Basic Ultrasound Physics

Basic Ultrasound Physics - Wave Wisdom

• US waves: Mechanical, longitudinal. Medical range: 2-20 MHz.
• Frequency (f): Determines resolution & penetration. ↑f = ↑resolution, ↓penetration.
• Wavelength (λ): $λ = c/f$. Inversely related to f.
• Propagation Speed (c): $c = f \times λ$. Constant in a medium. Avg. soft tissue: 1540 m/s. (📌 Cats Find Love)
• Acoustic Impedance (Z): $Z = \rho \times c$. Tissue resistance. Z-mismatches cause reflections (echoes). (📌 Zebras Prefer Carrots)
• Attenuation: Energy loss (absorption, scatter, reflection). ↑ with ↑f.

⭐ The propagation speed of ultrasound is highest in bone (~4080 m/s) and lowest in air/lung (~330 m/s).

Basic Ultrasound Physics - Probe Portraits

• Transducers (Probes): Convert electrical to ultrasound energy & vice-versa.
  • Piezoelectric Effect: Crystals (e.g., PZT) deform with current (transmit); echoes deform crystals (receive).
• Probe Types & Frequencies (📌 LCP: Linear-Lines, Convex-Cavities, Phased-Pumper):
  • Linear: 6-13 MHz. Superficial (vascular, nerves). Rectangular image. ↑ Axial resolution.
  • Curvilinear (Convex): 2-5 MHz. Deep (abdomen, FAST). Fan-shaped. ↑ Penetration.
  • Phased Array (Sector): 1-5 MHz. Cardiac, intercostal. Pie-shaped. Small footprint.
• Key Components:
  • Matching layer: Improves energy transmission.
  • Damping material: Shortens pulse, ↑ axial resolution.

⭐ Higher frequency probes offer better image resolution but have shallower penetration depth.

Basic Ultrasound Physics - Echo Enigmas

• Fundamentals:
  • Piezoelectric effect: Crystals generate/detect ultrasound.
  • Wave speed ($v = f\lambda$): Avg. 1540 m/s in soft tissue.
  • Acoustic Impedance ($Z = \rho c$): Mismatch at interfaces causes reflection (echoes).
  • Attenuation: Signal loss with depth; ↑frequency = ↑attenuation, ↓penetration.
• Common Artifacts - Echo Enigmas:
  • Acoustic Shadowing: Signal void posterior to high attenuators (e.g., bone, calculi).
  • Posterior Acoustic Enhancement: Increased echo intensity posterior to low attenuators (e.g., cysts).
  • Reverberation: Multiple, equally spaced linear echoes (e.g., A-lines).
  • Mirror Image: Artifactual structure deep to a strong, curved reflector (e.g., diaphragm).
  • Anisotropy: Echogenicity of tissues (e.g., tendons) varies with insonation angle. 📌 Angle matters!

⭐ The greater the acoustic impedance mismatch between two tissues, the stronger the reflected echo, forming brighter areas on the ultrasound image.

Basic Ultrasound Physics - Flow Frontiers

• Doppler Effect: Frequency shift of ultrasound waves reflected from moving structures (e.g., RBCs).
• Doppler Equation: $f_D = \frac{2 \cdot f_t \cdot v \cdot \cos\theta}{c}$
  • $f_D$: Doppler shift, $f_t$: transmitted frequency, $v$: velocity, $\theta$: angle, $c$: sound speed.
• Modes:
  • Continuous Wave (CW): No aliasing, range ambiguity.
  • Pulsed Wave (PW): Range specific, aliasing (Nyquist limit).
  • Color Doppler: Mean velocity map. 📌 BART: Blue Away, Red Towards.
  • Power Doppler: Sensitive to slow flow, no direction.

⭐ Nyquist Limit: In PW Doppler, aliasing occurs if Doppler shift > PRF/2 (Pulse Repetition Frequency divided by 2).

High‑Yield Points - ⚡ Biggest Takeaways

• Piezoelectric effect is fundamental for ultrasound transducers.
• Acoustic impedance (Z) mismatch (Z = ρc) is crucial for image formation via reflection.
• Higher frequency offers superior axial resolution but has ↓ penetration.
• Lower frequency provides ↑ penetration but has inferior axial resolution.
• Attenuation (energy loss) is directly proportional to frequency and depth.
• The Doppler effect enables assessment of blood flow velocity and direction.
• Key modes include B-mode (2D imaging), M-mode (motion), and Doppler (flow).