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Pictorial Essay

Two-Dimensional Mapping of Brain Surface Anatomy Helmuth Steinmetz1 and Yanxiong Huang

The gross morphology of the human brain is currently regaining some of the scientific interest it had 1 00 years ago [1-3]. Progress in neuroimaging explains this trend : MR morphometry has shown correlations between some aspects

A B

of anatomy and function [4], integration of MR and positron emission data has allowed the analysis of individual structural­functional relationships [5], and MR mapping of cortical anat­omy has improved neurosurgical strategies [5, 6].

Fig. 1.- A, MR reformation in an anteriorly tilted transaxial plane (see 8) shows landmarks of superior motor, premotor, and supplementary motor cortex. Primary motor cortex (Brodmann area 4) lies on precentral gyrus (a) anterior to central sulcus (2). Premotor and supplementary motor cortex (Brodmann area 6) lies anterior to area 4 mainly on superior (b) and medial (c) frontal gyrus. Note characteristic junction between superior precentral (3) and superior frontal (4) sulci. This junction is present in 86% of hemispheres [3].

B, Sagittal 1.17-mm slice from original FLASH data set indicates plane of reformation for A (see text) . Note position of central sulcus (2), which is consistently located immediately anterior to ascending marginal ramus of cingulate sulcus (1) [1 , 6, 7].

Received December 26, 1990; revision requested February 22, 1991; revision received March 19, 1991; accepted March 28, 1991 . ' Both authors: Department of Neurology, Heinrich-Heine-University, Moorenstr. 5, D-4000 Dusseldorf 1, Germany. Address repnnt requests to H. Steinmetz.

AJNR 12:997-1000, September /October 1991 0195- 61 08/91 /1 205- 0997 © American Society of Neuroradiology

Key to Numbers/Letters Used in Figures 1-6

1 marginal ramus of cingulate sulcus 2 central sulcus 3 superior precentral sulcus 4 superior frontal sulcus 5 postcentral sulcus 6 intraparietal sulcus 7 parietooccipital sulcus 8 inferior precentral sulcus 9 inferior frontal sulcus

1 0 anterior ascending sylvian rami 11 main (posterior horizontal) segment of sylvian

fissure 12 terminal (posterior ascending) segment of sylvian

fissure 13 horizontal segment of superior temporal sulcus 14 posterior ascending segment of superior tem­

poral sulcus 15 intermediate sulcus of Jensen a precentral gyrus b superior frontal gyrus c medial frontal gyrus d postcentral gyrus e superior parietal lobule f inferior parietal lobule g inferior frontal gyrus h superior temporal gyrus k supramarginal gyrus m angular gyrus

The cerebral sulci are the guiding structures in anatomic cortex mapping. Identification of specific sulci relies on their characteristic course, depth, junctional pattern , continuity or discontinuity, and on some constant topographic relationships between sulci [1 - 3, 7, 8]. An MR analysis of the sulcal pattern of the frontoparietal convexity illustrates these principles (Figs. 1-6). The images shown in part A of each figure are two-dimensional computer reformations (Mipron KAT 386; Kontron, D-8057 Eching, Germany) from a volumetric fast low-angle shot (3D-FLASH) MR data set. The 128 original images were acquired sagittally, with 1.17 -mm single-slice thickness (see part B of each figure) and 22 min measuring time. Technical factors of the FLASH sequence were 40/5/1 (TR/TEfexcitations), 40° flip angle, 25-cm field of view, 256 x 256 image matrix, 15-cm thickness of the excited volume and 128 partitions (1 .5-T Magnetom, circularly polarized head coil ; Siemens, D-8520 Erlangen, Germany). The volumetric data set thus contained more than 8 million voxels of 1 x 1 x 1 x 1.17 mm side length. As shown by the illustrations, it was sliced in arbitrary two-dimensional planes without signif­icant loss of resolution (Figs. 1-6). Computation times were 6 sec for each reformatted image, including interpolation to enhance image quality.

Specific sulci and gyri sometimes cannot be identified with certainty on conventional sections of the brain . This is why three-dimensional modeling of the brain surface has been used for individual sulcus mapping [5]. Three-dimensional rendering , however, requires time-consuming and operator­intensive two-dimensional image editing, and may not reflect anatomic reality owing to the methodological problems of MR image segmentation . Also, three-dimensional surface views do not reveal most of the brain cortex, of which more than

co~!~~:·-;A , ~R ~eformat(io1 n) in da posteriorly tilted transaxia l plane (see 8) shows landmarks of superior somatosensory and superior parietal association I . argma ramus an postcentral sulcus (5) separate Brodmann areas 3 1 2 and 5 on ostcentra l rus ( ) · ·

superior parietal lobule (e). The latter _area extends laterally to intraparietal su lcus (6J. Parletooccipitatsulcus (7) ;:parat=s ~~:~ssoc~atlon7 are: 7 ~n frfohm v1sual assoc1at1on area 19 postenorly (cuneus). Note characteristic junction between postcentral (5) and intraparietal (S) sulci ~nh~cahreoaccurasn .1 ne r8

10

3r.Y o em1spheres [2 , 3]. • ,.

B, Sagittal 1.17-mm slice from original FLASH data set indicates plane of reformation for A (see text).

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Fig. 3.-A, Slightly laterally tilted MR reformation (see 8) shows landmarks of superior precentral and postcentral re­gions of one hemisphere. (See Figs. 1 and 2 for ana­tomic description of sulcal and gyral topography.)

B, Sagittal 1.17-mm slice from original FLASH data set indicates plane of reformation for A (see text).

Fig. 4.-A, Farther laterally tilted MR reformation (see B and compare with Fig. 3) shows entire course of central sulcus (2) and adjacent pri­mary motor (a) and somato­sensory (d) cortices. Course of central sulcus is unbroken in 100% of hemispheres (3], which serves as another im­portant criterion for its identi­fication. Identification of su­perior and inferior precentral (3, 8) and postcentral (5) sulci thus becomes self-evident. Anterior to superior precentral sulcus (3) and between supe­rior (4) and inferior (9) frontal sulcus lies the frontal eye field on the medial frontal gyrus (c).

B, Sagittal 1.17-mm slice from original FLASH data set indicates plane of reformation for A (see text).

A

60% is buried in sulci . Large parts of the primary motor, primary sensory, or language-related cortices, for example, are hidden in the central sulcus, calcarine sulcus, or sylvian fissure. These areas are of particular interest for structural­functional correlations. Also, relationships between the depth extensions of sulci and subcortical lesions may be worth knowing prior to neurosurgery.

In our experience, two-dimensional brain sections refor­matted in planes tangential to the brain surface (Figs. 1 A-6A) provide fast and most detailed information about regional sulcus patterns, thus also serving as a reliable technique of anatomic sulcus identification. The sulcal and gyral pattern of the entire convexity can be recognized within minutes by tilting the plane of reformation step-wise around all three axes and moving back and forth between superficial and deeper

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sections. Simultaneous viewing of conventional and reformat­ted sections allows sulcus identification on both images, which is important for anatomically based definitions of PET­regions of interest [7).

The subtlety of anatomic details provided by two-dimen­sional sulcus mapping surpasses that of gross anatomic inspection. Anatomists of the 19th century spent large parts of their lives with minute descriptions of the different types of individual sulcal patterns [1-3). Today, it might be worthwhile to reconsider some questions they asked themselves, such as, Does it matter whether there are one, two, or three ascending sylvian rami cutting into Broca's area or its contra­lateral analogue [1 , 2]? Does it make a difference whether the intraparietal sulcus is prominent, deep, and unbroken or split into several small and scattered segments [2]? Is it

1000 STEINMETZ AND HUANG AJNR: 12, September/October. 1991

A B

A B

relevant whether the sylvian fissure ends in the anterior or posterior portion of the right or left inferior parietal lobule [8]?

Sulcus formation takes place in utero and presumably reflects the action of intracortical growth pressures during brain ontogenesis. A sulcus considerably increases the local amount of cortex. Of course, it is likely that most of the gross variability of the brain surface has no functional significance. Some features of sulcal anatomy, however, may be related to quantitative cytoarchitectonics, and, therefore, to functional variability [4]. On the basis of present knowledge, it is not premature to predict gross anatomic correlates of some ge­netically influenced behavioral conditions, such as right hem­isphere language dominance or developmental dyslexia. MR with two-dimensional rendering as shown in this article may be of help in answering a host of questions relating to ana­tomic variants , pathologic conditions, and surgical planning.

REFERENCES

Fig. 5.-A, Anteriorly and laterally tilted MR reformation (see 8) shows landmarks of inferior motor and pre­motor cortex and Broca's area. Broca's area lies anterior to inferior precentral sulcus (8) and below inferior frontal sulcus (9) on inferior frontal gyrus (g). It is divided by Y -shaped anterior as­cending sylvian rami (10) into triangular portion (Brodmann area 45 between the "arms" of the Y [10]) and opercular portion (area 44 between Y [10] and inferior precentral sulcus [8]) [6]. Note characteristic junction between inferior precentral sulcus (8) and inferior frontal sulcus (9), which occurs in approxi­mately 80% of hemispheres [1, 6].

11 =main horizontal stem of sylvian fissure, 13 = horizontal segment of su­perior temporal sulcus.

8, Sagittal 1.17-mm slice from origi­nal FLASH data set indicates plane of reformation for A (see text).

Fig. 6.-A, Posteriorly and laterally tilted MR reformation (see 8) shows landmarks of inferior parietal associa­tion cortex (Brodmann areas 40 and 39) and Wernicke's area. Supramarginal gyrus (k) and angular gyrus (m) cap upper ends of sylvian fissure (12) and superior temporal sulcus (14), respec­tively. They correspond to areas 40 and 39 and are separated by intermediate sulcus of Jensen (15) [8]. Intraparietal sulcus (6) separates superior parietal cortex (Brodmann area 7) from inferior parietal association cortex. The part of the superior temporal gyrus (h) be­tween the posterior horizontal portions ofsylvian fissure ( 11) and superior tem­poral sulcus (13) forms the core of Wer­nicke's area (posterior portion of Brad­mann area 22) [ 4 ). Compared with other brain regions, sulcal and gyral topog­raphy of the temporoparietal transition is more variable [8].

8, Sagittal 1.17-mm slice from origi­nal FLASH data set indicates plane of reformation for A (see text).

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