Things to Remember

  • Dose represents the energy deposited per unit mass; in CT, this is represented by the CTDI. Dose depends on mA, kV, and pitch.
  • The total energy deposted is the dose-length product. DLP = CTDI * scan length. The DLP can be converted into an estimate of the effective dose, which represents an average risk for stochastic radiation effects (i.e. cancer). However, the CTDI, DLP, and effective dose are not accurate for large or small patients.
  • For the same technique, large patients will have lower doses and smaller patients will have higher doses. This illustrates the importance of adjusting the scan settings (mA and kV) for pediatric and obese patients. Automated exposure control (tube current modulation) can perform some of these adjustments for you by using the CT topogram to estimate the patient's attenuation.
  • Decreasing the kV can improve contrast and decrease dose, but this is only effective in small and normal-sized patients. Very large patients will attenuate too much of the low-energy x-rays and their doses may actually increase.
  • Proper patient centering is critical to avoid excess dose (and noise) from the bowtie filter and AEC system.

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