Micrometer-scale resolution imaging of the anterior eye in vivo with optical coherence tomography

Anterior chamber optical coherence tomography study of human natural accommodation in a 19-year-old albino

Anterior segment optical coherence tomography is a new method to explore the anterior chamber. The target can be focused and defocused with positive or negative lenses to reproduce the conditions of natural accommodation. We studied accommodation in an albino patient because the absence of pigment allows the infrared beam to penetrate the iris and explore the modifications of the ciliary body and the crystalline lens during natural accommodation in a human subject.

Group index of the human cornea at 1.3-microm wavelength obtained in vitro by optical coherence domain reflectometry

The group index of the cornea, rather than the phase refractive index, is required for thickness calculations with optical coherence tomography. Recent advances with high-speed optical coherence tomography at 1.3 microm make index measurement at this wavelength of great interest. Group indices of three human corneas from an eye bank were measured in vitro with optical coherence domain reflectometry. Measurements were made in a calibrated cuvette filled with a preservation medium to maintain proper corneal hydration. Group indices were calculated from the optical path lengths measured. The corneal group index was 1.389 +/- 0.004 (average +/- standard deviation). The average group index of a balanced salt solution, an approximation to aqueous humor, was 1.343 +/- 0.001.

Ultrahigh-resolution optical coherence tomography

In the past two decades, optical coherence tomography (OCT) has been established as an adjunct diagnostic technique for noninvasive, high-resolution, cross-sectional imaging in a variety of medical fields. The rapid development of ultrabroad bandwidth light sources has recently enabled a significant improvement in OCT imaging resolution, demonstrating the potential of OCT to accomplish its original goal of performing noninvasive optical biopsies, i.e., the in vivo visualization of microstructural morphology in situ, which had previously only been possible with histopathology. In addition, these novel light sources might also enable the use of spectroscopic OCT, an extension of ultrahigh-resolution OCT, for enhancing image contrast as well as detecting spatially resolved functional, biochemical tissue information. State-of-the-art-light sources that now permit ultrahigh-resolution OCT covering the whole wavelength region from 500 to 1600 nm are reviewed and fundamental limitations of OCT image resolution are discussed. Ex vivo ultrahigh-resolution OCT tomograms are compared with histological results; first clinical in vivo ultrahigh-resolution OCT and preliminary spectroscopic OCT results are presented and their impact for future clinical and research applications is discussed.

Dynamic morphology of clear corneal cataract incisions

OBJECTIVE

Clear corneal cataract incisions without sutures allow for rapid visual rehabilitation after phacoemulsification but might be associated with an increased risk of postoperative infection. The goal of this study was to examine in vitro dynamic changes in unhealed clear corneal cataract incisions that might adversely affect the risk of intraocular infection.

DESIGN

Laboratory investigation.

METHODS

Self-sealing clear corneal incisions were created in cadaveric human and rabbit eyes, and intraocular pressure was controlled with an infusion cannula. Incisions were imaged in real time using optical coherence tomography as intraocular pressure was varied. Using an artificial anterior chamber, India ink was applied to the surface of cadaveric human corneas with clear corneal incisions to detect possible flow of surface fluid along the incision. Intraocular pressure was varied by raising and lowering the infusion bottle so as to simulate the variation in intraocular pressure that would occur with successive blinks.

RESULTS

Optical coherence tomography demonstrated variation of corneal wound morphology in response to changes in intraocular pressure. Higher intraocular pressures were associated with close apposition of the wound edges, with no tendency for wound leakage. At low intraocular pressures, however, wound edges tended to gape, starting at the internal aspect of the wound. One incision opened along the entire length, allowing fluid flow across the cornea. Histologic examination revealed India ink particles in all incisions for up to three fourths of the length of the wound.

CONCLUSIONS

Transient reduction of intraocular pressure might result in poor wound apposition in clear corneal incisions, with the potential for fluid flow across the cornea and into the anterior chamber, with the attendant risk of endophthalmitis.

Optical coherence tomography assessment of retinal nerve fiber layer thickness changes after glaucoma surgery

PURPOSE

To assess changes in retinal nerve fiber layer (NFL) thickness in glaucoma patients after filtration surgery by using optical coherence tomography (OCT).

DESIGN

Retrospective observational case series.

PARTICIPANTS

Thirty-eight eyes of 31 glaucoma patients who underwent trabeculectomy or a combined procedure of cataract extraction and trabeculectomy were evaluated retrospectively.

METHODS

Eyes were imaged with OCT before surgery (1 week to 6 months before surgery; mean +/- standard deviation [SD], 71.3 +/- 61.2 days) and after surgery (6-12 months after surgery; 247.2 +/- 63.5 days) to measure peripapillary NFL thickness.

MAIN OUTCOME MEASURES

Changes in mean and segmental NFL thickness with respect to age, postoperative change in intraocular pressure (IOP), preoperative visual field test global indices, and change in visual field global indices.

RESULTS

A significant increase in the overall mean NFL thickness was present after surgery (P < 0.0001). Segmental analysis found a significant increase in NFL thickness in the nasal, superior, and temporal quadrants. IOP decreased after surgery from 22.0 +/- 6.4 mmHg to 11.4 +/- 4.7 mmHg (mean +/- SD). Twenty-eight (73.7%) of 38 eyes had an IOP reduction >30%. The mean NFL thickness increase (0.5- microm/mmHg decrease of IOP) was significantly correlated with the IOP reduction (r = -0.41; P = 0.03). No correlation was found between NFL thickness changes and age, preoperative visual field global indices, or change in visual field global indices.

CONCLUSIONS

A significant increase of the mean NFL thickness, which was related to IOP reduction, was detected after glaucoma filtration surgery.

Precision of measurement of tissue optical properties with optical coherence tomography

Accurate and noninvasive measurement of tissue optical properties can be used for biomedical diagnostics and monitoring of tissue analytes. Noninvasive measurement of tissue optical properties (total attenuation and scattering coefficients, optical thickness, etc.) can be performed with the optical coherence tomography (OCT) technique. However, speckle noise substantially deteriorates the accuracy of the measurements with this technique. We studied suppression of speckle noise for accurate measurement of backscattering signal and scattering coefficient with the OCT technique. Our results demonstrate that the precision of measurement of backscattering signals with the OCT technique can be 0.2% for homogeneously scattering media and 0.7% for skin, if spatial averaging of speckle noise is applied. This averaging allows us to achieve the precision of tissue scattering coefficient measurements of approximately +/-0.8%. This precision can be further improved by a factor of 2-3, upon optimization of OCT operating parameters.

Analysis of macular volume in normal and glaucomatous eyes using optical coherence tomography

PURPOSE

To evaluate macular volume in normal and glaucomatous eyes using optical coherence tomography (OCT).

DESIGN

Case control study.

METHOD

The authors assessed 272 eyes of 164 subjects as part of an institutional study at New England Eye Center in Boston, Massachusetts; 202 eyes were in the study group and 70 eyes in the control group. Eyes were categorized as normal (70 eyes of 43 subjects), glaucoma suspect (70 eyes of 44 subjects), early glaucoma (70 eyes of 47 subjects), or advanced glaucoma (62 eyes of 43 subjects). Subjects underwent analysis with the commercially available OCT1 unit. Optical coherence tomography macular neurosensory retinal thickness maps were used to calculate macular volume for comparison to Humphrey visual field testing, intraocular pressure measurement, and stereo biomicroscopy of the optic nerve head and nerve fiber layer.

RESULTS

Using repeated measures regression, macular volume in normal (2.37 +/- 0.11 mm(3)) glaucoma suspect (2.33 +/- 0.16 mm(3)), and early glaucoma eyes (2.27 +/- 0.13 mm(3)) was significantly greater than in eyes with advanced glaucoma (2.12 +/- 0.23 mm(3), P =.0001, P =.0001, and P =.0008, respectively). Macular volume in normal eyes was significantly greater than in early glaucoma eyes (P =.01).

CONCLUSIONS

Optical coherence tomography retinal macular volume correlates with known structural defects of glaucoma, providing a potential objective and quantitative parameter for evaluation. Our data show a significant difference in macular volume between normal, glaucoma suspect, and early glaucoma eyes, compared with advanced glaucomatous eyes as well as between normal and early glaucomatous eyes. This correlates with a trend of decreasing macular volume in eyes with more advanced disease.

Experimental Imaging of Intracorneal Ring Segments With Optical Coherence Tomography

PURPOSE

To investigate the imaging of corneal structures with optical coherence tomography (OCT) after implantation of intracorneal ring segments (ICRS).

METHODS

In an experimental study with six porcine eyes, qualitative and quantitative imaging with corneal OCT using a wavelength of 1310 nm after implantation of ICRS was performed. The optical results were compared with light microscopy of the histological sections.

RESULTS

In corneal OCT, the ICRS revealed marked hyporeflective intrastromal areas, which correlated well with macroscopic and microscopic findings. Corneal OCT enabled precise images of the incision depth for the implantation of ICRS, and the exact intrastromal segment position.

CONCLUSIONS

Noncontact slit lamp-adapted corneal optical coherence tomography could be employed to clinically monitor corneal changes after implantation of ICRS, evaluate the depth of the segments to correlate refractive changes, and quantify the stromal wound healing response.

Intraoperative Optical Coherence Pachymetry During Laser in situ Keratomileusis-First Clinical Experience

PURPOSE

To investigate intraoperative optical coherence pachymetry during laser in situ keratomileusis (LASIK).

METHODS

In an initial clinical evaluation, three patients with myopia and myopic astigmatism were studied. Corneal thickness was assessed with optical pachymetry based on low-coherence interferometry during LASIK.

RESULTS

The attempted mean spherical equivalent refraction was -5.70 +/- 2.00 D with a mean calculated stromal ablation depth of 95 +/- 18 microm. Intraoperative optical coherence pachymetry was reproducible in all patients during the different stages of LASIK, demonstrating a mean flap thickness of 141 +/- 30 microm with a residual corneal stroma of 274 +/- 24 microm at the end of the laser ablation. The immediate postoperative corneal thickness revealed marked swelling.

CONCLUSIONS

This initial clinical evaluation proved that intraoperative optical coherence pachymetry may be an important safety feature for monitoring flap and residual stromal thickness during LASIK. It may be particularly helpful in the effort to avoid iatrogenic corneal ectasia in patients with thin corneas, higher refractive corrections, and LASIK enhancements.

Scanning laser polarimetry with variable corneal compensation and optical coherence tomography in normal and glaucomatous eyes

PURPOSE

To evaluate the relationship between visual function and retinal nerve fiber layer (RNFL) measurements obtained with scanning laser polarimetry with variable corneal compensation (SLP-VCC) and optical coherence tomography (OCT).

DESIGN

Cross-sectional analysis of normal and glaucomatous eyes in a tertiary care academic referral practice.

METHODS

A commercial GDx nerve fiber analyzer was modified to enable the measurement of corneal polarization axis and magnitude so that compensation for corneal birefringence was eye specific. Complete examination, SLP with fixed corneal compensation (FCC) and variable corneal compensation (VCC), optical coherence tomography (OCT) imaging of the peripapillary RNFL, and automated achromatic perimetry were performed in all subjects. Exclusion criteria were visual acuity less than 20/40, diseases other than glaucoma, and unreliable perimetry.

RESULTS

Fifty-nine patients (59 eyes; 29 normal, 30 glaucomatous) were enrolled (mean age, 56.7 +/- 20.3 years, range, 20-91). All eyes with glaucoma had associated visual field loss (average mean defect, -8.4 +/- 5.8 dB). Using SLP-FCC, nine of 12 retardation parameters (75%) were significantly less in glaucomatous eyes. Using SLP-VCC, 11of 12 retardation parameters (92%) were significantly less in glaucomatous eyes. Multiple regression models constructed for each retardation parameter with visual field demonstrated that the following VCC parameters were statistically significant whereas FCC parameters were not: ellipse average (FCC, P =.28, VCC, P =.001), superior average (FCC, P =.38, VCC, P <.001), inferior average (FCC, P =.10, VCC, P =.008), average thickness (FCC, P =.30, VCC, P =.031), and superior integral (FCC, P =.43, VCC, P =.001). Similar results were obtained for multiple regression models constructed with OCT-derived RNFL thickness: ellipse average (FCC, P =.99, VCC, P =.002), superior average (FCC, P =.90, VCC, P <.001), inferior average (FCC, P =.61, VCC, P =.007), and superior integral (FCC, P =.92, VCC, P <.001).

CONCLUSIONS

Compared with fixed compensation, mean-based SLP parameters generated with SLP-VCC have greater correlation with visual function and RNFL thickness assessments obtained with OCT.

Safety of Laser in situ Keratomileusis Performed Under Ultra-thin Corneal Flaps

PURPOSE

To report on the safety of laser in situ keratomileusis (LASIK) performed under ultra-thin corneal flaps (Micro-LASIK).

METHODS

We performed a retrospective review of 1131 eyes that underwent LASIK for myopia with the Nidek MK-2000 microkeratome, and Nidek EC-5000 excimer laser. The 130-microm head and 8.5-mm ring were used in 1042 eyes and the 160-microm head and 9.5-mm ring were used in 89 eyes. For 175 eyes, intraoperative ultrasonic pachymetry was used to measure central corneal thickness and central bed thickness. Flap thickness was calculated by subtracting bed thickness prior to laser ablation from central corneal thickness. All 175 of these eyes had keratectomies using the 130-microm head and 8.5-mm ring.

RESULTS

At last follow-up, 455 eyes (40%) achieved 20/20 or better, 798 eyes (70%) achieved 20/25 or better, and 1077 eyes (95%) achieved 20/40 or better uncorrected visual acuity; 922 eyes (82%) achieved within 1 line of their best spectacle-corrected visual acuity. The following clinically significant complications were observed: four epithelial defects (3.5%), zero irregular flaps (0%), seven stria (0.6%), one diffuse lamellar keratitis (0.1%), one epithelial ingrowth (0.1%), and zero infections (0.0%). Two eyes (0.2%) with stria had 20/40 best spectacle-corrected visual acuity, but lost more than 2 lines of best spectacle-corrected visual acuity. The average measured flap thickness was 87.3 +/- 15.4 microm.

CONCLUSION

LASIK can be performed safely under ultra-thin corneal flaps.

The macular thickness and volume in glaucoma: an analysis in normal and glaucomatous eyes using OCT

PURPOSE

To evaluate macular volume in normal and glaucomatous eyes using Optical Coherence Tomography (OCT).

METHODS

Fifty eyes of 30 patients (age: 49-68); 20 eyes normal, 15 eyes with early glaucoma and 15 eyes with advanced glaucoma have been studied with the commercially available OCT unit (OCT 2000) (Humphrey Zeiss, Dublin, CA, USA). All eyes were examined at a scan length of 3.44 mm vertically across the fovea. OCT macular retinal thickness maps were used to calculate macular volume.

RESULTS

We observed significant differences between groups. Normals (7.35 +/- 0.455 mm3) and early glaucoma (7.09 +/- 0.475 mm3), each had significantly greater volume than subjects with advanced glaucoma (6.678 +/- 0.455 mm3).

CONCLUSIONS

Volumetric analysis of macular thickness with OCT tomograms may be a useful method of documenting and monitoring patients with early glaucoma and advanced glaucoma. In our analysis, and according to the observations of other authors, OCT macular volumes correlates significantly with glaucoma status.

Optical coherence tomography for the detection of laser in situ keratomileusis in donor corneas

PURPOSE

In 2001, more than one million laser in situ keratomileusis (LASIK) procedures were performed worldwide. Considering the increasing number of refractive procedures, eye banks will be increasingly confronted with the problem of how to identify those donors with prior refractive surgery. To date, efficient screening methods to identify LASIK surgery in donor eyes have not been established. Therefore, the purpose of the current study was to determine whether optical coherence tomography (OCT) can be used to detect the presence of LASIK-induced changes in human corneas.

METHODS

Laser in situ keratomileusis was performed on 20 organ-cultured human cornea disks. The excimer laser ablation performed ranged from 0 to 12 diopters. The corneas were maintained in culture, and the visibility of flap-stromal interface by OCT was assessed up to 6 months after the LASIK procedure. Additionally, two donor corneas with the history of LASIK treatment before death were screened for structural changes.

RESULTS

Optical coherence tomography scans were able to detect the interface between the corneal flap and the residual stromal tissue in all corneas and at all examined time intervals. There were no differences in signal intensity among the different depths of ablation. The relative signal intensity of the interface compared with the averaged stromal intensity ranged from 2.1 to 6.0. In both donor corneas with suspected prior LASIK surgery, OCT scanning showed the characteristic stromal interface as found in the in vitro model.

CONCLUSIONS

Corneal examination by OCT could be an appropriate technique for eye banks to screen donor corneas for prior LASIK surgery.

Transscleral optical coherence tomography--an experimental study in ex-vivo human eyes

BACKGROUND AND OBJECTIVE

To evaluate the potentials of a 1310-nm optical coherence tomography (OCT) system to penetrate the highly backscattering sclera in enucleated human eyes and provide visualization of intraocular structures by transscleral imaging.

STUDY DESIGN/MATERIALS AND METHODS

OCT-images were generated by an experimental prototype (Medical Laser Center, Lübeck, Germany) using a superluminescence diode with a wavelength of 1310 nm. OCT-images were taken from two enucleated human eyes using 100-200 axial scans with 60 Hz line scan frequency and compared to subsequent histologic sections.

RESULTS

Transscleral OCT allowed penetration of the sclera and the anterior chamber angle could be completely identified. Some change within the anterior eye segment could be demonstrated with high accuracy. Additionally, limited demonstration of the ciliary body region was achieved. Due to limited signal intensity no detailed imaging of the pars plana and pars plicata region was possible. However, more posterior measurements allowed transscleral visualization of a retinal detachment.

CONCLUSIONS

OCT using lightsources with a wavelength longer than that used in conventional OCT provides a promising imaging technique at high resolution allowing transscleral imaging of the anterior eye segment.

Macular thickness measured by optical coherence tomography (OCT) in diabetic patients

PURPOSE

1) To compare macular thickness (MT) by optical coherence tomography (OCT) in diabetics and controls; 2) to assess the relationship between MT and stage of diabetic retinopathy (DR) and macular edema (ME); 3) to quantify MT changes after laser treatment for ME.

METHODS

One-hundred and thirty-seven patients with diabetes mellitus (216 eyes) were admitted to the study and examined by stereo-color fundus photos, retinal fluorangiography and OCT. DR was classified as: 1) no DR (46 eyes: 21.3%); 2) background DR (66 eyes: 30.6%); 3) pre-proliferative DR (50 eyes: 23.1%); 4) proliferative DR (54 eyes: 25%). The study group was then divided into three ME groups: 1) no edema (65 eyes: 30.1%); 2) not clinically significant ME (no CSME) (45 eyes: 20.8%); 3) clinically significant macular edema (CSME) (106 eyes: 49.1%). Three-month follow-up tomograms were taken to evaluate eyes laser-treated only for ME. The control group consisted of 50 eyes of 50 non-diabetic, age- and sex-matched subjects.

RESULTS

MT was 369.3 +/- 163.2 microm in diabetics and 161.9 +/- 12.9 microm in controls (p < 0.001). In the four DR groups it was: 1) 211.0 +/- 37.6 microm; 2) 370.8 +/- 159.6 microm; 3) 419.1 +/- 138.2 microm; 4) 456.1 +/- 162.0 microm (p<0.001). In the three ME groups, MT was: 1)227.8 +/- 53.4 microm; 2) 321.8 +/- 124.2 microm; 3) 476.2 +/-146.6 microm (p < 0.001). In the 52 eyes treated with laser photocoagulation of the posterior pole only and with a follow-up > 3 months, MT before and after treatment was 468.2 +/- 83. 17 microm and 372.1 +/- 120.63 microm.

CONCLUSIONS

MT was greater in diabetics than controls and tended to increase with DR and ME severity. OCT is a sensitive technique for detecting early diabetic macular abnormalities and quantifying their reduction after laser treatment.

Optical coherence tomography for in situ monitoring of laser corneal ablation

OBJECTIVE

To improve the precision of refractive surgery, a new approach for determination of the removed corneal thickness profile in situ with laser ablation by optical coherence tomography (OCT) is developed.

STUDY DESIGN/MATERIALS AND METHODS

The traditional method for precision (less than 10 microm) measurements of intraocular distances is based on the use of the reflected component of probing radiation. This component is characterized by a small range of operating angles between a probing beam and a normal to the surface under study. To enhance this range of operating angles we suggest using a light component backscattered from a biological object. This will enable precision measurements over the entire surface of the cornea without any changes in the orientation between a probing beam and the eye, a necessary condition for in situ monitoring of laser refraction correction in the eye. We suggest a specially developed algorithm of OCT signal processing to measure the corneal thickness by the backscattered light component for a single longitudinal scan (A scan). The corneal thickness profile is obtained by a series of such A scans acquired by successively scanning a probing beam along the corneal surface. The thickness profile of removed layer is determined by changes in the corneal thickness profile in the process of ablation. When the cornea is ablated by a beam with a fixed transverse profile, we propose using integral characteristics of the ablated layer profile, for example, the maximum ablation depth, as criteria of changes in refractive power of the eye. The measurement precision by these characteristics is considerably higher than by a single A scan. Since the cornea is a poorly scattering medium, the Fourier filtering is employed to increase reliability and precision of the method. Model experiments on monitoring the ablation process in a lavsan film and ex vivo human cornea are described. Preliminary experiments on in vivo measurements of human corneal thickness are performed.

RESULTS

In model experiments the precision of measurement of laser ablation depth by one A scan was 5-20 microm, depending on the signal-to-noise ratio (SNR), whereas the precision of measurement of laser ablation depth as the integral characteristic of the ablated layer profile was 0.3-5 microm. The experimental results showed that at small SNR Fourier filtering might considerably increase reliability and precision of measurements. When SNR is high, the measurement precision does not change. The precision of measurements of the corneal thickness in preliminary in vivo experiments was higher than in ex vivo experiments. This factor is very promising for application of the method suggested herein in refractive surgery.

Optical coherence tomography measurement of nerve fiber layer thickness and the likelihood of a visual field defect

PURPOSE

To determine if optical coherence tomography (OCT) measurements of nerve fiber layer (NFL) thickness can be used to predict the presence of visual field defects (VFD) associated with glaucoma.

DESIGN

Quota-sampled, cross-sectional study.

METHODS

Retrospective study of OCT NFL thickness measurements in 276 eyes of 276 subjects. All persons received OCT NFL thickness analysis; 136 eyes underwent frequency-doubling technology (FDT) perimetry; and 140 eyes underwent Swedish interactive threshold algorithm (SITA) perimetry. We defined a parameter called NFL(50), which is the NFL thickness value at which there was a 50% likelihood of a VFD with either SITA or FDT perimetry. We evaluated the use of NFL(50).

RESULTS

The mean NFL thickness with (n = 68) and without (n = 68) a VFD in the FDT group was 93.2 microm +/- 22.6 and 108.4 microm +/- 14.1, respectively. The mean NFL thickness with (n = 70) and without (n = 70) a VFD in the SITA group was 78.9 microm +/- 24.8 and 103.0 microm +/- 18.0, respectively. The FDT mean NFL(50) value was 98.5 microm. The SITA mean NFL(50) value was 87.0 microm. The area under the receiver operator characteristic (AROC) curve for mean NFL was 0.73, and the positive predictive value (PPV) for FDT mean NFL(50) was 72.2%. For SITA mean NFL, the AROC was 0.79 and the PPV for NFL(50) was 77.2%.

CONCLUSION

Nerve fiber layer thickness analysis using OCT may be clinically useful in identifying subjects who have visual field loss. However, the PPV suggests that OCT may need higher resolution and better reproducibility to enhance its sensitivity and specificity for population screening.

Corneal optical coherence tomography before and after phototherapeutic keratectomy for recurrent epithelial erosions(2)

PURPOSE

To study the representation of corneal structures with optical coherence tomography (OCT) before and after excimer laser phototherapeutic keratectomy (PTK) for recurrent epithelial erosions.

SETTING

Departments of Ophthalmology, Vivantes Klinikum Neukölln, Berlin, and Medizinische Universität, Lübeck, Germany.

METHODS

This prospective study comprised 15 eyes of 14 patients with recurrent epithelial erosions. The central corneal and epithelial thickness as well as the wound-healing response in the anterior corneal stroma were assessed with slitlamp-adapted OCT before and after PTK.

RESULTS

After PTK, the symptoms improved in all patients without loss of best corrected, glare, or low-contrast visual acuity. The mean central corneal OCT thickness was 540 microm +/-28 (SD) preoperatively, 492 +/- 36 microm immediately after epithelial debridement and PTK, and 519 +/- 25 microm after 7 weeks (P <.01). The mean central epithelial OCT thickness changed from 70 +/- 13 microm preoperatively to 60 +/- 7 microm after 7 weeks (P >.01). Changes in the light-scattering properties in the anterior subepithelial stroma revealed a hyperreflective area with a mean thickness of 46 +/- 13 microm after 7 weeks.

CONCLUSIONS

Using noncontact corneal OCT, corneal and epithelial thickness changes and the wound-healing response in the anterior corneal stroma could be evaluated after PTK in patients with recurrent epithelial erosions.

Relation between optical coherence tomography and optical pachymetry measurements of corneal swelling induced by hypoxia

PURPOSE

To determine the relation between optical coherence tomography (OCT) and optical pachymetry (OP) measurements of corneal swelling induced by hypoxia.

DESIGN

Experimental study.

METHODS

One randomly selected eye of 20 noncontact lens wearers (10 males and 10 females, age 35.6 +/- 9.6 years) was patched during 3 hours of soft contact lens (SCL) wear while the contralateral eye acted as control. Central corneal thickness of both eyes was measured before and after SCL wear using OCT and OP in randomized order.

RESULTS

Baseline central corneal thickness was 523.6 +/- 33.0 microm (mean +/- standard deviation [SD]) measured with OCT and 490.6 +/- 25.5 microm with OP. Immediately after contact lens removal, corneal thickness measured with OCT increased by 13.8 +/- 2.3% compared with 12.1 +/- 1.8% (paired t test: P <.001) measured with OP. Thereafter, corneal thickness decreased at the rate of 5.6% per hour for OCT and 5.4% per hour for OP. The difference in thickness between instruments before lens insertion, which was 33 microm compared with the difference after lens removal (edematous cornea), which ranged from 46 to 41 microm. The difference between instruments decreased during the corneal deswelling period after lens removal. The correlation coefficient between OCT and OP was 0.914 before lens insertion and 0.932 after lens removal.

CONCLUSION

This study has demonstrated the difference of corneal thickness measured with OCT and OP. Although both instruments are correlated highly in all conditions tested, OCT may overestimate corneal thickness in normal and edematous corneas.

Sterile structural imaging of donor cornea by optical coherence tomography

PURPOSE

The purpose of the study was to demonstrate a new noncontact method for sterile measurement of structure and thickness of donor cornea with use of optical coherence tomography (OCT).

METHODS

A commercially available OCT instrument designed for retinal measurements was used for noncontact assessment of human corneas. Structural changes occurring during organ culture were evaluated in 29 corneas. Comparison with histology was performed, and the ability of OCT to detect corneal scars and corneal thickness was investigated.

RESULTS

Corneal epithelium, stroma, and posterior curvature, as well as thickness, can be measured by standard OCT while the cornea remains in its storage bottle. Epithelial changes leading to a reduction of epithelial thickness, stromal structural changes, and hydration folds can be visualized. OCT scans correlate well with histology. Preexisting and developing corneal scars can be detected by OCT.

CONCLUSIONS

Corneal structural imaging can be performed under sterile conditions by OCT. This provides a method for improvement of corneal storage and a screening method for signs of photorefractive surgery and scarring in donor cornea.

In vivo confocal microscopy through-focusing to measure corneal flap thickness after laser in situ keratomileusis

PURPOSE

To measure flap thickness in laser in situ keratomileusis (LASIK) patients using in vivo confocal microscopy through-focusing (CMTF) and compare measured versus intended flap thickness achieved by 2 microkeratomes, the Automated Corneal Shaper(R) (ACS) (Chiron Bausch & Lomb) and the Hansatome (Bausch & Lomb).

SETTING

Department of Ophthalmology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, USA.

METHODS

Twenty-seven eyes of 27 patients were examined by in vivo CMTF 3 to 12 months after LASIK was performed with the ACS (12 patients) or Hansatome (15 patients) microkeratome. The central cornea was scanned, and the epithelium, flap, stroma, and total corneal thickness were measured. Normalized flap thickness (NFT) was also calculated to account for changes in epithelial thickness that may have occurred postoperatively.

RESULTS

The mean posterior stromal thickness was 341.1 microm +/- 53.9 (SD) (range 233 to 431 microm) in the ACS group and 320.3 +/- 42.3 microm (range 258 to 382 microm) in the Hansatome group. The mean nonnormalized flap thickness was 132.7 +/- 12.5 microm (range 11 to 151 microm) in the ACS group and 167.4 +/- 21.4 microm (range 141 to 209 microm) in the Hansatome group. The NFT was 129.6 +/- 9.5 microm and 158.4 +/- 22.1 microm, respectively. Both microkeratomes cut significantly less than intended (P <.05); however, the ACS cut a thinner-than-intended thickness in all cases, and the Hansatome cut thicker than intended in 13% of cases. The Hansatome also showed significantly greater variability in flap thickness than the ACS (P <.05).

CONCLUSIONS

A significant difference in precision was noted between the 2 microkeratomes. The findings emphasize the importance of performing thickness measurements and the usefulness of in vivo CMTF in making these determinations to ensure the safety and effectiveness of LASIK.

A comparison of laser photocoagulation with cryotherapy for threshold retinopathy of prematurity at 10 years: part 2. Refractive outcome

OBJECTIVE

To compare the refractive outcome of eyes treated with cryotherapy for threshold retinopathy of prematurity (ROP) with eyes treated with laser photocoagulation.

DESIGN

Extended follow-up of a randomized controlled clinical trial.

PARTICIPANTS

One hundred eighteen eyes from 66 patients were randomly assigned to receive either cryotherapy or laser photocoagulation for threshold ROP. Twenty-five patients (44 eyes treated) were available for follow-up examination 10 years later.

INTERVENTION

Cycloplegic autorefraction or retinoscopy was performed. Immersion ultrasound biometry (A-scan) was also performed, and an autokeratometer was used for keratometry. If an A-scan could not be tolerated or the patient was not cooperative, a B-scan through-the-lid biometry was performed. Corneal thickness was measured using optical coherence tomography. MAIN AND SECONDARY OUTCOME MEASURES: Refractive error. In addition, anterior chamber depth, lens thickness, and axial length were measured. Central corneal thickness measurements and keratometric readings were also obtained.

RESULT

Eyes treated with cryotherapy were significantly more myopic than those treated with laser photocoagulation. When comparing patients with bilateral treatment, the mean spherical equivalent (SE) of eyes treated with laser was -4.48 diopters (D) compared with a mean SE of -7.65 D for eyes treated with cryotherapy (n = 15 pairs of eyes, P = 0.019). Cryotherapy-treated eyes had a mean axial length of 21.7 mm versus 22.9 mm for laser-treated eyes (P = 0.024, n = 12 pairs of eyes). The anterior chamber depth and lens thickness averaged 2.86 mm and 4.33 mm, respectively, in the cryotherapy-treated eyes compared with 3.42 mm and 3.95 mm in the laser-treated eyes (P < 0.001, n = 12 pairs for both measurements). There were no statistical differences in anterior corneal curvature and central corneal thickness between the two treatment modalities. Crystalline lens power bore the strongest correlation to refractive outcomes in both laser-treated (r = 0.885, P < 0.001) and cryotherapy-treated eyes (r = 0.591, P = 0.026). Although keratometric readings were higher than normal values in these eyes, there was no correlation to the degree of myopia.

CONCLUSIONS

Laser-treated eyes were significantly less myopic than cyrotherapy-treated eyes. Lens power seemed to be the predominant factor contributing to the excess myopia.

Noncontact corneal pachymetry with slit lamp-adapted optical coherence tomography

PURPOSE

The purpose of this study was to determine the accuracy, the reproducibility, and the limits of agreement of noncontact central corneal thickness measurement with slit lamp-adapted optical coherence tomography (OCT).

DESIGN

Nonrandomized comparative clinical trial.

METHODS

In a prospective comparative observational study, a total of 108 consecutive patients (108 eyes) with normal corneas (92 eyes) and different corneal alterations (16 eyes) participated. Six sequential measurements of the central corneal thickness with slit lamp-adapted OCT and with ultrasound (US) pachymetry at 1640 ms(-1) were performed. The main outcome measures were accuracy, reproducibility assessed with precision and coefficient of variation (CV), and limits of agreement of central corneal thickness measurement.

RESULTS

The mean central corneal thickness values were 541 +/- 43 microm (OCT) and 549 +/- 44 microm (US) with a mean precision of +/- 5.8 microm (CV 1.08%) and of +/- 4.0 microm (CV 0.73%), respectively. The method comparison revealed equivalence (+/- 2SD) in the 5% range with a mean difference between both methods of 7.9 microm (1.45%). The relative error was 8.7 microm (1.6%), which corresponded to limits of agreement (+/- 2SD) ranging from -9.5 microm to 25.3 microm.

CONCLUSIONS

Central corneal pachymetry with slit lamp-adapted OCT revealed, for clinical purposes, an excellent accuracy and reproducibility with a high degree of agreement compared with US pachymetry. Thus, the presented OCT system seems to be a promising diagnostic modality to objectively measure corneal thickness in a convenient noncontact mode.

Optic nerve and retinal nerve fiber layer analyzers in glaucoma

There is mounting evidence that retinal nerve fiber layer (RNFL) loss precedes detectable visual field loss in early glaucomatous optic neuropathy. However, examination and photography of the RNFL is a difficult technique in many patients, particularly older individuals, and eyes with small pupils and media opacities. It is subjective, qualitative, variably reproducible, and often unreliable. Furthermore, optic nerve head and RNFL photography is time consuming, operator dependent, has limited sensitivity and specificity, and requires storage space. Imaging technologies have emerged which enable clinicians to perform accurate, objective, and quantitative measurements of the RNFL and optic nerve head topography. There is good agreement between such measurements and clinical estimates of optic nerve head structure and visual function. The reproducibility of these instruments suggests that they have the potential to detect structural change over time. This report will review the technological principles, reproducibility, sensitivity and specificity, capacity to detect glaucomatous progression, and limitations of currently available ocular imaging technologies.

The measurement of corneal epithelial thickness in response to hypoxia using optical coherence tomography

PURPOSE

To determine if corneal epithelial thickness increases in association with corneal edema induced by wearing soft contact lenses during eye closure.

DESIGN

Experimental study.

METHODS

One eye (randomly selected) of twenty noncontact lens wearers (10 males and 10 females, age 35.6 +/- 9.6 years) was patched during 3 hours of soft contact lens (SCL) wear and the contralateral eye acted as a control. Corneal and epithelial thickness of both eyes was measured before and after SCL wear using optical coherence tomography (OCT).

RESULTS

Immediately after contact lens removal, total corneal thickness was increased significantly by 13.8 +/- 2.3% (mean +/- SD) compared with baseline (P <.0001, paired t test) and after 100 minutes was still 4.5 +/- 2.3% thicker than baseline (P <.0001, paired t test). The control eyes showed no change in total corneal thickness (P >.05, paired t test). Immediately after contact lens removal, corneal epithelial thickness was increased by 1.7 +/- 4.8%, but this change was not statistically significant (P >.05, paired t test). Following contact lens removal, epithelial thickness changed significantly (Repeated measure analysis of variance [Re-ANOVA]: F((7,133)) = 4.91, p(H-F) < 0.001) over the next 100 minutes with thinning recorded at 60, 80, and 100 minutes (P <.05, paired t test). There was no significant change over time in epithelial thickness of the control eyes (Re-ANOVA: F(4, 76) = 0.91, p(H-F) = 0.464).

CONCLUSION

OCT demonstrated that corneal epithelial thickness does not increase in response to hypoxia from SCL wear and eye closure, in contrast to a significant increase in total corneal thickness.

Comparison of foveal thickness measured with the retinal thickness analyzer and optical coherence tomography

PURPOSE

To assess and compare the reliability and reproducibility of retinal thickness measurements for the retinal thickness analyzer (RTA) and optical coherence tomography (OCT) in normal and edematous retina.

METHODS

The authors measured the foveal thickness of 21 normal eyes and 9 eyes with macular edema with both methods in random order. With the RTA, the fovea was measured 10 times; with the OCT, six scans (one horizontal and five vertical cross-sections) of the fovea were obtained.

RESULTS

Mean foveal thickness of normal eyes measured 153 microm with OCT and 181 microm with RTA (median for both methods 150 microm). Coefficients of variation (CV) within the same subjects were 10% (OCT) and 9% (RTA) reducing to 9% (OCT) and 7% (RTA) when scans were repeated only five times for both methods. The RTA, however, yielded an interpatient CV of 33% (OCT 17%), which was caused by several falsely high readings in normal individuals. In eyes with retinal thickening the OCT measured a mean of 324 microm with 15% intra- and 58% interpatient CV. The RTA yielded a mean of 403 microm with CV of 18% and 73%, respectively.

CONCLUSION

Both methods yield reproducible measurements of foveal thickness in normal individuals and individuals with macular edema. However, falsely high measurements may occur with the RTA, reducing its reliability as compared to the OCT.

Retinal nerve fiber layer analysis: relationship between optical coherence tomography and red-free photography

PURPOSE

Comparison of retinal nerve fiber layer (RNFL) thickness measurements using optical coherence tomography (OCT) to the clinical standard red-free photography (evaluated semiquantitatively), in relation to functional visual field damage in primary open-angle glaucoma (POAG) patients, ocular hypertensives, and POAG suspects.

METHODS

Concurrent, cross-sectional study. In four age-matched groups (42 patients with early to moderate POAG, 34 ocular hypertensives, 22 POAG suspects, and 25 normal controls), RNFL was assessed with OCT, standardized red-free photographic scoring method and standard achromatic perimetry.

RESULTS

OCT RNFL thickness decreases with increased RNFL damage (detected with red-free photography). The global average OCT RNFL thickness correlated significantly with the photographic total RNFL score (r = .650, P = .0001). Both OCT and photographic scoring system were able to find significantly thinner RNFL in the glaucoma group as compared with normals (P = .0001 for both), ocular hypertensives (P = .0001 for both), and suspects (P = .0001 for both). However, neither OCT nor photography could significantly differentiate between ocular hypertensives, suspects, and normals. Both OCT and photography were significantly correlated with VF loss. For the percentage of VF points depressed <5%, the correlation was highest for OCT (r = -0.615 for OCT and r = -0.476 for photography). OCT had a higher diagnostic accuracy than photography (86% and 77%, respectively).

CONCLUSION

For RNFL thickness measurements, the presence of high correlations between OCT, photographic scores, and functional visual field loss suggest the validity of OCT measurements. The higher diagnostic accuracy of OCT RNFL measurements suggests its potential advantage for detection of early cases of glaucoma.

Imaging of posterior chamber phakic intraocular lens by optical coherence tomography

We demonstrate the applicability of optical coherence tomography to visualize the distance between the intraocular lens (IOL) and the lens capsule (lens vault) after implantation of the Staar Collamer posterior phakic IOL, also known as the implantable contact lens (ICL). Maintaining a substantial lens vault may be important to avoid cataract formation, but the lens vault was found to change over time; changes in accommodation and pupil size have also been noted. Optical coherence tomography allows precise assessment of the anatomic relationship between the ICL and the crystalline lens in a noncontact, noninvasive manner, permitting dynamic monitoring of changes in accommodation and pupil size.

Comparison of corneal pachymetry using ultrasound and Orbscan II

PURPOSE

To compare Orbscan II pachymetry with ultrasonic pachymetry in eyes having corneal refractive surgery.

SETTING

Instituto Oftalmológico de Alicante, Alicante, Spain.

METHODS

In this comparative prospective study, 72 eyes (36 patients) were divided into 4 groups: Group 1, 16 eyes with haze after photorefractive keratectomy (PRK); Group 2, 12 eyes with no haze after PRK; Group 3, 24 eyes that had uneventful laser in situ keratomileusis; Group 4, 20 normal eyes. The corneal thickness was measured in all eyes by the Orbscan II (version 3.27.10, Orbtek Inc.) and then by ultrasonic pachymetry (DHC 500, Technology Inc.).

RESULTS

The results between the 2 systems of pachymetry were significant in Group 1 (P < .0001) and not significant in Groups 2, 3, and 4 (P = .056, P = .051, and P = .68, respectively). A statistically significant correlation existed between the grade of haze and the decrease in Orbscan readings (C = -0.821, P < .0001).

CONCLUSION

The Orbscan II pachymetry measurements correlated with the ultrasound measurements in eyes with clear corneas. Haze after PRK decreased the Orbscan pachymetry.

Effects of pseudophakic lens capsule opacification on optical coherence tomography of the macula

PURPOSE

To report observations on the influence of posterior lens capsule opacification on optical coherence tomographic imaging quality and measurements of macular thickness.

METHODS

The retrospective study included 13 eyes in 12 patients of which 2 eyes had a healthy macula and 11 eyes had maculopathy. In all eyes posterior lens capsule fibrosis was present. A clinical examination including biomicroscopy of the macula and an in-vivo macular optical coherence tomography (OCT) before and after YAG-laser membranotomy had been performed.

RESULTS

Foveal retinal thickness before and after membranotomy were highly correlated (R = 0.98), and no systematic difference was found between values obtained before vs. after membranotomy (mean thickness 318.7 +/- 92.7 microm vs. 322.2 +/- 97.4 microm; p = 0.221). The signal-to-noise ratio increased in 11 out of 13 eyes after membranotomy (mean signal-to-noise ratio before 47.1 +/- 6.6 dB vs. 52.6 +/- 4.0 dB after; p = 0.004, the change ranging from -1.5 dB to 17 dB).

CONCLUSIONS

The qualitative effect of the posterior lens capsule opacification on the OCT-scan consists primarily of a loss of signal and consequently of intraretinal detail, whereas total foveal retinal thickness seems to be reliably assessed even when posterior lens capsule opacification interfering with the biomicroscopic evaluation of retinal edema are present. YAG-laser membranotomy results in a measurable improvement in the OCT signal-to-noise ratio, but OCT seems to yield reliable measurements of foveal retinal thickness before treatment. Consequently the OCT seems to reliably diagnose a macular thickening although not the type of this until after removal of the posterior lens capsule opacification.

In-vivo response to free electron laser incision of the rabbit cornea

BACKGROUND AND OBJECTIVE

We analyzed the in vivo ocular response to corneal incisions made by Medical Free Electron Laser (MFEL) as a function of scan rate and incision depth. Additionally, we compared biomicroscopy, optical coherence tomography (OCT), and light microscopy as ocular response diagnostic tools.

STUDY DESIGN/MATERIALS AND METHODS

Rabbit corneas were incised with pulsed MFEL radiation at 2.94 microm wavelength, scalpel incisions or focal cautery treatment were used as controls. The MFEL beam scan rate ranged from 0.2 to 1.0 mm/second. Ocular effects were monitored for 2 hours postoperatively using OCT and slit lamp examination. Ocular tissue was fixed for light microscopic examination.

RESULTS

Anterior chamber fibrin formation correlated with MFEL incision depth. Slower scan rates resulted in deeper incisions and greater fibrin formation. OCT was better than slit lamp biomicroscopy at identifying fibrin attachments. OCT and light microscopy proved to be excellent companion technologies.

CONCLUSIONS

Deep corneal incisions in the rabbit produced by the MFEL resulted in fibrin formation in the anterior chamber.

Real-time pachymetry during photorefractive keratectomy using optical low-coherence reflectometry

Optical low-coherence reflectometry (OLCR) was used as a noncontact method to measure the central corneal thickness of three patients intraoperatively during photorefractive keratectomy. Continuous on-line measurements were performed on the intact cornea immediately before the beginning of surgery, after the removal of the corneal epithelium, during laser tissue photoablation, and for 3 min after the ablation process. Corneal thinning due to evaporation was studied on a separate patient with the OLCR instrument, and it was found to be -0.14 microm/s during the first 5 min after epithelium removal. This baseline corneal thinning rate was used as a fit parameter to calculate actual from measured ablation depths. The measurements showed a maximum difference of +/-10 microm between planned ablations (34-92 microm) and measured ablation depths.

Diurnal variation of corneal and corneal epithelial thickness measured using optical coherence tomography

PURPOSE

To determine the feasibility of optical coherence tomography (OCT) measurement of overnight corneal swelling and de-swelling and, specifically, to examine overnight changes in the corneal epithelium and recovery during the day.

METHODS

Ten healthy volunteers were recruited. Corneal OCT imaging was performed at 10:00 PM (baseline), 8:30 AM, and every 2 hours thereafter until 4:30 PM. Either the right or left eye was taped shut (using a gauze pad and surgical tape) until the 8:30 AM session of the next day. Measurements were made along a 1-mm strip centered on the visual axis.

RESULTS

At baseline, there were no differences in corneal or epithelial thickness between control and experimental eyes (all p > 0.05). Immediately after patch removal, corneal and epithelial thicknesses of patched eyes were higher than the baseline (all p < 0.05). In patched eyes, the cornea and epithelium swelled 5.5% and 8.1% overnight, respectively (p > 0.05). Two hours later, the epithelial thickness of the experimental eyes recovered to baseline level (p > 0.05), but corneal thickness did not reach to baseline level until 4 hours after patch removal. For control eyes, there were no differences compared with baseline (all p > 0.05).

CONCLUSION

Both cornea and corneal epithelium experience proportionately similar amounts of overnight swelling. Recovery of overnight swelling may be slower for the cornea than for the epithelium. OCT provides valuable information about anterior segment morphometry.

Optical coherence tomography for the noninvasive evaluation of the cornea

PURPOSE

To determine the optical coherence tomographic (OCT) characteristics of normal corneas and to characterize the OCT images of abnormal corneal lesions.

METHODS

Eleven eyes from 10 patients were examined at the Cornea Service of the Nagoya University Hospital: 4 had corneal pathologies, 4 underwent keratoplasty, and 2 were normal controls; 1 enucleated eye was also examined. OCT (OCT 2000 Zeiss-Humphrey) was used to study the normal cornea and various corneal abnormalities. We compared the OCT images to the observations made by slit-lamp biomicroscopy.

RESULTS

Fluid spaces were detected as black images. A highly reflective reflex was observed at the interface of different tissues, and intensive backscattering (reflex) was seen when the incident ray hit the laminated layers vertically. Corneal opacities were not clearly imaged when they were diffuse and mild, or when they were arranged axially in a small area, as was the scar of the graft-host junction. It was possible to obtain images from the region of the cornea that was not clearly visible by slit-lamp examination because of a corneal opacity.

CONCLUSION

OCT is a noncontact and noninvasive technique that can be performed safely on diseased corneas. OCT can provide objective documentation of corneal disorders that cannot be obtained by slit-lamp examination. The use of OCT in conjunction with other conventional instruments should provide a more complete image of the cornea.

Potential new endoscopic techniques for the earlier diagnosis of pre-malignancy

Light interacts with tissue in a variety of ways, including absorption, fluorescence, elastic scattering and Raman scattering. These interactions enable a number of promising technologies for endoscopic diagnosis of pre-malignancy, including chromoscopy; fluorescence, scattering and Raman spectroscopies; and optical coherence tomography. Although still in various stages of technical development and clinical trials, these optical diagnostic techniques are demonstrating strong potential to significantly enhance the clinical endoscopist's ability to detect dysplasia in gastrointestinal mucosae.

Central corneal thickness measurement with a retinal optical coherence tomography device versus standard ultrasonic pachymetry

PURPOSE

To demonstrate the capability of a standard, commercially available optical coherence tomography device (originally designed to measure retinal thickness) to image the human cornea in vivo and to measure central corneal thickness (CCT) in normal and edematous corneas. The intrapatient precision and interpatient variability of this novel application was compared to standard ultrasonic pachymetry. Also, the correlation of both methods was investigated.

METHODS

CCT measurements using optical coherence tomography (OCT) and ultrasonic pachymetry (PACH) were obtained in 36 normal eyes and 16 eyes with corneal edema.

RESULTS

Direct in vivo imaging of the cornea and measurements of CCT by OCT could be performed in all eyes. In normal subjects, CCT(OCT) was 530+/-32 microm and CCT(PACH) was 581+/-34 microm. The two methods showed a highly significant correlation with a standardized regression coefficient of 0.988. The difference between both methods was constant over the range of CCT (mean difference, 49.4+/-5.9 microm). The mean intrapatient SD of CCT measurements was 4.90 microm and 4.96 microm for OCT and PACH, respectively. In patients with corneal edema, mean CCT(OCT) was 601+/-59 microm, and mean CCT(PACH) was 667+/-68 microm. The difference between the two techniques increased slightly with increasing corneal edema (mean difference, 66.9+/-10.9 microm).

CONCLUSION

Imaging of the human cornea can be performed by a standard retinal OCT device, and OCT measurement of CCT shows excellent correlation to values obtained by PACH, giving similar readings separated by a constant difference. In corneal edema, the difference between the two methods is additionally increased, but continues to demonstrate excellent consistency.

Corneal optical coherence tomography before and immediately after excimer laser photorefractive keratectomy

PURPOSE

To investigate the representation of the corneal structure with optical coherence tomography before and immediately after excimer laser photorefractive keratectomy.

METHODS

Twenty-four eyes of 24 patients with myopia and myopic astigmatism were prospectively studied. The corneal thickness and the corneal profile were assessed with slit-lamp-adapted optical coherence tomography preoperatively and immediately after excimer laser photorefractive keratectomy.

RESULTS

The attempted mean spherical equivalent of the refractive corrections was -6.7 +/- 3.6 (mean +/- SD) diopters with a mean calculated stromal ablation depth of 91 +/- 38 microm. The corneal optical coherence tomography was reproducible in all patients, demonstrating a mean decrease of central corneal thickness after epithelial debridement and excimer laser photorefractive keratectomy of 118 +/- 45 microm. The comparison of the calculated stromal ablation depth and the corneal thickness changes determined by corneal optical coherence tomography revealed a significant linear relationship with a correlation coefficient of 0.88 (P <.001). The flattening of the corneal curvature was confirmed in all patients with the optical coherence tomography system and correlated with the attempted refractive correction (r =.82, P <.001).

CONCLUSIONS

The slit-lamp-adapted optical coherence tomography system presented in this study allowed noncontact, cross-sectional, and high-resolution imaging of the corneal configuration. This initial clinical evaluation demonstrated that corneal optical coherence tomography could be a promising diagnostic modality to monitor corneal changes of thickness and curvature before and after excimer laser photorefractive keratectomy.

Central corneal thickness determined with optical coherence tomography in various types of glaucoma

AIMS

To evaluate central corneal thickness determined by optical coherence tomography (OCT) in various types of glaucoma, and its influence on intraocular pressure (IOP) measurement.

METHODS

Central corneal thickness (CCT) was determined by using OCT in 167 subjects (167 eyes). 20 had primary open angle glaucoma (POAG), 42 had low tension glaucoma (LTG), 22 had ocular hypertension (OHT), 10 had primary angle closure glaucoma (AC), 24 had pseudoexfoliation glaucoma (PEX), 13 had pigmentary glaucoma (PIG), and 36 were normal.

RESULTS

CCT was significantly higher in ocular hypertensive subjects (593 (SD 35) microm, p <0.0001) than in the controls (530 (32) microm), whereas patients with LTG (482 (28) microm, p < 0. 0001), PEX (493 (33) microm, p <0.0001), and POAG (512 (30) microm, p <0.05) showed significantly lower readings. There was no statistically significant difference between the controls and patients with PIG (510 (39) microm) and AC (539 (37) microm).

CONCLUSIONS

Because of thinner CCT in patients with LTG, PEX, and POAG this may result in underestimation of IOP, whereas thicker corneas may lead to an overestimation of IOP in subjects with OH. By determining CCT with OCT, a new and precise technique to measure CCT, this study emphasises the need for a combined measurement of IOP and CCT in order to obtain exact IOP readings.

Optical coherence tomography for evaluation of anatomical changes in the cornea after laser in situ keratomileusis

PURPOSE

To examine the use of optical coherence tomography (OCT) in evaluating anatomical changes after laser in situ keratomileusis (LASIK) and complications related to the interface and corneal flap.

SETTING

Istanbul University Eye Research Center and Department of Ophthalmology, Cerrahpasa Medical School, Istanbul, Turkey.

METHODS

Eleven eyes of 11 patients who had myopic LASIK were included in the study. Mean age of the 7 men and 4 women was 29.4 years +/- 6.9 (SD). Cases analyzed included uneventful LASIK (4 eyes), epithelial ingrowth (5 eyes), and flap striae (2 eyes). Corneas were examined by OCT (Humphrey Systems).

RESULTS

Optical coherence tomography resolved corneal flap and residual stromal layers in all cases. The mean thickness of the corneal flap and residual stroma was 138.2 +/- 16.5 microm and 321.7 +/- 32.1 microm, respectively. Interface between the corneal flap and residual stroma was shown by OCT. Optical coherence tomography revealed that the eye with flap striae had flap displacement undetected by biomicroscopy. Epithelial ingrowth was shown as a highly reflective area.

CONCLUSION

Optical coherence tomography appears to be a promising method for evaluating anatomical changes in the cornea after LASIK.

A uniform format for ocular imaging devices

PURPOSE

To develop a uniform file format of ocular imaging data, including, but not limited to, ultrasound biomicroscopy, optical coherence tomography, and nerve fiber analyzer, capable of being transmitted via Internet or intranet for collaborative research and telemedicine use.

METHOD

File filters were developed as dynamic link libraries (DLLs). These can read the original raw data format of each ocular imaging device. A data file format was also designed to describe these raw data uniformly in three different types of compression: noncompressed, run length compression, and differential pulse code modulation (DPCM). These three file formats were then tested in the following aspects: file size, speed of reading, and speed of writing.

RESULTS

Run length compression failed to compress raw data, while DPCM compressed raw data successfully (< or =35.5%). The speed of reading and writing files was slowest in DPCM. However, the actual time of reading and writing was fast enough (<0.6 s) for daily work regardless of file compression methods.

CONCLUSION

The format designed has robust potential to be the standard file format for transmission of any ocular imaging raw data.

High-resolution endoscopic imaging of the GI tract using optical coherence tomography

BACKGROUND

Optical coherence tomography (OCT) has demonstrated the microscopic structure of the gastrointestinal (GI) tract mucosa and submucosa in vitro. We evaluated a prototype OCT system and assessed the feasibility of OCT in the human GI tract.

METHODS

The 2.4 mm diameter prototype OCT probe, inserted through an endoscope, provides a 360-degree radial scan. Images (6.7 frames/sec) are displayed on a television monitor. Tissue contact is not required. In patients undergoing elective endoscopy, OCT images were obtained of normal mucosa (confirmed by biopsy).

RESULTS

Seventy-two sites were imaged (38 patients): esophagus (21), stomach (12), duodenum (11), terminal ileum (4), colon (15), and rectum (9). Varying the distance between the probe and the mucosal surface produced images of the GI wall of varying depth. When held about 1 mm above the mucosal surface, the images consisted of mucosal structures such as colonic crypts, gastric pits, and duodenal villi. With the probe held against the wall, the OCT image comprised several layers interpreted as mucosa, muscularis mucosae, and submucosa. Structures including blood vessels were evident within the submucosa. A probe with a 0.5 mm working distance to the focal point provided the best images. Reducing the frame rate to 4.0 per second facilitated image interpretation.

CONCLUSIONS

OCT is feasible in the human GI tract and provides interpretable high-resolution images of mucosa and submucosa.

Optical coherence tomography: an emerging technology for biomedical imaging and optical biopsy

Optical coherence tomography (OCT) is an emerging technology for performing high-resolution cross-sectional imaging. OCT is analogous to ultrasound imaging, except that it uses light instead of sound. OCT can provide cross-sectional images of tissue structure on the micron scale in situ and in real time. Using OCT in combination with catheters and endoscopes enables high-resolution intraluminal imaging of organ systems. OCT can function as a type of optical biopsy and is a powerful imaging technology for medical diagnostics because unlike conventional histopathology which requires removal of a tissue specimen and processing for microscopic examination, OCT can provide images of tissue in situ and in real time. OCT can be used where standard excisional biopsy is hazardous or impossible, to reduce sampling errors associated with excisional biopsy, and to guide interventional procedures. In this paper, we review OCT technology and describe its potential biomedical and clinical applications.

Optical coherence tomography evaluation of the corneal cap and stromal bed features after laser in situ keratomileusis for high myopia and astigmatism

OBJECTIVE

To study the corneal microstructure by optical coherence tomography (OCT) after laser in situ keratomileusis (LASIK) for high myopia with and without astigmatism.

DESIGN

Nonrandomized self-controlled comparative trial.

PARTICIPANTS

Sixty-three consecutive LASIK eyes with spherical equivalent refraction between -6.0 and -17.0 diopters (D) and astigmatism between 0.0 and -5.0 D were prospectively recruited for examination.

INTERVENTION

LASIK was performed with the Chiron Hansatome microkeratome (160-microm fixed plate) and Summit Apex Plus excimer laser using a 5.5/6.0/6.5-mm multizone pattern. Proper preoperative calculations were performed to ensure stromal beds thicker than 250 microm.

MAIN OUTCOME MEASURES

OCT imaging and measurement of corneal thickness was performed preoperatively. In addition, corneal cap and stromal bed thickness measurements were performed 1 day, 1 month, and 3 months postoperatively.

RESULTS

The average central corneal pachymetry was 538.9 +/- 26.2 microm preoperatively. Mean corneal cap thickness measured 124.8 +/- 18.5 microm 1-day postoperatively. Mean stromal bed thickness was 295.2 +/- 37.1 microm on the first postoperative day. Compared with the 1-day postoperative examination, the average stromal bed thickness increased significantly by 5.9 microm (P = 0.001) and 7.2 microm (P = 0.001) at the 1-month and 3-month postoperative examinations, respectively. Mean difference between actual (118.7 +/- 27.8 microm) and predicted (104.1 +/- 20.8 microm) central ablation depths was 14.6 +/- 16.7 microm (P = 0.0001). A weak but statistically significant positive association was found between preoperative refraction and the difference between expected and real ablation depth values (R = 0.26; P = 0.042). Posterior stromal beds were more than 250-microm thick in 58 eyes (89.9%) 1 day postoperatively. This safety requirement improved at the 1-month postoperative examination, when the partial regression accounted for slightly thicker stromal beds and only two cases (3.2%) exhibited posterior stromal tissue thinner than 250 microm. These two cases were seen only for corrections exceeding 12 D (P = 0.04).

CONCLUSIONS

OCT appears to be a useful tool for the evaluation of both the qualitative and quantitative anatomic outcome of LASIK. Corrections of higher degrees of ametropia run a higher risk of producing a thinner than expected central cornea. Particularly, corrections greater than 12 D may lead eventually to stromal beds thinner than 250 microm, despite proper preoperative calculations. Because corneal flaps are usually thinner than expected with the microkeratome used herein, adequate posterior corneal stroma is preserved in most instances.