CT dose

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CT dose is measured and reported via a variety of methods, put simply, it can be divided into three primary categories: exposure, absorbed dose, and effective dose.

It is important to note that to accurately determine a patients dose from a CT scan one must know the patient size and the radiation output. Using CTDI or CTDIvol should be thought of as a measure of how the CT was performed —not the amount dose a patient received .

Exposure 

Is the amount/concentration of radiation at a set point in a known amount of air, this is measured by using an ionization chamber. The measurement of exposure via the ionization chamber is in coulomb per kilogram (Ckg), it was previously measured in roentgen (R) .

Absorbed dose

This is also referred to as the radiation dose, it's the measure of energy absorbed per mass. When it comes to any discussion about how much 'dose' a patient received in CT this is generally what should (see note above regarding CTDI/CTDIvol) be referred to. It is measured in gray (Gy).

Effective dose

Also referred to as the equivalent dose, the effective dose is the measure of radiation calculated with the radiosensitivity of specific organs taken into account. It is measured in Sievert (Sv).

Dose metrics

Most often one will encounter varying metrics when examining the 'dose' of a CT scan that is :

  • CT dose index (CTDI)
    • measured in mGy 
    • standardized measure of dose output 
    • best used to compare CT scanners 
    • not a measure of absorbed dose or effective dose 
  • CTDIvol
    • ​CT dose index that measures radiation per slice of tissue using a reference phantom
    • only takes into account scanner output and therefore not a measure of absorbed or effective dose
  • dose length product (DLP)
    • measured in mGy*cm
    • product of the CTDIvol and scan length
    • factors in the length of the scan to show overall dose output
    • does not take the size of the patient into account and is not a measure of absorbed dose
    • not a measure of effective dose
  • size specific dose estimate (SSDE)
    • measured in mGy
    • takes into account the size of the patient 
    • measure of absorbed dose but not effective dose