1.ocular biometry (full) segi u

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Ocular Biometry

(Week 1)

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23 January 2014

PowerPoint® Slides

by Foo Say Kiang

Learning Objectives

• To understand the basic optic principles of the

instruments used to measure the parameters in the eye

• To learn how to operate the instruments and the purpose

of using the instruments

OPT3024 Ocular Optics I

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Learning Outcomes

• Recall the general purpose and clinical utility of a variety

of biometric techniques.

• Describe the basic principles of the biometric technique

• Understand and able to interpret the readings obtained

through the biometric instruments and use it for further

investigations


Subtopics

• Keratometry

• Pachometry & phakometry

• Ultrasonography

• Interferometry

• Pupillometry


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Keratometry

• For measuring the radius of curvature of the anterior

surface of the cornea

• Jesse Ramsdenm, an optical instrument maker

invented keratometer in 1769

• Helmholtz improved Ramsdem’s design and

developed an instrument similar to the manual

keratometer used today


Keratometer


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• Has variety clinical uses: ▫ Used in fitting contact lenses

▫ Serve as an objective method of monitoring corneal changes in

anomalies such as keratoconus

▫ Used to measure corneal astigmatism, can be used to predict the

total astigmatism of the eye (C = 1.25A – 0.50)

▫ Also used in research to evaluate the contribution of the cornea to

refractive development of the eye

▫ Can be used to check the radii of curvature of both hard and soft

contact lenses


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• The basic components of a keratometer are

a) an object to be reflected from the cornea

b) a lens system to give the examiner a

magnified view of the reflected image

c) a system to keep the reflected image in

focus

d) a system to measure image size



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• Figure 5.4


Optics of the Keratometer

• The size of a reflected image is a function of the radius of the curvature of the surface from which it is reflected

• This relationship can be determined by finding the magnification (m), which is the ratio of image size to object size (h’/h)

• Newton’s equation states that the magnification of a reflected image is equal to the focal length of the reflecting surface divided by the distance (x) from the object to the focal point, f/x


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• Figure 5.1


• The distance between the object and the anterior surface of the cornea is quite long relative to the focal length of the anterior corneal surface

• The virtual image formed by reflection from the anterior surface of the cornea is very close to the focal point (F) of the corneal surface

• d, the distance from the object to the image formed by reflection from the cornea, is close approximation to x

h’/h ≈ f/d


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• because the focal length of a mirror is equal to the

radius of the curvature divided by two,

h’/h ≈ (r/2)/d

r = (2d)(h’/h) = 2dm

r = 2dm


• d is constant as long as the image reflected from

patient’s cornea in focus

• h also constant

• r can be determined by measuring h’ (fixed mire

keratometer)

• r = (2d)(h’/h) = 2dm

• r α h’


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Measurement of the Reflected Image • A relatively large luminous circular object is

placed in front of the eye

• Reflection of this light from the anterior

corneal surface produces a first Purkinje

image , which is a greatly minified virtual

image circle located behind the cornea


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Measurement of the Reflected Image

• Keratometers use a telemicroscope to view the image

• An objective lens projects a real image of the virtual

image between the objective lens and the observer


• This image is then observed with an eyepiece

• The eye undergoes constant small involuntary movements, keratometer image also moves, even though the movements are small, they are large relative to the size of the Purkinje image

• To overcome the difficulty of measuring a moving target, keratometers employ a doubling principle

• Part of the image beam that travels through the keratometer to intercepted by a prism and is deflected. Another part of the beam bypasses the prism and is not deflected


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• Figure 5.2


• As the eye moves, both images still move, but they move together so that their separation remains constant

• If the amount of image separation is varied until it equals the image size, the image size could be calculated by noting the prism necessary to do this

• Instead of use prism of different powers to change the image separation, movement of one prism along the optical axis of the keratometer is used to vary effective prism power


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• The prism is moved until the two images

just overlap on one border

• The image dimension in the direction of

doubling is then equal to the amount of

deflection

• The keratometer dial that moves the prism

could then be calibrated to indicate the

amount of prism movement

• Calculations are avoided by calibrating the

dial to read the radius of curvature of the

cornea directly


• Different keratometers employ either variable or

fixed doubling

• Mostly variable doubling

• In instruments that have fixed doubling, object

size is varied to obtain a set criterion image size


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• Figure 5.4


• One pair of holes, oriented horizontally

isolates portions of the beam so that they go

separately

• The other pair of holes, oriented vertically

forms a Scheiner’s disc, used to assist in

focusing --- allows a single image to be seen

when the mires are in focus, but two images –

more sensitive than blur that results from

defocus


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• Examiner’s view of the reflected pattern

• Out of focus, rotate the control knob, changes the

distance of the objective lens from patient’s eye until the

images merge



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• Next adjusts the instrument for the proper axis of the astigmatism by rotating the entire optical assembly until the axes of the plus and minus signs that flank the prism-doubled mires line up with the central cross

• Then the examiner turns the two knob that move the doubling prisms in order to superimpose the two plus signs in the lower right and lower left images, and minus signs in the lower right and the top images



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• When alignment is achieved the scales on the

knobs yield the dioptric keratometer powers

• Two doubling prism in B&L keratometer with axes

90 degrees to one another allows both meridians to

be measured while the keratometer is rotated to

only one meridian


Calibration index

• The radius is the more direct measure

because it is found by using the keratometer

equation

• The total corneal power may be calculated by

modeling the cornea as a single refractive

surface whose radius is the actual anterior

corneal radius as determined by the

keratometer

• Most keratometers use a calibration index of

1.3375, although some use calibration index

of 1.336 or 1.332 OPT3024 Ocular Optics I

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If the cornea of the Gullstrand schematic eye No. 1

were examined with a keratometer with a calibration

index of 1.3375, the dioptric power read from the

instrument would be:

The dioptric power given by a keratometer is only an

estimate of the total dioptric power of the cornea

Bausch & Lomb Keratometer is the most widely used keratometer

Varies the amount of doubling but the object size constant


D 83.43m 0077.0

00.13375.1'

r

nnF

Area of the Cornea Measured

• Corresponds to the distance on the cornea between

the location where on the cornea between the

locations where the two plus signs and between

where the two minus signs are projected onto the

cornea

• The separations of these corneal regions vary

somewhat with the radius of the curvature of the

cornea


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• B&L keratometer – the separation between the

corneal points is between about 3.0 and 3.2 mm for

the most corneal radii

• This area is referred to as the corneal “cap”

• The validity of the keratometric values for the points

within the cap are uncertain and the validity for points

outside the cap are unknown


Corneal Astigmatism

• The meridian of the eye are expressed in degrees

from 0 to 180 degrees

• The meridians of greatest and least power are

called the principal meridians

• These meridians are generally 90 degrees apart –

regular astigmatism


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• With the rule (WTR) - greatest power (steepest) within 30 degrees of the vertical (90 degrees) meridian

• Against the rule (ATR) – greatest power within 30 degrees of the horizontal (180 degrees) meridian

• Oblique – one principal meridian is between 31 and 59 and the other is between 121 and 149

• Cylinder lens --- corrects the astigmatism


• The meridian of maximum power is called the power

meridian and the meridian of zero power is called the

axis meridian

• Exp: -1.00 x 180, with the rule or +1.00 x 90

• -1.00 x 90, against the rule

• 44 D @ 90, 45D @ 180,

corneal astig. = -1.00 x 90 , ATR


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The cornea is the main contributor to the astigmatism of the eye but rarely total astig. of the eye exactly equal the amount of the corneal astig.

a) The optical axis of the cornea does not coincide with the line of sight of the eye

b) The posterior surface of the cornea and crystalline lens may have astigmatism

c) The crystalline lens may be tilted within the eye

d) Keratometry is measured at the cornea while correcting lenses are placed in the spectacle plane

e) Calibration index used in the keratometer differs from the index of refraction of the cornea

* The tilt of the cornea and the crystalline lens probably

account for the most


• Total astigmatism can be predicted from the

corneal astigmatism

• Javal’s rule:

Total astigmatism

= 1.25 (CA) + (-0.50 D x 90)

• Grosvenor and colleagues simplified Javal’s rule:

Total astigmatism

= CA + (-0.50 D x 90)


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Pachometry

• Also pachymetry

• Measurement of corneal thickness

a) Monitoring corneal edema induced by contact lens wear and

various corneal conditions

b) Determine the depth of cuts or ablation in refractive surgery


• Ultrasonic pachometry uses a probe that produces

sound at a frequency high above the range of human

hearing is placed on the cornea

• Ultrasound directed into the cornea echoes from both

anterior and posterior surface of the cornea

• The difference in echo times is then measured to

calculate the time taken for the ultrasound to traverse

the cornea


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• Using a known sound velocity for corneal tissue,

corneal thickness is then calculated

• Ultrasonic pachometers give more accurate

measurements of corneal thickness than optical

pachometry

• Optical pachometry can be performed with an

attachment to a slit lamp biomicroscope but

ultrasonic pachometry needs a separate instrument


• Optical pachometry masks the illumination beam to form a slit of light so that only a cross section of cornea is illuminated

• The slit beam passes through two glass plates, a bottom plate is kept perpendicular to the observation axis and an upper plate can be rotated about a vertical axis

• When upper plate is rotated, the image of the upper corneal section is displaced horizontally relative to the lower corneal section


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• Figure 5.15


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• When turns the plate enough to align the back

surface of the upper half image of cornea with

front surface of the lower half image of the

cornea, the amount of displacement equals

the projected depth of the beam through the

cornea.

• The amount of rotation of the upper glass

plate yields a measure of the lateral projected

thickness (D) of the cornea. l ‘, is the apparent

thickness of the cornea, the distance from the

anterior corneal surface to the image of the

posterior corneal surface


• Apparent thickness, l’’ can be calculated from D and

θ

• The actual thickness l can be calculated from

apparent thickness


sin/' D

Fnn

FLL

'/'/

'

n = the index of refraction of the cornea

n’= the index of refraction of air

F = the refractive power of the anterior surface of the cornea

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• Figure 5.16



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Phakometry

A technique that employs direct observation or photography of the Purkinje images to measure the radii of curvature of the anterior and posterior surface of the crystalline lens

Two primary methods of phakometry: Tscherning’s method of ophthalmophakometry and comparison phakometry

Both based on the principle that the magnification of a reflected image is proportional to the radius of curvature of the reflecting surface


Phakometry


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• Several assumptions are made:

a) Because the Purkinje image II is very

difficult to see and photograph, the cornea is

assumed to be a single refracting surface

with its refractive power equal to the

keratometer power

b) Index of refraction values for the ocular

media are given standard values

c) The crystalline lens is treated as a

homogeneous medium with a single index of

refraction


d) The ocular refracting surfaces are assumed to

be spherical rather aspheric

e) The eye is assumed to be coaxial system with

the centers of curvature of the refracting

surfaces falling on a single line


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Comparison Phakometry

• A photograph is taken of the Purkinje images of a pair of lights

• The separation of the pair of lights is measured for each of the Purkinje images

• The apparent radius of a surface is the distance from the surface to the point where an image would be formed of the center of curvature of that surface



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• The apparent radius of curvature of the crystalline

lens surface is determined by :

• The apparent radius of curvature of the posterior

crystalline lens surface is determined in the same

way:


corneaanterior , curvature of radius

lensanterior radius,apparent

I Purkinje of size

III Purkinje of size

corneaanterior curvature, of radius

lensposterior radius,apparent

I Purkinje of size

IV Purkinje of size

• Using the solved values for the apparent radii

of curvature, the locations of the apparent

centers of curvature can be determined.

• Then the location of the actual centers of

curvature can be determined by where the

actual centers of curvature as objects would

have to be located in order to be imaged at

the locations of the apparent centers of

curvature

• Finally, the actual radii of curvature are the

distances from the actual surfaces to their

respective actual centers of curvature OPT3024 Ocular Optics I

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• The Gullstrand-Emsley schematic eye is used

• Radius of curvature of the anterior corneal surface & total power of cornea are obtained from keratometry

• The actual centers of curvature of the anterior and posterior lens surfaces are located by raytrace using the locations of the apparent centers of curvature as images



Refer to appendix 5.1 pg 132 – 134 (Goss DA et al.)

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• Anterior chamber depth and lens thickness must be

obtained from some method such as ultrasonography

• The actual radii of curvature are the distances

between the actual crystalline lens surfaces and their

actual centers of curvature


Tscherning’s Ophthalmophakometry

• Has been largely replaced by comparison

phakometry

• Methods of measurement was different

• Separation of the pair of lamps that produced

Purkinje image I was varied until two corneal

reflections had the same separation as the two

reflections from anterior and posterior crytalline lens

surfaces


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• The primary use for phakometry is for the study

of the contribution of the crystalline lens to

refractive errors

• Purkinje images have also been used to study

crystalline lens changes in accommodation and

night myopia


Ultrasonography

• Measurement of the distances between ocular surfaces

• Ultrasound is an acoustic wave with a frequency higher than the human audible range (human eye can detect tones with frequencies from 20 to 20,000 cycles per second)

• An ultrasound wave is produced by a transducer through the application of an alternating electrical current to a piezoelectric crystal


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• The crystal vibrates at a frequency that

matches the frequency of the driving current

• Transducer directs bursts of ultrasound pulses

into the eye at about one thousand bursts per

second

• The intervals between bursts are used to

register the echoes coming back into the

transducer after being reflected from surfaces

within the eye

• The crystal produces electrical energy when it

is mechanically vibrated by the returning

ultrasound waves


• The time between the echoes returning from two axially

separated tissue interfaces is measured, and converted

into distance measurements using known velocities of

ultrasound in the ocular tissues

d = (V)(t/2)


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• The range of ultrasound frequencies that have been used for various ophthalmic applications is about 5 million to 25 million cycles per second (megahertz)

• The frequencies used in ultrasonic pachometry cannot be used to measure axial length because attenuation is too great (attenuation is a progressive loss of ultrasound amplitude as a result of scattering and absorption)



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A-scan Ultrasonography

• Amplitude modulation, unidimensional display of the

amplitude of echoes

• Spikes on an oscilloscope trace correspond to echoes

from the cornea, the anterior and posterior surfaces of

the crystalline lens and the retina

• The main application of A-scan is the measurement of

intraocular distances



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• Intraocular distances are determined using

the formula for distance as a function of time

and velocity

• Commercially available ultrasound units will

give measurement for the distance from the

anterior surface of the cornea to the anterior

surface of the crystalline lens ( referred to as

anterior chamber depth), crystalline lens

thickness, and vitreous chamber depth.

• The sum of these will be the axial length of

the eye


• Used in studies of the relationship of the ocular

optical components to refractive error and in

calculations to predict the best lens implant

power for replacement of a crystalline lens

removed in cataract surgery

• It is important to align the ultrasound probe as

close as possible with the line of sight of the eye

• If not, the axial length will be too high

• Another error is press against the globe with the

probe rather than just touch the surface of the

cornea – give low measures of anterior

chamber depth and axial length OPT3024 Ocular Optics I

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B-Scan Ultrasonography

• Brightness mode or intensity modulation

ultrasonography

• Two dimensional, cross-sectional

representation of the eye

• The brightness of each spot in B-scan image is

proportional to the ultrasound energy reflected

from the corresponding tissue surface

• B-scan can be used to evaluate portions of the

eye that may be obscured from visual

inspection by media opacities



Video : How Does an A-Scan Become a B-Scan?

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• The primary uses of B-scan ultrasonography are the

detection and localization of conditions such as

intraocular tumors, retinal detachment, vitreous

hemorrhage, and other intraocular tissue anomalies,

also can be used for examination of orbital

conditions



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Partial Coherence Interferometry

• Uses interference of light to measure axial lengths in

the eye

• Has the potential to achieve measurements ten times

as precise as ultrasouond

• Zeiss IOLMaster

• Intrared light from superluminescent diode is directed

through an interferometer



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• A Fabry Perot interferometer consisting of two

parallel, partially transmitting mirrors is used

• One component of the IR beam goes directly

through the two mirrors without reflection

• The other component is reflected twice by the

two mirrors before it rejoins the direct component


• Figure 5.19


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• The reflected component of beam travels an

additional distance that is twice the separation

(d) between the two mirrors

• With a diode light source the difference in

distances traveled exceeds the difference in

distance over which the light remains coherent

(only about 9µm) so interference fringes are not

seen

• The two components of the beam are directed

into the eye along its optical axis OPT3024 Ocular Optics I

• If the beams reflected from anterior and posterior cornea receive an opposite phase shift, the phase shift induced by the interferometer will be nullified and interference fringes will result.

• When fringes are sensed by the fringe detector, the thickness (t) of the cornea equals the separation (d) of the interferometer mirrors divided by the refractive index of the cornea,

• t = d/n


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• Partial coherence interferometer has relatively high

order of dioptric resolution for axial length ( 0.03D)

• non-contact device with up to 0.01mm for axial length

measurements

• It takes approximately 0.4 sec for a measurement


Pupillometry

• Measurement of apparent pupil diameter

• Measure of the image (the entrance pupil) of the anatomical pupil as seen through the cornea, which functions as a magnifying lens

• The apparent pupil is about 12 % larger and 0.5 mm closer to the cornea than the anatomical pupil is

• Differences in the pupil size between the two eyes or anomalies in the pupil reaction to light or accommodation can be signs of ocular or neurological disease


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Simple Methods

• Holding ruler in front of the pupil

• Compare the pupil in question to a series of black half-

circles on a ruler (Pupil gauges in Rosenbaum cards)


• Broca’s pupillometer – uses two pinhole to

measure one’s own pupil diameter

• When two pinholes are held close to the eye, they

form blur circles on the retina.

• The edges of the two blur circles appear to just

touch when the separation of the two pinholes

equals the pupil diameter


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• Doubling methods, such as biprism also use same principle

• Ruler and comparison gauge pupil measurement is subject to variety of errors: intraobserver variability, lack of standardized illumination control, patient accommodation, papillary unrest, and difficulty visualizing the pupil under the true scotopic conditions


Instrumentation Methods

• Infrared can avoid the activation of the papillary reflex

• Infrared pupillometer: ▫ Colvard pupillometer

▫ Pupilscan 2

• Colvard pupillometer superimposes a millimeter scale

over a view of anterior segment, allowing examiner to

easily measure the pupil size to an accuracy of

approximately 0.5 mm


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• Pupilscan 2 provides a digital readout of the pupil

diameter to the nearest 0.1 mm derived from a

pixelized liquid crystal display of the pupil and iris

• The number of dark pixels is counted by a

microprocessor, and using a conversion factor, the

diameter of the pupil extrapolated


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• Some systems deriving pupil diameter from the amount of a broad beam of infrared being reflected or monitoring whether a small infrared scanned across the pupil and iris is reflected

• Dynamic bilateral infrared pupillometer – Procyon P2000SA

• Consists of dual eyepieces with eyecups to completely control illumination coupled to an infrared camera and a computer interface


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References

• GOSS DA & WEST RW (2002) Introduction to

the Optics of the Eye. Boston: Butterworth-

Heinemann.

• RABBETS, R (1998) Bennett & Rabett’s Clinical

Visual Optics. 3rd ed. Oxford: Butterworth-

Heinemann

• HENSON DB (January 1996) Optometric

Instrumentation. 2nd ed. Butterworth-Heinemann


Learning Outcomes

• Recall the general purpose and clinical utility of a variety

of biometric techniques.

• Describe the basic principles of the biometric technique

• Understand and able to interpret the readings obtained

through the biometric instruments and use it for further

investigations


1.ocular biometry (full) segi u

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