Anterior Segment Analyzer
Anterior Segment Analyzer
In addition to corneal topography, there are numerous imaging devices that provide additional information about the anterior segment. These instruments allow us to precisely image, measure, and map the anterior segment structures, which provides valuable information for refractive and lens based surgeries. The devices utilize a variety of technologies including ultrasound, optical coherence tomography, Scheimpflug imaging, and wavefront imaging.
Optical coherence tomography (OCT; Visante by Carl Zeiss Meditec):
uses reflected light to provide a two-dimensional picture. The principle is analogous to B-scan ultrasonography but with light instead of sound. OCT is also a non-contact test and provides higher resolution images. OCT technology can be used for anterior or posterior segment imaging but is wavelength dependent. Specifically, the optimal wavelength for anterior segment imaging is 1310 nm while posterior segment imaging is best at 820 nm (i.e., Stratus and Cirrus by Zeiss). The Visante is helpful for evaluating the cornea (thickness, LASIK flaps, incisions, wounds, dystrophies, scars), iris (tumors, trauma), angle (angle-closure glaucoma assessment, trabeculectomy patency, drainage device positioning), sulcus (size, implant location), and lens (implant position, accommodative IOL movement).
Scheimpflug imaging (Pentacam by Oculus, Galilei by Zeimer):
takes advantage of special cameras to achieve greater depth of focus, which produces sharp images of the anterior segment. For a normal camera, the object, lens, and image planes are parallel to each other, whereas for a Scheimpflug camera, these 3 planes are rotated so that they intersect at a single point or plane to produce increased depth of field. Therefore, these instruments are able to provide focused images of the entire anterior segment. The devices are also able to perform cataract densitometry, tomography, anterior chamber analysis, and generate corneal maps (pachymetry, topography and elevation of both the anterior and posterior corneal surfaces).
Wavefront aberrometry:
enables measurement of the eye’s higher order aberrations (HOAs). Traditional measurement of a patient’s refractive error is limited to sphere and cylinder (lower order aberrations), which can be corrected with spherocylindrical lenses. However, we now have the ability to quantitate and map HOAs, and we can correct them with wavefront lenses or surgery.
Wavefront aberrometers shine a monochromatic light beam into the eye and capture the reflected light travelling back through the eye from the retina. A wavefront refers to the physical representation of the optical quality of this light beam. A perfect wavefront is represented by a plane or flat wave. However, this ideal wavefront is disrupted by imperfections in a patient’s cornea and lens, which produces an irregular shape.
The Anterior Segment Analyzer use two main technologies used for analyzing wavefronts are Hartmann-Shack and ray tracing. Hartmann-Shack, the more common type of aberrometer, is faster (measures the wavefront in one shot), gives more data points, and has better repeatability than ray tracing, which uses consecutive measurements. Hartmann-Shack devices produce 240 data points for a 7 mm pupil. This data is analyzed to generate a wavefront image, but the method used to analyze the data influences the resulting image.The two main technologies used for analyzing wavefronts are Hartmann-Shack and ray tracing. Hartmann-Shack, the more common type of aberrometer, is faster (measures the wavefront in one shot), gives more data points, and has better repeatability than ray tracing, which uses consecutive measurements. Hartmann-Shack devices produce 240 data points for a 7 mm pupil. This data is analyzed to generate a wavefront image, but the method used to analyze the data influences the resulting image.
The Anterior Segment Analyzer There are two common algorithms for wavefront analyzis: Zernicke polynomials (mathematical functions are used to describe complex shapes) and Fourier analysis (sine waves are used to reconstruct the wavefront). The main limitation of the Zernike method is that it only utilizes a subset of the acquired data points (typically the amount necessary to produce a 6th order image). The optical aberrations are reported as the root mean square (RMS) by combining Zernike coefficients. This strategy works well for the more common lower-order shapes but is less accurate in eyes with highly aberrated wavefronts. Fourier analysis, on the other hand, uses all the Hartmann-Shack data points to derive the precise shape of the wavefront (equivalent to the 20th order Zernike image) and is therefore better for evaluating highly aberrated eyes.
Wavefront analysis is primarily used for evaluating laser vision correction candidates preoperatively so that HOAs can be reduced (i.e., by decreasing the preexisting HOAs as well as minimizing any surgically induced aberrations). The effect has been improved quality of vision and reduction of glare and halos. In addition, wavefront technology is being incorporated into IOL designs. These aspheric lenses compensate for corneal spherical aberration and thereby improve contrast sensitivity. Some surgeons are advocating measuring preop HOAs on cataract patients and then choosing the IOL design that will most fully correct the aberrations. Wavefront glasses and contact lenses may also be options for providing clearer, sharper vision.
UBM machines with various frequency probes are available from Opko, Quantel, and Reichert.