Prime Factors

• The x-ray beam

  Quantity
• Quantity of the x-ray beam

  Determined by the number of photons in the useful beam
• Exposure

  Measured in roentgens (R) or milliroentgens (mR)
• Quantity of the x-ray beam

  Affected by: mAs, Kilovoltage, Distance, Filtration
• Quantity of the x-ray beam

  • The primary influencer is mAs
• Quality of the x-ray beam

  Measurement of the beam's penetrability
• Quality of the x-ray beam

  • High energy/Short wavelength - more penetrability
  • Low energy/Long wavelength - less penetrability
• Half-Value Layer (HVL)


  Amount of material needed to reduce x-ray intensity to half its original value; mm Al. Eq.
• Energy, wavelength, & beam penetrability

  • High energy/Short wavelength = More penetrability
  • Low energy/Long wavelength = Less penetrability
• Quality of the x-ray beam

  Affected by: kVp
• Nature (quantity and quality) of the x-ray beam

  Controlled by: Inherent factors, Controllable factors
• Inherent factors of the x-ray tube

  • Anode target material
  • Inherent filtration
  • Generator voltage waveform
• Prime factors of imaging

  • Milliamperage
  • Time
  • Kilovoltage peak
  • Distance
• Milliamperage

  Measurement of x-ray tube current
• Tube Current

  The number of electrons travelling from the cathode towards the anode per second
• Tube current & electron quantity

  Increase in tube current = Increase in electron quantity
• Time

  Length of exposure, measured in seconds (s) and multiplied with milliamperage to find mAs
• Time & electron quantity

  Increase in exposure time = Increase in electron quantity
• mAs

  mA x s, where mA is milliamperage and s is exposure time in seconds
• Different variations of mA and time can still result in the same mAs
• Kilovoltage Peak (kVp)


  Controls both quantity and quality of the x-ray beam
• kVp & quantity

  The change in the quantity of x-rays is approximately directly proportional to the square of the ratio of the change in kVp
• Distance & x-ray intensity

  Increase in SID = Decrease in x-ray intensity to patient
• Distance & x-ray intensity

  Inverse Square Law: the intensity of radiation at a given distance from the source is inversely proportional to the square of the distance
• Distance & x-ray intensity

  Density Maintenance Formula: the exposure of radiation to the IR at a given distance from the source is directly proportional to the square of the distance
• Distance & x-ray intensity

  Increasing OID: Increases distortion, Decreases detail, Decreases density, Decreases contrast
• Law of Reciprocity


  The exposure of an image receptor should remain unchanged as long as the product of the intensity and the duration of the x-ray exposure remain unchanged
• Radiographic density


  The overall blackness of an image
• mAs & radiographic density/exposure

  Increasing mAs proportionally increases x-ray exposure, mAs is considered the primary controller of radiographic density/exposure
• Density


  Density is considered to be exposure
• kVp & density in film-screen

  Increasing kVp increases radiographic density, resulting in a decrease of contrast
• kVp & density in digital imaging

  Increasing kVp does not affect brightness or darkness, but does still result in a decrease of contrast from scatter
• Inherent factors - factors that are a part of the tube
• Controllable factors - factors that a radiographer manages .
• Three inherent factors of the x-ray tube :

  Anode target material

  Inherent filtration

  Generator voltage waveform

  These are not factors a radiologic technologist has control over .
• Tube Current - the number of electrons travelling from the cathode towards the anode per second .
• Increase of current = increase of electron quantity
• When the exposure time is increase :

  More electrons are able to be created .

  More electron go from the cathode towards the anode.
• Anode = positive
• Cathode = negative