Trans-cranial Doppler

Transcranial Doppler (TCD) ultrasound is a non-invasive method of obtaining bedside neurologic information that can supplement the physical examination in the comatose neurocritical care patient. This can be of particular value with an equivocal neurologic exam (e.g. pupils, Glasgow Coma Scale, Cushing’s response).

Though TCD was traditionally the domain of neuro-sonographers (as a non-imaging, Spectral Doppler only modality), we will focus on TCCD (transcranial colour-coded Duplex sonography), which combine both 2D/colour imaging as well as Spectral Doppler capabilities.

 

This page and video tutorial presents a systematic approach to incorporating the highest yield TCD techniques (e.g. vasospasm post subarachnoid hemorrhage [SAH], increased intracranial pressure [ICP], cerebral circulatory arrest) into critical care bedside practice. We will also explain the limitations of this modality, including when formal TCD is required. 

We will primarily focus on the transtemporal window for TCD, and focus on insonation of the middle cerebral artery (MCA). 

Section contributors: Dr. Vincent Lau

Why trans-cranial doppler?

With the increased ubiquity of point-of-care ultrasound machines, there is increasing adoption of goal-directed TCD at the bedside in the critical care environment. The value of this approach includes round-the-clock availability and a goal-directed approach allowing for repeatability, quick acquisition and bedside integration, which can help clinicians with decisions related to more definitive imaging studies as well as potential clinical interventions. 

How to perform

A phase-array probe (2-5 MHz) is used for this exam. 

Most modern ultrasound machines will have a TCD preset, which will by default, position the index marker to the left of the ultrasound screen. Users who do not have access to a machine with a TCD preset should use the cardiac preset, which is a reasonable substitute (just remember to flip the probe position). 

Begin the examination by finding the transtemporal window after applying ultrasound gel to the patient’s temporal bone at the level of the eye, just anterior to the patient’s ear. With the index marker pointed anteriorly (towards the patient’s eyes).

 

Using a sweeping motion, scan through the nearby brain for relevant structures. For example, the midbrain cerebral penducles/bilateral thalami are two hypoechoic structures which resemble a butterfly/heart. 

Potential pitfalls

POCUS TCD is primarily used as a rule-in test, rather than as a rule-out. If a rule-out is required, consider formal TCD.

The difference between the long axis of the MCA itself and the angle of insonation must be minimized (ideally to less than 10-15 degrees), otherwise measured velocities will be underestimated (mostly affecting vasospasm measurements).

Caution must be exercised with ICP estimation given its wide confidence intervals when compared directly to ICP monitors. The technique is generally considered to be more useful for following trends in PI and ICP, rather than trying to measure an absolute ICP value at any given moment. As such, TCD PI derived ICP values should be considered as estimates, and an intracranial invasive monitor should be sought to confirm in the appropriate clinical setting.

The perceived absence of intracranial vascular flow could easily be due to a poor acoustic window or inappropriately acquired/interpreted images, where clinicians could inappropriately misdiagnose brain death. These findings (particularly regarding cerebral circulatory arrest) should be substantiated with another examination (other ancillary testing like nuclear medicine perfusion scanning, CT-angiography or magnetic resonance imaging) before confirming the diagnosis.​

TCD/TCCDUS video tutorial

In this tutorial we will breakdown measurement and interpretation of TCD/TCCD.

References

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  10. Lindegaard K-F, Nornes H, Bakke SJ, Sorteberg W, Nakstad P. Cerebral vasospasm after subarachnoid haemorrhage investigated by means of transcranial Doppler ultrasound [Internet]. In: Proceedings of the 8th European Congress of Neurosurgery Barcelona, September 6–11, 1987. Springer; 1988 [cited 2017 Jan 16]. p. 81–84.Available from: http://link.springer.com/chapter/10.1007/978-3-7091-8975-7_16
  11. Lindegaard K-F, Nornes H, Bakke SJ, Sorteberg W, Nakstad P. Cerebral vasospasm diagnosis by means of angiography and blood velocity measurements. Acta Neurochir (Wien) 1989;100(1–2):12–24.

 

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