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Optical and Non-optical Methods of Measuring Axial Length of Eye

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.

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Introduction to optical and non-optical methods for measuring axial length presented by Rabindra Adhikary.

Definition of axial length with rapid growth statistics in infants. Key data: Birth - 16.8 mm, 12 months - 20 mm. Significant errors impact post-operative refraction.

Normal adult mean axial length: 23.67 mm; cataractous eyes: 23.65 mm. Noted diurnal fluctuations of 15-40μ.

Comparison of axial lengths for emmetropia and ametropia in different age groups with specific values.

Description of ultrasonic biometry, methodology using ultrasonic waves to determine axial length with accuracy ±0.1 mm.

Details on echogram peaks, their significance in measuring ocular structures, and factors affecting amplitude.

Applanation technique for accurate axial length measurement, discussing its method and potential measurement errors.

Description and methodology of immersion technique for biometry, highlighting its accuracy advantages.

Comparison results of axial length measurements by immersion and applanation techniques, significant findings on accuracy.

Identifies common errors in ultrasound measurement of axial length and limitations of immersion technique.

Introduction of optical methods like Partial Coherence Interferometry as a gold standard for axial length measurement.

Different types of interferometers and their relevance in optical biometers for accurate measurements.

The IOLMaster 500 biometer's features, including key measurements made for comprehensive ocular analysis.

Critical requirements for measurements with IOLMaster devices, emphasizing fixation and measurement variability.

Discusses limitations of IOLMaster 500, particularly in dense cataract cases and alternative ultrasound use.

Research findings comparing PCI optical biometry with ultrasound, emphasizing predictive accuracy.

Introduction and advantages of the IOLMaster 700, exploring its advanced technology in biometric measurements.

Comparison study results on IOLMaster 700 and IOLMaster 500 efficacy in various eye conditions.

Discusses the practical applications and future topics related to IOL calculations and best practices for biometry.

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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
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
• 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
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/
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
• 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
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
• 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
• 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
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
• 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
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
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.
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.
Immersion Technique
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
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.
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
Drawbacks of Immersion Technique • Time consuming, messy, expensive, risk of contamination
• 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).
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
Interference..(explained by Video)
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
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.
Source of Infrared Mirror Photodetector
Partial Coherence Interferometry
IOLMaster 500 • Partial Coherence Interferometry • FDA approved in 2000 AD (Carl Zeiss Meditec AG)
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
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.
• 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.
• 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
• If the AL difference between RE and LE exceeds 0.3 mm, a message appears to check the readings once again.
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)
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
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
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
• 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
• 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
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
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.
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
• 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
• 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.
• 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
Conversion Formula: • The above paper gives us the conversion formula • ALop = 0.015 + 0.996 X ALus
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
• 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.
• 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
• 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
• 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)
• 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
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.
• 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.
• 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/
• 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
• 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.
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/
• 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.
• Good Bye…Till I will come with the next Topics: 1. IOL Calculation • Old and new formulas 2. Pearls of getting best Biometry Outcomes

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Optical and Non-optical Methods of Measuring Axial Length of Eye

  • 1.
    Optical and Non-optical Methodsof Measuring Axial Length Rabindra Adhikary M. Optom, 1st Batch Tilganga Institute of Ophthalmology Pokhara University Facilitator: Rupesh Poudel 14th August 2019
  • 2.
    Axial Length • Thedistance 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 AxialLength 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 Comparisonin 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 theprinciple 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 ofphakic 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 lensecho 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 amplitudeof 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 • theultrasound 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 numberof 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 OpticalAL • 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 • Herea 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 • Unlikethe 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.
  • 16.
    https://www.researchgate.net/publication/293800665_Comparison_of_ocular_biometry_measurements_b y_applanation_and_immersion_A-scan_techniques • compared ocularbiometry 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 meansof 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 AxialLength 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 ImmersionTechnique • Time consuming, messy, expensive, risk of contamination
  • 20.
    • Recommendations: Theshell 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 ofmeasuring 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.
  • 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.
  • 27.
  • 28.
    IOLMaster 500 • PartialCoherence Interferometry • FDA approved in 2000 AD (Carl Zeiss Meditec AG)
  • 29.
    IOLMaster 500 • Firstoptical 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 toALM 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 thepatient 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 requires5 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 theAL difference between RE and LE exceeds 0.3 mm, a message appears to check the readings once again.
  • 34.
    IOLMaster: Modes andFeatures • 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 IOLMaster500 • 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 IOLMaster500 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 ofoptical 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 LS900 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 uses950 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 ScanOptical 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 AladinBiometer • 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: • Toinvestigate 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: • Therewas 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 ofoptical 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: • Theabove paper gives us the conversion formula • ALop = 0.015 + 0.996 X ALus
  • 47.
    IOLMaster 700 • SweptSource 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 a1-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 OCTin 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 Tocompare 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 differencesbetween 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 700was 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 theagreement 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 purposeof 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 153eyes (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: • Twobiometers 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 therepeatability 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-sourceOCT 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-opticalmethod • 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…TillI will come with the next Topics: 1. IOL Calculation • Old and new formulas 2. Pearls of getting best Biometry Outcomes