Fig. 3.1
(a) Direct fundus examination. (b) Standard fundus (en face) view right eye
Optical coherence tomography (OCT) was a major leap forward providing clinicians access to the cross-sectional details of the retina. Early OCT images, however, had limited resolution of the retinal layers and provided limited lesion localization. The evolution of OCT from time-domain (TD)-OCT to spectral-domain (SD)-OCT has led to better sensitivity and faster acquisition speeds of cross-sectional (B-scan) images. The improved resolution of OCT images has allowed better localization of retinal details, enabling more accurate diagnosis of pathology. OCT has revolutionized the diagnosis and treatment of retinal diseases offering a noninvasive virtual “optical biopsy” of the retina in real time (Fig. 3.2).
Fig. 3.2
The evolution of optical coherence tomography. (a) Time-domain OCT image (1997). (b) Time-domain OCT (2001). (c) Spectral-domain optical coherence tomography image from the macula of a normal eye. The following retinal layers are labeled: nerve fiber layer, ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, external limiting membrane, ellipsoid zone (previously referred to as the IS-OS junction), interdigitation zone, and retinal pigment epithelium
As OCT technology advanced, it became increasingly evident that localization of the cross-sectional information was almost as important as the features detected by the technique. In 1997, Podoleanu et al. introduced the concept of en face OCT paired with corresponding scanning laser ophthalmoscopic (SLO) imaging (Podoleanu et al. 1997). Signals from transverse scans of the retina were split between a confocal optical path to produce an SLO image and a Michelson interferometer which generated a series of en face OCT images at progressive depths.
En face imaging using TD-OCT employed lateral scanners with z-axis stages to control depth generating scans in both the transverse and axial directions. The transverse linear scans (T-scans) combined to create C-scan (coronal planar) images. B-scan (sagittal planar) cross-sectional planar images could also be constructed from this optical configuration. C-scan images when generated from axial scans as in the cases of SD-OCT or swept-source (SS)-OCT are constructed by post-processing 3D OCT volumes (Podoleanu 2013) (Fig. 3.3).
Fig. 3.3
Simultaneous time-domain scanning laser ophthalmoscopy (SLO)/en face (C-scan) OCT and B-scan OCT of a patient with central serous retinopathy. The split image view is used to show the relationship between the surface appearance and internal structure seen with the en face OCT
The A-scan data is processed through several steps which include zero padding, fast Fourier transformation (FFT), data resampling, spectral shaping, and apodization to render en face images from the 3D volume of A-scans (Bradu and Podoleanu 2014). The time it takes to obtain the transverse scans plus the time required to analyze the scans determines how long it takes to produce an en face image (Zhang and Kang 2010). Advancements in SD-OCT and swept-source (SS)-OCT technology combined with the use of graphic processing unit cards has helped minimize the time needed to image and process the data allowing for improved en face imaging quality and even approaching real-time en face imaging capabilities.
3.2 Unique Features of En Face Optical Coherence Tomography
There are several features of en face OCT imaging which provide unique clinical advantages. Familiarity of perspective is first on the list since en face OCT reveals the retinal surface and subsurface structural details in the orientation of the standard ophthalmoscope. With this perspective, the orientation of features is familiar and intuitive for the examiner to appreciate. The shapes and dimensions of pathological lesions revealed below the retinal surface are more easily interpreted than when transposed into a series of cross sections as provided by standard B-scan OCT imaging.
En face OCT enables precise registration with other imaging modalities, giving the clinician the ability to investigate features with a variety of structural and functional filters. The en face perspective, combined with vertical and horizontal B-scan OCT images, allow the clinician to survey a retinal or choroidal lesion using a convergence of orthogonal views. The ability to simultaneously study surface details provided by scanning laser ophthalmoscopy (SLO) along with en face OCT internal details is often helpful in educating the clinician as to the surface appearance of deeper retinal lesions (Fig. 3.4).
Fig. 3.4
En face OCT combines imaging of orthogonal planes. En face OCT (upper left) compared to vertical and horizontal B-scan OCT slices in a case of cystoid macular edema. The en face image reveals the full extent of the outer retinal disturbance
En face OCT reveals each layer of the retina as a separate surface. The contour of the slice will vary depending on which layer of the retina is used as the reference curvature. Choosing the internal limiting membrane vs the retinal pigment epithelium when segmenting the macula will have very different appearances (Figs. 3.5 and 3.6)
Fig. 3.5
SD-OCT (Avanti) en face of the segmented layers of the macula at progressively deeper layers
Fig. 3.6
High-resolution SD-OCT en face dissection of the central macula showing different retinal layers. The unique curvature of the fovea is not easily compensated for by the flattening processing resulting in multiple concentric zones of sequential layers
The en face perspective can often reveal small lesions that could be easily be missed if only single B-scan OCT slices are used. Since patients will typically fixate using the best part of their retina, small disturbances are preferentially avoided resulting in B-scan OCTs which can appear normal even in abnormal eyes. The expanse of the C-scan view can often pick up these features which can then be reexamined in a B-scan OCT cross-sectional images (Fig. 3.7).
Fig. 3.7
En face OCT identification of a small occult parafoveal intraretinal lesion that was overlooked on standard B-scan OCT
With en face OCT, the extent and expanse of a lesion can be better appreciated. The en face perspective can show the lateral as well as the vertical extent of lesions (Figs. 3.8 and 3.9).
Fig. 3.8
En face OCT and B-scan OCT of pigment epithelial detachment. Note the hypo-reflective center representing the low density serous fluid surrounded by a bright hyper-reflective overlying RPE signal
Fig. 3.9
En face images of central serous chorioretinopathy the demonstrating lateral extent of lesion and concentric rings produced by the cut through multiple retinal layers. The horizontal B-scan OCT is seen on top (upper right) directly above the SLO (middle left) of the macular surface. A corresponding fluorescein angiogram (middle second row image) shows a focal leak at the level of the RPE corresponding to an inferior temporal dip in the serous elevation. The lower level of images shows progressively deeper slices moving to the right through the serous elevation. At the RPE level (lower right), the isolated RPE disturbance is isolated and surrounded by the shadow of the overlying serous bleb
3.3 Vitreomacular Interface
Structural and dimensional details of vitreomacular interface disorders such as epiretinal membranes (ERM) and macular holes can be better delineated with en face OCT. Using en face OCT, certain patterns of idiopathic epiretinal membranes have been identified. Plaques were found to be the most common feature in patients with ERM. The plaques seen on en face OCT correspond generally to localized areas of tight adherence between the retina and the membrane on standard OCT sections. Retinal starfolds were not as common as plaques in patients with ERM and may possibly precede the development of these plaques. Diffuse retinal folds were least commonly seen on en face imaging in patients with an ERM. The identification of certain patterns of ERM seen on en face OCT complements the standard OCT images and can aid surgical planning of membrane peeling (Rispoli et al. 2012) (Fig. 3.10).
Fig. 3.10
Epiretinal membrane. En face OCT shows the disturbance to the anatomy which is most evident in the upper and lower left images, leaving the choroid untouched. The details of the plaque-like pattern of the ERM can be better appreciated with en face OCT
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