Normal Brain Anatomy on Magnetic Resonance Imaging




The exquisite detail provided by brain magnetic resonance imaging scans can make interpretation simultaneously straightforward and complicated, particularly to the novice. For this reason, it is essential to become familiar with normal structures before describing the pathologic condition. This article serves as a practical reference point to further enhance knowledge of the intracranial anatomy.


Brain magnetic resonance (MR) imaging studies provide multiple different imaging sequences in at least 2, and often 3, imaging planes. The different tissue signal characteristics and anatomic viewpoints are often complementary, and interpreting an MR imaging study of the brain can be a daunting task. The variety of pulse sequences and imaging planes makes understanding normal anatomy a necessity. Admittedly, many different approaches can be taken when interpreting images, but in this article just one approach to understanding normal anatomy is described. The normal anatomy of the brain has filled many textbooks, and a sincere effort has been made to provide a pertinent and concise reference suitable for review. Textbooks cited within the article serve as excellent references for those who wish to further their knowledge of brain anatomy.


Protocol


The overwhelming advantage of MR imaging is its ability to provide images with increased signal to noise ratios. Tissue characteristics with respect to different imaging sequences provide valuable clues when interpreting an MR image of the brain. Therefore, it is important to understand the accentuated tissue features on each scan. T1-weighted images provide good tissue discrimination and, in conjunction with the postcontrast scans, allow assessment for tissue enhancement. Precontrast and postcontrast scans must be obtained with identical imaging parameters to truly assess contrast enhancement ( Table 1 ). T1-weighted and T2-weighted images are complementary to each other because the T2-weighted images are sensitive to increased water content within tissues and to differences in susceptibility between tissues. Gradient echo images are susceptible to inhomogeneities in the magnetic field, accentuating blood products, iron, calcium, and manganese within tissues. This sequence is routinely performed during evaluation for stroke and trauma because hemorrhage is well seen. Fluid-attenuated inversion recovery (FLAIR) sequence can be used to obtain T2-weighted contrast while voiding the signal from cerebrospinal fluid, allowing a pathologic process to be identified with more confidence. Diffusion-weighted (DW) imaging assesses for the ability of water to diffuse in the local cell environment, which is particularly important in stroke and tumor imaging because areas of restricted diffusion demonstrate increased signal intensity. It is paramount to compare the DW images with the source apparent diffusion coefficient (ADC) images. Areas of restricted diffusion appear dark on the ADC maps, whereas areas of facilitated diffusion appear bright. Susceptibility-weighted images highlight differences in inherent tissue magnetic susceptibility and can be used for evaluation of deoxyhemoglobin in veins, hemorrhage, iron-containing tissues, and calcium deposition.



Table 1

Hospital of the University of Pennsylvania imaging parameters




















































































































Scan Repetition Time (ms) Echo Time (ms) Inversion Time (ms) Flip Angle (degrees)
1.5-T Brain MR Imaging
Sagittal T1 spin echo 450 10 NA 90
Axial T2 turbo spin echo 5490 73 NA 150
Axial gradient echo 800 26 NA 20
Axial FLAIR 8900 141 2500 180
Axial diffusion echo planar spin echo 4000 83 NA NA
Axial T1 precontrast spin echo 500 17 NA 90
Axial T1 postcontrast spin echo 500 17 NA 90
Coronal T1 postcontrast spin echo 500 17 NA 90
3.0-T Brain MR Imaging
Sagittal T1 FLAIR 2100 9 896.2 150
Axial FLAIR 9000 119 2500 150
Axial T1 precontrast spin echo 700 12 NA 90
Axial T2 gradient echo 700 19.9 NA 20
Axial T2 susceptibility weighted 27 20 NA 15
Axial T2 turbo spin echo 5440 96 NA 150
Axial diffusion echo planar spin echo 6400 109 NA NA
Axial T1 postcontrast spin echo 700 12 NA 90
Coronal T1 postcontrast spin echo 700 8.9 NA 90

Abbreviations: FLAIR, fluid-attenuated inversion recovery; NA, not available; TE, echo time; TI, inversion time; TR, repetition time.




Identifying the lobes of the brain


The frontal lobes are located anteriorly and extend posteriorly to the central (rolandic) sulcus, which partitions the frontal and parietal lobes. Several techniques can be used to identify the central sulcus, a universal point of reference. On the axial T2-weighted images near the vertex, the central sulci can be seen as a pair of mirror image transverse grooves ( Fig. 1 ), with the motor cortex always located anterior to this sulcus. The superior frontal sulcus is a horizontally oriented sulcus that terminates in the obliquely oriented precentral sulcus, and one can find the central sulcus as the sulcus posterior to the precentral sulcus. The precentral knob, the cortical location for hand function, is identified sitting just anterior to the central sulcus ( Fig. 2 ). The inferior central sulcus does not intersect the sylvian (lateral) fissure, rather it is contained by the junction of the precentral and postcentral gyri. On midline sagittal MR images, the central sulcus is somewhat more difficult to identify, but it is located anterior to the marginal ramus of the cingulate sulcus ( Fig. 3 ).




Fig. 1


Axial T2-weighted (repetition time, 3000 ms; echo time, 100 ms) image near the vertex demonstrates 2 mirror image horizontal sulci ( long black arrows ) demarcating the central sulcus. The precentral sulcus is thicker than the postcentral sulcus, a normal finding. Note the falx ( short black arrow ) and flow void within the superior sagittal sinus (SSS).



Fig. 2


Axial T2-weighted (repetition time, 3000 ms; echo time, 100 ms) image more inferior to Fig. 1 , demonstrating the omega-shaped precentral knob ( arrows ).



Fig. 3


Sagittal T1-weighted (repetition time, 442 ms; echo time, 9.3 ms) image in a paramidline location demonstrating the marginal sulcus ( white arrow ), cingulate sulcus (CS), callosal sulcus (cal sul), and cingulate gyrus (CING). The straight sinus flow void is also present (StS). Note also the tentorium cerebelli ( black arrow ), fornix (f), and optic tract (OT).


Inferiorly, the frontal lobe is separated from the temporal lobe by the sylvian fissure, which is easily seen on both axial and sagittal images ( Fig. 4 ). The middle cerebral arteries are located within the sylvian fissure and are seen as flow voids on T2-weighted images ( Fig. 5 ). The parietal lobes are bound anteriorly by the central sulcus. Superficially, there is no anatomic landmark separating the parietal and occipital lobes. However, toward the midline, the parietooccipital sulcus is well seen ( Fig. 6 ), demarcating their boundary.




Fig. 4


Sagittal T1-weighted (repetition time, 442 ms; echo time, 9.3 ms) far-lateral image demonstrating the sylvian fissure (SF) as well as the superior temporal (STG) and middle temporal gyri (MTG). The anterior (Ant) and posterior (Post) lobes of the cerebellum are separated by the primary fissure ( white arrow ).



Fig. 5


Axial T2-weighted image (repetition time, 3000 ms; echo time, 100 ms) at the level of the midbrain demonstrates flow voids of the middle cerebral arteries ( vertical white arrows ) within the sylvian fissures. The optic nerve (ON), optic chiasm (CHIASM), and optic tract (OT) are well seen at this level. The interpeduncular cistern (IPC) is bounded by the crus cerebri (CR). The basal veins of Rosenthal ( white arrow , BVR) travel around the brain stem in the ambient cistern. Note also the hippocampus (HIP), mammillary bodies ( white arrow , MB), uncus (UNCUS), and amygdala (AMY). More posteriorly, the cerebral aqueduct ( black circle ) and inferior colliculi (ic) can be seen.



Fig. 6


Sagittal T1-weighted (repetition time, 442 ms; echo time, 9.3 ms) midline image. The rostrum (R), genu (G), body (B), and splenium (S) of the corpus callosum are well seen with the third ventricle (3rd Vent) inferior to the body. The fornix (F) is seen arching posteriorly toward the splenium. Inferior to the fornix, the thalamus (Th) is seen just lateral to the third ventricle. The fornix originates in the subiculum and terminates in the mammillary bodies (mb). The pituitary gland (P) is located in the sella (s). Superior to the sella is the optic chiasm (c). The superior colliculus (sc), along with the inferior colliculus (ic), form the quadrigeminal plate in the tectum of the midbrain (MB), posterior to the cerebral aqueduct. A flow void within the basilar artery (basilar a.) is present anterior to the belly of the pons (pons). The triangular fourth ventricle (4) is seen anterior to the cerebellum (CB). The cerebellar tonsils (T) are present in the foramen magnum (FM). The subcallosal area (subC) of the cingulate gyrus (CG) continues around the corpus callosum where it becomes the isthmus of the corpus callosum inferior to the splenium and continues as the parahippocampal gyrus (not shown). Demarcating the parietal and occipital lobes, the parietooccipital sulcus ( black arrow ) sits posterior to the precuneus (P). The calcarine sulcus ( white arrow ) divides the medial occipital lobe into the cuneus and lingual gyrus. Notice the bright signal within the clivus (CL) and C2, indicating fatty marrow.


Superficial Surface Anatomy


Although there are variations of normal anatomy, the superficial surface of the brain follows a general pattern, identified best on sagittal images. Naidaich and colleagues provide a thorough description of the superficial frontal and temporal lobes. The frontal lobe contains 3 horizontal gyri and the obliquely oriented precentral gyrus. The superior frontal gyrus runs horizontally, parallel to the falx and interhemispheric fissure. The middle frontal gyrus is the largest of the horizontal gyri, running parallel to the superior frontal gyrus and undulating posteriorly, where it fuses with the precentral gyrus. The superior frontal sulcus divides the superior frontal gyrus and middle frontal gyrus, and the inferior frontal sulcus divides the middle frontal gyrus and inferior frontal gyrus. The inferior frontal gyrus is triangular and is separated from the frontal pole by the frontomarginal sulcus. The superior border of the inferior frontal gyrus is horizontal, whereas the inferior surface is triangular and divided into 3 parts: the pars orbitalis, pars triangularis, and pars opercularis.


The superficial temporal lobe contains 3 superficial gyri: the superior temporal gyrus, middle temporal gyrus, and inferior temporal gyrus. The superior temporal sulcus separates the superior and middle temporal gyri and courses parallel with the superior fontal gyrus until posteriorly, where it angles superiorly and is called the angular sulcus. The inferior temporal sulcus separates the middle temporal and inferior temporal gyri.


The postcentral gyrus demarcates the anterior border of the parietal lobe and is parallel to the precentral sulcus. The intraparietal sulcus divides the parietal lobe into superior and inferior lobes. The inferior lobe is divided into the supramarginal gyrus and the appropriately named angular gyrus, which envelopes the angular sulcus and represents the posterior aspect of the inferior parietal lobe. The inferior parietal lobe can show marked left to right asymmetry.


The occipital pole can be seen along the posterior superficial surface, but most occipital lobe structures are better seen on the midline sagittal views, discussed next.


Midline Structures


The easiest structure to identify on the midline sagittal image is the corpus callosum, consisting of the rostrum, genu, body, and splenium (see Fig. 6 ). The cingulate gyrus parallels the corpus callosum anteriorly until its marginal branch courses to the superior brain surface. The sulcus anterior to the marginal sulcus is named the central sulcus. Posteriorly, the isthmus of the cingulate gyrus is located between the splenium of the corpus callosum and the anterior calcarine sulcus and continues laterally as the parahippocampal gyrus where its superior border is demarcated by the hippocampal fissure. The hippocampal fissure continues above the body of the corpus callosum as the callosal sulcus (see Fig. 3 ). Anteriorly, the cingulate gyrus dives under the rostrum of the corpus callosum and becomes the subcallosal area. The posterior medial parietal lobe, or precuneus, is seen anterior to the parietooccipital sulcus. The calcarine sulcus divides the medial occipital lobe into the cuneus and lingual gyrus (see Fig. 6 ).


The lamina terminalis demarcates the anterior wall of the third ventricle and plays a role in cardiovascular and body fluid homeostasis.


The hippocampal formation, found in the medial temporal lobe, is composed of the subiculum, the dentate gyrus, the hippocampus, and their continuations around the corpus callosum and is best seen on coronal images. The parahippocampal gyrus forms the inferomedial border of the temporal lobe ( Fig. 7 ). The subiculum forms the medial and superior curvature of the parahippocampal gyrus and arcs into the hippocampal fissure. The hippocampal fissure is bordered superiorly by the dentate gyrus and inferiorly by the subiculum. The hippocampus forms a cap on the hippocampal fissure and bulges into the medial wall and floor of the temporal horn.




Fig. 7


Coronal T2-weighted image (repetition time, 6360 ms; echo time, 89 ms) demonstrating the inferior and medial border of the temporal lobe, the parahippocampal gyrus (PHG). The subiculum (S) and body of the hippocampus (H) are also demarcated. The hippocampal sulcus is seen on the left ( black arrow ), as are the collateral sulcus (cs) and the occipitotemporal sulcus (ots). Note the flow voids in the ambient cistern ( white arrow ) and sylvian fissure (SF). The superior, middle, and inferior temporal gyri (STG, MTG, ITG, respectively) are also seen. At this level, the bodies of the fornices (F) are present inferior to the corpus callosum (CC) and within the lateral ventricles (LAT). The cingulate gyrus (C) and interhemispheric fissure (IHF) are present in the midline.


The fornix, a white matter tract that connects the subiculum and the mammillary bodies, originates in the subiculum where its axons travel laterally to form a thin layer of white matter along the inferomedial temporal horn. The fornix continues posterior to the undersurface of the splenium, and most fibers arch upward, anterior, and inferior to the splenium to form the crura of the fornices, ending in the mammillary bodies (see Fig. 2 ).


The amygdala is a gray matter structure located just lateral to the uncus and anterior to the temporal horn and remains attached to the putamen superiorly. The tail of the caudate nucleus terminates in the amygdala. The anterior commissure is a white matter tract located in the anterior wall of the third ventricle at the junction of the lamina terminalis and the rostrum of the corpus callosum that connects the 2 temporal lobes and is easily seen on MR images.


Deep Structures


The deep central cerebral structures located between the insula and the sagittal midline are referred to as the central core ( Fig. 8 ). This core contains, among others structures, the extreme, external, and internal capsules, the claustrum, the putamen, the globus pallidus, the caudate nucleus, the amygdala, the diencephalon, and thalamic structures. It is related to motor and sensory functions, emotion, endocrine integration, and cognition. All the information passing between the brainstem and cortex passes through fibers in the central core. Anteriorly, the central core gray matter consists of the caudate nucleus and, to a lesser extent, the lentiform nucleus (putamen and globus pallidus), whereas the white matter consists primarily of the anterior limb of the internal capsule. More posteriorly, at the level of the foramen of Monro, the internal capsule contributes most of the white matter, whereas the lentiform nucleus contributes most of the gray matter, with a lesser contribution from the caudate nucleus. At the posterior insular level, most of the gray matter contribution arises from the thalamus and the white matter from the posterior limb of the internal capsule.


Sep 18, 2017 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Normal Brain Anatomy on Magnetic Resonance Imaging
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