Diagnostic Ophthalmic Ultrasound

Published on 09/03/2015 by admin

Filed under Opthalmology

Last modified 09/03/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 2752 times

Chapter 9 Diagnostic Ophthalmic Ultrasound

image For additional online content visit www.expertconsult.com

Ultrasound – past and present

In 1880, the Curie brothers first demonstrated that a difference in electric potential could be created by mechanically pressing opposing surfaces of a tourmaline crystal.17 This phenomenon is called the piezoelectric effect. This effect is the basis for ultrasound technology and was first applied in underwater sonar systems during World War II.8 During that same era, the medical community also adopted the use of ultrasound technology. Scientists realized the diagnostic potential of this technology when they were able to use acoustic wavelengths to study the consistency of a material without damaging the material itself.

In 1949, Ludwig used ultrasound to detect gallstones in patients. The first publication on the use of ophthalmologic ultrasound appeared in the medical literature in 1956.9 By the mid-1970s, ophthalmologists were using ultrasound to determine axial length in a clinical setting. These measurements facilitated calculations of intraocular lens power which led to a revolution in cataract surgery.10 Further innovations came when Baum and Greenwood introduced their two-dimensional B-mode image to ophthalmology.11 Soon afterwards, Bronson et al.12 developed a hand-held contact transducer for this type of image acquisition which led to the rapid dissemination of ultrasound devices within ophthalmology clinics. The B-mode images could be used to delineate accurately retinal detachments, vitreous membranes, and choroidal tumors. In the early 1990s, new technology made it possible to image the anterior segment of the eye with devices that captured images at higher frequencies of 35–50 MHz. This improved image resolution four- to fivefold and is still the gold standard for analysis of certain anterior-segment disease such as ciliary body effusions, infiltrates, and tumors.

Examination techniques

The ultrasound examination is performed with the patient in a reclined position. The frequency of the ultrasound cannot pass through air; therefore, a coupling medium is needed to transmit the sound waves from the transducer to the ocular tissues. A common coupling agent is methylcellulose (Fig. 9.1). The coupling agent is applied to the tip of the transducer probe, which is then placed on the patient’s anesthetized cornea.

B-mode technique

The B-scan is a two-dimensional cross-section image formed by mechanically sweeping the transducer over an angle of 50–60° with the probe oriented in a specific axis. A systematic approach should be used to acquire all images. One method is first to obtain axial scans of the globe by placing the probe in the center of the cornea with the transducer tip oriented toward 12 o’clock in order to image the posterior pole and optic nerve. Next, the transducer can be turned temporally 90° to obtain images through the macula. Finally, radial and transverse images of the globe can be obtained by placing the probe at each clock-hour around the limbus. Radial scans are acquired when the probe is placed perpendicular to the limbus and the transducer tip is oriented toward the cornea. Transverse scans are obtained by turning the probe 90°, orienting the transducer tip parallel to the limbus. The images obtained are the acoustic reflections from the opposing inner surface of the globe.

The reflected sound waves are recorded by the device and can be viewed as a two-dimensional image on the screen (Fig. 9.2). The ocular structures can be examined individually. The cornea is characterized ultrasonographically by two separate acoustic interfaces. The anterior chamber appears planoconvex in cross-section. The iris diaphragm cannot be satisfactorily imaged because of the limited lateral resolution power of the normal B-mode. A clear lens is acoustically empty and appears as an ellipsoid structure in axial sections. Similarly, normal vitreous does not give an acoustic signal; however, the presence of a detached posterior vitreous membrane presents an interface that can be imaged by increasing the amplification of the echo signal. The sclera is the most strongly reflecting structure on ocular ultrasonography.

High-frequency ultrasound technique

High-frequency echograms can be used for ultrasound biomicroscopy (UBM). The shorter wavelengths provide better resolution of the anterior structures of the eye, including the cornea, lens, aqueous (Fig. 9.3), and ciliary body (Fig. 9.4).13 High-frequency probes range from 50 to 100 MHz.1416 The 50-MHz probe provides the best balance between depth and resolution for UBM technique. One limitation of this technique is that the shorter wavelengths, from the higher frequency, have poor depth of penetration. UBM cannot visualize structures deeper than 4 mm from the surface.

UBM requires immersion of the transducer in a medium to transmit the higher-frequency wavelengths. Saline or methylcellulose can be used as the coupling agent and is held in place over the eye with the use of a custom cup during the examination. UBM is performed through open eyelids in order to obtain a good reflection signal. Images produced by UBM have a resolution of 30–40 µm, which is similar to that seen with a low-power microscope.17

The cornea is the first structure seen on UBM. The anterior-chamber depth can be measured from the posterior surface of the cornea to the anterior lens pole. The posterior lens pole cannot be imaged by UBM due to its distance from the anterior surface. The iris is seen as a flat uniform echogenic area. The iris and ciliary body converge in the iris recess and insert into the scleral spur. The area under the peripheral iris and above the ciliary processes is defined as the ciliary sulcus. The angle of the eye can be studied in cross-section by orienting the probe in a radial fashion at the limbus. The scleral spur is the most important landmark in the angle on UBM.

Doppler ultrasound

Doppler images are obtained by using frequency shifts from acoustic reflections to measure movements within a tissue and flow conditions within vessels. These frequency shifts can be observed in tissue volumes of less than 10 mm. False color can be added to the images based on ultrasound frequency to distinguish between higher and lower flow states, which aids in the interpretation of the final result (Fig. 9.5).

Ultrasound biometry

Basic physics formulae can be used to calculate the speed of sound as it passes through various ocular tissues. This number can then be used to calculate distance measurements within the eye (Fig. 9.6). In order to obtain accurate measurements, the specific speed of sound of the different intraocular media, such as the lens, aqueous, and vitreous, must be known.18 These formulae provide precise measurements that can be used to measure intraocular tumors or to deduce the axial length of the globe for intraocular lens power calculations.

Ultrasound in intraocular pathology

Changes in the shape of the globe

Vitreous

Echographic examination can provide information on vitreous structure which is particularly useful when visualization of the posterior pole is poor due to anterior media opacities. Ultrasonographic findings allow the examiner to differentiate dot-, strand-, and membrane-like reflections (Fig. 9.13). Table 9.1 summarizes the most frequent conditions associated with pathologic changes in the vitreous.

Table 9.1 Clinical conditions with ultrasonographically demonstrable vitreous changes

Persistent and hyperplastic primary vitreous

The primary vitreous contains the tunica vasculosa lentis, which is part of the fetal vasculature system. During development, the tunica vasculosa lentis emanates from the optic nerve head and supplies the posterior lens. This structure should involute prior to birth. Failure of the primary vitreous to regress fully is termed persistent hyperplastic primary vitreous. As mentioned earlier, this can be associated with microphthalmos and cataract formation in the newborn. The condition persistent hyperplastic primary vitreous can be ultrasonographically characterized by two features. The first is a strand of membrane that extends between the posterior surface of the lens and the area of the optic nerve head. The second is the reduced axial length of the globe from microphthalmos on ultrasound biometry (Fig. 9.16). If the anomaly is only mild, the lens may be clear at birth but may become cataractous when the posterior lens capsule ruptures.

Vitreous hemorrhages

An acute vitreous hemorrhage is an important indication for ultrasonography. Acute hemorrhages can fill the vitreous cavity with small opacities from the particles of the red blood cells. These opacities usually accumulate after a few hours in the lower circumference of the vitreous base (Fig. 9.17).

If a detachment of the posterior hyaloid membrane precedes a vitreous hemorrhage, the erythrocytes frequently precipitate on to a vitreous strand (Fig. 9.18). This strand may be responsible for the development of a retinal tear, and its traction can be demonstrated directly in acoustic sectioning (Fig. 9.19). A circumscribed thickening of the ocular wall in cross-section may indicate the presence of a retinal operculum (Fig. 9.20). This area should be localized echographically and then carefully scrutinized with ophthalmoscopy if possible.

In larger hemorrhages, the blood can also disseminate into multiple pre-existing vitreous compartments. In the early phase of this process, the erythrocytes will collect in the retrovitreal space (Fig. 9.21). The retrovitreal space may completely clear after a few days or weeks due to its high fluid exchange rate; however, blood on the vitreous framework absorbs much more slowly (Fig. 9.22).

Intraocular infections

Ocular infection that extends toward the anterior segment or results in a hypopyon formation will have changes within the anterior vitreous space that are demonstrable on ultrasound. A thickening of the retina or choroid can be seen if the inflammation penetrates to the outer layers of the globe (Fig. 9.26). After only a few hours, these changes may involve the entire vitreous body (Fig. 9.27). If panophthalmitis follows a perforating injury, ultrasound evaluation can detect a local reaction at the entrance point of the infection (Figs 9.28 and 9.29).

Intraocular foreign bodies

Intraocular foreign bodies induce a change in echo reflectivity which is based on the composition of the material (Figs 9.319.33). The change in the reflectivity on the image should be a helpful clue in the localization of the foreign body within the globe; however, this is not always the case since the foreign bodies can also create signal artifact on echograms that can make identifying their exact location difficult. For example, large metallic foreign bodies have significant artifacts from strong reflected signals that can distort their true location. In addition, foreign bodies from trauma can be associated with air bubbles within the vitreous that can mask the presence of the nearby foreign body within the acoustic shadow (Fig. 9.34).

Retina

Ultrasound can be used to assess the structure of the retina in order to discern anatomical changes such as retinal tears and detachments. It can also be used to identify changes in retinal thickness from infiltrative or exudative ocular diseases. The sound reflections in ultrasound images can delineate these pathological retinal changes even in eyes with opaque anterior media. This is a particularly useful tool since many diseases that affect the retina can also lead to vitreous changes that limit direct visualization. It is important to have a strong fundamental knowledge of ocular anatomy as well as a good technical approach for acquiring ultrasound images in order to make accurate assessments of retinal disease on ultrasound. This section will review the anatomical features of common retinal conditions as well as the special techniques needed to examine the retina with ultrasound.

Acute retinal detachment

In a retinal detachment, the neurosensory retina separates from the RPE layer. This development allows fluid to collect in the potential space between these two layers. The detached neurosensory retina appears as a membrane in the vitreous space on ultrasound. Partial retinal detachments may still maintain connections to the optic nerve or ora serrata since these areas have the strongest connections to the retina. Identification of these connections on ultrasound can distinguish a partial retinal detachment from a vitreous or choroidal detachment, which would have different anatomical connections (Fig. 9.35). A complete retinal detachment can form a funnel shape due to the retina folding in the center of the globe.

Complicated retinal detachments with severe pathology can make it difficult to identify all the structures on ultrasound (Fig. 9.36). For example, in severe trauma cases that are associated with proliferative vitreoretinopathy or in advanced diabetic disease associated with proliferative retinopathy, the membranes formed within the vitreous can appear similar to a true retinal detachment. The following questions can guide the ultrasound examination of the retina in order to differentiate these common causes of vitreous membranes:

These questions should be clarified in the following way: