Normal Anatomy and Common Variants

10.1055/b-0034-102654

Normal Anatomy and Common Variants

Normal Intracranial Anatomy

The frontal lobe is demarcated posteriorly from the parietal lobe by the central sulcus ( Fig. 1.1 ). On axial magnetic resonance (MR) images near the vertex, the central sulcus is readily identified. It is the major sulcus just behind the “L,” the intersection of two sulci formed in part by the superior frontal sulcus. The precentral gyrus lies just anterior to the central sulcus and the postcentral gyrus just posteriorly. As a generalization, the primary motor area (Brodmann area 4) is located in the precentral gyrus, and the primary somatesthetic (body′s sensations) area (Brodmann areas 1, 2, and 3) in the postcentral gyrus. The parietal lobe is demarcated from the occipital lobe posteriorly by the parieto-occipital sulcus (fissure). The anatomy of the nuclei and white matter tracts is beyond the scope of this book, but see Fig. 1.2 . The reader is referred to the many computer-based atlases, including The Human Brain in 1969 Pieces by Wieslaw Nowinski.

**Fig. 1.1** Normal lobar and gyral anatomy. Important landmarks include the central sulcus, which separates the frontal lobe anteriorly from the parietal lobe posteriorly, and the sylvian fissure (the lateral sulcus), which divides the frontal and parietal lobes above from the temporal lobe below. The parietal and occipital lobes are separated by the parieto-occipital sulcus. (Courtesy of Wieslaw Nowinski, DSc, PhD.)

**Fig. 1.2** Brain nuclei and white matter tracts, normal anatomy. Most relevant to MR interpretation are the locations of the caudate nucleus, putamen, globus pallidus (*light green*, immediately medial to the putamen and not labeled), hippocampus, and thalamus. Note also the location of the optic radiations. (Courtesy of Wieslaw Nowinski, DSc, PhD.)

The pituitary gland is divided into anterior and posterior lobes. The anterior pituitary is referred to as the adenohypophysis and the posterior pituitary as the neurohypophysis. The normal pituitary is less than 10 mm in height, and demonstrates intense enhancement following intravenous contrast administration, due to the lack of a blood–brain barrier. A common variant in appearance of the pituitary is a slight upward convex superior margin, which can be seen in young women. On T1-weighted scans, the posterior pituitary is seen to be hyperintense in up to half of the normal patient population, a finding that is more common in younger patients (but not in the elderly).

The internal auditory canal (IAC) is a bony foramen within the petrous portion of the temporal bone. It contains the 7th (facial) and 8th (vestibulocochlear) nerve complexes. The 7th nerve lies in the anterior superior quadrant of the IAC, when viewed in cross-section, and runs laterally to the geniculate ganglion. The cochlear division of the 8th nerve lies in the anterior inferior quadrant. The superior and inferior vestibular nerves, also divisions of the 8th nerve, which supply information concerning equilibrium, lie in the superior and inferior posterior quadrants. As visualized in the axial plane, the cochlea is anterior and the vestibule posterior. There are three semicircular canals: the lateral (which has a horizontal orientation), superior, and posterior. The fluid (endolymph) within the cochlea, vestibule, and semicircular canals is normally isointense to cerebrospinal fluid (CSF) on MR.

Normal Arterial Anatomy

Three major arteries supply the cerebral hemispheres ( Fig. 1.3 ). The anterior cerebral artery (ACA) supplies the anterior two-thirds of the medial cerebral surface and 1 cm of superior medial brain over the convexity. The recurrent artery of Heubner, which originates from the A1 or A2 segment of the ACA, supplies the caudate head, anterior limb of the internal capsule, and part of the putamen. The posterior limb of the internal capsule, portions of the thalamus, the caudate, the globus pallidus, and the cerebral peduncle are supplied by the anterior choroidal artery, which arises from the supraclinoid internal carotid artery. The middle cerebral artery (MCA) supplies the lateral portion of the cerebral hemispheres, the insula, and the anterior and lateral temporal lobes. The lenticulostriate arteries, which originate from the M1 segment of the MCA, supply the lentiform nucleus (globus pallidus and putamen) and the anterior limb of the internal capsule. The posterior cerebral artery (PCA) supplies the occipital lobe and the medial temporal lobe. The thalamoperforating and thalamogeniculate branches supply the medial portion of the thalami and the walls of the third ventricle. These small perforating branches arise from the P1 segment of the PCA with similar branches arising from the posterior communicating artery ( Fig. 1.4 ).

**Fig. 1.3** Normal arterial and venous anatomy. Anteroposterior and lateral projections are illustrated. (Courtesy of Wieslaw Nowinski, DSc, PhD.)

**Fig. 1.4** Supratentorial arterial territories. Axial diagrams of the brain at four levels depict the major arterial territories of the supratentorial region, specifically the anterior cerebral artery (*blue*), middle cerebral artery (*pink*), and posterior cerebral artery (*yellow*) territories. In *red* is the vascular territory supplied by the penetrating branches of the middle cerebral artery (the lenticulostriate arteries). In *brown* is the territory supplied by the penetrating branches of the posterior cerebral arteries (the posterior thalamoperforators) and posterior communicating arteries (the anterior thalamoperforators). In *green* is the territory supplied by the anterior choroidal artery, which supplies amongst other structures the posterior limb of the internal capsule, the optic tract, and the hippocampus and amygdala. (Courtesy of Wieslaw Nowinski, DSc, PhD.)

Three major but smaller vessels supply the cerebellum ( Fig. 1.5 ). The largest is the posterior inferior cerebellar artery (PICA), which supplies the tonsil, the inferior vermis, and the inferior cerebellum (with the exception of its most anterior extent). The anterior inferior cerebellar artery (AICA) supplies the anterior inferior portion of the cerebellum and is the smallest of the three vessels. It is commonly stated that the distribution of AICA is in continuum with PICA, with at times the distribution slightly larger or smaller. The superior cerebellar artery supplies the superior half of the cerebellum.

**Fig. 1.5** Arterial supply of the posterior circulation, visualized on an anatomic drawing of the base of the brain. The posterior inferior cerebellar artery (PICA) originates from the vertebral artery. The two vertebral arteries join to form the basilar artery, with the major paired branches (in order from caudal to cranial) being the anterior inferior cerebellar artery (AICA), the superior cerebellar artery (SCA), and the posterior cerebral artery (PCA)—the latter marking the termination of the basilar artery. (Courtesy of Wieslaw Nowinski, DSc, PhD.)

The circle of Willis is complete in only one quarter of the population. Variants include the following. A fetal origin of the posterior cerebral artery, with its origin from the internal carotid artery instead of the basilar artery, is seen in about one in five patients. The P1 segment of the posterior cerebral artery, which is the portion from the tip of the basilar artery to the junction with the posterior communicating artery (PCOM), is usually also hypoplastic in this circumstance. The PCOM is hypoplastic in one-third of patients. The anterior communicating artery (ACOM), which connects the two anterior cerebral arteries, is hypoplastic in 15%. The A1 segment of the anterior cerebral artery, which begins at the carotid terminus and continues to the juncture with the ACOM, is hypoplastic in 10%.

The external carotid artery is the smaller of the two terminal branches of the common carotid artery. It arises anterior and medial to the internal carotid artery, then courses posterior laterally. There are many muscular branches, with the early branching of the external carotid artery allowing rapid recognition of this vessel in distinction to the internal carotid artery.

The internal carotid artery was traditionally divided into four major segments: the cervical, the petrous (horizontal), the cavernous (juxtasellar), and the intracranial (supraclinoid) portions. Today, there are seven recognized segments (C1 to C7): the cervical, petrous, lacerum, cavernous, clinoid, ophthalmic, and communicating (terminal) segments. At its origin, the internal carotid artery is somewhat dilated, forming the carotid bulb. The petrous segment, C2, of the internal carotid artery has three sections: the ascending (vertical), the genu (bend), and the horizontal portions. The lacerum segment, C3, is still extradural. The cavernous segment, C4, is surrounded by the cavernous sinus. The meningohypophyseal artery arises from C4. The clinoid segment, C5, is very short, and begins after the artery exits from the cavernous sinus. C5 extends from the proximal dural ring to the distal dural ring. C6, the ophthalmic segment, extends from the distal dural ring (with this portion of the internal carotid artery thus considered intradural) to the origin of the PCOM. The ophthalmic artery arises from C6. C7 is that segment of the artery extending from the origin of the posterior communicating artery to the carotid terminus, where the vessel divides into the anterior and middle cerebral arteries. C6 and C7 together constitute the supraclinoid internal carotid artery.

There are many extracranial–intracranial vascular anastomoses. Two of these involve the ophthalmic artery. There are also multiple internal carotid–vertebral artery anastomoses, which represent persistent embryonic circulatory patterns. One of these is seen not uncommonly, as a normal variant, and is the persistent trigeminal artery ( Fig. 1.6 ). Pial–leptomeningeal anastomoses are also present, and are an important potential source of collateral blood flow in occlusive vascular disease.