1.5 Ultrasound and Tissue

Ultrasound is marked by a number of properties when it passes through tissue. Reflection occurs on border surfaces between tissues with different wave propagation properties (such as fat, muscle, and blood). The degree of reflection depends on the magnitude of this difference. The remaining ultrasound energy may either penetrate deeper or be absorbed by the tissue. Usually the ultrasound wave hits several different reflector surfaces. Interaction with tissue causes the ultrasound energy to diminish and become weaker as it penetrates deeper. This is also known as attenuation and is similar to sound that becomes fainter when one moves further away from it. You will read more about attenuation when we discuss 2D imaging and artifacts.

Reflection
Reflection at the tissue boundary - Multiple reflections and attenuation

As border zones are usually marked by more or less irregular surfaces, the ultrasound waves are scattered in all directions. This allows some of the reflected ultrasound waves to return to the transducer and be used to generate an image.

Multiple Reflections
Scatter of ultrasound waves, allowing waves to reflect back to the transducer
Definitions
Attenuation Attenuation is the energy loss of the ultrasound wave as it passes through tissue. This energy loss is caused by reflection/scatter and absorption.

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