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   Table of Contents      
OPHTHALMOLOGY PRACTICE
Year : 2002  |  Volume : 50  |  Issue : 4  |  Page : 339-353

Interpretation of computed tomography imaging of the eye and orbit. A systematic approach


Ophthalmic Plastic Surgery, Orbital Diseases and Ocular Oncology Service, L V Prasad Eye Institute, Hyderabad, India

Correspondence Address:
Milind N Naik
Ophthalmic Plastic Surgery, Orbital Diseases and Ocular Oncology Service, L V Prasad Eye Institute, Hyderabad
India
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Source of Support: None, Conflict of Interest: None


PMID: 12532506

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  Abstract 

Computed tomography (CT) has revolutionised the diagnosis and management of ocular and orbital diseases. The use of thin sections with multiplanar scanning (axial, coronal and sagittal planes) and the possibility of three-dimensional reconstruction permits thorough evaluation. To make the most of this technique, users must familiarize themselves with the pertinent CT principles and terminology. The diagnostic yield is optimal when the ophthalmologist and radiologist collaborate in the radiodiagnostic workup. In this article we describe a systematic approach to the interpretation of ocular and orbital CT scans.

Keywords: Computed tomography, interpretation, orbital disorders, tumour


How to cite this article:
Naik MN, Tourani KL, Sekhar G C, Honavar SG. Interpretation of computed tomography imaging of the eye and orbit. A systematic approach. Indian J Ophthalmol 2002;50:339-53

How to cite this URL:
Naik MN, Tourani KL, Sekhar G C, Honavar SG. Interpretation of computed tomography imaging of the eye and orbit. A systematic approach. Indian J Ophthalmol [serial online] 2002 [cited 2017 Apr 26];50:339-53. Available from: http://www.ijo.in/text.asp?2002/50/4/339/14751



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Computed tomography (CT) is an indispensable imaging tool in the evaluation of most orbital and some ocular lesions. This technique allows us to discern the location, extent and configuration of the lesion and its effect on adjacent structures. It also allows us to comment on the possible tissue mass composition. In addition, knowing the precise location of a lesion facilitates the planning of an appropriate surgical approach to minimise morbidity. A general ophthalmologist has need to review a CT scan fairly infrequently, especially if orbital diseases and ocular oncology are not areas of special interest. Moreover, there is still some insecurity among ophthalmologists regarding the use and interpretation of ocular and orbital CT scans.

This article is an attempt to consolidate the available ophthalmic and radiological literature "decontaminated" of technical jargon, and to provide a practical guide to the interpretation of ocular and orbital CT scans.


  The CT machine: Evolution and Principle Top


On their way through tissues, X-rays are attenuated due to absorption of energy. This "attenuation" is determined by the atomic number of the major tissue constituent. Different tissues provide different degrees of X-ray attenuation, and it is this property that forms the basis of all imaging techniques. Plain radiography involves X-rays that pass through the patient, and create an image directly on a photographic film. A three-dimensional structure is thus depicted on a two dimensional plane, giving rise to disturbing superimposition. Moreover, its sensitivity to small differences in the attenuation is low, i.e., its contrast resolution is poor. In traditional tomography, the X-ray tube and the film is moved simultaneously in such a way that only a thin plane through the patient is imaged sharply. Structures located in other planes become blurred due to gross movement.

The CT differs fundamentally from these two methods [Figure - 1]a. The X-ray tube of the CT machine emits a thin collimated fan-shaped beam of X-rays that are attenuated as they pass through the tissues, and are detected by an array of special detectors. Within these detectors, X-ray photons generate electrical signals, which are converted into images. High density areas are arbitrarily depicted as white whereas low density areas appear black. The CT images contain information from thin slices of tissue only, and are thus devoid of superimposition. The result is a contrast resolution far superior to projection X-ray techniques. Recent technological advances have greatly enhanced the applications of CT scan. Today, it can be used for imaging any part of the body, and its role in the diagnosis of ocular and orbital disorders is well established. Unfortunately, the widespread use of CT is accompanied by an excessive vocabulary of technical and radiological jargon. An intimate knowledge of this, however, is not necessary to interpret a CT scan, just as it is not necessary to understand the complexities of a personal computer to benefit from it.


  Requesting a CT Scan: Major Considerations Top


The list of indications for CT scanning of the orbit is exhaustive. However, with increasing demands for cost containment, one should request imaging only when absolutely indicated. The common indications for CT scanning of the orbit are given in [Table - 1].

CT scan is most informative when the ophthalmologist seeks active participation of the radiologist in the diagnostic work-up. The clinical information supplied by the referring ophthalmologist is used by the radiologist both in the selection of appropriate techniques for imaging, and in deriving the most specific conclusion. Failure to communicate the patient's clinical data and the need for special scans to the radiologist is the most common cause for falsely negative CT scans. The following pre-requisites should be considered and if necessary, discussed with the radiologist before an ocular or orbital CT scan is requested.

Slice thickness

Spatial resolution of a CT depends on slice thickness. The thinner the slice, the higher the resolution. The slice thickness can vary from 1-10 mm. Thin slices are good for spatial resolution, but require higher radiation dose, a greater number of slices, and eventually longer examination time. The choice of slice thickness therefore is a balance of these factors. Usually, 2mm cuts are optimal for the eye and orbit. In special situations (like evaluation of the orbital apex), thinner slices of 1mm can be more informative.

Imaging plane

Routine CT scan of the orbit involves axial as well as coronal views [Figure - 1]b. However, reformatted sagittal views along the axis of the inferior rectus muscle can have special application in evaluation of orbital floor blow-out fractures (vide infra), and should be preferred in such cases. A spiral CT is Preferable when reformatted sagittal cuts are required. The plane inclined at 30 to the orbito-meatal line [Figure - 1]c best depicts the optic canal and the entire anterior visual pathway.

Tissue windows

Tissues around the orbit form a spectrum of composition and density, ranging from air (within the para-nasal sinuses) to the bony orbit. Tissue window refers to the selection of a small range from this variable spectrum to decipher the finer details of the tissue of interest. Each tissue window has a specific window width and window level (vide infra). Thus we have bone window, soft tissue window, brain window and so on. A thorough evaluation of any tissue is possible only when it is scanned under appropriate window settings. Soft-tissue window [Figure - 2]a is best for evaluating orbital soft tissue lesions, whereas fractures and bony details are better seen with bone window settings [Figure - 2]b. Though tissue windows are selected by the radiologist, it is important for the ophthalmologist to be familiar with the concept.

Contrast enhancement

Contrast study involves imaging the area of interest after intravenous injection of a radiological contrast medium [Figure - 3]a and [Figure - 3]b. Fortunately, orbital fat provides intrinsic background contrast, and most orbital pathologies can be easily visualised without infusion of a contrast medium. In certain situations, however, a contrast medium is essential. A contrast-enhancing lesion is one which becomes bright or more intense after contrast medium infusion. An increase in its Hounsfield value (described later) is a more reliable indicator of contrast enhancement than increase in its brightness. Evaluation of optic chiasma, perisellar region and extra-orbital extensions of orbital tumours is best possible with contrast enhancement. Contrast enhancement also helps to define vascular and cystic lesions as well as optic nerve lesions, particularly meningioma and glioma. In short, the use of contrast enhancement is not routinely necessary for orbital pathologies unless they have intra-cranial extension, and its use is best left to the discretion of the radiologist.

Modification of CT procedure

Certain cases may require special modifications during the scanning procedure to aid diagnosis. For example, in a suspected case of orbital venous varix, in addition to the routine cuts, it is important to request for special scans (with contrast) while the patient performs a Valsalva maneuver.

Simultaneous brain CT

In certain situations it is mandatory to request CT brain along with that of the orbit. These include suspected neurocysticercosis with orbital involvement, head injury with orbital trauma, optic nerve meningiomas, bilateral heritable retinoblastomas (to rule out pinealoblastoma), and suspected perisellar lesions.


  Components of a CT Scan Top


A simple question anyone might ask at this stage is:
"After all, why should an ophthalmologist learn to interpret CT scans? Why not just read the radiologist's report and proceed?" There are two reasons why we need to learn to interpret orbital CT scans. First, orbital disease accounts for a small part of the radiologist's average CT scan load, and therefore provides him less opportunity to gain expertise in this area. Second, an ophthalmologist with a better anatomical and functional knowledge of orbital structures, and with a better knowledge of the patient's clinical profile, is at an advantage in interpreting ocular and orbital pathologies on a CT scan.

For a complete and systematic evaluation of any CT scan, it is mandatory to familiarise oneself with the CT plates. Unlike a plain radiograph, a CT scan usually provides three to four plates, each carrying multiple sequentially arranged images, with and without contrast. Since we need to view sagittal and coronal images together, it is ideal to have a large X-ray viewer board which can simultaneously display at least two plates. The various components of a CT plate can be categorised under the following headings.

Patient data

This includes the name, age, gender of the patient as well as the date of the CT scan [Figure - 2]a. It is always worthwhile to confirm that you are looking at the correct CT of the specific patient, done at the specific time (in case of serial CT scans) before you begin to interpret.

Type of CT scan

Note whether the plates provided are plain CT scans or contrast enhanced. Though the image brightness and the Hounsfield value enables us to identify the contrast images, it will be printed next to each image whether the scan is plain or contrast enhanced [Figure - 3].

Laterality

Though the eye depicted on the right side of the image usually depicts the right eye, it is important to note that these conventions are not universal. Therefore, the best way to confirm laterality is to look for the "R" or "L" mark which represents right or left respectively [Figure - 2]a.

Axial scan orientation

Axial scans are traditionally made to pass through a plane parallel to the Reid's baseline or the orbito-meatal line [Figure - 1]b. To orient yourself to the axial scans, always begin with the lateral scout view depicted as a plain radiogram that shows the exact location of planes chosen for axial slices. It is usually displayed as the first image on the plate, preceding the serial axial images. The scout view not only gives you an overview, but also enables you to confirm that the entire lesion of interest is included in the scanned area. Each axial slice is always displayed with the anterior (ventral) end facing up. By convention, the axial images are usually arranged progressing from an inferior plane to a superior plane within the orbit [Figure - 4]. A simple way to identify the level of the axial slice is to note that as we move from inferior to superior, the prominence of the nose flattens out anteriorly, and increasingly more brain parenchyma appears posteriorly. Slices that depict the lens represent the mid-level axial plane.

Coronal scan orientation

The coronal scans should ideally pass through a vertical plane perpendicular to that of the axial scans, but are usually angled slightly obliquely [Figure:1b]. This is done to avoid artifacts due to dental fillings if present, and does not significantly affect the anatomic relationships.

Evaluation of the coronal scans too, should begin with the lateral scout view. By convention, coronal images are arranged to progress from anterior plane to posterior plane within the orbit. One must remember, that because of the oblique direction of coronal scans, the first few anterior images do not show the orbital floor. If a given image depicts the globe, it is an anterior coronal section [Figure - 5]a. The image with maximum globe diameter roughly represents the equator of the eyeball. A posterior coronal section is devoid of the globe image, and demonstrates the optic nerve and extra ocular muscles [Figure - 5]b. The cross-sectional size of the orbital cavity reduces as we move to the posterior [Figure - 5].

We have described the axial and coronal sections separately only for convenience. In practice, one needs to interchangeably look at all of them for a thorough evaluation of the orbit.

Technical parameters

Several technical parameters are printed beside each CT image [Figure - 2]a. Though their location and number varies across machines, some common and important ones include slice thickness, scan time, Hounsfield number, window width, window level, table position and measurement scale.

Slice thickness represents the thickness of tissue scanned at one time, and is mentioned in millimeters.

Scan time represents the time taken (in seconds) to image each tissue slice. The ideal scan time is less than 1-2 seconds per image. A longer scan time (few seconds) can lead to motion artifacts (especially in children and uncooperative patients) and the findings need to be interpreted in that context.

Hounsfield units (HU) represent a scale of radiation attenuation values of tissues. The number assigned is called the Hounsfield number. This number can range from -1000 to +1000 HU or above, and a higher number represents greater attenuation of X-rays, indicating higher tissue density. The Hounsfield numbers of various ocular and periorbital tissues are shown in [Table - 2]. This can aid in the differential diagnosis. For example, a dermoid cyst [Figure - 6]a will have a Hounsfield number below zero due to its fat content, whereas a haematoma, will have a number of +70 to + 80 HU. These numbers though fairly accurate, can suffer inaccuracies due to artifacts, and therefore, should be used with caution.

Window width (WW) refers to the span of CT numbers on the Hounsfield scale that are selected to display the given image. It can vary from a few CT numbers to the entire range available on the system. Since the Hounsfield scale usually ranges from -1000 to +1000 HU or above, the maximum WW can be approximately 2000.Thus at a WW of 2000, air will be black and bone will be white. The rest of the tissues will be depicted in shades of gray between these two extremes of the spectrum. A wider WW thus depicts a large number of tissues, and bone details can be better appreciated.

Window level (WL) refers to the midpoint of the selected span of CT numbers, or in other words, represents a point midway between totally black and totally white. For example a WW of 100 with a WL of +50 [Figure - 2]a displays all tissues with Hounsfield value ranging from zero to +100 HU. Values above +100 HU will be white, those below zero will be black, and those between the two will have all shades of gray. This window setting is ideal for soft tissue evaluation. On the other hand, a WW of 2000 with a WL of +200 [Figure - 2]b displays all tissues with Hounsfield value ranging from -800 HU to +1200 HU. This setting is ideal for evaluation of bone. Intraocular structures show very low variations in tissue consistency and thus need a fairly narrow window setting, whereas structures within the orbit show a wide variation in tissue consistency, and require a wide window setting.

Table position (TP) refers to the position of the CT machine table for any given image, and can be helpful if we need to look back into the finer details of a particular area within the orbit. For example, if we want a more detailed evaluation of the orbital apex of a patient, we need simply to request for the thinnest possible cuts within the desired range of table position. This however is possible only if the images are still stored within the computer; the duration each radiologist stores these images varies.

Measurement scale is usually marked in centimeters beside each image [Figure - 2]a. Quick and accurate measurements can be made against this scale with the help of a geometric proportional divider.

Various other technical parameters that may be displayed such as kv (kilo voltage), FoV (field of vision), GT (Gantry tilt) and mA (milli-ampere) are of no clinical significance, and can be safely ignored.


  Systematic Evaluation of Ocular and Orbital Structures on CT Top


CT evaluation is most convenient and informative if performed on the CT machine screen itself. But this is not practical. One almost always has to read the hard copy plates sent by the radiologist. While a detailed description of each orbital structure on a CT is out of scope of this article, we aim to describe the important structures, and suggest that you read a CT plate under the following sub-headings so as to perform a systematic evaluation without missing any diagnostic clue. To get the best out of this section, you need to have an orbital CT plate in front of you (clipped on a viewing board, and not held in the air against a window!) as you read along. Alternatively, you can refer to [Figure - 4] and [Figure - 5] and other specific figures wherever mentioned.

The bony orbit

Begin your evaluation with the bony orbital wall. The axial view is preferred for evaluating the lateral and medial wall, superior orbital fissure, and the optic canal [Figure - 4]. The orbital floor and roof are best seen in coronal sections [Figure - 5]. Systematically evaluate the following features while assessing the bony orbit:

Orbital dimensions: Vertical and horizontal orbital dimensions should be measured (or at least compared) on coronal scans. As mentioned earlier, this is best done with a geometric proportional divider. Marked asymmetry is abnormal. Orbital dimensions can increase in any longstanding orbital mass due to increased intra-orbital pressure. Generally, extra-conal lesions cause localised expansion [Figure - 6]a, whereas intraconal lesions produce generalized expansion [Figure:6b] of the bony orbit. The orbit may be smaller in a patient with congenital anophthalmos, enucleation in infancy or following radiotherapy before the age of three years.

The Orbital roof: The roof is formed by the frontal bone anteriorly, and by the lesser wing of sphenoid posteriorly [Figure - 5]. The lacrimal gland fossa is a shallow excavation anterolaterally, and is difficult to appreciate on CT scan.

The Floor: It is the shortest orbital wall formed by the orbital plates of maxillary, zygomatic and palatine bones [Figure - 5]. Blow out fractures and bony erosions of the floor should be evaluated on coronal scans.

Medial orbital wall: It can be demonstrated by axial [Figure - 4] as well as coronal scans [Figure - 5], and is formed by the following four bones: frontal process of maxilla, lacrimal, ethmoid, and body of sphenoid. In lower axial scans, the bony nasolacrimal duct can be occasionally seen within the bony medial wall [Figure - 4]a.

The lateral wall: It forms a 45 angle with the mid-sagittal plane and is formed by the zygomatic bone anteriorly, and greater wing of sphenoid posteriorly. On axial scan it shows a slight convexity in its posterior part, then becomes flat in the mid portion, and finally becomes concave anteriorly [Figure - 4]b. Note that in [Figure - 6], this contour is preserved on the right side in each image, but the convexity in its posterior part is lost on the left due to bony expansion caused by the tumour.

Para-nasal sinuses: The maxillary sinus lies inferior to the orbit, and is best seen on coronal scan [Figure:5a]. The ethmoid sinuses lie medially, and are best depicted collectively on axial scans [Figure - 4]b or individually on coronal scans [Figure:5a].The orbital roof houses the frontal sinus superiorly, and is seen well on anterior-most coronal scans. The sphenoid sinus lies posteromedial to the orbit, and has a common wall with the optic canal. It can be well seen in axial [Figure - 4]b as well as coronal views [Figure - 5]c

The orbital apex: The orbital apex is anatomically defined as the region between the posterior ethmoidal foramen anteriorly, and the openings of the optic canal and the superior orbital fissure (SOF) posteriorly. The SOF is the largest communication between orbit and middle cranial fossa, and on mid-axial scans is seen to communicate with the cavernous sinus [Figure - 4]b. Enlargement of SOF [Figure - 6]c may be seen in optic nerve meningioma with intra-cranial extension, carotid cavernous fistula, infra-clinoid aneurysm, etc.

The optic foramen: In the upper axial scans, the optic canal lies medial to the anterior clinoid process [Figure - 4]b. Enlargement of the optic canal is seen in tumours of intra-canalicular part of optic nerve, whereas a decrease in diameter can be due to fibrous dysplasia, Paget's disease, or hyperostosis secondary to meningioma.

Inferior orbital fissure: It is seen in lower axial scans [Figure - 4]a and posterior coronal scans [Figure - 5]b and [Figure - 5]c, and can in rare instances get enlarged due to orbital extension of angiofibromas of the pterygo-palatine fossa.

The eyeball

The sclera, choroid and retina together form a well defined ring that enhances with contrast; they cannot be differentiated from each other [Figure - 4]b. The lens appears white, and the vitreous black.

Extra-ocular muscles

The extraocular muscles are well visualised on CT, and run parallel to the adjacent orbital wall. On axial cuts, only the horizontal recti are seen [Figure - 4]b. The superior and inferior recti, partially seen on axial scans, are visualised on coronal views [Figure - 5]a and [Figure - 5]b. The superior rectus and the levator palpebrae superioris are seen as a single soft tissue shadow on high axial scans [Figure - 4]c and coronal scans [Figure - 5]a and [Figure - 5]b. The superior oblique is best seen in the coronal view lying supero-medial to the superior rectus [Figure - 5]a, but can also be seen on upper axial scans as it passes through the trochlea [Figure - 4]c. The inferior oblique is the least defined muscle on CT scan; only the insertion is occasionally visible on axial views. The extraocular muscles and the fibrous tissue septa connecting them form the muscle cone and divide the orbit into intraconal and extraconal spaces, a division of radiodiagnostic importance.

Extraocular muscles should be evaluated for the following morphological characteristics on CT scan.

Size: There is an excellent symmetry between the extra-ocular muscles of both the orbits, and they are thus comparable in all respects. Actual measurements are not of much help, in view of fusiform configuration and artifactual asymmetry due to different scanning planes or positions of gaze. The most common muscle abnormalities relate to variations in muscle diameter. Enlargement is maximum in case of tumours or cysts; moderate enlargement is seen in thyroid ophthalmopathy (vide infra), vascular lesions, and myositis. On the other hand, decreased muscle diameter suggests atrophy from denervation or myopathy.

Shape: Diffuse enlargement suggests inflammation, venous congestion or infiltration, whereas focal enlargement suggests a neoplasm or a cyst. Tendon involvement suggests myositis (vide infra).

Laterality and distribution: Extra-ocular muscle enlargement may occur unilaterally or bilaterally. Within an affected orbit, a single or multiple muscles may be enlarged. Furthermore, multiple muscle involvement can be symmetrical or asymmetrical. Common causes of extra ocular muscle enlargement are enumerated in [Table - 3].

Muscle margin: Healthy extra-ocular muscles have sharp margins. Uniform configuration with distinct margins is seen in Graves' myopathy and vascular engorgement. On the other hand, diffuse infiltration for example by metastatic disease may cause irregular enlargement with indistinct borders.

Contrast enhancement: Normal muscles have moderate contrast enhancement, whereas marked enhancement is seen in thyroid ophthalmopathy or myositis. Contrast enhancement is variable in arterio-venous fistulas and neoplasms.

Extraconal tissues

The lids, conjunctiva, and the orbital septum together form an anterior soft tissue density which on axial scans is seen to extend from the pre-equatorial part of the globe to the lateral and medial orbital margins [Figure - 4]b. The lacrimal gland lies within its fossa supero-temporally, and can be seen on high-axial [Figure - 4]c as well as anterior coronal scans [Figure - 5]a.

Intraconal tissues

The two most important structures within the intraconal space in terms of CT visualisation are the optic nerve and the superior ophthalmic vein (SOV).

CT evaluation of optic nerve lesions is facilitated by 1.5 mm axial scans and contrast study. The middle third of the intraorbital part of optic nerve has a slight downward dip [Figure - 1]b. It may therefore appear thinner in this area as a result of partial volume averaging. The entire anterior visual pathway can be seen only when the imaging plane lies -30 to the Reid's Baseline [Figure - 1]c. The patient should fix in upgaze so as to stretch the optic nerve and make it straight. The intracanalicular portion of the nerve is poorly imaged on CT due to absence of intrinsic contrast material and partial volume averaging from the adjacent bone.

The two most commonly occuring optic nerve tumours are glioma and meningioma. Gliomas usually occur in children and have fusiform enlargement with sharp delineation from the surrounding tissue [Figure - 6]b. They are isodense with the optic nerve, and show variable enhancement with contrast. Meningiomas are usually found in adults, and show irregular enlargement along the optic nerve [Figure - 6]c. They tend to be hyperdense to the optic nerve, and show a more consistent contrast enhancement. Calcification within the optic nerve shadow is also commonly seen with meningioma.

From the neuro-ophthalmic point of view, the most common conditions that require imaging are optic neuritis, compressive optic neuropathies and papilloedema. Magnetic resonance imaging (MRI) is the imaging modality of choice in optic neuritis, especially when multiple sclerosis is suspected. Compressive optic neuropathy commonly has an intra-orbital cause, and papilloedema produces an enlargement of the optic nerve sheath.

The SOV is an important vascular structure to note. It begins in the superior nasal quadrant near the trochlea, courses posteriorly and laterally beneath the superior rectus, and exits the orbit through the superior orbital fissure. It is seen well on high axial scans [Figure - 4]c as well as coronal scans [Figure - 5]a Drainage is into the cavernous sinus. Normal diameter on coronal scans is 2 mm anteriorly and 3.5mm posteriorly above which it is considered abnormal.[1

]Sella and para-sellar regions

The optic chiasma is more readily identified than the canalicular or intracranial part of the optic nerve, because it is surrounded by cerebrospinal fluid (CSF) in the supra-sellar region. Any mass in the region of chiasma should be considered a pituitary adenoma unless proved otherwise. The chiasma, sella turcica and the pituitary gland are best visualised on coronal sections. A detailed description of these intracranial structures, however, is beyond the scope of this article.

The cavernous sinus lies behind the orbital apex, and is particularly well visualised with contrast enhancement. On axial cuts, the lateral border of a normal cavernous sinus should be straight or laterally concave. Convexity of the lateral border indicates an enlarged sinus [Figure - 7].


  Orbital Diseases and CT Interpretation Top


While topographical features are helpful in assessing each ocular and orbital structure, they must always be interpreted within the clinical context. Abnormalities in and around the orbit usually present as one of the following six pathophysiologic patterns. Most patients with ocular and orbital disease will fit into one of these, and in this section we describe CT findings pertinent to these broad clinical patterns.

Vascular disorders

The most common orbital vascular disorders are the venous varices, arteriovenous malformations, carotid cavernous fistulas, and aneurysms. Orbital varix is seen as a fusiform and globular density on CT scan. It has smooth, well-defined margins, and shows bright contrast enhancement. Increase in size during Valsalva maneuvre almost always confirms the diagnosis.[2] Arteriovenous malformations and carotid cavernous fistulas [Figure - 7] are characterised by ipsilateral enlargement of the cavernous sinus, superior ophthalmic vein and extraocular muscles, causing proptosis. Arterio-venous malformations also show irregular tortuosities with marked contrast enhancement, and intracranial component may also be evident on brain CT. Aneurysms usually occur at the origin of the ophthalmic artery from the internal carotid, but can also be intraorbital. The former are difficult to detect unless large, and may simulate a pituitary tumour. The latter are rare, but should appear as well defined contrast-enhancing mass lesions.

Orbital neoplasia

In any patient with features suggestive of orbital mass, the following aspects should be evaluated on CT to aid the diagnosis:

Assessment of proptosis: Extent of proptosis can be assessed by the technique described by Hilal and Trokel.[3] Using a mid-orbital axial scan, a straight line is drawn between the anterior margins of the zygomatic processes [Figure - 2]a. Normally it intersects the globe at or behind the equator. The distance between the anterior cornea and the inter-zygomatic line is normally 21mm or less. Asymmetry greater than 2mm or value above 21mm indicates proptosis.

Size of the tumour: can be easily measured with the geometric protractor at its widest dimensions.

Circumscription of the tumour: whether well delineated or diffuse.

Shape of the tumour, and whether it conforms to the shape of adjacent structures.

Margin of the tumour: whether smooth (benign lesion), or irregular (malignant lesion).

Effect on surrounding structures: displacement (benign lesion) or infiltration (malignant neoplasm).

Internal consistency: homogenous (benign lesion) or heterogenous (malignant lesion).

Surrounding bone: fossa formation (benign lesion), erosion (malignant lesion), or hyperostosis.

Exact location: It is necessary to localise the mass to a particular compartment of the orbit. Its relationship with the adjacent vital structures such as the optic nerve, extra ocular muscles, proximity to superior orbital fissure and optic foramen, and its posterior extent helps to plan the surgical approach. For example, if the nerve is stretched around the lateral border of the mass, a medial approach may be necessary instead of the lateral orbitotomy.

Extraorbital extension of the tumour.

CT evaluation of common orbital tumours is discussed under the following four headings:

Vascular tumours

Cavernous haemangioma [Figure - 8]a and haemangiopericytoma usually appears as a well demarcated contrast enhancing intraconal mass. Lymphangiomas [Figure - 8]b on the other hand appear as poorly defined masses with heterogeneous tumour density. They have irregular margins, and show little or no contrast enhancement. A capillary haemangioma usually appears as a well demarcated, homogenous, contrast enhancing, extraconal mass occupying the periorbital soft tissue and anterior orbit [Figure - 3].

Lacrimal fossa lesions

Inflammatory and lymphoid lesions of the lacrimal gland show diffuse enlargement conforming to the shape of the globe. In addition, there is marked contrast enhancement, but the adjacent bone is normal. In general, such lesions involve the entire lacrimal gland including the palpebral lobe, and thus often extend anteriorly. Moreover, such benign lesions usually do not cross the vertical midline of the orbit. There may be a ring of uveo-scleral enhancement of the adjacent ocular coats in such cases. Pleomorphic adenomas are nodular well delineated lesions with moderate contrast enhancement. They have smooth and well defined margins, and local bony fossa formation is common. Malignant neoplasm of lacrimal gland appears as an irregular mass with poorly defined margins and moderate contrast enhancement [Figure - 8]c. Intralesional calcification as well as surrounding bone destruction is common. Neoplastic lesions generally tend to extend posteriorly, and may cross the vertical midline of the orbital cavity. Dermoid cysts [Figure - 6]a are well delineated and may show calcification of the cyst rim. They have lucent internal consistency with a Hounsfield number corresponding to that of fat.

Secondary orbital tumours

Orbital extension of lid and ocular surface carcinomas shows an apparent mass with irregular poorly defined extension behind the orbital septum. Carcinomas from the paranasal sinuses also show a homogeneous mass with poorly defined margins and often extensive bone destruction. Mucocoeles [Figure - 9]a are well-defined lesions with smooth margins, homogenous consistency, and do not enhance. They usually displace bone margins at the sutures, but rarely can lead to bone erosion. Meningiomas along the sphenoidal wing frequently extend into the posterolateral wall of the orbit and are associated with hyperostosis.

Orbital metastases

The most common primary sites are breast carcinoma in females, lung carcinoma in males and round cell tumours in children. Secondaries appear as discrete homogenous masses with irregular margins and show only slight contrast enhancement. The greater wing of sphenoid is the most common site of bone metastases in the orbit. In children, the common site of the primary is neuroblastoma, or Wilm's tumour [Figure - 9]b. Orbital lymphomas are seen as homogeneous well-defined masses of soft tissue density on CT scan. They show mild to moderate contrast enhancement [Figure - 9]c.


  Orbital inflammatory diseases Top


Orbital cellulitis

A CT can help determine the underlying cause of orbital cellulitis. A paranasal sinus pathology is seen as an opacification or air-fluid level within the sinus. CT scan helps distinguish pre-septal from post-septal orbital cellulitis. In pre-septal cellulitis, the orbital septum acts as a barrier between inflamed adnexal tissue anteriorly, and normal appearing post-septal structures posteriorly. As the infection spreads posterior to the orbital septum, small stippled densities appear within the orbital fat (best appreciated with wide window settings). More discrete densities eventually develop, and there may be secondary thickening of extra-ocular muscles, especially the medial rectus. A frank orbital subperiosteal abscess [Figure - 10]a shows a typical ring enhancement on contrast study.

Orbital pseudotumour

Pseudotumour depicts a wide range of CT findings. It may appear as a well-defined mass, or mimic a malignancy. Contrast enhancement may occur in some of the cases. It may show an enlarged lacrimal gland, or thickening of the posterior scleral rim, with surrounding soft tissue involvement. Myositis usually involves a diffuse (occasionally irregular) enlargement of one or more muscles that conforms to the shape of the globe and the surrounding structures [Figure - 10]b. There are usually no bony changes, and involvement of tendinous insertion is common. Infiltrations within the orbital fat, perioptic area or sclerouveal thickening is highly suggestive of an inflammatory process. Sclerosing orbital pseudotumour can however mimic a lacrimal gland tumour.

Graves' ophthalmopathy

Graves ophthalmopathy typically shows unilateral or bilateral involvement of single or multiple muscles [Figure - 10]c. CT shows fusiform muscle enlargement with smooth muscle borders, especially posteriorly. The tendons are usually not involved and orbital fat is normal, but pre-septal oedema may be seen.

Ocular and orbital trauma

CT is helpful in ocular trauma because it provides excellent visualisation of soft tissues, bony structures and foreign bodies. Contrast study is necessary only in suspected carotid cavernous fistulas.

The following parameters should be assessed in all cases of ocular trauma:

Evaluation of fractures: their number, location, degree and direction of fracture fragment displacement, and demonstration of detached bony fragments in the orbital or intracranial cavity.

Evaluation of soft tissue injury: Muscle entrapment, haematoma, emphysema, etc.

Globe injury

Presence and location of foreign bodies

Orbital floor fractures [Figure - 11]a and [Figure - 11]b are common in the thinner posterior part of the floor. The CT findings may demonstrate a downward curvature of the orbital floor with bony discontinuity, and displacement of fragments into the maxillary sinus. Prolapse of orbital fat or inferior rectus, as well as opacification of maxillary sinus with or without fluid level may be seen. In medial wall fractures, orbital emphysema is also seen in addition to bony discontinuity. Optic canal fractures may be suspected in severe head injuries associated with impaired vision. Axial scans can only depict the lateral and medial wall of the optic canal, and special transverse complex scans might be required for evaluation of its roof. Presence of blood within the ethmoid sinus seen as soft tissue density on the CT, can be a helpful clue to detect optic canal fracture.

CT in retained foreign body [Figure - 11]c determines its location (extraocular or intraocular), and its relationship to the surrounding ocular structures. Both coronal and axial scans are required for exact localisation. Metal foreign bodies up to 0.5 mm can be detected, whereas stone, plastic or wood less than 1.5 mm size are usually not visualised. The suggested window setting for localisation of foreign bodies is at a WW of 500 HU, and WL of 50 HU.

Ocular lesions

Most of the ocular lesions can be reliably diagnosed with B-scan ultrasonography alone. All the same, a CT scan may help in diagnosis of certain ocular tumours. A retinoblastoma is seen as a well-defined high density mass with calcification [Figure - 12]a. CT scan, is not required in every case of retinoblastoma. It is warranted only in a patient of retinoblastoma who presents with proptosis, when there is a suspicion of optic nerve infiltration, or in a heritable tumour to rule out pinealoblastoma. CT scan helps differentiate between extrascleral extension of the tumour and orbital cellulitis secondary to tumour necrosis. The former shows a well-defined soft tissue density in continuity with the globe, and the latter shows a diffuse orbital haze. The differentiation is important, because extraocular extension needs chemotherapy (chemoreduction) whereas tumour necrosis induced cellulitis needs corticosteroids before enucleation. In addition, the former needs post-operative radiotherapy and the latter does not.

A choroidal malignant melanoma in the early stages appears as a localised thickening of sclero-uveal layer, and is seen as a well defined polypoidal mass if it is more than 3mm thick. CT can also demonstrate sizeable nodules of orbital extension, which may show slight contrast enhancement. CT however has no distinct advantage over ultrasound in size estimation and differential diagnosis.

CT scan also has no advantage over B-scan ultrasonography in evaluation of other ocular conditions like staphylomas, buphthalmos, microphthalmos and pthisis bulbi.

In short, applicability of CT scan in the diagnosis of ocular lesions is very limited.

Structural abnormalities of the orbit

A minor degree of orbital asymmetry is common and is of no clinical significance. Moderate amount of asymmetry is seen in conditions like maxillary hypoplasia, where the orbit is relatively retroplaced on the affected side. Craniofacial dysostosis like Crouzon's and Apert's disease are characterised by features of dysostosis (flat face with shallow orbits), hypertelorism (increased bony inter-orbital distance), and exorbitism (decreased orbital volume).

Hypertelorism is characterised by increased bony inter orbital distance, and is measured as the shortest distance between the two medial orbital walls on axial scans. The normal value is 16 mm at birth. In adults it increases to 25 mm for females and 28 mm for males.

Developmental anomalies like absence of greater or lesser wing of sphenoid can present as a part of von-Recklinghausen's disease [Figure - 12]b. Microphthalmos with cyst presents in childhood or early adulthood with an enlarged bony cavity [Figure - 12]c.

To conclude, CT scan is an important utility in the diagnosis and management of most orbital and several ocular disorders. An ophthalmologist should be able to systematically and objectively interpret an available CT

scan. We have described the relevant sectional anatomy of the eye and orbit as seen on CT scan, the basics of tomography, and the typical tomographic appearances of several common orbital and few ocular disorders. It is important for an ophthalmologist to first understand this basic information and then begin to routinely view each CT scan critically. A systematic approach will provide maximum information that may be useful in accurate clinical diagnosis and optimal patient management.[8]

 
  References Top

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2.
Winter J, Cenento R S, Bentson J R: Maneuver to aid in diagnosis of orbital venous varix by computed tomography. AJNR 1982;2:39-40.  Back to cited text no. 2
    
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Hilal SK, Trokel SL: Computerized tomography of the orbit using thin sections. Semin Roentgenol 1977;12:137-47.  Back to cited text no. 3
    
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Hammerschlag SB, Hesselink JR, Weber AL. Computed Tomography of the Eye and the Orbit. 1st ed. Appleton-Century-Crofts, Connecticut;1983.  Back to cited text no. 4
    
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Advanced Imaging techniques in Ophthalmology. Haik BG, editor.International Ophthalmology Clinics. 1986;Vol. 26.  Back to cited text no. 5
    
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Computed tomography in Ophthalmology. Grove AS, editor. International Ophthalmology Clinics. 1982;22.  Back to cited text no. 6
    
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Garrity JA, Forbes GS. Computed tomography of the orbit. In: Tasmman W, Jaeger EA, editors. Duane's Clinical Ophthalmology. Philadelphia: Lippincott-Raven;1997. Vol 2: pp 1-16.  Back to cited text no. 7
    
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Nungent R, Rootman J, Robertson W. Applied investigative anatomy. In: Virginia Barishek, Editor.Diseases of the Orbit-A Multidisciplinary Approach. Part 1. Philadelphia: Lippincott Company;1988. pp 35.  Back to cited text no. 8
    


    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12], [Figure - 13], [Figure - 14], [Figure - 15], [Figure - 16], [Figure - 17], [Figure - 18], [Figure - 19], [Figure - 20], [Figure - 21], [Figure - 22], [Figure - 23], [Figure - 24], [Figure - 25], [Figure - 26], [Figure - 27], [Figure - 28], [Figure - 29], [Figure - 30], [Figure - 31], [Figure - 32]
 
 
    Tables

  [Table - 1], [Table - 2], [Table - 3]


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