CN116794586A - Measurement method and measurement system for linearity of gradient coil - Google Patents
Measurement method and measurement system for linearity of gradient coil Download PDFInfo
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Abstract
The invention relates to the technical field of magnetic resonance imaging, in particular to a measuring method and a measuring system for linearity of a gradient coil, which are used for measuring and acquiring a background magnetic field through a magnetic field acquisition device; respectively and independently powering on a Gx coil, a Gy coil and a Gz coil of the gradient coil, and measuring and acquiring corresponding Gx working magnetic field, gy working magnetic field and Gz working magnetic field through a magnetic field acquisition device; according to the Gx working magnetic field, the Gy working magnetic field, the Gz working magnetic field and the background magnetic field, obtaining corresponding Gx center gradient strength, gy center gradient strength and Gz center gradient strength through spherical harmonic transformation; and determining the Gx coil linearity, the Gy coil linearity and the Gz coil linearity of the gradient coil according to the Gx working magnetic field, the Gx center gradient strength, the Gy working magnetic field, the Gy center gradient strength, the Gz working magnetic field, the Gz center gradient strength and the background magnetic field. The invention reduces the measurement cost, improves the measurement efficiency and ensures high measurement accuracy.
Description
Technical Field
The invention relates to the technical field of magnetic resonance imaging, in particular to a measuring method and a measuring system for linearity of a gradient coil.
Background
The linearity of the gradient coil is a key performance parameter of the gradient coil, and directly determines the spatial deformation degree of the magnetic resonance image. In order to predict the imaging quality of a magnetic resonance system, the linearity of the different imaging regions of the gradient coil needs to be evaluated in advance.
The gradient coils typically comprise three sets of coils Gx, gy and Gz, as key core components of the magnetic resonance apparatus, which generate varying spatial gradient magnetic fields for spatial encoding in the imaging region. The magnetic resonance image provides tissue structure information required for clinical diagnosis, and the spatial distortion degree of the image is an important measurement index of image quality. The spatial distortion of the image is mainly caused by magnetic field nonlinearities of the gradient coils and magnetic resonance artifacts.
The traditional method for measuring the linearity of the gradient coil is implemented on a magnetic resonance system, a characteristic water film is imaged by using a sequence, then characteristic data of the characteristic water film is read from the image, and the read characteristic data and the actual characteristic size of the characteristic water film are compared, so that the linearity of a corresponding area of the water film is obtained. The traditional measurement method must be implemented on a magnetic resonance system with imaging conditions, and the gradient coils cannot be tested independently before assembly, so that the space occupation is large, the measurement system is complex, a corresponding shielding room is required to be configured, the cost is high, and the steps required for solving the problems after the problems are found are complex.
Therefore, how to provide a method for measuring the linearity of the gradient coil device, which can independently measure the linearity of the gradient coil device, has high measurement accuracy and low measurement cost, and is simple to operate, is a problem to be solved by those skilled in the art.
Disclosure of Invention
The invention aims to provide a measuring method and a measuring system for linearity of a gradient coil, which are used for solving the problems of high cost, complex steps and low accuracy in the linearity measurement of a gradient coil device in the prior art.
In order to solve the technical problems, the invention provides a method for measuring the linearity of a gradient coil, which comprises the following steps:
measuring and acquiring a background magnetic field through a magnetic field acquisition device;
respectively and independently powering on a Gx coil, a Gy coil and a Gz coil of the gradient coil, and measuring and acquiring corresponding Gx working magnetic field, gy working magnetic field and Gz working magnetic field through the magnetic field acquisition device;
according to the Gx working magnetic field, the Gy working magnetic field, the Gz working magnetic field and the background magnetic field, obtaining corresponding Gx center gradient strength, gy center gradient strength and Gz center gradient strength through spherical harmonic transformation;
and determining the Gx coil linearity, the Gy coil linearity and the Gz coil linearity of the gradient coil according to the Gx working magnetic field, the Gx center gradient strength, the Gy working magnetic field, the Gy center gradient strength, the Gz working magnetic field, the Gz center gradient strength and the background magnetic field.
Optionally, in the method for measuring the linearity of the gradient coil, before measuring and acquiring the Gx working magnetic field, the Gy working magnetic field and the Gz working magnetic field, the method further includes:
respectively and independently powering on a Gx coil, a Gy coil and a Gz coil of the gradient coil, and measuring and acquiring corresponding Gx coil positioning magnetic field, gy coil positioning magnetic field and Gz coil positioning magnetic field through the magnetic field acquisition device;
obtaining a triaxial offset and an azimuth offset corresponding to the magnetic field acquisition device through spherical harmonic transformation according to the Gx coil positioning magnetic field, the Gy coil positioning magnetic field, the Gz coil positioning magnetic field and the background magnetic field;
the positioning of the magnetic field acquisition device is adjusted through the triaxial offset and the azimuth angle offset;
correspondingly, the step of respectively and independently electrifying the Gx coil, the Gy coil and the Gz coil of the gradient coil, and the step of measuring and obtaining the corresponding Gx working magnetic field, gy working magnetic field and Gz working magnetic field through the magnetic field acquisition device comprises the following steps:
and respectively and independently powering on the Gx coil, the Gy coil and the Gz coil of the gradient coil, and measuring and acquiring corresponding Gx working magnetic field, gy working magnetic field and Gz working magnetic field through the magnetic field acquisition device with adjusted positioning.
Optionally, in the method for measuring the linearity of the gradient coil, from obtaining the Gx coil positioning magnetic field, the Gy coil positioning magnetic field, and the Gz coil positioning magnetic field to adjusting the positioning of the magnetic field acquisition device by the triaxial offset and the azimuth offset includes:
measuring and obtaining the Gx coil positioning magnetic field;
according to the background magnetic field and the Gx coil positioning magnetic field, determining an x-axis offset and an azimuth angle offset through spherical harmonic transformation;
adjusting and moving the magnetic field acquisition device according to the x-axis offset and the azimuth angle offset;
after the adjustment of the x-axis offset and the azimuth angle offset is completed, measuring and acquiring the background magnetic field and the Gy coil positioning magnetic field;
determining a y-axis offset through spherical harmonic transformation according to the background magnetic field and the Gy coil positioning magnetic field;
adjusting and moving the magnetic field acquisition device according to the y-axis offset;
after the y-axis offset is adjusted, measuring and acquiring the background magnetic field and the Gz coil positioning magnetic field;
determining a z-axis offset through spherical harmonic transformation according to the background magnetic field and the Gz coil positioning magnetic field;
And adjusting and moving the magnetic field acquisition device according to the z-axis offset to finish positioning and adjusting the magnetic field acquisition device.
Optionally, in the method for measuring linearity of a gradient coil, determining the x-axis offset by spherical harmonic transformation according to the background magnetic field and the Gx coil positioning magnetic field includes:
according to the background magnetic field and the Gx coil positioning magnetic field, a corresponding spherical harmonic component is obtained through spherical harmonic transformation;
the x-axis offset is determined by:
dx=a00x/a11x
wherein a00x and a11x are corresponding spherical harmonic components, and dx is the x-axis offset;
and/or
The determining the y-axis offset by spherical harmonic transformation according to the background magnetic field and the Gy coil positioning magnetic field comprises:
according to the background magnetic field and the Gy coil positioning magnetic field, a corresponding spherical harmonic component is obtained through spherical harmonic transformation;
the y-axis offset is determined by:
dy=a00y/b11y
wherein a00y and b11y are corresponding spherical harmonic components, and dy is the y-axis offset;
and/or
The determining the z-axis offset by spherical harmonic transformation according to the background magnetic field and the Gz coil positioning magnetic field comprises:
according to the background magnetic field and the Gz coil positioning magnetic field, a corresponding spherical harmonic component is obtained through spherical harmonic transformation;
The z-axis offset is determined by:
dz=a00z/a10z
where a00z and a10z are the corresponding spherical harmonic components and dz is the z-axis offset.
Optionally, in the method for measuring linearity of a gradient coil, determining the azimuth offset by spherical harmonic transformation according to the background magnetic field and the Gx coil positioning magnetic field includes:
according to the background magnetic field and the Gx coil positioning magnetic field, a corresponding spherical harmonic component is obtained through spherical harmonic transformation;
the azimuth offset is determined by:
wherein b11x and a11x are the corresponding spherical harmonic components,is the azimuth offset.
Optionally, in the method for measuring linearity of a gradient coil, after adjusting and moving the magnetic field acquisition device according to the x-axis offset and the azimuth offset, the method further includes:
acquiring a new background magnetic field and a new Gx coil positioning magnetic field corresponding to the magnetic field acquisition device after adjustment movement, determining a new x-axis offset and a new azimuth offset according to the new background magnetic field and the new Gx coil positioning magnetic field through spherical harmonic transformation, judging whether the new x-axis offset and the new azimuth offset are smaller than a preset tolerance threshold, and continuously acquiring the new background magnetic field and the new Gx coil positioning magnetic field after adjustment movement of the magnetic field acquisition device according to the new x-axis offset and the new azimuth offset when the new x-axis offset and the new azimuth offset are not smaller than the preset tolerance threshold until the obtained new x-axis offset and new azimuth offset are smaller than the tolerance threshold;
Correspondingly, after the magnetic field acquisition device is adjusted and moved according to the y-axis offset, the method further comprises the following steps:
acquiring a new background magnetic field and a new Gy coil positioning magnetic field corresponding to the magnetic field acquisition device after adjustment and movement, determining a new y-axis offset according to the new background magnetic field and the new Gy coil positioning magnetic field through spherical harmonic transformation, judging whether the new y-axis offset is smaller than the tolerance threshold, and continuously acquiring the new background magnetic field and the new Gy coil positioning magnetic field after adjustment and movement of the magnetic field acquisition device according to the new y-axis offset when the new y-axis offset is not smaller than the preset tolerance threshold until the obtained new y-axis offset is smaller than the tolerance threshold;
correspondingly, after the magnetic field acquisition device is adjusted and moved according to the z-axis offset, the method further comprises the following steps:
acquiring a new background magnetic field and a new Gz coil positioning magnetic field corresponding to the magnetic field acquisition device after adjustment and movement, determining a new z-axis offset according to the new background magnetic field and the new Gz coil positioning magnetic field through spherical harmonic transformation, judging whether the new z-axis offset is smaller than the tolerance threshold, and continuously acquiring the new background magnetic field and the new Gz coil positioning magnetic field after adjustment and movement of the magnetic field acquisition device according to the new z-axis offset when the new z-axis offset is not smaller than the preset tolerance threshold until the obtained new z-axis offset is smaller than the tolerance threshold.
Optionally, in the method for measuring linearity of a gradient coil, the tolerance threshold includes an azimuth angle threshold, an x-axis threshold, a y-axis threshold, and a z-axis threshold;
the x-axis threshold, the y-axis threshold, and the z-axis threshold are not uniform.
Optionally, in the method for measuring linearity of a gradient coil, measuring and acquiring a positioning magnetic field and/or a working magnetic field corresponding to the gradient coil includes:
electrifying a Gx coil, a Gy coil or a Gz coil of the gradient coil, and acquiring a corresponding positioning magnetic field or a corresponding working magnetic field through the magnetic field acquisition device after waiting for a first preset time.
Optionally, in the method for measuring linearity of a gradient coil, the magnetic field acquisition device includes a fixed disk and a sensor array;
the fixed disk is used for fixing the sensor array;
the sensors included in the sensor array are distributed on the semicircular arc according to Gaussian integral point positions;
the magnetic field acquisition method corresponding to the magnetic field acquisition device comprises the following steps:
determining a plurality of sampling azimuth angles according to the fixed angle interval;
measuring sampling magnetic field data of a plurality of preset azimuth angles through the magnetic field acquisition device;
and determining a corresponding magnetic field according to the plurality of sampled magnetic field data.
A gradient coil linearity measurement system, wherein the gradient coil linearity measurement system executes steps of the gradient coil linearity measurement method, and the method comprises a superconducting magnet, a magnetic field acquisition device, a signal processor and a direct current power supply;
the superconducting magnet is used for providing the background magnetic field; the gradient coil is sleeved on the inner side of the cavity of the superconducting magnet;
the magnetic field acquisition device is positioned in the cavity of the superconducting magnet when in operation and is used for measuring and acquiring a corresponding magnetic field;
the direct current power supply supplies power to the gradient coil;
the signal processor is connected with the magnetic field acquisition device and is used for processing data acquired by the magnetic field acquisition device.
According to the method for measuring the linearity of the gradient coil, the background magnetic field is obtained through measurement of the magnetic field collecting device; respectively and independently powering on a Gx coil, a Gy coil and a Gz coil of the gradient coil, and measuring and acquiring corresponding Gx working magnetic field, gy working magnetic field and Gz working magnetic field through the magnetic field acquisition device; according to the Gx working magnetic field, the Gy working magnetic field, the Gz working magnetic field and the background magnetic field, obtaining corresponding Gx center gradient strength, gy center gradient strength and Gz center gradient strength through spherical harmonic transformation; and determining the Gx coil linearity, the Gy coil linearity and the Gz coil linearity of the gradient coil according to the Gx working magnetic field, the Gx center gradient strength, the Gy working magnetic field, the Gy center gradient strength, the Gz working magnetic field, the Gz center gradient strength and the background magnetic field. The invention can measure linearity without imaging equipment, realizes component level linearity measurement, thereby greatly reducing the requirement on measuring equipment, simplifying the measuring process, and greatly reducing the measuring cost. The invention also provides a measuring system for the linearity of the gradient coil with the beneficial effects.
Drawings
For a clearer description of an embodiment of the invention or of the prior art, the drawings that are used in the description of the embodiment or of the prior art will be briefly described, it being apparent that the drawings in the description below are only some embodiments of the invention, and that other drawings can be obtained from them without inventive effort for a person skilled in the art.
FIG. 1 is a schematic flow chart of an embodiment of a method for measuring linearity of a gradient coil according to the present invention;
FIG. 2 is a flow chart of an embodiment of a method for measuring linearity of a gradient coil according to the present invention;
FIG. 3 is a schematic diagram of a system for measuring linearity of a gradient coil according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of a partial structure of an embodiment of a system for measuring linearity of a gradient coil according to the present invention;
FIG. 5 is a schematic diagram of the magnetic field acquisition device of one embodiment of the system for measuring linearity of a gradient coil according to the present invention, when the magnetic field acquisition device is operated in the gradient coil;
fig. 6 is an azimuthal schematic diagram of an embodiment of a method for measuring linearity of a gradient coil according to the present invention.
In the drawing, it includes: 100. the device comprises a superconducting magnet 200, gradient coils 300, a magnetic field acquisition device 400, a signal processor 500, a direct current power supply 600, an upper computer 301, fixed discs 302 and a sensor array.
Detailed Description
In order that those skilled in the art will better understand the present invention, the following description will be given in detail with reference to the accompanying drawings and detailed description. It will be apparent that the described embodiments are only some, but not all, embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The core of the present invention is to provide a method for measuring linearity of a gradient coil 200, wherein a flow chart of one embodiment is shown in fig. 1, and the method is referred to as embodiment one, and includes:
s101: the background magnetic field is acquired by measurement of the magnetic field acquisition device 300.
Before explaining the present invention, a description is first given of the structure and principle of the magnetic field acquisition device 300. Referring to fig. 3, fig. 3 is a schematic structural diagram of an embodiment of a system for measuring linearity of a gradient coil 200, which is required for a method for measuring linearity of a gradient coil 200 according to the present invention, including a superconducting magnet 100, a gradient coil 200 (i.e. a coil to be measured), a magnetic field acquisition device 300, a signal processor 400, a dc power supply 500, and a host computer 600, wherein the gradient coil 200 generally includes three independent sets of coils Gx, gy, and Gz. The dc power supply 500 is connected to terminals of the corresponding coils of the gradient coil 200 for supplying constant dc currents to the Gx coil, gy coil, and Gz coil, respectively. The magnetic field acquisition device 300 is located in the central region of the gradient coil 200, and is used for measuring the background magnetic field generated by the superconducting magnet 100 and measuring the positioning magnetic field generated by the superconducting magnet 100 and the gradient coil 200 together. The signal processor 400 is connected to the magnetic field acquisition device 300, and is configured to analyze and process the signal acquired by the magnetic field acquisition device 300, and convert the signal into data that can be identified by the host computer 600. The upper computer 600 is connected to the signal processor 400, and is used for displaying and deriving final measurement data.
The magnetic field acquisition device 300 comprises a semicircular stationary disc 301 and a sensor array 302. The semicircular fixing plate 301 is used for supporting and fixing the sensor array 302, the semicircular fixing plate 301 is made of non-metal materials, such as bakelite, nylon, glass fiber composite materials, and the like, of course, it is also possible to only ensure that the magnetic field sensors are distributed on a semicircular arc, and the fixing plate 301 can also be in other shapes, such as rectangular, and the like. The sensor array 302 is used to measure a magnetic field and is formed by arranging a plurality of magnetic field sensors according to a certain geometric relationship. Fig. 4 shows a sensor array 302 structure according to the present embodiment, in which 12 magnetic field sensors are distributed according to gaussian integral point positions at a pitch radius r dsv The central angles θ of the magnetic field sensors relative to the z-axis are 11.02 °, 25.30 °, 39.65 °, 54.03 °, 68.42 °, 82.81 °, 97.19 °, 111.58 °, 125.97 °, 140.35 °, 154.70 ° and 168.98 °, respectively, so that 12-order accuracy is provided in the θ direction. For the case where higher order accuracy is required, 16, or 20, or 24 magnetic field sensors need to be provided, but all need to be arranged in a gaussian integral point position. More magnetic field sensors bring about cost increase, and a proper number of magnetic field sensors are required to be arranged according to the precision requirement in the actual implementation process.
As a preferred embodiment, the magnetic field acquisition device 300 includes a stationary plate 301 and a sensor array 302;
the fixed disk 301 is used for fixing the sensor array 302;
the sensors included in the sensor array 302 are distributed on a semicircular arc according to points of gaussian integral points;
correspondingly, the magnetic field acquisition method corresponding to the magnetic field acquisition device 300 comprises the following steps:
a1: a plurality of sampling azimuth angles are determined from the fixed angular intervals.
A2: sampled magnetic field data for a plurality of preset azimuth angles is measured by the magnetic field acquisition device 300.
A3: and determining a corresponding magnetic field according to the plurality of sampled magnetic field data.
According to the magnetic field measurement method of the steps A1 to A3, the method for measuring and acquiring the background magnetic field in the step specifically comprises the following steps: at the initial position of 0 degree angle azimuth, 12 magnetic field sensors collect and store magnetic field data of corresponding points; turning the magnetic field acquisition device 300 to the 30 ° angular orientation in fig. 6 (fig. 6 is a schematic diagram of the azimuth angle corresponding to the magnetic field acquisition device 300), and acquiring and storing magnetic field data; continuing to turn the magnetic field acquisition device 300 to the 60 °, 90 °, 120 °, 150 °, 180 °, 210 °, 240 °, 270 °, 300 °, and 330 ° orientations in fig. 4, respectively, magnetic field data is acquired and stored.
The specific structural schematic diagram of the magnetic field collecting device 300 is shown in fig. 4, and for convenience of description, fig. 4 and fig. 5 further define a local coordinate system xyz of the magnetic field collecting device 300, and fig. 5 is a schematic diagram of a positional relationship of the magnetic field collecting device 300 when working in the gradient coil 200, wherein an origin O is a center of a semicircular fixed disk 301, a z axis coincides with a semicircular straight edge of the fixed disk 301, an xz plane coincides with a plane where the sensor array 302 is located, a y axis is perpendicular to the plane where the sensor array 302 is located, and xyz meets a specification of a left-hand rectangular coordinate system. Before formally executing the test flow, the magnetic field acquisition device 300 is set in the initial state shown in fig. 3: ideally, the center of the semicircular fixing plate 301 coincides with the center of the gradient coil 200; the x-axis points in the 0 deg. angular direction of the gradient coil 200.
S102: the Gx coil, gy coil, and Gz coil of the gradient coil 200 are individually energized, and the corresponding Gx working magnetic field, gy working magnetic field, and Gz working magnetic field are obtained by measurement by the magnetic field acquisition device 300.
As a specific embodiment, the specific operation corresponding to this step is: connecting the dc power supply 500 to the post of the Gx coil of the gradient coil 200, adjusting the output current of the dc power supply 500 to the working current Ixm, waiting for several minutes after the current is stabilized, and then performing magnetic field acquisition to obtain Gx working magnetic field (Bzxm) data, wherein the operation is the same as the operation described in the acquisition of the background magnetic field through steps A1 to A3 described in the foregoing, and will not be repeated here. The current Ixm is a small current, typically 10-15A in magnitude. The wait for a few minutes after stabilization is to eliminate eddy current interference caused by current Ixm.
Connecting the dc power supply 500 to the post of the Gy coil of the gradient coil 200, adjusting the output current of the dc power supply 500 to the working current Iym, waiting for several minutes after the current is stabilized, and then performing magnetic field acquisition to obtain Gy working magnetic field (hereinafter abbreviated as Bzym) data, where the operation is the same as the operation described in the above-described acquisition of the background magnetic field through steps A1 to A3, and will not be repeated here. The current Iym is a small current, typically 10-15A in magnitude. The wait for a few minutes after stabilization is to eliminate eddy current interference caused by current Iym.
Connecting the dc power supply 500 to the terminal of the Gz coil of the gradient coil 200, adjusting the output current of the dc power supply 500 to the current Izm, waiting for several minutes after the current is stabilized, and then performing magnetic field acquisition to obtain Gz working magnetic field (Bzzm) data, where the operation is the same as the operation described in the foregoing for acquiring the background magnetic field through steps A1 to A3, and will not be repeated here. The current Izm is a small current, typically 10-15A in magnitude. The wait for a few minutes after stabilization is to eliminate eddy current interference caused by current Izm.
It is noted that the measured acquisition of the corresponding operating magnetic field of the gradient coil 200 and/or of the positioning magnetic field to be mentioned hereinafter comprises:
And electrifying the Gx coil, the Gy coil or the Gz coil of the gradient coil 200, waiting for a first preset time, and acquiring a corresponding positioning magnetic field or working magnetic field by the magnetic field acquisition device 300.
In other words, after the output current of the dc power supply 500 is adjusted to the operating current, the magnetic field collection device 300 is used to collect the current after a first preset time. The interference eddy current is generated in a period of time immediately after the coil is electrified, and the interference to the magnetic field data to be measured is avoided before the eddy current disappears for the first preset time, so that the accuracy of measurement is improved, and the technology is also suitable for the acquisition process of the Gy working magnetic field and the Gz working magnetic field in the foregoing, and the repeated description is not expanded at that time.
Still further, the first preset time ranges from 1 minute to 10 minutes, including an endpoint value, such as any one of 1.0 minutes, 5.8 minutes, or 10.0 minutes; yet further, during acquisition of the operating magnetic field by the magnetic field acquisition device 300, the operating current of the gradient coil 200 ranges from 10 amps to 15 amps, including at least one of the end point values, such as 10.0 amps, 12.3 amps, or 15.0 amps. The parameter range is an optimal range obtained after a large number of theoretical calculations and actual tests, and the value range of the first preset time can ensure that the interference vortex disappears and the waiting time is not wasted so as to reduce the detection efficiency; the magnetic field generated by the preset range of the operating current can be detected by the magnetic field acquisition device 300 when the coil operating requirement is satisfied. Of course, other parameters may be selected according to practical situations, and the invention is not limited herein. The above parameter ranges and methods are also applicable to the acquisition of the positioning magnetic field hereinafter, and will not be described in detail.
S103: and obtaining corresponding Gx center gradient strength, gy center gradient strength and Gz center gradient strength through spherical harmonic transformation according to the Gx working magnetic field, the Gy working magnetic field, the Gz working magnetic field and the background magnetic field.
Specifically, the method comprises the following steps:
performing spherical harmonic transformation to obtain Gx central gradient strength G xcent Center gradient intensity G of Gy ycent Gz center gradient intensity G zcent The linearity of the Gx coil, gy coil and Gz coil is then obtained from the formula. The specific operation comprises the following steps: from the measured Bz0m (refer to the background magnetic field) and Bzxm, harmonic components a11xm are obtained according to the following spherical harmonic transformation formulas (1) (2) (3):
（1）
（2）
（3）
wherein n is the order, m is the degree, θ is the elevation angle of the spherical coordinate system corresponding to the measurement point and the z axis,for the azimuth angle corresponding to the measurement point, P is the relevant legendre coefficient, and Bz is the measured magnetic field value.
Note that a11xm is the Gx center gradient strength G xcent ；
S104: and determining the Gx coil linearity, the Gy coil linearity and the Gz coil linearity of the gradient coil 200 according to the Gx working magnetic field, the Gx center gradient strength, the Gy working magnetic field, the Gy center gradient strength, the Gz working magnetic field, the Gz center gradient strength, and the background magnetic field.
In the above, the method for obtaining the coil linearity in the present step includes: the linearity of the Gx coil is obtained from the following formula (4) of the linearity of the Gx coil:
（4）
wherein, the liquid crystal display device comprises a liquid crystal display device,for the linearity of the Gx coil, +.>Equal to Bzxm-Bz0m, x i Is the central angle θ of the magnetic field sensor and the azimuth angle +.>X-coordinate (spherical coordinate system) of the determined acquisition point, r dsv Is the radius of the reference circle of the sensor array 302. Likewise, from the measured Bz0m and Bzym, the linearity of the Gy coil can be obtained; according to the measured Bz0m and Bzzm, the linearity of the Gz coil can be obtained, and the description is not repeated here.
Ideally, in the process of measuring linearity, the center of the semicircular fixing plate 301 should coincide with the center of the gradient coil 200; the x-axis should point to the 0 ° angular direction of the gradient coil 200, but in actual operation, the center of the semicircular fixed disk 301 or the x-axis is different from the ideal state, so in order to ensure the measurement accuracy, the posture of the magnetic field acquisition device 300 needs to be adjusted before measurement, so that the following preferred embodiments B1 to B4 are led out, and the flow chart of the specific embodiment in combination with B1 to B4 is shown in fig. 2.
As a preferred embodiment, before measuring and acquiring the Gx working magnetic field, the Gy working magnetic field, and the Gz working magnetic field, the method further includes:
b1: the Gx coil, gy coil, and Gz coil of the gradient coil 200 are individually energized, and the corresponding Gx coil positioning magnetic field, gy coil positioning magnetic field, and Gz coil positioning magnetic field are obtained by measurement by the magnetic field acquisition device 300.
The measurement and acquisition of the Gx coil positioning magnetic field, the Gy coil positioning magnetic field and the Gz coil positioning magnetic field may refer to the acquisition of the working magnetic field in the foregoing, and will not be described in detail herein.
B2: and obtaining a triaxial offset and an azimuth offset corresponding to the magnetic field acquisition device 300 through spherical harmonic transformation according to the Gx coil positioning magnetic field, the Gy coil positioning magnetic field, the Gz coil positioning magnetic field and the background magnetic field.
The three-axis offset, that is, the offset of the center of the semicircular fixed disk 301 in the x, y, and z directions with respect to the center of the gradient coil 200, in other words, the three-axis offset includes an x-axis offset, a y-axis offset, and a z-axis offset; also, as can be seen from the foregoing, since the gradient coil 200 generally includes three independent sets of coils Gx, gy and Gz, the measurement of the offset amounts in three directions also requires the separate acquisition of the magnetic field components corresponding to the above three coils when they are operated separately, in other words, the positioning magnetic field includes a Gx coil positioning magnetic field, a Gy coil positioning magnetic field and a Gz coil positioning magnetic field.
B3: the positioning of the magnetic field acquisition device 300 is adjusted by the triaxial offset and the azimuthal offset.
In the description of the three-axis offset, the positioning magnetic field and the background magnetic field, one embodiment of the steps from B1 to B3 includes:
c1: and measuring and acquiring the Gx coil positioning magnetic field.
The background magnetic field in this step is a background magnetic field generated by the superconducting magnet 100, hereinafter abbreviated as Bz0x, and the step of obtaining Bz0x includes: at the initial position of 0 degree angle, 12 magnetic field sensors collect corresponding point magnetic field data and store the data in the upper computer 600; rotating the magnetic field acquisition device 300 to a 90 degree angular orientation to acquire and store magnetic field data; and then the magnetic field acquisition device 300 is turned to azimuth angles corresponding to 180 degrees and 270 degrees (the angles of all the azimuth angles can be referred to fig. 6), and magnetic field data are acquired and stored respectively, so that Bz0x is obtained.
The following measurements obtain the Gx coil positioning magnetic field (Bzx for short), since Bzx requires the operation of the component coils corresponding to the gradient coil 200, specifically including: connecting the dc power supply 500 to the terminal of the Gx coil of the gradient coil 200, adjusting the output current of the dc power supply 500 to the working current Ix, and then performing magnetic field acquisition to obtain Bzx data, wherein the operation is the same as the operation of acquiring Bz0x, and the magnetic field acquisition device is sequentially rotated to acquire magnetic field data at azimuth angles of 300 to 0 °, 90 °, 180 ° and 270 °, which are not described herein again. The current Ix is a small current, and the magnitude of the current Ix is generally 10-15A. The wait for a few minutes after the stabilization is to eliminate the eddy current interference caused by the current Ix.
C2: and determining the x-axis offset and the azimuth offset through spherical harmonic transformation according to the background magnetic field and the Gx coil positioning magnetic field.
In this step, the specific method for obtaining the x-axis offset may include:
d1: and according to the background magnetic field and the Gx coil positioning magnetic field, obtaining corresponding spherical harmonic components by utilizing spherical harmonic transformation.
The specific spherical harmonic components in this step are obtained by the spherical harmonic transformation formulas of the formulas (1) to (3) in the foregoing, and will not be described in detail herein.
Specifically, in this step, spherical harmonic components a00x and a11x obtained by the formula (1) and the formula (2) are required.
D2: the x-axis offset is determined by the following equation (5):
dx=a00x/a11x （5）
where a00x and a11x are the corresponding spherical harmonic components and dx is the x-axis offset.
The x-axis offset is obtained by adopting the formula (5), so that the method is visual and convenient, less in calculation force is required, and the calculation efficiency is high.
Further, the specific method for obtaining the azimuth offset may include:
e1: and according to the background magnetic field and the Gx coil positioning magnetic field, obtaining corresponding spherical harmonic components by utilizing spherical harmonic transformation.
In this step, the required spherical harmonic component is obtained by the formula (2) and the formula (3) (specifically, the calculated azimuth offset amounts are b11x and a11 x), however, the required spherical harmonic component may be calculated at one time by combining the step E1 with the step D1, and then the required offset amounts may be calculated by performing the steps D2 and E2, respectively.
E2: determining the azimuth offset by the following equation (6):
（6）
wherein b11x and a11x are the corresponding spherical harmonic components,is the azimuth offset.
The azimuth angle offset is obtained by adopting the formula (6), so that the method is visual and convenient, less in calculation force is required, and the calculation efficiency is high.
And C3: the magnetic field acquisition device 300 is adjusted and moved according to the x-axis offset and the azimuth offset.
As can be seen from steps C1 to C3, the above three steps are only corrected and adjusted for the positioning error and the azimuth error of the magnetic field acquisition device 300 in the x-direction.
Further, after adjusting and moving the magnetic field acquisition device 300 according to the x-axis offset and the azimuth offset, the method further includes:
f1: and acquiring a new background magnetic field and a new Gx coil positioning magnetic field corresponding to the magnetic field acquisition device 300 after the adjustment and the movement are performed.
F2: and determining a new x-axis offset and a new azimuth offset through spherical harmonic transformation according to the new background magnetic field and the new Gx coil positioning magnetic field.
F3: and judging whether the new x-axis offset and the new azimuth angle offset are smaller than a preset tolerance threshold.
F4: and when the new x-axis offset and the new azimuth angle offset are not smaller than a preset tolerance threshold, continuously repeating the steps F1 to F3 until the obtained new x-axis offset and new azimuth angle offset are smaller than the tolerance threshold.
In this embodiment, after the adjustment of the magnetic field collecting device 300 is performed once, further verification is performed, the position of the magnetic field collecting device 300 in the x direction and the orientation of the azimuth angle of the magnetic field collecting device are continuously adjusted until the obtained offset is smaller than the preset tolerance value, and the verification step is added to further ensure the correctness of the position of the magnetic field collecting device 300.
In addition, it is not difficult to find that in this embodiment, each offset is calculated separately, and after a certain offset is calculated, the position and the azimuth angle of the magnetic field acquisition device 300 are adjusted in time, so that the interference caused by incorrect position and posture to calculation of other subsequent offsets is avoided, in this embodiment, steps C1 to C3 are taken as an example, in steps C1 to C3, the x-axis offset and the azimuth angle offset are calculated, and the magnetic field acquisition device 300 is adjusted, so that in calculation of y-axis offset in subsequent steps C4 to C6, the situation that the position of the magnetic field acquisition device 300 on the x-axis deviates, the acquired background magnetic field and the Gy coil positioning magnetic field are inaccurate, and then the situation that the y-axis offset is calculated inaccurately is avoided, and after the y-axis offset is obtained, the position of the magnetic field acquisition device 300 in the y-axis direction is adjusted in time, so that the measurement accuracy is also avoided, and the measurement accuracy is greatly improved.
And C4: and after the adjustment of the x-axis offset and the azimuth angle offset is completed, measuring and acquiring the background magnetic field and the Gy coil positioning magnetic field.
C5: and determining the y-axis offset through spherical harmonic transformation according to the background magnetic field and the Gy coil positioning magnetic field.
In this step, the specific method for obtaining the y-axis offset may include:
d3: and obtaining corresponding spherical harmonic components by utilizing spherical harmonic transformation according to the background magnetic field and the Gy coil positioning magnetic field.
The specific spherical harmonic components in this step are obtained by the spherical harmonic transformation formulas of the formulas (1) to (3) in the foregoing, and will not be described in detail herein.
Specifically, in this step, spherical harmonic components a00y and b11y obtained by the formula (1) and the formula (3) are required.
D4: the y-axis offset is determined by the following equation (7):
dy=a00y/b11y （7）
where a00y and b11y are the corresponding spherical harmonic components and dy is the y-axis offset.
The y-axis offset is obtained by adopting the formula (7), so that the method is visual and convenient, less in calculation force is required, and the calculation efficiency is high.
C6: the magnetic field acquisition device 300 is adjusted and moved according to the y-axis offset.
As can be seen from steps C4 to C6, the above three steps only correct and adjust the positioning error of the magnetic field acquisition device 300 in the y-direction. The three steps from C4 to C6 may be specifically performed by referring to the descriptions of steps C1 to C3, and will not be described in detail herein.
Accordingly, after the magnetic field acquisition device 300 is adjusted and moved according to the y-axis offset, the method further includes:
g1: and acquiring a new background magnetic field and a new Gy coil positioning magnetic field corresponding to the magnetic field acquisition device 300 after the adjustment and the movement are performed.
And G2: and determining a new y-axis offset through spherical harmonic transformation according to the new background magnetic field and the new Gy coil positioning magnetic field.
And G3: and judging whether the new y-axis offset is smaller than the tolerance threshold.
And G4: and when the new y-axis offset is not smaller than a preset tolerance threshold, continuously repeating the steps G1 to G3 until the obtained new y-axis offset is smaller than the tolerance threshold.
Steps G1 to G4 are also further verification after the adjustment of the magnetic field collecting device 300, so as to ensure the accuracy of the adjusted position, and please refer to the previous description of steps F1 to F4, which is not repeated here.
C7: and after the y-axis offset is adjusted, measuring and acquiring the background magnetic field and the Gz coil positioning magnetic field.
And C8: and determining the z-axis offset through spherical harmonic transformation according to the background magnetic field and the Gz coil positioning magnetic field.
In this step, the specific method for obtaining the z-axis offset may include:
D5: and according to the background magnetic field and the Gz coil positioning magnetic field, obtaining corresponding spherical harmonic components by utilizing spherical harmonic transformation.
The specific spherical harmonic components in this step are obtained by the spherical harmonic transformation formulas of the formulas (1) to (3) in the foregoing, and will not be described in detail herein.
Specifically, in this step, spherical harmonic components a00z and a10z obtained by the formula (1) and the formula (2) are required.
D6: determining the z-axis offset by the following equation (8):
dz=a00z/a10z （8）
where a00z and a10z are the corresponding spherical harmonic components and dz is the z-axis offset.
C9: and adjusting and moving the magnetic field acquisition device 300 according to the z-axis offset to finish positioning and adjusting the magnetic field acquisition device 300.
As is clear from steps C7 to C9, only the component of the magnetic field in the z-direction corrects and adjusts the positioning error of the magnetic field acquisition device 300 in the z-direction. The three steps from C7 to C9 may be specifically performed by referring to the descriptions of the steps C1 to C3, and will not be described in detail herein.
Accordingly, after adjusting and moving the magnetic field acquisition device 300 according to the z-axis offset, the method further includes:
h1: and acquiring a new background magnetic field and a new Gz coil positioning magnetic field corresponding to the magnetic field acquisition device 300 after the adjustment and the movement are performed.
H2: and determining a new z-axis offset through spherical harmonic transformation according to the new background magnetic field and the new Gz coil positioning magnetic field.
And H3: and judging whether the new z-axis offset is smaller than the tolerance threshold.
H4: and when the new z-axis offset is not smaller than a preset tolerance threshold, continuously repeating the steps H1 to H3 until the obtained new z-axis offset is smaller than the tolerance threshold.
Steps H1 to H4 are also further verification after the adjustment of the magnetic field collecting device 300, so as to ensure the accuracy of the adjusted position, and please refer to the description of steps F1 to F4 hereinabove, and are not repeated here.
Of course, further, the tolerance threshold includes an azimuth threshold, an x-axis threshold, a y-axis threshold, and a z-axis threshold;
the x-axis threshold, the y-axis threshold, and the z-axis threshold are not uniform.
In other words, the tolerance of the magnetic field collecting device 300 to the offset in each direction may be different, and may be adjusted according to actual needs, so that the technical solution provided in this embodiment has better universality, and is suitable for more special situations.
Correspondingly, the separately powering up the Gx coil, gy coil, and Gz coil of the gradient coil 200, and measuring and obtaining the corresponding Gx working magnetic field, gy working magnetic field, and Gz working magnetic field by the magnetic field acquisition device 300 includes:
B4: the Gx coil, the Gy coil and the Gz coil of the gradient coil 200 are respectively and independently electrified, and the corresponding Gx working magnetic field, gy working magnetic field and Gz working magnetic field are obtained through measurement of the magnetic field acquisition device 300 after adjustment and positioning.
Before formally measuring the linearity, the preferred embodiment firstly obtains the offset of the magnetic field acquisition device 300 in three axial directions and the offset between the azimuth angle and the preset standard 0 degrees by using the spherical harmonic transformation, and correspondingly adjusts the offset, so that the accuracy of the position of the magnetic field acquisition device 300 in the gradient coil 200 is ensured, the accuracy of magnetic field data obtained in the follow-up formal linearity measurement is further improved, and the accuracy of the linearity measurement result is further improved.
According to the method for measuring the linearity of the gradient coil 200, the background magnetic field is obtained through measurement of the magnetic field acquisition device 300; the Gx coil, the Gy coil and the Gz coil of the gradient coil 200 are respectively and independently electrified, and the corresponding Gx working magnetic field, gy working magnetic field and Gz working magnetic field are obtained through measurement of the magnetic field acquisition device 300; according to the Gx working magnetic field, the Gy working magnetic field, the Gz working magnetic field and the background magnetic field, obtaining corresponding Gx center gradient strength, gy center gradient strength and Gz center gradient strength through spherical harmonic transformation; and determining the Gx coil linearity, the Gy coil linearity and the Gz coil linearity of the gradient coil 200 according to the Gx working magnetic field, the Gx center gradient strength, the Gy working magnetic field, the Gy center gradient strength, the Gz working magnetic field, the Gz center gradient strength, and the background magnetic field. The invention can measure linearity without imaging equipment, realizes component level linearity measurement, thereby greatly reducing the requirement on measuring equipment, simplifying the measuring process, and greatly reducing the measuring cost.
The present invention also provides a system for measuring the linearity of the gradient coil 200, and a schematic structural diagram of a specific embodiment thereof referring to fig. 3 to 6, wherein the system for measuring the linearity of the gradient coil 200 performs the steps of the method for measuring the linearity of the gradient coil 200 according to any one of the above, and the system comprises a superconducting magnet 100, a magnetic field acquisition device 300, a signal processor 400 and a dc power supply 500;
the superconducting magnet 100 is configured to provide the background magnetic field; the gradient coil 200 is sleeved inside the cavity of the superconducting magnet 100;
the magnetic field collecting device 300 is located in the cavity of the superconducting magnet 100 during operation, and is used for measuring and obtaining a corresponding magnetic field;
the DC power supply 500 supplies power to the gradient coil 200;
the signal processor 400 is connected to the magnetic field acquisition device 300, and is configured to process data acquired by the magnetic field acquisition device 300.
The specific structure and the working principle of the system for measuring the linearity of the gradient coil 200 provided by the present invention refer to the foregoing description of the method for measuring the linearity of the gradient coil 200, which is not repeated herein, and it should be noted that the above-mentioned upper computer 600 is a terminal for storing and displaying the data processed by the signal processor 400, and in actual working, the upper computer 600 may be used, and other terminals may also be used for displaying and storing the data.
The system for measuring the linearity of the gradient coil 200 provided by the invention is used for executing the steps of the method for measuring the linearity of the gradient coil 200, which is realized by any one of the steps, and comprises a superconducting magnet 100, a magnetic field acquisition device 300, a signal processor 400 and a direct current power supply 500; the superconducting magnet 100 is configured to provide the background magnetic field; the gradient coil 200 is sleeved inside the cavity of the superconducting magnet 100; the magnetic field collecting device 300 is located in the cavity of the superconducting magnet 100 during operation, and is used for measuring and obtaining a corresponding magnetic field; the DC power supply 500 supplies power to the gradient coil 200; the signal processor 400 is connected to the magnetic field acquisition device 300, and is configured to process data acquired by the magnetic field acquisition device 300. The invention can measure linearity without imaging equipment, realizes component level linearity measurement, thereby greatly reducing the requirement on measuring equipment, simplifying the measuring process, and greatly reducing the measuring cost.
In this specification, each embodiment is described in a progressive manner, and each embodiment is mainly described in a different point from other embodiments, so that the same or similar parts between the embodiments are referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant points refer to the description of the method section.
It should be noted that in this specification, relational terms such as first and second, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.
Those of skill would further appreciate that the various illustrative elements and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both, and that the various illustrative elements and steps are described above generally in terms of functionality in order to clearly illustrate the interchangeability of hardware and software. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the solution. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.
The method and the system for measuring the linearity of the gradient coil provided by the invention are described in detail. The principles and embodiments of the present invention have been described herein with reference to specific examples, the description of which is intended only to facilitate an understanding of the method of the present invention and its core ideas. It should be noted that it will be apparent to those skilled in the art that various modifications and adaptations of the invention can be made without departing from the principles of the invention and these modifications and adaptations are intended to be within the scope of the invention as defined in the following claims.
Claims (10)
1. A method for measuring linearity of a gradient coil, comprising:
measuring and acquiring a background magnetic field through a magnetic field acquisition device;
respectively and independently powering on a Gx coil, a Gy coil and a Gz coil of the gradient coil, and measuring and acquiring corresponding Gx working magnetic field, gy working magnetic field and Gz working magnetic field through the magnetic field acquisition device;
according to the Gx working magnetic field, the Gy working magnetic field, the Gz working magnetic field and the background magnetic field, obtaining corresponding Gx center gradient strength, gy center gradient strength and Gz center gradient strength through spherical harmonic transformation;
and determining the Gx coil linearity, the Gy coil linearity and the Gz coil linearity of the gradient coil according to the Gx working magnetic field, the Gx center gradient strength, the Gy working magnetic field, the Gy center gradient strength, the Gz working magnetic field, the Gz center gradient strength and the background magnetic field.
2. The method of measuring linearity of a gradient coil as set forth in claim 1, further comprising, before measuring and acquiring said Gx working magnetic field, said Gy working magnetic field, said Gz working magnetic field:
respectively and independently powering on a Gx coil, a Gy coil and a Gz coil of the gradient coil, and measuring and acquiring corresponding Gx coil positioning magnetic field, gy coil positioning magnetic field and Gz coil positioning magnetic field through the magnetic field acquisition device;
Obtaining a triaxial offset and an azimuth offset corresponding to the magnetic field acquisition device through spherical harmonic transformation according to the Gx coil positioning magnetic field, the Gy coil positioning magnetic field, the Gz coil positioning magnetic field and the background magnetic field;
the positioning of the magnetic field acquisition device is adjusted through the triaxial offset and the azimuth angle offset;
correspondingly, the step of respectively and independently electrifying the Gx coil, the Gy coil and the Gz coil of the gradient coil, and the step of measuring and obtaining the corresponding Gx working magnetic field, gy working magnetic field and Gz working magnetic field through the magnetic field acquisition device comprises the following steps:
and respectively and independently powering on the Gx coil, the Gy coil and the Gz coil of the gradient coil, and measuring and acquiring corresponding Gx working magnetic field, gy working magnetic field and Gz working magnetic field through the magnetic field acquisition device with adjusted positioning.
3. The method of claim 2, wherein adjusting the positioning of the magnetic field acquisition device from acquiring the Gx coil positioning magnetic field, the Gy coil positioning magnetic field, and the Gz coil positioning magnetic field by the triaxial offset and the azimuthal offset comprises:
measuring and obtaining the Gx coil positioning magnetic field;
according to the background magnetic field and the Gx coil positioning magnetic field, determining an x-axis offset and an azimuth angle offset through spherical harmonic transformation;
Adjusting and moving the magnetic field acquisition device according to the x-axis offset and the azimuth angle offset;
after the adjustment of the x-axis offset and the azimuth angle offset is completed, measuring and acquiring the background magnetic field and the Gy coil positioning magnetic field;
determining a y-axis offset through spherical harmonic transformation according to the background magnetic field and the Gy coil positioning magnetic field;
adjusting and moving the magnetic field acquisition device according to the y-axis offset;
after the y-axis offset is adjusted, measuring and acquiring the background magnetic field and the Gz coil positioning magnetic field;
determining a z-axis offset through spherical harmonic transformation according to the background magnetic field and the Gz coil positioning magnetic field;
and adjusting and moving the magnetic field acquisition device according to the z-axis offset to finish positioning and adjusting the magnetic field acquisition device.
4. The method of claim 3, wherein determining the x-axis offset from the background magnetic field and the Gx coil positioning magnetic field by spherical harmonic transformation comprises:
according to the background magnetic field and the Gx coil positioning magnetic field, a corresponding spherical harmonic component is obtained through spherical harmonic transformation;
The x-axis offset is determined by:
dx=a00x/a11x
wherein a00x and a11x are corresponding spherical harmonic components, and dx is the x-axis offset;
and/or
The determining the y-axis offset by spherical harmonic transformation according to the background magnetic field and the Gy coil positioning magnetic field comprises:
according to the background magnetic field and the Gy coil positioning magnetic field, a corresponding spherical harmonic component is obtained through spherical harmonic transformation;
the y-axis offset is determined by:
dy=a00y/b11y
wherein a00y and b11y are corresponding spherical harmonic components, and dy is the y-axis offset;
and/or
The determining the z-axis offset by spherical harmonic transformation according to the background magnetic field and the Gz coil positioning magnetic field comprises:
according to the background magnetic field and the Gz coil positioning magnetic field, a corresponding spherical harmonic component is obtained through spherical harmonic transformation;
the z-axis offset is determined by:
dz=a00z/a10z
where a00z and a10z are the corresponding spherical harmonic components and dz is the z-axis offset.
5. The method of claim 3, wherein determining the azimuth offset from the background magnetic field and the Gx coil positioning magnetic field by spherical harmonic transformation comprises:
according to the background magnetic field and the Gx coil positioning magnetic field, a corresponding spherical harmonic component is obtained through spherical harmonic transformation;
The azimuth offset is determined by:
；
wherein b11x and a11x are the corresponding spherical harmonic components,is the azimuth offset.
6. The method of claim 3, further comprising, after adjusting the magnetic field acquisition device according to the x-axis offset and the azimuth offset:
acquiring a new background magnetic field and a new Gx coil positioning magnetic field corresponding to the magnetic field acquisition device after adjustment movement, determining a new x-axis offset and a new azimuth offset according to the new background magnetic field and the new Gx coil positioning magnetic field through spherical harmonic transformation, judging whether the new x-axis offset and the new azimuth offset are smaller than a preset tolerance threshold, and continuously acquiring the new background magnetic field and the new Gx coil positioning magnetic field after adjustment movement of the magnetic field acquisition device according to the new x-axis offset and the new azimuth offset when the new x-axis offset and the new azimuth offset are not smaller than the preset tolerance threshold until the obtained new x-axis offset and new azimuth offset are smaller than the tolerance threshold;
correspondingly, after the magnetic field acquisition device is adjusted and moved according to the y-axis offset, the method further comprises the following steps:
Acquiring a new background magnetic field and a new Gy coil positioning magnetic field corresponding to the magnetic field acquisition device after adjustment and movement, determining a new y-axis offset according to the new background magnetic field and the new Gy coil positioning magnetic field through spherical harmonic transformation, judging whether the new y-axis offset is smaller than the tolerance threshold, and continuously acquiring the new background magnetic field and the new Gy coil positioning magnetic field after adjustment and movement of the magnetic field acquisition device according to the new y-axis offset when the new y-axis offset is not smaller than the preset tolerance threshold until the obtained new y-axis offset is smaller than the tolerance threshold;
correspondingly, after the magnetic field acquisition device is adjusted and moved according to the z-axis offset, the method further comprises the following steps:
acquiring a new background magnetic field and a new Gz coil positioning magnetic field corresponding to the magnetic field acquisition device after adjustment and movement, determining a new z-axis offset according to the new background magnetic field and the new Gz coil positioning magnetic field through spherical harmonic transformation, judging whether the new z-axis offset is smaller than the tolerance threshold, and continuously acquiring the new background magnetic field and the new Gz coil positioning magnetic field after adjustment and movement of the magnetic field acquisition device according to the new z-axis offset when the new z-axis offset is not smaller than the preset tolerance threshold until the obtained new z-axis offset is smaller than the tolerance threshold.
7. The method of claim 6, wherein the tolerance threshold comprises an azimuth threshold, an x-axis threshold, a y-axis threshold, and a z-axis threshold;
the x-axis threshold, the y-axis threshold, and the z-axis threshold are not uniform.
8. A method of measuring linearity of a gradient coil as claimed in claim 3, wherein the measurement acquisition of the positioning magnetic field and/or the operating magnetic field corresponding to the gradient coil comprises:
electrifying a Gx coil, a Gy coil or a Gz coil of the gradient coil, and acquiring a corresponding positioning magnetic field or a corresponding working magnetic field through the magnetic field acquisition device after waiting for a first preset time.
9. The method for measuring linearity of a gradient coil as defined in claims 1 to 8, wherein said magnetic field acquisition means comprises a fixed disk and a sensor array;
the fixed disk is used for fixing the sensor array;
the sensors included in the sensor array are distributed on the semicircular arc according to Gaussian integral point positions;
the magnetic field acquisition method corresponding to the magnetic field acquisition device comprises the following steps:
determining a plurality of sampling azimuth angles according to the fixed angle interval;
measuring sampling magnetic field data of a plurality of preset azimuth angles through the magnetic field acquisition device;
And determining a corresponding magnetic field according to the plurality of sampled magnetic field data.
10. A system for measuring the linearity of a gradient coil, wherein the system for measuring the linearity of a gradient coil performs the steps of the method for measuring the linearity of a gradient coil according to any of claims 1 to 9, comprising a superconducting magnet, a magnetic field acquisition device, a signal processor and a dc power supply;
the superconducting magnet is used for providing the background magnetic field; the gradient coil is sleeved on the inner side of the cavity of the superconducting magnet;
the magnetic field acquisition device is positioned in the cavity of the superconducting magnet when in operation and is used for measuring and acquiring a corresponding magnetic field;
the direct current power supply supplies power to the gradient coil;
the signal processor is connected with the magnetic field acquisition device and is used for processing data acquired by the magnetic field acquisition device.
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