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

Introduction: what are X-rays Energy eV X rays = electromagnetic radiation (=photons) in the range – m <  < m –3  Hz < < 3  Hz –0.1 keV < E < 100 keV

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

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

X-ray interactions Photoelectirc effect Compton scattering e + e _ production Mass attenuation coefficient (cm 2 /g) Silicon

X-ray absorption Intensity of a beam traversing a material  attenuation I(x) = I 0 e -  x Absorption coefficient:  (E) = N  = (  N A )/ A Radiographs are based on the different absorption coefficient of different materials Bones absorb more X rays than soft tissue: appear white on the radiograph (photons darken the film) Bone: O43.5% Ca22.5% C15.5% P10.3% Other 8.2% Soft Tissue: O70.8% C14.3% H10.2% N 3.4% Other 1.3%

Conventional radiography: image receptors Direct-exposure X-ray film –emulsion of grains of AgBr (   1  m) suspended in gelatin –X-rays interact mostly with Ag and Br Ag and Br have a larger  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

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

Why silicon detectors? Main characteristics of silicon detectors: Small band gap (E g = 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 E g =1.12 V

Dual energy techniques GOAL: improve image contrast 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 ?

Example 1: dual energy mammography

E  keV: Signal from cancer tissue deteriorated by the adipose tissue signal E  keV Cancer tissue not visible, image allows to map glandular and adipose tissues

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…

Example 2: angiography at the iodine K-edge (II) 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

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 Bragg Diffraction on Highly Oriented Pyrolitic Grafite Crystal W anode tube Double slit collimator Two spatially separated beams with different energies  E-  E and E+  E obtained in 2 separate beams

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

Detecting system Chip RX64 → counts incident photons on each strip of the detector 4 cm 6.4 mm 10 strip = 1 mm micro-bondings 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 Knowing from which strip the electric signal comes from,the position of the incoming X-ray phonton is reconstructed.

Detector efficiency Front geometry –Strip orthogonal to the beam –70  m of Al light shield Edge-on geometry –Strip parallel to the beam –765  m of inactive Si –Better efficiency for E > 18 keV Efficiency calculation –X-ray absorbed if interacts in passive regions –X-ray detected if makes photoelectric effect in active regions

Experimental setup: RX64 chip Cracow U.M.M. design - ( 2800  6500  m 2 ) - CMOS 0.8 µm process (1) (1) 64 front-end channels a) preamplifier b) shaper c) 1 or 2 discriminators (2) (2) (1 or 2)x64 pseudo-random counters (20-bit) (3) (3) internal DACs: 8-bit threshold setting and 5-bit for bias settings (4) (4) internal calibration circuit (square wave 1mV-30 mV) (5) (5) control logic and I/O circuit (interface to external bus) Cracow U.M.M. design - ( 2800  6500  m 2 ) - CMOS 0.8 µm process (1) (1) 64 front-end channels a) preamplifier b) shaper c) 1 or 2 discriminators (2) (2) (1 or 2)x64 pseudo-random counters (20-bit) (3) (3) internal DACs: 8-bit threshold setting and 5-bit for bias settings (4) (4) internal calibration circuit (square wave 1mV-30 mV) (5) (5) control logic and I/O circuit (interface to external bus) Detector

System calibration setup in Alessandria Detector in Front config. Fluorescence target (Cu, Ge, Mo, Nb, Zr, Ag, Sn) Cu anode X-ray tube → X-ray energies = characteristic lines of target material

Cu K  Mo K  Ge K  Rb K  Ag K  Sn K  Ag K  Mo K  Sn K  SystemTpTp GAIN  V/el. ENCEnergy resolution 6 x RX640.7  s64≈170 el.≈0.61 keV 6 x RX64DTH0.8  s47≈ 200 el.≈0.72 keV 241 Am 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 calibration

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

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

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 K-edge subtraction imaging

X-ray tube with dual energy output Phantom Detector box with 2 collimators 1.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) 2.Detector box with two detectors aligned with two collimators 3.Step wedge phantom made of PMMA + Al with 4 iodine solution filled cavities of 1 or 2 mm diameter Angiography setup

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

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)

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

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