These angles are measured with respect to the normal line, represented perpendicular to the boundary. Refraction describes reflection where sound strikes the boundary of two tissues with different impedance, at an oblique angle.Īs sound passes the border between media, depending upon the relative refractive indices of the two media, the light will either be refracted to a lesser angle or a greater one.
This is termed diffuse reflection and is often seen on US image as various shades of grey located within the structure imaged.
Diffuse/scatter: When US waves hit an irregular surface between two tissues, such as in soft tissue, the reflections return to the transducer in a chaotic and disorganised way relative to the initial US beam. Often specular reflectors due to the significant difference in acoustic impedance between the two surfaces, appear very bright on the US image. Specular: When US waves hit a smooth, large surface, such as bone, the sound wave is reflected back in a singular uniform direction, this is referred to as a specular reflection. Reflection can be categorised as either specular or diffuse/scatter. The greater the acoustic impedance between the two tissue surfaces, the greater the reflection and the brighter the returning echo will appear on US, as a greater amount of energy is reflected back towards the transducer. At the moment an US wave hits this interface, part of the wave is reflected back (echo) towards the original medium, reducing the strength of the US wave moving forward. Reflection occurs when a sound wave transfers from one medium to another i.e. This loss of energy is referred to as attenuation. As an US wave is transmitted through a medium, particles vibrate and friction converts this energy into heat, reducing the US wave energy. Sound waves can be absorbed/attenuated, reflected, refracted and/or scattered.Ībsorption accounts for the main form of attenuation.
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