A-scan biometry is an ultrasound test used to measure the length of the eye, which is important for determining treatments for sight disorders. It works by emitting a sound beam into the eye and measuring the echoes that bounce back from different structures. The measurements of axial length, corneal curvature, and estimated lens position are used to calculate the ideal intraocular lens power needed after cataract surgery. Accuracy is important as even small errors in measurement can significantly impact the calculated lens power. Different formulas exist to relate the biometry measurements to the appropriate lens power, with newer regression formulas found to be most accurate.

1. A SCAN BIOMETRY
Mahantesh.B HOD of optometry

2. A SCAN Biometry
 A-scan ultrasound biometry, commonly
referred to as an A-scan, is routine type of
diagnostic test used in optometry or
ophthalmology.
 The A-scan provides data on the length of the
eye, which is a major determinant in common
sight disorders

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 In A-scan, thin, parallel sound beam is emitted
from the probe tip, with an echo bouncing back
into the probe tip as the sound beam strikes each
interface
 An interface is the junction between any two
media of different densities and velocities.
 Anterior corneal surface
 Aqueous/anterior lens surface
 Posterior lens capsule/anterior vitreous
 Posterior vitreous/retinal surface
 Choroid/anterior scleral surface.

4. calculation of IOL
 the process of measuring the power of the
cornea ( keratometry ) and the length of the
eye, and using this data to determine the ideal
intraocular lens power.

5. Ultrasound Principles
 Sound is defined as a vibratory disturbance
within a solid or liquid that travels in a wave
pattern
 When the sound frequency is between 20
hertz (Hz) and 20,000 Hz, the sound is audible
to the human ear.
 In ophthalmology, most A-scan and B-scan
probes use a frequency of approximately 10
million Hz (10 MHz) that is predesigned by the
manufacturer

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 This meets unique needs, because at times,
the probe is placed directly on the organ to be
examined, and its structures are quite small,
requiring excellent resolution.
 The velocity of sound is determined
completely by the density of the medium
through which it passes.
 Sound travels faster through solids than
through liquids, an important principle to
understand because the eye is composed of
both

7. calculate IOL Power required
 Accurate Corneal power (keratometry)
 Actual axial length
 Accurate estimated lens position (half a mm
shift in lens position can have a dramatic effect
on final vision)
 A good understanding of the various IOL
power calculation formulas is also required.

8. Keratometer
 A keratometer, also known as an
ophthalmometer, is a diagnostic instrument for
measuring the curvature of the anterior surface
of the cornea, particularly for assessing the axis
of astigmatism.

9. Keratometery
 Keratometry by Manual
 Topography
 Autokeratometer
 IOL master/ Lenstar 900

10. Source of keratometry errors
 Unfocused eye piece
 Failure to calibrate unit
 Poor patient fixation
 Dry eye
 Drooping eye lids
 Irregular cornea
 Contact lens user

11. Repeat Keratometery
 If Corneal curvature more than 47D or less than
40D.
 The difference in corneal cylinder is more than
one dioptre between eyes.
 The average keratometry (K1) 43.0(K2) 44.0D,
with one eye typically within 1D of each other.
 Difficult Situations
 Post Refractive Surgery
 Corneal Transplantation
 Corneal Scar
 Keratoconus etc.

12. REMEMBER
 Average Axial Length of Normal Eye 23.06 mm
 Majority 22.0 to 24.5 mm
 Error of 0.4mm in the measurement of axial
length may result in a one Dioptre change in
calculated IOL power
 Difference in AL measurement Between both
eyes +/- 0.3 mm

13. Method
 Contact –
Applanation
Method
Hand-Held Method
 Immersion

14. Applanation Method
 By the Applanation biometry method, an
ultrasound probe is placed directly on the
cornea, with attached slit lamp

15. Hand-Held Method

16. PROCEDURE
 Explain the procedure
 Use topical Anaesthesia
 Clean the probe
 A probe is placed on the patient’s cornea.
 The probe is attached to a device that delivers
adjustable sound waves.
 The measurements are displayed as spikes on
the screen of an Visual monitor .
 The appearance of the spikes and the distance
between them can be correlated to structures
within the eye and the distance between them.

17. Probe positioning
 The probe lightly touches the
cornea and is positioned, such
that the barrel of the probe is
aligned with the optical axis or
visual axis of the eye.
 The operator aims the probe
towards the macula of the eye.
 Alignment with the optical axis
will be indicated by high lens
spikes and a high retina spike on
the scan graph.

18. Corneal Compression
 If pressure is applied on the cornea, the axial
length measurement may be falsely too short.
 It can be monitored by observing the anterior
chamber depth, read out by an instrument.
 Most eyes will have an ACD readings between
2.5 to 4.0mm.
 The corneal compression error factor can be
avoided by using the immersion technique
 Error caused by 1 mm Corneal Compression
Average eye 2.5 D Long eye 1.75 D Short eye
3.75 D

19. Immersion
 Immersion A-scan Biometry • The immersion
technique requires the use of a Prager Scleral
Shell

20. Immersion A-scan Biometry
 The immersion technique is accomplished by
placing a small scleral shell between the
patient's lids, filling it with saline,
 immersing the probe into the fluid, being
careful to avoid contact with the cornea.
 More accurate than contact method because
corneal compression is avoided.
 Eyes measured with the immersion method
are, on average, 0.1-0.3 mm longer

21. formulae
 Theoretical formulae
 Regression formulae

22. Theoretical formulae
 Various formulae derived from the geometric
optics using schematic eye
 These formulae are based on 3 variables
1Axial length of the eye ball(AL)
2 keratometry reading(K)
3 estimated post operative AC depth(ACD)

23. Binkhorest formula
P=1336(4rd)/(a-d)(4rd-d)
P = IOL power in dioptrs
r = corneal radius in mm
a = axial length in mm
d = assumed post operative anterior chamber
depth+ corneal thickness

24. Regression formulae
 This formulae is based on regression
analysis of the post operative results of
implant power using variable of corneal
power and AL
 The commonly used SRK Formula and its
modification
 It was introduce by Sanders, Retzlaff and
Kraft
 Its based on the regression analysis
taking into account the retrospective
computer analysis of a large number of
post operative refraction

25. SRKl
P= A-(2.5L-0.9K)
P=IOL power
A= constant specific for each lenses
L= axial length
K= average keratometry in dioptrs

26. SRK ll
P= A-(2.5L-0.9K)
But A is modified on the basis of the axial length
If L is <20 mm then A+3.0
If L is 20-20.99 mm then A+2.0
If L is the 21-21.99 mm then A+1.0
If L is the 22-24.0 mm then A
If l is >24.50 = A-0.50

27. SRKT FORMULA
 It is regression formula empirically optimized
for post refractive ACD Retinal thickness and
corneal refractive index
 This combines the advantages of both the
theoretical and empirical analysis
 Significantly more accurate for extremely long
eyes

28. THANK YOU