A SCAN BIOMETRY | Principles, contact and immersion biometry |

00:39:49
https://www.youtube.com/watch?v=CrUJJDqDwPA

摘要

TLDRIn this lecture, Dr. Amrit discusses ultrasound biometry, a critical method in ophthalmology for calculating intraocular lens power during cataract surgery. The lecture outlines the prerequisites for accurate biometry, including measurements of axial length, corneal power, and anterior chamber depth. It highlights the importance of precision, as errors in these measurements can lead to significant postoperative refractive errors. The types of biometers are explained, focusing on ultrasound biometers (contact and immersion) and their operational principles based on the piezoelectric effect. The lecture also addresses common sources of error, the impact of sound velocity on measurements, and the significance of proper probe alignment. Overall, the session emphasizes the need for accurate biometry to meet rising patient expectations for refractive outcomes post-surgery.

心得

  • 👁️ Biometry is crucial for calculating intraocular lens power.
  • 📏 Accurate axial length measurement is vital to avoid refractive errors.
  • 🔍 Ultrasound biometers use sound waves for measurements.
  • ⚖️ Immersion biometers prevent corneal compression, improving accuracy.
  • 📊 Errors in measurements can significantly affect surgical outcomes.
  • 🔧 Proper alignment of the probe is essential for reliable results.
  • 📈 Gain settings can amplify signals but may introduce noise.
  • 🧪 Different eye media affect sound velocity and measurement accuracy.
  • 📝 Understanding the types of biometers helps in choosing the right tool.
  • 💡 Patient expectations for refractive outcomes have increased.

时间轴

  • 00:00:00 - 00:05:00

    Dr. Amrit introduces the topic of ultrasound biometry in ophthalmology, explaining that biometry applies mathematics to biology, particularly for calculating intraocular lens (IOL) power. He emphasizes the importance of accurate IOL power calculation for achieving excellent refractive outcomes in cataract surgery, which now aims for no refractive error post-surgery.

  • 00:05:00 - 00:10:00

    The prerequisites for IOL power calculation include measuring axial length, central corneal power, anterior chamber depth, lens thickness, effective lens position, white-to-white width, and corneal thickness. Errors in these measurements can lead to significant postoperative refractive errors, with specific percentages attributed to each measurement error.

  • 00:10:00 - 00:15:00

    The normal range for axial length is discussed, highlighting that longer eyes are more forgiving of measurement errors, while shorter eyes are less forgiving. The importance of obtaining data from both eyes is emphasized, particularly if there is a significant inter-eye difference in axial length.

  • 00:15:00 - 00:20:00

    Dr. Amrit explains the two types of biometers: ultrasound and optical. Ultrasound biometers can be contact or non-contact (immersion), while optical biometers use light. He notes that ultrasound biometers do not measure corneal power directly, unlike optical biometers, which integrate this measurement.

  • 00:20:00 - 00:25:00

    The working principle of ultrasound biometers is based on the piezoelectric effect, where mechanical deformation generates electricity. The ultrasound probe sends waves that reflect off various eye structures, and the machine calculates distances based on the time taken for these waves to return, using the formula distance = speed x time.

  • 00:25:00 - 00:30:00

    Different types of ultrasound biometers are discussed, including contact and immersion methods. The immersion method avoids corneal compression, providing more accurate readings. The importance of proper alignment of the probe and the quality of the interface for accurate readings is emphasized.

  • 00:30:00 - 00:39:49

    Common errors in ultrasound biometry include corneal compression, misalignment of the probe, and issues with the quality of the ocular interface. Dr. Amrit discusses how to identify and correct these errors, as well as the advantages of immersion scans over contact scans, particularly in reducing technician dependency and improving accuracy.

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思维导图

视频问答

  • What is biometry in ophthalmology?

    Biometry is the application of mathematics to biology, specifically in ophthalmology, it refers to the calculation of intraocular lens power.

  • What factors are measured in biometry?

    Key factors include axial length, central corneal power, anterior chamber depth, lens thickness, effective lens position, and white-to-white width.

  • What are the types of biometers?

    There are two main types: ultrasound biometers (contact and immersion) and optical biometers.

  • What is the significance of axial length measurement?

    Axial length measurement is crucial as errors can lead to significant postoperative refractive errors.

  • How does ultrasound biometry work?

    It works on the principle of the piezoelectric effect, where mechanical deformation generates ultrasonic waves that reflect off eye structures.

  • What is the difference between contact and immersion biometers?

    Contact biometers touch the cornea directly, while immersion biometers use a shell to avoid corneal compression.

  • What are common sources of error in biometry?

    Common errors include corneal compression, misalignment of the probe, and variations in eye anatomy.

  • How does sound velocity affect measurements?

    Sound travels at different velocities in various eye media, affecting the accuracy of distance calculations.

  • What is the role of gain in ultrasound biometry?

    Gain amplifies the ultrasound signal, but too much gain can introduce noise and reduce resolution.

  • Why is proper alignment of the probe important?

    Proper alignment ensures accurate measurements by maximizing the amplitude of reflected sound waves.

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  • 00:00:00
    hello and welcome to Insight oftalmology
  • 00:00:03
    this is Dr Amrit welcoming you to
  • 00:00:05
    another lecture today we are talking
  • 00:00:07
    about the ultrasound biometry so what is
  • 00:00:11
    biometry as the name suggests biometry
  • 00:00:13
    is the method of applying mathematics to
  • 00:00:16
    biology the term was originally used for
  • 00:00:19
    calculating the life expectancy however
  • 00:00:22
    in the field of oftalmology it deals
  • 00:00:25
    with the intraocular lens power
  • 00:00:27
    calculation the total refractive power
  • 00:00:30
    power of the eye primar depends upon the
  • 00:00:32
    coral
  • 00:00:33
    power it depends upon the lens power the
  • 00:00:39
    media and the axial length calculation
  • 00:00:42
    the present day cataract surgery doesn't
  • 00:00:44
    just aim at removing of the removal of
  • 00:00:47
    the cataractous lens but also at the
  • 00:00:50
    implantation of the intraocular lens and
  • 00:00:53
    giving the patient excellent refractive
  • 00:00:55
    outcomes all this is going to depend
  • 00:00:57
    upon our calculation of IAL power lens
  • 00:01:01
    Power Media and the axial length with
  • 00:01:05
    the technological advancement in the
  • 00:01:07
    cataract surgery and introduction in the
  • 00:01:09
    premium iol patients expectations have
  • 00:01:13
    actually risen and therefore now they
  • 00:01:15
    aim at getting no refractive error
  • 00:01:17
    following the cataract surgery any
  • 00:01:20
    refractive error is no longer tolerated
  • 00:01:23
    and therefore it's very very important
  • 00:01:25
    that we carry out our I power
  • 00:01:27
    calculation and biometry accurately
  • 00:01:31
    let us consider what are some of the
  • 00:01:33
    prerequisites for iul power calculation
  • 00:01:36
    in biometry we measure the axial length
  • 00:01:39
    we measure the central Coral power we
  • 00:01:42
    also measure what is known as the
  • 00:01:44
    anterior chamber depth apart from this
  • 00:01:47
    other things which are also measured are
  • 00:01:49
    the lens thickness the effective lens
  • 00:01:52
    position also know known as the ELP then
  • 00:01:55
    we have the white to white width then we
  • 00:01:58
    have Coral thickness is known as Petry
  • 00:02:01
    now let us discuss how much these
  • 00:02:03
    factors account as the sources of errors
  • 00:02:06
    in the intraocular lens power
  • 00:02:08
    calculation the first one is the
  • 00:02:11
    prediction of the effective lens
  • 00:02:13
    position and if you actually observe
  • 00:02:15
    this contributes the maximum percentage
  • 00:02:17
    responsible for inaccuracy in the I
  • 00:02:20
    power
  • 00:02:21
    calculation then we have the
  • 00:02:22
    post-operative refraction then we have
  • 00:02:25
    the axial length measurement errors
  • 00:02:27
    accounting for 17% of the ER inesis then
  • 00:02:31
    we have the keratometry measurements a
  • 00:02:35
    01 01 mm error in the axial length
  • 00:02:39
    calculation in an ie which has an
  • 00:02:42
    average axial length will lead to a post
  • 00:02:45
    operative refractive error of about 0.25
  • 00:02:48
    diopters so what you can say over here
  • 00:02:51
    is that one mm error in the axle length
  • 00:02:56
    will lead to about 2.5 diopters of of
  • 00:03:00
    postoperative refractive error in an i
  • 00:03:02
    which has normal or average axile length
  • 00:03:06
    next if you consider the coral power
  • 00:03:08
    errors if you do a Coral power error of
  • 00:03:11
    about one diopter in an i with average
  • 00:03:14
    length the post operative refractive
  • 00:03:16
    error would be about one diopter okay so
  • 00:03:20
    remember a 1 mm error in the axle length
  • 00:03:23
    will lead to about 2.5 diopters of
  • 00:03:25
    postoperative refractive error and in
  • 00:03:27
    one diopter error in the coral power
  • 00:03:30
    will lead to postoperative refractive
  • 00:03:32
    error of about one diopter okay remember
  • 00:03:36
    that let us talk about the normal axial
  • 00:03:39
    length the normal range of axial length
  • 00:03:41
    is about 22 to 24.5 mm and the average
  • 00:03:45
    is 23.5 mm this is important to know
  • 00:03:50
    because in eyes which have nor uh longer
  • 00:03:53
    Axel length than the average these eyes
  • 00:03:56
    are much more forgiving for example if
  • 00:03:58
    you take an eye which is about 30 mm
  • 00:04:01
    long in that IE a 1 mm error in the axle
  • 00:04:06
    length will lead to a postoperative
  • 00:04:08
    error of about 1.75 diopters compared to
  • 00:04:12
    the 2.5 diopter error that occurs in an
  • 00:04:15
    average length eye okay smaller eyes are
  • 00:04:18
    least forgiving for example in an ie
  • 00:04:21
    which is about 20 mm long and there is
  • 00:04:24
    an error of 1 mm the postoperative
  • 00:04:27
    refractor error is going to be much more
  • 00:04:29
    it is about 3.75 de opter which is much
  • 00:04:32
    greater than the 2.5 de opter error
  • 00:04:34
    which was occurring in a normal axal
  • 00:04:36
    length IE okay so what you can remember
  • 00:04:39
    is that the errors are more in smaller
  • 00:04:41
    eyes compared to the average eye
  • 00:04:43
    compared to the longer eyes and the
  • 00:04:45
    longer eyes are the most
  • 00:04:48
    forgiving now whenever we are doing axil
  • 00:04:51
    length calculation you always have to
  • 00:04:53
    obtain the data from two eyes and if the
  • 00:04:56
    inter eye difference of the axle length
  • 00:04:59
    is greater than3 mm then you have to be
  • 00:05:03
    suspicious we'll talk about this in a
  • 00:05:05
    while first let us try to discuss now
  • 00:05:08
    the types of biometers which are
  • 00:05:10
    available based on the energy that
  • 00:05:12
    you're using we have two types of
  • 00:05:14
    biometers we have an ultrasound biometer
  • 00:05:17
    and an optical biometer in ultrasound
  • 00:05:20
    biometers we using the sound waves and
  • 00:05:22
    in Optical biometers we are using what
  • 00:05:24
    is known as the light source okay light
  • 00:05:27
    as a source of energy now ultrasound
  • 00:05:30
    biometers again are of two types so we
  • 00:05:32
    have contact biometers in which the
  • 00:05:35
    probe touches the cornea directly and we
  • 00:05:38
    have a non-con biometer in which the
  • 00:05:40
    probe doesn't touch the cornea directly
  • 00:05:43
    instead the probe is actually immersed
  • 00:05:45
    in a scal shell which is placed on the
  • 00:05:47
    cornea and therefore this type of
  • 00:05:50
    ultrasound biometry is also known as the
  • 00:05:52
    immersion or non cont biom meter
  • 00:05:57
    okay let us see what are the types of
  • 00:05:59
    optical biometers which are available so
  • 00:06:01
    we have those which are based on the
  • 00:06:03
    principle of partial coherence
  • 00:06:06
    interferometry that is the I Master 500
  • 00:06:09
    we have those which are based based on
  • 00:06:11
    the optical low coherence reflector
  • 00:06:14
    metry like the lensar 900 OA 2000 gal G6
  • 00:06:19
    then we have swept Source OC like the I
  • 00:06:22
    master 700 now for the purpose of this
  • 00:06:25
    video we shall be limiting ourselves to
  • 00:06:27
    the ultrasound biometers like the
  • 00:06:30
    contact and the immersion biometer a
  • 00:06:33
    very important point that we must
  • 00:06:35
    remember is that the ultrasound
  • 00:06:37
    biometers doesn't uh really measure the
  • 00:06:41
    coral power okay unlike the optical
  • 00:06:44
    biometers so whenever you do an optical
  • 00:06:46
    biometry you don't need to really
  • 00:06:48
    measure the coral power separately the
  • 00:06:50
    biometer does it for you right whereas
  • 00:06:53
    when you do an ultrasound biometry you
  • 00:06:55
    actually need to separately calculate
  • 00:06:57
    the coral power using a keratometer or a
  • 00:07:02
    topographer now we have a series on
  • 00:07:04
    topography available on our channel in
  • 00:07:07
    which the first video is on the basics
  • 00:07:08
    of coral topography then we have uh on
  • 00:07:11
    the maps and then we have on how to read
  • 00:07:14
    a coral tomography print out all right
  • 00:07:16
    so you can check the links which are
  • 00:07:18
    present in the description
  • 00:07:20
    box next let us talk about how the
  • 00:07:23
    ultrasound biometers actually work so
  • 00:07:25
    they're basically working on the
  • 00:07:27
    principle of piso electric effect or p
  • 00:07:29
    Electric effect however you want to
  • 00:07:31
    pronounce it now the Greek word pison
  • 00:07:35
    basically stands for to squeeze or to
  • 00:07:39
    compress okay and piso electric effect
  • 00:07:42
    basically means mechanical deformation
  • 00:07:45
    of a piso electric material always leads
  • 00:07:48
    to generation of electricity so there
  • 00:07:51
    are certain materials like the quads
  • 00:07:54
    okay and what's going to happen in these
  • 00:07:56
    materials that whenever you apply Force
  • 00:07:58
    to deform these materials either squeeze
  • 00:08:00
    them or compress them is going to lead
  • 00:08:03
    to generation of electricity and also
  • 00:08:06
    vice versa is also true so what I mean
  • 00:08:08
    to say is if you apply electricity to
  • 00:08:11
    such materials it is going to lead to
  • 00:08:13
    the uh mechanical deformation of these
  • 00:08:16
    materials right so this is known as the
  • 00:08:19
    piso electric effect now this is
  • 00:08:21
    important because the ultrasound probe
  • 00:08:24
    actually is made up of these piso
  • 00:08:26
    electric crystals and these piso
  • 00:08:28
    electric crystals
  • 00:08:30
    they whenever there's application of
  • 00:08:32
    external electric energy to this probe
  • 00:08:35
    they will start vibrating they will get
  • 00:08:37
    deformed and as they get deformed and
  • 00:08:39
    they start vibrating they will generate
  • 00:08:42
    these ultrasonic waves right so these
  • 00:08:45
    ultrasonic waves are now going to travel
  • 00:08:47
    inside the eye and now they will get
  • 00:08:50
    reflected from the various structures
  • 00:08:52
    that these waves are going to encounter
  • 00:08:54
    like the cornea lens avitus retina and
  • 00:08:58
    the Scara and then then they will be
  • 00:09:00
    reflected back in the opposite direction
  • 00:09:02
    to the ultrasound machine and now since
  • 00:09:05
    the machine is again getting some sort
  • 00:09:07
    of energy the fiso electric crystals are
  • 00:09:09
    going to again vibrate and now they are
  • 00:09:12
    going to give us what is known as the
  • 00:09:14
    echo or these spikes right now these
  • 00:09:17
    Echoes and spikes are going to be
  • 00:09:18
    recorded on the screen right so you can
  • 00:09:21
    see clearly how the ultrasound works on
  • 00:09:23
    the principle of piso
  • 00:09:25
    electricity all right so you can see a
  • 00:09:28
    probe over here so this probe is going
  • 00:09:30
    to continuously uh this probe sorry is
  • 00:09:32
    going to send these ultrasonic waves now
  • 00:09:35
    one important point that you should know
  • 00:09:37
    is that these waves are traveling at a
  • 00:09:39
    particular velocity okay the sound has
  • 00:09:42
    this velocity U particular velocity in
  • 00:09:45
    various media in the eye right and these
  • 00:09:48
    waves are taking a particular time
  • 00:09:50
    period for traveling towards the object
  • 00:09:53
    and coming back towards from the object
  • 00:09:55
    to the probe right the machine is
  • 00:09:58
    basically going to operate in this pulse
  • 00:10:01
    system now pulse system basically means
  • 00:10:04
    that the probe is vibrating for some
  • 00:10:07
    time it's pulsating for some time and
  • 00:10:09
    then it takes a pause right for a few
  • 00:10:11
    micros seconds so that all these
  • 00:10:13
    returning Echoes can be received by the
  • 00:10:16
    probe tips and it is that it is during
  • 00:10:18
    that time that these signals are going
  • 00:10:20
    to be converted into spikes or Echoes
  • 00:10:23
    right now
  • 00:10:25
    usually the audible frequency to a human
  • 00:10:28
    ear is about about 20 to 20,000 Hertz
  • 00:10:31
    right however you can see the frequency
  • 00:10:34
    which a probe uses is about 10 mahz
  • 00:10:38
    right so it's a really high frequency
  • 00:10:41
    and the advantage of high frequency is
  • 00:10:43
    that it offers higher resolution all
  • 00:10:46
    right now the sound is going to travel
  • 00:10:49
    at a particular velocity within the eye
  • 00:10:51
    and we know that the waves are coming
  • 00:10:52
    back towards the probe they're taking
  • 00:10:54
    some time to go towards the object and
  • 00:10:57
    some time to come back towards the probe
  • 00:10:59
    the machine is going to calculate all
  • 00:11:01
    that time and once it has calculated
  • 00:11:04
    that time it is going to use the formula
  • 00:11:08
    that is speed into time equals distance
  • 00:11:11
    and it is using by using this formula
  • 00:11:13
    the machine is going to calculate the
  • 00:11:14
    various distances like the axal length
  • 00:11:18
    anterior chamber depth and the length
  • 00:11:21
    thickness okay so I hope you understood
  • 00:11:23
    how the axle length is calculated by the
  • 00:11:26
    machine all right now let us let talk
  • 00:11:29
    about some other principles of the
  • 00:11:31
    ultrasound you should know that the
  • 00:11:34
    sound usually travels faster in solids
  • 00:11:37
    compared to liquid okay and therefore
  • 00:11:40
    the velocity of the sound waves as
  • 00:11:42
    they're going to travel within the eye
  • 00:11:44
    is constantly changing in an a scan
  • 00:11:47
    biometer the sound is traveling through
  • 00:11:50
    the solid Gia then it has to travel in
  • 00:11:53
    the liquid Vitus then it has to travel
  • 00:11:56
    again in the solid lens again it travel
  • 00:11:59
    travels in the solid or the liquid or
  • 00:12:01
    you can say semif fluidic witus again it
  • 00:12:04
    travels in the retina choid Scara and
  • 00:12:07
    finally into the orbital tissue and
  • 00:12:09
    therefore we can say that the velocity
  • 00:12:11
    of the sound waves within the eye is
  • 00:12:13
    also constantly
  • 00:12:16
    changing right so you can observe over
  • 00:12:18
    here the speed of the sound waves within
  • 00:12:21
    the cornea is about 1641 m/s in lens
  • 00:12:25
    again it is somewhat similar in the
  • 00:12:27
    Aquis humor and in the humor because
  • 00:12:29
    they are more liquidy compared to the
  • 00:12:31
    cor and the lens and therefore the speed
  • 00:12:33
    over here is about 15 32
  • 00:12:36
    m/s now usually uh the machine averages
  • 00:12:40
    these values and therefore the average
  • 00:12:42
    sound velocity in a normal fake ey is
  • 00:12:45
    about 1550
  • 00:12:48
    m/s okay now the cornea is not routinely
  • 00:12:51
    factored in because the thickness of the
  • 00:12:53
    cornea is really less about 0.5 mm and
  • 00:12:57
    therefore the cornea uh the velocity of
  • 00:12:59
    the sound in the Cora is Lally not
  • 00:13:00
    factored in now the average sound
  • 00:13:03
    velocity in an eye which does not have
  • 00:13:05
    the lens that is an earth fic eye will
  • 00:13:08
    come to about 1532
  • 00:13:11
    m/s okay now what about the average
  • 00:13:13
    sound velocity in a pseudo ey now we
  • 00:13:16
    know that pseudo FIA means presence of
  • 00:13:18
    an intraocular lens now intraocular
  • 00:13:21
    lenses again could be of various
  • 00:13:24
    materials and therefore the average
  • 00:13:27
    sound velocity in a pseudo fic eye is is
  • 00:13:29
    going to be 1532 m/s plus or minus the
  • 00:13:34
    correction factor for that intraocular
  • 00:13:37
    material okay so we'll talk about the
  • 00:13:38
    correction factors also in a
  • 00:13:41
    while now a very important point that we
  • 00:13:44
    must all know is that within the eye we
  • 00:13:47
    have various interfaces that means for
  • 00:13:49
    example the air and the cornea is one
  • 00:13:52
    interface the cornea and the Aquis is
  • 00:13:54
    one interface between the Aquis and the
  • 00:13:56
    lens is another interface and then again
  • 00:13:59
    between the lens witus witus retina
  • 00:14:01
    retina and Scara there are different
  • 00:14:03
    different interfaces right so the echo
  • 00:14:05
    are received back into the probe after
  • 00:14:08
    being reflected from these interfaces
  • 00:14:11
    and these Echo are going to be converted
  • 00:14:13
    by the biometer into various spikes
  • 00:14:15
    which are arising from the Baseline as
  • 00:14:17
    you can see over here the greater is the
  • 00:14:20
    difference between the two Media or at
  • 00:14:22
    each interface for example between the
  • 00:14:24
    Aquis and the lens the greater is the
  • 00:14:27
    difference in these two media at the
  • 00:14:29
    interface the stronger is going to be
  • 00:14:32
    the spike and the stronger is going to
  • 00:14:33
    be the echo right and if the interface
  • 00:14:36
    difference is less the echo is going to
  • 00:14:38
    be weaker and the strong uh the strength
  • 00:14:42
    and the weakness of the echo basically
  • 00:14:43
    is represented in the form of amplitude
  • 00:14:46
    Okay so what you're doing in a biometer
  • 00:14:49
    is an a scan that is and over here a
  • 00:14:52
    basically stands for the amplitude
  • 00:14:55
    representation of the echo along the
  • 00:14:57
    path and it is a onedimensional
  • 00:14:59
    representation remember that all right
  • 00:15:02
    so now let us talk about these various
  • 00:15:04
    types of ultrasound biometers so we have
  • 00:15:06
    the contact biometer and the immersion
  • 00:15:08
    or non- cont biometer so let's see first
  • 00:15:12
    about the contact a scan also known as
  • 00:15:15
    the applanation a scan or biometry now
  • 00:15:18
    the term applanation I personally don't
  • 00:15:20
    like to call it that way because ideally
  • 00:15:23
    speaking you're not supposed to
  • 00:15:24
    applanate when carrying out a contact a
  • 00:15:26
    scan as well you're just supposed to
  • 00:15:28
    touch the probe to the patient's eye
  • 00:15:30
    okay compression or any sort of
  • 00:15:32
    applanation will lead to uh inaccurate
  • 00:15:35
    readings in contact biometry all right
  • 00:15:38
    the patient could be seated or the
  • 00:15:40
    patient could be lying down Supine
  • 00:15:42
    position you have to instill a topical
  • 00:15:44
    anesthetic solution and what you have to
  • 00:15:47
    do is you have to give the patient a
  • 00:15:48
    fixation Target like this okay and then
  • 00:15:52
    the examiner has to gently touch the
  • 00:15:54
    cornea without causing any indentation
  • 00:15:58
    okay and as you do so there will be
  • 00:16:00
    generation of these spikes as you can
  • 00:16:02
    see right the first spike is coming from
  • 00:16:05
    as you can see from the cornea and the
  • 00:16:07
    probe since the probe is in contact with
  • 00:16:09
    the cornea you get the first Spike then
  • 00:16:11
    you get one Spike from the anterior
  • 00:16:13
    surface of the lens then you get another
  • 00:16:15
    from the posterior surface of the lens
  • 00:16:17
    then you get one Spike from the retina
  • 00:16:18
    and one from the Scara okay let's talk
  • 00:16:21
    about that now over here the patient
  • 00:16:23
    could be sitting or the patient could be
  • 00:16:25
    lying down but make sure that the plane
  • 00:16:28
    of the iris is parallel to the floor
  • 00:16:30
    there should be no pillows or or no
  • 00:16:32
    headrest below the patient okay and
  • 00:16:34
    obvious and always make sure that you
  • 00:16:36
    give the fixation Target to the patient
  • 00:16:39
    when carrying out the contact bio
  • 00:16:41
    contact biometry make sure that you hold
  • 00:16:45
    the probe from The Wire okay this
  • 00:16:46
    actually reduces the compression force
  • 00:16:49
    on the cornea if you're going to
  • 00:16:51
    directly hold the probe from the uh end
  • 00:16:54
    of the probe it there are more chances
  • 00:16:56
    that you might actually compress the
  • 00:16:58
    cornea okay always make sure you give
  • 00:17:00
    the accurate
  • 00:17:02
    or adequate Target like patient can use
  • 00:17:05
    his own thumb or you can give a fixation
  • 00:17:07
    light as a Target and even the a scan
  • 00:17:09
    probe comes with a fixation light
  • 00:17:11
    usually it is red in color so you can
  • 00:17:13
    ask the patient to look towards the
  • 00:17:15
    light all right now a high quality
  • 00:17:18
    contact a scan of a fake eye basically
  • 00:17:21
    consists of five high amplitude spikes
  • 00:17:24
    okay remember now you might say there
  • 00:17:26
    are so many spikes over here yes they
  • 00:17:28
    are but what is important for us are the
  • 00:17:31
    high amplitude spikes so we have five
  • 00:17:34
    high amplitude spikes you can see the
  • 00:17:36
    first one which is coming from the
  • 00:17:38
    cornea and the probe together because
  • 00:17:41
    the probe and cornea are in touch with
  • 00:17:42
    each other then we have the one which is
  • 00:17:45
    coming from the anterior lens then we
  • 00:17:47
    have one coming from the posterior lens
  • 00:17:49
    then we have the spike which is coming
  • 00:17:51
    from the retina and Spike which is
  • 00:17:53
    coming from the Scara so all these are
  • 00:17:55
    high amplitude spikes then of course we
  • 00:17:57
    have smaller spikes which are coming
  • 00:17:59
    from the orbital tissues another thing
  • 00:18:01
    you should know note is that these all
  • 00:18:04
    these Spikes have to rise steeply from
  • 00:18:07
    the surface especially the retinal Spike
  • 00:18:09
    if you see it has to make this 90° angle
  • 00:18:13
    also there should be good resolution and
  • 00:18:15
    these spikes should be separate there
  • 00:18:17
    should be no intermixing of the retina
  • 00:18:19
    and the Scara spikes so that is very
  • 00:18:21
    important so Spike height or amplitude
  • 00:18:24
    is what provides us the information on
  • 00:18:27
    the quality about the quality of the a
  • 00:18:30
    scan and therefore since we are
  • 00:18:32
    basically relying our biometry on the
  • 00:18:34
    amplitudes and therefore this type of
  • 00:18:36
    biometry is known as an a scan or an
  • 00:18:39
    amplitude scan so I hope that is clear
  • 00:18:42
    all right so now let us talk about the
  • 00:18:44
    immersion or non- cont biometry now over
  • 00:18:47
    here what you can see is that there is a
  • 00:18:50
    sort of a shell that is placed on the
  • 00:18:52
    cornea and this shell is known as the
  • 00:18:54
    Prager shell or you can call it as a
  • 00:18:56
    Hansen shell the purpose of a
  • 00:18:59
    compression uh sorry the purpose of an
  • 00:19:01
    immersion biometer is that it avoids any
  • 00:19:05
    Coral compression because you can see
  • 00:19:07
    there's a there's some sort of distance
  • 00:19:08
    between the probe and the cornea over
  • 00:19:11
    here okay and therefore you're going to
  • 00:19:13
    see many spikes here you are going to
  • 00:19:15
    see a spike that is separately coming
  • 00:19:17
    from the probe then you see a corneal
  • 00:19:19
    Spike the corneal Spike seems to be
  • 00:19:21
    having these two peaks then you have an
  • 00:19:24
    anterior lens spike a posterior lens
  • 00:19:26
    Spike then you have a retina Spike and a
  • 00:19:28
    scler Spike and of course the orbital
  • 00:19:30
    Spike okay so in this also you make the
  • 00:19:34
    patient lie down spine make sure there's
  • 00:19:36
    no pillow underneath this and also you
  • 00:19:39
    have to again put anesthetic drops okay
  • 00:19:42
    make sure you avoid Coral compression
  • 00:19:44
    obviously this will be avoided in case
  • 00:19:46
    of this immersion biom meter ask the
  • 00:19:48
    patient to fix it align the probe with
  • 00:19:50
    the optical axis and now you have to
  • 00:19:52
    observe the various Echo and the spikes
  • 00:19:55
    which are observed okay so over here the
  • 00:19:58
    scleral shell is known as the Hansen or
  • 00:20:00
    the Prager shell and it is usually
  • 00:20:02
    filled up with saline you have to make
  • 00:20:04
    sure that no air bubbles enter the
  • 00:20:06
    saline and the probe should be immersed
  • 00:20:10
    in the saline and it should be about a
  • 00:20:12
    distance of about 5 to 10 mm away from
  • 00:20:15
    the cornea that is very very
  • 00:20:18
    important let us try to observe the
  • 00:20:20
    various waves that we see in an
  • 00:20:22
    immersion scan of a fake ey so over here
  • 00:20:25
    instead of five you're going to see six
  • 00:20:27
    waves so the first one is coming from
  • 00:20:29
    the probe the second one is from the
  • 00:20:31
    cornea then you have from the anterior
  • 00:20:34
    surface of the lens the posterior
  • 00:20:35
    surface of the lens the the retina and
  • 00:20:38
    the Scara of course there are some lower
  • 00:20:41
    amplitude waves as well these might be
  • 00:20:43
    coming from the vitus degeneration or
  • 00:20:45
    structures present within the vitus then
  • 00:20:48
    over here you have smaller wavelets
  • 00:20:50
    which are coming from the orbital fat
  • 00:20:52
    now two important things that you have
  • 00:20:54
    to remember which differentiates it from
  • 00:20:56
    a contact biometer in an immersion scan
  • 00:20:59
    you have the separate probe and coral
  • 00:21:01
    spikes the coral Spike over here
  • 00:21:04
    demonstrate two peaks okay you can see
  • 00:21:05
    this is bit so the first one is actually
  • 00:21:08
    coming from the epithelium and the
  • 00:21:09
    second one is coming from the
  • 00:21:11
    endothelium and you have to also note
  • 00:21:13
    that these two spikes which are coming
  • 00:21:15
    the two peaks of the coral Spike they
  • 00:21:18
    should be of equal amplitude in any case
  • 00:21:20
    if the these are not equal they should
  • 00:21:22
    actually U raise suspicion that the
  • 00:21:25
    probe is not aligned through the coral
  • 00:21:27
    vertex alignment of the probe is really
  • 00:21:30
    really important whether we talking
  • 00:21:32
    about contact biometry or the non-con
  • 00:21:35
    biometry so let's see why that happens
  • 00:21:38
    so if you're going to align the probe
  • 00:21:40
    basically like this to the uh whenever
  • 00:21:44
    the probe is nicely aligned parall uh
  • 00:21:47
    parall to the visual Axis or coagular to
  • 00:21:50
    the visual axis what's going to happen
  • 00:21:51
    is that all the sound waves which are
  • 00:21:53
    traveling from the probes they're going
  • 00:21:55
    to be reflected and they're going to be
  • 00:21:57
    traveling back
  • 00:21:59
    to the visual axis and all of them are
  • 00:22:00
    going to be received by the probe and
  • 00:22:03
    since all the sound wave is received by
  • 00:22:06
    the probe the spike amplitude is also
  • 00:22:08
    going to be good however if the probe is
  • 00:22:12
    not aligned correctly the waves are
  • 00:22:14
    going to be incident in an oblique
  • 00:22:15
    fashion and therefore some of the Waves
  • 00:22:17
    which are reflected they're not going to
  • 00:22:19
    make to the probe and instead they're
  • 00:22:20
    going to be reflected outside and this
  • 00:22:23
    will lead to compromise amplitude or
  • 00:22:25
    compromise signal um in the a scan
  • 00:22:28
    biometry okay and therefore it is very
  • 00:22:31
    important uh that the probe is oriented
  • 00:22:34
    nicely parallel to the visual axis at
  • 00:22:36
    the coral Vortex
  • 00:22:39
    okay next point is regarding the shape
  • 00:22:42
    and smoothness of the interface okay I
  • 00:22:45
    already told you what exactly is meant
  • 00:22:46
    by interface so over here we are talking
  • 00:22:49
    about the maula for example when the
  • 00:22:51
    maula is smooth okay all the waves are
  • 00:22:54
    going to be reflected nicely parallel to
  • 00:22:56
    the visual axis and they're going to be
  • 00:22:58
    uh
  • 00:22:59
    received by the probe and therefore the
  • 00:23:01
    amplitude is going to be nice and smooth
  • 00:23:03
    okay big nice amplitude however if the
  • 00:23:06
    macula is not smooth for example if
  • 00:23:08
    there's any irregularity in the surface
  • 00:23:10
    of the interface like if there is any
  • 00:23:13
    macula EMA if there's pigment epithelial
  • 00:23:15
    Detachment or if there's any epiretinal
  • 00:23:17
    membrane what is going to happen is that
  • 00:23:19
    there's also going to be reflection away
  • 00:23:21
    from the probe sometimes there might be
  • 00:23:23
    refraction of the returning sound waves
  • 00:23:25
    away from the probe and therefore a few
  • 00:23:28
    way only are going to make up to the tip
  • 00:23:30
    and therefore it lead to weaker Echoes
  • 00:23:33
    or weaker signals or low amplitude
  • 00:23:34
    spikes okay and therefore in order to
  • 00:23:37
    get a correct biometer readings it's
  • 00:23:39
    very important that you have a normal
  • 00:23:41
    maula as well all right for example over
  • 00:23:45
    here you can see this is basically a
  • 00:23:48
    good retinal spike a good retinal Spike
  • 00:23:50
    should actually rise sharply like this
  • 00:23:53
    it should be at 90°
  • 00:23:55
    angle all right now let's talk about
  • 00:23:58
    about the the concept of sound
  • 00:24:01
    absorption now we know that sound is
  • 00:24:04
    basically absorbed by everything through
  • 00:24:06
    which it passes before it travels to the
  • 00:24:08
    next interface so the greater is the
  • 00:24:10
    density of the structure through which
  • 00:24:12
    the sound is actually passing through
  • 00:24:14
    greater is going to be the amount of
  • 00:24:16
    absorption and therefore in a case of
  • 00:24:18
    extremely dense cataract like this
  • 00:24:20
    greater amount of sound energy is going
  • 00:24:22
    to be absorbed and there will be
  • 00:24:24
    attenuation of the signal that you're
  • 00:24:25
    going to get on your display okay and
  • 00:24:28
    that is a reason why in case of an
  • 00:24:30
    extremely dense cataract you're not
  • 00:24:32
    going to get really high amplitude
  • 00:24:33
    retinal spikes because the lens is going
  • 00:24:35
    to absorb most of the sound energy that
  • 00:24:38
    reaches the retinal surface okay so over
  • 00:24:41
    here you can see the waveforms are quite
  • 00:24:44
    shrunken actually you can see over here
  • 00:24:46
    multiple retinal uh multiple internal
  • 00:24:48
    spikes you don't have those classic five
  • 00:24:51
    spikes that you see and this happens
  • 00:24:53
    because of the uh various refractive
  • 00:24:56
    indexes uh which is present with within
  • 00:24:58
    the lens and the lens is also absorbing
  • 00:25:00
    most of the sound energy and therefore
  • 00:25:02
    you're not getting good amount of you're
  • 00:25:04
    not getting the proper morphology of the
  • 00:25:06
    spikes right so that is one problem that
  • 00:25:09
    can occur when you're carrying out
  • 00:25:10
    biometry in case of the uh dense
  • 00:25:14
    cataracts now this brings us to the
  • 00:25:16
    concept of gain in ultrasound so when
  • 00:25:18
    you're getting this uh dampened signals
  • 00:25:21
    because of a cataract is there a way
  • 00:25:24
    that we can amplify the signal yes you
  • 00:25:26
    can amplify the signal by increasing the
  • 00:25:28
    gain right so gain is that property
  • 00:25:31
    which is measured in DCB it is basically
  • 00:25:33
    the amplification of the ultrasound
  • 00:25:35
    signal that returns to the transducer
  • 00:25:38
    after passing through the tissue okay so
  • 00:25:41
    uh in cases of dense cataract we might
  • 00:25:43
    need to increase the gain these higher
  • 00:25:46
    gains will lead to increase amplitude it
  • 00:25:49
    increases the sensitivity and but the
  • 00:25:51
    only problem over here with increased
  • 00:25:54
    gains is that since the machine has now
  • 00:25:56
    become more sensitive there's going to
  • 00:25:58
    be more noise as well and therefore
  • 00:26:00
    you're going to suffer in terms of
  • 00:26:02
    resolution however in certain cases
  • 00:26:05
    where there is a fake here where there's
  • 00:26:07
    no lens to actually absorb you don't
  • 00:26:09
    need that much amount of signal and
  • 00:26:11
    there you can actually reduce your gain
  • 00:26:14
    so in AIC AIC patients you can actually
  • 00:26:18
    uh go for a lower gain setting now in
  • 00:26:21
    the first picture you can see the gain
  • 00:26:22
    is too high so when the gain is too high
  • 00:26:24
    the machine has become more sensitive
  • 00:26:26
    and it's picking up every random signal
  • 00:26:28
    and therefore you can see there's a lot
  • 00:26:30
    of noise over here instead of seeing two
  • 00:26:32
    two spikes one from retina one from
  • 00:26:34
    Scara you're seeing this broaden
  • 00:26:37
    thickened Spike with a flattened Peak
  • 00:26:39
    right this indicates that there is too
  • 00:26:41
    much gain so when there's too much gain
  • 00:26:43
    you're going to get a strong signal but
  • 00:26:45
    the resolution is going to be poor now
  • 00:26:48
    in such cases you have to actually
  • 00:26:50
    reduce the gain until the retinal and
  • 00:26:53
    the scar surfaces are seen as separate
  • 00:26:55
    spikes okay so if you uh reduce the gain
  • 00:26:59
    the resolution is going to be good but
  • 00:27:01
    the signal is going to be weaker okay so
  • 00:27:03
    that so you really have to decide what
  • 00:27:06
    exactly you want now ideally the retinal
  • 00:27:09
    and the scleral spike should be separate
  • 00:27:11
    and if you're carrying out an immersion
  • 00:27:12
    scan the peaks of the coral Spike that
  • 00:27:15
    is one coming from the epithelium one
  • 00:27:17
    coming from the endothelium they they
  • 00:27:19
    must also be separate if there's any
  • 00:27:21
    Fusion between these two then also it's
  • 00:27:23
    an indicator that you have to reduce the
  • 00:27:27
    uh you have to reduce the gain in order
  • 00:27:29
    to separate these two
  • 00:27:31
    peaks all right now let us talk about
  • 00:27:34
    the concept of electronic calipers or
  • 00:27:37
    the gates okay we all know what calipers
  • 00:27:40
    are so calipers are something which are
  • 00:27:42
    used to measure uh the distance between
  • 00:27:44
    two points right so electronically also
  • 00:27:47
    the machine introduces these uh Gates so
  • 00:27:50
    gates are these electronic calipers on
  • 00:27:52
    the display screen that measure between
  • 00:27:54
    the two points okay so biometers are
  • 00:27:57
    designed so that between each pair of
  • 00:27:59
    the gates a measurement is rendered for
  • 00:28:01
    example you're going to the biometer is
  • 00:28:03
    going to automatically place a gate over
  • 00:28:05
    here near the probe and the Cora then
  • 00:28:07
    it's going to place a gate where it
  • 00:28:08
    believes that the lens lens is spiking
  • 00:28:11
    uh lens anterior surface is spiking then
  • 00:28:14
    it's going to place a gate somewhere
  • 00:28:15
    near the posterior surface a gate near
  • 00:28:17
    the retina a gate near the Scara okay so
  • 00:28:20
    the biometer automatically places the
  • 00:28:22
    gate on what it believes to be the
  • 00:28:24
    central cornal Spike anterior lens Spike
  • 00:28:26
    posterior lens Spike retinal Spike so on
  • 00:28:28
    and so forth right and it is also
  • 00:28:30
    programmed to measure the distance
  • 00:28:32
    between each of these pairs at a given
  • 00:28:35
    velocity right so the ultrasound is
  • 00:28:38
    actually going to place these Gates
  • 00:28:39
    automatically and also it is at this
  • 00:28:41
    Gates that it is going to record the
  • 00:28:43
    time taken for the sound to travel from
  • 00:28:45
    one point to another and then the
  • 00:28:47
    machine is going to use this uh formula
  • 00:28:50
    distance equal to Velocity into time and
  • 00:28:52
    it's going to calculate the various
  • 00:28:53
    distances between these Gates and of
  • 00:28:56
    course we know how the velocity at
  • 00:28:58
    various parts of the uh I is Right
  • 00:29:02
    similarly also know about the average
  • 00:29:04
    sound velocities in the fake eye a fic
  • 00:29:07
    eye and in the pseudo fic eye as well
  • 00:29:11
    now if you know that the average sound
  • 00:29:12
    velocity in fake or fic and pseudophakic
  • 00:29:15
    ey is you should also know that there
  • 00:29:17
    are various modes which are present
  • 00:29:19
    within the biometer machine so in these
  • 00:29:21
    modes you can actually choose whether
  • 00:29:23
    you are dealing with a fake person that
  • 00:29:25
    is normal person or you're dealing with
  • 00:29:27
    an earth pic person or whether you are
  • 00:29:29
    doing a biometry in a case of a patient
  • 00:29:33
    who already has an i inside him so that
  • 00:29:36
    is known as a pseudo fic eye right so
  • 00:29:39
    again within the pseudo fic eye there
  • 00:29:40
    can be various settings which can be
  • 00:29:42
    available so there might be a setting
  • 00:29:44
    for the material that means you might be
  • 00:29:47
    able to choose whether the material is
  • 00:29:48
    PMA acrylic or the Silicon right so now
  • 00:29:52
    when you carry out a biometry in case of
  • 00:29:54
    pseudo gu you might actually be able to
  • 00:29:56
    see a lot of these spikes these spikes
  • 00:29:59
    basically are known as reverberation
  • 00:30:00
    artifact and these are more prominent
  • 00:30:03
    when the intraocular lens is basically
  • 00:30:05
    of pmma variety right so a pmma lens is
  • 00:30:09
    going to or an interocular lens is going
  • 00:30:11
    to give more reverberation artifacts
  • 00:30:14
    compared to a foldable I which is made
  • 00:30:16
    up of usually acrylic or the uh silicon
  • 00:30:19
    right so you can see the second picture
  • 00:30:22
    there is a foldable or I which is used
  • 00:30:24
    and therefore the reation artifacts are
  • 00:30:26
    much uh less over
  • 00:30:29
    here all right so I was talking about
  • 00:30:32
    the correction factor now this is used
  • 00:30:35
    when your biometer doesn't have a pseudo
  • 00:30:37
    fix setting or just has a pmma setting
  • 00:30:40
    now in those cases what you can do is
  • 00:30:42
    you can actually U put the mode as a
  • 00:30:45
    fake mode okay and then you can
  • 00:30:48
    calculate your axile length using the
  • 00:30:50
    ultrason biometer and after you have
  • 00:30:53
    calculated the axile length you can
  • 00:30:54
    actually add up the correction factor
  • 00:30:57
    for example if your patient has a pmma
  • 00:30:59
    intraocular lens inside the eye you will
  • 00:31:02
    carry out the biometry in the earth fic
  • 00:31:04
    mode and the Axel length say comes as 23
  • 00:31:08
    mm now in this person the correction
  • 00:31:11
    factor is plus 0.4 mm and therefore the
  • 00:31:15
    corrected axle length in this patient
  • 00:31:17
    would be 23 + 0.4 that is
  • 00:31:21
    23.4 MM okay similarly you can carry out
  • 00:31:24
    the correction factors for silicon and
  • 00:31:26
    also for the ACR
  • 00:31:28
    now let me tell you that if you already
  • 00:31:30
    have the settings for silicon and
  • 00:31:32
    acrylic iel then you don't need to take
  • 00:31:35
    these correction factor into
  • 00:31:37
    consideration all right now sometimes
  • 00:31:40
    there might be a patient who might be
  • 00:31:42
    having a silicone oil inside the eye now
  • 00:31:45
    silicon oil also can change the sound
  • 00:31:47
    velocity right it's it has high density
  • 00:31:50
    and uh for example we have 5,000 C
  • 00:31:53
    Strokes of silicon oil available in
  • 00:31:55
    which the velocity of the sound is going
  • 00:31:57
    to be 1 40 m/s and then we have a th000
  • 00:32:01
    c Scopes where the speed of sound is
  • 00:32:04
    about 980 m/ Second right as you can see
  • 00:32:07
    the sound waves are traveling slower in
  • 00:32:11
    the Silicon oil compared to the normal
  • 00:32:13
    eye and therefore if you don't really
  • 00:32:15
    use proper settings within your biometer
  • 00:32:19
    the the axi length that the biometer is
  • 00:32:22
    going to measure would be erroneously
  • 00:32:24
    long axial length all right so that's
  • 00:32:27
    one important point that you must
  • 00:32:28
    remember that silicon filled Globe is
  • 00:32:31
    going to lead to a false long axle
  • 00:32:33
    length measurement now to talk about
  • 00:32:36
    some common errors and challenging
  • 00:32:38
    situation while you're carrying out an
  • 00:32:40
    ultrasound biometry first thing first
  • 00:32:43
    whenever the inter eye difference is
  • 00:32:45
    greater than3 mm and when the
  • 00:32:48
    consecutive same eye readings are
  • 00:32:51
    different by more than 0.1 mm then in
  • 00:32:54
    those patients you have to be suspicious
  • 00:32:57
    the first first thing that you should do
  • 00:32:58
    is you have to cons uh consider the
  • 00:33:00
    medical history of the patient and find
  • 00:33:03
    out if there is any particular reason as
  • 00:33:05
    to why there are differences in the
  • 00:33:07
    reading the patient might be having a
  • 00:33:09
    macular pathology the patient might also
  • 00:33:11
    be having some problem like posterior
  • 00:33:13
    stoma which is going to give different
  • 00:33:16
    readings within the same eye all right
  • 00:33:20
    another common source of error which is
  • 00:33:22
    also the most common error seen in
  • 00:33:24
    contact technique is a corial
  • 00:33:26
    compression Okay so remember cornal
  • 00:33:28
    compression is seen in the contact
  • 00:33:29
    technique of biometry now this happens
  • 00:33:32
    because the eye is really soft and
  • 00:33:34
    pliable and the Cora can be indented
  • 00:33:36
    with minimal pressure also from the
  • 00:33:38
    probe tip and as a matter of fact the
  • 00:33:40
    softer the eye the lesser the
  • 00:33:42
    intraocular pressure and the greater is
  • 00:33:44
    the compression uh it's more easy to
  • 00:33:46
    compress a softer eye compared to a hard
  • 00:33:49
    eye and this will lead to uh the
  • 00:33:52
    problems within the axle length
  • 00:33:54
    measurement now since corneal
  • 00:33:56
    compression is going to compress in the
  • 00:33:58
    cornea the anterior chamber depth
  • 00:34:00
    measurement is also going to go down and
  • 00:34:02
    therefore in these cases it's very
  • 00:34:04
    important to keep an eye on the anterior
  • 00:34:06
    chamber depth if the depth is less you
  • 00:34:09
    know that there is some sort of cornal
  • 00:34:10
    compression which is going on and
  • 00:34:12
    therefore anterior chamber depth must
  • 00:34:14
    always be monitored and all the readings
  • 00:34:16
    with a shallow anterior chamber depth
  • 00:34:18
    must be deleted from your uh
  • 00:34:23
    measurements next uh error is the
  • 00:34:25
    misalignment of the probe I already told
  • 00:34:28
    you that it's very important to keep the
  • 00:34:30
    probe perpendicular to the coral vertex
  • 00:34:33
    and the Rays have to go and strike
  • 00:34:35
    perpendicularly to the macular surface
  • 00:34:37
    okay so that the rays are directed
  • 00:34:40
    parall to the visual axis this
  • 00:34:42
    perpendicularity is achieved whenever
  • 00:34:44
    this perpendicularity is achieved the
  • 00:34:46
    retinal Spike and the scar spikes will
  • 00:34:49
    be of high amplitude and the retinal
  • 00:34:51
    Spike will arise steeply from the
  • 00:34:53
    Baseline make sure that there are no
  • 00:34:55
    slopes no Jags no humps or steps on the
  • 00:34:57
    Ral spikes over here you can see that
  • 00:34:59
    this Ral spike is slightly sloping and
  • 00:35:03
    therefore this indicates that your
  • 00:35:04
    alignment of the probe is not correct
  • 00:35:06
    there's some misalignment so you need to
  • 00:35:08
    take another reading by proper placement
  • 00:35:11
    of the uh probe now sometimes the probe
  • 00:35:15
    the probe might be misaligned at the
  • 00:35:17
    lens surface okay so normally you have
  • 00:35:19
    the anterior lens Spike and the
  • 00:35:21
    posterior lens Spike so over here you
  • 00:35:23
    can see this is the anterior lens Spike
  • 00:35:27
    and this one over here is a posterior
  • 00:35:28
    lens Spike normally they should be of
  • 00:35:30
    equal amplitude if they are not of equal
  • 00:35:33
    amplitude it means that again the Rays
  • 00:35:36
    uh the sound waves are not uh striking
  • 00:35:39
    the lens appropriately that means there
  • 00:35:40
    is some sort of probe misalignment and
  • 00:35:43
    therefore you need to correct your
  • 00:35:45
    positioning now sometimes the probe can
  • 00:35:47
    be misaligned in such a way that the
  • 00:35:49
    Rays or the sound waves are going to
  • 00:35:51
    travel in uh through the optic nerve
  • 00:35:54
    instead of being instead of traveling to
  • 00:35:56
    the center of the maula right now in
  • 00:35:59
    such a case what is going to happen is
  • 00:36:01
    that there will be absence of the Scaris
  • 00:36:02
    spite now why does that occur we know
  • 00:36:05
    that the optic nerve basically is
  • 00:36:07
    actually a bundle of nerve fibers and
  • 00:36:09
    there's no Scara present within the
  • 00:36:11
    optic nerve and when the sound waves are
  • 00:36:13
    going to travel through the optic nerve
  • 00:36:14
    they're not going to encounter any
  • 00:36:16
    Scaris Spike now we don't want that we
  • 00:36:18
    basically are measuring the axal length
  • 00:36:21
    which is from the coral vertex up to the
  • 00:36:23
    internal limiting membrane in case of
  • 00:36:25
    ultrasound biom biometry and in case of
  • 00:36:28
    optical biometer we are measuring it
  • 00:36:30
    from the coral vertex up to the RP so
  • 00:36:33
    all these measurements are taken at the
  • 00:36:34
    macula right and therefore whenever you
  • 00:36:37
    see a waveform like this where the scar
  • 00:36:39
    spike is absent in those cases you
  • 00:36:42
    should know that there is misalignment
  • 00:36:44
    along the optic nerve all right another
  • 00:36:48
    thing is that make sure that there's no
  • 00:36:50
    uh big uh no thick gel sitting on the
  • 00:36:52
    patient's eye because then there's going
  • 00:36:54
    to be a miniscus between the probe tip
  • 00:36:56
    and the eye and you are going to get a
  • 00:36:58
    falsely long axial
  • 00:37:00
    length now another problem that occurs
  • 00:37:03
    uh while measuring the axial length is
  • 00:37:05
    extremely variable readings that you get
  • 00:37:07
    in case of posterior stomatis patients
  • 00:37:10
    and in high myopes now in high myopes
  • 00:37:12
    the globe is elongated and the uvia can
  • 00:37:15
    actually bulge into the Scara and this
  • 00:37:17
    happens mostly posteriorly now in these
  • 00:37:20
    cases the maula has a sloping Edge and
  • 00:37:24
    as I told you that the regularity and
  • 00:37:26
    smoothness of the interface is very
  • 00:37:28
    important in order to get good waves now
  • 00:37:31
    over here because there's a slope
  • 00:37:33
    configuration of the maula there will be
  • 00:37:35
    variable reflection of the sound waves
  • 00:37:37
    and therefore the waves that you see
  • 00:37:39
    over here would be of poor amplitude
  • 00:37:41
    there will be poor configuration
  • 00:37:43
    morphology and extremely variable
  • 00:37:45
    readings it's always better to carry out
  • 00:37:48
    an optical biometer biometry in case of
  • 00:37:51
    posterior stomas patient however if that
  • 00:37:54
    is not available you can carry out an
  • 00:37:56
    axi length measurement in a b scan as
  • 00:37:58
    well so in a b scan you do a horizontal
  • 00:38:01
    macular scan with the probe Mark present
  • 00:38:04
    um directed nely and the axle length
  • 00:38:08
    will be measured from the coral vertex
  • 00:38:10
    up to the maula over here what you see
  • 00:38:12
    is the optic nerve void so you take a
  • 00:38:15
    reading 4.5 mm below the optic nerve
  • 00:38:18
    void and this is how you can get the
  • 00:38:20
    axile length measure measurement in a
  • 00:38:22
    case of posterior stfy Lomas now we have
  • 00:38:25
    videos on B scan uh
  • 00:38:28
    B scan ultrasound as well on our channel
  • 00:38:31
    the link is going to be provided in the
  • 00:38:33
    description box now let us talk about
  • 00:38:35
    some of the advantages of an immersion
  • 00:38:37
    scan over the contact scan so here it is
  • 00:38:41
    more accurate than a contact scan
  • 00:38:43
    because the coral compression is
  • 00:38:45
    something which is avoided moreover you
  • 00:38:47
    will note that the immersion method
  • 00:38:50
    axile length are about 0.1 to 0.3 mm
  • 00:38:53
    longer than the contact method because
  • 00:38:55
    of course the uh the compression of the
  • 00:38:57
    cornea is avoided moreover the probe and
  • 00:39:01
    the corneal spikes are separate so you
  • 00:39:02
    can study the corneal spikes work the
  • 00:39:04
    cornea has two spikes over here okay
  • 00:39:08
    that is one from the endothelium one
  • 00:39:09
    from the epithelium it's a faster method
  • 00:39:12
    and moreover it reduces the technician
  • 00:39:14
    dependency now it doesn't mean that the
  • 00:39:17
    contact biometer is useless actually
  • 00:39:21
    speaking in the in good hands it
  • 00:39:23
    actually can give you really good values
  • 00:39:25
    and really comparable values but when
  • 00:39:27
    the technician dependency when the
  • 00:39:29
    technician is not as experienced and
  • 00:39:31
    therefore in those cases immersion scan
  • 00:39:33
    and an optical biometer is much better
  • 00:39:37
    choice all right so that's all for today
  • 00:39:39
    I hope you enjoyed the video if you did
  • 00:39:42
    don't forget to subscribe to our Channel
  • 00:39:44
    it really makes a difference and that's
  • 00:39:46
    all thank you and have a nice day
标签
  • biometry
  • ophthalmology
  • ultrasound
  • intraocular lens
  • cataract surgery
  • axial length
  • corneal power
  • anterior chamber depth
  • biometers
  • piezoelectric effect