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.