Franz Lexer

Wavelength-tuning interferometry of intraocular distances

We describe basic principles of wavelength-tuning interferometry and demonstrate its application in ophthalmology. The advantage of this technique compared with conventional low-coherence interferometry ranging is the simultaneous measurement of the object structure without the need for a moving reference mirror. Shifting the wavelength of an external-cavity tunable laser diode causes intensity oscillations in the interference pattern of light beams remitted from the intraocular structure. A Fourier transform of the corresponding wave-number-dependent photodetector signal yields the distribution of the scattering potential along the light beam illuminating the eye. We use an external interferometer to linearize the wave-number axis. We obtain high resolution in a model eye by slow tuning over a wide wavelength range. With lower resolution we demonstrate the simultaneous measurement of anterior segment length, vitreous chamber depth, and axial eye length in human eyes in vivo with data-acquisition times in the millisecond range.

Dynamic coherent focus OCT with depth-independent transversal resolution

Abstract We present a new OCT technique which renders the transversal resolution depth independent. This is achieved by an optical setup which shifts the focus of the beam illuminating the object through the object depth without changing the path length in the corresponding interferometer arm. Therefore, the coherence gate remains at the beam focus without any readjustment of the reference arm. Depth resolution was tested with the help of microscopy cover-plates and transversal resolution was tested with the help of Ronchi rulings. Resolution was 100 lines mm−1 over an object depth of 430 μm. For a first demonstration of the properties of this dynamic coherent focus scheme in a biologic system a section of a human cornea was used. We expect that this technique can further be improved to obtain transversal resolution down to the 1-μm range

Topical measurement of fundus pulsations

An instrument suitable for fundus pulsation measurements at preselected points of the retina is developed. Fundus pulsations are measured with a laser interferometer using a single-mode laser diode. Part of the laser light is reflected from the front side of the cornea and part of it is reflected from the retina. The interferences produced by the two remitted waves are detected with a linear CCD array. Evaluation of the interferogram yields the time course of the fundus pulsations. This laser interferometer is coupled to a fundus camera. The setup enables real-time inspection of the laser spot on the fundus. Measurements on seven healthy young subjects are presented. Fundus pulsations in the macular region are significantly smaller than those in the optical disk, but significantly larger than those in peripheral regions of the retina. The association between our results and the subretinal vasculature is discussed and possible future applications of our method are presented.

The effect of hyperoxia and hypercapnia on fundus pulsations in the macular and optic disc region in healthy young men

The purpose of this study was to investigate the influence of hyperoxia and hypercapnia on fundus pulsations in the macular and optic disc region and determine a potential autoregulative capacity of the choroid under these conditions. In a randomized cross-over study 100% O2 and a mixture of 5% CO2 with air was inhaled for 10 min by ten healthy male volunteers on different study days. Fundus pulsations were measured with a recently described laser interferometer. These results were compared to changes in systemic haemodynamics and Doppler-sonographic measurements of the radial artery. The fundus pulsation amplitude significantly decreased during hyperoxia and significantly increased during hypercapnia. These effects on fundus pulsations were stronger in the optic disc region than in the macular region. The systemic parameters showed only minor changes. Hence the amplitudes of fundus pulsations measured during hyperoxia and hypercapnia are assumed to be a consequence of metabolic autoregulative mechanisms. This autoregulative capacity is greater in the optic disc region than in the macular region, implying that the fundus pulsation amplitude in the papilla is influenced by both the choroidal and the retinal circulation.

A Randomized, Placebo-controlled, Double-blind Crossover Study of the Effect of Pentoxifylline on Ocular Fundus Pulsations

PURPOSE To estimate the short-term effects of pentoxifylline on ocular blood flow in healthy volunteers. METHODS In ten healthy subjects, either 200 or 400 mg of pentoxifylline or placebo was administered intravenously over 90 minutes in a randomized, double-blind, placebo-controlled, three-way crossover study design. Noninvasive measurements of blood pressure, pulse rate, flow variables in the radial artery, ocular fundus pulsations, and whole blood viscosity and filterability were performed at baseline and at 30-minute intervals until four hours after the start of drug infusion. Ocular fundus pulsation amplitude, which has been shown to estimate the pulsatile component of the ocular blood flow, was recorded with a laser interferometer in the macula, the optic disk, and a peripheral region. RESULTS Fundus pulsation amplitude significantly increased after infusion of pentoxifylline, with maximum effect 150 to 180 minutes after the start of the infusion. This effect was dose-dependent and more pronounced in the macula (+17%, P < .001 vs baseline, after 200 mg; and +27%, P < .001 vs baseline, after 400 mg of pentoxifylline) and the peripheral region (+14%, P < .001; and +26%, P < .001) than in the optic disk (+11%, P < .002; and +13%, P < .001). Viscosity but not filterability of whole blood dose-dependently decreased. Peak systolic flow velocity in the radial artery decreased after infusion of 400 mg of pentoxifylline. Blood pressure and pulse rate were unchanged during the observation period. CONCLUSIONS These findings demonstrate that pentoxifylline increases pulsatile ocular blood flow in healthy volunteers. Our results support the possibility that pentoxifylline could be used therapeutically in several eye diseases, such as diabetic retinopathy.

Fundus pulsation measurements in diabetic retinopathy

Abstract• Background: There is experimental evidence that retinal blood flow is impaired in patients with diabetes mellitus. Much less attention has been paid to choroidal blood flow. Hence it was the aim of the present study to investigate choroidal blood flow in diabetic retinopathy. • Methods: A new noninvasive laser interferometric technique was used to measure fundus pulsations in the macula. The fundus pulsation amplitude, which is the maximum distance change between cornea and retina during the cardiac cycle, is a measure of local pulsatile blood flow. The eyes (n = 214) were divided into four groups according to the modified Airlie House classification: (1) no retinopathy (control group), (2) background retinopathy, (3) moderate to severe preproliferative retinopathy, (4) proliferative retinopathy. In 83 eyes of different groups fundus pulsation measurements were repeated after 1–6 weeks. • Results: Fundus pulsation amplitudes were significantly smaller in group 4 than in the control group (P < 0.027). The reproducibility of the measurements was high and did not differ among the study groups. • Conclusions: Local fundus pulsations in the macula are reduced in proliferative diabetic retinopathy, which is compatible with previous findings of reduced choroidal blood flow in late stages of the disease. Laser interferometric measurement of fundus pulsations is non-contactile, assures optimal comfort for the patient and could be used for the long-term observation of patients with diabetes mellitus.

In-vivo intraocular ranging by wavelength tuning interferometry

Recently, wavelength tuning interferometry was suggested as an alternative technique for distance measurements. Compared to partial coherence interferometry, it has the advantages of needing no high precision mechanically moving components and the capability of measuring several distances simultaneously in very short time. We report on first measurements of intraocular distances in human eyes in vivo using a distributed Bragg reflector laser diode with a tuning range of 2 nm. We were able to measure the anterior chamber depth, the lens thickness, the vitreous depth, the axial eye length, and to estimate the thickness of the retina. The resolution is approximately 150 micrometer optical distance.

Measurement of the axial eye length by wavelength-tuning interferometry

An alternative approach to measure intraocular optical distances in vivo is wavelength tuning interferometry. The principle of this technique is that every wavelength shift of a laser diode is associated with a phase shift of the waves reflected at the cornea and the retina. This results in a change of the interference order of the fringe system that will occur when the two wavefronts interfere. By Fourier transform of the intensity data, we cannot only measure the axial eye length but also other intraocular distances. In our approach we used an external cavity, single mode laser diode at a center wavelength of 780 nm which is tunable over 15 nm. First measurements were carried out in a model eye using the mode hop free tuning range of 9 nm, at the maximum tuning rate of 0.33 nm/s. The precision we obtained for the axial eye length was 0.04 mm. The large tuning range cannot be used of in vivo measurements because of the slow scanning rate of 0.33 nm/s. With this scanning rate the signal frequency corresponding to the axial eye length is on the order of 30 Hz. This is within the frequency range of the intensity modulations caused by fundus pulsations due to the heart beat and can therefore not be separated from them. The in vivo measurements have to be performed by tuning the wavelength of the laser with a piezoelectric transducer to achieve signal frequencies in the kHz range. Since the tuning range in this case in only 0.18 nm the resolution is about 50 times worse than that achieved in the model eye.

Ortsaufgelöste Funduspulsationsmessung am Auge

Durch das Einstromen von arteriellem Blut in die okularen Gefase und die damit verbundene Gefaskaliberschwankung kommt es zu einer pulsatilen Anderung des Abstandes zwischen Cornea und Retina. Diese Distanzschwankungen konnen mittels Laserinterferometrie mit hoher Prazision gemessen werden und geben Auskunft uber den pulsatilen Anteil des Blutflusses in den okularen Gefasen. War es bisher nur moglich die Pulsationen in der Fovea centralis zu messen, kann nun durch die Ankopplung des Interferometers an eine Funduskamera praselektiv an verschiedenen Stellen der Netzhaut gemessen werden. Der Auftreffpunkt des Laserstrahls am Fundus kann am Bild der Funduskamera als heller Punkt identifiziert werden. Durch Vorgabe eines Fixierlichts kann das vermessene Auge relativ zum Laserstrahl positioniert werden und ein Mespunkt am Fundus ausgewahlt werden. Messungen an sieben gesunden Probanden zeigen, das die Funduspulsationsamplitude in der Makula signifikant geringer ist als jene im Bereich der Papille, jedoch deutlich hoher als jene in peripheren Netzhautbereichen. Weiters wurde das Mesverfahren angewendet um die Auswirkungen von 100% Sauerstoffatmung, sowie Zuatmung von 5% Kohlendioxid zu Luft auf die Funduspulsationsamplitude (FPA) zu studieren. Die Mesergebnisse werden diskutiert.

Fundus pulsation measurement by laser interferometry: a noninvasive technique for the assessment of hemodynamic drug effects

The pulse-synchronous pulsations of the eye fundus are measured by laser interferometry. The eye is illuminated by the beam of a single mode laser diode. The light is reflected at the front side of the cornea and the retina. The two remitted waves product interference fringes, from which the distance changes between cornea and retina can be determined. The interferometer is coupled to a fundus camera, so that fundus pulsations can be measured at preselected points on the retina with high transversal resolution. This technique was used to study the influence of phenylephrine (a peripherally vasoconstricting drug), isoproterenol (a predominantly positive inotropic drug), sodium nitroprusside (a peripherally vasocilating drug) on fundus pulsations in healthy volunteers. The effect of isoproterenol to increase pulse pressure amplitude was detectable even at low doses. Neither sodium nitroprusside nor phenylephrine had a significant influence on ocular fundus pulsations. These results show that measurement of fundus pulsations in the macula estimates the pressure pulse amplitude in choroidal vessels. Measurements of fundus pulsations at preselected points of the retina, show that fundus pulsations in the macular region are larger than in peripheral parts of the retina but smaller than in the optic disc region under baseline conditions.