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W Drexler

Publications and source records attributed to W Drexler.

At least 19 recordsLinked to original sources
PubMed2006

Optophysiology: depth-resolved probing of retinal physiology with functional ultrahigh-resolution optical coherence tomography.

Noncontact, depth-resolved, optical probing of retinal response to visual stimulation with a <10-microm spatial resolution, achieved by using functional ultrahigh-resolution optical coherence tomography (fUHROCT), is demonstrated in isolated rabbit retinas. The method takes advantage of the fact that physiological changes in dark-adapted retinas caused by light stimulation can result in local variation of the tissue reflectivity. fUHROCT scans were acquired from isolated retinas synchronously with electrical recordings before, during, and after light stimulation. Pronounced stimulus-related changes in the retinal reflectivity profile were observed in the inner/outer segments of the photoreceptor layer and the plexiform layers. Control experiments (e.g., dark adaptation vs. light stimulation), pharmacological inhibition of photoreceptor function, and synaptic transmission to the inner retina confirmed that the origin of the observed optical changes is the altered physiological state of the retina evoked by the light stimulus. We have demonstrated that fUHROCT allows for simultaneous, noninvasive probing of both retinal morphology and function, which could significantly improve the early diagnosis of various ophthalmic pathologies and could lead to better understanding of pathogenesis.

Animals↗
PubMed2005

Stimulus-driven versus pilocarpine-induced biometric changes in pseudophakic eyes.

PURPOSE: Most trials that study the lens movement of accommodative intraocular lens (IOLs) use pilocarpine to stimulate ciliary muscle contraction. The aim of this study is to assess in vivo whether a more physiologic, stimulus-driven accommodation is comparable to pilocarpine-induced IOL movement. DESIGN: Controlled patient- and examiner-masked clinical trial. PARTICIPANTS: The study population included 38 eyes with accommodative IOL implants (1CU) and a control group of 28 eyes with conventional open-loop IOLs. METHODS: A high-precision biometry technique, partial coherence interferometry, was used to measure IOL position. Anterior chamber depth was measured during physiologic (near point) and pharmacological (pilocarpine 2%) stimulation. In a subgroup of 14 1CU eyes, IOL position was determined repeatedly within 90 minutes after pilocarpine administration. A different subgroup was investigated as to the effect of cyclopentolate on IOL position. Best-corrected distance visual acuity (VA), best-corrected near VA, and distance-corrected near VA (DCNVA) were assessed using logarithm of the minimum angle of resolution charts. MAIN OUTCOME MEASURES: Anterior chamber depth change under pilocarpine and near-point-driven accommodation. RESULTS: Near-point accommodation did not induce movement of either the accommodating 1CU or the control IOLs. Pilocarpine induced a 201+/-0.137-mm anterior movement of the 1CU IOL (P<0.001), compared with no movement within the control IOL groups (P>0.05). There was no significant (P>0.05) difference in DCNVA between the accommodative and open-loop IOLs. No correlation between near point- or pilocarpine-stimulated IOL movement and DCNVA was found. Concerning the time course of movement after pilocarpine administration, most of the 1CU IOLs showed some movement 30 minutes after application. Cyclopentolate-induced ciliary muscle relaxation caused a posterior IOL movement, as compared with the relaxed state, when focusing on a distant target. CONCLUSION: Pilocarpine-induced ciliary muscle contraction seems to overestimate IOL movement relative to a monocular near-driven stimulus. Therefore, concerning IOL movement, pilocarpine may act as a superstimulus and may not adequately simulate daily life performance of accommodative IOLs. However, it may be helpful to evaluate the maximum potential of an accommodating IOL.

Accommodation, Ocular↗
PubMed2004

Adaptive-optics ultrahigh-resolution optical coherence tomography.

Merging of ultrahigh-resolution optical coherence tomography (UHR OCT) and adaptive optics (AO), resulting in high axial (3 microm) and improved transverse resolution (5-10 microm) is demonstrated for the first time to our knowledge in in vivo retinal imaging. A compact (300 mm x 300 mm) closed-loop AO system, based on a real-time Hartmann-Shack wave-front sensor operating at 30 Hz and a 37-actuator membrane deformable mirror, is interfaced to an UHR OCT system, based on a commercial OCT instrument, employing a compact Ti:sapphire laser with 130-nm bandwidth. Closed-loop correction of both ocular and system aberrations results in a residual uncorrected wave-front rms of 0.1 microm for a 3.68-mm pupil diameter. When this level of correction is achieved, OCT images are obtained under a static mirror configuration. By use of AO, an improvement of the transverse resolution of two to three times, compared with UHR OCT systems used so far, is obtained. A significant signal-to-noise ratio improvement of up to 9 dB in corrected compared with uncorrected OCT tomograms is also achieved.

Equipment Design↗
PubMed2004

Advances in broad bandwidth light sources for ultrahigh resolution optical coherence tomography.

Novel ultra-broad bandwidth light sources enabling unprecedented sub-2 microm axial resolution over the 400 nm-1700 nm wavelength range have been developed and evaluated with respect to their feasibility for clinical ultrahigh resolution optical coherence tomography (UHR OCT) applications. The state-of-the-art light sources described here include a compact Kerr lens mode locked Ti:sapphire laser (lambdaC = 785 nm, delta lambda = 260 nm, P(out) = 50 mW) and different nonlinear fibre-based light sources with spectral bandwidths (at full width at half maximum) up to 350 nm at lambdaC = 1130 nm and 470 nm at lambdaC = 1375 nm. In vitro UHR OCT imaging is demonstrated at multiple wavelengths in human cancer cells, animal ganglion cells as well as in neuropathologic and ophthalmic biopsies in order to compare and optimize UHR OCT image contrast, resolution and penetration depth.

Animals↗
PubMed2004

[Methodological advancements. Ultrahigh-resolution OCT].

Development of ultrabroad bandwidth light sources has recently enabled significant improvement of ophthalmic axial OCT imaging resolution, demonstrating the potential of ultrahigh resolution OCT (UHR OCT) to perform noninvasive optical biopsy, i.e., the in vivo visualization of microstructural morphology in situ, which had previously only been possible with histopathology. Therefore, UHR OCT allows detection of intraretinal changes that can be used for diagnosis of retinal disease in its early stages when treatment is most effective and irreversible damage can be prevented or delayed. Furthermore, it may provide a better understanding of the pathogenesis of several macular pathologies as well as contribute to the development of new therapy approaches. Future developments of ophthalmic OCT include high speed, three-dimensional retinal imaging, combining adaptive optics and UHR OCT, spatially resolved spectroscopic OCT, functional imaging, and OCT imaging with enhanced penetration into the choroid by employing novel wavelength regions.

Diagnosis, Differential↗
PubMed2003

Compact, low-cost Ti:Al2O3 laser for in vivo ultrahigh-resolution optical coherence tomography.

A compact, low-cost, prismless Ti:Al2O3 laser with 176-nm bandwidth (FWHM) and 20-mW output power was developed. Ultrahigh-resolution ophthalmic optical coherence tomography (OCT) ex vivo imaging in an animal model with approximately 1.2-microm axial resolution and in vivo imaging in patients with macular pathologies with approximately 3-microm axial resolution were demonstrated. Owing to the pump laser, this light source significantly reduces the cost of broadband OCT systems. Furthermore, the source has great potential for clinical application of spectroscopic and ultrahigh-resolution OCT because of its small footprint (500 mm x 180 mm including the pump laser), user friendliness, stability, and reproducibility.

Aluminum Oxide↗
PubMed2003

Compact, broad-bandwidth fiber laser for sub-2-microm axial resolution optical coherence tomography in the 1300-nm wavelength region.

A novel, compact, user friendly fiber laser with a broad emission bandwidth (MenloSystems, lambdac = 1375 nm, deltalambda = 470 nm, Pout = 4 mW) was used to achieve unprecedented sub-2-microm axial resolution optical coherence tomography (OCT) in nontransparent biological tissue in the 1300-nm wavelength region. Fresh human skin and arterial biopsies were imaged ex vivo with approximately 1.4-microm axial and approximately 3-microm lateral resolution and 95-dB sensitivity, demonstrating the great potential for clinical OCT applications of this stable, low-cost, and turn-on-key fiber laser.

Arteries↗
PubMed2001

Improved prediction of intraocular lens power using partial coherence interferometry.

PURPOSE: To evaluate the feasibility of using a new optical biometry technique, dual-beam partial coherence interferometry (PCI), to improve intraocular lens (IOL) power prediction in cataract surgery. SETTING: Department of Ophthalmology, Vienna General Hospital, and Institute of Medical Physics, University of Vienna, Vienna, Austria. METHODS: Preoperative axial length (AL) data obtained with PCI biometry and applanation ultrasound (US) biometry in 77 eyes of 51 patients was applied to 4 commonly used IOL power formulas. The refractive outcome and the mean absolute error (MAE) were calculated for each formula using both biometry methods. A linear multiple-regression model based on preoperative PCI biometry data was derived to predict the postoperative anterior chamber depth (ACD). The predictive power of this regression model was assessed by adding the predicted ACD to the SRK/T formula. Predicted residuals were calculated to evaluate the feasibility and stability of this modified IOL power formula. RESULTS: Using PCI instead of US biometry significantly improved the refractive outcome with all 4 IOL power formulas. The Holladay I and SRK/T formulas yielded an MAE of 0.44 diopter (D) using PCI AL data and 0.56 D and 0.57 D, respectively, using US biometry data. The SRK/T formula combined with the PCI regression model for postoperative ACD prediction performed slightly better (MAE 0.42 D) than the conventional SRK/T formula alone. Predicted residuals revealed an MAE of 0.46 D, proving the predictive performance of the new formula. CONCLUSIONS: Partial coherence interferometry biometry applied to several widely used IOL power formulas yielded significantly better IOL power prediction and therefore refractive outcome in cataract surgery than US biometry. Further improvement can be achieved by applying PCI to a modified SRK/T formula that predicts the postoperative ACD using PCI biometry data.

Biometry↗
PubMed2001

Correlation of collagen organization with polarization sensitive imaging of in vitro cartilage: implications for osteoarthritis.

OBJECTIVE: Optical coherence tomography (OCT), a new method of high resolution imaging, has shown feasibility for assessing articular cartilage to identify early changes in osteoarthritis (OA) and monitor therapy. OCT is analogous to ultrasound, measuring the intensity of backreflected infrared light rather than sound. The resolution of this technology is up to 25 times higher than existing methods. We investigated the correlation between changes observed by OCT and the degree of collagen organization in OA cartilage. METHODS: Polarization sensitive OCT (PSOCT) imaging was used to assess changes in cartilage collagen organization in vitro. RESULTS: The presence (or absence) of PSOCT changes correlated with collagen organization (or disorganization) on histology as assessed by picrosirius polarization microscopy (no significant difference). In multiple cases, cartilage was abnormal by both PSOCT and polarization microscopy, but was grossly normal by routine staining, showing cartilage thickness > 2 mm and no fibrillations. CONCLUSION: This in vitro study suggests PSOCT changes in cartilage are due to the state of collagen organization. The combination of high resolution structural imaging and birefringence detection make OCT a potentially powerful technology for early assessment of OA.

Cartilage, Articular↗
PubMed2000

Resolution-improved dual-beam and standard optical coherence tomography: a comparison.

BACKGROUND: The purpose of the study was to demonstrate the improved axial resolution and longitudinal stability of dual-beam optical coherence tomography (OCT) in comparison to conventional OCT setups used in commercially available OCT instruments. METHODS: The conventional OCT technique is based on an interferometric setup that is rather sensitive to axial eye motions. We have developed a special dual-beam OCT technique which eliminates the influence of axial eye motions. This is achieved by using the anterior corneal surface as the reference surface for the interferometric ranging. To improve the signal quality, the different wavefront curvatures of beams reflected at cornea and retina are matched by a diffractive optical element. To improve the axial resolution, a broadband synthesized light source with an effective bandwidth of 50 nm is used, and the group dispersion of the ocular media is compensated. Tomographic images were recorded in the fovea and the optic nerve head of healthy volunteers. For comparison purposes, approximately the same locations in the same eyes were imaged by a commercially available OCT instrument. RESULTS: Compared to the standard OCT technique, the dual-beam OCT images show considerably improved axial resolution. Especially in tomograms recorded at the fovea, dual-beam OCT resolves microstructural details that are not visible in the standard OCT images. Furthermore, the axial stability of dual-beam OCT enables the recording of exact geometrical contours of fundus layers. CONCLUSIONS: Dual-beam OCT is able to provide structural information on the ocular fundus that is not obtained with standard OCT. The long recording times of our instrument limit the transverse resolution to 100-150 microm at present.

Fovea Centralis↗
PubMed2000

Optical coherence tomography: advanced technology for the endoscopic imaging of Barrett's esophagus.

BACKGROUND AND STUDY AIMS: Endoscopic optical coherence tomography (OCT) is an emerging medical technology capable of generating high-resolution cross-sectional imaging of tissue microstructure in situ and in real time. We assess the use and feasibility of OCT for real-time screening and diagnosis of Barrett's esophagus, and also review state-of-the-art OCT technology for endoscopic imaging. MATERIALS AND METHODS: OCT imaging was performed as an adjunct to endoscopic imaging of the human esophagus. Real-time OCT (13-microm resolution) was used to perform image-guided evaluation of normal esophagus and Barrett's esophagus. Beam delivery was accomplished with a 1-mm diameter OCT catheter-probe that can be introduced into the accessory channel of a standard endoscope. Different catheter-probe imaging designs which performed linear and radial scanning were assessed. Novel ultrahigh-resolution (1.1-microm resolution) and spectroscopic OCT techniques were used to image in vitro specimens of Barrett's esophagus. RESULTS: Endoscopic OCT images revealed distinct layers of normal human esophagus extending from the epithelium to the muscularis propria. In contrast, the presence of gland- and crypt-like morphologies and the absence of layered structures were observed in Barrett's esophagus. All OCT images showed strong correlations with architectural morphology in histological findings. Ultrahigh-resolution OCT techniques achieved 1.1-microm image resolution in in vitro specimens and showed enhanced resolution of architectural features. Spectroscopic OCT identified localized regions of wavelength-dependent optical scattering, enhancing the differentiation of Barrett's esophagus. CONCLUSIONS: OCT technology with compact fiberoptic imaging probes can be used as an adjunct to endoscopy for real-time image-guided evaluation of Barrett's esophagus. Linear and radial scan patterns have different advantages and limitations depending upon the application. Ultrahigh-resolution and spectroscopic OCT techniques improve structural tissue recognition and suggest future potential for resolution and contrast enhancements in clinical studies. A new balloon catheter-probe delivery device is proposed for systematic imaging and screening of the esophagus.

Barrett Esophagus↗
PubMed1999

Changes in intraocular lens position after neodymium: YAG capsulotomy.

PURPOSE: To quantify changes in intraocular lens (IOL) position caused by neodymium: YAG (Nd:YAG) capsulotomy with 3 IOL styles. SETTING: Department of Ophthalmology, University of Vienna, Austria. METHODS: In a prospective study, anterior chamber depth (ACD) was measured by dualbeam partial coherence interferometry (PCI) in 32 pseudophakic eyes of 32 patients with posterior capsule opacification before and immediately after planned capsulotomy under mydriasis. Patients were divided into 3 groups with the following IOL styles: 1-piece poly(methyl methacrylate) (PMMA), 3-piece foldable, and plate haptic. RESULTS: The capsulotomy induced a backward IOL movement in all 32 eyes (mean 25 microns; range 9 to 55 microns). It was more pronounced in eyes with plate-haptic IOLs than in those with the other styles. Precision of ACD measurement by PCI was 4 microns. Changes in ACD correlated significantly with capsulotomy size but not with preoperative lens-capsule distance. CONCLUSION: Capsulotomy caused a backward movement of the IOL, which was more pronounced with plate-haptic IOLs than with 1-piece PMMA and 3-piece foldable IOLs. Since the magnitude of IOL movement in this study population was small, a hyperopic shift in refraction after capsulotomy will usually be small and not clinically relevant.

Adult↗
PubMed1998

Investigation of dispersion effects in ocular media by multiple wavelength partial coherence interferometry.

We report on quantitative measurements of group refractive indices and group dispersion in water and in human ocular media such as the cornea, the aqueous humor, the lens, artificial intraocular lenses, as well as a total value averaged over the media along the axial eye length of normal subjects and pseudophakic patients in vivo using dual beam partial coherence interferometry. Different optical thickness values due to the dispersion of the cornea are demonstrated using two spectrally displaced light sources. The displacement can be used to indirectly calculate the group dispersion of the human cornea in the spectral region between 810 nm and 860 nm. If the object under investigation is dispersive, resolution is limited due to a broadening of the detected signals. This broadening increases with group dispersion, i.e., the extent to which the group refractive index of the medium varies with wavelength and thickness of the tissue under investigation as well as with the spectral bandwidth of the light source. Measurements of the group dispersion in the cornea, lens and vitreous of pseudophakic and normal human eyes, show that the cornea and the lens are more dispersive than water-by a factor of about 5 and 2, respectively-in the investigated spectral region. The cornea is approximately threefold more dispersive than the human crystalline lens, the aqueous humor is less dispersive than water and the group dispersion of all ocular components together, averaged over the axial length of normal and pseudophakic eyes, was only slightly higher compared to that of water. Since the highly dispersive cornea and lens together have only a thickness of about one sixth of that of the axial eye length, it seems that their contribution to the group dispersive effect along the whole axial eye length is only small.

Aqueous Humor↗
PubMed1998

Partial coherence interferometry: a novel approach to biometry in cataract surgery.

PURPOSE: To compare biometry performed by an enhanced version of dual beam partial coherence interferometry and applanation ultrasound in a prospective study of 85 cataract eyes to improve refractive outcome of cataract surgery due to a more accurate calculation of intraocular lens power. METHODS: The SRK II formula using ultrasound biometry data was employed. Three months after surgery, partial coherence interferometry biometry was repeated and refractive outcome was determined. Preoperative partial coherence interferometry biometry data were used to determine the refractive power of the intraocular lenses retrospectively and to calculate the possible refractive outcome. RESULTS: Precision of partial coherence interferometry biometry was more than 10 times better than that of ultrasound. Therefore, the possible mean absolute error for postoperative refraction achieved with partial coherence interferometry biometry was 0.49 diopters (compared with 0.67 diopters with ultrasound biometry), resulting in an improvement of 27%. Axial eye length measured with the two techniques differed by a mean of 460 microm. The difference in lens thickness measured with partial coherence interferometry and ultrasound significantly correlated with cataract grade. A mean shortening of 120 microm of axial eye length following cataract surgery was also detected by partial coherence interferometry. CONCLUSIONS: The enhanced version of partial coherence interferometry offers biometry with unprecedented precision (<10 microm) and resolution (approximately 12 microm), therefore improving the refractive outcome in cataract surgery. This noninvasive technique provides a high degree of comfort for the patient, with no need for local anesthesia or pupil dilation and minimized risk of corneal infection.

Adult↗
PubMed1998

High precision biometry of pseudophakic eyes using partial coherence interferometry.

PURPOSE: To investigate the applicability of the scanning version of dual-beam partial coherence interferometry (PCI) for measuring the anterior segment and axial length of pseudophakic eyes in a clinical setting and to determine the achievable precision with this biometry technique. SETTING: Department of Ophthalmology, Vienna General Hospital, and Institute of Medical Physics, University of Vienna, Austria. METHODS: Partial coherence interferometry was performed in 39 pseudophakic eyes of 39 patients after implantation of a foldable acrylic intraocular lens (IOL). RESULTS: Effective lens position (ELP), IOL thickness and lens-capsule distance (LCD) were determined with a precision of 2 to 3 microns; corneal thickness and axial eye length, with a precision of 0.8 and 5.0 microns, respectively. The mean ELP of the IOL was 4.093 mm +/- 0.290 (SD). In 7 eyes (18%), a positive LCD of 68 +/- 40 microns was detected with PCI. Mean corneal thickness was 526.4 +/- 31.5 microns; mean IOL thickness, 791.5 +/- 40.2 microns; and mean axial length, 23.388 +/- 0.824 mm. CONCLUSION: The scanning version of PCI enables high precision (< or = 5 microns) and high resolution (approximately 12 microns) biometry of pseudophakic eyes that is better than conventional ultrasound by a factor of more than 20. For the first time, positive LCD, a possible risk factor for posterior capsule opacification, could be detected and quantified. Furthermore, this technique offers a high degree of comfort for the patient since it is a noncontact method with no need for local anesthesia or pupil dilation and has a reduced risk of corneal infection.

Acrylates↗
PubMed1998

Accurate determination of effective lens position and lens-capsule distance with 4 intraocular lenses.

PURPOSE: To measure effective lens position (ELP) of 4 intraocular lenses (IOLs) using high precision and high resolution dual-beam partial coherence interferometry (PCI) and to assess the tendency of these IOLs to produce a lens-capsule distance (LCD), a possible risk factor for posterior capsule opacification. SETTING: Department of Ophthalmology, Vienna General Hospital; Institute of Medical Physics, University of Vienna, Austria. METHODS: In a retrospective study, PCI was used to measure ELP and LCD in 139 pseudophakic eyes of 110 patients with 4 IOLs: acrylic 3-piece IOL (AcrySof MA60BM); silicone 3-piece IOL without a capsular tension ring (PhacoFlex SI30) and with a capsular tension ring (PhacoFlex SI30 and Morcher Type 14); silicone plate-haptic IOL (Staar AA4203VF); and a hydrogel plate-haptic IOL (logel 1103). RESULTS: The ELP and LCD were determined with a precision of approximately 3 to 4 microns. An LCD was detected in 21% eyes with the AcrySof, 20% of eyes with the SI30 without a capsular tension ring, 10% of eyes with a capsular tension ring, 21% of eyes with the Staar, and 17% of eyes with the logel. The LCDs detected by PCI, but not by slitlamp examination, were significantly smaller than those detected by both. CONCLUSION: The amount of LCD detected by PCI was approximately the same with all IOL types (approximately 20%) except the PhacoFlex SI30 with a capsular tension ring (10%).

Acrylates↗
PubMed1998

Eye elongation during accommodation in humans: differences between emmetropes and myopes.

PURPOSE: The pathophysiology and pathogenesis of myopia are still a matter of controversy. Exaggerated longitudinal eye growth is assumed to play an important role in the development of myopia. A significant correlation between refraction and amount of near-work has been reported. However, current knowledge of changes of axial eye length with accommodation is limited because clinical ultrasound biometry does not provide the precision and resolution required to thoroughly investigate these phenomena. METHODS: Partial coherence interferometry (PCI), a noninvasive biometric technique, uses laser light with short coherence length in combination with interferometry to achieve precision in the micrometer to submicrometer range and resolution of 10 microm. In the present study this technique was used to investigate axial eye length changes in 11 emmetropic and 12 myopic eyes during monocular fixation at the far and near point. In 7 subjects, the contralateral eye has also been measured to investigate interocular differences in eye elongation. RESULTS: All investigated eyes elongated during accommodation. This elongation was more pronounced in emmetropes than in myopes (P < 0.001). Mean accommodation-induced eye elongations of 12.7 microm (range, 8.6-19.2 microm) and 5.2 microm (range, 2.1-9.5 microm), corresponding to a dioptric change of approximately -0.036 D and -0.015 D, were obtained for emmetropes and myopes. No significant difference in accommodative amplitudes between groups (5.1 +/- 1.2 D [range, 3.8-7.1 D] versus 4.1 +/- 2.0 D [range, 1.0-7.1 D]; P = 0.14) was detected. No significant interocular difference in accommodation-induced eye elongation was revealed (P = 0.86). Also, a mean backward movement of the posterior lens pole of 38 microm (range, 9-107 microm) was observed in both study groups. CONCLUSIONS: The detected eye elongation can be explained by the accommodation-induced contraction of the ciliary muscle, which results in forward and inward pulling of the choroid, thus decreasing the circumference of the sclera, and leads to an elongation of the axial eye length. Finally, it was demonstrated that PCI, in contrast to clinical ultrasound, is capable of characterizing eye length changes during accommodation in humans.

Accommodation, Ocular↗