Ultrasonography of the Eye

Vol. Vo l.18 18,, No. No. 6

June Ju ne 19 1996 96

Continuing Education Article

FOCAL POINT # Ultrasonography is a safe, noninvasive way to evaluate the intraocular and retrobulbar tissue of opaque eyes.

KEY FACTS I

Transpalpebral ultrasonography may be the only way to examine the eye if the eyelid is severely swollen.

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A stand-off pad or extra coupling gel can be used to place the image of the anterior chamber below the near-field reverberation artifact.

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Sedation may cause extension of the nictitating membrane or rotation of the globe.

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Some horses require sedation and an auriculopalpebral nerve block, in addition to topical anesthetic, for ophthalmic ultrasonography.

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Bone, metal, or large q uantities of gas may make it difficult to visualize ocular lesions ultrasonographically.

Ultrasonography  of the Eye* Louisiana State University

Ohio State University  

Jamie Williams, MS, DVM

David A. Wi Wilkie, DV DVM, MS MS

R 

outine ophthalmic examination involves direct visualization of adnexal and intraocular structures. Opacity of the transmitting medium (e.g., cornea, aqueous humor, lens, or vitreous body) impairs ophthalmic examination. In such instances, ultrasonography can be used to complete the evaluation. Ophthalmic ultrasonography was first applied by Mundt and Hughs in 1956; they used amplitude (A-mode) technology. 1 In 1958, Brum and Green wood investigated the use of brightness (B-mode) ultrasonography for examination of ocular and orbital structures. 2 Since then, ultrasonography has proved to be valuable in the examination of  opaque eyes (i.e., when routine biomicroscopic or indirect ophthalmoscopic examination is impossible). B-mode scan ultrasonography presents a twodimensional image of the tissue. 3 Indications for ultrasonographic examination include ocular trauma, the need to measure axial length, intraocular or orbital foreign bodies or masses, intraocular hemorrhage, lens luxation, retinal detachment, and any opacity that prevents complete ophthalmoscopic examination.1,2,4–8 Ophthalmic ultrasonography has become part of the routine presurgical examination for intraocular and retrobulbar procedures in small and large animals at the Ohio State University Veterinary Veterinary Teaching Teaching Hospital.

PATIENT PREPARATION The equipment required to perform ophthalmic ultrasonography includes an ultrasound machine with an appropriate transducer (7.5- or 10-MHz), sterile acoustic coupling gel, topical ophthalmic anesthetic, and some form of sterile eyewash. Images should be recorded for later reference and included in the patient’s permanent record. The easiest way to preserve the images is to record them on thermal paper or radiographic film; however, a videotape may also be used. Images presented in this article are reproductions of images stored on radiographic film.  A topical ocular anesthetic (0.5% proparacaine hydrochloride) is applied to the cornea. Manual restraint is sufficient for ultrasonographic ophthalmic examination of most small animals. Sedation should be avoided because it may  *A companion article for veterinary technicians appeared in the June 1996 (Vol. 17, No. 6) issue of  Veterinary Technician ®.

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cause elevation of the nictiTRANSDUCER AND tating membrane and roSETTINGS tation of the globe, thus It is best to use a sector interfering with thorough scanner with a small scanexamination.8 Some horses, head diameter (footprint) to however, require sedation facilitate optimal placement and an auriculopalpebral on the patient’s eye. Transnerve block, in addition to ducers are available in a topical anesthetic, to facili  wide range of frequencies3; tate examination. however, a 7.5- or 10-MHz Sterile coupling gel faciliprobe is recommended for tates transmission of sound ophthalmic ultrasonografrom the transducer into the phy. optic tissue; therefore, a layTransducer frequency is er of sterile coupling gel is inversely proportional to the placed between the patient   wavelength of the sound and the transducer. If the Figure 1A beam.3,11 Depth of soundtransducer is to be applied beam penetration is proporto the cornea instead of to tional to wavelength. 3  Axial the eyelid, sterile coupling and lateral resolution have a gel is applied directly to the significant effect on the anesthetized cornea. Celluquality of the resulting imlose-based gels may be abraage. 3   Axial resolution is a sive and should be avoided function of pulse length. to reduce the risk of corneal Lateral resolution depends irritation. Applying the transon beam width, distance ducer directly to the gelfrom the transducer, and coated cornea provides a size of the transducer face better image of the posterior (footprint). 3,11,12   A 10-MHz globe and retrobulbar tissue transducer provides superior than does transpalpebral ulresolution but less depth of  9 trasonography. penetration than does a 7.5Some equine patients, MHz transducer. transducer. 3,7,8,11,12  A  however, do not tolerate apgood rule of thumb is to use plication of the probe to the Figure 1B the transducer of the highcornea. Transpalpebral imag- Figure 1—Ultrasonographic images of the eye of a healthy  est frequency that will allow ing is used for these pa- cat. Extra coupling gel was used as a stand-off. (  A ) Image visualization of the deepest tients. To avoid trapping air obtained with a 7.5-MHz transducer. The cornea and tissue of concern. (which is a barrier to ultra- most of the anterior chamber fall within the transducer arThe focal range of a 7.5sound) between the trans- tifact zone. Only the axial anterior and posterior lens cap- MHz transducer is 2 to 5 ducer and the patient, the sules are seen in the noncataractous lens. ( B ) Image centimeters. This transducer palpebral hair should be re- obtained with a 10-MHz transducer. The cornea and can produce good images of  moved or thoroughly wetted aqueous humor are easily seen below the transducer arti- eyes of small2,4,7,9,11 or large4 before the acoustic gel is ap- fact zone. C  = cornea, I  = iris, L  = lens, VIT  = vitreous animals. The 7.5-MHz body, and ON = ON = optic nerve. plied. transducer provides better Exposure of intraocular depth of penetration than contents to the coupling gel does the 10-MHz probe, should be avoided. Transpalpebral ultrasonography  but the anterior segment is lost in the near-field revermay therefore be necessary to examine eyes that have beration artifact (Figure 1A). 6–8 This problem can be at traumatic lesions (e.g., corneal laceration or uveal proleast partially overcome by the use of a tissue-equivalent 10 lapse). If the eyelids are severely swollen, transstand-off pad. 6–8 However, stand-off pads may produce palpebral ultrasonography may be the only possible linear reverberations that can become superimposed means of ophthalmic examination. over the image, thus complicating the interpretation. 3 AURICULOPALPEBRAL NERVE BLOCK

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TRANSPALPEBRAL IMAGING

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TRANSDUCER

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thalmic ultrasonography  have not been reported to occur in animals. Nevertheless, the transducer power should be kept at or below 20% when eyes are being imaged. Higher power settings unnecessarily subject the choroidal tissue and vasculature to increased pulses of ultrasound energy. energy. Setting the time-gain compensation too high unnecessarily amplifies weak  signals. Electronic amplifiFigure 2— Correct placement of the transducer on the cornea (left) and (left)  and the resulting ultra- cation of background noise sonographic image (right) . results in a diffuse scattering of hyperechoic signals throughout the image. 15   Another option is to apply  This amplification may  excess sterile coupling gel to cause a false impression of  the cornea and decrease the degeneration of the vitreous pressure applied with the body.16 transducer, thus allowing the gel to act as a stand-off. stand-off. 13 EiEXAMINATION ther method places the imEach eye is imaged in verage of the cornea and anteritical and horizontal planes or segment of the globe through the visual axis for a deeper on the screen, away  complete examination. Each from the near-field artifact view is optimized through zone. Transpalpebral imagminor adjustments in transing in horses may provide a ducer angle to obtain an opsimilar effect. timal image. Images of the  A 10-MHz transducer has right and left eye may then a focal range of approxibe compared for abnormalimately 3 to 4 centimeters.14 Figure 3A ty or asymmetry. After exDepth of penetration is adeamination, each eye is genquate for thorough visualtly flushed with eyewash or ization of retrobulbar tissue. sterile saline to remove the The 10-MHz transducer coupling gel and associated provides better resolution debris. than does the 7.5-MHz transducer. Although the NORMAL FINDINGS cornea may be lost in the The cornea is represented near-field artifact zone, as a curved hyperechoic inmore of the anterior chamterface immediately below ber will normally be visualthe transducer artifact zone ized (Figure 1B). Visualizaif a stand-off pad or addition of the anterior segment tional coupling gel has been can be improved with the Figure 3B used (Figure 2). The anuse of a stand-off pad or ad- Figure 3— ( A ) Ultrasonographic image (10 MHz) and echoic anterior and poste(B) schematic of a normal equine eye. The hyperchoic corpoditional coupling gel. rior chambers are located  Adverse effects from oph- ra nigra extend from the dorsal pupillary margin of the iris. between the cornea and the FOCAL RANGE

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DEPTH OF PENETRATION

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MACHINE SETTING

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TABLE I Normal Anterior-to-Posterior Dimensions of the Eye Dimension (mm)

ABNORMALITIES   Abnormalities are 17 17 Cat  Horse Cow   classified according to whether they in7.5–7.8 12.0–12.617 12.5 volve the anterior 1 11.9 segment, posterior segment, or retrobul21.0 43.717 35.3 bar or periorbital tis39.41 sue. They may be further categorized 24.717 18.3 as hypo- or hyper1 17.4 echoic, invasive or noninvasive, solid or cystic. Cystic and solid masses within the globe or in periocular tissue have been imaged. 2,4,13,18–20  A pigmented lesion in the anterior uvea might be a uveal cyst or an intraocular melanoma. 8,21 Differentiation of these lesions is critical because uveal cysts are benign and do not require treatment. A free-floating pigmented mass or a light-transmitting mass is considered to be a cyst because these characteristics are not associated with neoplasia. 21 However, masses that remain attached or that fail to transmit light may be either cysts or melanoma. These masses can be distinguished  with ultrasonography and histopathology. histopathology. Ultrasonographically, a cyst is a round, anechoic, ultrasound-transmitting structure. Acoustic enhancement of the far wall and deeper structures implies a fluidfilled structure.3,15 Intraocular melanoma, in contrast, is usually a solid lesion with internal echogenicity and   without ultrasound transmission.8 Because cysts and melanoma arise from the anterior uvea, a stand-off pad, extra coupling gel, or transpalpebral imaging may be required for thorough examination.

Dog 13 Structure Lens

Globe

Posterior lens

Mesoce Mesocepha phalic lic

Dolich Dolichoce ocepha phalic  lic 

7.6

7.6

19.6

21.0

8.8

9.6

surface to retina

anterior lens capsule. The axial anterior and posterior capsule of the lens are visualized. The equator of the lens capsule, however, cannot be visualized ultrasonographically because it is parallel or nearly parallel to the sound beam; waves reflecting from such surfaces miss the transducer and are therefore not imaged.3,6–8,14,15 The equatorial borders of the lens may  be evaluated by altering the transducer placement on the cornea so that these borders are more perpendicular to the incident angle of the sound wave. 6 The normal lens cortex and nucleus are anechoic. 6–8 The ciliary apparatus is the echogenic structure at the lens equator. In horses, because their eyes are large, the iris and corpora nigra (Figure 3) may also be seen. The corpora nigra are represented as an echogenic mass extending into the anechoic aqueous humor from the dorsal pupillary margin of the iris.   Anechoic vitreous body separates the posterior lens capsule from the echogenic posterior eye wall. In an ultrasonographic image of a healthy eye, the retina, choroid, and posterior sclera are not depicted as separate layers.6–8 The optic disk appears as a focal hyperechoic structure with a posterior hypoechoic area at the choroidal surface of the posterior eye wall. The triangular conus is the hypoechoic region that narrows as it extends from the posterior sclera into the deeper, more echogenic retrobulbar musculature and fat. The conus contains the optic nerve, vessels, and retrobulbar fat.  An optimal image (Figure 1B) contains the hyperechoic cornea and anechoic aqueous humor superficially. The thin hyperechoic anterior and posterior lens reflections with the echogenic ciliary body are superficial to the anechoic vitreous body. Deep to these structures are the echogenic choroid and retina with the central hyperechoic optic disk. Deeper still is the triangular, UVEAL CYST

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MELANOMA

hypoechoic conus. Table I lists biometric values.

Anterior Segment  Abnormalities of the anterior segment most commonly involve the iris, ciliary body, or lens. Masses of  the iris and ciliary body include cyst, tumor, and inflammatory granuloma. The most common intraocular neoplasm is lymphosarcoma 22; however, primary tumors of the anterior segment are most often melanoma, adenoma, or adenocarcinoma. 8 The size of the apparent mass can be measured on the ultrasonographic image, thus enabling the clinician to monitor response to therapy or progression of disease during subsequent examinations. In eyes with cataract, the entire lens capsule is visualI

INFLAMMATORY GRANULOMA

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ized ultrasonographically as and location relative to the a relatively smooth hyperiris and posterior eye wall. echoic structure with or withLens luxation may be readiout intralenticular hyperely visualized (Figure 6) and chogenicity (Figure 4). The classified as anterior or posaxial anterior–posterior diterior luxation on the basis mension of the lens may be of biometric measurements increased (intumescence) (posterior lens capsule to because of imbibition of fluposterior eye wall; Table I). id. An acquired decrease in Careful examination and inthe anterior–posterior axial terpretation of increased disdimension of the lens occurs tance between the posterior  with resorption of the liquelens and posterior eye wall is fied cortical material in hynecessary to differentiate anpermature cataracts. terior lens luxation from Microphakia (congenitally  Figure 4—Ultrasonographic image (10 MHz) of the left eye rupture of the posterior assmall lens) may occur as a of a 1-year-old male bichon frise with a cataract. The lens pect of the globe. solitary lesion or may be as- capsule is completely echogenic, and there is a slightly hysociated with other congeni- perechoic intralenticular signal. There is no retinal detach- Posterior Segment tal intraocular abnormali- ment. Vitreous Body  ties. Measurements of the If the time-gain compenanterior–posterior dimensation is properly set, multision of the globe are imporfocal hyperechoic interfaces tant in many instances (e.g., (signals) within the normaldetermining placement of  ly anech anechoic oic vitre vitreou ouss bod body  y  an intraocular implant). indicate hemorrhage, syneresis (degeneration of the vitPerilenticular inflammatory exudate may accompany  reous body), or asteroid anterior uveitis (especially if  hyalosis. As mentioned, setthe uveitis results from a ting the time-gain compengranulomatous granulomatous disease), thus sation too high creates a resulting in a fuzzy hyperefalse impression of hyperchoic border at the periphechogenicity in the posterior segment. ery of the lens capsule (Figure 5). Inflammatory material Liquefaction of the vitre(e.g., fibrin and hypopyon) ous body is a decrease in the may accumulate in the antehyaluronic gel and an inrior chamber as a result of  crease in the free water conFigure 5— Transpalpebral ultrasonographic image (10 anterior uveitis. In a nonotent of the vitreous body. MHz) of a cat with feline infectious peritonitis. Note the paque eye, this material is thickened, fuzzy hyperechoic lens capsule. The linear This shift in composition usually evident with pen- hyperechoic band extending caudally from the posterior results in focal changes in light examination of the an- lens capsule is a fibrin strand. A persistent hyaloid artery  acoustic impedance, thus terior segment, but severe may have a similar appearance if enough inflammatory ex- leading to the focal hyperanterior uveitis may be asso- udate is present. echoic signals seen in cases ciated with diffuse corneal of syneresis (Figure 7). edema, thus making ultra  Asteroid hyalosis is the sonographic examination necessary. Fibrin causes flocformation of calcium and phospholipid crystals within culent hyperechogenicities throughout the anterior the vitreous body, thus resulting in discrete areas of hychamber, whereas hypopyon typically occurs more venperechogenicity. The significance of syneresis in ani22 trally and is more uniform in echogenicity. echogenicity. mals is not well defined; however, its presence may preLens luxation or cataract may result from trauma. It dispose the animal to retinal detachment. 23  Asteroid is important to evaluate the lens of hyphemic eyes for hyalosis, which is common in old dogs but rare in horsinternal opacities (traumatic or preexisting cataract) es and cats, is generally not clinically significant. CATARACT

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ANTERIOR UVEITIS

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LENS LUXATION

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SYNERESIS

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The vitreous body may  also be evaluated for signs of hemorrhage (Figure 8). Hemorrhage in the vitreous body implies poor prognosis because it indicates damage to the ciliary body, body, retina, or 24 choroid (e.g., persistent hyaloid artery). Ultrasonographically, hemorrhage may be anechoic, hypoechoic, or hyperechoic, depending on duration and organization or clot formation. Hemorrhage may occur between the vitreal body  and retina (thus leading to vitreal detachment) or between the retina and posterior eye wall (thus resulting in retinal detachment).

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Figure 6—Ophthalmic ultrasonographic image (10 MHz)

of a dog with posterior luxation of the lens. The lens is located within the vitreous body, adjacent to the posterior eye wall. Internal linear hyperechogenicities are secondary  to the traumatic luxation of the lens.

detachment may elevate only a portion of the retina and may appear as only one hyperechoic strand of the gull-wing or V-shaped detachment. If the retina is also detached from the ciliary body, it may be displaced and folded on itself, thus appearing as an echogenic or hyperechoic fold near the optic disk. Hemorrhage between the retina and vitreous body can cause vitreal detachment,  which may be misdiagnosed as retinal detachment. Detachment of the vitreous body has been described as a distinctive inward bulging at the choroidal surface; the bulging may not terminate at the optic disk as with retinal detachment. Detachment of the vitreous body  usually produces a much thinner, less well-defined interface and may occur in conjunction with retinal detachment. 26 In humans, definitive diagnosis requires superimposition of an Amode vector over the Bmode scan. Detachment of  the vitreous body presents a single spike 30% to 50% as tall as the reference peak,  whereas retinal detachment forms a single spike equal to the reference peak height. 27

Retina  Retinal detachment may  be associated with congenital diseases or inherited disorders, may be idiopathic, may result from inflammation or systemic hypertension, may follow trauma or surgery, or may be associated with cataract. 23 The incidence of retinal detachment increases directly with the maturity of the cataract. 5 Retinal detachment classically appears as two welldefined linear or curvilinear hyperechoic structures with- Figure 7—Ophthalmic ultrasonogram (10 MHz) of an 8in the vitreous body and ex- year-old spayed Maltese. The classic gull-wing hyperechoic tending from the optic disk  detached retina extends into the vitreous body from its attachment at the optic disk. Note the echogenic vitreal detoward the ora ciliaris reti- bris (syneresis) between the strands of the detached retina. Posterior Eye Wall  na.6–8,25 This condition usuRupture of the posterior ally presents a gull-wing or eye wall (Figure 8) may occur in any animal but is more common in large animals V-shaped appearance within the vitreous body (Figure 7). Anechoic or hypoechoic signals usually separate the and is often secondary to blunt trauma. Ultrasonolinear hyperechoic retina from the posterior of the graphically, rupture of the posterior eye wall appears as globe. a uniform echogenicity extending from the vitreous  An anechoic subretinal space suggests the presence of  body to the retrobulbar tissue, without distinct visualnoncellular fluid (e.g., transudate), which may eventuization of the posterior eye wall. This appearance is due 24 ally be resorbed.   An echogenic subretinal space may  to the hemorrhage extending from the vitreous body  indicate hemorrhage, inflammation, or neoplasia, 25 thus into the retrobulbar tissue, through the damaged fi24 resulting in a less-favorable prognosis. Partial retinal brous tunic that was the posterior eye wall. HEMORRHAGE

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RETINAL DETACHMENT

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RUPTURE

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sound beam contacts any  Retrobulbar or highly reflective interface Periorbital Tissue and is almost completely reEvidence of periorbital or flected back to the transducretrobulbar disease is an iner. Dense cataracts may also dication for ultrasonographic 2,4,6–8,20,28 exhibit acoustic shadowing. examination. Solid The stream of echoes that masses may be differentiated seems to trail from a small from cysts, diffuse cellulitis, 6,14,20 quantity of gas is called a or vascular anomaly. comet-tail reverberation arti-  Retrobulbar masses may be  fact .3,11 Large accumulations diffuse or discrete and may  of gas resemble bone or metor may not deform the 4 al, with a hyperechoic signal globe. Retrobulbar inflamand strong acoustic shadowmation is usually diffuse, 15 ing. Combinations of these   whereas masses are more often discrete. Discrete Figure 8—Transpalpebral ultrasonogram (10 MHz) of the images may be seen in penemasses that deform the traumatized eye of a mature dog. The echogenic aqueous trating wounds of the eye. globe are generally believed humor is separated from the equally echogenic vitreous to be neoplastic (Figure 9), body by the anechoic lens. The posterior eye wall is not vi- CONCLUSION Ultrasonography is excelalthough a similar ultra- sualized, thus suggesting rupture of the globe. Rupture of  the globe was confirmed at surgery. lent for evaluating soft-tissue sonographic image may be structures; however, bone, produced by retrobulbar metal, or large quantities of  abscess or disease of the zygas may obscure lesions. Begomatic salivary gland. cause ultrasound does not Retrobulbar abscess may  penetrate bone, ultrasonogbe differentiated from a raphy is poorly suited for retrobulbar cyst by its thickevaluating complex or peneer, more irregular capsule trating osseous changes. Raand more echoic central 6,15,18 diography may better define region. In one study, a radiopaque intraocular or no ultrasonographic criteria periocular foreign body and   were pathognomonic; howmay more completely illusever, diffuse nondeforming trate any secondary periorlesions were most compatibital changes. Ultrasonograble with retrobulbar celluli4 phy may, however, more tis. completely pinpoint the lo An ultrasonographically  cation of the foreign body  guided fine-needle aspiraFigure 9— Ophthalmic ultrasonogram (10 MHz) of an 8before intervention. Theretion or biopsy of retrobulyear-old castrated domestic shorthair with a retrobulbar fore, radiography should acbar or periorbital masses mass distorting the medial (nasal) aspect of the posterior company ultrasonographic ultrasonographic may provide useful diagnoseye wall. The standard nasotemporal transducer angle has evaluation in cases of retrotic or prognostic informabeen adjusted to optimize the distortion of the globe. Fi- bulbar disease, ocular or perition. If the mass is solid, tisbrosarcoma was diagnosed by examination of fine-needle ocular penetrating wounds, sue or cells may be obtained aspirate. or suspected foreign bodies. for cytologic evaluation and In patients with ocular culture. If the lesion is cystrauma, information derived from ultrasonographic extic, the fluid may be drained and submitted for analysis amination helps determine the extent and severity of  and culture. the injury. It also helps provide a more accurate progUltrasonography is often helpful in identifying innosis and aids in the selection of treatment. Ultratraocular, periorbital, or retrobulbar foreign bodies. sonography may be the only useful way to examine an Bone or metal are hyperechoic and produce an aneye that is severely painful or opaque. Ultrasonographic echoic shadow in the tissue immediately deep to 3,15 measurements of identifiable structures may also be them. This acoustic shadowing occurs when the ultraRETROBULBAR ABSCESS

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FOREIGN BODY

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ARTIFACTS

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beneficial in assessing damage and identifying lens luxation or rupture of the globe. To evaluate ultrasonographic changes in ocular and periocular tissue, the veterinarian must have a thorough understanding of normal ocular anatomy and measurements. A basic knowledge of ultrasonography and ultrasonographic artifacts, combined with a knowledge of  ophthalmic disease, is also necessary. For patients without opacity of ocular transmitting media, ophthalmic ultrasonography is an excellent companion to, not a replacement for, routine complete ophthalmic examination. For animals with opaque or painful eyes or severely swollen eyelids, however, ultrasonography may be the most readily available and cost-effective means of  ophthalmic examination. B-mode ultrasonography is noninvasive, safe, fast, and efficient. It can be performed on most awake animals with little preparation of the patient.

About the Authors Dr. Williams is affiliated with the Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana. Dr. Wilkie is affiliated with the Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Ohio State University, Columbus, Ohio, and is a Diplomate of the American College of Veterinary Ophthalmologists.

REFERENCES 1. Cited in Rogers Rogers M, Cartee Cartee RE, Miller Miller W, Ibrahim Ibrahim AK: EvalEvaluation of the extirpated equine eye using B-mode ultrasonography. Vet Radiol 27:24–29, Radiol 27:24–29, 1986. 2. Cited in Dziezyc J, Hager DA, Millichamp NJ: Two-dimenTwo-dimensional real-time ocular ultrasonography in the diagnosis of  ocular lesions in dogs. JAAHA  dogs.  JAAHA 23:501–508, 23:501–508, 1987. 3. Kremkau Kremkau FW: Ultraso Ultrasound, und, in Kremkau Kremkau FW (ed): (ed): Diagnostic  Ultrasound : Principles, Instruments, and Exercises. Philadelphia, WB Saunders Co, 1989, pp 9–56, 65–99, 147–170. 4. Morgan RV: Ultrasonography Ultrasonography of retrobulbar retrobulbar diseases diseases of the dog and cat. JAAHA  cat. JAAHA 25:393–399, 25:393–399, 1989. 5. van der Woerdt A, A, Wilkie DA, Myer CW: Ultrasonographic Ultrasonographic abnormalities in the eyes of dogs with cataracts: 147 cases (1986–1992).  JAVMA 203:838–841,  JAVMA 203:838–841, 1993. 6. Eisenberg Eisenberg HM: Ultrason Ultrasonograp ography hy of the eye and orbit. orbit. Vet  Clin North Am 15:1263–1274, Am 15:1263–1274, 1985. 7. Selcer Selcer BA: The eye, eye, in Cann CC (ed): (ed): Practical Veterinary  Ultrasound . Philadelphia, Lea & Febiger, 1995, pp 43–50. 8. Mattoon JS, Nyland Nyland TG: Ocular ultrasonography, ultrasonography, in Mat-

toon JS, Nyland TG (eds): Veterinary Diagnostic Ultrasound . Philadelphia, WB Saunders Co, 1995, pp 178–197. 178–197. 9. Hager DA, Dziezyc Dziezyc J, Millichamp Millichamp NJ: NJ: Two-dimension Two-dimensional al real-time ocular ultrasonography in the dog: Technique and normal anatomy. Vet Radiol 8:60–65, Radiol 8:60–65, 1987. 10. Regnier A, Toutain PL: Ocular pharmacology pharmacology and therapeutic modalities, in Gelatt KN (ed): Veterinary Ophthalmology . Philadelphia, Lea & Febiger, 1991, pp 162–194. 162–194. 11. Herring DS, Bjornton G: Physics, facts, and artifacts of diagnostic ultrasound. Vet Clin North Am Small Anim Pract  15:1107–1122, 1985. 12. Rantanen NW, Ewing RL III: III: Principles Principles of ultrasound ultrasound application in animals. Vet Radiol 22:196–203, Radiol 22:196–203, 1981. 13. Cottrill NB, Banks WJ, Pechman RD: Ultrasonographic and biometric evaluation of the eye and orbit of dogs.  Am J  Vet Res 50:898–903, Res 50:898–903, 1989. 14. Dziezyc J, Hager Hager DA: Ocular ultrasonography ultrasonography in veterinary  medicine. Semin Vet Med Surg (Small Anim) 3:1–9, Anim)  3:1–9, 1988. 15. Park RD, Nyland Nyland TG, Lattimer Lattimer JC, et al: B-mode gray-scale gray-scale ultrasound: Imaging artifacts and interpretation principles. Vet Radiol 22:204–210, Radiol 22:204–210, 1981. 16. Coleman DJ, Jack RL, Franzen LA: High resolution resolution B-scan ultrasonography of the orbit. I. The normal orbit. Arch orbit.  Arch Oph-  thalmol 88:358–367, thalmol 88:358–367, 1972. 17. Samuelson Samuelson DA: Ophthalmic Ophthalmic embryology embryology and anatomy, anatomy, in Gelatt KN (ed): Veterinary Ophthalmology . Philadelphia, Lea & Febiger, 1991, pp 3–122. 18. Miller WW, Cartee RE: B-scan ultrasonography ultrasonography for the detection of space-occupying ocular masses.  JAVMA  187:66– 68, 1985. 19. Schoster JV, Dubielzig RR, Sullivan L: Choroidal melanoma melanoma in a dog. JAVMA  dog. JAVMA 203:89–91, 203:89–91, 1993. 20. Davidson HJ, Blanchard GL: Periorbital epidermoid cyst in the medial canthus of three dogs.  JAVMA  198:271–272, 1991. 21. 21. Corcoran KA, Koch Koch SA: Uveal cysts in dogs: 28 cases cases (1989– 1991). JAVMA  1991).  JAVMA 203(4):545–546, 203(4):545–546, 1994. 22. Collins BK, BK, Moore CP: Canine anterior uvea, in Gelatt KN (ed): Veterinary Ophthalmology . Philadelphia, Lea & Febiger, 1991, pp 357–395. 23. Curtis R, Barnett Barnett KC, Leon Leon A: Diseases Diseases of the canine posteposterior segment, in Gelatt KN (ed): Veterinary Ophthalmology . Philadelphia, Lea & Febiger, 1991, pp 461–525. 461–525. 24. Koplin Koplin RS, Gerste Gersten n M, Hodes Hodes B: Real Time Ophthalmic Ul-  trasonography and Biometry: A Handbook of Clinical Diagno-  sis . Thorofare, NJ, Slack Inc, 1985, pp 4–19, 53–63, 73–90. 25. Hoskins BN, BN, Koenig HA, Adaska J: What’s your diagnosis? diagnosis?  JAVMA 203:1273–1274,  JAVMA 203:1273–1274, 1993. 26. Zakow ZN, Berlin LA, Gutman FA: Ultrasonographic mapping of vitreoretinal abnormalities.   Am J Ophthalmol  96:622–631, 1983. 27. 27. Byrne Byrne SF, Gree Green n RL: Ultrasound of the eye and orbit . St. Louis, CV Mosby, 1992, pp 1–16, 53–93, 443–446. 28. Abrams K, K, Toal RL: What’s your your diagnosis? diagnosis?  JAVMA  196: 951–952, 1990.