Ultrasound Physics: Attenuation, Reflection, Refraction, and Echogenicity

Vocabulary flashcards covering ultrasound physics topics including attenuation, absorption, reflection types, acoustic impedance, Snell's law, critical angle calculations, echogenicity terms, attenuation rates, half-value layer, and dynamic range.

Attenuation

The loss of strength, amplitude, power, and intensity (magnitude) of a sound wave as it travels through a medium.

Absorption

The dominant form of attenuation that is direct and non-linear, increasing exponentially with sound frequency.

Specular Reflection

The strongest form of reflection occurring off a large, smooth, and flat surface relative to wavelength.

Scatter Reflection

Sound reflection produced when a beam strikes a large, rough surface.

Rayleigh Scatter

The weakest form of reflection (relevant in Doppler) that occurs off reflectors small relative to wavelength (such as red blood cells), where scattering increases directly and non-linearly with frequency.

Normal Line

An imaginary reference line constructed perpendicular ($90^\text{o}$) to the boundary line between two media.

Normal Incidence

Incidence occurring when the incident sound ray is parallel ($0^\text{o}$) to the normal line and perpendicular ($90^\text{o}$) to the medium boundary.

Oblique Incidence

Incidence occurring when the sound beam strikes a medium boundary at any angle other than $0^\text{o}$ or $90^\text{o}$.

Acoustic Impedance ($Z$)

A characteristic of a tissue calculated as $Z = \rho \times c$ (density multiplied by propagation speed), expressed in Rayls ($\text{kg} \times \text{m}^{-2} \times \text{s}^{-1}$).

Intensity Reflection Coefficient ($IRC$)

The percentage of sound intensity reflected at a boundary, calculated as $IRC = \frac{I_r}{I_i} = \frac{(Z_2 - Z_1)^2}{(Z_2 + Z_1)^2}$.

Intensity Transmission Coefficient ($ITC$)

The percentage of sound intensity transmitted across a boundary, equal to $100\% - IRC$ or $1 - IRC$.

Refraction

The change of direction or bending of sound when crossing a medium boundary, requiring oblique incidence and differing propagation speeds between media.

Snell's Law

The formula defining refraction across a boundary, given by $\frac{C_i}{C_t} = \frac{\tan(\theta_i)}{\tan(\theta_t)}$ or $\frac{C_i}{C_t} = \frac{\text{SIN}(\theta_i)}{\text{SIN}(\theta_t)}$, where the medium with the highest propagation speed has the largest angle.

Critical Angle

The incident angle that causes total internal reflection with refraction, defined by Snell's law when the transmission angle is $90^\text{o}$: $\frac{C_i}{C_t} = \frac{\text{SIN}(\theta_i)}{\text{SIN}(90^\text{o})}$.

Anechoic

Describing a region with no returning sound waves, producing a black display on the ultrasound screen.

Hypoechoic

Describing structures that produce less bright echoes relative to surrounding tissue.

Hyperechoic

Describing structures that produce higher brightness echoes relative to surrounding tissue.

Isoechoic

Describing structures that generate an equal amount of echoes, blending in with surrounding tissue.

Homogenous

An echotexture characterization where echo patterns are uniform throughout and blend together across different places.

Heterogenous

An echotexture characterization where echo patterns differ across regions and do not blend together.

Soft Tissue Attenuation Rate ($AR$)

The rate of sound attenuation in soft tissue, equal to $0.5\,\text{dB}/(\text{cm} \times \text{MHz})$.

Muscle Attenuation Rate ($AR$)

The rate of sound attenuation in muscle tissue, equal to $1.0\,\text{dB}/(\text{cm} \times \text{MHz})$.

Blood Attenuation Rate ($AR$)

The rate of sound attenuation in blood, equal to $0.125\,\text{dB}/(\text{cm} \times \text{MHz})$.

Attenuation Coefficient ($AC$)

The rate of attenuation multiplied by frequency, expressed as $AC = AR \times F$.

Half Value Layer ($HVL$)

The distance sound travels in tissue to experience a $3\,\text{dB}$ drop in intensity, measured in $\text{cm}$ with a diagnostic range of $0.3$ to $1\,\text{cm}$, calculated as $HVL = \frac{3\,\text{dB}}{AC}$.

Dynamic Range ($DR$)

The range of voltages that can be processed and handled by the system, measured in $\text{dB}$ and calculated as $DR = AC \times MID \times 2$.