Ординатура / Офтальмология / Английские материалы / Ultrasonography of the Eye and Orbit 2nd edition_Coleman, Silverman, Lizzi_2006
.pdf
Figure 3.115. A subretinal hemorrhage with the vector A-scan demonstrating low amplitude echoes from the
blood. (See also DVD.)
Figure 3.116. Acoustic discrimination within a melanoma caused by inhomogeneities, such as septae or blood
vessels. (See also DVD.)
Figure 3.117. B-scan ultrasonogram of a small melanoma at 10 and 20 MHz. Higher resolution at 20 MHz allows thickness to be more accurately measured and gives a better measurement of the posterior ocular coats. Even better resolution can be obtained with radiofrequency reconstruction, as seen in Figure 3.125.
P.99
Analytic Mathematical Modeling and Parameter Imaging
Tissue Characterization
Not only can gray scale be used to give texture to amplitude data on a B-scan, but analytic mathematical modeling can be used to analyze different frequencies of reflected echoes from a power spectrum, that is, frequencies calibrated against a perfect reflector for a range of tissue frequency responses, using a single frequency transducer. Analysis of the power spectrum is discussed in Chapter 2 for technical background.
Clinically, the resultant pixel representation of scatterer diameter or scatterer concentration relies on the B-scan. These parameter images of a tumor or tissue are digitally reconstructed from radiofrequency data acquired from the tumor or tissue. With this technique, we can compare different areas of a tumor or a tissue to determine probability of microarchitectural variations in the tumor that relate to such things as tumor lethality. The two parameters found most useful for these variations are scatterer concentration and scatterer density (Figure 3.118). A scan through a melanoma demonstrating scatterer size and concentration with pseudocolor is shown in Figure 3.119.
The areas that have been particularly useful with parameter image or tissue characterization are tumor identification with subclassification or stratification and tissue identification (141).
We studied 117 patients with ocular melanoma with the cooperation of the University of Iowa (Boldt and Weingeist) and the University of Illinois-Chicago (Folberg, Chen, and Vangveeravong). Patients were seen in Iowa prior to enucleation and the eyes were scanned and RF digitized data were collected. The enucleated eyes were evaluated by Dr. Folberg and his group at the University of Illinois-Chicago for the histologic presence of high risk extravascular matrix patterns. The ultrasound was analyzed independently at the Weill Cornell Ultrasound Lab by Dr. Silverman and Rondeau, using techniques previously described by Lizzi and Coleman (Figure 3.120). The results showed that this noninvasive technique can identify high-risk melanomas with 80.1% cross-validated correct classification (56).
Figure 3.118. Graph shows acoustic scatterer sizes as measured from tissues at different frequencies. By comparing the frequency response to the interrogated values of the tissue, we can determine the scatterer size and, in a similar way, concentration and density (see Chapter 2). (see color image)
Figure 3.119. A melanoma with gray scale on the left, and after processing the power spectrum for scatterer
diameter and concentration on the middle and right images. (see color image)
The value of this technique is being explored as a means of stratifying patients for treatment staging and monitoring of therapeutic modalities.
Tissue characterization is also of value in identifying tissue subgroups, such as the choroid and ciliary muscle. The identification of ciliary muscle is useful in studying physiologic effects of drugs and the effects of treatments, such as radiation on tumors.
Small Melanomas and Nevi
An area of concern and controversy is the diagnosis and possible treatment of very small melanomas. Although nevi and congenital hypertrophy of the retinal pigment epithelium (CHRPE) lesions are universally regarded as benign, the distinction between nevi and small melanomas can be subtle clinically. The distinction between small, dormant melanomas and potentially high-risk melanomas can be both subtle and daunting.
Conventional 10-MHz ultrasound has not been particularly helpful in this distinction, because both tend to show hyperreflectivity but too little thickness to allow conventional A-scan distinction. The use of higher frequencies, especially when complemented by parameter image analytic modeling, offers a better way of distinguishing these three categories. Many authors
P.100
have discussed the clinical differences between dormant and high-risk melanomas, but Shields et al. (143, 144, 145, 146) have written most extensively on this subject. Clinical signs of high-risk melanomas are orange (lipofuscin) pigment and subretinal fluid. These can be quantitatively augmented with high frequency ultrasound. Figure 3.121 shows the clinical and ultrasonic features that can be used to differentiate these three classes. The ultrasonic distinction rests on three quantitative features. These are choroidal replacement, subtumor or intratumor “fluid,” and precise measurement of growth.
Figure 3.120. Demonstrates the histology of a melanoma (lower left) compared to the ultrasonogram (upper left). The histology demonstrates the extravascular patterns (EVM) of the melanoma. The two right scans show the scatterers that correlate with these EVMs. The lower right shows the visual correlation of scatterers to EVMs indicating a “high-risk” melanoma. (see color image)
The histologic difference between a nevus and a tumor is shown in Figure 3.122. Melanin is highly reflective and thus easily seen with ultrasound. No fluid exists, and the choroid is normal. A dormant melanoma may be homogeneous in acoustic profile, has no fluid, and, generally, does not seem to replace the choroid. The “high-risk” melanomas in our experience are those that show
1.Lipofuscin pigment, subretinal fluid, and drusen, as emphasized by Shields and Shields (147, 148)
2.choroidal replacement
3.fluid either subretinal or intratumor
4.growth of at least 0.1 mm in thickness over a 3- to 6-month period.
Figures 3.123, 3.124 and 3.125 demonstrate these features, and Figure 3.126 is the schema demonstrating our clinical management.
The mathematical analysis used to distinguish the choroidal layer and the placement is the same as described previously in the section on age-related macular degeneration.
P.101
Figure 3.121. Clinical and ultrasound signs that have been shown helpful in differentiating nevi from dormant and high-risk melanomas.
Figure 3.122. Histologic preparations demonstrating a nevus, which has high concentration of melanin and relatively uniform vascular architecture, compared with a small melanoma showing uniform or homogeneous tumor tissue, oft-associated subretinal fluid, and the concentrated vascular pattern (box). (Courtesy of Robert Folberg, MD.) (see color image)
Figure 3.123. Patient with both a nevus, adjacent to the nerve, and a suspicious melanoma in the midperiphery. The B-scan ultrasonogram is taken at 20 MHz, with parameter image tissue staining to outline the choroid; in this case, presence is noted posterior both to the nevus and the melanoma. (see color image)
P.102
Figure 3.124. This patient also had a suspicious melanoma, although in this case, the choroid is replaced. Unlike a high-risk small melanoma, there is no evidence of fluid or growth. (see color image)
Doppler and Kinetic Properties of Tumors
Ossoinig (12) has emphasized the value of using the A-scan probe as a means of “ballotting” tumors to stimulate changes in their acoustic patterns, as well as to permit observation of their compressibility. This test is more useful in the orbit than in the globe, especially when dealing with cystic tumors. The detection of vascular echoes can be enhanced with this technique during ocular examination.
Color flow Doppler can show vascularity in large tumors, not only in the tumor but in its underlying choroid and orbit (149).
Acoustic Characteristics (B-scan)
The acoustic profile of a tumor on A-scan translates into a B-scan display as variations in the appearance of tissue texture. The phenomena of acoustic quiet zones, choroidal excavation, and acoustic shadowing are major sources of B-scan tumor differentiation.
Figure 3.125. This patient has what we regard as a high-risk melanoma, as a result of the presence of clinical and ultrasound signs. These include: subretinal fluid, choroidal replacement. This class of lesion is followed at short intervals for growth. (see color image)
Acoustic “Quiet Zone”
Malignant melanomas appear on B-scan as hyperechoic areas protruding into the anechoic vitreous cavity. Histologically, malignant melanomas are homogeneously cellular with varying degrees of vascularity. With increasing vascularity of the tumor tissue, there are many internal acoustic interfaces, so that more echoes are returned and the tumor thus appears hyperechoic. This hyperreflectivity in the more vascular uveal melanoma is apparent at transducer frequencies of 5, 10, 15, and 20 MHz. Polypoid-shaped melanomas almost always exhibit these characteristics of acoustic solidity in the “button,” whereas the base
or “collar” remains relatively anechoic. In relatively avascular melanomas (most often those with convex shape), the homogeneous cellularity of the tumor and lack of significant internal acoustic interfaces result in the appearance of an acoustic “quiet zone” or hypoechoic region within the tumor (Figure 3.127). This phenomenon is accentuated on the B-scan, although the A-scan tracing registers echoes of moderate though declining amplitude
P.103
throughout the tumor. If the tumor were actually physically fluid filled, the A-scan would show absence of echoes after the initial leading echo (as seen in a retinal detachment). This phenomenon of acoustic quiet zone, or hypoechoic region in relatively avascular melanomas, is most prominent at examining frequencies of 15 and 20 MHz. At 5 and 10 MHz, the tumors almost always appear echoic.
Figure 3.126. Schematic summarizing the clinical and ultrasonographic findings as they dictate separation of
suspicious and high-risk small melanomas, and the clinical management.
Choroidal Excavation
Involvement or replacement of the choroid by a melanoma can be shown dramatically by the “excavation” phenomenon. The area of tumor that has replaced the surrounding choroid demonstrates a dishor bowlshaped indentation into the smooth concave choroidal outline. It must be remembered that the choroid in the living eye is a highly vascular erectile tissue that may be as thick as 500 mm or more at the posterior pole (Figure 3.128). Excavation has been frequently noted in malignant melanoma, although not all melanomas exhibit this feature.
Figure 3.127. B-scan at 10 MHz of a homogeneous melanoma, showing very low amplitude or absent echoes in
the central part of the tumor.
Figure 3.128. A small melanoma seen at 20 MHz with the Quantel ultrasound apparatus, demonstrating the typical choroidal replacement on B-scan. As noted earlier, this feature can be accentuated by use of tissue staining. But in general, one sees a scaphoid indentation in the wall posterior to the tumor, of an accentuated curvature relative to the curvature of the normal choroid.
P.104
The incidence of choroidal excavation in a series of 110 intraocular tumors was evaluated (130). Choroidal excavation was absent in all cases of metastatic carcinoma and hemangioma and was noted only in malignant melanoma. Of the 89 malignant melanomas, 42% exhibited this characteristic and 58% did not. Choroidal excavation was not seen in any melanoma anterior to the equator. Fuller et al. (150) have, however, reported that excavation was seen in metastatic carcinoma in their series. The histologic similarity of metastatic carcinoma to certain melanomas, however, should perhaps lead us to expect a similar pattern. Subretinal hemorrhage and disciform macular degeneration, in our experience, do not show choroidal excavation.
With high-frequency scans, the choroid can be measured even in very small nevi/melanomas by use of “midband fit” scans (as described previously) that can differentiate sclera and choroid. Nevi, almost invariably, sit on residual choroid, whereas small melanomas displace the choroid.
Attenuation Defect or “Shadowing”
Attenuation of sound by one tissue mass can cause an acoustic attenuation defect or shadowing to appear in structures behind the mass. A solid mass will sometimes attenuate sound to such an extent that the area of retrobulbar fat behind it will seem fainter than the rest of the orbit, or the sound beam will not penetrate far into the orbit directly behind the tumor, causing a hypoechoic appearance (Figure 3.129). This absorption defect will not occur if the mass has good sound transmission properties. Apparently, no significant variation occurs in the shadowing produced by melanoma and metastatic tumors with our techniques. Hemangiomas show little evidence of shadowing, probably as a result of their lower attenuation coefficient.
Figure 3.129. A treated retinoblastoma, with considerable calcific change, produces a shadowing posterior to
the lesion.
Associated Ocular Changes