Optical Coherence Tomography
Robert W. Weisenthal
Optical coherence tomography (OCT) is a noninvasive technology that produces twodimensional, high-resolution, and high-definition cross-sectional images based upon low coherence interferometry. It has multiple uses in refractive surgery. As a screening tool, it can map the cornea and help identify the patient with abnormal corneal thinning or measure the depth of a corneal scar. Postoperatively, it can be used to accurately measure flap thickness as well as the residual stromal bed in consideration of retreatment. Diagnostically, it can help differentiate diffuse lamellar keratitis from pressure-induced keratitis. Epithelial ingrowth can be more carefully characterized with OCT.
There are two types of OCT systems used in ophthalmology, Time-domain OCT (TD-OCT) and Fourier-domain (FD-OCT), also named spectral or spectral-domain (SD-OCT). In TD-OCT, the light in the reference arm is directed into an oscillating mirror, which must be moved mechanically, limiting the scan speed in the range of several thousand per second. The recombined light is processed by conversion into electrical waveform by a detector, which then generates the axial scan. In contrast, in SD-OCT the reference mirror is stationary, so the frequency of the scan is limited only by the frame rate of the camera, increasing the speed of the image capture from tens of thousands to hundreds of thousands of scans per second. In addition, in SD-OCT, the recombined light from the reference and sample arms is passed through a spectrally separated detector (grate), which splits the light into a spectral interferogram. A computer then transforms the spectral interferogram into an axial scan. Overall, the use of SD-OCT improves the imaging speed dramatically, increases the signal-to-noise ratio, and improves the resolution of the images.
OCT Instruments
The first commercially available OCT system to image the anterior segment was the Visante (Carl Zeiss Meditec, Dublin, CA) with TD-OCT using a 1,300 nm wavelength beam (Table 3.1). The longer wavelength light had stronger water absorption, which produced less scatter. As a result, there was better visualization of turbid tissue such as cloudy cornea, sclera, iris, anterior chamber angle, and to a lesser extent, the ciliary body.
The Visante has a standard and a highresolution mode. The standard resolution is 16 mm in width and 6 mm in depth, while the high-resolution mode provides a more detailed image over a smaller area, 10 mm in width and 3 mm in depth. The Visante in the standard mode performs 256 scans in 0.125 seconds and in the high-resolution mode performs 512 scans in 0.25 seconds. The Visante OCT 3.0 software package includes a comprehensive anterior segment imaging and biometry mode that images the anterior chamber depth and width, anterior chamber angles, and crystalline lens rise (CLR). There is corneal imaging and a pachymetry mode with a 16 mm width and 6 mm depth scan. A refractive tools software module is capable of evaluating residual stromal bed thickness and the position of phakic intraocular lenses (IOLs). An iridocorneal tools module images the anterior chamber angle.
The advantages of the Visante include limbus to limbus visualization of the anterior segment (16 mm in width), ease of use, and
better depth of tissue penetration. The disadvantages of the Visante are that the image resolution is less than the SD-OCT, it has a slower capture time, making it more dependent upon patient fixation, it may be more difficult to localize isolated lesions, and it only provides images of the anterior segment.
better depth of tissue penetration. The disadvantages of the Visante are that the image resolution is less than the SD-OCT, it has a slower capture time, making it more dependent upon patient fixation, it may be more difficult to localize isolated lesions, and it only provides images of the anterior segment.
TABLE 3.1 Comparison of Anterior Segment Oct Instruments | ||||||||||||||||||||||||||||||||||||||||
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The Visante has also been integrated with the Atlas corneal topography unit in a product called the Visante Omni (Carl Zeiss Meditec, Dublin, CA) combining Placido disc corneal topography with corneal pachymetry. This provides the ability to measure the posterior float by using the thickness data from the OCT and subtracting it from the height data generated by the topography, which is useful in detecting early keratoconus.
The first commercially available SD-OCT instrument for the anterior segment was developed by Optovue (Optovue, Inc., Fremont, CA). The first device released was the RTVue SD-OCT in 2006. A portable version called the iVue compact SD-OCT became available in 2010. These units use a shorter wavelength light than the Visante (830 nm vs. 1,300 nm) and for this reason can better penetrate transparent tissue, making it possible to visualize both the anterior and posterior segments. In order to image the anterior segment, the Corneal Anterior Module (CAM) requires special lenses. The wide-angle lens provides a scan width of 6 mm and a transverse resolution (focused spot size) of 15 µm useful for screening. The high magnification CAM lens provides a scan width of 4 mm and a transverse resolution of 10 µm, which can image smaller elements such as acanthamoeba.
Since the Optovue units are based upon SD-OCT, they can generate 26,000 axial scans per second, which allows registration to the tissue rather than relying on patient fixation. SD-OCT also improves the image resolution to 5 µm as compared to the Visante at 17 µm. However, the imaging depth of the RTVue is limited to 2.3 mm as compared to 6 mm with the Visante reducing the visualization through translucent or opaque tissue. Another disadvantage of the Optovue units is that scanning is limited to only the central 6 mm of the cornea.
The 6.0 software available on the RTVue and iVue allows high-speed corneal scanning, which can generate a pachymetry map. It is also possible to generate three-dimensional volumetric maps. There is a corneal angle view, which can provide higher magnification of the images. These programs can map the epithelium, LASIK flap, and the residual stromal bed thickness. There is also special software that provides keratoconus screening and a program to measure the anterior and posterior corneal curvature in the central 3 mm of the cornea. This can be used to derive the true corneal power, which is useful for IOL
calculation after refractive surgery. These applications are illustrated below.
calculation after refractive surgery. These applications are illustrated below.
The Spectralis made by Heidelberg Engineering GmbH (Carlsbad, CA) is also based on SD-OCT. An anterior segment module has been commercially available since November 2011. It requires an add-on anterior segment lens similar to the Optovue units. The advantages of the Spectralis anterior segment module is the faster scan speed of SD-OCT technology combined with a wider scan diameter of up to 16 mm, similar to the Visante. The Spectralis unit has enhanced depth imaging with Heidelberg noise reduction as well as active eye tracking with the TruTrack active eye tracking. The axial resolution is 7 µm compared to 5 µm in the RTVue unit. The software includes preset scans of 8 to 11 mm designed to capture specific anatomical features, combined with an interactive zoom function for greater magnification and measurement tools. All three OCT instruments perform many of the clinical applications for refractive surgery; however, the examples in this chapter are illustrated with the Optovue unit.
▪ Clinical Applications
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