Ординатура / Офтальмология / Английские материалы / Ophthalmic Ultrasound A Diagnostic Atlas 2nd edition_ DiBernardo, Greenberg_2006

Almost a decade has passed since the first edition of this book was published, and ophthalmic ultrasound remains an integral part of ophthalmology practices around the world. Its role continues to be in the diagnosis and management of a wide variety of ocular and orbital disorders. Understanding both diseased and healthy ocular structures, as well as mastering the prescribed examination techniques, remains an essential task for echographers, no matter what their level of expertise may be.

Although the features of most equipment have remained constant, new technologies have been developed to further improve our diagnostic abilities. Thirty years ago the range of the probe frequencies used in ophthalmic ultrasound was limited to 8 to 12 MHz. Today there are different probes for different areas of the globe,

including 20, 50, and 100 MHz for the anterior segment and 20 MHz for the posterior segment. Three-dimensional imaging is being used in some practices as well.

In this second edition, we have improved and replaced many images. We have included some images in which we used a 20-MHz probe for the posterior segment in conjunction with 10-MHz probes when it was appropriate to do so. We have expanded the anterior segment chapter to include additional ultrasound biomicroscope (UBM) images, and we have separated orbital evaluation into three chapters dealing with the retrobulbar optic nerve, the extraocular muscles, and orbital lesions. Once again, rather than providing detailed references for each chapter, we have compiled a list of suggested reading to enhance what the reader has learned from each section.

Cathy W. DiBernardo, R.N., R.D.M.S., R.O.U.B., and Ellen F. Greenberg, C.O.T., R.D.M.S. R.O.U.B.

The examination techniques detailed in this atlas are based on those developed by Dr. Karl Ossoinig (University of Iowa) and further cultivated by Sandra Frazier Byrne, R.D.M.S. (Mars Hill, NC), and Ronald L. Green, M.D. (Estelle Doheny Eye Institute, Los Angeles, CA), and a multitude of echographers around the country.

This second edition was prepared with the support of Dr. Peter McDonnell, Chairman at the Wilmer Eye Institute. We wish to thank Maria Bernadete Ayres and Diane Chialant, R.N., R.D.M.S, C.O.T., for their generous assistance with the text and their contribution of many

spectacular images. We also wish to express our genuine appreciation to Michael McElwaine and David Emmert of the Wilmer Eye Institute Photography Department for their patience and assistance as we learned how to convert and manipulate digital images.

We continue to be indebted to all of the full-time and part-time ophthalmologists at the Wilmer Eye Institute and in the Baltimore community for supporting the Echography Department. We receive many referrals to our department, and we continue to be stimulated by the variety of ocular disorders that we see.

Over the past few decades, the use of ultrasound in ophthalmology has become an important and often necessary tool to aid in the diagnosis of intraocular and orbital disease. It is employed most frequently when ophthalmoscopic evaluation is limited or when differentiation of mass lesions or other pathology is needed. Imaging of the eye and orbit is facilitated by the use of high frequency sound (8–10 MHz, 20 MHz, and 50–100 MHz). The sound wave is transmitted and received through a probe that is placed directly on the eye or on the eyelid. As sound travels through the structures of the eye, reflected signals are returned to the probe, mechanical energy is converted to electrical energy, and signals or echoes are recorded on an oscilloscope.

The techniques of standardized echography and the descriptions of typical echographic features of selected disorders are described in this second edition atlas. The term standardized echography was first used by Dr. Karl Ossoinig, and refers to the combined use of a contact B-scan (brightness modulation) and a standardized A-scan (amplitude modulation) to evaluate ocular and orbital pathology using prescribed examination techniques. Dr. Ossoinig is credited with the development of the first standardized A-scan. The A-scan provides a one-dimensional image of spikes or deflections of varying amplitude along a baseline. Because of the amplification design of this equipment, differentiation of tissue is possible based on the reflectivity (height of spikes) and structure (distribution of spikes) produced by the cells of various tissue. Sound attenuation, consistency, vascularity, and precise measurements are also determined with the A-scan. The B-scan, on the other hand, allows for two-dimensional imaging of a series of dots and lines that form the echogram. B-scan imaging is most useful for determining the topographic features of the normal globe

structures as well as abnormal structures that may be present.

A simplified way of describing the need for both contact B-scan and standardized A-scan imaging is to compare the B-scan with color fundus photography and the A-scan with fluorescein angiography. The B-scan or fundus photograph may provide some useful information, but it may not be enough to confirm a diagnosis. Adding standardized A-scan or fluorescein angiography to the evaluation may provide additional information necessary to make a differential diagnosis.

Thorough echographic examination of the eye is critical to obtain accurate, reproducible results. Adhering to a methodical screening pattern will ensure that all segments of the globe have been examined and will help the echographer feel more confident about which probe position correlates with the area of the eye being examined at any given time.

The location of the equipment in the room and a reclining chair in close proximity to the machine are important considerations prior to beginning an ultrasound examination. The patient’s head should be positioned close to the screen, and the echographer should be comfortably seated within easy reach of the buttons and knobs. After explaining the procedure to the patient, an anesthetic drop should be administered to each eye. Even if the examination is only requested for one eye, it is helpful to numb both eyes in case comparison from one to the other is warranted. Having a fixation device that the patient can follow is also recommended. It is best to perform the examination with the probe placed directly on the globe. Doing so allows the echographer to control the patient’s gaze and it also decreases the amount of sound attenuation that occurs when the probe is placed on the lids. A coupling agent such as methylcellulose is needed to facilitate

produced indicates the time it takes for the sound to encounter an interface and return the signal back to the probe. This time value is converted to distance, and measurements in millimeters can be obtained.

When mass-like lesions are evaluated, a high amplitude signal will be displayed from the surface of the tumor. Any signals or spikes that are obtained from within the lesion are evaluated for height and distribution so a diagnosis can be made. The structure (distribution pattern) can be categorized as either regular or irregular and is determined by aiming the sound beam through the lesion in different directions. If the height and distribution of the internal spikes remain consistent, the lesion is regularly structured (e.g., choroidal melanoma). If the height of the spikes varies as the sound beam is moved, the lesion is irregularly structured (e.g., metastatic tumor).

The reflectivity can be classified in various ways: very low, low, low-medium, medium, medium-high, high, and irregular. The determination of the reflectivity is correlated to the density of the interfaces within a given lesion. For instance, choroidal melanomas are usually comprised of small, densely packed cells with a uniform size and distribution. As the sound passes through these small cells, little reflection of the sound beam is returned to the probe, producing low reflectivity. On the other hand, choroidal hemangiomas are comprised of large cells and the walls of these cells are more reflective, producing high reflectivity. Metastatic carcinomas generally have erratically dispersed, large and small cells and interfaces that cause irregular reflectivity.

The kinetic properties of normal and abnormal structures are features that are evaluated during the dynamic portion of the examination, using both the B-scan and standardized A-scan. Mobility or aftermovement is best appreciated during the B-scan screening by having the patient move the eyes. However, to obtain the full benefit of motion, the eyes should move in the same direction that the sound beam is moving (up and down for vertical transverse and left to right for horizontal transverse). If the patient has trouble moving the eyes or if the pathology does not readily move, changing the patient’s head position may be helpful.

Movement of a significant spike on standardized A-scan may be more difficult to appreciate, particularly on equipment that digitizes the images because there is a slight delay from the time of movement to the time it actually appears on the display screen. It is often useful to decrease the gain to improve the resolution of the spike in question. Vascularity within a lesion may be noted on the B-scan when large vessels are involved. However, fast flickering motion noted in the valleys of the spikes on standardized A-scan may be the best way to determine and/or confirm the presence of blood flow within a lesion. If the reflectivity of a lesion is very low, it may be necessary to increase the decibel level to appreciate vascular motion within the valleys of the spikes.

Basic Screening Techniques of the Normal Fundus

To begin a basic screening with the contact B-scan, the patient should be instructed to look up toward their eyebrows. The probe is placed on the eye inferiorly near the corneal limbus with the marker directed toward the nose. The probe is then shifted from the limbus to the fornix to evaluate the superior fundus from 9 o’clock to 3 o’clock. The patient is then instructed to look nasally. The probe is placed near the temporal limbus with the marker directed toward 12 o’clock. Again, the probe is shifted from limbus to fornix to evaluate the nasal fundus from 12 o’clock to 6 o’clock. This same process is repeated with the patient looking down (marker nasally) and temporally (marker superiorly). The system sensitivity (gain) should be adjusted (from high to low) throughout the screening process. With the gain set at a maximum level, small, fine, or scattered opacities or membranes within the vitreous cavity will be detected. Adjusting the gain to a lower setting effectively narrows the sound beam and improves the resolution. This process enhances the image of the ocular coats and enables the echographer to identify any areas of thickening or elevation of the retina, choroid, and sclera. Once transverse screening has been performed, it is useful to perform longitudinal scans of any areas of interest detected during the transverse screening. If no pathology was noted, longitudinal scans toward the nasal and temporal fundus are recommended. If the globe is determined to be within normal limits following B-scan screening, it is not necessary to proceed with additional evaluation using the standardized A-scan. If abnormalities are detected and cannot be differentiated with only B-scan evaluation, further evaluation with the standardized A-scan is warranted.

Labeling Echograms

Just as using prescribed examination techniques is important for evaluation of the fundus, labeling in a systematic fashion is equally important when reviewing the echograms after the examination is finished. To simplify labeling, it is easiest to use a system that denotes the area of the eye that has been imaged and the location along a particular meridian.

Typically for ophthalmoscopic evaluation, the eye is divided in terms of clock hours. This same process can be maintained for the echographic evaluation as well. By subdividing the locations even further, the echographer has the ability to provide exact locations of points of interest for the physician, which may in turn improve treatment.

When performing transverse B-scans, the sound is aimed opposite the probe position and the resulting

image should be labeled with the clock hour that is centered in the echogram. By beginning each sweep of a quadrant with the probe at the corneal limbus, the sound is directed posteriorly and the optic nerve should be noted. As the probe is shifted from the limbus to the fornix an estimation of the position along the fundus can be divided into seven positions (Table 1–1). As the echographer becomes more experienced, determining the location of pathology along a meridian becomes almost second nature.

Table 1–1 Echogram Identification

PPosterior (the area just as the nerve is noted)

EEquator (the inserting tendon of a rectus muscle is

OOra

Figure 1–1 Examination preparation. The set-up of the room, patient, and equipment are important factors to aid in obtaining an optimal examination. The patient should be reclining, and the eye being examined should be as close to the machine as possible.

Figure 1–2 Examination preparation. All of the necessary supplies should be readily available and within easy reach for the echographer.

Figure 1–3 Probes. The B-scan probe (top) has a focused beam and a marker for orientation. The standardized A-scan probe (bottom) is smaller and the sound beam is not focused.

Figure 1–4 Probe care. Conventional sterilization is not possible for either probe. Both should be cleaned between patients with alcohol, bleach, glutaraldehyde (Cidex), or hydrogen peroxide, and then rinsed with a wet cloth. If infection is suspected, it is best to cover the probe. This can be done using finger cots (right) or a small cover made from plastic wrap.

Suggested Readings

Byrne SF. Standardized echography of the eye and orbit. Neuroradiology 1986;28:618–640

Byrne SF, Green RL. Second Edition: Ultrasound of the Eye and Orbit. St. Louis: CV Mosby Yearbook; 2002

Ossoinig KC, Byrne SF, Weyer NJ. Standardized echography. Part II: Performance of standardized echography by the technician. Int Ophthalmol Clin 1979;19: 283–285

A

B

Figure 1–5 Basic screening, B-scan. (A) Transverse scan at maximum gain to evaluate fine, dispersed opacities within the vitreous. (B) Transverse scan at decreased gain; this narrows the sound beam and improves the resolution to evaluate the ocular coats (retina, choroid, and sclera).

P R

O

V

A

P

R

V

B

Figure 1–6 Basic screening, A-scan. (A) Standardized A-scan at tissue sensitivity: O, orbital signals; P, probe on the eye; R, retina and fundus spikes; V, normal vitreous baseline. (B) Standardized A-scan at a decreased gain setting to define the fundus spikes. P, probe on the eye; R, retina and fundus spikes (choroids and sclera); V, vitreous baseline.