Christoph K. Hitzenberger

Flow velocity measurements by frequency domain short coherence interferometry

A method to measure the longitudinal flow velocity component based on phase resolved frequency domain optical coherence tomography (FDOCT) is introduced. At a center wavelength of 800nm the accessible velocity components ranges from 2 micrometers /s up to 2 mm/s. The upper limit is set by half the maximum frame rate of the CCD detector array. The lower limit is determined by the minimum resolvable phase change in the system, which is set by the system phase noise of 1 deg. First tests of the method include the velocity measurement of a mirror mounted on an oscillating piezo translator, and the flow of 8 micrometers latex spheres dispersed in water through a glass capillary.

Measurement of optical distances by optical spectrum modulation

The spectral interferometry technique is outlined and its application to axial eye length measurement is discussed. First results obtained from glass plates are presented.

In-vivo dual-beam optical coherence tomography

Laser interferometry can be used to synthesize in vivo optical tomograms of the ocular tissue noninvasively. As a first example an in vivo partial coherence tomogram obtained from the optical nerve head of a human being is presented. The inherent high precision and sensitivity can make this technique into a very powerful diagnostic tool in ophthalmology.

Ocular partial-coherence interferometry

In the past interferometry has been applied in three main fields. Fringe interferometry has been used for the topographic measurement of surfaces. Interferometric length measurement techniques have been used for the measurement of distances, refractive indices, and wavelengths. In addition interferometric techniques have also been used in synthetic aperture imaging. Here we discuss various interferometric techniques which can be used in ophthalmology, describe the problems involved and present the results obtained so far.

Ocular partial-coherence tomography

Partial coherence tomography (or optical coherence tomography) uses signals obtained with partial interferometry techniques to synthesize tomographic images. Being still under evolution this technique has already been successfully applied in the clinic. We discuss the underlying principles and present some results obtained at the fundus of the eye.

Measurement of the anterior structures of the human eye by partial coherence interferometry

In the past few years a new technique for measuring intraocular distances has been developed, which is based on interferometry using partially coherent light beams and the Doppler principle. It has been shown that this technique is capable of measuring corneal thickness, axial eye length, and retinal thickness in human eyes in vivo with unprecedented precision. This technique has now been further extended to measure the anterior chamber depth and the thickness of the lens. A precision of 10 micrometers is obtained for both intraocular distances. This is more than one order of magnitude better than with conventional techniques.

Scanning laser interferometer for fundus profile measurement of the human eye

A special interferometric technique, which uses light of low coherence length and the Doppler principle, was developed to measure intraocular distances along the vision axis of the human eye in vivo. This laser Doppler interferometry (LDI) technique has been improved to measure the fundus profile and to obtain tomographic images of the human eye fundus, especially in the area of the optic nerve head. A horizontal fundus profile of a human eye between 25 degree(s) nasal and 20 degree(s) temporal consisting of 71 measurement points was recorded in vivo. Furthermore, a vertical scan across the optic disc of the same eye at 13 degree(s) nasal, and from 5 degree(s) superior to 5 degree(s) interior, was carried out in steps of 0.5 degree(s). High accuracy is achieved.

New Developments in Optical Coherence Tomography Technology

Optical coherence tomography (OCT) represents the fasted adopted retinal imaging modality in the history of ophthalmology. Three-dimensional (3D) retinal OCT at 1,060 nm (as opposed to 800 nm) enables wide-field 3D visualization of the entire choroid, fairly irrespective of the patient’s fundus pigmentation. 3D OCT at 1,060 nm (as opposed to 800 nm) promises improved clinical feasibility for retinal imaging in patients with opaque ocular media in the anterior eye segment (e.g., cataract or corneal haze). Combining adaptive optics and OCT technology might pave the way for in vivo cellular resolution retinal imaging for routine diagnosis in the eye clinic. Extensions of OCT are recently developed that enable noninvasive depth resolved functional imaging of the retina, providing blood flow or physiologic tissue information. These extensions of OCT should not only improve image contrast, but should also enable the differentiation of retinal pathologies via localized metabolic properties or functional state. Polarization sensitive (PS)-OCT provides intrinsic, tissue-specific contrast of birefringent and depolarizing tissue. PS-OCT can be used to identify and segment the retinal pigment epithelium (RPE) based on its depolarizing property, and to analyze the retinal nerve fiber layer (RNFL) based on its birefringence Doppler OCT (D-OCT) provides quantitative information about retinal perfusion, in addition to standard OCT structure tomograms. D-OCT promises to improve early diagnosis by detecting microcirculation abnormalities as precursors of retinal diseases, and in general, vascular diseases.

In vivo optical coherence tomography and topography of the fundus of the human eye

The dual beam scanning partial coherence interferometry technique, which was recently developed to measure the fundus profile of the human eye in vivo, is used for tomographic and topographic imaging and for the measurement of different retinal structures, e.g., the contour of the papilla and thickness profiles of retina and retinal nerve fiber layer. In case of well defined, sharp boundaries between two layers of different refractive indices, the absolute position of these retinal structures can be determined with a precision (standard deviation) of 5 micrometers . Compared to first results with this instrument, the measurement time for horizontal and vertical scans was reduced by a factor of 10. At present, the measuring time of a vertical scan over 10 degree(s) in steps of 0.5 degree(s) takes at least 20 seconds without and 80 seconds with signal averaging of 4 longitudinal scans over 1.5 mm distance. Further improvements to 5 - 20 seconds are discussed. The longitudinal resolution of this instrument is approximately 15 micrometers (full-width-half-maximum), depending on the coherence length of the light source; this is about 5 times better than previous measurements. Topographic and tomographic scans across the optic disk reveal an increase of thickness of the retinal nerve fiber layer at the inferior and superior rim of the disk, which are in good agreement with results published in the literature.

Polarization-sensitive optical coherence tomography in the anterior mouse eye (Conference Presentation)

Polarization-sensitive optical coherence tomography (PS-OCT) provides intrinsic contrast related to tissue microstructure. In the past, PS-OCT has been successfully used for imaging the anterior eye of humans in a variety of pathologic conditions. Here, we present PS-OCT imaging of the anterior eye in mice. Spectral domain PS-OCT centered at a wavelength of 840 nm was performed in anaesthetized laboratory mice. Three dimensional data sets were acquired at a 70 kHz A-line rate. PS-OCT images displaying phase retardation, birefringent axis orientation and degree of polarization uniformity (DOPU) were computed. Similar to human anterior segments, depolarization was observed in the corneal stroma and in structures containing melanin pigments such as the iris and the ciliary body. Birefringence was detected in the sclera close to the limbus. Aside from depolarizing foci observed within structures affected by cataract, the lens appeared mostly polarization preserving. Increased birefringence was observed in a scarred cornea. Given the similarity of the polarization characteristics in the murine eye and the human eye, PS-OCT lends itself as an ideal candidate for non-invasive imaging in preclinical studies in mouse models of anterior segment pathology.