Authors
Gregor Schmid
Topics
Ophthalmology and Visual Impairment StudiesCorneal surgery and disordersOptical measurement and interference techniques© The Author(s) 2024 403 J. Aramberri et al. (eds.), Intraocular Lens Calculations, Essentials in Ophthalmology, https://doi.org/10.1007/978-3-031-50666-6_27The GALILEI G6 is a noninvasive, noncontact optical diagnostic system designed for the assess ment of the anterior segment of the eye by means of processed images taken with an integrated rotating Dual-Scheimpflug tomography and Placido topography system. The Dual Scheimpflug system (two opposite cameras instead of one) allows significant reduction in measurement time without losing data coverage and automatic compensation of measurement decentration. Optical A-scans based on time domain partial-coherence interferometry enables the precise measurement of axial, intraocular dis tances, thereby adding the information needed to perform IOL power calculation. The precise acquisition of posterior corneal surface data reduces the risk of postoperative surprises. Together with the complete set of biometry data, including lens thickness measurement, the full dataset for making the optimal decision for sur geons and their patients is available.The GALILEI G6 is composed of a measurement head containing Placido disk and Dual Scheimpflug optics/mechanics/electronics, a main monitor, a PC, an elevation table, and an optical A-scan accessory (Fig. 27.1). The measurement head includes an optical front end for coupling the light beam from the optical A-scan accessory into the eye, optics for Placido and Dual-Scheimpflug imaging, mechan ics to rotate the cameras, as well as electronics for controlling measurement head rotation, light sources, and image acquisition. For data collec tion, the measurement head is rotated about the central instrument axis by 180°. During the rota tion, a series of Scheimpflug, Placido, and Topview images are taken of the cornea, iris, pupil, limbus, anterior chamber, and crystalline lens and transferred to the PC for processing and display. Topography and anterior segment tomog raphy are then calculated from those images. Figures 27.2 and 27.3 show examples of a Dual Scheimpflug image pair and a Topview/Placido image, respectively. The scanning process acquires an adjustable number (between 7 and 30, default: 17) of Scheimpflug and Topview images, including two Placido Topview images at 54° apart. On the Scheimpflug images, edges are detected (anterior cornea, posterior cornea, anterior lens, and iris). On the Placido images, the ring edges areFig. 27.1 GALILEI G6 Lens ProfessionalFig. 27.2 Pair of Dual-Scheimpflug images detected. In a separate process, the limbus and pupil are detected from a Topview image. The limbus and pupil do not influence any other cal culations performed by the system. From the Scheimpflug edges, height data is determined. The slope data from the Placido images are trans formed into conforming height data. Scheimpflug and Placido data are thereafter merged based on respective quality using a proprietary merging algorithm. The merged data are then used to cre ate surface fits from where indices are calculated and maps are generated. In addition, a color Topview camera permits taking color images of the front view of the eye (Fig. 27.4).Fig. 27.3 Topview image of Placido ring reflectionFig. 27.4 Color Topview imageFig. 27.5 Decentration affecting the images as viewed by the two Scheimpflug camerasFigure 27.5 illustrates how decentration and eye motion during a measurement can affect height data of the posterior surface, which directly affects pachymetry, as pachymetry is determined from anterior and posterior height data. When the slit light is well centered on the cornea, the left and right Scheimpflug cameras view the same corneal thickness as outlined by the blue and green lines. In the case of decentration to either side, the two Scheimpflug cameras view different corneal thicknesses. Note: the difference in sepa ration of the blue and green line pairs depends on the camera angle and the direction of displace ment from the center of the cornea. Combining the two camera views using ZIEMER’s patented Dual-Scheimpflug solution, the systematic error in the original captured image is automatically corrected by averaging the two opposed camera images. Averaging the two images corrects the decentration error caused by eye motion or misalignment, making the mea surement of the posterior edge independent of eye motion, allowing for accurate pachymetry and elevation data. Accurate anterior surface calculations techni cally require only one of the two Scheimpflug images, along with the Placido image. However, for posterior surfaces, both Scheimpflug images are needed to compensate for decentration due to eye motion. Therefore, accurate determination of corneal pachymetry, anterior chamber depth, and posterior corneal surface requires complete Dual Scheimpflug images. Loss of one of the two means that the corresponding image will be dis carded and the Scheimpflug quality percentage will drop accordingly. Comparing the GALILEI to a single Scheimpflug device, Aramberri et al. [1] reported that, while repeatability and reproducibility were good with both devices for all parameters and agreement was good with some relevant excep tions, the single-camera device was more precise for curvature, astigmatism, and corneal wavefront error measurements, and the dual-camera device was more precise for pachymetry measurements.Within the optical A-scan accessory, a collimated beam of an infrared, super-luminescent light emitting diode (SLED) is split by a beam splitter (BS) into a reference beam and a sample beam that are directed to a reference mirror and the patient’s eye along its visual axis, respectively (Fig. 27.2). Whenever the sample beam passes a transition between ocular layers with different refractive indices (e.g., corneal surfaces, crystal line lens surfaces, and retinal surfaces), a portion of the light is reflected back toward the beam splitter. The optical path length of the light reflected from ocular surfaces is compared to the optical path length of light that is reflected from the reference mirror which is adjusted by moving the reference mirror at a constant velocity (V). When these optical lengths match to within the coherence length (CL) of the SLED, an interfer ence signal is generated whose intensity is recorded with a detector and plotted as a function of the mirror position. The sample position is then deduced from the location on the plot’s x-axis of the interference peak (Fig. 27.6).Fig. 27.6 Time domain, partial coherence interferometry for precisely measuring axial, intraocular distancesOptical biometers, including the GALILEI G6, measure optical distances that represent geomet rical (actual) distances multiplied with the mea sured material’s refractive index. Thus, optical distances are converted to geometrical distances through division by the material’s refractive index. When converting optical axial length (AL) to geometrical AL, one faces the challenge that the components along the measurement axis (cornea, anterior chamber, crystalline lens, vitre ous chamber) have different refractive indices, and that refractive indices are dependent on the wavelength of the measuring light. With some optical biometers, the surfaces of the crystalline lens cannot be determined, such that an average refractive index must be employed for the con version of optical AL to geometrical AL. This works reasonably well if the measured AL is within a certain range of normal AL. For very long or very short ALs, however, significant mea surement errors may result because of altered refractive index contributions of the various axial components to the average refractive index. Such errors can be prevented by dividing the compo nents’ optical distances separately by their respective refractive indices and then adding the resulting, separate geometrical distances to obtain geometric AL. The Galilei G6 is capable of determining the surfaces of the crystalline lens, thereby measuring lens thickness, and there fore capable of converting optical distances segment-wise to geometrical distances. Two dif ferent AL are calculated and displayed by the GALILEI G6:
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PublishedJan 1, 2024
TypeBook-Chapter
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