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PASSAGE OF US THROUGH TISSUES

As the US moves through tissues, some of the wave is reflected back to the machine and some of it keeps passing through the tissues. Reflection gives us the shades of grey on the screen creating the image. 

As the wave moves through tissues, it causes movement and heating of tissue particles. This causes the US wave to degrade (attenuation). Similarly tissue characteristics may cause the US wave to change direction (refraction) or scatter. 

ATTENUATION

The degradation of the US wave is called attenuation. As the US wave passes through the tissues it decreases in intensity and amplitude. This occurs mainly due to absorption of energy from the US wave by the tissues and a small part due to reflection of the wave by structures in the form of echoes. 

The rate of attenuation of the US wave in tissues is called the attenuation coefficient and is calculated in decibels (dB) per cm. The important numbers to remember are: an attenuation coefficient of 3dB/cm leads to a 50% decrease in the intensity of the US wave per 1cm of tissue traversed and an attenuation coefficient of 10dB/cm leads a 90% decrease in the intensity of the US wave per 1cm of tissue travelled. 

Higher frequency US is attenuated faster than low frequency waves. This is why high frequency US cannot penetrate as deeply into the tissues. So a 10MHz high frequency transducer will have an attenuation coefficient of 5dB/cm which allows the US to only penetrate 6cm. On the other hand, a low frequency 2MHz transducer will have an attenuation coefficient if 1dB/cm leading to a maximum penetration depth of 30cm!

Similarly, some tissues are more attenuating than others due to higher levels of absorption. Bone and air have the highest attenuation coefficients. Blood and fluids have the lowest attenuation coefficients. This is why there is often shadowing behind bone and structures containing calcium: the US wave is degraded so much, it just can't go deeper to see past the bone/ calcium containing structure. 

Similarly, the image deeper to fluid filled structures will be brighter (or more hyperechoic) because the US wave hasn't been attenuated as much as it passes through the fluid. This is called posterior acoustic enhancement and occurs because to compensate for attenuation which occurs through soft tissue, the machine will amplify echoes returning from deeper structures. Thus, when  the US wave passes through a fluid filled structure (and does not attenuate as anticipated), this amplification falsely brightens the echoes posterior to the fluid filled structure.

REFRACTION

Refraction is where the US wave changes direction. 

This occurs when the wave meets a tissue interface at an oblique angle and the two tissues have different propagation speeds. You can work out which direction the US wave will go with a complex formula. But all you need to realise is that if the wave changes direction: artefacts will be generated. 

When the wave changes direction, it will hit structures which are outside its initial trajectory, and echoes from this misguided wave will return to a different crystal on the machine. This crystal will assume that the echo returned from its own previous pulse and will miscalculate the location of this object, Similarly, there maybe dropout of US information posterior to where the US wave changed direction (edge artefact).

If the speed of US propagation (c) is the same in two tissues or the angle of incidence is zero, refraction does not occur. 

SCATTER

Scatter creates the grainy appearance of soft tissue.

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