CN111773560B - Raster position calibration and verification method based on EPID - Google Patents

Raster position calibration and verification method based on EPID Download PDF

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CN111773560B
CN111773560B CN202010660774.2A CN202010660774A CN111773560B CN 111773560 B CN111773560 B CN 111773560B CN 202010660774 A CN202010660774 A CN 202010660774A CN 111773560 B CN111773560 B CN 111773560B
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grating
epid
image
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calculating
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CN111773560A (en
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王忠淼
文虎儿
姚毅
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Suzhou Linatech Medical Science And Technology
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N5/00—Radiation therapy
    • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
    • A61N5/1048—Monitoring, verifying, controlling systems and methods
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N5/00—Radiation therapy
    • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
    • A61N5/1042—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy with spatial modulation of the radiation beam within the treatment head
    • A61N5/1045—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy with spatial modulation of the radiation beam within the treatment head using a multi-leaf collimator, e.g. for intensity modulated radiation therapy or IMRT
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N5/00—Radiation therapy
    • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
    • A61N5/1048—Monitoring, verifying, controlling systems and methods
    • A61N5/1049—Monitoring, verifying, controlling systems and methods for verifying the position of the patient with respect to the radiation beam
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N5/00—Radiation therapy
    • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
    • A61N5/1048—Monitoring, verifying, controlling systems and methods
    • A61N5/1049—Monitoring, verifying, controlling systems and methods for verifying the position of the patient with respect to the radiation beam
    • A61N2005/1054—Monitoring, verifying, controlling systems and methods for verifying the position of the patient with respect to the radiation beam using a portal imaging system

Abstract

The invention solves the problems that the detection result of the existing grating position calibration and verification method based on the light field and the EPID is not accurate and objective enough and the operation is complicated, and provides a novel grating position calibration and verification method based on the EPID.

Description

Raster position calibration and verification method based on EPID
Technical Field
The invention belongs to the technical field of medical treatment, and particularly relates to a grating position calibration and verification method based on an EPID.
Background
The grating is an essential collimation device in modern radiotherapy equipment, and can have a very good conformal effect on a treatment target area through the movement of the grating blades; a multi-leaf collimator (MLC) is a common grating used in clinical radiotherapy, and the more precise the motion control of the MLC is, the closer the treatment effect calculated by a Treatment Planning System (TPS) can be. Since the TPS design plans for the grating movement positions all at the isocenter plane (ISO), the MLC is mounted above the isocenter plane, as shown in fig. 1. Therefore, the physical position of the MLC in the grating plane needs to be correlated with the position of the isocenter plane, and the boundary position of the MLC in the isocenter plane needs to be detected.
The traditional method is that a piece of coordinate paper is laid on an isocentric plane, an MLC is irradiated by a light field, and the position of the MLC on the isocentric plane is obtained by reading the position of the MLC projected on the coordinate paper. One common method at present is to use an electronic portal imaging system (EPID) to record an image of the radiation passing through the MLC at the isocenter plane, and then detect the position of the MLC through image processing techniques. Some details of this scheme are presented below and the shortcomings of the existing methods are analyzed:
to establish an MLC planeAnd the coordinate system of the Isocenter (ISO) plane, as shown in fig. 1. Firstly, the projection position of the MLC coordinate origin on the isocenter plane is required to be coincident with the coordinate origin of the isocenter plane. The current commonly used method for determining the origin of coordinates of the isocenter plane is to identify a laser light through a front pointer on the head of the accelerator, the laser light identifying a position of the isocenter plane (e.g., O in fig. 1)3) Then, the tungsten ball is placed at the position, an image at the moment is recorded by using the EPID under the condition that the accelerator emits beams, and finally, the center of the tungsten ball is extracted by using an image processing technology and is considered to be the coordinate origin of the isocentric plane. However, due to the influence of the penumbra, the edge of the tungsten ball image recorded by the EPID is blurred, and the center of the tungsten ball extracted by the image processing technology is not accurate enough.
In addition, to make the projection of the origin of coordinates of the MLC on the isocentric plane coincide with the center of the tungsten sphere, a common method at present is to open a pair of left and right leaves at the center of the grating, and let the left and right leaves at other positions have a relatively small interval to form a cross-shaped field, as shown in fig. 6. The tungsten ball was held stationary and the image at that time was recorded using the EPID with the accelerator beam out, as shown in fig. 7. And then observing whether the center of the tungsten ball is at the center of the cross, if not, adjusting the position of the MLC until the center of the tungsten ball is superposed with the center of the cross, thus ensuring that the origin of a physical coordinate system of the MLC corresponds to the origin of coordinates of the isocenter plane.
In summary, the above solutions face the following problems;
when the projection position of the MLC coordinate origin on the isocenter plane is determined, the center of the tungsten ball extracted by the image processing technology is required to be accurate enough, and after the cross-shaped field is opened by the grating, whether the origin of the grating coordinate is overlapped with the calculated center of the tungsten ball is determined by visual observation, so that the method is not accurate and objective.
Disclosure of Invention
In order to solve the technical problems that the detection result of the existing grating position calibration and verification method based on the light field and the EPID is not accurate and objective enough and the operation is complicated, the invention provides a novel grating position calibration and verification method based on the EPID.
In order to achieve the purpose, the technical scheme of the invention is as follows:
a grating position calibration and verification method based on EPID specifically comprises the following steps:
s1: determining the projection position of the origin of the physical coordinate system of the grating plane on the isocenter plane, which specifically comprises the following steps:
s1.1: only extending out the blades at the centers of the upper layer grating and the lower layer grating and moving a distance beyond the center position to form a cross shape; in the case of accelerator beam-out, recording the image at this time by using the EPID;
s1.2: extracting two boundary positions of the upper grating blade and the lower grating blade along the motion direction respectively by using an image processing technology, and calculating the average value of the two boundary positions to obtain the coordinate origin of the isocenter plane;
s2: determining the installation height of the EPID;
s3: let the upper grating and the lower grating of the grating plane move to n different positions and be recorded as x ═ x1，x2，...xn]TAnd recording an image at each position by using the EPID under the condition that the accelerator is out of the beam, and recording the acquired image as f1，f2，...fn；
S4: image f recorded for each EPIDiN, detecting the position of the MLC in the isocenter plane by adopting an image processing technology to obtain y ═ y ·1，y2，...yn]T。
On the basis of the technical scheme, the following improvements can be made:
preferably, in step S1.1,
if the number of the blades of the upper layer grating or the lower layer grating is odd, the blade at the center of the upper layer grating or the lower layer grating is only one blade;
if the number of the blades of the upper layer grating or the lower layer grating is an even number, the blades at the center of the upper layer grating or the lower layer grating are two adjacent blades.
Preferably, step S2 specifically includes the following steps:
calculating the width d of the blade on the EPID image by using the two boundary positions of the upper grating along the moving direction obtained in the step S11′MLCRecording the actual physical width of the central blade of the upper grating as d1MLC；
The mounting height SDD of the EPID is calculated by the following formula:
Figure GDA0003394632560000041
wherein: SCD1Is the distance from the radioactive source to the center of the upper grating;
or the like, or, alternatively,
the width d of the blade on the EPID image is calculated by using the two boundary positions of the lower grating along the moving direction obtained in step S12′MLCRecording the actual physical width of the central blade of the lower grating as d2MLC；
The mounting height SDD of the EPID is calculated by the following formula:
Figure GDA0003394632560000042
wherein: SCD2Is the distance from the radioactive source to the center of the lower grating.
Preferably, the detection method further comprises:
s5: for x ═ x1，x2，...xn]TAnd y ═ y1，y2，...yn]TAnd establishing a relation between the physical position x of the MLC in the grating plane and the position y of the isocenter plane, and recording the relation as a function x as g (y).
Preferably, in step S5, the function is obtained by one of the following methods: and obtaining a correlation coefficient by adopting polynomial fitting or adopting a piecewise linear interpolation method.
Preferably, the detection method further comprises:
s6: verifying the rationality of the established relation, specifically comprising the following steps:
s6.1: given a set of isocenter planes, yv＝[y1，y2，...ys]TCalculating the corresponding physical movement position x of the grating according to the function obtained in step S5v＝[x1，x2，...xs]T；
S6.2: let the gratings move to x respectivelyvAnd recording images of the grating at different positions by using EPID under the condition of beam outgoing of the accelerator, and detecting the position y of the grating at the isocenter plane by adopting an image processing technologymeasure＝[y1，y2，...ys]T；
S6.3: comparison of yvAnd ymeasureIf the error of the two is in a reasonable range, the detected position, the established physical position of the grating plane and the position relation of the isocenter plane are considered to be reliable;
if the error of the two is not in the reasonable range, the relational expression is considered to be unreliable, and the steps S1-S5 are repeated again; or, re-establishing the relation between the physical position x of the MLC at the grating plane and the position y of the isocenter plane.
Preferably, the image processing technology comprises: the boundary position of the grating is obtained by searching the position of the maximum slope of each line and/or each column on the EPID image and then calculating the average value of all the positions of the maximum slope.
As a preferred solution, the image processing technique in step S1.2 specifically includes the following:
(A) calculating the slope of each line on the image obtained in step S1.1, and finding out the position r with the maximum slopek(ii) a Wherein k is 1,21，m1Is the total number of lines of the image;
(B) for all rkCalculate the mean value rowcNamely:
Figure GDA0003394632560000051
(C) calculating the slope of each column on the image obtained in step S1.1, and finding the position c with the maximum slopek(ii) a Wherein k is 1,22，m2Is the total number of columns of the image;
(D) for all ckCalculate the mean value colcNamely:
Figure GDA0003394632560000061
(E) obtaining the origin of coordinates (row) of the isocentric planec，colc)。
Preferably, step S4 specifically includes the following steps:
(a) calculating the slope of each line on each image and finding the position r with the maximum slopek(ii) a Wherein k is 1, 2.. m, m is the total number of rows of the image;
(b) for all rkCalculate the mean value rowiNamely:
Figure GDA0003394632560000062
(d) the pixel size of EPID is pd × pd in terms of center position (row)c，colc) The position of the MLC in the isocenter plane can be calculated according to the following formula;
Figure GDA0003394632560000063
wherein: SAD is the distance from the source to the isocenter plane.
Preferably, the image processing technique further includes the following first steps: and performing smooth filtering and threshold processing on the image, and setting all pixels with pixel values larger than t (t is more than 0.5 and less than 1) times of the maximum pixel value in the image as t times of the maximum value.
The invention has the following beneficial effects:
the invention provides a novel grating position calibration and verification method based on EPID, which can effectively solve the position detection problem of a double-layer grating in an isocenter plane, is simple and convenient to operate, can improve the detection precision, and has extremely important significance for clinic.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present invention and therefore should not be considered as limiting the scope, and for those skilled in the art, other related drawings can be obtained according to the drawings without inventive efforts.
FIG. 1 is a schematic view of a space for installing a grating and a coordinate system definition;
FIG. 2 is an exemplary diagram of a ray of an EPID record after being rastered;
FIG. 3 is a horizontal section line of an exemplary graph of rays recorded by an EPID after they have passed through a grating;
FIG. 4 is a flowchart of a method for calibrating and verifying a raster position based on an EPID according to an embodiment of the present invention;
FIG. 5 is a flowchart of an algorithm for an image processing technique for detecting the position of an MLC according to an embodiment of the present invention;
FIG. 6 is a schematic view of MLC cross leaf position;
FIG. 7 is an EPID image of an MLC open cross field and illuminated tungsten ball;
FIG. 8 is an exemplary diagram of the positions of the detected rasters on the EPID image (white lines are the detected raster positions) according to an embodiment of the present invention;
fig. 9 is a schematic position diagram of a double-layer grating cross-shaped blade according to an embodiment of the present invention.
Detailed Description
Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and 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 equipment based on the invention is an electronic radiation field image system (EPID) and an orthogonal double-layer grating, the EPID can measure the intensity of rays which are emitted from a radiation source and penetrate through a patient in radiotherapy and convert the intensity of the rays into an electric signal to form a digital image, and the EPID is an important tool for verifying the positioning accuracy of the radiotherapy; the orthogonal double-layer grating is divided into an upper layer and a lower layer, and the motion directions of the two layers of gratings are orthogonal to each other and are perpendicular to the ray direction.
As shown in fig. 4, an embodiment of the present invention provides a grating position calibration and verification method based on an EPID, which specifically includes the following steps:
s1: determining the projection position of the origin of the physical coordinate system of the grating plane on the isocenter plane, which specifically comprises the following steps:
s1.1: only the blades at the centers of the upper layer grating and the lower layer grating extend out and move a distance beyond the center position to form a cross shape, as shown in fig. 9; in the case of accelerator beam-out, recording the image at this time by using the EPID;
s1.2: extracting two boundary positions of the upper grating blade and the lower grating blade along the motion direction respectively by using an image processing technology, and calculating the average value of the two boundary positions to obtain the coordinate origin of the isocenter plane;
s2: determining the installation height of the EPID;
s3: let the upper grating and the lower grating of the grating plane move to n different positions and be recorded as x ═ x1，x2，...xn]TAnd recording an image at each position by using the EPID under the condition that the accelerator is out of the beam, and recording the acquired image as f1，f2，...fn；
S4: image f recorded for each EPIDiN, detecting the position of the MLC in the isocenter plane by adopting an image processing technology to obtain y ═ y ·1，y2，...yn]T。
Compared with the method for determining the origin of coordinates of an isocenter plane by using a tungsten ball in the background art, the method is simpler and more accurate.
In order to further optimize the working effect of the invention, in other embodiments, the remaining features are the same, except that, in step S1.1,
if the number of the blades of the upper layer grating or the lower layer grating is odd, the blade at the center of the upper layer grating or the lower layer grating is only one blade;
if the number of the blades of the upper layer grating or the lower layer grating is an even number, the blades at the center of the upper layer grating or the lower layer grating are two adjacent blades.
In order to further optimize the implementation effect of the present invention, in other embodiments, the remaining features are the same, except that step S2 specifically includes the following steps:
calculating the width d of the blade on the EPID image by using the two boundary positions of the upper grating along the moving direction obtained in the step S11′MLCRecording the actual physical width of the central blade of the upper grating as d1MLC；
The mounting height SDD of the EPID is calculated by the following formula:
Figure GDA0003394632560000091
wherein: SCD1Is the distance from the radioactive source to the center of the upper grating;
or the like, or, alternatively,
the width d of the blade on the EPID image is calculated by using the two boundary positions of the lower grating along the moving direction obtained in step S12′MLCRecording the actual physical width of the central blade of the lower grating as d2MLC；
The mounting height SDD of the EPID is calculated by the following formula:
Figure GDA0003394632560000092
wherein: SCD2Is the distance from the radioactive source to the center of the lower grating.
It should be noted that step S2 may be performed in a conventional manner to obtain the mounting height of the EPID, or may be performed more preferably by using the method of the present invention.
However, the current common method is to place a known width d in the isocenter planeISOThen letting the accelerator beam out and acquiring an image by using the EPID, and acquiring the width d of the phantom by an image processing technologyEPIDThe mounting height of the EPID is calculated by the following formula
Figure GDA0003394632560000101
It has the following problems: when the installation height of the EPID is determined, additional die bodies need to be placed. The method for determining the mounting height of the EPID does not need to additionally arrange a die body, and is simpler.
In order to further optimize the implementation effect of the present invention, in other embodiments, the remaining features are the same, except that the detection method further includes:
s5: for x ═ x1，x2，...xn]TAnd y ═ y1，y2，...yn]TAnd establishing a relation between the physical position x of the MLC in the grating plane and the position y of the isocenter plane, and recording the relation as a function x as g (y).
Further, in step S5, the function is obtained by one of the following methods: and obtaining a correlation coefficient by adopting polynomial fitting or adopting a piecewise linear interpolation method.
Further, the detection method further comprises the following steps:
s6: verifying the rationality of the established relation, specifically comprising the following steps:
s6.1: given a set of isocenter planes, yv＝[y1，y2，...ys]TCalculating the corresponding physical movement position x of the grating according to the function obtained in step S5v＝[x1，x2，...xs]T；
S6.2: let the gratings move to x respectivelyvAnd recording images of the grating at different positions by using EPID under the condition of beam outgoing of the accelerator, and detecting the position y of the grating at the isocenter plane by adopting an image processing technologymeasure＝[y1，y2，...ys]T；
S6.3: comparison of yvAnd ymeasureIf the error of the two is in a reasonable range, the detected position, the established physical position of the grating plane and the position relation of the isocenter plane are considered to be reliable;
if the error of the two is not in the reasonable range, the relational expression is considered to be unreliable, and the steps S1-S5 are repeated again; or, re-establishing the relation between the physical position x of the MLC at the grating plane and the position y of the isocenter plane.
In order to further optimize the implementation effect of the present invention, in other embodiments, the rest of feature technologies are the same, except that the image processing technology is as follows: the boundary position of the grating is obtained by searching the position of the maximum slope of each line and/or each column on the EPID image and then calculating the average value of all the positions of the maximum slope.
Further supplementing the above embodiment, the image processing technique in step S1.2 specifically includes the following:
(A) calculating the slope of each line on the image obtained in step S1.1, and finding out the position r with the maximum slopek(ii) a Wherein k is 1,21，m1Is the total number of lines of the image;
(B) for all rkCalculate the mean value rowcNamely:
Figure GDA0003394632560000111
(C) calculating the slope of each column on the image obtained in step S1.1, and finding the position c with the maximum slopek(ii) a Wherein k is 1,22，m2Is the total number of columns of the image;
(D) for all ckCalculate the mean value colcNamely:
Figure GDA0003394632560000112
(E) obtaining the origin of coordinates (row) of the isocentric planec，colc)。
Further supplementing the above embodiment, as shown in fig. 5, step S4 specifically includes the following steps:
(a) calculating the slope of each line on each image and finding the position r with the maximum slopek(ii) a Wherein k is 1, 2.. m, m is the total number of rows of the image;
(b) for all rkCalculate the mean value rowiNamely:
Figure GDA0003394632560000121
(d) the pixel size of EPID is pd × pd in terms of center position (row)c，colc) The position of the MLC in the isocenter plane can be calculated according to the following formula;
Figure GDA0003394632560000122
wherein: SAD is the distance from the source to the isocenter plane.
Further supplementing the above embodiment, the image processing technique in the above steps S2, S4, S6 further includes the first step: and performing smooth filtering and threshold processing on the image, and setting all pixels with pixel values larger than t (t is more than 0.5 and less than 1) times of the maximum pixel value in the image as t times of the maximum value.
In the threshold processing step, since the pixel value of the EPID image reflects the intensity of the received ray, all pixels in the image with pixel values greater than t (0.5< t <1) times the maximum pixel value are set to be t times the maximum value, and the boundary position of the MLC will not be in these high-intensity areas. As shown in fig. 8, which is an exemplary diagram of the raster positions detected on an EPID image by the image processing technique of the present invention, white lines are the detected raster positions.
It should be noted that the image processing technique applied in steps S1, S4, and S6 of the present application may also be processed in the prior art to extract the boundary of the phantom, but the disadvantages of the prior image processing technique for detecting the MLC position will be analyzed in combination with the features of the EPID image.
As shown in fig. 2, the dark area in fig. 2 is an MLC blocking area, and the bright area is an area that is not blocked by the MLC, and a bright-dark transition area is also generated due to the influence of the penumbra; the cross section lines shown in fig. 3 can more intuitively reflect the relationship between the light intensities of different regions.
At present, a commonly used image processing technology for detecting the MLC position adopts a threshold method, i.e. an area with intensity lower than a certain threshold (generally taking the average value of light intensity) is considered to be an area blocked by the MLC, an area greater than or equal to the threshold is a non-MLC-blocked area, and the boundary of the two areas is determined as the boundary position of the MLC on the isocentric plane.
Threshold-based boundary detection techniques, where different thresholds detect different locations; but also noise in the image may interfere with the selection of the threshold.
In summary, the detection results of the existing grating position calibration and verification methods based on the light field and the EPID are not accurate and objective enough, and the operation is complicated. The invention provides a new grating position calibration and verification method based on EPID, the core of the method is to fully utilize the characteristics of orthogonal double-layer grating to determine the origin of coordinates of an isocenter plane and the installation height of EPID, and the operation is simple and convenient; and the boundary position of the grating is obtained by searching the position with the maximum slope of each line or each column on the EPID image and then calculating the average value of all the positions with the maximum slope. Meanwhile, the invention also establishes a relational expression of the physical position of the grating and the position of the grating of the isocentric plane, and provides an experimental scheme for verifying the rationality of the established relational expression.
The invention has the following beneficial effects:
the invention provides a novel grating position calibration and verification method based on EPID, which can effectively solve the position detection problem of a double-layer grating in an isocenter plane, is simple and convenient to operate, can improve the detection precision, and has extremely important significance for clinic.
The various embodiments above may be implemented in cross-parallel.
The above embodiments are merely illustrative of the technical concept and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the content of the present invention and implement the present invention, and not to limit the scope of the present invention, and all equivalent changes or modifications made according to the spirit of the present invention should be covered in the scope of the present invention.

Claims (5)

1. A grating position calibration and verification method based on EPID is characterized by comprising the following steps:
s1: determining the projection position of the origin of the physical coordinate system of the grating plane on the isocenter plane, which specifically comprises the following steps:
s1.1: only extending out the blades at the centers of the upper layer grating and the lower layer grating and moving a distance beyond the center position to form a cross shape; in the case of accelerator beam-out, recording the image at this time by using the EPID;
s1.2: extracting two boundary positions of the upper grating blade and the lower grating blade along the motion direction respectively by using an image processing technology, and calculating the average value of the two boundary positions to obtain the coordinate origin of the isocenter plane;
s2: determining the installation height of the EPID;
s3: the upper layer grating and the lower layer grating of the grating plane are moved to nDifferent positions are denoted as x ═ x1，x2，...xn]TAnd recording an image at each position by using the EPID under the condition that the accelerator is out of the beam, and recording the acquired image as f1，f2，...fn；
S4: image f recorded for each EPIDiN, detecting the position of the MLC in the isocenter plane by adopting an image processing technology to obtain y ═ y ·1，y2，...yn]T；
S5: for x ═ x1，x2，...xn]TAnd y ═ y1，y2，...yn]TEstablishing a relation between the physical position x of the MLC in the raster plane and the position y of the isocenter plane, where the relation is denoted as a function x ═ g (y), and in step S5, obtaining the function in one of the following manners: obtaining a correlation coefficient by adopting polynomial fitting or adopting a piecewise linear interpolation method;
the step S2 specifically includes the following steps:
calculating the width of the blade on the EPID image by using the two boundary positions of the upper layer raster along the moving direction obtained in step S1
Figure FDA0003394632550000011
The actual physical width of the central blade of the upper grating is recorded as
Figure FDA0003394632550000012
The mounting height SDD of the EPID is calculated by the following formula:
Figure FDA0003394632550000021
wherein: SCD1Is the distance from the radioactive source to the center of the upper grating;
or the like, or, alternatively,
calculating the position of the blade on the EPID image by using the two boundary positions of the lower grating along the moving direction obtained in step S1Width of (2)
Figure FDA0003394632550000022
The actual physical width of the central blade of the lower layer grating is recorded as
Figure FDA0003394632550000023
The mounting height SDD of the EPID is calculated by the following formula:
Figure FDA0003394632550000024
wherein: SCD2Is the distance from the radioactive source to the center of the lower grating;
the image processing technology comprises the following steps: the boundary position of the grating is obtained by searching the position with the maximum slope of each line and/or each column on the EPID image and then calculating the average value of all the positions with the maximum slope;
the step S4 specifically includes the following steps:
(a) calculating the slope of each line on each image and finding the position r with the maximum slopek(ii) a Wherein k is 1,2, … m, and m is the total number of lines of the image;
(b) for all rkCalculate the mean value rowiNamely:
Figure FDA0003394632550000025
(d) the pixel size of EPID is pd × pd in terms of center position (row)c,colc) The position of the MLC in the isocenter plane can be calculated according to the following formula;
Figure FDA0003394632550000026
wherein: SAD is the distance from the source to the isocenter plane.
2. The EPID-based grating position calibration and verification method according to claim 1, wherein, in the step S1.1,
if the number of the blades of the upper layer grating or the lower layer grating is odd, the blade at the center of the upper layer grating or the lower layer grating is only one blade;
if the number of the blades of the upper layer grating or the lower layer grating is an even number, the blades at the center of the upper layer grating or the lower layer grating are two adjacent blades.
3. The EPID-based raster position calibration and verification method of claim 1, further comprising:
s6: verifying the rationality of the established relation, specifically comprising the following steps:
s6.1: given a set of isocenter planes, yv＝[y1，y2，...ys]TCalculating the corresponding physical movement position x of the grating according to the function obtained in the step S5v＝[x1，x2，...xs]T；
S6.2: let the gratings move to x respectivelyvAnd recording images of the grating at different positions by using EPID under the condition of beam outgoing of the accelerator, and detecting the position y of the grating at the isocenter plane by adopting an image processing technologymeasure＝[y1，y2，...ys]T；
S6.3: comparison of yvAnd ymeasureIf the error of the two is in a reasonable range, the detected position, the established physical position of the grating plane and the position relation of the isocenter plane are considered to be reliable;
if the error of the two is not in the reasonable range, the relational expression is considered to be unreliable, and the steps S1-S5 are repeated again;
or, re-establishing the relation between the physical position x of the MLC at the grating plane and the position y of the isocenter plane.
4. The EPID-based raster position calibration and verification method according to claim 1, wherein the image processing technique in step S1.2 specifically includes the following:
(A) calculating the slope of each line on the image obtained in the step S1.1, and finding out the position r with the maximum slopek(ii) a Wherein k is 1,21，m1Is the total number of lines of the image;
(B) for all rkCalculate the mean value rowcNamely:
Figure FDA0003394632550000041
(C) calculating the slope of each column on the image obtained in the step S1.1, and finding out the position c with the maximum slopek(ii) a Wherein k is 1,2, … m2，m2Is the total number of columns of the image;
(D) for all ckCalculate the mean value colcNamely:
Figure FDA0003394632550000042
(E) obtaining the origin of coordinates (row) of the isocentric planec,colc)。
5. The EPID-based raster position calibration and verification method according to claim 1, wherein the image processing technique further comprises the first step of: and performing smooth filtering and threshold processing on the image, and setting all pixels with pixel values larger than t (0.5< t <1) times of the maximum pixel value in the image as t times of the maximum value.
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