Recent advances on X-ray imaging with a single photon counting system

1. Recent advances on X-ray imaging with a single photon counting system • Introduction • The system: microstrip detectors, RX64 ASICs, testing methods • Energy resolution and efficiency • Spatial resolution • Imaging results - mammography • Imaging results - angiography • Summary and outlook Luciano Ramello – Univ. Piemonte Orientale and INFN, Alessandria NURT 2003, October 27-31, 2003

2. G. Baldazzi1, D. Bollini1, A.E. Cabal Rodriguez2, C. Ceballos Sanchez2 ,W. Dabrowski3, • Diaz Garcia2, M. Gambaccini4, P. Giubellino5, M. Gombia1, P. Grybos3, M. Idzik3,5, J. Lopez Gaitan10, A. Marzari-Chiesa6, L.M. Montano Zetina7, F. Prino8, L. Ramello8, A. Sarnelli4, M. Sitta8, K. Swientek3, A. Taibi4, E. Tomassi6, A. Tuffanelli4, P. Van Espen9, P. Wiacek3 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; 10 Univ. de los Andes, Colombia

3. I. Introduction Introduction (1) • We are developing a single photon counting system for X-ray imaging in the 15-50 keV range • Spatial resolution is defined by the detector segmentation (presently 100 mm pitch strips) • Energy resolution is determined mainly by the low-noise front-end ASIC • Rate capability (converted photons/mm2/s) is defined by the timing characteristics of the ASIC and by the pixel size (presently 100 x 300 mm2) • Medical applications of this system are those requiring high dynamic range of counts, good energy resolution; furthermore, they must be compatible with scanning mode

4. I. Introduction Introduction (2) • 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 • 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

5. I. Introduction Introduction (3) • Subtraction imaging: removes background structures • Dual energy technique: isolates materials characterized by different energy dependence of the linear attenuation coefficient m [Alvarez and Macovski 1976] • Quasi-monochromatic beams: implement dual energy techniques in a small-scale installation, no synchrotron [see NIM A 365 (1995) 248 and Proc. SPIE Vol. 4682, p. 311 (2002)] • First application: dual energyangiography at iodine K-edge (33 keV), possible extension to gadolinium K-edge (50 keV) • Second application: dual-energy mammography (18+36 keV)

6. I. Introduction Silicon efficiency vs. X-ray energy Photoelectric conversion in the active volume • Front configuration • 70 mm Al shield (could be reduced) • 300 mm active Si • Edge configuration • 765 mm insensitive silicon • 10 mm (now) or 20 mm (later ?) active silicon simple calculation with cross-sections from XCOM data base of NIST

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

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

9. II. System Detector test: I-V measurements Leakage current (A) Keithley 237 provides reverse bias, HP 4145B measures currents, for bias line (serving 400 strips) and for guard ring. Reverse bias voltage (V) 400-strip detector from ITC-IRST, Trento, Italy: Ibias(60 V) = 18.9 nAIstrip(60 V)  47.2 pA Ibias(100 V) = 25.0 nA Istrip(100 V)  62.5 pA

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

11. II. System Strip-by-strip measurements Measuring strip current, Istrip • VB = 60 V • Contacts needed: • 0. Backplane • Strip i • Strip (i+1) • Bias line Measuring inter-strip resistance, Rstrip

12. II. System The RX64 ASIC (1) detector test capacitor Ct • RX64 - Cracow Univ. of Mining and Metallurgy design: single channel layout • charge-sensitive preamplifier • shaper • discriminator (2 discriminators in the latest version) • - pseudo-random counter (20-bit) [not shown]

13. II. System The RX64 ASIC (2) RX64 - Cracow U.M.M. design - (28006500 m2) - 64 front-end channels (preamplifier, shaper, 1 or 2 discriminators), - 64 pseudo-random counters (20-bit), - internal DACs: 1 or 2 for 8-bit threshold(s) setting and two 5-bit for bias settings - internal calibration circuit (square wave 1mV-30 mV), - control logic and I/O circuit (interface to external bus).

14. II. System RX64 ASIC testing Probe card testing before assembly on PCB becomes convenient when production yield is low: • Power consumption test • Test of the counter section • Full test of the analogue • performance of the 64 channels, • using both HIGH and LOW • discriminator/counter sets The test is performed using the same power supplies, cables, DAQ hardware and software as for the final assembled system

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

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

17. III. Energy resolution and efficiency Threshold uniformity (128 channels) • Calibration pulse of »5300 electrons (internal voltage step applied to Ctest = 75 fF) • Mean threshold (from gaussian fit) for 128 channels: • Threshold spread » 8% • Small syst. difference (» 4%) between chips

18. III. Energy resolution and efficiency 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

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

20. III. Energy resolution and efficiency Gain uniformity (128 channels) • Scan with 10 different amplitudes (4-22 mV) • Circuit response reasonably linear up to 8000 electrons (29 keV) for Tpeak= 0.5 ms <Gain> = 61.6±1.4mV/el. Small (3.5%) systematic difference between chips

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

22. III. Energy resolution and efficiency Gain and Noise summary (I) Detector with 128 equipped channels (2 x RX64): • RMS value of noise = 8.1 mV ÞENC = 131 electrons • RMS of comparator offset distribution = 3.2 mV: 2 times smaller than noise(common threshold setting for all channels)

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

24. III. Energy resolution and efficiency Calibration results (single strip) 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

25. III. Energy resolution and efficiency Gain and Noise summary (II)

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

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

28. III. Energy resolution and efficiency The conversion efficiency Quasi-monochromatic beam at 6 energies (18-36 keV) Fluorescence setup with 4 targets (15.7-25.0 keV) Preliminary analysis Detector was exposed to same beam flux in FRONT and EDGE mode The (not well kown) absolute beam flux cancels in the ratio: Counts(EDGE) / Counts(FRONT) Experimental results compare well with GEANT 3.21 simulations

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

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

31. IV. Position resolution The MicroBeam • Vertical scan of a 25 mm diameter Ni-Cr wire, tube at 15 kV • Si(Li) detector counts vs. wire position for Ni Ka peak: observed RMS of 28.5mm • Deduced beam RMS after deconvolution of wire is not much smaller • Beam RMS decreases with increasing tube kV (while beam halo becomes more important)

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

33. IV. Position resolution Position resolution results (1) Si microstrip beam profile: Centroid (strip units) vs. Beam Position (mm) Simulation of the Centroid vs. Beam Position

34. IV. Position resolution Position resolution results (2) Maximum deviation from straight line is ± 0.12 strips (12 mm) Later, beam halo has been reduced thanks to a 100 mm pinhole Preliminary analysis of latest data shows considerable reduction of maximum deviation from straight line

35. V. Mammographic imaging Dual Energy Mammography • Dual energy mammography 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

36. V. Mammographic imaging The dichromatic beam (1) • 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 DE/E) • q-2q goniometer • Bragg diffraction, first and second harmonics  energies E and 2E are obtained

37. V. Mammographic imaging 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) incident spectra at 3 energy settings … … spectra after 3 cm plexiglass (measured with HPGe detector)

38. V. Mammographic imaging 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

39. V. Mammographic imaging 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. m (cm-1) of the adipose, glandular and cancerous tissues in the breast • 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

40. V. Mammographic imaging The mammographic test (2) • Low energy and high energy images were acquired separately (no double threshold ASIC yet) with the 384-channel Si detector, covering a 38.4 mm wide slice of the phantom • 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 For more details, see poster by C. Ceballos on Tuesday 28/10

41. V. Mammographic imaging Mammography test results (1) The contrast cancellation angles for each pair of materials were obtained, both from experiment and from MCNP simulation

42. V. Mammographic imaging Mammography test results (2) The PE pattern alone is visible in measured data at projection angle 36.5 ° (PMMA-water cancellation) 1=detector 2=PMMA 3=water 4=PE Simulations are in fair agreement with data for PMMA-water cancellation angles at 2 out of the 3 energy pairs; we are investigating problems due to low statistics at high energies and to uncertainty on PE sample composition

43. VI. Angiographic imaging The angiographic test setup X-ray tube with dual energy output Phantom Detector box with 2 collimators • X-ray tube with dual-energy output • each measurement  1.4 • 10 6 photons / mm2 (in 2+2 seconds) • Phantom made of PMMA + Al • Detector box with two collimators Phantom with 4 iodine-filled cavities of diameter 1 or 2 mm

44. VI. Angiographic imaging Procedure for image analysis (I) 1. MeasureFlat fieldat both energies 2. Normalize counts between the two energies <N(31.5 keV)> / <N(35.5 keV)> = 2.432 3. Compute transmission in PMMA + Al

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

46. VI. Angiographic imaging Images vs. iodine concentration Cavity diameter = 1mm 370 mg / ml 92.5 mg / ml 23.1 mg / ml MCNP simulations: see C. Ceballos et al., AIP Conf. Proc. 682, 2003, pp. 185-191

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

48. VII. Conclusion Summary • A relatively simple linear X-ray detector for scanning mode radiography was developed • Energy resolution (1.3 keV FWHM at 22 keV) is well suited for the available quasi-monochromatic beams • Efficiency in edge mode (10 mm Si) is sufficient for D.E. mammography and angiography at iodine K-edge • Imaging results with phantoms show interesting SNR values, detailed simulations using MCNP and GEANT 3 were developed

49. VII. Conclusion Outlook • Exploit double threshold ASIC for D.E. Mammography (ASIC mass tests ongoing) • Build larger detectors for full-size imaging • Measure DQE and MTF with microbeam • Angiography: implement synchronization with ECG • Angiography: explore the Gadolinium option at 50 KeV

50. VII. Conclusion Thanks to ... • The organizers of NURT 2003 for this nice opportunity to present our results • The Italian Ministry for Education, University and Research (MIUR) • The Polish State Committee for Scientific Research • 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)