Matthias Salas

Visualization of micro-capillaries using optical coherence tomography angiography with and without adaptive optics.

The purpose of this work is to investigate the benefits of adaptive optics (AO) technology for optical coherence tomography angiography (OCTA). OCTA has shown great potential in non-invasively enhancing the contrast of vessels and small capillaries. Especially the capability of the technique to visualize capillaries with a lateral extension that is below the transverse resolution of the system opens unique opportunities in diagnosing retinal vascular diseases. However, there are some limitations of this technology such as shadowing and projection artifacts caused by overlying vasculature or the inability to determine the true extension of a vessel. Thus, the evaluation of the vascular structure and density based on OCTA alone can be misleading. In this paper we compare the performance of AO-OCT, AO-OCTA and OCTA for imaging retinal vasculature. The improved transverse resolution and the reduced depth of focus of AO-OCT and AO-OCTA greatly reduce shadowing artifacts allowing for a better differentiation and segmentation of different vasculature layers of the inner retina. The comparison is done on images recorded in healthy volunteers and in diabetic patients with distinct pathologies of the retinal microvasculature.

Multi-modal adaptive optics system including fundus photography and optical coherence tomography for the clinical setting

We present a new compact multi-modal imaging prototype that combines an adaptive optics (AO) fundus camera with AO-optical coherence tomography (OCT) in a single instrument. The prototype allows acquiring AO fundus images with a field of view of 4°x4° and with a frame rate of 10fps. The exposure time of a single image is 10 ms. The short exposure time results in nearly motion artifact-free high resolution images of the retina. The AO-OCT mode allows acquiring volumetric data of the retina at 200kHz A-scan rate with a transverse resolution of ~4 µm and an axial resolution of ~5 µm. OCT imaging is acquired within a field of view of 2°x2° located at the central part of the AO fundus image. Recording of OCT volume data takes 0.8 seconds. The performance of the new system is tested in healthy volunteers and patients with retinal diseases.

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.

Influence of wave-front sampling in adaptive optics retinal imaging

A wide range of sampling densities of the wave-front has been used in retinal adaptive optics (AO) instruments, compared to the number of corrector elements. We developed a model in order to characterize the link between number of actuators, number of wave-front sampling points and AO correction performance. Based on available data from aberration measurements in the human eye, 1000 wave-fronts were generated for the simulations. The AO correction performance in the presence of these representative aberrations was simulated for different deformable mirror and Shack Hartmann wave-front sensor combinations. Predictions of the model were experimentally tested through in vivo measurements in 10 eyes including retinal imaging with an AO scanning laser ophthalmoscope. According to our study, a ratio between wavefront sampling points and actuator elements of 2 is sufficient to achieve high resolution in vivo images of photoreceptors.

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.

Comparing digital and Shack-Hartmann wavefront sensing for in-vivo OCT imaging

A small lateral field of view of ∼ 150×150 μm² is scanned in vivo on human retina using a swept source OCT at a high B-scan rate of ∼1.3 kHz and used as a “guide star” to detect optical aberrations using sub-aperture based digital adaptive optics. The results are compared with Shack-Hartmann sensor measurements.

Optimizing the sampling density of a wave-front sensor in adaptive optics systems: application to scanning laser ophthalmoscopy

We present the optimization of an adaptive optics loop for retinal imaging. Generally, the wave-front is overdetermined compared to the number of corrector elements. The sampling of the sensor can be reduced while maintaining an efficient correction, leading to higher sensitivity, faster correction and larger dynamic range. An analytical model was developed to characterize the link between number of actuators, number of micro-lenses and correction performance. The optimized correction loop was introduced into a scanning laser ophthalmoscope. In vivo images of foveal photoreceptors were recorded and the obtained image quality is equivalent to the state of the art in retinal AO-imaging.

Investigation of the benefit of adaptive optics optical coherence tomography angiography for the human retina (Conference Presentation)

In this work we investigate the benefits of using optical coherence tomography angiography (OCTA) in combination with adaptive optics (AO) technology. It has been demonstrated that the contrast of vessels and small capillaries can be greatly enhanced by the use of OCTA. Moreover, small capillaries that are below the transverse resolution of the ophthalmic instrument can be detected. This opens unique opportunities for diagnosing retinal diseases. However, there are some limitations of this technology such as shadowing artifacts caused by overlying vasculature or the inability to determine the true extension of a vessel. Thus, the evaluation of the vascular structure and density can be misleading. To overcome these limitations we applied the OCT angiography technique to images recorded with AO-OCT. Due to the higher collection efficiency of AO-OCT in comparison with standard OCT an increased intensity contrast of vasculature can be seen. Using AO-OCTA the contrast of the vasculature to the surrounding static tissue is further increased. The improved transverse resolution and the reduced depth of focus of the AO-OCT greatly reduce shadowing artifacts allowing for a correct differentiation and segmentation of different vascular layers of the inner retina. The method is investigated in healthy volunteers and in patients with diabetic retinopathy.

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.