Equivalent dose is an amount that takes the damaging properties of different types of radiation into account. Not all radiation is alike.
Because all radiation used in diagnostic medicine has the same low-harm potential, the absorbed dose and the equivalent dose are numerically the same. Only the units are different. Effective dose is a calculated value, measured in mSv, that takes three factors into account:. Different body parts have different sensitivities to radiation. For example, the head is less sensitive than the chest. Effective dose relates to the overall long-term risk to a person from a procedure and is useful for comparing risks from different procedures.
The actual risk to a patient might be higher or lower, depending on the size of the patient and the type of procedure. Example of absorbed dose, equivalent dose and effective dose. If you have a CT of the abdomen, what is the dose to the abdomen? Absorbed dose and equivalent dose measurements can be used to assess short-term risk to tissues. Short term is weeks to months. For properly performed diagnostic examinations, there will be no short-term effects from the radiation exposure, so absorbed dose and equivalent dose are not very useful.
For patients, the most important dose quantity is effective dose, because it allows for simple comparisons of long-term risks. Radiation dose is not like medicine. A dose of radiation is not the same as a dose of medication.
There are several quantities in which dose is measured e. There are other dose quantities that have not been discussed. In the simplest cases, for gamma photon and beta electron radiation, the radiation weighting factor is 1, and therefore, for example, an absorbed dose of 1 mGy in an organ equals an equivalent dose of 1 mSv to that organ. This is the most frequently used dose in radiological protection. Unless you see mention of a specific organ, a "dose" in Sv or mSv is the effective dose.
In the simplest cases, for uniform whole-body exposure to gamma photon or beta electron radiation, the radiation weighting factor is 1, and the tissue weighting factors add up to 1, and therefore, for example, an absorbed dose of 1 mGy equals an effective dose of 1 mSv. ICRP Publication paragraphs , , and In radiation biology, clinical radiology, and radiological protection the absorbed dose, D, is the basic physical dose quantity, and it is used for all types of ionising radiation and any irradiation geometry.
It is defined as While it is defined at any point in matter, its value is obtained as an average over a mass Absorbed dose is a measurable quantity and primary standards exist to determine its value. When using the quantity absorbed dose in practical protection applications, doses are averaged over tissue volumes. It is assumed that, for low doses, the mean value of absorbed dose averaged over a specific organ or tissue can be correlated with radiation detriment for stochastic effects in that tissue with an accuracy sufficient for the purposes of radiological protection.
ICRP Publication paragraph The protection quantities are used to specify exposure limits to ensure that the occurrence of stochastic health effects is kept below unacceptable levels and that tissue reactions are avoided. The definition of the protection quantities is based on the average absorbed dose, D T,R in the volume of a specified organ or tissue T see Table 3 , due to radiation of type R see Table 2. The radiation R is given by the type and energy of radiation either incident on the body or emitted by radionuclides residing within it.
The protection quantity equivalent dose in an organ or tissue, H T , is then defined by where wR is the radiation weighting factor for radiation R. The sum is performed over all types of radiations involved. The unit of equivalent dose is J kg -1 and has the special name sievert Sv. The development of The sum is performed over all organs and tissues of the human body considered to be sensitive to the induction of stochastic effects.
These w T values are chosen to represent the contributions of individual organs and tissues to overall radiation detriment from stochastic effects. URL of Article. Quiz questions. Farr's physics for medical imaging. Saunders Ltd. Read it at Google Books - Find it at Amazon 2. Dendy PP, Heaton B. Physics for diagnostic radiology. Read it at Google Books - Find it at Amazon 3. ICRP publication Ann ICRP.
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