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

Figure 3.102. A typical 3-D portrayal of a melanoma, as shown in cross section in Figure 3.99.

Figure 3.103. Tumor adjacent to the optic nerve, causing a widening of the typical optic nerve shadow. In this

case, a vitreous hemorrhage obscured the tumor. Differentiation of melanocytoma from a perioptic melanoma

may not be possible with conventional 10-MHz ultrasound.

Figure 3.104. A small melanoma is demonstrated at 20 MHz. An area of subretinal fluid is noted anterior to the

tumor, and an area of sonolucence is noted posterior to the tumor, indicating possible orbit extension. (See also

DVD.)

Figure 3.105. An area of orbital sonolucence is seen posterior to a relatively flat ocular melanoma, consistent with a large orbital extension of the tumor.

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Figure 3.106. Ocular tumor growth of a relatively small ocular melanoma is demonstrated here over a period of

years.

Acoustic Profile

The acoustic profile of an ocular tumor includes the height of the echo from the leading boundary of the tumor (reflection coefficient), the attenuation of the sound beam as it is transmitted through the tumor (absorption coefficient or decay slope), the presence, spacing and height of reflecting surfaces within the tumor (internal tissue texture), and the variation of absorption and texture, with changes of frequency of the transducer. These acoustic parameters will be discussed individually and are interrelated, as are the morphologic characteristics.

Figure 3.107. The effects of radiation treatment on a ciliary body tumor, by use of parameter image staining, is shown. Tumors may not regress significantly in size but may show a progression of scattering elements. (see color image)

Boundary Properties

Echo amplitude is directly related to the change in impedance between the different tissue layers traversed by the examining ultrasound beam. High-amplitude echoes are produced at boundaries where there is great discontinuity in tissue sound velocities or tissue densities, producing an acoustic impedance mismatch. Examples of this occur at fluid-tissue boundaries or at boundaries between highly disparate tissues, such as lens and vitreous.

The leading edges of choroidal tumors produce high-amplitude echoes when the sound beam is perpendicular to the mass. For A-scan evaluation it is essential that

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this boundary echo be maximized to obtain proper relative values for the internal tissue echoes, as discussed in a later section, Attenuation Coefficient. The height of this leading edge is the high-amplitude echo from the vitreoretinal interface (Figure 3.109). The boundary properties of malignant melanomas, metastatic carcinomas, hemangiomas, and subretinal hemorrhages are thus similar and have not aided materially in differentiation. Identification of this feature, however, separates tumor masses from intravitreal hemorrhages, which, lacking regular boundaries, reflect low-amplitude echoes from their anterior surfaces.

Figure 3.108. Top: This figure demonstrates the use of ultrasound to follow the shape of a melanoma preand post-proton beam therapy. Bottom: Comparative broadband (left) and narrow band (right) images of the same post-treatment tumor. Narrowband image demonstrates the presence of a vitreous hemorrhage.

Attenuation Coefficient

The attenuation coefficient is a measure of the rate of energy loss in the ultrasound beam as it passes through tissue. The tissue absorbs ultrasound, and thus the height or strength of the echoes returned to the transducer diminishes. The amount of ultrasound energy absorbed is dependent on viscoelastic characteristics, and their absorption makes up over 90% of the loss of power. Ultrasound is also attenuated by reflection and scattering from tissue elements. In a heterogeneous tissue both absorption and scattering losses combine to create a gradual falloff (decay slope) in echo height following the initial boundary spike. This decay slope is specific to each type of tissue, and it is approximated for each tissue by a line connecting the peaks of echoes from within a tumor on A-scan. The decay slope of a heterogeneous tumor that has internal reflecting surfaces produced by blood vessels or variations in tissue type is less easily approximated than the slope of a tumor with only one type of internal scattering element or tissue.

Ossoinig (10) has termed the decay slope as an angle Kappa, referring to the angle between the echo peaks (i.e., attenuation) and a horizontal line parallel to the baseline. It should be noted that the decay slope in reality follows an exponential curve and appears as a line only when a logarithmic amplifier is used. Because the amplifier that Ossoinig uses (the S-shaped curve amplifier) approaches a logarithmic amplifier in character, the decay

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slope will be somewhat linear with this instrument. With linear amplifiers, which we use, the decay slope is exponential (see Chapter 2). This curve is best discerned with the radiofrequency display, which is one reason we use radiofrequency as well as video A-scan signals. The other major reasons for observing the radiofrequency signal are (a) the radiofrequency has minimal electronic processing and thus is less susceptible to amplifier overload or reject and is a more realistic and faithful display of tissue reflections than is the “envelope” or video trace; (b) tissue texture is better visualized with the radiofrequency than with the video display; and (c) digital image processing is more informative with the unadulterated RF signal.

Figure 3.109. A-scan through a melanoma demonstrating the internal tissue character once the A-scan has been oriented properly so that a maximal boundary echo is obtained. The figure shows the characteristic attenuation of a melanoma compared to subretinal hemorrhage.

Nevertheless, as Ossoinig et al. (127) and Coleman (36) have emphasized, the decay slope is essential in differentiating tumor types. Malignant melanomas show a high-amplitude leading portion with a steep decay (or “angle Kappa”), often reaching baseline as the tumor adjoins the sclera (Figures 3.110 and 3.111; see also DVD). Hemangiomas generally exhibit a relatively mild, uniform decay slope, lacking the final low-amplitude section seen with malignant melanoma (Figures 3.112 and 3.113). The average amplitude of the hemangioma is usually about 70% of the scleral echo height with our system but is normally about 95% to 100% of scleral echo height, with the system used by Ossoinig et al. (127).

Metastatic carcinomas, like hemangiomas, have relatively flat decay slopes but have lower internal amplitudes, usually about 50% of the scleral height (Figure 3.114; see also DVD). Subretinal hemorrhages show low-amplitude internal reflections, which are only 10% to 20% of the scleral height (Figure 3.115; see also DVD). The sclera is used as a reference for the height of choroidal tumor reflectance because it is posterior to the tumor and has already been subjected to tumor absorption. Thus, attenuation within the tumor by structures anterior to the tumor is subtracted from the scleral echo, and relative absorption differences between tumor and sclera are thereby maintained.

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Figure 3.112. Left: B-scan of choroidal hemangioma with typical dome shape and high internal echo levels. Right: High-resolution A-scan sustained high-amplitude echoes comparable in amplitude to orbital fat. The horizontal arrow indicates tumor position.

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Figure 3.113. Hemangioma on B-scan with the A-scan at 10 MHz (left), 20 MHz (middle), and 22 MHz (right).

Typical attenuation, taken on a vector through the hemangioma.

Internal Tissue Texture

Echoes occurring within a tumor following the boundary spike delineate tissue “texture” by their spacing and height. Internal echoes are the result of inhomogeneities in the tissue, such as blood vessels or poolings of fluid. These relatively small interfaces, known as scatterers, generally, do not contribute significantly to absorption, but their presence and position are useful in differentiation between tumors.

Melanomas often have high-amplitude discontinuities, usually large blood vessels, that produce echoes rising above the decay slope (Figure 3.116; see kinetic scan on DVD). These echoes may show time variations in amplitude and position within the tumor. Ossoinig (11) has described these variations as “spontaneous movements.” The rest of the radiofrequency echo complex in malignant melanoma appears as clustered, relatively coarse, widely spaced echoes mixed with closely spaced echoes. Subretinal hemorrhage, generally, appears as fine-textured, closely spaced echoes, and metastatic carcinoma appears as coarse-textured echoes.

Figure 3.114. Left: B-scan of metastatic carcinoma (lung primary) shows placoid shape with overlying retinal detachment. Note accentuation of Tenon's space and orbital involvement. Right: High resolution A-scan of marked vector segment shows that internal echo pattern is moderate in amplitude with negligible attenuation. (See also DVD.)

Frequency Variation

In discussing the previously mentioned internal tissue characteristics, we have assumed that only a single transducer with a given frequency is used. When the ultrasound frequency is changed (by using a different transducer), each of the internal tissue texture properties (i.e., echo amplitude, echo spacing, and acoustic absorption) may vary. We have found this variation in tumor

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“acoustic profile” between examining frequencies of 10 and 20 MHz (Figure 3.117) to be valuable in distinguishing melanomas from metastatic carcinomas, hemangiomas, and organized subretinal hemorrhages. Melanomas exhibit a sharper drop to baseline on A-scan (increased attenuation on B-scan) with higher frequencies. Metastatic carcinomas are usually solid at all frequencies (i.e., maintain internal echoes on A-scan). Hemangiomas vary according to size but are usually solid at all frequencies, and organized subretinal hemorrhages are usually anechoic at all frequencies. Even the different cytologic types of choroidal malignant melanoma may show different frequency-related variations, with mixed cell, or epithelioid tumors often showing hypoechogenicity with increased frequency, unlike spindle cell tumors (140). These frequency differences are part of the reason that techniques for obtaining the frequency spectrum of tissue echoes offer even greater tissue identification potential. Knowledge of frequency-tissue relationships as a means of augmenting the acoustic profiles of tumors should develop further as instrument and techniques are improved and experience is gained.