Overview
This lecture covers tissue harmonics in ultrasound imaging, focusing on the physics, creation, properties, and imaging benefits compared to standard (fundamental) frequencies.
Non-Linear Behavior and Harmonics
- Tissue harmonics and contrast harmonics both arise from non-linear behavior of sound waves.
- Tissue harmonics are created by the interaction of the ultrasound beam with body tissue.
- Harmonic frequencies are multiples of the fundamental frequency, which is produced by the transducer.
Fundamental and Harmonic Frequencies
- Fundamental frequency is the predictable sinusoidal output from the transducer.
- Harmonic frequencies form when compressions travel faster and rarefactions slower, distorting the wave.
- Harmonics are generated within tissues, not at the transducer.
- Harmonics increase as sound travels deeper, but high-order harmonics attenuate quickly.
Tissue Harmonics in Imaging
- Harmonics are weak in the near field, optimal in the mid field, and strongest (but unusable) in the far field.
- Attenuation limits the utility of higher harmonics for deep imaging.
- Strong beams, especially at the focal zone, are needed to generate harmonics.
Harmonic Imaging Techniques
- Harmonic imaging processes echoes at the second harmonic, ignoring other frequencies.
- Harmonic imaging results in a narrower beam, improving lateral resolution and reducing artifacts (e.g., grating lobes).
- Harmonics imaging produces a cleaner near field and overall improved image clarity.
Harmonic Image Processing Methods
- Standard harmonic filtering narrows bandwidth, degrading axial resolution.
- Pulse inversion harmonics sends two out-of-phase pulses; fundamental echoes cancel, harmonics combine and strengthen, minimal axial degradation but slight reduction in frame rate.
- Power modulation harmonics uses weak and strong pulses; fundamental echoes from weak are doubled and subtracted, isolating the strong beam’s harmonics.
Key Terms & Definitions
- Fundamental Frequency — original frequency emitted by the transducer.
- Harmonic Frequency — integer multiples of the fundamental frequency generated by tissue interaction.
- Attenuation — loss of sound wave energy as it passes through tissue.
- Lateral Resolution — ability to distinguish structures that are side-by-side.
- Pulse Inversion Harmonics — technique sending out-of-phase pulses to isolate harmonics.
- Power Modulation Harmonics — technique using weak and strong beams to separate harmonic signals.
Action Items / Next Steps
- Complete workbook activities on tissue harmonics.
- Review and answer nerd check questions on today’s content.