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reflected toward the source of the emitted wave (i.e., the probe). This reflected wave is referred to as an echo. Echoes are generated at adjoining tissue interfaces that have differential acoustic impedance. The greater the difference in acoustic impedance, the stronger the echo. For example, strong reflections occur at the interface between retinal tissue and vitreous, which is essentially water. When adjoining tissue interfaces have relatively small differences in acoustic impedance (e.g., vitreous gel and mild vitreous hemorrhage or clumped intravitreal white blood cells), weak reflections are seen. Pulse-echo technology uses synthetic crystal transducers to produce ultrasonic wavefront pulses and to retrieve echoes for electronic display processing.

33.How is the clinical ophthalmic ultrasound displayed?

The reflected echoes are received, amplified, electronically processed, and displayed in visual format as an A-scan or a B-scan (Fig. 5-6):

&A-scan ultrasonography, or the A- mode, is a one-dimensional, timeamplitude display. The horizontal baseline represents the distance and

depends on the time required for the sound beam to reach a given interface and for its echo to return to the probe. In the vertical dimension, the height of the displayed spike indicates the amplitude or strength of the echo.

& B-scan ultrasonography, or the B- mode, produces a two-dimensional, cross-sectional display of the globe and orbit. The image is displayed in variable shades of gray, and the shade depends on the echo strength.

Strong echoes appear white, and weaker reflections are seen as gray.

The A-scan is used predominantly for tissue characterization, whereas the B-scan is used to obtain architectural information. A-scans are also helpful in determining intraocular lens calculations for cataract surgery (see question 41).

34.What lesion features are evaluated during the ultrasound examination?

1.The topography (location, configuration, and extension) of a lesion is evaluated most often by the two-dimensional B-scan.

2.The quantitative features include the reflectivity, internal structure, and sound attenuation of a lesion.

&The reflectivity of a lesion is evaluated by observing the height of the spike on the A-scan and the signal brightness on the B-scan. The internal reflectivity refers to the amplitude of echoes within a lesion and correlates with its histologic architecture.

&The internal structure refers to the degree of variation in the histologic architecture within a mass lesion. Regular internal structure indicates a homogeneous architecture and is noted by minimal or no variation in the height of spikes on the A-scan and a uniform appearance of echoes on the B-scan. In contrast, an irregular internal structure is noted in a lesion with a heterogeneous architecture and is characterized by variations in the echo appearance.

&Sound attenuation occurs when the acoustic wave is scattered, reflected, or absorbed by a tissue and is noted by a decrease in the strength of echoes either within or posterior to a lesion. It is indicated by a decrease in spike height on the A-scan or a decrease in

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the brightness of echoes on the B-scan. Sound attenuation may produce decreased signal strength and a void posterior to the lesion that is referred to as shadowing. Substances such as bone, calcium, and foreign bodies typically produce sound attenuation (Fig. 5-7).

35.The dynamic features of or within a lesion can be detected on the B-scan.

&Aftermovement is determined by observing the motion of lesion echoes after cessation of eye movements. The rapid movement of a vitreous hemorrhage is distinguished from the slower, undulating movement of the retina in an acute rhegmatogenous retinal detachment.

&Vascularity is indicated by spontaneous motion of echoes within a lesion and represents blood flow within vessels.

36.How is ultrasound used in preoperative cataract evaluation?

The A-scan is used to measure the axial length of the globe, which is required in the formula to calculate the intraocular lens power. The B-scan is useful if the ocular media are opaque to assess for a retinal disorder that may affect visual outcome after cataract surgery.

37.How is ultrasound used to assess intraocular tumors?

Ultrasound may be used for diagnosis, to plan treatment, and to evaluate tumor response to therapy. Specifically the tumor shape, dimensions (such as

TABLE 5-2. U L T R A S O U N D P A T T E R N S I N T H E D I F F E R E N T I A L D I A G N O S I S O F C H O R O I D A L M E L A N O M A

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