Ultrasound principles and interpretation

Ultrasound Physics & Probes - Wave Wrangling

• Piezoelectric Effect: Transducer crystals convert electrical energy to mechanical sound waves & vice versa.
• Acoustic Impedance (Z): Resistance to sound travel. $Z = \text{density} (\rho) \times \text{propagation speed} (c)$. Echoes are generated at interfaces of tissues with different Z values.
• Frequency vs. Penetration Trade-off:
  • High frequency (>7 MHz): ↑ resolution, ↓ penetration (superficial structures).
  • Low frequency (2-5 MHz): ↓ resolution, ↑ penetration (deep structures).
• Probe Selection:
  • Linear: High frequency; vascular, thyroid.
  • Curvilinear (Convex): Low frequency; abdomen, OB/GYN.
  • Phased Array: Low frequency, small footprint; cardiac.
  • 📌 Linear for Lumps (superficial), Curvilinear for Core (deep).

⭐ The brightness of an echo (echogenicity) is determined by the difference in acoustic impedance between two tissues. Large differences (e.g., soft tissue-bone) create bright, hyperechoic interfaces.

Echogenicity & Terminology - Reading the Shadows

• Echogenicity: Tissue's ability to reflect ultrasound waves. Brightness on screen.

  • Hyperechoic: White/bright (e.g., bone, stones). High reflection.
  • Hypoechoic: Dark gray (e.g., solid organs, muscle). Low reflection.
  • Anechoic: Pure black (e.g., simple fluid, cysts). No reflection.
  • Isoechoic: Same brightness as surrounding tissue.

• Key Artifacts:

  • Acoustic Shadowing: Dark area behind a bright, dense object (e.g., gallstone) that blocks sound waves.
  • Posterior Acoustic Enhancement: Bright area behind a fluid-filled structure (e.g., cyst) as sound travels with less resistance.

⭐ In a FAST exam, anechoic (black) fluid collections in potential spaces like Morison's pouch are a critical finding suggestive of hemoperitoneum.

Doppler & M-Mode - Going with the Flow

• Doppler Effect: Measures frequency shift of returning echoes to assess blood flow velocity and direction.
  • Doppler Equation: $Δf = (2 f₀ v \cos θ) / c$
  • 📌 BART: Blue Away, Red Toward the transducer.
• Doppler Types:
  • Color: Average velocity & direction. Risk of aliasing.
  • Power: ↑ sensitivity for low flow (e.g., testicular torsion), no direction info.
  • Pulsed-Wave (PW): Site-specific velocity. Aliasing at high velocity.
  • Continuous-Wave (CW): High velocity, no aliasing. Lacks depth specificity.
• M-Mode (Motion Mode):
  • Single beam plotted against time. High temporal resolution.
  • Uses: Valve motion, wall thickness, fetal heart rate.

⭐ For accurate flow velocity, the Doppler angle (θ) should be < 60°. As θ approaches 90°, cosine θ approaches 0, and the detected velocity is falsely low or absent.

Key Artifacts - Glitches in the Matrix

• Artifacts are errors in imaging that do not correspond to actual anatomy. Understanding them is key to correct interpretation.

⭐ Acoustic enhancement (posterior enhancement) is a crucial feature used to characterize fluid-filled structures like simple cysts, distinguishing them from solid masses which typically cause shadowing or have no effect.

• Ultrasound uses sound waves; tissue echogenicity depends on acoustic impedance.
• Hyperechoic (bright) structures like bone reflect many waves; anechoic (dark) structures like cysts reflect none.
• Use high-frequency probes for superficial structures (better resolution) and low-frequency for deep structures (better penetration).
• Posterior acoustic enhancement occurs deep to fluid; acoustic shadowing occurs deep to solids like gallstones.
• Doppler assesses blood flow: Blue is away from the probe, Red is towards it (BART).