Clinical diagnostic forms of ultrasound are based on physical principles of pulse-echo technology and tissue acoustic impedance mismatch. Pulse-echo technology employs synthetic crystal transducers to produce ultrasonic wavefront pulses and retrieve echoes for electronic display and processing. Send-receive sequences are repeated thousands of times each second.

Echoes are generated at adjoining tissue interfaces that display differences in acoustic impedance: the greater the difference, the stronger the echo. Retinal tissue, for example, is sonically much different than vitreous (which is essentially water). Therefore, strong reflections occur at such interfaces. When adjoining tissues have relatively small differences in acoustic impedance, as with mild vitreous hemorrhages or clumped intravitreal cells, small or weak reflections are seen.

Similar interpretation principles are useful in evaluating types of ocular and orbital tumor tissue. Some lesions demonstrate low-amplitude internal reflectivity; few echoes are seen, and such lesions are described as ultrasonically homogeneous. In contrast, others demonstrate high-amplitude internal reflections; multiple strong retinal echoes are seen, and these lesions are described as ultrasonically heterogeneous. Either tissue type may in addition shadow deeper structures by absorbing or reflecting sonic energy. The presence or degree of shadowing often aids interpretation of ocular and orbital lesions by demonstrating sonic attenuation.

Once reflected, echoes are received, amplified, and electronically processed. They are displayed in visual format as A-scan or B-scan images. A-scan (A-mode or time-amplitude) ultrasonography is used predominantly for tissue characterization and interpretation. The horizontal axis of this display represents time or distance and is useful for linear measurements, whereas the vertical dimension reveals echo amplitude and is useful for characterizing tissues (Fig. 1). A-scan may be performed independently of B-scan. B-scan (B-mode or intensity-modulated) ultrasonography provides a cross-sectional image of the globe and orbit (Fig. 2). B-scans are used predominantly for topographic information. There are overlaps of information from A-scan and B-scan; most physicians rely on both forms for adequate ultrasound interpretation, and instruments are available that present B-scan and A-scan images simultaneously (Fig. 3). The display images are presented horizontally in contact scanning and vertically with waterbath (immersion) scanning^6–8 (Fig. 4).

The B-scan image, a two-dimensional crosssectional display of the globe and orbit, is more easily interpreted than A-scan, especially by examiners familiar with interpretation of ophthalmic pathology, where microscopic and gross crosssectional evaluation of the globe and orbit are routine and often topographically similar to B-scan cross-sections.

Several concepts are critical to accurate interpretation of B-scans. These concepts, although relatively easy to define, require considerable experience before the examiner feels comfortable with diagnosis in opaque media situations. These concepts are real time, gray scale, and threedimensional thinking.

Real time refers to the display of motion or movement during B-scan imaging. This capability is one of the greatest advantages of B-scan. Differences in mobility of a variety of movable intraocular abnormalities can be easily detected. For example, the rapid movement of vitreous hemorrhage is usually easy to appreciate and differentiate from the slower, undulating movements of a recent, rhegmatogenous retinal detachment. On occasion, movement of intraocular structures can be used for detecting abnormalities that lie immediately behind vitreous opacification and may be seen only when the vitreous opacities are moved away from the ocular wall, set in motion by command voluntary movements of the patient's globe (Fig. 5). Gray scale refers to the variable gray tone of display screen echoes. Strong returning signals (echoes) appear white, whereas weaker reflections are seen as gray, the shade depending on echo strength. Examples of strong reflectors of sound include retinal tissue, sclera, and calcifications of any type (Fig. 6). Weaker echoes from clotted or clumped cells within the vitreous are usually easy to differentiate from the stronger reflectors of sound (Fig. 7). Grading of echo strength may be performed only when the examining ultrasound beam is perpendicular to the object of interest, ensuring the strongest possible reflections. However, in the world of ultrasound interpretation, there are many exception to the rules. Tissues that are usually mild reflectors of sound, such as hemorrhage, can at times appear quite strong, especially in complex or multiple pattern disorders, as frequently occur in severe ocular trauma. These apparent inconsistencies often cause misinterpretation by the novice. With experience obtained by multiple examinations, careful follow-up, and clinical correlation whenever possible, diagnostic acumen rapidly improves.

3D thinking is the third B-scan concept necessary for accurate interpretation, and the most difficult to master. During B-scan examinations, the contact probe is almost always in motion, creating multiple, ever-changing two-dimensional B-scan cross-sections that must be mentally combined to create a 3D image of the globe and orbit. This technique is most easily mastered by those familiar with 3D spatial relations. The recent development of tomographic ultrasonography makes this concept easier to master because the student can use the computerized 3D image repeatedly, creating a wide variety of theoretical tomographic slices from the 3D image. Recognition of these patterns during future examinations becomes easier.

During contact B-scan imaging, each quadrant of the globe is scanned systematically from probe positions that avoid, if possible, any passage of the examining or returning sound through the artifact-inducing lens system (Fig. 8). For the beginner, practice with clear media patients, such as those with total or partial retinal detachment, is essential (Fig. 9). The ultrasonically derived vitreoretinal drawing can then be compared to standard optical examinations. As experience increases, similar but more difficult cases can be attempted. Specifically, B-scan examination of diabetic patients with traction retinal detachments and clear media should be attempted and compared to visually controlled drawings. These self-testing exercises, especially in complex cases, provide the examiner a feeling for his or her level of competence and reliability in 3D thinking. Opaque media cases can then be approached with greater confidence and diagnostic accuracy.

TUMORS

A-scan tumor characterization is often extremely helpful to the clinician. A thorough understanding of ophthalmic pathology is critical to interpretation and often predictive of typical A-scan tumor patterns.

Choroidal malignant melanoma, perhaps the most widely studied intraocular mass lesion, has the most reproducible and reliable A-scan pattern. Usually, the initial echo seen in A-scan is a high-amplitude spike secondary to the strong vitreoretinal surface echo overlying the tumor mass. Once the examining sonic beam has passed into the tumor tissue, a rapidly declining amplitude cadence is noted, a consequence of increasing ultrasonic tissue homogeneity. Clinical knowledge of the typical microscopic tumor pattern of tightly packed, homogeneous small cells makes anticipation of relatively low reflectivity possible (Fig. 10). This same low-amplitude reflectivity in B-scan imaging produces a picture that makes the melanoma mass appear hollow. Often, tumor-infiltrated choroid also appears dark (Fig. 11). This change in the normally highly reflective choroidal tissue is widely but inaccurately called choroidal excavation. The terms “hollowing” and “choroidal excavation” are misleading because these tumors are not hollow and the choroid is not excavated. Nevertheless, these terms have been used so frequently in past literature that any change in vocabulary is unlikely.

Tumors with great acoustic heterogeneity, such as choroidal hemangiomas, where adjoining cell and tissue layers have marked differences in acoustic impedance, create large echo amplitudes at each interface. These tumor types have typical high internal reflections at each major interface. These high internal reflections make the lesions appear solid white in B-scan displays and produce highamplitude spikes during A-scan imaging (Fig. 12).

Calcification in any type of tumor tissue creates a strong acoustic interface, resulting in high-amplitude A-scan patterns as well as white echoes in B-scan imaging. Behind the area of calcification, there is usually partial or complete shadowing of the sclera and orbital fat. Bony tumors of the choroid, some retinoblastomas, and drusen of the optic nerve head are typical examples where calcification may be found (Fig. 13).

Unfortunately, there are far more tumor types than there are different ultrasound patterns, and many mass lesions, both benign and malignant, have similar ultrasound appearances. Further, the echo patterns change from one area to another within the same mass lesion. Although much rhetoric swirls in the ophthalmic literature concerning the quality of a variety of instruments, the techniques used, and the expertise of the examiner, the ultrasonic patterns of many tumors simply do not demonstrate differentiating features. In such situations, the ultrasound examiner must hedge the diagnosis, compiling a list of possibilities rather than one. On occasion, interpretation is simply not possible with any degree of reliability.

Some ultrasonographers prefer to use A-scan alone without simultaneous B-scan imaging for their tissue pattern interpretation.¹¹ Separate A-scan units, some of which are packaged as an additional module housed in a simultaneous B-scan and A-scan instrument, are available. The separate units use modified amplifiers and are standardized against special test phantoms to permit comparison of results from one device to another.^12,13

TRAUMA AND FOREIGN BODY EVALUATIONS

Ultrasound examinations for ocular trauma and intraocular foreign bodies are among the most difficult, for a variety of reasons (Figs. 14 and 15). The examiner is often presented with recently injured, unstable, or open globes with multiple complex injuries. Extreme care is necessary to avoid undue pressure during ultrasound evaluation, and concerns for contamination are significant. Understandably, probe contact is often minimized, and unless additional sterile methylcellulose solution is used to improve signal transfer, less-than-optimal images result. These poor-quality images are difficult or impossible to interpret, especially in opaque media situations with multiple abnormalities. Further, patient noncompliance and the examiner's inexperience frequently lead to incomplete examination, limiting 3D analysis.

Despite the difficulties, there are several helpful concepts that aid in the examination of trauma cases:

1. Always examine the globe visually (by slit-lamp technique preferably before ultrasonography) to determine if ocular integrity has been disrupted anteriorly. The possibility of a posterior nonvisible ocular wall rupture must also be considered.
   
2. Clean the examining probe with soap, water, and alcohol, and use skin contact rather than scleral contact whenever possible.
   
3. Ultrasonically examine the normal globe first to gain patient confidence.
   
4. Use sterile methylcellulose solution to permit good signal transfer with less probe contact pressure.
   
5. Take your time and be extremely gentle.
   
6. Consider every trauma patient a potential foreign body patient.
   
7. Radiography for foreign bodies should be part of every evaluation of ocular trauma resulting in opaque media and preferably should be done before ultrasonography.
   

Ocular and orbital foreign body patients require the most careful ultrasonic evaluation, as well as multiple ancillary tests, to provide sufficient diagnostic information. Like ocular trauma, these injuries usually occur unexpectedly and involve high-velocity projectiles. Patient histories are understandably poor or intentionally misleading. Because most foreign bodies are metallic and visible radiographically, appropriate head position for anteroposterior and lateral x-rays of the involved eye and orbit is critical. Routine radiographic imaging quickly supplies information concerning the number, size, and shape of metallic foreign bodies, as well as the presence of any intraocular air. Small bubbles that enter the globe at the time of injury and remain within the vitreous can be confused ultrasonically with small metallic foreign bodies. The large acoustic impedance mismatch between vitreous and air is similar to that between vitreous and metal.

Computed tomography scans are extremely helpful in foreign body localization, although it is often difficult to pinpoint the exact intraocular or extraocular position of metallic foreign bodies close to the ocular wall. Metal detection devices add information about the magnetic or nonmagnetic properties of intraocular metallic foreign bodies. Until the magnetic qualities of any potential intraocular foreign bodies are determined, magnetic resonance imaging should not be performed.

Ultrasound evaluation in patients with intraocular foreign bodies provides extremely useful additional information concerning associated ocular injuries and another method for localization, especially with nonmetallic intraocular foreign bodies not visible using x-ray techniques. Ultrasonically, foreign bodies have great reflectivity once the examining beam is placed perpendicular to a reflective surface of the foreign body. These abnormalities remain visible even with extreme attenuation of the examining signal (Fig. 16). Many metallic foreign bodies, especially those that are round or spherical, demonstrate “ringing,” a string of reflections that extend posterior to the foreign body in the form of a cometlike tail. Ringing is an ultrasound artifact produced by multiple “ping-pong” reflections of sound pulses within the foreign body before they return to the examining probe. The string of returning echoes produces an unusual display image.

When combined with radiographic information and metal localization technique, ultrasonically derived information aids enormously in the clinical evaluation of the patient with an intraocular foreign object. However, ultrasound alone is not sufficient to rule out a foreign object.

PHTHISIS

Once the globe has become small and shrunken, ultrasound evaluation becomes nearly impossible. Often, calcifications of a variety of ocular tissues make signal transfer difficult. General landmarks become obscure, and interpretation is unreliable. Small tumor identification in such situations is usually beyond the scope of reason.

ORBITAL ULTRASONOGRAPHY

Orbital ultrasonography is far more complex than ocular evaluation for a number of reasons. The shape and depth of the orbit make access for diagnostic ultrasound techniques more difficult. Ocular examinations are usually performed with 10-megacycle probes or higher frequencies. Lower ultrasound frequencies (5 to 8 megacycles) are often used to penetrate to orbital tissue depths. Lower-frequency probes have less resolution capability. Further, recognition of abnormalities within orbital tissues is more difficult because orbital tissues are highly reflective, making the appearance of subtle gray-scale changes in B-scan or amplitude changes in A-scan more difficult to appreciate against highly reflective tissues. Nevertheless, a vast quantity of literature is available concerning orbital technique and tissue characterization.^8,9,14,15

In general, orbital ultrasonography is most useful in the evaluation of patients with exophthalmos related to a number of diseases, including primary and secondary tumors of the orbit, inflammatory diseases, and changes secondary to thyroid disease.

The displacement of orbital fat by more homogeneous tumor tissue is appreciated more readily in B-scan imaging. Tissue differentiation is better appreciated with simultaneous A-scan or standardized A-scan techniques. Two caveats are important: orbital interpretation is not for the novice, and examination of the deeper portions of the orbit, including the orbital apex, is usually best performed by other techniques, such as computed tomography or magnetic resonance imaging. Inflammatory conditions such as pseudotumor can involve a variety of orbital tissues, causing infiltration and thickening of the extraocular muscles, as well as space-occupying lesions within the orbital fat. Frequently, inflammatory exudate accumulates in the potential space between Tenon's capsule and the sclera. Such accumulations are easily detected during ultrasonography as dark, relatively echo-free spaces just outside the strong scleral echo.

Thyroid orbitopathy and exophthalmos are also frequently associated with thickening of the extraocular muscles, as well as other changes of the orbital contents.

POSTERIOR VITREORETINAL INTERFACE EVALUATION

Recent improvements in image quality and fused, real-time display have made ultrasound image interpretation easier for every ultrasonographer. Visualization of subtle changes such as movement and recognition of the posterior formed vitreous hyaloid are now possible, even in clear media situations. These clear vitreous structures, which are often exceedingly difficult to appreciate optically, can be recognized ultrasonically after a relatively short period of training (Fig. 17). Clinically, establishing the position of the posterior hyaloid is important in evaluating a variety of vitreous retinal disorders, such as macular holes, tractional detachments, and partial or complete vitreous separations.^16,17

For immersion scanning (waterbath technique), photographs can be used to preserve information; with 35-mm gray scale, review is possible. Immersion scanning, unlike contact scanning, provides images that are derived from an anteroposterior waterbath position. A micrometer device on the examining probe permits sequential still photographic documentation. Dynamic real-time scanning is more difficult with the immersion technique, which relies on rapid multiple manual sweeps of the hand-controlled probe to provide real-time imaging. Real-time changes are best appreciated with motor-driven probes and preserved for future reference on videotape.

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