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A silicon microstrip system with
the RX64DTH ASIC for dual
energy radiology
• Introduction
– Why digital?
– Why dual energy?
• Experimental setup
• Image processing and results
– Alvarez-Macovski algorithm
– Subtraction imaging with contrast medium
• Conclusion and outlook
The Collaboration
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;
9) CINVESTAV, Mexico City, Mexico
L.M. Montano;
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
Introduction: why dual energy ?
• Dual energy techniques
Based on different
energy dependence 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
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 at the
iodine K-edge
Iodine injected in
patient vessels acts as
radio-opaque contrast
medium
Dramatic change of
iodine absorption coeff. at
K-edge energy (33 keV)
Image subtraction
(2 images taken below and
above the K-edge energy)
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 (1)
Bragg Diffraction on Highly
Oriented Pyrolitic Grafite Crystal
1st and 2nd
Bragg
harmonics
 E and 2E are
obtained in the
same beam
n.h.c
E 
B 2d sin 
B
Collimator
W anode tube
Experimental setup: beam (2)
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
More on the dichromatic beam
incident
spectra
at 3 energy
settings …
… spectra
after 3 cm
plexiglass
(measured
with HPGe
detector)
Experimental seup: Single Photon
Counting System
X-rays
current pulses
N. I. I/O cards PCI-DIO96
and DAQCard-DIO-24
100 m
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
Experimental setup: silicon detector
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
Detector efficiency
• Efficiency calculation
– X-ray absorbed if interacts in passive regions
– X-ray detected if makes photoelectric effect in active regions
• 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
Experimental setup: RX64 chip
Cracow U.M.M. design - (28006500 m2) - 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
Experimental setup: PCB
detector
pitch 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 RX64
b) 6 x Dual threshold RX64
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
V/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
Dual energy imaging
• 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
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
100
200
300
pixels
Conc = 92.5 mg / ml
cavità 4 teor.
cavità 3 teor.
cavità 2 teor.
cavità 1 teor.
80
SNR
100
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
Conc = 23.1 mg / ml
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
Dual energy projection algorithm
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 
  E 
 E 
 a1
 a2



The logarithmic attenuation M = μξtξ of the material of thickness tξ
is measured at two different energies: low (El) and high (Eh):
M h   El   M l   Eh 

A1 





M

A

E

A

E
 Eh   El     Eh  El 

1 
l
2 
l
 l



M l  Eh   M h  El 

M h  A1 Eh   A2   Eh   A 
 2  Eh   El     Eh  El 

A1 and A2 represent the thicknesses of the two base materials which
would provide the same X-ray attenuation as material ξ.
Dual energy projection algorithm

The 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
If a monochromatic beam of intensity I0
goes through material ξ which is partly
replaced by another material ψ …
R
M1
C
90°

A1
ξ
I1
I2
Projecting along direction C, orthogonal to
R, with the contrast cancellation angle :
M2
2
ψ
… 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.
C
1 
I0
A2


C  A1 cos   A2 sin 
… it is possible to cancel the contrast
between materials ξ and ψ: both M1
and M2 will project to the same vector
Mammographic 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
E
_fat
_gland
_canc
20
.456
.802
.844
40
.215
.273
.281
E
20
40
μ_PE
μ_PMMA μ_water
.410
.680
.810
.225
.280
.270
 S. Fabbri et al., Phys. Med. Biol. 47 (2002) 1-13
Image processing (1)
High thr.
350
350
300
300
250
200
200
pixel
250
150
150
100
100
50
50
0
0
0
30
pixel
HE and LE images
16 keV
32 keV
1.
2.
3.
4.
pixels with huge n. of
counts (bad counter
conversion)
dead pixels
X-ray beam fluctuations
subtract high threshold
image from low
threshold one
correct for spatial
inhomogeneities of beam
and detector extracted
from flat-field profiles
pixel
Low thr.
Correct for:
5.
0
30
pixel
350
350
300
300
250
250
200
200
pixel
Measured (raw)
150
150
100
100
50
50
0
0
0
30
pixel
0
30
pixel
Image processing (2)
18 – 36 keV
350
350
300
300
300
300
250
250
250
250
200
200
200
200
pixel
350
pixel
350
pixel
pixel
16 – 32 keV
150
150
150
150
100
100
100
100
50
50
50
50
0
0
0
0
0
30
pixel
0
30
pixel
0
1= PMMA 2=water
3=PE 4=(water+PE)
30
pixel
0
30
pixel
Low statistics due to:
1) 2nd order harmonic
2) dectecting efficiency
Simulation with MCNP
Top View
1=detector
2=PMMA
3=water
4=PE
Side View
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
Experiment vs. Simulation (1)
simulation 16 – 32 keV
350
350
300
300
300
300
250
250
250
250
200
200
200
200
pixel
350
pixel
350
pixel
pixel
RX64DTH 16 – 32 keV
150
150
150
150
100
100
100
100
50
50
50
50
0
0
0
0
0
30
pixel
0
30
pixel
0
30
pixel
0
30
pixel
Experiment vs. Simulation (1)
3400
3400
Simul.16 keV Left Part
Meas.16 keV Left Part
3200
3200
3000
3000
2800
2800
2600
2600
2400
2400
2200
2200
0
100
200
3100
300
Simul.32 keV Left Part
Meas.32 keV Left Part
0
3000
2900
2900
2800
2800
2700
2700
2600
2600
100
200
300
100
200
300
3100
3000
0
Simul.16 keV Right Part
Meas.16 keV Right Part
Simul.32 keV Right Part
Meas.32 keV Right Part
0
100
200
300
Results (1): SNR vs. proj. angle
100
5x5 pixels area
Theoretical cancellation angles:
PMMA-water 36.5°
PE-water
40.5°
PMMA-PE
45°
SNR PE-Water
SNR PMMA-Water
SNR PMMA-PE
80
SNR
60
SNR = 9.6287
theta = 36.5deg
40
SNR = 3.1887
theta = 43deg
SNR = 4.7246
theta = 52.5deg
MCNP simulation
20
160
30
40
Angle (deg)
50
RX64DTH 16 – 32 keV
60
SNR_PE_225_23_WAT225_3_5x5
SNR_PMMA_2_20_WAT225_3_5x5
SNR_PMMA_20_2_PE225_23_5x5
70
140
120
SNR = 23.176
theta = 35deg
100
SNR
0
20
Cancellation angle for
a pair given by SNR=0
80
SNR = 14.521
theta = 44.5deg
SNR = 9.2112
theta = 39deg
60
40
20
0
20
30
40
Angle (deg)
50
60
70
Results (2): SNR summary
Energy
Canceled
Contrast
SNR
SNR
(keV)
materials
material
RX64*
RX64DTH
PMMA-water
PE
8.11
9.63
PE-water
PMMA
2.53
3.19
PE-PMMA
water
3.96
4.72
PMMA-water
PE
7.43
5.14
PE-water
PMMA
2.70
2.10
PE-PMMA
water
3.85
3.13
PMMA-water
PE
2.55
3.27
PE-water
PMMA
0.67
1.07
PE-PMMA
water
0.89
1.58
16-32
18-36
20-40
* Previous version of ASIC, exposure with about 2x more incident photons
Results (3): Projected images
RX64DTH 16 – 32 keV
52.5º
35º
44.5º
350
350
300
300
300
300
250
250
250
250
200
200
200
200
pixel
350
pixel
350
pixel
pixel
36.5º
simulation 16 – 32 keV
150
150
150
150
100
100
100
100
50
50
50
50
0
0
0
30
pixel
0
0
30
pixel
0
0
30
pixel
0
30
pixel
PMMA-water cancellation
PMMA-PE cancellation
Conclusion 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 threematerial 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