Neurological / Cranial POCUS

12.1 Adult TCD — transcranial Doppler

TCD is a functional vascular assessment, not structural brain imaging. It does not replace CT, MRI, or CTA. Use a 2 MHz TCD probe. Windows: temporal MCA/ACA/PCA, orbital ICA, foramen magnum VA/BA. Grade window quality 0–3. Acquisition should be stable over 5–10 respiratory cycles with correct depth and angle.

Core velocity parameters

  • MCA: approximately 50–80 cm/s.

  • ACA: approximately 40–70 cm/s.

  • PCA: approximately 20–40 cm/s.

  • ICA: approximately 40–80 cm/s.

  • VA/BA: approximately 20–40 cm/s.

Ranges vary by age, sex, center, system, depth, angle, respiration, and haemodynamics. Trend, ratio, and context are more important than a single absolute value.

Derived indices

  • Pulsatility index = (PV − DFV) / MFV.

  • Resistive index = (PV − DFV) / PV.

High PI suggests increased distal resistance: elevated ICP, oedema, small-vessel disease. Low PI suggests decreased resistance or hyperaemia: vasospasm, shunt. MCA PI is approximately 0.5–1.1, age- and center-dependent. RI has similar directional interpretation. PI is more commonly used.

Cerebrovascular reactivity

  • BHRI: baseline MCA MFV, approximately 40-second breath hold, peak MFV, BHRI = (MFV_breath-hold − baseline) / baseline × 100.

    • Greater than 30–35%: preserved.

    • 20–35%: borderline.

    • Less than 10–20%: severely impaired.

    • Center-specific.

  • Acetazolamide or vasodilator challenge: baseline, agent, post-agent MFV, CVR = (post − baseline) / baseline × 100.

    • Greater than 20% increase: preserved.

    • Blunted response or fall: impaired reserve.

    • Avoid in unstable, hypoxic, severe respiratory compromise, or contraindicated patients.

CVR is not autoregulation. CVR is response to stimulus. Autoregulation is flow stability across CPP changes.

SAH vasospasm monitoring

Monitor days 3–14 post-SAH, peak approximately days 5–10. Perform once or twice daily with same operator, timing, and conditions.

MCA MFV interpretation, center-specific:

  • Less than 80 cm/s: normal or mild.

  • 80–119 cm/s: mild.

  • 120–199 cm/s: moderate.

  • 200 cm/s or greater: severe.

Low PI supports vasospasm. Rising trend is more important than a single value. MCA/ICA ratio greater than 4–6 suggests moderate or severe vasospasm, study-specific. MCA/BA ratio greater than 6 in some settings suggests severe vasospasm. A 10–15% change after intervention may be clinically meaningful.

A single elevated velocity does not equal diagnosis. Integrate with neuro exam, blood pressure, oxygen, temperature, haemoglobin, and CTA/perfusion. Delayed cerebral ischaemia is the clinical concern, not velocity alone.

Microembolic signals

Typical criteria:

  • Duration less than 0.4 seconds.

  • Frequency greater than 400 Hz.

  • High amplitude, several times baseline.

  • Short-duration linear or spike shape.

Report number of MES per 5 or 10 minutes, side, and vessel.

MES burden per 10 minutes:

  • 0–10: lower.

  • 10–20: intermediate.

  • Greater than 20: higher.

There is no single universal therapeutic threshold. MES does not identify source. Correlate with ECG, carotid ultrasound, TTE/TEE.

TCD pitfalls

  • Poor temporal window.

  • Angle or depth error.

  • Vessel misidentification.

  • Single velocity overinterpretation.

  • Ignoring confounders: blood pressure, temperature, PaCO₂, haemoglobin, sedation, hypercapnia, hypocapnia.

  • Assuming TCD rules out structural disease.

  • Cardiac arrest scanning without brief protocol-driven approach.

  • Applying adult norms to children without pediatric reference data.

  • Confusing CVR with autoregulation.

  • MES artifact from probe movement, respiratory variation, cardiac pulsation, or poor contact.

Minimal TCD documentation

Patient, indication, date/time, vessels sampled, window quality, depth, side, MFV, PV, DFV, PI, MES count and interval, challenge if any, comparison to prior, interpretation, and limitations.

12.2 Neonatal/infant structural cranial US

B-mode structural imaging via fontanelle is distinct from adult TCD.

Parameters

  • EVAP/EAAP: coronal, at foramen of Monro, inner table to inner table, widest atrial transverse. Greater than 15 mm often ventriculomegaly; some protocols use greater than 12 mm, age- and technique-dependent.

  • Evans index: maximum frontal horn width divided by inner table skull diameter. Greater than 0.5 suggests ventriculomegaly.

  • IVH Papile grading: Grade I subependymal; Grade II IVH without dilation; Grade III IVH with dilation; Grade IV IVH with parenchymal or cystic change.

  • PVL: white matter echogenicity, cystic changes. MRI is superior for subtle PVL.

  • Posterior fossa: cisterna magna, fourth ventricle, posterior fossa cysts.

Views: coronal, axial/lateral, posterior fossa. Use minimal pressure over fontanelle. MRI is superior for subtle PVL, posterior fossa detail, and chronic injury. Fontanelle closure limits long-term use.