Ординатура / Офтальмология / Английские материалы / Ultrasonography of the Eye and Orbit 2nd edition_Coleman, Silverman, Lizzi_2006

Figure 3.19. Schematic demonstration of the difference between a sector and an arc scan for imaging a convex surface. The sector scanner will give a small reverse curve corneal echo, caused by the edge of the transducer beam striking the reverse slope of the cornea but imaged as if it were the center of the beam. This oblique angle also reduces the area of the cornea that can be displayed with a sector scan. The arc scan remains, generally, perpendicular over the area of travel of the transducer, thus producing a more accurate image. (See also DVD.)

ARTIFACTS ENCOUNTERED IN OCULAR ULTRASONOGRAPHY

Occasionally, artifacts arise in the course of ultrasonic evaluation of the eye, and familiarity with their appearance will avoid erroneous interpretation. Artifacts may be classified into four groups: (a) electronic artifacts, (b) reduplication echoes, (c) refraction artifacts, and (d) absorption effects. The sources of these artifacts are treated in Chapter 2. Examples of the types of clinically relevant artifacts are presented in the next sections.

Figure 3.20. 10-MHz B-scan with A-scan demonstrating reduced attenuation by bypassing the lid. C, cornea;

AL, anterior lens; PL, posterior lens; R, vitreoretinal interface.

ELECTRONIC ARTIFACTS

In certain scan situations, artifacts may arise from unsatisfactory electronic processing of the ultrasonic echoes. A typical artifact is referred to as snow, which is produced by background noise (“grass” on the A-scan trace) and resembles interference on a television screen. Background noise can usually be eliminated electronically by requiring incoming echo-generated energy to exceed a certain threshold level before triggering the B-scan presentation, thus rejecting low-amplitude background noise. This problem is rare with modern B-scan ultrasound systems.

REDUPLICATION ECHOES

These echoes (also known as multiple echoes) occur commonly and have been extensively analyzed by Kossoff (76). They usually appear along the axis of the cornea and lens. They occur when the transducer is aligned perpendicular to a tissue surface and high-amplitude echoes are reflected back to it. These echoes can then be reflected from the transducer back to the tissue and then rereflected, producing what is called a reduplication echo at a multiple of the distance between the transducer and the reflecting surface. An echo of this type is often seen in midvitreous when, using a water bath standoff, the transducer is positioned a short distance (e.g., 1 cm) from the eye. The artifact would then appear in midvitreous, that is, 1 cm posterior to the cornea, although this artifactual echo can be displaced farther back, even into the orbit fat by positioning the transducer farther away from the eye. Echoes bouncing back and forth between the transducer and the cornea may mimic abnormal tissue or foreign bodies. These echoes may be distinguished from real echoes by moving the transducer either toward or away from the eye. This causes a displacement of the reduplication echo relative to tissue, allowing it to be identified (Figure 3.21; see also DVD).

REFRACTION ARTIFACTS

Other artifacts relating to the transducer position are produced by refraction of ultrasound within ocular tissues. On B-scan, the relatively high lenticular propagation velocity can produce apparent abnormalities of the posterior pole that resemble tumor formations or thickening of the choroid (Figure 3.22). Purnell (29) has referred to these refraction abnormalities of the posterior pole as “Baum's bumps,” because they were originally described by Baum (77). In general, if a mass is seen at the posterior

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pole, scans should be made through different planes of preceding tissue to ascertain that the abnormality is not a reduplication echo or caused by refraction through the lens. Scans of the posterior pole should be made through the limbus or a more peripheral position, if practical, so that only normal sclera and vitreous precede the area of interest.

Figure 3.21. The arrows show reduplication artifacts from the anterior segment as a result of high gain used in

the electronic display. C, cornea; IP, iris plane.

ABSORPTION EFFECTS (SHADOWING)

Absorption of sound energy by anteriorly located structures may cause abnormal ocular ultrasonic patterns. The absence or attenuation of echoes in the posterior segment, giving an appearance of a defect in the ocular wall, is a typical example. Commonly encountered causes of such defects are dense cataract (Figure 3.23) and organized hemorrhage or a foreign body, for example, metal or silicone oil.

Figure 3.22. Baum's bumps on a contact B-scan. (See also DVD.)

Figure 3.23. Artifacts posterior to a calcific cataract are seen, producing distortion and hypoechoic areas

caused by deflection and absorption of the ultrasound beam.

Whenever an abnormal ocular B-scan pattern is encountered, the previously mentioned artifacts should be considered and ruled out. Recognition of these artifacts is aided by (a) careful monitoring of the A-scan, which permits recognition of many electronic artifacts; (b) repositioning of the transducer, if a reduplication echo or shadowing is suspected; and (c) analysis of any ocular abnormality that may cause absorption defects in the acoustic pattern and, perhaps, most important, a good history.

ABNORMALITIES OF OCULAR SIZE AND SHAPE

B-scan ultrasonography graphically portrays anomalies of ocular size and contours. B-scan ultrasonography, with its capability for two-dimensional, cross-sectional display, allows this information to be derived from a single scan plane. Figure 3.24 is a B-scan ultrasonogram of a patient with a posterior staphyloma as a result of high myopia. This aberration from the normal posterior contour of the globe appears acoustically as a concave dip in the globe wall. Figure 3.25 is the ultrasonogram of a patient with a coloboma of the choroid, in addition to a posterior staphyloma. The coloboma gives a pronounced aneurysm-like defect in the ocular wall, with a sharply defined rim.

B-scan portrayal of an enlarged globe and A-scan axial measurement documenting increased axial length allow differentiation of pseudoproptosis from true proptosis. This feature is discussed further in Chapter 5.

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Figure 3.24. B-scan demonstrating the posterior outpouching of the globe in the macular area, as seen in a

typical staphyloma.

ANAMORPHIC DISPLAY

The two-dimensional B-scan does not use the same scale for depth and cross section, which is essential to remember. Depth or range is dependent on sound transmission, whereas the lateral or cross section is dependent entirely on transducer sweep (see Chapter 1) and the electronic tracking of the sweep (Figure 3.26).

VERY HIGH FREQUENCY ULTRASOUND AND ULTRASOUND BIOMICROSCOPY

The very high frequency ultrasound, first introduced by Pavlin and Foster (59) as ultrasound biomicroscopy, or UBM, optimizes anterior segment imaging. Frequencies of 50 MHz and higher provide superb imaging of the cornea and anterior segment. Resolution of 30 microns or less can be achieved, and reproducibility with I-scan (digital signal processing) can approach 5 microns for the cornea thickness. Chapter 4 describes more fully corneal and anterior segment measurement in relation to very high frequency ultrasound and refractive surgery considerations.

Figure 3.25. B-scan demonstrating a coloboma at the posterior pole of an infant showing the relatively sharp

edges or clivus of the defect.

Figure 3.26. Schematic of the anamorphism of ultrasonic imaging produced by different scales for the axes depth and cross section. Depth relates to the speed of sound and density of tissue, whereas cross section relates solely to transducer displacement. This feature must be recognized and accounted for in any calculations that are taken off-axis to the transducer beam.

ANTERIOR SEGMENT ABNORMALITIES

Synopsis

ANTERIOR SEGMENT

Very high frequency ultrasound (VHFU), that is, 50 MHz and greater, is the preferable way to evaluate the cornea, iris, and ciliary body. Lower frequencies are required to outline the lens. Unless blood or fibrin converts the surfaces to a diffuse reflector, the entire outline is not seen.

VHF ultrasound, or UBM, provides excellent definition of the cornea and anterior segment, including iris and ciliary body tumor detection.

CORNEA

Size and Shape Abnormalities

As discussed earlier, anomalies of corneal size, such as megalocornea or microcornea, may be demonstrated with

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B-scan ultrasonography. Corneal curvatures can be directly measured from the B-scan display, and abnormalities of corneal shape (such as keratoconus) are demonstrable but generally require a high index of suspicion to merit the effort to map the posterior cornea and to measure corneal density.

Corneal Thickening

Thickening of the cornea can be determined by A-scan ultrasonic biometry (Chapter 4), and a form of A-scan is commonly used for “pachymetric” measurements. Pachymetry is becoming increasingly important to the glaucoma specialist in estimation of the intraocular pressure. Immersion B-scan ultrasonographic techniques, however, may also be used and can provide imaging and measurements over the entire structure, thus providing additional diagnostic information to the glaucoma or cornea surgeon (Figure 3.27).

Keratoprosthesis

In addition to the examination of the globe prior to keratoplasty, ultrasound is valuable in the evaluation of eyes prior to prosthokeratoplasty. As previously mentioned, clinical evaluation of eyes with opaque corneas is, at best, difficult, whereas ultrasound permits accurate determination of the status of the posterior segment of the globe. The axial length of the eye can be obtained ultrasonically, allowing the placement of accurate dioptric correction in the optic cylinder of the prosthesis. Although most eyes proposed for keratoprosthesis insertion are aphakic, in some cases a lens or lens remnant may be present. Ultrasonography determines the presence or absence of a lens and prepares the surgeon for a lens extraction at the time of keratoprosthesis placement, if the eye is phakic. If a cyclitic membrane is found ultrasonically prior to prosthokeratoplasty, it may be planned for surgical excision during prosthesis placement.

Figure 3.27. Top left: A 10-MHz immersion scan and Top right: a 50-MHz scan of the cornea, demonstrating the resolution of this higher frequency in a patient with corneal opacification. The scarring was caused by corneal abrasion. Lower left: A cross section of a thickened cornea showing the irregular outline of Descemet's. Lower right: The relationship of the angle and the cornea in the same patient.

Visual evaluation of a globe is difficult because of the very limited field of view (2 disc diameters) through the keratoprosthesis. The two-dimensional acoustic section of the globe provided by B-scan ultrasonography facilitates recognition of possible pathologic conditions.

Abnormal Eye with Keratoprosthesis

Coleman et al. (78) described results in 22 patients who were referred for ultrasonography because of unexplained visual loss after months or years with a satisfactory result from keratoprosthesis insertion. Nineteen of these patients were found to have ultrasonically demonstrable posterior segment abnormalities accounting for their visual loss. These abnormalities were classifiable into four groups: (a) cyclitic membrane, (b) choroidal detachment, (c) vitreous hemorrhage, and (d) retinal detachment. In some of the eyes, two of these conditions coexisted. Surgery can be more cogently planned, using gas and vitreosurgical methods, with foreknowledge of these anatomic conditions (79).

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Figure 3.28. A 50-MHz arc scan of a normal anterior segment, demonstrating in one frame the cornea, iris, ciliary body, and ciliary processes, as well as the anterior lens surface. This format allows measurement of all of the anterior segment dimensions more accurately than possible with collages, such as with the UBM.

ANTERIOR SEGMENT ULTRASOUND

ANTERIOR CHAMBER DEPTH

Accurate measurement of anterior chamber depth is obtained by A-scan ultrasonic biometry or by high frequency B-scan (50 MHz or VHFU). The A-scan beam should be positioned along the visual or optical axis to obtain a central representative and repeatable measurement. The optical axis is generally used because the use of maximal anterior and posterior lens echoes is easiest to align. The visual axis requires a target alignment transducer system, as described in Chapter 2. B-scan ultrasonography can provide accurate two-dimensional information, as well as anterior chamber depth, and can allow evaluation of the iris and sulcus plane depths. A normal anterior chamber is shown in Figure 3.28, and Figure 3.29 is a B-scan ultrasonogram of a phakic patient with a flat anterior chamber. The lens is displaced anteriorly, and the iris is seen to lie against the corneal surface. The echoes from the posterior cornea and anterior iris merge, and the interface between these two structures can be outlined. Figure 3.30, conversely, demonstrates a deep anterior chamber in a phakic eye following trauma with a cyclitic membrane producing hypotony. To provide the optimal accuracy in measuring chamber dimensions, careful three-dimensional alignment is required to avoid off-axis errors.

Figure 3.29. A 50-MHz arc scan of a phakic patient with a flat anterior chamber.

Figure 3.30. An abnormally deep anterior chamber in a patient following trauma, with a hypotony and

iridodialysis.

Hyphema

Hyphema (blood in the anterior chamber) appears as an echoic structure of variable echogenicity, depending on duration and clot lysis. Fresh hemorrhage will generally have low echogenicity, increasing with organization into a clot. Blood will sometimes be traceable to the site of bleeding and will often tend to accumulate in the angle inferiorly. Blood on the surface of the lens enhances imaging by producing a diffuse reflector, as seen in Figures 3.13 and 3.31.

Iris

Normal Iris

The normal iris is highly reflective and can be well imaged at 10 MHz, but it is far better outlined at 50 MHz. The melanin laden surface is reflective and is of interest

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largely for congenital anomalies, for physiologic studies, in trauma, and for evaluation of tumors. Iris conformation is significant in glaucoma as well, as in pupillary block.