If you are studying for a board exam or just learning TCD for the first time you need to have an intimate working knowledge of the Circle of Willis. This can be a challenging subject for some technologists. Hopefully this can help to simplify it.
For me, I found resourcing pictures of the inferior (underside looking up) view of the brain to be the easiest to conceptualize. I then work from posterior (back) to anterior (front). Attached to this post is a picture sourced from wikipedia of the inferior view.
To start grasping the concept, make yourself familiar with medical terminology and the anatomy of the skull and the brain. Understanding the words inferior, superior, distal, proximal, posterior and anterior will be crucial in visualization. Recognizing the various structures in both the skull and the brain will assist you in understanding basic concepts when applied to the Circle of Willis. Knowing these next basics will be particularly helpful. The foramen magnum of the skull, or essentially the largest hole in the skull at the base where all of the structures feeding from the brain to the body lie. The various lobes of the brain and where they lie. Lastly, basic knowledge of the homunculus; this does not have to be incredibly detailed. The homunculus is an easy concept if you break it down for yourself, know the difference between the motor strip and the sensory strip then know where the face, hands, and legs lie, the rest (mouth, tongue, eyes, shoulders etc) will fall in place. Once these basics are learned you can move on to the Circle of Willis with confidence that it will make sense. You will have less memorization and more working critical knowledge of the structure.
Starting posteriorly at an inferior view you have the vertebral arteries that move up through the transverse foramina of the cervical vertebrae (basically the small holes on either side of the upper cervical vertebrae). From there the arteries pass through the foramen magnum or the large hole in the base of the skull that connects the body with the head. These arteries then branch off into what we know as the PICA or posterior inferior cerebellar artery. Then as the vertebral arteries continue to move up, they branch off again creating the AICA or anterior inferior cerebellar artery. So you guessed it, these 4 branches off of the vertebral artery supply blood to both the anterior and posterior of that lowest and most posterior portion of the brain; the inferior portion of the cerebellum! Assuming you have studied your anatomy a stroke or occlusion in the PICA would directly affect the posterior inferior region of the cerebellum causing poor coordination and loss of muscle tone with altered balance. This could affect cranial nerve X (Vagus) and produce dysphagia (trouble swallowing). This could also affect the parasympathetic nervous system and produce Horner's syndrome (pupil constriction, anhidrosis (lack of sweat), ptosis (drooping eyelid). And a stroke or occlusion of the AICA supplying blood to the anterior portion of the cerebellum could cause damage to the spinothalamic tract. Lateral pontine syndrome, Cranial nerve VII palsy, vertigo or nystagmus or deafness, can produce ataxia or loss of coordination/tone/balance. This is all ipsilateral. However, there could be contralateral loss of pain and temperature sensations from the body.
As that second branch of AICA or anterior inferior cerebellar artery branches off, the vertebral artery also fuses to form the basilar artery. There are small branches off this basilar artery feeding blood to the spinal tract as well called posterior spinal arteries. There are more small branches off of this basilar artery that are called labyrinthe arteries or interior acoustics arteries. As you can guess these supply blood to the inner ear. The next branches off the basilar artery are the pontine branches, these supply, as the name suggests, to the pons. The next branch supplies blood to the superior (top) aspect of the cerebellum and it is called the superior cerebellar arteries. At the basilar artery level depending on where the stroke or occlusion occurs this can produce locked in syndrome, bilateral loss of corticospinal tracts, pt can still have vertical eye movements but remain conscious and aware as the pontine branches occlude and stop blood flow to corticospinal tracts.
After all these branches supplying blood to the lowest portions of the brains the basilar artery finally meets the Circle of Willis at the final branch of the basilar artery the PCA or posterior cerebral artery. As you guessed this branch supplies blood to the posterior aspect of the brain or occipital lobe and inferior aspect of the temporal lobes. The PCA supplies blood to the primary visual cortex in the occipital lobe so a stroke or occlusion here would cause homonymous and hemianopia. Or losing temporal visual field and nasal visual field. (inner and outer vision). Temporal visual field stays ipsilateral and the nasal visual field is contralateral to the occlusion.
The PCA branches fuse to the posterior communicating arteries or PCoA. This artery is interesting in the fact that it actually runs backwards to fuse with the PCA and contribute to the Circle of Willis but is just a small portion of the ICA or internal carotid artery. The PCoA fuses the posterior cerebral artery (PCA) to the internal carotid (ICA) to the distal middle cerebral artery (MCA). As you can guess the MCA supplies blood to the lateral portions of the middle of the brain. When we think of anatomy of the brain the lateral portion of the middle of the brain is where the motor and sensory strip lie. It is also where our Broca’s area lies. So a superior division of the MCA occlusion or stroke can lead to a contralateral loss of sensation and motor control of the face and upper extremities. Can also affect Broca's area and lead to Broca's aphasia or inability to produce speech. While the inferior division of the MCA, the temporal lobe is most affected or Wernicke's area. Remembering your homunculus is important in understanding why the lower extremities are not affected. The MCA does not supply blood to the anterior or superior portions of the brain, only the middle and on our homunculus the lower extremities are in the most superior regions of the brain.
Where the ICA and the MCA meet is where they join the anterior cerebral artery ACA. The anterior cerebral artery fuses at the anterior communicating artery ACoA completing the circle. The ACA supplies blood to the anterior portions of the brain including the frontal lobe and the anterior portions of the primary motor cortex and the primary somatosensory cortex. A stroke or occlusion in this region would cause a loss of sensation and motor control on the contralateral side particularly the lower extremities.
In conclusion, I hope supplying the material in this fashion can help someone as much as it helped me, whether you are trying your hand at TCDs or studying for a board exam it is imperative to have a working knowledge of how or why in order to critically think through presented questions or challenges. Memorization of these key fundamentals will not suit you on the long term in this challenging field.