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Section V. Frontal Lobe Overview
The frontal lobe is large with diverse functions and many classic anatomic signs on both axial and sagittal images. The major gyri include the precentral gyrus; superior, middle, and inferior frontal gyri; and supra-orbital gyri and gyrus rectus. Key functions include somatomotor, a large area of heteromodal association cortex dedicated to executive function, language (Broca's area), and areas of limbic function. The surface anatomy introduced here, in conjuction with that of the parietal lobe and Sylvian fissure will help localize the central sulcus.
3D surface images can provide an overview of the major borders, sulci, and gyri. Patterns of the sulci and many major signs that are useful on cross-sectional imaging are also seen.
Key Borders
Key Borders of the surface anatomy of the frontal lobe include the central sulcus and Sylvian fissure.
Key Structures
Key Structures of the surface anatomy of the frontal lobe, superior view. Notice the frontal lobe is comprised largely of the precentral gyrus and three strips of frontal gyri (superior, middle, and inferior frontal gyri).
Key Signs
Key Signs. The end-to-side termination of the superior frontal sulcus at the precentral sulcus defines the big T sign and outlines the L sign of the combined superior frontal and precentral gyri. Several other key signs that will be discussed are also evident.
Key Structures
Key Structures of the surface anatomy of the frontal lobe, lateral view. Again, the superior, middle, and inferior frontal gyri extend posteriorly to the precentral gyrus. Anteriorly, they converge to the frontal pole. Other named gyri are present at the anteriormost region of the frontal lobe and the basal frontal lobe.
Key Signs
key signs. The precentral, central, and postcentral sulci are the 3 prominent parallel sulci along the lateral convexity (triple sulcus sign). The complete ring of the pre and postcentral gyri are typically bridged by the subcentral gyrus (subcentral U sign). In distinction, the precentral sulcus can extend all the way to the Sylvian fissure. The inferior frontal sulcus usually has a T-bone termination with the precentral sulcus (little t sign). The anterior rami of the Sylvian fissure divide the inferior frontal gyrus (M sign).
Key Structures
Key Structures of frontal lobe surface anatomy, anterior view. The superior, middle, and inferior frontal gyri converge towards the frontal pole. Key components of the orbitofrontal region include the gyrus rectus and medial orbital gyrus, divided by the olfactory sulcus.
Cross sectional anatomy can be partially derived from the 3D surface images, but is best learned by looking at actual cross-sectional images instead of surface images or illustrations.
Axial Image, Near Vertex
Axial image, near vertex. The central sulcus is usually well-seen as it is continuous and extends medially, although differentiation from adjacent sulci can take practice. The pars marginalis typically has bilateral (or unilateral ) upturned edges (mustache) here.
Axial Image, Superior Level
Findings are similar to the vertex, but the central sulcus extends less medially, the big T sign isn't seen (no precentral sulcus), and the pars marginalis is flat (straight mustache).
Axial Image, Mid-Level
Approaching the operculum lining the Sylvian fissure. No prominent signs. The central sulcus has terminated. The anterior ascending ramus can be found by scrolling to confirm the communication with the main portion of the Sylvian fissure. The ascending ramus is just anterior to the precentral sulcus.
Coronal Image, mid-frontal lobe
The only standard plane to readily see both the superior and inferior frontal sulci. The cingulate gyrus has curved around the genu of the corpus callosum. The olfactory sulcus with a superolateral slant is well seen.
Sagittal Image, Lateral Convexity Level
The inferior frontal sulcus terminates at the precentral sulcus. The anterior rami of the Sylvian fissure divide the inferior frontal gyrus.
Key signs on Sagittal Image, Lateral Convexity Level
The key sulci and gyri of the lateral frontal convexity are reliably identifiable with use of several signs. The precentral, central, and postcentral sulci course obliquely across the lateral convexity in parallel fashion (triple sulcus sign). The precentral sulcus may reach the Sylvian fissure whereas the central sulcus usually terminates above it (subcentral U sign). The inferior frontal sulcus usually terminates at the precentral sulcus in T-bone fashion (little t sign). The anterior ascending ramus of the Sylvian fissure is always one sulcus anterior to the precentral sulcus (ascending sulcus sign) and, along with the anterior horizontal ramus, divide the inferior frontal gyrus into 3 smaller units (M sign). Be cautioned that the exact M-sign configuration is quite variable.
We start to see that patterns of sulci and signs define not only anatomic regions, but can help also us approximate expected locations of key clinically-relevant functional areas.
Approximations of Key Functional Areas of the Frontal Lobe Lateral Convexity
These include Broca's area in the inferior frontal gyrus (esp. pars orbitalis and pars opercularis) and the dorsolateral prefrontal cortex (posterior aspect of middle frontal gyrus). Assumes language dominant hemisphere.
Midline Sagittal Image
Note that the medial extent of the central sulcus is embedded in the Y-shaped paracentral lobule just anterior to the pars marginalis. In this location, the central sulcus has an posterior convex comma-shape. The pars marginalis is approximately parallel to the parieto-occipital fissure.
Axial Image, Basal Frontal Lobe
H-sign of the orbital sulcus, separating the lateral, anterior, medial (inferior to this level and not shown in this subject), and posterior orbital gyri.
Axial Image, Basal Frontal Lobe
H-signs of the orbital sulcus separating the orbital gyri. Note that a perfect H is not always present (may look like an 'X', 'K' etc).
Basal Frontal Lobe, Olfactory Sulcus Level
The olfactory sulcus separates the gyrus rectus from the medial orbital gyrus.
Coronal Image, Olfactory Sulcus Level
The olfactory sulcus has a superolateral slant and the height of the sulcus varies along the length. Measurement and assessment at a consistent level such as the posterior orbit may be useful.
Clinical correlates demonstrate the importance of recognition of imaging signs and functional regions.
Congenital Anosmia
Normal height of the olfactory sulcus vs decreased height with congenital anosmia. The vertical height varies along the AP-extent, shorter anteriorly. Note the typical superolateral slant.
Clinical Correlate: Olfactory Groove Meningioma
Olfactory sulcus region (groove) meningioma. Meningiomas in this location (although this one is central) may result in Foster-Kennedy Syndrome: anosmia (due to involvement of olfactory tract in the olfactory sulcus), ipsilateral optic nerve atrophy (direct compression), contralateral papilledema (increased ICP).
Clinical Correlation: Aphasia
Patient with metastasis and aphasia. The metastasis is centered in the pars opercularis of the inferior frontal gyrus of the language dominant hemisphere, splaying the anterior horizontal and anterior ascending rami of the Sylvian fissure. Vasogenic edema extends throughout the entire inferior frontal gyrus and the adjacent precentral gyrus.
Herpes Limbic Encephalitis
herpes limbic encephalitis involving key areas of limbic cortex: the temporal poles, medial temporal lobe, posterior orbital gyrus, medial orbital gyrus, and gyrus rectus.
Meningioma Overlying the Prefrontal Cortex
Meningioma overlying the the right prefrontal cortex with a large amount of vasogenic edema throughout the right frontal lobe white matter, in particular underlying the prefrontal cortex multimodal association area. Relatively large lesions/edema can develop within the prefrontal region with relatively mild clinical deficit compared to eloquent areas. The prefrontal cortex is a functional region and is not sharply demarcated by anatomic landmarks on clinical imaging.
Acute Infarct of the Motor Hand Cortex
Acute infarct hand knob region of the right precentral gyrus. Isolated infarcts in this area is associated with sudden onset of isolated contralateral hand weakness (an example of a small lesion causing a substantial typical clinical deficit).
Clinical Correlate: Motor Task Homunculus
Homunculus shown with multiple motor tasks in an fMRI exam in a normal volunteer. Note that lower extremity (blue) function is located medially, finger activities (red) are located off midline, lips (pink) and tongue (yellow) are located laterally).
Summary of Key Clinical Correlates of the Frontal Lobe
The primary somatomotor cortex lines the anterior bank of the central sulcus within the precentral gyrus, with function arranged in a homunculus.
Broca's area is located approximately within the mid to posterior inferior frontal gyrus (pars triangularis and pars opercularis).
The prefrontal cortex is a large functional area of multimodal association cortex involved with executive function.
The dorsolateral prefrontal cortex approximately corresponds to the middle frontal gyrus with function related to working memory and language.
Limbic cortex is present in the anterior basal frontal cortex.
The olfactory tract courses within the base of the olfactory sulcus.
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