CN109186452B - A high-precision positioning method for the axial position of non-spherical particles - Google Patents
A high-precision positioning method for the axial position of non-spherical particles Download PDFInfo
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Abstract
The invention discloses a high-precision positioning method for the axial position of non-spherical particles, which comprises the following steps: recording a real-time hologram by using a coaxial digital holographic microscope, reconstructing a scattered light field of non-spherical particles by numerical values, pre-positioning an axial position by using local maximum light intensity, and fitting the axial light intensity distribution by using Gaussian distribution to obtain an accurate axial position of each particle; the invention can axially track a plurality of non-spherical particles such as bacteria, cells, viruses and the like in real time, has positioning accuracy superior to 100nm, and is suitable for monitoring the dynamic behaviors of a plurality of particles simultaneously.
Description
Technical Field
The invention relates to the research field of particle axial position measurement, in particular to a high-precision positioning method for the axial position of non-spherical particles.
Background
The measurement of the real-time three-dimensional position, especially the axial position, of the microparticles is a prerequisite for observing and studying the dynamic behavior of the microparticles, and can be used for studying, for example, the interaction of nanoparticles, the adhesion process of microorganisms, various responses of cells and viruses to the surrounding environment, and the like.
The digital holographic microscope records an interference pattern-hologram formed by the coherence of object light and reference light, and then reconstructs the light field information of a measured object in the hologram through numerical value, thereby acquiring the real-time three-dimensional coordinates of the object. It does not need fluorescent mark, and is a nondestructive imaging method. The spatial variation range of the particle motion is generally tens to hundreds of microns, and the imaging depth of the digital holographic microscope can reach tens to hundreds of microns, so that the digital holographic microscope is very suitable for dynamically tracking the particle motion. However, the size of individual particles is on the order of microns, and to accurately record their real-time three-dimensional position, the positioning accuracy needs to reach sub-micron or even higher levels. The positioning accuracy, in particular the axial positioning accuracy, of digital holographic microscopes depends on the positioning method used, more precisely on the accuracy with which the algorithm finds the focal plane. Generally speaking, the axial positioning accuracy of digital holographic microscopes is far less than the lateral positioning accuracy, limited by the imaging principles and instrument hardware. At present, the positioning method of the digital holographic microscope at the axial position mainly comprises a cross-correlation method, a Mie scattering matching method, a Rayleigh-Sommerfeld algorithm and the like. The cross-correlation method comprises the steps of firstly establishing a database of holograms at different positions of a sample to be detected, then carrying out cross-correlation function calculation on the hologram recorded in an experiment and a picture in the database, and selecting an image with the maximum correlation coefficient to determine the real-time position of the sample. The axial positioning accuracy of the cross-correlation method depends on the spatial resolution of the images in the database, is time-consuming and is not suitable for positioning multiple particles simultaneously. The axial positioning accuracy of the mie scatterometry method can be below 10nm, but it can only be used to measure regularly shaped particles and is equally unsuitable for tracking multiple particles simultaneously. The Rayleigh-Sommerfeld algorithm is suitable for simultaneously measuring a plurality of particles with any shapes, but the axial positioning precision can only reach a submicron level, and the precision is not as high as that of the former two methods. The reported method for improving the axial positioning accuracy of the Rayleigh-Sommerfeld algorithm is to first obtain the light intensity distribution near the calculated axial position and then fit the light intensity distribution by polynomial distribution to obtain the optimized position. The method is already used for improving the axial positioning precision of the spherical colloidal particles, but the axial positioning precision of the non-spherical particles cannot be optimized. Non-spherical particles such as microorganisms, cells, etc. will take on different orientations and configurations in the medium, requiring extremely high demands on the axial positioning algorithm compared to spherical particles.
Disclosure of Invention
The invention mainly aims to overcome the defects of the prior art and provide a high-precision positioning method for the axial position of non-spherical particles, which comprises the steps of recording a real-time hologram by using a coaxial digital holographic microscope, numerically reconstructing a scattered light field of the non-spherical particles by using a Rayleigh-Sommerfeld algorithm, pre-positioning the axial position by using local maximum light intensity, fitting the axial light intensity distribution by using Gaussian distribution to obtain the accurate axial position of each particle, wherein the positioning precision can be better than 100 nm.
The purpose of the invention is realized by the following technical scheme:
a high-precision positioning method for the axial position of non-spherical particles comprises the following steps:
s1, recording a holographic image sequence on the sample to obtain an original holographic image, and recording a background image in a blank area of the sample, or carrying out light intensity averaging on the original holographic image to obtain a background image, wherein the sample is a particle;
s2, carrying out background subtraction on the original holographic image through the background image, and eliminating background noise to obtain a background-subtracted holographic image;
s3, carrying out numerical reconstruction on the background-subtracted holographic image to obtain information of three-dimensional reconstruction intensity distribution of the particles;
s4, obtaining initial three-dimensional positions of a plurality of particles by filtering the three-dimensional reconstruction intensity distribution information of the particles through a threshold value and searching a local maximum value of light intensity;
s5, selecting specific rectangular areas on the corresponding sections with different heights by taking the coordinate as the center according to the initial transverse coordinate of the particles, overlapping the light intensity of all pixel points in the rectangular areas to obtain the total light intensity of the particles with different heights, and further obtaining the distribution curve of the total light intensity in the axial direction;
and S6, fitting a distribution curve of the total intensity of the light intensity in the axial direction by adopting Gaussian distribution to obtain a central peak coordinate, namely the axial coordinate of the particle.
Further, the microparticles are non-spherical microparticles;
further, in step S1, the average light intensity specifically includes: and averaging the light intensity of the holographic image sequence to obtain a background image, wherein the calculation is as follows:
in the formula Ib(x, y) is the gray scale value of the pixel at the (x, y) position in the background image, N is the total frame number of the holographic image, t is the time, It(x, y) is the gray value of the pixel at the (x, y) position in the original holographic image at the time t;
further, after the background is deducted, the light intensity value of each pixel point in the holographic image is as follows:
Is(x,y)=It(x,y)-Ib(x,y)。
further, in step S3, the specific process is as follows: carrying out numerical reconstruction on the background-subtracted holographic image to obtain the information of the three-dimensional reconstruction intensity distribution of the particles, and calculating as follows:
U(r,z)=FT-1(FT(Is(r,0)·H(q,-z))),
wherein h (r, -z) is a propagation operator, r is an initial transverse coordinate of the particle, and z is an initial axial coordinate of the particle; i is an imaginary unit; k is the wave number; r is the light propagation distance; i issIs the light intensity of the sample; FT-1Is inverse Fourier transform; FT is Fourier transform; h (q, -z) is the Fourier transform of H (r, -z);
the value of the reconstruction axial interval is larger than the axial imaging range of the instrument, and the stepping is that the size of the pixel is divided by the magnification of the objective lens. (ii) a
Further, in step S4, the specific process is as follows: setting a light intensity threshold, filtering noise in a reconstructed light field, and then searching a local light intensity maximum point by point in a cube with side length close to an integral value of the particle size;
further, the light intensity threshold is less than 10%;
further, in step S5, the rectangular area is: taking the transverse coordinate of the particle as a center, and taking an integral value of the pixel size by the side length;
further, in the step S6, the fitting specifically includes performing, by using a fitting formula:
Iint=aexp(-(z-zc)2/b),
wherein, IintIs the sum of the light intensity in the rectangular area; a is peak height, b is peak width, zcAs the central peak coordinates.
Compared with the prior art, the invention has the following advantages and beneficial effects:
the invention adopts a coaxial digital holographic microscope to record a real-time hologram, reconstructs a scattered light field of non-spherical particles through Rayleigh-Sommerfeld algorithm numerical value, positions the axial position in advance by utilizing the maximum local light intensity, and fits the axial light intensity distribution by utilizing Gaussian distribution to obtain the accurate axial position of each particle, can track a plurality of non-spherical particles in real time in the axial direction, has positioning precision superior to 100nm, and is suitable for monitoring the dynamic behavior of a plurality of particles simultaneously.
Drawings
FIG. 1 is a flow chart of a method for high-precision positioning of the axial position of non-spherical particles according to the present invention;
FIG. 2 is a diagram illustrating the axial light intensity distribution of the scattered light field of the particle and the result of Gaussian fitting according to an embodiment of the present invention;
FIG. 3 is a graph showing the measurement results of the positioning accuracy of the axial position of the fine particles in the embodiment of the present invention;
FIG. 4 is a graph showing the distribution of the intensity of light in the direction perpendicular to the bottom surface of the particles and the corresponding Gaussian fit results in an embodiment of the present invention;
FIG. 5 is a graph showing the distribution of the axial light intensity parallel to the bottom surface particles and the corresponding Gaussian fitting results in an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to examples and drawings, but the present invention is not limited thereto.
Examples
A high-precision positioning method for the axial position of non-spherical particles is disclosed, as shown in figure 1, and comprises the following steps:
the first step is as follows: recording a holographic image sequence on a sample to obtain an original holographic image, wherein the sample is a particle; recording a background image in a blank area of a sample, or carrying out light intensity averaging on an original holographic image to obtain the background image, wherein the specific process comprises the following steps: and averaging the light intensity of the holographic image sequence to obtain a background image, wherein the calculation is as follows:
in the formula Ib(x, y) is the gray scale value of the pixel at the (x, y) position in the background image, N is the total frame number of the holographic image, t is the time, It(x, y) is the gray value of the pixel at the (x, y) position in the original holographic image at the time t;
recording a holographic image, specifically: dispersing non-spherical particles in a sample cell, and recording a holographic image sequence by using a coaxial digital holographic microscopic imaging system; the coaxial digital holographic microscopic imaging system comprises an LED light source, a high-power microscope objective and an sCMOS camera; the sample cell comprises a cover glass and a polydimethylsiloxane gasket; the non-spherical particles used in this example were E.coli; in order to realize the control and measurement of the movement of the escherichia coli, the escherichia coli is fixed on a cover glass through electrostatic force, and a sample cell is fixed on an electric displacement table; recording and storing a hologram and a background image of escherichia coli at an off-focus distance (d) of 10-40 μm by controlling an electric displacement table, wherein the background image is obtained at a sample blank consistent with the off-focus distance of the hologram, the step of the displacement table is 1 μm, and 100 frames of holograms are recorded at each height; to eliminate the influence of twinning phases, the holograms collected in the experiment were all below 10 μm in the focal plane, i.e. d0=10μm,0≤d-d0≤30μm;
The second step is that: background subtraction is carried out on the original holographic images at different positions through the background image, background noise is eliminated, and the light intensity value I of each pixel point in the hologram after the background subtraction is carried outs(x,y):
Is(x,y)=It(x,y)-Ib(x,y);
The third step: carrying out numerical reconstruction on the background-subtracted holographic image by using a Rayleigh-Sommerfeld algorithm to obtain the intensity distribution of the escherichia coli in a three-dimensional space, namely obtaining the information of the three-dimensional reconstruction intensity distribution of the escherichia coli, wherein the calculation formula is as follows:
U(r,z)=FT-1(FT(Is(r,0)·H(q,-z))),
wherein h (r, -z) is a propagation operator, r is an initial transverse coordinate of the particle, and z is an initial axial coordinate of the particle; i is an imaginary unit; k is the wave number; r is the light propagation distance; i issIs the light intensity of the sample; FT-1Is inverse Fourier transform; FT is Fourier transform; h (q, -z) is the Fourier transform of H (r, -z);
the reconstruction axial interval is (-60-0 μm), is larger than the maximum defocusing distance (40 μm below the focal plane) of the holographic image in the experiment, and is stepped to the pixel size of 0.1625 μm.
The fourth step: filtering the information of the three-dimensional reconstruction intensity distribution of the escherichia coli obtained by reconstructing the light field through a threshold value and searching a local maximum value of light intensity to obtain an initial three-dimensional position of the escherichia coli; firstly setting a light intensity threshold value which is 7 percent, filtering noise in a reconstructed light field, and then searching and recording the three-dimensional position of the local light intensity maximum value point by point in a cube with the side length close to the integral value of the particle size, namely the cube with the side length of 13 mu m;
the fifth step: according to the information of the three-dimensional reconstructed intensity distribution of the escherichia coli and the initial three-dimensional position of the escherichia coli, selecting rectangular areas with the side length of 1.625 mu m on the sections corresponding to different heights by taking the position as the center, and superposing the light intensities of all pixel points in the rectangular areas to obtain the total light intensity I of the particles at different heightsintTo obtain the total intensity of light intensity IintA profile in the axial direction, as shown in fig. 2;
and a sixth step: using Gaussian distribution to total intensity of light IintFitting the distribution curve in the axial direction to obtain a central peak position, namely the axial position of the escherichia coli;
the fitting process is carried out by a fitting formula:
Iint=aexp(-(z-zc)2/b),
wherein, IintIs the sum of the light intensity in the rectangular area; a is peak height, b is peak width, zcAs the coordinates of the central peak;
the axial positioning accuracy at each height is equal to the root-mean-square value of the error between the measured value (z) and the theoretical value (d), as shown in fig. 3, the weighted average (98nm) of the axial positioning accuracy at each height is the axial positioning accuracy of the algorithm to escherichia coli.
To further test the practicability of the algorithm, the algorithm was applied to the positioning of the axial positions of escherichia coli in different orientations in the solution, and fig. 4 and 5 are schematic diagrams of the axial light intensity distribution of escherichia coli perpendicular and parallel to the bottom surface and the corresponding gaussian fitting results, respectively.
The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.
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