2 Fundamentals of Retinal Optical Coherence Tomography
33
Further developments of OCT technology may impact the diagnosis of eye diseases and improve the management of the significant clinical and public health problems associated to visual impairment. Finally, there is growing evidence to incorporate the OCT technology into clinical settings managing cerebrovascular and neruological diseases [35]. However, a low-cost approach solution must be reached to
successfully introduce its application in telemedicine and population-based screening programs [36].
2.3 Interpretation of the Optical Coherence Tomography
Image
The OCT signal from a tissue layer is a combination of its reflectivity and the absorption and scattering properties of the overlying tissue layers. Strong reflections occur
at the boundaries between two materials of different refractive indices or may originate from a tissue that has a high scattering coefficient along with a disposition to
scatter light in the perfectly backward direction [3, 26]. Thus, an OCT image is a
map of the reflectivity of the sample. In most tissues, primary sources of reflection
are collagen fiber bundles, cell walls, and cell nuclei. Dark areas (i.e. areas with low
reflectivity) on the image represent homogeneous material with low reflectivities,
such as air or clear fluids. The imaging light is attenuated in the sample which leads
to an exponential decrease in the intensity of the image with depth. Blood attenuates
the signal faster than collagenous tissues while fat and fluids attenuate the signal the
least.
In OCT images, the signal strength is represented in false color or grey-scale. In
the case of false color representation, high backscatter appears red-orange and low
backscatter appears blue-black (see Fig. 2.4). Thus, tissues with different reflectivity
are displayed in different colors. It is important to note that OCT image contrast arises
from intrinsic differences in optical properties of tissues. Thus, coloring of different
structures represent different optical properties in a false-color image, and it is not
necessarily different tissue pathology (see Fig. 2.4). The exact relationship between
the histology of the tissue and the OCT map is still under investigation. Usually,
relative high layers correspond to areas of horizontal retinal elements such as the
RNFL at the retinal surface or the deeper plexiform layers and finally the outermost
single layer of RPE. Relative low reflectivity layers correspond to the nuclear layers
and a single layer of photoreceptor inner and outer segments (see Figs. 2.4 and 1.4).
Warm colors (red to white) represent areas of relatively high reflectivity, while cold
colors (blue to black) represent areas of relatively low reflectivity. However, from
a qualitative point of view, grey-scale OCT images are superior to the color-scale
pictures, avoiding misleading interpretations of the OCT reflectivity.
A typical example of color coded OCT images of the human macular for normal
and pathologic eyes is shown in Fig. 2.4. The OCT image shown in Fig. 2.4b is
from a subject with DME. This image demonstrates thickening of the macula with
33
Further developments of OCT technology may impact the diagnosis of eye diseases and improve the management of the significant clinical and public health problems associated to visual impairment. Finally, there is growing evidence to incorporate the OCT technology into clinical settings managing cerebrovascular and neruological diseases [35]. However, a low-cost approach solution must be reached to
successfully introduce its application in telemedicine and population-based screening programs [36].
2.3 Interpretation of the Optical Coherence Tomography
Image
The OCT signal from a tissue layer is a combination of its reflectivity and the absorption and scattering properties of the overlying tissue layers. Strong reflections occur
at the boundaries between two materials of different refractive indices or may originate from a tissue that has a high scattering coefficient along with a disposition to
scatter light in the perfectly backward direction [3, 26]. Thus, an OCT image is a
map of the reflectivity of the sample. In most tissues, primary sources of reflection
are collagen fiber bundles, cell walls, and cell nuclei. Dark areas (i.e. areas with low
reflectivity) on the image represent homogeneous material with low reflectivities,
such as air or clear fluids. The imaging light is attenuated in the sample which leads
to an exponential decrease in the intensity of the image with depth. Blood attenuates
the signal faster than collagenous tissues while fat and fluids attenuate the signal the
least.
In OCT images, the signal strength is represented in false color or grey-scale. In
the case of false color representation, high backscatter appears red-orange and low
backscatter appears blue-black (see Fig. 2.4). Thus, tissues with different reflectivity
are displayed in different colors. It is important to note that OCT image contrast arises
from intrinsic differences in optical properties of tissues. Thus, coloring of different
structures represent different optical properties in a false-color image, and it is not
necessarily different tissue pathology (see Fig. 2.4). The exact relationship between
the histology of the tissue and the OCT map is still under investigation. Usually,
relative high layers correspond to areas of horizontal retinal elements such as the
RNFL at the retinal surface or the deeper plexiform layers and finally the outermost
single layer of RPE. Relative low reflectivity layers correspond to the nuclear layers
and a single layer of photoreceptor inner and outer segments (see Figs. 2.4 and 1.4).
Warm colors (red to white) represent areas of relatively high reflectivity, while cold
colors (blue to black) represent areas of relatively low reflectivity. However, from
a qualitative point of view, grey-scale OCT images are superior to the color-scale
pictures, avoiding misleading interpretations of the OCT reflectivity.
A typical example of color coded OCT images of the human macular for normal
and pathologic eyes is shown in Fig. 2.4. The OCT image shown in Fig. 2.4b is
from a subject with DME. This image demonstrates thickening of the macula with