Biomedical subjects
Publications and source records attributed to A F Fercher.
A slit lamp laser interferometer is described for clinical in vivo eye length measurements. The basic components are a laser Doppler interferometer, a slit lamp attachment, and a commercial slit lamp. First measurements of the axial length of the eye yielded a standard deviation of 0.04 mm and less.
PURPOSE: To examine the applicability of the recently developed laser Doppler interferometry technique for measuring the axial length of cataract eyes in a realistic clinical situation. To determine the performance of the instrument as a function of cataract grade. To compare the results to those of ultrasound methods. METHODS: A total of 196 cataract eyes of 100 patients were examined. The axial eye length was determined by laser Doppler interferometry and by two different ultrasound techniques, the applanation technique and the immersion technique. The cataract grade was determined by a commercial instrument that measures backscattered light. RESULTS: Laser Doppler interferometry worked very well except in the cases of the highest cataract grades (4% of the eyes of this study were not measurable because of a too-high lens density). Only 3.5% of the other eyes were not measurable because of fixation problems of the patients. The precision of laser Doppler interferometry is not influenced by the cataract grade (except the highest grade). The standard deviation of the geometric eye length is approximately 20 microns. Linear regression analysis revealed a very good correlation of laser Doppler interferometry and ultrasonic measurements, but a systematic difference was found. The eye lengths measured by laser Doppler interferometry were about 0.18 mm longer than those measured by the immersion technique and about 0.47 mm longer than those measured by the applanation technique. CONCLUSION: These differences are attributed to the laser Doppler interferometry results including the retinal thickness and indentation of the cornea by the applanation technique. The main advantages of the laser Doppler interferometry technique are high precision, high accuracy, and more comfort for the patient because it is a noncontact method, anesthesia is unnecessary, and the risk of corneal infection is avoided.
The laser Doppler interferometry (LDI) technique, which was recently developed for axial eye length measurement, has been modified to measure the corneal thickness of the human eye in vivo. High accuracy is achieved. The standard deviation of the technique is about 7 microns, and improvement by a factor of 5 is possible. First comparisons with a usual slit lamp pachometer show a general agreement but a systematic difference of about 20 microns. Possible reasons for this discrepancy are discussed. Finally, the new method is compared to standard optical and ultrasound pachometry from a theoretical point of view, and advantages and drawbacks of the various techniques are discussed.
A new method has been developed for determining the axial length of the human eye. This method is based on laser interferometry with partially coherent light emitted by a multimode semiconductor laser diode. The measurement procedure is as follows. The subject looks into the laser beam (wavelength approximately equal to 780 nm, power approximately equal to 100 microW), which appears as a red spot. Hence the laser beam acts as a fixation target and measuring beam. This beam passes a Fabry-Perot interferometer in front of the eye. Reflected beams are generated at the cornea and retina. Since the coherence length of the laser used (approximately equal to 25 microns) is less than the difference in path length of the two reflected beams, they will usually not interfere with each other. However, if the distance between the interferometer plates is equal to the optical length of the eye, an interference pattern can be observed. (This is due to a second illuminating beam, which is generated by the interferometer and delayed by two times the interferometer plate distance.) During measurement, the plates are displaced relative to each other until the interference pattern is observed. At this point the plate distance, which can readily be measured, is equal to the optical eye length. This method has several advantages in comparison with the usual ultrasonic technique: it is a non-contact method; no anesthesia is needed; the optical eye length is determined directly; no assumptions need be made about sound velocities in the different eye media. High longitudinal and transversal resolution is achieved (10 microns or better possible). The first measurements obtained with the optical method were compared with the results yielded by the ultrasound technique.(ABSTRACT TRUNCATED AT 250 WORDS)
We present the preliminary findings of an investigation into the visualization of retinal blood-flow distribution by means of single-exposure laser-speckle photography. The technique relies on the speckle effect produced when laser light is scattered at a diffusing surface and on the fact that the speckle will be averaged out to some extent when the surface concerned is moving. Spatial filtering techniques are used enhance the resulting variations in speckle contrast. The method is noncontacting and noninvasive, properties shared with the laser Doppler anemometry technique. Although it is less quantitative than the Doppler technique, it does have the advantage of giving an overall map of blood velocities instead of point measurements only, and we offer it as a complementary technique.
The ability of ultra-high-resolution optical coherence tomography (UHR OCT) to discriminate between healthy and pathological human brain tissue is examined by imaging ex vivo tissue morphology of various brain biopsies. Micrometer-scale OCT resolution (0.9x2 microm, axialxlateral) is achieved in biological tissue by interfacing a state-of-the-art Ti:Al2O3 laser (lambda(c)=800 nm, delta lambda=260 nm, and P(out)=120 mW exfiber) to a free-space OCT system utilizing dynamic focusing. UHR OCT images are acquired from both healthy brain tissue and various types of brain tumors including fibrous, athypical, and transitional meningioma and ganglioglioma. A comparison of the tomograms with standard hematoxylin and eosin (H&E) stained histological sections of the imaged biopsies demonstrates the ability of UHR OCT to visualize and identify morphological features such as microcalcifications (>20 microm), enlarged nuclei of tumor cells (approximately 8 to 15 microm), small cysts, and blood vessels, which are characteristic of neuropathologies and normally absent in healthy brain tissue.
Optical coherence tomography (OCT) has been developed during the last 10 years as a new noninvasive imaging tool and has been applied to diagnose different ocular and skin diseases. This technique has been modified for cross-sectional imaging of dental structures. In this first preliminary study the technique was applied to obtain tomographic images of extracted sound and decayed human teeth in order to evaluate its possible diagnostic potential for dental applications. Classical OCT images based on reflectivity measurements and phase retardation images using polarization-sensitive OCT were recorded. It was demonstrated that polarization-sensitive OCT can provide additional information which is probably related to the mineralization status and/or the scattering properties of the dental material. One of the attractive features of OCT is that it uses near-infrared light instead of ionizing radiation. Furthermore, high transversal and depth resolution on the order of 10 microm can be obtained. Present limitations, e.g. the limited penetration depth, and possible solutions are discussed.