finals 2

• 1. rays

  Interact through matter as waves and particles
• 1. rays

  • Have very short wavelengths (10^-8 to 10^-9 meters)
  • The higher the energy, the shorter the wavelength (Inversely Proportional)
• Low energy x-rays

  Tend to interact with whole atoms
• Moderate energy x-rays

  Generally interact with electrons
• High energy x-rays

  Interact with nuclei
• Mechanisms of x-ray interaction with matter

  • Coherent Scattering
  • Compton Effect
  • Photoelectric Effect
  • Pair Production
  • Photodisintegration
• Coherent Scattering


  1. Incident x-ray interacts with target atom causing it to become excited

  2. Target atom releases excess energy as a scattered x-ray with wavelengths equal to incident x-ray

  3. Direction of scattered x-ray is different from incident x-ray
• Coherent Scattering


  • Results in change in direction of x-ray without change in energy
  • No energy transfer = no ionization
  • Most coherently scattered x-rays are scattered in the forward direction
  • Contributes slightly to image noise which reduces image contrast
  • Of little importance in diagnostic radiology
• Thompson Scattering


  Involves a single electron in the interaction
• Rayleigh Scattering


  Results from a cooperative interaction with all the electrons of an atom
• Compton Effect


  1. Incident x-ray ejects an outer shell electron, ionizing the atom

  2. X-ray continues in a different direction with less energy

  3. Energy of Compton scattered x-ray is equal to difference between energy of incident x-ray and energy of ejected electron

  4. Energy divided between scattered x-ray and Compton electron

  5. Scattered x-rays are absorbed photoelectrically

  6. Compton electron loses kinetic energy through ionization and excitation
• Compton Effect


  • Compton-scattered x-rays can be deflected in any direction including 180 degrees from incident x-ray
  • X-rays scattered back in direction of incident beam are called backscatter radiation
  • Probability of Compton scattering decreases as x-ray energy increases
• Compton Scattering

  • Reduces image contrast by increasing image fog
  • Of considerable importance in x-ray imaging
  • Results in occupational exposure for radiologic technologists
• Compton Scattering

  • Most likely to occur with outer shell electrons
  • Increases as x-ray energy increases
  • Increases as atomic number of absorber increases
  • Increases as mass density of absorber increases
• Photoelectric Effect


  1. X-rays are totally absorbed

  2. Photoelectrons escape with kinetic energy equal to difference between energy of incident x-ray and binding energy of electron

  3. Responsible for producing contrast on radiograph

  4. Characteristic x-rays are produced after photoelectric interaction
• Photoelectric Effect

  • In low atomic number atoms, photoelectron kinetic energy is nearly equal to energy of incident x-ray
  • In high atomic number atoms, photoelectron kinetic energy is lower
  • Characteristic x-rays may consist of secondary radiation and behave as scattered radiation, contributing nothing to diagnostic image and not penetrating to image receptor
• Photoelectric Effect


  • Only occurs if incident x-ray has energy equal to or greater than electron binding energy
• Effective atomic number of materials important to radiological science

  • Fat (6.3)
  • Soft Tissue (7.4)
  • Lung (7.4)
  • Bone (13.8)
  • Air (7.6)
  • Iodine (53)
  • Barium (56)
  • Concrete (17)
  • Molybdenum (42)
  • Tungsten (74)
  • Lead (82)
• Photoelectric Effect


  • Most likely to occur with tightly bound electrons
  • Increases proportionately with the cube of the atomic number
  • Increases proportionally with mass density of absorber
• Pair Production


  • Does not occur during x-ray imaging
  • Occurs when x-rays have sufficient energy to escape electron interaction and come close enough to nucleus of atom
  • Incident x-ray energy of 1.02 MeV produces an electron and a positron
• Pair Production


  • Observed in Positron Emission Tomography
  • Electron fills vacancy in atomic orbital shell, positron unites with free electron and mass is converted to energy in Annihilation Radiation
• Photodisintegration

  • Energy greater than 10 MeV, does not occur in diagnostic radiography
  • X-rays with energy above 10 MeV can be absorbed directly by nucleus, causing nucleus to emit nucleons or other nuclear fragments