This document discusses various optical and non-optical methods of measuring axial length of the eye. It begins by defining axial length and noting its importance in intraocular lens power calculations. It then describes ultrasonic (A-scan) biometry, the historical standard, and optical biometry techniques like partial coherence interferometry used in devices like the IOLMaster 500. Key advantages of optical techniques are discussed as well as limitations of ultrasound. Details are provided on performing both immersion and non-immersion ultrasound techniques and interpreting the results.

1. Optical and Non-optical
Methods of Measuring Axial
Length
Rabindra Adhikary
M. Optom, 1st Batch
Tilganga Institute of Ophthalmology
Pokhara University
Facilitator:
Rupesh Poudel
14th August 2019

2. Axial Length
• The distance from the anterior surface of the cornea to
the RPE1
• Axial length changes rapidly in the first 18 months of
life2
• Birth: 16.8 mm
• 12 months: 20 mm
• For every 1mm error in measuring axial length  3D
error in post-op refraction
1Clinically relevant biometry. Curr Opin Ophthalmol. Sahin A, Hamrah P.
• https://www.ncbi.nlm.nih.gov/pubmed/22081032
2Axial length in apparently normal pediatric eyes. Hussain RN, Shahid F, Woodruff G.
• https://www.ncbi.nlm.nih.gov/pubmed/23787457

3. • Mean Axial Length of Normal adult Eye1
• 23.67 mm
• Mean Axial Length of Cataractous Eye2
• 23.65 mm
• Diurnal Fluctuation occurs in axial length by3
• 15-40μ
1. Axial length and optic disc size in normal eyes
C Oliveira, N Harizman, C A Girkin, A Xie, C Tello, J M Liebmann, and R Ritch
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1857588/
2. Biometry of 7,500 Cataractous Eyes
J. Hoffer, MD
https://www.sciencedirect.com/science/article/pii/S0002939414749177
3. Diurnal Axial Length Fluctuation in Human Eyes
Richard A. Stone; Graham E. Quinn; Ellie L. Francis; Gui-shuang Ying; D. Ian Flitcroft; Parag Parekh
https://iovs.arvojournals.org/article.aspx?articleid=2123886

4. Axial Length Comparison in different age groups
Age
(years)
Emmetropia Ametropia
Myopia Hypermetropia Astigmatism
Male Female Male Female Male Female Male Female
0-10 22.28 21.93 22.81 22.82 21.22 21.29 22.32 22.03
SD 0.50 0.67 1.10 1.68 0.21 0.31 0.24 0.32
11-20 23.23 23.42 24.07 24.18 21.97 21.71 23.13 23.06
SD 0.48 0.46 0.67 0.40 1.09 0.56 1.01 1.07
21-40 22.71 22.99 23.58 23.56 22.06 22.01 23.23 22.16
SD 1.21 0.91 1.65 2.53 0.90 1.28 2.26 1.39
>40 22.33 22.99 23.64 23.95 21.68 21.61 22.06 21.5
SD 0.49 0.71 1.69 1.72 0.35 0.47 0.49 0.26
Axial Length, Anterior Chamber Depth-A Study in Different Age Groups and Refractive Errors
Veena Bhardwaj and Gandhi Parth Rajeshbhai
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3843406/

5. Ultrasonic Biometry ( A scan)
• Biometry=applying mathematics to biology, e.g.,
fingerprint biometry,
• From late 1960s
• Utilizes ultrasonic waves emitted by the transducer
• Frequency: 10 MHz
• The waves travel from the anterior corneal surface to
the vitreo-retinal interface and back, which is picked up
by the probe and the axial length is interpolated from
the time taken for the travel. The axial length is
measured in mm from the anterior cornea to the
Internal Limiting Membrane (ILM)
• Accuracy: ±0.1 mm (in experienced hands and a
relatively emmetropic eye)*
*http://ksos.in/ksosjournal/journalsub/Journal_Article_35_565.pdf

6. • utilizes the principle of signal reflection to measure the
difference between the various ocular structures and
the overall length of the eye.
• After the sound waves exit the transducer,
• they encounter each acoustic interface within the eye and
produce a series of echoes that are received by the probe.
• The time a signal is reflected back from an interface is
measured and divided by two and multiplied by speed
of signal in the corresponding medium.
• Formula
• Distance = speed X time/2
speed of sound
• Aphakic eyes: 1532 m/s
• Cataractous eye: 1550 m/s

7. Echogram peaks of phakic eye
a) Probe tip/cornea,
b) Aqueous fluid/anterior lens
c) Posterior lens/vitreous,
d) Vitreous/retina,
e) Retina/sclera,
f) Sclera/orbital fat.
X-axis:
Distance between ocular surface
Y-axis:
Magnitude of each echo
returned
https://link.springer.com/chapter/10.1007/3-540-30796-6_4

8. • Anterior lens echo is 90%
or more of maximum
height
• Posterior lens echo is
between 50%- 75% of
maximum
• Retina echo is 75% or
more of maximum
• scleral echo –well
identified
• Orbital fat- should
descend quick

9. • The amplitude of the peak depends upon:
• Difference in the density at the acoustic interface
• Greater difference produce higher echoes
• The angle of incidence at the interface
• Perpendicular waves produces maximum height of spike
• Measurement of A scan:
• Spike Height
• Regularity
• Reflectivity
• Sound attenuation

10. Applanation technique
• the ultrasound probe
• in direct contact with the cornea., centrally at the
pupilary area
• Probe  perpendicular
• Cornea to be anesthetized
• Inadvertent pressure may cause upto 0.3mm
corneal indentation
• Error of 1-1.5D  so getting obsolete

11. • A number of readings are taken either manually or in
auto-mode
• Readings from improper spikes—removed
• Average taken in the value of axial length
• Accuracy: ± 0.10
• Inter-eye variation: ± 0.3 mm
Holladay JT. Ultrasound and optical biometry. Cataract Refract Surg Today
Eur 2009.
https://crstodayeurope.com/wp-content/themes/crste/assets/downloads/1109_03.pdf

12. Anatomic vs Optical AL
• Ultrasound measures
Anatomic Axial
Length
• Ant cornea – ILM
• Photoreceptors Layer
lies 200μm (160-400
μm) beyond ILM1
• Need to add 200μm to
get Optical Axial
Length(OAL)
• 200 μm ≈0.56D error2
• Optical Methods
measure OAL
2https://www.nature.com/articles/6700157
1https://www.ncbi.nlm.nih.gov/pubmed/25473345

13. Immersion technique
• Here a waterbath is placed on the cornea using a
scleral shell(eg. Praeger Shell). The hard tip
transducer is applied to the shell. The rest of the
procedure is similar to the contact method.
• If done properly, immersion technique gives very
accurate and reproducible results in all types of
cataracts.

14. Immersion Technique
• Unlike the applanation echogram, the immersion
technique produces an additional spike probe tip within
the coupling fluid.
• The patient lies supine, looking up at the ceiling and the
scleral shell is placed between the eyelids and centered
over the cornea.
• The scleral shell is then filled with coupling gel (40-60
mixture of Goniosol and Dacriose)
• The probe tip is placed into the solution.
• Align the ultrasound beam with the macula by having
the patient look at the probe tip fixation light, then
simply take your readings as usual.

15. Immersion Technique

16. https://www.researchgate.net/publication/293800665_Comparison_of_ocular_biometry_measurements_b
y_applanation_and_immersion_A-scan_techniques
• compared ocular biometry values using applanation
and immersion techniques
• The study was a prospective cross-sectional
comparative study of different techniques of ocular
biometry from diagnostic equipment (biometry probe
10 MHz Sonomed A-scan (PACSCAN 300A, USA).
• Measurement variables were obtained among children
and adults undergoing cataract surgery.
• Scleral (Prager) shell was used for the immersion
technique followed by the contact technique by the
same examiner

17. RESULTS
• The means of the axial lengths biometry values
with immersion and contact technique were
23.66(±1.36) and 23.46 mm (±1.46)
• the axial length differences was 0.2 ±0.26 mm
(range 0.0-0.94 mm) and statistically significant
(95% CI, p =0.000).
• The Standard deviation SD(mm) of Individual Eye
Axial Length showed a mean of 0.03 ±0.04 (0.0-0.3
) mm for immersion and 0.14 ±0.12(0.0-0.6)mm for
contact technique.

18. Errors in Axial Length
Measurement with Ultrasound
Inaccurate assumption of speed for cataractous eyes
Indentation of the cornea (A scan)
Improper equipment calibration
Failure to recognize the appropriate pattern of echoes
The anatomic thickness of the retina may also cause errors in IOL
power calculation
Axial length measured in reference to anatomic axis not visual axis
(macula is located temporally)
•optical axis is tilted approximately 5° horizontally and 1° vertically
relative to the anatomic axis

19. Drawbacks of Immersion Technique
• Time consuming, messy, expensive, risk of contamination

20. • Recommendations: The shell and probe should be
soaked in alcohol or hydrogen peroxide for at least
5 minutes. The immersion shell should be allowed
to dry completely and flushed with balanced saline
solution (BSS).

21. Optical Methods of measuring axial
length
• In 1990s, alternative to ultrasound biometry introduced
• Also called Optical Coherence Biometry
• Principle:
• Partial Coherence Interferometry (PCI)
• Low Coherence Interferometry (LCI)
• Swept-source OCT (SS-OCT)
• Gold standard since last 20 years
• Increased operator independence
• Greater accuracy
Accuracy : ± 0.012 mm*
*Holladay JT. Ultrasound and optical biometry. Cataract Refract Surg Today Eur 2009.
https://crstodayeurope.com/wp-content/themes/crste/assets/downloads/1109_03.pdf

22. Interference..(explained by Video)

23. Interferometer
• Two-beam interferometers (4 types)
• The Fizeau Intearferometer
• The Michelson Interferometer
• used in optical biometers
• The Mach-Zehnder Interferometer
• The Sagnac interferometer
Interferometers. P . Hariharan Division of Applied Physics CSIRO Sydney , Australia
http://www.photonics.intec.ugent.be/education/IVPV/res_handbook/v2ch21.pdf

24. M=0,1,2,3,…
L1 changes continuously, L2 is fixed
Therefore this instrument allows you to observe distance changes
of the order of the wavelength of light.

25. Source of Infrared
Mirror
Photodetector

27. Partial Coherence Interferometry

28. IOLMaster 500
• Partial Coherence Interferometry
• FDA approved in 2000 AD
(Carl Zeiss Meditec AG)

29. IOLMaster 500
• First optical biometer introduced in 1999
• 780nm laser diode infrared light is used
• Measures keratometry analyzing the anterior corneal
curvature at 6 reference points at approx. 2.3mm optical
zone
• The ACD is measured using slit-lamp illumination and is
defined as a distance from the corneal epithelium and to
the anterior lens surface
• WTW is obtained by analyzing the image of the iris using an
infrared light source (wavelength 880 nm)
• All measurements are performed simultaneously.
• IOLMaster 500 is currently considered as a gold-standard
biometer.
https://www.ncbi.nlm.nih.gov/pubmed/9780097

30. ALM
• Switching to ALM mode will
automatically change the
magnification ratio: a smaller section
of the eye becomes visible with the
reflection of the alignment light and
a vertical line (1, Fig. 27).
• The patient should look at the red
fixation point in the center. A cross-
hair (3, Fig. 27) with a circle in the
middle will appear on the display.

31. • Ask the patient if he or she sees the fixation
point. If the patient fails to fixate properly, the
visual axis will not be correctly recognized, which
may result in measuring errors.
• poor visual acuity/high ametropia (> 4 D)?
• measure through the spectacles. If the procedure is
followed correctly, no measuring errors will be
produced.
• Measurements should not be taken while a patient is
wearing contact lenses, as it will result in measuring
errors.

32. • IOLMaster requires 5 measurements (max. 20 per
eye in a day)
• If a reading deviate from another by more than
0.05 mm, it will be displayed in red and the
message Multiple peaks will appear
• *Error in the display field denotes readings with an
SNR smaller than 1.6.
• The following considerations should be made:
• AL < 22mm (Wang-Koch Axial Length mod + Holladay1)
• AL > 25mm (Hoffer Q or Holladay 2 )
*https://www.ncbi.nlm.nih.gov/pubmed/21183100

33. • If the AL difference between RE and LE exceeds 0.3
mm, a message appears to check the readings once
again.

34. IOLMaster: Modes and Features
• Axial Length
• Corneal Curvature
• White-to-White (optional)
• Anterior Chamber Depth
• IOL Power Calculation
• Phakic (Anterior and Posterior Chamber) IOL Power
Calculation (optional)
• Lens Constant Personalization
• Post Refractive Surgery Corneal Power Calculation
(optional)
• Data Export (optional)

35. Limitation of IOLMaster 500
• To get the measurement, fixation is required.
• So, in dense cataract (VA<3/60) it is not possible to carry
out the biometry
• inaccurate measurements may result in cases of
other media opacities
• corneal opacity
• vitreous hemorrhage
• Nystagmus also may give rise to inaccurate readings
In these cases  Ultrasound Biometry

36. Measurement Range of
IOLMaster 500
Axial Length 14-40
Corneal Radii 5-10
ACD 1.5-6.5
White to white 8-16
Resolution Range
AL/CR/W-W 0.01 mm
Formula SRK II , SRK/T, Holladay, Hoffer Q, Haigis

37. 3 Advantages of optical method
over ultrasound method
1. Measures length along the visual axis
• cornea to fovea
2. Measures true distance that we want
• optical AL
3. Uses light instead of Sound
• The shorter the wavelength the more precise is the
measurement

38. • Lenstar LS 900 Optical Biometer
(Haag Streit)
• Optical Principle: OLCR
• Using a 820 µm super-luminescent diode as light
source
• allows the measurement of the AL, CCT, LT and
ACD
• The retinal thickness can also be determined
• requires subjective alignment of a cursor

39. • also uses 950 µm light to assess by image analysis
central corneal curvature using two rings of
diameter 1.65 mm and 2.30 mm of 16 light spot
each
• WTW and PS are obtained by fitting the best circle
with the lowest error square to the detected edge

40. NIDEK AL Scan Optical Biometer
• Optical Principle: PCI
• uses an 830 nm infrared laser diode.
• measures keratometry (K) at 36 measurement
points in two circles with diameters of 2.4 mm and
3.3 mm
• ACD and CCT are measured with an incorporated
Scheimpflug camera with a 470 nm monochromatic
light
• WTW and PS are obtained by analysing the image
of the iris edge

41. Oculus Pentacam AXL
• Introduced in 2015
• PCI
• Scheimpflug camera which rotates around the
eye
• AS tomography, ACD, CCT and WTW
measurements, corneal topography, anterior and
posterior corneal surface and spherical
aberrations  AL measurement is added
• Calculation of toric IOLs
• based on the total corneal refractive power taking into
account the influence of the posterior corneal surface.

42. Others
• Topcon Aladin Biometer
• Principle: OLCI
• Super-luminescent diode of 830 nm
• AL: 15-38 mm
• Built-in full Placido topography
• Point & shoot acquisition: all necessary measurements are
taken in under 5 seconds
• OA 2000 Optical Biometer
(Tomey Corp)
• Principle: Swept-source OCT
• Tomey Fourier domain A-scan technology
 measure almost all cases of dense cataract
• Cornea curvature: 5.0 ~ 11 mm
• AL: 14-40 mm

43. • PURPOSE:
• To investigate the relationship between optical biometry and
applanation ultrasound measurement of the axial length of
the eye.
• MATERIALS AND METHODS:
• This prospective study enrolled 55 (68 eyes) consecutive
patients scheduled for cataract surgery at Dhahran Eye
Specialist Hospital Every eye underwent two measurements
each with an optical biometer and with applanation
ultrasound. Only patients with cataract and no other
ophthalmic pathology or disease were enrolled. Comparison,
correlation and repeatability of axial length with both devices
were analyzed. Agreement between devices was evaluated. A
regression formula to convert measurements between
devices was investigated. A p value less than 0.05 was
statistically significant.
https://www.ncbi.nlm.nih.gov/pubmed/25473345

44. • RESULTS:
• There was strong repeatability (99.4%) and agreement
(r = 0.987) between both devices (p < 0.001); the
difference between devices was mainly in short eyes
(p = 0.031).
• CONCLUSION:
• Optical biometry and applanation ultrasound
measurements of axial length correlate well. However,
optical biometry is preferable in short eyes.

45. • Comparison of optical biometry and applanation ultrasound measurements of the axial length of
the eye, Fouad R. Nakhli, COMT
• https://www.ncbi.nlm.nih.gov/pubmed/25473345

46. Conversion Formula:
• The above paper gives us the conversion formula
• ALop = 0.015 + 0.996 X ALus

47. IOLMaster 700
• Swept Source Biometry
• Also called spectral-domain OCT
• Swept-source OCT scans sequentially through a series of
wavelengths to enhance the interference pattern, which
is then decoded with Fourier transformation into an A-
scan trace.
• Multiple adjacent A-scans can be combined to form a B-
scan image. Advantages of swept-source OCT include a
deeper range of imaging into the eye, less sensitivity
reduction with depth, and faster scanning speeds.
• Highly repeatable and accurate measurement upto date
• Full-eye length tomography
• Unusual eye geometires including lens decentration or tilt

48. • Performs a 1-mm central retinal scan to ensure
safety.
• Makes 2,000 scans/sec, Test time ≈ 50 sec
• The software’s telecentric keratometry provides
accurate corneal readings
• with its 950-nm light source, it penetrates the
sclera as well.
• Incredibly robust B-scan of almost any eye, with the
exception of white mature cataracts.

49. • uses OCT in 6 scan lines at 0, 300, 600, 900, 1200,
and 1500 to measure AL, ACD, lens thickness, and
CCT
• The K readings are calculated by analyzing the
anterior corneal curvature at 18 reference points in
hexagonal patterns at approximately 1.5, 2.4, and
3.2 mm optical zones
https://www.ncbi.nlm.nih.gov/pubmed/27531293

51. • Purpose To compare the measurements and failure
rates obtained with a new swept source optical
coherence tomography (OCT)-based biometry to
IOLMaster 500.
• Design Observational cross-sectional study and
evaluation of a new diagnostic technology.
• Methods 188 eyes of 101 subjects were included in the
study. Measurements of axial length (AL), anterior
chamber depth (ACD), corneal power (K1 and K2) and
the measurement failure rate with the new Zeiss
IOLMaster 700 were compared with those obtained
with the IOLMaster 500.
https://bjo.bmj.com/content/100/9/1201

52. • The differences between both methods were
assessed using the paired samples t test, and their
correlation was evaluated by intraclass correlation
coefficient (ICC).
• Results:
• The agreements between two devices were outstanding
regarding
• AL (ICC=1.0)
• ACD (ICC=0.920)
• K1 (ICC=0.992) and
• K2 (ICC=0.989)

53. • IOLMaster 700 was able to measure ACD AL, K1
and K2 in all eyes within high-quality SD limits of
the manufacturer
• IOLMaster 500 was able to measure ACD in 175
eyes
• Measurement not possible in 13 eyes
• ALM was not possible in 17 eyes with IOLMaster
500
• Nine of these eyes had posterior subcapsular cataracts
and eight had dense nuclear cataracts

54. conclusion
• Although the agreement between the two devices
was excellent, the IOLMaster 700 was more
effective in obtaining biometric measurements in
eyes with posterior sub-capsular and dense nuclear
cataracts.

55. • The purpose of this study is to compare the
predictive accuracy of intraocular lens (IOL)
calculations made with partial coherence
interferometry (PCI, IOLMaster, version 5) and
swept-source optical coherence tomography (SS-
OCT, Argos). Axial length (AL), mean keratometry
value (K), and anterior chamber depth (ACD) were
obtained using PCI and SS-OCT optical biometers.
Intraocular lens (IOL) power calculations were
made using the Barret-Universal II, Haigis, Hoffer Q,
SRK/T, and T2 formulas and compared the
predictive accuracy between biometers.

56. • In 153 eyes (153 patients), axial length
measurements made with PCI (24.65 ± 2.35 mm)
and SS-OCT (24.62 ± 2.29 mm) were significantly
different (P < 0.001). Corneal power (P = 0.97) and
anterior chamber depth (P = 0.51) were not
significantly different between biometer
• Conclusion:
• predictive accuracy of PCI and SS-OCT were nearly the
same. However, in medium-long eyes, the predictive
accuracy of SS-OCT for IOL calculations was higher.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6137182/

57. • Conclusion:
• Two biometers showed high repeatability and relatively
good agreements. The swept-source optical biometer
demonstrated better repeatability, penetration, and an
overall lower prediction error.
• Both the IOLMaster 700 and Galilei G6 showed good
repeatability, although the IOLMaster 700 showed
better repeatability than the Galilei G6.
• The proportion of eyes with an absolute prediction error
within 0.5 D was 85.0% for the IOLMaster 700 and was
80.0% for the Galilei G6 based on the SRK/T formula
https://www.ncbi.nlm.nih.gov/pubmed/29375908

58. • estimate the repeatability of biometric parameters
obtained with a new swept-source biometer and to
compare the agreement with that of partial
coherence interferometry (PCI) and optical low-
coherence reflectometry (OLCR).
• Swept-source biometry showed high repeatability
performance for all biometric parameters.

59. Accuracy/preference order-wise
1. Swept-source OCT based biometer
IOLMaster 700,
2. OLCR = PCI*
Lenstar LS 900, OA 2000, Topcon Aladin, IOLMaster 500
3. Acoustic Immersion
4. Acoustic Applanation
*Meta-analysis of optical low-coherence reflectometry versus partial
coherence interferometry biometry
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5324074/

60. • In non-optical method
• Axial length can also be measured by Imaging
technologies like CT scan and more precisely by MRI
• While these techniques are employed only when
patients are severely emaciated and are not done to
calculate axial length for IOL.
• They are done instead to find the extent of tumors (eg
retinoblastoma) or mass that are life threatening.

61. • Good Bye…Till I will come with the next Topics:
1. IOL Calculation
• Old and new formulas
2. Pearls of getting best Biometry Outcomes