CN108283502B - Focus moving type CT machine, scanning method and image reconstruction method - Google Patents

Focus moving type CT machine, scanning method and image reconstruction method Download PDF

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CN108283502B
CN108283502B CN201810143704.2A CN201810143704A CN108283502B CN 108283502 B CN108283502 B CN 108283502B CN 201810143704 A CN201810143704 A CN 201810143704A CN 108283502 B CN108283502 B CN 108283502B
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CN108283502A (en
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不公告发明人
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Suzhou Shengnuo Medical Technology Co ltd
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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
    • A61B6/02—Devices for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/03—Computerised tomographs
    • A61B6/032—Transmission computed tomography [CT]
    • 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
    • A61B6/40—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment with arrangements for generating radiation specially adapted for radiation diagnosis
    • 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
    • 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
    • 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
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T15/00—3D [Three Dimensional] image rendering
    • G06T15/005—General purpose rendering architectures

Abstract

The embodiment of the invention relates to a computed tomography device, which comprises a fixed frame (1), a rotating frame (2), a light source generating device (3), a signal detecting device (4), a scanned target supporting device (5) and a light source collimating device (8); the rotating frame (2) can rotate around a certain fixed point of an X-Y plane; the light source generating device (3) is arranged on the rotating frame (2), can output scanning light rays of a conical beam, and continuously moves a focus along the Z direction; the signal detection device (4) is arranged on the rotating frame (2) and is relatively fixed with the light source generation device (3); the signal detection device (4) is an area array structure suitable for collecting conical beams and comprises a plurality of photosensitive elements; the light source collimation device (8) translates along the Z direction along with the change of the focal position of the light source generation device (3).

Description

Focus moving type CT machine, scanning method and image reconstruction method
Technical Field
The invention relates to the technical field of medical computed tomography, in particular to a novel X-ray source dynamic focus control mode and a corresponding image reconstruction method in a tomography mode, and a CT (computed tomography) machine adopting the scanning mode and the reconstruction method.
Background
Ct (computed tomography), also known as computed tomography. With the maturity and development of the technologies such as the area array detector and the cone beam reconstruction method, the data acquisition of the CT machine is developed from a fan-shaped beam acquisition mode formed by single-layer detector physical units to a cone beam acquisition mode formed by multi-layer detector physical units. The imaging mode of the CT machine has also been gradually developed from the conventional single-slice and two-dimensional imaging mode, i.e., imaging in the rotation plane (referred to as X-Y plane herein), to a three-dimensional (volume) imaging mode, i.e., having the capability of continuous imaging in the direction of the rotation axis (referred to as Z direction herein).
Currently, the CT machine adopts the third generation architecture, that is, the X-ray source position (bulb) and the X-ray detector are installed in the opposite direction to each other, and the relative position during the rotation process is kept unchanged.
The current mainstream scanning modes of the CT machine comprise a tomography mode and a helical scanning mode, wherein the tomography mode is defined in that a bulb tube and a detector keep rotating in the data acquisition time, and a scanning bed keeps static; helical scanning is defined as the time during which data is acquired while the bulb and detector remain rotating while the couch continues to travel.
With the widespread application of three-dimensional image display methods (MPR, VR, etc.) in clinical diagnosis, the helical scan mode and the imaging mode of CT machines are becoming the mainstream diagnostic methods of CT machines.
The main advantages of helical imaging mode over tomographic imaging mode are its better three-dimensional imaging capability and scanning speed, but helical scanning has some data scanning redundancy in the beginning and end of a period, so the disadvantage of dose efficiency is very obvious, and the scanning repeatability (i.e. the time period interval for repeated imaging in a certain coverage, also called time resolution capability) is not as good as tomographic imaging mode. Especially, in clinical applications such as cardiac coronary angiography and perfusion imaging, the dosage is several times different.
With the improvement of the acquisition technology, the number of physical layers of detectors of the CT machine is gradually increased, and at present, detectors with more than 256 layers and a coverage range of a Z direction reaching 16cm are commonly used by CT products of mainstream manufacturers, so that the coverage range of the tomography reaches an organ level, and the scanning speed of the tomography in the application of the organ level is superior to that of spiral scanning. And the advantages of the tomography in the aspects of dose efficiency and time resolution capability enable the tomography mode to replace a helical scanning mode for three-dimensional imaging under certain application conditions.
In a conventional tomography mode, a focus of a light source moves on a circular track of a rotation plane (an X-Y plane), the relative position of the light source and the focus of the light source does not change in the rotation process, projection data based on the circular track of the 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 computed tomography device, a computed tomography method and an image reconstruction method, and aims to solve the technical problem of aliasing artifacts of the conventional CT scanning system.
The embodiment of the invention provides a computed tomography device, which comprises a fixed frame 1, a rotating frame 2, a light source generating device 3, a signal detecting device 4, a scanned target supporting device 5 and a light source collimating device 8, wherein the light source collimating device is arranged on the fixed frame; 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 arranged on the rotating frame 2, can output scanning light rays of a conical beam, and continuously moves a focus 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 light source collimation device 8 translates along the Z direction along with the change of the focal position of the light source generation device 3, and the focal position of the light source collimation device 8 relative to the light source generation device 3 is fixed; 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 collimation device 8 comprises a ray shielding unit a and a ray shielding unit B which are controlled independently, the relative positions of the ray shielding unit a and the ray shielding unit B are not changed, and the ray shielding unit a and the ray shielding unit B form a light through hole.
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 further provides a scanning control method using any one of the above computed tomography apparatuses, including the following steps:
s101: the light source generating device and the signal detecting device scan in an X-Y plane in a rotating mode so as to meet the requirement of a tomography track;
s102: when tomography is started, the position of the focus of the light source generating device on the anode target surface is continuously changed, and the position of the focus is continuously moved along the Z direction according to a preset track;
s103: the ray shielding unit A and the ray shielding unit B do translation along the Z direction under the control of the driving motor, and the displacement change meets the following requirements:
ΔAz＝ΔBz＝Δz (1)
s104: and the signal detection device receives scanning signals obtained according to the preset track and transmits the obtained data to a reconstruction computer for data processing and image reconstruction.
Further, the slice scanning trajectory in step S101 satisfies:
Figure RE-GDA0001590446060000041
wherein the content of the first and second substances,
Figure RE-GDA0001590446060000042
showing the locus of the focus in the tomographic mode, RfDenotes the focal rotation radius, and β is the projection angle.
Further, the predetermined trajectory in step S102 satisfies:
Figure RE-GDA0001590446060000043
wherein, Delta Z is the focus offset generated on the Z coordinate caused by the displacement of the focus on the anode target surface,
Figure RE-GDA0001590446060000044
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 adjacent photosensitive elements of the signal detection device in the Z direction;
Δ R satisfies the relationship:
Figure RE-GDA0001590446060000045
alpha denotes the angle at which the anode target surface of the bulb is present.
In addition, an embodiment of the present invention further provides a method for reconstructing an image according to data obtained by the scanning control method, which is characterized by 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:
β is the projection angle of the cone beam P at the current focal position, i.e. the OS line and the Y axis form an angle, γ is the angle formed by the ray path and the central channel when the focal point is not shifted, θ is the angle formed by the ray path and the Y axis, and t is the distance from the ray path to the origin O, the following relationships are satisfied:
θ＝β+γ+Δγ (6)
t＝(Rf+ΔR)sin(γ+Δγ) (7)
Figure RE-GDA0001590446060000051
the projection data of the cone beam generated by tomography sampling is recorded as P (beta, gamma, b), and b is the position of the ray path reaching the signal detection device;
interpolating and rearranging the cone-beam of the acquired raw projection data into a wedge-beam data set by equations (6) - (8):
P(β,γ,b)→P(θ,t,b) (9)
further, the filtering and weighting the data rearranged in the step S202 includes:
Figure RE-GDA0001590446060000052
wherein the content of the first and second substances,
Figure RE-GDA0001590446060000053
representing that projection data rearranged into wedge-shaped beams are filtered layer by layer in the channel arrangement direction;
Figure RE-GDA0001590446060000054
is to weight the projection data;
Figure RE-GDA0001590446060000055
cone angle compensation is performed on the radiation.
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
Figure RE-GDA0001590446060000056
Obtaining through back projection:
Figure RE-GDA0001590446060000057
for an angle θ, the ray position t, b through point (x, y, z) can be calculated as follows:
t＝x cosθ+y sinθ (12)
Figure RE-GDA0001590446060000058
b is the offset of the detector's Z position relative to the detector's center layer after the wedge beam has passed through (x, y, Z) to reach the rearranged wedge beam at a projection angle θ.
Compared with the prior art, the invention at least has the following technical effects:
the present invention performs data acquisition by the above-described CT apparatus and scanning mode, so that the sampling frequency in each projection direction in the Z direction is doubled. The effect equivalent to the Z-direction flying focus in the prior art is achieved, so that the Z-direction spatial resolution of the system can be effectively improved, and aliasing artifacts are improved. 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-performance three-dimensional imaging is realized.
By matching with the dynamic control method of the CT collimation device described by the invention, the side effect of greatly increasing the dosage when the Z-direction flying focus is used can be avoided. By the CT reconstruction method, three-dimensional imaging of scanning data in a correct mode can be guaranteed, and the method is a key step for guaranteeing improvement of Z-direction spatial resolution of a system and improvement of aliasing artifacts.
The focus is not concentrated on a fixed point on the anode target surface of the bulb tube in the scanning process, so that the heat dissipation and the focus stability of the bulb tube are facilitated, the reliability of the image quality is improved, and the scanning capability of the CT machine is improved.
In conclusion, by applying the technical scheme, the three-dimensional imaging capability and the scanning capability of the tomography mode of the CT machine can be further improved, and excellent image quality can be obtained under the condition that the dose efficiency and the repeated scanning capability of helical scanning are better.
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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 provided in 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, a scanned target supporting device 5, and a light source collimating device 8; 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 arranged on the rotating frame 2, can output scanning light rays of a conical beam, and continuously moves a focus 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 light source collimation device 8 translates along the Z direction along with the change of the focal position of the light source generation device 3, and the focal position of the light source collimation device 8 relative to the light source generation device 3 is fixed; 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.
The CT machine is provided with a movable scanned target supporting device, the support device is kept still during scanning so as to meet the characteristics of a tomography mode, the preference is defined as a vertical machine frame structure provided with a scanning bed, a rotating frame can rotate around a fixed point of an X-Y plane under electric drive, and an X-Y-Z coordinate system meets the definition of a right-hand system.
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 corresponding 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 move quickly. In order to implement the scanning control method described in the present invention. The X-ray generator, i.e., a high-voltage device and an X-ray tube, is capable of outputting a cone beam of X-rays and has a characteristic of moving a focal position in a direction of an anode target rotation axis (i.e., a Z-direction), and is mounted on a rotating frame of a CT machine. 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.
An X-ray detection device, which is mounted on a rotating frame of a CT machine and is opposite to the position of an X-ray bulb tube so as to ensure that a cone-beam light source can be received by a detector area array and the relative position of the X-ray bulb tube is not changed in the rotating process, in order to adapt to an area array structure for collecting cone-beam, the sizes of photosensitive elements in the channel direction (XY plane extension direction) and the layer direction (Z direction) are close to or consistent with each other, as shown in figure 2;
the X-ray collimation device is characterized in that the X-ray collimation device is matched with the characteristic that the focus of a light source continuously shifts, shielding slices are respectively controlled by two groups of precise motors, and dynamic displacement is carried out in the scanning process;
specifically, the light source collimation device 8 comprises a ray shielding unit a and a ray shielding unit B which are controlled independently, the relative positions of the ray shielding unit a and the ray shielding unit B are not changed, and the ray shielding unit a and the ray shielding unit B form a light through hole.
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 spiral track 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 and the signal detecting device scan in an X-Y plane in a rotating mode so as to meet the requirement of a tomography track;
s102: when tomography is started, the position of the focus of the light source generating device on the anode target surface is continuously changed, and the position of the focus is continuously moved along the Z direction according to a preset track;
s103: the ray shielding unit A and the ray shielding unit B do translation along the Z direction under the control of the driving motor, and the displacement change meets the following requirements:
ΔAz＝ΔBz＝Δz (1)
the collimator is composed of two independent ray shielding units A and B, and the ray shielding units are made of X-ray high-attenuation materials, so that the ray emission source only allows rays with a certain angle to pass through in the direction of the rotating shaft. Each shielding unit can translate along the Z direction under the control of the driving motor, and the relative positions of A and B are not changed.
S104: and the signal detection device receives scanning signals obtained according to the preset track and transmits the obtained data to a reconstruction computer for data processing and image reconstruction.
Further, the slice scanning trajectory in step S101 satisfies:
Figure RE-GDA0001590446060000091
wherein the content of the first and second substances,
Figure RE-GDA0001590446060000092
showing the locus of the focus in the tomographic mode, RfDenotes the focal rotation radius, and β is the projection angle. For simplicity of description, the Z coordinate of the plane in which the focal point is located is assumed to be 0.
Further, the predetermined trajectory in step S102 satisfies:
Figure RE-GDA0001590446060000101
as can be seen from equation (3), if the rotation speed is constant, the focal point will be displaced at a uniform speed in the Z direction during scanning. The Δ R exists because the anode target surface of the bulb has a certain angle (assumed to be α), and therefore, not only the focal point has Z-directional displacement, but also the distance (rotation radius) from the focal point to the rotation center changes, and the relationship between Δ Z and Δ R satisfies:
Figure RE-GDA0001590446060000102
az is the focus shift occurring in the Z coordinate due to the displacement of the focus 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 adjacent photosensitive elements of the signal detection device in the Z direction;
Δ R satisfies the relationship:
Figure RE-GDA0001590446060000103
alpha denotes the angle at which the anode target surface of the bulb is present.
Equations (2-4) describe the focal position trajectory for the new scan mode of the present invention.
The range of data acquisition can be less than one circle, just one circle or more than one circle; the CT machine and the scanning mode are adopted for X-ray signal acquisition, and the obtained data are transmitted to a reconstruction computer for data processing and image reconstruction.
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.
Further, the step S202 includes:
β is the projection angle of the cone beam P at the current focal position, i.e. the OS line and the Y axis form an angle, γ is the angle formed by the ray path and the central channel when the focal point is not shifted, θ is the angle formed by the ray path and the Y axis, and t is the distance from the ray path to the origin O, the following relationships are satisfied:
θ＝β+γ+Δγ(6)
t＝(Rf+ΔR)sin(γ+Δγ) (7)
Figure RE-GDA0001590446060000111
the projection data of the cone beam generated by tomography sampling is recorded as P (beta, gamma, b), and b is the position of the ray path reaching the signal detection device;
interpolating and rearranging the cone-shaped beams of the acquired original projection data into a wedge-shaped beam data set through equations (6) to (8) (namely, the rotating plane is a fan-shaped beam, and the Z direction is still in a divergent shape):
P(β,γ,b)→P(θ,t,b) (9)
further, the filtering and weighting the data rearranged in the step S202 includes:
Figure RE-GDA0001590446060000112
wherein the content of the first and second substances,
Figure RE-GDA0001590446060000113
representing that projection data rearranged into wedge-shaped beams are filtered layer by layer in the channel arrangement direction, and h (t) can use Shepp-Logan convolution kernel;
Figure RE-GDA0001590446060000114
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
Figure RE-GDA0001590446060000115
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
Figure RE-GDA0001590446060000116
Is weighted by considering
Figure RE-GDA0001590446060000117
Figure RE-GDA0001590446060000118
Is to perform cone angle compensation on the ray, wherein the different focus position pairs can be ignored
Figure RE-GDA0001590446060000119
Calculating the effect of the difference, i.e. during the calculation
Figure RE-GDA00015904460600001110
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
Figure RE-GDA00015904460600001111
Obtaining through back projection:
Figure RE-GDA00015904460600001112
for an angle θ, the ray position t, b through point (x, y, z) can be calculated as follows:
t＝x cosθ+y sinθ (12)
Figure RE-GDA00015904460600001113
and b is the offset of the Z position of the virtual detector (the detector of the rearranged wedge-shaped beam) relative to the central layer of the detector after the wedge-shaped beam passes through (x, y, Z), namely the position of the detector layer where the ray is located when the projection angle is theta. As reflected in equation (13), the key point in the backprojection step differs from the known approach in that the effect of accounting for the focus offset is corrected in calculating the layer direction position b of the pixel ray passing through the reconstructed point at each projection angle.
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 present invention performs data acquisition by the above-described CT apparatus and scanning mode, so that the sampling frequency in each projection direction in the Z direction is doubled. The effect equivalent to the Z-direction flying focus in the prior art is achieved, so that the Z-direction spatial resolution of the system can be effectively improved, and aliasing artifacts are improved. 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-performance three-dimensional imaging is realized.
By matching with the dynamic control method of the CT collimation device described by the invention, the side effect of greatly increasing the dosage when the Z-direction flying focus is used can be avoided. By the CT reconstruction method, three-dimensional imaging of scanning data in a correct mode can be guaranteed, and the method is a key step for guaranteeing improvement of Z-direction spatial resolution of a system and improvement of aliasing artifacts.
The focus is not concentrated on a fixed point on the anode target surface of the bulb tube in the scanning process, so that the heat dissipation and the focus stability of the bulb tube are facilitated, the reliability of the image quality is improved, and the scanning capability of the CT machine is improved.
In conclusion, by applying the technical scheme, the three-dimensional imaging capability and the scanning capability of the tomography mode of the CT machine can be further improved, and excellent image quality can be obtained under the condition that the dose efficiency and the repeated scanning capability of helical scanning are better.
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 (5)

1. A scanning control method of a computer tomography device comprises a fixed frame (1), a rotating frame (2), a light source generating device (3), a signal detecting device (4), a scanned target supporting device (5) and a light source collimating device (8);
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 continuously moves a focus along the Z direction;
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 light source collimation device (8) translates along the Z direction along with the change of the focal position of the light source generation device (3), and the focal position of the light source collimation device (8) relative to the light source generation device (3) is fixed;
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 light source collimation device (8) comprises a ray shielding unit A and a ray shielding unit B which are controlled independently, the relative positions of the ray shielding unit A and the ray shielding unit B are unchanged, and the ray shielding unit A and the ray shielding unit B form a light through hole;
the method comprises the following steps:
s101: the light source generating device (3) and the signal detecting device (4) scan in an X-Y plane in a rotating mode so as to meet the requirement of a tomography track;
s102: when tomography is started, the position of the focus of the light source generating device (3) on the anode target surface is continuously changed, and the focus position is continuously moved along the Z direction according to a preset track;
s103: the ray shielding unit A and the ray shielding unit B do translation along the Z direction under the control of the driving motor, and the displacement change meets the following requirements:
ΔAz＝ΔBz＝Δz (1)
s104: the signal detection device (4) receives scanning signals obtained according to the preset track and transmits the obtained data to a reconstruction computer (6) for data processing and image reconstruction;
the slice scanning trajectory in step S101 satisfies:
Figure FDA0002996948610000021
wherein the content of the first and second substances,
Figure FDA0002996948610000022
showing the locus of the focus in the tomographic mode, RfRepresents the focal rotation radius, beta is the projection angle;
the predetermined trajectory in the step S102 satisfies:
Figure FDA0002996948610000023
wherein, Delta Z is the focus offset generated on the Z coordinate caused by the displacement of the focus on the anode target surface,
Figure FDA0002996948610000024
Δ β is the angular interval spanned by the current projection angle relative to the starting projection angle;
Rfdwhen the radius of rotation is RfThe distance from the focus to the signal detection device (4);
b is the interval of the adjacent photosensitive elements of the signal detection device (4) in the Z direction;
Δ R satisfies the relationship:
Figure FDA0002996948610000025
alpha denotes the angle at which the anode target surface of the bulb is present.
2. A method for image reconstruction from data obtained by a scan control method according to claim 1, 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.
3. The method of claim 2, wherein: the step S202 includes:
β is the projection angle of the cone beam P at the current focal position, i.e. the OS line and the Y axis form an angle, γ is the angle formed by the ray path and the central channel when the focal point is not shifted, θ is the angle formed by the ray path and the Y axis, and t is the distance from the ray path to the origin O, the following relationships are satisfied:
θ＝β+γ+Δγ (6)
t＝(Rf+ΔR)sin(γ+Δγ) (7)
Figure FDA0002996948610000031
the projection data of the cone beam generated by tomography sampling is recorded as P (beta, gamma, b), and b is the position of the ray path reaching the signal detection device (4);
interpolating and rearranging the cone-beam of the acquired raw projection data into a wedge-beam data set by equations (6) - (8):
P(β,γ,b)→P(θ,t,b) (9)。
4. the method of claim 3, wherein: filtering and weighting the data rearranged in the step S202, including:
Figure FDA0002996948610000032
wherein the content of the first and second substances,
Figure FDA0002996948610000033
representing that projection data rearranged into wedge-shaped beams are filtered layer by layer in the channel arrangement direction;
Figure FDA0002996948610000034
is to weight the projection data;
Figure FDA0002996948610000035
cone angle compensation is performed on the radiation.
5. The method of claim 4, 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
Figure FDA0002996948610000036
Obtaining through back projection:
Figure FDA0002996948610000037
for an angle θ, the ray position t, b through point (x, y, z) can be calculated as follows:
t＝x cosθ+y sinθ (12)
Figure FDA0002996948610000038
b is the offset of the detector's Z position relative to the detector's center layer after the wedge beam has passed through (x, y, Z) to reach the rearranged wedge beam at a projection angle θ.
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