Practical Handbook of OCT (Retina, Choroid, Glaucoma) Bruno Lumbroso, Marco Rispoli
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Practical Suggestions to Obtain Clear and Clinically Useful OCT ImagesChapter 1

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Standard Optical Coherence Tomography Analysis
Standard analysis should include qualitative analysis and quantitative analysis of one or two line scans; cross scan study centered on fovea, the line scans should be centered on the point we want to study (if it is not located on the fovea); full retinal map analysis; tridimensional analysis; en face analysis, and progression analysis.
 
Gray Scale
We advise to study optical coherence tomography (OCT) images in scale of grays (black and white) rather than conventional pseudo colors. This allows to assess slight variations in the intensities of gray and make out details that could be otherwise easy to miss. Negative black and white images can sometimes help to highlight certain pathological patterns difficult to distinguish.
When using color images, the software arbitrarily gives a color for each degree of reflectivity, allowing us to see marked differences in color and giving an abnormally high contrast where we would otherwise observe a gradual change in reflectivity. Note that this higher contrast may be useful in some cases along with segmentation and delineation of retinal layers boundaries.
 
Averaging
Multiple cross-section scan averaging is an easy and practical method to improve the quality of OCT retinal images. Ten to thirty or more OCT images of the same area are taken, and then overlaid and averaged automatically by software.
Most OCT devices use this technique that improves the sharpness and makes images more intelligible, helping to diagnose and manage retinal diseases. Some devices like Optovue can average 100 or more images (Fig. 1.1).
Averaging OCT scans improves visualization of retinal structures and allows a better delineation of boundaries between layers. OCT signal is enhanced by increasing signal-to-noise ratio and is useful in defining more clearly retinal layers and lesions. Photoreceptor layers and some retinal pigment epithelium (RPE) lesions that could not be identified clearly in single-scan OCT images because of retinal images being unclear and fuzzy are seen much better. Using averaged images we see many pathological conditions in a much better way including diffuse retinal edema, cystoid macular edema, retinal hemorrhages, pigment epithelial detachment, choroidal structures and lesions beneath the pigment epithelium.
 
En Face Scans
En face technology allows to acquire and analyze en face frontal retinal scans adapted to the natural concavity of posterior pole of the eye. These frontal or transverse scans are part of the tridimensional study of the retina. They form a useful complement to the conventional cross-sectional B-scan OCT imaging that is much more intuitive and easier to understand. Clinical studies have confirmed the interest of en face OCT scans.
In age-related macular degeneration and in central serous chorioretinopathy, frontal scans allow the study of the dimensions and shape of pigment epithelium detachments, and a minute control of the photoreceptors. They help to assess shape thickness and smoothness of their walls.
In diabetic retinopathy, en face scan helps in understanding type and extension of diffuse and cystoid retinal edema, and hard exudates.
Transverse scans are essential in studying cystoid macular edema, allowing identification of the extension and shape of the edema pseudocysts, their evolution, their pattern and stage: early, advanced, regressive.
They are also useful in white dot syndromes and in acute zonal occult outer retinopathy. They show shape and ramification of the outer retinal tubulations.
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Figure 1.1: Image averaged 100 times. One hundred OCT images of the same area have been overlaid and averaged by software. This technique highly improves the sharpness and makes images clear and more intelligible.Courtesy: Andre Romano, Sao Paulo, Brazil (Optovue RTVue).
In cases of vitreoretinal interface lesions, epiretinal membranes, and macular holes, frontal en face imaging enables in vivo identification of the extension and dynamics of epiretinal traction. It offers images that facilitate understanding of the abnormalities in the vitreofoveal interface.
 
En Face One Click Mode
To make simpler and faster acquisition, and use of en face images, RTVue-100 software has recently proposed the “en face one click mode” (Fig. 1.2) that permits to obtain in a single move five scans: four “en face” images relative to the most clinically important frontal sections and a cross line scan.
The first en face scan is taken at retinal surface level and shows retina-vitreous interface alterations. The second is placed deeper in retina and shows eventually diffuse edema, cystoid edema and exudates. The third en face scan is parallel to RPE, and cuts through drusen and RPE detachments. The fourth scan is placed in the choroid, parallel to RPE at the level of Haller's vascular layer to show choroid condition.
The OCT cross line scan shows the exact level in the retina of the four frontal scans.
Thus in an immediate and easy way, OCT user will get instantaneously the four “en face” images most clinically useful to fine-tune diagnosis, and will know their exact position in relation to retina and choroid layers.
 
Segmentation
Some OCT devices allow detecting with precision retinal boundaries in normal retina. In general, spectral domain software is successful in automatically detecting and delineating retinal boundaries, resulting in more accurate retinal thickness maps, and helping in making the diagnosis and management of retinal diseases (Fig. 1.3).
Optical coherence tomography systems accurately detect the retinal boundaries in eyes with some degree of macular edema, but not as consistently in eyes with advanced diseases as diabetic retinopathy and age-related macular degeneration. Boundaries in these cases frequently are incorrectly identified by the software.
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Figure 1.2: Lumbroso vue RTVue-100 software permits to obtain in a single click five scans: four “en face” images and a cross line scan. The first en face scan is taken at retinal surface level and shows retina vitreous interface alterations, the second is placed deeper in retina and may study edema and exudates. The third scan parallel to RPE cuts through drusen and retinal pigment epithelium detachments. The fourth is set deeper in the choroid to show choroidal vessels condition. The fifth scan, cross line scan, shows exact depth of the “en face” images.
In case of loss of structure, manual segmentation is very difficult or impossible and automated segmentation is quite impossible. We have to learn and understand when segmentation is clinically useful and when it may mislead the diagnosis.
 
Progression Analysis in the Follow-up of Disease Evolution
Progression analysis software is a very useful tool for the follow-up of disease evolution with or without treatment. Maps showing thickness data can be compared from visit to visit.
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Figure 1.3: Segmentation, spectral domain Nidek device detects automatically retinal boundaries resulting in easier diagnosis and management of retinal diseases at their onset. In this scan five boundary lines have been selected (Nidek technologies).
It is also important to compare these maps with microperimetry maps. They are essential in monitoring disease progression.
Significance maps detailing thickness values that vary from database will bring precious evaluation data.
 
Artifacts in OCT Imaging
Spectral domain technology has allowed a decrease of artifacts in OCT imaging. The very fast acquisition (over 40 images per second) and the capability to average images produce normal OCT scans clear and well-defined. Eye movements and blinking can modify only one or two images of the 30 and over stack's images.
We can still find artifacts in procedures that need a longer acquisition time, like macular map and 3D. In these protocols the instrument acquires more than 100–140 images to compose a virtual cube. Usually macular cube acquisition needs about 3–4 seconds. Eye can miss fixation or blink. Software is normally able to adjust scans that are not perfectly aligned. These artifacts look like horizontal notches on the macular cube and may modify the result of algorithm in calculating macular segmentation and thickness measures.
If only few scans are imperfect, it is possible to correct manually the profile and adjust the 3D view; in other cases, it is necessary to repeat acquisition.
If the subject is unable to maintain fixation for 3 seconds, it will be possible to acquire a low resolution cube in 1–2 seconds. This cube is made by fewer scans (approximately 40–60).
 
Standard OCT Prints
Standard OCT prints should be sent to the referring ophthalmologist alongside the OCT report, and given to the patient. Prints should include at least one cross scan centered on fovea, one or more line scans centered on the point we want to study (if it is not located on the fovea), a full retinal map analysis, and an en face analysis. When possible, progression analysis maps will help with the follow-up. Standard OCT prints will always be completed by a detailed OCT report.