Primary thumb sensory cortex located at the lateral shoulder of the inverted omega-shape on the axial images of the central sulcus.

Useful landmarks on magnetic resonance (MR) images were identified for preoperative prediction of the relationship of a tumor to the primary sensory cortex of the thumb. Functional MR (fMR) imaging and magnetoencephalography were used to retrospectively localize the hand-digit sensorimotor area in four patients who underwent tumor resection around the central sulcus with intraoperative neurophysiological mapping. fMR imaging revealed the hand-digit motor cortex in the so-called "precentral knob" inside the characteristic inverted-omega on axial MR images. Equivalent current dipoles of the N20 m response in somatosensory evoked fields (SEFs) of the thumb, median nerve, and ulnar nerve stimuli were localized at the lateral portion of the inverted omega-shape from the lateral to medial directions. The SEF-based thumb sensory cortex was verified by intraoperative functional mapping with a neuronavigation system. The hand-digit somatosensory cortices were localized at the lateral shoulder of the inverted-omega, in the lateral anterior inferior position to the hand-digit motor cortices in the precentral knob. Axial MR imaging can provide useful preoperative planning information for the surgical treatment of tumors within or adjacent to the motor-somatosensory cortex.