xray1 new

xray1 new - 4.0 The Physics of Radiography Radiography...

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4.0 The Physics of Radiography Radiography developed very rapidly after Wilhelm Roentgen discovered the mysterious Xray in 1895. The xrays penetrate the body, but get attenuated differentially by different tissues and bones. Thus the emerging xrays when contain the information to form a “shadowgram” or “latent image” of overlapping organs. Any resulting image on film or digital detector has high resolution in the x and y directions but low resolution in the z (thickness) direction. The method is generally very fast and inexpensive with a very low dose and is one of the most ordered medical tests. The addition of using contrast material can allow the highlighting of blood vessels for additional information. •Ionization, electron-atom interactions, excitation and emission of xrays •Ionizing radiation—particles and electromagnetic radiation •Creation of EM radiation: Brehmsstrahlung radiation •Photoelectric effect, Compton scattering •Attenuation of EM radiation •Narrow-beam, broad beam evaluations of attenuation •Attenuation coefficients •Dose
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Projection X-Ray Disadvantage : Depth information lost Advantage : Cheap, simple ) z ( f ) , , ( μ density electron , z y x attenuation coefficient Measures line integrals of attenuation )dl μ( o d I I x,y,z e Film shows intensity as a negative ( dark areas, high x-ray detection ) Courtesy: Dr. W. Block, U. Wisconsin
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Radiograph of the hand of Röntgen’s wife, 1895. Sagittal Coronal Courtesy: Dr. W. Block, U. Wisconsin modern radiographs
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Assumptions: You know about the shell model and electrons and the binding energies You know about binding energies and the difference between collisional energy transfer and radiative transfer You know about Bremstrahlung and characteristic xrays You know about Raleigh, coherent, photoelectric, Compton scattering, pair production, binding energy.
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Important Points from Radiation Physics •Electrons in atoms are in electronic “shells” K, L, M,…. where each shell can contain 2n 2 electrons in sub-orbitals and n=1 for the K shell, n=2 for L, etc. •Binding energy (BE) of e - depends on Z and n, eg, K shell BE for 1-H is 13.6 eV, for 74-W is 69.5keV •Transitions between shells give characteristic x-rays, eg, in 74-W, L to K is (69.5-11)=58.5keV •Average BE for air=34eV •Electron interactions: distant atomic collisions, those that don’t ionize, promote atomic e - to higher energy levels that
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xray1 new - 4.0 The Physics of Radiography Radiography...

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