A silicon microstrip system with the RX64DTH ASIC for dual energy mammography

1
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

L. Ramello Università Piemonte Orientale and
INFN, Alessandria, Italy
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.
3
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

4
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

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 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 ???
6
Alvarez-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
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 100-200 mA s
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
9
Experimental 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

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
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
12
Image 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

13
Image processing (2)
1 PMMA 2water 3PE 4(waterPE)
16 32 keV
18 36 keV
14
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
1detector
2PMMA 3water 4PE
15
Experiment vs. Simulation (1)
RX64DTH 16 32 keV
16
Experiment vs. Simulation (2)
17
Results (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
18
Results (2) SNR summary
Energy Canceled Contrast SNR 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
PMMA-water PE 8.9 6.1
18-36 PE-water PMMA 1.9 2.2
PE-PMMA water 2.3 3.4
PMMA-water PE 2.5 2.2
20-40 PE-water PMMA 0.7 0.8
PE-PMMA water 0.9 1.0
Previous version of ASIC, exposure with about
2x more incident photons
19
Results (3) Projected images
RX64DTH 16 32 keV
simulation 16 32 keV
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

21
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)

22
Efficiency in edge-on mode
present detector
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
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