Dierck Hillmann

Axial resolution in swept-source-based Fourier-domain optical coherence tomography (FD-OCT) is limited by the sweep range of the source. However, broad swept-sources are not readily available, and passively combining two laser sources is not straightforward. In this paper, we develop a framework to overcome this limit by computationally combining independently sweeping sources to increase the bandwidth and subsequently the axial resolution of full-field Fourier-domain optical coherence tomography (FF-FD-OCT) systems. To this end, we demonstrate a dual-laser full-field FD-OCT system that uses two lasers, which sweep sequentially and are stitched phase-correctly in post-processing to obtain high-bandwidth spectra. After combining, we achieve an effective bandwidth of 145 nm at a central wavelength of 878 nm. The system has a high axial resolution of 3.1 μ m and can operate at an A-scan rate of 50 MHz. Our method requires a one-time calibration measurement to determine the non-linear sweeps from the lasers and the wavelength overlap, as well as a volume-by-volume phase matching procedure to compensate for sample motion. We demonstrate this for ex-vivo phantoms as well as in-vivo retinal data. Overall, the framework allows for extension to multiple lasers to further improve the axial resolution.

Linear digital filters are at the core of image reconstruction and processing for many coherent optical imaging techniques, such as digital holography (DH) or optical coherence tomography (OCT). They can also be efficiently implemented using fast Fourier transform (FFT) with appropriate transfer/filter functions that operate in the frequency domain. However, even with optimal filter design, they suffer from side effects such as sidelobe generation or resolution limitations, e.g., when using windowing for apodization. Here, we propose a novel, to the best of our knowledge, nonlinear (NL) filter and apply it to coherent (complex-valued) data. The NL filter preserves correct phase information, reduces sidelobes, and can be implemented indirectly by linear filters, making it almost as performant as linear filters. We demonstrate the usefulness of these filters in OCT data processing, showing the separation of background from signal, and an alternative to classical windowing that retains the full width at half maximum (FWHM) resolution of a rectangular window function, but with sidelobe suppression comparable to other window functions. Contrary to many alternatives, it even performs well in proximity to scattering structures that are subject to speckle noise. Because of its simplicity and advantages, we expect to see widespread applications of the technique beyond the demonstrated applications.

Holographic optical coherence tomography (OCT) is a powerful imaging technique, but its ability to reveal low-reflectivity features is limited. In this study, we performed holographic OCT by incoherently averaging volumes with changing diffuse illumination of numerical aperture (NA) equal to the detection NA. While the reduction of speckle from singly scattered light is only modest, we discovered that speckle from multiply scattered light can be arbitrarily reduced, resulting in substantial improvements in image quality. This technique also offers the advantage of suppressing noises arising from spatial coherence, and can be implemented with a partially spatially incoherent light source for further mitigation of multiple scattering. Finally, we show that although holographic reconstruction capabilities are increasingly lost with decreasing spatial coherence, they can be retained over an axial range sufficient to standard OCT applications.

Full-field swept-source optical coherence tomography (FF-SS-OCT) and laser Doppler holography (LDH) are two holographic imaging techniques presenting unique capabilities for ophthalmology. We report on interlaced FF-SS-OCT and LDH imaging with a single instrument. Effectively, retinal blood flow and pulsation could be quasi-simultaneously monitored. This instrument holds potential for a wide scope of ophthalmic applications.

Optical coherence tomography (OCT) has become one of the most important techniques in ophthalmic diagnostics, as it is the only way to three-dimensionally visualize morphological changes in the layered structure of the retina at a high resolution. In addition, OCT is applied for countless medical and technical purposes. Recent developments pave the way for small-footprint OCT systems at significantly reduced costs, thereby extending possible use cases. Now, it appears increasingly likely that, in the near future, OCT will find its way into many more industrial and medical applications, including disease monitoring at home.

A method for numerical estimation and correction of aberrations of the eye in fundus imaging with optical coherence tomography (OCT) is presented. Aberrations are determined statistically by using the estimate based on likelihood function maximization. The method can be considered as an extension of the phase gradient autofocusing algorithm in synthetic aperture radar imaging to 2D optical aberrations correction. The efficiency of the proposed method has been demonstrated in OCT fundus imaging with 6λ aberrations. After correction, single photoreceptors were resolved. It is also shown that wavefront distortions with high spatial frequencies can be determined and corrected. Graphical Abstract for Table of Contents: [Text. This work is dedicated to development a method for numerical estimation and correction of aberrations of the eye in fundus imaging with OCT. Aberration evaluation is performed statistically by using estimate based on likelihood function maximization. The efficiency of the proposed method has been demonstrated in OCT fundus imaging with 6λ aberrations. It has been shown that spatial high-frequency wavefront distortions can be determined]

Functional retinal imaging, especially of neuronal activity non-invasively in humans, is of tremendous interest. Although the activation of photoreceptor cells (PRCs) could be detected in humans, imaging the function of other retinal neurons had been so far hardly possible. Here, using phase-sensitive full-field swept-source optical coherence tomography (FF-SS-OCT), we show simultaneous imaging of the activation in the photoreceptor and ganglion cell layer/inner plexiform layer (GCL/IPL). The signals from the GCL/IPL are 10-fold smaller than those from the PRC and were detectable only using algorithms for suppression of motion artifacts and pulsatile blood flow in the retinal vessels. FF-SS-OCT with improved phase evaluation algorithms, therefore, allowed us to map functional connections between PRC and GCL/IPL, confirming previous ex vivo results. The demonstrated functional imaging of retinal neuronal layers can be a valuable tool in diagnostics and basic research.

Phase-sensitive coherent imaging exploits changes in the phases of backscattered light to observe tiny alterations of scattering structures or variations of the refractive index. But moving scatterers or a fluctuating refractive index decorrelate the phases and speckle patterns in the images. It is generally believed that once the speckle pattern has changed, the phases are scrambled and any meaningful phase difference to the original pattern is removed. As a consequence, diffusion and tissue motion that cannot be resolved, prevent phase-sensitive imaging of biological specimens. Here, we show that a phase comparison between decorrelated speckle patterns is still possible by utilizing a series of images acquired during decorrelation. The resulting evaluation scheme is mathematically equivalent to methods for astronomic imaging through the turbulent sky by speckle interferometry. We thus adopt the idea of speckle interferometry to phase-sensitive imaging in biological tissues and demonstrate its efficacy for simulated data and imaging of photoreceptor activity with phase-sensitive optical coherence tomography. We believe the described methods can be applied to many imaging modalities that use phase values for interferometry.

Computational adaptive optics (CAO) is emerging as a viable alternative to hardware-based adaptive optics—in particular when applied to optical coherence tomography of the retina. For this technique, algorithms are required that detect wavefront errors precisely and quickly. Here we propose an extension of the frequently used subaperture image correlation. By applying this algorithm iteratively and, more importantly, comparing each subaperture not to the central subaperture but to several randomly selected apertures, we improved aberration correction. Since these modifications only slightly increase the run time of the correction, we believe this method can become the algorithm of choice for many CAO applications.

Phase-sensitive coherent imaging exploits changes in the phases of backscattered light to observe tiny alterations of scattering structures or variations of the refractive index. But moving scatterers or a fluctuating refractive index decorrelate the phases and speckle patterns in the images. It is generally believed that once the speckle pattern has changed, the phases are scrambled and any meaningful phase difference to the original pattern is removed. As a consequence, diffusion and tissue motion below the resolution handicap phase-sensitive imaging of biological specimen. Here, we show that, surprisingly, a phase comparison between decorrelated speckle patterns is still possible by utilizing a series of images acquired during decorrelation. The resulting evaluation scheme is mathematically equivalent to methods for astronomic imaging through the turbulent sky by speckle interferometry. We thus adopt the idea of speckle interferometry to phase-sensitive imaging in biological tissues and demonstrate its efficacy for simulated data and for imaging of photoreceptor activity with phase-sensitive optical coherence tomography. The described methods can be applied to any imaging modality that uses phase values for interferometry.