6 Diagnostic Capability of Optical Coherence Tomography …
147
in an image or selected region of interest (ROI), the image irregularities can be
measured. Consequently, texture features, such as contrast and fractal dimension
could be analyzed for the macula and each intraretinal layer. The fractal dimension
(FD) of a profile or surface is a roughness measure regarded as a local property of
the system with higher values indicating rougher surface [136]. There are different
methods to determine the FD. The typical conventional approach used to calculate
the FD of an image is the box-counting method but the power spectrum method is
demonstrated to be more robust [137, 138].
The most common parameter investigated during the OCT examination is retinal
thickness. Reflectance is the direct measurement from which thickness is calculated
in OCT systems. The human retina is an almost transparent tissue that only reflects
about 1% of the incident light [139]. Retinal tissue is characterized by many small
random fluctuations in refractive index caused by the ultrastructure of the tissue
[140]. Therefore, incident light on tissue is deflected or scattered off this structure.
Therefore, differences in optical properties of normal and abnormal retinal tissue
may also provide additional information of disease development in pathological
eyes allowing OCT technology to be used for quantitative analysis of tissue optical
properties [43, 141, 142]. Accordingly, Bizheva et al. have shown previously that
optical properties of the retina may change due to their metabolic activity. They were
using OCT for this purpose and named the method optophysiology [34]. Huang et al.
have shown the early changes of reflectance of the RNFL in a rat model of glaucoma
preceding the pathological changes in the retina [52]. Gao et al. shown previously
that diabetes not only causes thinning of the inner retinal layers, but also reduces
the amplitude of the back-reflected signal from these layers [142]. Consequently,
diagnostic predictors based on reflectance changes may be of interest in MS as well
where pathological processes of the inner retina have been well described previously.
Varga et al. assessed the differences in texture descriptors and optical properties
of retinal tissue layers in 38 patients with MS and 24 healthy subjects [16]. Patient
group was divided based on the medical history, whether they previously had had
ON episode or not. Optical parameters such as contrast, FD, layer index and total
reflectance were measured. They found significant difference in contrast in the RNFL,
GCL+IPL, GCC, INL and OPL when comparing MS with ON to the other groups.
Higher fractal dimension values were observed in GCL+IPL and INL layers when
comparing healthy and MS with ON groups. A significant difference was found in
layer index in the RNFL, GCL+IPL and GCC layers in all comparisons. A significant
difference was observed in total reflectance in the RNFL, GCL+IPL and GCC layers
between the three examination groups. Overall, this study found that texture and
optical properties of the retinal tissue undergo pronounced changes in MS even
without ON. These results draw attention to the structural and optical changes in
the macular area in MS even without ON supporting the previous view of ongoing
neurodegeneration also present in the retina [31, 32, 120, 143]. The inner retinal
changes appear to be related to central nervous system changes, e.g. intracranial or
brain substructure volume reduction (i.e. brain atrophy) [31, 144, 145]. Therefore,
OCT may provide a possibility to better understand the neurobiological changes in
neurodegenerative diseases such as MS and may help to develop both diagnostic
147
in an image or selected region of interest (ROI), the image irregularities can be
measured. Consequently, texture features, such as contrast and fractal dimension
could be analyzed for the macula and each intraretinal layer. The fractal dimension
(FD) of a profile or surface is a roughness measure regarded as a local property of
the system with higher values indicating rougher surface [136]. There are different
methods to determine the FD. The typical conventional approach used to calculate
the FD of an image is the box-counting method but the power spectrum method is
demonstrated to be more robust [137, 138].
The most common parameter investigated during the OCT examination is retinal
thickness. Reflectance is the direct measurement from which thickness is calculated
in OCT systems. The human retina is an almost transparent tissue that only reflects
about 1% of the incident light [139]. Retinal tissue is characterized by many small
random fluctuations in refractive index caused by the ultrastructure of the tissue
[140]. Therefore, incident light on tissue is deflected or scattered off this structure.
Therefore, differences in optical properties of normal and abnormal retinal tissue
may also provide additional information of disease development in pathological
eyes allowing OCT technology to be used for quantitative analysis of tissue optical
properties [43, 141, 142]. Accordingly, Bizheva et al. have shown previously that
optical properties of the retina may change due to their metabolic activity. They were
using OCT for this purpose and named the method optophysiology [34]. Huang et al.
have shown the early changes of reflectance of the RNFL in a rat model of glaucoma
preceding the pathological changes in the retina [52]. Gao et al. shown previously
that diabetes not only causes thinning of the inner retinal layers, but also reduces
the amplitude of the back-reflected signal from these layers [142]. Consequently,
diagnostic predictors based on reflectance changes may be of interest in MS as well
where pathological processes of the inner retina have been well described previously.
Varga et al. assessed the differences in texture descriptors and optical properties
of retinal tissue layers in 38 patients with MS and 24 healthy subjects [16]. Patient
group was divided based on the medical history, whether they previously had had
ON episode or not. Optical parameters such as contrast, FD, layer index and total
reflectance were measured. They found significant difference in contrast in the RNFL,
GCL+IPL, GCC, INL and OPL when comparing MS with ON to the other groups.
Higher fractal dimension values were observed in GCL+IPL and INL layers when
comparing healthy and MS with ON groups. A significant difference was found in
layer index in the RNFL, GCL+IPL and GCC layers in all comparisons. A significant
difference was observed in total reflectance in the RNFL, GCL+IPL and GCC layers
between the three examination groups. Overall, this study found that texture and
optical properties of the retinal tissue undergo pronounced changes in MS even
without ON. These results draw attention to the structural and optical changes in
the macular area in MS even without ON supporting the previous view of ongoing
neurodegeneration also present in the retina [31, 32, 120, 143]. The inner retinal
changes appear to be related to central nervous system changes, e.g. intracranial or
brain substructure volume reduction (i.e. brain atrophy) [31, 144, 145]. Therefore,
OCT may provide a possibility to better understand the neurobiological changes in
neurodegenerative diseases such as MS and may help to develop both diagnostic