1. 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
NSS/MIC/RTSD Rome 2004 conference – Session JRM1 (joint RTSD – MIC session) – time available: 12 minutes + 3 for discussion

2. 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;

3. 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

4. Introduction: why dual energy ?
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

5. Example 1: dual energy mammography
E  keV:
Signal from cancer tissue deteriorated by the adipose tissue signal
E  keV
Cancer tissue not visible, image allows to map glandular and adipose tissues

6. 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)

7. 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, availability in hospitals
Linear array of silicon microstrips + 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

8. 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
Collimator
W anode tube

9. Experimental setup: beam (2)
Bragg Diffraction on Highly Oriented Pyrolitic Grafite Crystal
Two spatially separated beams with different energies
 E-DE and E+DE obtained in 2 separate beams
Double slit collimator
W anode tube

10. More on the dichromatic beam
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

11. Experimental seup: Single Photon Counting System
data, control
Silicon strip detector
Integrated circuit
100 m
current pulses
X-rays PC
N. I. I/O cards PCI-DIO-96
and DAQCard-DIO-24
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

12. 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

13. Detector efficiency Efficiency calculation Front geometry
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 mm of Al light shield
Edge-on geometry
Strip parallel to the beam
765 mm of inactive Si
Better efficiency for E > 18 keV

14. 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) 1 2 3 4 5
Detector

15. 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:
6 x Single threshold RX64
6 x Dual threshold RX64

16. System calibration setup in Alessandria
Fluorescence target
(Cu, Ge, Mo, Nb, Zr, Ag, Sn)
Detector in Front config.
Cu anode X-ray tube
→ X-ray energies = characteristic lines of target material

17. System calibration
Cu K
Mo K
Ge K
Rb Ka
Ag K
Sn K
Ag K
Mo K
Sn K
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 ms 64
≈170 el.
≈0.61 keV
6 x RX64DTH
0.8 ms 47
≈ 200 el.
≈0.72 keV

18. 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

19. Angiography setup
X-ray tube with dual energy output
Phantom
Detector box with 2 collimators
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)
Detector box with two detectors aligned with two collimators
Step wedge phantom made of PMMA + Al with 4 iodine solution filled cavities of 1 or 2 mm diameter

20. Angiographic test results (I)
E = 35.5 keV
E = 31.5 keV
logarithmic subtraction
Phantom structure not visible in final image

21. Angiographic test results (II)
Conc = 370 mg / ml
Conc = 92.5 mg / ml
Conc = 23.1 mg / ml
Possible decrease of iodine concentration keeping the same rad. dose

22. Results with a second phantom
Dual Energy Angiography
Digital Subtraction
Angiography
Iodine conc. = 95 mg/ml
smaller cavity (=0.4 mm) visible in DEA and not in DSA

23. 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 :
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.

24. 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  I0 I1 I2 ξ 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 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

25. Mammographic 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 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
 S. Fabbri et al., Phys. Med. Biol. 47 (2002) 1-13

26. Image processing (1) Measured (raw) Correct for: 16 keV 32 keV
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
16 keV
32 keV
HE and LE images
Low thr.
High thr.
RX64DTH images: see immagini_16-32-raw.pxp and immagini_18-36-raw.pxp

27. Image processing (2) 16 – 32 keV 18 – 36 keV Low statistics due to:
RX64DTH KeV, keV
1= PMMA 2=water
3=PE 4=(water+PE)
Low statistics due to:
2nd order harmonic
dectecting efficiency

28. 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
1=detector
2=PMMA
3=water
4=PE
Side View

29. Experiment vs. Simulation (1)
RX64DTH 16 – 32 keV
simulation 16 – 32 keV

30. Experiment vs. Simulation (1)

31. Results (1): SNR vs. proj. angle
RX64DTH 16 – 32 keV
Theoretical cancellation angles:
PMMA-water °
PE-water °
PMMA-PE °
MCNP simulation
Cancellation angle for
a pair given by SNR=0

32. Results (2): SNR summary
Energy
Canceled
Contrast
SNR
(keV)
materials
material
RX64*
RX64DTH
PMMA-water PE
8.11
9.63
16-32
PE-water
PMMA
2.53
3.19
PE-PMMA
water
3.96
4.72
7.43
5.14
18-36
2.70
2.10
3.85
3.13
2.55
3.27
20-40
0.67
1.07
0.89
1.58
* Previous version of ASIC, exposure with about 2x more incident photons

33. Results (3): Projected images
RX64DTH 16 – 32 keV
simulation 16 – 32 keV
PMMA-water cancellation
PMMA-PE cancellation

34. 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
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