Cross-sectional imaging techniques in veterinary ophthalmology

Cross-Sectional Imaging Techniques in Veterinary Ophthalmology Dominique Penninck, DVM, DVSc, Gregory B. Daniel, DVM, MS, Robert Brawer, DVM, and Amy S. Tidwell, DVM

Ultrasonography (US), computed tomography (CT), and magnetic resonance imaging (MRI) are useful, complementary crosssectional imaging modalities of the eye and orbit. High-resolution US provides excellent morphological information of ocular structures but offers limited information on the periocular tissues. CT and MRI provide valuable morphologic and topographic images of both ocular and periocular structures, thereby giving a more complete picture of the pathological process. US can be performed on awake patients, whereas CT and MRI require general anesthesia. In addition, US equipment is readily available and less costly than CT or MRI units. Fine-needle aspirations and biopsies under US or CT guidance can also be performed. This article reviews the technique and normal findings of ocular and orbital structures as displayed in each of these imaging modalities. Representative clinical cases are presented to illustrate the interpretation principles as well as to provide an illustrative reference for common ocular and orbital changes. Copyright © 2001 by W.B. Saunders Company

urvey radiographs are relatively insensitive for detection of pathological changes in the orbit or retrobulbar tissues because of the superimposition of overlying bone and the inability to differentiate between the various intraocular structures or the soft tissue and fat opacities in the retrobulbar region. Vascular contrast examinations have been used to delineate tumors or loci of inflammation, but these minimally invasive techniques are limited to the extraocular structures and often do not completely identify the extent of the disease. Over the last 2 decades, the development of cross-sectional diagnostic modalities such as uhrasonography (US), computed tomography (CT), and magnetic resonance imaging (MRI) have greatly contributed to the understanding and diagnosis of ocular and orbital changes. US is often considered as the imaging method of choice in veterinary ophthalmology to evaluate ocular diseases and the retrobulbar space. US is available to many practitioners and is a cost-effective screening procedure for evaluating ocular and retrobulbar changes. The image planes obtained from CT and MRI are similar, but

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From the Department of Clinical Sciences (Section of Radiology), School of Veterinary Medicine, Tufts University, North Grafton, MA. and the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN. Address reprint requests to Dominique Penninck, DVM, DVSc, Department of Clinical Sciences, Section of Radiology, School of Veterinary Medicine, Tufts University, 200 Westboro Road, North Grafton, MA 01536. E-mail: [email protected] Copyright © 2001 by W.B. Saunders Company 1096-2867/01/1601-0004535.00/0 doi:l 0.1053/svms.2001.22802 22

principles of image formation are completely different, with each technique having its own advantages and limitations. Often the choice of the modality depends on availability and cost-effectiveness. CT and MRI scanners are usually limited to large specialty practices and veterinary teaching hospitals, but they can be available to all veterinarians through referral. To optimize the choice of the imaging modality best suited for the examination of the orbit, it is necessary to first obtain adequate clinical information. The combination of clinical history and physical examination, together with the imaging findings, leads to a pertinent list of differential diagnoses. This article presents the complementarity of these modalities and illustrates the most common applications with representative images.

Ultrasonography Two-dimensional, real-time US is ideally suited for ocular and orbital evaluation, especially when opacity of the anterior segment (cornea, lens) precludes ophthalmic examination of deeper structure of the eye. 1-9 In addition, US has the advantages of being a nonionizing imaging rnodality and does not require anesthesia. Amplitude (A) and brightness (B) modes have been described in ophthalmic evaluation. 6,1°,n A- and B-modes can both provide useful biometric information, but A-mode in vivo measurements such as globe axial distance, anterior chamber depth, axial lens thickness, and vitreous axial length are considered more accurate. Ultrasonography can provide useful biometric, morphological, and vascular information, as well as guidance for aspiration/biopsy procedures. In this article, we present only B-mode applications and illustrations. The most common clinical indications for ultrasonography are exophthalmos (unilateral or bilateral), endophthalmos, corneal or lental opacification, abnormal retropulsion of the eye, displacement of the globe, ocular trauma, and pain in opening the mouth. A real-time sector scanner with 7.5 to 10 MHz transducer should be used. A few drops of topical anesthesia are instilled in the eye and, after the eyelids have been manually retracted, sterile couping gel is placed directly on the cornea. Sedation should be avoided because it causes rotation of the globe and elevation of the nictitating membrane. This corneal contact method is preferred to evaluate the posterior globe and extraorbital tissues. The water bath offset method has been recommended for the evaluation of superficial structures such as the anterior chamber, iris, and lens. The eyelid contact method is not advised, as it is associated with numerous artifacts that degrade the image. 3 However, this transpalpebral approach is sometimes the best choice if the

Clinical Techniques in Small Animal Practice, Vol 16, No 1 (February), 2001: pp 22-39

chorold sclera ciliary body

lens cep,ule ~ .

anterior

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posterior

lenscapsule retina

i" chamber

Fig 1. (A) Schematic drawing of the normal anatomic features of the eye of a dog. (B) Sagittal sonogram of a normal canine eye. (C) Close-up view (sagittal sonogram) of a normal anterior segment. One can clearly identify the cornea (c), lens (L), anterior (AC) and posterior (PC) chamber, vitreous body (VB), ciliary bodies (CB), iris, posterior wall, optic disc, optic nerve (ON; arrows).

cornea is damaged or if the eyelid is severely swollen. 4 Frontal (horizontal), sagittal (vertical), and oblique planes provide through evaluation of the eye. Detailed knowledge of the normal ultrasonographic anatomy of the eye and orbit is essential to further identify and characterize ocular and orbital pathology (Fig 1). The following structures are routinely identified and evaluated for size, shape, position, and echogenicity: anterior chamber, ciliary bodies, lens (with anterior and posterior capsules), vitreous body, posterior wall of the globe, optic disc, and optic nerve. Using a high-resolution probe, the cornea appears as 2 discrete, parallel echogenic lines, representing the anterior epithelial layer and the posterior endothelium, separated by an anechoic corneal stroma (Fig 1C). The anterior/ EYE IMAGING

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retrobutbarfat

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posterior chamber

extraocular m~scle

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posterior lens capsules appear as discrete hyperechoic convex/concave curvilinear interfaces. The ciliary bodies are echogenic projections, symmetrically positioned on each pole of the lens. In normal dogs, the iris and posterior chamber are usually difficult to distinguish from the adjacent ciliary bodies. Using high-resolution transducers, the posterior chamber can be seen as an anechoic, triangular space between the lens, ciliary body, and iris (Fig 1C). The iris appears as a bright interface on the surface of the anterior capsule of the lens. The posterior wall of the globe appears as a bright curvilinear surface. The choroid and retina cannot normally be identified as separate entities. The optic disc is a distinct, focal hyperechoic area, easily recognized in the central region of the posterior wall. The optic nerve is a

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Fig 2. Anterior uveitis in a dog. Close-up view showing focal increased echogenicity (arrowheads) present within the anterior chamber, c, cornea; L, lens.

wedge-shaped, poorly echogenic structure, located posterior to the optic disc (Fig 1B). The ultrasonographic ocular axial length (from mid-cornea to posterior wall) has been reported to range from 19 to 23 mm in dogs. 1° The equatorial diameter (anteroposterior dimension) of the lens ranges from 6.7 to 8.9 mm. The orbital structures accessible by ultrasound are the retrobulbar fat, part of the zygomatic salivary gland (inconsistently), part of the extraocular muscle, and the superficial edge of the frontal bone/zygomatic arch. Illustrations of common ocular and orbital changes are presented in the following section.

Anterior uveitis can be of infectious, autoimmune, or neoplastic etiology. In severe cases of uveitis, the presence of corneal edema prevents routine ophthalmologic examination. In these instances, US is necessary to assess the anterior segment. Inflammatory debris associated with anterior uveitis can appear as an ill-defined area of increased echogenicity within the anterior chamber (Fig 2). Cataracts are based on biomicroscopy and classified as immature (visible fundus), mature (fundus not visible), and hypermature (resorption or mineralization of the lens, or wrinkling of the anterior lens capsule). Cataract produces abnormal echoes within the lens (Fig 3). The pattern of distribution of these echoes most likely varies with the degree of maturity. In addition, change in ocular and/or lens size, as well as associated features such as retinal detachment with or without vitreal degeneration, have been described in a report on 147 dogs with cataracts. 12 Lens position can easily be assessed by ultrasound as it lies evenly between the ciliary bodies. Lens (sub)luxation corresponds to a partial or complete displacement of the lens from the peripheral ligamentous attachments (Fig 4). This dislocation can happen in the anterior chamber or in the vitreous cavity. Lens luxation can be a predisposing condition for cataract formation or inversely, cataract can predispose to lens luxation. Vitreous degeneration (asteroid hyalosis) is associated with liquefaction of the vitreous that results in changes of acoustic impedance. These changes appear as multiple point-like or echogenic lines within the vitreous cavity (Fig 5A), and they show marked aftermovement. When echogenic lines are present, careful evaluation of the posterior

Fig 3. Sonograms of cataracts wih different distribution pattern of internal echoes. (A) Close-up view of a lens with cataract. Curvilinear echoes are evenly distributed along the anterior and posterior capsules of the lens. (B) Amorphous and moderate distribution of internal echoes is noted within this lens. (C) Moderate internal echoes are noted within the lens. A tear-shaped echogenic structure is present on the outer surface of the posterior lens capsule. The exact origin of this structure is unknown. It could represent posterior lenticonus or ectopic lens material. Extrusion of lens material was considered less likely because of normal lens size.

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Fig 4. (A) Lens with mild cataract changes posteriorly luxated. (B) Lens with moderate cataract changes posteriorly luxated. In the near field, there are a few echogenic curvilinear strands, possibly representing fibrin tags.

~r

Fig 5. Vitreous degeneration appears in this eye as several small echogenic foci (arrows) "floating" within the vitreous cavity. EYE IMAGING TECHNIQUES

Fig 6. Complete retinal detachment. V-shaped curvilinear echogenic structures are identified within the vitreous body. They extend from the optic disc to the ora serrata. 25

loid artery) is a congenital, developmental disorder characterized by retrolenticular fibrovascular tissue formation and potential retention of the hyaloid vasculature, s3 The persistent hyaloid artery can be seen as a thin, discrete, hyperechoic linear stand, extending from the retrolental plaque to the area of the optic disc (Fig 8). Power Doppler evaluation is useful to assess blood flow in the retina, through remnant hyaloid vessel, and the lens. >aS Other associated changes such as retinal detachment, vitreal hemorrhage, and cataract can be diagnosed during the US examination of the affected eye(s). Papilledema and papillitis can appear as convex protrusion of the intrascleral portion of the optic nerve into the vitreous (Fig 9).6 Intraocular tumors can originate from any intraocular structures. The most commonly encountered tumors are melanotic tumors (anterior uvea, iris, ciliary body), adenoma/ adenocarcinoma, lymphoma, fibrosarcoma, and metastases. 16at Ocular tumors tend to be echogenic and have rounded borders (Fig 10). They often create displacement of normal ocular structures such as the lens or the ciliary bodies; however, they may be difficult to differentiate from an organized blood clot. Exophthalmos is a common clinical sign of retrobulbar disease. Recognition often depends on altered echogenicity

Fig 7. Vitreous hemorrhage. Moderate echogenic material is seen within the entire vitreous body. During real-time evaluation, there was evidence of after-movement motion. Note the echogenic changes of the lens, representing cataract.

wall of the globe is necessary to avoid confusing these changes with retinal detachment. Detection of the vitreous changes depends on the time gain compensation settings. 12 In cases of moderate to severe changes, it may be difficult to differentiate vitreous degeneration from vitreous hemorrhage. US is considered a useful presurgical screening method to diagnose retinal detachment in animals with opacification of the anterior segment (Fig 6). Complete retinal detachment is seen as a V-shaped, curvilinear membrane still attached to the optic disc and the ora serrata (caudal to the ciliary apparatus). Partial retinal detachment involves only a portion of the retina that is lifted from the posterior wall of the globe. In that case, an anechoic space underlying a discrete hyperechoic strand is seen on the posterior aspect of the globe. These conditions can be difficult to differentiate from a posterior vitreous detachment or a local choroid detachment. Unlike retinal detachment, posterior vitreous detachment is not attached at the optic disc. Vitreous hemorrhage is present in many conditions and is characterized by the presence of small to large echoes unevenly distributed within the vitreous cavity (Fig 7). It is sometimes difficult to distinguish vitreal hemorrhage from retinal or vitreous detachment, and both conditions can be present at the same time. Persistent hyperplastic primary vitreous (persistent hya26

Fig 8. Persistent hyperplastic primary vitreous. Retrolenticular fibrovascular tissue is present. A thin, hyperechoic linear strand extends from the retrolental plaque to the optic disc region. PENNINCK ET AL

ies often appear as bright interfaces of variable shape, with or without posterior acoustic shadowing (Fig 12). It is difficult to predict the nature of the foreign body based on its US characteristics. Metallic foreign bodies can be associated with comet-tail artifact, whereas wood chips, plant material, plastic, or glass fragments can be associated with subtle or strong shadowing. The wall of the lesion can be either discrete (organized capsule) or ill defined. Retrobulbar abscess cannot be accurately differentiated from retrobulbar tumors. However, ultrasound-assisted aspirate and/or biopsy can be performed to distinguish between these 2 conditions.

Fig 9. Papilledema. Focal echogenic thickening is noted at the optic disc region. However, the US examination misses the abnormalities in the retrobulbar space detected on the CT transverse images (see Fig 22}.

of the retrobulbar space as well as the shape of the posterior wall of the globe, is Retrobulbar tumors can range from poorly to moderately echogenic in appearance (Fig 11). Although it is difficult to assess integrity of the bony support of the orbit, adjacent bony invasion/lysis can often be suspected by the irregular bony margins. However, the extent and severity of the bony involvement are not adequately assessed as illustrated in a dog with an aggressive retrobulbar mass destroying part of the adjacent nasal cavity (Fig 11). In these cases, CT or MRI is recommended to fully assess the extent of the pathological process. Retrohulbar cellulitis can appear as a diffuse, nondeforming lesion of the retrobulbar space. The echogenicity of this lesion can vary from diffusely hyperechoic to the point of not identifying the optic nerve, to subtle hypoechoic areas. Retrobulbar abscess often appears as a hypoechoic mass with or without deformity of the posterior wall of the globe. During the evaluation of retrobulbar abscess, it is important to carefully screen for potential foreign bodies. Foreign bodEYE IMAGING TECHNIQUES

Fig 10. (A) A pigmented mucinous adenoma of the ciliary body was diagnosed in this 5-year-old golden retriever. A uniformly echogenic mass was seen compressing the iris and closing the drainage angle on the lateral aspect of the left eye. The vitreous body contained mucus, macrophages, lymphocytes, and plasma cells. (13) A malignant melanoma was diagnosed in this 8-year-old golden retriever. The uniformly echogenic mass was identified on the medial aspect of the lens involving the ciliary body, iris, choroid, and drainage angle. ")7

Fig 11. (A) Retrobulbar mass identified by US. The irregular ventromedial bony interface indicated bony involvement, but the exact extent of the lesion could not be assessed. (B) Transverse CT images of the same mass showed clearly the extensive bony destruction of the adjacent maxillary bone as well as the invasion of the corresponding nasal cavity. Based on the poor prognosis of this aggressive lesion, the owner elected euthanasia without any further diagnostic test. ((3) A uniformly echogenic retrobulbar mass is seen deforming the posterior wall of the globe in this 11-year-old cat. The mass measured about 1 cm in diameter. The histopathologic diagnosis was retrobulbar carcinoma. (D) A uniformly nearly anechoic mass, measuring 1.5 cm x 1.2 cm (between calipers) was identified in the retrobulbar space of this 6-year-old cat presented for unilateral exophthalmos. A US-guided fine needle aspirate of the mass revealed lymphosarcoma. Abbreviation: E, eye.

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Fig 12. (A) An echogenic mass measuring 1.5 cm in diameter (between calipers) was identified on the ventrolateral aspect of the retrobulbar space of this 13-year-old poodle presented for pain in opening the mouth. The mass did not appear to deform the globe. A retrobulbar abscess (Escherichia coli infection) was diagnosed via US-guided fine needle aspirates. (B) A large retrobulbar hypoechoic-to-anechoic mass was identified on the ventrolateral aspect of the right eye of this 3-year-old sheltie. The mass is deforming the globe. A discrete small echogenic structure, associated with acoustic shadowing (not visible on this picture), was seen in the center of this lesion and suspected to represent a foreign body. At surgery, a retrobulbar abscess secondary to a grass awn foreign body was confirmed. Abbreviation: E, eye.

US-guided procedure can be performed as a free-hand method or assisted with a guide secured on the probe. The free-hand method offers more flexibility in maneuvering, as the operator is quite limited by the bony orbit. A safe dorsal, ventral, or lateral window away from the ocular globe is then carefully selected. The animal is either heavily sedated or anesthetized, and a sterile local preparation of the skin is performed. The position of the patient is chosen accordingly to the location of the target lesion in relation to the affected eye. A 20- or 22-gauge spinal needle is often used. In cases of retrobulbar abscess, an 18-gauge needle may be needed to successfully drain the cavity; however, depending on the abscess location/extension, a wide drainage site through the oral cavity is often preferred.

Computed Tomography Computed tomography (CT) is an x-ray technique that produces cross-sectional images (Fig 13). Orbital structures can then be displayed without superimposition of overlying tissues, unlike in conventional radiography. The retrobulbar fat provides excellent image contrast on CT images and facilitates identification of the globe, optic nerve, extraocular structures, and cortical bone (Fig 14). 19 Passing a narrow beam of x-rays through thin sections of the body as the x-ray tube rotates around the patient produces the CT image. A ring of detectors on the opposite side of the patient from the x-ray tube measures the intensity of the existing beam. The computer incorporates the attenuation patterns using comEYE IMAGING TECHNIQUES

plex mathematical algorithms to create the images. 2° The table moves the patient a small distance for the next image plane. The time for image acquisition and reconstruction has significantly decreased with the availability of more powerful computers. The principles of subject densities and x-ray attenuation in CT are identical to traditional radiography. Bone causes a high degree of attenuation of the x-ray beam and will appear white. Air causes little attenuation of the x-ray beam and appears black. Unlike conventional radiographic images that can differentiate five main densities (gas, fat, water, bone, metal), the CT scanner can display hundreds of different densities from - 1000 (air) to 0 (water) to 1000 (bone) and far above 1000 for contrast agents and metal. Adjusting the gray scale of the display can enhance subtle differences in tissue densities. The two most common gray-scale adjustments used for orbital imaging are bone and soft tissue windows (Fig 15). New state-of-the-art CT scanners can produce images of the orbits as thin as 1 mm thick with excellent resolution. 2° Spiral or helical CT scanners have greatly reduced the imaging time. Instead of the tube rotating around the patient for each image plane or slice, the tube and detectors continuously rotate as the patient moves progressively through the gantry. Images can be reconstructed at any position within the scanned volume. Many of the newer computers can perform 3-dimensional reconstruction of the bony and/or soft tissue structures (Fig 16). The 3-dimensional image can be rotated on the computer screen to view various structures from numerous angles. The images are useful to understand the spatial relationship between different anatomic structures. They are especially useful to evaluate head 29

Fig 13, Dorsal plane CT images (left) and a dorsoventral radiograph (right) of a dog head. The CT images are 2 mm thick and are displayed using a soft tissue window, Note the superior resolution of the orbital structures on the tomographic CT images compared with the radiograph,

trauma and skull deformity and to delineate tumor borders before planning therapy. A major disadvantage of CT is that direct image formation can be obtained only in a plane parallel to the direction of the x-ray beam. Therefore, precise head positioning is required to ensure that the image plane passes through the central axis of the globe and retrobulbar cone of both eyes.

Fig 14. Dorsal plane CT image of a normal canine orbit. Normal structures that are seen include the vitreous body (A), optic nerve (B), medial rectus muscle (C), and lens (D). 30

Additional image planes can be created by computer reconstruction, but the quality of the reconstructed images is inferior to the directly acquired images (Fig 17). To optimize the quality of reconstructed images, the image slices should be thin and overlapping. A routine CT series of the orbits should include transverse, dorsal oblique, and/or sagittal oblique image planes. Image slice thickness is typically 1 to 3 mm. Thicker image slices can be used, but fine details such as the insertion of the extraocular muscles on the globe or the optic disc are then not distinctly seen. 21 The thin slices reduce the volume-averaging artifacts, which improves longitudinal resolution but reduces the signal/ noise ratio, thus making the images more cryptic. This can be partially offset by increasing the milliamperage. Most reports of orbital CT in veterinary medicine are limited to transverse images. 21 Multiple image planes are used to evaluate orbits and retrobulbar tissues. Evaluation of the optic nerve and extraocular muscles is best when the image plane is parallel to the optic nerve. To obtain such an image plane, the animal is positioned in dorsal recumbency with the animal's head or the CT gantry angled 20 ° to 30 ° from a plane parallel with the hard palate (Fig 18). Dorsal oblique plane images through the orbital cone in mesaticephalic dogs can then be obtained. The angle may vary depending on the head conformation. In brachycephalic breeds, this angle may be as much as 50°Y Sagittal oblique images can be made by positioning the dog in lateral recumbency with the head rotated as much as possible to a dorsoventral position. The nose is tipped dorsally and angled in such a fashion that an image plane of 60 ° from the midline of the skull is obtained. This view is best to display the optic nerves. 2~ Intravenous contrast administration has been recommended in cases of inflammatory disease or in cases of suspected intracranial extension of the disease. 2a The sheath of the optic nerve can be contrast enhanced by cisternography with a contrast dose of 0.3 mL iopamidol/kg. 21 Interpretation of the CT scan is based on morphologic changes in the globe position or shape, alterations m the retrobulbar tissues, or changes of the adjacent skull bones. The relationship to adjacent structures is important in differentiatPENNINCK ET AL

Fig 15. Dorsal plane CT images at the level of the orbits. These are the same images displayed with different windows. The image on the left is viewed with a soft tissue window ON 351, L 35), and the image on the right is viewed with a bone window (W 2800, L 325). Note the superior contrast of the orbital tissues using the soft tissue window and the discreteness of the bony margins and ethmoid turbinates with the bone window,

ing compressive versus infiltrative lesions. Extension into the nasal passage and intracranial cavity is an important diagnostic and prognostic criterion (Fig 19). CT reliably identifies the extent of disease of the eye and orbit. 23-25 Conventional radiographs cannot determine the extent of orbital masses unless there is significant bony lysis or production. CT can help differentiate inflammatory conditions from neoplastic diseases. Most masses in the orbital region

Fig 16. Three-dimensional CT image of a cat with an aggressive lytic bone lesion of the lacrimal, frontal, and maxillary bones, This image was reconstructed from transverse plane CT images. (Images courtesy of Dr. Jeryl C. Jones, Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine.) EYE IMAGING TECHNIQUES

are neoplastic. 26 In dogs and cats, the most common orbital tumor is adenocarcinoma and squamous cell carcinoma, respectively. ~6,26-2s Other tumor types include lymphosarcoma (Fig 20), osteosarcoma, fibrosarcoma, chondrosarcoma, meningioma, mast cell tumor, and basal cell carcinoma. = The presence of bony lysis is an important criterion for the diagnosis of malignancy. 16,26,2r,29 Carcinomas tend to cause the greatest degree of bony lysis in humans and animals. 16,26,2r,29 Destruction of the maxilla and maxillary zygoma has been reported as a consistent finding in squamous cell carcinomas. 26 The etiology of infiammatory/infectious diseases includes fungal or pyogranulomatous cellulitis/fascitis, ulcerative stomatitis, myositis, and foreign body reaction. Fungal cellulitis has also been associated with bony lysis, but the extent of lysis is typically less than that seen in tumors. 2>26Bony loss/distortion can occasionally be seen as the result of chronic compression (Fig 21). In similar cases, it is important to consider the whole picture, including the animal's signalment, the clinical history, and the imaging findings to narrow down the list of differential diagnoses.

Fig 17. Dorsal plane CT images through the orbits of a dog with a retrobulbar mass. The image on the left was directly acquired, and the image on the right is a computer-reconstructed image from transverse plane CT images. Note the superior resolution of the directly acquired image.

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Fig 18. Lateral image of a canine skull showing the direction of the dorsal oblique image plane. The image plane should pass through the central axis of the orbit and be parallel with the course of the optic nerve. This image plane is 25 ° to the hard palate.

Indentation of the globe is a sign of a mass effect most commonly associated with orbital malignancy. Globe indentation has been reported in cases of lymphoma, squamous cell carcinoma, and basal cell carcinoma. 26 The presence of globe indentation depends on the location and size of the tumor relative to the eye. Globe indentation/displacement however is not specific for neoplastic disease, as it may also be seen with nonneoplastic conditions resulting in periorbital swelling (such as retrobulbar abscess). CT is very sensitive in detecting mineralization within soft tissues; this change has been reported in cases of fibrosarcoma, chondrosarcoma, meningioma, and basal cell carcinoma. 23,26 CT is especially useful in fully evaluating orbital trauma (Fig 22). It allows simultaneous imaging of adjacent soft tissues, brain, nasal passage, and oral cavity to clearly define the extent of injury. Detection and localization of foreign material can also

Fig 20. Postcontrast transverse CT image of a 16-year-old golden retriever presented with right-sided exophthalmos. An enhancing retrobulbar mass without evidence of adjacent bony destruction was identified. A CT-guided, fine-needle aspirate confirmed the diagnosis of lymphosarcoma. Right is to the right of the image.

Fig 19. Transverse (top) and dorsal (bottom) plane CT images of a 9-year-old mixed breed dog with an aggressive mass medial to the right orbit. All images are 4 mm thick. The mass is extending into the right caudal nasal passage and the olfactory region of the brain.

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Fig 21. (A) A large fluid-filled mass was identified on the ventromedial aspect of the right eye of this 7-year-old golden retriever presented for a long-standing (>2 years) history of right-sided abnormal lacrimal discharge. The eye was moderately displaced caudally. (B) In addition, smooth bony loss/distortion was present along the right maxilla. Despite the marked bony deformity, there is no evidence of soft tissue invasion within the nasal cavity, A fine-needle aspiration under CT guidance was performed, and there was no evidence of neoplasia. It was speculated that the lacrimal duct had been previously damaged and expanded progressively into the maxilla.

be achieved with this imaging modality, depending on the nature of the foreign body. CT may be used to guide biopsy procedures of orbital and retrobulbar tissues when the location or small size of a lesion limits biopsy by other means or when additional information is needed for diagnosis, staging, or therapy planning. 3° CTguided bmpsy appears to be particularly useful for subtle

lesions exhibiting minimal mass effect such as in a retrobulbar mass causing minimal exophthalmos (Fig 23). These lesions may go undetected with ultrasonography, or their small size may restrict the space available for manipulation of the US transducer, guide, and needle. With CT guidance, inserting a needle immediately caudal to the bony and ligamentous orbital ring using a dorsal or lateral approach may

Fig 22. (A) Precontrast transverse image of a 1-year-old labrador retriever with a history of recent blunt trauma to the head. The ophthalmologic examination revealed extreme papilledema (confirmed by ultrasonography, see Fig 9). The eyeball was displaced in cranial, dorsal, and lateral directions by soft tissue swelling in the region of the extraocular muscles and optic nerve. (B) The postcontrast study of the same region showed marked and irregular enhancement of the soft tissue "mass." The irregular enhancement is most likely the result of contrast accumulation/pooling secondary to vascular disruption and/or local inflammation. There was no evidence of fracture. EYE IMAGING TECHNIQUES

3;3

pendicular plane (T2 relaxation). Because the hydrogen protons of various tissues differ in their chemical and physical states, their rates of relaxation and thus the intensity of emitted signals vary. These emitted radio signals are used to create the image by using complex mathematical formulas. 32 Inside the MRI gantry is an antenna, called a body coil, that can transmit and receive the RF signals. Specialized antennas, such as head coils, knee coils, or surface coils, may substitute for the body coil as a receiver antenna. The quality of the RF signal is better using these specialized antennas because they are close to the source of the signal. When the receiver coils are placed on the skin surface, they are referred to as surface coils. A typical orbital coil is a 3-inch circular surface coil. The main disadvantage of a surface coil is that the signal intensity drops off dramatically with increasing depth. As a rule of thumb, the maximum depth that can be imaged with a circular surface coil

Fig 23. Transverse CT image of the retrobulbar space of a dog with mild unilateral exophthalmos. There is soft tissue filling of the right sphenoidal sinus with localized bone destruction. Some of the retrobulbar fat planes are preserved. A 22-gauge spinal aspiration needle is seen coursing to the lesion. The white dots on the skin represent barium markers used to guide needle placement. Cytological diagnosis was adenocarcinoma.

access the retrobulbar soft tissues; the eye can be avoided with confidence by excluding its caudal limit from the transverse image of the target. Radiopaque markers such as barium are placed on the skin to determine the optimal needle pathway, and electronic cursors are used to determine the target depth. Although real-time evaluation of the procedure is not possible with CT (unless there is access to a fluoroscopy-assisted CT unit), location of the needle tip can be verified by repeating the scan with the needle in place? 1 Because of its excellent display of orbit and skull topography, CT may disclose the origin of the lesion, bone destruction, and bilateral involvement more readily than with US. For these reasons, CT imaging may be considered the preferred technique for aspiration biopsy guidance.

Magnetic Resonance Imaging Magnetic resonance imaging (MRI) generates cross-sectional images similar to CT but follows completely different principles of image formation. A major advantage of MRI is the excellent contrast between different tissues. MRI does not use ionizing radiation to generate the image; it is based on the chemical makeup and physical state of the tissues. The patient is placed within the bore of a strong magnetic field. The patient's hydrogen atoms (having an odd number of protons and thus a magnetic moment) will line up in the direction of the magnetic field. The hydrogen atoms are stimulated with radiofrequency (RF) energy pulses. The energy from the RF pulses is absorbed by the protons causing them to shift in their axial alignment perpendicular to the magnetic field. After the pulse is turned off, the stimulated protons realign longitudinally with the main magnetic field (TI relaxation) and lose coherence in the per-

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Fig 24. (A) Dorsal oblique plane MRI of the orbits of a normal dog. Three different spin echo sequences are shown. Note the differences in soft tissue contrast and in intensity of the different ocular structures. (13) Close-up of a Tl-weighted image (post injection of gadolinium DTPA) of a feline eye. Note the different intensities of the main ocular structures. The optic nerve is only partially seen on this coronal plan. a, anterior chamber; b, posterior chamber; VB, vitreous body; c, cornea; F, retrobulbar fat. PENNINCK ET AL

Fig 25. Protocol for MRI slice orientation. The initial localizing scan is obtained in the transverse plane (upper left image). The dorsal orbital rims are used as an anatomic marker to align the subsequent dorsal plane Iocalizer slices. A dorsal plane image from the second localizing scan is shown in upper middle image. From this dorsal plane image, a parasagittal image is selected through the orbit (upper right image) that is parallel to the falx cerebri and nasal septum. This parasagittal image is then used to select the angle of an oblique dorsal plane image through the eye (lower right image). The dorsal oblique image (lower middle image) can then be used to choose the angle of subsequent oblique sagittal plane images (lower left image),

Fig 26. A Tl-weighted oblique sagittal plane MRI of the left orbit of a 4-year-old beagle with panuveitis. Signal intensity is substantially increased in the area of the anterior chamber and iris (a), and retina-choroid (c). Signal intensity is also mildly increased in the vitreous cavity, b, lens. (From Morgan et ah Magnetic resonance imaging of ocular and orbital disease in 5 dogs and a cat. Vet Radiol Ultrasound 37:185192, 1996, with permission) EYE IMAGING TECHNIQUES

Fig 27. A Tl-weighted dorsal-plane image of a 14-year-old pomeranian with a mixed mammary adenocarcinoma. Note the mass (X) in the posterior nasal aspect of the left eye. (From Morgan et ah Magnetic resonance imaging of ocular and orbital disease in 5 dogs and a cat. Vet Radiol Ultrasound 37:185-192, 1996, with permission). ;35

is equal to the radius of the coil. 20,32 Head coils or even knee coils 33 surround the entire head of small patients allowing both eyes to be viewed simultaneously. By including the entire head in the image, it is easier to select the image planes and to ensure the proper angle of the image slice. Whereas CT provides superior bone detail and is better at detecting calcifications, MRI offers better soft tissue discrimination with muhiplanar capabilities. The most common image sequence for orbital imaging is the spin echo sequence. The three common spin echo pulse sequences are the Tl-weighted, proton-density weighted, and Ta-weighted images. 33 The Tl-weighted image has the best morphologic detail of the spin echo sequences. 2°,32 Tl-weighted images are made with repetition times (TR) of 500 to 800 msec and echo times (TE) of 20 to 30 msec. The hallmark of a Tl-weighted image of the orbit is the bright signal intensity of the orbital fat. This is due to the fast T~ decay constants of fat resulting in a rapid T 1 relaxation. 2°,32 Structures of intermediate signal intensity include the extraocular muscles, optic nerve, and iris. Structures of low signal intensity (black) include the lens. The vitreous has a signal intensity that is in between the lens and extraocular muscles. The lens capsule has a signal intensity between the fat and extraocular muscles 33,34 (Fig 24). The proton density-weighted images (also known as spin density) are an intermediate-weighted image sequence. Proton density-weighted images are the result of long TR (-->2000 msec) and short TE (-----30msec). This sequence produces images with fair to good soft tissue contrast and spatial resolution. The signal intensity is based on the hydrogen ion concentration within the tissue. The relative order of signal intensity from high to low is vitreous and aqueous, extraocular muscles, fat, and sclera 34,35 (Fig 24). The T2-weighted image has the best soft tissue contrast of the three sequences, but the spatial resolution is less than with the other sequences. Ta-weighted images are made with a TR of ->2,000 msec and a TE of 80 to 100 msec. The hallmark of a Ta-weighted image of the orbit is the bright signal intensity of vitreous. The absolute intensity of these signals depends on the magnetic strength of the scanner. In addition, fat has variable signal intensity depending on the length of the TR. The longer the TR, the darker the fat will appear. Because of the lower signal intensity of fat with a longer TR, the optic nerve is often easier to see with T2-weighted images. T2-weighted images are also helpful in differentiating a fluid-filled versus soft tissue lesion, as lesions that are solid are of lower signal intensity than fluid-filled lesions. The relative order of signal intensity on T2-weighted images is, from high to low, vitreous and aqueous, brain, extraocular muscles/optic nerve, and faU<35 (Fig 24). Chemical fat saturation techniques are useful in evaluating the orbit. Precontrast Tl-weighted sequences should be obtained without fat saturation, as fat saturation reduces the inherent contrast between the orbital fat and the lesion. After intravenous gadolinium contrast administration, however, enhancing lesions tend to blend with the high signal orbital fat, making fat saturation necessary for better outlining of the lesion. 36 Fat suppression may also be achieved by using an inversion recovery technique called STIR (short time inversion recovery). On these images, lesions are typically hyperintense and nicely contrasted by the suppressed fat. An artifact of light and dark bands can occur near fat/soft 36

tissue interfaces using all spin echo sequences but is most apparent on T2-weighted images. 2°,32 This is called a chemical shift artifact. The chemical shift artifact may obscure some detail in MRI orbital imaging. Another inversion-recovery technique is the FLAIR (fluid attenuated inversion recovery), sequence which suppresses the signal from fluid such as cerebrospinal fluid. The FLAIR technique may be used to image structures near the optic chiasm or the brain and may distinguish solid from cystic lesions. Proper positioning of the patient is important. If the image plane does not pass through each orbit at the same angle, the orbits will appear asymmetric. This asymmetry may be misinterpreted as a lesion. MRI has a considerable advance over CT in that images can be obtained directly in any plane without repositioning the patient. Precise alignment requires the image planes to be planned in 3 dimensions. This can be accomplished using the sequences illustrated in Fig 25. 34,35 The initial localizing scan is performed in the transverse plane. A second localizing scan is aligned from the initial transverse localizer using the dorsal orbital rims as an anatomic marker to align the dorsal plane image. From the dorsal plane image, the falx cerebri and nasal septum are used to align straight sagittal images. Sagittal images through the orbital cone are then used to select the angle of an oblique dorsal plane image through the eye. This image plane aligns the central horizontal axis of the cornea with the optic chiasm. Typically this image plane is 30 ° to 33 ° dorsal to a plane parallel to the hard palate 34 (Fig 25). Alignment of the oblique sagittal images is selected from the dorsal oblique image. The image plane parallel with the central axis of the orbital cone is typically 28 ° to 33 ° lateral to midline in the dog and cat. 3+ It should be noted that the angles will vary from animal to animal based on the head conformation. From these sequences, anatomic structures can be identified and compared with the

Fig 28. Postcontrast Tl-weighted dorsal plane image of the brain in a cat with cavernous sinus syndrome. An enhancing mass (M) is present along the ventral aspect of the cranium. The mass obliterated the right cavernous sinus and extended into the orbit through the orbital fissure. There was no conclusive evidence of invasion through the optic canal. No further diagnostic tests were pursued. PENNINCK ET AL

Fig 29. (A) Tl-weighted, (B) proton-density, and (C) T 2weighted oblique dorsal-plane MRI of a 10-year-old chihuahua with a right retrobulbar optic nerve meningioma, X = mass. a, optic nerve; b, area of optic chiasm. (From Morgan et al- Magnetic resonance imaging of ocular and orbital disease in 5 dogs and a cat. Vet Radiol Ultrasound 37:185192, 1996, with permission).

contralateral eye for symmetry. Intravenous administration of gadolinium can provide additional information on the type of lesion, as it accentuates the vascular supply. Interpretation of the MR1 scan is based on the degree and location of soft tissue swelling, deviation and/or shape changes of the globe, changes in signal intensity of structures on the various imaging sequences, pattern of contrast enhancement, and identification and extent of mass lesions. MRI of intraocular structures can be useful and complementary to ultrasonographic evaluation 36 (Figs 26 and 27). In humans, many ophthalmic conditions such as cataracts; retinal detachments; intraocular foreign bodies; inflammatory, degenerative, congenital diseases; and neoplasms are identified and characterized with this modality. 31,3r Besides the detailed information on the ocular and orbital structures, MRI provides the best morphologic assessment of the optic nerves and chiasm 3r (Figs 28 and 29). Since cortical bone is of low signal intensity (black) on all image sequences, invasion into bone is based on replacement of the bone by soft tissue 38 (Fig 30). Identification and evaluation of paraorbital structures such as zygomatic salivary gland are EYE IMAGING TECHNIQUES

important to diagnose orbital mucocele. 39 This uncommon condition can develop secondary to trauma, duct obstruction, or chronic adenitis (Fig 31). MRI is contraindicated in clinical cases with suspicion of metallic foreign body because the magnetic field may induce movement of the foreign body and cause secondary hemorrhage. In these cases, plain radiographs of the orbits are recommended to screen for metallic foreign bodies. MRI of the eye and orbit has only been described recently in veterinary medicine; only a few clinical reports and illustrations are currently available in the l i t e r a t u r e . 24,33-3s,39-4° In a report by Dennis, 4° 25 small animal patients with signs of orbital disease were investigated using MRI. Sixteen of the patients had tumors that appeared as discrete masses of medium signal intensity on Tl-weighted images, and variable, mainly hyperintense on T2-weighted images. After contrast medium injection, tumors showed mild, diffuse enhancement. The extent of the 16 tumors evaluated was depicted more clearly with MRI than with radiography or US. Based on this report, osteolysis and extension of tissue beyond the orbit appear as reliable hallmarks of malignancy. ;37

Fig 30. An expansile mass (M) involving both nasal cavities was noted to invade both retrobulbar spaces and the brain of this 11-year-old airedale terrier. There was clear evidence of massive bony distortion and destruction, as normally hypointense cortical bone was replaced by a large medium-intensity mass. Note the deformity of the caudal aspect of the right globe. The moderately displaced left globe is not seen on this image. The nasal biopsy revealed an undifferentiated nasal carcinoma. (Courtesy of Dr. R.V. Morgan).

In addition, unlike radiographs, MRI images allow distinction between frontal sinus fluid accumulation and solid tissue, as fluid appears hypointense on Tl-weighted images and hyperintense on T2-weighted images. 4° Cellulitis was often associated with diffuse connective tissue swelling and muscle changes without loss of normal architecture. Abscesses and foreign bodies reactions were reliable identified as markedly enhancing (on postcontrast Tl-weighted images) fluidfilled cavities. However, a wooden foreign body was not detected, 4° which was not fully surprising, as a previous experimental study performed in dogs showed that hydrated wooden foreign bodies cannot be reliably detected by CT or MRI, as they mimic soft tissue. 4a With the progressive increase in the number of MRI units in veterinary schools and in referral practices, there will be a significant development in the use and applications of ocular and orbital MRI in companion animals.

Summary US, CT, and MRI are excellent, complementary cross-sectional diagnostic methods that enable determination of the location and extent of diseases affecting the ocular and orbital tissues. Despite its limited spatial resolution and the obstacle of the bony orbit, US is a valid, noninvasive, safe, fast procedure to evaluate ocular diseases. When both CT and MRI are available, the choice between these 2 modalities can be difficult, as each procedure has its advantages and limitations. CT is the modality of choice for examination of bony structures and for detection of calcification; MRI provides excellent contrast resolution of soft tissue, with muhiplanar capability and no radiation ex38

Fig 31. Tl-weighted oblique dorsal-plane MRI through the right ventral orbit of a 6-year-old Shetland sheepdog with a zygomatic salivary mucocele. A semispherical mass (a) deforming the lower eyelid anterior to the orbit is continuous with a cavitated zygomatic salivary gland (b). (The white circle overlying the right coronoid process of the mandible is an artifact.) (B) A Tl-weighted oblique sagittal-plane MRI of the right orbit. Note the rounded mass anterior and ventral to the right eye (a). The zygomatic salivary gland is poorly defined and of mixed signal intensity (b). (From Morgan et al: Magnetic resonance imaging of ocular and orbital disease in 5 dogs and a cat. Vet Radiol Ultrasound 37:185-192, 1996, with permission).

posure. The information obtained by one or more of these modalities significantly aids in reaching a diagnosis, elaborating a treatment plan, and evaluating response to therapy.

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