ocular ultrasonography/ ophthalmic ultrasonography (ocular usg/ ophthalmic usg/ ophthalmic...

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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