Ultrasound Physics: Wavelength and Propagation Speed

A set of vocabulary flashcards covering the physical principles of wavelength and propagation speed in diagnostic ultrasound, including tissue-specific values and mathematical relationships.

Wavelength

The distance or length of one complete cycle of a sound wave.

Units of Wavelength

Measured in units of length, such as $\text{mm}$ and meters.

Clinical Wavelength Range

In clinical imaging, the wavelength in soft tissue ranges from $0.1$ to $0.8\,mm$.

Determinants of Wavelength

Wavelength is the only parameter determined by both the sound source and the medium.

Wavelength and Period Differentiation

Wavelength refers to the distance of a single cycle, while period refers to the time it takes to complete a single cycle.

Wavelength and Frequency Relationship

Inversely related; as frequency increases, wavelength decreases. If frequency is doubled, wavelength is halved.

Soft Tissue Wavelength (1 MHz)

Sound with a frequency of $1\,MHz$ has a wavelength of $1.54\,mm$ in soft tissue.

Wavelength Formula in Soft Tissue

$$\text{wavelength (mm)} = \frac{1.54\,mm/\mu s}{\text{frequency (MHz)}}$$

Image Quality and Wavelength

Shorter wavelengths created by high frequency sound produce higher quality images with greater detail.

Propagation Speed

The rate at which a sound wave travels through a medium, measured in $m/s$ or $mm/\mu s$.

Clinical Speed Range

In the human body, the speed of sound ranges from $500\,m/s$ to $4000\,m/s$ depending on the tissue.

Determinants of Propagation Speed

Speed is determined only by the medium through which sound travels; it is not affected by the frequency of the sound wave.

Speed of Sound in Soft Tissue

The average speed is $1540\,m/s$, also reported as $1.54\,mm/\mu s$ or $1.54\,km/s$.

General Rule for Speed in Media

Sound travels fastest in solids, slower in liquids, and slowest in gases.

Speed of Sound in Lung Tissue

$$500\,m/s$$

Speed of Sound in Bone

$$3500\,m/s$$

Propagation Speed Formula

$$\text{speed (m/s)} = \text{frequency (Hz)} \times \text{wavelength (m)}$$

Stiffness (Bulk Modulus)

The ability of an object to resist compression; it is directly related to speed.

Density

The relative weight of a material; it is inversely related to speed (as density increases, speed decreases).

Elasticity and Compressibility

Terms that describes the opposite of stiffness; highly compressible materials have slower sound speeds.

Rules of Thumb for Speed

Stiffness and speed move in the same direction (S and S); Density and speed move in different directions (D and S).

Primary Influence on Speed

Although both affect speed, stiffness has the greatest influence on propagation speed.

Adjustable Parameters

Amplitude, power, and intensity (the "bigness" parameters) are the only parameters that can be changed by the sonographer.