Unit 17A: Tissue Harmonics

Key Concepts:

• Tissue Harmonics and Contrast Agents both rely on non-linear behavior.
  • Tissue harmonics result from sound beams interacting with soft tissue.
  • Contrast harmonics result from sound beams interacting with bubble contrast.
• Unit 17 is divided into two parts:
  • 17A: Tissue Harmonics
  • 17B: Contrast Harmonics

17A.1: Fundamental and Harmonic Frequencies

• Fundamental Frequency:
  • When predictable and linear.
  • Emitted by the transducer, expected and sinusoidal.
• Harmonic Frequencies:
  • Created when waveforms are non-linear and unpredictable.
  • Derived as multiples (odd and even) of the fundamental frequency.
  • Examples: 2 MHz fundamental frequency:
    • 2nd Harmonic: 4 MHz
    • 3rd Harmonic: 6 MHz
• Process:
  • Compressions travel faster and refractions slower.
  • This speed variation creates harmonic frequencies.
• Creation:
  • Harmonics are created in the tissue, not at the transducer.
  • Different harmonic frequencies return to the transducer.

17A.2: Physics of Tissue Harmonics

• Propagation:
  • Harmonics grow stronger as they propagate deeper into the tissue.
• Field Effects:
  • Near Field: Minimal harmonic change.
  • Mid Field: Best harmonics for imaging.
  • Far Field: Strongest harmonics but attenuated quickly.
• Imaging:
  • Strong beams essential for harmonic creation.
  • Best harmonics come from areas of interest when the focus is well-placed.
  • Harmonics improve lateral resolution.
  • Harmonics reduces artifacts from grading lobes.

Harmonic Imaging

• Amplification:
  • Machine amplifies specific frequencies within the bandwidth, typically listening for the second harmonics.
• Advantages:
  • Narrow beams improve lateral resolution.
  • Eliminates artifacts due to grading lobes.
  • Less distortion in the near field.
• Pulse Inversion Harmonics:
  • Sends two out-of-phase pulses per scan line.
  • Destructive interference cancels fundamental echoes, enhancing harmonics.
  • Slightly decreases temporal resolution (e.g., frame rate).
• Power Modulation Harmonics:
  • Uses weak and strong beams to isolate and double harmonic returns.

Summary

• Tissue harmonics are created inside the body tissue.
• Most machines use pulse inversion techniques for improved imaging.
• Harmonics enhance image quality by improving spatial resolution and reducing noise/artifacts.
• Important to use harmonics appropriately, balancing with potential loss in temporal resolution.

Activities:

• Workbook exercises and questions titled "Nerd Check" to review content.

Note: Remember to adjust focus for optimal harmonic imaging, recognizing when to switch back to fundamental frequencies for deeper imaging needs.