Transcranial Duplex Scanning (TCCS): Velocity Norms, Lindegaard Ratio, and Vasospasm Assessment

TCD and TCCS: Two Methods

Historically, transcranial Doppler (TCD) appeared first — a "blind" spectral Doppler without imaging: the vessel is identified by the access approach, insonation depth, flow direction, and response to maneuvers. Since 1990, transcranial color-coded duplex scanning (TCCS/TCCD) has been added — with B-mode and color flow mapping, where the arteries at the base of the brain are visualized against the midline structures and the sample volume is positioned precisely.

The practical distinction is important: in TCD the insonation angle is not corrected, and velocities are measured "as is" (assuming the vessel runs along the beam). Velocities obtained on duplex equipment may be lower than on non-duplex equipment; stroke risk thresholds may need to be lowered depending on the center's protocol (AIUM, 2023).

Access Windows and Vessel Identification

TCD/TCCS allows assessment of the middle, anterior, and posterior cerebral arteries through the temporal acoustic windows, the vertebral and basilar arteries through the suboccipital window, and the carotid siphons through the transorbital window. The submandibular (transcervical) window: the probe is placed under the angle of the mandible, at a depth of 40–50 mm, with a low-resistance spectrum of the distal ICA — used for the Lindegaard ratio. Vessel identification is based on depth and flow direction.

Velocity Norms

Mnemonic for the distribution of normal velocities: MCA > ACA > PCA > OA = VA. A marked disruption of this hierarchy is a reason to suspect pathology or a measurement error. Summary data on normal velocities and stenosis thresholds for intracranial arteries are provided in the ASN/WFUMB consensus (2024). Mean flow velocities (MFV) and their ratios are the primary criterion for diagnosing cerebrovascular pathology.

MFV Thresholds for Intracranial Stenosis

For screening intracranial stenosis, MCA MFV thresholds of ≥80 / ≥100 / ≥120 cm/s are used (Neurology, 2002). PI/RI indices are significant only for specific tasks: indirect assessment of ICP (PI is more reliable than RI), vasospasm (Lindegaard/Sviri ratios); they do not by themselves grade stenosis.

Lindegaard Ratio and Vasospasm

Vasospasm is the narrowing of the large arteries at the base of the brain, most often as a complication of aneurysmal subarachnoid hemorrhage (SAH). When MCA MFV is in the range of >120 and <200 cm/s, MFV in the extracranial ICA is assessed through the submandibular window and the Lindegaard ratio (LR) is calculated to differentiate vasospasm from cerebral hyperemia.

Diagnostic characteristics of TCD for MCA vasospasm: sensitivity 90 % (95 % CI 77–96 %), specificity 71 % (95 % CI 51–84 %), PPV 57 % (95 % CI 38–71 %), NPV 92 % (95 % CI 83–96 %). With clinical suspicion of vasospasm and MFV <120 cm/s, CT perfusion or angiography is still performed (Robba C. et al., 2019).

Recorded Parameters

The TCD device records at least CBFV — systolic (Vs), diastolic (Vd), and mean (Vm) flow velocities, as well as calculated indices, in particular the pulsatility index PI = (Vs − Vd)/Vm. Spectral analysis includes peak systolic velocity, end-diastolic velocity, systolic upstroke (acceleration) time, pulsatility index, and time-averaged maximum mean velocity. Spectra are obtained in 2–5 mm steps with a sample volume size of 3–6 mm.

Clinical Indications

TCD/TCCS is used to assess stroke risk in children with sickle cell anemia, to detect and monitor vasospasm after spontaneous SAH, to diagnose intracranial stenosis/occlusion and cerebral circulatory arrest. The method is the gold standard for real-time detection, localization, and quantification of cerebral embolism, useful for monitoring thrombolysis of acute intracranial occlusions, detecting extracranial ICA stenosis, microembolism, right-to-left cardiac shunt, and assessing cerebrovascular reactivity.