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A scan biometry

a scan ultra sound biometry calculation for intra ocular lens

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A scan biometry

  1. 1. A SCAN BIOMETRY Mahantesh.B HOD of optometry
  2. 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
  3. 3. continue  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. 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. 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
  6. 6. continue  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. 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. 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. 9. Keratometery  Keratometry by Manual  Topography  Autokeratometer  IOL master/ Lenstar 900
  10. 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. 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. 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. 13. Method  Contact – Applanation Method Hand-Held Method  Immersion
  14. 14. Applanation Method  By the Applanation biometry method, an ultrasound probe is placed directly on the cornea, with attached slit lamp
  15. 15. Hand-Held Method
  16. 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. 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. 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. 19. Immersion  Immersion A-scan Biometry • The immersion technique requires the use of a Prager Scleral Shell
  20. 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. 21. formulae  Theoretical formulae  Regression formulae
  22. 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. 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. 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. 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. 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. 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. 28. THANK YOU

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