Ординатура / Офтальмология / Английские материалы / Imaging of Orbital and Visual Pathway Pathology_Muller-Forell_2005

34

a

W. Wichmann and W. Müller-Forell

11.1

1.2

tongue

b

2.25

11.2

pterygomaxillary fissure

2.2

Optic Nerve Sheath and Fasciae

(Figs. 2.3, 2.4, 2.11, 2.17, 2.25–2.27)

The orbit is lined by a periosteal layer, the socalled periorbita, which is loosely adherent to the bone. Anteriorly, the periorbita blends with the periosteum of the orbital margin and to the orbital septum, while in the posterior region it fuses with the dural sheath of the optic nerve to form a unique sheet, the dura of the optic canal, which is tightly adherent to the bone. The orbital septum (Fig. 2.15b) is a thin fibrous membrane, becoming thicker laterally, with attachment to, although not exactly following, the orbital margin. The septum, directly in contact with the orbital fat and separating it from the lacrimal glands, borders the orbits anteriorly. The orbital septum separates the extraorbital preseptal space from the postseptal orbital space. In this way, the orbital septum represents an important neuroradiological landmark which indicates whether a superficial lesion involves the orbit or not (Hoffmann et al. 1998).

The globe is enveloped by a thin membranous socket, termed Tenon’s capsule, that separates it from the retro-orbital fat. Tenon’s capsule is perforated posterolaterally, in order to allow the passage of the optic nerve, the ciliary vessels and nerves, as well as the tendons of the four recti muscles. The fibers of the capsule fuse with the optic sheath and the tubular muscular fasciae. Directly posterior to Tenon’s capsule, the muscular sheath of the four recti muscles form a short fascial ring, called the intermuscular septum, best seen on coronal view (Figs. 2.27, 2.28). This ring encloses the anterior aspect of the so-called intraconal space, and the more posterior region of the intraconal space is defined incompletely by the four rectus muscles.

The subdivision of the orbital space into four compartments is a reliable, proven help in the differential diagnosis of the various space-occupying,

vascular, inflammatory or infiltrative lesions that may involve the orbit. The four rectus muscles and their fasciae form an incomplete cone including the muscles themselves. Using this cone, the retroorbital space can be subdivided into an intraconal and extraconal space; pathologic processes of the nasal and paranasal area, the temporal fossa, and the intracranial region may invade the latter because of the close neighborhood (Fig. 2.29) (chapters 2.4, 6.2, 6.3). To facilitate the differential diagnosis of retro-orbital lesions, the first step of image analysis should attribute the discovered lesion to one of these spaces. The other orbital compartments are the globe (chapters 2.3, 6.1) and the optic nerve (chapters 2.5, 6.4).

2.3 Globe

Although CT is able to differentiate the lens and its ciliary bodies from the fluid of the anterior chamber and of the vitreous, respectively, MRI is able to acquire high resolution images of the cornea, the fluid of the anterior and posterior chambers, the iris, the lens with the lens capsule, the vitreous body, and components of the uveal tract (Breslau et al. 1995). After i.v. administration of gadolinium, the vascular tunic (choroid, ciliary body, and iris) and in particular the macula are depicted as a strongly contrast-enhancing structure (Figs. 2.16, 2.17). The different histological composition of these tissues, especially their different water content, explains the biophysical characteristics responsible for the soft-tissue contrast on MRI. Although there is analogous contrast enhancement of the vascular tunic on CT after i.v. application of iodinate contrast medium, in contrast to MRI it cannot be differentiated exactly from the sclera.

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b

|

Anatomy

9.7

10.11

10.143.2

14.2

13.1

c

e

a

c

10.15

3.2

14.2 13.12

13.1 3.4

41

d

Fig. 2.17c–e

b

Fig. 2.18.a Axial T1-weighted native MRI of the superior orbital region. b Corresponding contrast-enhanced (FS) view. c Corresponding diagram: 7.2 = frontal sinus, 10.3 = superior rectus muscle, 10.5 = superior oblique muscle, 10.8 = trochlea of superior oblique muscle, 10.13 = superior ophthalmic vein, 10.15 = lacrimal gland, 12.2 = chiasm, 13.1 = temporal lobe, 13.2 = frontal lobe, 13.12 = pituitary stalk, 14.9 = middle cerebral artery (MCA)