1. The precentral gyrus is the most posterior boundary of the frontal lobe. Its anterior boundary is the precentral sulcus and the posterior boundary is the central sulcus. The superior boundary is the superior end of the cortex. The inferior boundary at the medial end is the cingulate sulcus and the inferior boundary at the lateral end is the sylvian fissure. It is helpful to have a 3D object model to reference the boundaries of the precentral gyrus. (Fig. 1, Fig. 2, Fig. 3)
  2. Mask in the axial view starting superiorly at a slice where the central, precentral, and paracentral sulci are clear. Trace the precentral sulcus to the paracentral sulcus, which is the upper boundary of the paracentral lobule. Trace the central sulcus and angle down to the paracentral lobule’s lower boundary. If the lobule’s lower boundary is not clear, reference the previous slice to best approximate its location. Including the paracentral lobule, mask everything within these boundaries. Continue this step superiorly. (Fig. 4, Fig. 5)
  3. Moving more superiorly, the paracentral sulcus may disappear. When this occurs, refer to the previously masked slice and approximate the anterior end of the precentral gyrus. Continue this step until reaching the brain’s most superior point. (Fig. 6, Fig. 7)
  4. Return to the first slice masked inferiorly. Moving inferiorly, continue Step 2 until reaching the cingulate sulcus. The cingulate sulcus can be found easily on either the 3D object or in the sagittal view. (Fig. 8, Fig. 9, Fig. 10)
  5. Once the cingulate sulcus is reached, the paracentral lobule is no longer masked. Trace the precentral sulcus to its interior endpoint, and draw a straight line to the internal end of the central sulcus. Mask everything within these boundaries. If the precentral sulcus disappears, follow the next sulcus above it. Continue this step until reaching the sylvian fissure. Check the 3D object model to verify the sylvian fissure’s location. (Fig. 11, Fig. 12, Fig. 13, Fig. 14, Fig. 15)