5.1 Introduction

The human brain is structured on the basis of regional functional specialization and integration of these regions into task-defined networks. Regions of cortex and subcortical nuclei behave as functional modules, each with a distinct cytoarchitecture. All perceptual, executive, and motor functions recruit a subset of these functional modules into a network determined by the specific needs of the task. Thus, operations of these cortical functional “modules” are best viewed as serial or parallel subprocesses required for task execution. The white matter tracts serve as pathways of information flow across these modules.

Indirect, lesion-based data have been instrumental in defining the overall functional neuroanatomic organization of the brain. One of the highlights of brain organization is the remarkable consistency of regional functional specialization across human subjects and, to a slightly lesser extent, all primates. For example, the primary hand motor cortex can be reliably localized by identifying the “knob” or inverted “omega” in the precentral gyrus (pre-CG), and phonological information processing in posterior half of the superior temporal sulcus (STS). This chapter focuses on anatomic neuroimaging-based localization of sensorimotor and language regions, which are two of the most critical functions neurosurgeons aim to preserve. Surgical interventions in or adjacent to these “eloquent” brain regions carry higher risk of postoperative neurological deficits that majorly impact quality of life. Although there have been major advances in our ability to map brain function including widespread application of blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI), sound understanding of functional neuroanatomy remains critical in surgical decision-making. Interpretation and meaningful reporting of clinical fMRI cannot be performed without the knowledge of appropriate anatomic landmarks.

5.2 Primary Sensorimotor (Pericentral, Perirolandic, S1M1) and Supplementary Motor Cortex

Resections in this region involve risk of permanent motor and sensory deficits that seriously limit everyday function. Primary sensorimotor cortex has a consistent morphology that is readily and reliably identified by imaging landmarks (Fig. 5‑1, Fig. 5‑2, Fig. 5‑3, Fig. 5‑4, Fig. 5‑5). Fortunately, visualization of these landmarks is subject to little variation despite changes in scan orientation.

Several anatomic imaging landmarks and morphologic appearances can be used to reliably identify the primary sensorimotor cortex. In their practice, the authors begin by identifying in the axial images the superior frontal sulcus (SFS) located immediately paramedian to the anterior interhemispheric fissure. The SFS ends posteriorly by joining the precentral sulcus (pre-CS), which runs immediately anterior and parallel to the central sulcus (CS). The latter separates the anterior pre-CG from posterior postcentral gyrus (post-CG). CS is then confirmed by tracing it medially toward the interhemispheric fissure where it curves slightly posteriorly and ends in 94 to 96% of cases by pointing at the horizontal bracket formed by pars marginalis (pars “bracket” sign; Fig. 5‑1a, Fig. 5‑3). ^{1 ,​ 2 ,​ 3} Pars marginalis or the marginal ramus is a superior directed branch of the cingulate sulcus on the interhemispheric surface that terminates by curving over the apex of the cerebral convexity immediately posterior to the CS (Fig. 5‑1a, c). A highly reliable and readily applicable landmark along the CS is the precentral “knob,” an inverted omega (ʊ) or less commonly a horizontal epsilon (ω) shape protuberance from the posterior face of pre-CG. This is located on the pre-CG, immediately lateral to the parasagittal plane passing through the SFS and is a site for hand motor area (Fig. 5‑1a, Fig. 5‑2). ¹ On the parasagittal image passing through the plane of insula, the precentral knob corresponds to the “precental hook” that snugly fits into a concavity of hand sensory region of the post-CG (Fig. 5‑1b, Fig. 5‑5b). ¹ Finally, the anteroposterior thickness of the pre-CG is always greater than that of the post-CG. ³ Together, the above features are highly reliable in confirming the location of the primary sensorimotor cortex.

Even when the regional anatomy is effaced and distorted by tumor mass effect and edema, the above methodology is invariably successful in identifying the CS (Fig. 5‑3, Fig. 5‑4, Fig. 5‑5). In the presence of subcortical vasogenic edema in the perirolandic region, a twofold difference in cortical thickness between the anterior and posterior banks of the CS uniquely identifies the CS on T2-weighted images despite the marked distortion of sulcal anatomy (Fig. 5‑4). ^{4 ,​ 5} The face sensorimotor cortex can be fairly reliably localized about 2 cm inferolateral to the hand knob along the CS. Similarly, the primary sensorimotor localization of foot corresponds to the posterior paracentral lobule (PCL), located on the medial surface of the cerebrum, immediately anterior to the pars marginalis (Fig. 5‑2). Thus, the somatotopy of the primary sensorimotor cortex is laid out from lateral to medial. Compared to the precentral knob, which is highly specific for the hand motor region, anatomic approaches have limited accuracy for localization of the remaining primary sensorimotor homunculus. Despite excellent landmarks for identification of the S1M1, the anatomic approach for its localization can be rarely limited by occasional anatomic variants, extreme pathological distortions of anatomy, and functional reorganization induced by lesions.

The supplementary motor area (SMA) is located on the medial surface of the cerebral hemisphere in the anterior PCL and posterior end of the medial (superior) frontal gyrus (Fig. 5‑1c). On the axial plane, as the name implies, the PCL can be easily identified as a lobule at the medial end of the CS, located between the pars marginalis posteriorly and superior frontal gyrus (SFG) anteriorly (Fig. 5‑1a, c, Fig. 5‑2). The somatotopy in SMA is from anterior to posterior: the SMA for lower extremity lies immediately anterior to the corresponding M1; the two are often inseparable on BOLD fMRI. Immediately anterior to the SMA lies the pre-SMA with a similarly oriented, albeit, coarser somatotopy. Pre-SMA appears to be involved in procedural aspects of cognitive processing, including higher order speech processing. ⁶