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

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;

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

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

Example 1: dual energy mammography E  15-20 keV: Signal from cancer tissue deteriorated by the adipose tissue signal E  30-40 keV Cancer tissue not visible, image allows to map glandular and adipose tissues

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

Experiment vs. Simulation (1)

Results (1): SNR vs. proj. angle RX64DTH 16 – 32 keV Theoretical cancellation angles: PMMA-water 36.5° PE-water 40.5° PMMA-PE 45° MCNP simulation Cancellation angle for a pair given by SNR=0

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

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

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