What are the special units used to quantify radiation?

A becquerel is a measure of radioactivity that indicates how many nuclei decay (disintegrate) per second during natural radioactive decay. In medical fields, it is used as a unit for administering drugs with a radioactivity level that gradually changes, such as drugs for nuclear medicine scans. A gray indicates the amount of energy (absorbed dose) absorbed by human bodies, or substances from radiation. It is used to evaluate the dose that a patient is actually exposed to during radiotherapy or radiological examinations. A sievert is a unit used to indicate the effective dose for radiation protection or legal/regulatory purposes. (Effective dose is based on the size of the risk of biological effects due to radiation exposure.)