Patient dosimetry


X-ray imaging

Factors that increase dose:

  • Beam properties
    • Higher tube current (mA) and exposure time (s)
    • Wider collimation (reduces scatter and irradiated area)
    • Larger field of view (FOV)
    • Higher kVp (however, higher kVp produces more penetrating beam which allows a lower mA to be used which may end up reducing patient dose)
  • Scanner properties
    • No filtration
    • Use of a grid
    • Reduced receptor sensitivity
  • Patient properties
    • Closer to focal spot (x-ray source)
    • Larger patient habitus (larger skin surface to absorb maximum dose)

Fluoroscopy

Factors that increase dose:

  • Beam properties
    • Lower kVp (a less penetrating beam means more radiation absorbed, particularly on skin)
    • Continuous (vs pulsed)
    • Using a higher dose level setting
    • Larger area of collimation
    • Keeping x-ray tube over same anatomical area (maximum skin dose can be reduced by rotating and penetrating patient from different angles, called "dose spreading")
  • Scanner properties
    • Use of a grid
    • Increased electrical magnification
    • Increased geometric magnification (i.e. moving patient closer to source)
  • Patient properties
    • Larger patient habitus (as for x-ray imaging)

CT imaging

Factors that increase dose:

  • Beam properties
    • Higher tube current (mA)
    • Longer exposure time
    • Not using mA modulation 
    • Wider collimation (however, if collimation too small system will compensate for reduced signal by increasing mAs / kVp)
  • Scanner properties
    • Decreasing pitch (normally dose and pitch inversely proportional. However, some scanners automatically correct for pitch by maintaining same 
    • Use of noise reduction algorithm allows lower dose to be used
  • Patient properties
    • Smaller patient (more x-rays will penetrate to the centre and deposit a higher dose N.B. a larger patient will receive more total x-rays but dose is measured per unit mass)

Nuclear imaging

Factors that increase dose:

  • Increased amount of injected radioactivity

Next chapter: Appendix


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