Instrumentation and Techniques

USG Physics - Sound Wave Secrets

• Ultrasound: Longitudinal mechanical wave; frequency >20 kHz. Medical USG: 2-20 MHz.
• Velocity ($v$) = Frequency ($f$) × Wavelength ($\ ext{λ}$). $v = f \ ext{λ}$.
• Acoustic Impedance ($Z$): Tissue's sound resistance. $Z = \text{Density} (\rho) \times \text{Propagation Speed} (c)$. Unit: Rayl.
• Interactions: Reflection (image basis), refraction, scattering (e.g., Rayleigh from RBCs), absorption (heat).
• Attenuation: Sound intensity ↓ with depth. Proportional to frequency (approx. 0.5 dB/cm/MHz in soft tissue).
• Piezoelectric Effect: Transducer (PZT crystals) converts electrical ↔ mechanical energy.
  • Curie Temperature: Max temp for piezoelectricity.

⭐ Higher frequency offers better axial resolution but suffers from increased attenuation, limiting penetration depth.

Transducers & Beam - The Probe's Power

• Function: Converts electrical to ultrasound energy & vice-versa (Piezoelectric effect; PZT crystals).
  • Matching Layer: $1/4 \lambda$ thick. ↓ Acoustic impedance mismatch (PZT > tissue), ↑ transmission.
  • Damping Material: ↓ Spatial Pulse Length (SPL), ↑ axial resolution, ↑ bandwidth.
• Beam Shape:
  • Near Field (Fresnel): Converging. Length $L \approx D^2 / (4\lambda)$.
  • Focal Zone: Narrowest part; best lateral resolution.
  • Far Field (Fraunhofer): Diverging.
• Common Probes: Linear (vascular), Convex (abdomen), Phased Array (cardiac).

⭐ Higher frequency = ↑ Resolution (axial, lateral), ↓ Penetration. Lower frequency = ↓ Resolution, ↑ Penetration.

Imaging Modes & Display - Pixel Perfect Pictures

• A-Mode (Amplitude): 1D display. Echo strength as spikes vs. depth. Ophthalmic use.
• B-Mode (Brightness): 2D grayscale. Echo strength as dot brightness. Foundation of US imaging.
• M-Mode (Motion): "Ice-pick" view. Displays motion of structures along a single scan line over time. Echocardiography.
• Digital Scan Converter (DSC):
  • Converts analog signals to digital; stores image data.
  • Pixel: Smallest image unit. ↑Pixels = ↑Spatial Resolution.
  • Bit Depth: No. of gray shades per pixel. ↑Bit depth = ↑Contrast Resolution.

⭐ B-mode (Brightness mode) forms the basis of most ultrasound examinations, creating 2D anatomical images that are fundamental for diagnosis across various medical specialties.

Doppler Ultrasound - Chasing the Flow

• Principle: Detects flow via Doppler shift ($f_D$) from moving RBCs. $f_D = (2 \cdot f_t \cdot v \cdot \cos\theta) / c$.
• Doppler Angle ($\theta$): Angle (beam to flow). Optimal 0°, acceptable <60°. At 90°, no shift.
• Types:
  • CW Doppler: High velocity, no range gate. No aliasing.
  • PW Doppler: Range gate. Aliasing if velocity > Nyquist limit (PRF/2).
  • Color Doppler: Mean velocity & direction. 📌 BART: Blue Away, Red Towards.
  • Power Doppler: Slow flow detection, less angle-dependent.
• Spectral Display: Velocity vs. time. Window: laminar; broadening: turbulent.

⭐ For accurate velocity, maintain Doppler angle <60°. At 90°, flow appears absent.

Artifacts & Safety - Glitches & Guidelines

• Common Artifacts:
  • Reverberation: Multiple echoes (comet tail: metal; ring down: gas).
  • Shadowing: ↓ signal (clean: stones; dirty: gas).
  • Enhancement (PAE): ↑ signal posterior to fluid (cysts).
  • Mirror Image: Duplication across strong reflector.
  • Side Lobe: Spurious echoes in anechoic structures.
  • Anisotropy: Tendon echogenicity varies with angle.
• Safety (ALARA Principle):
  • Thermal Index (TI): Tissue heating. Keep TI < 1.0.
  • Mechanical Index (MI): Cavitation risk. Keep MI < 1.9.
  • Obstetric US: Prudent use; limit 1st trimester Doppler.

⭐ Posterior acoustic enhancement is a key feature distinguishing cysts from solid masses.

High-Yield Points - ⚡ Biggest Takeaways

• Piezoelectric effect is crucial for transducer operation, converting energy types.
• Higher frequency yields superior resolution but shallower penetration.
• Acoustic impedance mismatch between tissues generates echoes, forming images.
• Doppler effect enables assessment of blood flow direction and velocity.
• Key artifacts include acoustic shadowing, posterior acoustic enhancement, and reverberation.
• Transducer selection (linear, curvilinear, phased array) depends on the application and depth.
• Gain and TGC optimize image brightness and compensate for attenuation.