mGy to mSv Radiation Dose Units 101 (Absorbed Dose, Equivalent, Effective Dose)

Conversion for mGy to mSv in units is converting from the absorbed dose (mGy) that is measured on the scanner to the equivalent dose first and then to the effective dose. The equivalent dose accounts for the radiation type and how damaging that radiation type is. For x-ray/CT imaging this part is easy as the radiation weighting factor is 1.0, so to convert from absorbed dose to equivalent dose you just need to multiply by 1.0. Finally to convert to the effective dose you need to take into account the radiosensetivsity of the organs involved. For each organ you need to multiply the equivalent dose times the tissue weighting factor.

For an illustrated guide see our post: https://howradiologyworks.com/doseunits

Chapters
00:00. Intro
00:25 Absorbed Dose
02:00 Equivalent Dose
03:00 Effective Dose

Absorbed dose is a measure of the energy deposited per unit mass of tissue. The SI units are Gray (Gy) which is 1 Joule of energy per kilogram (J/kg). Often, in radiology equipment, we’re looking at doses that are much lower than Gray, so we often talk about units of milliGray for instance of 1/1000 of a Gray.

The absorbed dose is different from the exposure in that it is a measurement in a tissue like material and we are interested in the energy absorbed within the material.

In this table we have the conversion between rads and Gy(mGy).

Traditional Unit SI Unit
rad Gy
100 erg/g 1 J/kg
1 rad 10 mGy
100 rads 1 Gy
100 mrads 1 mGy
Depending on the type of radiology equipment different methods for estimating the absorbed dose may be used. It is not feasible to insert ionization chambers into the body during the exams so estimates of the absorbed dose have been developed.

On the other hand, for CT the absorbed dose is measured in tissue like phantoms by inserting ion chambers into the phantom itself during the measurements.

The damage caused by radiation to individuals depends of type of radiation that is incident on the body and the anatomy that is irradiated.
Equivalent dose is calculated by multiplying the absorbed radiation dose by a weighting factor specific to each type of radiation.

The need to have these radiation weighting factors is described in the description of LET and RBE. As different types of radiation have varying biological effects even if the radiation dose is the same.

The relative weighting that converts from Absorbed Dose to Effective Dose are given in this table.

Radiation Type Radiation Weighting Factor
(WR) (ICRP 2007)
Photons (x-rays) 1
Electrons 1
Protons 1
Alpha particles 20
Neutrons Energy Dependent
Luckily for those of us who are primarily concerned with x-ray radiography and CT the conversion is very easy since the weighting factor is 1.0. So the Absorbed dose and the Equivalent Dose will have the same value but with different units.

When the dose has been converted to Equivalent dose it is measured in Sieverts (Sv) rather than in Gray(Gy).

Patients may be exposed to other types of radiation with different relative bioliogical impact, for example, alpha radiation, will have more sever effects given the same radiation dose. Thus, the need to track the Equivalent Dose in addition to the physical unit of the Absorbed Dose.

One final weighting which will be discussed in the next section is to account for which body parts have been irradiated in reporting the radiation dose.

Effective Dose
Not all organs are equally radiosensitive and a means is needed to account for this varied radio sensitivity across organ and tissue types. For instance hereditary effects [add link to header in radiation biology] are only possible in the gonads when germline cells receive radiation damage so a relatively high weight is given to the gonads.

Additionally, in the somatic (non-germline cells) there is varying radiosensitivity which is directly dependent upon how frequency the different tissue types are reproduced within the body.

For instance bone marrow cells are continuously being reproduced and thus will have a higher sensitivity to radiation.

The ICRP has determined the effective weighting factors:

Organ Tissue Weighting
Factor (ICRP 2007)
Gonads 0.08
Red Bone Marrow 0.13
Colon 0.19
Lung 0.16
Stomach 0.12
Breasts 0.12
Bladder 0.04
Liver 0.04
Esophagus 0.04
Thyroid 0.04
Skin 0.01
Bone surface 0.01
Salivary glands 0.01
Brain 0.01
Rest of body 0.12
Total 1
If we want to calculate the Effective Dose, we take our Equivalent Dose and then we multiply it by a weighting for each organ that is irradiated. So for each of the organs which is exposed, we have a weighting factor.

Multiplying the Equivalent Dose that each organ receives with weighting factor and adding up all of the contributions gives an Effective Dose.

The effective dose is an important quantity to understand and it is applicable to estimate potential risk to a large population.