ASN CNSA cheat sheet

Doppler shift formula: f_d = (2 · f_0 · v · cos θ) / c, where c ≈ 1540 m/s in soft tissue; shift is proportional to transmit frequency and to cos of the Doppler angle.

Doppler angle must be ≤60° and ideally ≤45° for velocity accuracy; at 60° a 1° angle error produces roughly a 2% velocity error, and error grows steeply beyond 60°.

Transcranial Doppler uses a 2 MHz (pulsed or power-M-mode) probe for temporal, orbital, and suboccipital windows; 1 MHz probes improve penetration for thick skulls.

Normal middle cerebral artery mean flow velocity in adults is approximately 55 ± 12 cm/s; velocities >120 cm/s suggest vasospasm and >200 cm/s suggest severe vasospasm.

Lindegaard ratio = MCA mean velocity / ipsilateral extracranial internal carotid mean velocity; ratio >3 supports vasospasm over hyperemia.

Pulsatility index (Gosling) = (peak systolic − end diastolic) / time-averaged mean velocity; elevated PI indicates increased distal resistance.

Resistive index (Pourcelot) = (peak systolic − end diastolic) / peak systolic; normal renal/ophthalmic values are roughly 0.55–0.70.

Temporal window is found at the zygomatic arch superior to the tragus; the MCA runs toward the probe (positive shift) at 30–60 mm depth and the ACA runs away at 60–80 mm.

Pulsed-wave Doppler at 1.5–2 MHz with a small sample volume allows depth-selective insonation of individual intracranial vessels; color/power Doppler aids vessel identification.

Carotid duplex criteria for ≥70% ICA stenosis: peak systolic velocity ≥230 cm/s, end-diastolic velocity ≥100 cm/s, and ICA/CCA PSV ratio ≥4.0 (University of Washington–derived criteria).

Normal ICA PSV is <125 cm/s with no spectral broadening; a ratio of ICA to CCA PSV <2 and PSV <125 cm/s indicates <50% stenosis.

Near-occlusion (string sign) shows markedly reduced or undetectable ICA flow with a high-resistance external carotid pattern; velocity criteria may underestimate severity.

Vertebral artery flow is normally low-resistance and continuous; reversal or absent flow suggests subclavian steal or proximal stenosis.

The circle of Willis connects anterior (ACA via ACoA) and posterior (PCA via PCoA) circulations; a complete circle occurs in only about 20–25% of people.

The ophthalmic artery normally flows toward the probe (low resistance); flow reversal or blunted flow indicates severe ipsilateral ICA stenosis or occlusion.

Transcranial Doppler criteria for sickle cell disease: time-averaged mean velocity ≥200 cm/s in the MCA or terminal ICA indicates high stroke risk and warrants transfusion therapy.

Microembolic signals on TCD are brief, high-intensity, unidirectional signals lasting <300 ms with a characteristic chirp; they indicate active embolization from a proximal source.

Carotid endarterectomy benefit is greatest for symptomatic ≥70% stenosis (NASCET); asymptomatic stenosis benefit is smaller and depends on perioperative risk.