Assessment of a liquid lens enabled in vivooptical coherence microscope

Abstract

The optical aberrations induced by imaging through skin can be predicted using formulas for Seidel aberrations of a plane-parallel plate. Knowledge of these aberrations helps to guide the choice of numerical aperture (NA) of the optics we can use in an implementation of Gabor domain optical coherence microscopy (GD-OCM), where the focus is the only aberration adjustment made through depth. On this basis, a custom-designed, liquid-lens enabled dynamic focusing optical coherence microscope operating at 0.2 NA is analyzed and validated experimentally. As part of the analysis, we show that the full width at half-maximum metric, as a characteristic descriptor for the point spread function, while commonly used, is not a useful metric for quantifying resolution in non-diffraction-limited systems. Modulation transfer function (MTF) measurements quantify that the liquid lens performance is as predicted by design, even when accounting for the effect of gravity. MTF measurements in a skinlike scattering medium also quantify the performance of the microscope in its potential applications. To guide the fusion of images across the various focus positions of the microscope, as required in GD-OCM, we present depth of focus measurements that can be used to determine the effective number of focusing zones required for a given goal resolution. Subcellular resolution in an onion sample, and high-definition in vivo imaging in human skin are demonstrated with the custom-designed and built microscope.

Three-dimensional imaging of normal skin and nonmelanoma skin cancer with cellular resolution using Gabor domain optical coherence microscopy

“…Using the 100 μm separation between focal planes, over a depth of 60 μm there is 2-μm lateral resolution, and the remaining imaging depth (i.e., 40 μm) supports 3-μm lateral resolution measured as a minimum of 20% contrast criterion. 41 The lateral scale ðx; yÞ of images was determined with the scanned length by the two galvanometer based mirrors. The optical depths (z) of the structure were first measured with optical path differences between the reference arm and the structures in the sample and the optical depths were then rescaled to the physical depths of the images by dividing them with the average refractive index 1.4 of epidermis and dermis at 800 nm.…”

Design of a handheld optical coherence microscopy endoscope

“…Additionally, recently a Gabor-based fusion technique demonstrated 2-μm lateral resolution. 6,7 Each of these techniques requires increased hardware and/or software complexity to achieve uniform lateral resolution over a large depth range.…”

Parallelized multi–graphics processing unit framework for high-speed Gabor-domain optical coherence microscopy

“…The microscope objective probe with 2-mm field-of-view incorporates a liquid lens, which allows dynamic-focusing in order to image different depths of the sample while maintaining a lateral resolution of 2 μm within the imaging depth of up to ∼2 mm by design. 28,29 A custom dispersion compensator and a custom spectrometer with a high-speed CMOS line camera (spl4096-70 km, Basler Inc., Exton, Pennsylvania) are used to acquire the spectral information. 60,61 With a set depth of focus of 100 μm, the liquid lens is refocused six times to image 600 μm in depth in skin tissue, yielding six volumes of data to acquire and to process.…”

“…28 Provided the increased lateral resolution by an order of magnitude compared with conventional OCT, the images acquired with GD-OCM have a shallower depth of focus around the focal plane controlled by the liquid lens, typically in the order of 60 to 100 μm. 29 Therefore, in order to image a volume up to 0.6 mm in depth for example, the liquid lens is dynamically refocused six times to image six volumes, which are then fused in postprocessing to produce a volumetric image with 2-μm resolution throughout the 0.6-mm depth. 30 The large dataset and the multiple computational tasks associated with GD-OCM compound the need for fast processing; performing the processing steps on conventional architectures takes about two orders of magnitude longer than the acquisition steps.…”