6.1 Introduction

During the past couple of decades, ophthalmic ultrasound has become an important and often necessary tool to aid in the diagnosis and management of intraocular pathology. Whenever ophthalmoscopic evaluation is limited or obscured, ophthalmic ultrasound should be utilized to evaluate the posterior segment.

Common indications for diagnostic ultrasound include opacification of the cornea, cataract, vitreous hemorrhage secondary to systemic disease or trauma, and evaluation and differentiation of noted pathology, such as mass lesions. ^{1 ,​ 2} This chapter describes the echographic evaluation and findings in vitreoretinal disease using the techniques of standardized echography. ^{3 ,​ 4}

Standardized echography was first introduced in ophthalmology in the early 1970s by Dr. Karl Ossoinig. It is considered to be the combination of a contact B-scan and standardized A-scan in which a set of prescribed examination techniques is used. ⁴ Ophthalmic ultrasound (both B- and A-scan) utilizes a relatively high frequency (8–10 MHz), which produces improved resolution in superficial structures. ¹ As the sound travels through the eye, it is reflected by the intraocular structures, and signals or echoes are returned to the screen. B-scan or brightness modulation provides two-dimensional images of a series of dots and lines (Fig. 6-1a). A-scan or amplitude modulation provides a one-dimensional image of vertical deflections from a baseline (Fig. 6-1b). The brightness or height of the signals returned depends on the density of the structure producing the signal, the angle of incidence of the sound beam, the acoustic impedance of the tissues through which it is passing, and the gain (decibel) level used on the machine. ^{1 ,​ 2 ,​ 3 ,​ 4}

B-scan provides the topographic information about intraocular structures. This includes shape, location, extension, and mobility and frequently a gross estimation of height or thickness of tissue can be obtained. The standardized A-scan has a specially designed S-shaped amplifier that allows for differentiation of tissue. ^{5 ,​ 6} The standardized A-scan provides information necessary to secure a diagnosis, including structure (size and distribution of cells), reflectivity (height of spikes), sound attenuation (absorption), mobility, and vascularity.

6.2 Basic Screening Techniques

It is best to begin with a maximum gain setting on the B-scan for evaluating the vitreous. Evaluation at a decreased decibel level may lead to misdiagnosis because scattering of the reflected sound occurs secondary to the small size of opacities that may be present relative to the diameter of the sound beam.

As mentioned previously, using a consistent screening sequence aids the echographer in obtaining a reliable, thorough examination. It also helps the screener feel confident in understanding what probe position correlates with the area of the eye being examined.

The best resolution is obtained when the probe is placed directly on the globe, as opposed to an examination through the lids. Every interface that the sound travels through decreases the amount of energy that enters the globe. Performing the examination on the globe also enables the echographer to control the patient’s gaze.

The probe is placed on the globe opposite the area to be examined, and the initial line on the B-scan corresponds to the probe on the eye. The marker on the probe acts as the orientation point and corresponds to the upper portion of the echogram. To evaluate the superior and inferior fundus, the marker should be directed toward the nose (horizontal transverse). To evaluate the nasal and temporal fundus, the marker should be directed up toward 12 o’clock position (vertical transverse). The best detail of pathology is obtained in the central portion of the echogram. If the pathology is not located at one of the major meridians (12, 3, 6, or 9 o’clock position), an oblique transverse scan can be used to center the pathology in the echogram. With these cuts, the marker should be directed up at an angle (Fig. 6-2). With transverse scans, the sound beam is aligned perpendicularly to the fundus. This type of scan allows for the evaluation of a thin cross-section of tissue of six clock hours (Fig. 6-3).

To be sure that all regions of the globe are evaluated adequately, the screening of each quadrant should begin with the probe face directed at the limbus for evaluation of the posterior pole. As the echographer slowly shifts the probe into the fornix, the entire fundus to the periphery is visualized. Once cross-sectional evaluation of the eye has been obtained, it is useful to evaluate any areas of interest using a longitudinal (radial) approach; in this type of scan, the sound beam is aligned parallel to the fundus. Longitudinal scans allow for evaluation of a single meridian from its most posterior aspect to the far periphery (Fig. 6-4). This is accomplished by directing the marker at the corneal limbus opposite the area to be examined. Axial scans provide a pleasing, generally understandable picture; however, the information obtained from these scans can be limited, and because they require placing the probe directly on the cornea, the risk for abrasion increases with their use (Fig. 6-5).

Once the eye has been screened with the B-scan, it may be necessary to incorporate the standardized A-scan, particularly if pathology is present that cannot be differentiated with the B-scan alone. To evaluate normal and abnormal tissue accurately with the standardized A-scan, the gain should be set at a decibel level known as tissue sensitivity. ^{5 ,​ 6 ,​ 7} Each probe and machine combination has a separate, standard tissue setting that makes the equipment sensitive to variations in tissue and allows for differentiation from one tissue type to another. Tissue sensitivity is determined either by the echographer using a tissue model that mimics live tissue or by the manufacturer of the equipment.

As with the B-scan, the A-scan probe is placed on the eye opposite the area to be examined. There is no marker on the A-scan because the sound beam is a parallel beam (as opposed to the B-scan, which has a focused beam). However, it should still be shifted from the limbus to the fornix for adequate evaluation of the fundus. As stated previously, the A-scan produces a single-dimension image that consists of a series of deflections (spikes) from a baseline. The height of the spikes produced as the sound encounters an interface is directly related to the density of the interface. Consequently, a maximally high spike indicates a dense interface. The space between the spikes indicates the time it takes for the sound to encounter an interface and return the signal back to the probe. This time value is then converted into distance, and measurements in millimeters can be obtained.

When mass-like lesions are evaluated, the surface of a tumor will display a maximally high signal. If signals or spikes are obtained from within the lesion, the structure and reflectivity are evaluated. The structure of a lesion can be categorized as either regular or irregular and is determined by aiming the sound through the lesion in different directions. If the height and distribution of the internal spikes remain consistent, the lesion is regularly structured (e.g., melanoma). If the height of the spikes varies when the sound beam is aimed through different areas, the lesion is irregularly structured (e.g., metastatic tumor).

The internal reflectivity of a mass lesion can be classified in various ways (low, medium-low, medium, medium-high, high, and irregular) and is determined by the density, size, and shape of interfaces within the lesion. Melanomas are comprised of small, densely compact cells of uniform proportion and distribution. ⁸ As sound passes through these small cells, little reflection of the sound occurs, so that reflectivity is low. Conversely, choroidal hemangiomas have large cells (blood-filled cavities) and the walls of these cavities are more reflective, producing high reflectivity. ⁸ Metastatic carcinomas have erratically dispersed large and small cells and interfaces, thus causing irregular reflectivity. ⁸

A quick comment about B-scans with a “vector” A-scan image superimposed over the B-scan image. While almost all equipment currently on the market has this “vector” capability, those performing ultrasound examinations should be aware that this technique holds no diagnostic value and, therefore, should not be utilized to evaluate the reflectivity of structures. The vector A-scan is simply a one-dimensional duplication of the B-scan image. If the gain is turned up, the spikes will be high, and conversely if the gain is decreased, the spikes will be low.

The kinetic properties of pathologic structures are features that are evaluated during the dynamic portion of the examination using both the B- and A-scans. Mobility (aftermovement) is most obvious and appreciated during the B-scan screening when patients move their eyes in the same direction that the sound beam is moving (up and down for vertical transverse and left or right for horizontal transverse). If the patient has trouble with eye movement or the pathology does not readily move, the patient’s head position can be shifted. Vascularity may be noted on the B-scan when large vessels are present. Detection of fast, flickering motion in the valleys of spikes on A-scan may be the best way to determine the presence of blood flow.

6.3 Evaluation of the Vitreous

A maximally high gain (decibel level) should be used to evaluate the vitreous cavity. It is important to note that dispersed opacities and dispersed hemorrhage appear identical acoustically. Having accurate clinical information can help in the distinction of one from the other. For instance, if a patient presents with an anterior chamber hyphema and opacities are noted within the vitreous cavity on ultrasound examination, the probability that opacities are red blood cells is increased and the echographer can feel comfortable reporting the echographic findings as vitreous hemorrhage. If, on the other hand, a patient presents with a dense cataract and the clinical history is limited but opacities are noted within the vitreous echographically, the echography report should describe the findings as simply “vitreous opacities.”

6.3.1 Normal Vitreous

In a normal globe, especially that of a child or young adult, the vitreous cavity is devoid of any acoustic signals and appears black or echolucent on the B-scan. On standardized A-scan, the baseline remains flat throughout the scan (Fig. 6-1). ⁶ During the normal aging process, the vitreous gel begins to solidify and opacities in varying amounts may form (Fig. 6-6). There may be significant contracture of the vitreous gel and the posterior hyaloid surface may separate from the retina.

6.3.2 Asteroid Hyalosis

Asteroid hyalosis is a unilateral condition characterized by the formation of calcium soaps within the vitreous gel. These soaps appear as bright, round signals on B-scan. On standardized A-scan, each opacity produces its own echo spike (Fig. 6-7). Generally these opacities exhibit distinct singular movement on both A- and B-scans. ⁹

6.3.3 Endophthalmitis

This infection in the eye can be fungal or bacterial in nature and is most often seen following surgery or penetrating injury. ¹⁰ Patients may present with pain and decreased vision, and clinically a hypopyon is noted in the anterior chamber. Echographically, the findings vary depending on the severity of the infection. Generally, opacities are noted and membrane formation is present. In the most severe cases, retinal detachment and/or choroidal detachments may be present (Fig. 6-8). ¹¹ Serial ultrasounds may be useful in cases of endophthalmitis to evaluate and confirm the progression of the infection or in cases in which differentiation of pathology is difficult and confirmation of the findings is necessary.

6.3.4 Vitreous Hemorrhage

Patients with vitreous hemorrhage secondary to trauma or systemic disease probably account for the majority referred for ultrasound examination. Hemorrhage in the vitreous appears as small, white echoes on B-scan, and generally, the more echoes seen, the greater the density. It is possible to differentiate fresh hemorrhage (Fig. 6-9) from clotted hemorrhage (Fig. 6-10). ^{11 ,​ 12} When a posterior vitreous detachment is present, it appears as an extensive, smooth membrane on B-scan. Although the posterior hyaloid face can produce a significant spike on standardized A-scan, it is generally less than 100% tall. It is important for the echographer to note any areas of the fundus where the posterior hyaloid remains adherent to the retina. Frequently, blood may collect beneath the posterior hyaloid (Fig. 6-11). This subhyaloid blood can be dispersed or clotted, and having patients move their eyes or turn their head will move the blood away from the pooled area so retinal detachment can be ruled out. ^{9 ,​ 11}

Pearls

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  Although the posterior hyaloid face can produce a significant spike on standardized A-scan, it will generally be less than 100% tall. This is in contrast to the spike of a detached retina, which is almost always 100% tall.

6.4 Evaluation of the Retina

The retina is a somewhat dense membrane, and when the direction of the sound beam is perpendicular to it, it produces a steeply rising, high (100% tall compared with the initial spike) spike. ^{9 ,​ 11 ,​ 13} On B-scan, the typical echographic features of an extensive or total retinal detachment include the following: a dense, thick, often folded membrane that inserts into the optic disc (when totally detached). ^{9 ,​ 11 ,​ 13} Generally, the retina will exhibit some mobility, depending on the length of time it has been detached. The longer the detachment has been present, the less mobile and more folded it becomes. Dense vitreous membranes and retinal detachment can look very similar echographically, so it is very important for the echographer to recognize the findings that can help to distinguish one from the other (Fig. 6-12). ¹⁴

6.4.1 Retinal Tears

When a patient presents with sudden onset of decreased vision secondary to vitreous hemorrhage, ultrasound can be useful in detecting small retinal tears. ^{9 ,​ 15} Small tears are most frequently located in the superior quadrant, but they can be noted anywhere in the peripheral fundus. ^{9 ,​ 15} On B-scan, small tears appear as highly reflective tufts of elevated tissue. In almost all cases, a thin vitreous membrane adherent to the flap can be detected. ¹⁵ On standardized A-scan, the flap tear will produce a maximally high spike separate from the other fundus spikes (Fig. 6-13). It is important to note if the adjacent retina is detached (Fig. 6-14). Observation while the blood clears is generally the treatment of choice, followed by cryotherapy or laser treatment once the view improves. Ultrasound-guided cryotherapy may be a viable early treatment option in some patients. ^{15 ,​ 16}

Pearls

• 
  

  When a patient presents with sudden onset of decreased vision secondary to vitreous hemorrhage, ultrasound can be useful in detecting relatively small retinal tears.

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1. 7 Electrophysiology and Miscellaneous Noninvasive Tests (Including Scanning Laser Ophthalmoscopy, Fundus Autofluorescence, and Adaptive Optics)
2. 3 Examination of the Fundus
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4. 5 Optical Coherence Tomography

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