Transcranial Doppler is the most technically demanding examination in most vascular laboratories. You are insonating vessels you cannot see, through bone, identifying them by depth, flow direction, and the response to compression manoeuvres rather than by anatomy on screen.

It is also uniquely valuable: TCD is the only bedside, repeatable, real-time monitor of cerebral haemodynamics, and in two settings — vasospasm after subarachnoid haemorrhage, and stroke risk screening in sickle cell disease — it changes management directly.

Equipment

Probe: Low-frequency phased array, 2 MHz. Higher frequencies do not penetrate the skull.

Modes:

Non-imaging (blind) TCD — pencil probe, relies entirely on depth and flow direction. Still used for monitoring and emboli detection.
Transcranial colour-coded duplex (TCCD) — B-mode and colour guidance. Substantially easier to learn and allows angle correction.

Set a low wall filter, appropriate depth for the window, and adequate power output. Sample volume is typically 5–10 mm.

The Four Acoustic Windows

The skull is largely opaque to ultrasound. You are exploiting thin regions and natural foramina.

1. Transtemporal

The workhorse window. Above the zygomatic arch, anterior to the ear.

Three sub-windows — posterior, middle, anterior. Start posterior, just above the zygoma and anterior to the ear, and work forward.

Vessels: MCA, ACA, PCA, terminal ICA.

Failure rate matters. An inadequate temporal window occurs in a meaningful minority of patients — more commonly in older patients, women, and certain ethnic groups, because of skull thickness. This is a limitation of the technique, not a failure of the operator, and should be stated plainly in the report.

2. Transorbital

Through the closed eyelid, angled medially.

Vessels: Ophthalmic artery, ICA siphon.

Reduce power output substantially — the eye is sensitive to acoustic energy, and this is a hard safety requirement, not a suggestion. Use the lowest output that yields a signal and minimize dwell time.

3. Suboccipital (Transforaminal)

Through the foramen magnum. Patient's head flexed forward, probe below the occiput aimed at the bridge of the nose.

Vessels: Vertebral arteries, basilar artery.

4. Submandibular

Below the angle of the mandible, angled upward.

Vessels: Distal extracranial ICA — needed for the Lindegaard ratio.

Vessel Identification

Without direct visualization, identity rests on three things: window, depth, and flow direction relative to the probe.

Vessel	Window	Typical depth	Flow direction
MCA	Transtemporal	30–65 mm	Toward probe
ACA (A1)	Transtemporal	60–80 mm	Away from probe
PCA (P1)	Transtemporal	60–75 mm	Toward probe
Terminal ICA	Transtemporal	55–65 mm	Toward probe
Ophthalmic	Transorbital	40–50 mm	Toward probe
ICA siphon	Transorbital	55–65 mm	Bidirectional
Vertebral	Suboccipital	40–75 mm	Away from probe
Basilar	Suboccipital	80–120 mm	Away from probe
Distal ICA	Submandibular	40–60 mm	Away from probe

The MCA/ACA bifurcation at around 60–65 mm shows simultaneous bidirectional flow and is the anatomical anchor of the temporal window. Find it first, then walk shallower along the MCA.

Compression manoeuvres confirm identity in non-imaging studies — brief common carotid compression alters flow in a predictable, vessel-specific way. Use with caution and avoid entirely in patients with carotid disease or plaque.

Normal Velocities

Mean flow velocity (MFV) in adults, approximate:

Vessel	Normal MFV
MCA	55 ± 12 cm/s
ACA	50 ± 11 cm/s
PCA	40 ± 10 cm/s
Basilar	40 ± 10 cm/s
Vertebral	38 ± 10 cm/s

Velocities decrease with age and increase with anaemia, fever, pregnancy, and hypercapnia. Children have substantially higher normal velocities than adults — do not apply adult values to paediatric studies.

Pulsatility index (PI) = (PSV − EDV) / MFV. Normal is roughly 0.5–1.1.

Elevated PI suggests increased distal resistance — raised intracranial pressure, distal microvascular disease
Reduced PI with a damped, delayed waveform suggests a proximal stenosis or occlusion

Vasospasm After Subarachnoid Haemorrhage

The commonest indication in most centres. Vasospasm typically develops between days 3 and 14 after aneurysmal SAH, and TCD is used for serial monitoring because it is repeatable at the bedside.

MCA Velocity Thresholds

MCA MFV	Interpretation
< 120 cm/s	Normal
120–150 cm/s	Mild vasospasm
150–200 cm/s	Moderate vasospasm
> 200 cm/s	Severe vasospasm

The Lindegaard Ratio

Velocity rises in vasospasm — but it also rises in hyperaemia, which is common in these patients. The ratio distinguishes them:

Lindegaard ratio = MCA MFV / extracranial ICA MFV

The extracranial ICA velocity comes from the submandibular window, which is why that window matters.

Ratio	Interpretation
< 3	Hyperaemia, not vasospasm
3–6	Mild to moderate vasospasm
> 6	Severe vasospasm

This is the single most important calculation in SAH monitoring. An MCA velocity of 180 cm/s means completely different things at a ratio of 2 versus a ratio of 7 — the first is a well-perfused brain, the second is a brain at risk of infarction.

Rate of rise matters too. An increase of more than about 50 cm/s per day is concerning even before absolute thresholds are crossed.

For the posterior circulation, an analogous basilar-to-vertebral ratio is used.

Sickle Cell Disease Screening

TCD screening in children with sickle cell disease is one of the clearest examples of imaging preventing disease, following the landmark stroke-prevention trial evidence.

Measurement uses time-averaged mean maximum velocity (TAMMV) in the MCA or distal ICA:

TAMMV	Risk category
< 170 cm/s	Normal
170–199 cm/s	Conditional — repeat sooner
≥ 200 cm/s	Abnormal — high stroke risk

Children with abnormal velocities have a substantially elevated stroke risk, and chronic transfusion therapy markedly reduces it. Use the correct measurement (TAMMV, not peak systolic) and follow your protocol's repeat intervals — this is a screening programme, and its value depends on consistency.

Other Applications

Acute stroke: Assessing intracranial occlusion and recanalization. The TIBI grading system describes residual flow from absent (grade 0) through to normal (grade 5).

Emboli detection: Microembolic signals appear as short-duration, high-intensity transient signals within the Doppler spectrum, with a characteristic audible chirp. Used in carotid disease and during cardiac and carotid surgery.

Right-to-left shunt detection ("bubble study"): Agitated saline injected intravenously while monitoring the MCA. Microbubbles appearing in the cerebral circulation, particularly after Valsalva, indicate a shunt such as a patent foramen ovale.

Cerebral circulatory arrest: In brain death assessment, TCD may show characteristic patterns — oscillating to-and-fro flow, or short systolic spikes. This is an ancillary test performed under strict protocol, never a standalone determination.

Vasomotor reactivity: Response to a CO₂ or breath-holding challenge assesses cerebrovascular reserve.

Key Images to Capture

MCA spectral waveform with depth, MFV, PSV, EDV, PI
ACA, PCA, terminal ICA — bilaterally
Vertebral and basilar arteries via the suboccipital window
Extracranial ICA via the submandibular window where the Lindegaard ratio is needed
Documentation of window adequacy on each side
Serial comparison table where monitoring

Reporting Language

"Adequate bilateral temporal windows. Right MCA mean flow velocity 186 cm/s at 52 mm depth (previous study 128 cm/s). Right extracranial ICA MFV 28 cm/s, giving a Lindegaard ratio of 6.6. Left MCA MFV 92 cm/s, ratio 2.9. Pulsatility index 0.7 bilaterally. Findings consistent with severe right MCA vasospasm with a significant interval rise. Recommend urgent clinical correlation."

SonoBuddy is a reference tool. TCD findings in acute neurological patients require immediate direct communication with the clinical team.

Transcranial Doppler: Windows, Velocities, and Vasospasm Detection