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PURPOSE: To examine epithelial and stromal layers by three-dimensional very high-frequency (VHF) digital ultrasound scanning before and after implantation of Intacs (intracorneal ring segments [ICRS]). METHODS: Three-dimensional scanning was performed in five eyes before and 3 months after Intacs insertion. Digital signal processing techniques provided high-resolution B-scan imaging and I-scan traces for high-precision (1-microm) three-dimensional pachymetry. Thickness maps of individual corneal layers were constructed of the epithelium, stroma, and full cornea before and after surgery. Difference maps for epithelium and stroma were produced to examine anatomical changes in the thickness profile induced in each layer and correlate these to refractive changes. RESULTS: B-scan examination revealed stromal and epithelial anatomy anterior and adjacent to the Intac. Ring depth could be measured topographically. There was stromal lamellar displacement by the ring segments that produced a concave anterior stromal groove within an annulus central to the ring. Epithelial filling of this concavity was shown in three dimensions in such a way as to produce orthogonally asymmetrical flattening of the corneal surface, thus potentially accounting for induced astigmatism. Mapping of the central stroma demonstrated thickening, potentially also accounting for astigmatic changes ascribable to orthogonal asymmetry. CONCLUSIONS: VHF digital ultrasound scanning provided imaging and three-dimensional thickness mapping of corneal layers, enabling anatomical evaluation of the changes induced in the cornea by Intacs.

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PURPOSE: To test and demonstrate measurement precision, imaging resolution, 3D thickness mapping, and clinical utility of a new prototype 3D very high-frequency (VHF) (50 MHz) digital ultrasound scanning system for corneal epithelium, flap, and residual stromal thickness after laser in situ keratomileusis (LASIK). METHODS: VHF ultrasonic 3D data was acquired by arc-motion, meridional scanning within a 10-mm zone. Digital signal processing techniques provided high-resolution B-scan imaging, and I-scan traces for high-precision pachymetry in 4 eyes. Thickness maps of individual corneal layers were constructed. Reproducibility of epithelial, flap, and full corneal pachymetry was assessed for single-point and 3D thickness mapping by repeated measures. Thickness mapping of the epithelium, stroma, flap, and full cornea were determined before and after LASIK. Preoperative to postoperative difference maps for epithelium, flap, and stroma were produced to demonstrate anatomical changes in the thickness profile of each layer. RESULTS: Surface localization precision was 0.87 microm. Central reproducibility for single-point pachymetry of epithelium was 0.61 microm; flap, 1.14 microm; and full cornea, 0.74 microm. Reproducibility for central pachymetry on 3D thickness mapping was 0.5 microm for epithelium and 1.5-microm for full cornea. B-scans and 3D thickness maps after LASIK demonstrated resolution of epithelial, stromal component of the flap, and residual stromal layers. Large epithelial profile changes were demonstrated after LASIK. Topographic variability of flap thickness and residual stromal thickness were significant. CONCLUSIONS: VHF digital ultrasound arc-B scanning provides high-resolution imaging and high-precision three-dimensional thickness mapping of corneal layers, enabling accurate anatomical evaluation of the changes induced in the cornea by LASIK.

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OBJECTIVE: To identify avoidable factors that can lead to serious complications of laser refractive surgery (photorefractive keratectomy [PRK] and laser assisted in situ keratomileusis [LASIK]). DESIGN: Noncomparative case series. PARTICIPANTS: Twenty-seven eyes of 19 patients who had undergone either LASIK or PRK with severe complications accrued retrospectively. INTERVENTION: Review of clinical records. MAIN OUTCOME MEASURES: Symptoms, refractive outcome, and assessment of avoidable factors contributing to the complication. RESULTS: Patients were analyzed in four groups: group 1, scarring with ectasia; group 2, unrecognized keratoconus; group 3, flap related LASIK complications; and group 4, multiple retreatments. The 8 eyes with scarring and ectasia presented with the worst vision, 20/400 uncorrected visual acuity and 20/200 best spectacle-corrected visual acuity, with avoidable factors considered as high or difficult prescriptions with multiple retreatments. Four eyes in two patients with possible forme fruste keratoconus showed worsening irregular astigmatism. Laser assisted in situ keratomileusis flap complications included six eyes with partial laser treatment under an incomplete flap with subsequent severe irregular astigmatism. Six eyes in three patients who had undergone an average of three multiple retreatments showed decreased vision with irregular astigmatism. CONCLUSIONS: Certain severe complications of laser refractive surgery likely can be avoided by using caution when treating high prescriptions, particularly with retreatments, recognizing early keratoconus and avoiding laser treatment under a partial flap in LASIK.

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PURPOSE: To determine whether the refractive change obtained using the Orbscan-derived total optical power (TOP) map is in concordance with the manifest refractive change produced by laser in situ keratomileusis (LASIK). SETTING: LASIK Vision Canada, Vancouver, BC, Canada (an ambulatory surgical center for refractive surgery). METHODS: Twenty eyes of 10 consecutive bilateral LASIK patients were included in the study. Orbscan topographical analysis and manifest refraction were performed preoperatively and at least 1 month postoperatively. The change in manifest refraction (corrected to the corneal plane) before and after LASIK was correlated with the corneal power change averaged within the 2.0, 3.0, 4.0, and 5.0 mm diameter zones of TOP and axial power maps. RESULTS: The central 4.0 mm zone TOP map gave the best correlation between manifest refractive change and Orbscan-measured corneal power change (r2 = 0.835, P < .004). The correlation was higher with TOP maps than with anterior axial power maps. CONCLUSION: The corneal power change measured by the Orbscan TOP maps correlated highly with the changes in manifest refraction after LASIK.

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OBJECTIVE: To examine the utility of very high-frequency (VHF) ultrasound scanning in determining the anatomic changes and correlates of optical complications in lamellar refractive surgery. STUDY DESIGN: Case series. PARTICIPANTS: Cases analyzed included marked asymmetric astigmatism postautomated lamellar keratoplasty (ALK), image ghosting despite normal videokeratography post-ALK, uncomplicated myopic laser in situ keratomileusis (LASIK), and hyperopic LASIK with regression. METHODS: A prototype VHF ultrasound scanner (50 MHz) was used to obtain sequences of parallel B-scans of the cornea. Digital signal processing techniques were used to measure epithelial, stromal, and flap thickness values in a grid encompassing the central 4 to 5 mm of the cornea, enabling pachymetric mapping of each layer with 2-micron precision. MAIN OUTCOME MEASURE: The appearance of the corneas in VHF ultrasound images and thickness values of individual corneal layers determined from VHF ultrasound data. RESULTS: VHF ultrasound resolved the epithelial, stromal cap, or flap and residual stromal layers 1 year after lamellar surgery. Asymmetric stromal tissue removal was differentiated from stromal cap irregularity. Epithelium acted to compensate for asymmetry of the stromal surface about the visual axis and for localized surface irregularities. Irregularities in the epithelial-stromal interface accounted for image ghosting present despite apparently normal videokeratography. Epithelial thickening was shown after uncomplicated myopic LASIK. Hyperopic LASIK demonstrated relative epithelial thickening localized to the region of ablation accounting for refractive regression. CONCLUSIONS: VHF ultrasound shows promise as a sensitive method of determining the anatomic correlates of optical complications in lamellar refractive surgery.

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PURPOSE: The aim of this investigation was to derive a mathematical model for Bowman's layer, the interface between the epithelium and stroma, in the human cornea. METHODS: The central epithelial thickness distribution within 14 normal human corneas was measured in vivo using high frequency ultrasonic digital signal processing with a measurement precision of 2 microns. The results per eye were averaged and incorporated into existing algorithms for the estimation of the shape of the anterior surface of Bowman's layer using terminology in accordance with Baker's equation. RESULTS: The average radius of Bowman's layer was 7.34 mm (SE +/- 0.17 mm). Descriptions of this boundary ranged from a steepening or prolate ellipse to a hyperbola. However, the typical Bowman's layer is hyperbolic with a shape factor, p = -0.22 (SE +/- 1.81). CONCLUSION: The results support previous cadaver studies where Bowman's layer was found to be steeper than the anterior corneal surface but disagree with the concept that the average Bowman's layer is akin to a prolate ellipse. The hyperbolic nature of the average Bowman's layer has the potential to influence the optical performance of the eye.

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PURPOSE: To use very high frequency ultrasound scanning for in situ analysis of a new phakic posterior chamber intraocular lens (No-Touch; International Visions Inc, Cincinnati, Ohio). METHODS: In this pilot study, very high frequency ultrasound (50 MHz) wide-angle (15 mm) full anterior segment scans were obtained in two patients who had undergone phakic posterior chamber intraocular lens implantation into legally blind eyes with normal anterior segment anatomy. RESULTS: Very high frequency ultrasound B-scan images delineated the phakic posterior chamber intraocular lens within the posterior chamber. The relations to the sulci were clearly imaged. Anatomic relations of the phakic posterior chamber intraocular lens optic and haptics were visualized in both static (light/dark) and kinetic (distance/accommodative) states. CONCLUSION: Very high frequency ultrasound wide-angle scanning provides a unique tool to noninvasively evaluate the eye preoperatively and the static and kinetic relations of this new refractive device within the posterior chamber.

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OBJECTIVE: The aim of this report is to describe the technology of three-dimensional (3-D) ultrasonic imaging and its impact on improved diagnosis and monitoring of ocular disease. DESIGN: The authors reviewed techniques for acquiring and displaying 3-D ultrasound data of the eye. PARTICIPANTS: The authors applied these techniques to representative individual cases, including a choroidal hemorrhage, a ciliary body melanoma, a ciliary body detachment, a displaced posterior chamber intraocular lens, and topographic analysis of a normal cornea. INTERVENTION: A computer-controlled motion system was used to perform very high-frequency (VHF) (50-MHz) and conventional (10-MHz) digital 3-D ultrasound data collection. The scanning system allowed digitization of ultrasound data from a series of parallel planes. The 3-D data could be manipulated interactively to obtain two-dimensional images in any plane through the scan volume. The 3-D images were constructed by volume rendering and could be positioned for viewing from a variety of perspectives. The 3-D ultrasound parameter images representing acoustic scatterer properties were generated by spectrum analysis of digitized echo data. Color maps representing the contour and thickness of the epithelium and stroma of the central corneal were generated by digital signal processing of 3-D echo data. RESULTS: Quantitative volume measurement and biometric techniques enhanced the diagnostic and treatment planning information content in 3-D ultrasound images. The location and extent of hemorrhage and clots within the suprachoroidal space were shown with solid modeling. Volume changes in ciliary body melanoma over time were documented and 3-D ultrasound parameter image changes associated with radiation therapy observed. In ciliary body detachment, the extent of the detachment was shown. Solid modeling of a posterior chamber intraocular lens showed misplacement of the haptic in relation to the lens capsule remnants. Keratopachymetric maps showed the range and variance of thickness and local radius of curvature measurements in the cornea. CONCLUSIONS: Quantitative volume measurement and biometric tools combined with segmentation of 3-D ultrasound images improve diagnostic and treatment planning informational content of 3-D ultrasound images through improved localization of tissue structures.

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We developed a programmable motion system for high-frequency (50 MHz) ultrasonographic imaging of the cornea. The system permits a variety of scanning modes, including arc scans matched to the approximate radius of curvature of the cornea. This approach allows high-precision biometry (+ or - 2 microm) over a much larger region of the cornea than the 3 mm central zone obtainable with conventional rectilinear scanning. Using 60 degree wide meridional arc scans covering each clock-hour of a rabbit cornea, we obtained data over an 8 mm wide central zone and produced maps representing the thickness of the corneal epithelium.

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AIMS: To elucidate changes in the anatomy of the anterior chamber associated with iridoschisis, a rare form of iris atrophy, and their potential contribution to angle closure glaucoma. METHODS: Both eyes of a 71-year-old woman with bilateral iridoschisis and fibrous dysplasia and her asymptomatic 50-year-old daughter were scanned with a very high frequency (50 MHz) ultrasound system. RESULTS: The symptomatic patient exhibited diffuse changes in the iris stoma with an intact posterior iris pigmented layer in both eyes. These changes were clinically compatible with the lack of iris transillumination defects. Additionally, iris bowing with a resultant narrowing of the angle occurred. The asymptomatic daughter showed discrete, but less severe iris stromal changes. CONCLUSION: This is the first detailed study of high frequency ultrasonic imaging of the iris in iridoschisis. The observed structural changes suggest angle narrowing by forward bowing of the anterior iris stroma may be a mechanism of IOP elevation in this condition. The ultrasonic detection of iris changes in the asymptomatic daughter of the symptomatic patient and the association of iridoschisis with fibrous dysplasia suggest a possible genetic component in the pathogenesis of this condition.

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BACKGROUND: The diagnosis of pupillary block glaucoma requires sufficient clarity of the ocular media. This is particularly important for assessment of both the presence and patency of an iridotomy, and the determination of central anterior chamber depth. METHODS: High frequency ultrasonography was used in three patients with suspected pupillary block to determine iris configuration, posterior chamber volume, and ciliary body conformation. RESULTS: All patients demonstrated high frequency ultrasonographic findings consistent with pupillary block: iris bombé, a formed posterior chamber, and a lack of anterior rotation of the ciliary processes. CONCLUSION: High frequency ultrasound imaging appears to be a valuable adjunct in making or corroborating the diagnosis of pupillary block glaucoma.

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High frequency (50-MHz) ultrasound allows greater resolution (approximately 30 microns) and improved tissue differentiation of the anterior ocular structures than conventional (8-10 MHz) ultrasonic techniques. Spectral analysis of tissue acoustic backscatter is sensitive to both the concentration and size of tissue inhomogeneities. We studied the healing process of experimentally induced corneal scars using high frequency ultrasound spectral analysis. Scatterer size and concentration parameter images of scarred corneas showed distinctive patterns that were correlated with histology over time. This quantitative technique offers a new, noninvasive, in vivo method for the assessment of the internal microarchitecture of surgically altered corneal tissue and its healing dynamics.

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Radial keratotomy is a surgical procedure to correct myopia that involves placing corneal incisions of precise partial thickness to induce flattening. It has yielded positive but sometimes unpredictable results. Many surgical variables influence the final result. Among them, incision depth is probably the most difficult to control and evaluate. In this study, we used very high frequency (50 MHz) ultrasound (HFU) to image radial keratotomy incisions in post-radial keratotomy human corneas to obtain high definition images of the cornea. The images allowed us to measure the depth of incisions as a percentage of corneal thickness.

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We used high-frequency ultrasound B-scanning with digital signal processing for pachymetric analysis of corneal scars to help determine the optimal management strategy. Four patients were selected for this report. By an automated motor system, 12 consecutive, parallel ultrasound B-scans, each 2.5 to 3 mm wide at 0.25 mm intervals, were obtained from the central corneal area of three patients with anterior corneal scarring. In a fourth patient with near complete corneal scarring obscuring the view of the anterior chamber, a set of 15 mm wide B-scans was obtained. Digitized ultrasound signals were used to produce high-resolution images and I-scans enabling a pachymetric precision of +/- 2 microns (SD). Epithelial, scar, and corneal thickness measurements were made along each scan to determine the most significant zone of pathology. Pachymetry of the cornea and the individual layers was used to assess the suitability for either photorefractive or penetrating keratectomy. B-scan imaging of the full anterior segment provided useful information for the preoperative planning of anterior segment reconstruction and the prognostic evaluation of penetrating keratoplasty. This method provides a powerful tool for the corneal surgeon in management planning.

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PURPOSE: The authors developed a system for producing topographic pachymetric maps of the corneal epithelium and anterior scar tissue. METHOD: The system uses high-frequency ultrasound scanning enhanced by digital signal processing. Ultrasonic echo data from consecutive parallel B-scans of the cornea spaced at 250-microns intervals are digitized and stored. Using the I-scan (obtained by computing the analytic signal magnitude of the deconvolved ultrasound signal), layer thickness measurements are made with a precision of 2 microns (standard deviation) at 120-microns intervals along each scan plane. The data are stored as an array, z(x,y), mapping thickness, z, onto horizontal and vertical (x,y) spatial coordinates. Pachymetric maps are then constructed by plotting local thickness, represented by a color scale, against measurement point position. RESULTS: Examples of a normal cornea, a contact lens-wearing cornea, Reis-Bückler dystrophy, and postphotorefractive keratectomy are presented. Areas with significant subepithelial scarring and general epithelial thickening in a subject with Reis-Bückler dystrophy are mapped. Unevenness in the epithelial thickness profile of the cornea in a subject after photorefractive keratectomy is shown, relative to the fellow (untreated) cornea. CONCLUSION: This technique provides the corneal surgeon with a new tool for the topographic evaluation of the thickness of anterior corneal layers in normal and pathologic corneas with high precision. In addition, the technique is not limited to optically transparent tissue.

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PURPOSE: The authors determine the mean central corneal and epithelial thickness in a group of normal human subjects using a new high-frequency ultrasound technique, incorporating digital signal processing. METHOD: Both eyes of ten volunteers (age range, 23-44 years) were scanned through a normal saline standoff. Digitized ultrasonic echo data were mathematically transformed to produce a plot, the I-scan, which optimally localizes acoustic interfaces to provide improved measurement precision. System precision was determined by analysis of variance of repeated measures. Central epithelial thickness was obtained by averaging multiple measurements. Central corneal thickness was determined by fitting measurements of apparent corneal thickness in consecutive parallel B-scans to a mathematically modeled cornea. A speed of sound of 1640 m/second was used. RESULTS: Epithelial pachymetric precision using A-scan and I-scan was 4.8 and 2.0 microns (standard deviation), respectively. The mean epithelial thicknesses for the right and left eyes were 50.7 +/- 3.7 microns and 50.3 +/- 3.4 microns, respectively. The mean corneal thicknesses in the right and left eyes were 514.6 +/- 38.4 microns and 516.2 +/- 37.8 microns, respectively. The root mean-square differences in epithelial and corneal thickness between the left and right eyes of each subject were 1.3 and 7.7 microns, respectively (neither was statistically significant). CONCLUSION: This system provides a pachymetric precision superior to current optical and ultrasound methods. Epithelial and corneal pachymetry is obtained noninvasively by a method that is not limited to optically clear media.

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PURPOSE: High-frequency (50-MHz) ultrasound allows high-resolution imaging of anterior ocular structures. Spectrum analysis of acoustic backscatter is sensitive to the concentration, size, and density of tissue inhomogeneities. The authors sought to determine whether acoustic imaging and spectrum analysis of hyphema would allow them to distinguish organized from fluid hyphema and recent from old hemorrhage in the eye. METHODS: Trauma-induced hyphemas were followed by slit-lamp photography and high-frequency ultrasonography in six New Zealand white rabbits. The blood collections were analyzed using the normalized power spectra of the digitized radio frequency ultrasound data and compared with in vitro references. RESULTS: The 50-MHz acoustic images permitted differentiation between fluid (diffuse) and clotted (organized) blood. Spectrum analysis allowed quantitative characterization of the degree of blood organization. Significant changes were observed in spectral properties during the time course of absorption both for initial and after-rebleeding hemorrhages. The characteristics of a human postsurgical hyphema also were examined and found to be similar to those seen in the experimental model. CONCLUSION: Spectrum analysis of high-frequency ultrasound data was able to distinguish organized from recent hemorrhage, which is clinically helpful for planning hyphema therapy.

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OBJECTIVE: To report objective grading and analysis of excimer laser-induced scarring using high-frequency ultrasound. METHODS: High-frequency ultrasound (50 MHz) corneal examination was performed on eight New Zealand white rabbits at different time points (10, 17, 24, and 74 days following phototherapeutic keratectomy). We used biometry and signal-processing techniques to determine corneal, epithelial, and scar thicknesses and to quantify the acoustic backscatter. RESULTS: Excimer laser-induced scarring showed an irregularly distributed acoustic hyperreflectivity that decreased through day 74. Corneal thickness remained reduced after ablation (mean, 318 microns compared with 419 microns for controls). Epithelial thickness averaged 62 microns and scar thickness, 87 microns. Scar peak and average anterior stroma acoustic backscatter ratios decreased from day 10 to day 74 (19.65 to 2.76, and 6.42 to 1.32, respectively). Histopathologic study showed increased keratocyte activity at early time points that correlated with acoustic backscatter ratios and imaging pattern. CONCLUSIONS: High-frequency ultrasound signal processing is a noninvasive method that quantitatively grades excimer laser-induced corneal scarring.

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