1. L. Ramello MIC-RTSD, Rome, 20 October 2004
A silicon microstrip system with the RX64DTH ASIC for dual energy mammography
Introduction
Alvarez-Macovski algorithm
Experimental setup
Image processing
Results: SNR and projected images
Conclusion and outlook
NSS/MIC/RTSD Rome 2004 conference – Session JRM1 (joint RTSD – MIC session) – time available: 12 minutes + 3 for discussion
L. Ramello – Università Piemonte Orientale
and INFN, Alessandria, Italy
L. Ramello MIC-RTSD, Rome, 20 October 2004

2. The Collaboration
L. Ramello1, C. Avila2, D. Bollini3, A.E. Cabal Rodriguez4,
C. Ceballos Sanchez4, W. Dabrowski5, A. Diaz Garcia4,
M. Gambaccini6, P. Giubellino7, P. Grybos5,
J. Lopez Gaitan2, A. Marzari-Chiesa8, L.M. Montano9,
F. Prino7, J.C. Sanabria2, A. Sarnelli6, K. Swientek5,
A. Taibi6, A. Tuffanelli6, P. Van Espen10, P. Wiacek5
1 University of Eastern Piedmont and INFN, Alessandria, Italy;
2 Univ. de los Andes, Colombia;
3 University and INFN, Bologna, Italy; 4 CEADEN, Havana, Cuba;
5 AGH Univ. of Science and Technology, Cracow, Poland;
6 University and INFN, Ferrara, Italy;
7 INFN, Torino, Italy; 8 University of Torino, Torino, Italy;
9 CINVESTAV, Mexico City, Mexico;
10 University of Antwerp, Antwerp, Belgium.
L. Ramello MIC-RTSD, Rome, 20 October 2004

3. L. Ramello MIC-RTSD, Rome, 20 October 2004
Introduction (1)
Digital mammography has well known advantages over conventional screen-film mammography
Dual energy mammography [Lehmann, Alvarez & Macovski, Med. Phys. 8 (1981) 659] 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
L. Ramello MIC-RTSD, Rome, 20 October 2004

4. L. Ramello MIC-RTSD, Rome, 20 October 2004
Introduction (2)
Quasi-monochromatic beams with ordinary X-ray tube and Highly Oriented Pyrolitic Graphite crystals (instead of truly monochromatic synchrotron radiation)
Linear array of silicon microstrips
Binary readout but with two discriminators (and counters) per channel
Straightforward digital output: only integrated counts for each pixel are readout
Scanning is necessary to build 2D image
L. Ramello MIC-RTSD, Rome, 20 October 2004

5. The Alvarez-Lehmann-Macovski idea
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 :
The logarithmic attenuation M = μξtξ of the material of thickness tξ
is measured at two different energies: low (El) and high (Eh):
A1 and A2 represent the thicknesses of the two base materials which
would provide the same X-ray attenuation as material ξ.
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 
L. Ramello MIC-RTSD, Rome, 20 October 2004

6. Alvarez-Macovski cont’dξ ψ
If a monochromatic beam of intensity I0 goes through material ξ which is partly replaced by another material ψ …  M1 R 1 C 
90° 
… 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 : M2
C= A1cos+A2sin  A2 2
… it is possible to cancel the contrast between materials ξ and ψ: both M1 and M2 will project to the same vector  A1
L. Ramello MIC-RTSD, Rome, 20 October 2004

7. Experimental setup (1): beam
Beam cross section:
8 mm x 68 mm
only 0.3 mm x
40 mm used in
this experiment
W anode tube
operated at 49 kV
mA s
L. Ramello MIC-RTSD, Rome, 20 October 2004

8. Experimental setup (2): phantom
Three components: polyethylene (PE), PMMA and water to simulate the attenuation coeff. m (cm-1) of the adipose, glandular and cancerous tissues in the breast
 S. Fabbri et al., Phys. Med. Biol. 47 (2002) 1-13 E
m_fat
m_gland
m_canc 20
.456
.802
.844 40
.215
.273
.281 E
μ_PE
μ_PMMA
μ_water 20
.410
.680
.810 40
.225
.280
.270
L. Ramello MIC-RTSD, Rome, 20 October 2004

9. Experimental setup (3): detector
guard ring
first strip (AC contact)
DC contact (to p+ implant)
bias line
400 strips (only 384 equipped with ASICs) of 100 μm pitch, 10 mm length
AC coupling: Bias Line with FOXFET biasing
300 μm Si thickness
765 μm inactive Si in edge-on orientation
Designed and fabricated by ITC-IRST, Trento, Italy
L. Ramello MIC-RTSD, Rome, 20 October 2004

10. Experimental setup (4): ASIC
RX64DTH ASIC with 64 channels
Two discriminators + two 20-bit counters
per channel, fully digital input/output
Energy resolution of 0.8 keV RMS
For more details: see talk by P. Grybos, NSS-N8-5 Monday 17h
L. Ramello MIC-RTSD, Rome, 20 October 2004

11. Experimental setup (5): system
detector
pitch adapter
ASIC
Two operation modes:
Threshold scan to optimize low and high threshold settings for each ASIC
Imaging mode: repeated irradiation and mechanical scanning step to build 2D image
For angiography: see poster by G. Baldazzi, M5-438 Thu. 11h
L. Ramello MIC-RTSD, Rome, 20 October 2004

12. L. Ramello MIC-RTSD, Rome, 20 October 2004
Image processing (1)
RX64DTH
18 keV
raw
18 keV
corrected
36 keV
corrected
correct for:
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
RX64DTH images: see immagini_16-32-raw.pxp and immagini_18-36-raw.pxp
L. Ramello MIC-RTSD, Rome, 20 October 2004

13. L. Ramello MIC-RTSD, Rome, 20 October 2004
Image processing (2)
1= PMMA 2=water
3=PE 4=(water+PE)
16 – 32 keV
18 – 36 keV
RX64DTH KeV, keV
L. Ramello MIC-RTSD, Rome, 20 October 2004

14. L. Ramello MIC-RTSD, Rome, 20 October 2004
Simulation with MCNP
MCNP-4C simulation with
ENDF/B-VI library
Photons and electrons are tracked
through the phantom materials
and the detector, including the
inactive region in front of the strips
Energy deposition in each strip is
recorded, an histogram of counts
vs. strip number is filled
1=detector
2=PMMA
3=water
4=PE
L. Ramello MIC-RTSD, Rome, 20 October 2004

15. Experiment vs. Simulation (1)
RX64DTH 16 – 32 keV
simulation 16 – 32 keV
L. Ramello MIC-RTSD, Rome, 20 October 2004

16. Experiment vs. Simulation (2)
L. Ramello MIC-RTSD, Rome, 20 October 2004

17. Results (1): SNR vs. proj. angle
Theoretical cancellation angles:
PMMA-water °
PE-water °
PMMA-PE °
MCNP simulation
RX64DTH 16 – 32 keV
Cancellation angle for
a pair given by SNR=0
L. Ramello MIC-RTSD, Rome, 20 October 2004

18. Results (2): SNR summary
Energy
Canceled
Contrast
SNR
(keV)
materials
material
RX64*
RX64DTH
PMMA-water PE
4.3
10.1
16-32
PE-water
PMMA
1.6
3.4
PE-PMMA
water
2.7
5.0
8.9
6.1
18-36
1.9
2.2
2.3
2.5
~2.2
20-40
0.7
~0.8
0.9
~1.0
* Previous version of ASIC, exposure with about 2x more incident photons
L. Ramello MIC-RTSD, Rome, 20 October 2004

19. Results (3): Projected images
RX64DTH 16 – 32 keV
simulation 16 – 32 keV
PMMA-water cancellation
PMMA-PE cancellation
L. Ramello MIC-RTSD, Rome, 20 October 2004

20. 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 three-material phantom
We have demonstrated the feasibility of contrast cancellation between two materials, enhancing the visibility of small features in the third one
OUTLOOK:
Increase photon statistics at high energy, optimize exposure conditions
Tests with a more realistic mammographic phantom
L. Ramello MIC-RTSD, Rome, 20 October 2004

21. L. Ramello MIC-RTSD, Rome, 20 October 2004
Thanks to …
The organizers of NSS-MIC-RTSD 2004 for this nice opportunity to present our results
The Italian Ministry for Education, University and Research (MIUR)
The Polish State Committee for Scientific Research (Grant N. 3T11B01427)
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)
L. Ramello MIC-RTSD, Rome, 20 October 2004

22. Efficiency in edge-on mode
present detector
L. Ramello MIC-RTSD, Rome, 20 October 2004

23. Energy resolution (RX64 ASIC)Cu
E (K) = 8.0 KeV Mo
E (K) = 17.4 keV
E (K) = 19.6 keV Sn
E (K) = 25.3 keV
E (K) = 28.5 keV Ge
E (K) = 9.9 keV Ag
E (K) = 22.1 keV
E (K) = 24.9 keV Rb
E (Ka) = 13.4 keV
For more details: see talk by P. Grybos, NSS-N8 Monday 18/10
L. Ramello MIC-RTSD, Rome, 20 October 2004

24. More on the dichromatic beam …
A. Tuffanelli et al., Dichromatic source for the application of dual-energy tissue cancellation in mammography, SPIE Medical Imaging 2002 (MI )
incident
spectra
at 3 energy
settings …
… spectra after 3 cm plexiglass
(measured with HPGe detector)
Measured fluxes (photons / mm^2 / mA / s) for INCIDENT spectra:
<E> = 17.8 keV : 1.7x10^4; <E> = 36.1 keV : 2.4x10^3
L. Ramello MIC-RTSD, Rome, 20 October 2004

25. L. Ramello MIC-RTSD, Rome, 20 October 2004
it’s possible to tune dichromatic beam energies to breast thickness, to obtain equal statistics at both energies  better signal-to-noise ratio
L. Ramello MIC-RTSD, Rome, 20 October 2004