Acoustic Waves and Properties Flashcards

Which of the following acoustic wave types does diagnostic ultrasound primarily use for imaging?

Answer: Longitudinal (compressional) waves

Diagnostic ultrasound uses longitudinal waves in which particle displacement occurs parallel to the direction of wave propagation, as these travel efficiently through soft tissue.

Total internal reflection of an ultrasound beam occurs when the angle of incidence exceeds:

Answer: The critical angle

When the incident angle exceeds the critical angle at an interface where speed increases, refraction is impossible and all energy is reflected back.

Acoustic shadowing in ultrasound images is caused by:

Answer: Strong reflection or high attenuation by a structure, reducing echoes beyond it

Structures that strongly reflect or absorb sound (e.g., gallstones, bone) prevent beam transmission, creating a dark shadow posterior to them.

The spatial pulse length (SPL) is determined by:

Answer: Number of cycles × wavelength

SPL = number of cycles per pulse × wavelength; shorter SPL (fewer cycles, shorter wavelength) means better axial resolution.

Compared to soft tissue, fat has a propagation speed that is:

Answer: Lower (~1450 m/s)

Fat propagates sound at approximately 1450 m/s, slower than the 1540 m/s assumed by scanners, which can cause focal-zone registration errors.

The duty factor of a pulsed ultrasound system is defined as:

Answer: Pulse duration divided by pulse repetition period

Duty factor = pulse duration (on-time) / pulse repetition period (on+off time), indicating the fraction of time the transducer is actively transmitting.

Posterior acoustic enhancement (increased echoes deep to a structure) occurs when sound passes through:

Answer: A fluid-filled structure with low attenuation

Fluid-filled cysts attenuate sound less than surrounding tissue, so the beam emerges stronger, producing brighter echoes (enhancement) posterior to the cyst.