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 Focusisocentre distance 
2. Weighted CTDI (CTDI_{w})
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  CTDI_{w} = 1/3 CTDI_{centre} + 2/3 CTDI_{periphery} 
Units  mGy 
3. Volume CTDI (CTDI_{vol})
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 

Definition 
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  CTDI_{vol} = CTDI_{w} / pitch 
Units  mGy^{} 
4. Dose length product (DLP)
Now we know the CDTIvol, 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 = CTDI_{vol} 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 
Equation 
In the latest ICRP103 guideline the equation used to calculate effective dose is: E = ΣT (W_{T}) x ΣR (W_{R}D_{T,R}) or E = Σ W_{T}H_{T} Key:

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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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 ∝ kVp^{2} i.e. doubling the kVp will increase the dose by a factor of 4 (approximately).