Transcript Results about imaging with silicon strips for angiography

Recent advances on X-ray imaging with
a single photon counting system
I.
II.
III.
IV.
V.
VI.
VII.
Introduction
The system: microstrip detectors,
RX64 ASICs, testing methods
Energy resolution and efficiency
Spatial resolution
Imaging results - mammography
Imaging results - angiography
Summary and outlook
Luciano Ramello – Univ. Piemonte
NURT 2003, October 27-31, 2003
Orientale and INFN, Alessandria
G. Baldazzi1, D. Bollini1, A.E. Cabal Rodriguez2,
C. Ceballos Sanchez2 ,W. Dabrowski3,
A. Diaz Garcia2, M. Gambaccini4, P. Giubellino5,
M. Gombia1, P. Grybos3, M. Idzik3,5,
J. Lopez Gaitan10, A. Marzari-Chiesa6,
L.M. Montano Zetina7, F. Prino8, L. Ramello8,
A. Sarnelli4, M. Sitta8, K. Swientek3,
A. Taibi4, E. Tomassi6, A. Tuffanelli4,
P. Van Espen9, P. Wiacek3
1 University and INFN, Bologna, Italy; 2 CEADEN, Havana, Cuba;
3 University of Mining and Metallurgy, Cracow, Poland; 4 University
and INFN, Ferrara, Italy; 5 INFN, Torino, Italy; 6 University of
Torino, Torino, Italy; 7 CINVESTAV, Mexico City, Mexico;
8 University of Eastern Piedmont and INFN, Alessandria, Italy;
9 University of Antwerp, Antwerp, Belgium; 10 Univ. de los Andes,
Colombia
I. Introduction
Introduction (1)





We are developing a single photon counting system for Xray imaging in the 15-50 keV range
Spatial resolution is defined by the detector segmentation
(presently 100 mm pitch strips)
Energy resolution is determined mainly by the low-noise
front-end ASIC
Rate capability (converted photons/mm2/s) is defined by the
timing characteristics of the ASIC and by the pixel size
(presently 100 x 300 mm2)
Medical applications of this system are those requiring high
dynamic range of counts, good energy resolution;
furthermore, they must be compatible with scanning mode
I. Introduction
Introduction (2)

One-dimensional silicon array for scanning mode
imaging:
• Good spatial resolution with reduced number of channels
• Spatial resolution in silicon limited by Compton scattering and
parallax error, pitch smaller than about 50-100 micron not really
useful

Advantages of digital single photon X-ray imaging:
• Higher detection efficiency with respect to screen-film systems
• Edge-on orientation (parallel incidence) preferred for
energies above 18 keV
• Double energy threshold with simultaneous exposure possible
• Easy processing, transferring and archiving of digital images
I. Introduction
Introduction (3)



Subtraction imaging: removes background structures
Dual energy technique: isolates materials characterized
by different energy dependence of the linear attenuation
coefficient m [Alvarez and Macovski 1976]
Quasi-monochromatic beams: implement dual energy
techniques in a small-scale installation, no synchrotron
[see NIM A 365 (1995) 248
and Proc. SPIE Vol. 4682, p. 311 (2002)]
First application: dual energy angiography at iodine K-edge
(33 keV), possible extension to gadolinium K-edge (50 keV)


Second application: dual-energy mammography (18+36 keV)
I. Introduction
Silicon efficiency vs. X-ray energy
Photoelectric conversion in the active volume


Front configuration
• 70 mm Al shield
(could be reduced)
• 300 mm active Si
Edge configuration
• 765 mm insensitive
silicon
• 10 mm (now) or 20
mm (later ?) active
silicon
simple calculation with cross-sections from XCOM data base of NIST
I. Introduction
GaAs: a better alternative ?
Photoelectric conversion in the active volume



Front configuration
for GaAs, Edge
configuration for Si
GaAs is the best
choice for 20 keV
mammography
Si in edge mode (10
mm) is almost
equivalent to GaAs
for angiography
II. System
Silicon microstrip detectors

AC coupling:
Bias Line with
FOXFET biasing
 Guard ring
essential to
collect surface
currents
 Designed and
fabricated by
ITC-IRST,
Trento, Italy
DC contact (to p+ implant)
guard
ring
bias line first strip (AC contact)
II. System
Detector test: I-V measurements
Corrente
di fuga (A)
(A)
Leakage current
2
-8
10
6
4
Corrente di guard ring
Corrente di bias line
2
-9
10
6
4
2
Temperatura = 25.5 °C
-10
Keithley 237 provides
reverse bias,
HP 4145B measures
currents, for bias line
(serving 400 strips) and
for guard ring.
10
0
20
40
60
80
100
Tensionebias
di polarizzazione
Reverse
voltage (V) inversa (V)
400-strip detector from ITC-IRST, Trento, Italy:
Ibias(60 V) = 18.9 nA

Istrip(60 V)  47.2 pA
Ibias(100 V) = 25.0 nA

Istrip(100 V)  62.5 pA
II. System
Detector test: C-V measurements
0.35
2
1/C (pF
-2
)
0.30
0.25
V0 = (23.16 ± 1.06) Volt
0.20
0.15
0.10
0.05
0.00
0
Full depletion voltage is
 constant across detector
40
60
80
100
28
26
V0(Volt)
Keithley 237 provides reverse bias,
HP 4284A injects sinusoidal
signal to measure C:
• V = 500 mV
• f = 100 kHz
20
Reversedi bias
voltage inversa
(V) (V)
Tensione
polarizzazione
24
22
20
18
2
4
6
Posizione
8
10
II. System
Strip-by-strip measurements
Measuring strip current, Istrip
70
65
•
•
Istrip (pA)
60
55
50
45
40
0
100
200
Numero strip
300
400
VB = 60 V
Contacts needed:
0. Backplane
1. Strip i
2. Strip (i+1)
3. Bias line
Measuring inter-strip resistance, Rstrip
500
Rstrip(G)
400
Rstrip 
300
200
100
0
100
200
Numero strip
300
400
2dV
I strip (dV  )  I strip (dV  )
II. System
The RX64 ASIC (1)
detector
test capacitor
Ct
RX64 - Cracow Univ. of Mining and Metallurgy design:
single channel layout
- charge-sensitive preamplifier
- shaper
- discriminator (2 discriminators in the latest version)
- pseudo-random counter (20-bit) [not shown]
II. System
The RX64 ASIC (2)
RX64 - Cracow U.M.M. design - (28006500 mm2)
- 64 front-end channels (preamplifier, shaper, 1 or 2 discriminators),
- 64 pseudo-random counters (20-bit),
- internal DACs: 1 or 2 for 8-bit threshold(s) setting and two 5-bit for
bias settings
- internal calibration circuit (square wave 1mV-30 mV),
- control logic and I/O circuit (interface to external bus).
II. System
RX64 ASIC testing
Probe card testing before
assembly on PCB becomes
convenient when production
yield is low:
• Power consumption test
• Test of the counter section
• Full test of the analogue
performance of the 64 channels,
using both HIGH and LOW
discriminator/counter sets
The test is performed using
the same power supplies, cables,
DAQ hardware and software
as for the final assembled system
II. System
System assembly
Manual wire bonding (detector - chip)
Automatic wire
bonding
(detector - pitch
adapter - chip)
III. Energy resolution and efficiency
Noise and gain evaluation method
x0
= 291.4 ± 0.446
sigma = 11.34 ± 0.51
15
150
100
200
10
50
150
5
100
0
0
240
260
280
300
320
Soglia (mV)
340
Conteggi
Conteggi
200
240
50
260
280
300
320
340
Soglia (mV)
0
240
260
280
300
320
340
Soglia (mV)
1
Obtain Counts vs.
Discriminator
Threshold
(threshold scan)
2
Smoothing of Counting
Curve

Error function Fit,
or …
3
Differential Spectrum

Gaussian Fit

extract mean and s
III. Energy resolution and efficiency
Threshold uniformity (128 channels)

Calibration pulse of
5300 electrons
(internal voltage step
applied to Ctest = 75 fF)

Mean threshold (from
gaussian fit) for 128
channels:
• Threshold spread
 %
• Small syst. difference
( 4%) between chips
III. Energy resolution and efficiency
Linearity vs. injected charge (1)
40
calib DAC = 4
calib DAC = 6
calib DAC = 8
30
calib DAC = 10
calib DAC = 12
calib DAC = 14
calib DAC = 16
calib DAC = 18
calib DAC = 20
20
10
0
100
200
300
400
500
600
Soglia (mV)
Differential spectra obtained with internal calibration:
each value of the Calibration DAC produces
on the test capacitor Ct (75 fF) a pulse of given charge
III. Energy resolution and efficiency
Linearity vs. injected charge (2)
600
a = 5.1 ± 1.8
b = 0.064966 ± 0.000486
m (mV)
500
400
300
200
2000
3000
4000
5000
6000
7000
8000
9000
Elettroni
in ingresso
Injected
charge
(electrons)
• the RX64 chip is strictly linear up to 5500 electrons input charge
(i.e. up to 20 keV X-ray energy)
• a straight line fit within linearity range gives offset (a) & gain (b)
III. Energy resolution and efficiency
Gain uniformity (128 channels)


Scan with 10 different amplitudes
(4-22 mV)
Circuit response reasonably linear
up to 8000 electrons (29 keV) for
Tpeak= 0.5 ms
<Gain> = 61.61.4 mV/el.
Small (3.5%) systematic
difference between chips
III. Energy resolution and efficiency
Rate capability of the RX64
Gain
Efficiency
30
100
100
(a)
(b)
25
80
Gain [mV/keV]
Efficiency [%]
20
60
40
Tp=1.0ms
Tp=0.7ms
Tp=0.5ms
20
0 01k
15
10
Tp=1.0ms
Tp=0.7ms
Tp=0.5ms
5
0
10k
Counting rate [1/s]
100k
10 k
1k
10k
Counting rate [1/s]
100 k
Counting rate [1/s]
10 k
100k
100 k
Counting rate [1/s]
Test with random signals, 8 keV
Three different shaping times T(peak): 1.0, 0.7, 0.5 ms
Sufficient performance for imaging applications up to 100 kHz / strip
III. Energy resolution and efficiency
Gain and Noise summary (I)
Module
T(peak)
Gain
ENC (el.)
Det. + 2 x RX64
Short
61.6
131
6 x RX64
Short
63.7
176
6 x RX64
Long
82.8
131
Fanout + 6 x RX64
Short
63.7
184
Fanout + 6 x RX64
Long
82.8
148
Detector with 128 equipped channels (2 x RX64):
• RMS value of noise = 8.1 mV  ENC = 131 electrons
• RMS of comparator offset distribution = 3.2 mV:
2 times smaller than noise (common threshold setting for all channels)
III. Energy resolution and efficiency
Calibration setups for X-ray detector
241Am
source with rotary
target holder
Cu-anode X-ray tube with
fluorescence targets
Board with detector
Pb collimator
Fluorescence
target
X-ray tube
III. Energy resolution and efficiency
Calibration results (single strip)
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
Counts
150
100
50
0
100
200
300
400
500
Threshold (mV)
Cu
E (K) = 8.0 KeV
Ge
E (K) = 9.9 keV
Mo
E (K) = 17.4 keV
E (K) = 19.6 keV
Rb
E (K) = 13.4 keV
Sn
E (K) = 25.3 keV
E (K) = 28.5 keV
Ag
E (K) = 22.1 keV
E (K) = 24.9 keV
III. Energy resolution and efficiency
Gain and Noise summary (II)
Sn
450
Retta calibrazione con la sorgente
Retta calibrazione con il tubo
m (mV)
400
Ag
Ag
350
Mo
300
Mo
250
Rb
Ge
200
Cu
150
8
10
12
14
16
18
20
22
24
Energia (keV)
6 x RX64 + fanout +
detector, T(peak) Long
GAIN
ENC30
ENC50
improved amplif.
setting
62.8 mV/el.
154 el.
179 el.
X-ray tube
63.7 mV/el.
151 el.
182 el.
internal calib.
64.6 mV/el.
141 el.
164 el.
241Am
source
III. Energy resolution and efficiency
Matching between channels
RX64 chip: 64 channels measured simultaneously with common threshold
(absolutely essential for practical applications)
III. Energy resolution and efficiency
The Double Threshold chip
ENC = 196 electrons
First RX64-DT chip measured: spectra obtained with moving
hardware window of 14 mV (5 LSB threshold DAC) by 1 LSB steps.
III. Energy resolution and efficiency
The conversion efficiency
Quasi-monochromatic
beam at 6 energies
(18-36 keV)
Fluorescence setup
with 4 targets
(15.7-25.0 keV)
Preliminary analysis
Detector was exposed to same beam flux in FRONT and EDGE mode
The (not well kown) absolute beam flux cancels in the ratio:
Counts(EDGE) / Counts(FRONT)
Experimental results compare well with GEANT 3.21 simulations
IV. Position resolution
The micro X-ray beam





X-ray tube (Mo anode) with
capillary output at MiTAC,
Antwerp University
Si(Li) detector to measure
fluorescence at 90 degrees
CCD camera with same focal
plane as X-ray beam
optional Mo/Zr filters to reduce
intensity and change energy
spectrum
X, Y, Z movements with 1 mm
precision
IV. Position resolution
Measuring the position resolution





X-ray tube (Mo anode)
operated at 15 kV and 40 kV
Silicon detector in front
configuration (Al protection
removed)
Mo or Zr filter
Horizontal scan (in/out of
beam focus) by 1 mm steps
to check focus
Vertical scan (across strips)
by 10 mm steps to measure
position resolution
IV. Position resolution
The MicroBeam




Vertical scan of a 25 mm
diameter Ni-Cr wire, tube
at 15 kV
Si(Li) detector counts
vs. wire position for Ni K
peak: observed RMS of
28.5 mm
Deduced beam RMS after
deconvolution of wire is
not much smaller
Beam RMS decreases with
increasing tube kV (while
beam halo becomes more
important)
IV. Position resolution
Beam profile in microstrip detector

The minimum size of the beam is maintained for a
depth of focus of 3-4 mm
IV. Position resolution
Position resolution results (1)
102.5
Si microstrip beam profile:
Centroid (strip units) vs.
Beam Position (mm)
y=99.711 + 0.0098132x
Hit centroid (strip)
102.0
101.5
101.0
100.5
100.0
0
50
100
150
200
Beam Position (mm)
Simulation of the Centroid
vs. Beam Position
250
300
IV. Position resolution
Position resolution results (2)
Maximum deviation from straight
line is ± 0.12 strips (12 mm)
Centroid - fit (strip units)
0.10
0.05
0.00
-0.05
Later, beam halo has been reduced
thanks to a 100 mm pinhole
-0.10
0
50
100
150
200
Beam position (mm)
Preliminary analysis
of latest data shows
considerable reduction
of maximum deviation
from straight line
250
300
V. Mammographic imaging
Dual Energy Mammography



Dual energy mammography allows to
remove the 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
to implement this we need:
• a dichromatic beam
• a position- and energy-sensitive detector
V. Mammographic imaging
The dichromatic beam (1)
W-anode X-ray tube operated at  50 kV
 Highly oriented pyrolithic graphite (HOPG)
mosaic crystal (Optigraph Ltd., Moscow) 
higher flux than monocrystals (also higher DE/E)

q-2q goniometer
Bragg diffraction,
first and second
harmonics
 energies E and
2E are obtained


V. Mammographic imaging
The dichromatic beam (2)
A. Tuffanelli et al., Dichromatic source for the application of dual-energy tissue
cancellation in mammography, SPIE Medical Imaging 2002 (MI 4682-21)
incident
spectra
at 3 energy
settings …
… spectra
after 3 cm
plexiglass
(measured
with HPGe
detector)
V. Mammographic imaging
Use of dichromatic beam
it’s possible to tune dichromatic beam energies to
breast thickness, to obtain equal statistics at both
energies  better signal-to-noise ratio
V. Mammographic imaging
The mammographic test (1)

A three-component phantom made of polyethylene,
PMMA and water [S. Fabbri et al., Phys. Med. Biol.
47 (2002) 1-13] was used to simulate the attenuation
coeff. m (cm-1) of the adipose, glandular and
cancerous tissues in the breast
 By measuring the logarithmic transmission of the
incident beam at two energies, with a projection
algorithm [Lehmann et al., Med. Phys. 8 (1981) 659]
the contrast between two chosen materials vanishes
E (keV)
20
40
m_fat
.456
m_gland
.802
m_canc
.844
.215
.273
.281
PE
PMMA
.410 .680
.225 .280
water
.810
.270
V. Mammographic imaging
The mammographic test (2)

Low energy and high energy images were acquired
separately (no double threshold ASIC yet) with the
384-channel Si detector, covering a 38.4 mm wide
slice of the phantom
 After correction for flat-field and bad channels, the
dual-energy algorithm was applied to the logarithmic
images at the two energies, changing the projection
angle to find the contrast cancellation angles for pairs
of materials
For more details, see poster by C. Ceballos on Tuesday 28/10
V. Mammographic imaging
Mammography test results (1)
The contrast cancellation angles for each pair of materials were
obtained, both from experiment and from MCNP simulation
V. Mammographic imaging
Mammography test results (2)
The PE pattern alone is visible in
measured data at projection angle
36.5 ° (PMMA-water cancellation)
1=detector
2=PMMA
3=water
4=PE
Simulations are in fair agreement with data for PMMA-water cancellation angles
at 2 out of the 3 energy pairs; we are investigating problems due to low statistics at
high energies and to uncertainty on PE sample composition
VI. Angiographic imaging
The angiographic test setup
X-ray tube with dual energy output
Phantom
Detector box with 2 collimators
1.
X-ray tube with dual-energy output
-
each measurement  1.4 • 10 6
photons / mm2 (in 2+2 seconds)
2.
Phantom made of PMMA + Al
3.
Detector box with two collimators
Phantom with 4 iodine-filled
cavities of diameter 1 or 2 mm
VI. Angiographic imaging
Procedure for image analysis (I)
1. Measure Flat
field at both
energies
Flatfield normal.
Flatfield normal.
1.1
1.0
0.9
0.8
E = 31.5 keV
0.7
0.6
1.1
1.0
0.9
0.8
E = 35.5 keV
0.7
0.6
0
100
200
300
0
100
200
canali
300
canali
2. Normalize counts between the two energies
<N(31.5 keV)> / <N(35.5 keV)> = 2.432
1.0
Trasmissione teorica
Trasmissione teorica
3. Compute transmission in PMMA + Al
E = 31.5 keV
0.8
0.6
0.4
0.2
0.0
0
100
200
300
pixels
400
500
1.0
E = 35.5 keV
0.8
0.6
0.4
0.2
0.0
0
100
200
300
pixels
400
500
VI. Angiographic imaging
3
Conteggi (x10 )
15
10
5
0
6
15
5
pixels
16
14
12
10
8
6
4
2
pixels
3
Conteggi ( x10 )
Procedure for image analysis (II)
4
3
2
10
5
1
0
0
100
200
pixels
300
0
15000
100
200
pixels
300
6000
Conteggi
Conteggi
12000
9000
E = 31.5 keV
6000
4000
E = 35.5 keV
2000
3000
0
0
log conteggi
-0.6
300
15
pixels
log conteggi
-0.4
200
pixels
10
5
-0.8
0
-0.2
100
200
pixels
300
200
pixels
300
lnN35.5   lnN31.5 
-0.4
-0.6
-0.8
0
100
logarithmic subtraction
0.0
0.0
-0.2
100
0
100
200
pixels
300
VI. Angiographic imaging
Images vs. iodine concentration
Cavity diameter = 1mm
92.5 mg / ml
23.1 mg / ml
10
5
-0.8
15
0.1
0.0
-0.1
-0.2
10
5
-0.3
0
100
200
300
15
5
100
200
300
0
pixels
-0.4
-0.6
0.0
-0.1
-0.2
-0.3
-0.8
200
pixels
300
200
300
0.15
0.10
log conteggi
log conteggi
-0.2
100
pixels
0.1
0.0
100
10
0
0
pixels
0
0.15
0.10
0.05
0.00
-0.05
-0.10
-0.15
0
0
log conteggi
log conteggi
-0.6
15
pixels
-0.4
log conteggi
-0.2
pixels
log conteggi
0.2
0.0
pixels
370 mg / ml
0.05
0.00
-0.05
-0.10
-0.15
0
100
200
pixels
300
0
100
200
300
pixels
MCNP simulations: see C. Ceballos et al., AIP Conf. Proc. 682, 2003, pp. 185-191
Signal-to-Noise ratio
VI. Angiographic imaging
SNR defined as ratio between CONTRAST (Cs) and fluctuations in
a given area (here 1x1 pixel) of the image (Cn): SNR = Cs/Cn
d = 1 mm
50
cav ità
cav ità
cav ità
cav ità
teor.
teor.
teor.
teor.
cav ità
cav ità
cav ità
cav ità
4
3
2
1
30
20
10
0
100
200
Concentrazione (mg/ml)
80
SNR
0
100
SNR
SNR
SNR
40
4
3
2
1
300
cav ità
cav ità
cav ità
cav ità
4
3
2
1
teor.
400
teor.
teor.
teor.
cav ità
cav ità
cav ità
cav ità
4
3
2
1
d = 2 mm
60
40
20
0
0
100
200
300
Concentrazione(mg/ml)
(mg/ml)
Concentration
400
VII. Conclusion
Summary

A relatively simple linear X-ray detector for
scanning mode radiography was developed
 Energy resolution (1.3 keV FWHM at 22 keV) is
well suited for the available quasi-monochromatic
beams
 Efficiency in edge mode (10 mm Si) is sufficient
for D.E. mammography and angiography at
iodine K-edge
 Imaging results with phantoms show interesting
SNR values, detailed simulations using MCNP
and GEANT 3 were developed
VII. Conclusion
Outlook





Exploit double threshold ASIC for D.E.
Mammography (ASIC mass tests ongoing)
Build larger detectors for full-size imaging
Measure DQE and MTF with microbeam
Angiography: implement synchronization
with ECG
Angiography: explore the Gadolinium
option at 50 KeV
VII. Conclusion
Thanks to ...






The organizers of NURT 2003 for this nice
opportunity to present our results
The Italian Ministry for Education, University and
Research (MIUR)
The Polish State Committee for Scientific
Research
INFN Torino for allowing access to technical staff
and bonding facilities
ICTP Trieste for travel and subsistence support to
Cuban researchers
The European Community for travel and
subsistence support for students under the ALFA II
programme (contract AML/B7-311/97/0666/II-0042)