Attenuation coefficient

Last revised by Lachlan McKay on 23 Feb 2024

The attenuation coefficient is a measure of how much the incident energy beam (e.g. ultrasound or x-rays) is weakened by the material it is passing through.

Photon interactions

When a photon passes through a matter, it can either penetrate the matter without any interactions (penetration), be completely absorbed by matter (absorption), or deposit its energy and deflected from its original path (scatter). The three main mechanisms of photon interactions in body tissue are 3:

The attenuation of X-rays or gamma rays in tissues is generally affected by several factors, namely the energy of the photon, the thickness of the bodily tissue, atomic number, and density of the tissue 3.

In photon interactions, there are two types of attenuation coefficients 2:

The linear attenuation coefficient will vary with substance density, whereas mass attenuation coefficient will not. As an example, water and ice would have different linear attenuation coefficients but the same mass attenuation coefficient.

A monoenergetic beam has a fixed energy and the energy will not change with any depth of attenuation, although the intensity of beam can be reduced. Meanwhile, in polyenergetic beam, filtration will occur where low energy photons will be filtered out, leaving the higher energy photons. This phenomenon is known as "beam hardening" 5.

Acoustic interactions

In ultrasound, the attenuation coefficient (α) is estimated from the gradient of attenuation against the frequency of ultrasound with units of dB MHz-1 cm-1. In soft tissue, ultrasound attenuation is 0.5 to 0.75 dB MHz-1 cm-1 4. In fatty liver and fibrotic liver, the attenuation coefficient increases. In the case of breast tissue, the attenuation coefficient decreases in fatty tissue and medullary carcinoma. It increases fibrotic tissue and ductal carcinoma of breasts 1.

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