Ординатура / Офтальмология / Английские материалы / Ultrasonography of the Eye and Orbit 2nd edition_Coleman, Silverman, Lizzi_2006
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Figure 5.4 Plain film x-ray of shotgun pellets around the face and orbit. The orbital pellet had traversed the
globe. Shotgun pellets may be steel, lead, or composite material. Magnetic testing can be useful, using either a similar shell supplied by the patient or family or, at surgery, using a pellet from a superficial nonocular site. (Courtesy of Murk-Hein Heinemann, MD, New York, NY.)
Figure 5.5 Plain film x-ray of a nail that penetrated the orbit and cranial vault but did not puncture the globe.
(Courtesy of Alan Maberley, MD, Vancouver, BC.)
Figure 5.6 Plain film x-ray of an intraocular broken paper clip. The shape is outlined better than with any other imaging modality, but the globe relationships are lacking and can be seen with ultrasound. (Courtesy of Gwen Sterns, MD, Rochester, NY.)
Orbital evaluation with ultrasound is typically performed at 10 MHz, with higher frequencies limited to the posterior sclera, optic nerve, and proximal orbital region. The ultrasonic orbital evaluation consists of serial tomographic sections, using both static and kinetic A-and B-scanning. In general, horizontal scans are made serially across the eye and orbit at roughly 2-mm intervals, with the eye in the six major gaze positions. Ossoinig (3) has emphasized the importance of additional meridional scanning of the orbit to portray structures partially obscured by the orbital rim. Lower frequency transducers (such as 5 MHz) may be used to outline the posterior extent and apex of the orbit. A lower frequency may also be useful, if ocular pathology prevents adequate penetration at the normal examining frequency of 10 MHz. However, these transducers are not generally
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available in ophthalmic ultrasound equipment and have low resolution.
Figure 5.7 Plain film x-ray of a van radio antenna that penetrated the orbit and cranium, demonstrating the value of x-rays in outlining the shape of the foreign body. The tip of the antenna was removed through a cranial burr hole before the antenna was withdrawn. (Courtesy of Stephen Trokel, MD, New York, NY.)
Kinetic scanning, that is, having the patient move his or her eye from side to side and up and down, while fast sector scans are performed, is important in orbital diagnosis. Kinetic scanning helps indicate the degree of adherence of a mass to the mobile anatomic features of the orbit, such as the optic nerve, and to the fixed structures, such as the orbital wall.
Comparative evaluation of the patient's other orbit is not usually performed, because variations from the normal orbital pattern in most disease states are easily discerned. However, in patients with subtle pathologic changes, such as disease of the optic nerve, a comparative evaluation of the companion orbit may aid in differentiation.
When a lesion is palpable (e.g., a cyst or mass in the adnexa) it is useful to use direct contact A- and B-scan techniques. The contact probe can be more easily manipulated to confirm the extent and location of the mass. In addition, the contact A- or B-scan can be used to identify a lesion that may be concealed by the bony overhang of the superior orbital rim. These techniques are particularly useful with lacrimal gland tumors. A contact probe to compress various orbital lesions has been described by Ossoinig (3) and is useful in characterizing the cystic or solid components of orbital masses.
A thorough knowledge of orbital anatomy and the pathologic situations likely to arise in the orbit greatly enhances the ability to interpret information that can be obtained from the ultrasonic evaluation. For these reasons, ultrasonic orbital examination is best performed by an ophthalmologist or a technician who is well trained for ophthalmic ultrasonography.
DIAGNOSTIC PARAMETERS
The ultrasound echo patterns seen in the orbital plane arise from the acoustic impedance mismatch between adjacent tissues. Measuring acoustic characteristics of orbital tissues, Buschmann (4) reported sound velocities of 1,462 meters per second for fat compared with 1,615 meters per second for optic nerve and 1,631 meters per second for muscle. The difference in velocity between these tissue components is the acoustic impedance mismatch that causes partial reflection of an ultrasonic wave as it meets the tissue interfaces.
Within a heterogeneous tissue, such as retrobulbar fat, are many smaller tissue elements, including vessels, nerves, and fat globules with many fibrous septa. These multiple tissue interfaces produce individual echoes and result in a nearly uniform confluence of echoes representing the fat pad. The relatively uniform and organized overall tissue structure of the optic nerve and extraocular muscles has markedly less internal impedance mismatches and produces only low-amplitude echoes within the tissue substance. In addition, the optic nerve and, to a lesser extent, extraocular muscles and major structural components are organized in tissue planes parallel to the ultrasonic beam, minimizing echoes that are detectable along the transmitter-receiver beam path. The same principle of reflections from internal structural elements of a tissue applies to tumors and other abnormalities and is a major criterion for differentiation of tissue types. Orientation to the beam is, thus, the major reason that amplitude of the tissue impedance differences alone cannot be absolutely reliable from the A-scan.
TYPES OF DIAGNOSTIC INFORMATION
Clinicians have long relied on radiographic techniques for orbital evaluation. Plain films and computed tomography depict bony abnormalities well—fractures, erosion, or hyperostosis. Vascular contrast studies demonstrate arteriovenous lesions and intracranial abnormalities causing exophthalmos. Some indications of orbital soft tissue lesions may also be gained from these studies, but the findings are often not definitive (5). Direct injection of contrast material into the orbit
(contrast orbitography) has been used but has fallen out of favor because of morbidity and diagnostic unreliability. The most common diagnostic tests performed today for orbital imaging are plain film radiography, computed tomography, magnetic resonance imaging, and ultrasonography. Additional tests include plain film angiography and venography, MR angiography (MRA), color Doppler imaging (CDI), and dacryocystography. Radiography methods
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and computed tomography subject patients to the known risks of radiation exposure. Magnetic resonance imaging is contraindicated in the case of ferromagnetic foreign objects in the body. CT and MR provide similar soft tissue details in the orbit. CT provides clearer bone details and is sensitive to calcification. MR is better at defining the globe and intraocular structures as well as the optic nerve along its path.
MR is the best overall way of imaging abnormalities of the orbit. Definition and resolution are superb, but the test is still far more expensive than ultrasound.
Ultrasonography, in contrast, provides unique high-resolution information regarding tumors and inflammatory orbital changes using an inexpensive, reliable, and easily repeatable technique. Bony changes and vascular abnormalities, however, are not well demonstrated, although soft tissue is generally well seen. Ultrasonography can serve as an invaluable complement to the usual radiographic, CT, and MR studies, and can often demonstrate an abnormality (because of its sensitivity and high resolution) when all other tests are negative.
Medical and surgical therapies are aided considerably by ultrasonographic findings. For instance, the effect of steroid administration in presumed pseudotumors can be monitored by following the inflammatory signs ultrasonically. Surgical approaches to a tumor, for example, plaque therapy, can be planned with full knowledge of the location, size, extent, tissue composition, and circumscribed or invasive character of the tumor. In general, ultrasonography, followed by contrast MR or CT, should be the first test used for orbital evaluation. Ultrasound is a sensitive test and rarely misses any significant orbital abnormality. This inherent sensitivity is occasionally misleading (in that inflammatory tissues can resemble neoplasms) so further imaging with MR or CT is desirable, if the ultrasonographic findings indicate a pathologic situation.
INDICATIONS FOR ORBITAL ULTRASOUND
The indications for orbital ultrasonography are summarized in Table 5.1. Ultrasound is specifically useful in documenting clinically, evaluable pathologic states such as myositis, Graves disease, or optic neuropathy. In general, ultrasound is indicated when orbital pathology is suspected. Ultrasound findings may aid in the management and treatment of these orbital pathologic conditions, particularly inflammatory conditions.
TABLE 5.1 Indications for Ocular Ultrasonography
Unilateral or bilateral exophthalmos
Retinal striae
Unexplained optic atrophy
Papilledema without evident cause
Suspected orbital foreign body
B-SCAN
HORIZONTAL SCAN PLANE THROUGH THE OPTIC NERVE
With the ultrasonic scan plane passing through the optic nerve, the normal retrobulbar echo pattern is a W-shaped, acoustically opaque (white) area (Figure 5.8). This opaque W-shaped area is bounded anteriorly by the globe and is indented posteriorly by an acoustically empty (black) notch that widens toward the orbital apex. This notch, or triangle, is formed by the optic nerve and associated structures.
The source of the echoes that give rise to the relatively uniform, isoechoic W-shaped retrobulbar echo pattern is not definitely known. Purnell (6) postulated that orbital fat lobules provide the major source of these echoes, and his view is generally accepted. Acoustic discontinuities occur throughout this loculated tissue between intracellular lipids and cell membranes and between cell membranes and loose connective tissue septa. (Similar echo patterns can be produced experimentally in fine-mesh silicone sponge.) M-scans of the orbit have shown marked pulsatile vascular activity, and acoustic discontinuities in the blood vessel network in the muscle cone probably contribute to this echo pattern.
Figure 5.8 A typical 10-MHz horizontal B-scan of the eye and orbit through the lens and the optic nerve demonstrating the typical W-shaped pattern of the orbital fat where the nerve is located (see also DVD).
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In all normal orbits, the optic nerve consistently appears as an acoustically empty triangular notch in the retrobulbar fat pattern. The optic nerve appears acoustically hypoechoic because of homogeneous tissue structure and because the nerve fibers, septa, and meninges usually lie parallel to the examining ultrasonic beam, thus causing no reflections. The anterior angle of the optic nerve triangle or notch is usually less than 90 degrees (with a range of normally 40 to 70 degrees). Widening or any rounding of this angle may be an indication of pathologic enlargement of the optic nerve or its sheaths.
HORIZONTAL SCAN PLANE ABOVE OR BELOW THE OPTIC NERVE
When scans of the orbit are made inferior or superior to the optic nerve, the acoustically hypoechoic (black) optic nerve triangle is, of course, absent. The retrobulbar pattern appears as a uniform, acoustically isoechoic (white) crescent lying immediately posterior to the globe (Figure 5.9). This crescent becomes progressively wider as the center of the orbit is approached.
VERTICAL AND MERIDIONAL SCAN PLANES
Although horizontal scan planes are generally used, performing both vertical and meridional scans of the orbit is important, because areas of pathology that might be missed in serial horizontal scans may be demonstrated more readily. The vertical scans closely resemble horizontal scans. On an axial scan, the optic nerve notch is demonstrable, and orbital structures can be accentuated by having the patient look in different fields of gaze.
Figure 5.9 A horizontal 10-MHz B-scan directed above the nerve, giving the typical cup-shaped orbital fat outline. The orbital walls (ow) are shown but the necessary high gain blurs the muscle (m) boundaries.
VARIATIONS WITH POSITIONS OF GAZE
The normal retrobulbar fat pattern has been described, with the eye in a straight-ahead direction of gaze. Gaze to the far right or left results in a foreshortening of the retrobulbar pattern (decreasing its thickness) and bending of the optic nerve pattern toward the direction of gaze (shown dynamically on DVD). The movement of the optic nerve with changing positions is diagnostically important. If a mass lesion is found in the orbit, a fast sector scan performed as the patient moves his or her eye may demonstrate the relationship of the mass to the mobile optic nerve.
VARIATIONS WITH AGE
Age has relatively little effect on the B-scan ultrasonographic orbital pattern. The retrobulbar pattern in infants is, of course, smaller than in adults but is identical in outline. Our impression is that the retrobulbar fat pattern in infants and children usually appears denser and more uniformly white than in adults, possibly indicating fat distribution in smaller micelles.
EXTRAOCULAR MUSCLES AND ORBITAL WALL
The outer limit of the acoustically opaque (white) W-shaped retrobulbar pattern is formed by the rectus muscles and intermuscular septa. In B-scans made exactly in the horizontal meridian, a rectus muscle outline can often be seen and traced forward to the globe at its insertion.
The ultrasonic patterns formed by the normal orbital walls in different horizontal scan planes are shown for reference in Figure 5.10.
A small portion of one orbital wall (usually the more perpendicular accessible lateral orbital wall) is usually seen on B-scan ultrasonograms, but, if a large portion of the orbital wall or both medial and lateral walls are seen, edema or inflammation of the ocular muscles or periorbital tissue is suggested. The orbital apex is rarely seen in clinical ultrasonograms but can be approximated by tracing the optic nerve and the rectus muscle/orbital wall echoes to their juncture.
Demonstration of the orbital wall is not a dependable ultrasonographic finding but is a feature dependent on the transducer frequency (more likely to be seen with the lower-frequency transducers, i.e., 5 MHz); depth of ultrasonic beam penetration, for example, focal zone of the transducer; and receiver gain. The following are imperative: (a) to obtain familiarity with the area of orbital wall seen in normal orbits with a given instrument, (b) to use a given transducer, and (c) to calibrate the receiver gain before conclusions can be made, with regard to abnormal prominence of the orbital wall on one or both sides in a scan of a possibly abnormal orbit.
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Figure 5.10 Ultrasonic sections taken horizontally through a demonstration skull to illustrate the typical acoustic
appearance of the orbital walls.
FREQUENCY-RELATED VARIATION
The orbital ultrasonographic appearance varies according to the transducer frequency selected. Figure 5.11 demonstrates a comparison of the normal orbital patterns found, using 10and 20-MHz transducers. As a rule, the lower the transducer frequency, the deeper the orbital penetration achieved.
ARTIFACTS ENCOUNTERED IN ORBITAL ULTRASONOGRAPHY
Numerous artifacts may occur during B-scan ultrasonography of the orbit, and knowledge of these possible artifacts will help avoid erroneous interpretation of ultrasonograms of the orbit. As discussed in Chapter 3, artifacts may be classified into two groups: (a) reduplication artifacts and (b) absorption defects.
REDUPLICATION ARTIFACTS
These artifactual echoes (also known as axial multiple echoes) occur commonly and usually appear in the central orbit along the axis of the cornea and lens, often
in the region of the optic nerve triangle. They represent a “second bounce” of echoes, usually from anteriorly located ocular surfaces. A metal lid speculum can be another common cause of reduplication echo artifacts, especially with immersion ultrasound. These echoes can be distinguished from “real” echoes by moving the transducer either away from or toward the eye in the immersion system. This causes movement of the reduplication echo relative to the tissue, allowing them to be readily identified (Figure 5.11). In CDI, reduplication effects from foreign bodies or other strong, reflective surfaces can cause the so-called twinkle artifact, where false flow is seen (7).
ABSORPTION DEFECTS
Absorption of ultrasound energy by structures located in or in front of the eye may cause abnormal orbital
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ultrasonic patterns. Because these absorption defects, or “shadows,” in the retrobulbar fat pattern may simulate orbital tumors, one must be aware of their existence. Ocular tumors (Figure 5.12) or dense calcified lenses (Chapter 3, Figure 3.23) are commonly encountered ocular causes of such orbital defects.
Figure 5.11 A comparison of the orbital fat patterns with transducer center frequency. A: 20-MHz scan of the normal orbit, with high resolution but inferior penetration. B and C: 10-MHz scan of the normal orbit, demonstrating good resolution and good penetration. D: 7.5-MHz linear array scan of normal orbit demonstrating poor resolution, but excellent penetration with angular resolution. Right: 7.5-MHz CDI scan of normal orbit showing retinal and optic nerve flow. (see color image)
When an abnormal orbital B-scan pattern is encountered, the above artifacts should be ruled out. Recognition is aided by careful monitoring of the A-scan, permitting detection of many electronic artifacts, movement of the transducer toward or away from the eye if a reduplication echo is suspected, and analysis of the ocular B-scan for structures that may cause absorption defects in the orbital pattern.
Figure 5.12 A 10-MHz B-scan of a choroidal osteoma (arrow) showing an orbital shadow or artifact (A) as a
result of reflection from calcification.
GENERAL CLASSIFICATION OF ORBITAL ABNORMALITY
Ultrasonically, orbital abnormalities can be classified into structural anomalies, mass lesions, inflammatory or congestive changes, or foreign bodies. Each of these categories has several well-defined subdivisions. A-mode findings in orbital pathology have been described extensively by Ossoinig (8) and Byrne and Green (1). B-scan patterns were classified by Purnell (6) and elaborated on by Coleman et al. (9, 10, 11, 12, 13, 14, 15, 16). The flow diagram proposed by Coleman (17) (Figure 5.13) is useful for the examiner in approaching an unknown orbital problem.
Pseudoproptosis of an eye may be accounted for by either a large globe or a shallow bony orbit (Figure 5.14). A-scan measurement of globe diameters will establish any significant difference between the eyes. A posterior staphyloma may be completely outlined with B-scan. With progressively increasing exophthalmos, even with a unilaterally large globe, complete ultrasonography of orbital structures should always be done to evaluate coincident disease.
MASS LESIONS
A distinct distortion of the retrobulbar fat, optic nerve, or rectus muscles by any sort of abnormal contour ultrasonically indicates a mass lesion in the orbit. Purnell
(6) first described ultrasound B-scan patterns of the orbit that we have reclassified into four general diagnostic patterns
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of mass lesions that are identifiable with B-scan ultrasonography: cystic, solid, angiomatous, and infiltrative (Figure 5.15). Several features of the abnormal area can be used to categorize the lesion, including its contour, sound transmission, internal echoes, and location (Figure 5.16). Close correlation exists between the ultrasonographic findings and the morphologic and histologic characteristics of mass lesions.