In addition to producing images, ultrasound can be used to...

In addition to producing images, ultrasound can be used to heat tissues of the body for therapeutic purposes. An emitter is placed against the surface of the skin; the amplitude of the ultrasound wave at this point is quite large. When a sound wave hits the boundary between soft tissue and bone, most of the energy is reflected. The boundary acts like the closed end of a tube, which can lead to standing waves. Suppose $0.70 \mathrm{MHz}$ ultrasound is directed through a layer of tissue with a bone $0.55 \mathrm{cm}$ below the surface. Will standing waves be created? Explain.

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Key Concepts

Ultrasound Wave Propagation

This concept refers to how ultrasound waves travel through different media. In medical applications, ultrasound waves propagate through soft tissues where their speed, attenuation, and scattering properties are determined by the medium’s characteristics.

Acoustic Impedance and Impedance Mismatch

Acoustic impedance is a property of a medium that describes how much resistance it offers to the passage of a sound wave. When an ultrasound wave encounters a boundary between two media, such as soft tissue and bone, a significant mismatch in acoustic impedance leads to a large proportion of the wave being reflected back.

Wave Reflection at Boundaries

When a sound wave meets a boundary where there is an impedance mismatch, part of the wave is reflected back into the original medium. This reflection is analogous to sound waves impacting a rigid barrier, similar to a closed-end condition in a tube, and is key to understanding energy distribution within the tissue.

Standing Wave Formation

Standing waves occur when incident and reflected waves of the same frequency interfere with each other. This interference can lead to regions of constructive and destructive interference, known as nodes and antinodes, particularly when the reflected wave is strong, as is the case when encountering a boundary with high reflection.

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