results about imaging with silicon strips for angiography and mammography i. introduction ii. the...
TRANSCRIPT
Results about imaging with silicon strips for Angiography and Mammography
I. Introduction II. The system: microstrip detectors,
RX64 ASICsIII. Energy resolution and efficiencyIV. Spatial resolutionV. Imaging results - mammographyVI. Imaging results - angiographyVII. Summary and outlook
Luciano Ramello – Univ. Piemonte Orientale and INFN, Alessandria VII MSMP, March 24-26, 2003
G. Baldazzi1, D. Bollini1, A.E. Cabal Rodriguez2, W. Dabrowski3, A. Diaz Garcia2, M. Gambaccini4, P. Giubellino5, M. Gombia1, P. Grybos3, M. Idzik3,5, 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, R. Wheadon5, 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
I. Introduction
Introduction (1) 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 (2) Subtraction imaging: removes background structures Dual energy technique: isolates materials characterized
by different energy dependence of the linear attenuation coefficient Alvarez and Macovski 1976
Quasi-monochromatic beams: implement dual energy techniques in a small-scale installation
[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)
Another application: dual-energy mammography (18+36 keV)
Silicon efficiency vs. X-ray energyI. Introduction
Front configuration • 70 m Al shield
(might be reduced)• 300 m active Si
Edge configuration• 765 m insensitive
silicon • 10 or 20 mm
active Si
Photoelectric conversion in the active volume
cross-sections from XCOM data base of NIST
GaAs: a better alternative ?I. Introduction
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
guardring bias line first strip (AC contact)
DC contact (to p+ implant)
I-V measurements
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
10-10
2
4
6
10-9
2
4
6
10-8
2
Cor
rent
e di
fuga
(A)
100806040200Tensione di polarizzazione inversa (V)
Corrente di guard ring Corrente di bias line
Temperatura = 25.5 °C
Keithley 237 provides reverse bias,
HP 4145B measures currents.
Reverse bias voltage (V)
Lea
kag
e cu
rren
t (A
)
II. System
C-V measurements
Keithley 237 provides reverse bias,HP 4284A injects sinusoidal signal to measure C:• V = 500 mVV = 500 mV• f = 100 kHzf = 100 kHz
28
26
24
22
20
18
V0(
Vol
t)
108642Posizione
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
1/C
2 (p
F-2
)
100806040200Tensione di polarizzazione inversa (V)
V0 = (23.16 ± 1.06) Volt
Full depletion voltage is constant across detector
Reverse bias voltage (V)
II. System
Strip-by-strip measurements
• VVBB = 60 V = 60 V• Contacts needed:Contacts needed: 0. Backplane0. Backplane
1.1. Strip Strip i2.2. Strip Strip (i+1)3.3. Bias lineBias line
Measuring strip current, IMeasuring strip current, Istripstrip
Measuring inter-strip resistance, RMeasuring inter-strip resistance, Rstripstrip
70
65
60
55
50
45
40
I strip
(pA
)
4003002001000Numero strip
500
400
300
200
100
Rst
rip(G
)
4003002001000Numero strip
)()(
2
dVIdVI
dVR
stripstripstrip
II. System
The RX64 ASIC
RX64 - Krakow UMM design - (28006500 m2) consists of:
- 64 front-end channels (preamplifier, shaper, discriminator),
- 64 pseudo-random counters (20-bit),
- internal DACs: one 8-bit threshold setting and and two 5-bit for bias,
- internal calibration circuit (square wave 1mV-30 mV),
- control logic,
- I/O circuit (interface to external bus).
II. System
System assemblyManual wire bonding (detector - chip)
II. System
Automatic wire bonding (detector - pitch adapter - chip)
Noise and gain evaluation method200
150
100
50
0
Co
nte
gg
i
340320300280260240Soglia (mV)
200
150
100
50
0
Con
tegg
i
340320300280260240Soglia (mV)
15
10
5
0
340320300280260240Soglia (mV)
x0 = 291.4 ± 0.446sigma = 11.34 ± 0.51
1Obtain Counts vs.
Discriminator Threshold
(threshold scan)
2Smoothing of Counting
Curve
Error function Fit, or …
3Differential Spectrum
Gaussian Fit
extract mean and
III. Energy resolution and efficiency
Threshold uniformity (128 channels)
Calibration pulse of 5300 electrons (internal voltage step applied to C
test = 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
30
20
10
0
600500400300200100Soglia (mV)
calib DAC = 4 calib DAC = 10 calib DAC = 16 calib DAC = 6 calib DAC = 12 calib DAC = 18 calib DAC = 8 calib DAC = 14 calib DAC = 20
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
500
400
300
200
(m
V)
90008000700060005000400030002000Elettroni in ingresso
a = 5.1 ± 1.8b = 0.064966 ± 0.000486
• the RX64 chip is strictly linear up to the RX64 chip is strictly linear up to 55005500 electrons input charge electrons input charge (i.e. up to (i.e. up to 20 keV X-ray energy20 keV X-ray energy))• aa straight line fit straight line fit within linearity range gives offset (a) & gain (b)within linearity range gives offset (a) & gain (b)
Injected charge (electrons)
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 T
peak= 0.5 s
<Gain> = 61.6 V/el.
Small (3.5%) systematic difference between chips
III. Energy resolution and efficiency
Rate capability of the RX64
1k 10k 100k0
20
40
60
80
100(a)
Counting rate [1/s]
Effi
cien
cy [%
]
Tp=1.0s Tp=0.7s Tp=0.5s
1k 10k 100k0
5
10
15
20
25
30(b)
Tp=1.0s Tp=0.7s Tp=0.5s
Counting rate [1/s]
Gai
n [m
V/k
eV]
Efficiency Gain
Counting rate [1/s]
Test with random signals, 8 keV
Three different shaping times T(peak): 1.0, 0.7, 0.5 s
Sufficient performance for imaging applications up to 100 kHz / strip
Counting rate [1/s]
100
0100 k 100 k10 k10 k
III. Energy resolution and efficiency
Gain and Noise summary (I)
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)
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
III. Energy resolution and efficiency
Calibration setups for X-ray detectorCu-anode X-ray tube with Cu-anode X-ray tube with
fluorescence targetsfluorescence targets
241241Am source with rotary Am source with rotary target holdertarget holder
III. Energy resolution and efficiency
Pb collimator
Fluorescencetarget X-ray tube
Board with detector
Calibration results (single strip)
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
CuE (K) = 8.0 KeV
GeE (K) = 9.9 keV
RbE (K) = 13.4 keV
MoE (K) = 17.4 keVE (K) = 19.6 keV
AgE (K) = 22.1 keVE (K) = 24.9 keV
SnE (K) = 25.3 keVE (K) = 28.5 keV
III. Energy resolution and efficiency
Gain and Noise summary (II)450
400
350
300
250
200
150
(m
V)
24222018161412108Energia (keV)
CuGe
Mo
Ag
Sn
Rb
Mo
Ag
Retta calibrazione con la sorgente Retta calibrazione con il tubo
6 x RX64 + fanout + detector, T(peak) Long
GAIN ENC30
improved amplif. setting
ENC50
241Am source 62.8 V/el. 154 el. 179 el.
X-ray tube 63.7 V/el. 151 el. 182 el.
internal calib. 64.6 V/el. 141 el. 164 el.
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
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
ENC = 196 electrons
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 m 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 m steps to measure position resolution
IV. Position resolution
Beam dimension
Vertical scan of a 25 m dia. Ni-Cr wire
Si(Li) detector counts at Ni K peak: observed raw RMS of 38 ± 5 m
Deduced beam RMS of 28 m (PRELIMINARY)
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 of Si detector
0.10
0.05
0.00
-0.05
-0.10
Ce
ntr
oid
- f
it (s
trip
un
its)
300250200150100500
Beam position (m)
102.5
102.0
101.5
101.0
100.5
100.0
Hit
cen
tro
id (
stri
p)
300250200150100500
Beam Position (m)
y=99.711 + 0.0098132x Si microstrip beam profile:Centroid (strip units) vs.Beam Position (m)
Maximum deviation from straightline is ± 0.12 strips (12 m)
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 E/E)
V. Mammographic imaging
-2 goniometer Bragg diffraction,first and second harmonics energies E and 2E are obtained
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)
V. Mammographic imaging
incidentspectraat 3 energysettings …
… spectra after 3 cm plexiglass
(measured with HPGe detector)
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. (cm-1) of the adipose, glandular and cancerous tissues in the breast
V. Mammographic imaging
E (keV) fat gland. canc. PE PMMA water
20 .456 .802 .844 .410 .680 .810
40 .215 .273 .281 .225 .280 .270
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
The mammographic test (2) Low energy and high energy images were
acquired separately (no double threshold 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
V. Mammographic imaging
Mammography test results (1)Determination of contrast cancellation angle: SNR between PMMA and water is zero for = 33°, where
PE has a SNR of 16.2
V. Mammographic imaging
Mammographic test results (2)V. Mammographic imaging
= 33° = 42.5°Low E High E
6 mm dia.cylinders
PE + water
PE
PMMAbase material
The angiographic test setup
Phantom with 4 iodine-filled Phantom with 4 iodine-filled cavities of diameter cavities of diameter 1 1 or or 2 mm2 mm
1.1. X-ray tube with dual-energy outputX-ray tube with dual-energy output
- each measurement each measurement 1.4 • 10 6 photons photons / mm/ mm2 2 (in 2+2 seconds)
2.2. Phantom made of PMMA + AlPhantom made of PMMA + Al
3.3. Detector box with two collimatorsDetector box with two collimators
X-ray tube with dual energy output
Detector box with 2 collimators
Phantom
VI. Angiographic imaging
Procedure for image analysis (I)
1. 1. MeasureMeasure Flat Flat fieldfield at both at both energiesenergies
1.1
1.0
0.9
0.8
0.7
0.6F
latfi
eld
norm
al.
3002001000canali
E = 31.5 keV
1.1
1.0
0.9
0.8
0.7
0.6
Fla
tfiel
d no
rmal
.
3002001000canali
E = 35.5 keV
2. 2. Normalize counts between the two energiesNormalize counts between the two energies
3. 3. Compute transmission in PMMA + AlCompute transmission in PMMA + Al1.0
0.8
0.6
0.4
0.2
0.0Tra
smis
sio
ne
te
oric
a
5004003002001000pixels
E = 31.5 keV
1.0
0.8
0.6
0.4
0.2
0.0Tra
smis
sio
ne
te
oric
a
5004003002001000pixels
E = 35.5 keV
<N(31.5 keV)> / <N(35.5 keV) = 2.432
VI. Angiographic imaging
Procedure for image analysis (II)15
10
5
0
pixe
ls
3002001000pixels
161412108642
Con
tegg
i ( x
103 )
15000
12000
9000
6000
3000
Con
tegg
i
3002001000pixels
15
10
5
0
pixe
ls
3002001000pixels
6
5
4
3
2
1
Con
tegg
i (x1
03 )
6000
4000
2000
Co
nte
gg
i
3002001000pixels
15
10
5
0
pixe
ls
3002001000pixels
-0.8
-0.6
-0.4
-0.2
0.0
log
con
tegg
i
-0.8
-0.6
-0.4
-0.2
0.0
log
co
nte
gg
i
3002001000pixels
E = 31.5 keV E = 35.5 keV
logarithmic subtraction
5.315.35 lnln NN
VI. Angiographic imaging
Images vs. iodine concentration
-0.8
-0.6
-0.4
-0.2
0.0
log
co
nte
gg
i
3002001000pixels
-0.3
-0.2
-0.1
0.0
0.1
log
co
nte
gg
i
3002001000pixels
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
log
cont
eggi
3002001000pixels
Cavity diameter = 1mm
15
10
5
0
pix
els
3002001000pixels
-0.8
-0.6
-0.4
-0.2
0.0
log
co
nte
gg
i
15
10
5
0
pixe
ls3002001000
pixels
-0.3
-0.2
-0.1
0.0
0.1
0.2
log
cont
eggi 15
10
5
0
pixe
ls
3002001000pixels
-0.15
-0.10
-0.05
0.00
0.05
0.100.15
log
cont
eggi
370 mg / ml 92.5 mg / ml 23.1 mg / ml
VI. Angiographic imaging
MCNP simulations: see poster by A. Cabal, C. Ceballos et al.
Signal-to-Noise ratioSNR SNR defined as ratio betweendefined as ratio between CONTRAST (C CONTRAST (Css) and fluctuations in ) and fluctuations in
a given area (here 1x1 pixel) of the image (a given area (here 1x1 pixel) of the image (CCnn): ): SNR = Cs/Cn
50
40
30
20
10
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
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
d = 1 mm
d = 2 mm
SNR
SNR
Concentration (mg/ml)
VI. Angiographic imaging
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
VII. Conclusion
Outlook
Double threshold ASIC (produced, first tests OK) for D.E. mammography
Larger detectors for full-size imaging Measure DQE and MTF with microbeam Angiography: synchronization with ECG Angiography: explore the Gadolinium
option Extensive MC simulations of the different
setups under way