Microscopic imaging method using a correction factor

Abstract

Microscopic imaging procedure with the steps: Illuminating a sample (1) with an illuminating radiation and detecting detection radiation caused by the illuminating radiation along a detection axis, at a first time as a wide-field signal (WF) and at a second time as a composite signal (CI) formed from a superposition of a confocal signal and a wide-field signal (WF), Extracting the confocal signal by subtracting the wide-field signal (WF) from the composite signal (CI), where a correction factor is used, characterized in that a current correction factor is determined for each image taken and / or for each sample imaged (1) the extraction of the confocal signal is carried out using the current correction factor and that to determine the current correction factor in a correction plane, which is selected along the detection axis at a distance from a surface (Ao) of the sample (1), at least one correction wide-field image and one correction composite image are acquired, the image data of which are determined, and the current correction factor is determined on the basis of the determined image data; wherein the distance of the correction plane is selected to be so large that no structures of the surface (Ao) penetrate the correction plane, where the distance is selected from a range of four to six half-widths of the point spread function of the lens used to capture the detection radiation.

Description

[0001] The invention relates to a microscopic imaging method according to the preamble of the independent claim.

[0002] In the technical field of microscopy, for example, the high-resolution three-dimensional imaging of biological / technical samples or sample regions, the challenge is to expose the sample to the lowest possible intensities of illumination radiation while simultaneously providing high intensities of detection radiation for high-resolution detection and imaging. At the same time, the image data and the final imaging should be provided as quickly as possible, ideally in real time.

[0003] The approach of correlative spinning disk microscopy is known from DE 10 2014 004 249 A1. In a method disclosed therein for determining the topography of a sample using correlative spinning disk microscopy, the following steps are performed: A stage and / or a focus drive are moved vertically while alternatingly acquiring first and second images of a sample placed on the stage. A vertical focus position is stored as metadata for each image. Two first or two second images are interpolated in a further step, resulting in an intermediate image. The intermediate image is combined with the second or first image at that position to generate a confocal image for a specific vertical position.

[0004] The use of suitable masks in the illumination and / or detection beam path to generate composite images and non-confocal images is known from WO 97 / 31282 A1 and DE 100 14 331 A1. A confocal image is extracted from these two image types by appropriate calculation. This enables very fast imaging. At the same time, a high proportion of the detection light is utilized, so it is sufficient to illuminate the sample with low-intensity illumination radiation.

[0005] The publications by Neil et al. (Neil, MA A et al. (1997), A light efficient optically sectioning microscope; Journal of Microscopy 189: 114 - 117) and Wilson et al. (Wilson, T. et al. (1996), Confocal microscopy by aperture correlation; OPTICS LETTERS 21: 1879 - 1881) also suggest ways to extract confocal signals from composite images and wide-field images.

[0006] The following formula can be used to calculate the confocal image: Confocal signal=composite signal−n*widefield signal. The factor n is a correction factor that is determined once and sample-independently for an imaging system and applied unchanged. The correction factor n compensates for system-specific deviations, such as different transmission values ​​of the optical paths of the composite signal and the wide-field signal. This approach can be used, for example, to acquire stacks of images (z-stacks) along a detection axis, also referred to as the z-axis, and combine them to create a three-dimensional image, for example, a topography of the sample.

[0007] The invention is based on the object of proposing an improved possibility of three-dimensional imaging, in particular using low illumination intensities.

[0008] The object is achieved by a microscopic imaging method according to claim 1. Advantageous further developments can be found in the dependent claims.

[0009] The microscopic imaging method comprises the step of illuminating a sample with illumination radiation and detecting detection radiation caused by the illumination radiation along a detection axis. At a first time, the detection radiation is detected as a wide-field signal and, at a second time, as a composite signal formed by superimposing a confocal image, or rather its signals, and a wide-field signal. A confocal image is obtained by subtracting the wide-field image from the composite image, using a correction factor. The wide-field image signals are subtracted from the composite signals, taking the correction factor into account.

[0010] The method is characterized by determining a current correction factor for each image taken and / or each imaged sample. The extraction of the respective confocal image is performed using the current correction factor.

[0011] Surprisingly, it has been shown that a one-time determination of a correction factor for the imaging system is completely sufficient for the originally intended applications, but that the method according to the invention not only reduces imaging errors but also allows new classes of materials to be investigated.

[0012] The core of the invention is therefore to break away from the sample-independent correction factor and to determine it dynamically for each measurement or each sample.

[0013] The sample surface is detected by the analysis algorithm based on a recorded intensity maximum of the detection radiation. Under certain circumstances, for example, in the presence of high contrast in the sample and / or autofluorescence of the sample to be imaged, the ratio of the intensities between the wide-field image and the composite image, or between their signals, can change. As a result of a change in the intensity ratio of the wide-field image and the composite image, an intensity maximum can be erroneously detected above or below the actual surface of the sample, or the position of the surface can be incorrectly calculated. In this case, the erroneously recorded intensity is so high that it is not recognized as artifacts and therefore cannot be eliminated, for example, using a noise-cut filter. As a result, depressions or elevations on the sample are calculated that do not actually exist.

[0014] To analyze the errors described above, calculated images were stacked along the z-axis (z-stacks) and these z-stacks were viewed from the side (so-called orthoview). The distribution of the intensities along the z-axis was evaluated. It was recognized that scattered light can be detected over a relatively wide z-range (see Fig. 1) is fairly evenly or consistently distributed.

[0015] Based on this finding, options for determining the current correction factor are proposed, which are explained in more detail below.

[0016] In the method according to the invention, at least one correction wide-field image and one correction composite image are acquired to determine the current correction factor in a correction plane selected along the detection axis at a distance from a sample surface. The image data (signals) of the correction wide-field images and the correction composite images, also referred to as correction images, are determined, and a current correction factor is determined, for example, calculated or derived, based on the determined image data.

[0017] To acquire image data, a focus of the detection lens or detection system is directed to the expected position (expected z-position) of the surface, thus defining a focal plane. The distance is the distance between the focal plane and the correction plane.

[0018] According to the invention, the distance of the correction plane is chosen so large that no surface structures penetrate the correction plane. Thus, the distance can be selected from a range between four and six half-widths of the point spread function of the detection lens or detection system used to capture the detection radiation. The distance can be, for example, four, four and a half, five, or six half-widths.

[0019] For each measurement, a correction wide-field image and a correction composite image are generated and captured at a sufficient distance from the surface. From these two correction images, the sample-specific and current correction factor is then determined, which is then inserted into the formula given above.

[0020] Variations in the distance of the correction plane from the sample surface are possible within a certain range, since the scattered light component is distributed relatively consistently along the z-axis. Variations in the generation of the composite image are also possible, for example, by using a slit mask. It is also possible to use a spinning disk with a poorly matched pinhole size, a microdisplay with a poorly matched pinhole size, or a laser scanning microscope (LSM) with a poorly matched pinhole size.

[0021] There are several ways to determine the current correction factor. This can be determined from the average brightness of the correction images. The gray values ​​of each correction image are averaged, and the calculated average is used.

[0022] Instead of simply arithmetic averaging the gray values, the images can also be evaluated pixel by pixel. Each image element or pixel can be assigned its own current correction factor.

[0023] Since the pixel-by-pixel generation of a current correction factor and its use in image generation can lead to discontinuities and thus artifacts, suitable computational filters, such as median filters or Gaussian filters, are subsequently applied to the image data to smooth them if necessary. The current correction factor is thus determined for each pair of image elements in the correction images. Each pair of image elements is formed by a specific image element of the correction wide-field image and the corresponding image element of the correction composite image.

[0024] A further advantageous embodiment of the method consists in the fact that the two correction images are acquired both above and below the focal plane with the expected position of the sample surface—i.e., in a correction plane in front of and behind a current focal plane—and both pairs of correction images are incorporated into the calculation. This embodiment is particularly useful for sufficiently transparent samples.

[0025] To carry out the method according to the invention, a recording workflow is advantageous which includes the recording of at least two correction images for determining the current correction factor.

[0026] In a further embodiment, the corrected wide-field image is calculated from the z-stack and used to determine the correction factor. For example, an average is calculated using three-dimensional data from the z-stack.

[0027] The advantages of the invention include the elimination of the one-time calibration of the sample-independent correction factor during commissioning. Furthermore, due to the significantly improved signal quality, filtering steps during evaluation are no longer required, which are used in prior art methods to remove random or systematic positive and / or negative signal peaks (outliers).

[0028] A further advantage of the method according to the invention is that even with the classical aperture correlation, an adapted correction factor can be determined automatically, which significantly improves the user-friendliness and the quality of the image data.

[0029] The method can be used particularly advantageously with image acquisition devices in which wide-field images and composite images are acquired separately and successively and, in particular, are stored and processed as a batch image.

[0030] The method according to the invention can be used particularly advantageously for imaging the topographies of sample surfaces. The method is particularly applicable in the field of materials testing and quality control. For example, machined metallic surfaces can be imaged with significantly improved quality. Reflections and / or sharp transitions on the surface lead to signal enhancements only to a minimal extent or not at all with the method according to the invention.

[0031] The use of the method according to the invention opens up the possibility of examining highly reflective samples such as painted samples, light-sensitive samples such as biological samples and autofluorescent samples such as many types of paper.

[0032] The invention is explained in more detail below with reference to illustrations and exemplary embodiments. They show: Fig. 1 a schematic representation of a section through a z-stack (Orthoview); Fig. 2 a schematic representation of a first possibility for determining a current correction factor; Fig. 3 a schematic representation of a surface of a sample with selected signal curves according to the prior art; Fig. 4 a schematic representation of the surface of the sample with selected signal curves according to the method according to the invention; Fig. 5 is a diagram of signal waveforms along a z-stack of a machined metal surface according to the prior art; Fig. 6 is a diagram of signal curves along the z-stack of the machined metal surface according to the method of the invention; Fig. 7 is a diagram of signal curves along a z-stack of a metallic lacquered surface of a sample according to the prior art; Fig. 8 shows a diagram of signal curves along the z-stack of the surface of the sample provided with metallic paint according to the method of the invention; Fig. 9 is a diagram of signal curves along a z-stack of a surface of a paper sample according to the prior art; and Fig. 10 a diagram of signal curves along the z-stack of the surface of the paper sample according to the method of the invention.

[0033] The Fig. 1 shows an example of a so-called orthoview of a z-stack of images of a sample 1. The images were each calculated using the formula Confocal signal = composite signal * widefield signal calculated. The position of surface Ao of sample 1 is indicated by an arrow and a dashed line. Furthermore, the intensity maxima are plotted along the x-axis. The x-axis is orthogonal to the z-axis and somewhat parallel to surface Ao.

[0034] The surface Ao is characterized by strong reflections and associated intensity maxima at approximately the same z-coordinate over large sections of the x-axis. In a section in the left part of the curve Imax, the intensity maxima are found at higher z-coordinates. This may mean that in this area of ​​sample 1, which corresponds to the respective x-coordinates, there is a depression or a protrusion, depending on how the coordinate system is configured in the Fig. 1 was laid in advance.

[0035] One way to determine the current correction factor n is shown schematically in Fig. 2. The amplitude values ​​of the wide-field signal WF and the composite signal CI (composite image) are plotted as examples along the z-axis. A maximum (peak) of the amplitude values ​​of the composite signal CI can be seen in one focal plane. This intensity maximum Imax characterizes, for example, the surface Ao of sample 1 (see, for example, Fig. 1). In a correction plane with the z-coordinate zn, the amplitude values ​​xWF of the wide-field signal WF and the amplitude values ​​xCI of the composite signal CI are determined. The current correction factor n can be determined from the ratio of the amplitude values ​​xWF and the amplitude values ​​xCI.

[0036] The Fig. Figure 3 schematically shows the measurement results for a sample 1 having a groove-shaped depression 2 traversed on its surface Ao. The image data calculated using a method according to the prior art show high intensity peaks Imax along the edges of the depression 2. As a result, a raised edge of the depression 2 is depicted in a resulting image, although such a raised edge does not actually exist.

[0037] In contrast, the intensity peaks Imax along the edges of the depression 2 are significantly lower when the image data were determined using the method according to the invention ( Fig. 4). Depression 2 is shown with no or only a very small raised rim, which corresponds to the actual topography of sample 1.

[0038] In the further Fig. In Figures 5 to 10, the image data obtained by the method according to the invention are plotted as height values ​​along the z-axis over the x-axis. Instead of plotting the height values ​​along the x-axis, the height values ​​could also be plotted over the y-axis (not shown), which is orthogonal to the x-axis and the z-axis.

[0039] In the Fig. Figure 5 shows image data of the topography of a sample 1 made of a metallic material. Sample 1 was machined by milling. The individual paths of the milling tool can be identified by the periodic sequence of amplitude values ​​around the zero point.

[0040] The image data of the same sample 1 show intensity peaks Imax in the areas of periodic direction changes, which do not reflect the actual profile of the surface Ao ( Fig. 6). The fact that the intensity peaks Imax are extremely narrow indicates the presence of imaging errors rather than actual surface shapes.

[0041] The same is true in the Fig. 7 and Fig. 8. A surface Ao of sample 1 coated with a metallic lacquer is detected and displayed using the method according to the invention with a topography with a fluctuation range of approximately 1.5 to 2 µm around the zero position ( Fig. 7). In contrast, the topography of sample 1, created using a method according to the state of the art, shows fluctuation ranges of approximately 5 to more than 10 µm ( Fig. 8).

[0042] A sample 1 made of paper behaves very similarly. Fig. Figure 9 shows the comparatively rough surface Ao of sample 1 with a fluctuation range of about 10 µm around the zero position.

[0043] In contrast, the erroneously occurring intensity peaks when using a state-of-the-art method lead to fluctuation ranges of 30 to 40 µm around the zero position.

[0044] This exemplary embodiment demonstrates that samples 1 made of paper or with an (auto-)fluorescent surface Ao can be examined for their topography using the method according to the invention. In contrast, methods according to the prior art do not produce satisfactory results or require additional steps to reduce the intensity peaks. Reference symbol 1 sample 2 Deepening Ao surface (of sample 1) CI composite signal Imax Intensity maximum, intensity peak WF wide-field signal xCI amplitude value (of the composite signal CI) xWF amplitude value (of the wide-field signal WF) zn position (of the correction plane)

Claims

[1] Microscopic imaging procedure with the steps: Illuminating a sample (1) with an illuminating radiation and detecting detection radiation caused by the illuminating radiation along a detection axis, at a first time as a wide-field signal (WF) and at a second time as a composite signal (CI) formed from a superposition of a confocal signal and a wide-field signal (WF), Extracting the confocal signal by subtracting the wide-field signal (WF) from the composite signal (CI), where a correction factor is used, characterized by , that a current correction factor is determined for each image taken and / or for each sample imaged (1) the extraction of the confocal signal is carried out using the current correction factor and that to determine the current correction factor in a correction plane, which is selected along the detection axis at a distance from a surface (Ao) of the sample (1), at least one correction wide-field image and one correction composite image are acquired, the image data of which are determined, and the current correction factor is determined on the basis of the determined image data; wherein the distance of the correction plane is selected to be so large that no structures of the surface (Ao) penetrate the correction plane, where the distance is selected from a range of four to six half-widths of the point spread function of the lens used to capture the detection radiation. [2] Method according to claim 1, characterized by that at least one correction wide-field image and one correction composite image are captured in a correction plane in front of and behind a current focal plane. [3] Method according to one of the preceding claims, characterized by that the current correction factor is determined from the average brightnesses of the correction images, correction wide-field image and correction composite image. [4] Method according to one of the preceding claims, characterized by that the current correction factor is determined for each pair of image elements in the correction images.

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