Ординатура / Офтальмология / Английские материалы / Optical Coherence Tomography in Age-Related Macular Degeneration_Coscas_2009

Spectral-Domain OCT (SD-OCT) instruments provide ophthalmologists with a significant amount of new information.

The images obtained with SD-OCT are much sharper and more precise that those obtained with Time-Domain OCT (TD-OCT).

With SD-OCT, so much information is provided in a single scan that interpretation of this data must be based on clearly defined logical criteria.

Technology

Time-Domain versus Spectral-Domain

Time-Domain analysis consists of measuring the various times taken by the semi-coherent light beam to travel in the sample arm and the reference arm.

In contrast, Spectral-Domain analysis uses the basic harmonies of the light reaching the detector.

Interpretation

Interpretation of these new images is less intuitive than that of TD-OCT.

Normal or physiological images contain many structures and additional characteristics, some of which remain difficult to interpret.

Images obtained in pathological situations are not only more detailed, but often demonstrate structures and features not recently visualized by other modalities.

Some structures visualized as segmented and heterogeneous on TD-OCT may appear dense and homogeneous on SD-OCT.

It is still difficult to know whether these new features really correspond to pathological lesions or are simply related to optical and electronic phenomena.

For example, the external limiting membrane is much more clearly visualized on SD-OCT than on light microscopy histological preparations.

Similarly, the structure of the retinal pigment epithelium (RPE) appears to be stratified and has a different optical density from the appearance known up until now.

A good knowledge of anatomy and histology is essential to more clearly understand these new images.

In vivo layer-by-layer optical segmentation images of the retina are also entirely new and their interpretation requires a learning process.

Time-Domain Analysis

Time-Domain OCT instruments have two acquisition strategies.

▬Point-by-point acquisition on the longitudinal axis: with a series of A-scans over a defined distance (Zeiss OCT 1, 2, and 3 Stratus*).

B-scans are constructed by a series of vertical segments of A-scans. The reference mirror is moved continuously to detect the plane of analysis.

▬ Acquisition in a coronal plane

(Ophthalmic Technologies, Inc. [OTI*] OCT/SLO).

The instrument uses a second low-frequency galvanometric mirror in the axis perpendicular to the A-scan to obtain segmental scanning.

The B-scan is constructed with a stack of horizontal scans.

Spectral-Domain Analysis

Spectral-Domain analysis uses the first acquisition strategy except that the galvanometric mirror is fixed.

This results in a very large number of A-scans per second (more than 25,000), associated with complete elimination of artifacts.

This rapid scanning rate allows acquisition of:

▬A stack of closely spaced horizontal (raster sections),

▬3D reconstruction of the retina,

▬In vivo optical dissection.

The Spectral B-Scan

The resolution of SD-OCT systems is about 4 to 5 microns, although this resolution is not demonstrated in vivo

SD-OCT scanning is objectively superior to that of previous systems and reveals different appearances in terms of reflectivity and structure.

SD-OCT systems also allow the acquisition of about 6 100 serial scans, from above downwards of the posterior pole, providing a dynamic tomographic image of the macular region (at a visual angle of about 25

degrees).

As a result of this technology, SD-OCT can be compared to ultrasound or a dynamic imaging technique assessing alterations of the various structures of the posterior pole and collecting the most significant images.

Presentation of the Results

The use of a grayscale for OCT images has two consequences (Figures 1 and 2):

OCT examinations conducted without any clinical information about the eye examined, may acquire sections that do not comprise the pathological zone, leading to false-negative results.

Retinal Mapping

Retinal mapping can be obtained in two ways:

▬Very rapid acquisition (about one second) with a grid comprising relatively distant segments;

▬Or a slower method (about two seconds) but with higher resolution on the vertical axis and closely spaced serial scans.

3D Reconstruction

Three-dimensional reconstruction is based on about 50 to 150 B-scans per second (Figures 2 and 3).

The software allows automatic subtraction of the various iso-reflective layers and manual dissection of the 3D cube.

sue and the IS/OS interface and changes of the neuro epithelium (high sensitivity).

This can be explained theoretically by the fact that the human eye has a higher sensitivity for grayscale discrimination than for the chromatic scale.

▬However, false-color imaging highlights the presence of more marked alterations, allowing better definition of simple constituents (relative increase of the specificity of the technique).

The retinal specialist plays an essential role in dynamic study of retinal tissue and in selecting the most effective mode of visualization to demonstrate a specific disease.

For all of these structures, the technician’s role must be limited to simple, routine acquisition, under the ophthalmologist’s supervision.

scans (Figure 4).

Any artifacts can be eliminated manually on the individual scans constituting the map, by slightly altering the scan lines defined automatically by the software.

C-Scan

C-scans correspond to coronal reconstruction of the stack of B-scans.

This method results in a marked reduction of resolution and induces several artifacts, as B-scans may not always be perfectly aligned.

However, this type of C-scan has the advantage of demonstrating certain features by allowing examination of the retina in a plane that is not parallel to the tissue.

92 Chapter 6 · Spectral-Domain OCT/cSLO

Advantages of SD-OCT

SD-OCT visualizes certain layers that have not yet been clearly identified.

The reflectivity of lesions and known physiological structures varies with their depth and their density.

The choroid, for example, is clearly visualized by SDOCT but is difficult to visualize with TD-OCT.

6SD-OCT technology clearly visualizes deeper structures, allowing precise analysis of the choroidal plane.

SD-OCT also appears to be more suitable to examine a structure or a lesion with a particular density in the vertical axis

Contrast

Overall, SD-OCT images can be considered to have a lower contrast than TD-OCT images, making it more difficult to examine hypo-reflective structures, such as the vitreous, vitreoretinal interface, intraretinal fluid, and elements situated close to the RPE-choriocapillaris complex.

Artifacts

Analysis of SD-OCT examinations reveals an almost complete absence of artifacts on the various sections.

A minor difficulty concerns calibration of the reference plane (the focal analysis plane), as two tissue images are acquired.

This analysis is easy to perform on the normal retina but can be more difficult on the pathological retina.

Microperimetry

In AMD, microperimetry can be used to evaluate the extent and density of scotomas and precisely record the presence of micro scotomas that are difficult to identify by other methods.

It can also be used to localize the fixation point, evaluate its stability, and localize this point in relation to the scotoma.

OTI* SD-OCT allows immediate and simultaneous evaluation of retinal function and morphological data concerning retinal thickness and edema.

The apparatus software allows a combination of these functional tests and visualization of morphological abnormalities.

Conclusion

In summary, the OCT/SLO-OTI appears to be particularly useful in two fields.

First, it can be used to acquire 3D images that provide important information for diagnosis and which are very useful for teaching purposes.

Second, and possibly even more useful, OCT/SLO-OTI allows a combination of microperimetry and OCT to overlay retinal thickness mapping and functional studies of the retina.

Microperimetry provides an exact point-by-point correspondence between fundus images and perimetry results.

Microperimetry combined with OCT allows real-time retinal function studies with ophthalmoscopic control of the retinal surface (Menke 2006 1 ).

An important field of application is the study of retinal sensitivity in various macular diseases.

Many studies are in process in different diseases such as diabetic retinopathy (before and after treatment) (Carpineto 20072 Vujosevic 20063 Okada 20064), and other diseases ( Carpineto 2005 5 Ojima 20086 Charbel 20087), as well as definition of the fixation point in patients with low vision.

This easy-to-use instrumentation combining OCT and microperimetry can therefore provide detailed analysis of fixation and correlations between anatomical lesions and retinal dysfunction.