Transcranial Doppler ultrasonography: From methodology to major clinical applications

Abstract

Non-invasive Doppler ultrasonographic study of cerebral arteries [transcranial Doppler (TCD)] has been extensively applied on both outpatient and inpatient settings. It is performed placing a low-frequency (≤ 2 MHz) transducer on the scalp of the patient over specific acoustic windows, in order to visualize the intracranial arterial vessels and to evaluate the cerebral blood flow velocity and its alteration in many different conditions. Nowadays the most widespread indication for TCD in outpatient setting is the research of right to left shunting, responsable of so called "paradoxical embolism", most often due to patency of foramen ovale which is responsable of the majority of cryptogenic strokes occuring in patients younger than 55 years old. TCD also allows to classify the grade of severity of such shunts using the so called "microembolic signal grading score". In addition TCD has found many useful applications in neurocritical care practice. It is useful on both adults and children for day-to-day bedside assessment of critical conditions including vasospasm in subarachnoidal haemorrhage (caused by aneurysm rupture or traumatic injury), traumatic brain injury, brain stem death. It is used also to evaluate cerebral hemodynamic changes after stroke. It also allows to investigate cerebral pressure autoregulation and for the clinical evaluation of cerebral autoregulatory reserve.

Keywords: Acute subarachnoid hemorrhage; Cryptogenic STroke; Ischemic stroke; Lindegaard ratio; Microembolic signals; Middle cerebral artery; Paradoxical embolism; Patent foramen ovale; Transcranial Doppler ultrasonography; Vasospasm.

Figure 1

Circle of Willis and Ultrasonographic study by transcranial Doppler ultrasound. A: Circle of Willis; B: Transmission of ultrasound beam through skull using pulsed Doppler sectorial probe with a 2.0-3.5 MHz emission frequency. Probe is positioned on temporal window. MCA: Middle cerebral artery.

Figure 2

Transcranial Doppler color Doppler study of intracranial arteries. MCA: Middle cerebral artery; PCA: Posterior cerebral artery; ACA: Anterior cerebral artery; Mes: Mesencephalon.

Figure 3

Transcranial Doppler spectral Doppler study of intracranial middle cerebral artery.

Figure 4

Transthoracic echocardiography showing high grade right to left shunt with evident micro-bubbles in the left heart after intravenous contrast administration (A and B).

Figure 5

Right to left shunt with microembolic signals. A: Low grade shunt; B: Moderate grade shunt; C: High grade shunt (shower); D: Curtain effect.

Figure 6

Contrast enhanced transcranial Doppler as a first line screening tool in the setting of a cryptogenic ischemic stroke. TCD: Transcranial Doppler; c-TCD: Contrast enhanced TCD; TEE: Transesophageal echocardiography; RLS: Right-to-left shunting; PFO: Patent foramen ovale; ASA: Atrial septal aneurysm; MES: Microembolic signals; VKA: Vitamin K antagonist.