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  • Maharajgunj Medical Campus, Nepal

    Bikash Sapkota

    B. Optometry

    16th Batch


    Introduction History Physics Principles & instrumentation Terminologies Indications &

    contraindications Methods

    - A-Scan- B-Scan



    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



    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


    Refined techniques for measuring axial length, AC depth, lens thickness

    Bronson in 1974 made contact B scan machine


    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




    v= *

    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


    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


    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 is approx. 0.2mm

    Good resolution of minute ocular & orbital structures

    f 1/ resolution 1/penetration



    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


    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


    - 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



    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 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


    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


  • PRINCIPLE Pulse- Echo System

    Emission of multiple short pulses of ultrasound waves with brief interval to detect, process and display the turning Echoes







  • 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



    Used in A scan echography

    Beam has parallel border


    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




    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,


  • 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 un