A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

The diffraction tomography theorem is adapted to one-dimensional length measurement. The resulting spectral interferometry technique is described and the first length measurements using this technique on a model eye and on a human eye in vivo are presented.

Measurement of intraocular distances by backscattering spectral interferometry

We describe a high-speed optical coherence domain reflectometer. Scan speeds of 40 mm/s are achieved with a dynamic range of >90 dB and a spatial resolution of 17 μm. Two applications are presented: the noninvasive measurement of anterior eye structure in a rabbit in vivo and the characterization of reflections and interelement spacing in a multielement lens.

High-speed optical coherence domain reflectometry

Originally developed for diagnostic ultrasound imaging, piezoelectric transducers are the most widespread technology employed in optoacoustic (photoacoustic) signal detection. However, the detection requirements of optoacoustic sensing and imaging differ from those of conventional ultrasonography and lead to specifications not sufficiently addressed by piezoelectric detectors. Consequently, interest has shifted to utilizing entirely optical methods for measuring optoacoustic waves. All-optical sound detectors yield a higher signal-to-noise ratio per unit area than piezoelectric detectors and feature wide detection bandwidths that may be more appropriate for optoacoustic applications, enabling several biomedical or industrial applications. Additionally, optical sensing of sound is less sensitive to electromagnetic noise, making it appropriate for a greater spectrum of environments. In this review, we categorize different methods of optical ultrasound detection and discuss key technology trends geared towards the development of all-optical optoacoustic systems. We also review application areas that are enabled by all-optical sound detectors, including interventional imaging, non-contact measurements, magnetoacoustics, and non-destructive testing.

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Looking at sound: optoacoustics with all-optical ultrasound detection.

Imaging speed is one of the most important parameters that define the performance of optical coherence tomography (OCT) systems. During the last two decades, OCT speed has increased by over three orders of magnitude. New developments in wavelength-swept lasers have repeatedly been crucial for this development. In this review, we discuss the historical evolution and current state of the art of high-speed OCT systems, with focus on wavelength swept light sources and swept source OCT systems.

High-speed OCT light sources and systems [Invited]

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

https://www.pnas.org/content/pnas/113/46/13138.full.pdf

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

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.

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

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.

Imaging pulse wave propagation in human retinal vessels using full-field swept-source 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.

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

A full-field speckle interferometry method for non-contact and prospectively high speed Photoacoustic Tomography is introduced and evaluated as proof of concept. Thermoelastic pressure induced changes of the objects topography are acquired in a repetitive mode without any physical contact to the object. In order to obtain high acquisition speed, the object surface is illuminated by laser pulses and imaged onto a high speed camera chip. In a repetitive triple pulse mode, surface displacements can be acquired with nanometre sensitivity and an adjustable sampling rate of e.g. 20 MHz with a total acquisition time far below one second using kHz repetition rate lasers. Due to recurring interferometric referencing, the method is insensitive to thermal drift of the object due to previous pulses or other motion. The size of the investigated area and the spatial and temporal resolution of the detection are scalable. In this study, the approach is validated by measuring a silicone phantom and a porcine skin phantom with embedded silicone absorbers. The reconstruction of the absorbers is presented in 2D and 3D. The sensitivity of the measurement with respect to the photoacoustic detection is discussed. Potentially, Photoacoustic Imaging can be brought a step closer towards non-anaesthetized in vivo imaging and new medical applications not allowing acoustic contact, such as neurosurgical monitoring or burnt skin investigation.

Hendrik Spahr

Papers

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

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

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

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

Full-field speckle interferometry for non-contact photoacoustic tomography