Real-Time Emboli Monitoring

Emboli are small particles that travel through the bloodstream and lodge in downstream vessels. They can be particles such as plaque, blood clots, fat, bacteria, tumor cells, amniotic fluid, or even air bubbles. Strokes occur when embolic particles lodge in a small artery in the brain, blocking blood flow to that artery’s distribution. The area that has been affected by the loss of blood flow is called an infarct.

Depending on the source of the emboli and the patient’s clinical history, there are several possible treatments for active embolization.  Anti-platelet medications like Aspirin decrease platelet aggregation.  Anti-coagulants such as Heparin slow down plasmatic coagulation.  Antibiotics are used for bacterial embolization, and r-tPA (recombinant tissue plasminogen activator) is a thrombolytic agent. Surgical treatment such as endarterectomy removes plaque from an artery, such as an internal carotid artery, removing the source of embolization.

Transcranial Doppler (TCD) can be used to monitor for emboli and, if identified, can help direct appropriate anticoagulation, with the goal of preventing strokes.

Using TCD for Emboli Monitoring

TCD is the only method of detecting real-time arterial emboli.  MRI and CT tell you that an embolism already occurred by identifying an infarct (a small localized area of dead tissue resulting from failure of blood supply), but they cannot see them happen in real time.  Real-time detection enables faster intervention to prevent additional emboli and therefore stroke.  The indications for performing emboli monitoring include patients with known carotid stenosis, atrial fibrillation, prosthetic cardiac valves, and endocarditis, as well as during and after carotid endarterectomies.1

This MRI shows a cerebral infarct that has already happened
TCD shows emboli passing through an artery as it happens

TCD monitoring exams involve listening to one or two vessels at a time for 20-30 minutes. Microembolic signals are seen both in the TCD M-mode and within the spectral waveform.  They appear as brief, high-intensity marks and have a characteristic ‘chirp’ sound. This TCD exam shows spectral waveforms and M-mode with multiple high intensity embolic signals.

TCD exam with multiple embolic signals.

Performing an Emboli Monitoring Exam

Any main cerebral artery can be monitored for emboli using TCD.  Vessels identified through the temporal window (MCA, ACA, PCA, and terminal ICA) can be monitored uni- and bilaterally using the autonomous, robotic NovaGuideTM 2 Intelligent Ultrasound.  Other vessels, such as the vertebral and basilar artery identified through the transoccipital window, can be monitored using a hand-held transducer.  Monitoring the ophthalmic and internal carotid siphon through the transorbital window is not recommended due to the power output and the sensitivity of the eye tissue.

Once the desired vessel has been identified, it should be monitored for 20-30 minutes.  Any emboli detected should be counted, which can be done during the exam or in post-processing.  Emboli rates are reported as ‘emboli per hour’, so if monitoring was performed for 20 minutes, the emboli count would be multiplied by three to get a ‘per hour’ rate.  For example, if there were 12 emboli detected in a vessel during 20 minutes of monitoring, the rate would be 12 x 3, or 36 emboli per hour.

Emboli vs. Artifact

A few simple guidelines can help distinguish between emboli and artifact on TCD.

  • Emboli travel within the artery, so the high-intensity signal will only be within the M-mode and spectral waveform, without crossing the spectral zero baseline or going outside the M-mode band or spectral waveform.
  • M-mode shows segments of the artery, so the temporal effect of the high-intensity signal will be apparent with a positive sloping line for emboli traveling toward the transducer and a negative sloping line for emboli traveling in a vessel away from the transducer.
Positive slope
Negative slope
  • The audible ‘chirp’ made by a true embolus differs from a static or clicking sound made by an artifact.
  • Artifact typically have straight lines rather than sloped, and may be accompanied by a short band of color drop-out in the M-mode. They can also be bidirectional and cross the spectral zero baseline.
  • Optimizing the image, by adjusting the gain, filter, scale, sample location, baseline, and volume, can make identifying emboli easier.
Artifact examples
Unoptimized image (left) vs. Optimized image (right)

Categorizing Emboli

TCD cannot distinguish between the different types of embolic material, however knowing the patient’s clinical history and disease state can assist with that determination. If emboli are detected in a patient with a prosthetic cardiac valve or high-grade carotid stenosis for example, the emboli are likely particulate.  If the patient is having a bubble study to look for a right-to-left cardiac shunt and agitated saline is injected, it is most likely that air emboli are being detected.  If the patient is in a surgical procedure and the proximal vasculature is being manipulated when emboli are detected, the manual manipulation is the likely source of either air or particulate emboli.  Finally, if emboli are detected in a patient with known endocarditis, bacterial clusters are the likely emboli makeup.  A good clinical history combined with a positive emboli monitoring exam can direct providers as to the most appropriate treatment.

Citations

  1. Ali, M.F. Transcranial Doppler ultrasonography (uses, limitations, and potentials): a review article. Egypt J Neurosurg 36, 20 (2021). Link
  2. https://www.health.harvard.edu/mind-and-mood/could-a-silent-stroke-erode-your-memory
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