ARDMS SPI Acoustic Waves and Properties 5 — Questions and Answers
Question 1: Which of the following acoustic wave types does diagnostic ultrasound primarily use for imaging?
- Transverse (shear) waves
- Longitudinal (compressional) waves (Correct answer)
- Surface (Rayleigh) waves
- Plate (Lamb) waves
Correct 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.
Question 2: Total internal reflection of an ultrasound beam occurs when the angle of incidence exceeds:
- The Brewster angle
- The critical angle (Correct answer)
- 45 degrees
- 90 degrees
Correct 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.
Question 3: Acoustic shadowing in ultrasound images is caused by:
- Strong reflection or high attenuation by a structure, reducing echoes beyond it (Correct answer)
- Reverberation artifacts in the near field
- Side lobe energy striking off-axis structures
- Grating lobes from phased array elements
Correct 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.
Question 4: The spatial pulse length (SPL) is determined by:
- Pulse repetition frequency × duty factor
- Number of cycles × wavelength (Correct answer)
- Propagation speed ÷ frequency
- Transducer diameter × focal depth
Correct answer: Number of cycles × wavelength
SPL = number of cycles per pulse × wavelength; shorter SPL (fewer cycles, shorter wavelength) means better axial resolution.
Question 5: Compared to soft tissue, fat has a propagation speed that is:
- Higher (~1700 m/s)
- Lower (~1450 m/s) (Correct answer)
- Identical (~1540 m/s)
- Highly variable with temperature
Correct 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.
Question 6: The duty factor of a pulsed ultrasound system is defined as:
- Pulse duration divided by pulse repetition period (Correct answer)
- Frequency divided by propagation speed
- Peak intensity divided by average intensity
- Number of pulses per second
Correct 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.
Question 7: Posterior acoustic enhancement (increased echoes deep to a structure) occurs when sound passes through:
- A highly attenuating structure such as bone
- A fluid-filled structure with low attenuation (Correct answer)
- A calcified region
- A gas-containing interface
Correct 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.
