MR IMAGING ANATOMY OF THE OPTIC PATHWAYS

0033-8389/99 $8.00

IMAGING IN OPHTHALMOLOGY I1

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MR IMAGING ANATOMY OF THE OPTIC PATHWAYS Jean C. Tamraz, MD, PhD, Claire Outin-Tamraz, MD, and Roger Saban, PhD

sion-recovery (IR), or three-dimensional gradient From the retina, the visual pathways extend echo (SPGR 23/min/35") pulse sequences, as well anteroposteriorly as the optic nerve, chiasm, optic as long TR FSE T2-weighted (4500/102 ms) or tracts, and optic radiation and terminate in the STIR (5000/17/120 ms) sequences in order to reach striate cortex on the medial aspect of the occipital optimum contrast resolution. lobes. Along their orbitocranial route, the visual pathways maintain a nearly axial (horizontal) oriHISTORICAL BACKGROUND entation from the eyes to the calcarine fissure. For this reason, the neuro-ocular plane (NOP), taken During the second half of the second century, as the cephalic reference plane, appears to be an Galen,26whose experiences on the nervous system appropriate plane to study the visual pathways.59, of animals led him to be considered the first physi11, 12, 15, 3*38, 4a56 Most authors agree that the NOP is ologist, described the optic nerves as pneumatic the best cephalic orientation for studying the antecanals carrying sensation from the eyes to the rior optic 48-51 In our opinion, it is brain.'6 These canals were assumed to connect each an excellent plane for brain imaging as well as eye to the corresponding cerebral ventricle without evaluating the retrochiasmatic visual p a t h ~ a y s . 4 ~ ~ ~ consideration for crossing over between the any Consequently, we believe the three-dimensional right and left side. Galen, however, thought that anatomy of the skull and brain can be defined and the canals join each other on the midline and take outlined without any topographic disorganization the aspect of an "X" before they separate immediof brain structures. Anteroposterior and craniocauately afterward. He believed that this kind of exdal anatomic relationships are respected.4as1This is change, at the level of the chiasm, allowed the not the same, of course, when other more oblique pneuma to go to the opposite eyeball and double reference planes are used for brain imaging. In its strength if the other eye is destroyed. On the keeping with the purpose of this issue of the Radioother hand, Galen attempted to justify the arrangelogic Clinics of North America, we review the imment of the optic nerves and chiasm by an explaaging anatomy of the entire visual pathways, with nation of the binocular visual fields based on the an attempt to focus on the MR imaging sections geometry of converging cones whose apices were that best delineate the anatomic details of these located at the pupils. very long sensory tracts. We also review MR imThe first representation of the chiasm showing a aging findings of certain congenital oculo-orbital total crossing over of the optic nerves is found malformations. MR imaging studies using superaround 1266 in an Arabic book of ophthalmology conductive magnet (SIGNA 1.5-T, General Electric, by the Syrian Khalifah Ibn Abi Al-Mahasin AlMilwaukee, WI) were performed according to new Halabi.28The picture (Fig. l) shows the brain and axial and coronal reference lines to evaluate the its ventricles where the five senses are considered visual pathways. Acquisition data (3 mm thick) to be located as reported by Ibn Sina in his Canon, were obtained using mainly T1-weighted, invera three-volume textbook, around the year 1000.

From the Department of Magnetic Resonance and Neuroimaging, Hotel-Dieu de France, Universite Saint-Joseph (JCT, CO-T), Beirut, Lebanon; and the Laboratoire d'Anatomie Comparke, Museum National d'Histoire Naturelle (RS),

Paris, France RADIOLOGIC CLINICS OF NORTH AMERICA VOLUME 37 NUMBER 1 * JANUARY 1999

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Figure 1. Khalifa (1266) gives the first representation of the chiasm showing a total crossing over of the optic

nerves.

This illustration also shows the hollow nerves in which the visual spirit is emanated from the brain. Moreover, the optic nerve is represented with its two sheaths that go over into the boats of the eye. The structures of the eye are also detailed. In 1543, Vesalius61in his first textbook of modern anatomy (Fabrica), in which he referred to Galen’s description of optic nerves, showed the first exact reproduction of the inferior surface of the brain and the chiasm (Fig. 2)5l He doubted the existence of a cavity inside the optic nerves, except possibly for the chiasm. He believed, however, that there is no real crossing-over of the nerves but a simple juxtaposition at the chiasmatic level. Ironically, however, Leonard0 da Vinci,’6 half a century before, was the first to describe the decussation of optic nerves in his anatomic drawings. Varolius60 in 1573 published the first book devoted to the optic nerve in which he demonstrates the thalamic origin of this nerve from the lateral aspect of the third ventricle (Fig. 3). The concepts of Galen were still very widely present at the middle of the seventeenth century. Descartes19 still believed in the mechanistic representation of vision, that the optic nerves do not decussate at the level of the chiasm. Each of them, originating from a precise region of the lateral ventricle, follow a parallel chiasmatic route before

terminating in the retina in a precise well-defined manner (Fig. 4). Images are thus transmitted to the pineal gland, separately for each eye, before they are memorized inside the brain. A few years later, Willis@ published the first textbook on brain anatomy in 1664. He demonstrated that the optic nerve is composed of a fascicle of nervous fibers originating from the thalamus, instead of a hollow tube. He discovered the tracts connecting the internal structures to the cortex and recognized the existence of higher centers responsible for voluntary motility. In 1685, C ~ l l i n s ’published ~ a very good representation of the visual system (Fig. 5). He clearly illustrated the relationship of the optic pathways with the optic thalamus. Vicq d’AzyP in 1786 demonstrated the diversity of cortical structures inside the occipital lobe. In his book Traite d’rinatomie ef de Physiologie devoted to the brain, he showed a cut of the cerebral hemisphere passing through the optic pathways. This cut (Fig. 6), similar to what we obtain presently with MR imaging, shows as indicated in its legends the optic nerves, the optic nerves junction, the optic tract, the enlargement or the posterior tubercle of the optic tectum, and the implantation of the brain ”legs.” Gratiolet in 185428in his Memoire sur les plis cerebraux de I’Homme et des Primates described the continuity of the fibers of the visual tract projecting as terminal fibers to the vicinity of the calcarine fissure. In 1869, MeynerP demonstrated the role of the lateral geniculate body in vision and its connections with the temporal and the occipital

Figure 2. Vesale (1543) shows the first exact reproduction of the inferior surface of the brain and the chiasm.

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ANATOMY AND MR IMAGING OF THE EYES AND VISUAL PATHWAYS Cephalic Orientations and Visual Pathways

Most of the brain atlases are described according to the stereotaxic bicommissural anterior and posterior commissures line of Talaira~h.~~, 42, 4547 Such a cephalic orientation, very close to the orbitomeatal line (1,4 degrees 2 2, 7),44 is too much oblique with respect to the visual pathways.39,40, 44 A more accurate cephalic orientation is therefore desirable to visualize best the entire optic pathways. Many authors proposed reference planes and most of them tried to calculate the different angulations with respect to the international widely used orbitomeatal line, or according to the anthropologic baseline?, 27, 41* 57-59, 63 It appears that a better compromise is a reference plane suitable for the optic pathways (mainly the intracranial) as well as the brain at the same time. We attempt to describe in the following our technique for evaluation of optic pathways. The NOP

Figure 3. Varole (1573) shows the optic nerves and chiasm along with the optic radiations.

lobes via the optic radiations. Finally, Flechsig= was the first to elaborate the time schedule of myelination in the fetus, showing that just before birth myelination commences in the optic nerve and the geniculocalcarine tract, now easily observed on spin echo and IR pulse sequences using MR imaging. This may be considered as the beginning of the era of neuroanatomy.’,28

In his writings on the projections of the head, Broca2 stated: “the direction of gaze is the only characteristic of the living by which it may be determined that the head is horizontal. When man is standing and his visual axis is horizontal he is in his natural attitude.” One century after his death, CT demonstrates the validity of Broca’s2 opinion about horizontally oriented visual pathways. The NOP originally described for the CT study of the optic nerve in papillederna4l appears to be an optimal cephalic orientation for scanning the visual pathways (Fig. 7).12 Anatomic Correlations. From an anatomic point of view, the NOP is defined as the plane passing through the lens, the heads of the optic nerves,

Figure 4. Descartes (1 664) established the first diagram concerning the projection of the retinal images.

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Figure 5. Collins (1685) shows a very good representation of the optic tracts and their connections to the optic thalami and a dissection of the temporal horns of the lateral ventricles.

MR IMAGING ANATOMY OF THE OPTIC PATHWAYS

Figure 6. Vicq d'Azyr (1786) shows a real anatomic cut passing through the retro-chiasmal optic pathways, the optic tracts, and the temporal white core along the temporal horns and the hippocampal formations.

and the optic canals with the patient maintaining primary gaze. Such an orientation provides the optimal plane for CT or MR imaging examination of the intraorbital structures. The partial volume effect on the optic nerves is particularly reduced to a minimum (Fig. 8).12The mean angle between the NOP and the Frankfurt-Virchow plane (FVP) is about 7 degrees? The orbitomeatal plane, which is the classic radiologic reference is inclined at approximately 15 degrees relative to the FVP. Fenart et a12' presented an exhaustive work about the relation of the orbital axis plane to several craniofacial reference lines. It is interesting to note that the NOP provides a meridional cut through the globe and through the horizontal rectus muscles, from the annulus of Zinn to their tendinous attachment on the eyeball. Cuts inferior and superior to the NOP show the vertical rectus muscles (see Fig. 8). During the last 20 years more than 30,000 orbitocranial CT examinations and 15,000 MR imaging examinations performed taken along and parallel to the NOP prove its excellent practical application (Figs. 8, 9). In the chiasmal region, the appearance of the CT or the MR image cut along the NOP is determined by the anatomy of this region, the height of the sella turcica, and the type of obliquity of the chiasm. The retrochiasmal visual pathways are well demonstrated on this cephalic orientation along

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the NOP (Figs. 8,lO). This seems obvious considering the fact that the visual pathways are axial and transverse to the axis of the body, extending from the retina to the calcarine fissure. So, the entire visual pathways are included in successive cuts taken along the NOP depending on the slice thickness. Some parts of the visual pathways, however, depart somewhat from the ideal horizontal plane: the chiasm, for example, because of its variable positioning in the optic-chiasmatic cistern; the optic radiations because they are thicker than one single cut and the calcarine fissure in its ascending portion (see Fig. 7). Of course, the angle of the visual pathways with respect to the base of the skull does change with age due to the well-known occipital des~ent.'~ Once maturation is completed, however, the angle between the visual pathways and the skull remains constant. Therefore, the angle between the NOP and FVP becomes constant in the adult. So, one would say with Delmas and PertuisetI8 "the vision of man raised to encounter the horizon." On the other hand, considering that the optic placodes are one of the earliest embryologic developments of the central nervous system, formed by symmetric outpouchings at about the 32th day, it is not surprising that it might provide a neuro-ocular reference plane for the mature brain? Practically, the NOP is easily determined and allows rapid alignment of the head in the gantry. The line joining the centers of the pupils, which is precisely horizontal in the normal, determines the anterior portion of the plane. The second point is found about 33 mm above the tragus. External cutaneous landmarks, experimentally determined and materialized by canthomeatal line, are helpful to orient the patient's head in routine practice. To validate the accuracy of this cephalic orientation, an atlas of cross-sectional anatomy of the head with MR imaging correlation was prepared,", 48 supported by S~hering-France,4~ and edited by Masson. Three heads were oriented and sectioned every 6 mm in the sagittal, axial (NOP), and coronal planes, according to the method of Delmas and Pertuiset.ls All the cuts are annotated using the international nomenclature (Parisiensa nomina anatomica) along with an English and French alphabetical index. The anatomic correlations, as observed from the successive anatomic c~ts,4~-~I, 53 demonstrate the close parallelism of the NOP as to the direction of the temporal horns and lobes, and its perpendicularity to the axis of brain stem in most instances. Coronal cuts were obtained along the posterior commissure-obex, brain stem reference line (Cp-Ob line) (Figs. 11, 12, 13, 14).56 The NOP remains an excellent cephalic orientation (reference plane) for investigations and biometrical studies as well as for follow-up of diseases of the eyes and the intraorbital optic nerves and optic pathways in the axial and coronal planes.1o,48, 49 Sagittal oblique cuts oriented along the optic nerve axis or the optic canal may be of great help in special conditions. Text continued on page 11

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Color Plate 1 Figure 7. The neuro-ocular plane (NOP). Anatomic correlations showing the cephalic landmarks of this reference plane: the lenses, the optic cups, and the optic canals (12). A, The NOP cut showing the optic nerve (80) and the optic radiation (98). €3, The NOP cut showing the optic nerve (80), optic chiasm (15), and optic radiation (98). C and 0,Transverse sections through the rostra1 mesencephalon showing the manner in which diencephak nuclei (pulvinar [go], corpus geniculatum laterale 1311, corpus geniculatum rnediale [32]) surround the dorsal and lateral portions of the mesencephalon, included in the same cut passing through the superior colliculi (22), mammilary bodies (32), and the origin of the optic tracts (120) at the level of the chiasrn (thus very closely to what is observed using the Ch-Cp reference line).

Figure 11. The Cp-Ob (commissura posterior-obex) brainstem reference line. A, Midsagittal anatomical cut showing the Cp-Ob line (thick arrows) and its close relationship with the direction of both the medial longitudinal fasciculi (39) and the cortico-spinal (long arrows) and pyramidal tracts (119). B, Coronal anatomic cut taken along the Cp-Ob plane showing several structures, including the internal capsule (13), crus cerebri (33), and pons (93). Cand D, see page 13.

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D Figure 7.

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B Figure 11.

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D Figure 16.

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D Figure 18.

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C Figure 20.

MR IMAGING ANATOMY OF THE OPTIC PATHWAYS

Color Plate 2 Figure 16. A and B, see page 18. C and 0,The cisternal optic nerve beneath the frontal orbital gyri and the sellar and cavernus sinuses. C, 2 = internal carotid artery; 56 = gyrus rectus; 59 = hypophysis; 69 = temporal lobe; 79 = olfactory nerve; 80 = optic nerve; 103 = cavernous sinus. 0,2 = internal carotid artery; 36 = diaphragma sellae; 50 = orbitofrontal gyri; 56 = gyrus rectus; 69 = temporal lobe; 79 = olfactory nerve; 80 = optic nerve; 103 = cavernous sinus; 125 = lateral ventricle.

Figure 18. Coronal anatomy of the chiasm and the cisternal optic tracts. A and B, The chiasm is shown with its main relationship: the inferior aspect of the frontal lobes superiorly, the supraclinoid internal carotid arteries (2) and the olfactory tracts (79) laterally, the sellar region with the pituitary gland and the stalk inferiorly, and the third ventricular antero-inferior wall at its posterior border. C, The origin and infundibular recesses above the clivus. 0,The cisternal optic tracts (12c) at the lateral aspect of the hypothalamus above the proximal segments of the posterior cerebral arteries (5m) are shown. The oculomotor nerves also are well depicted between the posterior cerebral arteries and the superior cerebellar arteries below, both bifurcating from the tip of the basilar artery. See the close relation of the tract to the sublenticular area. A, 2 = internal carotid artery; 3 = anterior cerebral artery; 15 = optic chiasrn; 59 = hypophysis; 66 = frontal lobe; 78 = oculomotor nerve; 79 = olfactory nerve; 86 = caudate nucleus; 103 = cavernous sinus; 106 = sphenoidal sinus: 125 = lateral ventricle. B, 2 = internal carotid artery; 3 = anterior cerebral artery; 15 = optic chiasm; 29 = corpus callosum; 59 = hypophysis; 66 = frontal lobe; 69 = temporal lobe; 78 = oculomotor nerve; 83 = neurohypophysis;86 = caudate nucleus. C, 4 = middle cerebral artery; 19 = claustrum; 29 = corpus callosum; 51 = insular gyri; 69 = temporal lobe; 86 = caudate nucleus; 94 = gray striatal bridges; 96 = putamen; 120 = optic tract. D, 1 = basilar artery; 5 = posterior cerebral artery; 13 = internal capsule; 19 = claustrum; 23 = anterior commissure; 28 = amygdaloid body; 29 = corpus callosum; 48 = lateral segment of globus pallidus; 51 = insular gyri; 52 = lingual gyrus; 55 = parahippocampal gyrus; 78 = oculomotor nerve; 86 = caudate nucleus; 93 = pons; 96 = putamen; 108 = anterior perforated substance; 120 = optic tract; 127 = inferior horn of lateral ventricle.

Figure 20. Anatomy of the perimesencephalic optic tracts and the lateral geniculate bodies. A, Coronal cut showing the intimate relationship of the tract (120) to the corticospinal tract (119) medially, the pallidum above, and the anterior perforated substance more laterally. This cut passes through the interventricularforamina and the interpeduncularcistern. B, See page 21. C,Coronal cut through the thalami showing the medial (31) and lateral (30) geniculate bodies appended beneath and laterally to the posterior inferior aspect of the thalami. The curved arrow represents the origin of the optic radiation (peduncle). 0,See page 21. A, 13 = internal capsule; 19 = claustrum; 29 = corpus callosum; 46 = fornix; 48 = lateral segment of globus pallidus; 49 = medial segment of globus pallidus; 55 = parahippocampal gyrus; 58 = hippocampus; 66 = frontal lobe; 69 = temporal lobe; 93 = pons; 94 = gray striatal bridges; 96 = putamen; 120 = optic tract; 125 = lateral ventricle; 127 = inferior horn of lateral ventricle; 129 = third ventricle. C,13 = internal capsule; 29 = corpus callosum; 30 = lateral geniculate body; 31 = medial geniculate body; 35 = decussation of brachia conjunctiva; 66 = frontal lobe; 69 = temporal lobe; 84 = anterior nuclear group of thalamus; 85 = lateral nuclear group of thalamus; 86 = caudate nucleus; 89 = dorsomedial nucleus; 98 = optic radiation; 118 = thalamus; 120 = optic tract; 125 = lateral ventricle.

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Figure 8. The neuro-ocular plane (NOP). Anatomic correlations using MR imaging. The NOP as obtained on 3-mm axial cut with TP-weighting pulse sequence (with invert video), showing the anatomic landmarks defining the plane of section that best displays the intra-orbital optic nerves: The lenses (1); the optic cups (2); and the optic canals (3). Also shown are the medial and lateral horizontal recti muscles. Note that the brain is cut typically at the level of the inferior colliculi and of the decussation of the superior cerebellar peduncles comprising the upper culmen cerebelli (A). The orientation of the cut through the brain may show slight variations, depending on the slice thickness, the orientation pattern of the chiasm and the cranial typology, and may involve the chiasm and the superior colliculi as obtained on the anatomic cut of the atlas and the MR cut shown (6).

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contained in the NOP itself, well delineated by the uveoscleral rim (see Figs. 7, 8, 9).17 lntraorbital Optic Nerve

Figure 9. Oculo-orbital biometry. Linear measurements are performed in the neuro-ocular plane according to the methodology proposed by Cabanis et aL5 1 = external bicanthal referential plane and distance; 2 = pre-bicanthal segment; 3 = retro-bicanthal segment; 4 = axial diameter of the eyeball; 5 = interocular distance; 6 = maximal interplanar distance: 7 = intra-orbital nerve diameter. The Eyes

Surface-coil MR imaging of the eye has greatly improved the anatomic resolution of images obtained from the eyeball.30Spatial resolution of less than 1 mm can now be achieved. The lens shows a moderate T1 with a short T2, whereas the vitreous body, the major component of the eyeball, reflects the long T1 and T2 of its water content.

The intraorbital segment of the optic nerve begins as the optic nerve head exits the sclera and extends to the orbital opening of the optic canal, measuring about 2 to 3 cm in length and 3 to 5 mm in diameter (Figs. 7, 9, 15). The nerve is surrounded by the pia mater; the arachnoid; and the dura, which is the outermost sheath (see Figs. 7, 9). The dura fuses anteriorly with Tenon’s capsule and posteriorly with the periosteum. The optic nerve sheaths are continuous with the leptomeninges of the brain, transmitting the cerebrospinal fluid (CSF). On MR images, the optic nerve demonstrates signal characteristics similar to the cerebral white matter on T1-weighted and T2-weighted sequences. It is outlined by the low intensity signal of the surrounding CSF on T1-weighted MR image and high intensity on the T2-weighted sequences. In proton-density pulse sequences there is no distinction between the optic nerve and its surrounding sheaths, sometimes giving a false-positive impression of enlargement of the optic nerves (see Fig. 8). The dense fibrous dural sheath may not be distinguished from the CSF rim on the T1weighted image but.can be seen on T2-weighted pulse sequence. lntracanalar Optic Nerve

The optic canal measures about 9 mm in length. The angle between the axis of the optic canal and the median sagittal plane is about 40 degrees. The angle between its axis and the FVP is about 15.5 degrees in adults and 42 degrees in newborns.29 Its orbital opening is elliptical, with the widest diameter oriented vertically. The intracranial opening is also elliptical but with the widest diameter oriented horizontally (see Figs. 9, 16, 17). This latter appearance may be misinterpreted as optic nerve enlargement on axial MR imaging slices. Moreover, the optic canal is separated from the superior orbital fissure by a bony ridge, which is sometimes misdiagnosed as the optic canal on inadequately oriented axial cuts.

The Optic Nerves

lntracranial Optic Nerve

The optic nerve is about 5 cm long if measured from the eye to the chiasm. Four parts may be distinguished: (1) intraocular, (2) intraorbital, ( 3 ) intracanalar, and (4) intracranial.

The intracranial portion of the optic nerve varies in length from 3 to 16 mm, being flattened in coronal sections and measuring about 4.5 mm in its great diameter. The cistemal optic nerve is covered by the inferior aspect of the posterior part of the frontal lobe (see Figs. 7, 8, 16, 17). MR imaging sections (3 to 4 mm) oriented parallel to the Cp-Ob reference line are optimum enough to evaluate the shape and size of these nerves. Using the brain stem reference line (Cp-Ob) provides the opportunity to perform reproducible sections (see

lntraocular Optic Nerve

The intraocular part of the optic nerve is about 1 mm long and 1.5 mm in diameter. Its anterior surface represents the visible optic disc in the ocular fundus. Actually, the optic nerve heads are

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Figure 10. The chiasmato-commissuralline. Chiasmal point-inferior border of posterior commissure line (CH-PC) defining a chiasmato-mamillo-postcommissural plane (anatomic correlation using MR imaging). A, In vivo midsagittal T1-weighted gradient-echo slice showing the orientation of CH-PC line compared with the AC-PC (bicommissurai) line, which is much more oblique (-24"j and the PC-06 (posterior commissure-obex) reference line, which is almost perpendicular to PC-06. 6, Parasagittal cut showing the close parallelism to the first temporal or parallel sulcus (1) and to the lateral or sylvian fissure (2). C,Axial MR cut of a brain specimen showing the anatomic landmarks of the CH-PC plane (the chiasmal point and posterior commissure at the midbrain-diencephalic junction, comprising the cisternal and pericrural optic tracts). (From Tamraz J, Saban R, Cabanis EA, et al: Definition d'un plan de reference cephalique en imagerie par resonance. CR Acad Sci Paris, 3:115-121, 1990; with permission.)

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Figure 11. A and 6,See Color Plate 1. Inversion recovery T1-weighted 3 mm (C)and T2-weighted STIR 5 mm coronal cut (0) taken parallel to the PC-06 reference line. The close relation with the direction of both the medial longitudinal fasciculi (31) and its parallelism to the corticospinal and pyramidal tracts (119) are well shown in their entirety, at least from the midbrain-diencephalicregion to the medulla.

Fig. 12). The chiasm and the optic tracts of the same sections may be well delineated (see Figs. 12, 13). The Optic Chiasrn

The chiasm is a flattened quadrilateral bundle of nerve fibers located at the junction of the anterior wall of the third ventricle and its floor. It is formed by the fusion of both optic nerves with their partial decussation. It averages 15 (10 to 20) x 8 (4 to 13) x 4 (3 to 5 mm) in size and is located in the chiasmatic cistern behind the tuberculum sallae and the chiasmatic sulcus (see Fig. 18). Its position varies in relation to the sella turcica and the pituitary gland. In 79% of cases the chiasm overlies the posterior two thirds of the sella. In 12% of cases the chiasm is found over the center of the sella. It lies over and behind the dorsum sellae in 4% of cases, and rests in the chiasmatic sulcus in only 5% of cases. The variable position of the chiasm and its prefixed or postfixed situation account for the variations in its appearance in the axial plane as shown with CT or MR imaging. Moreover, the position of the chiasm itself varies according to the shape of the skull and the cephalic index, being more rostra1 and dorsal in brachycephaly than in dolichocephaly. So, the actual position of the optic chiasm and its obliquity vary widely among normal persons. For that reason, sagittal MR imaging sections best evaluate the location, shape, and thickness of the chiasm. The anatomic relationship of the chiasm is well shown on MR images, being the third ventricle and its optic recess superiorly; the internal carotid arteries

on each side; the anterior cerebral arteries and the anterior communicating artery in front; and posteriorly the tuber cinereum, the infundibular recess, and the pituitary stalk within the interpeduncular fossa (Figs. 18, 19). The chiasmatic (suprasellar) cistern separates the chiasm from the diaphragma sellae and the pituitary gland in the sella turcica (see Fig. 18). Optic Tract and Lateral Geniculate Body

The optic tract begins in the posterolateral angle of the chiasm, runs laterally and backward between the anterior perforated substance and the tuber cinereum (Figs. 19, ZO), constitutes the anterolateral boundary of the interpeduncular fossa, and then sweeps around the upper part of the cerebral peduncles.2* Along this portion of its course, each optic tract is hidden by the subjacent uncus and parahippocampal gyrus (see Fig. 20). The optic tracts run in close association with the posterior cerebral arteries along their perimesencephalic course (Figs. 8, 14, 21) and end in the lateral geniculate bodies at the posterolateral aspect of the thalamus (Fig. 22). Highly developed in primates, the lateral geniculate body is a small ovoid cap-shaped mass of gray and white matter located on the posterolateral aspect of the pulvinar (see Figs. 20,21). Its long axis is sagittally oriented, with an anterior pole blending with the optic tract (see Fig. 20). Each lateral geniculate body receives nearly 80% of the fibers from the corresponding optic tract. Some of the fibers do not end in it, but pass over it to reach the superior colliculus. The lateral geniculate bodies are the end-stations for

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Figure 12. Coronal three-dimensional SPGR T1-weighted MR cuts (3 mm thick), reconstructed parallel and anterior to the PC-OB referential (A), displaying the anterior optic pathways and (From showing the cisternal optic nerves (B and C) (arrow) and the chiasm (double arrows) (0). Tarnraz J, Saban R, Cabanis EA, et al: Definition d’un plan de reference cephalique en imagerie par resonance. CR Acad Sci Paris 3:115-112, 1990; with permission.)

the anterior visual pathways and the origins of the optic radiations, through which they are connected to the calcarine cortex. Perimesencephalic optic tracts, lateral geniculate body, and optic peduncle necessitate high-contrast MR imaging slices in order to be depicted appropriately, as can be obtained with fast spin echo, IR, or STIR sequences (Figs. 21, 24, 25, 26).

The Optic Radiations The anterior visual fibers are relayed to the occipital cortex as the optic radiation (of Gratiolet), which extends from the lateral geniculate body to the striate cortex.31The geniculocalcarine tract leaving the lateral geniculate body as the optic peduncle is well-shown on MR imaging sagittal cuts (see Figs. 20, 22). It forms a prominent ribbonlike lamina about 2 cm wide in the temporal,

parietal, and occipital lobes. Its fibers are grouped into fiber bundles arranged in parallel fashion with definite topographic origin from the lateral geniculate body and end on the visual cortex. The optic radiation then divides into three main anatomicfunctional bundles that occupy the external sagittal stratum and can be discerned mainly on T2weighted coronal slices using either the spin echo or the STIR pulse sequence with long TR (Figs. 23, 27). These cuts ought to be perpendicular to the long axis of the temporo-occipital lobe as obtained with the CP-OB brain stem reference line due to their relationship to the inferior horn of the lateral ventricle (see Fig. 27). From the anatomic point of view, the dorsal and lateral bundles spread directly posterior through the posterior, temporal, and parietal lobes. The ventral bundle makes a loop into the temporal lobe anteriorly and laterally, above and around the temporal horn of the lateral ventricle, before it spreads backward to reach the striate

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Figure 13. A-0, Coronal three-dimensional SPGR T1-weighted MR cuts show the cisternal optic tracts (arrows) and their regional relationships, from their origin posterior to the chiasm (A) to the level of the interventricular foramen-mamillary body cut (O), just before the tracts undergo their pericrural route, along with the posterior cerebral artery (see Fig. 16). 6,The cisternal optic tracts at the level of the anterior commissure and at the level of the anterior columns of fornix (C).

cortex just as the other two bundles do. The anterior deviation of the inferior optic radiation, known as Meyev’s loop, is about 0.5 to 1 cm lateral to the tip of the inferior horn of the lateral ventricle, provided that the visual fibers are not encountered in the first 5 cm from the temporal pole (see Fig. 7). Considering the MR imaging correlation and signal appearance, the optic radiations are readily depicted in normal conditions (as well as in the pathologic states, dissected for instance by vasogenic edema) in the coronal plane as well as in the axial, on T2:weighted sequences (hypointensity component of the bulky fibers along the temporooccipital horns of the lateral ventricles) as shown in Figure 27. The interpretation of this relatively lower signal of the optic radiations, as compared with the rest of the loosely organized white matter core of the hemispheres, could be due to the orientational dependence of T2 relaxation of the tracts with the static magnetic field direction (B,) causing

at least partly a relative shortening of the T2 relaxation time of these radiations, which are roughly orthogonal to the coronal cuts (parallel to CP-OB line), which could be considered as somehow oriented in the Bo direction in our routine positioning of the patient’s head in the magnet (NOP cutaneous landmark^).^^ The vertical or lateral bundle, comprising more than half of the optic radiation, corresponds to the macular fibers originating from both homonymous hemimaculas. The upper half of the segment represents the upper quadrants and the lower half of the lower quadrants. These fibers supply the striate cortex over the pole of the occipital lobe. The dorsal bundle includes fibers originating from the medial part of the lateral geniculate body and corresponding to the upper extramacular portions of both homonymous hemiretinas. This bundle projects to the upper lip of the calcarine fissure. The ventral horizontal bundle, well seen on coronal T2-weighted MR imaging cuts perpendicular

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Figure 14. A-D, Coronal three-dimensional SPGR T1-weighted MR cuts display the pericrural optic tracts (arrows) in their route towards the lateral geniculate bodies found at the PC-06 reference plane level (see Fig. 21).

to the great axis of the temporal horn, includes fibers originating from the external part of the lateral geniculate body and corresponding to the lower extramacular or peripheral portions of the homonymous hemiretinas. This bundle projects to the lower lip of the calcarine fissure. The Striate Cortex

The striate, or visual, cortex, also referred to as area 17 of Brodmann, occupies the superior and inferior lips of the calcarine fissure.3l It is limited posteriorly by the lunate sulcus when present, and does not extend beyond the occipital pole in humans. The cortex of the visual sensory area is identified histologically by a white line called the line of Gennari, which is a layer of the myelinated terminals of optic radiations fibers. The parietooccipital sulcus limits the striate cortex anteriorly, extending somewhat further below the calcarine fissure (see Figs. 21, 22, 23). An average of 67% of the visual projection cortex is buried in the depth

of the calcarine fissure and its branches. The calcarine fissure is usually restricted to the medial surface of the hemisphere, well delineated on the MR imaging midsagittal cut (see Fig. 10). It begins near the occipital pole, then runs anteriorly with a slightly curved course before it joins the parietooccipital sulcus at an acute angle. Its anterior portion forms the inferolateral limit of the isthmus. At its junction with the parieto-occipital sulcus, the floor of the calcarine fissure is crossed by the buried anterior cuneolingual gyms (see Fig. 22), which is evidenced on sagittal or coronal cuts. The posterior part of the calcarine fissure is an axial sulcus set in the long axis of the visual cortex, but the anterior part is a limiting sulcus producing an elevation in the medial wall of the posterior horn of the lateral ventricle, named the calcar avis (see Fig. 20). The wedge-shaped area located above the calcarine fissure is the cuneus, whose surface is generally indented by one or two small sulci. The lingual gyrus lies between the calcarine fissure superiorly and the collateral sulcus inferiorly (see

MR IMAGING ANATOMY OF THE OPTIC PATHWAYS

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Figure 15. Coronal anatomy of the orbit and the intraorbital optic nerves (perpendicular to the NOP orientation). Antero-posterior cuts passing through the optic nerve head at the posterior pole of the globe (A), the midorbital cavity and the intraorbital optic nerve and sheaths (80, 122) (B),and the posterior juxta-apical orbital cavity, optic nerve and muscle cone (C and 0). A, 8 = posterior pole of eyeball; 27 = orbital fat; 63 = cribriform plate; 66 = frontal lobe; 70 = levator palpebrae superioris muscle; 72 = superior oblique muscle; 73 = inferior rectus muscle; 74 = lateral rectus muscle; 75 = medial rectus muscle; 76 = superior rectus muscle; 80 = optic nerve. B,27 = orbital fat; 72 = superior oblique muscle; 73 = inferior rectus muscle; 74 = lateral rectus; 75 = medial rectus muscle; 76 = superior rectus muscle; 79 = olfactory nerve; 80 = optic nerve; 104 = ethmoidal sinus; 123 = superior ophthalmic vein. C,43 = inferior orbital fissure; 50 = orbitofrontal gyri; 56 = gyrus rectus; 74 = lateral rectus muscle; 76 = superior rectus muscle; 77 = abducent nerve; 104 = ethmoidal sinus; 122 = outer sheath of optic nerve; 123 = superior ophthalmic vein. 0, 43 = inferior orbital fissure; 50 = orbitofrontal gyri; 73 = inferior rectus muscle; 74 = lateral rectus; 75 = medial rectus muscle; 76 = superior rectus muscle; 77 = abducent nerve; 78 = oculomotor nerve; 80 = optic nerve; 104 = ethmoidal sinus; 123 = superior ophthalmic vein.

Fig. 20). The latter begins near the occipital pole and extends anteriorly, roughly parallel to the calcarine fissure. MR Imaging Investigation of Neuroophthalmologic Disorders

Because of the close anatomic relationship between the eyes and brain, one should be aware that neuro-ophthalmologic signs or symptoms are of diagnostic and localizing significance.Considering the fact that more than 50% of patients with brain tumors present with some form of impair-

ment of the visual pathways or of the ocular motor system, one may realize the importance of the working relationship of the neuro-ophthalmologist and the neuroradiologist in the evaluation of neurologic di~0rders.l~.MR imaging and CT examination strategy depends very closely on the clinical and paraclinical neuro-ophthalmologic data (mainly visual acuity, ocular fundus examination, visual field, and visual evoked potentials tests). Imaging of neuro-ophthalmologic disorders should be designed to evaluate three main anatomic as well as functional areas in the visual pathways: (1) the orbits, (2) the optochiasmatic region, and (3) the optic radiations.

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Figure 16. Coronal anatomy of the anterior optic pathways: the intra-canalar optic nerve (A); the intra-cranial optic nerve at the cranial aperture of the optic canal (B); the cisternal optic nerve beneath, and the frontal orbital gyri, and the sellar and cavernous sinuses are shown (C and 0,Color Plate 2). A, 12 = optic canal; 44 = superior orbital fissure; 56 = gyrus rectus; 80 = optic nerve; 95 = anterior clinoid process; 106 = sphenoidal sinus. B, 56 = gyrus rectus; 80 = optic nerve; 95 = anterior clinoid process; 106 = sphenoidal sinus

We believe the NOP is the ideal cephalic orientation for the examination of the intraorbital optic nerves. Coronal cuts perpendicular to the NOP best show and help to evaluate any subtle modification in size or in signal intensity (best seen using IR and STIR) of the intraorbital segment itself as well as the perioptic sheaths (Fig. 28). Sagittal oblique slices oriented in the long axis of the optic canals, if necessary, best evaluate the intracanalar as well as the cisternal optic nerve. Optic-tract enlargement, even if easily evidenced in the axial plane when tumoral, is better evaluated on coronal cuts when subtle modifications of the perimesencephalic part are suspected (see Fig. 28). The lateral geniculate bodies and the adjacent part of the optic tract having an important relation to the pulvinar and the crura cerebri are best studied and depicted on coronal cuts parallel to the brain stem CP-OB reference line. The lateral geniculate bodies are found anterior to this plane in most instances depending on the slice thickness (see Figs. 14, 21). Parasagittal cuts are also very informative (see Fig. 20D), showing the lateral geniculate body above the hippocampal formation and the temporal horn of the lateral ventricle, all sectioned with regard to their longitudinal anteroposterior long axis. In general, coronal sections oriented according to the CP-OB line and extending from the tectal plate to the genu of the corpus callosum suffice to study reliably and efficiently the anterior intracranial optic pathways from the optic canal to the origin of the geniculocalcarine tracts (see Figs. 12, 14). Imaging the lateral geniculate bodies in the sagittal plane also permits a perpendicular evaluation of the bulky peduncle of the optic radiation as it spreads later-

ally above the inferior horn of the lateral ventricle (see Figs. 20, 25). Considering its temporal, parietal, and occipital lateroventricular course as the geniculocalcarine tract, the axial and coronal cuts performed according to the previously demonstrated reference line are obviously the most helpful. Vertical topographic correlations as compared with the perimetry best benefit from the coronal cuts perpendicular to the long axis of the inferior horns of the lateral ventricle and parallel to the CP-OB line (see the successive cuts, Fig. 27). The calcarine fissures are readily shown on the midsagittal cut of the brain, which can easily be used to evaluate the medial aspect of the cerebral hemisphere. The striate cortex, lying in the depth of the fissure and forming its upper and lower lips, may be depicted on coronal as well as axial cuts. Its close relationship to the occipital horns of the lateral ventricle and the visibility of the optic radiations on T2-weighted coronal cuts at its vicinity may help to recognize it (see Figs. 22, 23, 27). Actually, there is no ideal cephalic orientation for studying the calcarine fissure, being variable among individuals. Its inclination as noticed on the midsagittal cut is of help best to recognize its location on both axial or coronal cuts. CONGENITAL OCULO-ORBITO-CRANIAL MALFORMATIONS: THE SPECTRUM OF CYCLOPIA Since the exhaustive preliminary works of Etienne Geoffroy Saint-Hilaire, father of systematic teratology defining cyclopia, ethmocephaly, and cebocephaly, and his son Isidore Geoffroy SaintHilaire, many authors are still fascinated by the

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Figure 17. Coronal TP-weighted MR imaging cuts, 3 mm thick, of the intracanalar (arrows) and cisternal optic nerves (arrowheads), performed parallel to PC-OB. A and 6,The intracanalicular optic nerves (arrows). C and 0, The cisternal optic nerves (arrowheads) at the cranial aperture of the optic canal (C) and in the suprasellar cistern (0).

striking morphologic abnormalities associated with the spectrum of holoprosencephalic facies. The maldevelopment of eyes and orbits is probably the most impressive feature of these craniofacia1 dysmorphisms and tends to be almost pathognomonic of holoprosencephaly. We had the opportunity to study with MR imaging some formalin specimen of monstruous fetuses (using SE, T2-weighted, 4-mm contigu-

ous slices) from the historical collection of the Laboratory of Comparative Anatomy of the Museum National d’Histoire Naturelle (Paris) in a collaborative effort. (Courtesy of R. Saban, MD, and J. Reperant, MD). The preliminary results are shown in Figures 29-30 in order to illustrate the oculo-orbital malformations as observed in cyclopia. The term koloprosencepkaly has been suggested Texf continued on page 24

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Figure 19. Coronal MR image cuts of the chiasm and the cisternal optic tracts, performed parallel to PC-06 reference plane. A, Coronal cut through the sella turcica (Ti -weighted spin-echo after Gd cavernous sinus enhancement) showing the chiasm, the pituitary stalk, and the hypophysis and its relation with the cavernous sinus. The intracavernous carotid arteries show a very close relationship with the abducent nerve. More laterally along the lateral wall of the cavernous sinuses, from above to below, the oculomotor nerve (arrow, L), the trochlear nerve (arrow on the right side) and inferiorly, the maxillary (double arrows). 6 and C,STIR (invert video image) coronal cut through the cisternal optic tracts, anterior cut passing through the ventral striatum and the amygdala (6) and posterior cut at the level of the anterior columns of fornix, the substantia innominata and the hippocampal head (C) displaying also the basilar artery and its bifurcation.

MR IMAGING ANATOMY OF THE OPTIC PATHWAYS

Figure 20. Anatomy of the perirnesencephalic optic tracts and the lateral geniculate bodies. A, see Color Plate 2. B, The optic tract (120) is shown on a parasagittal cut (109). C,see Color Plate 2. 0, Sagittal cut showing the lateral geniculate body (30) and the adjacent optic tract (120) anteriorly, the cerebral peduncle and the retrolenticular part of the internal capsule superiorly along with the auditory radiation, and the pulvinar posteriorly. The lateral geniculate body is found above the middle part of the subiculum (stripe of gray matter along the upper margin of the parahippocampal gyrus). B, 2 = internal carotid artery; 29 = corpus callosum; 33 = crus cerebri; 34 = cuneus; 52 = lingual gyrus; 57 = cerebellar hemisphere; 85 = lateral nuclear group of thalamus; 86 = caudate nucleus; 87 = dentate nucleus of cerebellum; 93 = pons; 103 = cavernous sinus; 107 = substantia nigra; 109 = calcarine sulcus; 114 = parieto-occipital sulcus; 117 = tentorium cerebelli; 120 = optic tract; 125 = lateral ventricle. 0,4 = middle cerebral artery; 91 = calcar avis; 13 = internal capsule; 23 = anterior commissure; 26 = corona radiata; 28 = amygdaloid body; 30 = lateral geniculate body; 48 = lateral segment of globus pallidus; 53 = lateral occipitotemporal gyrus; 55 = parahippocampal gyrus; 58 = hippocampus; 66 = frontal lobe; 90 = pulvinar; 94 = gray striatal bridges; 96 = putamen; 120 = optic tract; 125 = lateral ventricle.

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Figure 21. MR image of the pericrural optic tracts and the lateral geniculate bodies. A = Coronal cut using STIR T2weighted pulse sequence showing the relatively low signal of the optic tracts (arrow) along the lateral aspect of the crus cerebri. B, More posterior coronal cut (invert video), passing through the PC-06 reference plane and displaying constantly the lateral geniculate bodies (cgl). Note the projection of CH-PC reference plane tangential to the superior border of the lateral geniculate bodies.

Figure 22. Anatomy of the geniculo-calcarinetracts and the striate cortex. A, Coronal cut through the atria of the lateral ventricles containing the choroid plexuses, separated by the splenium of the corpus callosum (29) and the subsplenial gyri. 6-0,Successive coronal cuts in the occipital lobe showing the situation of the calcarine fissure and its relationship with the posterior horn of the lateral ventricle and the topography of the optic radiation lateral to the tapetum somehow distant from the lateral wall of the ventricle (arrows). €, Sagittal cut through the length of the dentate gyrus and the temporal horn, which arches above the hippocampal formation and communicates with the atrium. The tail of the caudate nucleus forms a stripe of gray matter along the upper margin of the temporal horn. Ventral to the putamen are the sublenticular and the retrolenticular parts of the internal capsule, which contains the optic radiations along with the auditory radiations. The elevation in the wall of the posterior horn of the lateral ventricle, produced by the deep calcarine sulcus on the medial surface of the occipital lobe, is the calcar avis. F; The optic tract (120) is closely related to the crus cerebri (33), which it encircles to terminate in great part in the lateral geniculate body. Note its location beneath the globus pallidus and the posterior limb of the internal capsule dorsally. Beneath the tract is the uncus. On the right part of the image, the pararnesial aspect of the occipital lobe shows the cuneus, triangular gyrus between the parietooccipital sulcus, and the calcarine sulcus; inferior to the latter is the lingual gyrus, which is continuous anteriorly with the parahippocampal gyrus. A, 29 = corpus callosum; 57 = cerebellar hemisphere; 87 = dentate nucleus of cerebellum; 98 = optic radiation; 109 = calcarine sulcus; 110 = collateral sulcus; 124 = internal cerebral veins; 125 = lateral ventricle; 130 = vermis. 6,9 = calcar avis; 57 = cerebellar hemisphere; 87 = dentate nucleus of cerebellum; 98 = optic radiation; 109 = calcarine sulcus; 110 = collateral sulcus; 125 = lateral ventricle; 130 = vermis. C,34 = cuneus; 52 = lingual gyrus; 57 = cerebellar hemisphere; 67 = occipital lobe; 98 = optic radiation; 109 = calcarine sulcus; 110 = collateral sulcus; 126 = posterior horn of lateral ventricle; 130 = vermis. D,57 = cerebellar hemisphere; 67 = occipital lobe. €, 53 = lateral occipitotemporal gyrus; 54 = medial occipitotemporal gyrus; 58 = hippocampus; 66 = frontal lobe; 86 = caudate nucleus; 96 = putamen; 98 = optic radiation; 125 = lateral ventricle. F; 5 = posterior cerebral artery; 13 = internal capsule; 23 = anterior commissure; 33 = crus cerebri; 34 = cuneus; 52 = lingual gyrus; 57 = cerebellar hemisphere; 66 = frontal lobe; 68 = parietal lobe; 81 = trigeminal nerve; 85 = lateral nuclear group of thalamus; 86 = caudate nucleus; 90 = pulvinar; 109 = calcarine sulcus; 114 = parietooccipital sulcus; 117 = tentorium cerebelli; 120 = optic tract; 125 = lateral ventricle.

MR IMAGING ANATOMY OF THE OPTIC PATHWAYS

Figure 22. See legend on opposite page

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Figure 23. MR imaging of the optic radiations and the calcarine fissure as shown in the axial plane in an infant using STIR TPweighted sequence (inverted images). Note the relationship of the geniculo-calcarinetract with the inferior horns of the lateral ventricle and the roughly parallel orientation of the cuts with respect of the anterior-posterior extent of the radiations (arrows). The topographic organization of the optic radiations is well displayed on the coronal cuts, performed parallel to the PC-06 reference plane as shown in Figure 27.

by Demyer et allsbto include the following conditions in order of most to least severe: cyclopia, ethmocephaly, cebocephaly, median cleft lip with orbital hypotelorism (see Figs. 29-31) or premaxillary agenesis, and philtrum-premaxillary anlage with orbital hypotelorism. Intermediate forms between these characteristic facies are also common. Typical cyclopia, the most severe dysmorphism, shows a median monophthalmia (see Fig. 29) or synophthalmia (Fig. 30) or anophthalmia. But the sine qua non for the diagnosis is the presence of a single median orbit. Commonly, the face may show a proboscis, which may be single or double protruding from the glabella, just above the median eye, but might be absent. True cyclopia has complete arhinia with no nose, no nasal bones, but a lomolog of a nose: the median proboscis. Patients with a proboscis lateralis and two orbits should not be considered as cyclopian and tend to survive. The incidence of cyclopia is estimated to average 1 per 40,000 births. The degree of facial dysmorphism is strongly correlated with the severity of brain malformation because the facial and characteristically the oculo-orbital phenotype reflect the underlying central nervous system pathology. This is why ”the face predicts the brain.” Ethmocephaly is characterized by hypoteloric eyes located in two separate orbits and a median proboscis between the two eyes as a rudimentary nose, which may be displaced upward. The median facial bones are hypoplastic or absent as in cyclopia. This type may be considered as a transi-

tional form between cyclopia and cebocephaly and seems to be the rarest type (Fig. 31). Cebocephaly, defined by Saint-Hilaire, designates a facies characterized by an orbital hypotelorism with two separate eyes into separate orbits and a nose with a single nostril. Its single median canal ends blindly. Cebocephaly has an incidence of about 1 per 15,000 births. Considering the prognosis, patients with cyclopia and ethmocephaly do not survive the neonatal period in most cases. Cebocephalic patients may survive for days, weeks, or several months. Ultrasonography may be used to diagnose the malformation as early as the 19th week of gestation. Concerning the brain malformation in holoprosencephaly, one may distinguish three main categories: (1) the alobar holoprosencephaly corresponding to an abnormally small forebrain vesicle with absence of cleavage in two cerebral hemispheres; (2) the semilobar holoprosencephaly, which is an intermediate form between the alobar and the lobar types, showing a single ventricle with rudimentary lobes and an incomplete interhemispheric fissure; and ( 3 ) the lobar holoprosencephaly, which includes a distinct interhemispheric fissure, with a possible midline cortical continuity mainly in the frontal lobes, and a communication of the lateral ventricles due to the absence of septum pellucidum. Embryologically, holoprosencephaly is due to a failure of cleavage of the prosencephalon into cerebral and optic vesicles. The associated Text continued on page 34

MR IMAGING ANATOMY OF THE OPTIC PATHWAYS

Figure 24. Vascular relationshipsof the visual pathways. A, The ophthalmic artery (arrow) underlying the intracanalicular optic nerve. 13,The proximal segment of the anterior cerebral arteries (arrow) encircling the superior aspect of the cisternal optic nerve. C and 0,The supra-clinoid parts of the internal carotid arteries and their bifurcations (arrow) lateral to the chiasm in the chiasmal cistern. Illustrationcontinued on following page

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Figure 24 (Continued). f-I, The proximal segment of the posterior cerebral arteries (arrow) from their origin at the tip of the basilar artery, extending along their perirnesencephalic route in close contact with the optic tract.

Figure 25. A and 6,Sagittal MR cuts passing through the level of the pericrural tract and the lateral geniculate body, in a T1-weighted IR pulse sequence. The pericrural optic tract is better outlined on IR pulse sequences than on SPGR or even SE pulse sequences and well depicted at the level of the uncus (A, arrowhead) and the hippocampus (B, arrowhead).

MR IMAGING ANATOMY OF THE OM’IC PATHWAYS

Figure 26. Coronal MR cuts in the STIR T2-weighted sequence (A and B), 5 mm thick and inversion recovery T1-weighted sequence (C-€),3 mm thick, showing the best contrast between the cisternal optic path (arrowheads)and the adjacent CSF. Morphology and subtle pathology of the pre-geniculate intracranial optic pathways are best outlined on such high contrast sequences. C,Chiasm. D,Origin of optic tracts. E and F; cisternal optic tracts.

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Figure 27. Coronal MR STIR T2-weighted (5 mm thickness) MR images obtained in an infant, parallel to the PC-OB reference line, and showing the shape and topography of the optic radiations (arrows) from their origin from the lateral geniculate body found in the referential cut (A) to its terminal projection on the calcarine fissure (€ and F). See the close relationship of the temporal component to the temporal horn of the lateral ventricle (A) and the progressive verticalization of the radiations from front to back in relation to the inferior part of the atria of the lateral ventricles (B),then the occipital horns (C and 0). Note the lateral displacement of the optic radiations at the level of the splenium of the corpus callosum, separated from the lateral wall of the posterior ventricular horn by the interposed tapetum (C and 0). On the most posterior cuts, the radiations are shown to roughly encircle characteristically the deepest part of the calcarine fissure (15and F).

Figure 28. See legend and continuation of illustration on following page

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Figure 28. MR coronal cuts illustrating some characteristic pathologic conditions involving the anterior optic pathways. A, Normal coronal STIR T2-weighted cut of the intraorbital optic nerves (arrow) and sheaths (bright signal of the CSF in the perioptic subarachnoid space; double arrows). B, Coronal T1-weighted spin-echo successive cuts showing an asymmetrical enlargement of the left anterior pathways involving the intra-orbital (Bl), intracanalicular (B2), and cisternal (a3 and 54)segments of the left optic path in a patient presenting an optic glioma. C and D, Optic nerve glioma in another patient explored in the sagittal oblique plane according to the optic nerve axis, showing on a STIR sequence, the intracranial extension of the glioma into the cranial cavity, involving the anterior part of the cisternal optic nerve (C). D,Same case in the coronal plane, using spin-echo T1-weighted with Gd infusion, showing the enhancement of the enlarged left cisternal optic nerve (arrow) when compared with the normal right optic nerve (asterisk). E and F; Spin-echo T2-weighted coronal oblique cuts perpendicular to the optic canal long axis, showing the enlarged gliomatous cisternal optic nerve (arrow) very well disclosed beneath the fronto-orbital gyri and medial to the anterior clinoid process. G, FLAIR sequence showing the bright signal of the chiasm in the chiasmal cistern, caused by a chiasmal neuritis (double arrows), very nicely depicted in a patient with multiple sclerosis and showing other disseminated demyelinating lesions (arrows). H, Compression of the chiasm (arrow) by a small meningioma of the jugum sphenoidale (double arrows) well demonstrated after constrast infusion. l and J, Extrinsic compression of the chiasm (asferisk) by pituitary adenomas with suprasellar extension (double arrows), explored in the coronal plane using spin-echo T1-weighted (I) and T2-weighted (J) pulse sequences in different patients.

Figure 29. Cyclopia. The face shows some typical clinical features: a single median aperture (hidden orbit), no nose but no proboscis, no philtrum, and a mouth. MR disclosed in the sagittal (A), coronal (B),and axial (Cand D)planes, the single bony orbit, the optic canal and the median well-developed eye, the lens and optic nerve, and the cerebral malformations (alobar holoprosencephaly), with the holotelencephalon tilted forward (see classification in Demyer18a). The anatomic specimen courtesy of the Laboratoire d’Anatomie of the Museum National d’Histoire Naturelle, Paris. (Courtesy of J. Reperant, MD, and R. Saban, PhD), and the MR imaging was performed in the Neuroradiology Department (Courtesy of E.A Cabanis, MD, PhD), the Quinze-Vingts hospital, Paris. (From Tamraz J: Morphometric encephalique par resonance magnetique: Applications a la pathologie chromosomique hurnaine, a I’anatornie compare@et a la teratologie (thesis). Universite Rene Descartes, Paris, 1991; with permission.)

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Figure 30.Cyclopia. The face shows a single median orbit containing an eye with probably a partially doubled cornea and two lenses (synophthalmia), a median proboscis is shown protruding from the glabella above the eye, and a mouth but no philtrum. MR imaging evidenced the single bony orbit, the synophthalmia and the two lenses (B), as well as the lack of nasal bones. The median proboscis shows a single outer opening and a dead-end canal (A). The anatomic specimen was provided by the Laboratoire d’Anatomie of the Museum National dHistoire Naturelle, Paris (Courtesy of J. Reperant, MD, and R. Saban, PhD) and the MR imaging was performed in the Neuroradiology Department. (Courtesy of E.A Cabanis, MD, PhD) of the Quinze-Vingtshospital, Paris. (From Tamraz J: Morphometric encephalique par resonance magnetique: Applications a la pathologie chromosomique humaine, a I’anatomie comparee et a la teratologie (thesis). Universite Rene Descartes, Paris, 1991; with permission.)

MR IMAGING ANATOMY OF THE OPTIC PATHWAYS

Figure 31. Monstrous fetus (Janiceps). The face (A) shows the ethmocephalic facies type with two separate .palpebral fissures and hypoteloric orbits and a median proboscis between the two eyes. MR shows the median proboscis (0) and absence of the median nasal bones (0)and the incompletely separated orbits as well as both eyes and optic nerves in the coronal plane (6) and the axial NOP cut (C and F; arrow). The face (6) shows a hypotelorism with a median cleft lip (absence of the medial third of the upper lip) and a flat nasal bridge with nasal alae. MR shows the hypoteloric eyes and orbits and the ethmoido-nasal bones (A and F; double arrows.) See the arrangement of both couple of orbits and both posterior fossa, facing each other in the axial plane passing through the orbits (F). See the corresponding frontal cut through both posterior fossa showing both brainstem somehow parallel and symmetrically oriented on each part of the (C)common midsagittal plane (E). Illustration continued on following page

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Figure 31. See legend on preceding page

malformations of the brain and face, in humans and animals, may result from a primary defect in the prechordal mesoderm, which fails to produce the normal facial structures (the frontonasal prominence from which derives the median sector of the face) and causes abnormal brain development.18a Numerous reports of holoprosencephaly associated with either teratogenic agents or genetic aberrations are found in the literature, in addition to sporadic cases without any identifiable cause. Since the first reports concerning trisomy of a Dgroup chromosome and deletion of the short arm of an E-group chromosome, most of the available reports about phenotype-karyotype correlation concern trisomy 13 and monosomy 18p. To these may be added duplication of the distal short arm of chromosome 3 and deletion of the distal long arm of chromosome 7, among other rare syndromes. CONCLUSION

The optic pathways are roughly axial and symmetric, beginning at the retina and extending from anterior to posterior to end in the visual cortex of

the occipital lobe. For purposes of reproducibility it is important to choose an accurate reference plane and examination algorithm. The choice of the cephalic reference to adopt and the orientation of the slices to use helps to respond to three main objectives. First, the cuts should be accurately oriented in order optimally to disclose the morbid process, either intrinsic or extrinsic with respect to the visual path. Second, the cephalic reference plane used should allow a reliable and reproducible follow-up of the lesion if necessary in order to evaluate its natural history or conversely to appreciate the effect of therapy. Finally, the imaging technique ought to be scrupulously adapted to obtain the best anatomic result (free of eye movements and flow artifacts, as much as possible) and the most accurate contrast, which are necessary to depict even the subtle abnormality in or around the optic pathways. ACKNOWLEDGMENTS We would like to dedicate this work to our colleagues of the department of neuroradiology at the Centre Hospi-

MR IMAGING ANATOMY OF THE OPTIC PATHWAYS talier National d’Ophthalmologie des Quinze-Vingts, Professor E. A. Cabanis, head of the department, Drs. A. Abanou, R. Benrabah, D. Bleynie, R. Cavezian, M. T. IbaZ i z h , A. Lopez, A. Majdalani, C. Outin, J.L. Stievenart, C. Stoffels and M. Thibierge, members of the staff. We wish to express our warmest gratitude for their teaching and confidence, to all heads of ophthalmological department of our Institution: Prs D. Godde-Jolly, H. Hamard, J. Haut, S. Limon and M. Massin. We are sincerely grateful to Pr. R. Saban and Pr. J. RepCrant, head of the laboratory of comparative anatomy at the Museum National d’Histoire Naturelle who gave us the opportunity to work on the formalin brains and the historical specimen of the Museum in Paris. Finally, we are indebted to the A.R.S.E.P. (M.S. Research Association) and to General Electric-CGR (European Research Center, buc) and thank Mrs. S. Favino for her secretarial assistance.

References 1. Arrington GE: A History of Ophthalmology. New York, MD Publications, 1959 2. Broca P: Sur le plan horizontal de la tete et sur la methode trigonometrique. Bull SOCAnthropol 8:4% 96, 1873 3. Cabanis EA, Haut J, Iba-Zizen MT Exophtalmometrie tomodensitometrique et biometrie TDM oculoorbitaire. Bull SOCOphtalmol 80:6346, 1980 4. Cabanis EA, Iba-Zizen MT, Coin JL: Les voies visuelles u n “nouveau” plan d’orientation de la tete (Plan Neuro-Oculaire). Bull SOCOphtalmol 81:433439, 1981 5. Cabanis EA, Iba-Zizen MT, Pineau H, et a 1 Le plan neuro-oculaire (PNO) en tomodensitometrie: Determination d‘un “nouveau“ plan horizontal de reference cephalique, oriente selon les voies visuelles. Biometrie Humaine 172148,1982 6. Cabanis EA, Iba-Zizen MT, Tamraz J, Stoffels C: IRM de la tete et du cou orientee selon le plan neuro-oculaire (PNO): Une condition d’efficacite anatomique. J Bioph Med Nucl8:48-50, 1984 7. Cabanis EA, Iba-Zizen MT, Tamraz J, Muzac S Topometric oculo-orbitaire: Aspect dynamique normal et pathologique. Bull SOC Ophtalmol :31-56, 1982 8. Cabanis EA, Laugier A, Iba-Zizen MT, et al: Anatomie de la tete en imagerie par resonance magnetique. Le Concours Medical 34:3191-3197, 1985 9. Cabanis EA, Pineau H, Iba-Zizen MT, et al: CT scanning in the ”neuro-ocular plane”: The optic pathways as a ”new” cephalic plane. Neuro-Ophthalmology 1:237-251, 1981 10. Cabanis EA, Salvolini U, Rodallec A, et al: Computed tomography of the optic nerve: Part 11. Size and shape modifications in papilledema. J Comput Assist Tomogr 2:150--155,1978 11. Cabanis EA, Tamraz J, Iba-Zizen MT Correlations anatomiques normales dans 3 dimensions selon l’orientation du plan neuro-oculaire a 0.5 Tesla. In Cabanis EA, Doyon D, et a1 (eds): Atlas d’IRM de l’Encephale et de la Moelle. Paris, Masson, 1988, pp 11-120 12. Cabanis EA, Tamraz J, Iba-Zizen MT Imagerie par resonance magnetique (IRM)de la tete a 0.5 Tesla. Atlas de correlations anatomiques normales dans 3 dimensions, selon l’orientation du plan neurooculaire (PNO). Feuillets de Radiologie 26:308-416, 1986

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