Fabrice Harms

Effects of Zernike Wavefront Aberrations on Visual Acuity Measured Using Electromagnetic Adaptive Optics Technology

PURPOSEnThis study measured the changes in visual acuity induced by individual Zernike ocular aberrations of various root-mean-square (RMS) magnitudes.nnnMETHODSnA crx1 Adaptive Optics Visual Simulator (Imagine Eyes) was used to modify the wavefront aberrations in nine eyes. After measuring ocular aberrations, the device was programmed to compensate for the eyes wavefront error up to the 4th order and successively apply different individual Zernike aberrations using a 5-mm pupil. The generated aberrations included defocus, astigmatism, coma, trefoil, and spherical aberration at a level of 0.1, 0.3, and 0.9 microm. Monocular visual acuity was assessed using computer-generated Landolt-C optotypes.nnnRESULTSnCorrection of the patients aberrations improved visual acuity by a mean of 1 line (-0.1 logMAR) compared to best sphero-cylinder correction. Aberrations of 0.1 microm RMS resulted in a limited decrease in visual acuity (mean +0.05 logMAR), whereas aberrations of 0.3 microm RMS induced significant visual acuity losses with a mean reduction of 1.5 lines (+0.15 logMAR). Larger aberrations of 0.9 microm RMS resulted in greater visual acuity losses that were more pronounced with spherical aberration (+0.64 logMAR) and defocus (+0.62 logMAR), whereas trefoil (+0.22 logMAR) was found to be better tolerated.nnnCONCLUSIONSnThe electromagnetic adaptive optics visual simulator effectively corrected and generated wavefront aberrations up to the 4th order. Custom wavefront correction significantly improved visual acuity compared to best-spectacle correction. Symmetric aberrations (eg, defocus and spherical aberration) were more detrimental to visual performance.

Adaptive optics with pupil tracking for high resolution retinal imaging

Adaptive optics, when integrated into retinal imaging systems, compensates for rapidly changing ocular aberrations in real time and results in improved high resolution images that reveal the photoreceptor mosaic. Imaging the retina at high resolution has numerous potential medical applications, and yet for the development of commercial products that can be used in the clinic, the complexity and high cost of the present research systems have to be addressed. We present a new method to control the deformable mirror in real time based on pupil tracking measurements which uses the default camera for the alignment of the eye in the retinal imaging system and requires no extra cost or hardware. We also present the first experiments done with a compact adaptive optics flood illumination fundus camera where it was possible to compensate for the higher order aberrations of a moving model eye and in vivo in real time based on pupil tracking measurements, without the real time contribution of a wavefront sensor. As an outcome of this research, we showed that pupil tracking can be effectively used as a low cost and practical adaptive optics tool for high resolution retinal imaging because eye movements constitute an important part of the ocular wavefront dynamics.

Multimodal full-field optical coherence tomography on biological tissue: toward all optical digital pathology

Optical Coherence Tomography (OCT) is an efficient technique for in-depth optical biopsy of biological tissues, relying on interferometric selection of ballistic photons. Full-Field Optical Coherence Tomography (FF-OCT) is an alternative approach to Fourier-domain OCT (spectral or swept-source), allowing parallel acquisition of en-face optical sections. Using medium numerical aperture objective, it is possible to reach an isotropic resolution of about 1x1x1 ìm. After stitching a grid of acquired images, FF-OCT gives access to the architecture of the tissue, for both macroscopic and microscopic structures, in a non-invasive process, which makes the technique particularly suitable for applications in pathology. Here we report a multimodal approach to FF-OCT, combining two Full-Field techniques for collecting a backscattered endogeneous OCT image and a fluorescence exogeneous image in parallel. Considering pathological diagnosis of cancer, visualization of cell nuclei is of paramount importance. OCT images, even for the highest resolution, usually fail to identify individual nuclei due to the nature of the optical contrast used. We have built a multimodal optical microscope based on the combination of FF-OCT and Structured Illumination Microscopy (SIM). We used x30 immersion objectives, with a numerical aperture of 1.05, allowing for sub-micron transverse resolution. Fluorescent staining of nuclei was obtained using specific fluorescent dyes such as acridine orange. We present multimodal images of healthy and pathological skin tissue at various scales. This instrumental development paves the way for improvements of standard pathology procedures, as a faster, non sacrificial, operator independent digital optical method compared to frozen sections.

Control of an electromagnetic deformable mirror using high speed dynamics characterization and identification

The transient response of a deformable mirror to be used in a closed-loop adaptive-optics imaging system is modeled and evaluated. A theoretical model is developed that describes the motion of the mirror membrane. This allows an adaptive control to achieve reduced overshoot and short settling time. Applicability of the model is tested on a mirao 52-e electromagnetic deformable mirror using a specially designed high-speed adaptive-optics test bench. This test bench permits precise mirror motion measurements up to 10u2009kHz.

A pupil tracking system for adaptive optics retinal imaging

High-resolution imaging of the retina is a challenge due to the optical aberrations introduced by the eye, a living system in constant change and motion. Adaptive Optics (AO) is particularly suited to the continuous, dynamic correction of aberrations as they change over time. In particular, eye pupil displacements induce fast-changing wave front errors which lead to a need for faster wave front sensors. We propose a new approach for ocular adaptive optics by adding a Pupil Tracking System (PTS) into the AO loop. This system is different from the existing eye tracking devices by its speed, high precision in a short range and therefore its suitability for integration in an AO loop. Performance tests done using an artificial eye with a pupil diameter of 7 mm have shown promising results. These tests have demonstrated that the device achieves an accuracy of <15 μm in a ±2 mm range of eye movements with a standard deviation <10 μm, and requires less than 12 ms for each detection.

Performance assessment of a pupil tracking system for adaptive optics retinal imaging

Adaptive Optics (AO) is particularly suitable for correction of aberrations that change over time - a necessity for high resolution imaging of the retina. The rapidly changing aberrations originating from eye movements require wavefront sensors (WFS) with high repetition rates. Our approach is enhancing aberration correction by integrating a Pupil Tracking System (PTS) into the AO loop of the retinal imaging system. In this study we assessed the performance of the PTS developed for this purpose. Tests have demonstrated that the device achieves an accuracy of <15 μm in a ±2 mm range of eye movements with a standard deviation <10 μm. PTS can tolerate ±5 mm defocus with an increase of 4 μm in mean standard deviation. In vivo measurements done with temporarily paralyzed pupils have resulted in a precision of approximately 13 μm.

Comparison of reconstruction approaches for plenoptic imaging systems

Plenoptic cameras provide single-shot 3D imaging capabilities, based on the acquisition of the Light-Field, which corresponds to a spatial and directional sampling of all the rays of a scene reaching a detector. Specific algorithms applied on raw Light-Field data allow for the reconstruction of an object at different depths of the scene. Two different plenoptic imaging geometries have been reported, associated with two reconstruction algorithms: the traditional or unfocused plenoptic camera, also known as plenoptic camera 1.0, and the focused plenoptic camera, also called plenoptic camera 2.0. Both systems use the same optical elements, but placed at different locations: a main lens, a microlens array and a detector. These plenoptic systems have been presented as independent. Here we show the continuity between them, by simply moving the position of an object. We also compare the two reconstruction methods. We theoretically show that the two algorithms are intrinsically based on the same principle and could be applied to any Light-Field data. However, the resulting images resolution and quality depend on the chosen algorithm.

Developments of EUV/x-ray wavefront sensors and adaptive optics at Imagine Optic

Imagine Optic (IO) is actively developing EUV to X-ray wavefront (WF) sensors since 2003 for applications on metrology of EUV to X-ray beams emitted by synchrotrons, free-electron lasers, plasma-based soft X-ray lasers and high harmonic generation. Our sensors have demonstrated their high usefulness for metrology of EUV to X-ray optics from single flat or curved mirrors to more complex optical systems (Schwarzschild, Kirkpatrick-Baez static or based on bender technology or with activators). Our most recent developments include the realization of a EUV sensor adapted to strongly convergent or divergent beams having numerical aperture as high as 0.15, as well as the production of a hard X-ray sensor working at 10 keV and higher energies, providing repeatability as good as 4 pm rms. We present a review of the developed sensors, as well as experimental demonstrations of their benefits for various metrology and WF optimization requirements.

Rapid full-field OCT assessment of clinical tissue specimens(Conference Presentation)

FFOCT (Full Field Optical Coherence Tomography) is a novel optical technology that gives access to very high resolution tomography images of biological tissues within minutes, non-invasively. This makes it an attractive tool to bridge the gap between medical imaging modalities (MRI, ultrasound, CT) used for cancer lesion identification or targeting and histological diagnosis. Clinical tissue specimens, such as surgical cancer margins or biopsies, can potentially be assessed rapidly, by the clinician, in the aim to help him decide on the course of action. A fast FFOCT prototype was built, that provides 1cm2 images with 1 µm resolution in 1 minute, and can accommodate samples up to 50mm diameter. Specific work was carried out to implement a large sample holder, high-speed image acquisition system, optimized scanning, and accelerated GPU tiles stitching. Results obtained on breast, urology, and digestive tissues show the efficiency of the technique for the detection of cancer on clinical tissue specimens, and reinforce the clinical relevance of the technique. The technical and clinical results show that the fast FFOCT system can successfully be used for a fast assessment of cancer excision margins or biopsies providing a very valuable tool in the clinical environment.

Adaptive optics loop for en-face optical coherence tomography and laser scanning confocal microscopy

The capabilities of a novel deformable mirror and wave-front sensor combination to correct aberrations in microscopy are analyzed. The deformable mirror, (Mirao52-D, Imagine Eyes) is incorporated with a Shack-Hartmann sensor (HASO, Imagine Optic) within a complex imaging system able to produce simultaneous en-face Optical Coherence Tomography and Laser Scanning Confocal Microscopy images as well as B-scan OCT images. A large angle imaging along one of the scanning directions is demonstrated using the AO loop to correct for the interface optics aberration. The image is split into three panels, and each panel is imaged using its own set of corrections. The three images are subsequently collaged into a final image and preliminary promising results are presented.