Neurovascular anatomy is even more complicated than it may initially seem, but learning the major structures can be simplified with some simple concepts and patterns.

First, the neurovascular section builds on all prior sections. Specifically, vascular structures have important relationships with the brain surface anatomy, dura, CSF spaces, skull base foramina and canals, head and neck spaces and structures, and the spine. In some cases, the names of the vascular structures and housing anatomy match, making memorization and recognition easy-the olfactory sulcus cistern houses the olfactory vein is found in the olfactory sulcus cistern, the calcarine artery courses within the calcarine sulcus, the jugular vein traverses the jugular foramen, the artery of the foramen rotundum is found in the foramen rotundum and so forth. Extracranial and intracranial arteries and their key branches seem numerous, but are also often named for the destination anatomy-the superior thyroidal artery to the thyroid, the posterior aurical artery to the posterior external ear, the petrosal branch of the middle meningeal artery supplies the dura over the petrous ridge, etc.

While the spatial resolution of CTA and MRA remain lower than DSA, many details of distal intracranial and extracranial branches can be discerned on close inspection with modern technical and are often the key to optimal interpretation.

The concept of anatomy by expectation promulgated by spine interventionalists (prior section) is analogous to longstanding techniques used by neuroradiologists. For example, the location of vascular structures on cerebral angiograms can indicate the expected location of adjacent parenchyal or CSF structures. This was used to detect intracranial mass effect prior to the advent of cross-sectional imaging, but remains highly useful for understanding neurovascular anatomy today (even if often forgotten).

Finally, the key vascular territories of the brain and spinal cord parenchyma can be learned by studying infarct patterns and the function can be learned by the related deficits/syndromes.

Imaging interpretation is facilitated by recognition of several key properties of the intracranial arteries. First, these arteries are rich in collateral connections. Theses collaterals are not always sufficient to prevent infarct in the setting of acute stroke, but can become more robust overtime in the setting of chronic hemodynamic compromise. The type and degree of collateralization can be important to assess in both acute and chronic pathologies. Such collaterals can include ECA-ICA, lenticulostriate/thalamoperforator, leptomeningeal, transdural, and trans-osseous routes. An occluded ICA is often reconstituted via collateral flow to the ophthalmic artery by several ECA branches. Additionally, flow through the circle of Willis provides redundancy of arterial supply to the brain.

There is a high degree of congenital variation including: origin, size, course, branching pattern, arterial territory, persistence/regression of embryonic arteries, and degree of collateralization. Essential variants to learn include posterior communicating artery (PCOM) variants such as hypoplastic/absent or persistent fetal PCOM, aberrant ICA, persistent stapedial artery, and artery of Percheron. The concept of the AICA-PICA loop, indicating that either of these two may be dominant on any given side, is also important.

A given artery tends to supply a certain area or arterial territory, although the precise size of the territories vary from patient to patient (but are more constant than venous territories). The regions between territories are border zones. In this section, the arterial territories are depicted by associated pathology, predominantly infarcts.

Evaluation of all neurovascular anatomy involves viewing a high-resolution dataset in a volume viewer, assessing all major vessels in the 3 major imaging planes at minimum, supplementing with oblique or double oblique planes as needed. 3D surface rendered images and MIPS can be useful adjunctions. With modern high-resolution images, it is imperative to assess arteries well beyond the circle of Willis to arrive at an optimal interpretation.

The basal and pial arteries are associated with specific CSF spaces/cisterns, including cisterns, ventricles, and sulcal subarachnoid regions Perforator arteries course within perivascular spaces and supply territories within the brainstem or deep gray matter/internal capsule region.

Lack vasa vasorum. Circle-many variants. Course within specific cisterns and distal arteries may course along specific sulci..

Many important ICA-ECA collaterals.

Must assess beyond the COW. Stenosis or thrombo-emboli.

Many causes of steno-occlusive disease. ICAD underestimated.

Particularly important to assess in all 3 planes--superiorly or inferior projected aneurysm, eccentric plaque, etc.

M1-3, A1-3, P1-3,

Vascular territories, deep and superficial borderzones.

Major supply of AVMs (as opposed to most AVFs).

Important variants (fetal PCA, abberent ICA, PICA-AICA loop, early branch M1, absent/diminutive pCOM)

Moyamoya disease is a disorder of idiopathic progressive stenosis of the ICA termini, proximal MCAs and proximal ACAs (red arrows). The distribution is a key factor for diagnosis. ''Puff-of-smoke' perforator collaterals and other collaterals may be seen. Beyond luminal imaging, parenchymal imaging, perfusion, and cerebrovascular reactivity may provide additional information of disease status.