B-scan Ultrasonography 239

240Diagnostic Procedures in Ophthalmology

different homogeneity of density or elasticity within a tissue. It can be considered to be the redirection of the incident ultrasound energy into many directions. There is no particular direction ascribed to the reflected energy, but a continuum of directions. In true scattering no energy is lost from the pulse, but the energy is redirected.

Absorption

In the ocular tissues, an ultrasound pulse loses energy due to conversion of the vibrational energy of the pulse to other energy forms such as heat. The mechanisms of absorption in media are not properly understood; different tissues exhibit different frequency dependent absorptions. Ophthalmic ultrasonography utilizes 8-10 MHz sound waves. As it travels through the eye, it is reflected by the intraocular structures, and the echoes or the signals are returned to the screen.

Ultrasound Unit

An ultrasound unit is composed of four basic elements: the pulser, the receiver, and the display unit are all contained within the same chassis and connected to the transducer, located at the tip of the probe by an electrically shielded cable. The pulser produces electric pulse at a rate of 1000 pulses per second. Each pulse excites the electrodes of the piezo-electric crystal of the transducer, generating sound waves. The returning echoes are received by the transducer and transformed into electric signals. These signals are processed in the receiver and demodulator, and then displayed on the screen of the display unit.

Ophthalmic ultrasonography commonly uses two modes of display—the A-scan, and B-scan.

A-scan or amplitude modulation scan provides one dimensional image of vertical deflections from a base line. The A-scan provides information

regarding structure (size, distribution), reflectivity (height of spikes), sound attenuation (absorption), and vascularity.

B-scan or brightness modulation scan provides two dimensional images of a series of dots and lines. B-scan provides the topographic information of shape, location, extension, mobility, and gross estimation of thickness of the tissue. While independently each mode of ultrasonography do provide a wealth of information, the combination of both A- and B- scan (Vector A-scan) is invaluable in a variety of occasions where diagnostic dilemma exists. In vector display the A-scan pattern corresponds to the vector’s direction. The vector B-scan uses a focused 10 MHz transducer in contrast to 8 MHz unfocused transducer used in standardized A-scan.

Three-dimensional ultrasound tomography of the eye is a new advanced ultrasound technique and digital computer technology where ocular pathology can now be viewed in “3D”. The sonographer scans the eye using a regular B-scan ultrasound probe which is inserted into the motorized scanner assembly. This in turn rotates the probe allowing the computer to acquire over 200 B-scan images in 5 to 10 seconds. Using digital technology, the 2D images and their global positions are recorded, reconstructed and displayed in real time as part of a 3D volume.

Three-dimensional ultrasound tomography provides the following benefits over the current oneand two-dimensional evaluations:

1.Improved visualization: Information is presented in a format that reflects the 3D nature of the pathology under examination. The multiple acquired scans in 3D imaging also reduce the risk of missing a small pathology that can be overlooked if the probe is not properly aimed toward it.

2.Volume measurements: The 3D examination provides volume measurement capabilities

that surpass any of the volume estimation methods available with conventional 2D ultrasound techniques. Accurate volume measurements of intraocular tumors allow the physician to monitor changes over a certain period of time, i.e. growth of a small choroidal tumor, decrease in size of a disciform macular degeneration, or the response of a melanoma to radiation, laser or drug therapy.

3.Profile A-scan analysis: Using an S-shaped amplifier that allows an evaluation of the internal echo-spikes an accurate linear measurement in any chosen direction can be made.

4.Analysis of the volume-of-interest: With multidirectional slicing can show a tomographic display of intraocular pathology.

5.Surface rendering with a three-dimensional view: The surfaces and boundaries of the ocular pathology can be made under examination.

Screening Techniques

It is best to begin with a maximum gain (80 decibels) setting on the B-scan, with the patient lying on his back. The eye is anesthetized with topical paracaine when the transducer can be placed on the sclera; alternately, the probe can be placed on the closed eyelid and in such a situation the eye need not be anesthetized. The probe is placed on the globe opposite the area to be examined. The marker on the probe acts as the orientation point and corresponds to the upper portion of the echogram. To evaluate the superior and inferior fundus the marker is directed towards the nose (horizontal transverse), and to evaluate the nasal and temporal fundus, the marker is directed at 12 o’clock meridian (vertical transverse). The best detail of pathology is obtained in the central portion of the echogram;

B-scan Ultrasonography 241

if the pathology is not located in one of the major meridians (3, 6, 9, 12 o’clock) an oblique transverse scan can be used to evaluate the pathology. In order to completely scan the eye it is prudent to first direct the probe face at the limbus, and then slowly shift to the fornix. Thus one could evaluate from the posterior pole to the periphery in each quadrant. Once the crosssectional evaluation is completed, the area of interest is scanned by longitudinal scan. Longitudinal scans allow for evaluation of a single meridian from its most posterior aspect to the far periphery. This is accomplished by directing the marker at the corneal limbus opposite the area to be examined. Axial scan provides a pleasing, generally understandable picture; however, it requires placement of the probe directly on the cornea and thus the risk of corneal abrasion increases.

A-scan produces a series of deflections from the base line. The amplitude of the spike is directly related to the density of the interface, and the space between the spikes indicates the time it takes for the sound to encounter an interface and return as a signal.

Screening Technique with a 3D Unit

To begin scanning with 3D-unit, the operator either presses a foot switch or may press the on-screen scan button. The scanner assembly rotates for several seconds (5, 7.5, or 15 according to the chosen scan type), and then return to its starting position. The system emits a beep tone at the start and end of actual scanning period, during which fixation must be maintained. After scanning, the images become static, the recorded initial scan plane images replace the line B-scan display. Several buttons appear on the right side of screen, which allow the operator to review the recorded images and send to the 3D reviewing mode.

242Diagnostic Procedures in Ophthalmology

3D Scan Review

Activating the 3D viewing function produces a screen, where central part of the screen shows the polyhedron (cube), with its sides positioned at the extreme borders of the scanned volume. The view is initially oriented as seen by the ultrasound probe, i.e. the near field is in front and the far field is behind. The 3D viewing software includes many functions.

The Normal Eye

Examination of a normal globe at high system sensitivity reveals two echographic areas, separated by an echo free area. The echographic area at the beginning of the scan represents reverberations at the tip of the probe and has no clinical significance. When the scan resolution is good,onecouldseetheposteriorconvexstructure of the crystalline lens. The large echo free area representsthevitreouscavity.Theechogenicarea after the vitreous represents the retina, choroid, sclera, and the orbital tissue behind it. The retina isseenasaconcavesurfaceproximally.Theoptic nerve shadow is seen as a triangular shadow within the orbital fat (Fig. 15.1).

Evaluation of the Vitreous

A maximum high gain should be used for evaluation of the vitreous humor. The normal vitreous cavity is devoid of any acoustic signals and appears black or sonolucent on the B-scan. On the A-scan the baseline remains flat throughout the scan. During normal aging the vitreous begins to degenerate, and varying amounts of opacities are seen. Also there may be significant contracture of the vitreous gel leading to complete separation of the posterior hyaloid.

Asteroid Hyalosis

Asteroid hyalosis, a unilateral condition characterized by formation of calcium soaps within the vitreous cavity, appears as bright round signals on B-scan, and medium amplitude spikes in A-scan, with an echo free space just in front of the retina that represents the echofree vitreous gel (Fig. 15.2). This is in contrast to an eye with emulsified silicone oil, where there is no echo-free space. Generally, these opacities exhibit distinct movement on movement of the eye.

244Diagnostic Procedures in Ophthalmology

Vitreous Hemorrhage

The ultrasonic pattern of vitreous hemorrhage depends on the density, location, extent, and associated fibrous changes. The density of hemorrhage is best estimated from the A-scan amplitude and the area of vitreous hemorrhage from B-scan. Hemorrhage in the vitreous appears as small white echoes on B-scan and low amplitude spikes on A-scan. With greater density of vitreous hemorrhage, usually greater opacities are seen on the B-scan. A fresh diffuse and unclotted hemorrhage produces very little or no echo response so that many time the vitreous might appear sonolucent. Membranes are easily differentiated from blood clots by their patterns, and the height of the echoes.

One can also image the location of vitreous hemorrhagesuchasconfinedwithinthePVD,pre- andpost-hyaloidlocation,ordiffuselydispersed (Fig. 15.4). One can also differentiate old clotted blood from fresh hemorrhage. We have earlier reported that the overall accuracy of ultrasonic diagnosis of vitreous hemorrhage and retinal detachment in opaque media vis-a-vis the intraand postoperative findings were nearly 92%.3

Subhyaloid hemorrhage typically does not clot. On echography, high gain settings are often

required to detect mild subhyaloid hemorrhage. However, dense subhyaloid hemorrhage shows high echo reflectivity (Fig. 15.5).

Endophthalmitis

Ultrasonography of the eye with endophthalmitis depends on the degree and severity of infection and the extent of vitreous involvement. Generally opacities are noted, and membrane formation becomes apparent in severe cases. Choroidal thickening, choroidal detachment, retinal detachment and retained IOFB are possible associated findings (Fig. 15.6).

Evaluation of the Retina

The retina appears as a dense membrane on B-scan, and in normal circumstances one can not differentiate retina from the choroid. On A-scan it typically gives a 100% tall spike.

Retinal Detachment

Retinal detachment appears as tall (100% amplitude) spike separated from the choroidoscleral layer; it is attached, however, to the optic nerve and the ora serrata. By serial scanning

Fig. 15.9: Longitudinal B-scan shows formation of intraretinal cysts (white arrow) and retinal detachment with high reflective surface spikes on corresponding A-scan. Often intraretinal cysts may mimic a tractional retinal detachment

tion. It appears as a breach of tissue on B-scan, and on A-scan it appears as a highly reflective tissue separate from the other fundus spikes (Fig. 15.10). Giant retinal break with detachment appears as a rolled out tissue on B-scan with clear breach of tissue. In general, however, detecting retinal tears on ultrasonography is not easy and it is never as specific or sensitive as on optical evaluation. It is useful in situations when fresh vitreous hemorrhage due to retinal tear obscures the fundus view; in these situations the retinal tears are mostly located in the upper half of the retina.

B-scan Ultrasonography 247

Fig. 15.10: Retinal tear: B-scan showing a breach of retinal tissue. Vitreous is attached to this breach of tissue suggesting the element of traction in causing retinal tear

Traction Retinal Detachment

Traction detachment is a common finding in vascular retinopathies, chiefly diabetic retinopathy. It is caused by the strong adhesion of vitreous membranes, bands, or the posterior hyaloid face to the retina and subsequent traction. The vitreoretinal adhesion could be focal causing a tent-like, or broad, causing a table-top traction of the retina (Fig. 15.11); the detached retina appears to have a concave configuration, in contrast to the convex configuration of the rhegmatogenous retinal detachment.

Exudative Retinal Detachment

The configuration of the detachment is convex and bullous. It is usually secondary to tumors, inflammatory conditions, e.g. Vogt-Koyanagi- Harada disease (VKH), or vascular disorders such as hypertensive choroidopathy, and toxemia of pregnancy. In VKH syndrome there is a diffuse choroidal thickening with low to medium echo reflectivity (Fig. 15.12).

Retinoschisis

This condition most often involves the inferotemporal peripheral fundus. It may be

Fig. 15.13: Retinoschisis: Transverse B-scan shows a moderately elevated thin smooth dome-shaped membrane echo (arrow) located in the inferotemporal periphery. Very thin 100% spike is also seen on A scan

unilateral or bilateral. On B-scan it appears as smooth, thin, dome-shaped membrane that does not insert in the optic disc (Fig. 15.13). On A- scan, 100% high spike is produced, which may

B-scan Ultrasonography 249

demonstrate slight vertical after movement. It differs from retinal detachment by its more focal, smooth and thin character. A choroidal detachment is thicker than retinoschisis and may have a double peaked spike.

Cysticercosis

There is a characteristic echographic appearance with a sharply outlined, oval cyst within the vitreous cavity and/or in the subretinal space (Fig. 15.14). The scolex of the parasite is seen as a very highly reflective, echo-dense nodule that is located adjacent to the inner wall of the cyst.

Evaluation of the Choroid

The retinochoroidal layer has a smooth concave configuration on B-scan and gives a tall spike on A-scan.

Choroidal Thickening

Thickening of choroid can be localized or diffuse, and is seen in a number of conditions. They include posterior uveitis, sympathetic ophthalmia, Vogt-Koyanagi-Harada disease, late stage of endophthalmitis and uveal effusion syndrome.

Fig. 15.17: Dislocated lens: B-scan shows a globular structure in the posterior vitreous signifying a dislocated lens. Acoustic shadowing is seen, implying that the lens could be cataractous or calcified

Dislocated Lens

Dislocated lens appears as a round or oval globular structure in the posterior vitreous, and

B-scan Ultrasonography 251

strands of vitreous might be attached to the dislocated lens (Fig. 15.17).

Intraocular Foreign Body

Ultrasonography can detect both metallic and non-metallic foreign bodies. Metallic foreign bodies produce very bright signals on B-scan that persist on lowering the gain (Fig. 15.18). When the sound beam is focused on the metallic foreign body, much of the sound waves are absorbed by the foreign body, thus creating a shadowing artifact on the adjacent orbit. Round metallic foreign bodies classically produce reverberation artifact just behind the foreign body, and the sound signals gradually reduce as it progresses to the orbit. On A-scan metallic foreign bodies produce high (100%) reflective echoes, and reduplication echoes are seen as progressively decreasing amplitude spikes behind the

Fig. 15.21: Choroidal melanoma: B-scan shows a collar- button-shaped mass from the choroid into the vitreous cavity

movements of the lesion spikes during examination in A-scan. While the most common shapes are a dome or collar-button, they can also be diffuse. A collar-button shape signifies rupture of Bruch’s membrane, and it is usually associated with retinal detachment (Fig. 15.21). There can be other signs such as acoustic hollowing (decreased reflectivity at the tumor base due to uniform echotexture of the tumor), choroidal excavation at the tumor base and posterior scleral bowing (noted in younger individuals).

Metastatic Choroidal Carcinoma

On B-scan metastatic choroidal carcinomas appear diffuse; they have a typical bumpy,

B-scan Ultrasonography 253

irregular contour, with a central elevation. They have medium to high reflectivity, with minimal to none internal vascularity. The large interface between the choroidal tissue and the carcinoma mass is responsible for the high reflectivity. On A-scan, irregular spikes of medium to high amplitudes are seen.

Choroidal Hemangioma

These tumors appear as a flat, echogenic, solid, subretinal mass, often located at the posterior pole, with minimal sound attenuation, with or without concomitant exudative retinal detachment (Fig. 15.22).

On A-scan, it has a regular acoustic structure with very high (95-100%) internal reflectivity, that results from the large interfaces formed by the vessel surfaces. By reducing the gain, the vascularity of the tumor can be better appreciated.

Retinoblastoma

On B-scan retinoblastoma, if large, is seen as an irregular echogenic mass involving the vitreous, retina, and/or the subretinal space. Area of calcification is seen as area of high echogenicity. This causes strong sound attenuation, and is seen as an area of echolucency behind the calcification (Fig. 15.23). This is because the

high reflectivity at or within the optic nerve head. They are best seen with transverse and longitudinal B-scan approaches, which bypass the lens, and demonstrate the calcified nodules better than the axial approach (Fig. 15.27).

Optic Nerve Head Coloboma

Coloboma involving the optic disk is easily imaged by B-scan. These can be small and shallow. The sharp edge of the defect margin differentiates a coloboma from a staphyloma on ultrasonography (Fig. 15.28).

Immersion B-scan

Immersion B-scan is used to study the anterior segment structures (Fig. 15.29). A water bath is used to incorporate the delay zone.

Ophthalmic ultrasonography is an invaluable tool in diagnosis and evaluation of the posterior segment of the eye. Knowledge of various features and appropriate clinical correlation is essential to gain maximum information from this technology.

B-scan Ultrasonography 257

Fig. 15.28: Optic nerve head coloboma: Horizontal B- scan showing sharp defect over the optic disk area suggestive of coloboma of the optic disk

Fig.15.29: Left: Immersion B- scan shows a total cataract with intact posterior capsule. Right: Immersion B-scan showing partially absorbed cataractous lens. Note the thickness of the lens and increased reflectivity of the posterior capsule

258Diagnostic Procedures in Ophthalmology

References

1.Mundt GH, Huges WF. Ultrasonic in ocular diagnosis. Am J Ophthalmol 1956;41:488-98.

2.Das T, Namperumalsamy P: Ocular ultrasound in preoperative evaluation of posterior segment of the eye. Indian J Ophthalmol 1983;31:1022-24.

3.Das T, Namperumalsamy P. Ultrasonographic characterisation of vitreous hemorrhage and retinal detachment. Afro-Asian J Ophthalmol

1985;4:10-16.

4.The Retina Society Terminology Committee. The classification of retinal detachment with proliferative vitreoretinopathy. Ophthalmology 1983;90:121-25.

5.Das T, Namperumalsamy P. Ultrasonic characterisation of proliferative vitreoretinopathy.

Afro-Asian J Ophthalmol 1987;5:180-85.

6.Das T, Namperumalsamy P. Ultrasonography in ocular trauma. Indian J Ophthalmol 1987;35: 121-25.

7.Das T, Namperumalsamy P. Ultrasonic evaluation of retinoblastoma. Afro-Asian J Ophthalmol 1986;5:4-10.

Bibliography

1.Coleman DJ, Lizzi FL, Jack RL (Eds). Ultrasonography of the eye and orbit. Philadelphia, Lea and Febiger, 1977.

2.Shammas HJ. Atlas of Ophthalmic Ultrasonography and Biometry. St Louis: CV Mosby Co, 1984.