Recent advances on X-ray imaging with a single photon counting systemIntroduction The system: microstrip detectors, RX64 ASICs, testing methodsEnergy resolution and efficiencySpatial resolutionImaging results - mammographyImaging results - angiographySummary and outlookLuciano Ramello Univ. Piemonte Orientale and INFN, AlessandriaNURT 2003, October 27-31, 2003

I. Principles

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. Wiacek31 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

I. Principles

We are developing a single photon counting system for X-ray imaging in the 15-50 keV rangeSpatial resolution is defined by the detector segmentation (presently 100 mm pitch strips)Energy resolution is determined mainly by the low-noise front-end ASICRate 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 I. IntroductionIntroduction (1)

I. Principles

One-dimensional silicon array for scanning mode imaging:Good spatial resolution with reduced number of channelsSpatial resolution in silicon limited by Compton scattering and parallax error, pitch smaller than about 50-100 micron not really usefulAdvantages of digital single photon X-ray imaging:Higher detection efficiency with respect to screen-film systemsEdge-on orientation (parallel incidence) preferred for energies above 18 keVDouble energy threshold with simultaneous exposure possibleEasy processing, transferring and archiving of digital imagesI. IntroductionIntroduction (2)

I. Principles

Subtraction imaging: removes background structuresDual 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)]I. IntroductionIntroduction (3) First application: dual energy angiography at iodine K-edge (33 keV), possible extension to gadolinium K-edge (50 keV)

Second application: dual-energy mammography (18+36 keV)

I. Principles

Silicon efficiency vs. X-ray energyFront configuration 70 mm Al shield (could be reduced)300 mm active SiEdge configuration765 mm insensitive silicon 10 mm (now) or 20 mm (later ?) active siliconI. IntroductionPhotoelectric conversion in the active volumesimple calculation with cross-sections from XCOM data base of NIST

I. Principles

GaAs: a better alternative ?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 angiographyI. IntroductionPhotoelectric conversion in the active volume

I. Principles

Silicon microstrip detectorsAC coupling: Bias Line with FOXFET biasingGuard ring essential to collect surface currentsDesigned and fabricated by ITC-IRST, Trento, ItalyII. Systemguardringbias linefirst strip (AC contact)DC contact (to p+ implant)

I. Principles

Detector test: I-V measurements400-strip detector from ITC-IRST, Trento, Italy:

Ibias(60 V) = 18.9 nAIstrip(60 V) 47.2 pA

Ibias(100 V) = 25.0 nAIstrip(100 V) 62.5 pAKeithley 237 provides reverse bias, HP 4145B measures currents, for bias line (serving 400 strips) and for guard ring.Reverse bias voltage (V)Leakage current (A)II. System

I. Principles

Detector test: C-V measurementsKeithley 237 provides reverse bias,HP 4284A injects sinusoidal signal to measure C: V = 500 mV f = 100 kHzFull depletion voltage is constant across detectorReverse bias voltage (V)II. System

I. Principles

Strip-by-strip measurements VB = 60 VContacts needed: 0. BackplaneStrip iStrip (i+1)Bias lineMeasuring strip current, IstripMeasuring inter-strip resistance, RstripII. System

I. Principles

The RX64 ASIC (1)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]II. Systemdetectortest capacitor Ct

I. Principles

The RX64 ASIC (2)RX64 - Cracow U.M.M. design - (28006500 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).II. System

I. Principles

RX64 ASIC testing II. SystemProbe card testing before assembly on PCB becomesconvenient when productionyield 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 setsThe test is performed using the same power supplies, cables, DAQ hardware and software as for the final assembled system

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System assemblyManual wire bonding (detector - chip)II. SystemAutomatic wire bonding (detector - pitch adapter - chip)

I. Principles

Noise and gain evaluation method1Obtain Counts vs. Discriminator Threshold(threshold scan)2Smoothing of Counting CurveError function Fit, or 3Differential SpectrumGaussian Fitextract mean and sIII. Energy resolution and efficiency

I. Principles

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 Threshold uniformity (128 channels)III. Energy resolution and efficiency

I. Principles

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 chargeIII. Energy resolution and efficiency

I. Principles

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

I. Principles

Scan with 10 different amplitudes (4-22 mV)Circuit response reasonably linear up to 8000 electrons (29 keV) for Tpeak= 0.5 msGain uniformity (128 channels) = 61.61.4 mV/el.Small (3.5%) systematic difference between chipsIII. Energy resolution and efficiency

I. Principles

Rate capability of the RX64EfficiencyGainCounting rate [1/s]Test with random signals, 8 keVThree different shaping times T(peak): 1.0, 0.7, 0.5 msSufficient performance for imaging applications up to 100 kHz / stripCounting rate [1/s]1000100 k100 k10 k10 kIII. Energy resolution and efficiency

I. Principles

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

ModuleT(peak)Gain ENC (el.)Det. + 2 x RX64Short61.61316 x RX64Short63.71766 x RX64Long82.8131Fanout + 6 x RX64 Short63.7184Fanout + 6 x RX64Long82.8148

I. Principles

Calibration setups for X-ray detectorCu-anode X-ray tube with fluorescence targets 241Am source with rotary target holderIII. Energy resolution and efficiency

I. Principles

Calibration results (single strip)CuE (K) = 8.0 KeVGeE (K) = 9.9 keVRbE (Ka) = 13.4 keVMoE (K) = 17.4 keVE (K) = 19.6 keVAgE (K) = 22.1 keVE (K) = 24.9 keVSnE (K) = 25.3 keVE (K) = 28.5 keVIII. Energy resolution and efficiency

I. Principles

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

6 x RX64 + fanout + detector, T(peak) LongGAINENC30improved amplif. settingENC50241Am source62.8 V/el.154 el.179 el.X-ray tube63.7 V/el.151 el.182 el.internal calib.64.6 V/el.141 el.164 el.

I. Principles

Matching between channelsRX64 chip: 64 channels measured simultaneously with common threshold(absolutely essential for practical applications)III. Energy resolution and efficiency

I. Principles

_1053693385.doc

_1053693463.doc

The Double Threshold chipFirst 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 efficiencyENC = 196 electrons

I. Principles

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

I. Principles

The micro X-ray beamX-ray tube (Mo anode) with capillary output at MiTAC, Antwerp UniversitySi(Li) detector to measure fluorescence at 90 degreesCCD camera with same focal plane as X-ray beamoptional Mo/Zr filters to reduce intensity and change energy spectrumX, Y, Z movements with 1 mm precisionIV. Position resolution

I. Principles

Measuring the position resolutionX-ray tube (Mo anode) operated at 15 kV and 40 kVSilicon detector in front configuration (Al protection removed)Mo or Zr filterHorizontal scan (in/out of beam focus) by 1 mm steps to check focusVertical scan (across strips) by 10 mm steps to measure position resolutionIV. Position resolution

I. Principles

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

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Beam profile in microstrip detectorThe minimum size of the beam is maintained for a depth of focus of 3-4 mmIV. Position resolution

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

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Position resolution results (2)IV. Position resolutionMaximum deviation from straightline is 0.12 strips (12 mm)Later, beam halo has been reducedthanks to a 100 mm pinholePreliminary analysis of latest data shows considerable reductionof maximum deviationfrom straight line

I. Principles

Dual Energy MammographyDual energy mammography allows to remove the contrast between the two normal tissues (glandular and adipose), enhancing the contrast of the pathologySingle exposure dual-energy mammography reduces radiation dose and motion artifactsto implement this we need:a dichromatic beama position- and energy-sensitive detector

V. Mammographic imaging

I. Principles

The dichromatic beam (1)W-anode X-ray tube operated at 50 kVHighly oriented pyrolithic graphite (HOPG) mosaic crystal (Optigraph Ltd., Moscow) higher flux than monocrystals (also higher DE/E)V. Mammographic imaging q-2q goniometer Bragg diffraction,first and second harmonics energies E and 2E are obtained

I. Principles

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 incidentspectraat 3 energysettings spectra after 3 cm plexiglass

(measured with HPGe detector)

I. Principles

Use of dichromatic beam its possible to tune dichromatic beam energies to breast thickness, to obtain equal statistics at both energies better signal-to-noise ratioV. Mammographic imaging

I. Principles

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 breastV. Mammographic imaging 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

E (keV)m_fatm_glandm_cancPEPMMAwater20.456.802.844.410.680.81040.215.273.281.225.280.270

I. Principles

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 phantomAfter 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 materialsV. Mammographic imaging For more details, see poster by C. Ceballos on Tuesday 28/10

I. Principles

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

I. Principles

Mammography test results (2)V. Mammographic imaging 1=detector2=PMMA3=water4=PEThe PE pattern alone is visible in measured data at projection angle 36.5 (PMMA-water cancellation)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

I. Principles

The angiographic test setupPhantom with 4 iodine-filled cavities of diameter 1 or 2 mmX-ray tube with dual-energy outputeach measurement 1.4 10 6 photons / mm2 (in 2+2 seconds)Phantom made of PMMA + AlDetector box with two collimatorsX-ray tube with dual energy outputDetector box with 2 collimatorsPhantomVI. Angiographic imaging

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Procedure for image analysis (I)1. Measure Flat field at both energies2. Normalize counts between the two energies3. Compute transmission in PMMA + Al / = 2.432VI. Angiographic imaging

I. Principles

Procedure for image analysis (II)E = 31.5 keVE = 35.5 keV logarithmic subtractionVI. Angiographic imaging

I. Principles

Images vs. iodine concentrationCavity diameter = 1mm370 mg / ml92.5 mg / ml23.1 mg / mlVI. Angiographic imagingMCNP simulations: see C. Ceballos et al., AIP Conf. Proc. 682, 2003, pp. 185-191

I. Principles

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

I. Principles

SummaryA relatively simple linear X-ray detector for scanning mode radiography was developedEnergy 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-edgeImaging results with phantoms show interesting SNR values, detailed simulations using MCNP and GEANT 3 were developed VII. Conclusion

I. Principles

OutlookExploit double threshold ASIC for D.E. Mammography (ASIC mass tests ongoing)Build larger detectors for full-size imagingMeasure DQE and MTF with microbeamAngiography: implement synchronization with ECGAngiography: explore the Gadolinium option at 50 KeVVII. Conclusion

I. Principles

Thanks to ...The organizers of NURT 2003 for this nice opportunity to present our resultsThe Italian Ministry for Education, University and Research (MIUR) The Polish State Committee for Scientific ResearchINFN Torino for allowing access to technical staff and bonding facilitiesICTP Trieste for travel and subsistence support to Cuban researchersThe European Community for travel and subsistence support for students under the ALFA II programme (contract AML/B7-311/97/0666/II-0042)VII. Conclusion

I. Principles

NURT 2003: 45 minutes for talk + 15 minutes for questions/discussion.The Proceedings on CD are on file nurt03.tex (closed on 30 september 2003).http://physics.nist.gov/PhysRefData/Xcom/Text/XCOM.htmlFigure: D:\Xrays\effic\eff_si.eps(.gif)At iodine K-edge, GaAs is better than Si (10 mm) and equivalent to Si (20 mm); GaAs in general has less Compton, so images should be less noisythan those taken with Si. Of course, GaAs in front mode allows 2D imaging if subdivided into pixels.Figure: D:\Xrays\effic\eff_gaas.eps(.gif)

Figure: D:\Xrays\Talks\Mexico03\irst_det.epsFigures: from E. Tomassi thesis presentation.Figures: from E. Tomassi thesis presentation.Figures: from E. Tomassi thesis presentation.Figures: D:\Xrays\pcb\probe*.eps(.gif)Figures: D:\Xrays\Talks\Mexico03\fanout.eps + Pedro/IWORID02 presentationFigures: from E. Tomassi thesis presentationCal12.jpg (at CalDAC = 12, that is approx. 5300 electrons) data from the 128 channel detectorChip 1, channel 20; AmplDAC = 50, ShapDAC = 60.Chip 1, channel 20; AmplDAC = 50, ShapDAC = 60.Cal_chan.jpg + Slope.jpg. (128-channel detector)Figure: gaus109.epsThe suffixes 30 and 50 refer to two different settings of the preamplifier DAC (AmpDAC = 30 gives less noise: corresponds to higher Rfed in preamplifier).

Figure: D:\Xrays\Talks\Mexico03\RX64DT_spectrum.eps -> rxdt_spe.jpg Figure: si_effratio_1.gif + si_effratio_2.gif (in D:\Xrays\effic\)TOP figure: see D:\Xrays\RX400\Antwerpen\fluospec1.pxp -> NiCr-spec-Mo15.eps(.bmp)BOTTOM figure: see D:\Xrays\RX400\Antwerpen\NiCr-Mo-15kV-corr.pxp -> NiCr-Mo-15.eps(.bmp)The ADC calibration for the TOP figure is given by: gain = 15.3 eV/channel, zero_ADC = -63 eV.OLD BOTTOM FIGURE WAS microbea.jpg with DATA incorrectly normalized (NOTE: ni15prof.jpg reports the gaussian fit giving 38 um sigma).The PRESENT BOTTOM FIGURE is NiCr-Mo-15.bmp (see also NiCr-Mo-15.eps) from NiCr-Mo-15kV-corr.pxpSee also the separate analysis of the 4 peaks (Ni, Cr, Kalfa and Kbeta) in Picchi_Ni-Cr.pxp of 30 april 2003.Data taken in december 2002 with 15kV, Mo filter.Simulation figure: D:\Xrays\RX400\Antwerpen\simu.eps(.jpg) Pinhole is inserted into a 2 mm diameter indium disk.Preliminary analysis of July 2003 Antwerps data: see D:\Xrays\RX400\Antwerpen\pinhole_23jul03.txt Schemadet.eps (Francesco ha la figura originale in formato XFIG)NOTA: qui sarebbe piu appropriato il fantoccio mammografico.Measured fluxes (photons / mm^2 / mA / s) for INCIDENT spectra: = 17.8 keV : 1.7x10^4; = 36.1 keV : 2.4x10^3

Figure: snr-exp-sim-18-36.epssnr-exp-sim-18-36.eps

Punto 1: FlatLow11.eps, FlatHigh11.eps (sono una versione smoothed rispetto alle figure usate qui)Punto 3: FantoccioLow.eps, FantoccioHigh.epsLe immagini nella parte superiore sono corrette per il flat field ma non ancora per il diverso profilo di assorbimento del PMMA + Al alle due energie.

Typical ENC (400-strip detector, RX64 chip) of 150 rmse -> 0.54 keV RMS -> 1.28 keV FWHM.Measured at Ag K-alpha (22.1 keV) -> 5.8 % FWHM.