Doppler Ultrasound Principles and Applications

Doppler Effect & Basics - Echoes on the Move

• Doppler Effect: Frequency change of sound from moving reflectors (RBCs).
  • Positive shift: towards transducer (↑ frequency).
  • Negative shift: away from transducer (↓ frequency).
• Doppler Equation: $f_d = (2 \cdot f_0 \cdot v \cdot \cos{\theta}) / c$
  • Where $f_d$=Doppler shift, $f_0$=tx freq, $v$=velocity, $c$=sound speed.
• Angle of Insonation ($\theta$):
  • Optimal: $\theta < \textbf{60}^\circ$ for accuracy.
  • $\theta = \textbf{90}^\circ$: no Doppler shift ($\cos{90}^\circ = 0$).
• 📌 BART: Blue Away, Red Towards (Color Doppler).

⭐ The Doppler shift is directly proportional to the velocity of blood and the cosine of the angle of insonation. Maintaining an angle $\theta < \textbf{60}^\circ$ is crucial for accuracy.

Spectral Doppler Analysis - Signals Unpacked

• Spectral Display: Plots velocity against time; brightness indicates signal strength (RBCs).
• Waveform Features:
  • Peak Systolic Velocity (PSV), End Diastolic Velocity (EDV).
  • Systolic upstroke: Rate of acceleration.
  • Spectral window: Clear below systole (laminar flow).
  • Spectral broadening: Filling of window (turbulence, e.g., stenosis).
• Key Indices:
  • Resistive Index (RI): $(PSV - EDV) / PSV$. Measures downstream resistance. Normal renal RI: $\textbf{0.5-0.7}$.
  • Pulsatility Index (PI): $(PSV - EDV) / TAMV$. Measures flow pulsatility.

⭐ A 'tardus parvus' waveform (slow systolic upstroke, low peak systolic velocity) distal to a stenosis indicates its hemodynamic significance.

Vascular Applications - Vessel Voyagers

• Carotid Arteries: Stenosis grading.
  • PSV $\textbf{>125 cm/s}$ for $\textbf{>50%}$ stenosis.
  • PSV $\textbf{>230 cm/s}$ for $\textbf{>70%}$ stenosis.
  • ICA/CCA ratio, EDV also assessed.
• Peripheral Arteries (PAD):
  • Waveform changes: Triphasic → Biphasic → Monophasic.
  • Site/severity of stenosis.
• Venous System (DVT):
  • Key: Loss of compressibility.
  • Absent/continuous flow, no augmentation.

  ⭐ In Deep Vein Thrombosis (DVT) assessment, lack of venous compressibility is the most reliable sign, often accompanied by absent or abnormal Doppler flow.

• Other: AV fistula/graft monitoring, transplant vessel patency.

Diverse Applications - Beyond the Pipes

• Obstetrics:
  • Fetal surveillance: Umbilical Artery (S/D $\textbf{<3}$ post $\textbf{30 weeks}$), MCA (brain-sparing), Ductus Venosus.
  • Uterine Artery (PI/RI) for pre-eclampsia risk.
  • Ovarian/Testicular torsion: Detects ↓/absent flow.
• Cardiology:
  • Valvular disease: Quantifies stenosis/regurgitation.
  • Shunt detection: Identifies ASD, VSD.
  • Assesses diastolic function.
• Abdominal:
  • Portal hypertension: Shows flow reversal.
  • Renal Artery Stenosis (↑PSV).
  • Transplant assessment (renal, liver).
  • Detects inflammation (hyperemia).

⭐ In fetal hypoxia, the 'brain sparing effect' manifests as a decreased Pulsatility Index (PI) in the Middle Cerebral Artery (MCA) due to vasodilation.

Artifacts & Pitfalls - Doppler's Deceptions

• Aliasing: Doppler shift > Nyquist limit ($PRF/2$). Wrap-around appearance.
  • 📌 Correction: Baseline shift, ↑PRF/Scale, ↓frequency transducer, ↑Doppler angle (<60°), CW Doppler.
• Mirror Image: Strong reflector or angle >90°. Duplicate vessel, reversed flow.
• Twinkling: Behind rough surfaces (stones). Rapid alternating color.
• Spectral Broadening: Turbulent flow, large sample volume. Widened waveform.
• Range Ambiguity: High PRF. Prior pulse echoes.
• Blossoming: Color gain too high. Color spills.

⭐ Aliasing artifact occurs when the Doppler frequency shift exceeds the Nyquist limit. It appears as a wrap-around of the spectral waveform or color display.

High‑Yield Points - ⚡ Biggest Takeaways

• Doppler effect: Change in sound frequency due to relative motion between source and receiver.
• Doppler shift: Proportional to blood flow velocity and cos(θ); optimal imaging angle < 60°.
• Color Doppler: Shows flow direction (BART: Blue Away, Red Towards) and mean velocity of blood.
• Power Doppler: Highly sensitive for slow flow, angle-independent, but no directional information.
• Spectral Doppler (PW/CW): Quantitative flow analysis; PW Doppler prone to aliasing if Nyquist limit (PRF/2) exceeded.
• Key Applications: DVT detection, carotid stenosis, fetal monitoring, organ transplant evaluation.