Enface Thickness Mapping and Reflectance Imaging of Retinal Layers in Diabetic Retinopathy

Abstract

Purpose: To present a method for image segmentation and generation of enface thickness maps and reflectance images of retinal layers in healthy and diabetic retinopathy (DR) subjects.

Methods: High density spectral domain optical coherence tomography (SDOCT) images were acquired in 10 healthy and 4 DR subjects. Customized image analysis software identified 5 retinal cell layer interfaces and generated thickness maps and reflectance images of the total retina (TR), inner retina (IR), outer retina (OR), and the inner segment ellipsoid (ISe) band. Thickness maps in DR subjects were compared to those of healthy subjects by generating deviation maps which displayed retinal locations with thickness below, within, and above the normal 95% confidence interval.

Results: In healthy subjects, TR and IR thickness maps displayed the foveal depression and increased thickness in the parafoveal region. OR and ISe thickness maps showed increased thickness at the fovea, consistent with normal retinal anatomy. In DR subjects, thickening and thinning in localized regions were demonstrated on TR, IR, OR, and ISe thickness maps, corresponding to retinal edema and atrophy, respectively. TR and OR reflectance images showed reduced reflectivity in regions of increased thickness. Hard exudates appeared as hyper-reflective spots in IR reflectance images and casted shadows on the deeper OR and ISe reflectance images. The ISe reflectance image clearly showed the presence of focal laser scars.

Conclusions: Enface thickness mapping and reflectance imaging of retinal layers is a potentially useful method for quantifying the spatial and axial extent of pathologies due to DR.

Fig 1. An example of a SDOCT B-scan image at the fovea in a healthy subject displaying segmentation of five retinal interfaces comprised of 1) vitreous and internal limiting membrane (ILM), 2) inner nuclear layer (INL) and outer plexiform layer (OPL), 3) outer nuclear layer (ONL) and inner segment ellipsoid (ISe), 4) ISe and retinal pigment epithelium (RPE), and 5) RPE and choroid.

Fig 2

Top: SLO image with the corresponding SDOCT B-scan imaged through the fovea in the right eye of a control subject. Bottom: Thickness maps and reflectance images of the total retina, inner retina, outer retina, and the inner segment ellipsoid (ISe) layer are displayed. Color bars for thickness maps represent thickness in microns. Thickness maps were consistent with normal retinal anatomy. Outer retina and ISe reflectance images display shadowing of the retinal vasculature.

Fig 3

Top: SLO image with the corresponding SDOCT B-scan imaged through the fovea in the left eye of a DR subject with a history of PDR and DME (Case 1). The SLO image shows focal laser scars and the SDOCT B-scan displays cystoid DME, hard exudates, and inner segment ellipsoid (ISe) discontinuity (arrow). Bottom: Total retina (TR) and outer retinal (OR) thickness and deviation maps display central retinal thickening. The inner retina (IR) thickness map shows a distorted foveal depression and the inner segment ellipsoid (ISe) thickness map shows a ring-shaped area of thinning which is consistent with the pattern of focal laser scars seen in the SLO image. Color bars for thickness maps represent thickness in microns. TR and OR reflectance images show reduced reflectivity in regions of increased thickness. Hard exudates appear as hyper-reflective spots in the IR reflectance image and cast shadows on the deeper OR and ISe reflectance images. The ISe reflectance image clearly shows the presence of focal laser scars.

Fig 4

Top: SLO and SDOCT B-scan in a PDR subject with a history of DME in the left eye (Case 2). SDOCT B-scan shows irregular retinal architecture, hard exudates, and cystoid DME at the fovea. Bottom: Total retina (TR) and outer retina (OR) thickness and deviation maps reveal substantial regions of retinal thickening. The inner segment ellipsoid (ISe) thickness map shows thinning supero-temporal to the fovea. Color bars for thickness maps represent thickness in microns. The ISe reflectance image displays spots of reflectance inhomogeneity due to laser scars in the same region.

Fig 5

Top: SLO and SDOCT B-scan images in a PDR subject with a history of DME and retinal atrophy (Case 3). The SDOCT B-scan shows areas of inner retinal (IR) thinning with preservation of the outer retina (OR) and inner segment ellipsoid (ISe) centrally. Bottom: Total retina (TR) and IR thickness and deviation maps showed large regions of retinal thinning. The OR and ISe thickness maps demonstrate preservation of the normal retinal anatomy near the fovea with scattered areas of thinning in the periphery, due to focal laser scars. Color bars for thickness maps represent thickness in microns. OR and ISe reflectance images display scattered focal hypo-reflective spots due to laser scars superiorly and temporally.

Fig 6

Top: SLO and SDOCT B-scans of the left eye in a NPDR subject with a history of DME (Case 4). On the SDOCT B-scan, there is a temporal region of cystoid DME and hard exudates in the outer retina (OR) resulting in distortion of the foveal depression. The inner segment ellipsoid (ISe) layer appears intact. Bottom: Total retina (TR) and OR thickness and deviation maps show thickening at the fovea. The inner retina (IR) thickness and deviation maps show preservation of the normal foveal depression with nonspecific scattered areas of thinning inferiorly. ISe thickness and deviation maps show a central region of increased thickness that corresponds to the normal anatomy. Color bars for thickness maps represent thickness in microns. TR and OR reflectance images show reduced reflectivity in focal regions of increased thickness. The IR reflectance image demonstrates scattered hyper-reflectance spots from hard exudates. The ISe reflectance image shows hypo-reflectance spots that correspond to shadows from hard exudates in the inner retina.