Title: A silicon microstrip system with the RX64DTH ASIC for dual energy mammography
1A 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
L. Ramello Università Piemonte Orientale and
INFN, Alessandria, Italy
2The 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.
3Introduction (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
4Introduction (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
5The 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 µ?t? of the
material of thickness t? 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 ?.
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 ???
6Alvarez-Macovski contd
If a monochromatic beam of intensity I0 goes
through material ? which is partly replaced by
another material ?
?
then the vertexes of log. attenuation vectors
M2 (material ?) and M1 (mat. ? ?) lie on a line
R which is defined only by the properties of
materials a, ß, ? and ?.
Projecting along direction C, orthogonal to R,
with the contrast cancellation angle ?
M2
A2
?2
it is possible to cancel the contrast between
materials ? and ? both M1 and M2 will project to
the same vector
?
A1
7Experimental 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 100-200 mA s
8Experimental 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
9Experimental setup (3) detector
- 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
10Experimental 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
11Experimental 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
12Image processing (1)
18 keV raw
18 keV corrected
36 keV corrected
RX64DTH
- 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
13Image processing (2)
1 PMMA 2water 3PE 4(waterPE)
16 32 keV
18 36 keV
14Simulation 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
1detector
2PMMA 3water 4PE
15Experiment vs. Simulation (1)
RX64DTH 16 32 keV
16Experiment vs. Simulation (2)
17Results (1) SNR vs. proj. angle
Theoretical cancellation angles PMMA-water
36.5 PE-water 40.5 PMMA-PE
45
RX64DTH 16 32 keV
Cancellation angle for a pair given by SNR0
18Results (2) SNR summary
Previous version of ASIC, exposure with about
2x more incident photons
19Results (3) Projected images
RX64DTH 16 32 keV
simulation 16 32 keV
20Conclusion 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
21Thanks 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)
22Efficiency in edge-on mode
present detector
23Energy 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
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 4682-21)
incident spectra at 3 energy settings
spectra after 3 cm plexiglass (measured with
HPGe detector)
25- its possible to tune dichromatic beam
energies to breast thickness, to obtain equal
statistics at both energies ? better
signal-to-noise ratio