A orbits:

Part A orbits:

Introduction

Orbit is a paired pyramidal cavity in the facial skeleton, between the maxillary sinus inferiorly, the frontal sinus superiorly and ethmoid and sphenoid sinuses medially. The apex opens into the cranial cavity via the fissures and optic canal. It contains the eyeball, optic nerve, extraocular muscles, lacrimal apparatus and neurovascular structures. It is aptly called the “window to the brain”, as patients with neurological disorders often present to the ophthalmologist!

Imaging of the orbit involves the study of the soft tissues, viz., the eyeball, optic nerve, lacrimal glands and extraocular muscles which are best imaged by magnetic resonance imaging (MRI) and bony orbit best studied using computed tomography (CT) with 3D/volume rendering imaging.

The study of the imaging anatomy is an essential part of orbit imaging, and therefore, radiologists should be familiar with the important imaging landmarks and their normal CT and MRI appearance.

Soft-tissue anatomy

Eyeball (globe):
- • The globe occupies one-third of the orbital volume. It is divided into the anterior and posterior segments by the lens. The globe comprises the ocular coats, aqueous and vitreous humor, lens, iris and ciliary body. The various structures of the globe are best imaged using surface coil high-resolution MRI.

The ocular coats

• The ocular coats consist of outer fibrous (collagen–elastic) layer comprising the sclera and cornea, middle vascular layer uvea (choroid, ciliary body and iris) and inner sensory layer, retina. These layers are not identified separately in CT, but are well identified on MRI due to their differences in signal characteristics.
• The sclera and cornea are seen as a low signal rim in both T1 and T2 WI and better defined in T2 WI against the bright intraocular fluid and orbital fat.
• The sclera is surrounded by a fascial sheath, the Tenon’s capsule. It separates the eyeball from the orbital fat. It is pierced by the tendons of the extraocular muscles before reaching the sclera and optic nerve and sheath. The space between the Tenon’s capsule and sclera is the episcleral space. It cannot be identified on routine MRI.
• The chorioretinal layer in which the normal retina is 0.1 mm cannot be differentiated from the highly vascularized choroids (0.1–0.2 mm thick) on routine scans of 2–3 mm slice thickness and therefore identified as a single layer in T1 WI displaying intermediate signal against the dark vitreous.
• The ciliary body attaches to the lens via the zonular fibres and appears hyperintense on T1-weighted images and hypointense in T2-weighted images.
• The aqueous humour is plasma-like fluid in the eye that nourishes the eye and is seen anterior to the lens. The vitreous humour is gel-like fluid seen between the lens and the retina. It is 99% water and therefore is hypointense on T1-weighted images and hyperintense on T2-weighted images.
Blood supply to the eye:
- Arterial supply is from the central retinal artery, the short and long posterior ciliary arteries and the anterior ciliary arteries.
- The venous drainage is into the vortex vein, central retinal vein into the superior and inferior ophthalmic veins into the cavernous sinus.

Extraocular muscles (Table 3.25.1)

The four rectus and two oblique muscles are responsible for the ocular movements. The four rectus muscles, viz., the superior, inferior, medial and lateral originate from the annulus of Zinn at the apex and insert into the sclera just behind the limbus.

TABLE 3.25.1

Normal Orbital Measurements in Adults

Globe (mm)	Axial Length	22–25 mm
Extraocular muscles thickness (mm)	Inferior rectus	3.6–4 0.0 mm
	Medial rectus	3.8–4.5 mm
	Lateral rectus	3.0–3.5 mm
	Superior rectus	3.7–4.0 mm
Optic nerve sheath complex		4 – 5 mm
Orbit dimensions- transverse		36–37 mm
Vertical		37–38 mm
Orbit volume		30 cc

The inferior oblique muscle originates from the maxillary bone just behind the rim, posterolateral to the lacrimal fossa, and inserts in the posterolateral surface of the globe.

The superior oblique muscle originates from the periosteum of the body of the sphenoid, above the annulus, and courses along the superomedial wall; the tendon then passes through a fibrocartilaginous ring, trochlea which acts as pulley and then reflects posterior, inferior and lateral and passes beneath the superior rectus tendon and inserts in the superolateral aspect of the globe.

The extraocular muscles show intermediate signal in T1- and hypointense T2-weighted images with respect to the orbital fat.

All the muscles except the lateral rectus and superior oblique are supplied by the third(oculomotor) cranial nerve. The superior oblique is supplied by the fourth(trochlea) nerve and lateral rectus by the sixth (abducens) nerve.

The levator palpebrae superioris is not involved in ocular motility but involved in lid retraction. It originates from periorbita, above the annulus of Zinn, and inserts in the tarsal plate.

The blood supply of the extraocular muscles is from the muscular branches of the ophthalmic artery.

Connective tissue system

• The orbital septum is a connective tissue that attaches to the periosteum of the orbital rim, posterior lacrimal crest (medially) and centrally fuses with the lid margins. It is better delineated on MRI as a linear hypointense structure in T2 WI.
• The tarsal plate is better appreciated in sagittal images and appears T1 hyperintense due to its lipid content.
• The extraocular muscles are linked to each other by the intermuscular septum. It is best seen on coronal images and has similar signal as the extraocular muscles. It divides the orbit into intraconal and extraconal space.

Nerves

• The motor nerves, viz., the superior and inferior branch of the oculomotor nerve, abducens nerve and thin trochlear nerve are identified in the coronal images.
• The sensory nerves, viz., branches of the ophthalmic division of the trigeminal nerve (frontal, lacrimal and nasociliary) are identified on MR images.
• The frontal nerve and its branches are identified on axial and coronal images superior to the levator palpebrae superioris.
• The lacrimal nerve is best seen in coronal sections.
• The infraorbital nerve is seen in the infraorbital canal.

The optic nerve

The optic nerve can be divided into four parts:

1. Intraocular optic nerve: It is approximately 1 mm in length.
2. The intraorbital optic nerve has an S-shaped course from the optic canal to the globe. The normal mean pial diameter of the intraorbital segment of the optic nerve measures between 3.2 mm (anterior) and 2.6 mm (posterior); the mean dural diameter measures 5.2 mm (anterior) and 3.9 mm (posterior).
3. The intracanalicular optic nerve is 5 mm long. The ophthalmic artery is below the optic nerve within the canal.
4. The intracranial optic nerve is 1 cm in length and is surrounded by CSF.

Lacrimal system

The lacrimal gland is an almond-shaped structure superolateral to the globe. It is divided into the orbital and palpebral lobes by the LPS aponeurosis. The orbital lobe lies deep to the orbital septum in the lacrimal fossa. It shows isointense signal in T1-weighted images and hypointense signal in T2-weighted images with respect to orbital fat. It is composed of both epithelial and lymphoid elements similar to the salivary glands.

Lacrimal apparatus

Lacrimal apparatus is responsible for the drainage of the lacrimal fluid (tears) into the nasal cavity. It consist of the superior, inferior, common canaliculi, lacrimal sac and nasolacrimal duct.

The upper and the lower canaliculi run along the medial margin of the respective eyelids and measure 6–10 mm on length. They open into the common canaliculus. The common canaliculus is 2–5 mm long and enters the lacrimal sac via a small recess in the lateral wall of the lacrimal sac, known as the sinus of Maier.

At the opening of the common canaliculus into the lateral wall of the lacrimal sac, two small mucous membrane folds are seen, which have valvular functions. The upper fold, the valve of Rosenmuller, and the lower fold, the valve of Huschke, are responsible for mucocoele formations when there is impaired drainage into the nasolacrimal duct causing the sac pressure to increase and closure of these valves.

The lacrimal sac is situated in the lacrimal fossa between the anterior and posterior lacrimal crest and is the dilated superior end of the nasolacrimal duct. The lacrimal sac opens into the nasolacrimal canal. There is a small constriction at the lower end of the sac caused by a mucous membrane fold called the valve of Krause.

The nasolacrimal duct is an extension of the lacrimal sac and runs vertically downwards in the bony nasolacrimal canal located in the lateral wall of the nasal cavity and open in the inferior meatus. The valve of Hasner, located at the end part of the nasolacrimal duct, acts as a functional barrier for the backward tear outflow.

The lacrimal sac and nasolacrimal duct are best seen on T2-weighted images when fluid filled. The lacrimal canaliculi are visualized on high-resolution surface coil MRI.

Bony anatomy

The orbit is a conical cavity which has a base, an apex and four walls. The base opens in the face and has four borders.

The following bones constitute the formation of the orbital rim:

1. Superior rim – frontal bone
2. Inferior rim – maxilla and zygoma
3. Medial rim – lacrimal, frontal and maxilla
4. Lateral rim – zygoma and frontal bone
- • The apex lies near the medial end of superior orbital fissure (SOF) and contains the optic canal which opens into the suprasellar cistern.
- • The roof (superior wall) is formed by the orbital plate frontal bone and the lesser wing of sphenoid. The orbital surface has trochlear fovea medially and lacrimal fossa laterally.
- • The floor (inferior wall) is formed by the orbital plate of maxilla, orbital process of the zygomatic and palatine bones. It has a groove/canal for the infraorbital nerve which opens in the face at the infraorbital foramen, 1 cm below the rim.
- • The medial wall is formed by the frontal process of maxilla, lacrimal bone, orbital plate of ethmoid and part of the body of the sphenoid. The medial wall is thin and therefore frequently fractured during trauma, and for the same reason, infections/neoplasms from the ethmoid spread rapidly to the orbit. The anterior and posterior ethmoid foramina are located in the medial wall, transmitting the anterior and posterior ethmoid vessels and nerves, respectively.
- • The lateral wall is the thickest wall and is formed by the orbital process of zygomatic bone and the orbital plate of greater wing of sphenoid. The bones meet at the sphenozygomatic suture.

Foramen and fissures

The infraorbital foramen contains the infraorbital nerve and artery. It connects the orbit to pterygopalatine and infratemporal fossa.

The superior orbital foramen is seen as a small notch on the medial third of the superior orbital rim through which passes the supraorbital nerve and artery.

Superior orbital fissure (SOF)
- It is a comma-shaped opening between the lesser and greater wing of the sphenoid. Medial wall is formed by the optic strut which separates the optic canal from the SOF. Anteriorly, it opens into the orbit and posteriorly into the cavernous sinus and is therefore an important landmark for spread of infection and tumours into the orbit and cranial cavity. It contains cranial nerves third, fourth, sixth branches of the V1 division (lacrimal and frontal) of the Vth nerve and superior ophthalmic vein.
Inferior orbital fissure
- It is a slit-like opening between the floor and the lateral wall of the orbit. It connects the orbit to the pterygopalatine fossa and cavernous sinus. It contains the infraorbital nerve and zygomatic nerve (from the maxillary division of trigeminal nerve), inferior ophthalmic vein and several emissary veins connecting it to the pterygoid venous plexus, infraorbital artery and vein.
Optic canal
- It is a cylindrical canal traversing through the lesser wing of the sphenoid transmitting the optic nerve, ophthalmic artery and sympathetic nerves. The anterior end opens into the orbital apex, and posterior end opens into the cranial cavity/suprasellar cistern. The normal optic canal is approximately 1.5 cm long and approximately 4–5 mm wide.
Anatomical variation and types of optic canal
- The optic canal can be classified into four types based on its relation to the sphenoid sinus.
  - • Type 1: most common (76%): the optic canal is immediately adjacent to the lateral or superior wall of the sphenoidal sinus, without impression on the sinus wall.
  - • Type 2(15%) canal causes an impression on the lateral sphenoidal sinus wall.
  - • Type 3 (6%) canal courses through the sphenoidal sinus rather than simply running adjacent to the sinus.
  - • Type 4 (3%) nerve courses immediately lateral to the posterior ethmoidal and sphenoidal sinuses.

Fig. 3.25.1Axial T1 weighted image of the eye at the level of the inferior.

Fig. 3.25.2Axial T1 weighted image at the level of the inferior oblique insertion.

Fig. 3.25.3Axial T1 weighted image at the level of the inferior ophthalmic vein.

Fig. 3.25.4Axial T1 weighted image just inferior to the optic nerve.

Fig. 3.25.5Axial T1 weighted image at the level of the optic nerve.

Fig. 3.25.6Axial T1 weighted image just superior to the optic nerve.

Fig. 3.25.7Axial T1 weighted image at the level of the anterior ethmoid artery.

Fig. 3.25.8Axial T1 weighted image at the level of the trochlea.

Fig. 3.25.9Axial T1 weighted image at the level of the superior rectus.

Fig. 3.25.10Axial T1 weighted image above the superior rectus muscle.

Fig. 3.25.11Coronal T2 weighted image at the level of the orbital septum.

Fig. 3.25.12Coronal T2 weighted image at the level of the inferior oblique muscle.

Fig. 3.25.13Coronal T2 weighted image of the eye at the level of inferior oblique insertion.

Fig. 3.25.14Coronal T2 weighted image at the level of the trochlea.

Fig. 3.25.15Coronal T2 weighted image just beyond the equator.

Fig. 3.25.16Coronal T2 weighted image at the level of the optic nerve head.

Fig. 3.25.17Sagittal T2 weighted image of the eye at the level of the lacrimal gland.

Fig. 3.25.18Sagittal T2 weighted image at the level of the lateral orbit.

Fig. 3.25.19Sagittal T2 weighted at the inferior rectus muscle.

Fig. 3.25.20Sagittal T2 weighted image at the level of inferior rectus.

Fig. 3.25.21Sagittal T2 weighted image at the level of the optic nerve.

Fig. 3.25.22Sagittal T2 weighted image at the level of the medial rectus.

Fig. 3.25.23Sagittal T2 weighted image at the medial orbit.

Fig. 3.25.24Axial CT scan at the level of the maxillary sinus.

Fig. 3.25.25Axial section at the level of inferior orbit.

Congenital malformations of the eyeball and orbit

The eyeball develops from the brain as an outpouching called the optic vesicle. The malformations of the eye can therefore be associated with brain malformations, occur in isolation or be part of multisystem anomalies or syndromes.

Congenital malformations of the eye include the following:

1. Absent globe – anophthalmia
2. Small globe – microphthalmia
3. Congenital cystic eye
4. Cryptophthalmos
5. Cyclops or synophthalmia

Anophthalmia

As the name suggests, it is complete absence of the globe. It is extremely rare and may actually be an extreme microphthalmia such that there is a small remnant and therefore termed as clinical anophthalmia.
CT/MRI findings:
- • Absence of globe
- • Hypoplastic or absent optic nerve (extreme cases)
- • Hypoplastic extraocular muscles
- • Small bony orbit
- • Brain imaging – midline defects and hydrocephalus

Microphthalmia

• Microphthalmia is defined as an eye that has an axial length of less than 21 mm in an adult or less than 19 mm in a 1-year-old child.
• It can occur as an isolated disorder or with other ocular and craniofacial anomalies.
• Other causes of microphthalmia include congenital rubella syndrome, persistent hyperplastic primary vitreous and retinopathy of prematurity.
Microphthalmia is of two types:
1. Small anatomically correct eye – simple
2. Small malformed eye complex

It may be unilateral or bilateral. It may or may not be associated with colobomatous cysts. Those with colobomatous cysts result from failure of fusion of the fetal optic fissure.

Computed tomography/magnetic resonance findings

• A small globe with small bony orbit.
• Extraocular muscles and optic nerve are hypoplastic.
• Colobomatous cyst may vary from small to large almost occupying the orbit.
• The colobomatous cyst may cause a conical deformity on the posterior globe
• The cysts can be very large and at times larger than the microphthalmic eye, resulting in bony remodeling.
• Often intraocular calcification may be seen which may represent calcified lens.
- Cryptophthalmos:
- Cryptophthalmos consists of partial or complete failure of development of the eyelids, eyebrow, palpebral fissure, eyelashes and conjunctiva.
  - • Patients will have hidden eyes because the skin of the eyelids is partially or fully sealed.
  - • It may be unilateral or bilateral.
  - • It may be associated with skin-like cornea, an incompletely developed anterior segment or a rudimentary cyst-like globe.
  - • Cryptophthalmos may be associated with systemic anomalies.

Imaging – Incompletely developed anterior segment, or a rudimentary cyst-like globe.

Congenital cystic eye

Congenital cystic eye results due to failure of the optic vesicle to invaginate during the fourth week of embryogenesis. It can be associated with other nonocular abnormalities such as facial clefts, choanal atresia, malformation of the sphenoid bone, agenesis of corpus callosum and midbrain deformities.

Imaging:

• The normal globe is not visualized.
• Cystic structure not resembling the normal globe is seen in the orbit. If large, can cause remodelling of the bony orbit. A rudimentary connection to a thinned optic nerve may be seen. A nodular focus may be seen adjacent to the cyst wall.
• The cyst may have an attached stalk. If the stalk is patent, the size of the cyst remains small due to communication of the cyst with the cranial cavity.
• Extraocular muscles and optic nerve are usually absent.

Differential Diagnosis – Microphthalmia with colobomatous cyst.

Inflammatory lesions of the eye

Endophthalmitis and panophthalmitis

Endophthalmitis is an inflammation of the eye without involvement of the sclera, and panophthalmitis is inflammation of all the ocular coats and is usually caused by an infection.

• Sterile or noninfectious endophthalmitis is caused by retained lens material and toxic agents.
• Endophthalmitis is of two types: endogenous (i.e. metastatic) and exogenous:
1. 1. Endogenous endophthalmitis results from the haematogeneous spread of organisms from a distant source of infection (e.g. endocarditis).
2. 2. Exogenous endophthalmitis results from direct inoculation as a complication of ocular surgery, foreign bodies or penetrating ocular trauma.
• The inflammation can also spread to involve the orbital soft tissue.
• Bacterial endophthalmitis usually presents acutely with pain, redness, lid swelling and decreased visual acuity.
• Fungal endophthalmitis may have an indolent course over days to weeks. It is usually seen in penetrating trauma with plant substance or soil-contaminated.

Imaging

• CT scan is usually performed to look for intraocular foreign bodies in patients with history of trauma and patients with suspected orbital cellulitis.
• Thickening of the sclera and uveal tissues with increased density in the vitreous and periocular soft-tissue structures may be seen.
• MRI is indicated when orbital cellulitis and cavernous sinus spread is suspected.
• The vitreous shows hyperintense signal better appreciated in T1 WI and FLAIR sequence.
• Thickened ocular coats.
• Ultrasound typically shows choroidal thickening and echoes in the vitreous which support the diagnosis.
• Retained lens material may also be seen in postcataract surgery inflammation.

Scleritis

Scleritis is a chronic, painful and potentially blinding inflammatory disease commonly seen in the fourth to sixth decades with a female preponderance. It can be classified as anterior (necrotizing or nonnecrotizing) or posterior (nodular or diffuse).

Systemic associations include autoimmune disorders, viz., rheumatoid arthritis, systemic lupus erythematosus, relapsing polychondritis, spondyloarthropathies, Wegener granulomatosis, polyarteritis nodosa and giant cell arteritis.

Clinical presentation – Redness and pain are the most common presenting symptoms.

CT and MRI findings:
- • Posterior scleritis shows diffuse thickening of the posterior sclera with periocular fat stranding and fluid in the Tenon’s space.
- • The thickened sclera is dark in T2 WI.
- • Postcontrast study shows uniform enhancement of the thickened sclera.
- • Nodular posterior scleritis appears as a focal mass-like thickening of the sclera posterior to the equator, which enhances uniformly.
Intraocular cysticercosis:
- • The eye due to its rich vascularity is a prime location for infestation by the larval form of Taenia solium. It can involve the vitreous or the subretinal space.
- • The patient presents with chemosis and congestion.
- • Ultrasound is the modality of choice.
- • Neuroimaging is performed to look for neurocysticercosis.

MRI findings – FIESTA or CISS imaging is sensitive to identify intraocular cyst with scolex. Typical cyst with scolex is not always identified on imaging.

Vogt–Koyanagi–Harada syndrome

Vogt–Koyanagi–Harada (VKH) syndrome is a rare multiorgan autoimmune disorder affecting the pigmented tissues of the eye, ear, skin and central nervous system. The patients usually present with uveitis, patchy vitiligo, alopecia, hearing loss, tinnitus, headaches and meningismus.

Imaging: MRI is the investigation of choice.

• The ocular findings are that of choroidal thickening, retinal detachment and subretinal fluid. Postcontrast study shows uniform enhancement of the thickened choroid.
• Brain imaging might reveal periventricular white matter lesions and leptomeningeal enhancement.

Intraocular tuberculosis

The uveal tissue is the most common ocular tissue affected by tuberculosis. Diagnosis is clinical, and imaging is done in case of granuloma or abscess and when diagnosis is in doubt. Granulomas larger than 3 mm can be appreciated on surface coil imaging. They are intermediate in T1 WI and hypointense in T2 WI. They show uniform contrast enhancement. There may be associated thickening of the ocular coats. Brain imaging should be performed for intracranial granulomas.

Intraocular abscess

Intraocular abscess can result from bacterial, fungal or tuberculous infection. In bacterial infection, the presentation is acute, and tuberculous and fungal have a rather subacute or indolent course.

Imaging

MRI – dome-shaped lesions with central T1 hypointense and T2 hyperintense signal and an isointense rim. Diffusion-weighted imaging (DWI) shows central restricted diffusion. Contrast imaging shows thick-walled uniform enhancement and nonenhancing necrotic center. Associated ocular coat thickening and periocular inflammation may be present.

Intraocular tumours – adults

Melanoma

It is the most common intraocular malignancy encountered in clinical practice. Common sites are choroid, ciliary body and iris. The posterior uveal melanoma is usually well-defined dome-shaped lesion and can occur anywhere from the ciliary body to the posterior pole. Factors that predict an unfavourable prognosis are tumour size, extraocular extension, extension to the ciliary body and intense pigmentation. The choroidal melanoma is dome or mushroom shaped. Liver is the common site for metastases; hence, tumour workup should include liver imaging.

Imaging features

Computed tomography

• Well-defined dome-shaped lesions are usually hyperdense and homogeneously enhance with contrast.
• It is difficult to characterize the lesion as other intraocular lesions such as choroidal haemangioma and metastases have similar CT features.

Magnetic resonance imaging

• Uveal melanoma has characteristic MRI appearance. They are typically dome or mushroom shaped and have characteristic T1 hyperintense and T2 hypointense signal with respect to vitreous. The T1 hyperintense signal is attributed to the T1 shortening property of melanin.
• This property helps in differentiating it from other intraocular tumours as they usually display intermediate signal in T1 WI and isointense signal in T2-weighted images with respect to the vitreous.
• Amelanotic melanomas usually pose a diagnostic challenge as they lack the characteristic T1 hyperintense signal.
• Contrast-enhanced MRI is done to look at the enhancement pattern and identify extraocular extension and intracranial lesions.
• Studies have been done to look at the enhancement pattern and identify the malignant lesions based on the intensity of enhancement. It also helps to see the response to treatment.
• Postbrachytherapy, they found reduction in the contrast enhancement.
• Doppler study can also be done to look for tumour vascularity. The nonmalignant lesions usually demonstrate poor flow.

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Tags: Comprehensive Textbook of Clinical Radiology Volume I

Mar 25, 2024 | Posted by admin in CARDIOVASCULAR IMAGING | Comments Off

Radiology Key

Fastest Radiology Insight Engine

A orbits:

Part A orbits:

Introduction

Soft-tissue anatomy

The ocular coats

Extraocular muscles (Table 3.25.1)

Connective tissue system

Nerves

The optic nerve

Lacrimal system

Lacrimal apparatus

Bony anatomy

Foramen and fissures

Congenital malformations of the eyeball and orbit

Anophthalmia

Microphthalmia

Computed tomography/magnetic resonance findings

Congenital cystic eye

Inflammatory lesions of the eye

Endophthalmitis and panophthalmitis

Imaging

Scleritis

Vogt–Koyanagi–Harada syndrome

Intraocular tuberculosis

Intraocular abscess

Imaging

Intraocular tumours – adults

Melanoma

Imaging features

Computed tomography

Magnetic resonance imaging

Stay updated, free articles. Join our Telegram channel

Full access? Get Clinical Tree

Radiology Key

Fastest Radiology Insight Engine

A orbits:

Part A orbits:

Introduction

Soft-tissue anatomy

The ocular coats

Extraocular muscles (Table 3.25.1)

Connective tissue system

Nerves

The optic nerve

Lacrimal system

Lacrimal apparatus

Bony anatomy

Foramen and fissures

Congenital malformations of the eyeball and orbit

Anophthalmia

Microphthalmia

Computed tomography/magnetic resonance findings

Congenital cystic eye

Inflammatory lesions of the eye

Endophthalmitis and panophthalmitis

Imaging

Scleritis

Vogt–Koyanagi–Harada syndrome

Intraocular tuberculosis

Intraocular abscess

Imaging

Intraocular tumours – adults

Melanoma

Imaging features

Computed tomography

Magnetic resonance imaging

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