George Asimellis

Development of a method for automated quantitative analysis of ores using LIBS

Abstract This paper reports the development of a method for real-time automated quantitative analysis of mineral ores using a commercial laser-induced breakdown spectroscopy instrument, TRACER™ 2100, fitted with a recently developed computer controlled auto-sampler. The auto-sampler permits the execution of methods for performing calibrations and analysis of multiple elements on multiple samples. Furthermore, the analysis is averaged over multiple locations on each sample, thus compensating for heterogeneous morphology. The results for phosphate ore are reported here, but similar methods are being developed for a range of ores and minerals. Methods were developed to automatically perform metallic element calibrations for supplied phosphate ore samples containing known concentrations of the following minerals: P2O5, CaO, MgO, SiO2 and Al2O3. A spectral line for each desired element was selected with respect to the best combination of peak intensity and minimum interferences from other lines. This is a key step, because of the observed matrix dependence of the technique. The optimum combination of the time interval between the laser firing (plasma formation), signal detection, and the duration of the optical detection was then determined for each element, to optimize spectral line intensity and resolution. The instrument was capable of analyzing the required elements in the phosphate ore samples supplied with 2–4% relative standard deviations for most elements. Calibrations were achieved for P, Ca, Mg, Al and Si with linear regression coefficients of 0.985, 0.980, 0.993, 0.987 and 0.985, respectively. Preparation and analysis time for each sample was less than 5 min.

Keratoconus Management: Long-Term Stability of Topography-Guided Normalization Combined With High-Fluence CXL Stabilization (The Athens Protocol)

PURPOSE To investigate refractive, topometric, pachymetric, and visual rehabilitation changes induced by anterior surface normalization for keratoconus by partial topography-guided excimer laser ablation in conjunction with accelerated, high-fluence cross-linking. METHODS Two hundred thirty-one keratoconic cases subjected to the Athens Protocol procedure were studied for visual acuity, keratometry, pachymetry, and anterior surface irregularity indices up to 3 years postoperatively by Scheimpflug imaging (Oculus Optikgeräte GmbH, Wetzlar, Germany). RESULTS Mean visual acuity changes at 3 years postoperatively were +0.38 ± 0.31 (range: -0.34 to +1.10) for uncorrected distance visual acuity and +0.20 ± 0.21 (range: -0.32 to +0.90) for corrected distance visual acuity. Mean K1 (flat meridian) keratometric values were 46.56 ± 3.83 diopters (D) (range: 39.75 to 58.30 D) preoperatively, 44.44 ± 3.97 D (range: 36.10 to 55.50 D) 1 month postoperatively, and 43.22 ± 3.80 D (range: 36.00 to 53.70 D) up to 3 years postoperatively. The average Index of Surface Variance was 98.48 ± 43.47 (range: 17 to 208) pre-operatively and 76.80 ± 38.41 (range: 7 to 190) up to 3 years postoperatively. The average Index of Height Decentration was 0.091 ± 0.053 μm (range: 0.006 to 0.275 μm) preoperatively and 0.057 ± 0.040 μm (range: 0.001 to 0.208 μm) up to 3 years postoperatively. Mean thinnest corneal thickness was 451.91 ± 40.02 μm (range: 297 to 547 μm) preoperatively, 353.95 ± 53.90 μm (range: 196 to 480 μm) 1 month postoperatively, and 370.52 ± 58.21 μm (range: 218 to 500 μm) up to 3 years postoperatively. CONCLUSIONS The Athens Protocol to arrest keratectasia progression and improve corneal regularity demonstrates safe and effective results as a keratoconus management option. Progressive potential for long-term flattening validates using caution in the surface normalization to avoid overcorrection.

Revisiting keratoconus diagnosis and progression classification based on evaluation of corneal asymmetry indices, derived from Scheimpflug imaging in keratoconic and suspect cases

Purpose To survey the standard keratoconus grading scale (Pentacam®-derived Amsler–Krumeich stages) compared to corneal irregularity indices and best spectacle-corrected distance visual acuity (CDVA). Patients and methods Two-hundred and twelve keratoconus cases were evaluated for keratoconus grading, anterior surface irregularity indices (measured by Pentacam imaging), and subjective refraction (measured by CDVA). The correlations between CDVA, keratometry, and the Scheimpflug keratoconus grading and the seven anterior surface Pentacam-derived topometric indices – index of surface variance, index of vertical asymmetry, keratoconus index, central keratoconus index, index of height asymmetry, index of height decentration, and index of minimum radius of curvature – were analyzed using paired two-tailed t-tests, coefficient of determination (r2), and trendline linearity. Results The average ± standard deviation CDVA (expressed decimally) was 0.626 ± 0.244 for all eyes (range 0.10–1.00). The average flat meridian keratometry was (K1) 46.7 ± 5.89 D; the average steep keratometry (K2) was 51.05 ± 6.59 D. The index of surface variance and the index of height decentration had the strongest correlation with topographic keratoconus grading (P < 0.001). CDVA and keratometry correlated poorly with keratoconus severity. Conclusion It is reported here for the first time that the index of surface variance and the index of height decentration may be the most sensitive and specific criteria in the diagnosis, progression, and surgical follow-up of keratoconus. The classification proposed herein may present a novel benchmark in clinical work and future studies.

In Vivo 3-Dimensional Corneal Epithelial Thickness Mapping as an Indicator of Dry Eye: Preliminary Clinical Assessment

PURPOSE To evaluate in vivo epithelial thickness in dry eye by anterior segment optical coherence tomography. DESIGN Observational, retrospective case-control study. METHODS Two age-matched groups of female subjects, 70 eyes each, age ≈ 55 years, were studied in clinical practice setting: a control (unoperated, no ocular pathology) and a dry eye group (clinically confirmed dry eye, unoperated and no other ocular pathology). Corneal epithelium over the entire cornea was topographically imaged via a novel anterior segment optical coherence tomography (AS-OCT) system. Average, central, and peripheral epithelial thickness as well as topographic epithelial thickness variability were measured. RESULTS For the control group, central epithelial thickness was 53.0 ± 2.7 μm (45-59 μm). Average epithelium thickness was 53.3 ± 2.7 μm (46.7-59.6 μm). Topographic thickness variability was 1.9 ± 1.1 μm (0.7-6.1 μm). For the dry eye group, central epithelial thickness was 59.5 ± 4.2 μm (50-72 μm) and average thickness was 59.3 ± 3.4 μm (51.4-70.5 μm). Topographic thickness variability was 2.5 ± 1.5 μm (0.9-6.9 μm). All pair tests of respective epithelium thickness metrics between the control and dry eye group show statistically significant difference (P < .05). CONCLUSIONS This study, based on very user-friendly, novel AS-OCT imaging, indicates increased epithelial thickness in dry eyes. The ease of use and the improved predictability offered by AS-OCT epithelial imaging may be a significant clinical advantage. Augmented epithelial thickness in the suspect cases may be employed as an objective clinical indicator of dry eye.

Customized hydrogel contact lenses for keratoconus incorporating correction for vertical coma aberration

Purpose:  To study the effects on visual performance of a novel custom hydrogel contact lens, which employs a correction for vertical coma aberration, in keratoconic eyes.

In Vivo Three-Dimensional Corneal Epithelium Imaging in Normal Eyes by Anterior-Segment Optical Coherence Tomography: A Clinical Reference Study

Purpose: To evaluate the safety and efficacy of real-time measurement of corneal epithelial thickness and investigate the distribution characteristics in a large normal-eye population using a clinically available spectral-domain anterior-segment optical coherence tomography (AS OCT) system. Methods: Corneal epithelial thickness distribution and topographic thickness variability were clinically investigated using AS OCT imaging in 373 patients with normal, healthy eyes. Descriptive statistics investigated 3 sets of subgroups, male (n = 171) and female (n = 202), younger (n = 194) and older (n = 179), right eyes (n = 195) and left eyes (n = 197). Results: Pupil center epithelial thickness repeatability was an average 0.88 ± 0.71 &mgr;m; a similar repeatability was noted for the superior, inferior, maximum, and minimum epithelial thickness. On average, the pupil center epithelial thickness was 53.28 ± 3.34 &mgr;m, superior 51.86 ± 3.78 &mgr;m, inferior 53.81 ± 3.44 &mgr;m, minimum 48.65 ± 4.54 &mgr;m, maximum 56.35 ± 3.80 &mgr;m, and topographic variability was 1.78 ± 0.78 &mgr;m. Small differences were noted between male (average center 54.10 ± 3.34 &mgr;m) and female (52.58 ± 3.19 &mgr;m) subjects. The topographic thickness variability seems to increase with age: younger group, 1.65 ± 0.83 &mgr;m; older group, 1.93 ± 0.90 &mgr;m (P = 0.173). Conclusions: We present a comprehensive investigation of corneal epithelial thickness distribution characteristics in a healthy, untreated human eye population by using in vivo, clinically available Fourier-domain AS OCT. The 3-dimensional epithelial maps reveal epithelial nonuniformity and provide a novel benchmark for future and comparative studies.

Correlation between epithelial thickness in normal corneas, untreated ectatic corneas, and ectatic corneas previously treated with CXL; is overall epithelial thickness a very early ectasia prognostic factor?

Video abstract Video

Epithelial remodeling after femtosecond laser-assisted high myopic LASIK: comparison of stand-alone with LASIK combined with prophylactic high-fluence cross-linking.

Purpose: The aim of this study was to evaluate the possible topographic epithelial profile thickness changes (remodeling) after high myopic femtosecond laser in situ keratomileusis (LASIK) with concurrent prophylactic high-fluence cross-linking (CXL) in comparison with standard femtosecond LASIK. Methods: Preoperative and 6-month postoperative 3-dimensional epithelial thickness distribution maps were investigated through clinical spectral domain anterior-segment optical coherence tomography in 2 groups of femtosecond laser–assisted myopic LASIK cases. Group A represented 67 eyes treated additionally with concurrent prophylactic CXL (LASIK-Xtra); group B represented 72 eyes subjected to stand-alone femtosecond LASIK. Optical coherence tomography measurements of the epithelial thickness over the center 2-mm-diameter disk, mid-peripheral 5-mm rim, and overall (the entire 6-mm-diameter disc area) were investigated. Results: The comparison of matched myopic correction subgroups indicated statistically significant differences in the epithelial thickness increase specifically between high myopia subgroups. For example, in group A (LASIK-Xtra), the mid-peripheral epithelial thickness increase was +3.79 and +3.95 &mgr;m for the “−8.00 to −9.00 diopter” and “−7.00 to −8.00 diopter” subgroups, which compare with increased thickness in group B (stand-alone LASIK), of +9.75 &mgr;m (P = 0.032) and +7.14 &mgr;m (P = 0.041), respectively, for the same subgroups. Conclusions: Application of prophylactic CXL concurrently with high myopic LASIK operation results in a statistically significant reduced epithelial increase in comparison with stand-alone LASIK. This comparison is observed between matched high myopic correction subgroups. This difference may correlate with higher regression rates and/or may depict increased biomechanical instability in stand-alone LASIK.

Introduction of quantitative and qualitative cornea optical coherence tomography findings induced by collagen cross-linking for keratoconus: a novel effect measurement benchmark

Purpose To introduce a novel, noninvasive technique to determine the depth and extent of anterior corneal stroma changes induced by collagen cross-linking (CXL) using quantitative analysis of high-resolution anterior-segment optical coherence tomography (OCT) post-operative images. Setting Private clinical ophthalmology practice. Patients and methods Two groups of corneal cross-sectional images obtained with the OptoVue RTVue anterior-segment OCT system were studied: group A (control) consisted of unoperated, healthy corneas, with the exception of possible refractive errors. The second group consisted of keratoconic corneas with CXL that were previously operated on. The two groups were investigated for possible quantitative evidence of changes induced by the CXL, and specifically, the depth, horizontal extent, as well as the cross-sectional area of intrastromal hyper-reflective areas (defined in our study as the area consisting of pixels with luminosity greater than the mean +2 × standard deviation of the entire stromal cross section) within the corneal stroma. Results In all images of the second group (keratoconus patients treated with CXL) there was evidence of intrastromal hyper-reflective areas. The hyper-reflective areas ranged from 0.2% to 8.8% of the cross-sectional area (mean ± standard deviation; 3.46% ± 1.92%). The extent of the horizontal hyper-reflective area ranged from 4.42% to 99.2% (56.2% ± 23.35%) of the cornea image, while the axial extent (the vertical extent in the image) ranged from 40.00% to 86.67% (70.98% ± 7.85%). There was significant statistical difference (P < 0.02) in these values compared to the control group, in which, by application of the same criteria, the same hyper-reflective area (owing to signal noise) ranged from 0.00% to 2.51% (0.74% ± 0.63%). Conclusion Herein, we introduce a novel, noninvasive, quantitative technique utilizing anterior segment OCT images to quantitatively assess the depth and cross-sectional area of CXL in the corneal stroma based on digital image analysis. Mean cross-sectional area showing evidence of CXL was 3.46% ± 1.92% of a 6 mm long segment.

Comparison of Placido disc and Scheimpflug image-derived topography-guided excimer laser surface normalization combined with higher fluence CXL: the Athens Protocol, in progressive keratoconus

Background The purpose of this study was to compare the safety and efficacy of two alternative corneal topography data sources used in topography-guided excimer laser normalization, combined with corneal collagen cross-linking in the management of keratoconus using the Athens protocol, ie, a Placido disc imaging device and a Scheimpflug imaging device. Methods A total of 181 consecutive patients with keratoconus who underwent the Athens protocol between 2008 and 2011 were studied preoperatively and at months 1, 3, 6, and 12 postoperatively for visual acuity, keratometry, and anterior surface corneal irregularity indices. Two groups were formed, depending on the primary source used for topoguided photoablation, ie, group A (Placido disc) and group B (Scheimpflug rotating camera). One-year changes in visual acuity, keratometry, and seven anterior surface corneal irregularity indices were studied in each group. Results Changes in visual acuity, expressed as the difference between postoperative and preoperative corrected distance visual acuity were +0.12 ± 0.20 (range +0.60 to −0.45) for group A and +0.19 ± 0.20 (range +0.75 to −0.30) for group B. In group A, K1 (flat keratometry) changed from 45.202 ± 3.782 D to 43.022 ± 3.819 D, indicating a flattening of −2.18 D, and K2 (steep keratometry) changed from 48.670 ± 4.066 D to 45.865 ± 4.794 D, indicating a flattening of −2.805 D. In group B, K1 (flat keratometry) changed from 46.213 ± 4.082 D to 43.190 ± 4.398 D, indicating a flattening of −3.023 D, and K2 (steep keratometry) changed from 50.774 ± 5.210 D to 46.380 ± 5.006 D, indicating a flattening of −4.394 D. For group A, the index of surface variance decreased to −5.07% and the index of height decentration to −26.81%. In group B, the index of surface variance decreased to −18.35% and the index of height decentration to −39.03%. These reductions indicate that the corneal surface became less irregular (index of surface variance) and the “cone” flatter and more central (index of height decentration) postoperatively. Conclusion Of the two sources of primary corneal data, the Scheimpflug rotating camera (Oculyzer™) for topography-guided normalization treatment with the WaveLight excimer laser platform appeared to provide more statistically significant improvement than the Placido disc topographer (Topolyzer™). Overall, the Athens protocol, aiming both to halt progression of keratoconic ectasia and to improve corneal topometry and visual performance, produced safe and satisfactory refractive, keratometric, and topometric results. The observed changes in visual acuity, along with keratometric flattening and topometric improvement, are suggestive of overall postoperative improvement.