B-Scan Ocular Ultrasound: Practice Essentials, Ultrasound Principles and Physics, Instrumentation (2024)

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]

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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.

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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.

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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.

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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 ofopacity 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.

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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.

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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.

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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 hom*ogenous, 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 basem*nt 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.