b scan in ophthalmology

7/29/2019 B Scan in ophthalmology

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B-Scan Ocular Ultrasound

Ophthalmic ultrasonography uses high-frequency sound waves, which aretransmitted from a probe into the eye. As the sound waves strike intraocular

structures, they are reflected back to the probe and converted into an electricsignal. The signal is subsequently reconstructed as an image on a monitor, whichcan be used to make a dynamic evaluation of the eye or can be photographed todocument pathology.

Sound is emitted in a parallel, longitudinal wave pattern, similar to that of light.The frequency of the sound wave is the number of cycles, or oscillations, persecond measured in hertz (Hz). For sound to be considered ultrasound, it musthave a frequency of greater than 20,000 oscillations per second, or 20 KHz,rendering it inaudible to human ears. 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 oftissue penetration (the shorter the wavelength, the more shallow the penetration).However, as the wavelength shortens, the image resolution improves.

Given that ophthalmic examinations require little in the way of tissue penetration(an eye being 23.5 mm long on average) and much in the way of tissueresolution, ultrasound probes used for ophthalmic B-scan are manufactured withvery high frequencies of about 10 million oscillations per second, or 10 MHz. Incontrast, ultrasound probes used for purposes such as obstetrics use lowerfrequencies for deeper penetration into the body, and, because the structuresbeing imaged are larger, they do not require the same degree of resolution.

Recently, high-resolution ophthalmic B-scan probes (ultrasound biomicroscopy orUBM) of 20-50 MHz have been manufactured that penetrate only about 5-10 mminto the eye for incredibly detailed resolution of the anterior segment.

Velocity

The velocity of the sound wave is dependent on the density of the mediumthrough which the sound travels. Sound travels faster through solids than liquids,an important principle to understand since the eye is composed of both. Thereare known velocities of different components of the eye, with sound travelingthrough both aqueous and vitreous at a speed of 1,532 meters/second (m/s) and

through the cornea and lens at an average speed of 1,641 m/s.

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 intothe probe. This is known as an echo; the greater the density difference at thatinterface, the stronger the echo, or the higher the reflectivity.


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In A-scan ultrasonography, a thin, parallel sound beam is emitted, which passesthrough the eye and images one small axis of tissue; the echoes of which arerepresented as spikes arising from a baseline (see Image 1). The stronger theecho, the higher the spike. For example, the vitreous is less dense than thevitreous hyaloid, which, in turn, is much less dense than the retina. Therefore, the

spike obtained as the sound strikes the interface of the vitreous and hyaloid isshorter than the spike obtained when the sound strikes the hyaloid-retinalinterface.

In B-scan ultrasonography, an oscillating sound beam is emitted, passingthrough the eye and imaging a slice of tissue; the echoes of which arerepresented as a multitude of dots that together form an image on the screen(see Image 2). The stronger the echo, the brighter the dot. Using the sameexample, the dots that form the posterior vitreous hyaloid membrane are not asbright as the dots that form the retinal membrane. This is very useful indifferentiating a posterior vitreous detachment (a benign condition) from a more

highly reflective retinal detachment (a blinding condition).

Angle of incidence

The angle of incidence of the probe is critical for both A-scan and B-scanultrasonography. When the probe is held perpendicular to the area of interest,more of the echo is reflected directly back into the probe tip and sent to thedisplay screen. When held oblique to the area imaged, part of the echo isreflected away from the probe tip and less is sent to the display screen. Themore oblique the probe is held from the area of interest, the weaker the returningecho and, thus, the more compromised the displayed image.

On A-scan, the greater the perpendicularity, the more steeply rising the spike isfrom baseline and the higher the spike. On B-scan, the greater theperpendicularity, the brighter the dots on the surface of the area of interest. Thesize and shape of the surface at each interface also affect that reflection. When asurface is large and flat and the probe is held in a perpendicular manner, thecomplete echo returns to the probe for display. If the surface is curved orirregular in shape, part of the echo is reflected away and less echo returns to theprobe for display, even when the probe is held in a perpendicular manner. If theinterface is very small, as in a vitreous opacity, so much of the sound is scatteredthat the returning echo is very weak.

Because various parts of the eye and various pathologies are different in sizeand shape, understanding this concept and anticipating the best possible displayfor that eye are important. Perpendicularity to the area of interest should bemaintained to achieve the strongest echo possible for that structure.

Absorption
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Ultrasound is absorbed by every medium through which it passes. The moredense the medium, the greater the amount of absorption. This means that thedensity of the solid lid structure results in absorption of part of the sound wavewhen B-scan is performed through the closed eye, thereby compromising theimage of the posterior segment. Therefore, B-scan should be performed on the

open eye unless the patient is a small child or has an open wound. By performingon the open eye, the patient is also now able to look in extreme down gaze,which is impossible when the eye is closed and rotated upward.

Likewise, when performing an ultrasound through a dense cataract as opposedto the normal crystalline lens, more of the sound is absorbed by the densecataractous lens and less is able to pass through to the next medium, resulting inweaker echoes and images on both A-scan and B-scan. For this reason, the bestimages of the posterior segment are obtained when the probe is in contact withthe sclera rather than the corneal surface, bypassing the crystalline lens orintraocular lens implant. Finally, when calcification of tissue is present, there is so

much absorption and such a strong reflection of the echo back to the probe thatthere is no signal posterior to that medium. This is referred to as shadowing

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 briefpause (microseconds) for the receiving of echoes and processing to the displayscreen. The amplification of the display can be altered by adjusting the gain,which is measured in decibels (dB). Adjusting the gain in no way changes thefrequency or velocity of the sound wave but acts to change the sensitivity of theinstrument's display screen. When the gain is high, weaker signals are displayed,such as vitreous opacities and posterior vitreous detachments. When the gain is

low, the weaker signals disappear, and only the stronger echoes, such as theretina, remain on the screen. However, there is better resolution, or detail, of thearea of interest when the gain is lowered. Typically, all examinations begin onhighest gain so that no weak signals are missed; then, the gain is reduced asnecessary for good resolution of the stronger signals.

The probe face is usually oval in shape and when placed on the globe isrepresented by the initial white line on the left side of the display screen. Thevitreous cavity is displayed in the center of the echogram, and the posterior poleis displayed on the right side of the echogram. There is a marker (usually a dot orline) on the side of the probe handle near its face, on one side of the short end ofthe oval. Knowing the orientation of the marker at all times is extremely importantbecause it represents the upper portion of the echogram. The back-and-forthmotion of the transducer occurs along the long portion of the oval; thus, the sliceemitted occurs in the direction of the marker.

In other words, if the area of interest is at the 3-o'clock position, the probe face isheld on the globe at the 9-o'clock position with the marker aimed upward. Thecenter of the probe is aiming at the 3-o'clock position, which appears in the


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center of right side of the echogram, the area of best resolution. The top of theright side of the echogram represents the 12-o'clock position since that is theorientation of the marker, and the bottom of the echogram on the right representsthe 6-o'clock position since that is the portion opposite the marker. Therefore, theslice of tissue on the right side of the display is 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 atthe 9-o'clock position but 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'clockposition is at the top and the 12-o'clock position is at the bottom.

Transverse probe positions

The transverse probe position most commonly is used. This techniquedemonstrates the lateral extent of the pathology and encompassesapproximately 6 clock hours (see Image 4). Because of the area covered, thisorientation is used for basic screening examinations when there is no view of the

posterior segment.

With the eye anesthetized, the patient should be instructed to look in the directionof the area of interest. The probe face is positioned on the opposite scleralsurface parallel to the limbus, regardless of probe location around the globe, withthe marker aimed either superiorly or nasally. Consequently, the marker isoriented superiorly when examining the nasal or temporal globe (3-o'clock or 9-o'clock positions) and toward the nose when examining the superior or inferiorglobe (12-o'clock or 6-o'clock positions). When the probe is aimed at an obliqueclock hour, such as 10:30 or 5:00, the marker should be oriented in the superiorportion of the oblique angle.

Knowing the position of both the probe face and the marker in relation to thepatient's gaze is critical to understanding the position and orientation of pathologywithin the eye. By knowing these, what lies in the center, top, and bottom on theright side of the echogram, and all meridians in between, is understood. Forexample, if scanning the superior portion of the right globe, the patient looksupward, and the probe is placed on the sclera at the 6-o'clock position, with themarker aimed toward the nose at the 3-o'clock position. The cross section of theeye seen on the display screen corresponds to the following aspects of theglobe: 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 thebottom. Thus, the 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 topto bottom on the right side.

For oblique positions, such as the 1:30 position in the left eye, the patient shouldbe instructed to look up and left. The probe is held on the opposite sclera(inferonasal) with the marker oriented superiorly in the oblique angle. Thus, fromthe top of the display to the bottom, the clock hours represented are the 10:30,
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11:30, 12:30, 1:30, 2:30, 3:30, and 4:30 positions, respectively. Any clock hourcan be centered easily, and, by adding and subtracting about 3 clock hours, all ofthe clock hours represented can be estimated. The designation of eachtransverse scan is by that clock hour in the center on the right side, followed byan 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 forposterior/equator, EP for equator/posterior, E for equator, EA for anterior to theequator, O for ora serrata, and CB for ciliary body.

A limbus-to-fornix approach is a technique used to examine each meridian fromthe posterior pole to the periphery. The probe is swept from the limbus to thefornix 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 theposterior pole. When brought toward the fornix, the slice is now aimed moreperipherally, and the further it can be moved into the fornix, the more anterior thescan. By sweeping back-and-forth, limbus-to-fornix, in each transverse meridian,

several clock hours are being examined not only at once but also from theposterior 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 scansdemonstrate only 1 clock hour per echogram radially, but that clock hour isrepresented from the posterior pole out to the anterior equator or ora serrata (seeImage 5).

This technique is an adjuvant to the transverse probe examination in manysituations but most importantly for intraocular tumors and retinal tears. Thetransverse probe position assesses the lateral width of an intraocular tumor,whereas the longitudinal probe position evaluates the radial extent and theproximity to the optic nerve. Because the flap of a retinal tear is directed radiallytoward the posterior pole from the periphery, a longitudinal scan is the only wayto image the flap. If only transverse cuts are used, a retinal tear can easilyremain undetected and therefore untreated, resulting in subsequent retinaldetachment. When a tumor is being measured, the height can be measured oneither a transverse scan or a longitudinal scan, but the width of the lesion mustbe measured in both the lateral direction and the radial direction so that thelargest width can be detected for treatment (eg, determining radiation plaquesize).

As with transverse scans, the patient is instructed to look in the direction of thearea of interest, and the probe face is placed on the opposite scleral surface.However, in longitudinal scanning, the probe face is rotated so that it isperpendicular to the limbus, with the marker directed toward the limbus, ortoward the area of interest, regardless of the clock hour being examined. This


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results in the optic nerve shadow being represented at the bottom on the rightside of each longitudinal echogram, and the posterior pole just above the nerveshadow. The anterior portion of the clock hour is represented at the top of theright side. The designation of the longitudinal scan is simply the clock hour beingexamined followed by an "L."

A limbus-to-fornix approach should be used in longitudinal scanning toadequately center the pathology into the area of best resolution. For instance, ifthe pathology is located near the posterior pole, by placing the probe near thelimbus, that area will be centered. However, if the pathology is located in the farperiphery, the probe will need to be shifted farther into the fornix to achieveadequate centration. The nerve shadow will shift downward, and, depending onhow far into the fornix the shift, the shadow may be moved so far inferiorly that itis no longer visible on the display screen.

Axial probe positions

The term axial probe position as used in B-scan echography is different inmeaning to the term axial as used in A-scan biometry. In biometry, this term isused for the measurement of the length of the eye along the visual axis, orthrough the vertex of the cornea, center of the lens, and the center of the macula.

In B-scan echography, the term axial refers to the centering of the posterior lenscurve to the left of the echogram and the optic nerve shadow to the right of theechogram rather than the macula (see Image 6). To accomplish this, the patientmust be looking in primary gaze, and the probe should be centered on thecorneal vertex. 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 toimage the nerve in the right center of the echogram. The orientation of themarker depends on the desired meridian. Because the ultrasound slice is emittedfrom the probe tip in the direction of the longest oval of the probe face along theline of the marker, any clock hour can be imaged in the upper and lowerquadrants of the right side by changing marker orientation.

Note that because the sound is now traveling through the lens, some absorptionwill occur, compromising the fundus image. If an intraocular lens is present onaxial 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 thenose, or the 3-o'clock position for the right eye or the 9-o'clock position for the lefteye. This results in the slice cutting through the nerve horizontally, with the nasalmeridian (ie, 3-o'clock position right eye, 9-o'clock position left eye) at the top ofthe 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 basicscreening purposes because the nerve and macula are both in the display.


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Because the macula is located just temporal to the optic nerve, the macula islocated 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 thebottom in either the right or 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 markershould be directed toward that meridian. If the meridian desired is located belowmidline, the marker should be oriented opposite that meridian. For example, if thedesired meridian is a tumor at the 11-o'clock position, rotate the marker towardthe 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 tumorresides 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 thescan, with the 5-o'clock position tumor below the nerve, at the bottom

Basic screening refers to an examination performed when there is no view intothe eye because of opaque media, such as corneal edema or scarring, extremelydense cataracts, or vitreous hemorrhages, and the determination of the status ofthe posterior segment is required. In these cases, the highest gain setting mustbe used to visualize any weak signals, such as vitreous opacities and posteriorvitreous detachments, or to gauge the extent of vitreous hemorrhages. If anypathology such as retinal or choroidal detachments is found, then the gain maybe reduced for better resolution of the stronger signals from these structures

once the basic screening is completed and documented.

Technique

It is vital that during basic screening the entire globe be examined, from theposterior pole out to the far periphery. Using a limbus-to-fornix approach, eachquadrant is evaluated carefully. 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 theechogram in transverse approaches. Because approximately 6 clock hours areimaged at once, by examining each quadrant, 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 shouldbe obtained. Next, document the posterior pole with a horizontal axial scan,which incorporates both the optic nerve and the macula in one echogram. If noadditional pathology is detected, these 5 echograms complete the examination.

Centering pathology found during basic screening


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If any posterior pathology is detected during basic screening, it should becentered on the right side of the echogram to achieve greatest resolution. This isaccomplished by determining the clock hour represented in the center, top, andbottom of the right side on the transverse scan where it was discovered, and thendetermining where this pathology lies in relation to those clock hours. Once

determined, the patient should be instructed to redirect his or her gaze to thatmeridian, with the probe then placed on the opposite scleral surface.Perpendicularity to the pathology is achieved when it is centered and when thevertex of the pathology is a brighter white. The gain is now reduced until thegreatest resolution is achieved, and photographic documentation is producedwith proper labeling.

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

Localization of the macula

The 4 methods of localizing and centering of the macula are as follows:horizontal, vertical, transverse, and longitudinal. Depending on the eye, onemethod may be preferable to another, or a combination of methods may bedesired.

The horizontal method involves placing the probe on the corneal vertex with themarker nasally, as with a horizontal axial scan; but, rather than tilting to centerthe nerve, the probe should be aimed straight ahead to center the macula. Thenerve 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 (seeImage 7). These scans should be labeled HMAC.

The vertical method involves again placing the probe on the corneal vertex, butthe marker is in the 12-o'clock position. Rather than tilting to center the opticnerve, as with a vertical axial scan, the probe should be aimed straight back tocenter the macula (see Image 8). The nerve will not appear in these scansbecause this is a vertical (instead of horizontal) slice of the macula. These scansshould be labeled VMAC.

The transverse method involves the patient fixating slightly temporally and

placing the probe onto the nasal sclera with the marker at the 12-o'clock position.Using the optic nerve 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 positionat the posterior pole in the left eye (see Image 9). This scan bypasses the lens,thereby preventing absorption or reverberation artifacts from an intraocular lens.These scans should be labeled TMAC.
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The longitudinal method also bypasses the lens. This method involves directingthe patient's gaze slightly temporally, with the probe on the nasal sclera and themarker oriented toward the limbus or temporally toward the macula. This is ahorizontal scan of the macula, with the nerve at the bottom-right of the echogramand the macula just superior to the nerve, with the lateral rectus muscle visible

coursing through the orbit (see Image 10). These scans should be labeledLMAC.

Because both the nerve and the macula are imaged, the longitudinal methodmay be preferable if the patient has a cataract or intraocular lens in place; in thisorientation, the lens is bypassed while still producing an image with both the opticnerve and the macula in one echogram.

Immersion technique

Because the area of best resolution is in the center on the right side of an

echogram, examining the anterior segment with a standard 10 MHz contractprobe can be accomplished only with the use of a scleral shell. This shifts theanterior segment to the right and into the area of focus of the sound beam,improving resolution of anterior segment pathology. The shell is filled withmethylcellulose or some other viscous solution to a meniscus, avoiding airbubbles within the shell. The probe is placed on top of the shell. This producesan echolucent area on the left side of the echogram corresponding to the shelland methylcellulose, and it shifts the anterior segment to the right side of thedisplay screen (see Image 11).

With contact B-scan, the patient looks toward the pathology, and the probe is

held opposite to adequately center the pathology, whereas with immersion B-scan, the patient looks opposite the pathology to center the area of interestdirectly under the shell. Diagnostic A-scan also can be performed through theshell, directly over the lesion, for tissue differentiation.

High-resolution technique

High-resolution B-scan probes have been developed for higher quality imaging ofthe anterior segment. As the frequency of the sound wave increases, theresolution increases, but the depth of tissue penetration decreases. These high-resolution probes range from 20 MHz to 50 MHz, with penetration depths of

about 10 mm to 5 mm, respectively; therefore, they may be used only for imagingthe anterior segment of the eye. The images rendered from these probes are farsuperior to that of the standard immersion technique because of the higherresolution (see Image 12).

These probes may have an external oscillating transducer and can be placed in aspecial scleral shell, although great care must be taken so that the probe neverslips into the shell far enough for the transducer to come in contact with the


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cornea as it oscillates. Another method is to slip a small handheld pen-typetonometer cover over the tip of the probe and fill it with tap water to form aprotective layer of water between the transducer and the patient's eye. By placinga small amount of methylcellulose on the vertex of the cover for soundconduction, the tip of the cover rests on the eye directly over the pathology for

imaging. Care should again be taken not to push hard enough against the eye forthe transducer to contact the eye.

For both immersion and high-resolution imaging of the anterior segment, themarker is oriented as with contact scanning for transverse and longitudinal cuts.

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 canbe 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(see Image 13).

Other conditions or diseases of the vitreous that can be detected with ultrasoundinclude asteroid hyalosis, another benign condition of the vitreous where calciumsalts accumulate in the vitreous cavity. The calcium is relatively dense and,therefore, produces multiple pinpoint, highly reflective vitreous opacities (seeImage 14).

Vitreous hemorrhage can occur in several different situations, such as aftertrauma or a retinal tear or as a complication of diabetes mellitus or a retinal veinocclusion. The echographic pattern of a vitreous hemorrhage depends on its ageand severity. Fresh mild hemorrhages appear as small dots or linear areas of lowreflective mobile vitreous opacities (see Image 7), whereas in more severe olderhemorrhages, blood organizes and forms membranes. The membranes formlarge interfaces that are visualized echographically as a vitreous filled withmultiple large opacities that are higher in their reflectively. Vitreous hemorrhagesmay also layer inferiorly due to gravitational forces.

Membrane formation also can occur after trauma, particularly after penetrating or

perforating eye injuries. A membranous track often develops along the path ofthe offending object. In penetrating injuries, this track may end in the vitreouscavity or at an impact site opposite the entry site. In perforating injuries, the trackspans the eye from the entry site to the exit site. Therefore, following the trackmay lead to an intraocular foreign body and/or retinal pathology at an impact orexit site. Intraocular foreign bodies can be detected easily with ultrasound. Evenif already detected with some other imaging modality, such as computerizedtomography or magnetic resonance imaging, ultrasound can more precisely


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localize the foreign object. This can be extremely vital information because it candetermine how the surgeon approaches the case.

Retina

A retinal tear can be detected with ultrasound when using longitudinalapproaches. On occasion, retinal tears are accompanied by vitreoushemorrhages, which preclude visualization of the etiologic tear. In suchinstances, one often can see the posterior vitreous hyaloid or a vitreous strandattached to the retinal flap (see Image 15). These tend to occur in the farperiphery, where the vitreous is most firmly attached to the retinal surface,particularly superotemporally. A shallow cuff of subretinal fluid may accompanythe tear and make the diagnosis more evident.

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 nerveposteriorly (see Image 16). Initially, a retinal detachment is relatively mobile (witheye movement). However, with time, proliferative vitreoretinopathy (epiretinalmembrane formation) can occur, and the retina becomes more stiff and takes onmore of a funnel configuration.

Retinoschisis is a condition where there is a split between specific layers of theretina. Clinically, differentiating a retinoschisis from a retinal detachment isdifficult. Ultrasound can further assist in the differentiation because retinoschisisis more focal, smooth, dome-shaped, and thin.

B-scan ultrasonography commonly is used for the initial and follow-up evaluationof retinoblastoma. Retinoblastoma, a highly malignant retinal cancer found ininfants and young children, commonly has focal areas of calcification within thetumor. Ultrasound can easily detect the calcium, represented as highly reflectivefoci within the tumor or vitreous (see Image 17). When small, the tumors aresmooth, dome shaped, and are low to medium in internal reflectivity. As thetumors grow, they become more irregular in configuration and more highlyreflective as the amount of calcium accumulates. This pediatric cancer can beunilateral and unifocal, unilateral and multifocal, or bilateral. Ultrasound hasbecome a very useful and very cost effective way to follow these tumors astreatment is delivered. Baseline tumor size measurements and tumor locations

are obtained, and these parameters are monitored closely during and aftertreatment.

Typically, the presence of leukocoria (a white pupil) alerts the parent or thepediatrician to this disease. However, multiple other pediatric retinal diseases areassociated with leukocoria, such as persistent hyperplastic primary vitreous(PHPV), retinopathy of prematurity (ROP), Coats disease, andmedulloepithelioma. PHPV, also called persistent fetal vasculature (PFV), is


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almost always a unilateral condition where the primary vitreous (particularly thehyaloid vessel) fails to regress and continues to extend from the optic nerve tothe posterior lens capsule. Echographically, one can detect the very thinpersistent hyaloidal vessel coursing from the disc to the lens when longitudinalapproaches are used (see Image 18). Other echographic features may include a

retrolental membrane, a small globe (small axial length), and, in severe cases, anassociated traction or total retinal detachment.

ROP is a bilateral disease that may be asymmetric in its severity but is commonlyquite symmetric. There are various stages of this disease; however, the mostadvanced stage (stage V) often has a white pupillary reflex. Stage V disease isdefined as a total retinal detachment due to peripheral contraction offibrovascular proliferative tissue and commonly has a funnel configuration. Theconfiguration 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 diseasecan be the most difficult to differentiate from retinoblastoma. However, ultrasoundis very useful because of the lack of calcium and the presence of cholesterol inthe subretinal space. In the areas of telangiectasia, the retina is commonlythickened.

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

Choroid

Echographically, the choroid is much thicker than the retina. When the retina andchoroid are still apposed, one can see a double spike on diagnostic A-scan, ahighly reflective spike representing the vitreoretinal interface, and a slightly lessreflective spike representing the retinochoroidal interface. A choroidaldetachment may occur spontaneously, after trauma, or following a variety ofintraocular 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 centralvitreous cavity (see Image 19). When choroidal detachments are hemorrhagicrather than serous (as commonly seen in traumatic situations), the subchoroidal

space is filled with a multitude of dots in contrast to the echolucent subchoroidalspace of a serous choroidal detachment.

The most common tumor of the choroid is malignant melanoma. Although thesecan arise in the ciliary body or iris, they most commonly are seen in the choroid.Like retinoblastoma, ultrasound has become invaluable in the diagnosis andfollow-up evaluation of uveal malignant melanomas. This homogenous highlycellular tumor results in low-to-medium internal reflectivity and regular internal


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structure. Diagnostic A-scan and B-scan can detect internal vascularity in mostmelanomas.

A nearly pathognomonic finding is a collar button configuration (ie, mushroomshape), 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 the Bruchmembrane, a basement membrane found between the choroid and the retina(see Image 20). Typically, a choroidal melanoma has a smooth, dome shape(see Image 4). Diffuse melanomas have a relatively flat shape and an irregularcontour but maintain low-to-medium internal reflectivity.

When a portion of a melanoma outgrows its blood supply, that portion of thetumor may necrose and bleed internally, or into the subretinal, vitreous, orsuprachoroidal space. If the hemorrhage is extensive, the blood may preventechographic 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. Since largermelanomas produce significant internal sound attenuation, there is a lowerreflectivity at the base of the tumor, which is referred to as acoustic hollowing.

Occasionally, choroidal evacuation is seen at the base of the tumor. This isbelieved to represent the tumor invading the deeper choroidal structures. Amelanoma can progress further and extend through the scleral wall, referred toas extrascleral extension. This usually occurs along emissary canals. Ultrasoundis probably the only reliable method of detecting small posterior extrascleralextensions.

Such information is critical to management decision making and prognosis. If amelanoma is treated with brachytherapy, intraoperative echographic localizationof the plaque in relation to the tumor has significantly improved treatmentsuccess. Finally, if eye-sparing treatments can be performed, such asbrachytherapy, proton beam irradiation, or transpupillary thermal therapy,ultrasound is invaluable in monitoring the tumor response in both size andinternal reflectivity. A favorable response is a progressively regressing tumor withincreasingly higher internal reflectivity. Obviously, an unfavorable response is atumor that continues to grow.

Benign melanocytic tumors include nevi and melanocytomas. Like a melanoma,

the pigmentation of a nevus can range from no pigmentation (amelanotic) to adeep brown pigmentation (melanotic). A melanocytoma typically is heavilypigmented. They, too, have a dome-shaped configuration but, in contrast tomelanoma, are highly reflective and do not have internal vascularity.Unfortunately, small melanomas may show an absence of low internal reflectivity,and, consequently, it may be difficult to differentiate a small benign lesion from asimilar sized malignant one.


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Metastatic tumors can spread to the choroid due to its highly vascular nature.These tumors have a much different echographic appearance. Clinically, thesetumors are creamy or yellow in color and multilobulated. Echographically, thesetumors usually have an irregular lumpy contour, an irregular internal structure, amedium-to-high internal reflectivity, and little evidence of internal vascularity (see

Image 21). Although exudative detachments occur with uveal melanomas, similarsized metastatic tumors generally have more extensive detachments.Extrascleral extension also can be seen with these tumors and, therefore, is nothelpful in the differentiation of the tumor.

Choroidal hemangioma is a benign vascular tumor of the choroid. These tumorscan produce localized exudative retinal detachments and subsequent vision loss.Clinically, these tumors are orange dome-shaped tumors. Echographically, achoroidal hemangioma is dome-shaped and has a high internal reflectivity (seeImage 22). An overlying serous retinal detachment can be seen with B-scan. Amore diffuse form of a choroidal hemangioma is seen in Sturge-Weber

syndrome. In these patients, the tumor is more extensive and less elevated.

Calcific choroidal tumors are easily differentiated and detected with B-scan. Achoroidal osteoma clinically appears as a yellow lesion, commonly located nearthe optic nerve. These tumors are not significantly elevated. On ultrasound, theyhave very high internal reflectivity due to the calcium. Their contour is usually flatand smooth, but, on occasion, these tumors are lumpy in appearance. Markedshadowing occurs posterior to the tumor due to the calcium absorbing the soundenergy (see Image 3).

Ciliary body

The ciliary body is visualized best with high-resolution scanning; however, theimmersion method may be used, or even the contact method can be used toevaluate the more posterior aspects of the ciliary body. A ciliary body detachmentcan 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-mediumreflective cleft is seen in the subciliary space (see Image 23).

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

leiomyomas.

Sclera

Diagnostic ultrasonography is probably the best way to evaluate scleralthickening. Scleral thickening occurs in cases of nanophthalmos (very smalleyes), ocular hypotony, phthisis bulbi, and scleritis. In scleritis, the degree ofscleral thickening can vary from mild to severe, and it can be focal or diffuse.
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Commonly, associated edema adjacent to the sclera is present. This manifestsitself as an echolucent area in the Tenon space. When posterior and adjacent tothe optic nerve, it forms a T-sign (see Image 24). Other associated findingsinclude a thickened highly reflective sclera, retinal detachments, andciliochoroidal detachments.

Patients who are myopic may have focal areas of thinning sclera. These areascan form staphylomas, or out-pouching (see Image 25). Ultrasound is the bestimaging modality for staphylomatous changes. In trauma, occult scleral rupturescan be difficult to appreciate on clinical examination. Ultrasound typically cannotdetect the actual rupture; however, several echographic clues can assist theclinician. These clues include hemorrhage in the immediate episcleral space, athickened or detached choroid, a detached retina in the area of concern, vitreoushemorrhage, or vitreous incarcerated into the rupture.

Optic nerve

Optic disc cupping usually can be seen on clinical examination. However, ifmedia opacities prevent examination, the contour (including the degree ofcupping) can be detected with ultrasound (see Image 26). Similarly, optic nervecolobomas are imaged easily with ultrasound.

When seen clinically, differentiating papilledema (optic disc edema) frompseudopapilledema is critical since the former is associated with elevatedintracranial pressure, while the latter may have no systemic relevance. Optic discdrusen are calcific nodules buried within the optic nerve head and can simulatepapilledema. On ultrasound, these nodules are highly reflective and exist at or

within the optic nerve head (see Image 27). In true papilledema, increasedintracranial pressure (ICP) is transmitted along the subdural space within theoptic nerve. Clinical entities that can cause elevated intracranial pressure includepseudotumor cerebri and intracranial tumors. When the ICP is mildly elevated,the optic nerve is slightly widened. In the more severe cases, one can see anecholucent circle within the optic nerve sheath (separating the sheath from theoptic nerve). This is the so-called crescent sign (see Image 28).

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 opticnerve in primary gaze and again after the patient shifts gaze 30 degrees from

primary. In cases of increased ICP, the nerve and sheath are stretched as theglobe turns 30 degrees, and the subarachnoid fluid is distributed over the extentof the nerve, resulting in measurements less than when in primary gaze. If nerveenlargement is due to parenchymal infiltration or thickening of the optic nervesheath, then the measurement will not change as the globe turns from primary.

An optic nerve glioma is a neoplastic process that infiltrates the optic nerveparenchyma. On ultrasound, this is a smooth, fusiform mass with low-to-medium
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and regular internal reflectivity. An optic nerve sheath meningioma is an exampleof a tumor of the optic nerve sheath. In contrast to a glioma, this neoplasticprocess typically has a medium-to-high, irregular internal reflectivity with possibleareas of calcification.

Summary

With an understanding of ultrasound principles, thorough examinationtechniques, and knowledge of ultrasound characteristics of a variety ofintraocular pathologies, B-scan ultrasound of the eye is a vital part of anophthalmologist's diagnostic armamentarium. Without this tool, the clinician maynot be able to detect or manage a variety of ocular diseases. However, as withany technical skill, B-scan ultrasound requires training, time, and experience toachieve a high level of both confidence and quality imaging

b scan in ophthalmology

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