finals 3

• Differential absorption


  Process whereby some of the x-ray beam is absorbed in the tissue and some passes through the anatomic part and is received by the image receptor

  
• Image Receptors


  Devices that receive the radiation leaving the patient to create an image

  
• Differential Absorption


  • Occurs because of Compton Scattering, Photoelectric Effect, and x-rays transmitted through the patient

  
• Three important types of x-rays to make a radiograph

  • Those scattered by Compton Interaction (FOG)
  • Those absorbed by the photoelectrically (Radiopaque)
  • Those transmitted through the patient without interaction (Radiolucent)

  
• Increased differential absorption


  Decreased kVp
• Increased differential absorption


  Increased image contrast
• Increased differential absorption


  Increased patient dose (because kVp is decreased, more low energy x-rays do not pass through the patient)
• As X-ray energy increases

  • Fewer Compton interactions
  • Many photoelectric interactions
  • More transmissible through tissue

  
• As tissue atomic number increases

  • No change in Compton interactions
  • Many photoelectric interactions
  • Less x-ray transmission

  
• As tissue mass density increases

  • Proportional increase in Compton interaction
  • Proportional increase in photoelectric interaction
  • Proportional reduction in x-ray transmission
• Compton-scattered x-rays


  • Contribute no useful information to the image
  • The image receptor does not recognize the scattered x-ray as representing an interaction off the straight line from the target
  • Scattered x-rays result in image noise

  
• Photoelectric interactions


  • Provide information to the image receptor
  • Do not reach the image receptor as this interaction is absorbed by the body
  • Represent anatomy with high x-ray absorption characteristics (radiopaque)
  • Produce the light areas in the radiograph

  
• Differential Absorption and Image Formation

  A radiographic image is created by passing an x-ray beam through the patient and interacting with an image receptor, such as an imaging plate in computed radiography. The variations in absorption and transmission of the exiting x-ray beam structurally represent the anatomic area of interest.

  
• Beam Attenuation


  • As the x-ray beam passes through anatomic tissue, it loses some of its energy
  • Fewer x-ray photons remain in the beam after it interacts with the tissue
  • Attenuation is the reduction in the energy or number of photons in the primary beam
  • Occurs as a result of x-ray photons interacting with anatomic structures

  
• Absorption


  • Some x-ray photons are absorbed as the energy of the primary beam is deposited within the atoms comprising the tissue
  • Complete absorption occurs when the incoming x-ray photon has enough energy to remove an inner-shell electron
  • An ejected electron is called a photoelectron and quickly loses energy by interacting with nearby tissues
  • A secondary emission of x-ray photons happens when an outer shell electron fills in an inner shell void; this secondary x-ray photon has very low energy and is unlikely to exit the patient

  
• Transmission


  • Term used when the incoming x-ray photon passes through the anatomic part without any interaction with the anatomic structures
  • The combination of absorption and transmission creates an image that represents the anatomic parts
  • Scattered radiation produces image fog and compromises the image if it strikes the image receptor
  • Exit Radiation is the term used for when the attenuated x-ray beam leaves the patient's body

  
• Factors affecting attenuation

  • Tissue Thickness
  • Type of Tissue
  • Tissue Density
  • Beam Quality
• Tissue Thickness


  • Increasing anatomic structure thickness increases beam attenuation by absorption of scattering
  • X-rays are attenuated by approximately 50% for each 4-5 cm (1.6 to 2 inches) of tissue thickness
  • More x-rays are needed to produce a radiographic image for a thicker anatomic part
• Type of Tissue


  • Increased tissue anatomic number also increases x-ray beam attenuation
  • Example: Bones would need more x-ray photons to penetrate than lungs
  • X-ray absorption is more likely to occur in tissues with higher anatomic number
• Tissue Density


  • Refers to the compactness of the anatomic particles comprising an anatomic part
  • Example: Muscle and Fat are similar in anatomic number. However, the anatomic particles of a muscle is more compact than of fat
  • Increased tissue density also increases beam attenuation
• Beam Quality


  • The penetrability of an x-ray beam affects its interaction with anatomic tissue
  • Higher energy x-rays are more likely to be transmitted through anatomic tissue without interacting with the tissue's anatomic structures
  • Increased x-ray beam quality (energy) decreases beam attenuation

  
• Summary of factors affecting attenuation

  • Tissue Thickness: Increase attenuation, decrease transmission
  • Tissue Anatomic Number: Increase attenuation, decrease transmission
  • Tissue Density: Increase attenuation, decrease transmission
  • X-ray Beam Quality: Decrease attenuation, increase transmission
• Scatter Radiation


  • Results from Compton effect (incoming photons in the diagnostic range loses energy during interactions with the atoms comprising the tissue)
  • An incoming photon loses its energy and ejects an outer shell electron (secondary or Compton electron) from a tissue atom
  • The remaining low energy x-ray photon changes direction and may leave the anatomic part to interact with the image receptor

  
• Scatter Radiation


  • Increases fog in the image and provides no useful diagnostic information
  • Increasing the x-ray energy will result in the decrease of photon absorption, but increase Compton scattering percentage

  
• Reducing Scatter Radiation on Image

  • Use of Beam Restriction
  • Use of Grid
  • Use of Air Gap Technique