Autofocusing and edge detection schemes in cell volume measurements with quantitative phase microscopy

Rapid flowing cells localization enabled by spatiotemporal manipulation of their holographic patterns

Lab-on-a-Chip microfluidic devices present an innovative and cost-effective platform in the current trend of miniaturization and simplification of imaging flow cytometry; they are excellent candidates for high-throughput single-cell analysis. In such microfluidic platforms, cell tracking becomes a fundamental tool for investigating biophysical processes, from intracellular dynamics to the characterization of cell motility and migration. However, high-throughput and long-term cell tracking puts a high demand on the consumption of computing resources. Here, we propose a novel strategy to achieve rapid 3D cell localizations along the microfluidic channel. This method is based on the spatiotemporal manipulation of recorded holographic interference fringes, and it allows fast and precise localization of cells without performing complete holographic reconstruction. Conventional holographic tracking is typically based on the phase contrast obtained by decoupling the calculation of optical axial and transverse coordinates. Computing time and resource consumption may increase because all the frames need to be calculated in the Fourier domain. In our proposed method, the 2D transverse positions are directly located by morphological calculation based on the hologram. The complex-amplitude wavefronts are directly reconstructed by spatiotemporal phase shifting to calculate the axial position by the refocusing criterion. Only spatial calculation is considered in the proposed method. We demonstrate that the computational time of transverse tracking is only one-tenth of the conventional method, while the total computational time of the proposed method decreases up to 54% with respect to the conventional approach. The proposed approach can open the route for analyzing flow cytometry in quantitative phase microscopy assays.

Optical edge detection with adjustable resolution based on cascaded Pancharatnam-Berry lenses

We designed a versatile optical edge detection setup with two cascaded Pancharatnam-Berry lenses (PBLs) placed at the Fourier plane of a 4f system. When the two PBLs are parallel and close to each other, owing to the moiré-like effect, one-dimensional edge detection with adjustable resolution is achieved by introducing a transverse displacement of one PBL. Furthermore, two-dimensional edge detection with adjustable resolution can also be realized by tuning the longitudinal distance between the PBLs, and the transverse displacement is exploited to adjust the edge resolution in specified directions. The proposed scheme is verified by a proof-of-principle experiment in which the resolution-adjustable edges of different targets and cells were clearly observed, showing its flexibility and potential application in image processing and high-contrast microscopy.

Background-free wide-field fluorescence imaging using edge detection combined with HiLo

Fluorescence microscopy has been widely used in the field of biological imaging, but the disturbance of background noise has always been an unavoidable phenomenon. To obtain a background free image, a virtual HiLo based on edge detection (V-HiLo-ED) method for background removing is proposed, which is different from the existing popular software algorithms that obtain the background-free image by subtracting the estimated background, but the background-free image is directly reconstructed by estimating the foreground. Compared with two other popular software-based methods, the wavelet-based background and noise subtraction algorithm (WBNS) and the rolling ball algorithm (RBA), the V-HiLo-ED owns a better quality on achieving background-free imaging. Compared with hardware-based method such as HiLo method, V-HiLo-ED exhibits almost the same performance but faster speed. In combination with light sheet microscopy, the V-HiLo-ED further improves the signal-to-noise ratio of images with thick light-sheet. These experiment results indicate that the V-HiLo-ED owns the potentiality in many other image applications such as endoscopy.

Exploiting a holographic polarization microscope for rapid autofocusing and 3D tracking

We report a fast autofocusing and accurate 3D tracking scheme for a digital hologram (DH) that intrinsically exploits a polarization microscope setup with two off-axis illumination beams having different polarization. This configuration forms twin-object images that are recorded in a digital hologram by angular and polarization multiplexing technique. We show that the separation of the two images on the recording plane follows a linear relationship with the defocus distance and indicates the defocus direction. Thus, in the entire field of view (FOV), the best focus distance of each object can be directly retrieved by identifying the respective separation distance with a cross-correlation algorithm, at the same time, 3D tracking can be performed by calculating the transverse coordinates of the two images. Moreover, we estimate this linear relationship by utilizing the numerical propagation calculation based on a single hologram, in which the focus distance of one of the objects in the FOV is known. We proved the proposed approach in accurate 3D tracking through multiple completely different experimental cases, i.e., recovering the swimming path of a marine alga (tetraselmis) in water and fast refocusing of ovarian cancer cells under micro-vibration stimulation. The reported experimental results validate the proposed strategy's effectiveness in dynamic measurement and 3D tracking without multiple diffraction calculations and any precise knowledge about the setup. We claim that it is the first time that a holographic polarization multiplexing setup is exploited intrinsically for 3D tracking and/or fast and accurate refocusing. This means that almost any polarization DH setup, thanks to our results, can guarantee accurate focusing along the optical axis in addition to polarization analysis of the sample, thus overcoming the limitation of the poor axial resolution.

Single-shot common-path off-axis dual-wavelength digital holographic microscopy

A single-shot common-path off-axis self-interference dual-wavelength digital holographic microscopic (DHM) system based on a cube beam splitter is demonstrated to expand the phase range in a stepped microstructure and for simultaneous measurement of the refractive index and physical thickness of a specimen. In the system, two laser beams with wavelengths of 532 nm and 632.8 nm are used. These laser beams are combined to transilluminate the object under study, then the object beam is divided into two beams by using a beam splitter oriented in such a way that both the beams propagate in almost the same direction, with an appropriate lateral separation between them. One of the object beams is spatially filtered at its Fourier plane, using a pinhole to generate a reference spherical beam free from the object information. The reference beam interferes with the object beam to form a digital hologram at the faceplate of the image sensor. The phase information is extracted from a single recorded digital hologram using the phase aberration compensation method that is based on principal component analysis (PCA). Owing to the common-path configuration, the system shows high temporal phase stability and it is less vibration-sensitive compared to counterparts such as a Mach-Zehnder type DHM. The performance of the dual-wavelength DHM system is verified in two different application fields by conducting the experiments using microsphere beads and living plant cells.

Implementation and Optimization of a Dual-confocal Autofocusing System

This paper describes the implementation and optimization of a dual-confocal autofocusing system that can easily describe a real-time position by measuring the response signal (i.e., intensity) of the front and the rear focal points of the system. This is a new and systematic design strategy that would make it possible to use this system for other applications while retrieving their characteristic curves experimentally; there is even a good chance of this technique becoming the gold standard for optimizing these dual-confocal configurations. We adopt two indexes to predict our system performance and discover that the rear focal position and its physical design are major factors. A laboratory-built prototype was constructed and demonstrated to ensure that its optimization was valid. The experimental results showed that a total optical difference from 150 to 400 mm significantly affected the effective volume of our designed autofocusing system. The results also showed that the sensitivity of the dual-confocal autofocusing system is affected more by the position of the rear focal point than the position of the front focal point. The final optimizing setup indicated that the rear focal length and the front focal length should be set at 200 and 100 mm, respectively. In addition, the characteristic curve between the focus error signal and its position could successfully define the exact position by a polynomial equation of the sixth order, meaning that the system can be straightforwardly applied to an accurate micro-optical auto-focusing system.

3D image distortion problem in digital in-line holographic microscopy and its effective solution

Digital in-line holographic microscopy (DIHM) has attracted attention because of its simple but powerful three-dimensional (3D) imaging capability. To improve the spatial resolution, 3D image reconstruction algorithms use numerical magnification, which generates distortions in the generated images. We propose a method to overcome this problem by using the simple relation between the object and image positions in 3D space. Several holograms were taken while translating a resolution target at different axial positions by a motorized stage. We demonstrated the effectiveness of our method by reconstructing the 3D positions of 3-μm-diameter polymer beads on a tilted slide glass from a single measured hologram.

Opposite-view digital holographic microscopy with autofocusing capability

Digital holographic microscopy (DHM) has its intrinsic ability to refocusing a sample by numerically propagating an object wave from its hologram plane to its image plane. In this paper opposite-view digital holographic microscopy (OV-DHM) is demonstrated for autofocusing, namely, digitally determining the location of the image plane, and refocusing the object wave without human intervention. In OV-DHM, a specimen is illuminated from two sides in a 4π-alike configuration, and two holograms are generated and recorded by a CCD camera along two orthogonal polarization orientations. The image plane of the sample is determined by finding the minimal variation between the two object waves, and consequently refocusing is performed by propagating the waves to the image plane. Furthermore, the field of view (FOV) of OV-DHM can be extended by combining the two object waves which have an angle in-between. The proposed technique also has the potential to reduce speckle noise and out-of-focus background.

Double-field-of-view, quasi-common-path interferometer using Fourier domain multiplexing

We present a quasi-common-path interferometer with a double field of view (FOV). The laser beam of an imaging system is separated into three parts using three mirrors; the first and second beams are used to image two different areas of a sample, while the third beam functions as a reference beam. The reference beam is prepared by making clear area in a sample and projecting it on an image sensor. A double FOV is obtained by Fourier domain multiplexing, whereby two interferometric images corresponding to two different areas of a sample are modulated with two different spatial carrier frequencies. The feasibility of this technique is experimentally demonstrated by imaging two different areas of a test target with a single image sensor.

Large step-phase measurement by a reduced-phase triple-illumination interferometer

We present a reduced-phase triple-illumination interferometer (RPTII) as a novel single-shot technique to increase the precision of dual-illumination optical phase unwrapping techniques. The technique employs two measurement ranges to record both low-precision unwrapped and high-precision wrapped phases. To unwrap the high-precision phase, a hierarchical optical phase unwrapping algorithm is used with the low-precision unwrapped phase. The feasibility of this technique is demonstrated by measuring a stepped object with a height 2100 times greater than the wavelength of the source. The phase is reconstructed without applying any numerical unwrapping algorithms, and its noise level is decreased by a factor of ten.

Optimum phase shift for quantitative phase microscopy in volume measurement

Volume measurement of a phase object is one of the most distinctive capabilities of quantitative phase microscopy (QPM). However, the accuracy of a measured volume is limited by the different noises of a measurement system and the finite bandpass filter used in the phase extraction algorithm. In this paper, we analyze the inherent errors in volume measurement with QPM and propose the optimum condition that can minimize these errors. We find that phase information of a sample in the frequency domain nonlinearly oscillates as a function of the phase shift corresponding to the sample and its medium, and that the phase information of a sample inside the bandpass filter can be maximized by a proper phase shift. Through numerical simulations and actual experiments, we demonstrate that the error in phase volume measurement can be effectively reduced by the enhancement of the phase signal inside the bandpass region using an optimum amount of phase, which can be controlled by changing either the medium index or the wavelength of illumination.

Reduced-phase dual-illumination interferometer for measuring large stepped objects

We present a reduced-phase dual-illumination interferometer (RPDII) that measures the topography of a sample with large step height variation. We experimentally demonstrate the basic principle and the feasibility of this novel single-shot quantitative phase imaging. Two beams of this interferometer illuminate a sample at different incident angles, and two phases of the different incident angles and their phase difference are simultaneously recorded using three spatial frequencies. The relative phase difference between two beams of an RPDII can be controlled by adjusting the angle such that the maximum phase difference is smaller than 2π, and thus there is no phase wrapping ambiguity in the reconstructed phase. One 4f optical system with a transmission grating is used to illuminate the sample with two collimated beams incident at different angles. The feasibility of this technique is demonstrated by measuring the thicknesses of two stepped metal layers with heights of 150 and 660 μm. Although the change in stepped height is more than 1000 times the wavelength of the laser used in our interferometer, the thicknesses of these two metal layers are successfully obtained without the use of an unwrapping algorithm.

Dual-wavelength diffraction phase microscopy for simultaneous measurement of refractive index and thickness

We present a quantitative phase microscopy scheme that simultaneously acquires two phase images at different wavelengths. The simultaneous dual-wavelength measurement was performed with a diffraction phase microscope (DPM) based on a transmission grating and a spatial filter that form a common-path imaging interferometer. With a combined laser source that generates two-color light continuously, a different diffraction order of the grating was utilized for each wavelength component so that the dual-wavelength interference pattern could be distinguished by the distinct fringe frequencies. Our dual-wavelength phase imaging allowed us to extract information on the physical thickness and the refractive index for a specimen immersed in a highly dispersive surrounding medium. We found that our dual-wavelength DPM (DW-DPM) provides an accurate measurement of the volume and the refractive index of a microscopy sample with good measurement stability that results from the common-path geometry.

Particle tracking by full-field complex wavefront subtraction in digital holography microscopy

The 3D tracking of micro-objects, based on digital holography, is proposed through the analysis of the complex wavefront of the light scattered by the micro-samples. Exploiting the advantages of the off-axis full-field holographic interferometry, the tracking of multiple objects is achieved by a direct wavefront analysis at the focal plane overcoming the limitation of the conventional back focal plane interferometry in which only one object at a time can be tracked. Furthermore, the method proposed and demonstrated here is a step forward with respect to other holographic tracking tools. The approach is tested in two experiments, the first investigates the Brownian motion of particles trapped by holographic optical tweezers, while the second relates to the cell motility in a 3D collagen matrix, thus showing its usefulness for lab-on-chip systems in typical bioassay testing.

On the holographic 3D tracking of in vitro cells characterized by a highly-morphological change

Digital Holography (DH) in microscopic configuration is a powerful tool for the imaging of micro-objects contained into a three dimensional (3D) volume, by a single-shot image acquisition. Many studies report on the ability of DH to track particle, microorganism and cells in 3D. However, very few investigations are performed with objects that change severely their morphology during the observation period. Here we study DH as a tool for 3D tracking an osteosarcoma cell line for which extensive changes in cell morphology are associated to cell motion. Due to the great unpredictable morphological change, retrieving cell's position in 3D can become a complicated issue. We investigate and discuss in this paper how the tridimensional position can be affected by the continuous change of the cells. Moreover we propose and test some strategies to afford the problems and compare it with others approaches. Finally, results on the 3D tracking and comments are reported and illustrated.

Autofocusing of digital holographic microscopy based on off-axis illuminations

An auto-focusing method for digital holographic microscopy has been proposed by employing two off-axis illumination beams. When specimens are illuminated by two plane waves in different directions, it is found that the farther the reconstruction plane is from the image plane, the wider the two reconstructed images are separated from each other. Thus, the image plane can be determinated by finding the minimum of the variation between the two reconstructed object waves on both the amplitude and phase distributions. The feasibility of the proposed method is demonstrated by the corresponding simulation and experiment.

Autofocusing based on wavelength dependence of diffraction in two-wavelength digital holographic microscopy

An autofocusing method for two-wavelength digital holographic microscopy (TWDHM) based on the wavelength dependence of the diffraction process is proposed. Red and green lights are employed for the illumination of the TWDHM, and the generated holograms are recorded simultaneously by a color CCD camera. Due to the wavelength dependency of the diffraction process, the farther the reconstruction plane is from the image plane, the larger the difference is between the red and green light distributions. Thus, the image plane can be determined by finding the minimum of the variation between the red and green lights on their amplitude distributions. The feasibility of the proposed method is demonstrated by simulation and experiment.

Detrended fluctuation analysis of membrane flickering in discocyte and spherocyte red blood cells using quantitative phase microscopy

Dynamic analyses of vibrational motion in cell membranes provide a lot of information on the complex dynamic motilities of a red blood cell (RBC). Here, we present the correlation properties of membrane fluctuation in discocyte and spherocyte RBCs by using quantitative phase microscopy (QPM). Since QPM can provide nanometer sensitivity in thickness measurement within a millisecond time scale, we were able to observe the membrane flicking of an RBC in nanometer resolution up to the bandwidth of 50 Hz. The correlation properties of the vibrational motion were analyzed with the detrended fluctuation analysis (DFA) method. Fractal scaling exponent α in the DFA method was calculated for the vibrational motion of a cell surface at various surface points for normal discocyte and abnormal spherocyte RBCs. Measured α values for normal RBCs are distributed between 0.7 and 1.0, whereas those for abnormal spherocyte RBCs are within a range from 0.85 to 1.2. We have also verified that the vibrational motion of background fluid outside of a cell has an α value close to 0.5, which is a typical property of an uncorrelated white noise.

Automatic focusing in digital holography and its application to stretched holograms

The searching and recovering of the correct reconstruction distance in digital holography (DH) can be a cumbersome and subjective procedure. Here we report on an algorithm for automatically estimating the in-focus image and recovering the correct reconstruction distance for speckle holograms. We have tested the approach in determining the reconstruction distances of stretched digital holograms. Stretching a hologram with a variable elongation parameter makes it possible to change the in-focus distance of the reconstructed image. In this way, the proposed algorithm can be verified at different distances by dispensing the recording of different holograms. Experimental results are shown with the aim of demonstrating the usefulness of the proposed method, and a comparative analysis has been performed with respect to other existing algorithms developed for DH.

Dynamic analysis of pathogen-infected host cells using quantitative phase microscopy

We present the real-time quantitative analysis of Vibrio vulnificus-infected host cells using quantitative phase microscopy (QPM) based on interferometric techniques. This provides the ability to retrieve the phase or optical path-length distribution over the cell with nanometer path-length sensitivity from a single interferogram image. We have used QPM to study dynamic cell morphologic changes and to noninvasively quantify the cell volumes of rat basophilic leukemia RBL-2H3 cells infected with V. vulnificus strains: wild type (MO6-24∕O) and RtxA1 toxin mutant (CMM770). During the process of V. vulnificus infection in RBL-2H3 cells, the dynamic changes of quantitative phase images, cell volumes, and areas were observed in real time using QPM. In contrast, dramatic changes were not detected in RBL-2H3 cells infected with the noncytotoxic RtxA1 toxin mutant. The results showed good correlation between QPM analysis and biochemical assays, such as lactate dehydrogenase assay or β-hexosaminidase release assay. We suggest that QPM is a powerful quantitative method to study the dynamic process of host cells infected with pathogens in a noninvasive manner.

Silver nanoparticle-induced degranulation observed with quantitative phase microscopy

Monitoring a degranulation process in a live mast cell is a quite important issue in immunology and pharmacology. Because the size of a granule is normally much smaller than the resolution limit of an optical microscope system, there is no direct real-time live cell imaging technique for observing degranulation processes except for fluorescence imaging techniques. In this research, we propose optical quantitative phase microscopy (QPM) as a new observation tool to study degranulation processes in a live mast cell without any fluorescence labeling. We measure the cell volumes and the cross sectional profiles (x-z plane) of an RBL-2H3 cell and a HeLa cell, before and after they are exposed to calcium ionophore A23187 and silver nanoparticles (AgNPs). We verify that the volume and the cross sectional line profile of the RBL-2H3 cell were changed significantly when it was exposed to A23187. When 50 microg/mL of AgNP is used instead of A23187, the measurements of cell volume and cross sectional profiles indicate that RBL-2H3 cells also follow degranulation processes. Degranulation processes for these cells are verified by monitoring the increase of intracellular calcium ([Ca(2+)](i)) and histamine with fluorescent methods.