Effective dose E is used as a measure of the radiation dose for expressing relative risk to humans. The exposures related to different types of medical x-ray examinations can be compared using the quantity effective dose. Why effective dose? The effective dose E, often expressed in millisievert (mSv), is generally used as a measure of radiation dose of patients and personnel in medical diagnostics . Unless indicated otherwise, 'dose' often refers to effective dose. Using the quantity effective dose, the exposures for different types of medical x-ray examinations can be compared. The effective dose is not a measurable quantity, but is calculated from the physically absorbed dose (with Gray (Gy) as a unit) in the exposed tissues/organs. The point is that different tissues/organs are not equally sensitive to the possible adverse effects of radiation, such as cancer induction and mutations. For this reason, the different tissues/organs have been assigned tissue weighting fac...