diode•The impinging ionizing particles generate electron-hole pairs •The impinging photons which interact in the detector volume create an electron (via Photoelectric, Compton or Pair Production)
•The electron ionizes the surrounding atoms generating electron-hole pairs
• Electron and holes drift to the electrodes under the effect of the electric field present in the detector volume. •The electron-hole current in the detector induces a signal at the electrodes on the detector faces.Metal contact
n+-type implant
n-type bulk
Charged particle -V
+V
electron
hole
P+-type implant
photon
photoelectron
ReversebiasE
Why reverse biased diode?Why reverse biased diode?•The amount of charge deposited in the silicon detector is very small
≈5500 electrons are produced by a 20 keV photons making photoelectric effect in the silicon
Forward-biased junction: the signal would be masked by the fluctuations of the current which the applied field makes flow even in high resistivity, hyper-pure silicon.Reverse-biased junction: allows to obtain the necessary electric field and only a very small “dark” current also at room temperature.
-V
+V
depleted region
Increasing the polarization voltage, it is possible to extend the depletion layer down to the backplane.
To have full efficiency, the polarization voltage must be high enough to deplete the full detector thickness (typically 300 m)
junction
NOT GOOD
Silicon Microstrips detectorsSilicon Microstrips detectors• A micro-strip detector is a silicon detector segmented in long, narrow elements.
•Each strip is an independent p-n reverse-biased junction• Provides the measurement of one coordinate of the particle’s crossing point with high precision (down to 1 m).
N-type substrate
P+n+
Al
P+
SiO2
AC coupling to electronics
SiO2AlDC coupling to electronics
Experimental setup: silicon Experimental setup: silicon detectordetector
Parameter Value
Depth 300 μm
Strip length 10 mm
Number of strips 400
Strip pitch 100 μm
Depletion voltage 20-23 V
Leakeage curr. (22º C) 50-60 pA
Inactive region thickn. 765 μm
Designed and fabricated by ITC-IRST, Trento, Italy
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)
11 22
3344
55Det
ecto
r
Detector efficiencyDetector efficiency
• Front geometry– Strip orthogonal to the beam– 70 m of Al light shield
• Edge-on geometry– Strip parallel to the beam– 765 m of inactive Si– Better efficiency for E > 18 keV
• Efficiency calculation– X-ray absorbed if interacts in passive regions– X-ray detected if makes photoelectric effect in active regions
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
bjec
t
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
Pasos
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
Scan
nin
g
System calibration setup in System calibration setup in AlessandriaAlessandria
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
Cou
nts
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 TpGAINV/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
• K-edge subtraction imaging with contrast medium Cancel background structures by subtracting 2 images taken at energies just
below and above the K-edge of the contrast medium Suited for angiography at iodine (gadolinium) K-edge
- Cancel background structures to enhance vessel visibility Possible application in mammography (study vascularization extent)
- Hypervascularity characterizes most malignant formations • Dual energy projection algorythm
Make the contrast between 2 chosen materials vanish by measuring the logarithmic transmission of the incident beam at two energies and using a projection algorithm [Lehmann et al., Med. Phys. 8 (1981) 659]
Suited for dual energy mammography– remove contrast between the two normal tissues (glandular
and adipose), enhancing the contrast of the pathology– Single exposure dual-energy mammography reduces
radiation dose and motion artifacts
Dual energy imagingDual energy 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
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0 50 100 150 200 250 300 350
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0,2 Conc(I) = 370 mg/ml Measurement Simulation
ln[c
ount
(E=3
5.5K
ev)]
- ln[
coun
t(E=3
1.5K
ev)]
Strip Number
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161412108642
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103 )
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0,0
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1,0 Conc(I) = 370 mg/ml E = 31.5 KeV
Measurement Simulation
Coun
ts /
Max
.Cou
nts
Strip Number
E = 31.5 keVE = 31.5 keV
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654321C
onte
ggi (
x103 )
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1,0
Strip Number
Measurement Simulation
Conc(I) = 370 mg/ml E = 35.5 KeV
Coun
ts /
Max
.Cou
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)
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eggi
Conc = 92.5 mg / mlConc = 92.5 mg / ml
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-0.15-0.10-0.050.000.050.100.15
log
cont
eggi
Conc = 23.1 mg / mlConc = 23.1 mg / ml
100
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60
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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)
Results with a second Results with a second phantomphantom
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ixel
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PhantomDigital Subtraction
Angiography
Dual Energy Angiography
smaller cavity (=0.4 mm) visible in DEA and not in DSA
Iodine conc. = 95 mg/ml
Dual energy projection Dual energy projection algorithmalgorithm
The mass attenuation coefficient μ of any material at a given energy E is expressed as a combination of the coefficients of any two suitable materials and :
E
aEaE
21
The logarithmic attenuation M = μξtξ of the material of thickness tξ is measured at two different energies: low (El) and high (Eh):
lhlh
lhhl
lhlh
hllh
hhh
lll
EEEEEMEMA
EEEEEMEM
A
EAEAM
EAEAM
2
1
21
21
A1 and A2 represent the thicknesses of the two base materials which would provide the same X-ray attenuation as material ξ.
C
C90°
M1
R
1
M2
2
If a monochromatic beam of intensity I0 goes through material ξ which is partly replaced by another material ψ …
I0
I1 I2
ξψ
… then the vertexes of log. attenuation vectors M2 (material ξ) and M1 (mat. ξ + ψ) lie on a line R which is defined only by the properties of materials α, β, ξ and ψ. Projecting along direction C, orthogonal
to R, with the contrast cancellation angle :
… it is possible to cancel the contrast between materials ξ and ψ: both M1 and M2 will project to the same vector
A2
A1
Dual energy projection Dual energy projection algorithmalgorithmThe logarithmic attenuation M in a given pixel can be represented
as a vector having components A1 and A2 in the basis plane, the modulus will then be proportional to the gray level of that pixel
sincos 21 AAC
Mammographic phantomMammographic phantom• Three components: polyethylene (PE), PMMA and
water to simulate the attenuation coeff. (cm-1) of the adipose, glandular and cancerous tissues in the breast
S. Fabbri et al., Phys. Med. Biol. 47 (2002) 1-13
E _fat _gland _canc
20 .456 .802 .844
40 .215 .273 .281
E μ_PE μ_PMMA μ_water
20 .410 .680 .810
40 .225 .280 .270
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Image processing (1)Image processing (1)
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Low thr. High thr.
Measured (raw)
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l
16 keV
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32 keVHE and LE imagesCorrect for:
1. pixels with huge n. of counts (bad counter conversion)
2. dead pixels3. X-ray beam fluctuations4. subtract high threshold
image from low threshold one
5. correct for spatial inhomogeneities of beam and detector extracted from flat-field profiles
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16 – 32 keV 18 – 36 keV
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1= PMMA 2=water3=PE 4=(water+PE)
Image processing (2)Image processing (2)
Low statistics due to:1) 2nd order harmonic2) dectecting efficiency
Simulation with MCNPSimulation with MCNP
1=detector2=PMMA3=water4=PE
MCNP-4C simulation with ENDF/B-VI library• Photons and electrons
tracked through the phantom and the detector (including the inactive region in front of the strips)
• Energy deposition in each strip recorded
• histogram of counts vs. strip number filled
Top View
Side View
Experiment vs. Simulation (1)Experiment vs. Simulation (1)RX64DTH 16 – 32 keV
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simulation 16 – 32 keV
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Experiment vs. Simulation Experiment vs. Simulation (1)(1)
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Simul.32 keV Left Part Meas.32 keV Left Part
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Results (1): SNR vs. proj. Results (1): SNR vs. proj. angleangle100
80
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0
SN
R
706050403020Angle (deg)
SNR PE-Water SNR PMMA-Water SNR PMMA-PE
5x5 pixels area
SNR = 9.6287 theta = 36.5deg
SNR = 4.7246 theta = 52.5degSNR = 3.1887
theta = 43deg
RX64DTH 16 – 32 keV
MCNP simulation160
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R
706050403020Angle (deg)
SNR_PE_225_23_WAT225_3_5x5 SNR_PMMA_2_20_WAT225_3_5x5 SNR_PMMA_20_2_PE225_23_5x5
SNR = 23.176 theta = 35deg
SNR = 14.521 theta = 44.5deg
SNR = 9.2112 theta = 39deg
Cancellation angle for a pair given by SNR=0
Theoretical cancellation angles: PMMA-water 36.5° PE-water 40.5° PMMA-PE 45°
Results (2): SNR summaryResults (2): SNR summaryEnergy Canceled Contrast SNR SNR
(keV) materials material RX64* RX64DTH
PMMA-water PE 8.11 9.63
16-32 PE-water PMMA 2.53 3.19PE-PMMA water 3.96 4.72PMMA-water PE 7.43 5.14
18-36 PE-water PMMA 2.70 2.10PE-PMMA water 3.85 3.13PMMA-water PE 2.55 3.27
20-40 PE-water PMMA 0.67 1.07PE-PMMA water 0.89 1.58
* Previous version of ASIC, exposure with about 2x more incident photons
Results (3): Projected Results (3): Projected imagesimagesRX64DTH 16 – 32 keV simulation 16 – 32 keV
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35º
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44.5º
PMMA-water cancellationPMMA-PE cancellation
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36.5º
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52.5º
Conclusion and OutlookConclusion and Outlook• We have developed a single photon counting silicon detector equipped
with the RX64DTH ASIC, with two selectable energy windows• The energy resolution of 0.8 keV (rms) is well adapted for dual energy
mammography and angiography• We have performed mammography imaging tests with a three-material
phantom– We have demonstrated the feasibility of contrast cancellation between two
materials, enhancing the visibility of small features in the third one• We have performed angiography imaging tests with 2 different phantoms
and iodine contrast medium– We have demonstrated the feasibility of logarithmic subtraction between two
images, enhancing contrast of small vessels also with lower iodate solution concentrations
• OUTLOOK: – Increase photon statistics at high energy, optimize exposure conditions– New detector materials, CZT?– Tests with a more realistic mammographic phantom