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Maharajgunj Medical Campus, Nepal
Bikash Sapkota
B. Optometry
16th Batch
PRESENTATION LAYOUT
Introduction History Physics Principles & instrumentation Terminologies Indications &
contraindications Methods
- A-Scan- B-Scan
Interpretation
INTRODUCTION
Sound has been used clinically as an alternative to light in the diagnostic evaluation of variety of conditions
Advantage of sound over light is it can pass through opaque tissue
An important tool in terms of diagnosis and management
Is a non-invasive investigation of choice to study eye in opaque media
Ultrasound Waves are acoustic waves that have frequencies greater than 20 KHz
The human ear can respond to an audible frequency range, roughly 20 Hz - 20 kHz
DEFINITION
HISTORY
• In 1956
• First time: Mundt and Hughes, American Oph.
• A-scan (Time Amplitude ) to demonstrate various oculardisease
• Oksala et al in Finland
• Ultrasound Basic Principle (Pulse-Echo Technique)
• Studied reflective properties of globe
• In 1958, Baum and Greenwood
• Developed the first two-dimensional(immersion) (B-scan)ultrasound instrument for ophthalmology
• In the early 1960s, Jansson and associates, in Sweden,
• Used ultrasound to measure the distances between structuresin the eye
In the 1960s, Ossoinig, an Austrian ophthalmologist First emphasized the importance of standardizing
instrumentation and technique Developed standardized A-scan
In 1972, Coleman and associates made First commercially available immersion B -scan
instrument
Refined techniques for measuring axial length, AC depth, lens thickness
Bronson in 1974 made contact B scan machine
ADVANTAGES OF USG
Easy to use
No ionizing radiation
Excellent tissue differentiation
Cost effectiveness
Primary uses in ophthalmology
Posterior segment evaluation in hazy media / orbit
- Structural integrity of eye but no functional integrity
Detection and differentiation of intraocular and orbital lesions
Tissue thickness measurements
Location of intra ocular foreign body
Ocular biometry for IOL power calculations
PHYSICS Ultrasound is an acoustic wave that consists of an oscillation
of particles that vibrate in the direction of the propagation
Longitudinal waves Consist of alternate compression and rarefaction of
molecules of the media
Oscillation of particles is characterized by velocity,
frequency & wavelength
8
VELOCITY
Velocity=wavelength*frequency
v=λ * μ
Depends on the density of the media
Takes 33 micro sec to come back from posterior pole to transducer
About 1500 m/sec average velocity in phakic eye and 1532 m/sec in aphakic eye
SOUND WAVE VELOCITIES THROUGH VARIOUS MEDIA
Medium Velocity (m/sec)
Water 1,480
Aqueous/ Vitreous 1,532
Silicon Lens 1,486
Crystalline Lens 1,641
PMMA Lens 2,718
Silicon Oil 986
Tissue 1,550
Bone 3,500
FREQUENCY
Ophthalmic ultrasonography uses frequency ranging from 6 to 20 MHz
High frequency provide better resolution
8 MHz in A scan
10 MHz in B scan
Low frequency (1-2 MHz)used in body scanning gives better penetration
WAVELENGTH
Wavelength is approx. 0.2mm
Good resolution of minute ocular & orbital structures
f α1/λ α resolution α 1/penetration
FREQUENCY VS PENETRATION
REFLECTIVITY
When sound travels from one medium to another medium of different density, part of the sound is 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 USG echoes are represented as spikes arising from a baseline
The stronger the echo, the higher the spike
In B-scan USG 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
ABSORPTION
Ultrasound is absorbed by every medium through which it passes
The more dense the medium, the greater the amount of absorption
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
B-scan should be performed on the open eye unless
the patient is a small child or has an open wound
When performing an USG through a dense cataract,
- more of the sound is absorbed by the dense cataractous
lens
- less is able to pass through to the next medium
- resulting in weaker echoes and images on both A-scan
and B-scan
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 IOL implant
ECHOLOCATION
ULTRASOUND ECHO
Ultrasound wave Refraction & reflection
Echo (reflected portion of wave) Produced by acoustic interfaces
Created at the junction of two media that have different acoustic impedances
- Determined by sound velocity & density
Acoustic impedance = sound velocity × density
Factors influencing the returning echo
( Height in A-Scan & Brightness in B-Scan )
1. Angle of the sound beam
2. Interface
3. Size and shape of interfaces
ANGLE OF INCIDENCE
Angle at which a sound beam encounters an ocular structure
Sound beam directed perpendicularly to a structure
maximum amount of sound will be reflected back to
the probe
The farther away from the ideal angle
the lower the amplitude
INTERFACE
Relative difference between various tissues that the sound beam encounters
Strong or weak echoes due to the significance of tissue interface
For example:
- The difference in interface between vitreous and fresh
blood is very slight resulting in small echo
- The difference between a detached retina and the
vitreous is great producing a large echo
Smooth surface like retina will give strong reflection
Smooth and rounded surface scatters the beam
Coarse surface like ciliary body or membrane with folds tend to scatter the beam without any single strong reflection
Small interface produces scattering of reflection
TEXTURE AND SIZE OF INTERFACE
PRINCIPLE Pulse- Echo System
Emission of multiple short pulses of ultrasound waves with brief interval to detect, process and display the turning Echoes
ELECTRIC
CURRENT
TRANSDUCER
US WAVE
SURFA
CE
Ophthalmic USG 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
EMITED SOUND BEAM
NON-FOCUSED BEAM
Used in A scan echography
Beam has parallel border
FOCUSED BEAM
Used in B scan
Examination takes place in a focal zone
The beam is slightly diffracted
PROBE Consists of piezoelectric transducer
Device which converts electrical energy to sound energy [Pulse ] and vice versa [Echo]
Basic Components –
Piezoelectric plate
Backing layer
Acoustic matching layer
Acoustic lens
INSTRUMEN
TATION
PIEZOELECTRIC ELEMENT
Essential part generates ultrasonic waves
Coated on both sides with electrodes to which a voltage is applied
Oscillation of element with repeat expansion and contraction generates a sound wave
Most common: Piezoelectric ceramic ( Lead zirconate,
titanate)
Shape of the Crystal
Planer crystal
- Produce relatively parallel sound beam (A- Scan)
Acoustic lens
- Produce focused sound beam (B-scan)
- Improves lateral resolution
Backing layer (Damping material: metal powder with
plastic or epoxy)
Located behind the piezoelectric element
Dampens excessive vibrations from probe
Improves axial resolution
Acoustic matching layer
Located in front of piezoelectric element
Reduces the reflections from acoustic impedance between
probe and object
Improves transmission
Axial Resolution(longitudinal resolution or azimuthal resolution )
Resolution in the direction parallel to the ultrasound beam
The resolution at any point along the beam is the same; therefore axial resolution is not affected by depth of imaging
Increasing the frequency of the pulse improves axial resolution
Lateral Resolution
Ability of the system to distinguish two points in the direction perpendicular to the direction of the ultrasound beam
Affected by the width of the beam and the depth of imaging
Wider beams typically diverge further in the far field and any ultrasound beam diverges at greater depth, decreasing lateral resolution
Lateral resolution is best at shallow depths and worse with deeper imaging
RECEIVER (computer unit)
Receives returning echoes
Produces electrical signal that undergoes complex processing
Amplification, Compensation, Compression, Demodulation and Rejection
GAIN
Relative unit of Ultrasound intensity
Expressed in Decibel (db)
Adjust of gain doesn't change the amount of energy emitted by transducer
- but change in intensity of the returning echoes for
display
Higher the gain – Greater the sensitivity of the instrument in displaying weaker echoes (i.e Vitreous opacities)
Lower the gain – Weaker the depth of sound penetration
TERMINOLO
GIES
Acoustic impedance mismatch
- Resistance of tissue to passage of sound waves
- Difference of two tissues at the interface
Homogeneous ( Vitreous)- Sound passes through tissue with no returning signal
Heterogeneous (Orbital Fat) - Different levels of acoustic impedance mismatch within tissue
Anechoic : No Echo
Attenuation : Sound is absorbed & scattered
Shadowing : Sound is strongly reflected, nothing passes
through it (drusen of optic nerve head, air bubble)
Reverberation : Collection of Reflected sounds bouncing
back and forth between tissue boundaries
(foreign body in eyeball )
Indications of Ocular B Scan
Enophthalmos
Unilateral or Bilateral Exophthalmos
Globe Displacement
Lid Abnormalities -Ptosis, Retraction, Swelling, Ecchymosis
Indications of Orbital B Scan
Palpable or Visible Masses
Chemosis
Motility Disturbances; Diplopia
Pain
DISPLAY MODES
Modes
M-Mode
A-Mode
B-Mode
A MODE DISPLAY
Time amplitude USG
One dimensional acoustic display
Tissue boundary
- displayed graphically as function of distance along a selected axis
Spacing of the spike time taken for the sound beam to reach the given interface and
its echo to reach the probe
Amplitude of echo on the display is proportional to the sound energy reflected at specific tissue boundary
8 MHz
Probe emits unfocused beam
40
The term “A-Scan” is often used to describe this mode, but it is not an appropriate term, since the transducer is fixed
in one position during biometric procedure and is not scanning
USESAxial length measurements
Intraocular and intraorbital pathologies Detection
Differentiation
Localization
A-MODE USG BIOMETRY
Axial length measurement
To obtain the power of IOL
Calculation of the total refracting power of the eye
PROBE POSITION
Just touch the cornea
Aligned with optical axis of eye
- Aimed towards the macula
Corneal compression
- A 0.4mm compression causes 1 D error in the calculated
IOL power
- Contact Vs immersion method
43
Tall echo – cornea, one peak – contact probe, double peak – immersion probe
Tall echoes – ant. & post. lens capsules
Tall sharply rising echo – retina
Medium tall to tall echo – sclera
Medium to low echoes – orbital fat
A SCAN CHARACTERISTICS
M MODE DISPLAYMotion mode or time motion mode
Dilation and constriction of blood vessel
Accommodation fluctuation
Vascular pulsation in ocular tumor
Motion of detached retina
- PVD vs RD
B MODE DISPLAY
Intensity modulated USG
B Stands for Brightness modulation
Presents a cross sectional or 2D image
True scanning
Probe emits focused beam
10 MHz
Each echooRepresented as a dot on display screen
oStrength of the echo brightness of the dot
NORMAL B-SCAN
• Initial line on left: probe tip
• Right side: fundus opposite to probe
• Upper part: portion of the globe where probe marker is directed
INTERPRETATION
Based upon three concepts Real Time
Gray Scale
Three-Dimensional analysis
REAL TIME
Images can be visualized at approximately 32 frames/sec, allowing motion of the globe and vitreous to be easily detected
B scan allows real time evaluation of any ocular pathology
Real time ultrasonic information frequently aids in vitreoretinal surgery
GRAY SCALE
Displays the returning echoes as a 2D image
Strong echoes are displayed brightly at high gain and remain visible even when the gain is reduced
Weaker echoes are seen as lighter shades of gray that disappear when the gain is reduced
Comparing echo strengths during examination is the basis for qualitative tissue analysis
THREE-DIMENSIONAL ANALYSIS
Developing a mental 3D image or anatomical map frommultiple 2D B-scan images is the most difficult conceptto master
This is essential, because it provides the vital architecturalinformation that is the basis for B-scan diagnosis
Especially important in the preoperative evaluation of complex retinal detachments and intraocular or orbital tumors
EXAMINATION TECHNIQUES FOR THE
GLOBE-B SCAN
Axial Transverse Longitudinal
Probe Orientations
AXIAL
Probe directly over cornea and directed axially
Pt. fixating in primary gaze
Posterior lens surface and optic nerve head are placed in the centre of the echogram
Optic nerve head is used as an echographic centre section
Easiest to perform
Mainly two varieties of axial scans
Horizontal axial scan
Marker at 3 0’clock RE and 9 0’clock LE
Macular region is placed just below the optic disk
Vertical axial scan
Marker at 12 0’clock
Macula is not seen in this scan
Oblique axial scan
Marker always superior
Sections of all other clock hours
can be performed
POINTS TO BE NOTED
Higher decibel gain levels are needed to show structures at the posterior segment
Because of scatter and strong sound attenuation created by the lens
- In pseudophakia strong artifacts created by the lens
implant hampers the adequate visualization
SignificanceEasy orientation and demonstration of posterior pole lesions and attachments of membranes to optic nerve head
TRANSVERSE
EYE – looking in the direction of observer’s interest PROBE –parallel to limbus and placed on the opposite
conjunctival surface PROBE MARKER
superior (if examining nasal or temporal) or nasal(if examining superior or inferior)
6 clock hrs examined at a time Limbus-to-fornix approach is used to detect from posterior
pole to periphery Quadrant examination Gives lateral extent of the lesion
The clock hr which the marker faces is always at the topof the scan
The area of interest in a properly done transverse scan is always at the centre of the right side of scan
Nasal
Bridge
LONGITUDINAL EYE - looking in the direction of observer’s interest PROBE – perpendicular to the limbus and placed on the
opposite conjunctival surface PROBE MARKER- directed towards the limbus Optic nerve shadow always at the bottom on the right side 1 clock hr per time examination Determines the antero-posterior (axial) extent of the lesion Significance
- Best for peripheral tears and documentation of macula
Nasal
Bridge
EXAMINATION PROCEDURE Positioning the patient
Topical anesthesia
Techniques Contact Techniqueo Probe is placed directly
on the globe
Immersion Technique oMethylcellulose - a
coupling medium (B-Scan)
Sources of Error in contact technique Corneal compression (Shorter Axial length)
- 1mm error in Axial length – 2.5 to 3.0 Ds error in IOL
Power
Misalignment of sound beam
Source of error in immersion technique Small air bubbles in the fluid gives falsely long AL
measurement
LOCALIZATION OF MACULA
Macula
Localization
Vertical
Horizontal
Longitudinal
Transverse
HORIZONTAL
Probe placed on the corneal vertex
Marker nasally (as with a horizontal axial scan)
The probe should be aimed straight ahead to center the macula
The macula will be centered to the right of the echogram, with the posterior lens surface centered to the left
VERTICAL
Probe placed on the corneal vertex
Marker is in the 12-o'clock position
The nerve will not appear in these scans because this is a vertical (instead of horizontal) slice of the macula
TRANSVERSE
Patient fixes slightly temporally
Probe on nasal sclera with marker at 12’o clock
Optic nerve as the centre of imaged clock macula is at 9’o clock in right eye and 3’o clock in left
Bypasses the lens
LONGITUDINAL
Probe held on sclera, bypassing crystalline lens
Optic nerve is seen at the bottom with macula just above
ORBITAL SCREENING
Orbit highly reflective owing to heterogeneity of orbital fat which produce large acoustic interface
B scan- bright zone
A scan- highly packed tall spike fading from left to right
Three major portionsOrbital soft tissue assessment
Extraocular muscle evaluation
Retrobulbar optic nerve examination
TWO APPROACHES
Transocular (through the globe)- For lesions located within the posterior & midaspects of the orbital cavity
Paraocular (next to the globe)- For lesions located within the lids or anterior orbit
Three methods: Axial, transverse & longitudinal
Transverse Longitudinal
A-SCAN B-SCAN
AMPLITUDE MODULATION SCAN. BRIGHTNESS MODULATION SCAN.
FREQUENCY OF ULTRASOUND IS
8 MHERTZ.
FREQUENCY OF ULTRASOUND IS
10 MHERTZ.
ONE DIMENTIONAL IMAGE OF
SPIKES OF VARYING AMPLITUDES
ALONG A BASELINE.
TWO DIMENTIONAL IMAGING OF
SERIES OF DOTS AND LINES THAT
FORM THE ECOGRAM.
EMITS UNFOCOUSED BEAM. EMITS FOCUSED BEAM.
PROVIDES QUANTITATIVE
INFORMATIONS.
PROVIDES TOPOGRAPHIC
INFORMATIONS.
IS A BASIS OF OCULAR BIOMETRY. ALLOWS REAL TIME EVALUATION
OF ANY OCULAR PATHOLOGY.
ANTERIOR SEGMENT EVALUATION
Immersion Technique
High Resolution Technique
IMMERSION TECHNIQUE
Examining the anterior segment with a standard 10 MHz contact probe can be accomplished only with the use of a scleral shell
This shifts the anterior segment to the right and into the area of focus of the sound beam, improving resolution of anterior segment pathology
The shell is filled with methylcellulose or some other viscous solution to a meniscus, avoiding air bubbles within the shell
The probe is placed on top of the shell
This produces an echolucent area on the left side of the echogram corresponding to the shell and methylcellulose
Diagnostic A-scan also can be performed through the shell, directly over the lesion, for tissue differentiation.
Immersion B-scan image of an iris
melanoma extending into the ciliary body
High-resolution B-scan images of
an iris melanoma.
HIGH RESOLUTION TECHNIQUE
Ultrasound biomicroscopy
Probes ranges from 20MHz to 50 MHz, with penetration depths of about 10 mm to 5 mm respectively
The zone of focus is quite small
Scleral shell technique is used
Image quality far superior to immersion technique
OCT vs UBM
NORMAL USGCHARACTERISTICS
Lens : Oval highly reflective structure
Vitreous : Echolucent
Retina , Choroid , Sclera : Each is single highly reflective structure
Optic Nerve : Wedge shaped acoustic void in retrobulbar region
Extra ocular muscles : Echolucent to low reflective fusiform
structures
- The SR- LPS complex is the thickest, IR is the thinnest
- IO is generally not seen except in pathological conditions
Orbit : Highly reflective due to orbital fat
TOPOGRAPHIC ECHOGRAPHY
Point-like e.g. fresh V.H
Membrane-like e.g. R.D
Mass-like e.g. choroidal melanoma
Opacities produce dots or short lines Membranous lesions produce an echogenic line
INTERPRETATIONSAND
CLINICAL EXAMPLES
Fresh:oDot-like: Echolucent or low reflectivity
Old:
oMembrane-like: Varying reflectivity & dense inferiorly
Fresh VH Old VH
Multiple, densely packed, homogeneously distributed echodense dots of medium to high reflectivity with a clear preretinal space suggestive ofAsteroid 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
PVD at high gain (90dB)PVD (arrowheads) and retina (arrow)
PVD at low gain (39 dB)
As the gain is reduced, the PVD (arrowheads) disappears in contrast to the retina (arrow), which remains visible even at low gain settings
KISSING CHOROIDALS
Smooth, dome shaped ,
thick, less mobile with
double high spike suggestive
of Choroidal Detachment
PVD RD CD
Topographic
Smooth, with or
without disc insertion
Smooth or folded with disc insertion
Smooth without disc insertion
Quantitative
< 100 % spike 100 % spike Double 100 % spike
Kinetic Marked Moderate None
DIFFERENTIATING FEATURES OF RD
Rhegmatogenous RD Tractional RD Exudative RD
Convex elevation ,
Undulating folds, PVR
Concave
elevation,Fibrous
tractional band
Convex elevation,
Shifting fluid changes
Configuration with
postural change
PHPV: Longitudinal B-scan demonstrates taunt, thickened vitreous band adherent to the slightly elevated optic disc
Globular/Oval echoic structure in posterior vitreous signifying a
Dislocated Lens
Retinoblastoma: Transverse B-scans demonstrate a large, dome- shaped lesion with
marked internal calcification
Extremely thin IOFBs (< 100 mm) can be differentiated, localized Metallic IOFBs are echo dense—even at low gain settings—and often produce
shadowing of intraocular structures and the orbit
Transverse B-scan shows marked vitreous opacities and membrane
formation consistent with endophthalmitis
PAPILLOEDEMA
Transverse B-scan shows markedelevation of the optic disc
OPTIC DISC DRUSEN
Longitudinal B-scan shows highly calcified, round drusen
at the optic nerve head with
shadowing
T sign collection of fluid in subtenon space suggestive of Posterior Scleritis High reflective thickening of retinochoroid layer and sclera
Posterior Staphyloma in High Myopia
Shallow excavation of posterior pole
Smooth edges
REFERENCES
Clinical Procedures in Optometry by J. D. Barlett, J. B. Eskridge & J. F. Amos
Ophthalmic Ultrasound: A Diagnostic Atlas by C. W. DiBernardo & E. F. Greenberg
Internet
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