Since its development in 1991, optical coherence tomography (OCT) has become an indispensable tool in ophthalmology. This advanced technology provides valuable information by enabling us to precisely image, measure, and analyze ocular structures. OCT is essential for detecting and treating retinal diseases, and it can be very helpful for diagnosing and managing patients with anterior segment disorders and glaucoma.
OCT is a non-invasive, diagnostic imaging technique that utilizes low-coherence interferometry to create in vivo cross-sectional pictures of ocular structures. The principle is similar to that of ultrasonography but uses light reflectance instead of sound. OCT scans produce high-resolution images of both the anterior and posterior segments of the eye. However, obtaining the best image depends on the wavelength of light: higher for anterior segment (1310 nm; for greater absorption and less penetration) than for posterior segment (830 nm) imaging.
Anterior segment OCT devices aid in evaluating the cornea (thickness, LASIK flaps, incisions, wounds, dystrophies, scars), iris (tumors, trauma), angle (angle size, trabeculectomy patency, drainage device positioning), sulcus (size, implant location), and lens (implant position, accommodative IOL movement).
Posterior segment OCT is helpful for assessing the macula (detecting macular edema, vitreomacular traction, epiretinal membranes, macular holes, neurosensory retina and pigment epithelial detachments, drusen, and quantitating central foveal thickness, measuring retinal cell layers) as well as the optic nerve and retinal nerve fiber layer (quantitating RNFL thickness in glaucoma and glaucoma suspect patients).
OCT technology has evolved significantly since the first devices became commercially available. The initial time domain machines (TD-OCT) with slower acquisition times (400 axial scans/second using 6 radial slices 30 degrees apart) and lower resolution (10-15 microns) had potential issues with fixation drift, interscan repeatability, and missing pathology. Newer frequency domain (Spectral Domain (SD-OCT) or Fourier Domain (FD-OCT)) devices have faster acquisition time (18,000-50,000 A-scans/second provides continuous image of 6 mm area) and higher resolution (3-6 microns) so there is less motion artifact and fewer missing areas. The new technology also is able to create 3D images.
The main advantages of OCT are that it is a non-invasive, non-contact, painless, rapid test that is easy for patients and has no radiation exposure. The principle disadvantage is the quality of the image is affected by media opacities (i.e., corneal opacities, cataracts, and vitreous hemorrhage).
If your office is considering purchasing a new OCT instrument, here are some of the current systems:
Cirrus HD-OCT 5000 (ZEISS): This posterior segment OCT machine is an FD-OCT and therefore it provides higher resolution retinal images than the previous Stratus time-domain device which Zeiss no longer manufactures. Features of the Cirrus 5000 family include FastTrac retinal tracking system, Fovea Finder, AutoCenter, 3-D cubes, En face reports,and Advanced Visualization for retinal diseases, RNFL, ONH, PanoMap Wide-Field Display, and Ganglion Cell Analysis for glaucoma. The new Anterior Segment Premier Module enables beautiful imaging capabilities for the cornea, angle, and iris with the ChamberView, HD Angle, Wide Angle-to-Angle, and HD Cornea scans. This technology has replaced the Visante OCT device for anterior segment imaging. The Cirrus photo combines the HD-OCT technology with a full mydriatic/non-mydriatic fundus camera to capture color fundus pictures, fundus autofluorescence and fluorescein angiography images.
SPECTRALIS (Heidelberg Engineering): This multi-modal device integrates an SD-OCT with confocal scanning laser ophthalmoscopy (cSLO). Notable features are TruTrack active eye tracking, multi-modality imaging, Heidelberg Noise Reduction, AutoRescan, BluePeak blue laser autofluorescence, FoDi Fovea-to-Disc alignment technology, and HEYEX networking solutions. The Anterior Segment Module (add-on lens and software) enables the device to capture images of the cornea, sclera, and anterior chamber angle.
3D OCT-2000 (Topcon): This SD-OCT system also contains a high-resolution fundus camera with a color touch screen display. The Pin-Point Registration function identifies the location of the OCT image in the fundus photo.
RTVue Premier (Optovue): This SD-OCT device performs anterior and posterior segment imaging. Retinal imaging can be performed with two different modes (vitreoretina or chorioretina) depending on the location of the pathology, real time eye tracking is accomplished with Vtrac, and anterior segment features include total cornea power measurement, epithelial thickness mapping and corneal pachymetry mapping.
RS-3000 Advance (Nidek): This high speed SD-OCT system also incorporates cSLO to allow for comprehensive analysis of the retina and optic nerve. The optimization button automatically adjusts the SLO fundus image focus and the OCT depth alignment to rapidly capture both images at once. The auto-tracking function provides for highly reproducible follow-up exams.