ARDMS SPI Principles of Image Resolution 5 — Questions and Answers
Question 1: A 5 MHz transducer in soft tissue (c = 1540 m/s) produces a wavelength of approximately:
- 0.03 mm
- 0.31 mm (Correct answer)
- 3.1 mm
- 0.62 mm
Correct answer: 0.31 mm
λ = c/f = 1540/5,000,000 ≈ 0.308 mm ≈ 0.31 mm.
Question 2: Which statement correctly compares axial and lateral resolution at a specific depth?
- Axial resolution is always poorer than lateral resolution
- Axial resolution is typically better than lateral resolution (Correct answer)
- Axial and lateral resolution are always equal
- Lateral resolution improves with increasing depth
Correct answer: Axial resolution is typically better than lateral resolution
Axial resolution (determined by SPL) is generally superior to lateral resolution because the beam width is almost always wider than the SPL.
Question 3: Coded excitation (e.g., chirp pulses) is used in ultrasound primarily to:
- Increase frame rate at low frequency
- Improve signal-to-noise ratio and penetration while maintaining resolution (Correct answer)
- Eliminate harmonic content
- Widen the beam for panoramic imaging
Correct answer: Improve signal-to-noise ratio and penetration while maintaining resolution
Coded excitation transmits a long modulated pulse and uses pulse compression on receive to achieve high SNR with good axial resolution.
Question 4: Spatial compounding improves image quality by:
- Averaging frames from multiple steering angles to reduce speckle and improve contrast resolution (Correct answer)
- Transmitting at multiple frequencies simultaneously
- Widening the transmit beam for faster acquisition
- Applying harmonic filtering to the receive signal
Correct answer: Averaging frames from multiple steering angles to reduce speckle and improve contrast resolution
Compounding averages images steered from several angles, reducing coherent speckle and improving contrast and boundary definition.
Question 5: Which parameter does NOT directly affect axial resolution?
- Pulse duration
- Number of cycles in the pulse
- Transducer frequency
- Active aperture width (Correct answer)
Correct answer: Active aperture width
Active aperture width affects lateral resolution (beam width), not axial resolution, which depends solely on spatial pulse length.
Question 6: The depth of field (depth of focus) of an ultrasound beam refers to:
- The maximum imaging depth at a given frequency
- The range of depths over which lateral resolution remains acceptable (Correct answer)
- The distance between the near field and the transducer face
- The slice thickness in the elevation plane
Correct answer: The range of depths over which lateral resolution remains acceptable
Depth of field describes the axial extent around the focal point where lateral resolution stays within an acceptable limit.
Question 7: Which approach can improve both axial resolution and penetration simultaneously in modern ultrasound?
- Using a single high-frequency narrow-band pulse
- Broadband multi-frequency transducers combined with coded excitation (Correct answer)
- Reducing the number of active elements
- Increasing pulse repetition frequency
Correct answer: Broadband multi-frequency transducers combined with coded excitation
Broadband transducers with coded excitation allow penetration at lower frequencies while compressing the pulse for good axial resolution.
ARDMS SPI Principles of Image Resolution 5
A 5 MHz transducer in soft tissue (c = 1540 m/s) produces a wavelength of approximately: