ARDMS SPI Attenuation, Reflection, and Refraction 5 — Questions and Answers
Question 1: A sonographer tilts the transducer away from perpendicular when scanning a tendon. What effect does this have on the echogenicity of the tendon?
- The tendon appears hypoechoic due to anisotropy (Correct answer)
- The tendon appears hyperechoic due to increased reflection
- The tendon appears the same regardless of angle
- The tendon shows posterior shadowing
Correct answer: The tendon appears hypoechoic due to anisotropy
Tendons exhibit anisotropy; off-axis imaging reduces specular reflection from their parallel fibers, making them appear hypoechoic.
Question 2: Critical angle refraction in ultrasound occurs when:
- The refracted beam travels parallel to the interface (90° refraction) (Correct answer)
- The incident beam strikes perpendicular to the interface
- Attenuation equals the impedance mismatch
- The frequency exceeds 10 MHz
Correct answer: The refracted beam travels parallel to the interface (90° refraction)
At the critical angle, refraction bends the transmitted beam to 90°, meaning it travels along the interface and no energy enters medium 2.
Question 3: What is the primary mechanism of attenuation in blood?
- Scattering by red blood cells (Correct answer)
- Absorption by hemoglobin
- Reflection at vessel walls
- Refraction at the plasma-cell boundary
Correct answer: Scattering by red blood cells
Red blood cells act as Rayleigh scatterers, and scattering is the dominant mechanism of attenuation in blood.
Question 4: A mirror-image artifact occurs because of:
- Strong specular reflection at a curved or flat interface causing double registration (Correct answer)
- Refraction bending the beam to display two copies of a structure
- Attenuation differences creating ghost images
- Side-lobe energy producing duplicate echoes
Correct answer: Strong specular reflection at a curved or flat interface causing double registration
A strong specular reflector (e.g., diaphragm) re-reflects echoes back to a structure, causing its mirror image to appear on the other side.
Question 5: How does the presence of a gas bubble affect ultrasound beam propagation?
- It causes near-total reflection and distal shadowing (Correct answer)
- It enhances transmission due to low density
- It increases refraction but not reflection
- It selectively absorbs low frequencies
Correct answer: It causes near-total reflection and distal shadowing
Gas has very low acoustic impedance, creating an extreme mismatch with tissue that reflects almost all incident energy and shadows structures behind it.
Question 6: What happens to wavelength as ultrasound travels from soft tissue into bone?
- Wavelength increases because propagation speed is higher in bone (Correct answer)
- Wavelength decreases because propagation speed is lower in bone
- Wavelength remains the same; only frequency changes
- Wavelength decreases because frequency increases in bone
Correct answer: Wavelength increases because propagation speed is higher in bone
Bone has a higher propagation speed (~4080 m/s) than soft tissue; since frequency is constant, wavelength = c/f increases.
Question 7: Which imaging scenario is MOST likely to produce a refraction artifact causing a false double image of a structure?
- Scanning through the rectus abdominis muscle edge into deeper tissue (Correct answer)
- Scanning a cyst in a homogeneous liver
- Using a linear transducer on a flat superficial tendon
- Imaging a gallstone with acoustic shadowing
Correct answer: Scanning through the rectus abdominis muscle edge into deeper tissue
The curved edge of the rectus abdominis acts as a refracting lens, splitting the beam and creating a duplicated image of deeper structures.
ARDMS SPI Attenuation, Reflection, and Refraction 5
A sonographer tilts the transducer away from perpendicular when scanning a tendon.
What effect does this have on the echogenicity of the tendon?