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 (if we stick with the same mAs, more photons overall and more with higher energy. However, some higher energy photons pass straight through the patient)
  • 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)


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")
    • Whether fluoroscopy or acquisition mode is selected
  • Scanner properties
    • Use of a grid
    • Increased electrical magnification (zoom modes)
    • Increased geometric magnification (i.e. moving patient closer to source)
    • Decreased distance between the tube and the detector/II
  • Patient properties
    • Larger patient habitus (as for x-ray imaging)

CT imaging

Factors that increase dose:

  • Beam properties
    • Higher tube current (mA)
    • Higher kV
    • 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
  • Each radioisotope will deposit different doses
  • Reduced drinking and urination results in slower loss of activity from the bladder, increasing patient dose


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Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.


FRCR Physics Notes: Medical imaging physics for the First FRCR examination

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