Optical coherence tomography

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Optical coherence tomography

  1. 1. Optical Coherence Tomography OCT Dennis M. West, CRA
  2. 2. What is OCT? <ul><li>Diagnostic imaging technique that examines living tissue non-invasively. It is based on a complex analysis of the reflection of low coherence radiation from the tissue under examination. </li></ul><ul><li>Real time cross sectional analysis </li></ul>
  3. 3. <ul><li>OCT allows both qualitative and quantitative analysis of the retina </li></ul><ul><li>Qualitative analysis includes description by location, a description of form and structure, identification of anomalous structures, and observation of the reflective qualities of the retina </li></ul>
  4. 4. <ul><li>Quantitative analysis involves measurements of the retina, specifically retinal thickness and volume, and nerve fiber layer thickness.  This is possible because the OCT software is able to identify and &quot;trace&quot; two key layers of the retina, the NFL and RPE </li></ul>
  5. 5. How does it work? <ul><li>128 to 768 axial samples (A-scans) in a single &quot;scan pass“ </li></ul><ul><li>Each A-scan has 1024 data points and is 2mm long (deep).  </li></ul>
  6. 6. Resolution <ul><li>When all of the A-scans are combined into one image, the image has a resolving power of about 10 microns vertically and 20 microns horizontally </li></ul><ul><li>Compare that to the resolution of a good ophthalmic ultrasound at 100 microns </li></ul>
  7. 7. <ul><li>OCT Ultrasound </li></ul>                                                                              
  8. 8. Protocols <ul><li>The Zeiss OCT 3 has several built-in protocols for scanning the retina and the optic nerve head. </li></ul><ul><li>A protocol is simply a pre-determined procedure or method </li></ul>
  9. 10. Scan Protocol Types <ul><li>Line </li></ul><ul><li>Circle </li></ul><ul><li>Radial Lines </li></ul>
  10. 11. <ul><li>The &quot;line&quot; scan simply scans in a single, straight line.  The length of the line can be changed as well as the scan angle. </li></ul>
  11. 12. <ul><li>The &quot;circle&quot; scans in a circle instead of a line. </li></ul>
  12. 13. <ul><li>The &quot;radial lines&quot; scans 6 consecutive line scans in a star pattern </li></ul>
  13. 14. Not All OCT Scans Are Created Equally <ul><li>The &quot;fast&quot; scan protocols of the OCT 3 reduce the time needed for multiple scans </li></ul><ul><li>The scan time reduction is intended to minimize the error created by patient movement </li></ul><ul><li>Fast scans grab fewer A-scans in the 6 mm length of the scan.  The normal 6 mm scan contains 512 A-scans, whereas the fast 6 mm scan contains only 128 A-scans, resulting in a lower resolution image </li></ul>
  14. 15. Fast OCT 3 scan The same eye scanned with maximum resolution
  15. 16. Retinal Anatomy Compared to OCT <ul><li>The vitreous is the black space on the top of the image </li></ul><ul><li>We can identify the fovea by the normal depression </li></ul><ul><li>The nerve fiber layer (NFL) and the retinal pigment epithelium (RPE) are easily identifiable layers as they are more highly reflective than the other layers of the retina </li></ul>
  16. 17. <ul><li>This higher reflectivity is represented by the &quot;hotter&quot; colors (red, yellow, orange, white) in the false color representation of the OCT 3. </li></ul><ul><li>The middle layers of the retina, between the NFL and RPE, are much less easily identifiable in the scan. </li></ul>
  17. 19. Regions <ul><li>For purposes of analysis, the OCT image of the retina can be subdivided vertically into four regions </li></ul><ul><li>the pre-retina </li></ul><ul><li>the epi-retina </li></ul><ul><li>the intra-retina </li></ul><ul><li>the sub-retina </li></ul>
  18. 20. The pre-retinal profile <ul><li>A normal pre-retinal profile is black space </li></ul><ul><li>Normal vitreous space is translucent </li></ul><ul><li>The small, faint, bluish dots in the pre-retinal space is &quot;noise&quot; </li></ul><ul><li>This is an electronic aberration created by increasing the sensitivity of the instrument to better visualize low reflective structures. </li></ul>
  19. 21. Anomalous structures <ul><li>pre-retinal membrane </li></ul><ul><li>epi-retinal membrane </li></ul><ul><li>vitreo-retinal strands </li></ul><ul><li>vitreo-retinal traction </li></ul><ul><li>pre-retinal neovascular membrane </li></ul><ul><li>pre-papillary neovascular membrane </li></ul>
  20. 22. The over-all retinal profile
  21. 23. <ul><li>A pre-retinal membrane with traction on the fovea </li></ul>
  22. 24. a pigment epithelial detachment is causing the convexity
  23. 25. Aside from the retinal detachment, notice the underlying concave curvature of the retina, suggesting the long eye of a significant myope
  24. 26. The foveal profile <ul><li>The normal foveal profile is a slight depression in the surface of the retina </li></ul>
  25. 27. Deformations in the foveal profile <ul><li>macular pucker </li></ul><ul><li>macular pseudo-hole </li></ul><ul><li>macular lamellar hole </li></ul><ul><li>macular cyst </li></ul><ul><li>macular hole, stage 1 (no depression, cyst present) </li></ul><ul><li>macular hole, stage 2 (partial rupture of retina, increased thickness) </li></ul><ul><li>macular hole, stage 3 (hole extends to RPE, increased thickness, some fluid) </li></ul><ul><li>macular hole, stage 4 (complete hole, edema at margins, complete PVD) </li></ul>
  26. 28. Macular cyst
  27. 29. Macular hole, stage 2
  28. 30. Macular hole, stage 3
  29. 31. Macular hole, stage 4, operculum suspended by the hyaloid membrane
  30. 32. The macular profile <ul><li>The macular profile can, and often does,  include the fovea as it's center </li></ul>
  31. 33. Deformations in the macular profile <ul><li>Serous retinal detachment (RD) </li></ul><ul><li>Serous retinal pigment epithelial detachment (PED) </li></ul><ul><li>Hemorrhagic pigment epithelial detachment </li></ul>
  32. 34. Serous retinal pigment epithelial detachment (PED)
  33. 35. Intra-retinal anomalies in the macular profile <ul><li>Choroidal neovascular membrane </li></ul><ul><li>Diffuse intra-retinal edema </li></ul><ul><li>Cystoid macular edema </li></ul><ul><li>Drusen </li></ul><ul><li>Hard exudates </li></ul><ul><li>Scar tissue </li></ul><ul><li>Atrophic degeneration </li></ul><ul><li>Sub-retinal fibrosis </li></ul><ul><li>RPE tear </li></ul>
  34. 36. Choroidal neovascular membrane
  35. 37. Cystoid macular edema cause by diabetic maculopathy
  36. 38. Sub-retinal fibrosis
  37. 39. OCT and Fluorescein Angiography in retinal diagnosis <ul><li>FAs provide excellent characterization of retinal blood flow over time, as well as size and extent information on the x and y axis (north-south, east-west) </li></ul><ul><li>The OCT gives us information in the z (depth) axis, telling us what layers of the retina are affected </li></ul>
  38. 42. OCT scans for qualitative analysis of the retina <ul><li>The Fast Macular Thickness Scan (FMTS, FMTM, or FMT scan) </li></ul><ul><li>The Line Scan </li></ul><ul><li>The Cross Hair Scan (3mm and 6mm) </li></ul>
  39. 43. The Fast Macular Thickness Scan <ul><li>The Fast Macular Thickness Scan consists of 6 radial line scans in a spoke pattern.  It is a low resolution scan that was designed for quantitative analysis (thickness and volume) </li></ul>
  40. 44. <ul><li>The FMT scan is placed over the area of interest, which is usually the macula.  When scanning the macula, the patient simply looks at the fixation target.  The center of the FMT scan lines up with the fixation target by default </li></ul><ul><li>A scan is saved and then reviewed with any of the retina analysis tools </li></ul>
  41. 45. <ul><li>Each of the 6 scans can be viewed individually by clicking on the thumbnails on the left of the scan selection screen </li></ul>
  42. 46. The Line Scan <ul><li>The line scan is particularly useful because of it's flexibility.  The length of the line can be changed, the angle of the line can be changed, and the line can be dragged with the mouse to any position or angle on the video screen </li></ul>
  43. 47. The Cross Hair Scan <ul><li>Cross Hair Scan performs a high resolution horizontal line scan and then automatically flips to a vertical line scan without having to exit the protocol </li></ul><ul><li>This is a common technique used in B-scan ultrasonography </li></ul>
  44. 48. Scan analysis protocols for qualitative analysis <ul><li>Line scans can be viewed with a variety of analysis tools (see the OCT manual).  I have found the &quot;Align process&quot; to be the most useful, with the &quot;Proportional&quot; analysis a good choice if the align process is not needed </li></ul>
  45. 49. The Align Process <ul><li>This tool &quot;straightens&quot; motions artifacts </li></ul>
  46. 50. Proportional analysis <ul><li>Proportional analysis produces an image with its true horizontal and vertical proportions </li></ul>
  47. 51. Retinal Thickness Analysis <ul><li>Using the retinal thickness analysis tool, the software then traces a line along the NFL layer and a line along the RPE layer. </li></ul><ul><li>The software then measures the distance between the two lines and a graph is produced which compares the measured thickness to the thickness of a normal retina </li></ul>
  48. 52. Each of the six scans can be reviewed by clicking on the slider bar to the left, and any or all of them can be printed out for the patient's record
  49. 53. <ul><li>Retinal thickness analysis does not measure retinal elevation </li></ul><ul><li>for example this eye with a pigment epithelial detachment (PED) pictured below.  The arrow on the left would represent retinal elevation, from the choroid, through the fluid space of the PED, to the nerve fiber level.  The arrow on the right shows what the analysis measures, defined by the distance from the RPE (which is detached) to the NFL </li></ul>
  50. 55. Serial FMT scans over time <ul><li>One of the most useful functions of the OCT is the ability to take volume measurement over time.  For example,  a FMT scan before treatment for AMD, and FMT scans at various intervals after treatment.  Successful treatment should be followed by a decrease in retinal thickness and volume </li></ul>
  51. 56. Retinal Thickness/Volume Change Analysis <ul><li>Two FMT scans on the same eye, but taken on different dates, can be selected at the same time while holding down the &quot;ctrl&quot; key.  &quot;Retinal Thickness/Vol Change is chosen from the analysis tab. </li></ul>
  52. 57. <ul><li>The analysis will give you a &quot;change map&quot;, showing the difference between the two scans </li></ul>
  53. 58. Glaucoma Scans <ul><li>When evaluating the glaucoma suspect or the glaucoma patient, two parameters that the ophthalmologist is interested in are the characteristics of the optic nerve cup and the thickness of the nerve fiber layer surrounding the optic nerve head </li></ul><ul><li>The Fast Optic Disc scan </li></ul><ul><li>The Fast RNFL Thickness scan </li></ul>
  54. 59. The Fast Optic Disc scan   <ul><li>The optic cup profile can be evaluated by capturing a &quot;Fast Optic Disc&quot; scan </li></ul><ul><li>The patient fixes on the target, which is automatically placed at the edge of the scan window so that the optic nerve is viewed toward the center of the video window.  The operator then moves the scan so that the star pattern is centered on the optic nerve head.  Centering can be aided by clicking on the scan window to view the white centering lines. </li></ul>
  55. 61. <ul><li>The optic nerve scan can be analyzed with the &quot;optic nerve head analysis&quot; protocol </li></ul>
  56. 62. The Fast RNFL Thickness scan <ul><li>Nerve fiber layer thickness can be evaluated with the &quot;Fast RNFL Thickness&quot; scan.  This is a circular scan that requires the operator to place the circle so that the center of the circle is centered on the optic nerve head. </li></ul>
  57. 63. <ul><li>The analysis software places lines on the top and bottom of the nerve fiber layer and the distance between the two lines is interpreted to be the thickness of the nerve fiber layer </li></ul>
  58. 64. <ul><li>Care must be take to make sure that the image is captured with the circle centered on the optic nerve </li></ul><ul><li>The placement of the circle can make a big difference in the analysis of the nerve fiber layer thickness </li></ul>
  59. 65. <ul><li>These two scans (OD) are of a normal eye.  The scan in the first analysis is well centered and the RNFL thickness falls within the normal range.  The scan in the second analysis is of the same eye (OD), but the scan is not well centered.  The analysis is abnormal (black arrows). </li></ul>
  60. 66. Is it Perfect? <ul><li>Scanning with the OCT suffers from a lack of registration and questionable repeatability.  Until improvements in the hardware and software improve or eliminate these problems, operator skill will play a major roll in the quality of OCT scanning. </li></ul>
  61. 67. What makes a good OCT scan? <ul><li>A good quality OCT scan has good reflectivity from edge to edge. </li></ul><ul><li>  The &quot;hotter&quot; colors (orange, red, white, yellow) are maximized </li></ul><ul><li>Generally, the retina should be in the lower portion of the scan window so that the vitreous can be images as well. </li></ul>
  62. 68. Scanning Tips <ul><li>Communicate with the doctor regarding the size and location of the pathology of interest. </li></ul><ul><li>Refer to other images of the pathology, e.g. color photos and FA. </li></ul><ul><li>Review past OCT exams and repeat scan types used before. </li></ul><ul><li>Dilate the eye well?????? </li></ul><ul><li>The patient must keep the forehead against the bar and the chin in the chinrest, with teeth together.  Use the marker on the headrest to align the patient vertically.  The outer canthus should be even with the line </li></ul>
  63. 70. Scanning Tips <ul><li>Use the two buttons near the joystick for freezing and saving scans.  This saves you from having to juggle the joystick and the mouse. </li></ul><ul><li>Minimize patient fatigue by keeping scan time to a minimum.  Never scan an eye for more than 10 minutes ( FDA regulation ). </li></ul><ul><li>Keep the cornea lubricated.  Use artificial tears and have the patient blink when you are not saving a scan pass. </li></ul><ul><li>Move the instrument on the x and y axis (using the joystick) to work around opacities </li></ul>
  64. 71. <ul><li>Reflectivity may be further enhanced by moving the focus knob on the side of the OCT unit.  </li></ul>
  65. 72. Alignment and focus <ul><li>Alignment and focus are used to maximize the quality of the scan.  Alignment begins with centering and zooming in on the &quot;football&quot; shaped reflex in the video image.   The initial lens-to-subject distance is achieved when the retinal image fills the video screen.  This is similar to the image you see when doing retina photography.  At this point, your attention should shift to the scan window </li></ul>
  66. 74. <ul><li>If an image is not visible in the scan window, you should click on the &quot;z-offset optimize&quot; button on the &quot;scan parameter&quot; tab.  Once the scan image is visible, it can be move with the z-offset arrows. </li></ul>
  67. 75. <ul><li>At this point, the &quot;optimize polarize&quot; button should be clicked.  </li></ul>
  68. 76. <ul><li>This should automatically refine the focus on the retina, and you should see an increase in the &quot;hot&quot; colors in the scan, as illustrated below.  The left image is before optimization, the right is after. </li></ul>
  69. 77. <ul><li>At this point, the &quot;scan mode&quot; button is clicked so that you have a full resolution image of the scan(s) in the scan window.  From this point on, maintaining a good scan image is a matter of adjusting the unit with the joystick if necessary to compensate for movement by the patient that may degrade alignment.  Encourage the patient to blink until you are ready to freeze the image </li></ul>
  70. 78. Freezing and selecting the scan <ul><li>The image can be frozen with a video image with a flash or without a flash </li></ul><ul><li>When ready to freeze the image, tell the patient not blink and let the scan go through several passes before freezing the image </li></ul><ul><li>The software saves the last 8 scan passes for review.  Always use the review button (on the bottom row of buttons).  </li></ul>
  71. 79. When you click on the thumbnail at the bottom, the current set of scans is displayed in the windows above.  The software also tells you what the signal strength was for that particular scan, on a scale from 1 to 10, with 10 being maximum signal strength. You can only save one scan from the group
  72. 80. Noise <ul><li>The OCT scan can sometimes be improved by changing the &quot;noise&quot; and &quot;range&quot; settings on the &quot;OCT Image&quot; tab.  The default settings are indicated by blue markers on the scale </li></ul>
  73. 81. Noise <ul><li>Most instruments produce the best scans when set with the default values.  Increasing the noise level will produce &quot;hotter' colors in the scan, but the noise artifacts will also increase.  Noise artifacts are those &quot;snowflakes&quot; that you see in the dark areas of the scan (e.g. the vitreous) </li></ul>
  74. 82. What’s New <ul><li>OCT with SLO </li></ul><ul><li>OCT with HRA (FA and ICG) </li></ul><ul><li>Increase in resolution to 5 microns </li></ul><ul><li>Overlays, 3D imaging </li></ul>
  75. 83. Questions? References:   Brancato R. and Lumbroso B. Guide to Optical Coherence Tomography Interpretation. Rome: Innovation-News-Communication, 2004.   Schuman J., Puliafito C., and Fujimoto J. Ocular Coherence Tomography of Ocular Diseases. Thorofare NJ: Slack Inc., 2004.

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