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Chapter 5:X-Ray Production - Human Health Campus

IAEAI nternational Atomic Energy AgencySlide set of 121 slides based on the Chapter authored byR. Nowotnyof the IAEA publication (ISBN 978-92-0-131010-1):Diagnostic Radiology Physics: A Handbook for Teachers and Students Objective:To familiarize the student with the principles of X ray Production and the characterization of the radiation output of X ray 5:X-Ray ProductionSlide set prepared by Maherfollowing initial work byS. EdyveanIAEACHAPTER 5 TABLE OF Fundamentals of X Ray X Ray Energizing & Controlling the X Ray X Ray Tube & Generator Collimation & Factors Influencing X Ray Spectra & FiltrationBibliographyDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, 2 IAEACHAPTER 5 TABLE OF Fundamentals of X Ray Characteristic X Ray X Ray Components of the X Ray Choice of Line-Focus principle (Anode angle) Stationary and rotating Thermal Tube Tube housingDiagnostic Radiology Physics: a Handbook for Teachers and Students

been developed using Monte Carlo methods For practical purposes a Semi Empirical approach gives satisfactory results, useful in simulations 5.2 FUNDAMENTALS OF X-RAY PRODUCTION ... a Handbook for Teachers and Students –chapter 5, 21. IAEA The Filament is heated by a current that controls the thermionic emission of electrons, which in turn ...

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Transcription of Chapter 5:X-Ray Production - Human Health Campus

1 IAEAI nternational Atomic Energy AgencySlide set of 121 slides based on the Chapter authored byR. Nowotnyof the IAEA publication (ISBN 978-92-0-131010-1):Diagnostic Radiology Physics: A Handbook for Teachers and Students Objective:To familiarize the student with the principles of X ray Production and the characterization of the radiation output of X ray 5:X-Ray ProductionSlide set prepared by Maherfollowing initial work byS. EdyveanIAEACHAPTER 5 TABLE OF Fundamentals of X Ray X Ray Energizing & Controlling the X Ray X Ray Tube & Generator Collimation & Factors Influencing X Ray Spectra & FiltrationBibliographyDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, 2 IAEACHAPTER 5 TABLE OF Fundamentals of X Ray Characteristic X Ray X Ray Components of the X Ray Choice of Line-Focus principle (Anode angle) Stationary and rotating Thermal Tube Tube housingDiagnostic Radiology Physics.

2 A Handbook for Teachers and Students Chapter 5, 3 IAEACHAPTER 5 TABLE OF Energizing & Controlling the X-Ray Filament Generating the Tube Single-phase Three-phase High-frequency Capacitive discharge Constant voltage Comparison of generator Exposure Timing (AEC) Falling X-Ray Tube & Generator X Ray Tube HousingDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, 4 IAEACHAPTER 5 TABLE OF Collimation & Collimator & Light Inherent Added Compensation Factors Influencing X Ray Spectra & Quantities Describing X Ray Tube Voltage & Tube Voltage Anode FiltrationBibliographyDiagnostic Radiology Physics.

3 A Handbook for Teachers and Students Chapter 5, INTRODUCTIONB asis of Radiological ImagingThe differential absorption of X rays in tissues and organs, due to their atomic compositionX-Ray ProductionPrinciples have remained the same since their discovery however many design refinements have been introducedThis ChapterOutlines the principles of X ray Production and characterizes the radiation output of XRTsDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, FUNDAMENTALS OF X-RAY Production The Production of X Raysinvolves the bombardment of a thick target with energetic electronsElectrons undergo a complex sequence of collisions and scattering processes during the slowing down process which results in the Production of Bremsstrahlungand Characteristic RadiationA Simplified treatment of this process, based on classical theory, is provided in this section Diagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, FUNDAMENTALS OF X-RAY BremsstrahlungEnergetic Electronsare mostly slowed down in matter by.

4 Collisions and Excitation interactionsIf an electron comes close to an atomic Nucleusthe attractive Coulomb forces causes a change of the electron s trajectoryAn accelerated electron or an electron changing its direction emits electromagnetic radiation and given the name Bremsstrahlung(braking radiation)Diagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, FUNDAMENTALS OF X-RAY BremsstrahlungThe energy of the emitted photon is subtracted from the kinetic energy of the electronThe Energyof the Bremsstrahlung photon depends on the AttractiveCoulomb forces and hence on the Distanceof the electron from the nucleusDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, 9 IAEAC lassical TheoryConsider that electron bombardment of a thin target yields a constant energy fluence ( ) from zero up to the initial electron kinetic energy (T) FUNDAMENTALS OF X-RAY BremsstrahlungDiagnostic Radiology Physics.

5 A Handbook for Teachers and Students Chapter 5, 10 IAEAA thick target can be thought ofas a sandwich of many thin targetlayers each producing arectangular distribution of energyfluenceThe superposition of all theserectangular distributions forms atriangularenergy fluencedistribution for a thick target, theIdeal SpectrumThe ideal spectrum does not include any attenuation FUNDAMENTALS OF X-RAY BremsstrahlungDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, 11 IAEAA ccording to this model An increase in electron energy increases the number of thin layers each radiating X raysThe triangular area grows proportional to the square of the electron energyTherefore, the Radiation Outputof an XRT is proportional to U2U: tube voltagerelationship holds if spectral changes due to attenuation and emission of characteristic radiation are FUNDAMENTALS OF X-RAY BremsstrahlungDiagnostic Radiology Physics.

6 A Handbook for Teachers and Students Chapter 5, FUNDAMENTALS OF X-RAY Production Characteristic RadiationA Fast Electroncolliding with an electron of an atomic shell could knock out the electron once its KE exceeds the binding energy of the electron in that shellThe binding energy is Highestin the most innerK-shell and decreases for the outer shells (L, M, ..)The Scatteredprimary electron carries away the difference of kinetic energy and binding energyThe vacancy in the shell is then filled with an electron from an outer shell accompanied by the emission of an X Ray Photonwith an energy equivalent to the Differencein binding energies of the shells involvedDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, 13 IAEAFor each element binding energies and the Monoenergeticradiation resulting from such interactions, are unique and Characteristicfor that elementBinding energy, keVEnergies of characteristic X rays, keVElement L-shellK-shell of characteristic radiation the energy available could be transferred to an electron which is ejected from the shell (Auger Electron) - Production probability decreases with FUNDAMENTALS OF X-RAY Production Characteristic RadiationDiagnostic Radiology Physics.

7 A Handbook for Teachers and Students Chapter 5, FUNDAMENTALS OF X-RAY Production X-ray Spectruma) Ideal Bremsstrahlungspectrum for a tungsten anode(tube voltage 90 kV)b) An Actualspectrum at the beam exit port with characteristic X rays (anode angle: 20 , inherent filtration: 1 mm Be)c) The spectrum Filteredwith an equivalent of mm AlDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, 15 IAEAThe electrons are slowed down and stopped in the Targetwithin a range of a few tens of mX rays are not generated at the surface but within the targetresulting in Attenuationof the X ray beamSelf-Filtrationappears most prominent at the low-energy end of the spectrumCharacteristic Radiationshows up if the kinetic energy of the electron exceeds the binding FUNDAMENTALS OF X-RAY Production X-ray SpectrumDiagnostic Radiology Physics.

8 A Handbook for Teachers and Students Chapter 5, 16 IAEAL-Radiationis totally absorbed by a typical filtration of mm AlThe K-Edgein the photon attenuation of tungsten can be noticed as a drop of the continuum at the binding energy of keVFor tungsten targets the fraction of K-radiation contributing to the total energy fluence is <10%for 150 kV tube FUNDAMENTALS OF X-RAY Production X-ray SpectrumDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, 17 IAEAThe Radiative Mass Stopping Powerof electrons is proportional to Z Integrationalong the electron path gives the total X ray energy fluence as ~ Z I U where I: electron current and U: tube voltageIf a high Bremsstrahlung yield is required, metals with high Zare preferableTungsten (Z=74) is commonly chosen as it also withstands high temperatures (2757 C at 10-2Pa vapour pressure) FUNDAMENTALS OF X-RAY Production X-ray SpectrumDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, 18 IAEAE fficiencyfor the conversion of electrical power to Bremsstrahlung radiation is proportional to U ZAt 100 kV the efficiency is as low as ~ is the cause for most of the technical problems in the design of XRTs as practically all electrical power applied in the acceleration of electrons is converted to FUNDAMENTALS OF X-RAY Production X-ray SpectrumDiagnostic Radiology Physics.

9 A Handbook for Teachers and Students Chapter 5, 19 IAEAThe ideal spectrum appears triangular with the Energy Fluencetaken as the quantity describing the spectral intensityThe Photon Fluenceis a more practical quantity for calculations using spectral data and is therefore used in the following sectionsMore refined models for the generation of X ray spectra have been developed using monte Carlo methodsFor practical purposes a Semi Empiricalapproach gives satisfactory results, useful in FUNDAMENTALS OF X-RAY Production X-ray SpectrumDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, X-RAY Components of the X Ray TubeThe Production of both Bremsstrahlungand Characteristic Radiationrequires energetic electrons hitting a targetPrinciple components of an X ray tube are an Electron Sourcefrom a heated tungsten filament with a focusing cup serving as the tube Cathode, an Anodeor Targetand a Tube Envelopeto maintain an interior vacuumDiagnostic Radiology Physics.

10 A Handbook for Teachers and Students Chapter 5, 21 IAEAThe Filamentis heated by a current that controls the thermionic emission of electrons, which in turn determines the number of electrons flowing from cathode to anode (Tubeor Anode Current) <10 mA in fluoroscopy and 100 to >1000 mA in single exposuresThe accelerating Potential Differenceapplied between cathode and anode controls both X ray energy and 40 to 150 kV for general diagnostic radiology and 25 to 40 kV in mammographyThus Twomain circuits operate within the XRT: Filament circuit Tube voltage X-RAY Components of the X Ray TubeDiagnostic Radiology Physics: a Handbook for Teachers and Students Chapter 5, X-RAY TUBES CathodeThe Arrangementof the filament, the focusing cup, the anode surface and the tube voltage generates an electric field accelerating the electrons towards the focal spot at the anodeThe effect of an Unbiasedfocusing cup on the electric field and electron trajectoriesThe typical Bimodaldistribution of the current density can be seen in a pinhole image of the focusNumbers indicate potential difference near the cup in kVDiagnostic Radiology Physics.


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