CN111879798B - Nano CT projection position drift correction method and device based on acquisition sequence subdivision - Google Patents
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Abstract
The invention belongs to the technical field of nano CT projection image correction, and particularly relates to a nano CT projection position drift correction method and device based on acquisition sequence subdivisionDegree, collecting M Zhang Touying at each collection angle; calculating the collection angle as theta n The position deviation of M Zhang Touying, correcting the position drift of the projection according to the position deviation, and performing multi-frame accumulation on the corrected CT image; the collection angle is theta 1 The position deviation of the middle M Zhang Touying and the 1 st projection is used as an initial transfer correction value, and the acquisition angle is theta 2 Performing secondary correction on the projection; the initial transmission correction value and the acquisition angle are theta 2 Accumulating the position deviation of the middle M Zhang Touying and the 1 st projection as a transfer correction value to obtain an acquisition angle theta 3 Performing secondary correction on the projection; and sequentially completing the position drift correction of the projections at all the acquisition angles to obtain projection data for three-dimensional reconstruction. The invention can correct the position deviation of the CT image and can improve the signal-to-noise ratio of the nano 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 projection position drift correction method and device based on acquisition sequence subdivision.
Background
X-ray has been developed as one of three major discoveries of human uncovering research microcosmic world frontier, and nano CT technology has been developed in recent decades with the development and progress of various related technologies. As a novel imaging method, the method can nondestructively observe the internal structure of a sample under high resolution, has the advantages of high spatial resolution, high density resolution and the like, and has very wide application prospects in the fields of microelectronic industry, life science, energy and material science, bionics and the like.
The imaging of cone beam nano-CT has high requirements on the mechanical stability and environmental stability of the system. When CT scanning is performed, projection data that can be directly reconstructed three-dimensionally can theoretically be obtained only if the position change is smaller than the target resolution of the entire system. Most of the mechanical platforms of the nano CT are made of aluminum materials, the aluminum materials are very sensitive to temperature change, and when the temperature changes by 1 ℃, the aluminum blocks with the size of 100mm can be enlarged by 1.484um. The height of the nanoct mechanical system used is about 400mm, so the temperature rises by 1 ℃, and the sample held by the mechanical system will rise by 6um. Nanoct typically has a magnification of about several tens to several hundreds, and thus every 0.1 ° temperature change will result in a change of several or dozen or so pixel positions of the sample projection. Therefore, temperature changes during tens of hours or even tens of hours of scanning may cause the projection to deviate from the original position, which affects the final reconstruction result.
The focal spot position of the nano CT light source is not fixed, and the focal spot of the light source fluctuates as shown in fig. 1. Although the low energy characteristic of the nano CT light source ensures a small focal spot size (i.e., beam concentration) of the light source, the photon flux is small, which causes problems of reduced contrast, increased noise, reduced image quality, and the like. To obtain a high signal-to-noise ratio projection, the exposure time of the imaging needs to be increased. But both the positional fluctuations of the focal spot itself and the geometric deviations of the imaging caused by temperature changes become larger with increasing exposure time. Therefore, it is necessary to adopt a proper nano CT image acquisition method to improve the signal-to-noise ratio of the projection image while restoring the original position of the projection image.
Disclosure of Invention
In order to solve the problem of CT projection position drift caused by mechanical deformation, equipment shake and focal spot drift, the invention provides a nano CT projection position drift correction method and device based on acquisition sequence subdivision, which can correct the position deviation of a CT image and improve the signal-to-noise ratio of the nano CT image.
In order to solve the technical problem, the invention adopts the following technical scheme:
the invention provides a nanometer CT projection position drift correction method based on acquisition sequence subdivision, which comprises the following steps:
step 1, collecting a sample projection image at N angles in a circular track mode, and collecting M Zhang Yangpin projection at each collection angle;
step 4, setting the initial transmission correction value and the acquisition angle as theta 2 Accumulating the position deviation of the middle M Zhang Touying and the 1 st projection as a transfer correction value to obtain an acquisition angle theta 3 Performing secondary correction on the projection image after the primary correction;
and 5, performing position drift correction on the projections at all the acquisition angles by analogy to obtain projection data for three-dimensional reconstruction.
Further, before step 1, the method further comprises:
and determining scanning parameters and a threshold value of the photon counting detector according to the sample to be detected, and preprocessing the photon counting detector.
Furthermore, scanning parameters are selected according to the material, density and size factors of the sample to be detected, so that the threshold value of the photon counting detector is determined, and then the photon counting detector is preprocessed.
Furthermore, the projection of N angles is collected in a circular track mode, and the collection angle is theta 1 ,θ 2 ……θ N (ii) a Acquiring M Zhang Yangpin projection under each acquisition angle, wherein the exposure time is t 1 ,t 2 ……t M 。
Further, the step 2 specifically includes:
calculating the acquisition angle theta by adopting a sub-pixel image registration algorithm n M-1 projection and the 1 st projection
N =1,2,3 … N, correcting the position shift of the 2 nd projection to the M projections according to the position deviation value, and performing multi-frame accumulation on the corrected CT images as an acquisition angle theta n The primary corrected projection image of (a).
Further, the collection angle is theta 1 Position deviation of the middle M Zhang Touying and the 1 st projection
Initial transfer correction value delta theta as drift error 2 For collection angle theta 2 The projection image after the primary correction of (2) is subjected to a secondary correction.
Further, the correction value is initially transmitted
At an angle theta to the collection 2 Position deviation of the middle M Zhang Touying and the 1 st projection
Adding up the transfer correction value as a drift error, i.e.
For the collection angle theta 3 The projection image after the primary correction of (2) is subjected to a secondary correction.
The invention also provides a device for correcting the nanometer CT projection position drift based on the collection sequence subdivision, which comprises:
the sample projection acquisition module is used for acquiring a sample projection image at N angles in a circular track mode, and acquiring M Zhang Yangpin projection at each acquisition angle;
a primary correction module for projected image, which calculates the acquisition angle theta by using sub-pixel image registration algorithm n The position deviation of M Zhang Touying, correcting the position drift of the projection according to the position deviation, and performing multi-frame accumulation on the corrected CT image to obtain a projection image after primary correction;
secondary correction module for projected image with collection angle theta 1 The position deviation of the middle M Zhang Touying and the 1 st projection is used as the initial transfer correction value of the drift error, and the collection angle is theta 2 Performing secondary correction on the projection image after the primary correction; the initial transmission correction value and the acquisition angle are theta 2 Accumulating the position deviation of the middle M Zhang Touying and the 1 st projection as a transfer correction value to obtain an acquisition angle theta 3 Performing secondary correction on the projection image after the primary correction;
and the three-dimensional reconstruction projection data acquisition module is used for completing the position drift correction of the projections at all the acquisition angles to obtain projection data for three-dimensional reconstruction.
Compared with the prior art, the invention has the following advantages:
in the existing CT reconstruction process, three-dimensional reconstruction is generally performed by estimating system parameters through a projection image sequence of a marker fixed on a sample or a projection image sequence of the marker acquired again under the same experimental parameters. The geometric dimension of a sample to be detected in nano CT is generally less than 1 millimeter, and the geometric dimension of a marker is dozens or hundreds of micrometers, so that the proper sample and marker are very difficult to manufacture.
Because the photon flux of the nano CT light source is small, the exposure time of a single projection is required to be increased in order to obtain a projection image with higher signal-to-noise ratio, and the definition of the projection image is reduced due to the position drift of the CT image caused by temperature change and focal spot drift in long-time acquisition.
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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 used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
FIG. 1 is a diagram of fluctuations in focal spot position of a light source;
FIG. 2 is a flowchart of a method for correcting a shift of a projection position of a nano CT based on a subdivision of an acquisition sequence according to an embodiment of the present invention;
FIG. 3 is a schematic view of a nano-CT sampling of the subdivision of the acquisition sequence according to an embodiment of the present invention;
FIG. 4 shows an exemplary embodiment of the present invention with a collection angle θ n The projected primary correction schematic;
FIG. 5 shows an exemplary embodiment of the present invention with a collection angle θ 2 The projected secondary correction diagram of (1);
FIG. 6 is a comparison graph of the star card projection (exposure time 200 seconds) and the star card projection (exposure time 20 seconds, corrected for positional deviation and accumulated) according to an embodiment of the present invention;
fig. 7 is a comparison graph of a reconstruction result slice (left) without positional deviation correction and a reconstruction result slice (right) after positional deviation correction according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer and more complete, the technical solutions in the embodiments of the present invention will be described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention, and based on the embodiments of the present invention, all other embodiments obtained by a person of ordinary skill in the art without creative efforts belong to the scope of the present invention.
As shown in fig. 2, the method for correcting the shift of the projection position of the nano CT based on the subdivision of the acquisition sequence of the present embodiment includes the following steps:
s101, selecting scanning parameters according to factors such as the material and the density of a sample to be detected, determining a threshold value of a photon counting detector, and then preprocessing the photon counting detector;
step S102, as shown in FIG. 3, the projection of N angles is collected in a circular track manner, and the collection angle is θ 1 ,θ 2 ……θ N (ii) a Acquiring M Zhang Yangpin projection under each acquisition angle, wherein the exposure time is t 1 ,t 2 ……t M ；
Step S103, as shown in FIG. 4, calculating the collection angle theta by using a sub-pixel image registration algorithm 1 The position deviation of the rear M-1 projection and the 1 st projection is recorded as
Performing position drift correction on the 2 nd projection to the M th projection according to the position deviation value, and performing multi-frame accumulation on the corrected M-1 projections and the 1 st projection as an acquisition angle theta 1 Projection image I of 1 ；
Step S104, the collection angle is theta 1 Position deviation of the middle M Zhang Touying and the 1 st projection
As an initial value of the transmitted correction of the drift error, i.e.
Step S105, as shown in FIG. 5, calculating the acquisition angle θ by using a sub-pixel image registration algorithm 2 The position deviation of the rear M-1 projection and the 1 st projection is recorded as
Performing position drift correction on the 2 nd projection to the M th projection according to the position deviation value, and performing multi-frame accumulation on the corrected M-1 projections and the 1 st projection as an acquisition angle theta 2 Corrected projection image I 2 ′；
Step S106, as shown in FIG. 5, transfers the correction value initially according to step S104
For projection image I 2 ' carry out a quadratic correction as the acquisition angle θ 2 Projection image I of 2 ；
Step S107, the initial transfer correction value
At an angle theta to the collection 2 Position deviation of the middle M Zhang Touying and the 1 st projection
Adding up the transfer correction value as a drift error, i.e.
Step S108, analogizing in sequence to the collection angle theta 3 ,θ 4 ……θ N And carrying out secondary correction on the projection image after primary correction to obtain projection data for three-dimensional reconstruction.
The following is a specific example to better explain the present invention.
The method for correcting the nanometer CT projection position drift based on the acquisition sequence subdivision comprises the following steps:
step S201, the X-ray source generates a continuous X-ray spectrum with a short wavelength limit lambda 0 And the wavelength λ of the radiation with the highest intensity m Only with respect to the tube voltage. The attenuation degrees of different substances to X-rays are different, so that the voltage of the scanning tube can be determined according to the information such as the material, the density, the size and the like of a sample to be detected.
Step S202, the energy level range of the photon counting detector adopted in the experiment is 5 keV-36 keV, and the threshold value setting range is 2.7 keV-18 keV. The threshold of the photon counting detector is generally about half of the X-ray energy in order to reduce the noise of the acquired image, and thus can be determined according to the X-ray energy level used in the experiment. After the voltage of the scanning tube and the threshold value of the detector are determined, the contrast of a sample projection image under different exposure time is observed, and when the projection image has better contrastExposure time of t 0 Then the exposure time for acquiring M Zhang Touying at each angle is t = (t) 1 ,t 2 …t M ),(t m ≥t 0 ) And t is 2 …t M-1 ≥(t 1 ,t M )。
In step S203, since consistency of response of the electronic element for manufacturing the photon counting detector to the X-ray cannot be guaranteed, the photon counting detector needs to be preprocessed, which is specifically as follows:
step S2031, collecting K frames of exposure time as t under the same experiment parameters m The gray value of the pixel point of the bright-field projection image is
The average bright-field image is then calculated according to equation (1):
(x, y) represents the coordinates of the pixel points.
Step S2032, assuming that the number of rows and columns of the pixel points of the photon counting detector are p and q respectively, the average gray level of the average bright field image is:
step S2033, the variance average of the average bright field image is:
setting the judgment value of the gain correction coefficient as
The gain factor of the photon counting detector is:
step S204, collecting sample projections under N angles in a circular track mode, collecting M Zhang Yangpin projections at each angle, and collecting the projections at an angle theta 1 ,θ 2 ……θ N Exposure time of t 1 ,t 2 ……t M As shown in fig. 3.
Step S205, using the collection angle as theta n The first projection of (1) is a standard projection, wherein N =1,2,3 … N, and the position deviation of the M-1 projections and the 1 st projection is calculated by adopting a sub-pixel image registration algorithm. Suppose the position deviation of the m-th projection is (x) m ,y m ) Then there is
f m (x,y)＝f 1 (x-x m ,y-y m ) (5)
Fourier transform of equation (5) yields:
F m (u,v)＝F 1 (u,v)exp(-j2π(ux m +vy m )) (6)
the normalized cross-power spectrum is calculated as:
in the formula
Is F 1 (u, v) complex conjugation. Inverse transformation of the cross-power spectrum yields the pulse function:
δ m (x-x m ,y-y m ) (8)
so that the positional deviation (x) can be obtained m ,y m ). By analogy, the collection angle theta can be obtained n Position deviation of lower M Zhang Touying
As shown in fig. 4. According to the position deviation value, carrying out position drift correction on the 2 nd projection to the M projections, and carrying out multi-frame correction on the CT images after correctionAre accumulated to obtain a projection image I' n 。
Step S106, the collection angle is theta 1 The position deviation of the projection of the middle M Zhang Touying and the projection of the 1 st picture is used as the initial transfer correction value of the drift error, and the acquisition angle is theta 2 The projected image of (a) is secondarily corrected as shown in fig. 5.
Step S107, the acquisition angle is known to be theta by analogy n The transfer correction value of (a) is:
to the projection image I' n Carrying out secondary correction to obtain a projection image I for three-dimensional reconstruction n 。
The effectiveness of the method was verified by the following two specific experiments, setting the experimental parameters as shown in tables 1 and 2.
TABLE 1 Simens Star card comparative experiment parameters
| Exposure time/s | 200 | 20 |
| Number of collected sheets | 1 | 10 |
| voltage/kV | 60 | 60 |
| Current/. |
100 | 100 |
| power/W | 6 | 6 |
TABLE 2 comparative experiment parameters of glass beads
| Experiment of | A | B |
| Exposure time/s | 19 | 2、5、5、5、2 |
| Sampling interval/. | 1° | 1° |
| voltage/kV | 60 | 60 |
| Current/. |
100 | 100 |
| power/W | 6 | 6 |
The siemens star card is collected, the collection parameters are shown in table 1, 10 projection accumulated projections with exposure time of 200 seconds and exposure time of 20 seconds are compared as shown in fig. 6, the left image is a star card projection image with exposure time of 200 seconds, details of the star card are blurred, and the contrast is low. The right image is 20 seconds of single exposure time, 10 projected images are subjected to position deviation correction and multi-frame accumulation, and all lines of the star card can be clearly displayed.
The scanning test is carried out on the glass beads, the experimental parameters are shown in table 2, the reconstruction result slice of the comparative experiment 2 is shown in fig. 7, the microspheres in the left image have obvious geometric artifacts, the edges of the microspheres are blurred, the microspheres in the right image have no geometric artifacts, and the edges of the microspheres are clear and sharp.
The method for correcting the nanometer CT projection position drift based on the acquisition sequence subdivision can effectively solve the problem of projection position offset of the nanometer CT in the long-time scanning process and improve the signal-to-noise ratio of the nanometer CT projection.
The embodiment also provides a nanometer CT projection position drift correction device based on collection sequence subdivision, which comprises a sample projection collection module, a projection image primary correction module, a projection image secondary correction module and a three-dimensional reconstruction projection data acquisition module.
The sample projection acquisition module is used for acquiring a sample projection image at N angles in a circular track mode, and acquiring M Zhang Yangpin projection at each acquisition angle;
a primary correction module for the projected image, which calculates the acquisition angle theta by using a sub-pixel image registration algorithm n The position deviation of M Zhang Touying, correcting the position drift of the projection according to the position deviation, and performing multi-frame accumulation on the corrected CT image to obtain a projection image after primary correction;
secondary correction module for projected image with collection angle theta 1 The position deviation of the middle M Zhang Touying and the 1 st projection is used as the initial transfer correction value of the drift error, and the collection angle is theta 2 Once correction ofCarrying out secondary correction on the corrected projection image; the initial transmission correction value and the acquisition angle are theta 2 Accumulating the position deviation of the middle M Zhang Touying and the 1 st projection as a transfer correction value to obtain an acquisition angle theta 3 Performing secondary correction on the projection image after the primary correction;
and the three-dimensional reconstruction projection data acquisition module is used for completing the position drift correction of the projections at all the acquisition angles to obtain projection data 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 understand that: all or part of the steps for realizing the method embodiments can be completed by hardware related to program instructions, the program can be stored in a computer readable storage medium, and the program executes the steps comprising the method embodiments when executed; and the aforementioned storage medium includes: various media that can store program codes, such as ROM, RAM, magnetic or optical disks.
Finally, it is to be noted that: the above description is only a preferred embodiment of the present invention, and is only used to illustrate the technical solutions of the present invention, and not to limit the protection scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention.
Claims (8)
1. A nanometer CT projection position drift correction method based on acquisition sequence subdivision is characterized by comprising the following steps:
step 1, collecting a sample projection image at N angles in a circular track mode, and collecting M Zhang Yangpin projection at each collection angle;
step 2, adopting a sub-pixel image registration algorithm meterCalculating the collection angle as theta n The position deviation of M Zhang Touying, correcting the position drift of the projection according to the position deviation, and performing multi-frame accumulation on the corrected CT image to obtain a projection image after primary correction;
step 3, the collection angle is theta 1 The position deviation of the projection of the middle M Zhang Touying and the projection of the 1 st picture is used as the initial transfer correction value of the drift error, and the acquisition angle is theta 2 Performing secondary correction on the projection image after the primary correction;
step 4, setting the initial transmission correction value and the acquisition angle as theta 2 Accumulating the position deviation of the middle M Zhang Touying and the 1 st projection as a transfer correction value to obtain an acquisition angle theta 3 Performing secondary correction on the projection image after the primary correction;
and 5, performing position drift correction on the projections at all the acquisition angles by analogy to obtain projection data for three-dimensional reconstruction.
2. The acquisition sequence subdivision-based nanoct projection position drift correction method according to claim 1, further comprising, before step 1:
and determining scanning parameters and a threshold value of the photon counting detector according to the sample to be detected, and preprocessing the photon counting detector.
3. The acquisition sequence subdivision-based nano CT projection position drift correction method as claimed in claim 2, wherein scanning parameters are selected according to material, density and size factors of a sample to be detected, so as to determine a threshold value of the photon counting detector, and then the photon counting detector is preprocessed.
4. The acquisition sequence subdivision-based nanoct projection position drift correction method of claim 1, wherein N-angle projections are acquired in a circular trajectory manner, and the acquisition angle is θ 1 ,θ 2 ……θ N (ii) a Acquiring M Zhang Yangpin projection under each acquisition angle, wherein the exposure time is t 1 ,t 2 ……t M 。
5. The acquisition sequence subdivision-based nanoct projection position drift correction method according to claim 4, wherein the step 2 specifically comprises:
calculating the acquisition angle theta by adopting a sub-pixel image registration algorithm n M-1 projection and the 1 st projection
According to the position deviation value, carrying out position drift correction on the 2 nd projection to the M projections, and carrying out multi-frame accumulation on the corrected CT images as an acquisition angle theta n The primary corrected projection image of (a).
6. The acquisition sequence subdivision-based nanoct projection position drift correction method of claim 5, wherein an acquisition angle is θ 1 Position deviation of the middle M Zhang Touying and the 1 st projection
Initial transfer correction value delta theta as drift error 2 For collection angle theta 2 The projection image after the primary correction of (2) is subjected to a secondary correction.
7. The acquisition sequence subdivision-based nanoct projection position drift correction method of claim 6, wherein an initial transfer correction value is used
At an angle theta to the collection 2 Position deviation of the middle M Zhang Touying and the 1 st projection
Adding up the transfer correction value as a drift error, i.e.
For the collection angle theta 3 The projection image after the primary correction of (2) is subjected to a secondary correction.
8. A nanometer CT projection position drift correction device based on acquisition sequence subdivision is characterized by comprising:
the sample projection acquisition module is used for acquiring a sample projection image at N angles in a circular track mode, and acquiring M Zhang Yangpin projection at each acquisition angle;
a primary correction module for the projected image, which calculates the acquisition angle theta by using a sub-pixel image registration algorithm n The position deviation of M Zhang Touying, correcting the position drift of the projection according to the position deviation, and performing multi-frame accumulation on the corrected CT image to obtain a projection image after primary correction;
secondary correction module for projected image with collection angle theta 1 The position deviation of the middle M Zhang Touying and the 1 st projection is used as the initial transfer correction value of the drift error, and the collection angle is theta 2 Performing secondary correction on the projection image after the primary correction; the initial transmission correction value and the acquisition angle are theta 2 Accumulating the position deviation of the middle M Zhang Touying and the 1 st projection as a transfer correction value to obtain an acquisition angle theta 3 Performing secondary correction on the projection image after the primary correction;
and the three-dimensional reconstruction projection data acquisition module is used for completing the position drift correction of the projections at all the acquisition angles to obtain projection data for three-dimensional reconstruction.
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| JP5472907B2 (en) * | 2008-12-24 | 2014-04-16 | 株式会社東芝 | Image diagnostic apparatus and image diagnostic method |
| CN104181177B (en) * | 2013-05-24 | 2017-08-22 | 上海联影医疗科技有限公司 | CT detector location bearing calibrations |
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