basics of USG B Scan

1. USG B SCAN
Dr.Gyanendra Lamichhanae
Vitreo retinal Fellow, Gunma University ,JAPAN
Lumbini Eye Institute, Bhairahawa

2. 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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4. • B-scan ultrasonography is an important
noninvasive technique for the clinical
assessment of various ocular and orbital
diseases
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5. 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
Bats use ultrasounds to navigate in the
darkness
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6. 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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7. • 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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8. • 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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9. 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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10. 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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11. 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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13. • 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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14. 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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15. • 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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16. • 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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17. 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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18. • 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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19. 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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20. • 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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21. • 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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22. 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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23. Normal USG
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24. 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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25. Asteriod Hyalosis Vitreous Haemorrhage
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26. 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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28. 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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29. RD vs PVD
In 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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30. 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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33. • 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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39. Posterior Scleritis
Subtenon fluid
OPTIC NERVE
T-sign 39

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46. THANK YOU
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