Dr.Gyanendra LamichhanaeVitreo retinal Fellow, Gunma University ,JAPANVitreo retinal Fellow, Gunma University ,JAPAN

Lumbini Eye Institute, BhairahawaLumbini Eye Institute, Bhairahawa

USG B SCAN

What is ultrasound

• sound pressure with a frequency greater than the upper limit of human hearing.

• Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults

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• B-scan ultrasonography is an important noninvasive technique for the clinical assessment of various ocular and orbital diseases

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HISTORY

• 1793: Lazzaro Spallanzani (Italy) discovered that bats orient themselves with the help of sound whistles while flying in darkness. This was the basis of modern ultrasound application

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Bats use ultrasounds to navigate in the darkness

History contd…….

• World war II: a device based on piezoelectric effect developed by Paul Langevin (France) ,able of emitting & receiving ultrasound under water used as sonar.

• 1956: first documented use of ocular USG, Mundt and Hughes used A scan technique to detect intraocular tumour.

• 1972: First use of hand held B scan by Bronston & workers ,which was applied directly to the closed lid without a water bath

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• Principles of ultrasound:• By definition, an ultrasound wave has a frequency greater than 20

kHz (20,000 oscillations/ second)

• As the frequency of USG increases, the wavelength decreases and wavelength of an ultrasound determines its depth of tissue penetration and resolution

Wavelength α Depth of penetration of the ultrasound

• So, Larger is the frequency of US = shorter is its wavelength =

shallower is its penetration = better is the resolution of resultant echo graph.

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• That’s why USG probes used for Ocular USG are of higher frequency(10MHz)as it needs much less tissue penetration (an eye is 23.5 mm long on average) & higher resolution.

• In contrast, ultrasound probes used for purposes such as obstetrics, use lower frequencies (1-5Hz) for deeper penetration into the body, and, because the structures being imaged are larger, they do not require the same degree of resolution

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Ultrasound Principles and Physics

Ophthalmic ultrasonography uses high-frequency sound waves,

transmitted from a probe into the eye.

As the sound waves strike intraocular structures,

they are reflected back to the probe and converted into an electric signal.

The signal is subsequently reconstructed as an image on a monitor,

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Velocity

• The velocity of the sound wave is dependent on the density of the medium through which the sound travels.

• Sound travels faster through solids than liquids, an important principle to understand since the eye is composed of both.

• There are known velocities of different components of the eye, with sound traveling through both aqueous and vitreous at a speed of 1,532 meters/second (m/s) and through the cornea and lens at an average speed of 1,641 m/s

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Reflectivity

• When sound travels from one medium to another medium of different density, part of the sound is reflected from the interface between those media back into the probe.

• This is known as an echo; the greater the density difference at that interface, the stronger the echo, or the higher the reflectivity

• In A-scan ultrasonography, a thin, parallel sound beam is emitted, which passes through the eye and images one small axis of tissue; the echoes of which are represented as spikes arising from a baseline. The stronger the echo, the higher the spike

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• In B-scan ultrasonography, an oscillating sound beam is emitted, passing through the eye and imaging a slice of tissue; the echoes of which are represented as a multitude of dots that together form an image on the screen.

The stronger the echo, the brighter the dot.

example, the dots that form the posterior vitreous hyaloid membrane are not as bright as the dots that form the retinal membrane.

This is very useful in differentiating a posterior vitreous detachment (a benign condition) from a more highly reflective retinal detachment (a blinding condition) because retina is more dense than vitreous.

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Angle of incidence

• The angle of incidence of the probe is critical for both A-scan and B-scan ultrasonography.

• When the probe is held perpendicular to the area of interest, more of the echo is reflected directly back into the probe tip and sent to the display screen.

• When held oblique to the area imaged, part of the echo is reflected away from the probe tip and less is sent to the display screen.

• The more oblique the probe is held from the area of interest, the weaker the returning echo and, thus, the more compromised the displayed image.

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• On A-scan, the greater the perpendicularity, the more steeply rising the spike is from baseline and the higher the spike.

• On B-scan, the greater the perpendicularity, the brighter the dots on the surface of the area of interest

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• Because various parts of the eye and various pathologies are different in size and shape, understanding this concept and anticipating the best possible display for that eye are important.

• Perpendicularity to the area of interest should be maintained to achieve the strongest echo possible for that structure

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Absorption

• Ultrasound is absorbed by every medium through which it passes.

• The more dense the medium, the greater the amount of absorption.

• This means that the density of the solid lid structure results in absorption of part of the sound wave when B-scan is performed through the closed eye, thereby compromising the image of the posterior segment

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• Therefore, B-scan should be performed on the open eye unless the

patient is a small child or has an open wound.

• Likewise, when performing an ultrasound through a dense cataract as opposed to the normal crystalline lens, more of the sound is absorbed by the dense cataractous lens and less is able to pass through to the next medium, resulting in weaker echoes and images on both A-scan and B-scan. For this reason, the best images of the posterior segment are obtained when the probe is in contact with the sclera rather than the corneal surface,

bypassing the crystalline lens or intraocular lens implant.

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Instrumentation

• Ophthalmic ultrasound instruments use what is known as a pulse-echo system, which consists of a series of emitted pulses of sound, each followed by a brief pause (microseconds) for the receiving of echoes and processing to the display screen.

• The amplification of the display can be altered by adjusting the gain, which is measured in decibels (dB). Adjusting the gain in no way changes the frequency or velocity of the sound wave but acts to change the sensitivity of the instrument's display screen.

• When the gain is high, weaker signals are displayed, such as vitreous opacities and posterior vitreous detachments.

• When the gain is low, the weaker signals disappear, and only the stronger echoes, such as the retina, remain on the screen.

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• Typically, all examinations begin on highest gain so that no weak signals are missed; then, the gain is reduced as necessary for good resolution of the stronger signals

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• The probe face is usually oval in shape and when placed on the globe is represented by the initial white line on the left side of the display screen. The vitreous cavity is displayed in the center of the echogram, and the posterior pole is displayed on the right side of the echogram

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Indications

• when direct visualization of intraocular structures is difficult or impossible.

• Situations that prevent normal examination

lid problems (eg, severe edema, partial or total tarsorrhaphy), corneal opacities (eg, scars, severe edema),

hyphema, hypopyon,

miosis, pupillary membranes

dense cataracts

vitreous opacities (eg, hemorrhage, inflammatory debris).

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Normal USG

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Clinical Applications

Differentiation between VH & asteroid Hyalosis:

• AH is highly ecogenic,they are still visible when the gain setting is reduced upto 60dB whereas VH which usually disappears by 60 dB

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Asteriod Hyalosis Vitreous Haemorrhage

Vitreous Inflammation

USG is very helpful in assessing the severity and extent of intraocular inflammation in a patient suspected of having endophthalmitis.

VITRITIS appears in B-scan as scattered particle or large aggregates.

sometimes in absence of external inflammatory signs, it is important to differentiate between endophthalmitis and vitreous hemorrhage. VH is generally associated with PVD and layering of blood in inferior portion of the eye to produce sheet-like echoes

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PVD

• In PVD with normal eye, the reflectivity is very low, high gain(90dB) setting is required the reflectivity disappears lowering the sensitivity,under 70 dB.

• It should be kept in mind that PVD with hemorrhage shows

extremely high reflectivity.

• Kinetic echography typically shows a very undulating movement that continue after the eye movements stops, which differentiates PVD from less mobile retinal and choroidal detachments

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RD vs PVDIn presence of opaque media,the differentiation between PVD and RD is

challenging. Few points are-

1) RD is usually uniformly high reflective and of even thickness whereas tilting of probe in different direction may reveal uneven thickness & reflectivity of membranes in PVD,

2) The image of PVD will disappear from the screen at higher gain setting

(70dB) than a RD(40-50dB)

4) PVD may appear as a line with multiple discontinuities or may be completely detached from ON.Rhegmatogenous RD:appear as a mobile membrane attached anterior to the ora serrata and posterior to the ON head

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4) On kinetic Echography,a PVD has much more after movements when compared to RD.

The mobility of RD depends on

duration of the detachment. Recent bullous RD may be highly mobile,whereas chronic RD with proliferative Vitreoretinopathy appear stiff.

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• Choroidal Detachment: CD appear as smooth, convex elevations from the posterior eye wall. In massive CD, choroids from opposite fundus areas may touch in the middle of the vitreous cavity-“Kissing Choroid”

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Posterior Scleritis

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OPTIC NERVE

Subtenon fluid

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THANK YOU

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