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

Synopsis

TWO DIAGNOSTIC APPROACHES

1.B-scan with A-scan ultrasonography. A-scan generated along vector of B-scan with same focused transducer. Class A: A-scan is calibrated and has radiofrequency display for “quantitative” digital display.

Class B: A-scan vector uses same amplifier that generated the B-scan. It is a simple rectified trace and has not been calibrated for amplitude variation.

2.Standardized echography, using a separate B-scan and A-scan with a nonfocused transducer. The A-scan is calibrated against a tissue standard.

As has been discussed in Chapter 2, there are two primary techniques for ocular ultrasonic examination: A-scan and B-scan, with supplementary display techniques of M-scan, I-scan, 3-D scans, and kinetic scanning. With these techniques, different systems for ultrasonically evaluating patients have evolved. A-scan and B-scan are not mutually exclusive methods of diagnosis. A thorough knowledge of A-scan or B-scan ocular ultrasonography can provide reliable diagnostic information, but a combination of both A- and B-scan diagnostic methods is optimal and almost universally used. In our laboratory, for diagnostic purposes we rely primarily on B-scan, and we use a constant A-scan monitor to obtain maximum quantitative echo amplitude information from the calibrated RF digitized data.

B-scan provides the two-dimensional display that provides the cross-section basis for comparison of characteristic echo amplitude variation (or third dimension) provided by the A-scan. The “third dimension” of the levels is displayed on the B-scan as gray scale but varies with amplifier characteristics and gain settings. Amplitude comparisons can be very useful on B-scan, but amplitude character of a tissue is more accurately obtained by observing the A-scan monitor simultaneously with the B-scan to identify the orientation of the A-scan vector.

Figure 3.11. Quantel Cinescan 20-MHz sector scanner providing excellent resolution of the posterior pole on

B-scan.

Ideally, B-scan ultrasound with good dynamic range, or gray scale, can present a tomogram, or a thin cross section of the eye, with highly accurate resolution of tissue surfaces, such as the cornea, the anterior chamber, or tumor characteristics. It also displays reflectivity patterns as a cross section within the tissue being observed. Amplification of echoes or dynamic range is best shown using logarithmic amplification or the “s” shaped amplification of Ossoinig (12). M-scan is a technique that has been beneficial in demonstrating consistent or reproducible pulsations, such as the respiratory or vascular pulsations of certain tissues, or the

magnetic properties of foreign bodies, but is used only occasionally in ocular diagnosis.

Ossoinig (11) has emphasized the value of A-scan ultrasonography in providing quantitative echo information and developed an “s” shaped amplifier to compress the dynamic range of echoes and emphasize echo amplitude variations with a separate and independent A-scan evaluation, using a nonfocused A-scan transducer. However, most ophthalmologists can more readily interpret two-dimensional B-scan patterns and rely on the A-scan from the same instrument and transducer to interpret the pathology. This has given rise to two separate “schools” of ophthalmic ultrasonography. Most practitioners use the B-scan and obtain a vector A-scan, using the same transducer that produces the B-scan.

Our approach has been to use an amplifier that generates an RF signal. This signal can be rectified to produce a characteristic A-scan along a vector on the B-scan. The amplitude is calibrated for the transducer (the same focused transducer that produces the B-scan when the trace

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is modulated). With this technique of calibration from the RF trace, which is termed quantified A-scan, a very accurate amplitude is determined. We find this method preferable because it provides the most accurate amplitude quantification while allowing it to be compared to the precise tissue area as localized on the B-scan. However, in most commercial systems, the A-scan is generated along the vector, from the displayed B-scan pixel intensities. These instruments are less expensive and may be adequate for routine examination, but the A-scan amplitude comparisons are obviously less desirable. The “standardized echography” school uses a separate A-scan with nonfocused transducer standardized against a tissue standard using the “s” shaped amplifier developed by Ossoinig (70, 71, 72). This method is more time-consuming because the examiner may switch from B- to A-scan and cannot identify the tissue of reference as easily as when it is selected from the simultaneous B-scan. In addition, angle of incidence of the beam and the tissue that affects amplitude are less confidently identified.

Excellent courses and books on standardized echography are available that detail this technique, notably Frazier-Byrne and Green (73) and DiBernardo and Schachat (74). Although ideological differences thus exist, both methods essentially use the B-scan for orientation and rely on an A-scan for tissue quantification and identification.

The combined A-scan and B-scan technique, because of its clinically demonstrated value and availability, has come to play a critical diagnostic role in ophthalmology, particularly when the media is opaque or a lesion is occult.

This book will outline the method of diagnosis, which uses the B-scan to provide the broad, topographic information about tissue geometry and morphology and the A-scan and other digital analytic techniques to provide specific comparative information regarding the reflectivity and backscatter from tissue structures, as well as accurate measurement of their dimensions.

Figure 3.12. A typical 10-MHz B-scan with a vector selected A-scan. The power and other settings for modification of the scan are shown, as is the time variable gain graphic on the lower right.

DIAGNOSTIC PARAMETERS

The typical ophthalmic examination proceeds in two stages to identify the anatomic ocular features listed in Table 3.1. First, general tissue features, such as size and position are established. Second, specific features, particularly anomalous structures, are identified by examining echo characteristics that are indicative of finer, more discriminative morphologic features.

Although the general architecture of the eye and orbit is readily discerned in B-scans (Figure 3.12), finer features can be interpreted only with an understanding of how

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tissue structure influences ultrasonic reflectivity. As noted in Chapters 1 and 2, different tissues transmit, absorb, and reflect in various manners, depending upon factors such as density, elasticity, and internal structural features. At boundaries between tissues, ultrasound is reflected to a degree determined by the acoustic impedance mismatch between the tissues and by the size, orientation, and roughness of the boundary. To use an analogy, a mirror that is very smooth and that lies perpendicular to a flashlight beam will reflect most of the energy back toward the flashlight (a specular reflector) but very little reflectance back to the flashlight, if the light is off-axis. If the mirror is roughened (a diffuse reflector) or smaller than the total beam or angled away from the beam, it will reflect proportionately less light back to the flashlight. Conversely, a diffuse reflector will reflect some energy even off-axis, allowing curved surfaces, such as the lens, to be better outlined if

blood or fibrin converts the smooth to a diffuse reflective surface (Figure 3.13).

TABLE 3.1 Diagnostic Parameters

Gross Morphologic Features

Location

Size

Outline/Contour/Shape

Associated Ocular Changes

Changes with Time

Fine Morphologic Features

Boundary Layer Properties

Acoustic Impedance

Roughness of Surface

Internal Tissue Properties

Internal Texture (Homogeneous or Heterogeneous)

Type of Internal Structural Elements

Spatial Distribution of Internal Structural Elements

Acoustic Absorption

Internal tissue characteristics also influence ultrasonic transmission and reflection. If a tissue has a homogeneous structure (e.g., lens, optic nerve, or a solid tumor, such as a malignant melanoma), there are few internal reflective surfaces, giving a “cystic” or sonolucent hollow or hypoechoic appearance on B-scans. This appearance contrasts sharply with the dense, speckled hyperechoic appearance generated by reflections from internal features of heterogeneous structures, such as in hemangioma, angioma, or vitreous hemorrhage. In these heterogeneous structures, echo amplitude and spatial distributions depend on the type and distribution of the internal structural elements (e.g., blood vessels, calcific deposits, or necrotic regions). In addition, the falloff of echo amplitude with increasing depth is indicative of attenuation of the ultrasound beam through absorption and scattering. (In homogeneous structures, attenuation can be manifested by a “shadowing,” or blocking of detail, in more posterior tissues.) The attenuation of ultrasound frequency-related scattering in tissue can be used to identify specific ocular tissues and pathologic conditions. New developments in instrumentation promise to provide even more gray scale and computer-enhanced information than is presently available for ultrasound-based tissue diagnosis.

Figure 3.13. Immersion ultrasound examinations of two traumatized eyes, one with an older 10-MHz instrument

(left) showing how blood can outline the entire lens. On the right, a newer contact B-scan used in immersion demonstrates how blood can help outline the lens by converting it to a diffuse reflector.

TYPES OF DIAGNOSTIC INFORMATION

Synopsis

INDICATIONS FOR DIAGNOSTIC ULTRASOUND

Measurement of distances or volumes (corneal thickness, ocular biometry)

Opaque media—no view (cataract, blood, etc.)

Occult areas of globe (retroiridal)

Trauma—foreign bodies

Retinal/choroidal detachment

Orbit and optic nerve

We have found it convenient to consider ultrasonic diagnosis in terms of the (a) unique, (b) supplemental, and (c) documentary information available from ultrasound. Unique information indicates that which is obtained with opaque ocular media or, for example, in occult ciliary body tumors. Supplemental information is exemplified by tumor diagnosis. Although a mass may be visualized ophthalmoscopically, differentiation by means of clinical appearance is often inaccurate or misleading. The ultrasonic characteristics of a mass (e.g., shape, height, and acoustic transmission properties) may be added to information that is obtained visually. Documentary information refers to the ability of ultrasound

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to measure accurately the thickness of the cornea or lens, the length of the globe, the dimensions of a tumor, the motion of the lens during accommodation, or ocular properties that vary under the influence of drugs. All of these are measurements that are not easily, or not at all, obtainable by other means. Although the unique information is the most dramatic and most often described utilization of ultrasound, (e.g., detection of tumors in opaque eyes), the supplemental and documentary uses, such as in characterizing tumors or measuring eye length for an intraocular lens, are of equal or, in many instances, greater importance.

INDICATIONS FOR OCULAR ULTRASOUND

A summary of the indications for the ophthalmic use of ultrasound is presented in Table 3.2. In addition, there are specific uses of ultrasound, such as detecting loci of choroidal effusion in patients with flat chamber, preoperatively studying the character and form of vitreous hemorrhages prior to vitrectomy, measuring the size and volume of tumors prior to radiation, plaque, proton beam or other treatment, and measuring axial dimensions to determine keratoprosthetic or intraocular lens dioptric powers.

In general, where visual techniques fail to provide sufficient information as to the structural configuration of the eye, ultrasonic imaging is indicated. It is safe, economical, and rapid. Even where it fails to provide optimal imaging, as, for example, when MR or CT is superior for orbital evaluation, or CT is superior for identifying foreign bodies, follow-up evaluation may be useful and most economically provided by ultrasound.

THE NORMAL EYE

A-SCAN ULTRASONOGRAPHY

Synopsis

A-SCAN MEASUREMENTS FOR INTRAOCULAR LENS

Known velocities for each tissue traversed by the round trip “time of flight” echo-detected, convert the time measurement to distance (mm) using the formula: Distance = Velocity × (time/2)

Maximal amplitude of lens and posterior pole surfaces indicate alignment with the optical axis.

TABLE 3.2 Indications for Ocular Ultrasonography

Opaque Media

(Corneal Leukoma, Hyphema, Hypopyon, Cataract, Vitreous Hemorrhage)

Occluded or Markedly Miotic Pupil

Ophthalmoscopically Visible Mass Lesion

Suspicion of Tumor Underlying Retinal Detachment

Ocular Trauma

Ocular Foreign Body

Axial A-scan Ultrasonography

The axial ultrasonogram is obtained by using the visual or optical axis as the path of the examining ultrasound beam, so that echoes are obtained from structures along the path of the central cornea and on posterior through the lens to the retina. Echoes arise from the ocular tissue interfaces that produce acoustic impedance mismatches. These echoes are displayed as vertical deflections on a display device. In the optical axial echogram of the normal eye (Figure 3.14), high amplitude echoes are produced by the corneal surfaces, by the lens surfaces, and by the vitreoretinal interface. The vitreoretinal interface echo is followed by a complex of echoes representing retina, choroid, sclera, and retrobulbar fat. The echoes in the retrobulbar fat diminish gradually to baseline as the sound is absorbed. Certain parts of the eye are normally acoustically homogeneous at typical ophthalmic ultrasound frequencies. These include the cornea, anterior chamber, the lens, the vitreous, and, to some extent, the optic nerve. These areas appear as baseline (zero echo or anechoic) segments between echo groups from their surfaces.

Optical axis measurements are obtained when the echo amplitude is maximized as a result of the orthogonal or perpendicular relationship of the transducer beam and the tissue, that is, cornea, lens surfaces, and retina. This feature is used to insure alignment of the optic axis when taking axial measurements for computation of lens power (Figure 3.15).

The visual axis may be more important in some situations but can be obtained only by having the patient visually align his or her eye with a target or light in the center of the transducer beam. Because this is subjective and required only in special situations, there are only a few transducers so specially designed, and optical axes are the norm for A-scan measurement.

The velocity of sound constants for all of the anatomic structures in the ultrasound beam path, that is, cornea, anterior chamber, lens, and vitreous, are required to convert the round trip “time of flight” measurements to distance in millimeters (time/2 × velocity = distance) (Table 3.3). Several formulas based on optical models of the eye, regression, or some combination of the two, can then be used to indicate proper lens power for intraocular lens (IOL) implantation.

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Figure 3.14. An A-scan along the optical axis at 10 MHz showing the amplitude variation of the optical surfaces. Note that the posterior lens surface is of lower amplitude than the anterior surface because of lens absorption and the concave surface reducing the area incident to the beam. This feature is useful in maximizing true optical axis measurements because on-axis measurement is assured when the posterior lens echo is maximal.

B-SCAN ULTRASONOGRAPHY

Synopsis

B-scan alignment can be “on axis” (through the lens) or diascleral. The lens and lids attenuate sound, more so with higher frequencies.

High frequency has better resolution but less tissue depth penetration or sensitivity, and lower frequencies give less resolution but deeper tissue sensitivity.

Artifacts are produced by anatomic and electronic causes and must be accounted for in examinations.

B-mode ultrasonographic systems have been described in Chapter 2. B-scan ultrasonography provides a two-dimensional “acoustic section” of the globe along any desired scan plane. As in A-scan ultrasonography, the appearance of the normal eye varies according to the scan plane selected.

B-scans are anamorphic displays. Depth or distance is related to sound transmission, whereas cross section or lateral position is related only to the orientation of the transducer.

Axial B-scan Ultrasonography (10-MHz Sector Scan)

A typical 10-MHz B-scan ultrasonogram along an axial scan plane (Figure 3.16) shows both the anterior and posterior surfaces of the cornea, separated by a sonolucent interval representing the corneal stroma. With sector scanners the cornea will have a “reverse curve” because of the sector movement and the width of the transducer beam in the near field. The anterior chamber appears as a uniformly, acoustically clear (hypoechoic) area. The anterior surface of the iris is usually demonstrable. The echoes from the posterior iris surface usually merge with those from the anterior lens surface. However, with a dilated pupil, the anterior lens curvature is more prominently seen. The interior of the normal lens also appears as an acoustically homogeneous (hypoechoic) space. The posterior curvature of the lens is usually well demonstrated, at least centrally with a sector scanner, but the equator is not seen because of its oblique orientation to the beam. The vitreous compartment normally appears as an anechoic or sonolucent cavity with no internal sound reflections. The vitreoretinal interface forms a smooth, concave curvature. Echoes from the retina merge with echoes from the choroid and the sclera, and in the normal eye these contiguous echoes cannot be well separated at normal examining frequencies of 10 and 15 MHz. These boundaries between the retina, choroid, and sclera can be better identified using digital signal

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processing techniques. The scleral fat boundary (Tenon's capsule) is, however, well seen acoustically. In B-scan ultrasonography, the area between the ora serrata and the equator of the globe is poorly demonstrated with axial orientation of the transducer, because, as with the equator of the lens, these areas are more parallel to the sound beam. Thus, for a complete ultrasonic examination, multiple scans with the scanner placed meridionally at each clock hour and using diascleral scans that bypass the lens is necessary. In an axial B-scan, the retrobulbar fat forms a W-shaped pattern, with a black notch formed by the relatively homogeneous optic nerve. The orbital fat appears as a highly reflective mass with extraocular muscle bellies forming the outline for the fat. The normal orbital B-scan appearance is discussed in Chapter 5 on orbital diagnosis.

Figure 3.15. Left: An A-scan demonstrating maximized posterior lens echoes along the optical axis. It is shown as a vector on the B-scan to help demonstrate the positioning of the ultrasound beam. The anterior lens echo is clipped or saturated, explaining the difference in appearance to Figure 3.14.

TABLE 3.3 Reported Mean Velocities of Ultrasound in Ocular Tissues

Figure 3.16. A typical immersion B-scan at 10 MHz demonstrating the excellent imaging of the vitreous and retina, as well as moderate imaging of the lens. The cornea has only a small area visualized because of the mismatch of sector scanning and the corneal curvature. The echoes in the vitreous are a “multiple” of the anterior segment. The reverse arc of the cornea is explained in Figure 3.19.

Diascleral (Off-axis) A-scan Ultrasonography

To obtain the diascleral ultrasonogram, the examining ultrasound beam must pass peripheral to the cornea and the lens to avoid the absorption of the ultrasound beam by the lens. The transducer is placed on or anterior to the sclera, and aligned toward the posterior pole. The normal diascleral ultrasonogram (Figure 3.17) consists of a high-amplitude echo complex, representing sclera, followed by a long acoustically empty (anechoic) interval, representing the normal vitreous cavity. The final echo complex produced by retina, choroid, sclera, and retrobulbar fat is similar to that seen in the axial ultrasonogram. Oksala (75) first pointed out that the echoes from the posterior ocular wall in the diascleral ultrasonogram are

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higher and broader than those obtained in the axial ultrasonogram, because there is no sound absorption from the lens. Tumor measurement and such amplitude features that aid diagnosis of retinal detachment often benefit from this variation of beam orientation.

Figure 3.17. A diascleral B-scan demonstrating improved sensitivity of the posterior pole when the ultrasound

beam is not partially absorbed by the lens.

Transducer Frequency Variation

As we have discussed in Chapters 1 and 2, there is a balance, or “trade-off,” between resolution and penetration. Figure 3.18 shows B-scans of the same normal eye, taken at 10 and 20 MHz, to illustrate the higher resolution obtainable with the 20-MHz transducer. The penetration of the ultrasound beam from a 20-MHz transducer, however, is much less than that obtained with a 10-MHz transducer, which depicts more of the orbital fat and optic nerve. In general, the 10-MHz examining frequency is the best compromise for initial examination, with higher (or lower) frequency transducers then substituted for the study of specific tissues or areas.

Figure 3.18. B-scans of a normal eye at 10 MHz (left) and 20 MHz (right), which demonstrate improved

resolution at higher frequency with concomitant reduced sensitivity.

Effect of Scanning Mode

Figure 3.19 shows the difference between a sector scan and an arc scan of the eye, using a single transducer (see also DVD). An arc scan is better at outlining the contours of the anterior segment and the equator of the globe than is the sector scan. The arc scanner and sector scanner are equivalent at the posterior pole, because at the lower 10to 20-MHz frequencies needed for the posterior pole, the sector and reverse arc are the same. (The pivot point of the arc scanner is placed in midvitreous so that the reverse arc follows the contour of the posterior pole.)

Effect of Lid Attenuation

We prefer to use a water bath standoff with a lid speculum in most diagnostic work; however, both immersion and contact B-scan ultrasonography can be performed through the closed eyelid. Figure 3.20 demonstrates the marked attenuation of ultrasound energy caused by passage through the lids. In addition to the marked absorption of the sound beam, ocular structures immediately posterior to the lid are obscured. For optimum B-scan ultrasonography, a water bath standoff of some type with lids open, usually with a speculum, is recommended. The contact B-scan is easier to use and for routine evaluation, such as evaluating possible retinal detachment or choroidal elevations, may be the preferable technique because of ease of use.

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