The Insula: Anatomic Study and MR Imaging Display at 1.5 T

The transverse gyrus and the orbitofrontal insula relationship. Base view of the orbital surface of the left frontal lobe after resection of part of the gyrus rectus (GR) and the anterior portion of the temporal lobe (T). Medial is to the reader’s left. I indicates olfactory bulb; II, optic chiasm and tract. The H-shaped orbital sulcus (arrowheads) defines the MOG, POG, anterior orbital gyrus (AOG), and lateral orbital gyrus (LOG). At the posteromedial aspect of the orbitofrontal surface, the posterior portion of the MOG merges with the medial portion of the POG to form the prominent PMOL. PMOL gives rise to the transverse insular gyrus (t) that extends laterally to form the pole (po) of the insula just anteroinferomedial to the apex (asterisk) of the insula. The CS (dashed line) curves inferiorly immediately behind and below the apex and the pole en route to join the stem of the sylvian fissure. The ASG (a) and the PSG (p) converge to form the apex of the insula anterior to the CS. In this image, the deliberate slight rotation used to illustrate the course of the transverse gyrus from the PMOL to the pole also rotates the apex medially, so the apex does not appear to lie as far lateral in position as it would in a true base view.

A and B, Anatomic relationship of the opercula to the insula at earlier (A) and later (B) stages of dissection of the left cerebral hemisphere of a gross anatomic specimen from a 71-year-old man. The partes orbitalis (or), triangularis (tr), and opercularis (op) of the inferior frontal gyrus (I) overlie the anterior lobule of the insula. The anterior horizontal ramus (single open arrow) of the sylvian fissure leads to the anterior PS (APS). The anterior ascending ramus (dual open arrows) of the sylvian fissure leads to the superior PS (SPS) (8). The pars opercularis (op) joins with the inferior end of the precentral gyrus (4) to form most of the frontal operculum. In this specimen, they form a “bulky” union that invaginates into the convexity surface of the anterior lobule and depresses the MSG (m) slightly below the surface of the insula. Further posteriorly, the subcentral gyrus (11) and the anterior limb of the supramarginal gyrus (6) form the rest of the frontoparietal operculum that overlies the posterior lobule. The HG (H) overlies the PLG (P), so the anteromedial surface of HG abuts onto the lateral surface of the PLG. Deeper dissection (in B) shows the origin of the HG immediately posterior to the insula, posterosuperolateral to the PLG. In this specimen, the postcentral sulcus defines separate ALG (A) and PLG (P). PLG is dominant and forms the pole of the posterior lobule at the limen.

Relationship of the insula to the temporal operculum. Gross anatomic specimen from a 44-year-old woman, viewed from above and lateral after removal of the frontal and parietal opercula to expose the superior surfaces of the ASG (a), MSG (m), PSG (p), the CS, and the ALG (A). The HG (H) arises immediately posterior to the insula and curves anterolaterally around the lateral surface of the PLG (arrow), largely obscuring it in this view. The anteromedial surface of the HG abuts upon and conforms to the surface of the PLG as it crosses the superior surface of the temporal lobe. PP indicates planum polare; PT, planum temporale.

A–D, Imaging anatomy of the normal insula in a 51-year-old man. Sequential sagittal T2-weighted MR imaging sections from lateral (A) to medial (D) demonstrate the anterior (APS), superior (SPS), and inferior (IPS) PS, the ASG (a), MSG (m), and PSG (p) of the larger anterior lobule, and the ALG (A) and the branched configuration of the PLG (P) of the smaller posterior lobule. The CS (d) courses across the convexity between the precentral gyrus (4) and the postcentral gyrus (5) and then curves across the insula between the corresonding gyri of the insula (ie, between p and A). The overlying opercula interdigitate with the insular gyri with no significant distortion of insular anatomy. The HG (H) lies immediately behind the PLG and marks the posteroinferior border of the insula. M indicates middle frontal gyrus; I, inferior frontal gyrus; 6, supramarginal gyrus; 8, superior temporal gyrus; and 11, subcentral gyrus. Other abbreviations as in Figs 1–4.

A and B, Imaging anatomy of the posterior lobule on sagittal T2-weighted MR images in a 13-year-old girl (A) and a 26-year-old man (B). In A, the ALG (A) and PLG (P) are well formed and distinct, correlating with Fig 1C. In B, ALG (A) and PLG (P) are poorly separated. PLG appears to bud off the posterior aspect of the ALG, giving the posterior lobule a branched configuration, such as seen in Fig 1B. Note the convergence of the short insular gyri to form the apex (asterisk in B) as seen in Fig 1B, the course of the CS posteroinferior to the apex en route to the stem of the sylvian fissure, the relationship of the pole of the posterior lobule to the limen, and the relationship of the HG (H) to both the PLG and the entire posterior lobule.

A and B, Sagittal T2-weighted MR images of the left hemisphere in a 34-year-old woman show the opercular-insular relationship. The union of the pars opercularis (op) of the inferior frontal gyrus with the inferior end of the precentral gyrus (4) forms a very bulky “pseudomass” (Ø) that invaginates into the convexity surface of the anterior lobule, depresses the insular surface at the MSG, and splays the ASG (a) apart from the PSG (p). (See also Figs 1 and 3.)

Talairach-Tournoux relationship of the insula. Sagittal T2-weighted MR image in a 26-year-old woman. The two vertical white lines indicate the insular intersections of the coronal planes erected perpendicular to the AC-PC baseline at AC (VAC) and at PC (VPC). VAC nearly always intersects the anterior lobule, most frequently in relation to the upper portion of the precentral sulcus between the MSG (m) and the PSG (p). VPC typically defines the posterior margin of the insula.