Published on March 24, 2008
A silicon microstrip system with the RX64DTH ASIC for dual energy mammography: 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 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. The Collaboration Introduction (1): 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 Introduction (2): 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 The Alvarez-Lehmann-Macovski idea: 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 Alvarez-Macovski cont’d: Alvarez-Macovski cont’d M2 2 If a monochromatic beam of intensity I0 goes through material ξ which is partly replaced by another material ψ … … 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 : … it is possible to cancel the contrast between materials ξ and ψ: both M1 and M2 will project to the same vector A2 A1 Experimental setup (1): beam: 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 Experimental setup (2): phantom: 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 Experimental setup (3): detector: 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 Experimental setup (4): ASIC: Experimental setup (4): ASIC For more details: see talk by P. Grybos, NSS-N8-5 Monday 17h RX64DTH ASIC with 64 channels Two discriminators + two 20-bit counters per channel, fully digital input/output Energy resolution of 0.8 keV RMS Experimental setup (5): system: Experimental setup (5): system For angiography: see poster by G. Baldazzi, M5-438 Thu. 11h 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 Image processing (1): Image processing (1) 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 18 keV raw 18 keV corrected 36 keV corrected Image processing (2): Image processing (2) 16 – 32 keV 18 – 36 keV 1= PMMA 2=water 3=PE 4=(water+PE) Simulation with MCNP: Simulation with MCNP 1=detector 2=PMMA 3=water 4=PE 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 Experiment vs. Simulation (1): Experiment vs. Simulation (1) RX64DTH 16 – 32 keV Experiment vs. Simulation (2): Experiment vs. Simulation (2) Results (1): SNR vs. proj. angle: Results (1): SNR vs. proj. angle RX64DTH 16 – 32 keV Cancellation angle for a pair given by SNR=0 Theoretical cancellation angles: PMMA-water 36.5° PE-water 40.5° PMMA-PE 45° Results (2): SNR summary: Results (2): SNR summary * Previous version of ASIC, exposure with about 2x more incident photons Results (3): Projected images: Results (3): Projected images RX64DTH 16 – 32 keV simulation 16 – 32 keV Conclusion and Outlook: 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 Thanks to …: 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) Efficiency in edge-on mode: Efficiency in edge-on mode present detector Energy resolution (RX64 ASIC): Energy resolution (RX64 ASIC) For more details: see talk by P. Grybos, NSS-N8 Monday 18/10 Cu E (K) = 8.0 KeV Ge E (K) = 9.9 keV Rb E (Ka) = 13.4 keV Mo E (K) = 17.4 keV E (K) = 19.6 keV Ag E (K) = 22.1 keV E (K) = 24.9 keV Sn E (K) = 25.3 keV E (K) = 28.5 keV Slide24: 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) Slide25: it’s possible to tune dichromatic beam energies to breast thickness, to obtain equal statistics at both energies better signal-to-noise ratio
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