Abstract

Opposed-view digital holographic microscopy (OV-DHM) with autofocusing and out-of-focus background suppression was demonstrated and applied to measure the refractive index (RI) of suspended HeLa cells. In OV-DHM, a specimen is illuminated from two sides in a 4π-like configuration. The generated two opposite-view object waves, which have orthogonal polarization orientations, interfere with a common reference wave, and the generated holograms are recorded by a CMOS camera. The image plane of the sample was determined by finding the minimal variation between the two object waves. The out-of-focus background was suppressed by averaging the two object waves. Simultaneous determination of both the cell thickness and the phase retardation was avoided by using a spheroidal model for the detached cell obtained from confocal microscopy. Thus, the RI of suspended HeLa cells was measured from phase images of OV-DHM, with the thickness of the cells estimated by using a constant axial-to-lateral ratio. This measurement strategy reveals the RI with an accuracy of 10% of the RI difference between cells and surrounding medium.

© 2017 Optical Society of America

1. INTRODUCTION

Digital holographic microscopy (DHM) is a non-invasive, high-resolution, whole-field imaging technique for microscopic specimens, particularly translucent samples [16]. The phase imaging capability of DHM provides intrinsic contrast for transparent biological samples and also permits quantitative analyses of their 3D structures and refractive index (RI). DHM usually uses a monochromatic plane wave for illumination; consequently, both lateral resolution and axial sectioning are worse in comparison to a conventional microscope employing Koehler illumination. DHM also suffers from speckle noise due to the employed coherent illumination. Off-axis illumination [710], structured illumination [6,11], and speckle illumination [1216] were introduced to improve the lateral and axial sectioning capability and to suppress speckle noise in DHM or, more generally, phase imaging. Low-coherence and incoherent illuminations were also used to reduce speckle noise of DHM [17,18]. Another variant, dark-field opposed-view digital holographic microscopy (OV-DHM), collects scattered light concurrently from both opposite views and, therefore, improves the contrast of internal structures, as well as the signal-to-noise ratio [19,20]. Recently, OV-DHM was shown not only to reduce out-of-focus background and speckle noise, but also to provide autofocusing and field-of-view (FOV) extension [21]. Similar to other non-conventional illumination-based autofocusing methods [19,20], the image plane of a sample is identified in OV-DHM by finding the minimal variation between the two object waves propagating in opposite directions. Consequently, the sample can be refocused by propagating the object waves to their image planes in the computer. The FOV can be extended by combining two object waves propagating at an angle (10mrad) with respect to each other.

DHM, which is based on interferometry and thus has optical path length measurement accuracy on the nanometer scale, is often employed to investigate the thickness or the RI of biological samples. Of note, the RI of cells is of great significance because it provides fundamental information about the composition and organizational structure of cells [22]. For a cell with refractive index ncell, suspended in a medium with refractive index nmedium, the phase shift of light of wavelength λ passing through the cell with respect to the medium is

Δφ=2πλΔnd,
where Δn=ncellnmedium. Measurement of the RI of cells requires the determination of both the cell thickness d and the phase difference Δφ. In practice, there is no convenient way to simultaneously determine both d and Δφ because a cell imaged in a confocal microscope for d determination is not easily recovered after moving the sample to a wide-field DHM microscope for Δφ determination. Other strategies have been reported to measure the RI of cells, such as digital holographic tomography (DHT), in which a pollen cell was scanned by the illumination beam in an angular range of [0,180°] [23]. The RI of a cell was also measured in comparison to an air bubble by holding both between two cover slips and assuming identical thicknesses [24,25]. A combination of confocal microscopy with transport-of-intensity (TIE)-based phase retrieval was utilized to separately measure the cell thickness d and phase difference Δφ [22]. Finally, dual-wavelength DHM was employed to measure the RI of cells, utilizing dispersion of the surrounding medium of the cells [26].

In this paper, we determine the RI of suspended HeLa cells by using OV-DHM. The analysis is based on a description of the cells by a spheroidal model with constant cross-sectional aspect ratio (axial/lateral), which we derived from laser scanning confocal microscopy measurements. The proposed strategy is an easy-to-use approach that—unlike other RI determination methods—avoids the difficulty of imaging the same cell in confocal and DHM modes by employing the spheroidal model. Furthermore, we demonstrate that OV-DHM has the ability to efficiently suppress out-of-focus background. This background suppression as well as the autofocusing ability of OV-DHM contribute to minimizing errors of the RI determination due to defocusing and speckle noise.

2. OV-DHM TECHNIQUE

A schematic diagram of our home-built OV-DHM setup is shown in Fig. 1. It is based on a common-path Sagnac interferometer, comprised of a polarization-maintaining beam splitter (PBS) and two mirrors, M1 and M2. A 561-nm continuous-wave (CW) laser is coupled into a 1×2 fiber splitter, the two outputs of which are used as the reference wave and the light irradiating the object from opposite sides. The laser output from the object-wave end of the fiber splitter is split by the PBS into two components with orthogonal (horizontal and vertical) polarizations, respectively. The horizontally (vertically) polarized component passes through the Sagnac configuration in a clockwise (counterclockwise) fashion. Two telescope systems with 20× magnification, MO1L1 and MO2L2, are inserted between mirrors M1 and M2 to image the sample in opposite views. The distance between the objectives MO1 and MO2 is about twice their working distance. The sample is placed close to the middle plane between MO1 and MO2 (Plan 25×/0.4, Nanjing Yingxing Optical Instrument Company, Nanjing, China). The two object waves O1 and O2 arising from light passing through the sample in opposite directions are further magnified by the two telescopes by a factor of 1.5 and superimposed with a common reference wave R via a non-polarizing BS. The reference wave R is linearly polarized at an angle of 45° with respect to the polarizations of O1 (horizontal) and O2 (vertical). Two holograms, I1=|O1+R|2 and I2=|O2+R|2, are measured sequentially, by rotating polarizer P to the horizontal and vertical directions, respectively, with a complementary metal–oxide–semiconductor (CMOS) camera (1920×1200pixels, 5.86μm/pixel, 54 fps, DMK 23UX174, Imaging Source, Bremen, Germany). The CMOS camera is aligned so as to collect in-focus images of the middle plane between MO1 and MO2 for both clockwise and anti-clockwise directions. A small angle of the reference wave R of 10±0.1mrad with respect to the object waves allows separation of the real image, twin image, and dc terms of the generated carrier frequency hologram in the Fourier domain, while maintaining the highest spectral components of the object waves with diffraction-limited resolution [27]. An exemplary hologram of OV-DHM and its spectrum are shown in Figs. 2(a) and 2(b), respectively. We mention in passing that this OV-DHM configuration can be further upgraded with two CCD cameras to record both holograms I1 and I2 simultaneously.

 

Fig. 1. Experimental OV-DHM setup. BS, beamsplitter; CCD, charge-coupled device; L1 and L2, tube lenses; MO1 and MO2, microscope objectives; PBS, polarization-maintaining beamsplitter; M1 and M2, mirrors; O1 and O2, object waves linearly polarized along the horizontal (0°) and vertical (90°) directions, respectively; P, polarizer; R, reference wave linearly polarized at 45°.

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Fig. 2. Exemplary hologram and spectrum of OV-DHM. (a) Overview and close-up of OV-DHM hologram; (b) spectrum of the hologram in (a). κx and κy are the carrier-frequencies of the off-axis hologram in the x and y directions, respectively. The dashed rectangle in (b) indicates the spectral region selected for hologram reconstruction.

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Due to the symmetry of the OV-DHM configuration, a sample at a defocus distance Δd in hologram I1 (clockwise) will have an opposite defocus distance Δd in hologram I2 (counterclockwise). Without loss of generality, two object waves, Or1 and Or2, can be reconstructed and refocused to a plane at an arbitrary distance Δd(Δd) from the hologram I1(I2) by using the angular spectrum method [28]:

Or1=FT1{FT{I1RD}·Wfilter·exp[ikΔd1(λ)2(λ)2]},Or2=FT1{FT{I2RD}·Wfilter·exp[ikΔd1(λ)2(λ)2]}.
Here, k=2π/λ denotes the wavenumber; FT{·} and FT1{·} denote Fourier and inverse Fourier transform operators, respectively. ξ and η are coordinates in the frequency domain. RD=exp[2πi(κxx+κyy)] is a digitalized reference wave, which has a linear phase term to shift the spectrum of the real image to the center of the frequency domain; κx and κy are the carrier frequencies of the off-axis hologram, determined from spectrum analysis, as shown in Fig. 2(b). Wfilter(ξ,η) is a window function that selects the real images of I1RD and I2RD in the frequency domain and removes dc terms and twin images. Of note, ideally, the CMOS camera ought to be aligned so as to collect in-focus images of the middle plane Pmid between MO1 and MO2 for both clockwise and anti-clockwise directions. However, if the image/conjugate plane of Pmid has defocusing distances Δz1 and Δz2 to the CMOS camera along the clockwise and anti-clockwise directions, respectively, such misalignment can be compensated digitally by replacing Δd with ΔdΔz1 (for Or1) and ΔdΔz2 (for Or2) in Eq. (1).

A key issue in refocusing an image from the out-of-focus hologram plane is to find the distance Δd between the hologram plane and the image plane, since a slight out-of-focus displacement will reduce the accuracy of the phase measurement. Hitherto, there have been many reports on image plane detection, based on amplitude analysis [29], spectral norms [30], non-conventional illumination [6,7,31], and other approaches [32,33]. In OV-DHM, focus determination follows the following strategy: Because holograms I1 and I2 have opposite defocus, the difference between the intensity of the two object waves IDiff(x,y)=|Or1(Δd)|2|Or2(Δd)|2 is minimal for the correct distance Δd=M2Δz employed in the reconstruction, with Δz being the defocus in the sample domain and M the effective magnification factor of the OV-DHM system. Accordingly, the focus criterion can be defined as

Cri(Δd)=[1XY1X1Y(IDiffI¯Diff)2]1/2,
with X and Y denoting the number of pixels in the x and y directions; I¯diff is the mean value of IDiff. In the implementation, the focus plane can be determined by finding the minimum of the criterion function in Eq. (3). Generally, for cells located in different axial planes, the focus criterion can be applied to different regions and, consequently, enables us to refocus laterally separate regions of a hologram to different focal planes, providing 3D information of the sample [25]. Compared with conventional image plane determination methods, this method does not rely on the type of a sample, i.e., it can be used for samples with both amplitude and phase modulations.

For a specific refocused plane, only in-focus cells will have the same images in Or1 and Or2, whereas out-of-focus components are different [see Fig. 2(d)]. Thus, out-of-focus components as well as speckle noise can be suppressed by averaging Or1 and Or2. Of note, out-of-focus background suppression can be further enhanced if the two object waves are not coaxial but propagate at a slight angle (10mrad). In this case, only in-focus cells appear at the same locations and are enhanced upon averaging, whereas out-of-focus cells are shifted laterally and are averaged out.

3. OUT-OF-FOCUS BACKGROUND SUPPRESSION OF OV-DHM

The auto-focusing and out-of-focus background suppression of OV-DHM were demonstrated with phase imaging of human cervical cancer (HeLa) cells. HeLa cells (LGC Standards GmbH, Wesel, Germany) were maintained at 37°C and 5% CO2 in Dulbecco’s modified Eagle’s medium (DMEM), containing 10% fetal bovine serum (FBS) and antibiotics (60μg/mL penicillin and 100ng/mL streptomycin, both from Invitrogen, Carlsbad, CA). After 24 h incubation, the cells were detached by using 500 μL trypsin and mixed with an agarose-phosphate buffered saline solution (3%, weight/weight) at 37°C. After cooling to room temperature (24°C), a transparent gel formed with cells entrapped. Figures 3(a) and 3(b) show amplitude and phase images of Or1 (left) and Or2 (right) after refocusing to the focal plane, with Δz1/2=210/+210μm, obtained by the focus criterion in Fig. 3(e) for selected region 1, indicated by the white rectangle in Fig. 3(b). For comparison, Fig. 3(c) shows phase images of Or1 (left) and Or2 (right) refocused to the focal plane with Δz1/2=45/+45μm, obtained by the focus criterion in Fig. 3(f) for region 2 [green rectangle in Fig. 3(c)].

 

Fig. 3. OV-DHM imaging on HeLa cells suspended in an agarose-water gel (3%, weight/weight). Reconstructed amplitude (a) and phase (b) images of the opposite-view object waves Or1 (left) and Or2 (right) reconstructed with Δz1=210μm (Δz2=210μm). (c) Phase images of Or1 (left) and Or2 (right) reconstructed with Δz1=45μm (Δz2=45μm). Scale bar in (c), 40 μm. (d) Comparison of out-of-focus background in Or1, Or2, and (Or1+Or2)/2 for the same region [indicated by the red rectangle in (b)]. Standard deviations of the three images, Or1, Or2, and (Or1+Or2)/2 in (d) are 0.49, 0.4, and 0.21, respectively. (e) and (f) Focus criterion curves of selected (e) region 1 [white rectangle in (b)] and (f) region 2 [green rectangle in (c)]. In the figure, the abbreviations “Amp.” and “Pha.” refer to amplitude and phase; “Obj.1,” “Obj.2,” and “Aver.” indicate the two opposite-view object waves along the clockwise and anti-clockwise directions and the average of the two (in phase).

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Figures 3(a)3(c) show clearly that only in-focus cells have identical images in Or1 and Or2. Out-of-focus cells are different in Or1 and Or2 images and thus can be suppressed by averaging Or1 and Or2. To quantify the ability of OV-DHM to suppress out-of-focus background, an area [red rectangle in Fig. 3(b)] was selected in Or1 and Or2, and (Or1+Or2)/2 and compared in Fig. 3(d). The visibility of the out-of-focus background in the images of Or1, Or2, and (Or1+Or2)/2 was quantified by the standard deviation, calculated from the three rectangular regions (50μm×80μm) in Fig. 3(d), yielding 0.49, 0.40, and 0.21, respectively. Accordingly, OV-DHM has the capability of reducing out-of-focus background by averaging the two object waves.

4. REFRACTIVE INDEX MEASUREMENT ON CELLS BY OV-DHM

Live HeLa cells adhere to the bottom of cell containers and thus have a flat, carpet-like appearance [34]. If they die or are detached from surfaces by using trypsin, they round up and become ellipsoids [35], shaped by the tension of cell membrane and gravity. A triaxial ellipsoid is described by the equation x2/a2+y2/b2+z2/c2=1, with a, b, and c being the lengths of the principal half axes along x, y, and z, respectively. In the agarose preparation, the cells are compressed along the principal axis z, chosen to be perpendicular to the glass slide supporting the cells, whereas the two other extensions are nearly equal, giving the cells an almost spheroidal shape [35]. For the OV-DHM experiment, the sample is introduced into the setup such that the z axis is along the axial direction of the microscope. Depending on the tension of the cell membrane and the density of the cells with respect to the surrounding medium, the cellular extensions may vary [36,37], but the ratio S=c/r0, with c and r0=(a+b)/2 being the axial and lateral extensions, is essentially fixed for a given cell type.

The 3D ellipsoid model of suspended HeLa cells was studied by using our home-built laser-scanning confocal fluorescence microscope [38,39]. In the experiment, HeLa cells were seeded on eight-well Lab-Tek II chambered cover glass (Thermo Fischer Scientific, Waltham, MA) and cultured in DMEM for 24 h. Afterwards, the cell membranes were stained with CellMask Green (Invitrogen, Darmstadt, Germany) in DMEM for 5 min. The cells were thoroughly washed twice with phosphate-buffered saline, and then embedded into agarose-water gel with the protocol described above. Then, the cells were imaged under the confocal microscope using a bandpass filter (600/37 nm) for fluorescence detection. A series of 3D images was obtained by scanning different regions (90μm×90μm×30μm) of the sample, each with 128×128×128 voxels. Exemplarily, Figs. 4(a) and 4(b) show orthogonal views and a 3D view of cells from a selected region. The suspended cells are approximately round in xy view, while they are elliptical in the xz and yz views. For quantitative analysis, the cross-sectional intensities along two lines through the center of a cell along y and z [red and blue lines in Fig. 4(a)] are shown in Fig. 4(c). From these data, the overall axial (lateral) extension of the cell is 12.3±0.1 (21.8±0.1) μm. The axial-to-lateral aspect ratio S of the cell is thus 0.56±0.005. This procedure was performed for 50 cells; the statistics of axial/lateral (c/r0) and lateral/lateral (a/b) ratios are shown in Fig. 4(d), yielding averages of 0.56±0.04 and 1.05±0.03, respectively. Here r0=(a+b)/2 is the average lateral extension, and the error bars indicate standard deviations of all measurements. The data show that a spheroidal model (i.e., an ellipsoid with a=b) is a good approximation for the suspended HeLa cells, with an almost constant aspect ratio between the axial and lateral extensions. In general, the spheroidal model can be further simplified to a 3D spherical model (a=b=c) if cells or tissues are prepared in a rotatory cell culture system [35]. More generally, 3D bioprinting [37] allows one to produce a designed shape for live cells or tissues and thus give more freedom to extend the model.

 

Fig. 4. 3D elliptical model of suspended HeLa cells. (a) Perpendicular views of the HeLa cells. (b) 3D view of cells in volume rendering based on a maximum intensity projection. The inset in (b) shows schematically the 3D elliptical model of a suspended cell. (c) Intensities along the blue and red lines in panel (a). (d) Ratios c/r0 and a/b of 50 suspended cells. The range 25–75% of all data points is included in the boxes; the lines in the boxes show the median. The means are indicated by small squares; vertical lines represent the maximum spread of the data.

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With the spheroidal model of the suspended HeLa cells, the cell thickness d along the axial direction can be computed from the lateral extension r0 and the aspect ratio S. By using Eq. (1) together with the thickness of the cell along the axial direction for a point (x,y) in the lateral plane, d(x,y)=2z(x,y), we obtain

Δφ(x,y)=2πλΔn[2Sr02x2y2].
From the phase image, Δφ(x,y), measured by OV-DHM, we thus obtain Δn by fitting the phase distribution of a suspended cell with Eq. (4), so the RI of the cell equals nmedium+Δn.

The same HeLa cell sample used for fluorescence imaging was also employed for the RI measurement. Figure 5(a) shows the wrapped phase image of cells reconstructed from the object wave Or1 in OV-DHM, after refocusing by a distance Δz1=45μm. The correct refocusing distance, determined by finding the minimal variation between Or1 and Or2 for the selected cell, helps to minimize the phase error due to image defocus. Even a slight defocus will introduce a moderate error on the phase measurement [40]. The phase distributions of Or1,Or2, and (Or1+Or2)/2 along the red dashed line across the cell in Fig. 5(a) were extracted and are shown in Fig. 5(b). It is apparent that the green curve is less noisy due to the averaging operation on Or1 and Or2. Fitting this curve with Eq. (4) and setting y=0 yields r0=10.00±0.05μm and Δn=0.0462±0.0005. The RI of the agarose-water gel (3%, weight/weight) was 1.3481±0.0004, measured by using a refractometer (PAL-RI, ATAGO, Tokyo, Japan) at 24°C. Therefore, the RI of the cell was 1.394±0.003 at 24°C. Ten cells were analyzed by the same procedure, yielding ncell=1.39±0.01, with the error denoting the standard deviation within the ensemble. The obtained ncell agrees with the result ncell=1.399±0.004 measured by dual-wavelength DHM [26], implying that the OV-DHM approach provides a precise RI determination for the HeLa cells.

 

Fig. 5. Refractive index measurement on suspended HeLa cells. (a) Wrapped phase image of HeLa cells reconstructed from Or1 in OV-DHM. (b) Phase distributions of Or1 (black), Or2 (red), and (Or1+Or2)/2 (green) along the line across the cell center, indicated by the red dashed line in (a). The violet solid curve is the fit of the green curve using Eq. (4).

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5. CONCLUSIONS

This paper presents a unique technique for RI determination of cells by using OV-DHM. Simultaneous determination of both d and Δφ is circumvented by using a spheroidal model for detached cells, which was obtained by confocal microscopy. The proposed strategy is not entirely stand-alone because it needs a confocal microscope to quantify the parameters of the spheroidal model. However, the difficulty of imaging the same cell in confocal and DHM modes is avoided by using the spheroidal model. OV-DHM was employed to determine the phase Δφ of a light passing through a cell. The image plane within the sample was identified by finding the minimal variation between the two object waves; consequently, refocusing was performed by propagating the waves to the common image plane. Out-of-focus background can be suppressed by averaging the two object waves Or1 and Or2 because in-focus contributions are enhanced by the averaging operation, whereas out-of-focus contributions have different defocus distances and thus are at least partially cancelled, as are noise contributions (e.g., coherent noise). The autofocusing and out-of-focus light-suppressing ability of OV-DHM contributes to improving the accuracy of RI determination.

3D profiles of suspended HeLa cells in agarose gel were measured by laser-scanning confocal microscopy, revealing a spheroidal cell model with a constant axial-to-lateral aspect ratio. Based on this model, the RI can be determined by using a stand-alone phase-imaging approach (including, but not limited to, OV-DHM), since the thickness of the cell can be calculated from the phase image for all lateral positions. The RI depends on the composition of the cell, and the surrounding medium and will be maintained under constant conditions, no matter whether the cell is detached or not.

Our RI measurement strategy based on an ellipsoid model is not limited to HeLa cells studied here but also suitable for many other cell types. Different model shapes, e.g., spheres, can also be imposed by using the newly developed 3D bioprinting techniques [37].

Funding

Helmholtz Program Science and Technology of Nanosystems (STN); Deutsche Forschungsgemeinschaft (DFG) (GRK 2039); National Natural Science Foundation of China (NSFC) (61605150, 61475187, 61575154, 61107003, U1304617); 863 Program (2013AA014402); Fundamental Research Funds for the Central Universities (JB160511, XJS16005, JBG160502).

Acknowledgment

We thank Prof. Wolfgang Osten and Dr. Giancarlo Pedrini from University Stuttgart for discussions of opposed-view digital holographic microscopy.

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31. P. Gao, B. L. Yao, R. Rupp, J. W. Min, R. L. Guo, B. H. Ma, J. J. Zheng, M. Lei, S. H. Yan, D. Dan, and T. Ye, “Autofocusing based on wavelength dependence of diffraction in two-wavelength digital holographic microscopy,” Opt. Lett. 37, 1172–1174 (2012). [CrossRef]  

32. P. Memmolo, C. Distante, M. Paturzo, A. Finizio, P. Ferraro, and B. Javidi, “Automatic focusing in digital holography and its application to stretched holograms,” Opt. Lett. 36, 1945–1947 (2011). [CrossRef]  

33. J. Gillespie and R. A. King, “The use of self-entropy as a focus measure in digital holography,” Pattern Recogn. Lett. 9, 19–25 (1989). [CrossRef]  

34. E. Ruoslahti, “Fibronectin in cell adhesion and invasion,” Cancer Metastasis Rev. 3, 43–51 (1984). [CrossRef]  

35. M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016). [CrossRef]  

36. M. M. Kozlov and L. V. Chernomordik, “Membrane tension and membrane fusion,” Curr. Opin. Struct. Biol. 33, 61–67 (2015). [CrossRef]  

37. V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014). [CrossRef]  

38. P. Gao, B. Prunsche, L. Zhou, K. Nienhaus, and G. U. Nienhaus, “Background suppression in fluorescence nanoscopy with stimulated emission double depletion,” Nat. Photonics 11, 163–169 (2017). [CrossRef]  

39. P. Gao and G. U. Nienhaus, “Precise background subtraction in stimulated emission double depletion nanoscopy,” Opt. Lett. 42, 831–834 (2017). [CrossRef]  

40. M. T. Rinehart, H. S. Park, and A. Wax, “Influence of defocus on quantitative analysis of microscopic objects and individual cells with digital holography,” Biomed. Opt. Express 6, 2067–2075 (2015). [CrossRef]  

References

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  12. G. Kim, N. Nagarajan, E. Pastuzyn, K. Jenks, M. Capecchi, J. Shepherd, and R. Menon, “Deep-brain imaging via epi-fluorescence Computational Cannula Microscopy,” Sci. Rep. 7, 44791 (2017).
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  15. Y. Park, W. Choi, Z. Yaqoob, R. Dasari, K. Badizadegan, and M. S. Feld, “Speckle-field digital holographic microscopy,” Opt. Express 17, 12285–12292 (2009).
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    [Crossref]
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    [Crossref]
  28. J. J. Zheng, Y. L. Yang, M. Lei, B. L. Yao, P. Gao, and T. Ye, “Fluorescence volume imaging with an axicon: simulation study based on scalar diffraction method,” Appl. Opt. 51, 7236–7245 (2012).
    [Crossref]
  29. F. Dubois, C. Schockaert, N. Callens, and C. Yourassowsky, “Focus plane detection criteria in digital holography microscopy by amplitude analysis,” Opt. Express 14, 5895–5908 (2006).
    [Crossref]
  30. W. Li, N. C. Loomis, Q. Hu, and C. S. Davis, “Focus detection from digital in-line holograms based on spectral l1 norms,” J. Opt. Soc. Am. A 24, 3054–3062 (2007).
    [Crossref]
  31. P. Gao, B. L. Yao, R. Rupp, J. W. Min, R. L. Guo, B. H. Ma, J. J. Zheng, M. Lei, S. H. Yan, D. Dan, and T. Ye, “Autofocusing based on wavelength dependence of diffraction in two-wavelength digital holographic microscopy,” Opt. Lett. 37, 1172–1174 (2012).
    [Crossref]
  32. P. Memmolo, C. Distante, M. Paturzo, A. Finizio, P. Ferraro, and B. Javidi, “Automatic focusing in digital holography and its application to stretched holograms,” Opt. Lett. 36, 1945–1947 (2011).
    [Crossref]
  33. J. Gillespie and R. A. King, “The use of self-entropy as a focus measure in digital holography,” Pattern Recogn. Lett. 9, 19–25 (1989).
    [Crossref]
  34. E. Ruoslahti, “Fibronectin in cell adhesion and invasion,” Cancer Metastasis Rev. 3, 43–51 (1984).
    [Crossref]
  35. M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
    [Crossref]
  36. M. M. Kozlov and L. V. Chernomordik, “Membrane tension and membrane fusion,” Curr. Opin. Struct. Biol. 33, 61–67 (2015).
    [Crossref]
  37. V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
    [Crossref]
  38. P. Gao, B. Prunsche, L. Zhou, K. Nienhaus, and G. U. Nienhaus, “Background suppression in fluorescence nanoscopy with stimulated emission double depletion,” Nat. Photonics 11, 163–169 (2017).
    [Crossref]
  39. P. Gao and G. U. Nienhaus, “Precise background subtraction in stimulated emission double depletion nanoscopy,” Opt. Lett. 42, 831–834 (2017).
    [Crossref]
  40. M. T. Rinehart, H. S. Park, and A. Wax, “Influence of defocus on quantitative analysis of microscopic objects and individual cells with digital holography,” Biomed. Opt. Express 6, 2067–2075 (2015).
    [Crossref]

2017 (4)

G. Kim, N. Nagarajan, E. Pastuzyn, K. Jenks, M. Capecchi, J. Shepherd, and R. Menon, “Deep-brain imaging via epi-fluorescence Computational Cannula Microscopy,” Sci. Rep. 7, 44791 (2017).
[Crossref]

J. J. Zheng, P. Gao, and X. P. Shao, “Opposite-view digital holographic microscopy with autofocusing capability,” Sci. Rep. 7, 425 (2017).
[Crossref]

P. Gao, B. Prunsche, L. Zhou, K. Nienhaus, and G. U. Nienhaus, “Background suppression in fluorescence nanoscopy with stimulated emission double depletion,” Nat. Photonics 11, 163–169 (2017).
[Crossref]

P. Gao and G. U. Nienhaus, “Precise background subtraction in stimulated emission double depletion nanoscopy,” Opt. Lett. 42, 831–834 (2017).
[Crossref]

2016 (1)

M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
[Crossref]

2015 (3)

M. M. Kozlov and L. V. Chernomordik, “Membrane tension and membrane fusion,” Curr. Opin. Struct. Biol. 33, 61–67 (2015).
[Crossref]

M. T. Rinehart, H. S. Park, and A. Wax, “Influence of defocus on quantitative analysis of microscopic objects and individual cells with digital holography,” Biomed. Opt. Express 6, 2067–2075 (2015).
[Crossref]

J. J. Zheng, G. Pedrini, P. Gao, B. L. Yao, and W. Osten, “Autofocusing and resolution enhancement in digital holographic microscopy by using speckle-illumination,” J. Opt. 17, 085301 (2015).
[Crossref]

2014 (3)

2013 (3)

A. Faridian, G. Pedrini, and W. Osten, “High-contrast multilayer imaging of biological organisms through dark-field digital refocusing,” J. Biomed. Opt. 18, 086009 (2013).
[Crossref]

P. Gao, G. Pedrini, and W. Osten, “Structured illumination for resolution enhancement and autofocusing in digital holographic microscopy,” Opt. Lett. 38, 1328–1330 (2013).
[Crossref]

Y. Cotte, F. Toy, P. Jourdain, N. Pavillon, D. Boss, P. Magistretti, P. Marquet, and C. Depeursinge, “Marker-free phase nanoscopy,” Nat. Photonics 7, 418 (2013).
[Crossref]

2012 (4)

2011 (3)

2009 (2)

P. F. Almoro and S. G. Hanson, “Object wave reconstruction by speckle illumination and phase retrieval,” J. Eur. Opt. Soc. 4, 09002 (2009).
[Crossref]

Y. Park, W. Choi, Z. Yaqoob, R. Dasari, K. Badizadegan, and M. S. Feld, “Speckle-field digital holographic microscopy,” Opt. Express 17, 12285–12292 (2009).
[Crossref]

2008 (4)

2007 (4)

2006 (3)

2005 (2)

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

M. G. L. Gustafsson, “Nonlinear structured-illumination microscopy: wide-field fluorescence imaging with theoretically unlimited resolution,” Proc. Natl. Acad. Sci. USA 102, 13081–13086 (2005).
[Crossref]

2004 (1)

2003 (1)

1989 (1)

J. Gillespie and R. A. King, “The use of self-entropy as a focus measure in digital holography,” Pattern Recogn. Lett. 9, 19–25 (1989).
[Crossref]

1984 (1)

E. Ruoslahti, “Fibronectin in cell adhesion and invasion,” Cancer Metastasis Rev. 3, 43–51 (1984).
[Crossref]

Allman, B. E.

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

Almoro, P. F.

P. F. Almoro and S. G. Hanson, “Object wave reconstruction by speckle illumination and phase retrieval,” J. Eur. Opt. Soc. 4, 09002 (2009).
[Crossref]

P. F. Almoro, G. Pedrini, and W. Osten, “Aperture synthesis in phase retrieval using a volume-speckle field,” Opt. Lett. 32, 733–735 (2007).
[Crossref]

Arienti, C.

M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
[Crossref]

Badizadegan, K.

Bellair, C. J.

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

Bevilacqua, A.

M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
[Crossref]

Bjornsson, C.

V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
[Crossref]

Boss, D.

Y. Cotte, F. Toy, P. Jourdain, N. Pavillon, D. Boss, P. Magistretti, P. Marquet, and C. Depeursinge, “Marker-free phase nanoscopy,” Nat. Photonics 7, 418 (2013).
[Crossref]

Bredebusch, I.

B. Kemper, S. Kosmeier, P. Langehanenberg, G. von Bally, I. Bredebusch, W. Domschke, and J. Schnekenburger, “Integral refractive index determination of living suspension cells by multifocus digital holographic phase contrast microscopy,” J. Biomed. Opt. 12, 054009 (2007).
[Crossref]

B. Kemper, D. Carl, J. Schnekenburger, I. Bredebusch, M. Schafer, W. Domschke, and G. von Bally, “Investigation of living pancreas tumor cells by digital holographic microscopy,” J. Biomed. Opt. 11, 034005 (2006).
[Crossref]

Brooker, G.

Brueck, S. R. J.

Callens, N.

Capecchi, M.

G. Kim, N. Nagarajan, E. Pastuzyn, K. Jenks, M. Capecchi, J. Shepherd, and R. Menon, “Deep-brain imaging via epi-fluorescence Computational Cannula Microscopy,” Sci. Rep. 7, 44791 (2017).
[Crossref]

Carl, D.

B. Kemper, D. Carl, J. Schnekenburger, I. Bredebusch, M. Schafer, W. Domschke, and G. von Bally, “Investigation of living pancreas tumor cells by digital holographic microscopy,” J. Biomed. Opt. 11, 034005 (2006).
[Crossref]

Charriere, F.

Charrière, F.

Chernomordik, L. V.

M. M. Kozlov and L. V. Chernomordik, “Membrane tension and membrane fusion,” Curr. Opin. Struct. Biol. 33, 61–67 (2015).
[Crossref]

Choi, W.

Colomb, T.

Cotte, Y.

Y. Cotte, F. Toy, P. Jourdain, N. Pavillon, D. Boss, P. Magistretti, P. Marquet, and C. Depeursinge, “Marker-free phase nanoscopy,” Nat. Photonics 7, 418 (2013).
[Crossref]

Cuche, E.

Curl, C. L.

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

Dai, G. H.

V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
[Crossref]

Dan, D.

Dasari, R.

Dasari, R. R.

Davis, C. S.

Deflores, L. P.

Delbridge, L. M. D.

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

Depeursinge, C.

Dirksen, D.

Distante, C.

Domschke, W.

B. Kemper, S. Kosmeier, P. Langehanenberg, G. von Bally, I. Bredebusch, W. Domschke, and J. Schnekenburger, “Integral refractive index determination of living suspension cells by multifocus digital holographic phase contrast microscopy,” J. Biomed. Opt. 12, 054009 (2007).
[Crossref]

B. Kemper, D. Carl, J. Schnekenburger, I. Bredebusch, M. Schafer, W. Domschke, and G. von Bally, “Investigation of living pancreas tumor cells by digital holographic microscopy,” J. Biomed. Opt. 11, 034005 (2006).
[Crossref]

Dubois, F.

Faridian, A.

Feld, M. S.

Ferraro, P.

Finizio, A.

Gao, P.

P. Gao, B. Prunsche, L. Zhou, K. Nienhaus, and G. U. Nienhaus, “Background suppression in fluorescence nanoscopy with stimulated emission double depletion,” Nat. Photonics 11, 163–169 (2017).
[Crossref]

P. Gao and G. U. Nienhaus, “Precise background subtraction in stimulated emission double depletion nanoscopy,” Opt. Lett. 42, 831–834 (2017).
[Crossref]

J. J. Zheng, P. Gao, and X. P. Shao, “Opposite-view digital holographic microscopy with autofocusing capability,” Sci. Rep. 7, 425 (2017).
[Crossref]

J. J. Zheng, G. Pedrini, P. Gao, B. L. Yao, and W. Osten, “Autofocusing and resolution enhancement in digital holographic microscopy by using speckle-illumination,” J. Opt. 17, 085301 (2015).
[Crossref]

W. Osten, A. Faridian, P. Gao, K. Körner, D. Naik, G. Pedrini, A. K. Singh, M. Takeda, and M. Wilke, “Recent advances in digital holography [Invited],” Appl. Opt. 53, G44–G63 (2014).
[Crossref]

P. Gao, G. Pedrini, and W. Osten, “Structured illumination for resolution enhancement and autofocusing in digital holographic microscopy,” Opt. Lett. 38, 1328–1330 (2013).
[Crossref]

P. Gao, B. L. Yao, J. W. Min, R. L. Guo, B. H. Ma, J. J. Zheng, M. Lei, S. H. Yan, D. Dan, and T. Ye, “Autofocusing of digital holographic microscopy based on off-axis illuminations,” Opt. Lett. 37, 3630–3632 (2012).
[Crossref]

J. J. Zheng, Y. L. Yang, M. Lei, B. L. Yao, P. Gao, and T. Ye, “Fluorescence volume imaging with an axicon: simulation study based on scalar diffraction method,” Appl. Opt. 51, 7236–7245 (2012).
[Crossref]

P. Gao, B. L. Yao, R. Rupp, J. W. Min, R. L. Guo, B. H. Ma, J. J. Zheng, M. Lei, S. H. Yan, D. Dan, and T. Ye, “Autofocusing based on wavelength dependence of diffraction in two-wavelength digital holographic microscopy,” Opt. Lett. 37, 1172–1174 (2012).
[Crossref]

P. Gao, B. Yao, I. Harder, J. Min, R. Guo, J. Zheng, and T. Ye, “Parallel two-step phase-shifting digital holograph microscopy based on a grating pair,” J. Opt. Soc. Am. A 28, 434–440 (2011).
[Crossref]

García, J.

V. Mico, Z. Zalevsky, and J. García, “Common-path phase-shifting digital holographic microscopy: a way to quantitative phase imaging and superresolution,” Opt. Commun. 281, 4273–4281 (2008).
[Crossref]

Gillespie, J.

J. Gillespie and R. A. King, “The use of self-entropy as a focus measure in digital holography,” Pattern Recogn. Lett. 9, 19–25 (1989).
[Crossref]

Guo, R.

Guo, R. L.

Gustafsson, M. G. L.

M. G. L. Gustafsson, “Nonlinear structured-illumination microscopy: wide-field fluorescence imaging with theoretically unlimited resolution,” Proc. Natl. Acad. Sci. USA 102, 13081–13086 (2005).
[Crossref]

Hanson, S. G.

P. F. Almoro and S. G. Hanson, “Object wave reconstruction by speckle illumination and phase retrieval,” J. Eur. Opt. Soc. 4, 09002 (2009).
[Crossref]

Harder, I.

Harris, P. J.

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

Harris, T.

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

Hu, Q.

Iwai, H.

Javidi, B.

Jenks, K.

G. Kim, N. Nagarajan, E. Pastuzyn, K. Jenks, M. Capecchi, J. Shepherd, and R. Menon, “Deep-brain imaging via epi-fluorescence Computational Cannula Microscopy,” Sci. Rep. 7, 44791 (2017).
[Crossref]

Jourdain, P.

Y. Cotte, F. Toy, P. Jourdain, N. Pavillon, D. Boss, P. Magistretti, P. Marquet, and C. Depeursinge, “Marker-free phase nanoscopy,” Nat. Photonics 7, 418 (2013).
[Crossref]

Karande, P.

V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
[Crossref]

Kemper, B.

P. Langehanenberg, B. Kemper, D. Dirksen, and G. von Bally, “Autofocusing in digital holographic phase contrast microscopy on pure phase objects for live cell imaging,” Appl. Opt. 47, D176–D182 (2008).
[Crossref]

B. Kemper and G. von Bally, “Digital holographic microscopy for live cell applications and technical inspection,” Appl. Opt. 47, A52–A61 (2008).
[Crossref]

B. Kemper, S. Kosmeier, P. Langehanenberg, G. von Bally, I. Bredebusch, W. Domschke, and J. Schnekenburger, “Integral refractive index determination of living suspension cells by multifocus digital holographic phase contrast microscopy,” J. Biomed. Opt. 12, 054009 (2007).
[Crossref]

B. Kemper, D. Carl, J. Schnekenburger, I. Bredebusch, M. Schafer, W. Domschke, and G. von Bally, “Investigation of living pancreas tumor cells by digital holographic microscopy,” J. Biomed. Opt. 11, 034005 (2006).
[Crossref]

Kim, G.

G. Kim, N. Nagarajan, E. Pastuzyn, K. Jenks, M. Capecchi, J. Shepherd, and R. Menon, “Deep-brain imaging via epi-fluorescence Computational Cannula Microscopy,” Sci. Rep. 7, 44791 (2017).
[Crossref]

Kim, M. K.

King, R. A.

J. Gillespie and R. A. King, “The use of self-entropy as a focus measure in digital holography,” Pattern Recogn. Lett. 9, 19–25 (1989).
[Crossref]

Körner, K.

Kosmeier, S.

B. Kemper, S. Kosmeier, P. Langehanenberg, G. von Bally, I. Bredebusch, W. Domschke, and J. Schnekenburger, “Integral refractive index determination of living suspension cells by multifocus digital holographic phase contrast microscopy,” J. Biomed. Opt. 12, 054009 (2007).
[Crossref]

Kozlov, M. M.

M. M. Kozlov and L. V. Chernomordik, “Membrane tension and membrane fusion,” Curr. Opin. Struct. Biol. 33, 61–67 (2015).
[Crossref]

Kuehn, J.

Kuznetsova, Y.

Langehanenberg, P.

P. Langehanenberg, B. Kemper, D. Dirksen, and G. von Bally, “Autofocusing in digital holographic phase contrast microscopy on pure phase objects for live cell imaging,” Appl. Opt. 47, D176–D182 (2008).
[Crossref]

B. Kemper, S. Kosmeier, P. Langehanenberg, G. von Bally, I. Bredebusch, W. Domschke, and J. Schnekenburger, “Integral refractive index determination of living suspension cells by multifocus digital holographic phase contrast microscopy,” J. Biomed. Opt. 12, 054009 (2007).
[Crossref]

Lee, V.

V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
[Crossref]

Lei, M.

Li, W.

Loomis, N. C.

Ma, B. H.

Ma, J.

Magistretti, P.

Y. Cotte, F. Toy, P. Jourdain, N. Pavillon, D. Boss, P. Magistretti, P. Marquet, and C. Depeursinge, “Marker-free phase nanoscopy,” Nat. Photonics 7, 418 (2013).
[Crossref]

Magistretti, P. J.

Marian, A.

Marquet, P.

Memmolo, P.

Menon, R.

G. Kim, N. Nagarajan, E. Pastuzyn, K. Jenks, M. Capecchi, J. Shepherd, and R. Menon, “Deep-brain imaging via epi-fluorescence Computational Cannula Microscopy,” Sci. Rep. 7, 44791 (2017).
[Crossref]

Mico, V.

V. Mico, Z. Zalevsky, and J. García, “Common-path phase-shifting digital holographic microscopy: a way to quantitative phase imaging and superresolution,” Opt. Commun. 281, 4273–4281 (2008).
[Crossref]

Min, J.

Min, J. W.

Montfort, F.

Nagarajan, N.

G. Kim, N. Nagarajan, E. Pastuzyn, K. Jenks, M. Capecchi, J. Shepherd, and R. Menon, “Deep-brain imaging via epi-fluorescence Computational Cannula Microscopy,” Sci. Rep. 7, 44791 (2017).
[Crossref]

Naik, D.

Nienhaus, G. U.

P. Gao and G. U. Nienhaus, “Precise background subtraction in stimulated emission double depletion nanoscopy,” Opt. Lett. 42, 831–834 (2017).
[Crossref]

P. Gao, B. Prunsche, L. Zhou, K. Nienhaus, and G. U. Nienhaus, “Background suppression in fluorescence nanoscopy with stimulated emission double depletion,” Nat. Photonics 11, 163–169 (2017).
[Crossref]

Nienhaus, K.

P. Gao, B. Prunsche, L. Zhou, K. Nienhaus, and G. U. Nienhaus, “Background suppression in fluorescence nanoscopy with stimulated emission double depletion,” Nat. Photonics 11, 163–169 (2017).
[Crossref]

Nugent, K. A.

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

Osten, W.

Park, H. S.

Park, Y.

Pastuzyn, E.

G. Kim, N. Nagarajan, E. Pastuzyn, K. Jenks, M. Capecchi, J. Shepherd, and R. Menon, “Deep-brain imaging via epi-fluorescence Computational Cannula Microscopy,” Sci. Rep. 7, 44791 (2017).
[Crossref]

Paturzo, M.

Pavillon, N.

Y. Cotte, F. Toy, P. Jourdain, N. Pavillon, D. Boss, P. Magistretti, P. Marquet, and C. Depeursinge, “Marker-free phase nanoscopy,” Nat. Photonics 7, 418 (2013).
[Crossref]

Pedrini, G.

Piccinini, F.

M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
[Crossref]

Polico, R.

M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
[Crossref]

Popescu, G.

Prunsche, B.

P. Gao, B. Prunsche, L. Zhou, K. Nienhaus, and G. U. Nienhaus, “Background suppression in fluorescence nanoscopy with stimulated emission double depletion,” Nat. Photonics 11, 163–169 (2017).
[Crossref]

Rappaz, B.

Rinehart, M. T.

Roberts, A.

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

Rosen, J.

Ruoslahti, E.

E. Ruoslahti, “Fibronectin in cell adhesion and invasion,” Cancer Metastasis Rev. 3, 43–51 (1984).
[Crossref]

Rupp, R.

Santi, S.

M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
[Crossref]

Schafer, M.

B. Kemper, D. Carl, J. Schnekenburger, I. Bredebusch, M. Schafer, W. Domschke, and G. von Bally, “Investigation of living pancreas tumor cells by digital holographic microscopy,” J. Biomed. Opt. 11, 034005 (2006).
[Crossref]

Schnekenburger, J.

B. Kemper, S. Kosmeier, P. Langehanenberg, G. von Bally, I. Bredebusch, W. Domschke, and J. Schnekenburger, “Integral refractive index determination of living suspension cells by multifocus digital holographic phase contrast microscopy,” J. Biomed. Opt. 12, 054009 (2007).
[Crossref]

B. Kemper, D. Carl, J. Schnekenburger, I. Bredebusch, M. Schafer, W. Domschke, and G. von Bally, “Investigation of living pancreas tumor cells by digital holographic microscopy,” J. Biomed. Opt. 11, 034005 (2006).
[Crossref]

Schockaert, C.

Schwarz, C. J.

Shao, X. P.

J. J. Zheng, P. Gao, and X. P. Shao, “Opposite-view digital holographic microscopy with autofocusing capability,” Sci. Rep. 7, 425 (2017).
[Crossref]

Shepherd, J.

G. Kim, N. Nagarajan, E. Pastuzyn, K. Jenks, M. Capecchi, J. Shepherd, and R. Menon, “Deep-brain imaging via epi-fluorescence Computational Cannula Microscopy,” Sci. Rep. 7, 44791 (2017).
[Crossref]

Singh, A. K.

Singh, G.

V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
[Crossref]

Situ, G.

Stewart, A. G.

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

Takeda, M.

Tesei, A.

M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
[Crossref]

Toy, F.

Y. Cotte, F. Toy, P. Jourdain, N. Pavillon, D. Boss, P. Magistretti, P. Marquet, and C. Depeursinge, “Marker-free phase nanoscopy,” Nat. Photonics 7, 418 (2013).
[Crossref]

Tran, T. N.

V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
[Crossref]

Trasatti, J. P.

V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
[Crossref]

Vaughan, J. C.

von Bally, G.

B. Kemper and G. von Bally, “Digital holographic microscopy for live cell applications and technical inspection,” Appl. Opt. 47, A52–A61 (2008).
[Crossref]

P. Langehanenberg, B. Kemper, D. Dirksen, and G. von Bally, “Autofocusing in digital holographic phase contrast microscopy on pure phase objects for live cell imaging,” Appl. Opt. 47, D176–D182 (2008).
[Crossref]

B. Kemper, S. Kosmeier, P. Langehanenberg, G. von Bally, I. Bredebusch, W. Domschke, and J. Schnekenburger, “Integral refractive index determination of living suspension cells by multifocus digital holographic phase contrast microscopy,” J. Biomed. Opt. 12, 054009 (2007).
[Crossref]

B. Kemper, D. Carl, J. Schnekenburger, I. Bredebusch, M. Schafer, W. Domschke, and G. von Bally, “Investigation of living pancreas tumor cells by digital holographic microscopy,” J. Biomed. Opt. 11, 034005 (2006).
[Crossref]

Wax, A.

Wilke, M.

Xu, X. W.

V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
[Crossref]

Yan, S. H.

Yang, Y. L.

Yao, B.

Yao, B. L.

Yaqoob, Z.

Ye, T.

Yoo, S. S.

V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
[Crossref]

Yourassowsky, C.

Yuan, C. J.

Zalevsky, Z.

V. Mico, Z. Zalevsky, and J. García, “Common-path phase-shifting digital holographic microscopy: a way to quantitative phase imaging and superresolution,” Opt. Commun. 281, 4273–4281 (2008).
[Crossref]

Zamagni, A.

M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
[Crossref]

Zanoni, M.

M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
[Crossref]

Zheng, J.

Zheng, J. J.

Zhou, L.

P. Gao, B. Prunsche, L. Zhou, K. Nienhaus, and G. U. Nienhaus, “Background suppression in fluorescence nanoscopy with stimulated emission double depletion,” Nat. Photonics 11, 163–169 (2017).
[Crossref]

Appl. Opt. (5)

Biomed. Opt. Express (2)

Cancer Metastasis Rev. (1)

E. Ruoslahti, “Fibronectin in cell adhesion and invasion,” Cancer Metastasis Rev. 3, 43–51 (1984).
[Crossref]

Curr. Opin. Struct. Biol. (1)

M. M. Kozlov and L. V. Chernomordik, “Membrane tension and membrane fusion,” Curr. Opin. Struct. Biol. 33, 61–67 (2015).
[Crossref]

Cytometry A (1)

C. L. Curl, C. J. Bellair, T. Harris, B. E. Allman, P. J. Harris, A. G. Stewart, A. Roberts, K. A. Nugent, and L. M. D. Delbridge, “Refractive index measurement in viable cells using quantitative phase-amplitude microscopy and confocal microscopy,” Cytometry A 65A, 88–92 (2005).
[Crossref]

J. Biomed. Opt. (3)

A. Faridian, G. Pedrini, and W. Osten, “High-contrast multilayer imaging of biological organisms through dark-field digital refocusing,” J. Biomed. Opt. 18, 086009 (2013).
[Crossref]

B. Kemper, D. Carl, J. Schnekenburger, I. Bredebusch, M. Schafer, W. Domschke, and G. von Bally, “Investigation of living pancreas tumor cells by digital holographic microscopy,” J. Biomed. Opt. 11, 034005 (2006).
[Crossref]

B. Kemper, S. Kosmeier, P. Langehanenberg, G. von Bally, I. Bredebusch, W. Domschke, and J. Schnekenburger, “Integral refractive index determination of living suspension cells by multifocus digital holographic phase contrast microscopy,” J. Biomed. Opt. 12, 054009 (2007).
[Crossref]

J. Eur. Opt. Soc. (1)

P. F. Almoro and S. G. Hanson, “Object wave reconstruction by speckle illumination and phase retrieval,” J. Eur. Opt. Soc. 4, 09002 (2009).
[Crossref]

J. Opt. (1)

J. J. Zheng, G. Pedrini, P. Gao, B. L. Yao, and W. Osten, “Autofocusing and resolution enhancement in digital holographic microscopy by using speckle-illumination,” J. Opt. 17, 085301 (2015).
[Crossref]

J. Opt. Soc. Am. A (2)

Nat. Photonics (2)

P. Gao, B. Prunsche, L. Zhou, K. Nienhaus, and G. U. Nienhaus, “Background suppression in fluorescence nanoscopy with stimulated emission double depletion,” Nat. Photonics 11, 163–169 (2017).
[Crossref]

Y. Cotte, F. Toy, P. Jourdain, N. Pavillon, D. Boss, P. Magistretti, P. Marquet, and C. Depeursinge, “Marker-free phase nanoscopy,” Nat. Photonics 7, 418 (2013).
[Crossref]

Opt. Commun. (1)

V. Mico, Z. Zalevsky, and J. García, “Common-path phase-shifting digital holographic microscopy: a way to quantitative phase imaging and superresolution,” Opt. Commun. 281, 4273–4281 (2008).
[Crossref]

Opt. Express (2)

Opt. Lett. (12)

B. Rappaz, F. Charriere, C. Depeursinge, P. J. Magistretti, and P. Marquet, “Simultaneous cell morphometry and refractive index measurement with dual-wavelength digital holographic microscopy and dye-enhanced dispersion of perfusion medium,” Opt. Lett. 33, 744–746 (2008).
[Crossref]

P. Gao, B. L. Yao, R. Rupp, J. W. Min, R. L. Guo, B. H. Ma, J. J. Zheng, M. Lei, S. H. Yan, D. Dan, and T. Ye, “Autofocusing based on wavelength dependence of diffraction in two-wavelength digital holographic microscopy,” Opt. Lett. 37, 1172–1174 (2012).
[Crossref]

P. Memmolo, C. Distante, M. Paturzo, A. Finizio, P. Ferraro, and B. Javidi, “Automatic focusing in digital holography and its application to stretched holograms,” Opt. Lett. 36, 1945–1947 (2011).
[Crossref]

P. Gao and G. U. Nienhaus, “Precise background subtraction in stimulated emission double depletion nanoscopy,” Opt. Lett. 42, 831–834 (2017).
[Crossref]

G. Popescu, L. P. Deflores, J. C. Vaughan, K. Badizadegan, H. Iwai, R. R. Dasari, and M. S. Feld, “Fourier phase microscopy for investigation of biological structures and dynamics,” Opt. Lett. 29, 2503–2505 (2004).
[Crossref]

P. Gao, G. Pedrini, and W. Osten, “Structured illumination for resolution enhancement and autofocusing in digital holographic microscopy,” Opt. Lett. 38, 1328–1330 (2013).
[Crossref]

P. Gao, B. L. Yao, J. W. Min, R. L. Guo, B. H. Ma, J. J. Zheng, M. Lei, S. H. Yan, D. Dan, and T. Ye, “Autofocusing of digital holographic microscopy based on off-axis illuminations,” Opt. Lett. 37, 3630–3632 (2012).
[Crossref]

C. J. Schwarz, Y. Kuznetsova, and S. R. J. Brueck, “Imaging interferometric microscopy,” Opt. Lett. 28, 1424–1426 (2003).
[Crossref]

J. Rosen and G. Brooker, “Digital spatially incoherent Fresnel holography,” Opt. Lett. 32, 912–914 (2007).
[Crossref]

M. K. Kim, “Adaptive optics by incoherent digital holography,” Opt. Lett. 37, 2694–2696 (2012).
[Crossref]

P. F. Almoro, G. Pedrini, and W. Osten, “Aperture synthesis in phase retrieval using a volume-speckle field,” Opt. Lett. 32, 733–735 (2007).
[Crossref]

F. Charrière, A. Marian, F. Montfort, J. Kuehn, T. Colomb, E. Cuche, P. Marquet, and C. Depeursinge, “Cell refractive index tomography by digital holographic microscopy,” Opt. Lett. 31, 178–180 (2006).
[Crossref]

Pattern Recogn. Lett. (1)

J. Gillespie and R. A. King, “The use of self-entropy as a focus measure in digital holography,” Pattern Recogn. Lett. 9, 19–25 (1989).
[Crossref]

Proc. Natl. Acad. Sci. USA (1)

M. G. L. Gustafsson, “Nonlinear structured-illumination microscopy: wide-field fluorescence imaging with theoretically unlimited resolution,” Proc. Natl. Acad. Sci. USA 102, 13081–13086 (2005).
[Crossref]

Sci. Rep. (3)

G. Kim, N. Nagarajan, E. Pastuzyn, K. Jenks, M. Capecchi, J. Shepherd, and R. Menon, “Deep-brain imaging via epi-fluorescence Computational Cannula Microscopy,” Sci. Rep. 7, 44791 (2017).
[Crossref]

J. J. Zheng, P. Gao, and X. P. Shao, “Opposite-view digital holographic microscopy with autofocusing capability,” Sci. Rep. 7, 425 (2017).
[Crossref]

M. Zanoni, F. Piccinini, C. Arienti, A. Zamagni, S. Santi, R. Polico, A. Bevilacqua, and A. Tesei, “3D tumor spheroid models for in vitro therapeutic screening: a systematic approach to enhance the biological relevance of data obtained,” Sci. Rep. 6, 19103 (2016).
[Crossref]

Tissue Eng. C (1)

V. Lee, G. Singh, J. P. Trasatti, C. Bjornsson, X. W. Xu, T. N. Tran, S. S. Yoo, G. H. Dai, and P. Karande, “Design and fabrication of human skin by three-dimensional bioprinting,” Tissue Eng. C 20, 473–484 (2014).
[Crossref]

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Figures (5)

Fig. 1.
Fig. 1. Experimental OV-DHM setup. BS, beamsplitter; CCD, charge-coupled device; L 1 and L 2 , tube lenses; MO 1 and MO 2 , microscope objectives; PBS, polarization-maintaining beamsplitter; M 1 and M 2 , mirrors; O 1 and O 2 , object waves linearly polarized along the horizontal (0°) and vertical (90°) directions, respectively; P, polarizer; R, reference wave linearly polarized at 45°.
Fig. 2.
Fig. 2. Exemplary hologram and spectrum of OV-DHM. (a) Overview and close-up of OV-DHM hologram; (b) spectrum of the hologram in (a). κ x and κ y are the carrier-frequencies of the off-axis hologram in the x and y directions, respectively. The dashed rectangle in (b) indicates the spectral region selected for hologram reconstruction.
Fig. 3.
Fig. 3. OV-DHM imaging on HeLa cells suspended in an agarose-water gel (3%, weight/weight). Reconstructed amplitude (a) and phase (b) images of the opposite-view object waves O r 1 (left) and O r 2 (right) reconstructed with Δ z 1 = 210 μm ( Δ z 2 = 210 μm ). (c) Phase images of O r 1 (left) and O r 2 (right) reconstructed with Δ z 1 = 45 μm ( Δ z 2 = 45 μm ). Scale bar in (c), 40 μm. (d) Comparison of out-of-focus background in O r 1 , O r 2 , and ( O r 1 + O r 2 ) / 2 for the same region [indicated by the red rectangle in (b)]. Standard deviations of the three images, O r 1 , O r 2 , and ( O r 1 + O r 2 ) / 2 in (d) are 0.49, 0.4, and 0.21, respectively. (e) and (f) Focus criterion curves of selected (e) region 1 [white rectangle in (b)] and (f) region 2 [green rectangle in (c)]. In the figure, the abbreviations “Amp.” and “Pha.” refer to amplitude and phase; “Obj.1,” “Obj.2,” and “Aver.” indicate the two opposite-view object waves along the clockwise and anti-clockwise directions and the average of the two (in phase).
Fig. 4.
Fig. 4. 3D elliptical model of suspended HeLa cells. (a) Perpendicular views of the HeLa cells. (b) 3D view of cells in volume rendering based on a maximum intensity projection. The inset in (b) shows schematically the 3D elliptical model of a suspended cell. (c) Intensities along the blue and red lines in panel (a). (d) Ratios c / r 0 and a / b of 50 suspended cells. The range 25–75% of all data points is included in the boxes; the lines in the boxes show the median. The means are indicated by small squares; vertical lines represent the maximum spread of the data.
Fig. 5.
Fig. 5. Refractive index measurement on suspended HeLa cells. (a) Wrapped phase image of HeLa cells reconstructed from O r 1 in OV-DHM. (b) Phase distributions of O r 1 (black), O r 2 (red), and ( O r 1 + O r 2 ) / 2 (green) along the line across the cell center, indicated by the red dashed line in (a). The violet solid curve is the fit of the green curve using Eq. (4).

Equations (4)

Equations on this page are rendered with MathJax. Learn more.

Δ φ = 2 π λ Δ n d ,
O r 1 = FT 1 { FT { I 1 R D } · W filter · exp [ i k Δ d 1 ( λ ) 2 ( λ ) 2 ] } , O r 2 = FT 1 { FT { I 2 R D } · W filter · exp [ i k Δ d 1 ( λ ) 2 ( λ ) 2 ] } .
Cri ( Δ d ) = [ 1 X Y 1 X 1 Y ( I Diff I ¯ Diff ) 2 ] 1 / 2 ,
Δ φ ( x , y ) = 2 π λ Δ n [ 2 S r 0 2 x 2 y 2 ] .

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