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Dual energy radiology • Introduction – Conventional radiology – Why digital? – Why dual energy? • Experimental setup • Image acquisition • Image processing and results Introduction: what are X-rays • X rays = electromagnetic radiation (=photons) in the range 10-11 10-8 – m<l< m – 31016 Hz < n < 3 1019 Hz – 0.1 keV < E < 100 keV Energy 10-9 10-6 10-3 eV 1 103 106 X-ray generation ACTIVITY 1Bq = 1decay / second • Sorgenti • Radiazione di sincrotrone • Tubi a raggi X 1Ci = 37GBq X-ray tube • Electrons emitted by cathod and accelerated towards the anode (W, Mo) •Then in the anode do: Breemstrahlung Ionization K L Anode heating At diagnostic energies more than 99% of eenergy goes into heating; less than 1% is used for X-rays! K-shell e- extraction L K Continuous spectrum Characteristic lines Max. energy eV X-ray interactions Mass attenuation coefficient (cm2/g) Silicon Photoelectirc effect Compton scattering _ e+ e production X-ray absorption • Intensity of a beam traversing a material attenuation I(x) = I0 e-mx • Absorption coefficient: m(E) = N s = (srNA)/ A • Radiographs are based on the different absorption coefficient of different materials Bone: O Ca C P Other 43.5% 22.5% 15.5% 10.3% 8.2% Soft Tissue: O 70.8% C 14.3% H 10.2% N 3.4% Other 1.3% Bones absorb more X rays than soft tissue: appear white on the radiograph (photons darken the film) 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 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 (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 mm speed of the order of 10 ns Eg=1.12 V Introduction: why dual energy ? • Dual energy techniques Based on different energy dependence of the absorption coefficient of different materials • GOAL: improve image contrast Enhance detail visibility (SNR) Decrease dose to the patient Decrease contrast media concentration Example 1: dual energy mammography Example 1: dual energy mammography 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 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, Linear array of silicon microstrips + availability in hospitals 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 Two spatially separated beams with different energies E-DE and E+DE obtained in 2 separate beams Bragg Diffraction on Highly Oriented Pyrolitic Grafite Crystal n.h.c E B 2d sin B Double slit collimator W anode tube Experimental setup: Single Photon Counting System X-rays current pulses N. I. I/O cards PCI-DIO96 and DAQCard-DIO-24 100 mm data, control Silicon strip detector Integrated circuit PC • 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 Detecting system Silicon microstrip detector each strip is an independent detector which gives an electric signal when an Xray photon crosses it and interacts with a silicon atom 4 cm 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. Experimental setup: RX64 chip Cracow U.M.M. design - (28006500 mm2) - 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) 2 Detector 1 5 4 3 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 System calibration Mo K Counts 150 Sn K Ge K 100 Ag K Cu K Mo K Ag K Rb K 50 0 100 200 Source Am+Rb target Source Am+Mo target Source Am+Ag target Tube+Cu target Tube+Ge target Tube+Mo target Tube+Ag target Tube+Sn target Sn K 300 400 500 Threshold (mV) 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) System Tp GAIN mV/el. ENC Energy resolution 6 x RX64 0.7 ms 64 ≈170 el. ≈0.61 keV 6 x RX64DTH 0.8 ms 47 ≈ 200 el. ≈0.72 keV Imaging test 1-dimensional array of strips → 2D image obtained by scanning Test Object 5 mm Collimator (0.5 mm) Detector Cd-109 source (22.24 keV) Imaging test Pasos Scanning 1-dimensional array of strips → 2D image obtained by scanning 210 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 24,00 21,00 18,00 15,00 12,00 9,000 6,000 3,000 0 0 10 20 30 40 Canales 50 60 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 Kedge 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 Angiography setup 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 pixels 15 10 5 6 15 5 pixels 3 Conteggi (x10 ) Angiographic test results (I) 4 3 10 2 5 1 0 0 200 pixels 100 0 0 300 E = 31.5 keV 100 E = 35.5 keV 1,0 Counts / Max.Counts 0,8 0,6 logarithmic subtraction 0,4 C1 ln N35.5 C2 ln N31.5 0,2 Measurement Simulation 0,0 0 50 100 150 200 250 300 Counts / Max.Counts Conc(I) = 370 mg/ml E = 31.5 KeV 1,0 Conc(I) = 370 mg/ml E = 35.5 KeV 0,6 0,4 0,2 Measurement Simulation 0,0 0 50 100 350 -0.4 -0.6 150 200 250 Strip Number 15 10 5 -0.8 0 0 100 200 pixels 300 ln[count(E=35.5Kev)] - ln[count(E=31.5Kev)] -0.2 pixels log conteggi 0.0 Measurement Simulation Conc(I) = 370 mg/ml 0,2 0,0 -0,2 -0,4 -0,6 -0,8 -1,0 0 50 100 150 200 Strip Number 300 0,8 Strip Number Phantom structure not visible in final image 200 pixels 250 300 350 300 350 Angiographic test results (II) 5 -0.8 0.0 -0.2 10 5 -0.3 0 0.15 0.10 0.05 0.00 -0.05 -0.10 -0.15 0 0 100 200 300 100 200 300 5 0 pixels Conc = 370 mg / ml 10 0 0 pixels 15 pixels 0.1 -0.1 log conteggi -0.6 10 15 pixels -0.4 15 pixels -0.2 log conteggi 0.2 0.0 100 200 300 pixels Conc = 92.5 mg / ml Conc = 23.1 mg / ml Signal contrast Sig .Counts Bckgr.Counts SNR Noise contrast Bckgr. fluctuations 100 cavità 4 teor. cavità 3 teor. cavità 2 teor. cavità 1 teor. SNR 80 cavità 4 cavità 3 cavità 2 cavità 1 60 40 20 0 0 100 200 Concentrazione (mg/ml) 300 400 Possible decrease of iodine concentration keeping the same rad. dose Results with a second phantom Phantom 0 100 200 300 140 120 0 100 200 300 140 140 300 um pixel 100 120 300 um pixel 100 120 80 100 60 80 80 60 40 60 40 20 40 20 20 0 0 100 200 300 140 0 140 0 0 120 100 300 um pixel Digital Subtraction Angiography Dual Energy Angiography Iodine conc. = 95 mg/ml smaller cavity (=0.4 mm) visible in DEA and not in DSA 100 200 100 um pixel 300 120 100 80 80 60 60 40 40 20 20 0 0 100 200 100 um pixel 300 Application to art painting analysis Detect the presence of cadmium in a painting Cd red Test object Cd K-edge = 26.7 keV Cu red 60 50 40 30 60 E = 24.2 keV 20 10 0 0 100 200 300 logarithmic subtraction 50 40 30 20 60 10 50 0 40 0 30 20 E = 27.5 keV 10 0 0 100 200 300 100 200 300 After subtraction: • Cd grains contrast enhanced • Cu wires contrast decreased