Say Hello to Willy

Based on the circle of Willis, Willy hopes to raise awareness about the importance of transcranial Doppler ultrasound (TCD).

Cartoon character named Willy

The Where's Willy Challenge

The circle of Willis supplies up to 80% of the blood to the brain and is the most important source of collateral circulation. When an artery becomes blocked or damaged due to significant extracranial disease like stenosis, hemorrhage, or stroke, collateral circulation provides alternative routes for blood flow.  

Transcranial Doppler ultrasound is the only non-invasive method for continuous, real-time monitoring of blood flowing through this important structure. TCD can detect emboli, the narrowing of blood vessels, vasospasm due to hemorrhage, and much more. Due to the unique and important information provided by TCD, it is a required technology for all comprehensive stroke centers.

As part of our effort to raise awareness of the importance of TCD in the healthcare setting, we are announcing the Where's Willy Challenge.

Visit us at any of the conferences listed below to receive your free Willy sticker. Take a photo with Willy at your facility and post it on social media with the tags #whereswilly and #novasignal. Together we can spread the word about the importance of TCD!

2023 Conference Calendar

An image showing conferences NovaSignal is attending in 2023

Not attending any of these conferences but still want to support the cause? 

Download Willy's picture and print him in whatever form you would like!

What is the Circle of Willis?

The circle of Willis is an important junction of arteries at the base of the brain. The major extracranial arteries which feed the circle of Willis are the bilateral internal carotid arteries and the bilateral vertebral arteries.

The structure of the circle of Willis includes:

  • Internal carotid artery (ICA)
  • Anterior cerebral artery (ACA)
  • Posterior cerebral artery (PCA)
  • Posterior communicating artery (PCoA)
  • Anterior communicating artery (ACoA)
  • Basilar artery (BA)

Connecting arteries to the circle of Willis include:

  • Middle cerebral artery (MCA)
  • Anterior cerebral artery A2 (ACA A2)
  • Posterior cerebral artery P2 (PCA P2)
Circle of Willis with labeled arteries
Circle of Willis in the brain

What Does the Circle of Willis Do?

The circle of Willis plays a critical role in ensuring the brain receives adequate blood supply. It allows blood to flow from the anterior and posterior hemispheres, as well as from the left and right hemispheres of the brain. In other words, without the circle of Willis blood could not travel across the brain.

The arteries that make up the circle of Willis supply up to 80% of the blood to the brain.

The circle of Willis is the most important source of collateral circulation in the presence of significant extracranial disease like stenosis, hemorrhage, or stroke. Collateral circulation provides alternative routes for blood flow when another artery becomes blocked or damaged. The natural structure of the circle allows for bi-directional blood flow in the event of significant changes in pressure in any brain segment. This means blood flow can actually change direction when necessary. These collateral pathways can assure adequate blood supply to all areas of the brain, reducing brain damage or other negative after effects.

Circle of Willis Anomalies

A complete circle of Willis is seen in only ~30% of the population.

Some common structural variations include:

  • A hypoplastic or atretic anterior cerebral artery (A1) segment occurs in ~25% of the population.
  • A posterior cerebral artery that arises from the internal carotid artery instead of the basilar artery (fetal origin) occurs in ~22% of the population.

Additional variations include:

  • A single vessel that divides into two channels, then merges back into one. This is called fenestration.
  • The presence of two distinct arteries where there is usually only one. This is called duplication.
  • The presence of two anterior cerebral arteries which fuse into one vessel distally. These are called azygos arteries.
Circle of Willis anomalies

Transcranial Doppler and the Circle of Willis

Transcranial Doppler (TCD) is a non-invasive, painless ultrasound technique that utilizes high-frequency ultrasound waves to measure the rate and direction of blood flow inside the intracranial blood vessels. The test examines and records the speed of blood flowing in the circle of Willis to facilitate the diagnosis of a wide range of conditions.

TCD is the only non-invasive method for continuous, real-time monitoring of cerebral hemodynamics.

A robotic TCD machine on table

TCD Applications

A brain with text describing various clinical areas that TCD is used


  1. (2020, May 14). What is the circle of Willis? Medical News Today. Retrieved January 24, 2023, from
  2. Roy Munialo Machasio, Rose Nyabanda, Timothy Musila Mutala, "Proportion of Variant Anatomy of the Circle of Willis and Association with Vascular Anomalies on Cerebral CT Angiography", Radiology Research and Practice, vol. 2019, Article ID 6380801, 7 pages, 2019.
  3. (2022, April 25). Circle of Willis. MedlinePlus. Retrieved January 24, 2023, from
  4. Purkayastha S, Sorond F. Transcranial Doppler ultrasound: technique and application. Semin Neurol. 2012;32(4):411-420. doi:10.1055/s-0032-1331812