A Patterned Approach to Cerebral Gyri and Sulci for the General Radiologist

Note: Figures 3A, 4, 5, 6, 7, 8, 16 and 18 are reprinted with permission from Alastair J.E. Moore, MD, medical illustrator.

Evaluation of CT and MRI studies of the brain in routine clinical practice >not only requires an accurate identification and description of abnormal findings, but precise localization is essential as well. Localization is not only critical for the neurosurgeon/neurologist but having this knowledge and being cognizant of the functional deficits related to a specific anatomic location allows the interpreting radiologist to focus attention on specific regions of the brain. This pictorial review will provide the basic tools a radiologist needs to generate an accurate and precise description of neurologic-based diseases by providing a pattern recognition approach and will focus on gyral and sulcal anatomy. The MRI and CT images in this pattern approach will be supplemented with high-resolution, computer-generated 3-D images.

Variability in gyral/sulcal anatomy is the rule, as no two individuals have the exact same pattern. There are also certain normal asymmetries of the cerebral hemispheres that one will often encounter. Despite this, there is a general schema (pattern) from which the interpreter can call upon to adapt to unexpected individual variances that may not look exactly like the textbook picture. The typical patterns of surface anatomy will be presented along with functional attributes when pertinent.

Basic Concepts of Cerebral Anatomy

The traditional concept has been that the brain consisted of 4 lobes: the frontal, temporal, parietal, and occipital lobes. Current concepts and nomenclature have expanded the number of lobes to 6, which now includes the insular and limbic lobes.¹ The limbic lobe/system refers to a functional unit of the brain involved with emotions and instinctual behaviors.

General Considerations

The frontal, parietal, temporal, occipital and insular lobes surround the sylvian fissure (lateral sulcus) along the lateral convexity in an anterior to posterior orientation. The sylvian fissure has various branches including the anterior horizontal and anterior ascending ramus, the anterior and posterior subcentral sulci, and the posterior ascending ramus. A small posterior descending ramus is often present (Figure 1). A common normal hemispheric asymmetry involving the sylvian fissure exists. In a significant portion of individuals, the posterior ascending ramus of the sylvian fissure on the right has a steeper, more vertical orientation than on the left (Figures 2A, 2B). The midline interhemispheric fissure separates one cerebral hemisphere from the other. The basal portion of the anterior frontal lobes sits atop the orbital plates of the frontal bones in the anterior cranial fossa. The temporal lobes sit upon the floor of the middle cranial fossa and the occipital lobes lie atop the tentorium cerebelli above the posterior cranial fossa.

Typical Patterns of Cerebral Gyri and Sulci

Typical Pattern – Frontal Lobe

The largest lobe is the frontal lobe extending from the frontal pole to the central sulcus and inferiorly bordered by the cingulate gyrus (part of limbic lobe) along its medial surface (Figure 3A, 3B). Over the lateral convexity surface are 3 main gyri oriented longitudinally: superior, middle, and inferior frontal gyri (Figures 1, 4-10). The superior frontal gyrus is separated from the middle frontal gyrus by the superior frontal sulcus, which extends posteriorly to dead end or end in a “T” at the superior precentral sulcus. The middle frontal gyrus is the largest frontal gyrus and is separated from the inferior frontal gyrus by the inferior frontal sulcus. Sometimes the middle frontal gyrus is subdivided into superior and inferior sections by an inconstant middle frontal sulcus. An interruption of the pre-central sulcus at the level of the middle frontal gyrus allows the middle frontal gyrus to merge with the anterior margin of the pre-central gyrus. The smallest frontal gyrus and most unusual in shape is the inferior frontal gyrus. It has a triangular or “M” shape and is divided into an anterior pars orbitalis, a middle pars triangularis and a posterior pars opercularis (Figure 11). Pars triangularis and pars opercularis in the language dominant hemisphere (usually the left) correspond to the classical Broca’s region² responsible for the motor production of language. Injury to the same regions in the language nondominant hemisphere results in disorders of production of prosody (the intonation/inflections/rhythm of speech) providing meaning to spoken language.³ The anterior horizontal ramus of the sylvian fissure separates pars orbitalis from pars triangularis while the anterior ascending ramus of the sylvian fissure separates pars triangularis from pars opercularis. The inferior frontal gyrus is separated posteriorly from the pre-central gyrus by the inferior pre-central sulcus. Along the lateral convexity surface, the central sulcus (posterior margin of the frontal lobe) extends obliquely from posterior-superior to anterior-inferior, never reaching the sylvian fissure along its inferior margin. Instead, its inferior aspect is surrounded by the “U”-shaped subcentral gyrus (primary gustatory cortex). A characteristic morphology of the pre- and postcentral gyri on parasagittal images allows identification of the primary motor and somatosensory gyri and the central sulcus. This morphologic appearance can be reproduced by interlocking one’s hands with the fingers and thumb curled in a “C”-shape with the curled left fingers below the curled right fingers and the left thumb under the right thumb (Figures 12A, 12B). The posteriorly projecting prominence of the pre-central gyrus represents the hand motor knob (motor function for the contralateral hand). The pre-central gyrus (primary motor strip) is thicker/larger than the postcentral gyrus (primary sensory strip). In the axial imaging plane, the hand motor knob region of the pre-central gyrus can be identified by its characteristic upside-down omega shape. Along the medial/interhemispheric aspect of the frontal lobe, the superior/medial frontal gyrus lies above the cingulate sulcus. The posterior aspect of the cingulate sulcus swings upward in a vertical direction and at the vertex splits the medial end of the postcentral gyrus into a bifid configuration. This upward, vertical segment is called pars marginalis or the posterior ascending ramus of the cingulate sulcus. The central sulcus along the lateral convexity reaches the superomedial surface of the hemisphere and curves posteriorly, a few millimeters anterior to and pointing toward pars marginalis (Figures 3A, 3B, 13). A variably inconstant vertically oriented paracentral sulcus, which can arise from and project superiorly from the posterior cingulate sulcus or extend inferiorly from the vertex or both, marks the anterior margin of the paracentral lobule, the posterior margin being pars marginalis and the inferior border being the posterior cingulate sulcus. The superior end of the paracentral sulcus is at the level of, or just anterior to, the superior end of the precentral sulcus. The cingulate gyrus represents the inferior extent of the paracentral lobule (Figure 3A, 3B, 13). Along the posterior medial aspect of the superior frontal gyrus, just anterior to the pre-central gyrus in the paracentral lobule near the level of the pre-central sulcus lies the supplementary motor area (SMA) (Figures 8, 14A, 14B). This region is functionally involved in the planning and execution of complex movements of the extremities including the correction of posture during motor activation.⁴ SMA activation results in bilateral (but contralateral greater than ipsilateral) effects. Injury to the SMA results in the SMA syndrome characterized by contralateral weakness and speech arrest with near complete recovery in weeks to months. This has important clinical implications for patients undergoing surgery in this region. The basal surface of the frontal lobes sitting above the floor of the anterior cranial fossa consists of a series of orbital gyri (orbitofrontal gyri). The gyrus rectus (straight gyrus) is the most constant of these gyri and is located just above the cribriform plates on each side of the midline. The olfactory sulci lie along the lateral aspect of the gyri recti and lie superior to the olfactory bulbs and tracts. Lateral to the olfactory sulci are the orbital gyri, which are arranged around an “H”-shaped pattern of sulci (“H”-shaped sulcus of Rolando) (Figure 15, 16).^5-7 These include the anterior, posterior, medial and lateral orbital gyri. The posteromedial orbital lobule is where the posterior and medial orbital gyri converge.

Typical Pattern – Parietal Lobe

The parietal lobe consists of the postcentral gyrus and the superior and inferior parietal lobules (Figures 1, 4-6, 8, 11). The superior parietal lobule is located superomedial to the inferior parietal lobule. It extends posteriorly from the postcentral gyrus to the parieto- occipital sulcus along the medial, interhemispheric aspect and is separated from the inferior parietal lobule by the intraparietal sulcus more laterally. The mesial surface of the superior parietal lobule is the precuneus (between the parieto-occipital sulcus and pars marginalis). The precuneus is separated from the cingulate gyrus of the limbic lobe by the subparietal sulcus (Figures 3A, 3B, 13). The intraparietal sulcus is obliquely oriented in the anterior to posterior direction (Figure 1, 4, 6, 14A). The inferior parietal lobule consists of the supramarginal and angular gyri. The supramarginal gyrus surrounds the posterior ascending ramus of the sylvian fissure. Posterior to the supramarginal gyrus lies the angular gyrus (Figures 1, 4, 11). There is a clear demarcation of the parietal lobe from the occipital lobe along the interhemispheric region by the parieto-occipital sulcus, which runs obliquely from anterior-inferior to posterior-superior (Figures 3A, 3B, 13). However, along the lateral convexity no clear demarcation exists between the parietal and occipital lobes and a commonly used imaginary line to delineate this boundary is the lateral parietotemporal line (created by connecting a line from the preoccipital notch along the lateral undersurface of the cerebral hemisphere) to the point at which the parieto-occipital sulcus reaches the superomedial convexity surface (Figure 17). This lateral parietotemporal line also provides a demarcation between the posterior temporal lobe and the occipital lobe. There is also no clear demarcation between the parietal lobe and the temporal lobe over the lateral convexity. Another imaginary line used to delineate this demarcation is the temporo-occipital line, which is drawn from the posterior sylvian fissure to the midpoint along the lateral parietotemporal line (Figure 17).⁸

Typical Pattern – Occipital Lobe

The occipital lobe has the most inconstant arrangement of sulci and gyri, and inconsistent descriptions in the literature of its surface pattern creating confusion in delineating its morphologic anatomy. The occipital lobe can be divided into superior, middle and inferior occipital gyri converging posteriorly at the occipital pole (Figures 4, 18). When the intraparietal sulcus extends inferiorly along its posterior aspect into the occipital lobe, it is referred to as the intra-occipital sulcus.⁵ The superior occipital gyrus (oriented vertically) is located closest to the interhemispheric fissure lying medial to the intra-occipital sulcus and posterior-inferior to the superior parietal lobule with the middle occipital gyrus lateral to the intra-occipital sulcus. The middle and inferior occipital gyri are arranged longitudinally along the posterolateral surface of the hemisphere, paralleling the inferior surface of the hemisphere and separated by a less well-defined inferior occipital sulcus, which is often seen as the posterior extension of the inferior temporal sulcus. The inferior occipital gyrus can be seen as a posterior continuation of the inferior temporal gyrus. The middle occipital gyrus is often the largest occipital gyrus and may be subdivided into superior and inferior portions by an inconstant lateral or middle occipital sulcus.⁹ It lies posterior to the junction between the temporal and parietal lobes. The transverse occipital sulcus separates the parietal lobe from the middle occipital gyrus.¹⁰ Along the basal surface of the cerebral hemisphere, an imaginary line (basal parietotemporal line) drawn from the inferior parieto-occipital sulcus to the preoccipital notch demarcates the temporal from the occipital lobes (Figure 13).

Typical Pattern – Temporal Lobe

The temporal lobe lies inferior to the sylvian fissure. It extends posteriorly from the temporal pole in the anterior middle cranial fossa. There is no clear anatomic structure delineating the posterior borders of the temporal lobe from the parietal and occipital lobes. As discussed above, imaginary lateral parietotemporal, temporo-occipital and the basal parietotemporal lines (Figures 13, 17) provide direction in locating these posterior boundaries. The lateral convexity of the temporal lobe consists of the superior, middle and inferior temporal gyri and are separated by the superior and inferior temporal sulci (Figures 1, 4, 19A, 19B). The superior temporal sulcus has also been named the parallel sulcus since it runs parallel to the sylvian fissure. The posterior aspect of the superior temporal sulcus can be single or bifid and is called the angular sulcus since it leads to the angular gyrus. The flat superior surface of the temporal lobe is called the superior temporal plane.⁹ The transverse temporal gyrus (Heschl’s gyrus – primary auditory cortex) extends along the superior temporal plane from posteromedial to anterolateral (Figures 19A, 19B, and 20A, 20B). The posterior peri-sylvian area corresponding to Wernicke’s area (receptive language) is not a highly localized region¹¹ and includes the posterior aspect of the superior temporal gyrus and the adjacent supramarginal and angular gyri (inferior parietal lobule).⁸ The undersurface (basal surface) of the temporal lobe also consists of three gyri. From medial to lateral, these are the parahippocampal gyrus, the lateral occipitotemporal gyrus (fusiform gyrus) and the inferior temporal gyrus (Figure 13, 19A, 19B). The fusiform gyrus is implicated in the function of facial recognition with inability to recognize familiar faces (prosopagnosia) occurring with bilateral or unilateral lesions to the fusiform gyrus.¹² The collateral sulcus separates the parahippocampal gyrus from the lateral occipitotemporal gyrus (fusiform gyrus) and the lateral occipitotemporal sulcus separates the lateral occipitotemporal gyrus (fusiform gyrus) from the inferior temporal gyrus. The inferior temporal gyrus shares the lateral undersurface and inferolateral margins of the temporal lobe. The posterior aspect of the parahippocampal gyrus is continuous with the isthmus of the cingulate gyrus (curving around the splenium of the corpus callosum) (Figure 13). Along the basal surface of the temporal lobe, the posterior aspect of the lateral occipitotemporal gyrus (fusiform gyrus) diverges laterally where the medial occipitotemporal gyrus (anterior aspect of the lingual gyrus of the occipital lobe) intercalates itself for a short distance between the more medial parahippocampal/isthmus of cingulate gyrus and the divergent lateral occipitotemporal gyrus. The collateral sulcus maintains its position medial to the lateral occipitotemporal gyrus posteriorly while the anterior calcarine sulcus separates the medial occipitotemporal gyrus (lingual gyrus) from the parahippocampal/isthmus of the cingulate gyrus (Figure 13, 16).

Typical Pattern – Insular Lobe

The insular lobe (Figures 12B, 20A, 20B) is identified on gross inspection of the brain by separating the operculum (lips) of the frontal, parietal and temporal lobes. When this is performed, one is looking at the insular cortex consisting of a larger anterior lobule and a smaller posterior lobule separated by the central sulcus of the insula. Most commonly, the anterior lobule consists of 3 short gyri with well-formed anterior and posterior short gyri and a less well-formed middle short gyrus. The precentral sulcus of the insula is located between the middle and posterior short gyri. The smaller posterior lobule of the insula most often consists of an anterior long and posterior long gyrus separated by the postcentral sulcus of the insula. The posterior long gyrus is oriented parallel to the superior temporal plane and lies subjacent to Heschel’s gyrus (transverse temporal gyrus). The circular sulcus (anterior, superior and inferior peri-insular sulcus) surrounds the insular lobe.

Typical Pattern – Limbic Lobe/Limbic System

The limbic lobe/system lies along the medial surface of the hemisphere and includes the subcallosal area, the olfactory cortex, the “C”-shaped cingulate gyrus (that follows the contour of the corpus callosum) and parahippocampal gyri (with associated hippocampus and amygdala), connected by the isthmus of the cingulate gyrus that wraps around the splenium of the corpus callosum (Figures 3A, 3b, 13). Its boundaries include the cingulate sulcus, the subparietal sulcus and the collateral sulcus. There is an overlap in that the parahippocampal gyrus is included in descriptions as part of both the temporal and limbic lobes.

Conclusion

Feeling comfortable with gyral and sulcal anatomy (surface anatomy) of the brain is challenging but critically important when interpreting cross-sectional imaging studies. Acquisition of this knowledge is best accomplished through a routine and repetitive practice of reviewing this information from case to case, which over time will become assimilated and reflexive. Knowledge of the typical patterns will allow you to problem solve when encountering atypical patterns of surface anatomy. You will find a sense of confidence in the ability to localize lesions, which will be appreciated by your neurology and neurosurgical colleagues who rely on your interpretive skills.

References

1. Borden NM, Forseen SE. Supertentorial Compartment: Cerebral Hemispheres. In: Imaging Anatomy of the Human Brain: A Comprehensive Atlas Including Adjacent Structures. demosMedical; 2015:464.
2. Afifi AK, Bergman RA. Functional Neuroanatomy: Text and Atlas. 2nd ed. McGraw HIll Professional; 2005.
3. Patel S, Oishi K, Wright A, et al. Right hemisphere regions critical for expression of emotion through prosody. Front Neurol 2018;9(APR). doi:10.3389/fneur.2018.00224
4. Naidich TP, Hof PR, Yousry TA, Yousry I. The motor cortex: anatomic substrates of function. Neuroimaging Clin N Am 2001;11(2):171-193, vii-viii. Accessed June 21, 2020. http://www.ncbi.nlm.nih.gov/pubmed/11489733
5. Ribas GC. The cerebral sulci and gyri. Neurosurg Focus 2010;28(2). doi:10.3171/2009.11.FOCUS09245
6. Tamaraz J, Comair Y. Atlas of Regional Anatomy of the Brain Using MRI. Springer-Verlag Berlin Heidelberg; 2000. doi:10.1007/3-540-30672-2
7. Naidich T, Castillo M, Cha S, Smirniotopoulos J. Inferior Surface: Frontal Lobes. In: Imaging of the Brain. Elsevier; 2012:139.
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Figure Legend Key

Frontal Lobe Gyri

F1 = Superior Frontal Gyrus

F2 = Middle Frontal Gyrus

F3 = Inferior Frontal Gyrus

PO = Pars Orbitalis

PT = Pars Triangularis

POp = Pars Opercularis

H1 = Anterior Orbital Gyrus

H2 = Posterior Orbital Gyrus

H3 = Medial Orbital Gyrus

H4 = Lateral Orbital Gyrus

Pmol = Posteromedial Orbital Lobule

gr = Gyrus Rectus

M1 = Pre-Central Gyrus (Primary Motor Strip)

Frontal Lobe Sulci

SFS = Superior Frontal Sulcus

MFS = Middle Frontal Sulcus

IFS = Inferior Frontal Sulcus

Olfs = Olfactory Sulcus

HR = “H”-Shaped Sulcus of Rolando

PrU = Upper Pre-Central Sulcus

PrL = Lower Pre-Central Sulcus

PrCs = Pre-Central Sulcus

Temporal Lobe Gyri

T1 = Superior Temporal Gyrus

T2 = Middle Temporal Gyrus

T3 = Inferior Temporal Gyrus

T4 = Lateral Occipital Temporal Gyrus (Fusiform Gyrus)

T5 = Parahippocampal Gyrus

unc = Uncus of Temporal Lobe

Temporal Lobe Sulci

STS = Superior Temporal Sulcus (Parallel Sulcus)

AngS = Angular Sulcus

ITS = Inferior Temporal Sulcus

lots = Lateral Occipital Temporal Sulcus

cols = Collateral Sulcus

hg = Heschl’s (Transverse Temporal) Gyrus

Parietal Lobe Gyri

Se1 = Postcentral Gyrus (Primary Somatosensory Strip)

P1 = Superior Parietal Lobule

P2 = Supramarginal Gyrus

P3 = Angular Gyrus

ipl = Inferior Parietal Lobule

Parietal Lobe Sulci

pm = Pars Marginalis (Ascending Ramus of Cingulate Sulcus)

Pocs = Postcentral Sulcus

PoU = Upper Postcentral Sulcus

PoL = Lower Postcentral Sulcus

IPS = Intra-Parietal Sulcus

sups = Subparietal Sulcus

Occipital Lobe Gyri

O1 = Superior Occipital Gyrus

O2 = Middle Occipital Gyrus

O3 = Inferior Occipital Gyrus

Cu = Cuneus

ling = Lingual Gyrus (Medial Occipital-Temporal Gyrus)

Occipital Lobe Sulci

IOS = Intra-Occipital Sulcus (Superior Occipital Sulcus)

LOS = Lateral (Middle) Occipital Sulcus

TOS = Transverse Occipital Sulcus

InOS = Inferior Occipital Sulcus

pos = Parieto-Occipital Sulcus

cal = Calcarine Sulcus

cala = Anterior Calcarine Sulcus

Insular Lobe Gyri

ali = Anterior Lobule of Insula

as = Anterior Short Gyrus

ms =Middle Short Gyrus

ps = Posterior Short Gyrus

pli = Posterior Lobule of Insula

al = Anterior Long Gyrus

pl = Posterior Long Gyrus

prgi = Pre-Central Gyrus of Insula

pogi = Postcentral Gyrus of Insula

Insular Lobe Sulci

csi = Central Sulcus of Insula

prsi = Pre-Central Sulcus of Insula

posi = Postcentral Sulcus of Insula

apis = Anterior Peri-insular Sulcus (Circular Sulcus) of insula

spis = Superior Peri-Insular Sulcus (Circular Sulcus) of Insula

ipis = Inferior Peri-Insular Sulcus (Circular Sulcus) of Insula

Limbic Lobe Gyri

cg = Cingulate Gyrus

iscg = Isthmus of Cingulate Gyrus

T5 = Parahippocampal Gyrus

sca = Subcallosal Area

Sylvian Fissure (Lateral Fissure)

S1 = Sylvian fissure

S2 = Posterior ascending ramus of sylvian fissure

S3 = Anterior subcentral sulcus

S4 = Posterior subcentral sulcus

S5a = Anterior ascending ramus

S5b = Anterior horizontal ramus

Miscellaneous

cc = Corpus callosum

cgs = Cingulate sulcus

CS = Central sulcus (Rolandic Fissure)

FP = Frontal pole

IHF = Interhemispheric fissure

OP = Occipital pole

pcl = Paracentral lobule

pcls = Paracentral sulcus

PreCu = Precuneus

Scg = Subcentral Gyrus

sma = Supplementary motor area

TP = Temporal pole

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