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Introduction Conventional radiology Why digital? Why dual energy? Experimental setup Image acquisition Image processing

Dual energy radiology. Introduction Conventional radiology Why digital? Why dual energy? Experimental setup Image acquisition Image processing and results. Energy. 10 -9. 10 -6. 10 -3. eV. 1. 10 3. 10 6. Introduction: what are X-rays.

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Introduction Conventional radiology Why digital? Why dual energy? Experimental setup Image acquisition Image processing

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  1. Dual energy radiology • Introduction • Conventional radiology • Why digital? • Why dual energy? • Experimental setup • Image acquisition • Image processing and results

  2. Energy 10-9 10-6 10-3 eV 1 103 106 Introduction: what are X-rays • X rays = electromagnetic radiation (=photons) in the range • 10-11 m < l < 10-8 m • 31016 Hz < n < 3 1019 Hz • 0.1 keV < E < 100 keV

  3. ACTIVITY X-ray generation • Sorgenti • Radiazione di sincrotrone • Tubi a raggi X

  4. Breemstrahlung K-shell e- extraction Ionization L K L K Characteristic lines Continuous spectrum Max. energy eV Anode heating At diagnostic energies more than 99% of e- energy goes into heating; less than 1% is used for X-rays! X-ray tube • Electrons emitted by cathod and accelerated towards the anode (W, Mo) • Then in the anode do:

  5. X-ray interactions Mass attenuation coefficient (cm2/g) Silicon Photoelectirc effect Compton scattering e+e _ production

  6. Bones absorb more X rays than soft tissue: appear white on the radiograph (photons darken the film) X-ray absorption • Intensity of a beam traversing a material  attenuation I(x) = I0 e-mx • Absorption coefficient: m(E) = N s = (srNA)/ A • Radiographs are based on the different absorption coefficient of different materials Bone: O 43.5% Ca 22.5% C 15.5% P 10.3% Other 8.2% Soft Tissue: O 70.8% C 14.3% H 10.2% N 3.4% Other 1.3%

  7. Conventional radiography: image receptors • Direct-exposure X-ray film • emulsion of grains of AgBr (  1 mm) suspended in gelatin • X-rays interact mostly with Ag and Br • Ag and Br have a larger s than the elements in gelatine • A latent image is built up of sensitised BrAg grains • The latent image is then developed (senitised grains converted to silver) • Problem: very low efficiency •  0.65% of incident X-rays are detected • Screen-film combinations • Phosphor screen to absorb X-ray photons and re-emit part of its energy in the form of light fluorescent photons • The light photons expose the film (emulsion of AgBr in gelatine) • The interaction of light photons with the AgBr is a photochemical reaction • The silver distribution forms the latent image • Problem: compromise between detection efficiency and unsharpness (=loss of edge details) • The larger the screen thickness, the larger the efficiency, but also the unsharpness

  8. Why digital radiology? • Digital radiography has well known advantages over conventional screen-film systems • Enhance detecting efficiency w.r.t. screen-film • Image analysis • Easy data transfer

  9. Eg=1.12 V Why silicon detectors? • Main characteristics of silicon detectors: • Small band gap (Eg = 1.12 V) •  good resolution in the deposited energy • 3.6 eV of deposited energy needed to create a pair of charges, vs. 30 eV in a gas detector • Excellent mechanical properties • Detector production by means of microelectronic techniques • small dimensions • spatial resolution of the order of 10 m • speed of the order of 10 ns

  10. Based on different energy dependence of the absorption coefficient of different materials Enhance detail visibility (SNR) Decrease dose to the patient Decrease contrast media concentration Introduction: why dual energy ? • Dual energy techniques • GOAL: improve image contrast

  11. Example 1: dual energy mammography

  12. 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 1: dual energy mammography

  13. Example 2: angiography • Angiography = X-ray examination of blood vessels • determine if the vessels are diseased, narrowed or blocked • Injection of a contrast medium (Iodine) which absorbs X-ray differently from surrounding tissues • Coronary angiography • Iodine must be injected into the heart or very close to it • A catheter is inserted into the femoral artery and managed up to the heart • Long fluoroscopy exposure time to guide the catheter • Invasive examination • Why not to inject iodine in a peripheral vein? • Because lower iodine concentration would be obtained, requiring longer exposures and larger doses to obtain a good image • But, if the image contrast could be enhanced in some way…

  14. Iodine injected in patient vessels acts as radio-opaque contrast medium Dramatic change of iodine absorption coeff. at K-edge energy (33 keV) Subtraction of 2 images taken with photons of 2 energies (below and above the K-edge) → in the resulting image only the iodine signal remains and all other materials are canceled Example 2: angiography at the iodine K-edge (II)

  15. 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 • To implement dual energy imaging we need: • a dichromatic beam • a position- and energy-sensitive detector

  16. Experimental setup: beam 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

  17. X-rays N. I. I/O cards PCI-DIO-96 and DAQCard-DIO-24 current pulses 100 m data, control Silicon strip detector Integrated circuit PC Experimental setup: Single Photon Counting System • 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

  18. Silicon microstrip detector each strip is an independent detector which gives an electric signal when an X-ray photon crosses it and interacts with a silicon atom Chip RX64 → counts incident photons on each strip of the detector 6.4 mm 10 strip = 1 mm micro-bondings Knowing from which strip the electric signal comes from,the position of the incoming X-ray phonton is reconstructed. Detecting system 4 cm

  19. Detector 1 2 5 4 3 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)

  20. 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

  21. Mo K Sn K Ge K Ag K Cu K MoK Ag K Rb Ka Sn K System calibration 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)

  22. 5 mm Imaging test 1-dimensional array of strips → 2D image obtained by scanning Test Object Collimator (0.5 mm) Detector Cd-109 source (22.24 keV)

  23. Scanning Imaging test 1-dimensional array of strips → 2D image obtained by scanning

  24. K-edge subtraction imaging • Map the concentration of a particular element in a sample • X-ray energies chosen so that the element under study has the K-edge discontinuity between them • Cancel background structures by subtracting 2 images taken at the 2 energies • For best background cancellation the 2 energies must be close to each other • Best choice: energies just above and below the K-edge of the interesting material • Art painting analysis • Isolate one typical material (ec. Zn, Cd) to date a painting • Medical imaging with contrast medium • Suited for angiography at iodine K-edge • Cancel background structures to enhance vessel visibility • Possible application at the Gadolinium K-edge (50.2 keV) • Possible application in mammography (study vascularization extent) • Hypervascularity characterizes most malignant formations

  25. X-ray tube with dual energy output Phantom Detector box with 2 collimators Angiography setup • 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

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

  27. Conc = 370 mg / ml Conc = 92.5 mg / ml Conc = 23.1 mg / ml Possible decrease of iodine concentration keeping the same rad. dose Angiographic test results (II)

  28. Dual Energy Angiography Digital Subtraction Angiography smaller cavity (=0.4 mm) visible in DEA and not in DSA Results with a second phantom Phantom Iodine conc. = 95 mg/ml

  29. Cd red Test object Cu red E = 24.2 keV logarithmic subtraction E = 27.5 keV Application to art painting analysis  Detect the presence of cadmium in a painting Cd K-edge = 26.7 keV • After subtraction: • Cd grains contrast enhanced • Cu wires contrast decreased

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