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Other ultrasound principles that affect our image include absorption, scatter, and angle of reflection. As the sound waves enter the body, some of them are absorbed by the tissues and are never reflected back to the probe. These waves are lost and do not contribute to the image. Furthermore, many of the waves are scattered by the tissues and their surface irregularities and either return to the probe (receiver) in a distorted path or do not return at all. As a result, the ultrasound waves are “misinterpreted” by the processor, and the image and its resolution are affected.

Figure 2.2. Acoustic impedance (106 kg/m2sec) of common body tissues or substances. This figure illustrates the degree of difference in acoustic impedance between substances that helps determine sound wave transmission. The greater the difference, the greater is reflection or loss of transmission. One can see how ultrasound is ideally suited for most soft tissue and why it is not suited for imaging bone or air‐filled structures (Reef 1998).

Figure 2.3. Attenuation (db/cm/MHz) in common tissues. Attenuation of sound energy within tissues varies with the frequency of the sound and is affected by reflection, scattering, and absorbance. Note how bone and air have the greatest attenuation values (Reef 1998).