CN111882624B - Nanometer CT image motion artifact correction method and device based on multiple acquisition sequences - Google Patents
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Abstract
The invention belongs to the technical field of nanometer CT projection image correction, and particularly relates to a method and a device for correcting motion artifact of a nanometer CT image based on a multi-acquisition sequence, wherein the method comprises the steps of setting the number of circle CT image acquisition and the number of projection image acquired in each circle, and carrying out circle CT image acquisition by using the parameters; reading a temperature change curve of a stage monitoring point in a temperature detection system, and selecting a standard circular track CT projection sequence; preprocessing all the collected projection images; calculating the position deviation matrixes of other circular track CT projection sequences and standard circular track CT projection sequences under the same angle; correcting motion artifact of other circular track CT projection sequences according to the obtained position deviation matrix; and carrying out multi-frame accumulation on the CT images subjected to deviation correction under the corresponding angles to obtain the CT projection images finally used for three-dimensional reconstruction. The invention can effectively correct CT image motion artifact and improve the signal-to-noise ratio of the nanometer CT image.
Description
Technical Field
The invention belongs to the technical field of nano CT projection image correction, and particularly relates to a nano CT image motion artifact correction method and device based on a multi-acquisition sequence.
Background
The nanometer Cone Beam CT (CBCT) system mainly comprises an X-ray source, a flat panel detector, a precision machine and a computer, can obtain three-dimensional structure information of the interior of an object under the conditions of non-contact and non-damage, and is widely applied to the fields of nondestructive testing, medical diagnosis, bionic materials and the like. The resolution of cone beam nano CT imaging depends on the relative positions of the sample and the detector, and theoretically, only the position change is smaller than the target resolution of the whole system, so that projection data which can be directly reconstructed in three dimensions can be obtained.
The mechanical platform of nano CT is mostly made of aluminum material, and the aluminum material is very sensitive to temperature change, and when the temperature is increased from 25 ℃ to 26.6 ℃, a 100mm aluminum block is enlarged by 2.375um. The magnification of nano-CT is typically about tens to hundreds times, so that every 0.1 ℃ temperature change will bring about a change in the position of several or tens of pixels projected by the sample. Temperature changes during projection image acquisition of up to tens of hours or even tens of hours can cause the projections to deviate from position, affecting the final three-dimensional reconstruction result. The system is integrated with a constant temperature system to reduce the influence of the environmental temperature change on the system, but because the internal space of the system is large, the temperature fluctuation range in the system is known to be +/-2 ℃ according to the temperature change curve of the temperature detection system. It is therefore necessary to design a suitable motion artifact correction method to correct the acquired image.
The focal spot position of the light source used in nano-CT is not fixed, and in long-time projection acquisition, the focal spot of the light source fluctuates by ±100nm, as shown in fig. 1. The low energy characteristic of the nano CT light source ensures the small focal spot size (namely light beam concentration) of the light source, is favorable for improving the photon number in unit area and obtaining higher spatial resolution, but also brings the problems of contrast reduction, noise increase, image quality reduction and the like. In order to obtain a projection with a high signal-to-noise ratio, the exposure time of the imaging can only be increased. But both the fluctuation of the position of the focal spot itself and the geometrical deviation of the imaging with temperature changes become larger with increasing exposure time. It is therefore desirable to reduce geometric variations using suitable nano-CT image acquisition methods.
Disclosure of Invention
In order to solve the problem of CT image position deviation caused by temperature change, mechanical jitter and focal spot drift, the invention provides a nano CT image motion artifact correction method and device based on a multi-acquisition sequence, which can effectively correct CT image motion artifact and improve the signal to noise ratio of a nano CT image.
In order to solve the technical problems, the invention adopts the following technical scheme:
the invention provides a nano CT image motion artifact correction method based on a multi-acquisition sequence, which comprises the following steps:
step 1, setting the number of circles of circle track CT image acquisition and the number of projection images acquired in each circle, and acquiring the circle track CT image by using the parameters;
step 2, reading a temperature change curve of a stage monitoring point in a temperature detection system, and selecting a standard circular track CT projection sequence;
step 3, preprocessing all the collected projection images;
step 4, calculating the position deviation matrixes of other circular track CT projection sequences and standard circular track CT projection sequences under the same angle;
step 5, correcting motion artifact of other circular track CT projection sequences according to the obtained position deviation matrix;
and 6, carrying out multi-frame accumulation on the CT images subjected to deviation correction under the corresponding angles to obtain the CT projection images finally used for three-dimensional reconstruction.
Further, before step 1, the method further includes:
and acquiring a plurality of projection images with different exposure time, evaluating whether a sample to be detected is penetrated by X-rays or not and the signal to noise ratio factor of the projection images, selecting proper exposure time according to the factors, and acquiring corresponding bright field data.
Further, the nano CT continuously performs CT image acquisition of K circular tracks, and the number of projection images acquired by each circular track and the exposure time are different.
Further, in step 2, a circular track CT projection sequence within a period corresponding to the minimum temperature change is selected from the read temperature change curves as a standard circular track CT projection sequence.
Further, preprocessing all the acquired projection images, including:
and carrying out gain correction on the projection image data according to the collected bright field data, and filtering out salt and pepper noise of the projection image data.
Further, the acquired images of the other circular track CT projection sequences and the acquired images of the standard circular track CT projection sequences are similar to only translational change under the same angle, and the position deviation is calculated through a sub-pixel image registration algorithm based on Fourier transformation, so that a position deviation matrix is obtained.
Further, taking the circular track CT projection sequence with the largest number of acquired images as a reference, and carrying out multi-frame accumulation on projections under the same angle to obtain a CT projection image finally used for three-dimensional reconstruction.
The invention also provides a nano CT image motion artifact correction device based on the multi-acquisition sequence, which comprises:
the circular track CT image acquisition module is used for setting the number of circles of circular track CT image acquisition and the number of projection images acquired in each circle, and acquiring the circular track CT image according to the parameters;
the standard circular track CT projection sequence selection module is used for reading a temperature change curve of a monitoring point of the objective table in the temperature detection system and selecting a standard circular track CT projection sequence;
the preprocessing module is used for preprocessing all the acquired projection images;
the position deviation calculation module is used for calculating the position deviation matrix of the CT projection sequences of other circular tracks and the CT projection sequences of standard circular tracks under the same angle;
the motion artifact correction module is used for carrying out motion artifact correction on other circular track CT projection sequences according to the obtained position deviation matrix;
and the three-dimensional reconstruction CT projection image enhancement module is used for carrying out multi-frame accumulation on the CT images subjected to deviation correction under the corresponding angles to obtain the CT projection images finally used for three-dimensional reconstruction.
Compared with the prior art, the invention has the following advantages:
the invention can effectively reduce the requirements on temperature control equipment and experimental environment, and can obtain clear three-dimensional reconstruction results under the condition of not changing the existing CT equipment and external environment variables. The geometric dimension of the sample to be detected of the nano CT is generally smaller than 1 millimeter, so that the marker is very difficult to manufacture on the sample, and the method does not need to manufacture the marker on the sample, thereby effectively reducing the experiment difficulty.
The projection images of the multi-circle circular tracks are collected, sub-pixel position deviation correction is carried out, multi-frame accumulation is carried out on the projection images subjected to the deviation correction under the same angle, the actual positions of images can be quickly recovered, motion artifacts are effectively corrected, meanwhile, the signal to noise ratio of CT images is improved, the position drift of the CT images caused by temperature change and focal spot drift in long-time image collection is avoided, and the definition is reduced.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a source focal spot position fluctuation map;
FIG. 2 is a flow chart of a method for correcting motion artifacts of a nano CT image based on a multi-acquisition sequence according to an embodiment of the present invention;
FIG. 3 is a schematic view of a kth circle trace acquisition in accordance with an embodiment of the present invention;
FIG. 4 is a graph of photon threshold discrimination counts in accordance with an embodiment of the present invention;
FIG. 5 is a schematic diagram illustrating motion artifact correction for a kth circular trajectory CT projection sequence according to an embodiment of the present invention;
fig. 6 is a graph comparing a reconstructed slice (left) without offset correction and a reconstructed slice (right) after offset correction according to an embodiment of the present invention.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments, and all other embodiments obtained by those skilled in the art without making any inventive effort based on the embodiments of the present invention are within the scope of protection of the present invention.
As shown in fig. 2, the method for correcting motion artifacts of a nano CT image based on a multi-acquisition sequence of the present embodiment includes the following steps:
step S101, collecting a plurality of projection images with different exposure time, evaluating whether a sample to be detected is penetrated by X-rays or not and a signal to noise ratio factor of the projection images, selecting proper exposure time according to the signal to noise ratio factor and collecting corresponding bright field data;
step S102, setting the number of circles of circle CT image acquisition and the number of projection images acquired in each circle, and acquiring the circle CT image by using the parameters;
in the embodiment, the nano CT continuously performs CT image acquisition of K circular tracks, and the number of projection images acquired by each circular track and the exposure time are different.
Step S103, reading a temperature change curve of a stage monitoring point in the temperature detection system, and selecting a circular track CT projection sequence within a period corresponding to the minimum temperature change as a standard circular track CT projection sequence;
step S104, preprocessing all the acquired projection images, including:
and carrying out gain correction on the projection image data according to the collected bright field data, and filtering out salt and pepper noise of the projection image data.
Step S105, calculating the position deviation matrix of the CT projection sequences of other circular tracks and the CT projection sequences of standard circular tracks under the same angle;
in this example, the acquired images of the other circular track CT projection sequences and the acquired images of the standard circular track CT projection sequences are similar to only translational changes under the same angle, and the position deviation is calculated by a fourier transform-based sub-pixel image registration algorithm, so as to obtain a position deviation matrix.
Step S106, correcting motion artifact of other circular track CT projection sequences according to the obtained position deviation matrix;
and S107, taking the CT projection sequence of the circular track with the maximum number of acquired images as a reference, and carrying out multi-frame accumulation on the CT images subjected to deviation correction under the corresponding angles to obtain the CT projection images finally used for three-dimensional reconstruction.
The following list a specific example in order to better explain the present invention.
The method for correcting the motion artifact of the nano CT image based on the multi-acquisition sequence of the embodiment comprises the following steps:
step S201, the penetration depth of monochromatic X-rays for different wavelengths can be expressed as:
middle l 0 For attenuation of X-rays from initial intensity to e -1 Thickness of sample passing through at multiple times, mu m Represents the absorption coefficient of the substance, and ρ is the density of the substance. The nano CT light source emits multi-energy X-rays, and the X-ray intensity I transmitted through the sample to be detected is as follows:
wherein I is the intensity of transmitted X-rays, I 0 For the intensity of incident X-rays, S (E) is the spectral distribution of the rays, μ (E) is the attenuation coefficient corresponding to the material, and L is the thickness of the transmitted sample.
In practical applications, the attenuation coefficient of the sample to be detected cannot be estimated, and whether the sample is penetrated is generally estimated according to the formula (3), and the voltage, current and exposure time of the light source can be adjusted according to the estimated attenuation coefficient. Let t be the exposure time 0 The contrast and signal-to-noise ratio of the sample projection are at the minimum of the acceptable range.In order to obtain three-dimensional volume data with good spatial resolution and density resolution in an experiment, a corresponding increase in exposure time is required, provided that the exposure time employed in the experiment is t= (t 1 t 2 … t K ),(t k >t 0 ). After the exposure time is determined, the J Zhang Liangchang projection is acquired for the corresponding time.
P in the formula minval P being the minimum gray value in the projected image maxval Is the gray value in the projected image at the initial ray intensity.
In step S2021, as shown in fig. 1, the focal spot position of the nano CT light source fluctuates greatly in the initial experiment, and the shielding box needs to be opened when the sample to be detected is fixed, which affects the temperature balance in the shielding box to a certain extent, so that the first circular track CT projection sequence is not used as the standard sequence. The temperature change in the shielded box is small in a relatively short time, so the number of acquired projection images and the exposure time should be properly reduced when setting the circular trajectory CT projection sequence to be calibrated.
Step S2022, setting nano CT to continuously perform K circle CT image acquisitions, as shown in FIG. 3, wherein the ith projection image acquisition angle of the kth circle trackThe method comprises the following steps:
m, N in k Is a positive integer, M represents the number of angles acquired by the kth circular track, N k Represents the projection number, theta, under each acquisition angle m ＝mθ，△θ＝θ/N k θ=360°/M. The number of acquired k-th circular tracks is M k The exposure time of the single projection is t k After experimental parameter setting, circular track CT image is carried outAnd (5) image acquisition.
Step S203, after the projection image data acquisition is completed, the temperature change curve of the objective table monitoring point in the temperature detection system is read, and the circular track CT projection sequence in the period with the minimum temperature change is selected as the standard circular track CT projection sequence, namely the kth circular track CT projection sequence is the standard sequence.
Step S204, after completing data acquisition, preprocessing all acquired projection data, and selecting different preprocessing methods according to the types of the detectors, where the embodiment uses photon counting detectors as an example for explanation.
In step S2041, each pixel unit pair of the photon counting detector has a certain energy photon response, and generates a pulse signal, and noise generated by low energy photons can be reduced by setting an appropriate energy threshold, as shown in fig. 4.
In step S2042, in the actual production of the photon counting detector, the performance parameters of all the probe elements cannot be completely consistent, so that the gray values of the images obtained by the photon counting detector are not the same even under the same X-ray irradiation. To make the response of each probe element uniform, reducing the inherent noise in the image, correcting all projection data using equation (5);
wherein f 0 (x, y) represents the gray value of the pixel point (x, y) in the original projection image, f flat (x, y) represents the gray value of the pixel (x, y) in the bright field projection image, R (x, y) represents the average value of the gray values of the pixel (x, y) in the bright field projection image, R 0 Is the median value of R (x, y), and f (x, y) represents the gray value of the pixel point (x, y) in the corrected projection image.
Step S2043, performing dead point correction and median filtering on the corrected projection image.
In step S205, a sub-pixel image registration algorithm is used to calculate the position deviation between the projection image of the kth circular track CT projection sequence and the projection image of the kth' standard circular track CT projection sequence. The acquisition angle in the standard acquisition sequence isIs f i (x, y), the projection image of the kth circular track acquisition sequence under the same angle is f i k (x, y), then there is:
fourier transforming equation (7) yields:
calculating a normalized cross power spectrum:
f in the formula i * (u, v) is F i Complex conjugate of (u, v). Inverse transforming the cross-power spectrum can result in a pulse function:
thereby obtaining positional deviationAnd the position deviation matrix under all acquisition angles can be obtained by analogy.
Step S206, performing motion artifact correction on other circular track CT projection sequences by the position deviation matrix obtained in step S205, wherein as shown in FIG. 5, the nearest neighbor principle is adopted for the kth circular track circle, and the motion artifact correction is performed on the position deviation matrix at θ m And theta m+1 Projection image acquired at an acquisition angle therebetween using θ m Angular positional deviationMotion artifact correction is performed.
In step S207, when the projection image is used for three-dimensional reconstruction, the circular track CT projection sequence with the largest acquired projection image is generally selected, i.e., the circular track CT projection sequence with the largest M is selected. In order to make full use of other projection data, a multi-frame noise reduction mode can be used for eliminating random noise, and the signal to noise ratio of a projection image to be reconstructed is improved. Assuming that there are L projection images at a certain acquisition angle, the first projection image can be expressed as:
f(x,y)＝g l (x,y)+n l (x,y) (11)
g in l (x, y) is a noiseless projection image g at the same angle l (x, y) are equal, n l (x, y) is gaussian noise, and after superposition averaging can be expressed as:
and after the multi-frame noise reduction is completed, reading the projection image to perform three-dimensional reconstruction to obtain the volume data of the sample to be detected.
The effectiveness of the method is verified by specific experiments, and the relevant parameters are shown in table 1.
Table 1 experimental parameters
The slice of the sample reconstruction result without bias correction for the 3 rd projection is shown in the left graph of fig. 6, and obvious geometric artifacts can be seen, and the detailed information is blurred. The slice of the sample reconstruction result after the motion artifact correction by using the 3 rd circle CT image is shown in the right graph of FIG. 6, has no geometric artifact, and the sample detail is clear and distinguishable. The nano CT image motion artifact correction method based on the multi-acquisition sequence can effectively correct CT image motion artifacts and improve the signal-to-noise ratio of nano CT images.
The embodiment also provides a nano CT image motion artifact correction device based on the multi-acquisition sequence, which comprises a circular track CT image acquisition module, a standard circular track CT projection sequence selection module, a preprocessing module, a position deviation calculation module, a motion artifact correction module and a three-dimensional reconstruction CT projection image enhancement module.
The circular track CT image acquisition module is used for setting the number of circles of circular track CT image acquisition and the number of projection images acquired in each circle, and acquiring the circular track CT image according to the parameters;
the standard circular track CT projection sequence selection module is used for reading a temperature change curve of a monitoring point of the objective table in the temperature detection system and selecting a standard circular track CT projection sequence;
the preprocessing module is used for preprocessing all the acquired projection images;
the position deviation calculation module is used for calculating the position deviation matrix of the CT projection sequences of other circular tracks and the CT projection sequences of standard circular tracks under the same angle;
the motion artifact correction module is used for carrying out motion artifact correction on other circular track CT projection sequences according to the obtained position deviation matrix;
and the three-dimensional reconstruction CT projection image enhancement module is used for carrying out multi-frame accumulation on the CT images subjected to deviation correction under the corresponding angles to obtain the CT projection images finally used for three-dimensional reconstruction.
It should be noted that, in this document, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
Those of ordinary skill in the art will appreciate that: all or part of the steps for implementing the above method embodiments may be implemented by hardware related to program instructions, and the foregoing program may be stored in a computer readable storage medium, where the program, when executed, performs steps including the above method embodiments; and the aforementioned storage medium includes: various media in which program code may be stored, such as ROM, RAM, magnetic or optical disks.
Finally, it should be noted that: the foregoing description is only illustrative of the preferred embodiments of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.
Claims (4)
1. The nanometer CT image motion artifact correction method based on the multi-acquisition sequence is characterized by comprising the following steps of:
step 1, collecting a plurality of projection images with different exposure time, evaluating whether a sample to be detected is penetrated by X-rays or not and the signal to noise ratio factor of the projection images, selecting proper exposure time according to the factors and collecting corresponding bright field data;
step 2, setting the number of circles of circle track CT image acquisition and the number of projection images acquired in each circle, and acquiring the circle track CT image by using the parameters; nanometer CT continuous processKThe CT image acquisition of the secondary circular track ring, wherein the number of projection images acquired by each circular track and the exposure time are different;
step 3, reading a temperature change curve of a stage monitoring point in the temperature detection system, and selecting a standard circular track CT projection sequence;
step 4, preprocessing all the acquired projection images, including: gain correction is carried out on projection image data according to the collected bright field data, and salt and pepper noise of the projection image data is filtered;
step 5, calculating a position deviation matrix of other circular track CT projection sequences and standard circular track CT projection sequences under the same angle, including: the acquired images of other circular track CT projection sequences and the acquired images of the standard circular track CT projection sequences are similar to only translational change under the same angle, and position deviation is calculated through a sub-pixel image registration algorithm based on Fourier transformation to obtain a position deviation matrix;
step 6, correcting motion artifact of other circular track CT projection sequences according to the obtained position deviation matrix;
and 7, carrying out multi-frame accumulation on the CT images subjected to deviation correction under the corresponding angles to obtain the CT projection images finally used for three-dimensional reconstruction.
2. The method for correcting motion artifacts of a nano CT image based on a multi-acquisition sequence according to claim 1, wherein in step 3, a circular trajectory CT projection sequence within a period corresponding to a minimum temperature change is selected from the read temperature change curves as a standard circular trajectory CT projection sequence.
3. The method for correcting motion artifact of nano CT image based on multi-acquisition sequence according to claim 1, wherein the CT projection sequence with the largest number of acquired circular tracks is used as a reference, and projections under the same angle are accumulated for multiple frames to obtain a CT projection image finally used for three-dimensional reconstruction.
4. A nano CT image motion artifact correction device based on a multi-acquisition sequence, comprising:
the projection image acquisition module is used for acquiring a plurality of projection images with different exposure time, evaluating whether a sample to be detected is penetrated by X rays or not and the signal to noise ratio factor of the projection images, selecting proper exposure time according to the factors and acquiring corresponding bright field data;
the circular track CT image acquisition module is used for setting the number of circles of circular track CT image acquisition and the number of projection images acquired in each circle, and acquiring the circular track CT image according to the parameters; nanometer CT continuous processKThe CT image acquisition of the secondary circular track ring, wherein the number of projection images acquired by each circular track and the exposure time are different;
the standard circular track CT projection sequence selection module is used for reading a temperature change curve of a monitoring point of the objective table in the temperature detection system and selecting a standard circular track CT projection sequence;
the preprocessing module is used for preprocessing all the acquired projection images and comprises the following steps: gain correction is carried out on projection image data according to the collected bright field data, and salt and pepper noise of the projection image data is filtered;
the position deviation calculation module is used for calculating a position deviation matrix of other circular track CT projection sequences and standard circular track CT projection sequences under the same angle, and comprises the following steps: the acquired images of other circular track CT projection sequences and the acquired images of the standard circular track CT projection sequences are similar to only translational change under the same angle, and position deviation is calculated through a sub-pixel image registration algorithm based on Fourier transformation to obtain a position deviation matrix;
the motion artifact correction module is used for carrying out motion artifact correction on other circular track CT projection sequences according to the obtained position deviation matrix;
and the three-dimensional reconstruction CT projection image enhancement module is used for carrying out multi-frame accumulation on the CT images subjected to deviation correction under the corresponding angles to obtain the CT projection images finally used for three-dimensional reconstruction.
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