B-Scan Ocular Ultrasound

B-scan ultrasonography is an important adjuvant for the clinical assessment of various ocular and orbital diseases. With understanding of the indications for ultrasonography and proper examination technique, one can gather a vast amount of information that cannot be obtained through clinical examination alone. ^[1]  

B-scan ultrasound is most useful when direct visualization of intraocular structures is difficult or impossible. Situations that prevent normal examination include lid problems (eg, severe edema, partial or total tarsorrhaphy), keratoprosthesis, corneal opacities (eg, scars, severe edema), hyphema, hypopyon, miosis, pupillary membranes, dense cataracts, and vitreous opacities (eg, hemorrhage, inflammatory debris). ^[2]

In such cases, diagnostic B-scan ultrasound can accurately image intraocular structures and provide valuable information on the status of the lens, vitreous, retina, choroid, and sclera. However, in many instances, ultrasound is used for diagnostic purposes even though pathology is clinically visible. Such instances include differentiating iris and ciliary body lesions; differentiating vitreous opacities and membrane formations; differentiating and measuring intraocular tumors; identifying serous versus hemorrhagic choroidal detachments; identifying rhegmatogenous versus tractional or exudative retinal detachments; ruling out scleritis; and identifying disc drusen versus papilledema.

With an understanding of ultrasound principles, thorough examination techniques, and knowledge of ultrasound characteristics of a variety of intraocular pathologies, the ophthalmologist includes B-scan ultrasound of the eye as a vital part of the diagnostic armamentarium. Without this tool, the clinician may not be able to detect or manage a variety of ocular diseases. However, as with any technical skill, B-scan ultrasound requires training, time, and experience to achieve a high level of confidence and high-quality imaging.

Ophthalmic ultrasonography uses high-frequency sound waves, which are transmitted from a probe into the eye. As sound waves strike intraocular structures, they are reflected back to the probe and are converted into an electrical signal. This signal is subsequently reconstructed as an image on a monitor that can be used to perform a dynamic evaluation of the eye or photographed to document pathology.

Sound is emitted in a parallel, longitudinal wave pattern. The frequency of the sound wave is the number of cycles, or oscillations, per second, measured in hertz (Hz). For sound to be considered ultrasound, it must have a frequency greater than 20,000 oscillations per second, or 20 KHz, rendering it inaudible to human ears. ^[3] The higher the frequency of the ultrasound, the shorter the wavelength (distance from the peak of one wave to the peak of the next wave). A direct relationship exists between wavelength and depth of tissue penetration (the shorter the wavelength, the more shallow the penetration). However, as the wavelength shortens, image resolution improves.

Given that ophthalmic examinations require little in the way of tissue penetration (with an eye measuring 23.5 mm long on average) and much in the way of tissue resolution, ultrasound probes used for ophthalmic B-scan are manufactured with very high frequencies of about 10 million oscillations per second, or 10 MHz. In contrast, ultrasound probes used for purposes such as obstetrics use lower frequencies for deeper penetration into the body, and because the structures being imaged are larger, they do not require the same degree of resolution. High-resolution ophthalmic B-scan probes (ultrasound biomicroscopy [UBM]) of 20 to 50 MHz that penetrate only about 5 to 10 mm into the eye for incredibly detailed resolution of the anterior segment have been manufactured. ^[4]

Velocity

The velocity of the sound wave is dependent on the density of the medium through which sound travels. Sound travels faster through solids than through liquids—an important principle to understand because the eye is composed of both. Velocities of different components of the eye are known, with sound traveling through both aqueous and vitreous at a speed of 1,532 meters/second (m/s), and through the cornea and the lens at an average speed of 1,641 m/s. ^[3]

Reflectivity

When sound travels from one medium to another medium of different density, part of the sound is reflected from the interface between those media back into the probe. This is known as an echo; the greater the density difference at that interface, the stronger the echo, or the higher the reflectivity. ^[3]

In A-scan ultrasonography, a thin, parallel sound beam is emitted, which passes through the eye and images 1 small axis of tissue, the echoes of which are represented as spikes arising from baseline. The stronger the echo, the higher the spike. For example, the vitreous is less dense than the vitreous hyaloid, which, in turn, is much less dense than the retina. Therefore, the spike obtained as sound strikes the interface of the vitreous and the hyaloid is shorter than the spike obtained when sound strikes the hyaloid-retinal interface.

(See the image below.)

In B-scan ultrasonography, an oscillating sound beam is emitted, passing through the eye and imaging a slice of tissue, the echoes of which are represented as a multitude of dots that together form an image on the screen. The stronger the echo, the brighter the dot. In the same example, the dots that form the posterior vitreous hyaloid membrane are not as bright as the dots that form the retinal membrane. This is very useful in differentiating a posterior vitreous detachment (a benign condition) from a more highly reflective retinal detachment (a blinding condition).

(See the image below.)

Angle of incidence

The angle of incidence of the probe is critical for both A-scan and B-scan ultrasonography. When the probe is held perpendicular to the area of interest, more of the echo is reflected directly back into the probe tip and is sent to the display screen. When the probe is held oblique to the imaged area, part of the echo is reflected away from the probe tip, and less is sent to the display screen. ^[3] The more obliquely the probe is held from the area of interest, the weaker is the returning echo and the more compromised is the displayed image.

On A-scan, the greater the perpendicularity, the more steeply rising is the spike from baseline and the higher is the spike. On B-scan, the greater the perpendicularity, the brighter are the dots on the surface of the area of interest. The size and the shape of the surface at each interface also affect that reflection. When a surface is large and flat and the probe is held in a perpendicular manner, the complete echo returns to the probe for display. If the surface is curved or irregular in shape, part of the echo is reflected away and less echo is returned to the probe for display, even when the probe is held in a perpendicular manner. If the interface is very small, as in a vitreous opacity, so much of the sound is scattered that the returning echo is very weak.

Because various parts of the eye and various pathologies differ in size and shape, understanding this concept and anticipating the best possible display for that eye are important. Perpendicularity to the area of interest should be maintained to achieve the strongest echo possible for that structure.

Absorption

Ultrasound is absorbed by every medium through which it passes. The denser the medium, the greater is the amount of absorption. ^[3] This means that the density of the solid lid structure results in absorption of part of the sound wave when B-scan is performed through the closed eye, thereby compromising the image of the posterior segment. B-scan should be performed on the open eye unless the patient is a small child or has an open wound. When it is performed on the open eye, the patient is now able to look in extreme downgaze, which is impossible when the eye is closed and is rotated upward. Because the probe is placed directly on the conjunctiva, and because the probe face and the globe must not be separated by any air, a liberal amount of gel-type tear solution should be placed on the probe face prior to examination. Care should be taken that only those solutions designed for ophthalmic use are incorporated, so eye irritation does not occur.

Likewise, when ultrasound is performed through a dense cataract as opposed to the normal crystalline lens, more sound is absorbed by the dense cataractous lens and less is able to pass through to the next medium, resulting in weaker echoes and images on both A-scan and B-scan. For this reason, the best images of the posterior segment are obtained when the probe is in contact with the conjunctiva rather than the corneal surface, bypassing the crystalline lens or the intraocular lens implant. Finally, when calcification of tissue is present, absorption is so great and reflection of the echo back to the probe is so strong that there is no signal posterior to that medium. This is referred to as shadowing.

(See the image below.)

Ophthalmic ultrasound instruments use what is known as a pulse-echo system, which consists of a series of emitted pulses of sound, each followed by a brief pause (microseconds) for receiving echoes and processing to the display screen. Amplification of the display can be altered by adjusting the gain, which is measured in decibels (dB). Adjusting the gain in no way changes the frequency or velocity of the sound wave but acts to change the sensitivity of the instrument's display screen. When gain is high, weaker signals such as vitreous opacities and posterior vitreous detachments are displayed. When gain is low, weaker signals disappear and only stronger echoes such as the retina remain on the screen. However, resolution, or detail, of the area of interest is better when gain is lowered. Typically, all examinations begin on highest gain so that no weak signals are missed; then gain is reduced as necessary for good resolution of stronger signals. ^[3]

Sterile dry eye gel should be dispensed onto the surface of the probe face, which is usually oval in shape and, when placed on the anesthetized globe, is represented by the initial white line on the left side of the display screen. The vitreous cavity is displayed in the center of the echogram, and the posterior pole is displayed on the right side of the echogram. A marker (usually a dot or a line) is present on the side of the probe handle near its face, on one side of the short end of the oval. Knowing the orientation of the marker at all times is extremely important because it represents the upper portion of the echogram. The back-and-forth motion of the transducer occurs along the long portion of the oval; thus, the slice is emitted in the direction of the marker.

In other words, if the area of interest is at the 3-o'clock position, the probe face is held on the globe at the 9-o'clock position, with the marker aimed upward. The center of the probe is aimed at the 3-o'clock position, which appears in the center of the right side of the echogram—the area of best resolution. The top of the right side of the echogram represents the 12-o'clock position because that is the orientation of the marker, and the bottom of the echogram on the right represents the 6-o'clock position because that is the portion opposite the marker. Therefore, the slice of tissue on the right side of the display extends from the 12-o'clock position to the 6-o'clock position, with the 3-o'clock position in the center. If the probe is held at the 9-o'clock position but is rotated so the marker is now aimed inferiorly, the 3-o'clock position remains in the center of the display, but now the 6-o'clock position is at the top and the 12-o'clock position is at the bottom.

Transverse probe positions

The transverse probe position is used most commonly. This technique demonstrates the lateral extent of the pathology and encompasses approximately 6 clock hours. ^[3] Because of the covered area, this orientation is used for basic screening examinations when there is no view of the posterior segment.

(See the image below.)

With the eye anesthetized, the patient should be instructed to look in the direction of the area of interest. The probe face is coated in gel-type tear solution and is positioned on the opposite conjunctival surface parallel to the limbus, regardless of probe location around the globe, with the marker aimed superiorly or nasally. Consequently, the marker is oriented superiorly when the nasal or temporal globe is examined (3-o'clock or 9-o'clock position) and toward the nose when the superior or inferior globe is examined (12-o'clock or 6-o'clock position). When the probe is aimed at an oblique clock hour, such as 10:30 or 5:00, the marker should be oriented in the superior portion of the oblique angle.

Knowing the position of both the probe face and the marker in relation to the patient's gaze is critical to understanding the position and orientation of pathology within the eye. When these are known, what lies in the center, top, and bottom on the right side of the echogram and all meridians in between is understood. For example, if the superior portion of the right globe is scanned, the patient looks upward, and the probe is placed on the conjunctiva at the 6-o'clock position, with the marker aimed toward the nose at the 3-o'clock position. The cross section of the eye seen on the display screen corresponds to the following aspects of the globe: the 12-o'clock position will be centered on the right side of the echogram, the 3-o'clock position will be at the top, and the 9-o'clock position will be at the bottom. Thus, clock hours represented are the 3-o'clock, 2-o'clock, 1-o'clock, 12-o'clock, 11-o'clock, 10-o'clock, and 9-o'clock positions, respectively, from top to bottom on the right side.

For oblique positions, such as the 1:30 position in the left eye, the patient should be instructed to look up and left. The probe is held on the opposite conjunctiva (inferonasal) with the marker oriented superiorly in the oblique angle. Thus, from the top of the display to the bottom, clock hours represented are the 10:30, 11:30, 12:30, 1:30, 2:30, 3:30, and 4:30 positions, respectively. Any clock hour can be centered easily, and by adding and subtracting about 3 clock hours, all clock hours represented can be estimated. The designation of each transverse scan is by that clock hour in the center on the right side, followed by an estimation of how far in the periphery the slice is at that clock hour. The labeling system for this estimation is as follows: P for posterior pole, PE for posterior/equator, EP for equator/posterior, E for equator, EA for anterior to the equator, O for ora serrata, and CB for ciliary body. ^[3]

The limbus-to-fornix approach is a technique used to examine each meridian from the posterior pole to the periphery. The probe is swept from the limbus to the fornix as far as possible, pivoting on axis to follow the curvature of the globe. When the probe is placed at the limbus, the sound beam slice is aimed at the posterior pole. When it is brought toward the fornix, the slice is now aimed more peripherally, and the farther it can be moved into the fornix, the more anterior is the scan. By sweeping back and forth, limbus-to-fornix, in each transverse meridian, several clock hours are examined not only at once but also from the posterior pole out to the anterior portion of the globe.

Longitudinal probe positions

Whereas transverse probe positions demonstrate the lateral extent of pathology, longitudinal probe positions represent the radial extent. Longitudinal scans demonstrate only 1 clock hour per echogram radially, but that clock hour is represented from the posterior pole out to the anterior equator or the ora serrata. ^[3]

(See the image below.)

This technique is an adjuvant to the transverse probe examination in many situations, but most importantly for intraocular tumors and retinal tears. The transverse probe position assesses the lateral width of an intraocular tumor, whereas the longitudinal probe position evaluates the radial extent and proximity to the optic nerve. Because the flap of a retinal tear is directed radially toward the posterior pole from the periphery, a longitudinal scan is the only way to image the flap. If only transverse cuts are used, a retinal tear can easily remain undetected and therefore untreated, resulting in subsequent retinal detachment. When a tumor is measured, height can be measured on either a transverse scan or a longitudinal scan, but width of the lesion must be measured in both the lateral direction and the radial direction, so the largest width can be detected for treatment (eg, determining radiation plaque size).

As with transverse scans, the patient is instructed to look in the direction of the area of interest, and the probe face is placed on the opposite conjunctival surface. However, in longitudinal scanning, the probe face is rotated so it is perpendicular to the limbus, with the marker directed toward the limbus, or toward the area of interest, regardless of the clock hour being examined. This results in the optic nerve shadow being represented at the bottom on the right side of each longitudinal echogram, and the posterior pole just above the nerve shadow. The anterior portion of the clock hour is represented at the top of the right side. The designation of the longitudinal scan is simply the clock hour being examined, followed by an "L."

The limbus-to-fornix approach should be used in longitudinal scanning to adequately center the pathology into the area of best resolution. For instance, if the pathology is located near the posterior pole, when the probe is placed near the limbus, that area will be centered. However, when the pathology is located in the far periphery, the probe will need to be shifted farther into the fornix to achieve adequate centration. The nerve shadow will shift downward, and, depending on how far into the fornix the shift occurs, the shadow may be moved so far inferiorly that it is no longer visible on the display screen.

Axial probe positions

The term "axial probe position" as used in B-scan echography is different in meaning from the term "axial" as used in A-scan biometry. In biometry, this term is used for measurement of the length of the eye along the visual axis, or through the vertex of the cornea, the center of the lens, and the center of the macula.

In B-scan echography, the term "axial" refers to centering of the posterior lens curve to the left of the echogram and the optic nerve shadow to the right of the echogram rather than the macula. ^[3]

(See the image below.)

If this is to be accomplished, the patient must be looking in primary gaze and the probe should be centered on the corneal vertex. Because the probe is being placed on the cornea rather than on the conjunctiva, additional gel-type solution should be used for cushioning to protect the corneal surface. Because the optic nerve inserts into the globe just nasal to the macula, the probe should be tilted to aim the sound beam slightly nasally to image the nerve in the right center of the echogram. The orientation of the marker depends on the desired meridian. Because the ultrasound slice is emitted from the probe tip in the direction of the longest oval of the probe face along the line of the marker, any clock hour can be imaged in the upper and lower quadrants of the right side by changing marker orientation.

Note that because sound is now traveling through the lens, some absorption will occur, compromising the fundus image. If an intraocular lens is present on axial scanning, artifact reverberations will occur in the vitreous cavity, as with A-scan biometry.

A horizontal axial scan is accomplished by rotating the marker to aim toward the nose or the 3-o'clock position for the right eye, or the 9-o'clock position for the left eye. This results in the slice cutting through the nerve horizontally, with the nasal meridian (ie, 3-o'clock position right eye, 9-o'clock position left eye) at the top of the right side and the temporal meridian (ie, 9-o'clock position right eye, 3-o'clock position left eye) at the bottom. This is the most useful axial scan for basic screening purposes because both the nerve and the macula are in the display. Because the macula is located just temporal to the optic nerve, the macula is located just inferior to the nerve shadow on the echogram.

A vertical axial scan is produced by rotating the marker superiorly toward the 12-o'clock position in either eye. The slice will now cut through the nerve vertically, with the 12-o'clock position at the top on the right and the 6-o'clock position at the bottom in either the right eye or the left eye.

For oblique axial scans, the marker is rotated to include the clock hours desired. By convention, if the meridian desired is located above midline, the marker should be directed toward that meridian. If the meridian desired is located below midline, the marker should be oriented opposite that meridian. For example, if the desired meridian is a tumor at the 11-o'clock position, the marker should be rotated toward the 11-o'clock position and the tumor will appear in the upper-right of the scan, with the 5-o'clock position at the bottom, below the nerve. However, if the tumor resides at the 5-o'clock position, the marker should be rotated toward the 11-o'clock position and the 11-o'clock position will appear at the top-right of the scan, with the 5-o'clock position tumor below the nerve, at the bottom.

(See the image below.)

Basic screening refers to an examination performed when there is no view into the eye because of opaque media such as corneal edema or scarring, extremely dense cataracts, or vitreous hemorrhages, and determination of the status of the posterior segment is required. In these cases, the highest gain setting must be used to visualize any weak signals such as vitreous opacities and posterior vitreous detachments, or to gauge the extent of vitreous hemorrhages. If any pathology such as retinal or choroidal detachments is found, gain may be reduced for better resolution of stronger signals from these structures once basic screening is completed and documented.

Technique

During basic screening, the entire globe must be examined, from the posterior pole out to the far periphery. Through a limbus-to-fornix approach, each quadrant is evaluated carefully. ^[3] The 4 major quadrants include the 12-o'clock, 3-o'clock, 6-o'clock, and 9-o'clock positions, each centered on the right side of the echogram in transverse approaches. Because approximately 6 clock hours are imaged at once, when each quadrant is examined, the areas examined will overlap, thereby reassuring the examiner that the entire periphery of the globe is visualized. A photo or printed documentation of each of the 4 quadrants should be obtained. Next, the posterior pole should be documented with a horizontal axial scan, which incorporates both the optic nerve and the macula in a single echogram. If no additional pathology is detected, these 5 echograms complete the examination.

Centering pathology found during basic screening

If any posterior pathology is detected during basic screening, it should be centered on the right side of the echogram to achieve greatest resolution. This is accomplished by determining the clock hour represented in the center, top, and bottom of the right side on the transverse scan where it was discovered, and then determining where this pathology lies in relation to those clock hours. Once this has been determined, the patient should be instructed to redirect his or her gaze to that meridian, with the probe then placed on the opposite scleral surface. Perpendicularity to the pathology is achieved when it is centered, and when the vertex of the pathology is a brighter white. Gain is now reduced until the greatest resolution is achieved, and photographic documentation is produced with proper labeling.

Additional scans may be required, such as longitudinal scans to document the radial aspect of the pathology, axial scans to document location of the pathology from the optic disc, and diagnostic A-scans for tissue differentiation.

Localization of the macula

The 4 methods used to localize and center the macula include the following: horizontal, vertical, transverse, and longitudinal. Depending on the eye, one method may be preferable to another, or a combination of methods may be desired.

The horizontal method involves placing the probe on the corneal vertex with the marker nasally, as with a horizontal axial scan, but rather than tilting to center the nerve, the probe should be aimed straight ahead to center the macula. The nerve shadow will now shift upward slightly, and the macula will be centered to the right of the echogram, with the posterior lens surface centered to the left. These scans should be labeled HMAC. ^[3]

(See the image below.)

The vertical method involves again placing the probe on the corneal vertex, but the marker is in the 12-o'clock position. Rather than tilting to center the optic nerve, as with a vertical axial scan, the probe should be aimed straight back to center the macula. The nerve will not appear in these scans because this is a vertical (instead of a horizontal) slice of the macula. These scans should be labeled VMAC. ^[3]

(See the image below.)

The transverse method involves fixating the patient slightly temporally and placing the probe onto the nasal sclera with the marker at the 12-o'clock position. With the optic nerve used as the center of the imagined clock, the macula is at the 9-o'clock position at the posterior pole in the right eye, and at the 3-o'clock position at the posterior pole in the left eye. This scan bypasses the lens, thereby preventing absorption or reverberation artifacts from an intraocular lens. These scans should be labeled TMAC. ^[3]

(See the image below.)

The longitudinal method also bypasses the lens. This method involves directing the patient's gaze slightly temporally, with the probe on the nasal sclera and the marker oriented toward the limbus or temporally toward the macula. This is a horizontal scan of the macula, with the nerve at the bottom-right of the echogram and the macula just superior to the nerve, and with the lateral rectus muscle visibly coursing through the orbit. These scans should be labeled LMAC. ^[3]

(See the image below.)

Because both the nerve and the macula are imaged, the longitudinal method may be preferable if the patient has a cataract or an intraocular lens in place; in this orientation, the lens is bypassed while an image is produced with both the optic nerve and the macula in a single echogram.

High-resolution technique

High-resolution B-scan probes have been developed for higher-quality imaging of the anterior segment and have proved useful for many pathologies, including lesions of the iris and ciliary body, sulcus-to-sulcus measurements, angle measurements, and imaging lenses. As the frequency of the sound wave increases, the resolution increases, but the depth of tissue penetration decreases. These high-resolution probes range from 20 to 50 MHz, with penetration depths of about 10 to 5 mm, respectively; therefore, they may be used only for imaging the anterior segment of the eye. Images rendered from these probes are far superior to those obtained by the standard immersion technique because of higher resolution. However, the zone of focus with these high-resolution transducers is small; thus, the examiner must adjust the depth of the transducer within the saline to obtain best image quality.

(See the image below.)

These probes may have an external oscillating transducer and can be placed in a special scleral shell, although care must be taken so the probe never slips into the shell far enough for the transducer to come in contact with the cornea as it oscillates. Another method is to slip the probe into a disposable balloon-type apparatus filled with distilled water to form a protective layer of water between the transducer and the patient's eye. ^[3] When a small amount of methylcellulose is placed on the vertex of the cover for sound conduction, the tip of the cover rests on the eye directly over the pathology for imaging.

Again, care should be taken to not push hard enough against the eye for the transducer to contact the eye, or for the bag of water to indent the globe, particularly if scanning is done to measure the angle. When these high frequencies are used, any membrane in front of the transducer causes a degree of sound attenuation; best image quality is obtained with the scleral shell technique.

With high-resolution imaging of the anterior segment, the marker is oriented, as with contact scanning, for transverse, longitudinal, and axial cuts of the posterior segment. The main difference is that the probe is placed directly over the area of interest; and depending on the manufacturer, the depth of the transducer in the water must be adjusted to achieve maximum focus.

Vitreous

In a young healthy eye, the vitreous is relatively echolucent. However, as the eye ages, the vitreous undergoes syneresis, and low reflective vitreous opacities can be detected. A posterior vitreous separation (a benign condition of the aging eye) may occur and is represented as a mobile, fine, thin, low reflective line on B-scan. ^[5]

(See the image below.)

Other conditions or diseases of the vitreous that can be detected with ultrasound include asteroid hyalosis—another benign condition of the vitreous in which calcium salts accumulate in the vitreous cavity. The calcium is relatively dense and therefore produces multiple pinpoint, highly reflective vitreous opacities.

(See the image below.)

Vitreous hemorrhage can occur in several different situations such as after trauma or a retinal tear, or as a complication of diabetes mellitus or a retinal vein occlusion. The echographic pattern of a vitreous hemorrhage depends on its age and severity. Fresh mild hemorrhages appear as small dots or linear areas of low reflective mobile vitreous opacities, whereas in more severe, older hemorrhages, blood organizes and forms membranes. The membranes form large interfaces that are visualized echographically as a vitreous filled with multiple large opacities that are higher in their reflectivity. Vitreous hemorrhages may also layer inferiorly due to gravitational forces.

(See the image below.)

Membrane formation can occur after trauma, particularly after penetrating or perforating eye injuries. A membranous track often develops along the path of the offending object. For penetrating injuries, this track may end in the vitreous cavity or at an impact site opposite the entry site. For perforating injuries, the track spans the eye from the entry site to the exit site. Therefore, following the track may lead to an intraocular foreign body and/or retinal pathology at an impact or exit site. Intraocular foreign bodies can be detected easily with ultrasound. Even if it has already been detected with some other imaging modality, such as computed tomography or magnetic resonance imaging, ultrasound can more precisely localize the foreign object. This can reveal extremely vital information, because it can determine how the surgeon approaches the case.

In a study of the reliability of ocular ultrasonography for presurgical evaluation of various vitreo-retinal conditions, including trauma, diabetic vitreous hemorrhage, endophthalmitis, and other causes of vitreous hemorrhage, overall sensitivity and specificity were 92.31% and 98.31%, respectively, for identification of rhegmatogenous retinal detachment, and 96.2% and 100%, respectively, for posterior vitreous detachment. For eyes with trauma, sensitivity was 90.9% and specificity 97.7% in identifying the status of the retina. ^[5]

Retina

A retinal tear can be detected with ultrasound when longitudinal approaches are used. On occasion, retinal tears are accompanied by vitreous hemorrhages, which preclude visualization of the etiologic tear. In such instances, one can often see the posterior vitreous hyaloid or a vitreous strand attached to the retinal flap. These tend to occur in the far periphery, where the vitreous is most firmly attached to the retinal surface, particularly superotemporally. A shallow cuff of subretinal fluid may accompany the tear and may make the diagnosis more evident.

(See the image below.)

In a prospective study conducted in patients with retinal tears (RT) diagnosed by B-scan ultrasound, researchers sought to describe the predominant location of RT, factors influencing their location, and the vitreous status of eyes with RT by using ultrasonography. Study results showed a superior location of RT diagnosed by ultrasonography in more than two thirds of cases associated with significantly shorter axial length than in other locations. Study authors concluded that this finding could increase the sensitivity of ultrasound for RT detection and could help to improve the ultrasonography learning curve of ophthalmologists in training and surgical decision-making when the retina is inaccessible because of opacity media. ^[6]

When a retinal detachment is present, the examiner sees a highly reflective, undulating membrane. In patients with total retinal detachments, the typically folded surface attaches to the ora serrata anteriorly and the optic nerve posteriorly. Initially, a retinal detachment is relatively mobile (with eye movement). However, over time, proliferative vitreoretinopathy (epiretinal membrane formation) can occur, and the retina becomes stiffer and takes on more of a funnel configuration.

(See the image below.)

Retinoschisis is a condition in which there is a split between specific layers of the retina. Clinically, differentiating a retinoschisis from a retinal detachment is difficult. Ultrasound can assist in differentiation because retinoschisis is more focal, smooth, dome shaped, and thin.

In a study that used high-resolution ultrasound B-scan to differentiate retinoschisis from retinal detachment, in eyes with retinoschisis the outer retina demonstrated the presence of 2 hyperreflective lines corresponding to interfaces of the outer plexiform layer and the retinal pigment epithelium, whereas eyes with retinal detachment demonstrated 2 hyperreflective lines in the detached portion, corresponding to the nerve fiber layer and the outer plexiform layer interfaces; the attached portion revealed the presence of the third hyperreflective interface. These findings correlated well with spectral-domain optical coherence tomography (SD-OCT). ^[7]

B-scan ultrasonography is used commonly for initial and follow-up evaluation of retinoblastoma. Retinoblastoma, a highly malignant retinal cancer found in infants and young children, commonly has focal areas of calcification within the tumor. Ultrasound can easily detect the calcium, represented as highly reflective foci within the tumor or the vitreous.

(See the image below.)

When small, the tumors are smooth, dome shaped, and low to medium in internal reflectivity. As tumors grow, they become more irregular in configuration, and more highly reflective as the amount of calcium accumulates. This pediatric cancer can be unilateral and unifocal, unilateral and multifocal, or bilateral. Ultrasound has become a very useful and very cost-effective way to follow these tumors as treatment is delivered. Baseline tumor size measurements and tumor locations are obtained, and these parameters are monitored closely during and after treatment.

Typically, the presence of leukocoria (a white pupil) alerts the parent or the pediatrician to this disease. However, multiple other pediatric retinal diseases, such as persistent hyperplastic primary vitreous (PHPV), retinopathy of prematurity (ROP), Coats disease, and medulloepithelioma, are associated with leukocoria. PHPV, also called persistent fetal vasculature (PFV), is almost always a unilateral condition in which the primary vitreous (particularly the hyaloid vessel) fails to regress and continues to extend from the optic nerve to the posterior lens capsule. Echographically, one can detect the very thin, persistent hyaloidal vessel coursing from the disc to the lens when longitudinal approaches are used. Other echographic features may include a retrolental membrane, a small globe (small axial length), and in severe cases, associated traction or total retinal detachment.

(See the image below.)

ROP is a bilateral disease that may be asymmetric in its severity but is commonly quite symmetric. This disease has various stages; however, the most advanced stage (stage V) often has a white pupillary reflex. Stage V disease, defined as a total retinal detachment due to peripheral contraction of fibrovascular proliferative tissue, commonly has a funnel configuration. The configuration of this detachment is detected easily with ultrasound.

Coats disease is a unilateral condition characterized by retinal vascular telangiectasia and, when severe, an exudative retinal detachment. This disease can be the most difficult to differentiate from retinoblastoma. However, ultrasound is very useful because of lack of calcium and the presence of cholesterol in the subretinal space. In areas of telangiectasia, the retina is commonly thickened.

A medulloepithelioma is a rare tumor that primarily arises in the ciliary body of children. Typical ultrasound features include a dome-shaped configuration, high internal reflectivity, moderate vascularity, and multiple cystic spaces.

Choroid

Echographically, the choroid is much thicker than the retina. When the retina and the choroid are still apposed, one can see a double spike on diagnostic A-scan, a highly reflective spike representing the vitreoretinal interface, and a slightly less reflective spike representing the retinochoroidal interface. ^[8] A choroidal detachment may occur spontaneously, after trauma, or following a variety of intraocular surgeries. On ultrasound, the detachment is smooth, dome shaped, and thick. Virtually no movement is seen with eye movement. When extensive, one can see multiple dome-shaped detachments, which may "kiss" in the central vitreous cavity. When choroidal detachments are hemorrhagic rather than serous (as is commonly seen in traumatic situations), the subchoroidal space is filled with a multitude of dots in contrast to the echolucent subchoroidal space of a serous choroidal detachment.

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The most common tumor of the choroid is malignant melanoma. Although this tumor can arise in the ciliary body or in the iris, it is seen most commonly in the choroid. As with retinoblastoma, ultrasound has become invaluable in the diagnosis and follow-up evaluation of uveal malignant melanomas. This homogenous, highly cellular tumor results in low to medium internal reflectivity and regular internal structure. ^{[3, 9]} Diagnostic A-scan and B-scan can detect internal vascularity in most melanomas.

A nearly pathognomonic finding is a collar button configuration (ie, mushroom shape), but this shape is seen in less than 25% of cases. Histologically, the collar button represents the portion of the tumor that has broken through Bruch's membrane—a basement membrane found between the choroid and the retina.

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Typically, a choroidal melanoma has a smooth, dome shape. Diffuse melanomas have a relatively flat shape and an irregular contour but maintain low to medium internal reflectivity.

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When a portion of a melanoma outgrows its blood supply, that portion of the tumor may necrose and bleed internally, or into the subretinal, vitreous, or suprachoroidal space. If the hemorrhage is extensive, blood may prevent echographic detection of the tumor. In such cases, follow-up examination is vital. When the tumor bleeds internally, the examiner may see highly reflective pockets within the tumor and a consequently irregular internal structure. Because larger melanomas produce significant internal sound attenuation, reflectivity is lower at the base of the tumor; this is referred to as acoustic hollowing.

Occasionally, choroidal evacuation is seen at the base of the tumor. This is believed to represent the tumor invading deeper choroidal structures. A melanoma can progress further and extend through the scleral wall; this is referred to as extrascleral extension and usually occurs along emissary canals. Ultrasound is probably the only reliable method of detecting small posterior extrascleral extensions.

Such information is critical for management decision-making and prognosis. If a melanoma is treated with brachytherapy, intraoperative echographic localization of the plaque in relation to the tumor significantly improves treatment success. Finally, if eye-sparing treatments can be provided, such as brachytherapy, proton beam irradiation, or transpupillary thermal therapy, ultrasound is invaluable in monitoring tumor response in both size and internal reflectivity. A favorable response is seen as a progressively regressing tumor with increasingly higher internal reflectivity. Obviously, an unfavorable response is seen in a tumor that continues to grow.

Benign melanocytic tumors include nevi and melanocytomas. Similar to a melanoma, the pigmentation of a nevus can range from no pigmentation (amelanotic) to deep brown pigmentation (melanotic). Melanocytomas typically are heavily pigmented. They, too, have a dome-shaped configuration but, in contrast to melanoma, are highly reflective and do not have internal vascularity. Unfortunately, small melanomas may show absence of low internal reflectivity; consequently, it may be difficult to differentiate a small benign lesion from a malignant one of similar size.

Metastatic tumors can spread to the choroid due to their highly vascular nature. These tumors have a much different echographic appearance. Clinically, they are creamy or yellow in color and multilobulated. Echographically, these tumors usually have an irregular lumpy contour, an irregular internal structure, medium to high internal reflectivity, and little evidence of internal vascularity. Although exudative detachments occur with uveal melanomas, metastatic tumors of similar size generally have more extensive detachments. Extrascleral extension can be seen with these tumors and therefore is not helpful in differentiation of the tumor.

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Choroidal hemangioma is a benign vascular tumor of the choroid. This tumor can produce localized exudative retinal detachments and subsequent vision loss. Clinically, this is an orange, dome-shaped tumor. Echographically, a choroidal hemangioma is dome shaped and has high internal reflectivity. An overlying serous retinal detachment can be seen with B-scan. A more diffuse form of a choroidal hemangioma is seen in Sturge-Weber syndrome. In these patients, the tumor is more extensive and is less elevated.

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Calcific choroidal tumors are easily differentiated and detected with B-scan. A choroidal osteoma clinically appears as a yellow lesion, commonly located near the optic nerve. These tumors are not significantly elevated. On ultrasound, they have very high internal reflectivity due to calcium. Their contour is usually flat and smooth, but on occasion, these tumors are lumpy in appearance. Marked shadowing occurs posterior to the tumor due to absorption of sound energy by calcium.

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Ciliary body

The ciliary body is visualized best with high-resolution scanning; however, the immersion method, or even the contact method, can be used to evaluate more posterior aspects of the ciliary body. A ciliary body detachment can extend into the peripheral choroid and can be seen on contact B-scan, although it is displayed best on high-resolution scanning. A low to medium reflective cleft is seen in the subciliary space.

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Ciliary body tumors are similar to those seen in the choroid. The most common ciliary body tumors are melanomas; however, a variety of other tumors, including metastatic tumors, medulloepitheliomas, and leiomyomas, do arise in the ciliary body.

Sclera

Diagnostic ultrasonography is probably the best method for evaluating scleral thickening. Scleral thickening occurs in cases of nanophthalmos (very small eyes), ocular hypotony, phthisis bulbi, and scleritis. In scleritis, the degree of scleral thickening can vary from mild to severe, and thickening can be focal or diffuse. Associated edema is commonly noted adjacent to the sclera. This manifests as an echolucent area in the Tenon space. When posterior and adjacent to the optic nerve, it forms a T-sign. Other associated findings include a thickened, highly reflective sclera, retinal detachments, and ciliochoroidal detachments.

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Patients who are myopic may have focal areas of thinning sclera. These areas can form staphylomas, or out-pouchings. Ultrasound is the best imaging modality for staphylomatous changes. With trauma, occult scleral ruptures can be difficult to appreciate on clinical examination. Ultrasound typically cannot detect the actual rupture; however, several echographic clues can assist the clinician. These clues include hemorrhage in the immediate episcleral space, a thickened or detached choroid, a detached retina in the area of concern, vitreous hemorrhage, or vitreous incarcerated into the rupture.

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Optic nerve

Optic disc cupping usually can be seen on clinical examination. However, if media opacities prevent examination, the contour (including the degree of cupping) can be detected with ultrasound. Similarly, optic nerve colobomas are imaged easily with ultrasound.

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When they are seen clinically, differentiating papilledema (optic disc edema) from pseudopapilledema is critical because the former is associated with elevated intracranial pressure and the latter may have no systemic relevance. Optic disc drusen, calcific nodules buried within the optic nerve head, can simulate papilledema. On ultrasound, these nodules are highly reflective and exist at or within the optic nerve head.

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In true papilledema, increased intracranial pressure (ICP) is transmitted along the subdural space within the optic nerve. Clinical entities that can cause elevated ICP include pseudotumor cerebri and intracranial tumors. When ICP is mildly elevated, the optic nerve is slightly widened. In more severe cases, one can see an echolucent circle within the optic nerve sheath (separating the sheath from the optic nerve). This is the so-called crescent sign.

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The presence of increased fluid within the sheath is confirmed best with the 30-degree test, which is a dynamic A-scan test that measures the width of the optic nerve in primary gaze and again after the patient shifts gaze 30 degrees from primary. In cases of increased ICP, the nerve and the sheath are stretched as the globe turns 30 degrees, and subarachnoid fluid is distributed over the extent of the nerve, resulting in lesser measurements than are found in primary gaze. If nerve enlargement occurs as the result of parenchymal infiltration or thickening of the optic nerve sheath, the measurement will not change as the globe turns from primary.