Letâ€™s get a little more familiar (and look at a cool image) with the path blood takes as it leaves the heart and travels up to the brain. If you donâ€™t recall what we covered inÂ Blood Flow Through the Brain Part 1, take a moment to review.
Remember the above picture? Now letâ€™s take a look at what it really looks like, courtesy of an MRI scanner:
Pretty cool, huh? Itâ€™s a bit confusing but hang in there and letâ€™s really nail those anatomic structures and take a look at what it looks like in real life. Starting at the bottomâ€¦see the horseshoe looking structure about parallel with the â€˜Aâ€™? Thatâ€™s theÂ aortaÂ as it arches out of the heart on towards the rest of the body. With the heart being the blob at the bottom, blood moves up starting from the left side and arching up towards the right and then down towards the abdominal organs.
Coming up from the aorta are two lateral curving projections outward. These are theÂ subclavian arteriesÂ running beneath the clavicles and out to the arms. Now look at the two tangles, one on the right & one on the left, of vessels shooting straight up. Youâ€™re looking at theÂ carotidÂ &Â vertebralarteries.
Now keep looking directly upwards horizontal with the heart. See those two that are coming together and meet almost in the center of the head at the midline? Thatâ€™s where the vertebral arteries come together to create theÂ basilar artery,Â also called theÂ vertebrobasilar system.Â Directly to the right and left, those large corkscrew-looking vessels, are theÂ carotid arteries.
Pop quiz: Which vessels comprise theÂ posterior circulation? Carotids or Vertebrobasilar?
A: Vertebrobasilar. The carotids give rise to theÂ anterior circulation.
As such remember as you are looking at the images that the carotids are actually in front while the vertebral and basilar are in the back. View the MRI as if you were looking face to face with the patient. Follow those carotids up and the basilar up, almost to the top of the image, and draw a horizontal line in your mind. Right about the area where all those vessels cross that line is where theÂ Circle of Willislies. Itâ€™s hard to visualize from this view but weâ€™ll cover it in-depth soon.