CN108283503B - CT machine, scanning method and image reconstruction method - Google Patents
CT machine, scanning method and image reconstruction method Download PDFInfo
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- 230000001960 triggered effect Effects 0.000 claims description 3
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- A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
- A61B6/02—Devices for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
- A61B6/03—Computerised tomographs
- A61B6/032—Transmission computed tomography [CT]
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
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- A61B6/4021—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment with arrangements for generating radiation specially adapted for radiation diagnosis involving movement of the focal spot
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
- A61B6/42—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment with arrangements for detecting radiation specially adapted for radiation diagnosis
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
- A61B6/52—Devices using data or image processing specially adapted for radiation diagnosis
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Abstract
The embodiment of the invention relates to a computed tomography device, a scanning method and an image reconstruction method, which comprise a fixed frame (1), a rotating frame (2), a light source generating device (3), a signal detecting device (4) and a scanned target supporting device (5), wherein the rotating frame can rotate around a certain fixed point of an X-Y plane, and the scanned target supporting device is fixed along the Z direction; the light source generating device is arranged on the rotating frame, can output scanning light rays of a conical beam, and quickly switches focus positions along the Z direction during adjacent sampling periods while the center position of the light source continuously moves along the Z direction; the signal detection device is arranged on the rotating frame, is opposite to the light source generation device in position, and does not change in relative position with the light source generation device in the rotating process so as to ensure that the conical beam light source can be received by the signal detection device area array; the signal detection device is an area array structure suitable for collecting the cone-shaped beam.
Description
Technical Field
The invention relates to the technical field of medical computed tomography, in particular to a scanning mode and a corresponding image reconstruction method of a CT (computed tomography) machine for generating high-resolution images, and the CT machine adopting the scanning mode and the reconstruction method.
Background
Ct (computed tomography), also known as computed tomography. With the development of the technology, the conventional slice imaging mode, i.e. imaging in the rotation plane (referred to as X-Y plane herein), is gradually developed into the volume imaging mode, i.e. continuous imaging in the direction of the rotation axis (referred to as Z direction herein). This is mainly brought about by the maturity of technologies such as volume continuous scanning mode, area array detector and cone beam reconstruction method.
With the wide application of volume (three-dimensional) imaging methods (MPR, VR, etc.) in clinical diagnosis, the CT machine focuses on the improvement of spatial resolution in the X-Y plane, and also focuses on the improvement of Z-direction resolution, especially in the application scenarios of inner ear and cardiac imaging bone joint, the Z-direction resolution is required to approach or reach the level in the X-Y plane.
Improving the Z-direction resolution and reducing the interference caused by aliasing artifacts as much as possible are important issues of current CT technology.
Currently, the CT machine is widely adopted in the third generation architecture, that is, the light source position and the detector are installed in the opposite directions, and the relative position is not changed during the rotation process.
In order to realize the volume scanning of the CT machine and achieve or approach the Z-direction resolution to the X-Y plane resolution, the detector needs to adopt an area array structure in which the channel direction (X-Y plane) and the slice direction (Z direction) are distributed simultaneously. When data is collected, the bulb and the detector rotate at a high speed in a plane.
In a traditional tomography mode, a focus of a bulb moves on a circular track of a rotation plane (an X-Y plane), the relative position of the bulb and an X-ray bulb is not changed in the rotation process, projection data based on the circular track of a cone beam are acquired, and a reconstruction algorithm can perform three-dimensional imaging by using a known FDK reconstruction method.
The limitation of the technical scheme is that in each projection angle, the sampling interval in the Z direction of the acquisition mode is larger than the width of the detector unit in the slice direction, which is far from meeting the sampling theorem, so that not only is the resolution loss caused, but also the image reconstructed by using three-dimensional reconstruction algorithms such as FDK and the like generates serious aliasing artifacts (common clinical manifestations are windmill artifacts), thereby bringing interference and inconvenience to the diagnosis of doctors. The problem does not exist only in the plane where the focal track is located, but also in planes at other Z positions, and although the problem does not exist in the imaging of the plane where the focal track is located, the cost is the loss of resolution, which also brings the problem that the spatial resolution distribution of the three-dimensional image in the Z direction is uneven to a certain extent. These all limit the application of three-dimensional imaging in tomographic mode.
In a word, no better scheme exists in the prior art, the system can exert the limit resolving power in the Z direction, and simultaneously can effectively eliminate aliasing artifacts, so that the clinical application prospect of the CT machine on volume imaging is greatly hindered.
Disclosure of Invention
The embodiment of the invention provides a CT (computed tomography) machine, a scanning method and an image reconstruction method, which aim to solve the technical problem of aliasing artifacts of the conventional CT scanning system.
The embodiment of the invention provides a computer tomography device, which comprises a fixed frame 1, a rotating frame 2, a light source generating device 3, a signal detecting device 4 and a scanned target supporting device 5: the rotating frame 2 can rotate around a fixed point in an X-Y plane; the scanned target supporting device 5 is fixed along the Z direction to meet the fault scanning track, and an X-Y-Z coordinate system meets the definition of a right-hand system; the light source generating device 3 is installed on the rotating frame 2, can output scanning light rays of a conical beam, and rapidly switches focus positions along the Z direction during adjacent sampling periods while continuously moving the center position of the light source along the Z direction; the signal detection device 4 is mounted on the rotating frame 2, is opposite to the light source generation device 3 in position, and does not change in relative position with the light source generation device 3 in the rotating process, so as to ensure that the conical beam light source can be received by the signal detection device 4 in an area array; the signal detection device 4 is an area array structure suitable for collecting the cone beam and comprises a plurality of photosensitive elements; the image reconstruction computer 6 is connected with the signal detection device 4 and is used for receiving and processing the scanning data to realize reconstruction calculation; and an image display device 7 for displaying the image processed by the reconstruction computer 6.
Further, the light source generating device 3 is an X-ray light source, and includes a high voltage device and an X-ray bulb tube.
An embodiment of the present invention provides a scanning control method using any one of the above computed tomography apparatuses, including the following steps:
s101: the light source generating device 3 and the signal detecting device 4 rotate and scan in an X-Y plane, the scanned target supporting device 5 is fixed in the Z direction to meet the requirement of a tomography track, sampling is triggered in an equal angle mode during tomography, and the total number of projection sampling is an even number;
s102: when the tomography starts, the position of the focus of the light source generating device (3) on the anode target surface is continuously and rapidly switched, and the central position of the light source continuously moves along the Z direction at a constant speed, and scans along the Z direction according to a preset track in a sampling period;
s103: the signal detection device 4 receives signals obtained by scanning according to the preset track, and transmits the obtained data to a reconstruction computer for data processing and image reconstruction.
Further, the predetermined trajectory in step S102 satisfies:
wherein, β is a projection angle at the nth sampling, and the projection angle is defined as an angle formed by a ray path where a focal point and a rotation center are located and the Y-axis direction;
Rfa radius of rotation representing an original focus state;
Δ Z is the focal shift in the Z coordinate caused by the translation of the focal spot on the anode target surface
Δ β is the angular interval spanned by the current projection angle relative to the starting projection angle;
Rfdis the distance from the focal point to the signal detection means;
b is the interval of the photosensitive element of the signal detection device in the Z direction;
Δ R represents the amount of change in the distance from the focal point to the center of rotation, and satisfies the following relationship:
where α represents the angle at which the anode target surface of the bulb exists.
An embodiment of the present invention provides a method for reconstructing an image according to data obtained by any one of the above-mentioned scan control methods, including the following steps:
s201: carrying out necessary preprocessing on the scanning data;
s202: rearranging the data preprocessed in the step S201;
s203: filtering and weighting the data rearranged in the step S202;
s204: back-projecting the data filtered and weighted in the step S203;
s205: the data after the above-described inverse projection in step S204 is post-processed to obtain an image that can be used for diagnosis.
Further, the step S202 includes:
s2021: when different projection angles are adopted, the rotation radius and the channel angle are changed, and rearrangement interpolation is carried out according to the changed values;
s2022: the output wedge beam data sets are divided into two groups, one group of wedge beam data is generated by interpolation of odd-numbered sampled cone beam data sets, and the other group of wedge beam data is generated by interpolation of even-numbered sampled cone beam data sets.
Further, the step S202 includes:
beta is the projection angle of the cone beam P at the current focus position, namely the angle formed by the OS line and the Y axis, gamma is the angle formed by the ray path and the central channel under the ideal focus state, theta is the projection angle of the ray path and the Y axis, namely the parallel beam, and t is the distance from the ray path to the origin O, and the following geometrical relations are satisfied:
θ＝β+γ+Δγ (6)
t＝(Rf+ΔR)Sin(γ+Δγ) (7)
the cone beam projection data generated by the scanning mode downsampling is recorded as P (beta, gamma, b), and b is the position of a ray path reaching the detector;
rebinning the cone-beam of acquired raw projection data into a wedge-beam dataset:
Pnmod2＝1(β,γ,b)→Pnmod2＝1(θ,t,b) (9)
Pnmod2＝0(β,γ,b)→Pnmod2＝0(θ,t,b) (10)
the notation of nmod2 is to take the remainder of 2 for n, i.e., the acquired cone beams are re-ordered separately by dividing them into two groups, one group for odd samples and one group for even samples.
Further, the filtering and weighting the data rearranged in the step S202 includes:
wherein the content of the first and second substances,
namely, filtering the projection data rearranged into the wedge-shaped beam layer by layer in the channel arrangement direction;
is to weight the projection data;
representing cone angle compensation for the ray.
Further, the back-projecting the data filtered and weighted in step S203 includes:
the image of a point (x, y, z) in space is assembled by a wedge-shaped beam
Obtaining through back projection:
for an angle θ, the ray position t, b through point (x, y, z) can be calculated as follows:
t＝xcosθ+ysinθ (13)
and b is the offset of the Z position of the detector relative to the central layer of the detector when the wedge-shaped beam passes through (x, y, Z) and reaches the rearranged wedge-shaped beam when the projection angle is theta.
Compared with the prior art, the invention has at least the following beneficial technical effects.
The invention performs data acquisition by the CT device and the scanning mode, so that the sampling frequency in each projection direction in the Z direction is increased by 4 times. The sampling frequency is higher than that required to restore the system's ultimate resolution. Therefore, the system can exert the capability of limiting resolution of the system, and the data can not be subjected to aliasing phenomenon. If the used detector units have similar sizes in the channel direction and the layer direction, a good isotropic resolution effect can be obtained, namely high-quality three-dimensional imaging is realized.
By the CT reconstruction method, three-dimensional imaging of scanning data in a correct mode can be guaranteed, and the CT reconstruction method is a key step for ensuring that Z-direction resolution obtains a limit value and eliminating aliasing artifacts.
In conclusion, the technical scheme of the invention can fundamentally solve the problems of resolution and aliasing artifacts caused by the inherent defect of insufficient sampling in the Z direction of the existing CT machine, and the scanning mode of the invention is based on a tomography mode, can improve the Z-direction resolution to the system limit level under the condition of dosage efficiency and repeated scanning capability superior to that of spiral scanning, and can eliminate the Z-direction aliasing artifacts.
The invention also has the advantages that the focus is not concentrated on a fixed point on the anode target surface of the bulb tube in the scanning process, thereby being beneficial to the heat dissipation and the focus stability of the bulb tube, further improving the stable reliability of the image quality and improving the scanning throughput of the CT machine.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without inventive exercise.
FIG. 1 is a schematic diagram of a computed tomography apparatus according to an embodiment of the present invention;
FIG. 2 is a schematic illustration of a cone beam geometry for a method according to an embodiment of the invention;
FIG. 3 is a ray geometry diagram of a method according to an embodiment of the invention;
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention clearer, the present invention will be described in further detail with reference to the accompanying drawings, and it is apparent that the described embodiments are only a part of the embodiments of the present invention, not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The terminology used in the embodiments of the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used in the examples of this application and the appended claims, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise, and "a plurality" typically includes at least two.
It should be understood that the term "and/or" as used herein is merely one type of association that describes an associated object, meaning that three relationships may exist, e.g., a and/or B may mean: a exists alone, A and B exist simultaneously, and B exists alone. In addition, the character "/" herein generally indicates that the former and latter related objects are in an "or" relationship.
It should be understood that although the terms first, second, third, etc. may be used to describe XXX in the embodiments of the present application, these XXX should not be limited to these terms. These terms are used only to distinguish XXX. For example, a first XXX may also be referred to as a second XXX, and similarly, a second XXX may also be referred to as a first XXX, without departing from the scope of embodiments of the present application.
The words "if", as used herein, may be interpreted as "at … …" or "at … …" or "in response to a determination" or "in response to a detection", depending on the context. Similarly, the phrases "if determined" or "if detected (a stated condition or event)" may be interpreted as "when determined" or "in response to a determination" or "when detected (a stated condition or event)" or "in response to a detection (a stated condition or event)", depending on the context.
It is also noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a good or system that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such good or system. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other like elements in a commodity or system that includes the element. In the signal extraction of laser radar echo signal image, through the calculation of introducing the eigenvalue, widen the signal part edge in the stripe signal picture, with the noise part more prominent simultaneously, then through setting up the gate width and increasing the threshold value to get rid of the noise, and leave the signal edge completely, no matter be weak signal connection region or the fracture zone of signal middle part, can both preserve completely. Meanwhile, the fringe echo signal is presented in a gray scale image form, the fringe echo signal has very obvious unsmooth degree in the edge of a target and a noise area, distortion of different degrees can occur in the edge area when a characteristic value is calculated, according to the point, the edge of the target can be enhanced by introducing calculation of the characteristic value, the difference between a noise point and a signal is more obvious, and the edge of the signal can be expanded, so that the noise can be more thoroughly removed in the process of extracting the signal, meanwhile, the center of the signal and the edge detail part of the signal can be perfectly reserved, and the efficient and complete extraction of the echo signal is realized.
Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
Example 1
As shown in fig. 1-2, a computed tomography apparatus according to an embodiment of the present invention includes a fixed frame 1, a rotating frame 2, a light source generating device 3, a signal detecting device 4, and a scanned target supporting device 5: the rotating frame 2 can rotate around a fixed point in an X-Y plane; the scanned target supporting device 5 is fixed along the Z direction to meet the fault scanning track, and an X-Y-Z coordinate system meets the definition of a right-hand system; the light source generating device 3 is installed on the rotating frame 2, can output scanning light rays of a conical beam, and rapidly switches focus positions along the Z direction during adjacent sampling periods while continuously moving the center position of the light source along the Z direction; the signal detection device 4 is mounted on the rotating frame 2, is opposite to the light source generation device 3 in position, and does not change in relative position with the light source generation device 3 in the rotating process, so as to ensure that the conical beam light source can be received by the signal detection device 4 in an area array; the signal detection device 4 is an area array structure suitable for collecting the conical beam and comprises a plurality of photosensitive elements with the same size along the channel direction and the layer direction;
the image reconstruction computer 6 is connected with the signal detection device 4 and is used for receiving and processing the scanning data to realize reconstruction calculation; and an image display device 7 for displaying the image processed by the reconstruction computer 6.
In particular, the light source generating device 3 may be an X-ray light source, including a high voltage device and an X-ray bulb. The CT machine fixing frame adopts a vertical structure, the X-ray light source can use a bulb tube with model 2280 or 2251 of Dunlee company in the United states and a correspondingly matched high-voltage and X-ray generating device of a Spellman company, and the bulb tube can control the position of an X-ray focus on an anode by utilizing an electromagnetic field and can be switched rapidly. In order to implement the scanning control method described in the present invention. An X-ray generating device, namely a high-voltage device and an X-ray bulb tube, can output X-ray of a cone beam, has the characteristic of rapidly switching focus positions along the Z direction during adjacent sampling periods and continuously moving the central position of a light source along the Z direction, is arranged on a rotating frame of a CT machine,
the X-ray detection device is arranged on a rotating frame of the CT machine and is opposite to the X-ray bulb tube so as to ensure that a conical beam light source can be received by the detector area array and the relative position of the X-ray bulb tube is not changed in the rotating process, as shown in figure 2; in addition, the light source generating device 3 is not exclusive of an X-ray light source, and other generating devices and corresponding data receiving devices (detectors) capable of generating light output such as gamma rays, visible light, ultraviolet light, infrared light, etc. can be used.
The signal detection device 4 is assembled into a cylindrical area array structure by adopting modules. I.e. the configuration shown in fig. 2. The scanned target support 5 may be a scanning table. The number of samples in one rotation is between 2000 and 2400, the number is even, and an equiangular sampling mode is adopted. Of course, besides the arc-shaped structure of module splicing, an area array structure suitable for cone-beam collection, such as a flat panel detector, can be selected.
The CT machine may be replaced by an X-ray scanning device having a C-arm structure with a rotation structure and a longitudinal displacement function, such as a cone beam CT machine widely used in dental oral diagnosis and other CT devices capable of realizing a tomographic trajectory acquisition system.
Example 2
As shown in fig. 3, an embodiment of the present invention further provides a scanning control method using any one of the above computed tomography apparatuses, where the computed tomography apparatus is not described again. The scan control method may include the steps of:
s101: the light source generating device 3 and the signal detecting device 4 rotate and scan in an X-Y plane, the scanned target supporting device 5 is fixed in the Z direction to meet the requirements of a tomography track, sampling is triggered in an equal angle mode during tomography, the total number of projection sampling is an even number and is defined as N, and the acquisition range can be less than one circle, exactly one circle or more than one circle;
s102: when the tomography is started, the position of the focus of the light source generating device on the anode target surface is continuously and rapidly switched, the central position of the light source continuously and uniformly moves along the Z direction, and the scanning is performed along the Z direction according to a preset track in one sampling period.
Specifically, the Z direction indicates the moving direction of the focal point, in the first sampling period, the focal point moves from position 1 to position 5, the central position of the light source is a, when the focal point returns from position 5 to enter the second sampling period, the focal point moves from position 2 to position 6, the central position of the light source is B, and so on, the focal point switches between position 1 and position 5, position 2 and position 6, position 3 and position 7, and position 4 and position 8, and meanwhile, the center of the light source moves at a constant speed along A, B, C, D.
S103: the signal detection device 4 receives signals obtained by scanning according to the preset track, and transmits the obtained data to a reconstruction computer for data processing and image reconstruction.
Further, the predetermined trajectory in step S102 satisfies:
wherein, β is a projection angle at the nth sampling, and the projection angle is defined as an angle formed by a ray path where a focal point and a rotation center are located and the Y-axis direction;
the radius of rotation of the original focus state (the focus position is a circular track in the X-Y plane when the focus is not shifted and changed) is defined as RfHere, for the sake of simple description, it is assumed that the Z coordinate of the plane in which the focus is located in the original focus state is 0.
Δ Z is the focal shift in the Z coordinate caused by the translation of the focal spot on the anode target surface
Δ β is the angular interval spanned by the current projection angle relative to the starting projection angle;
Rfdis the distance from the focal point to the signal detection means;
b is the interval of the photosensitive element of the signal detection device in the Z direction;
Δ R represents the amount of change in the distance from the focal point to the center of rotation, and satisfies the following relationship:
where α represents the angle at which the anode target surface of the bulb exists.
Example 3
As shown in fig. 3, an embodiment of the present invention further provides a method for reconstructing an image according to data obtained by any one of the above-mentioned scanning control methods, where the CT machine is the CT machine described in embodiment 1, and the scanning method is the CT machine described in embodiment 2, and is not described herein again. The method for reconstructing the image of the data obtained by the scanning control method comprises the following steps:
s201: carrying out necessary preprocessing on the scanning data;
s202: rearranging the data preprocessed in the step S201;
s203: filtering and weighting the data rearranged in the step S202;
s204: back-projecting the data filtered and weighted in the step S203;
s205: the data after the above-described inverse projection in step S204 is post-processed to obtain an image that can be used for diagnosis.
Specifically, the step S202 includes:
s2021: when different projection angles are adopted, the rotation radius and the channel angle are changed, and rearrangement interpolation is carried out according to the changed values;
s2022: the output wedge beam data sets are divided into two groups, one group of wedge beam data is generated by interpolation of odd-numbered sampled cone beam data sets, and the other group of wedge beam data is generated by interpolation of even-numbered sampled cone beam data sets.
Specifically, the step S202 includes:
the geometrical relationship in the rotation plane is shown in fig. 3, where β is the projection angle of the cone beam P at the current focus position, i.e. the angle formed by OS and Y-axis, i.e. the fan beam projection angle, γ is the angle formed by the ray path and the central channel (OS) in the ideal focus state, θ is the angle formed by the ray path and the Y-axis, i.e. the parallel beam projection angle, and t is the distance from the ray path to the origin O, the following geometrical relationships exist:
θ＝β+γ+Δγ (6)
t＝(Rf+ΔR)sin(γ+Δγ) (7)
the cone beam projection data generated by the scanning mode downsampling is recorded as P (beta, gamma, b), and b is the position of a ray path reaching the detector;
rebinning the cone-beam of acquired raw projection data into a wedge-beam dataset:
Pnmod2＝1(β,γ,b)→Pnmod2＝1(θ,t,b) (9)
Pnmod2＝0(β,γ,b)→Pnmod2＝0(θ,t,b) (10)
the notation of nmod2 is to take the remainder of 2 for n, i.e., the acquired cone beams are re-ordered separately by dividing them into two groups, one group for odd samples and one group for even samples.
Further, the filtering and weighting the data rearranged in the step S202 includes:
wherein the content of the first and second substances,
namely, filtering the projection data rearranged into the wedge-shaped beam layer by layer in the channel arrangement direction; h (t) Shepp-Logan convolution kernels can be used.
The projection data is weighted: the purpose of this step is to ensure the normalization of the weights on each ray path involved in the back-projection, and at the same time, the weights can be normalized by the cone angle of the ray
Different weights are adopted, so that the cone angle artifact problem caused by an approximation algorithm is suppressed to a certain degree. This step may be performed by a known method. Can also be omitted
Is weighted by considering
Is to perform cone angle compensation on the ray, wherein the different focus position pairs can be ignored
Calculating the effect of the difference, i.e. during the calculation
The focus position is calculated as if it is not shifted, and it is considered that the focus position is not shifted in the Z direction.
Further, the back-projecting the data filtered and weighted in step S203 includes:
the image of a point (x, y, z) in space is assembled by a wedge-shaped beam
Obtaining through back projection:
for an angle θ, the ray position t, b through point (x, y, z) can be calculated as follows:
t＝xcosθ+ysinθ (13)
and b is the offset of the Z position of the detector relative to the central layer of the detector when the wedge-shaped beam passes through (x, y, Z) and reaches the rearranged wedge-shaped beam when the projection angle is theta.
Ray cone angle of the system
When the angle is small, for example, not more than 2 °, the influence of Δ R can be ignored, that is, Δ R is set to 0, so as to increase the calculation speed.
The CUDA programming can be carried out on the data in the back projection calculation process by using double display cards of the Invita company so as to realize parallel calculation and ensure the execution efficiency.
The invention performs data acquisition by the CT device and the scanning mode, so that the sampling frequency in each projection direction in the Z direction is increased by 4 times. The sampling frequency is higher than that required to restore the system's ultimate resolution. Therefore, the system can exert the capability of limiting resolution of the system, and the data can not be subjected to aliasing phenomenon. If the used detector units have similar sizes in the channel direction and the layer direction, a good isotropic resolution effect can be obtained, namely high-quality three-dimensional imaging is realized.
By the CT reconstruction method, three-dimensional imaging of scanning data in a correct mode can be guaranteed, and the CT reconstruction method is a key step for ensuring that Z-direction resolution obtains a limit value and eliminating aliasing artifacts.
In conclusion, the technical scheme of the invention can fundamentally solve the problems of resolution and aliasing artifacts caused by the inherent defect of insufficient sampling in the Z direction of the existing CT machine, and the scanning mode of the invention is based on a tomography mode, can improve the Z-direction resolution to the system limit level under the condition of dosage efficiency and repeated scanning capability superior to that of spiral scanning, and can eliminate the Z-direction aliasing artifacts.
The invention also has the advantages that the focus is not concentrated on a fixed point on the anode target surface of the bulb tube in the scanning process, thereby being beneficial to the heat dissipation and the focus stability of the bulb tube, further improving the stable reliability of the image quality and improving the scanning throughput of the CT machine.
The above-described apparatus embodiments are merely illustrative, wherein the units described as separate components may or may not be physically separate. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of the present embodiment.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (7)
1. A scanning control method of a computer tomography device is characterized in that the computer tomography device comprises a fixed frame (1), a rotating frame (2), a light source generating device (3), a signal detecting device (4) and a scanned target supporting device (5),
the rotating frame (2) can rotate around a certain fixed point of an X-Y plane; the scanned target supporting device (5) is fixed along the Z direction to meet the fault scanning track, and an X-Y-Z coordinate system meets the definition of a right-hand system;
the light source generating device (3) is arranged on the rotating frame (2), can output scanning light rays of a conical beam, and rapidly switches focus positions along the Z direction during adjacent sampling periods while continuously moving the center position of the light source along the Z direction; the position of the focus of the light source generating device on the anode target surface is continuously and rapidly switched, and the central position of the light source continuously moves at a constant speed along the Z direction; in a sampling period, the focus moves from a position 1 to a position 5, the central position of the light source is A, when the focus returns from the position 5 to enter the next sampling period, the focus moves from a position 2 to a position 6, the central position of the light source is B, and so on, the focus is switched among the position 1, the position 2, the position 6, the position 3, the position 7 and the position 4, and meanwhile, the center of the light source moves at a constant speed along the Z direction of A, B, C, D;
the signal detection device (4) is arranged on the rotating frame (2), is opposite to the light source generation device (3), and does not change relative position with the light source generation device (3) in the rotating process, so that the conical beam light source can be received by the signal detection device (4) in an area array; the signal detection device (4) is an area array structure suitable for collecting conical beams and comprises a plurality of photosensitive elements;
the image reconstruction computer (6) is connected with the signal detection device (4) and is used for receiving and processing the scanning data to realize reconstruction calculation;
an image display device (7) for displaying the image processed by the reconstruction computer (6);
the scanning control method comprises the following steps:
s101: the light source generating device (3) and the signal detecting device (4) rotate and scan in an X-Y plane, the scanned target supporting device (5) is fixed in the Z direction to meet a tomography track, sampling is triggered in an equal angle mode during tomography, and the total number of projection sampling is an even number;
s102: when the tomography starts, the position of the focus of the light source generating device (3) on the anode target surface is continuously and rapidly switched, and the central position of the light source continuously moves along the Z direction at a constant speed, and scans along the Z direction according to a preset track in a sampling period; in a sampling period, the focus moves from a position 1 to a position 5, the central position of the light source is A, when the focus returns from the position 5 to enter the next sampling period, the focus moves from a position 2 to a position 6, the central position of the light source is B, and so on, the focus is switched among the position 1, the position 2, the position 6, the position 3, the position 7 and the position 4, and meanwhile, the center of the light source moves at a constant speed along the Z direction of A, B, C, D;
s103: the signal detection device (4) receives signals obtained by scanning according to the preset track and transmits the obtained data to a reconstruction computer for data processing and image reconstruction;
the predetermined trajectory in the step S102 satisfies:
wherein, β is a projection angle at the nth sampling, and the projection angle is defined as an angle formed by a ray path where a focal point and a rotation center are located and the Y-axis direction;
Rfa radius of rotation representing an original focus state;
Δ Z is the focal shift in the Z coordinate caused by the translation of the focal spot on the anode target surface
Δ β is the angular interval spanned by the current projection angle relative to the starting projection angle;
Rfdis the distance from the focal point to the signal detection means;
b is the interval of the photosensitive element of the signal detection device in the Z direction;
Δ R represents the amount of change in the distance from the focal point to the center of rotation, and satisfies the following relationship:
where α represents the angle at which the anode target surface of the bulb exists.
2. The scan control method according to claim 1, wherein: the light source generating device (3) is an X-ray light source and comprises a high-voltage device and an X-ray bulb tube.
3. A method for image reconstruction from data obtained by a scan control method according to any one of claims 1-2, comprising the steps of:
s201: carrying out necessary preprocessing on the scanning data;
s202: rearranging the data preprocessed in the step S201;
s203: filtering and weighting the data rearranged in the step S202;
s204: back-projecting the data filtered and weighted in the step S203;
s205: the data after the above-described inverse projection in step S204 is post-processed to obtain an image that can be used for diagnosis.
4. The method of claim 3, wherein: the step S202 includes:
s2021: when different projection angles are adopted, the rotation radius and the channel angle are changed, and rearrangement interpolation is carried out according to the changed values;
s2022: the output wedge beam data sets are divided into two groups, one group of wedge beam data is generated by interpolation of odd-numbered sampled cone beam data sets, and the other group of wedge beam data is generated by interpolation of even-numbered sampled cone beam data sets.
5. The method of claim 4, wherein: the step S202 includes:
beta is the projection angle of the cone beam P at the current focus position, namely the angle formed by the OS line and the Y axis, gamma is the angle formed by the ray path and the central channel under the ideal focus state, theta is the projection angle of the ray path and the Y axis, namely the parallel beam, and t is the distance from the ray path to the origin O, and the following geometrical relations are satisfied:
θ＝β+γ+Δγ (6)
t＝(Rf+ΔR)sin(γ+Δγ) (7)
the cone beam projection data generated by the down sampling in the current scanning mode is recorded as P (beta, gamma, b), and b is the position of the ray path on the detector;
rebinning the cone-beam of acquired raw projection data into a wedge-beam dataset:
Pnmod2＝1(β,γ,b)→Pnmod2＝1(θ,t,b) (9)
Pnmod2＝0(β,γ,b)→Pnmod2＝0(θ,t,b) (10)
the representation of n mod2 is the remainder of taking n to 2, i.e., the acquired cone beams are re-ordered into two groups, odd samples for one group and even samples for one group.
6. The method of claim 5, wherein: filtering and weighting the data rearranged in the step S202, including:
wherein the content of the first and second substances,
namely the channelFiltering the projection data rearranged into the wedge-shaped beam layer by layer in the arrangement direction;
is to weight the projection data;
representing cone angle compensation for the ray.
7. The method of claim 6, wherein: the back projection of the data filtered and weighted in step S203 includes:
the image of a point (x, y, z) in space is assembled by a wedge-shaped beam
Obtaining through back projection:
for an angle θ, the ray position t, b through point (x, y, z) can be calculated as follows:
t＝xcosθ+ysinθ (13)
and b is the offset of the Z position of the detector relative to the central layer of the detector when the wedge-shaped beam passes through (x, y, Z) and reaches the rearranged wedge-shaped beam when the projection angle is theta.
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