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

Figure 3.45. 50-MHz anterior segment scan demonstrating lesions placed for treatment of presbyopia by laser

surgery.

Physiologic Measurements

Physiologic changes can be measured both statistically and in real time, using ultrasound. In addition to measurements of blood flow that can be imaged with Doppler or swept-scan techniques, physiologic changes in the choroid, ciliary body, and lens position and shape are routinely measured with ultrasound. The effect of accommodation, pressure, and light can be shown, and the effects of pharmacologic agents can be documented.

The development of an instrument to demonstrate a forward or translational movement of the lens in accommodation led to the first use of an electronic interval to measure axial length (33), which is useful in determining lens power for surgery.

Studies of accommodation in our laboratory led us to the catenary diaphragm theory of accommodation that better explains the paraboloid anterior lens curvature in accommodation and helps explain how accommodating intraocular lenses can work, as well as why presbyopic surgery techniques are possible (67,81).

The definition possible with early radiofrequency 20-MHz A-scan ultrasound is shown in Figure 3.46, which was used to demonstrate a mass or translational forward lens movement. Using VHF ultrasound, we have demonstrated how the anterior lens curvature is similar to the paraboloid curvature proposed by Koretz et al. (82). This aspheric lens surface and its depth-of-field advantages explain many of the inconsistencies noted with other theories, such as the capsular or Helmholtz theory.

Pupil diameter changes with light stimulation are shown in Figure 3.47. The position of the iris during accommodation (Figure 3.48), relative to the cornea-scleral

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angle and the lens, can be critical in planning possible intraocular lens placement (83,84). Measurements of anterior segment dimensions and possible placement of explants or surgical or laser incision for presbyopia are shown in Figure 3.49.

Figure 3.46. 20-MHz A-scan demonstrating both the RF and video traces that can allow very accurate

measurement of ocular dimensions. Note on the RF that the first quarter cycle of sound can be positive as the sound enters the cornea and negative as it leaves the cornea, denoting the change of media and speed of sound.

Figure 3.47. Iris position during light illumination and darkness. These and other physiologic measurements of

the iris can easily be made with very high frequency ultrasound.

Figure 3.48. Position of the iris and its relation to the lens during accommodation and unaccommodation.

Anterior lens curvature can also be measured.

Figure 3.49. The position of the lens equator relative to external landmarks can be measured and predicted from B-scan imaging. Here the circle demonstrates the expected position of the lens equator for possible presbyopic surgery correction; similar prediction is possible postcataract extraction or IOL position.

Effect of Pharmacologic Agents

Dilation, and the degree of dilation or contraction of the pupil, can be demonstrated with B-scan ultrasound, and, ordinarily, the pupil size can be accurately estimated (Figure 3.50). Positioning the transducer parallel to the iris in contact B-scan ultrasonography can show the actual movement of the iris sphincter in a graphic manner. Studies of the effect of drugs, such as pilocarpine or other agents on both the pupil and ciliary body, provide unique measurements of pharmacologic effects on anatomic and vascular structures (Figure 3.51) (44,85).

Figure 3.50. B-scan demonstrating the pupillary opening, which can be measured in both accommodation or pharmacologic or other physiologic conditions. See also Figures 3.47, 3.48.

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Figure 3.51. Ciliary body area with color enhancement of the scatterer dimensions in the ciliary body in studies of pharmacologic effects on the ciliary body. (see color image)

Lens

The normal lens has been described previously and is normally a clear (anechoic) space as a result of homogeneity of the lens cells. The surfaces have a high acoustic index but are mostly specular reflectors, which may not be seen unless the transducer is orthogonal. On B-scan, the arc scan easily shows the anterior surface but may show only a “highly reflective center” of the posterior surface. The reverse is true for a sector scan (Figure 3.52). For structures deeper than 5 to 6 mm, lower 10to 25-MHz scanning frequencies are required. Fibrin or blood can convert the surfaces to a diffuse reflector and permit a better outline (Figure 3.13).

Figure 3.52. A 10-MHz sector scan shows only a small segment of a reverse image of the cornea (as a result of transducer beam width) and of the anterior lens surface. The posterior lens surface is better appreciated as a result of its concave “fit” of the lens surface and the sector. The arc scan is much better at showing anterior lens surface but does not show the posterior lens surface well.

Absence and Displacement

Variations of normal lens position may be depicted ultrasonically. The absence of the lens from its proper position in patients suffering from trauma should initiate a thorough search of the vitreous compartment for a displaced lens, as seen in Figure 3.53.

Cataract

The ultrasonographic appearance of a cataractous lens differs from the normal lens in that optical opacities also produce acoustic inhomogeneities. The A-scan trace through the lens changes from a picture of acoustic homogeneity and sonolucent areas (with echo return only from the anterior and posterior lens surfaces) to an acoustic heterogeneity, where numerous echoes are seen within the nucleus and cortex of the lens. The position of these echoes indicates the area of acoustic change, which usually corresponds to the optical changes, and the degree of visual loss. Figure 3.54 demonstrates the separation of nucleus and cortex and shows a posterior cortical cataract. B-scan ultrasonograms taken of a cataract demonstrate multiple intralenticular echoes. Pathologic changes responsible for these

acoustic alterations include nonuniform lens fiber swelling and water cleft formation.

Pre-intraocular and Post-intraocular Lens Implant

Ultrasonography is useful in documentation of the status of the anterior segment of the globe and accurate determination of the axial length of the eye, which permits selection of dioptric correction with an intraocular lens.

The diameter of the cornea-iris angle and of the ciliary body sulcus is not uniform. Meridional scans will allow the axis of maximum diameter to be evlauated. This may be useful for lens haptic placement to prevent lens movement, called propellering.

Figure 3.53. In this traumatized eye, the crystalline lens was completely dislocated and can be seen as a rounded mass in a suitable plane. Differentiation from a tumor is usually not a problem and is facilitated by having the patient move his or her eye, thus inducing lens movement.

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Figure 3.54. Cataract shown at 10 MHz in a patient with a flat anterior chamber (left), and 50 MHz image of cataractous lens in another patient showing high internal reflectivity within the lens (right).

The angle-to-angle and sulcus-to-sulcus measurements can be critical for sizing of IOL placement. Rondeau et al. (86) have shown that the coronal section of the eye at the angle and at the sulcus is not round but generally oval. In addition, the white-to-white measurement that has been advocated is not an accurate substitute for actual measurement, with errors reported up to 2 mm. The oval shape of the coronal section is usually longer vertically, but the axis does not correspond exactly with the axis of astigmatism (Figure 3.55).

Figure 3.55. Top: 1. pupillary alignment vector for the scan series. 2. angle-to-angle measurement plane. 3. sulcus-to-sulcus measurement plane. Bottom: The scan geometry for the semimeridional scan series is demonstrated with lines representing individual scans. We have demonstrated that this coronal measurement is commonly ellipsoidal, and the long axis can be determined for optimal placement of lens haptics to avoid “propellering.”

A B-scan ultrasonogram of an eye with an intraocular lens in correct place is shown in Figure 3.56. Similarly, eccentric placement or dislocation can be shown as well as haptic malposition, as shown in Figures 3.57 and 3.58. This can be helpful in directing surgical intervention. Certainly, “sizing” is the critical element in developing better lens designs, and VHF ultrasound is the preferred method for its accuracy in measuring all ocular anatomies.

POSTERIOR SEGMENT ULTRASOUND

Synopsis

Vitreous hemorrhage as a result of diabetes, trauma, or other causes can appear of variable density. Blood in the formed vitreous can be best seen with a narrow band transducer, using kinetic scanning for movement.

Light hemorrhage, endophthalmitis, or the vitreous changes in uveitis or central nervous system (CNS) lymphoma may be easily detected but acoustically indistinguishable.

Retinal detachment hallmarks are high amplitude surface, always attached at the optic nerve and (except for giant tears) at the ora serrata. Retina movement on kinetic scanning can indicate recent (fluid movement) or old (fixed, rigid) detachments.

Retinal detachment over tumors is characteristically bullous followed by a smooth tumor surface, which may or may not be attached to retina.

Posterior pole abnormalities, like proliferating membranes, small melanomas and nevi, or age-related macular degeneration (AMD), are better visualized at 20 to 30 MHz.

Severe vitreous hemorrhage may produce visual loss by chemical changes (i.e., hemosiderosis of the retina, hemosiderogenic syneresis of the vitreous) and mechanical

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