equivalent dose in radiology

However , the absorbed dose in a medium for all ionizing radiation is measured in energy joules per kilogram ( J / kg ) absorbed by per unit mass . But the biologiacl effect produced by the same or equivalent dose of different types of radiation is  not the same. 

     This is due to the difference in the specific ionization that occurs in the medium due to the difference in the biological effect generated by different radiation . Due to different radiation, specific ionization in the medium varies.

A separate unit sievert (Sv) was defined to detect the biologic effect generated by ionizing radiation.  

1 Sievert = 1J / kg

   The dose that produces the biological effect is called Equivalent dose. Sievert is a unit of dose measurement. In the case of this equivalent dose of radiotherapy, it is important for radiation protection. Its Conventional Unit, is Rem.

High ICRP has proposed a radiation weighting factor (Wr) to show the relationship between equivalent       dose and Absorbed dos e. This shows the relative effectiveness of the dose of radiation causing biological damage. 

The  product of absorbed dos e and  radiation weighting factor is   called Equivalent dose . 

Dose equivalent (rem) = Wr X rad

If absorbed dose is displayed in Gy, equivalent dose is displayed in sievert. 

Sv = Wr X Gy

     Wr is a dimensionless or dimensionless  amount and  depends on the LET . Wr is 1 for X-ray and Gamma ray. The value of Wr for Thermal neutron is 5. The value of Wr is 20 for fast neutron and heavy particle (ɑ-particle). High LET radiation can cause more biological damage , so their Wr is more.