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What is the Attenuation Coefficient?

Different parts of the body that are scanned using an ultrasound will have a different attenuation coefficient.
Article Details
• Written By: John Lister
• Edited By: Bronwyn Harris
2003-2015
Conjecture Corporation
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The attenuation coefficient is a ratio which compares the decrease in the intensity of an energy beam passing through an object with the distance it passes through that object. Knowing the coefficient can make it much easier to calculate the affects of changing any of the factors in this process.

There are two meanings of coefficient in science and mathematics. The first is used in mathematics to show a multiplier. For example, in algebra with the expression 5x2, 5 is the coefficient of x2. This meaning is not the one used in attenuation coefficient, and it is important to be aware of this as using this meaning would cause inaccuracy and confusion.

The second meaning of coefficient is a quantitative measure of either an effect or a property. It is the ratio by which a change in one property will change another property. For example, a material may expand by 5cm2 for every one degree Fahrenheit increase in its temperature. This ratio is known as the thermal expansion coefficient.

Attenuation, in the context of physics, is a loss of intensity in an energy beam as it passes through a substance or object. Perhaps the easiest example of attenuation to picture is with sunlight. As sunlight passes through a pair of sunglasses, the brightness intensity is reduced.

The attenuation coefficient is thus a ratio comparing the loss of intensity to the distance that the energy beam passes through the material. The units used to express the intensity will depend on the precise energy beam in question. Knowing the attenuation coefficient can help scientists to predict how changes to one part of an interaction, such as thickening the sunglasses, can affect another part of the interaction, such as the brightness of the light which passes through to the eyes.

There are many potential uses of the attenuation coefficient. For example, it can be used in the field of X-rays when working out how different materials affect the intensity needed in the X-ray beam. It can also be used in working out how well a wall absorbs sound. Knowing the attenuation coefficient of a particular material will allow architects to choose the right thickness of the wall to absorb a set level of sound.

The attenuation coefficient is also used in ultrasound. Different parts of the body, such as blood and bones, have widely different attenuation coefficients. Knowing these coefficients and then measuring the decrease in the ultrasound beamâ€™s intensity as it passes through a known distance can give some insight into the differing body parts it is passing through.