CT dose

Units of dose

We can think of the different dose measurements as a stepwise progression, each time adding an additional variable into the equation.

1. CT Dose Index (CTDI)

First, we measure the dose to the detectors from a single gantry rotation to give us the CTDI.

CT dose index

Definition Dose to the phantom from single gantry rotation
Units mGy
Affected by Collimator
Focus-isocentre distance

2. Weighted CTDI (CTDIw)

The dose is not equal across the scan plane. It is higher in the periphery than in the centre. We need to adjust for this by making the average periphery dose make up 2/3 of the dose to give us the weighted CTDI.

There are separate calculations for imaging the head, body and paediatric patients. In adults we use a head phantom (16 cm) and a body phantom (32 cm) with dosimeters placed at the periphery and centre in order to calculate the weighted average of doses.

Weighted CTDI

Definition Adjusted for spatial variation of dose
Equation CTDIw = 1/3 CTDIcentre + 2/3 CTDIperiphery
Units mGy

3. Volume CTDI (CTDIvol)

We don't scan single slices. The concentration of the dose along a patient is determined by the pitch. The higher the pitch, the larger the gaps between slices and the lower the dose. Taking into account the pitch gives us the volume CTDI.

Volume CTDI


Accounts for effect of pitch. Higher pitch = lower dose as less overlapping

However, many manufacturers autocompensate for changes in pitch by adjusting mA to keep the noise and dose constant.

Equation CTDIvol = CTDIw / pitch
Units mGy

4. Dose length product (DLP)

Now we know the CTDlvol, we multiply this by the distance along the patient we have scanned to give us the dose length product. It is proportional to the radiation risk to the patient.

Dose length product

Definition Total dose to phantom / patient along the distance scanned
Equation DLP = CTDIvol x distance scanned
Units mGy*cm

5. Effective dose (E)

We now have the total dose along the patient. But radiation does not affect all organs equally. Each organ has a sensitivity to radiation that needs to be taken into account. We display this as the effective dose.

Effective dose

Definition Physical effect of total dose on patient determined by the sensitivity of imaged area to radiation

In the latest ICRP103 guideline the equation used to calculate effective dose is:

E = ΣT (WT) x ΣR (WRDT,R) or



  • HT or WTDT,R is the equivalent dose in a tissure or organ (T)
  • WT is the tissue weighting factor
Units Millisieverts (mSv) or J.kg-1 - note that the units have changed as this is the effective dose to patients.


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Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.


FRCR Physics Notes: Medical imaging physics for the First FRCR examination

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Factors affecting dose

  • Tube current
    • Doubling mA = doubling of CTDI, DLP and E
  • Rotation time
    • Doubling rotation time = doubling of CTDI, DLP and E
  • Pitch
    • Doubling pitch = halving of CTDI, DLP and E
  • kVp
    • Dose is approximately ∝ kVp2 i.e. doubling the kVp will increase the dose by a factor of 4 (approximately).

Σ  Summary
CT dose definitions

Next chapter: Ultrasound imaging

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