Tim Bonin

Holoscopy — Holographic optical coherence tomography

We demonstrate Holoscopy -- a combination of full-field swept-source optical coherence tomography and digital holography. By using a simple Michelson interferometer setup, a rapidly tunable laser and combining scalar diffraction theory with standard Fourier-domain OCT signal processing we obtain depth invariant imaging quality.

Common approach for compensation of axial motion artifacts in swept-source OCT and dispersion in Fourier-domain OCT

Swept-source optical coherence tomography (SS-OCT) is sensitive to sample motion during the wavelength sweep, which leads to image blurring and image artifacts. In line-field and full-field SS-OCT parallelization is achieved by using a line or area detector, respectively. Thus, approximately 1000 lines or images at different wavenumbers are acquired. The sweep duration is identically with the acquisition time of a complete B-scan or volume, rendering parallel SS-OCT more sensitive to motion artifacts than scanning OCT. The effect of axial motion on the measured spectra is similar to the effect of non-balanced group velocity dispersion (GVD) in the interferometer arms. It causes the apparent optical path lengths in the sample arm to vary with the wavenumber. Here we propose the cross-correlation of sub-bandwidth reconstructions (CCSBR) as a new algorithm that is capable of detecting and correcting the artifacts induced by axial motion in line-field or full-field SS-OCT as well as GVD mismatch in any Fourier-domain OCT (FD-OCT) setup. By cross-correlating images which were reconstructed from a limited spectral range of the interference signal, a phase error is determined which is used to correct the spectral modulation prior to the calculation of the A-scans. Performance of the algorithm is demonstrated on in vivo full-field SS-OCT images of skin and scanning FD-OCT of skin and retina.

Slit-lamp-adapted fourier-domain OCT for anterior and posterior segments: preliminary results and comparison to time-domain OCT.

Purpose: To evaluate the diagnostic potential of a slit-lamp (SL)-adapted Fourier-domain (= spectral radar, SR) optical coherence tomography (OCT)—SL-SR-OCT—instrument as an in vivo imaging device for use in examinations of the anterior and posterior segments. Materials and Methods: In a pilot study, 88 eyes from 70 healthy volunteers and patients were examined using a prototype Fourier-domain SL-SR-OCT system. Results were compared to those from the following commercially available systems: the 1310-nm SL-OCT (Heidelberg Engineering, Heidelberg, Germany) for anterior segment and the Stratus OCT (Zeiss Meditec, Jena, Germany) for posterior segment imaging. Our SL-SR-OCT provides 1025 axial scans, 5000 Hz line-scan frequency, scan length of up to 8 mm, axial depth in air of 3.5 mm, and resolution of 9 μm. For posterior visualization, a hand-held 78-diopter ophthalmoscopic lens was used. Results: Our SL-SR-OCT system allowed simultaneous scanning with direct biomicroscopic and SL imaging of anterior and posterior segment structures. Anatomical structures and pathological changes were displayed with high resolution and excellent contrast. Measurements of corneal and retinal thickness were possible. In comparison to images obtained by the SL-OCT, our SL-SR-OCT boasted a higher resolution, thus providing more clinically relevant details of the corneal epithelium, internal structure of filtering blebs, etc. Complete imaging of the chamber angle was limited, however, due to the backscattering properties of the sclera at 830 nm. For posterior segment imaging, excellent delineation of the macula and optic nerve head details, with a distinct portrayal of macular pathology and retinal edema, was possible with SL-SR-OCT. Conclusion: SL-SR-OCT enables detailed imaging of physiological and pathological anterior and posterior segment structures. As a multi-purpose device, it offers a wide spectrum of applications, with high-quality OCT-imaging, in a comfortable setting without the need to move the patient.

Off-axis full-field swept-source optical coherence tomography using holographic refocusing

We demonstrate a full-field swept-source OCT using an off-axis geometry of the reference illumination. By using holographic refocusing techniques, a uniform lateral resolution is achieved over the measurement depth of approximately 80 Rayleigh lengths. Compared to a standard on-axis setup, artifacts and autocorrelation signals are suppressed and the measurement depth is doubled by resolving the complex conjugate ambiguity. Holographic refocusing was done efficiently by Fourier-domain resampling as demonstrated before in inverse scattering and holoscopy. It allowed to reconstruct a complete volume with about 10μm resolution over the complete measurement depth of more than 10mm. Off-axis full-field swept-source OCT enables high measurement depths, spanning many Rayleigh lengths with reduced artifacts.

Imaging of temperature distribution and retinal tissue changes during photocoagulation by high speed OCT

Considerable improvement in the reproducibility of retinal photocoagulation is expected if degree and extend of the heat-induced tissue damage can be visualized on-line during the treatment. Experimental laser treatments of the retina with enucleated pig eyes were investigated by high speed phase-sensitive OCT. OCT could visualize the increase of tissue scattering during the photocoagulation in a time-resolved way. Immediate and late tissue changes were visualized with more than 15 µm resolution. Changes of the reflectance in the OCT images had a similar sensitivity in detecting tissue changes than macroscopic imaging. By using Doppler OCT slight movements of the tissue in the irradiated spot were detected. At low irradiance the thermal expansion of the tissue is observed. At higher irradiance irreversible tissue changes dominate the tissue expansion. OCT may play an important role in understanding the mechanisms of photocoagulation. This may lead to new treatment strategies. First experiments with rabbits demonstrate the feasibility of in-vivo measurements.

Fiber spectral domain optical coherence tomography for in-vivo rat brain imaging

A well established navigation method is one of the key conditions for successful brain surgery: It should be accurate, safe and online operable. Recent research shows that Optical Coherence Tomography is a potential solution for this application by providing a high resolution and small probe dimension. In this study a fiber Spectral-Domain OCT system with a super luminescent diode with the center wavelength of 840 nm providing 13.6 μm axial resolution was used. A single mode fiber (Ø 125 μm) was employed as the detecting probe. The information acquired by OCT was reconstructed into grayscale images by vertically aligning several A-scans from the same trajectory with different depth, i.e. forward scanning. For scans of typical white matter, the images showed a higher reflection of light intensity with lower penetration depth as well as a steeper attenuation rate compared to the scans typical for grey matter. Since the axial resolution of this OCT system is very high, some microstructures lying on the striatum, hippocampus and thalamic nucleus were visible in these images. The research explored the potential of OCT to be integrated into a stereotactic surgical robot as a multi-modal navigation method.

Ultra highspeed in-vivo Fourier domain full-field OCT of the human retina

In-vivo full field (FF) optical coherence tomography (OCT) images of human retina with up to 6.8 million A-lines/s are presented by using a rapidly tunable laser source in combination with an ultra-high speed CMOS camera. It is shown that Fourier domain (FD) full field OCT could provide a way to overcome limitations in imaging speed which are posed by the maximal possible exposure (MPE) of the retina. With a 100~Hz sweep rate FF-OCT was fast enough to acquire OCT images without motion artifacts, but with rather low sensitivity of 77 dB limited by an undesired incoherent background. Nevertheless, FF-OCT may become an attractive alternative for ultrafast retinal imaging boosting image speed by a lack of moving parts and the use of considerably higher irradiation power, if it is possible to to increase the sensitivity by reducing incoherent straylight.

Comparison of fast swept source full-field OCT with conventional scanning OCT

Recently, in-vivo full eld (FF) optical coherence tomography (OCT) with an ultra-high speed camera has been presented for fast in vivo retinal imaging. By parallel A-scans acquisition, imaging with 1,5 million A-scans/s was shown with an extended illumination of the retina. In this paper, the image quality of FF-OCT images will be compared to conventional scanning OCT systems. The eect of the absence of a confocal aperture leading to crosstalk between adjacent image points will be shown and an experimental analysis of the systems lateral point spread function (PSF) in dependence of depth will be given and discussed.