Optical Coherence Tomography - principle and uses in ophthalmology
By DR TAPAN JAKKAL DEPT OF OPHTHALMOLOGY G.M.C.H AURANGABAD Optical Coherence Tomography (OCT)
Optical coherence tomography <ul><li>Optical Coherence Tomography, or OCT, is a noncontact, noninvasive imaging technique used to obtain high resolution cross-sectional images of the retina and anterior segment. </li></ul><ul><li>Three-dimensional imaging technique with ultrahigh spatial resolution </li></ul><ul><li>Measures reflected light from tissue discontinuities </li></ul><ul><li>Based on interferometry </li></ul><ul><ul><li>involves interference between reflected light and a reference beam. </li></ul></ul>
INTRODUCTION <ul><li>OCT of the retina is like doing a vertical biopsy section of the retina. Instead of a knife, light is used. Instead of viewing a stained section under a microscope, we are presented with a "false-color" view with micron level resolution. </li></ul><ul><li>There is no physical contact with the eye. OCT of the retina is the most important diagnostic tool for retinal pathology since the advent of fluorescein angiography. </li></ul>
Optical coherence tomography- The process is similar to that of ultrasonography, except that light is used instead of sound waves. Analog to ultrasound
Construction of tomographic image Transverse Scanning Backscatter Intensity Axial Scanning (Depth)
<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, or 1/10th of a millimeter. </li></ul><ul><li>The image on the right has 1/10th of the pixels per inch that the image on the left does. The image on the right would represent the resolution of the ultrasound as compared to the resolution of the OCT on the left. </li></ul>
Difficulties and limitations <ul><li>Limited by intraocular media opacities , which attenuate measurement beam and reflected light </li></ul>
OCT Image of Normal Fovea The OCT image above can be compared to what we know about retinal anatomy from conventional microscopic sections. The vitreous is the black space on the top of the image. We can identify the fovea by the normal depression. The nerve fiber layer (NFL) and the retinal pigment epithelium (RPE) are easily identifiable. These layers are more highly reflective than the other layers of the retina. This higher reflectivity is represented by the "hotter" colors (red, yellow, orange, white) in the false color representation of the OCT . The middle layers of the retina, between the NFL and RPE, are much less easily identifiable in the scan.
Qualitative and Quantitative Analysis <ul><li>The OCT allows both qualitative and quantitative analysis of the retina. </li></ul><ul><li>Qualitative analysis involves describing or identifying morphological changes and anomalous structures in the retina. Morphology is the study of forms and structures of organisms. </li></ul><ul><li>Quantitative analysis is possible because the OCT software is able to identify and "trace" two key layers of the retina, the NFL and RPE. The software can then measure the distance between these two layers, which represents retinal thickness. </li></ul>
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>and the sub-retina </li></ul>
Profiles <ul><li>OCT retinal morphology (form and structure) can be subdivided into four "profiles": Each profile has it's own set of deformations and anomalous structures. </li></ul><ul><li>1. pre-retinal profile </li></ul><ul><li>2. overall retinal profile </li></ul><ul><li>3. foveal profile </li></ul><ul><li>4. macular profile </li></ul>
The pre-retinal profile <ul><li>A normal pre-retinal profile is black space, as pictured below, because the normal vitreous space is translucent, meaning it has minimal reflective properties. The small, faint, bluish dots in the pre-retinal space is "noise". This is an electronic aberration created by increasing the sensitivity of the instrument to better visualize low reflective structures. </li></ul>
Anomalous structures that have been observed in the pre-retinal profile include the following: 1. pre-retinal membrane 2. epi-retinal membrane 3. vitreo-retinal strands 4. vitreo-retinal traction 5. pre-retinal neovascular membrane 6. pre-papillary neovascular membrane A pre-retinal membrane with traction on the fovea is pictured below.
The over-all retinal profile The normal over-all retinal profile has a slightly concave curvature that you would expect from observing the surface of a globe. Abnormal profiles would include exaggerated concavity and convexity. Retinal folds would also result in an abnormal over-all profile.
The following OCT image demonstrates an abnormal convexity in the over-all retinal profile. In this case, a pigment epithelial detachment is causing the convexity.
The image below demonstrates an abnormal concavity to the over-all retinal profile. Aside from the retinal detachment, notice the underlying concave curvature of the retina, suggesting the long eye of a significant myope.
The foveal profile The normal foveal profile is a slight depression in the surface of the retina, as pictured below.
Deformations that have been observed in the foveal profile include the following: 1. macular pucker 2. macular pseudo-hole 3. macular lamellar hole 4. macular cyst 5. macular hole, stage 1 (no depression, cyst present) 6. macular hole, stage 2 (partial rupture of retina, increased thickness) 7. macular hole, stage 3 (hole extends to RPE, increased thickness, some fluid) 8. macular hole, stage 4 (complete hole, edema at margins, complete PVD)
The macular profile The macular profile can, and often does, include the fovea as it's center. Therefore, a common OCT scan length of 6 mm would include 3 mm of the macula on each side of the fovea.
Deformations that have been observed in the macular profile include the following: 1. serous retinal detachment (RD) 2. serous retinal pigment epithelial detachment (PED) 3. hemorrhagic pigment epithelial detachment A serous PED is pictured below. We know that it is a PED because the fluid (black space around the arrow) is pushing up underneath the retinal pigment epithelium, identified by the relatively highly reflective (red and orange) line (arrow).
Intra-retinal anomalies that have been identified in the macular profile include: 1. choroidal neovascular membrane 2. diffuse intra-retinal edema 3. cystoid macular edema 4. drusen 5. hard exudates 6. scar tissue 7. atrophic degeneration 8. sub-retinal fibrosis 9. RPE tear
Cystoid Macular Edema OCT is capable of detecting small, fluid-filled, cystic spaces within the macula.
Central Serous Chorioretinopathy Central serous chorioretinopathy is characterized by the presence of fluid between the RPE and neurosensory retina.
Diabetic Retinopathy Exudates appear as accumulation of dense material within the neurosensory retina.
Artifacts <ul><li>Artifacts in the OCT scan are anomalies in the scan that are not accurate images of actual physical structures, but are rather the result of an external agent or action. </li></ul><ul><li>Notice the large gap in the middle of the scan below. This is an artifact caused by a blink during scan acquisition. The was a high resolution scan, which takes about a second for the scan pass, which is plenty of time to record a blink. </li></ul>
The scan below has waves in the retinal contour. These are not retinal folds, but rather movement of the eye during the scan pass .
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>
6.Use the two buttons near the joystick for freezing and saving scans. This saves you from having to juggle the joystick and the mouse. 7.Minimize patient fatigue by keeping scan time to a minimum. Never scan an eye for more than 10 minutes (FDA regulation). 8.Keep the cornea lubricated. Use artificial tears and have the patient blink when you are not saving a scan pass. 9.Move the instrument on the x and y axis (using the joystick) to work around opacities.
Anterior segment optical coherence tomography (OCT) <ul><li>High-speed anterior segment optical coherence tomography (OCT) offers a non- contact method for high resolution cross- sectional and three-dimensional imaging of the cornea and the anterior segment of the eye. </li></ul><ul><li>Anterior Segment Optical Coherence Tomography enhances surgical planning and postoperative care for a variety of anterior segment applications by expertly explaining how abnormalities in the anterior chamber angle, cornea, iris, and lens can be identified and evaluated </li></ul>
On the leading edge of anterior segment imaging: <ul><li>Mapping of corneal thickness and keratoconus evaluation </li></ul><ul><li>Measurement of LASIK flap and stromal bed thickness </li></ul><ul><li>Visualization and measurement of anterior chamber angle and diagnosis of narrow angle glaucoma </li></ul><ul><li>Measuring the dimensions of the anterior chamber and assessing the fit of intraocular lens implants </li></ul><ul><li>Visualizing and measuring the results of corneal implants and lamellar procedures </li></ul><ul><li>Imaging through corneal opacity to see internal eye structures </li></ul>
Image shows an anterior-chamber angle as viewed with gonioscopy and the OCT The latter replaces subjective evaluation with objective measurement.
A narrow angle is apparent with OCT imaging, in this case 9.5°.
With the increase in popularity of anterior chamber imaging, and anterior segment OCT proving to be the best tool for high resolution biometry, Anterior Segment Optical Coherence Tomography is a must-have for anterior segment, refractive, cornea, and glaucoma surgeons.