The orbital contents

The sublingual gland (Fig. 1. 21) This small gland lies submucosally just anterior to the deep lobe of the submandibular gland and drains via several ducts (up to 20) directly into the floor of the mouth posterior to the opening of the submandibular gland. Some of its ducts may unite and join the submandibular gland.

Radiology of the salivary glands

Sialography (Fig. 1. 24) The ducts of the parotid and submandibular glands may be cannulated and injected w i th radio-opaque contrast to outline the ductal system. The ducts of the parotid gland arch around the mandible because of the way in which the gland is moulded to the adjacent structures. This is best seen on the AP view. The parotid duct is seen on the lateral view. The submandibular gland and duct system may be seen on the lateral projection. The ducts of the sublingual gland are not amenable to canalization.

Cross-sectional imaging CT (see Figs 1. 23, 1. 35 and 1. 42) and MRI are of particular value for tumours of the glands, to assess involvement of surrounding structures. CT may be performed after sialography to improve visualization of the ducts.

High-resolution MR images may actually demonstrate the facial nerve w i t h in the parotid gland. It is of slightly lower intensity than the surrounding gland on T1-weighted images.

Ultrasound

This may be performed through the skin or intraorally w i th high-frequency transducers.

Nuclear imaging Because the salivary gland accumulates and secretes technetium-99m (99mTc) used in nuclear imaging, this can be used to image several glands at once without cannulating the ducts. Graphs of uptake and excretion of the agent by individual glands may be computed.

THE ORBITAL CONTENTS (Fig. 1. 25) The orbit contains the lacrimal gland, the globe, the extraocular muscles (including levator palpebrae), the optic nerve and the ophthalmic vessels. The whole is embedded in fat. The orbit is limited anteriorly by the orbital septum. This is a thin layer of fascia that extends from the orbital rim to the superior and inferior tarsal plates, separating the orbital contents from the eyelids. A fascial layer, the periorbita, lines the bony cavity of the orbit and this is continuous w i th the dura mater of the brain through the superior orbital fissure and optic canal.

The globe of the eye is composed of a transparent anterior part covered by the cornea, and an opaque posterior part covered by the sclera. These are joined at the corneoscleral junction, known as the limbus. The anterior and posterior extremities of the globe are known as the anterior and posterior poles. The midcoronal plane of the globe is the equator. A further layer of fascia, Tenon's capsule, covers the sclera from the limbus to the exit of the optic nerve from the eye. This facial layer fuses w i th the fascia of the extrinsic ocular muscles at their insertions. Anteriorly, a mucous membrane known as the conjunctiva covers the anterior aspect of the eye. It is reflected from the inner surface of the eyelids and fuses w i th the limbus.

There are six extrinsic ocular muscles that insert into the sclera. The four rectus muscles, the superior, inferior, medial and lateral recti, arise from a common tendinous ring called the annulus of Zinn. This is attached to the lower border of the superior orbital fissure. These muscles insert into the corresponding aspects of the globe, anterior to its equator. The superior oblique arises from the sphenoid bone superomedial to the optic foramen. It passes through a tendinous ring, the trochlea, which is attached to the frontal bone in the superolateral part of the orbit, acting as a pulley. It then passes posteriorly to insert into the upper outer surface of the globe, posterior to the equator.

The inferior oblique arises from the anterior part of the floor of the orbit and inserts into the lower outer part of the globe, behind the equator.

The levator palpebrae superioris is also within the anterior fascial limit of the orbit, arising from the inferior surface of the lesser wing of sphenoid (see Fig. 1. 10) and inserting into the tarsal plate of the upper eyelid behind the orbital septum.

The arterial supply of the orbit is from the ophthalmic artery. This enters the orbit through the optic canal and gives off the central retinal artery, which runs in the optic nerve into the back of the eye. It supplies the orbital contents and its anterior branches anastomose w i th branches of the external carotid in the eyelids.

The venous drainage of the orbit is through the superior and inferior ophthalmic veins into the cavernous sinus. The superior ophthalmic vein is formed by the union of the angular vein and the supraorbital vein at the supero¬ medial angle of the orbit. These veins drain the periorbital skin and thus provide a possible pathway for infection, causing potentially lethal cavernous sinus thrombosis. The superior ophthalmic vein runs posterolaterally, then medially, and drains to the cavernous sinus via the superior orbital fissure. The inferior ophthalmic vein passes posteriorly and may join the cavernous sinus alone or w i th the superior vein.

The optic nerve is a direct extension of the brain. It is myelinated and has external coverings of dura, arachnoid and pia, forming its own subarachnoid space continuous w i th that of the brain. It is 4 mm thick and has four parts. The intraocular part begins at the optic disc. The intraorbital part runs posteriorly within the muscle cone formed by the four recti and is lax to allow movement of the globe. The intracanalicular part lies in the narrow optic canal with the ophthalmic artery, and the intracranial part is between the intracranial opening of the optic canal and the optic chiasm.

Internal anatomy and coverings of the eye (Fig. 1. 26) The globe of the eye is composed of three layers. The outermost consists of the tough white sclera posteriorly and the transparent cornea anteriorly. The junction of the sclera and cornea is called the limbus.

The middle layer is a vascular layer known as the uveal tract. It consists of choroid posteriorly, and the ciliary body and iris anteriorly. The ciliary body is a fibrous ring continuous w i th both the choroid and iris, and gives rise to the ciliary muscle, which alters the shape of the lens of the eye, allowing accommodation.

The innermost layer is the retina, which contains the rods and cones. The retina ends anteriorly a short distance behind the ciliary body, and its anterior limit is known as the ora serrata. Posteriorly, nerve fibres converge to form the optic nerve at the optic disc. The nerve pierces both the choroid and sclera as it passes posteriorly The sclera is continuous w i th the dural covering of the optic nerve. The macula, which has the greatest concentration of cones and is responsible for central vision, lies temporal to the optic disc.

The anterior segment of the eye is that part anterior to the lens. It is divided into two chambers. The anterior chamber is between the cornea and iris, and the posterior chamber is between the iris and lens. The two chambers are filled w i th aqueous humour and are continuous through the aperture of the iris (the pupil).

The posterior segment is behind the lens and is filled w i th a gelatinous fluid known as the vitreous body. The outer part of the vitreous is condensed to form the so-called vitreous or hyaline membrane. A potential space exists between the vitreous and the retina known as the subhyaloid space. In pathological conditions fluid or blood may accumulate in this space.

Radiology of the orbit and eye

(Figs 1. 27 and 1. 28)

Plain films The orbital margins may be assessed by plain radiography and are well seen on OF20, OM and OM30 views of the facial bones. The floor of the orbit is undulating and not well defined. Lateral radiography of the anterior part of the eye may be performed on small dental films using a low exposure, and demonstrates the cornea and eyelids. CT has replaced tomography and may be required to assess the floor of the orbit for trauma.

Ultrasound Ultrasound of the eye using high-frequency transducers (5-20 MHz) can demonstrate its internal anatomy (Fig. 1. 27). The higher-frequency transducer visualizes the anterior segment and the lower-frequency transducers (5-10 MHz) image the posterior segment. Scans may be performed

1. Nasal septum 10. Optic canal 2. Nasal bone 11. Anterior clinoid process 3. Ethmoid air cells (pneumatized) 4. Globe of left eye 12. Superior orbital fissure 5. Sclera 13. Middle cranial fossa 6. Optic nerve 14. Greater wing of sphenoid 7. Medial rectus muscle 15. Frontal process of zygomatic bone 8. Lateral rectus muscle 16. Temporal fossa/temporalis muscle 9. Superior ophthalmic vein

Fig. 1. 27 Ultrasound of eye

(a) transverse image showing posterior structures

B

(b) longitudinal image showing anterior structures

1. Vitreous body 4. Retrobulbar fat 2. Retinal surface 5. Lateral rectus muscle 3. Optic nerve 6. Lateral wall of bony orbit

1. Anterior chamber 4. Posterior aspect of lens 2. Iris 5. Vitreous body 3. Anterior aspect of lens

in any plane, but are usually obtained in transverse (axial) and longitudinal (sagittal) planes. The aqueous and vitreous chambers are anechoic spaces. The cornea and lens are echogenic and easily defined. The inner walls of the eye the choroid, retina and sclera - are not distinguishable from each other and are seen as a line of low-amplitude echoes. The retrobulbar fat is also echogenic, and the extraocular muscles and optic nerve appear as echo-free structures w i t h in it.

Computed tomography CT is an excellent modality for demonstrating the extraocular contents of the orbit (Figs 1. 28 and 1. 12). The lacrimal gland, extraocular muscles, globe, optic nerve and superior ophthalmic vein are routinely seen on sections obtained at 4 mm intervals. The lens has a low water content and is dense on CT. The bony walls of the orbit are demonstrated, and the foramina of the orbit and related anatomy are readily assessed. Coronal images are best for assessment of the orbital floor, especially in trauma.

Magnetic resonance imaging MRI demonstrates the soft tissues of the orbit. It may be performed in any plane. It is of particular value in demonstrating the optic nerve, allowing excellent visualization of the entire nerve, including the intracanalicular segment on vertical oblique images along the nerve's long axis. On coronal images the third, fourth and sixth nerves and the first division of the fifth can be seen just below the anterior clinoid process. Much of the internal anatomy of the eye can also be distinguished, as can the orbital septum, extraocular muscles, nerves and vessels. Images of the intraorbital part of the optic nerve are performed w i th fat saturation pulses to help distinguish the optic nerve and its sleeve of dura and CSF from the surrounding highsignal fat.

The lacrimal apparatus (Figs 1. 29 and 1. 30) This consists of the lacrimal gland, which lies in the superolateral part of the orbit and produces the tears, and the lacrimal canaliculi, lacrimal sac and nasolacrimal duct, which drain the secretions to the nose.

The lacrimal gland lies lateral to the levator palpebrae. This muscle grooves the gland, dividing it into an almondsized orbital lobe posteriorly and a smaller palpebral lobe, which extends anteriorly under the lateral part of the upper eyelid. The orbital lobe lies in a bony depression called the lacrimal fossa. The gland secretes tears into the space between the upper eyelid and eye (the upper fornix) through several small ducts.

On the medial margins of each eyelid are openings known as the lacrimal puncta. Tears drain through these openings into the superior and inferior lacrimal canaliculi.

1. Lacrimal gland 2. Ethmoid infundibulum draining to middle meatus 3. Upper end of right nasolacrimal duct 4. Inferior turbinate 5. Crista galli 6. Middle turbinate 7. Inferior part of left nasolacrimal duct 8. Opening of nasolacrimal duct