 |
Module 37 |
|||
|
|
|
|||
|
Module 37: |
Optical Coherence Tomography |
|||
|
|
Part 1: Qualitative Analysis and scanning techniques | |||
|
|
||||
| This module contains many images. The module is not divided into sections so that subsequent images can load while you are reading the module. If you have a dial-up internet connection, you may have to wait for some images to load. | ||||
|
The OCT retina scan compared to retinal anatomy Qualitative and quantitative analysis Pre-retinal profile Overall retinal profile Foveal profile Macular profile OCT and fluorescein angiography in retinal diagnosis OCT scan protocols vs. scan analysis OCT scans for qualitative analysis of the retina Fast macular thickness scan Line scan Cross hair scan Posterior pole scans Scan analysis protocols for qualitative analysis Align Process Proportional Normalize Normalize and Align Gaussian and Median Smoothing |
||||
|
This module talks about Optical Coherence Tomography (OCT) in general, but refers specifically to the Zeiss Stratus OCT 3 as applied to the retina and the optic nerve head.
In a nutshell, 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. There is no cutting involved. 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.
The OCT uses an interferometer that measures the time it takes for light to be reflected back from structures in the retina, as compared to the time it takes for light to be reflected back from a reference mirror at specific distances. The process is similar to that of ultrasonography, except that light is used instead of sound waves.
The Zeiss OCT 3 can make from 128 to 768 axial samples (A-scans) in a single "scan pass". Each A-scan has 1024 data points and is 2mm long (deep). The following illustration of a scan pass is an animation that is best viewed on a computer screen.
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. Ten microns is 1/100th of a millimeter. Compare that to the resolution of a good ophthalmic ultrasound at 100 microns, or 1/10th of a millimeter. The difference is illustrated below. 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.
|
||||
 |
 |
|||
|
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. The protocols are represented by descriptive icons in the software, as pictured below.
|
||||
| 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. |
|
|||
| All of the scan protocols are derived from either a circle or a line scan. For example, a line scan can be used in a cross-hair pattern or an X pattern. The circle pattern can be used to create a series of concentric circle scans. The patterns can be placed anywhere in the posterior pole, most typically over the fovea and the optic nerve. You also have the option of designing your own protocol. Several of the most useful scan protocols will be discussed later in the module. One of the most useful scan protocols is the "radial lines" scan pictured below. | ||||
| The "radial lines" scans 6 consecutive line scans in a star pattern. |
|
|||
|
The OCT retina scan compared to retinal anatomy
Before comparing the OCT scan to retinal anatomy, be aware that not all OCT scans are created equally, as far as resolution is concerned. The "fast" scan protocols of the OCT 3 reduce the time needed for multiple scans from 3+ seconds to about 2 seconds. The scan time reduction is intended to minimize the error created by patient movement. The downside of the fast scan is that fewer A-scans are grabbed 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 as shown below.
Fast OCT 3 scan
The same eye scanned with maximum resolution
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, or dip, in the retinal surface. 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 3. The middle layers of the retina, between the NFL and RPE, are much less easily identifiable in the scan. Compare the schematic below to the OCT image.
Qualitative and Quantitative Analysis
The Zeiss OCT 3 allows both qualitative and quantitative analysis of the retina.
Qualitative analysis involves describing or identifying morphological changes and anomalous structures in the retina. Morphology is the study of forms and structures of organisms. An anomaly is a deviation from what is normal (an irregularity).
Quantitative analysis is possible because the OCT 3 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. Multiple thickness measurements can be interpolated into a volume measurement 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.
For purposes of analysis, the OCT image of the retina can be subdivided vertically into four regions:
as shown on the next image.
OCT retinal morphology (form and structure) can be subdivided into four "profiles": Each profile has it's own set of deformations and anomalous structures.
1. pre-retinal profile 2. overall retinal profile 3. foveal profile 4. macular profile
The pre-retinal profile
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.
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.
When scanning the macula, the operator should always try to locate the foveal depression. Anatomically, the fovea is the center of the macula. Physiologically, the fovea provides our sharp 20/20 central vision. The foveal depression is an important indicator of the health of the macula. The absence of the depression is indicative of a significant disease process. Scanning techniques for locating the foveal depression will be discussed later in this module.
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)
Some examples follow:
macular cyst:
macular hole, stage 2:
macular hole, stage 3:
macular hole, stage 4, the tissue suspended by the hyaloid membrane is called an operculum:
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.
The often used "fast macular scan" scans 6 radial lines through the fovea in a star burst pattern. However, OCT macular scans do not have to be centered on the fovea. The various scan patterns can be moved to any location in the posterior pole.
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
Some examples are pictured below:
choroidal neovascular membrane:
cystoid macular edema cause by diabetic maculopathy:
sub-retinal fibrosis
Qualitative analysis of the OCT scan includes observation of the reflective qualities of the retinal structures. The OCT software assigns "cooler" colors (green, blue) to structures with lower reflectivity. It assigns "warmer" colors (yellow, orange, red) to more highly reflective structures. White represents the most highly reflective structures, and black represents the least reflective structures.
On the OCT scan of a normal retina, the NFL and RPE are highly reflective, the middle retinal layers are medium reflective, and the photoreceptors (just above the RPE layer) are low reflective.
Reflectivity helps to identify anomalous structures. The sub-retinal fibrosis pictured below is identified by it's location, shape, and highly reflective structure.
The most highly reflective anatomical layers are the NFL and the RPE. Highly reflective anomalous structures include areas of dense pigmentation, scar tissue (image above), neovascularization, and hard exudates.
The most obvious areas of low reflectivity are the vitreous space at the top of the scan, and the black space at the bottom of the scan. The area at the bottom of the scan is black because all of the light has been absorbed or reflected by the retinal structures above. Light does not penetrate very far past the choroidal tissues just below the RPE layer.
Low reflective anomalous structures are areas of edema (fluid). These may be in the form of intraretinal cavities, cysts, diffuse intraretinal edema, or exudative detachments (image below).
Black areas in the OCT scan may also be caused by shadowing. Shadowing occurs when a dense structure prevents light from penetrating below the structure, just like you or I cast a shadow on the ground in bright sunlight.
Shadow "cones" can appear below normal retinal blood vessels, a dense hemorrhage, exudates, a densely pigmented formation (e.g. RPE hypertrophy, choroidal nevus), and a thick choroidal neovascular membrane.
The image below illustrates shadowing. There is a dense intraretinal hemorrhage and a dense subretinal neovascular membrane that are both causing shadowing.
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.
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.
The scan below has waves in the retinal contour. These are not retinal folds, but rather movement of the eye during the scan pass.
This next scan has reduced brightness and detail on the right side. This is not intra-retinal edema. This is caused by a blockage of the light as it passes into the eye. The OCT was not centered well in the pupil, with some of the light striking the edge of the pupil.
OCT and Fluorescein Angiography in retinal diagnosis
The advent of fluorescein angiography (FA) revolutionized the diagnosis and treatment of retinal (particularly macular) diseases. In many ways, the advent of the OCT has been a second revolution. FAs and OCTs present diagnostic information in distinctly different formats, but some of the information has "overlapping" diagnostic value. Although sodium fluorescein is a relatively benign diagnostic intravenous dye, the procedure is invasive and it is presents elements of discomfort and risk to the patient. It would obviously be better patient care if diagnosis and treatment of a retinal disease could be adequately managed with OCT imaging instead of fluorescein angiography.
As discussed in the modules on fluorescein angiography, 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. The information overlaps somewhat concerning the presence of fluid in and under the retina. Both modalities give us information about how much fluid is present and what layers are affected, as is illustrated by the color fundus photo, FA, and OCT of a pigment epithelial detachment.
As will be discussed in the next section on quantitative analysis, the OCT has the added benefit of being able to give us retinal thickness and volume measurements that can be compared over time. The PED above is a good example of a macular disease that can be diagnosed and followed with the OCT alone. The PED has a characteristic appearance when viewed in stereo with a fundus lens at the slit lamp. The diagnosis can be confirmed with an OCT scan, which also has a characteristic appearance of a fluid space pushing up under the RPE layer. The treatment course can be followed with serial OCT qualitative analysis, and with retinal thickness and volume measurements.
Macular cysts and holes are anomalies that can be diagnosed and treated without fluorescein angiography. The OCT is now the modality of choice for holes and cysts because the scan can clearly demonstrate the stages of hole formation. Before OCT, it was useful to use fluorescein angiography to confirm the presence of fluid adjacent to the hole. The OCT now gives us that information also.
Macular cyst and hole diagnosis/confirmation is not automatic with the OCT. It is possible to miss the anomaly in a scan session. This patient was suspected of having a macular cyst and an OCT scan was performed. The patient had good fixation and it was assumed that horizontal and vertical scans through the fixation point would pick up any cyst, with the following result:
Upon not getting the expected result, the eye was rescanned after a "scan search" of the area. The cyst was found nearby and documented as follows (vertical and horizontal scans):
A scanning protocol can be followed that will improve the accuracy of the session regarding finding and documenting pathology.
Diabetic maculopathy is another pathology that can be followed with the OCT scan alone.
The intra-retinal macular edema is plainly evident in this scan. Retinal thickness and volume measurements can be obtained serially with the OCT to follow the patient's progress after treatment.
The OCT is particularly useful when following the patient with choroidal neovasularization (CNV) post-treatment. Intra-retinal edema (fluid) is a marker for continuing leakage of the net. With fluorescein angiography, it is sometimes difficult to tell if photos late in the dye sequence are showing fluid or just staining of scar tissue, as pictured here:
An OCT scan of the lesion confirms that the FA is showing late staining of a scar, with little intraretinal edema.
OCT scan protocols vs. OCT scan analysis protocols
There are two major "tabs" or groupings on the OCT 3 scan screen: "scan" and "analysis". The "scan" tab lists all the available scan protocols. These are simply the methods of scanning as discussed earlier, which include line scans, radial line scans, and circle scans. The "analysis" tab lists the analysis protocols, which are different ways that the software processes and presents the scan image and the scan data. Analysis is "post-processing" that is performed on a scan that is saved and selected from the scan list for the particular patient.
OCT scans for qualitative analysis of the retina
OCT scan protocols that are particularly useful for qualitative analysis of the retina include the following:
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). Quantitative analysis will be discussed in more detail in the next module.
The FMT scan is also useful as a screening tool for qualitative analysis of the retina. 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. To view each of the six scans, click on the "scan selection" button at the bottom of the analysis window.
Each of the 6 scans can be viewed individually by clicking on the thumbnails on the left of the scan selection screen (black arrow below).
The scan angles that demonstrate the pathology the best can then be noted (red arrow). Since the FMT scan is a low resolution scan, a high resolution line scan can be performed at the angle of interest. As pictured below, when using the line scan, the angle can be changed in the "scan parameter" window.
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. After the eye is screened with the FMT scan, the line scan can be used to provide a high resolution scan of the area of interest. The scan angle from the FMT scan is simple inputted into the angle for the line scan. The line scan can be "dragged" on the video window with the mouse to "survey" an area to look for pathology.
The Cross Hair Scan
Most pathology can be adequately characterized by using two scans 90 degrees apart. This is a common technique used in B-scan ultrasonography. The OCT 3 provides a Cross Hair Scan that performs a high resolution horizontal line scan and then automatically flips to a vertical line scan without having to exit the protocol.
As with the line scan, the length of the line in the cross hair scan can be changed (arrow in the picture below). Commonly used lengths are 3 and 6 mm (3 is the default value). The 6mm line gives a "wider angle" view, but the resolution is slightly lower because the number of scans is the same (512) for each length.
Some practitioners prefer the cross hair as a standard technique, rather than taking the time to screen with the FMT scan. This scan protocol works very well for a lesion of known dimensions and location. If you are unsure of the location of the pathology, or even the existence of pathology, it is best to screen with the FMT scan, or to drag a line scan over the area of interest.
The 7mm posterior pole scan
The fovea is a very important anatomical location on the retina. It is the location of our central vision. It is the area of the retina that has 20/20 acuity. When evaluating macular disease with the OCT, at least one scan through the fovea is a pre-requisite. The FMT scan, a line scan, or the cross-hair scan will provide a scan through the fovea if the patient has foveal fixation.
The is only one way to tell for sure if you are scanning through the fovea, by the appearance of the foveal depression on the scan, as pictured below.
What if there is no foveal depression, as is the case with this eye, pictured below?
How would you know that you are scanning through the anatomical location of the fovea? You can't always rely on patient fixation. Sometimes the patient cannot fixate well because of the disease process. Sometimes the patient can fixate, but the fixation is "eccentric", meaning the eye is not fixing with the fovea.
Greg Hoffmeyer at Duke University was one of the first to use the OCT technology extensively. He recognized this problem early on and he came up with a scan protocol to locate the scan through the foveal area even when there is poor patient fixation and/or no foveal depression.
Anatomically, the fovea is located a little distance below a horizontal line drawn through the center of the optic nerve head.
If you draw a 7mm line from the center of the temporal edge of the optic nerve head, at a downward 3 degree angle, the line will pass through the anatomical location of the normal fovea.
This is Greg's "7mm Posterior Pole Scan". Some practitioners use this scan routinely. It is always good practice to use this scan if you are unsure that you have a scan through the fovea. The scans have to be different for the right eye and the left eye in order to have the correct angle. Be sure to choose the scan marked for the eye you are scanning. The default position of the scan is usually sufficient, but the scan can be dragged so that the optic nerve end of the line is aligned with the center of the temporal edge of the optic nerve.
Scan analysis protocols for qualitative analysis
Line scans can be viewed with a variety of analysis tools (see the OCT manual). We have found the "Align process" to be the most useful, with the "Proportional" analysis a good choice if the align process is not needed. We have not found the other line analysis protocols to be clinically useful. Some other practitioners use the "Normalize" protocol routinely. All of the protocols present the "analyzed" image on top, and the standard image on the bottom. If you are viewing a cross hair scan, the horizontal and vertical images can be viewed by using the the slider on the left of the screen.
The Align Process
This tool "straightens" motions artifacts, as pictured below.
If there are no motion artifacts in the scan, the align process tends to flatten out the natural curves in the image, as pictured below.
Proportional
Proportional analysis produces an image with its true horizontal and vertical proportions. The OCT ordinarily presents line images that are stretched vertically, so that you can see more detail in the vertical axis.
Normalize
Normalize analysis produces an image with equal brightness and with an equal color range when compared to other scans. This process eliminates noise, but in the process also removes some data from the scan image.
Normalize and Align
Normalize and Align analysis combines the two functions into one image.
Gaussian Smoothing and Median Smoothing
The smoothing analysis functions "average out the noise and blend the colors of the scan image", according to the manual, whatever that means.
The module assumes that you have some basic knowledge of how to use the instrument. The basic knowledge is best gained by reading the manual and by observing someone else perform scans. Although there are many "automatic" features to the OCT 3, an experienced operator is needed to maximize the potential of the instrument. Here are some tips collected from conversations with experienced operators.
There is more discussion on scanning techniques in the module on OCT quantitative analysis.
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. |
||||
| Back to top Go to Post-Tests | ||||
