Optical Coherence Tomography OCT Dennis M. West, CRA

What is OCT? 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. Real time cross sectional analysis

OCT allows both qualitative and quantitative analysis of the retina 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

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 "trace" two key layers of the retina, the NFL and RPE

How does it work? 128 to 768 axial samples (A-scans) in a single "scan pass“ 128 to 768 axial samples (A-scans) in a single "scan pass“ Each A-scan has 1024 data points and is 2mm long (deep). Each A-scan has 1024 data points and is 2mm long (deep).

Resolution 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 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 Compare that to the resolution of a good ophthalmic ultrasound at 100 microns Compare that to the resolution of a good ophthalmic ultrasound at 100 microns

OCT Ultrasound OCT Ultrasound

Protocols The Zeiss OCT 3 has several built-in protocols for scanning the retina and the optic nerve head. The Zeiss OCT 3 has several built-in protocols for scanning the retina and the optic nerve head. A protocol is simply a pre-determined procedure or method A protocol is simply a pre-determined procedure or method

Scan Protocol Types Line Line Circle Circle Radial Lines Radial Lines

The "line" scan simply scans in a single, straight line. The length of the line can be changed as well as the scan angle.

The "circle" scans in a circle instead of a line.

The "radial lines" scans 6 consecutive line scans in a star pattern

Not All OCT Scans Are Created Equally The "fast" scan protocols of the OCT 3 reduce the time needed for multiple scans The "fast" scan protocols of the OCT 3 reduce the time needed for multiple scans The scan time reduction is intended to minimize the error created by patient movement The scan time reduction is intended to minimize the error created by patient movement 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 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

Fast OCT 3 scan The same eye scanned with maximum resolution

Retinal Anatomy Compared to OCT The vitreous is the black space on the top of the image The vitreous is the black space on the top of the image We can identify the fovea by the normal depression We can identify the fovea by the normal depression 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 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

This higher reflectivity is represented by the "hotter" colors (red, yellow, orange, white) in the false color representation of the OCT 3. This higher reflectivity is represented by the "hotter" colors (red, yellow, orange, white) in the false color representation of the OCT 3. The middle layers of the retina, between the NFL and RPE, are much less easily identifiable in the scan. The middle layers of the retina, between the NFL and RPE, are much less easily identifiable in the scan.

Regions For purposes of analysis, the OCT image of the retina can be subdivided vertically into four regions the pre-retina the pre-retina the epi-retina the epi-retina the intra-retina the intra-retina the sub-retina the sub-retina

The pre-retinal profile A normal pre-retinal profile is black space A normal pre-retinal profile is black space Normal vitreous space is translucent Normal vitreous space is translucent The small, faint, bluish dots in the pre- retinal space is "noise" 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. This is an electronic aberration created by increasing the sensitivity of the instrument to better visualize low reflective structures.

Anomalous structures pre-retinal membrane pre-retinal membrane epi-retinal membrane epi-retinal membrane vitreo-retinal strands vitreo-retinal strands vitreo-retinal traction vitreo-retinal traction pre-retinal neovascular membrane pre-retinal neovascular membrane pre-papillary neovascular membrane pre-papillary neovascular membrane

The over-all retinal profile

A pre-retinal membrane with traction on the fovea

a pigment epithelial detachment is causing the convexity

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

Deformations in the foveal profile macular pucker macular pucker macular pseudo-hole macular pseudo-hole macular lamellar hole macular lamellar hole macular cyst macular cyst macular hole, stage 1 (no depression, cyst present) macular hole, stage 1 (no depression, cyst present) macular hole, stage 2 (partial rupture of retina, increased thickness) macular hole, stage 2 (partial rupture of retina, increased thickness) macular hole, stage 3 (hole extends to RPE, increased thickness, some fluid) macular hole, stage 3 (hole extends to RPE, increased thickness, some fluid) macular hole, stage 4 (complete hole, edema at margins, complete PVD) macular hole, stage 4 (complete hole, edema at margins, complete PVD)

Macular cyst

Macular hole, stage 2

Macular hole, stage 3

Macular hole, stage 4, operculum suspended by the hyaloid membrane

The macular profile The macular profile can, and often does, include the fovea as it's center

Deformations in the macular profile Serous retinal detachment (RD) Serous retinal detachment (RD) Serous retinal pigment epithelial detachment (PED) Serous retinal pigment epithelial detachment (PED) Hemorrhagic pigment epithelial detachment Hemorrhagic pigment epithelial detachment

Serous retinal pigment epithelial detachment (PED)

Intra-retinal anomalies in the macular profile Choroidal neovascular membrane Choroidal neovascular membrane Diffuse intra-retinal edema Diffuse intra-retinal edema Cystoid macular edema Cystoid macular edema Drusen Drusen Hard exudates Hard exudates Scar tissue Scar tissue Atrophic degeneration Atrophic degeneration Sub-retinal fibrosis Sub-retinal fibrosis RPE tear RPE tear

Choroidal neovascular membrane

Cystoid macular edema cause by diabetic maculopathy

Sub-retinal fibrosis

OCT and Fluorescein Angiography in retinal diagnosis 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) The OCT gives us information in the z (depth) axis, telling us what layers of the retina are affected

OCT scans for qualitative analysis of the retina The Fast Macular Thickness Scan (FMTS, FMTM, or FMT scan) The Fast Macular Thickness Scan (FMTS, FMTM, or FMT scan) The Line Scan The Line Scan The Cross Hair Scan (3mm and 6mm) The Cross Hair Scan (3mm and 6mm)

The Fast Macular Thickness Scan 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)

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 A scan is saved and then reviewed with any of the retina analysis tools

Each of the 6 scans can be viewed individually by clicking on the thumbnails on the left of the scan selection screen

The Line Scan 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

The Cross Hair Scan 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 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 This is a common technique used in B-scan ultrasonography

Scan analysis protocols for qualitative analysis Line scans can be viewed with a variety of analysis tools (see the OCT manual). I have found the "Align process" to be the most useful, with the "Proportional" analysis a good choice if the align process is not needed

The Align Process This tool "straightens" motions artifacts

Proportional analysis Proportional analysis produces an image with its true horizontal and vertical proportions

Retinal Thickness Analysis Using the retinal thickness analysis tool, the software then traces a line along the NFL layer and a line along the RPE layer. 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

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

Retinal thickness analysis does not measure retinal elevation 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

Serial FMT scans over time 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

Retinal Thickness/Volume Change Analysis Two FMT scans on the same eye, but taken on different dates, can be selected at the same time while holding down the "ctrl" key. "Retinal Thickness/Vol Change is chosen from the analysis tab.

The analysis will give you a "change map", showing the difference between the two scans

Glaucoma Scans 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 The Fast Optic Disc scan The Fast Optic Disc scan The Fast RNFL Thickness scan The Fast RNFL Thickness scan

The Fast Optic Disc scan The Fast Optic Disc scan The optic cup profile can be evaluated by capturing a "Fast Optic Disc" scan 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.

The optic nerve scan can be analyzed with the "optic nerve head analysis" protocol

The Fast RNFL Thickness scan Nerve fiber layer thickness can be evaluated with the "Fast RNFL Thickness" 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.

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

Care must be take to make sure that the image is captured with the circle centered on the optic nerve The placement of the circle can make a big difference in the analysis of the nerve fiber layer thickness The placement of the circle can make a big difference in the analysis of the nerve fiber layer thickness

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).

Is it Perfect? 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.

What makes a good OCT scan? A good quality OCT scan has good reflectivity from edge to edge. A good quality OCT scan has good reflectivity from edge to edge. The "hotter" colors (orange, red, white, yellow) are maximized The "hotter" colors (orange, red, white, yellow) are maximized Generally, the retina should be in the lower portion of the scan window so that the vitreous can be images as well. Generally, the retina should be in the lower portion of the scan window so that the vitreous can be images as well.

Scanning Tips Communicate with the doctor regarding the size and location of the pathology of interest. Communicate with the doctor regarding the size and location of the pathology of interest. Refer to other images of the pathology, e.g. color photos and FA. Refer to other images of the pathology, e.g. color photos and FA. Review past OCT exams and repeat scan types used before. Review past OCT exams and repeat scan types used before. Dilate the eye well?????? Dilate the eye well?????? 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 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

Scanning Tips Use the two buttons near the joystick for freezing and saving scans. This saves you from having to juggle the joystick and the mouse. Use the two buttons near the joystick for freezing and saving scans. This saves you from having to juggle the joystick and the mouse. Minimize patient fatigue by keeping scan time to a minimum. Never scan an eye for more than 10 minutes (FDA regulation). Minimize patient fatigue by keeping scan time to a minimum. Never scan an eye for more than 10 minutes (FDA regulation). Keep the cornea lubricated. Use artificial tears and have the patient blink when you are not saving a scan pass. Keep the cornea lubricated. Use artificial tears and have the patient blink when you are not saving a scan pass. Move the instrument on the x and y axis (using the joystick) to work around opacities Move the instrument on the x and y axis (using the joystick) to work around opacities

Reflectivity may be further enhanced by moving the focus knob on the side of the OCT unit. Reflectivity may be further enhanced by moving the focus knob on the side of the OCT unit.

Alignment and focus Alignment and focus are used to maximize the quality of the scan. Alignment begins with centering and zooming in on the "football" 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

If an image is not visible in the scan window, you should click on the "z-offset optimize" button on the "scan parameter" tab. Once the scan image is visible, it can be move with the z-offset arrows. If an image is not visible in the scan window, you should click on the "z-offset optimize" button on the "scan parameter" tab. Once the scan image is visible, it can be move with the z-offset arrows.

At this point, the "optimize polarize" button should be clicked. At this point, the "optimize polarize" button should be clicked.

This should automatically refine the focus on the retina, and you should see an increase in the "hot" colors in the scan, as illustrated below. The left image is before optimization, the right is after.

At this point, the "scan mode" 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

Freezing and selecting the scan The image can be frozen with a video image with a flash or without a flash The image can be frozen with a video image with a flash or without a flash When ready to freeze the image, tell the patient not blink and let the scan go through several passes before freezing the image When ready to freeze the image, tell the patient not blink and let the scan go through several passes before freezing the image The software saves the last 8 scan passes for review. Always use the review button (on the bottom row of buttons). The software saves the last 8 scan passes for review. Always use the review button (on the bottom row of buttons).

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

Noise The OCT scan can sometimes be improved by changing the "noise" and "range" settings on the "OCT Image" tab. The default settings are indicated by blue markers on the scale

Noise Most instruments produce the best scans when set with the default values. Increasing the noise level will produce "hotter' colors in the scan, but the noise artifacts will also increase. Noise artifacts are those "snowflakes" that you see in the dark areas of the scan (e.g. the vitreous)

What’s New OCT with SLO OCT with SLO OCT with HRA (FA and ICG) OCT with HRA (FA and ICG) Increase in resolution to 5 microns Increase in resolution to 5 microns Overlays, 3D imaging Overlays, 3D imaging

Questions? References: Brancato R. and Lumbroso B. Guide to Optical Coherence Tomography Interpretation. Rome: Innovation-News- Communication, Schuman J., Puliafito C., and Fujimoto J. Ocular Coherence Tomography of Ocular Diseases. Thorofare NJ: Slack Inc., 2004.