5.3 Intracranial Hemodynamics & Clinical TCD Applications

Interpret TCD in the correct protocol

Transcranial Doppler (TCD) measures intracranial blood-flow velocity. It does not directly measure cerebral volume flow, tissue oxygenation, or the exact percentage of every intracranial stenosis. Vessel identification uses the acoustic window, depth, direction, neighboring signals, and compression or other protocol-specific maneuvers where appropriate. Imaging and non-imaging techniques have different acquisition assumptions, so their thresholds must not be mixed.

Record the vessel, side, depth, waveform quality, and the velocity parameter actually measured. Peak systolic velocity, a time-averaged spectral-envelope velocity, and a time-averaged mean across the spectrum are not interchangeable. Published clinical algorithms depend on specific measurements. A number without its parameter and technique can lead to the wrong conclusion.

Vasospasm after subarachnoid hemorrhage

After aneurysmal subarachnoid hemorrhage (aSAH), TCD trends can help identify vasospasm and support evaluation of delayed cerebral ischemia. Serial studies are more useful when vessel depth, technique, and measurement method are comparable. Increased velocity may reflect narrowing, hyperemia, anemia, systemic changes, or technical differences. Interpret it with the neurological examination and the clinical team's other data.

For the middle cerebral artery (MCA), commonly used mean-velocity bands are below 120 cm/s, 120–199 cm/s, and at least 200 cm/s, with increasing concern for vasospasm in the appropriate setting. These bands are not transferable to every intracranial artery. A low velocity does not guarantee normal perfusion in a patient with severe disease or poor insonation.

Lindegaard ratio

The Lindegaard ratio compares the MCA mean velocity with the ipsilateral extracranial ICA mean velocity obtained using the appropriate protocol:

$$\text{LR} = \frac{\text{MCA mean velocity}}{\text{extracranial ICA mean velocity}}$$

A ratio below about 3 favors hyperemia rather than focal MCA vasospasm; ratios 3–6 support vasospasm, and values above 6 are strongly concerning for severe vasospasm when supported by the MCA velocity and clinical context. Use the ratio and absolute velocity together. A high ratio caused by a very low denominator is not an independent proof of severe narrowing.

For MCA mean velocity 195 cm/s and extracranial ICA mean velocity 30 cm/s, LR = 6.5. This is concerning for vasospasm and requires clinical correlation. The absolute MCA value is below the usual ≥200 severe band, so report the discordance rather than claiming both measurements independently prove a severe grade.

The 2023 AHA/ASA aSAH guideline emphasizes euvolemia. Do not describe routine prophylactic “triple-H therapy” of hypertension, hypervolemia, and hemodilution as the current default. Treatment of symptomatic ischemia is directed by the neurocritical-care team, not by a sonographer's velocity category alone.

Sickle-cell stroke-risk screening

The STOP method uses non-imaging TCD to measure time-averaged maximum velocity (TAMMV) in the distal ICA or proximal MCA. For children with appropriate sickle-cell genotypes, commonly taught categories are normal <170 cm/s, conditional 170–199 cm/s, and abnormal ≥200 cm/s. The parameter is not PSV. Use the validated screening protocol and identify whether the examination is imaging or non-imaging.

The 2020 ASH guideline describes confirmation with two non-imaging values ≥200 cm/s or one value >220 cm/s for the treatment threshold. Imaging TCD uses different thresholds. An abnormal screen triggers specialist evaluation and protocol-directed management; it should not be presented as proof of an exact fixed stenosis or as an automatic transfusion order from one 200 cm/s value.

A child with one non-imaging TAMMV of 205 cm/s has an abnormal screening category. The clinical team confirms and manages the result according to the protocol. A PSV of 205 cm/s, in contrast, cannot be substituted into the TAMMV algorithm. Document the measurement method clearly so the correct pathway is followed.

Collateral pathways and embolic monitoring

With carotid disease, the anterior communicating artery can supply cross-filling, the posterior communicating artery can connect posterior and anterior circulation, and external-to-internal pathways can reverse ophthalmic flow. Direction changes suggest recruitment but must be interpreted with the entire vascular anatomy. A single reversed signal without reliable vessel identification is not adequate evidence of a collateral pathway.

Microembolic signals are brief high-intensity Doppler events that require protocol-based recognition and artifact discrimination. Movement, probe contact, and other noise can mimic emboli. Monitoring duration, insonated vessel, and signal characteristics matter. A contrast TCD right-to-left shunt study can demonstrate microbubble passage but does not by itself locate the shunt or prove that it is a patent foramen ovale rather than another right-to-left pathway. Echocardiographic or other evaluation may be needed.

Cerebral circulatory arrest: the diagnostic boundary

As intracranial perfusion fails, characteristic patterns can include oscillating flow with little net forward flow and brief systolic spikes. Absent signal alone is insufficient, because a poor acoustic window can prevent insonation in a living patient. Technical adequacy and the required anterior/posterior circulation sampling must be established.

The 2023 AAN/AAP/CNS/SCCM guideline allows TCD as an ancillary test for adults when ancillary testing is indicated and performed under the appropriate protocol. It should not be used as an ancillary test for children under that guideline. Brain death/death by neurologic criteria is a clinical determination with prerequisites, examination, and apnea-testing requirements. A sonographer must not equate one abnormal trace with legal death.

When recording a concerning pattern, preserve representative waveforms and communicate promptly to the responsible clinician. State limitations, physiological conditions, and the protocol used. The authority to determine death and the validity of ancillary testing remain with appropriately qualified clinicians and applicable policy.

ASH cerebrovascular guideline, 2023 aSAH guideline, and 2023 neurologic death guideline — checked October 18, 2026.

Match the measurement to the question