ARDMS SPI Attenuation, Reflection, and Refraction 3 — Questions and Answers
Question 1: What artifact is produced when refraction causes the beam to miss its intended target?
- Lateral displacement artifact (Correct answer)
- Shadowing
- Enhancement
- Reverberation
Correct answer: Lateral displacement artifact
Refraction can displace the apparent position of a structure laterally, creating a lateral displacement artifact.
Question 2: At normal incidence, what determines the fraction of sound intensity reflected at an interface?
- The difference in acoustic impedance between the two media (Correct answer)
- The frequency of the transducer
- The depth of the interface
- The beam diameter at the interface
Correct answer: The difference in acoustic impedance between the two media
The reflection coefficient at normal incidence depends solely on the acoustic impedances of the two media.
Question 3: Which of the following interfaces reflects nearly 100% of incident sound energy?
- Soft tissue–air interface (Correct answer)
- Soft tissue–fat interface
- Muscle–liver interface
- Blood–muscle interface
Correct answer: Soft tissue–air interface
The acoustic impedance mismatch between soft tissue and air is extreme, causing nearly total reflection and no transmission.
Question 4: The angle of refraction is related to the angle of incidence by:
- Snell's law (Correct answer)
- The impedance mismatch formula
- The attenuation coefficient equation
- Beer's law
Correct answer: Snell's law
Snell's law (sin θ₁/c₁ = sin θ₂/c₂) governs the relationship between the angle of incidence and angle of refraction.
Question 5: Acoustic impedance (Z) is calculated as:
- Density × propagation speed (Correct answer)
- Frequency × wavelength
- Pressure / particle velocity squared
- Attenuation coefficient / frequency
Correct answer: Density × propagation speed
Acoustic impedance Z = ρ × c, where ρ is the tissue density and c is the propagation speed.
Question 6: Which artifact results from acoustic enhancement behind a fluid-filled cyst?
- Posterior acoustic enhancement (Correct answer)
- Acoustic shadowing
- Side-lobe artifact
- Mirror image artifact
Correct answer: Posterior acoustic enhancement
Fluid attenuates sound less than surrounding tissue, so structures behind a cyst appear brighter — posterior acoustic enhancement.
Question 7: If the acoustic impedance of two media are identical, what happens at their interface?
- No reflection occurs (Correct answer)
- Total reflection occurs
- Refraction is maximized
- Attenuation doubles
Correct answer: No reflection occurs
When Z₁ = Z₂, the reflection coefficient is zero, so all sound is transmitted with no reflection.
ARDMS SPI Attenuation, Reflection, and Refraction 3
What artifact is produced when refraction causes the beam to miss its intended target?