Carlton Ch. 14

Exposure vs. Dose

  • Exposure: Radiation intensity in air, measured in roentgens (R).
  • Dose: Amount of radiation absorbed, measured in rad.
  • Diagnostic use of ionizing radiation: risk vs. benefit decision.

Estimating Entrance Skin Exposure (ESE)

  • Maximum exposure calculated at minimum SOD.
  • Overestimate exposure rather than underestimate.
  • Requires calculation of mR/mAs, varies between x-ray machines/tubes.

Fluoroscopic R/min Charts

  • ESE for fluoroscopic equipment measured in R/min.
    • Standard Fluoroscopy: 11.5 R/min (10 cGy/min).
    • High-Level Control Fluoroscopy: 23 R/min (20 cGy/min).

Reducing Patient Dose

  • Communication: Gain patient confidence for cooperation.
  • Positioning:
    • Different projections yield different ESE and absorbed dose values.
    • AP female pelvis vs PA: Lower ovarian exposure. PA skull vs AP: Lower lens of eye exposure.
  • Immobilization: Reduces retakes due to motion artifacts, improves image quality.

Technical Factors

  • Prime factors interrelationship: Kilovoltage, milliamperage, time, distance, focal spot size, filtration.
  • Other factors: Field size, gonad shielding, subject part density, grids.

Kilovoltage Range

  • Increase in kVp without mAs compensation increases dose.
  • Increase in kVp with mAs compensation decreases dose.
  • Optimum kVp: Match k-edge of detector material.

Milliamperage and Time

  • Increase in mAs without compensation increases dose.
  • Lowest possible mAs reduces patient dose.

Distance

  • Increase in SID or SOD results in ESE decrease.
  • Decrease in OID increases SOD, therefore, ESE decreases.

Filtration

  • Increased filtration with kVp modification decreases ESE.
  • Increased filtration with mAs modification (to maintain image quality) increases ESE.

Field Size

  • Decrease in primary beam size decreases patient dose.
  • Significant field size decrease may require marginal mAs increase.
  • Reduction in patient exposure from collimation > marginal increase in mAs.

Grids

  • Higher ratio grid requires increased mAs, increases patient dose.
  • Use lowest ratio grid necessary without jeopardizing image quality.

Digital Image Receptor Systems

  • Respond to 0.01 mR to 100 mR; wide dynamic range.
  • Extreme exposures unacceptable, produces digital data drop, excessive scatter.
  • Avoid overexposing despite post-processing ability.