New York Eye and Ear Infirmary of Mount Sinai's Retinal Center was the first to acquire the latest version of the OTI OCT/SLO, which was made commercially available in October 2006. This new device is different from the original OCT/SLO in that it uses spectral-domain (Fourier) technology instead of time-domain technology (used in the original OCT/SLO as well as in the other commercially available OCT devices). Spectral OCT/SLO imaging allows us to obtain images with much better resolution than the original OCT/SLO (8 microns vs. 10-15 microns), at speeds of up to 32 times faster (64 Hz vs 2 Hz). This high-speed, high-resolution imaging minimizes movement artifacts, and allows several different imaging strategies to be performed quickly and with greater accuracy.
Longitudinal (B-scan) OCT Imaging
Imaging through a segment of the retina with the Spectral OCT/SLO generates an image of such high resolution that the different layers of the retina are clearly seen. Because of this, subtle pathologies are visualized earlier, and this may lead to early diagnosis and intervention in some cases.
The superiority of the Spectral OCT/SLO scans over the time-domain OCT/SLO scans is evident in the longitudinal scans below:
Figure 1: Time-domain OCT/SLO longitudinal scan of a normal retina. The retinal layers, though visible, are not clearly delineated.
Figure 2: Spectral-domain OCT/SLO longitudinal scan of same subject as above, clearly showing the different retinal layers.
Figure 3: Longitudinal scan of a patient with diabetic retinopathy acquired using the Stratus OCT (time-domain unit by Carl Zeiss Meditec, USA). Cystic changes as well as an epiretinal membrane are evident.
Figure 4: Same patient as above, imaged with the Spectral OCT/SLO. Note that the cystic changes are well-defined, demonstrating that the cystic spaces shown with the Stratus OCT (Figure 3) are actually composed of several smaller cysts. The epiretinal membrane is seen to clearly extend from one end of the scan to the other. Additional information not obtained with the Stratus OCT includes vitreous debris (seen as white dots in the vitreous) as well as a posterior vitreous detachment (PVD).
The Spectral OCT/SLO is able to generate a 3-dimensional retinal thickness map from 64 longitudinal scans taken in succession over 2 seconds over a user-defined area. This is in contrast to the Stratus OCT, which generates its 3-dimensional retinal thickness map from 6 radial scans acquired in 2 seconds.
Figure 5: Retinal Thickness map generated from the Zeiss Stratus OCT. The six radial lines from which the retinal thickness is calculated is shown in the fundus picture on the upper right hand side. Retinal thickness is displayed both in color scale (with red being the thickest) and numerically (in 9 general areas) below the OCT image. This image is taken from the same eye as in Figures 3 and 4.
Figure 6: Retinal thickness map generated from the OTI Spectral OCT/SLO. The area in which the retinal thickness measurements were taken is clearly depicted in the SLO image. In contrast to the Zeiss Stratus OCT map, both the color scale and numerical measurements are shown simultaneously over the fundus image, thus making it easy to determine exactly where in the retina the areas of thickening occur. Also, because more scans are used to generate this map, the thickness readings over each pixel of the retinal map can be demonstrated (unlike the Stratus map where the thickness is reported in 9 general areas).
Unique to the OTI Spectral OCT/SLO is the capability to do functional testing of the retina. Visual field testing over a smaller, more specific area of the retina is called microperimetry, which tests the ability of the the patient to perceive lights of varying intensities. Results are mapped over the SLO fundus image, and may also be overlaid upon a retinal thickness map. In this way, one can assess the relationship between retinal function and thickness at particular points in the retina.
Figure 7: Microperimetry testing using the OTI Spectral OCT/SLO. Numerical as well as color representation of dimmest stimulus seen is overlaid upon SLO image. Lower decibel reading indicates brighter lights. Patient's fixation is excellent, as shown on the right hand side of the SLO image.
Figure 8: Retinal thickness map of patient in Figure 7, showing a marked central swelling of the retina in a diagonal orientation.
Figure 9: Figures 7 and 8 can be superimposed by aligning the respective retinal vessels of both images. In doing so we can see how, in this particular case, the poorest retinal function correlates well with the thickest part of the map.
Optic Nerve Topography and RNFL Analysis
Retinal nerve fiber layer (RNFL) thickness analysis is important in assessing the damage from early glaucoma, as the RNFL has been shown to become thinner before other changes from glaucoma become evident.
The OTI Spectral OCT/SLO can generate a topographic map of the optic nerve area in the same way that it is generated in the macula, using 128 longitudinal scans over a 5 mm area. From this data, information about the retinal nerve fiber layer thickness can be extracted, either as a thickness map or as a measurement of the RNFL thickness along a fixed diameter around the optic nerve. In contrast, the Zeiss Stratus OCT makes its RNFL measurements along a fixed scan around the optic nerve.
Figure 10: Zeiss Stratus OCT RNFL Scan. Measurements are made along a fixed circular scan around the optic nerve, seen on the upper right hand of the image above.
Figure 11: The OTI Spectral OCT/SLO generates an RNFL thickness map over the entire optic nerve region, so that the RNFL thickness throughout the 5mm area can be seen.
Figure 12: Based on data generated from Figure 11, an RNFL thickness along a 3.4 mm ring (similar to the diameter of the Zeiss Stratus OCT RNFL Scan) can be extracted. Because the 3.4 mm ring is extracted from a thickness map, the ring may be moved to examine different areas around the optic nerve without having to scan the patient multiple times.