ASOCT NEW SLIDES.pptx

asoct

1. Presenter : dR . ramesh bhandari 1st year resident Department of ophthalmology Bpklcos , MMC, iom 1 ANTERIOR SEGMENT OCT AND KERATOMETRY

2. LAYOUT OF PRESENTATION : INTRODUCTION HISTORY PRINCIPLE TYPES APPLICATIONS ADVANTAGES AND LIMITATIONS 2

3. INTRODUCTION Optical coherence tomography (OCT) A non-invasive Non-contact 2-Dimensional imaging system Provides high resolution (10-15 micrometer) Cross-sectional images Imaging of anterior and posterior segment structures “ Optical ” – meaning light. “ Coherence ” – constant phase difference (interference) “ Tomography ” – imaging in cross section Handbook of OCT Brett E. Bouma Guillermo J. Tearney 3

4. HISTORY: The use of OCT in ocular tissue imaging was first reported by Huang et al in 1991. 1 st and 2 nd generation OCT -- axial (depth) resolution of approx. 12 – 15 micro meter. 3 rd generation OCT (OCT -3) uses 500 axial scans taken in 1 second and has increased the resolution to 7 – 8 micrometer. New ultra-high resolution OCT achieves a resolution of 2 – 3 micrometer but is not yet commercially available 4

5. HISTORY- OCT TIMELINE 5

6. The first demonstration of corneal and anterior segment OCT imaging was described by Joseph Izatt and others in Prof. Fujimoto’s laboratory in 1994. 6

7. 7 Radhakrishnan S. Rollins AM, Roth JE, et al. Real-time optical coherence tomography of the anterior segment at 1310 nm. Arch Ophthalmology 2001 19:1179-85

8. PRINCIPLE Based on the principle of Michelson interferometer . Utilizes interferometry and Low coherence radiation in near – infrared range . OCT images obtained by measuring- Echo time Intensity of reflected light 8

9. 9 A broadband light of wavelength near – infrared (1310nm ) is projected by a diode source axially to a beam splitter (tilted at 45 degree). Among the splitted beam, 1 st beam is projected by 78D lens to the tissue of interest e.g. cornea ( probe/object beam ). Another beam travels to a reference mirror at known reference distance ( reference beam )

10. Both the light beams are back - reflected and are projected back to the “beam splitter”. From here, both the beams are reflected towards the “detector” . However , light reflected are scattered differently from tissue with different optical properties. The time delay of the light reflected from various structures is compared with the time delay of light reflected from reference mirror . Resulting in a signal generation and is displayed on the OCT monitor. 10

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13. TYPES OF OCT 13

14. TIME- DOMAIN OCT Uses a single photo detector, and an A-scan is created by moving a reference mirror to change the optical path of the reference beam in order to match different axial depths in the target tissue. This setup limited the scanning speed to a few thousand A-scans per second and has narrow range of depth. 14

15. SPECTRAL- DOMAIN OCT Is able to acquire an entire A-scan by using an array of detectors and the reference mirror is kept stationary. The spectral pattern of interference between target and reference reflections is measured. Thus reflections from all layers are detected simultaneously. SD-OCT can provide20000-52000 A-scans per second with a 5-7 µm resolution 15

16. 16 S.N. TIME- DOMAIN OCT SPECTRAL- DOMAIN OCT 1. Captures 1 pixel at a time. Can capture 2000 pixels simultaneously 2. More time consuming Less time consuming. 3. Axial resolution 10 microns Axial resolution 6-7 microns 4. Transverse resolution 20 microns Transverse resolution 10 microns 5. Motion artifact may be present. No motion artifact 6. Image obtained are relatively dim and less clean Brighter and cleaner image obtained. 7. Reference mirror moves Reference mirror remains stationary.

17. SPECTRAL-DOMAIN OCT: – Spectralis (Heidelberg) Cirrus (Zeiss) RTVue (Optovue) Optovue and Cirrus : Anterior Segment imaging capabilities in addition to posterior segment. Spectralis : Require special lens and anterior segment module for anterior segment imaging. 17

18. The development of newer technology in AS-OCT has following features:- L onger wavelength (1310nm) Telocentric transverse scanning Very high speed axial scanning with a grating based rapid scanning optical delay (RSOD ) This system provides : A speed of 4000 A-scan/sec, 17-um axial resolution ( in tissue) and scan dimension of up to 15mm(width) X 8mm ( depth) 18

19. Advantages of Longer Wavelength Less than 7% of the 1310 nm light incident on the cornea reaches the retina compared to 93% transmission for 830 nm light – that means higher power level can be used safely . Higher exposure limit : 15mW for the 1310 nm wavelength compared to 0.7 mw for 830 nm. Reduced scattering in opaque tissues such as the limbus, sclera and iris . Penetration is six times deeper in highly scattering tissue such as the sclera. Allows deeper penetration of the limbus for visualization of the scleral spur and angle recess, important landmarks for narrow angle glaucoma diagnosis and anterior chamber biometry. 19

20. Differences between AS-OCT And UBM 20

21. USES OF AS-OCT:- Mapping of corneal thickness on patients with keratoconus, corneal scars and corneal dystrophies, glaucoma. Measurement of LASIK flap and stromal bed thickness pre and postoperative. Visualizing and measuring the result of corneal implants and lamellar procedures. Imaging through corneal opacity to see internal eye structure. 21

22. Images of the angle are performed to quantify the angle for angle closure glaucoma and attempt to identify the angle structures . Detection of pathological processes such as dry eye syndrome, ocular surface conditions, tumors, and infections Measuring the dimensions of anterior chamber and assessing the fitness of intraocular lens implants Imaging the lens informs Ophthalmologists about the location of intraocular lenses (IOLs ) 22

23. AS-OCT is an excellent preoperative and postoperative tool to evaluate and manage patients with: Blebs, Intrastromal corneal rings, Full-thickness penetrating Keratoplasty (PK), Descemet-Stripping Endothelial Keratoplasty (DSEK), Deep L amellar Endothelial Keratoplasty (DLEK), IOLs and laser-assisted in situ Keratomileusis (LASIK). 23

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27. ASOCT AND KERATOCONUS AS-OCT demonstrates alterations in corneal epithelial thickness and distribution in keratoconus. T he depth of the demarcation line following corneal collagen cross-linking . 27

28. In keratoconus , focal thinning and corneal asymmetry are evaluated on AS‑OCT. Focal corneal thinning is a more specific indicator of keratoconus . Several parameters for detecting asymmetry and thinning have been reported.[15] Pachymetric diagnostic parameters include (a) minimum‑median, (b) inferior‑superior, (c) inferotemporal‑superonasal , (d) minimum, and (e) vertical location of the minimum.[16] Focal thinning was captured by minimum‑median and minimum parameters. Asymmetric thinning was captured by inferior‑superior, inferotemporal‑superonasal parameter, and by the vertical location of the minimum locations (superior to the corneal vertex had positive values and locations inferior to the vertex had negative values). Swept source Fourier domain AS‑OCT is found to discriminate healthy eyes from subclinical keratoconus .[17] AS‑OCT is found useful to find the depth of demarcation following corneal collagen cross‑linking.[15] In keratoconus patients, AS‑OCT is found useful for qualitative evaluation of the cornea before and after implantation of the intrastromal ring.[ 28

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32. 32 AS-OCT can be implemented to evaluate changes in the geometric properties of keratoconic corneas after the insertion of intracorneal ring segments and also assess their position and depth in the cornea.

33. AS‑OCT is useful to evaluate the Descemet’s membrane tear, dimensions of intrastromal clefts, and corneal thickness in acute corneal hydrops of keratoconus ]. AS‑OCT is also useful in assessing the response of treatment following interventions such as injection of sulfur hexafluoride (SF6)/ perfluoropropane (C3F8) gas into the anterior chamber. 33

34. Anterior segment‑optical coherence tomography of acute corneal hydrops showing Descemet’s membrane tear and intrastromal cleft Figure 2: Anterior segment‑optical coherence tomography of same patient showing healing in response to injection of C3F8 gas 34

35. AS-OCT AND DRY EYE SYNDROME OCT can be a quick, noninvasive method to measure the tear meniscus height (TMH) and treatment response in dry eye patients. Studies show that lower tear meniscus parameters measured with SD-OCT correlate well with the Schirmer test, break-up time, and subjective symptoms. Czajkowski G, Kaluzny BJ, Laudencka A. Tear meniscus measurement by spectral optical coherence tomography . Optom Vis Sci. 2012;89(3):336-42 . 35

36. AS-OCT AND OCULAR SURFACE LESIONS OCT provides a noninvasive technique to document the extent and depth of corneal/ conjunctival intraepithelial neoplasia and pterygium. A. Slit lamp photograph of a pterygium. B. AS-OCT image of the pterygium shows a dense,hyper -reflective , fibrillary subepithelial lesion that is between the corneal epithelium and Bowman ’ s layer. 36

37. 37 Pterygium AS- OCT with subepithelial thickening, shadowing,and hyperreflectivity . The epithelium is preserved without any thickening or hyperreflectivity and lacks a transition point as would be seen with ocular surface neoplasias . Large nodular ocular surface squamous neoplasia

38. 38 (A) Slit lamp photo with corresponding AS- OCT of an irregular ocular surface squamous neoplasia with significant shadowing. ( B) Slit lamp photo graph of the same patient after four cycles of 5- fluorouracil with normalization of corneal epithelium and subepithelial scarring .

39. AS-OCT AND CORNEAL INFECTIONS 39 Slit lamp photo of a pigmented corneal scar from fungal keratitis. ( B) Corresponding AS- OCT with subepithelial stromal hyperreflectivity (arrow) with one- third thickness of the anterior stroma with residual scarring inferiorly , less hyper reflective in nature in the remaining two- thirds of the stroma .

40. AS-OCT serves as a powerful tool for the non-invasive diagnosis of OSSN and can be used to determine the need for treatment initiation as well as monitoring of the disease course . Other lesions that can be characterized by AS-OCT include conjunctival nevus, melanomas , lymphomas and amyloidosis. 40 Slit lamp photograph displaying a cystic nevus . On AS-OCT, this lesion is a well-circumscribed subepithelial lesion containing cystic spaces

41. AS-OCT can be used to image several dystrophic and degenerative conditions of the cornea. 41 Slit lamp photo of lattice corneal dystrophy. ( B) Corresponding AS- OCT with the arrow highlighting the amorphous hyperreflective material in the central stroma .

42. 42 (a) Slit lamp photograph of a central Salzmann’s nodule. (b) On AS-OCT, the nodule is seen as a localized area of hyperreflective material that has replaced the anterior stroma and Bowman’s layer underneath normal epithelium. (c) Slit lamp photograph of band keratopathy in the peripheral cornea. ( d) AS-OCT imaging shows a thin band of hyperreflectivity along Bowman’s layer with underlying shadowing.

43. AS-OCT AND CORNEAL DEPOSITS AS‑OCT is useful to see early drug deposits and early KayserFleischer rings of Wilson disease which is missed by slit‑lamp biomicroscope . Kayser –Fleischer ring is seen as hyperreflectivity at the level of Descemet’s membrane in the peripheral cornea Amiodarone ‑induced keratopathy is observed as highly reflective and bright intracellular inclusions in the epithelial basal layer. 43

44. AS-OCT AND REFRACTIVE SURGERY V isualization of flap thickness, flap interface. Look for any flap displacement . M easuring residual stromal thickness following LASIK surgery. AS‑OCT has been used as a rescue tool for difficult lenticular extraction in SMILE surgery to identify the cause for retained lenticle . 44 LASIK FLAP

45. T ool to monitor the success and complications of several anterior segment surgical procedures including Descemet stripping automated endothelial keratoplasty (DSAEK), Descemet membrane endothelial keratoplasty (DMEK ),Penetrating Keratoplasty , L aser-assisted in situ keratomileusis (LASIK ). D etecting early graft detachment after DMEK surgery and also to find interface fluid between the host cornea and the graft . E xcellent intra-operative adjunct for the anterior segment surgeon particularly during lamellar keratoplasty . Post-operatively, high quality AS-OCT images can allow clinicians to assess graft adherence, graft centration, graft thickness and even epithelial remodeling after DSAEK surgery , all of which can affect the optical quality of corneas post-operatively. 45

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47. Descemet´s membrane detached in a central and peripheral pendant wavy shape, with stromal edema.after cataract or DMEK surgery. S uch as injection of sulfur hexafluoride(SF6 )/ perfluoropropane (C3F8) gas into the anterior chamber 72 hrs,Postoperative clear cornea, with gas bubble present. ( B ) 4 weeks, AS -OCT- adhered Descemet .   Pseudoexfoliation syndrome (PXF or PEX) is  an age-related systemic syndrome  that targets mainly ocular tissues through the gradual deposition of fibrillary white flaky material from the lens, mainly on the lens capsule, ciliary body, zonules , corneal endothelium, iris and pupillary margin Cataract surgery in settings of PXF carries a significant risk of complications in the  form of capsule rupture, vitreous loss, nucleus luxation, and IOL dislocation 47

48. The aqueous outflow system is an important component in the maintenance of intraocular pressure (IOP). Aqueous humor is produced from plasma within the capillaries of the ciliary processes by three distinct mechanisms: diffusion, ultrafiltration and secretion. Aqueous humor then flows through the pupil from the posterior chamber into the anterior chamber. From the anterior chamber, aqueous then drains via the trabecular and the uveoscleral pathway. 90 % of the outflow. As part of this route, aqueous humor flows through the trabecular meshwork (TM) towards Schlemm's canal (SC). The trabecular meshwork is a porous structure consisting of laminar beams with a core of elastic and collagenous fibers (Tamm, 2009). Drainage through the TM is passive, virtually acting as a filter. The SC is an endothelium-lined circular vessel that shares many similarities with lymphatic vessels. From the SC, 25 to 35 collector channels anastomose with the aqueous veins, where the aqueous humor is drained. The major site of resistance to flow is not fully known but may include juxtacanalicular tissue region and the inner wall of SC. In the uveoscleral outflow pathway, aqueous passes through the ciliary body, into the suprachoroidal space and is finally drained by the venous circulation. The IOP is dependent on the dynamic balance between production and outflow of aqueous humor and an increase in outflow resistance will lead to an increase in IOP Later the same group reported significantly reduced cross-sectional area of SC (3942mm2) in patients with primary open angle glaucoma 48

49. AS-OCT allows to evaluate the anterior lens capsule in pseudoexfoliation patients and predict postoperative (IOL) tilt, and assist in IOL power calculations to improve visual outcome after cataract surgery . AS‑OCT is useful to evaluate the wound characteristics of clear corneal incisions of cataract surgery, analyzing incision angle, incision length, corneal thickness, epithelial side closure of the incision or epithelial gaps, endothelial side closure or endothelial gaps and Descemet’s membrane detachment . Lee H, Kim EK, Kim HS, Kim TI. Fourier‑domain optical coherence tomography evaluation of clear corneal incision structure according to blade material. J Cataract Refract Surg 2014;40:1615‑24. 49

50. AS-OCT AND AQUEOUS OUTFLOW SYSTEM The first OCT image of SC and the TM was presented by Sarunic and co-workers in 2008 using a SS OCT system operating at 1310 nm and proving an axial resolution of approximately 9mm. Visualizing aqueous outflow pathway also help in guiding glaucoma surgery and evaluating treatment success. 50

51. AS-OCT demonstrated to be useful in the evaluation : Trabeculectomies Bleb morphology Patency/position of tubes of glaucoma drainage devices 51

52. Qualitative and quantitative assessment of anterior chamber angle (ACA), anterior chamber, iris and lens are accomplished with AS-OCT. Angle closure with AS OCT is determined by any contact between the iris and the angle wall anterior to the scleral spur. The identification of the scleral spur is an important landmark to be assessed, although the Schwalbe's line (SL) has been proposed as another possible landmark since it has better identification in FD OCT devices. (Cheung et al .,2011 ) 52

53. A study comparing AS-OCT with goniscopy AS-OCT detected more closed angles than gonioscopy Disparity to attributed: Possible distortion of the anterior segment by contact gonioscopy Differences in illumination Nolan W, See JL, (‘hew PT, et al. Detection of primary angle- closure using anterior segment optical coherence tomography in Asian eyes. Ophthalmology 2007:114:33-9 53

54. llustration of AS-OCT measurements of parameters providing by the Zhongshan Angle Assessment Programme . TISA500 is defined as the trapezoidal area with the following boundaries: anteriorly, the angle opening distance 500; posteriorly, a line drawn from the SS perpendicular to the plane of the inner sclera wall to the opposing iris; superiorly, the inner corneoscleral wall; inferiorly, the iris surface. ARA is defined as the area of triangle between anterior iris surface, corneal endothelium, and a line perpendicular to the corneal endothelium drawn to the iris surface 750 µm from the SS. IT750 is the iris thickness at 750 µm from the SS. Iarea is defined as the cumulative cross-sectional area of the full length (from spur to pupil) of the iris. ACW is defined as the horizontal SS to SS distance. ACA is defined as the cross-sectional area of the anterior segment bounded by the endothelium, anterior surface of iris and anterior surface of lens (within the pupil). LV is defined as the perpendicular distance from the horizontal line between the 2 SS to the anterior pole of the lens. PD is the shortest distance between the pupil edges of the iris cross sections. ACA, anterior chamber area; ACW, anterior chamber width; ARA, angle recess area; Iarea , iris area; IT750, iris thickness at 750 µm; LV, lens vault; PD, pupil diameter; SS, sclera spur; TISA500, trabecular iris space area at 500 µm. 54

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56. ADVANTAGES OF AS- OCT Best axial resolution available so far Scans various ocular structures Tissue sections comparable to histopathology sections Easy to operate and non-invasive. Short scanning time 56

57. LIMITATIONS OF AS-OCT Inability to visualize structures posterior to the iris and ciliary body due to blockage of wavelength by pigment . Each scan much be taken in range and in focus must be examined for blinks and motion artifacts Axial motion is corrected with computer correlation software transverse motion cannot be corrected 57

58. Astigmatism is quantified by by the axes and refractive power of the meridian with the highest refractive power (steep meridian) and lowest refractive power (at meridian). In regular astigmatism, the at and steep meridian are perpendicular to each other by denition . In irregular astigmatism, they are not. Regular astigmatism is divided into three categories: with-the-rule astigmatism, against-the-rule astigmatism and oblique astigmatism. Most of the population have withthe - rule astigmatism where the meridian with the highest refractive power (steep meridian) is more or less vertical (cf. Sec. 2.5). Against-the-rule astigmatism refers to eyes where the steep meridian is more or less horizontal. In oblique astigmatism, the steep axis is somewhere in between . 58

59. KERATOMETRY Also called as Ophthalmometry . Basically comprises: Kerato - Cornea and Metry - Measurement A technique used to measure the curvature of the anterior surface of the cornea across a fixed chord length , usually 2-4 mm , which lies within the optical spherical zone of cornea. 59

60. HISTORY In 1980 , the development of autokeratometer

61. CLINICAL USES: Determines curvature of the cornea Estimates the amount and direction of corneal astigmatism IOL power calculation (Pre-op cataract surgery workup) Monitors pre and post-op astigmatism Helps to diagnose and monitor keratoconus and other corneal diseases Contact lens fitting 61

62. OPTICAL SYSTEM OF KERATOMETER 62

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64. PRINCIPLE OF KERATOMETER Keratometry is based on the fact that the anterior surface of cornea acts as a convex mirror and the size of the image formed varies with its curvature. Greater the curvature of cornea lesser is the image size Therefore, from the size of the image formed by the anterior surface of cornea (1 st Purkinje image) the radius of curvature calculated as: Where , r is the radius of curvature of the reflective cornea, u is the distance from the object to the cornea, I is the size of the image, and O is the size of the object 64 r = 2u(I/O)

65. Optical principle involved is the relationship between the size of an object and size of the image of that object reflected from the surface Radius of curvature determined by the apparent size of the image of bright object (mires) viewed by the reflection from anterior corneal surface which acts as a convex mirror 65 AB is the object and A' B' is the image. By measuring the size of the object and image, curvature of the convex surface can be calculated .

66. The final step is to convert the radius of curvature into an estimate of the cornea’s dioptric refractive power; P is the refractive power of the cornea, n’ is the refractive index of the cornea, n is the refractive index of air (which is close to 1.0), and r is the measured radius of curvature of the cornea An “averaged” corneal refractive index of 1.3375 is used 66 P = (n’ – n)/r

67. DOUBLING PRINCIPLE 67

68. Measurement of image height Doubling device - Plano prism Lateral displacement of doubled image = IMAGE HEIGHT Prism is moved along the optical axis until two images are just touching At this point, the prismatic displacement is exactly equal to the size of the image The larger the image size is, the greater the amount of doubling must be to achieve contact of two image TYPES Fixed doubling Variable doubling 68

69. FIXED DOUBLING – Bausch & Lomb, Topcon & Magnon VARIABLE Image size and mire separation FIXED Object height and doubling device distance VARIABLE DOUBLING – Haag streit & Javal - Schiotz FIXED Image size & mire separation VARIABLE Object size & doubling device distance 69

70. Basically there are two types of keratometer on the basis of operation ; Manual Keratometer a . Bausch and Lomb Keratometer b . Javal - schiotz keratometer c . Zeiss ophthalmometer d . Haag streit ophthalmometer e . Topcon OM-4 keratometer Auto keratometer a. IOL Master b. Pentacam c. Humphrey autokeratometer d. Corneal topographer 70

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