The landmarks for delineating inferior frontal cortex include:

  • Precentral sulcus
  • Inferior frontal sulcus
  • Lateral orbital sulcus
  • Sylvian fissure
  • Circular sulcus of the insula

Exceptions include:

  1. Middle Frontal Gyrus Continuous With Inferior Frontal Cortex
  2. Double Inferior Frontal Gyrus
  3. Pars Opercularis Connected to Precentral Gyrus
  4. Extra Triangular Gyrus Between Precentral Sulcus and Inferior Frontal Gyrus

Inferior Frontal Cortex

  1. The inferior frontal cortex (IFC) is traced in the axial plane. The IFC is defined as cortex anterior to the precentral sulcus, inferior to the inferior frontal sulcus and superior and posterior to the lateral orbital sulcus (Fig. 1). The IFC includes three subregions, pars opercularis, pars triangularis, and pars orbitalis, which can be identified using the vertical and horizontal rami of the sylvian fissure (Fig. 1).
  2. Tracing begins on the most superior axial slice in which the inferior frontal sulcus can be delineated or where the pars orbitalis appears (Fig. 2). To find this slice, locate the highest point of pars opercularis on the cortical object model (Fig. 1).
  3. In the axial viewing plane, color the region that is both between the precentral sulcus and inferior frontal sulcus and also lateral to the point where these two sulci meet (
    Fig. 3    ). When the inferior frontal sulcus and precentral sulcus do not meet, draw a straight line between the most medial point of the inferior frontal sulcus to the most medial point of the precentral sulcus (Fig. 4). As one moves more inferiorly, the circular sulcus of the insula replaces this line as the most medial boundary (Fig. 5). Normally the lateral orbital sulcus is continuous with the inferior frontal sulcus and serves as the anterior boundary in more inferior slices (Fig. 1 and Fig. 6). Tracing ends in the most inferior axial section in which the inferior frontal gyrus can be distinguished (Fig. 7).

Exceptions:

Middle Frontal Gyrus Continuous With Inferior Frontal Cortex
Occasionally, the middle frontal gyrus is continuous with the inferior frontal cortex. When the middle frontal gyrus is connected to the inferior frontal cortex at the pars opercularis or pars triangularis, the inferior frontal sulcus is not clearly defined.

When the middle frontal gyrus is connected to the pars opercularis, a geometric line is drawn connecting the two points of the inferior frontal sulcus on either side of the junction of the middle frontal gyrus with the inferior frontal gyrus (Fig. 8). Since the inferior frontal sulcus is not clearly defined in the axial slices of this region, the cortical object model must be used to distinguish this geometric line (Fig. 8 and Fig. 9). A similar method is used when the middle frontal gyrus is connected to the pars triangularis (Fig. 10).

When the middle frontal gyrus connects to the pars orbitalis, the inferior frontal sulcus does not meet the lateral orbital sulcus. In this case, a geometric line is drawn from the most superior tip of the lateral orbital sulcus to the nearest point of the inferior frontal sulcus (Fig. 11). This line serves as the most anterior boundary of the inferior frontal cortex.

Double Inferior Frontal Gyrus
An extra gyrus may appear superior to pars triangularis (Fig. 12). Since this gyrus is inferior to the inferior frontal sulcus, it is included as inferior frontal cortex.

Pars Opercularis Connected to Precentral Gyrus
When the precentral sulcus does not meet the sylvian fissure, the pars opercularis is connected to the precentral gyrus. In this case, a straight line is drawn from the most inferior point of the precentral sulcus to the nearest point of the sylvian fissure (Fig. 12).

Extra Triangular Gyrus Between Precentral Sulcus and Inferior Frontal Gyrus
An extra gyrus, usually having a triangular shape, may appear between the precentral sulcus and the inferior frontal gyrus. Since this gyrus is anterior to the precentral sulcus, it is included as inferior frontal cortex. It is labeled differently to distinguish it from the rest of the inferior frontal cortex (Fig. 13).