Medial Surface Central Sulcus on Axial Imaging Cs Cs Pm Pocs

Medial Surface Central Sulcus on Axial Imaging Cs Cs Pm Pocs

eye movements to the opposite direction Medial surface (Figure 5-2) CT The cingulate sul- prcs CENTRAL SULCUS cus terminates posteri- PL orly in the pars SFG pM marginalis (pM) (plu- cins sps ral: partes marginales). cins CinG PCu On axial imaging, the MRI pMs: are visible on 95% s callosu rpu m pos of CTs and 91% of co MRIs4, are usually the most prominent of the paired grooves strad- Cu dling the midline, and LG they extend a greater distance into the pons hemispheres4. On axial CT, the pM is located slightly posterior to the widest biparietal diameter4; on the typi- cally more horizontally Figure 5-2 Medial aspect of the right hemisphere “CT” & “MRI” bars depict typical axial slice orientation for CT & MRI scans. oriented MRI slices the See Table 5-1 and Table 5-2 for abbreviations pM assumes a more posterior position. The pMs curve posteriorly in lower slices and anteriorly in higher slices (here, the paired pMs form the “pars bracket” - a characteristic “handlebar” configuration straddling the mid- line). Central sulcus on axial imaging See Figure 5-3. Identification is important to localize the motor strip (contained in the PreCG). The central sulcus (CS) is visible on 93% of CTs and 100% of MRIs4. It curves posterior- ly as it approaches the interhemispher- ic fissure (IHF), and often terminates in the paracentral lobule, just anterior SFG cs to the pars marginalis (pM) within the cs pars bracket (see above)4 (i.e. the CS of- PreCG ten does not reach the midline). PL PostCG Pointers: pocs • parieto-occipital sulcus (pos) (or fissure): more prominent over the medial surface, and on axial imag- ing is longer, more complex, and more posterior than the pars pM marginalis5 • post-central sulcus (pocs): usually bifurcates and forms an arc or pa- renthesis (“lazy-Y”) cupping the pM. The anterior limb does not en- ter the pM-bracket and the posteri- Figure 5-3 CT scan (upper cut) showing gyri/sulci. or limb curves behind the pM to See Table 5-1 and Table 5-2 for abbreviations enter the IHF NEUROSURGERY 5.1. Surface anatomy 85 Table 5-1 Cerebral sulci Table 5-2 Cerebral gyri and lobules (abbreviations) (abbreviations) cins cingulate sulcus AG angular gyrus cs central sulcus CinG cingulate gyrus ips-ios intraparietal-intraoccip- Cu cuneus ital sulcus LG lingual gyrus los lateral occipital sulcus MFG, SFG middle & superior frontal gyrus pM pars marginalis OG orbital gyrus pocn pre-occipital notch PCu precuneous pocs post-central sulcus PreCG, PostCG pre- and post-central gyrus pof parieto-occipital fissure PL paracentral lobule (upper SFG and PreCG pos parieto-occipital sulcus and PostCG) prcs pre-central sulcus IFG inferior frontal gyrus POp pars opercularis sfs, ifs superior, inferior frontal PT pars triangularis sulcus POr pars orbitalis sps superior parietal sulcus sts, its superior, inferior tem- STG, MTG, ITG superior, middle & inferior temporal gyrus poral sulcus SPL, IPL superior & inferior parietal lobule tos trans occipital sulcus SMG supramarginal gyrus 5.1.2. Surface anatomy of the cranium CRANIOMETRIC POINTS See Figure 5-4. Pterion: region vertex where the following bones are approximat- bregma P ed: frontal, parietal, ARIET L AL temporal and sphenoid stephanion TA ON (greater wing). Esti- pterion FR mated as 2 finger- cs stl breadths above the zy- ophyron sqs lambda gomatic arch, and a glabella thumb’s breadth be- nasion hind the frontal pro- S TEMPORAL rhinion ls W L cess of the zygomatic A G pms IT bone (blue circle in Fig- sms IP C ure 5-4). C OID O Asterion: junc- NASAL G T ZY AS oms tion of lambdoid, occip- M prosthion inion itomastoid and MAXILLA parietomastoid su- asterion tures. Usually lies inferior alveolar point opisthion within a few millime- ters of the posterior-in- gnathion BLE gonion ferior edge of the or menton MANDI junction of the trans- verse and sigmoid si- Figure 5-4 Craniometric points & cranial sutures. nuses (not always Named bones appear in all upper case letters. reliable6 - may overlie Abbreviations: GWS = greater wing of sphenoid bone, NAS = nasal bone, stl = either sinus). superior temporal line, ZYG = zygomatic. Vertex: the top- Sutures: cs = coronal, ls = lambdoid, oms = occipitomastoid, pms = parietomas- most point of the skull. toid, sms = squamomastoid, sqs = squamosal Lambda: junc- tion of the lambdoid and sagittal sutures. Stephanion: junction of coronal suture and superior temporal line. 86 5. Neuroanatomy and physiology NEUROSURGERY Glabella: the most forward projecting point of the forehead at the level of the su- praorbital ridge in the midline. Opisthion: the posterior margin of the foramen magnum in the midline. Bregma: the junction of the coronal and sagittal sutures. Sagittal suture: midline suture from coronal suture to lambdoid suture. Although often assumed to overlie the superior sagittal sinus (SSS), the SSS lies to the right of the sagittal suture in the majority of specimens7 (but never by > 11 mm). The most anterior mastoid point lies just in front of the sigmoid sinus8. RELATION OF SKULL MARKINGS TO CEREBRAL ANATOMY Taylor-Haughton lines Taylor-Haughton (T-H) 2 cm lines can be constructed on an 1/2 angiogram, CT scout film, or skull x-ray, and can then be re- s u c constructed on the patient in the l u O.R. based on visible external s 9 l 3/4 landmarks . T-H lines are a r t shown as dashed lines in Figure n e 5-5. c 1. Frankfurt plane, AKA re ©2001 Mark S Greenberg, M.D. baseline: line from inferi- su All rights reserved. fis Unauthorized use is prohibited. or margin of orbit ian ylv through the upper mar- s gin of the external audi- tory meatus (EAM) (as distinguished from Re- EAM id’s base line: from infe- Frankfurt rior orbital margin plane through the center of the EAM)10 (p 313) posterior ear line 2. the distance from the na- condylar line sion to the inion is mea- sured across the top of the calvaria and is divid- ed into quarters (can be Figure 5-5 Taylor-Haughton lines done simply with a piece and other localizing methods of tape which is then folded in half twice) 3. posterior ear line: perpendicular to the baseline through the mastoid process 4. condylar line: perpendicular to the baseline through the mandibular condyle 5. T-H lines can then be used to approximate the sylvian fissure (see below) and the motor cortex (also see below) Sylvian fissure AKA lateral fissure Approximated by a line connecting the lateral canthus to the point 3/4 of the way posterior along the arc running over convexity from nasion to inion (T-H lines). Angular gyrus Located just above the pinna, important on the dominant hemisphere as part of Wernicke’s area. Note: there is significant individual variability in the location2. Angular artery Located 6 cm above the EAM. Motor cortex Numerous methods utilize external landmarks to locate the motor strip (pre-central gyrus) or the central sulcus (Rolandic fissure) which separates motor strip anteriorly from primary sensory cortex posteriorly. These are just approximations since individual variability causes the motor strip to lie anywhere from 4 to 5.4 cm behind the coronal suture11. The central sulcus cannot even be reliably identified visually at surgery12. • method 1: the superior aspect of the motor cortex is almost straight up from the EAM near the midline • method 213: the central sulcus is approximated by connecting: NEUROSURGERY 5.1. Surface anatomy 87 A. the point 2 cm posterior to the midposition of the arc extending from nasion to inion (illustrated in Figure 5-5), to B. the point 5 cm straight up from the EAM • method 3: using T-H lines, the central sulcus is approximated by connecting: A. the point where the “posterior ear line” intersects the circumference of the skull (see Figure 5-5) (usually about 1 cm behind the vertex, and 3-4 cm be- hind the coronal suture), to B. the point where the “condylar line” in- tersects the line representing the sylvian fissure cs • method 4: a line drawn 45° to Reid’s base line B starting at the pterion D1 FM F A points in the direction of V3 O the motor strip14 (p 584-5) Aq T Twining D2 V4 D3 RELATIONSHIP OF VENTRICLES D4 TO SKULL Figure 5-6 shows the rela- tionship of non-hydrocephalic ventricles to the skull in the lat- opisthion eral view. Some dimensions of in- baseline terest are shown in Table 5-315. sigmoid sinus In the non-hydrocephalic sella turcica adult, the lateral ventricles lie 4- 5 cm below the outer skull sur- face. The center of the body of the Figure 5-6 Relationship of ventricles to skull landmarks* lateral ventricle sits in the midp- upillary line, and the frontal * Abbreviations: (F = frontal horn, B = body, A = atrium, O = oc- horn is intersected by a line pass- cipital horn, T = temporal horn) of lateral ventricle. FM = fora- men of Monro. Aq = sylvian aqueduct. V3 = third ventricle. V4 ing perpendicular to the calvaria → along this line16. The anterior = fourth ventricle. cs = coronal suture. Dimensions D1-4 horns extend 1-2 cm anterior to see Table 5-3 the coronal suture. Average length of third ventricle ≈ 2.8 cm. Table 5-3 Dimensions from Figure 5-6 Dimension Description Lower limit Average Upper limit (see Figure 5-6) (mm) (mm) (mm) D1 length of frontal horn anterior to FM 25 D2 distance from clivus to floor of 4th ventricle at 33.3 36.1 40.0 level of fastigium* D3 length of 4th ventricle at level of fastigium* 10.0 14.6 19.0 D4 distance from fastigium* to opisthion 30.0 32.6 40.0 * the fastigium is the apex of the 4th ventricle within the cerebellum 88 5.

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