The culminating effect of tissue on sound as it travels through the body is attenuation. Scattering is dependent for four different factors: the dimension of the scatterer, the number of scatterers present, the extent to which the scatterer differs from surrounding material, and the ultrasound frequency. In the image below of the left saphenous vein (SV), common femoral vein (CFV), superficial femoral (SFA) and profunda femoris (PFA) arteries, Rayleigh scattering is present within each of the blood vessels. When the sound wave is greater than the structure it comes in contact with, it creates a uniform amplitude in all directions with little or no reflection returning to the transducer. This is common with red blood cells (RBC), where the average diameter of an RBC is 7μm, and an ultrasound wavelength may be 300μm (5 MHz). Rayleigh scattering occurs at interfaces involving structures of small dimensions. In the video below, a needle is inserted into a phantom at an acute angle causing refraction of the sound beam back to the transducer at an oblique angle, resulting in a weaker “second” needle to be seen to the right of the actual needle. Because sound is not reflected directly back to the transducer, the image being depicted may not be clear, or potentially altered, “confusing” the ultrasound system since it assumes that sound travels in a straight line. If the velocity is greater in the second medium, refraction occurs away from the originating beam (B). If the propagation velocity is greater in the first medium, refraction occurs towards the center, or perpendicular (A). The angle of refraction is dependent on two things the angle the sound wave strikes the boundary between the two tissues and the difference in their propagation velocities. The reflections generated do not return directly back to the transducer. Refraction is governed by Snell’s Law and describes reflection where sound strikes the boundary of two tissues at an oblique angle. The different accoustic impedances of the structures located within the muscle result in the various shades of grey seen on the BMode image.
The pectoris major muscle (PM) located between the white arrows is an example of diffuse reflection. The large smooth surface of the bone causes a uniform reflection because of the significant difference in the acoustic impedance between it and the adjoining soft tissue. The tibia, (yellow arrows) is a good example of a specular reflector. However, because of the numerous surfaces, sound is able to get back to the transducer in a relatively uniform manner.
Conversely, soft tissue is classified as a diffuse reflector, where adjoining cells create an uneven surface causing the reflections to return in various directions in relation to the transmitted beam. The greater the acoustic impedance between the two tissue surfaces, the greater the reflection and the brighter the echo will appear on ultrasound. Specular reflectors are large, smooth surfaces, such as bone, where the sound wave is reflected back in a singular direction. Reflection can be categorized as either specular or diffuse.
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