Practical suggestions to obtain clear and clinically useful optical coherence tomography imagesCHAPTER 1
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 pseudocolors. This allows to assess slight variations in the intensities of gray and makes 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. About 10–30 or more OCT images of the same area are taken and then overlaid and averaged automatically by software.
All 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 (Figure 1).
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.
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.
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.
Figure 1: Image averaged 100 times. 100 optical coherence tomography (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 (Optovue RTVue).
Courtesy: Andre Romano, Sao Paulo, Brazil.
It offers images that facilitate understanding of the abnormalities in the vitreofoveal interface.
OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY SCANS
Since 2014, a new technology, OCT angiography, has rapidly developed and replaced in many cases the invasive fluorescein and indocyanine green (ICG) angiography. The method is easy, fast, innocuous, it gives no side effects, and can be repeated many times without problems for the patients.
In age-related macular degeneration and in central serous chorioretinopathy, angio-OCT scans allow the study of the dimensions and shape new vessels. They have induced new classifications and allow an easy follow-up.
In diabetic retinopathy, in vascular occlusions, they help in understanding type and extension of capillary occlusions, cystoid retinal edema. They are also most useful in choroid and retinal inflammation.
‘EN FACE’ ONE CLICK MODE
To make simpler and faster acquisition and use of en face images, the ‘en face one click mode’ (Figure 2) 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 retinal pigment epithelium (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, 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
Optical coherence tomography devices allow detecting with precision retinal boundaries and retinal layers in normal retina. Structural OCT 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 (Figure 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. 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.
Figure 2: Lumbrosovue 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 retinal pigment epithelium (RPE) cuts through drusen and RPE 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.
Figure 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).
In many cases, it will be useful for the expert OCT operator to make manually the segmentation.
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. 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 OPTICAL COHERENCE TOMOGRAPHY IMAGING
Optical coherence tomography 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.
In today OCT devices, we see few artifacts. We can still find artifacts in procedures that need a longer acquisition time, like macular map and three-dimensional (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 (~ 40–60).
STANDARD OPTICAL COHERENCE TOMOGRAPHY 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.6