Andreas Wartak

Total retinal blood flow measurement by three beam Doppler optical coherence tomography.

We present measurements of total retinal blood flow in healthy volunteers using a three beam Doppler optical coherence tomography (D-OCT) technique. This technology has the advantage of a precise determination of the flow vector without the use of any a-priori information on the vessel geometry. Circular D-OCT scans around the optic disc were recorded and venous as well as arterial total blood flow was determined and compared for each subject. The reproducibility of the method was assessed in 6 subjects by repeated measurements. Only small deviations of around 6% between the measurements were found which indicates the high precision of the proposed method.

Active-passive path-length encoded (APPLE) Doppler OCT

We present a novel active-passive path-length encoded (APPLE) swept source Doppler optical coherence tomography (DOCT) approach, enabling three-dimensional velocity vector reconstruction of moving particles without prior knowledge of the orientation of motion. The developed APPLE DOCT setup allows for non-invasive blood flow measurements in vivo and was primarily designed for quantitative human ocular blood flow investigations. The systems performance was demonstrated by in vitro flow phantom as well as in vivo retinal vessel bifurcation measurements. Furthermore, total retinal blood flow - a biomarker aiding in diagnosis and monitoring of major ocular diseases such as glaucoma, diabetic retinopathy or central/branch retinal vein occlusion - was determined in the eyes of healthy human volunteers.

Multi-directional optical coherence tomography for retinal imaging

We introduce multi-directional optical coherence tomography (OCT), a technique for investigation of the scattering properties of directionally reflective tissue samples. By combining the concepts of multi-channel and directional OCT, this approach enables simultaneous acquisition of multiple reflectivity depth-scans probing a mutual sample location from differing angular orientations. The application of multi-directional OCT in retinal imaging allows for in-depth investigations on the directional reflectivity of the retinal nerve fiber layer, Henle’s fiber layer and the photoreceptor layer. Major ophthalmic diseases (such as glaucoma or age-related macular degeneration) have been reported to alter the directional reflectivity properties of these retinal layers. Hence, the concept of multi-directional OCT might help to gain improved understanding of pathology development and progression. As a first step, we demonstrate the capabilities of multi-directional OCT in the eyes of healthy human volunteers.

Conical scan pattern for enhanced visualization of the human cornea using polarization-sensitive OCT

Conventional imaging of the human cornea with optical coherence tomography (OCT) relies on telecentric scanning optics with sampling beams that are parallel to the optical axis of the eye. Because of the shape of the cornea, the beams have in some areas considerable inclination to the corneal surface which is accompanied by low signal intensities in these areas and thus an inhomogeneous appearance of corneal structures. In addition, alterations in the polarization state of the probing light depend on the angle between the imaging beam and the birefringent axis of the sample. Therefore, changes in the polarization state observed with polarization-sensitive (PS-) OCT originate mainly from the shape of the cornea. In order to minimize the effects of the corneal shape on intensity and polarization-sensitive based data, we developed a conical scanning optics design. This design provides imaging beams that are essentially orthogonal to the corneal surface. Thus, high signal intensity throughout the entire imaged volume is obtained and the influence of the corneal shape on polarization-sensitive data is greatly reduced. We demonstrate the benefit of the concept by comparing PS-OCT imaging results of the human cornea in healthy volunteers using both scanning schemes.

Sequential multi-channel OCT in the retina using high-speed fiber optic switches

A sequential multi-channel OCT prototype featuring high-speed fiber optical switches to enable inter A-scan (A-scan rate: 100 kHz) sample arm switching was developed and human retinal image data is presented.

Large field of view adaptive optics scanning laser ophthalmoscopy and optical coherence tomography (Conference Presentation)

Adaptive Optics (AO) retinal imaging is revealing microscopic structures of the eye in a non-invasive way. Due to anisoplanatism, conventional AO systems are efficient on small 1°x1° field of view (FoV). We present a lens-based AO scanning laser ophthalmoscope (SLO) set-up with 2 deformable mirrors (DM), providing high-resolution retinal imaging on a 4°x4° FoV, for an eye pupil diameter of 7 mm. The first DM is in a pupil plane and is driven using a Shack-Hartmann (SH). The second DM is conjugated to a plane located 0.7 mm in front of the retina, to correct for aberrations varying within the FoV. Its shape is optimized using sensorless AO technique. The performance of this set-up was characterized in-vivo by measuring the eyes of four healthy volunteers. The obtained image quality was satisfactory and uniform over the entire FoV. Foveal cones could be resolved and no image distortion was detected. Furthermore, a 10°x10° FoV image was acquired at the fovea of one volunteer, by stitching 9 images recorded at different eccentricities. Finally, different layers of the retina were imaged. In addition to the photoreceptors mosaic, small capillaries and nerve fibers were clearly identified. The presented AO-SLO instrument provides high-resolution images of the retina on a relatively large FoV in reasonable time. With 2 DMs, one SH and no guide star, the system stays quite simple. The imaging performance of the set-up was validated on 4 healthy volunteers and we are currently imaging patients with different eye diseases.

Design of a head phantom produced on a 3D rapid prototyping printer and comparison with a RANDO and 3M lucite head phantom in eye dosimetry applications

An anthropomorphic head phantom including eye inserts allowing placement of TLDs 3 mm below the cornea has been produced on a 3D printer using a photo-cured acrylic resin to best allow tissue equivalence. Thus Hp(3) can be determined in radiological and interventional photon radiation fields. Eye doses and doses to the forehead have been compared to an Alderson RANDO head and a 3M Lucite skull phantom in terms of surface dose per incident air kerma for frontal irradiation since the commercial phantoms do not allow placement of TLDs 3 mm below the corneal surface. A comparison of dose reduction factors (DRFs) of a common lead glasses model has also been performed. Eye dose per incident air kerma were comparable between all three phantoms (printed phantom: 1.40, standard error (SE) 0.04; RANDO: 1.36, SE 0.03; 3M: 1.37, SE 0.03). Doses to the forehead were identical to eye surface doses for the printed phantom and the RANDO head (ratio 1.00 SE 0.04, and 0.99 SE 0.03, respectively). In the 3M Lucite skull phantom dose on the forehead was 15% lower than dose to the eyes attributable to phantom properties. DRF of a sport frame style leaded glasses model with 0.75 mm lead equivalence measured were 6.8 SE 0.5, 9.3 SE 0.4 and 10.5 SE 0.5 for the RANDO head, the printed phantom, and the 3M Lucite head phantom, respectively, for frontal irradiation. A comparison of doses measured in 3 mm depth and on the surface of the eyes in the printed phantom revealed no difference larger than standard errors from TLD dosimetry. 3D printing offers an interesting opportunity for phantom design with increasing potential as printers allowing combinations of tissue substitutes will become available. Variations between phantoms may provide a useful indication of uncertainty budgets when using phantom measurements to estimate individual personnel doses.

Few-mode fiber detection for tissue characterization in optical coherence tomography

A few-mode fiber based detection for OCT systems is presented. The capability of few-mode fibers for delivering light through different fiber paths enables the application of these fibers for angular scattering tissue character- ization. Since the optical path lengths traveled in the fiber change between the fiber modes, the OCT image information will be reconstructed at different depth positions, separating the directly backscattered light from the light scattered at other angles. Using the proposed method, the relation between the angle of reflection from the sample and the respective modal intensity distribution was investigated. The system was demonstrated for imaging ex-vivo brain tissue samples of patients with Alzheimer’s disease.

Total retinal blood flow and reproducibility evaluation by three beam optical Doppler tomography

We present a three beam optical Doppler tomography (ODT) technique suitable for 3-D velocity and flow measurements to evaluate total retinal blood circulation from and to the optic nerve head (ONH). The system consists of three independent ODT channels. Superluminescent diodes with a central wavelength of 840 nm and a spectral bandwidth of 50 nm were used. The sources are coupled to collimators resting in a specially designed mount to ensure a well-defined beam geometry, necessary for the full reconstruction of the three dimensional velocity vector. The reconstruction works without prior knowledge on the vessel geometry, which is normally required for ODT systems with less than three beams. The beams share a common bulk optics Michelson interferometer, while the detection comprises three identical spectrometers with a line scan rate of 50 kHz. 20 eyes of healthy volunteers were imaged with the 3 beam ODT, employing a circular scan pattern around the ONH. The mean total blood flow was calculated for arteries (47.1 ± 2.4 μl/min (mean ± SD)) and veins (47.1 ± 2.7 μl/min μl/min) independently. The two results showed no significant difference (paired t-test, p < 0.96), rendering both equally reliable for total flow measurements. Furthermore the reproducibility of the method was evaluated for the total flow and flow, velocities within each individual vessel of 6 eyes. The average variation for total flow measurements is sufficiently low to detect deviations of ~ 6% indicating high precision of the proposed method.

Phase sensitive adaptive optics assisted SLO/OCT for retinal imaging(Conference Presentation)

Adaptive optics (AO) is essential in order to visualize small structures such as cone and rod photoreceptors in the living human retina in vivo. By combining AO with optical coherence tomography (OCT) the axial resolution in the images can be further improved. OCT provides access to the phase of the light returning from the retina which allows a measurement of subtle length changes in the nanometer range. These occur for example during the renewal process of cone outer segments. We present an approach for measuring very small length changes using an extended AO scanning laser ophthalmoscope (SLO)/ OCT instrument. By adding a second OCT interferometer that shares the same sample arm as the first interferometer, phase sensitive measurements can be performed in the en-face imaging plane. Frame averaging decreases phase noise which greatly improves the precision in the measurement of associated length changes.