RVT Ultrasound Physics and Instrumentation 1 — Questions and Answers
Question 1: The piezoelectric effect in ultrasound transducers refers to the ability of certain crystals to:
- Convert electrical energy into mechanical (sound) energy and vice versa (Correct answer)
- Amplify returning echo signals without distortion
- Filter out low-frequency noise from the Doppler signal
- Measure blood flow velocity independent of angle
Correct answer: Convert electrical energy into mechanical (sound) energy and vice versa
The piezoelectric effect describes how piezoelectric crystals (e.g., PZT) convert electrical pulses into mechanical vibrations (ultrasound) and convert returning echoes back into electrical signals.
Question 2: Which relationship best describes how ultrasound frequency affects axial resolution?
- Higher frequency produces shorter wavelengths and better axial resolution (Correct answer)
- Lower frequency produces shorter wavelengths and better axial resolution
- Frequency has no effect on axial resolution
- Higher frequency improves lateral resolution but worsens axial resolution
Correct answer: Higher frequency produces shorter wavelengths and better axial resolution
Higher frequency transducers produce shorter wavelengths, which improves axial resolution (ability to distinguish two structures along the beam axis), though at the cost of tissue penetration.
Question 3: The Nyquist limit in pulsed-wave Doppler is defined as:
- Half the pulse repetition frequency (PRF) (Correct answer)
- Twice the pulse repetition frequency (PRF)
- The maximum tissue depth that can be imaged
- The minimum detectable blood flow velocity
Correct answer: Half the pulse repetition frequency (PRF)
The Nyquist limit is PRF/2; when the Doppler shift frequency exceeds this value, aliasing occurs and velocities are incorrectly displayed on the opposite side of the baseline.
Question 4: Acoustic impedance (Z) of a tissue is determined by:
- The product of tissue density and the speed of sound in that tissue (Correct answer)
- The ratio of transmitted frequency to reflected frequency
- The depth of the structure divided by the time of flight
- The attenuation coefficient multiplied by path length
Correct answer: The product of tissue density and the speed of sound in that tissue
Acoustic impedance Z = ρ × c, where ρ is tissue density and c is the propagation speed of sound; mismatches at tissue interfaces determine the magnitude of reflection.
Question 5: Ultrasound attenuation in soft tissue is approximately:
- 0.5 dB/cm/MHz (Correct answer)
- 5 dB/cm/MHz
- 0.05 dB/cm/MHz
- 2 dB/cm/MHz
Correct answer: 0.5 dB/cm/MHz
Soft tissue attenuates ultrasound at roughly 0.5 dB/cm/MHz, meaning a 5 MHz beam loses about 2.5 dB of intensity per centimeter traveled.
Question 6: The assumed propagation speed of sound used by diagnostic ultrasound equipment in soft tissue is:
- 1,540 m/s (Correct answer)
- 1,000 m/s
- 340 m/s
- 3,300 m/s
Correct answer: 1,540 m/s
Ultrasound systems use the fixed assumption of 1,540 m/s in soft tissue to calculate depth from round-trip travel time; deviations cause range errors.
Question 7: Which of the following transducer types produces a sector-shaped image that is useful for accessing small acoustic windows?
- Phased array (Correct answer)
- Linear array
- Curvilinear array
- Annular array
Correct answer: Phased array
Phased array transducers electronically steer a narrow footprint beam to create a wide sector image, making them ideal for small acoustic windows such as between ribs.
RVT Ultrasound Physics and Instrumentation 1
The piezoelectric effect in ultrasound transducers refers to the ability of certain crystals to: