Radiation Dose

• Radiation exposure is related to the amount of ionization of air produced by an x-ray beam. It is a radiation source-related term and is a measured quantity.
• Radiation dose is a body-related term and is calculated from the exposure. For a given measure of radiation exposure, we can specify the amount of radiation energy deposited in the patient’s body as a result of that exposure.

Factors Affecting Radiation Dose

Radiation dose must be optimized. Insufficient radiation dose results in increased noise and degradation of image quality. An increase in radiation dose above a certain level does not further improve image quality; it merely deposits more radiation in the patient’s body. Radiation dose can be modified by adjusting the tube voltage, tube current, scan time, scan coverage, collimation or detector configuration, pitch, and table movement.

Tube voltage and radiation dose

• The amount of tube voltage (kVp) affects radiation dose.
  • Reduction of the tube voltage will decrease the output of the x-ray tube and reduce the radiation dose to the patient.
  • Inappropriate reduction of the voltage may result in a marked increase in CT tissue attenuation and noise, particularly in large patients.
• Most CT scanners offer a limited number of choices for kVp values when performing CT scans.
  • Routine body CT for adult patients is generally performed at 120 to 140 kVp.
  • The use of 80 kVp is a well accepted level when attempting to reduce radiation doses in pediatric patients.

Tube current and radiation dose

• The tube current (mA) is usually adjustable over a wide range; from 10 mA to 800 mA.
• The effect of tube current on image quality is more straightforward than that of tube voltage.
  In general, image quality improves with an increase in tube current.
• Radiation dose and image noise are directly affected by the product of tube current and scan time.

Pitch and radiation dose

• For single-detector–row CT scanners, overall radiation dose and scan duration decrease proportionally with increasing pitch (or table speed) if the tube voltage and current are kept constant.
  Noise remains constant with increasing pitch.
• For multidetector-row CT scanners, the relationship between pitch and radiation dose is not straightforward.
  • The tube current can be increased to compensate for increased noise at higher pitch values.
  • Increased pitch does not translate directly into reduced radiation dose.