ATTENUATION is the Loss and Absorption of Ultrasound Energy Through a Medium
Attenuation is a fairly easy concept to understand compared to impedance. It just describes how rapidly does a medium reduce the intensity of an ultrasound wave as it passes through it. The two mediums with the highest amounts of attenuation are actually AIR and BONE!
As you can see attenuation is not simply dependent on the density of the material like impedance is. Look at ultrasound physics table below to see the relationship between tissue density, impedance, and attenuation:
| Tissue or Material | Speed of Sound (m/s) | Acoustic Impedance (kg/[s m2]) × 10^6 | Density (g/cm3) | Attenuation (dB/cm/MHz) |
|---|---|---|---|---|
| Air | 330 | 0.0004 | 0.0012 | 12 |
| Fat | 1450 | 1.38 | 0.95 | 0.63 |
| Blood | 1575 | 1.66 | 1.055 | 0.18 |
| Liver | 1570 | 1.69 | 1.06 | 0.94 |
| Bone | 4080 | 7.75 | 1.9 | 15 |
This is the reason that ultrasound waves can’t pass through air or bone. The ultrasound waves either get reflected back (impedance mismatch) or gets absorbed (high attenuation).
Attenuation will account for the “Shadowing” artifact seen in bone or gall stones.
