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. When a sound wave hits the boundary between soft tissue and air, or between soft tissue and bone, most of the energy is reflected; only $0.11 \%$ is transmitted. This means that standing waves can be set up in the body, creating excess thermal energy in the tissues at an antinode. Suppose 0.75 MHz ultrasound is directed through a layer of tissue with a bone $0.50 \mathrm{~cm}$ below the surface. Will standing waves be created? Explain.

Key Concepts

Wavelength Calculation in Tissue

The wavelength of ultrasound in tissue is determined by the frequency of the ultrasound and the speed of sound in the tissue. Given a frequency, the wavelength is calculated as the speed of sound in the medium divided by the frequency. In typical tissue, where the speed of sound is around 1540 m/s, the wavelength can be quite short. Knowing the wavelength is essential for predicting where nodes and antinodes in a standing wave will form, relative to the distance of the reflective boundary.

Standing Wave Formation

A standing wave is formed when two waves of the same frequency and amplitude traveling in opposite directions interfere. In tissues, a reflective boundary (like a bone) can reflect most of the incoming ultrasound energy, and the superposition of the incident and reflected waves leads to a pattern of nodes and antinodes. The antinodes, where constructive interference occurs, can experience localized heating, which is important for therapeutic applications.

Acoustic Impedance and Reflection

Acoustic impedance is a property of a medium that affects how much of an ultrasound wave is reflected when encountering a boundary between two media. A large mismatch in impedance, such as between soft tissue and bone or air, results in most of the energy being reflected back rather than being transmitted. This reflection is key in the formation of standing waves when the incident and reflected waves interfere.

Ultrasound Wave Propagation

Ultrasound is a type of sound wave characterized by frequencies above 20 kHz. In medical applications, ultrasound waves are used both for imaging and therapeutic purposes. Their behavior in tissues—including propagation speed, wavelength, and attenuation—is critical in understanding how energy is delivered and interacted with biological tissues.

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