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DIAGNOSTIC RADIOLOGY

DIAGNOSTIC RADIOLOGY. Part 1 : X-ray Beam Aim : To become familiar with the basic knowledge in radiation physics and image formation process. Contents. Introduction to the atomic basic structure Quantities and units Bremsstrahlung production Characteristic X-rays.

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DIAGNOSTIC RADIOLOGY

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  1. DIAGNOSTIC RADIOLOGY Part 1 : X-ray Beam Aim: To become familiar with the basic knowledge in radiation physics and image formation process.

  2. Contents • Introduction to the atomic basic structure • Quantities and units • Bremsstrahlung production • Characteristic X-rays Add module code number and lesson title

  3. Topic 1 :Introduction to the atomic basic structure

  4. Electromagnetic spectrum E keV 1.5 3 eV 0.12 keV 1 10 102 103 104 X and  rays IR UV light 100 10 1 0.1 0.01 0.001 4000 8000  Angström IR : infrared, UV = ultraviolet

  5. The atomic structure • The nuclear structure • protons and neutrons = nucleons • Z protons with a positive electric charge (1.6 10-19 C) • neutrons with no charge (neutral) • number of nucleons = mass number A • The extranuclear structure • Z electrons (light particles with electric charge) • equal to proton charge but negative •  The atom is normally electrically neutral

  6. Topic 2 :Quantities and units

  7. Basic units in physics (SI system) • Electric charge : 1 coulomb [C] • Power : 1 watt [W] (1 J/s) • 1 mAs = 0.001 C • electron-volt [eV] : 1.603 10-19 J • Proton electric charge = 1.6 10-19 C • mass of proton : 1.672 10-24 g

  8. Atom characteristics A, Z and associated quantities • Hydrogen A = 1 Z = 1 EK= 13.6 eV • Carbon A = 12 Z = 6 EK= 283 eV • Phosphor A = 31 Z = 15 EK= 2.1 keV • Tungsten A = 183 Z = 74 EK= 69.5 keV • Uranium A = 238 Z = 92 EK= 115.6 keV

  9. Topic 3 :Bremsstrahlung production

  10. Electron-nucleus interaction (I) • Bremsstrahlung: • radiative energy loss (E) by electrons slowing down on passage through a material is the deceleration of the incident electron by the nuclear Coulomb field radiation energy (E) (photon) is emitted.

  11. N N Bremsstrahlung spectrum E E n(E) n1E1 n2E2 n1 n3E3 n2 n3 Emax E1 E1 E2 E2 E3 E3 Electrons strike the nucleus

  12. Electron-nucleus interaction (II) • With materials of high atomic number • the energy loss is higher • The energy loss by Bremsstrahlung • > 99% of kinetic E loss as heat production • it increases with increasing electron energy X-rays are dominantly produced by Bremsstrahlung

  13. Bremsstrahlung continuous spectrum • Energy (E) of Bremsstrahlung photons may take any value between “zero” and the maximum kinetic energy of incident electrons • Number of photons as a function of E is proportional to 1/E • Thick target  continuous linear spectrum

  14. Bremsstrahlung spectra dN/dE dN/dE (spectral density) E0 E0 E E From a “thick” target From a “thin” target E = energy of emitted photons E0= energy of electrons

  15. E Bremsstrahlung Bremsstrahlung after filtration keV keV 50 100 150 200 X-ray spectrum energy • Maximum energy of Bremsstrahlung photons • kinetic energy of incident electrons • In X-ray spectrum of radiology installations: • Max (energy) = X-ray tube peak voltage

  16. Ionization and associated energy transfers Example: electrons in water • ionization energy : 16 eV (for a water molecule) • other energy transfers associated to ionization • Excitation energy(each requires only a few eV) • thermal transfers(at even lower energy) • W = 32 eV is the average loss per ionization • it is characteristic of the medium • independent of incident particle and of its energy

  17. Topic 4 :Characteristic X-rays

  18. Spectral distribution of characteristic X-rays (I) • Starts with ejection of e- mainly from k shell (also possible for L, M,…) by ionization • e- from L or M shell fall into the vacancy created in the k shell • Energy difference is emitted as photons • A sequence of successive electron transitions between energy levels • Energy of emitted photons is characteristic of the atom

  19. Energy (eV) K1 100 80 60 40 20 - 20 - 70 - 590 - 2800 - 11000 - 69510 K2 6 5 4 3 2 0 P K1 O N L L K2 M L L 0 10 20 30 40 50 60 70 80 (keV) K Spectral distribution of characteristic X-rays (II)

  20. Summary • The elemental parts of the atom constituting both the nucleus and the extranucleus structure can contribute to photon production. • Electrons and photons have different types of interactions with matter • Two different forms of x-rays production, Bremsstrahlung and characteristic radiation contribute to the image formation process. Add module code number and lesson title

  21. Where to Get More Information • Attix FH. Introduction to radiological physics and radiation dosimetry. New York, NY: John Wiley & Sons, 1986. 607 pp. ISBN 0-47101-146-0. • Johns HE, Cunningham JR. Solution to selected problems from the physics of radiology 4th edition. Springfield, IL: Charles C. Thomas, 1991. • Wahlstrom B. Understanding Radiation. Madison, WI: Medical Physics Publishing, 1995. ISBN 0-944838-62-6. Add module code number and lesson title

  22. DIAGNOSTIC RADIOLOGYPart 2X-ray production Aim: To become familiar with the technological principles of the X-ray production. IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

  23. Introduction A review is made of: • The main elements of the of x-rays tube: cathode and anode structure • The technology constraints of the anode and cathode material • The rating charts and x-ray tube heat loading capacities Add module code number and lesson title

  24. Contents • Basic elements of an X-ray source assembly • Cathode structure • Anode structure • Rating chart Add module code number and lesson title

  25. Topic 1: Basic elements of the x-ray assembly source • Generator : power circuit supplying the required potential to the X-ray tube • X-ray tube and collimator: device producing the X-ray beam

  26. X-ray tube components • Cathode : heated filament which is the source of the electron beam directed towards the anode • tungsten filament • Anode (stationary or rotating): impacted by electrons, emits X-rays • Metaltube housingsurrounding glass X-ray tube (electrons are traveling in vacuum) • Shielding material(protection against scattered radiation) Add module code number and lesson title

  27. X-ray tube components 1: long tungsten filament 2 : short tungsten filament 3 : real size cathode 1: mark of focal spot Add module code number and lesson title

  28. Topic 2:Cathode structure (I) • Cathode includes filament(s) and associated circuitry • tungsten material : preferred because of its high melting point (3370°C) • slow filament evaporation • no arcing • minimum deposit of W on glass envelope • To reduce evaporation the emission temperature of the cathode is reached just before the exposure • in stand-by, temperature is kept at ± 1500°C so that 2700°C emission temperature can be reached within a second Add module code number and lesson title

  29. Cathode structure (I) • Modern tubes have two filaments • a long one : higher current/lower resolution • a short one : lower current/higher resolution • Coulomb interaction makes the electron beam divergent on the travel to the anode, thus: • lack of electrons producing X-rays • larger area of target used • focal spot increased lower image resolution • Focalization of electrons is crucial Add module code number and lesson title

  30. Topic 3 : Anode structure(X-ray tube characteristics) • Anode mechanical constraints • Material : tungsten, rhenium, molybdenum, graphite • Focal spot : surface of anode impacted by electrons • Anode angle • Disk and annular track diameter (rotation frequency from 3,000 to 10,000 revolutions/minute) • Thickness  mass and material (volume)  heat capacity • Anode thermal constraints • Instantaneous power load (heat unit) • Heat loading time curve • Cooling time curve Add module code number and lesson title

  31. Anode angle (I) • The Line-Focus principle • Anode target plate has a shape that is more rectangular or ellipsoidal than circular • the shape depends on : • filament size and shape • focusing cup’s and potential • distance between cathode and anode • Image resolution requires a small focal spot • Heat dissipation requires a large spot • This conflict is solved by slanting the target face Add module code number and lesson title

  32. Anode angle (II) Angle  Angle  Actual focal spot size Actual focal spot size Incident electron beam width Incident electron beam width Increased apparent focal spot size Apparent focal spot size Film Film THE SMALLER THE ANGLE THE BETTER THE RESOLUTION Add module code number and lesson title

  33. Anode heel effect (I) • Anode angle (from 7° to 20°) induces a variation of the X-ray output in the plane comprising the anode-cathode axis • Absorption of photons by anode body is more in low emission angle • The magnitude of influence of the heel effect on the image depends on factors such as : • anode angle • size of film (FOV) • focus to film distance • Anode aging increases heel effect Add module code number and lesson title

  34. Anode heel effect (II) • The heel effectis not always a negativefactor • It can be used to compensate for different attenuation through parts of the body • For example: • thoracic spine (thicker part of the patient towards the cathode side of the tube) • mammography Add module code number and lesson title

  35. Focal spot size and imaging geometry • Focal spot finite size image unsharpened • Improving sharpness small focal spot size • For mammography focal spot size  0.4 • Small focal spot size reduced tube output (longer exposure time) • Large focal spot allows high output (shorter exposure time) • Balance depends on organ movement (fats moving organs may require larger focus) Add module code number and lesson title

  36. Topic 4:X-ray tube rating chart (I) • Tube cooling characteristics and focal spot size • {mA - time} relationship at constant kV • intensity decreases with increasing exposure time • Note: higher power reduced exposure time  reduced motion unsharpness Add module code number and lesson title

  37. Heat loading capacities • A procedure generates an amount of heat depending on: • kV used, tube current (mA), length of exposure • type of voltage waveform • number of exposures taken in rapid sequence • Heat Unit (HU) [joule] : unit of potentialxunit of tube currentxunit of time • The heat generated by various types of X-ray circuits are: • 1 phase units : HU = kV x mA x s • 3 phase units, 6 pulse : HU = 1.35 kV x mA x s • 3 phase units, 12 pulse: HU = 1.41 kV x mA x s • J = HU x 0.71 Add module code number and lesson title

  38. X-ray tube rating chart (II) • Manufacturers combine heat loading characteristics and information about the limits of their x-ray tubes in graphical representations calledTube Rating Charts • Example : • Tube A : a 300 mA, 0.5 s, 90 kV procedure would damage the system operated from a1 half wave rectified generator (unacceptable) • Tube B : a 200 mA, 0.1 s, 120 kV procedure comply with the technical characteristics of the system operated from a3fully rectified generator (acceptable) Add module code number and lesson title

  39. X-ray tube rating chart (IV) 700 600 500 400 300 200 100 X-ray tube B 3 full-wave rectified 10.000 rpm 1.0 mm effective focal spot 70 kVp 50kVp Tube current (mA) 90 kVp 125 kVp Acceptable 0.01 0.05 0.1 0.5 1.0 5.0 10.0 Exposure time (s) Add module code number and lesson title

  40. Anode cooling chart (I) • Heat generated is stored in the anode, and dissipated through the cooling circuit • A typical cooling chart has : • input curves (heat units stored as a function of time) • anode cooling curve • The following graph shows that : • a procedure delivering 500 HU/s can go on indefinitely • if it is delivering 1000 HU/s it has to stop after 10 min • if the anode has stored 120.000 HU, it will take  5 min to cool down. Add module code number and lesson title

  41. 240 220 200 180 160 140 120 100 80 60 40 20 1000 HU/sec Imput curve 500 HU/sec 350 HU/sec Heat units stored (x 1000) 250 HU/sec Cooling curve 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Elapsed time (min) Anode cooling chart (II) Maximum Heat Storage Capacity of Anode Add module code number and lesson title

  42. Summary • The main parts of the system contributing to the desired X-ray production: • provide the required source of power • deliver an appropriate x-ray spectrum • ensure the optimum adjustment of exposure to warrant the image quality Add module code number and lesson title

  43. Where to Get More Information • Equipment for diagnostic radiology, E. Forster, MTP Press, 1993 • The Essential Physics of Medical Imaging, Williams and Wilkins. Baltimore:1994 • Manufacturers data sets for different x-ray tubes Add module code number and lesson title

  44. DIAGNOSTIC RADIOLOGY Part 3: X-ray Equipment and Parts Aim:To become familiar with the technological principles of the X-ray production. IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

  45. Introduction A review is made of: • The system for x-ray beam production : generator, rating chart, automatic exposure control system • The mode of operation of X-ray equipment Add module code number and lesson title

  46. Contents • Rating chart • X-ray generator • Automatic Exposure Control (AEC) • X-ray equipment operation and mode Add module code number and lesson title

  47. Topic 1 : X-ray generator

  48. X-ray generator (I) It supplies the X-ray tube with : Current to heat the cathode filament Potential to accelerate electrons Automatic control of exposure (power application time) Energy supply  1000  X-ray beam energy (of which 99.9% is dissipated as thermal energy) Add module code number and lesson title

  49. X-ray generator (II) • Generator characteristics have a strong influence on the contrast and sharpness of the radiographic image • The motion unsharpness can be greatly reduced by a generator allowing an exposure time as short as achievable • Since the dose at the image plane can be expressed as : D = k0 . kVpn . I . T • kVp : peak voltage (kV) • I : mean current (mA) • T : exposure time (ms) • n : ranging fromabout 3at 150 kVto 5at 50 kV Add module code number and lesson title

  50. X-ray generator (III) • Peak voltage value has an influence on thebeam hardness • It has to be related to medical question • What is the anatomical structure to investigate ? • What is the contrast level needed ? • The ripple “r” of a generator has to be as low as possible r = [(kV - kVmin)/kV] x 100% Add module code number and lesson title

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