Helge Sudkamp

In vivo optical imaging of physiological responses to photostimulation in human photoreceptors

Significance Using a full-field optical coherence tomography system, we measured changes in the time that light requires to pass through photoreceptor outer segments, when the retina is stimulated by a light pulse. This effect can be used to monitor the activity of single cones in the living human eye. Objective monitoring of photoreceptor activity using such intrinsic optical signals could have important diagnostic applications in ophthalmology and neurology and might provide insight to facilitate basic research. Noninvasive functional imaging of molecular and cellular processes of vision may have immense impact on research and clinical diagnostics. Although suitable intrinsic optical signals (IOSs) have been observed ex vivo and in immobilized animals in vivo, detecting IOSs of photoreceptor activity in living humans was cumbersome and time consuming. Here, we observed clear spatially and temporally resolved changes in the optical path length of the photoreceptor outer segment as a response to an optical stimulus in the living human eye. To witness these changes, we evaluated phase data obtained with a parallelized and computationally aberration-corrected optical coherence tomography system. The noninvasive detection of optical path length changes shows neuronal photoreceptor activity of single cones in living human retina, and therefore, it may provide diagnostic options in ophthalmology and neurology and could provide insights into visual phototransduction in humans.

Imaging pulse wave propagation in human retinal vessels using full-field swept-source optical coherence tomography

We demonstrate a new noninvasive method to assess biomechanical properties of the retinal vascular system. Phase-sensitive full-field swept-source optical coherence tomography (PhS-FF-SS-OCT) is used to investigate retinal vascular dynamics at unprecedented temporal resolution. The motion of retinal tissue that is induced by expansion of the vessels therein is measured with an accuracy of about 10 nm. The pulse shapes of arterial and venous pulsations, their temporal delays, as well as the frequency-dependent pulse propagation through the capillary bed, are determined. For the first time, imaging speed and motion sensitivity are sufficient for a direct measurement of pulse waves propagating with more than 600 mm/s in retinal vessels of a healthy young subject.

Aberration-free volumetric high-speed imaging of in vivo retina.

Certain topics in research and advancements in medical diagnostics may benefit from improved temporal and spatial resolution during non-invasive optical imaging of living tissue. However, so far no imaging technique can generate entirely diffraction-limited tomographic volumes with a single data acquisition, if the target moves or changes rapidly, such as the human retina. Additionally, the presence of aberrations may represent further difficulties. We show that a simple interferometric setup–based on parallelized optical coherence tomography–acquires volumetric data with 10 billion voxels per second, exceeding previous imaging speeds by an order of magnitude. This allows us to computationally obtain and correct defocus and aberrations resulting in entirely diffraction-limited volumes. As demonstration, we imaged living human retina with clearly visible nerve fiber layer, small capillary networks, and photoreceptor cells. Furthermore, the technique can also obtain phase-sensitive volumes of other scattering structures at unprecedented acquisition speeds.

In-vivo retinal imaging with off-axis full-field time-domain optical coherence tomography.

With a simple setup, mainly composed of a low coherence light source and a camera, full-field optical coherence tomography (FF-OCT) allows volumetric tissue imaging. However, fringe washout constrains its use in retinal imaging. Here, we present a novel motion-insensitive approach to FF-OCT, which introduces path-length differences between the reference and the sample light in neighboring pixels using an off-axis reference beam. The temporal carrier frequency in scanned time-domain OCT is replaced by a spatial carrier frequency. Volumetric in-vivo FF-OCT measurements of the human retina were acquired in only 1.3 s, comparable to the acquisition times of current clinically used OCT devices.

Reduction of frame rate in full-field swept-source optical coherence tomography by numerical motion correction [Invited]

Full-field swept-source optical coherence tomography (FF-SS-OCT) was recently shown to allow new and exciting applications for imaging the human eye that were previously not possible using current scanning OCT systems. However, especially when using cameras that do not acquire data with hundreds of kHz frame rate, uncorrected phase errors due to axial motion of the eye lead to a drastic loss in image quality of the reconstructed volumes. Here we first give a short overview of recent advances in techniques and applications of parallelized OCT and finally present an iterative and statistical algorithm that estimates and corrects motion-induced phase errors in the FF-SS-OCT data. The presented algorithm is in many aspects adopted from the phase gradient autofocus (PGA) method, which is frequently used in synthetic aperture radar (SAR). Following this approach, the available phase errors can be estimated based on the image information that remains in the data, and no parametrization with few degrees of freedom is required. Consequently, the algorithm is capable of compensating even strong motion artifacts. Efficacy of the algorithm was tested on simulated data with motion containing varying frequency components. We show that even in strongly blurred data, the actual image information remains intact, and the algorithm can identify the phase error and correct it. Furthermore, we use the algorithm to compensate real phase error in FF-SS-OCT imaging of the human retina. Acquisition rates can be reduced by a factor of three (from 60 to 20 kHz frame rate) with an image quality that is even higher compared to uncorrected volumes recorded at the maximum acquisition rate. The presented algorithm for axial motion correction decreases the high requirements on the camera frame rate and thus brings FF-SS-OCT closer to clinical applications.

In vivo full-field time-domain optical coherence tomography using a spatially coherent off-axis reference (Conference Presentation)

Time domain OCT measures the interference between sample and reference radiation as a function of the reference arm length. In full-field-OCT (FF-OCT) a camera is used instead of a scanned beam for a parallel detection of the interference pattern and thus acquiring a complete en face image. Because multiple images have to be acquired to resolve the phase ambiguity, this method is prone to motion artifacts. We present a novel motion-insensitive approach to FF-OCT. Spatially coherent illumination and an off-axis reference beam is used to introduce path-length differences between reference and sample light in neighboring pixels. This spatial carrier frequency replaces the temporal carrier frequency in scanned TD-OCT. The setup is based on a Mach-Zehnder interferometer with a super-luminescent diode and a CMOS area camera. The Sensitivity of the system was determined to be 75 dB. The field of view was 1.42 x 1.42 mm. Each frame had 237x237 lateral channels at an axial resolution of 9 µm in tissue. By step-wise changing the length of the reference arm between the en face scans, volumetric in vivo FF-OCT measurements of the human retina have been acquired within 1.3 s. OCT with a spatially coherent off-axis reference beam is suitable for in vivo imaging of human retina. The quality of the images is sufficient to discriminate the different tissue layers.

Imaging of physiological responses to photostimulation in human photoreceptors with full-field swept-source OCT (Conference Presentation)

The non-invasive measurement of cellular physiological responses to photostimulation in living retina may have significant clinical value and give new insight into the vision process. Optical coherence tomography (OCT) has been reported to detect suitable intrinsic optical signals (IOS) in retinal photoreceptor layers upon their stimulation. Commonly, changes in backscattering intensity were observed ex vivo and immobilized animals in vivo. However, in humans measurements were time-consuming and cumbersome. Promising results were achieved when observing phase signals to detect intrinsic optical signals. But to achieve sufficient phase stability to image an entire area of photoreceptors turned out to be challenging. Here, we report full-field swept-source OCT to be sufficiently stable to detect the phase signals after projecting a stimulation image onto the living human retina. We extracted time-courses and signal dependencies from the measured datasets. For long stimuli, we were even able to assign responses to single cones. This functional imaging of photoreceptor activity could potentially be used to detect loss of photoreceptor function prior to visible morphological changes, which is associated with numerous retinal diseases.

Off-axis reference beam for full-field swept-source OCT and holoscopy

In numerous applications, Fourier-domain optical coherence tomography (FD-OCT) suffers from a limited imaging depth due to signal roll-off, a limited focal range, and autocorrelation noise. Here, we propose a parallel full-field FD-OCT imaging method that uses a swept laser source and an area camera in combination with an off-axis reference, which is incident on the camera at a small angle. As in digital off-axis holography, this angle separates autocorrelation signals and the complex conjugated mirror image from the actual signal in Fourier space. We demonstrate that by reconstructing the signal term only, this approach enables full-range imaging, i.e., it increases the imaging depth by a factor of two, and removes autocorrelation artifacts. The previously demonstrated techniques of inverse scattering and holoscopy can then numerically extend the focal range without loss of lateral resolution or imaging sensitivity. The resulting, significantly enhanced measurement depth is demonstrated by imaging a porcine eye over its entire depth, including cornea, lens, and retina. Finally, the feasibility of in vivo measurements is demonstrated by imaging the living human retina.

Imaging vascular dynamics in human retina using full-field swept-source optical coherence tomography(Conference Presentation)

We demonstrate a new non-invasive method to assess the functional condition of the retinal vascular system. Phase-sensitive full-field swept-source optical coherence tomography (PhS-FF-SS-OCT) is used to investigate retinal vascular dynamics at unprecedented temporal resolution. Motion of retinal tissue, that is induced by expansion of the vessels therein, is measured with an accuracy of about 10 nm. The pulse shape of arterial and venous pulsation, their temporal delay as well as the frequency dependent pulse propagation through the capillary bed are determined. For the first time, imaging speed and motion sensitivity are sufficient for a direct measurement of pulse waves propagating with more than 600 mm/s in retinal vessels of a healthy young subject.