Neurosonology (RPNI): Intracranial Hemodynamics and Waveform Interpretation
Core concepts
Neurosonology questions test whether you can translate a Doppler waveform into a physiologic statement about the vessel and the brain it supplies. Know the insonation windows (transtemporal, transorbital, suboccipital, submandibular), the depth and direction conventions for MCA, ACA, PCA, basilar, and vertebral arteries, and normal mean flow velocities (roughly 40–60 cm/s in the MCA in adults, dropping with age). Understand resistance patterns: low-resistance internal carotid and vertebral beds versus the higher-resistance external carotid. Be fluent in the vocabulary of stenosis - focal velocity elevation, spectral broadening, turbulence, post-stenotic disturbance - and in vasospasm after subarachnoid hemorrhage, where velocities rise segmentally without a focal plaque. Also know autoregulation, the meaning of the pulsatility index, and how CO2 and blood pressure shift the waveform.
Traps
Questions love directionality: the MCA normally flows toward the probe, the ACA away. A reversed ACA suggests collateral supply across the anterior communicating artery. Confusing velocity elevation from stenosis with elevation from hyperemia (low resistance, high diastolic flow) is a classic distractor. Another trap is treating a single high velocity as diagnostic without a velocity ratio or contralateral comparison.
Rule of thumb
A focal velocity more than doubling the adjacent segment, with turbulence and a post-stenotic change, supports stenosis; diffuse symmetric elevation with low pulsatility suggests hyperemia. For vasospasm, a mean MCA velocity above about 120 cm/s raises concern, and a Lindegaard-type ratio (MCA to extracranial ICA) above 3 strengthens it.
Worked example
A 52-year-old with SAH day 5 has MCA mean velocity 165 cm/s and extracranial ICA 45 cm/s. Ratio ≈ 3.7: interpret as probable vasospasm, not stenosis, because there is no focal plaque and the elevation is segmental and bilateral in trend.