Ultrasound Physics: Attenuation, Mechanisms, and Diagnostic Effects

Vocabulary practice cards covering ultrasound attenuation mechanisms (absorption, reflection, refraction, scattering), tissue attenuation coefficients, half-value layer formulas, wave angles, and acoustic artifacts.

Attenuation

The loss of energy of an ultrasound wave as it propagates through the body.

Absorption

The conversion of ultrasound energy into heat within the medium, acting as the dominant mechanism of attenuation in soft tissue.

Specular Reflectors

Boundaries or reflectors characterized by long, straight interfaces that are larger than the wavelength of the ultrasound beam.

Scattering Reflectors

Reflectors that are smaller than the ultrasound beam's wavelength, causing sound energy to disperse across multiple directions.

Rayleigh Scattering Reflectors

Reflectors that are much smaller than the ultrasound wavelength (such as red blood cells), scattering acoustic energy uniformly in all directions.

Refraction

The bending of the transmitted ultrasound beam across an interface resulting from a change in propagation velocity between media and a non-zero incident angle.

Angle of Incidence

The angle formed between the wave direction (or wavefront) and the line normal (or interface).

Normal Incidence

A condition where the ultrasound wave direction travels parallel to the line normal and strikes the tissue interface at a perpendicular ($90^\circ$) angle.

Angle of Reflection

The angle between the reflected beam direction and the line normal, which is always equal to the angle of incidence ($\emptyset_{\text{incident}} = \emptyset_{\text{reflection}}$).

Angle of Transmission

The angle between the transmitted beam direction and the line normal, which is equal to the incident angle ($\emptyset_{\text{incident}} = \emptyset_{\text{transmission}}$) if no refraction occurs.

Thermal Bioeffects

Biological effects in tissue caused by the absorption of sound energy and its subsequent conversion into heat.

Viscosity (Ultrasound Medium)

The internal molecular attraction of a medium; higher viscosity requires more acoustic energy to displace molecules, increasing sound attenuation.

Acoustic Shadowing

A diagnostic ultrasound artifact characterized by a localized band of reduced echo intensity posterior to a highly attenuating structure, such as a gallstone.

Acoustic Enhancement

A diagnostic ultrasound artifact appearing as increased echogenicity posterior to a weakly attenuating structure (such as fluid), occurring because fluids absorb less energy than adjacent soft tissue without adding energy to the beam.

Output Power (Transmit Power)

The transducer setting controlling acoustic power delivered to the patient; increasing it compensates for attenuation but is kept as the last adjustment due to bioeffect concerns.

Time Gain Compensation (TGC)

Receiver controls that allow selective amplification of returning ultrasound signals at varying depths to compensate for tissue attenuation.

Liver Steatosis

Fatty infiltration of the liver that causes excessive beam attenuation, typically requiring a lower transducer frequency to achieve necessary depth penetration at the cost of spatial resolution.

Beam Divergence

The spreading (diffraction) of the ultrasound beam posterior to the focal zone, which enlarges the cross-sectional beam area, reduces intensity, and increases attenuation.

Attenuation Coefficient

The approximate rate at which sound energy decreases as it propagates through a specific medium, expressed in decibels per centimeter per megahertz ($\text{dB}/(\text{cm}\cdot\text{MHz})$).

Soft Tissue Attenuation Coefficient

An attenuation rate of approximately $0.5\,\text{dB}/(\text{cm}\cdot\text{MHz})$.

Muscle Attenuation Coefficient

An attenuation rate of approximately $1.0\,\text{dB}/(\text{cm}\cdot\text{MHz})$.

Blood Attenuation Coefficient

An attenuation rate of approximately $0.125\,\text{dB}/(\text{cm}\cdot\text{MHz})$.

Total Attenuation Formula

$$\text{Total Attenuation (dB)} = \text{Attenuation Coefficient} \times \text{Path Length (cm)} \times \text{Frequency (MHz)}$$

Half-Value Layer (HVL)

The distance sound must travel in a medium to reduce its intensity by one-half ($-3\,\text{dB}$), calculated in soft tissue as $\text{HVL} = \frac{6}{\text{Frequency (MHz)}}$.

-3 dB Attenuation Rule

A reduction in sound intensity or power by a factor of $2$ (one-half of its original value).

-6 dB Attenuation Rule

A reduction or change in ultrasound wave amplitude by a factor of $2$.