Vascular Ultrasonography

Doppler Fundamentals - Wave Wisdom

• Doppler Effect: Frequency shift from moving RBCs.
• Doppler Equation: $f_D = \frac{2 \cdot f_t \cdot v \cdot \cos\theta}{c}$
  • $f_D$: shift; $f_t$: transmitted freq; $v$: velocity; $c$: sound speed (1540 m/s).
  • $\theta$: Doppler angle. Optimal $\le \textbf{60}^\circ$; $\textbf{90}^\circ \implies$ no shift.
• Modes:
  • CW: No aliasing/range gate. For high velocities.
  • PW: Range-gated; aliasing if $f_D >$ Nyquist Limit ($PRF/2$).
• Color Doppler: Mean velocity & direction. 📌 BART: Blue Away, Red Towards.
• Power Doppler: ↑ sensitivity for low flow; no direction/velocity info.
• Spectral Doppler: Velocity-time waveform.

⭐ Velocity accuracy depends on Doppler angle ($\theta$); $> extbf{60}^\circ$ causes significant error. At $\textbf{90}^\circ$, no shift detected.

Arterial Assessment - Plaque Patrol

• Plaque Morphology:
  • Echogenicity: Hypoechoic (lipid-rich, higher risk ⚠️), isoechoic, hyperechoic (fibrous), calcified (acoustic shadowing).
  • Surface: Smooth, irregular, ulcerated (embolic risk ↑).
  • Location & Extent.
• Stenosis Quantification:
  • B-mode: Direct visualization.
  • Color Doppler: Flow acceleration, aliasing.
  • PW Doppler: ↑PSV, spectral broadening. PSV ratio: $PSV_{stenosis} / PSV_{pre-stenosis}$.
• Carotid Stenosis Grading (SRU Consensus Criteria Simplified):
  • <50%: PSV <125 cm/s.
  • 50-69%: PSV 125-230 cm/s, ICA/CCA PSV ratio 2.0-4.0.
  • ≥70% (but not near occlusion): PSV >230 cm/s, ICA/CCA PSV ratio >4.0.
  • Near Occlusion: Markedly reduced lumen, low velocity flow ("string sign").
  • Total Occlusion: No flow.
• Peripheral Arterial Disease (PAD):
  • Waveform changes: Triphasic (normal) → Biphasic (mild/mod) → Monophasic (severe).

⭐ Distal to significant stenosis, a "Tardus Parvus" waveform (slow systolic upstroke, low amplitude) is characteristic.

Venous Evaluation - Clot Quest

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Specialized Scans - Vascular Virtuoso

• Transcranial Doppler (TCD):
  • Assesses intracranial arteries (MCA, ACA, PCA, VA, BA).
  • Indications: Vasospasm (post-SAH; Lindegaard ratio >3), sickle cell stroke risk (MCA TAMMV >200 cm/s), brain death, PFO detection.
• Contrast-Enhanced Ultrasound (CEUS):
  • Microbubble contrast improves vessel/perfusion imaging, lesion characterization.
  • Uses: TIPS patency, endoleak detection (post-EVAR).
• Intravascular Ultrasound (IVUS):
  • Catheter-based, detailed vessel wall/plaque imaging. Crucial for guiding interventions (stenting).
• Specific Applications:
  • Popliteal Artery Entrapment (PAES): Dynamic scan with flexion.
  • Thoracic Outlet Syndrome (TOS): Positional Doppler.
  • Erectile Dysfunction: Penile Doppler (PSV <30 cm/s = arterial insufficiency).

⭐ TCD in sickle cell: MCA TAMMV >200 cm/s indicates high stroke risk, guiding prophylactic transfusions.

High‑Yield Points - ⚡ Biggest Takeaways

• Doppler effect is fundamental for assessing blood flow direction and velocity.
• Color Doppler visualizes flow: BART (Blue Away, Red Towards transducer).
• Spectral Doppler quantifies flow velocity and displays characteristic waveforms (e.g., triphasic).
• Primary diagnostic tool for DVT (deep vein thrombosis) - non-compressibility of vein is key.
• Crucial for carotid artery stenosis assessment (PSV, EDV, ICA/CCA ratio).
• Evaluates peripheral arterial disease (PAD), often correlating with Ankle-Brachial Index (ABI).
• Monitors renal transplant vascularity for complications like stenosis or thrombosis.