1. Results about imaging with silicon strips for Angiography and Mammography I. Introduction II. The system: microstrip detectors, RX64 ASICs III. Energy resolution and efficiency IV. Spatial resolution V. Imaging results - mammography VI. Imaging results - angiography VII. Summary and outlook Luciano Ramello – Univ. Piemonte Orientale and INFN, Alessandria VII MSMP, March 24-26, 2003

2. G. Baldazzi 1, D. Bollini 1, A.E. Cabal Rodriguez 2, W. Dabrowski 3, A. Diaz Garcia 2, M. Gambaccini 4, P. Giubellino 5, M. Gombia 1, P. Grybos 3, M. Idzik 3,5, A. Marzari-Chiesa 6, L.M. Montano Zetina 7, F. Prino 8, L. Ramello 8, A. Sarnelli 4, M. Sitta 8, K. Swientek 3, A. Taibi 4, E. Tomassi 6, A. Tuffanelli 4, P. Van Espen 9, R. Wheadon 5, P. Wiacek 3 1 University and INFN, Bologna, Italy; 2 CEADEN, Havana, Cuba; 3 University of Mining and Metallurgy, Cracow, Poland; 4 University and INFN, Ferrara, Italy; 5 INFN, Torino, Italy; 6 University of Torino, Torino, Italy; 7 CINVESTAV, Mexico City, Mexico; 8 University of Eastern Piedmont and INFN, Alessandria, Italy; 9 University of Antwerp, Antwerp, Belgium

3. I. Introduction Introduction (1) w One-dimensional silicon array for scanning mode imaging: Good spatial resolution with reduced number of channels Spatial resolution in silicon limited by Compton scattering and parallax error, pitch smaller than about 50-100 micron not really useful w Advantages of digital single photon X-ray imaging: Higher detection efficiency with respect to screen-film systems Edge-on orientation (parallel incidence) preferred for energies above 18 keV Double energy threshold with simultaneous exposure possible Easy processing, transferring and archiving of digital images

4. I. Introduction Introduction (2) w Subtraction imaging: removes background structures  Dual energy technique: isolates materials characterized by different energy dependence of the linear attenuation coefficient  Alvarez and Macovski 1976  w Quasi-monochromatic beams: implement dual energy techniques in a small-scale installation [see NIM A 365 (1995) 248 and Proc. SPIE Vol. 4682, p. 311 (2002)] è First application: dual energy angiography at iodine K-edge (33 keV), possible extension to gadolinium K-edge (50 keV) è Another application: dual-energy mammography (18+36 keV)

5. Silicon efficiency vs. X-ray energy I. Introduction w Front configuration 70  m Al shield (might be reduced) 300  m active Si w Edge configuration 765  m insensitive silicon 10 or 20 mm active Si Photoelectric conversion in the active volume cross-sections from XCOM data base of NIST

6. GaAs: a better alternative ? I. Introduction Photoelectric conversion in the active volume w Front configuration for GaAs, Edge configuration for Si w GaAs is the best choice for 20 keV mammography w Si in edge mode (10 mm) is almost equivalent to GaAs for angiography

7. II. System Silicon microstrip detectors w AC coupling: Bias Line with FOXFET biasing w Guard ring essential to collect surface currents w Designed and fabricated by ITC-IRST, Trento, Italy guard ring bias line first strip (AC contact) DC contact (to p+ implant)

8. I-V measurements 400-strip detector from ITC-IRST, Trento, Italy: I bias (60 V) = 18.9 nA  I strip (60 V)  47.2 pA I bias (100 V) = 25.0 nA  I strip (100 V)  62.5 pA Keithley 237 provides reverse bias, HP 4145B measures currents. Reverse bias voltage (V) Leakage current (A) II. System

9. C-V measurements Keithley 237 provides reverse bias, HP 4284A injects sinusoidal signal to measure C: V = 500 mV V = 500 mV f = 100 kHz f = 100 kHz Full depletion voltage is  constant across detector Reverse bias voltage (V) II. System

10. Strip-by-strip measurements V B = 60 V V B = 60 V Contacts needed:Contacts needed: 0. Backplane 0. Backplane 1.Strip 1.Strip i 2.Strip 2.Strip (i+1) 3.Bias line Measuring strip current, I strip Measuring inter-strip resistance, R strip II. System

11. The RX64 ASIC RX64 - Krakow UMM design - ( 2800  6500  m 2 ) consists of: - 64 front-end channels (preamplifier, shaper, discriminator), - 64 pseudo-random counters (20-bit), - internal DACs: one 8-bit threshold setting and and two 5-bit for bias, - internal calibration circuit (square wave 1mV-30 mV), - control logic, - I/O circuit (interface to external bus). II. System

12. System assembly Manual wire bonding (detector - chip) II. System Automatic wire bonding (detector - pitch adapter - chip)

13. Noise and gain evaluation method 1 Obtain Counts vs. Discriminator Threshold (threshold scan) 2 Smoothing of Counting Curve  Error function Fit, or … 3 Differential Spectrum  Gaussian Fit  extract mean and  III. Energy resolution and efficiency

14. Threshold uniformity (128 channels)  Calibration pulse of  5300 electrons (internal voltage step applied to C test = 75 fF) w Mean threshold (from gaussian fit) for 128 channels: Threshold spread  % Small syst. difference (  4%) between chips III. Energy resolution and efficiency

15. Linearity vs. injected charge (1) Differential spectra obtained with internal calibration: each value of the Calibration DAC produces on the test capacitor Ct (75 fF) a pulse of given charge III. Energy resolution and efficiency

16. Linearity vs. injected charge (2) the RX64 chip is strictly linear up to 5500 electrons input charge the RX64 chip is strictly linear up to 5500 electrons input charge (i.e. up to  20 keV X-ray energy) (i.e. up to  20 keV X-ray energy) a straight line fit within linearity range gives offset (a) & gain (b) a straight line fit within linearity range gives offset (a) & gain (b) Injected charge (electrons) III. Energy resolution and efficiency

17. Gain uniformity (128 channels) w Scan with 10 different amplitudes (4-22 mV)  Circuit response reasonably linear up to  8000 electrons (29 keV) for T peak = 0.5  s = 61.6   V/el. Small (3.5%) systematic difference between chips III. Energy resolution and efficiency

18. Rate capability of the RX64 Efficiency Gain Counting rate [1/s] Test with random signals, 8 keV Three different shaping times T(peak): 1.0, 0.7, 0.5  s Sufficient performance for imaging applications up to 100 kHz / strip Counting rate [1/s] 100 0 100 k 10 k III. Energy resolution and efficiency

19. Gain and Noise summary (I) Detector with 128 equipped channels (2 x RX64): RMS value of noise = 8.1 mV  ENC = 131 electrons RMS of comparator offset distribution = 3.2 mV: 2 times smaller than noise (common threshold setting for all channels) ModuleT(peak)GainENC (el.) Det. + 2 x RX64Short61.6131 6 x RX64Short63.7176 6 x RX64Long82.8131 Fanout + 6 x RX64Short63.7184 Fanout + 6 x RX64Long82.8148 III. Energy resolution and efficiency

20. Calibration setups for X-ray detector Cu-anode X-ray tube with fluorescence targets 241 Am source with rotary target holder III. Energy resolution and efficiency Pb collimator Fluorescence target X-ray tube Board with detector

21. Calibration results (single strip) Cu E (K  ) = 8.0 KeV Ge E (K  ) = 9.9 keV Rb E (K  ) = 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 III. Energy resolution and efficiency

22. Gain and Noise summary (II) 6 x RX64 + fanout + detector, T(peak) Long GAINENC 30 improved amplif. setting ENC 50 241 Am source 62.8  V/el. 154 el.179 el. X-ray tube 63.7  V/el. 151 el.182 el. internal calib. 64.6  V/el. 141 el.164 el. III. Energy resolution and efficiency

23. Matching between channels RX64 chip: 64 channels measured simultaneously with common threshold (absolutely essential for practical applications) III. Energy resolution and efficiency

24. The Double Threshold chip First RX64-DT chip measured: spectra obtained with moving hardware window of 14 mV (5 LSB threshold DAC) by 1 LSB steps. III. Energy resolution and efficiency ENC = 196 electrons

25. IV. Position resolution The micro X-ray beam w X-ray tube (Mo anode) with capillary output at MiTAC, Antwerp University w Si(Li) detector to measure fluorescence at 90 degrees w CCD camera with same focal plane as X-ray beam w optional Mo/Zr filters to reduce intensity and change energy spectrum  X, Y, Z movements with 1  m precision

26. IV. Position resolution Measuring the position resolution w X-ray tube (Mo anode) operated at 15 kV and 40 kV w Silicon detector in front configuration (Al protection removed) w Mo or Zr filter w Horizontal scan (in/out of beam focus) by 1 mm steps to check focus  Vertical scan (across strips) by 10  m steps to measure position resolution

27. IV. Position resolution Beam dimension  Vertical scan of a 25  m dia. Ni-Cr wire  Si(Li) detector counts at Ni K  peak: observed raw RMS of 38 ± 5  m w Deduced beam RMS of 28  m (PRELIMINARY)

28. IV. Position resolution Beam profile in microstrip detector w The minimum size of the beam is maintained for a depth of focus of 3-4 mm

29. IV. Position resolution Position resolution of Si detector Si microstrip beam profile: Centroid (strip units) vs. Beam Position (  m) Maximum deviation from straight line is ± 0.12 strips (12  m)

30. Dual Energy Mammography w Dual energy mammography allows to remove the contrast between the two normal tissues (glandular and adipose), enhancing the contrast of the pathology w Single exposure dual-energy mammography reduces radiation dose and motion artifacts w to implement this we need: a dichromatic beam a position- and energy-sensitive detector V. Mammographic imaging

31. The dichromatic beam (1) w W-anode X-ray tube operated at  50 kV  Highly oriented pyrolithic graphite (HOPG) mosaic crystal (Optigraph Ltd., Moscow)  higher flux than monocrystals (also higher  E/E) V. Mammographic imaging   -2  goniometer w Bragg diffraction, first and second harmonics  energies E and 2E are obtained

32. The dichromatic beam (2) A. Tuffanelli et al., Dichromatic source for the application of dual-energy tissue cancellation in mammography, SPIE Medical Imaging 2002 (MI 4682-21) V. Mammographic imaging incident spectra at 3 energy settings … … spectra after 3 cm plexiglass (measured with HPGe detector)

33. Use of dichromatic beam it’s possible to tune dichromatic beam energies to breast thickness, to obtain equal statistics at both energies  better signal-to-noise ratio V. Mammographic imaging

34. The mammographic test (1)  A three-component phantom made of polyethylene, PMMA and water [S. Fabbri et al., Phys. Med. Biol. 47 (2002) 1-13] was used to simulate the attenuation coeff.  (cm -1 ) of the adipose, glandular and cancerous tissues in the breast V. Mammographic imaging E (keV)fatgland.canc.PEPMMAwater 20.456.802.844.410.680.810 40.215.273.281.225.280.270 w By measuring the logarithmic transmission of the incident beam at two energies, with a projection algorithm [Lehmann et al., Med. Phys. 8 (1981) 659] the contrast between two chosen materials vanishes

35. The mammographic test (2) w Low energy and high energy images were acquired separately (no double threshold yet) with the 384-channel Si detector, covering a 38.4 mm wide slice of the phantom w After correction for flat-field and bad channels, the dual-energy algorithm was applied to the logarithmic images at the two energies, changing the projection angle to find the contrast cancellation angles for pairs of materials V. Mammographic imaging

36. Mammography test results (1) Determination of contrast cancellation angle: SNR between PMMA and water is zero for  = 33°, where PE has a SNR of 16.2 V. Mammographic imaging

37. Mammographic test results (2) V. Mammographic imaging  = 33°  = 42.5° Low EHigh E 6 mm dia. cylinders PE + water PE PMMA base material

38. The angiographic test setup Phantom with 4 iodine-filled cavities of diameter 1 or 2 mm 1.X-ray tube with dual-energy output -each measurement  photons / mm 2 -each measurement  1.4 10 6 photons / mm 2 (in 2+2 seconds) 2.Phantom made of PMMA + Al 3.Detector box with two collimators X-ray tube with dual energy output Detector box with 2 collimators Phantom VI. Angiographic imaging

39. Procedure for image analysis (I) 1. Measure Flat field at both energies 2. Normalize counts between the two energies 3. Compute transmission in PMMA + Al / <N(35.5 keV) = 2.432 VI. Angiographic imaging

40. Procedure for image analysis (II) E = 31.5 keV E = 35.5 keV logarithmic subtraction VI. Angiographic imaging

41. Images vs. iodine concentration Cavity diameter = 1mm 370 mg / ml 92.5 mg / ml 23.1 mg / ml VI. Angiographic imaging MCNP simulations: see poster by A. Cabal, C. Ceballos et al.

42. Signal-to-Noise ratio SNR defined as ratio between CONTRAST (C s ) and fluctuations in a given area (here 1x1 pixel) of the image (C n ): SNR defined as ratio between CONTRAST (C s ) and fluctuations in a given area (here 1x1 pixel) of the image (C n ): SNR = C s /C n d = 1 mm d = 2 mm SNR Concentration (mg/ml) VI. Angiographic imaging

43. VII. Conclusion Summary w A relatively simple linear X-ray detector for scanning mode radiography was developed w Energy resolution (1.3 keV FWHM at 22 keV) is well suited for the available quasi- monochromatic beams w Efficiency in edge mode (10 mm Si) is sufficient for D.E. mammography and angiography at iodine K-edge w Imaging results with phantoms show interesting SNR values

44. VII. Conclusion Outlook w Double threshold ASIC (produced, first tests OK) for D.E. mammography w Larger detectors for full-size imaging w Measure DQE and MTF with microbeam w Angiography: synchronization with ECG w Angiography: explore the Gadolinium option w Extensive MC simulations of the different setups under way