CN113933772B - Method for detecting gradient magnetic field focus in magnetic resonance imaging system - Google Patents
Method for detecting gradient magnetic field focus in magnetic resonance imaging system Download PDFInfo
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Abstract
The invention discloses a method for detecting a gradient magnetic field focus in a magnetic resonance imaging system. The method utilizes the characteristic that the magnetic field intensity of the gradient magnetic field center is unchanged when positive and negative gradients are applied, and combines the relationship between the magnetic field intensity and proton resonance frequency to measure the offset of the gradient magnetic field center in each direction, thereby judging whether three paths of gradient magnetic fields have a common center, namely judging whether a gradient magnetic field focus exists or not, and determining the position of the gradient magnetic field focus. The detection method provided by the invention does not need an additional detection instrument, has simple and feasible detection flow, can measure the offset of the gradient magnetic field center in each direction, can provide a reference basis for correcting the assembly error of the gradient coil, and can also provide useful information for correcting the space coordinate in the image reconstruction process. The method is particularly suitable for magnetic resonance imaging in which data is acquired in a non-grid point scanning mode.
Description
Technical Field
The invention belongs to the technical field of magnetic resonance imaging, and relates to a method for detecting a gradient magnetic field focus in a magnetic resonance imaging system.
Background
Magnetic resonance imaging techniques have become a very useful tool in medical diagnostics. Typically, in a magnetic resonance imaging system, when a sample (e.g., human tissue) under test is subjected to a static magnetic field B 0 （B 0 Direction as z-axis direction of rectangular coordinate system), nuclei (nuclear spins) in the sample are balanced by B 0 Polarization to produce a macroscopic magnetization vector M 0 The method comprises the steps of carrying out a first treatment on the surface of the The M is 0 Rotated to the horizontal plane (xy plane) under excitation of a radio frequency pulse and then precessed about the Z axis. A receiving coil is placed around the sample to be measured which induces a magnetization vector precession signal. After the magnetic resonance signals acquired by the receiving coil are amplified and analog-to-digital converted, the magnetic resonance signals enter a computer for image reconstruction. Generally, for imaging, magnetic resonance imaging systems also require the generation of three orthogonal gradient magnetic fields for three-dimensional spatial localization of the magnetic resonance signals.
In the system installation process, the situation that errors exist in the assembly of the gradient coils can occur, and then the three paths of gradient magnetic fields have no focus, or the focus of the gradient magnetic fields is not positioned in the center of a scanning area, namely, the focus of the gradient magnetic fields is positioned inaccurately, and the like. In the conventional data acquisition mode, the original data is acquired row by row or column by column, and then the original data is subjected to fourier transformation to obtain an image. In this mode, even if the three gradient magnetic fields in the magnetic resonance imaging system have no focus or the gradient magnetic field focus is not positioned correctly, the image is not affected. However, with the popularization of ultra-short echo time magnetic resonance imaging technology in recent years, more and more unconventional data acquisition modes are used for magnetic resonance imaging. For example, in the radial data acquisition mode, raw data is acquired along the trajectory of radiation. In the unconventional data acquisition mode, the image reconstruction can adopt a projection reconstruction method, or can adopt a reconstruction method of firstly gridding the original data and then carrying out Fourier transformation. However, no matter which reconstruction method is adopted, if the three paths of gradient magnetic fields have no focus or the focus of the gradient magnetic fields is positioned inaccurately, larger errors are introduced in the reconstruction process. Thus, whether a set of magnetic resonance imaging systems is suitable for scanning in an irregular data acquisition mode requires a predetermined determination of whether a gradient magnetic field focus exists in the set of systems and a determination of the position of the gradient magnetic field focus.
Disclosure of Invention
The invention aims to provide a method for detecting a gradient magnetic field focus in a magnetic resonance imaging system aiming at the existing problems. The method utilizes the characteristic that the magnetic field intensity of the gradient magnetic field center is unchanged when positive and negative gradients are applied, and combines the relationship between the magnetic field intensity and proton resonance frequency to measure the offset of the gradient magnetic field center in each direction, thereby judging whether three paths of gradient magnetic fields have a common center, namely judging whether a gradient magnetic field focus exists or not, and determining the position of the gradient magnetic field focus. The detection method provided by the invention does not need an additional detection instrument, has simple and feasible detection flow, can measure the offset of the gradient magnetic field center in each direction, can provide a reference basis for correcting the assembly error of the gradient coil, and can also provide useful information for correcting the space coordinate in the image reconstruction process. The method is particularly suitable for magnetic resonance imaging in which data is acquired in a non-grid point scanning mode.
In order to achieve the above purpose, the invention adopts the following technical scheme:
the method utilizes the characteristic that the magnetic field intensity of the gradient magnetic field center is unchanged when positive and negative gradients are applied, and combines the relationship between the magnetic field intensity and proton resonance frequency to measure the offset of the gradient magnetic field center in each direction, so as to judge whether three paths of gradient magnetic fields have a common center, namely judge whether the gradient magnetic field focus exists or not, and determine the position of the gradient magnetic field focus; the method specifically comprises the following steps:
step 1: carrying out three-dimensional imaging scanning by using a standard water model, confirming that the gradient linearity of an imaging area meets the standard requirement, confirming that gradients generated by three gradient systems are mutually perpendicular, and then taking out the standard water model;
step 2: placing a detection water ball at the center of an imaging area; the position is taken as an origin, and the origin is marked by x=0, y=0 and z=0;
step 3: measuring the central offset of the gradient magnetic field in the x direction, updating the x component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 3.1: measuring and detecting the resonance frequency of a sample in the water ball when positive and negative gradient magnetic fields with gradient strength Gx are applied respectively, and marking the resonance frequency as f_px and f_nx;
step 3.2: calculating the x-direction gradient magnetic field center offset delta_x=abs (f_px-f_nx)/2γgx;
step 3.3: updating the x component of the origin coordinate to x= -delta_x, and placing the detection water ball at the new origin x=0, y=0, z=0 position;
step 4: measuring the central offset of the gradient magnetic field in the y direction, updating the y component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 4.1: measuring the resonance frequency of the sample in the detection water ball when positive and negative gradient magnetic fields with gradient strength Gy are applied, and marking as f_py and f_ny;
step 4.2: calculating the y-direction gradient magnetic field center offset delta_y=abs (f_py-f_ny)/2γgy;
step 4.3: updating the y component of the origin coordinate to y= -delta_y, and placing the detection water ball at the new origin x=0, y=0, z=0 position;
step 5: measuring the central offset of the gradient magnetic field in the z direction, updating the z component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 5.1: measuring and detecting the resonance frequency of a sample in the water ball when positive and negative gradient magnetic fields with gradient strength Gz are applied, and marking the resonance frequency as f_pz and f_nz;
step 5.2: calculating the z-direction gradient magnetic field center offset delta_z=abs (f_pz-f_nz)/2γgz;
step 5.3: updating the z component of the origin coordinate to be z= -delta_z, and placing the detection water ball at a new origin x=0, y=0, z=0 position;
step 6: measuring the central offsets delta_x and delta_y of the gradient magnetic fields in the x direction and the y direction again; if delta_x and delta_y are both within the error allowable range, the focus of the three-way gradient magnetic field is at the new original point x=0, y=0 and z=0; if delta_x or delta_y exceeds the error allowable range, the three gradient magnetic fields have no common center, namely the imaging system has no gradient magnetic field focus;
wherein:
said abs () represents the calculated absolute value and γ represents the gyromagnetic ratio of the proton;
the standard water model is used for checking the linearity of gradients and the water model with three paths of gradients mutually perpendicular in the standard configuration of the magnetic resonance imaging system; the inside of the water mould is filled with copper sulfate solution with standard component proportion and is evenly divided into grids;
the gradient linearity meets the standard requirement, namely, reaches the nominal value of the gradient linearity in the magnetic resonance imaging system;
the detection water ball comprises: the copper sulfate solution with standard component proportion is filled in the ball body, and the volume of the ball body is determined according to the requirement of a user on precision;
the error allowance range: determining according to the requirement of a user on precision;
the sequence of the step 3, the step 4 and the step 5 is arbitrary; the two directions detected again in step 6 are the two directions detected first among the three detection directions.
The beneficial effects of the invention are as follows: the detection method provided by the invention does not need an additional detection instrument, has simple and feasible detection flow, can measure the offset of the gradient magnetic field center in each direction, can provide a reference basis for correcting the assembly error of the gradient coil, and can also provide useful information for correcting the space coordinate in the image reconstruction process. The method is particularly suitable for magnetic resonance imaging in which data is acquired in a non-grid point scanning mode.
Drawings
FIG. 1 is a block diagram of a magnetic resonance imaging system according to the present invention;
FIG. 2 is a schematic diagram of a standard water model structure according to the invention;
FIG. 3 is a flowchart of the coordinate updating in embodiment 1 of the present invention;
FIG. 4 is a flowchart of the coordinate updating in embodiment 2 of the present invention;
fig. 5 is a flowchart of the coordinate updating in embodiment 3 of the present invention.
Detailed Description
The invention is further described below with reference to the drawings and examples.
Fig. 1 is a block diagram of a magnetic resonance imaging system according to the present invention. In a magnetic resonance imaging system, a magnet 101 has a cavity for placing a sample. Gradient coils 102 are placed around the cavity for generating gradient magnetic fields to spatially localize the sample. A radio frequency transmitting coil 103 and a radio frequency receiving coil 104 are arranged around the cavity, the transmitting coil is used for transmitting radio frequency pulses to excite magnetization vectors of the sample, and the receiving coil is used for receiving magnetization vector precession signals. The gradient coil 102 is connected to a gradient current amplifier 112, and the transmit coil 103 and the receive coil 104 are connected to a radio frequency power amplifier 113 and a preamplifier 114, respectively.
Based on instructions given by the computer 130, the pulse sequence storage circuit 125 controls the gradient waveform generator 122 and the transmitter 123 according to the pulse sequence stored therein. The gradient waveform generator 122 outputs a gradient pulse signal having a predetermined timing and waveform, which is amplified by the gradient current amplifier 112, and then generates a gradient magnetic field in the magnet cavity through the gradient coil 102. The transmitter 123 outputs a radio frequency pulse signal having a predetermined timing and envelope, which is amplified by the radio frequency power amplifier 113 and then excites nuclear spins in the sample by the radio frequency transmit coil 103.
The rf receive coil 104 detects the magnetization vector precession signal, which is amplified by the pre-amplifier 114 and input to the receiver 124. The receiver 124 detects and digital-to-analog converts the amplified signal under the control of the pulse train storage circuit 125 to obtain a digital signal. The digital signals are transmitted to computer 130 for reconstruction into an image. The display/printer 126 is used to display/print the scanned image.
In the field of magnetic resonance imaging, in particular in the field of magnetic resonance image processing, digital signals obtained by detection and digital-to-analog conversion are also referred to as raw data; since the original data constitutes the space K, the original data is also called space K data. And obtaining an image after the space K data is subjected to image reconstruction. Fourier transform is the most common image reconstruction method in magnetic resonance imaging. Generally, the space K must be filled to obtain an image.
As the magnetic resonance imaging scan proceeds, the spatial K data is acquired row by row or column by column in a conventional data acquisition mode. After the space K is filled, fourier transform is performed to obtain an image. Depending on the nature of the fourier transform, in conventional data acquisition mode, even if the three gradient magnetic fields have no common center, only the phase of the image is affected. Since most of the magnetic resonance imaging uses a mode diagram rather than a phase diagram, the mode diagram obtained in the conventional data acquisition mode is not affected by the lack of focus of the gradient magnetic field.
With the popularization of ultra-short echo time magnetic resonance imaging technology in recent years, more and more unconventional data acquisition modes are used for magnetic resonance imaging. For example, in the radial data acquisition mode, raw data is acquired along the trajectory of radiation. In the unconventional data acquisition mode, the image reconstruction can adopt a projection reconstruction method, or can adopt a reconstruction method of firstly gridding the original data and then carrying out Fourier transformation. However, no matter which reconstruction method is adopted, if the three paths of gradient magnetic fields have no focus or the focus of the gradient magnetic fields is positioned inaccurately, larger errors are introduced in the reconstruction process. Thus, whether a set of magnetic resonance imaging systems is suitable for scanning in an irregular data acquisition mode requires a predetermined determination of whether a gradient magnetic field focus exists in the set of systems and a determination of the position of the gradient magnetic field focus.
In the magnetic resonance imaging system, the magnetic field directions of three gradient magnetic fields are the same and are the same as that of the static magnetic field B 0 The magnetic field directions (z direction) of the three paths of gradient magnetic fields are consistent, and the gradient directions of the three paths of gradient magnetic fields are mutually perpendicular. That is, the magnetic field directions of the gradient magnetic fields in the three directions of x, y and z are all z, and the gradient directions thereof are x, y and z, respectively. Taking the x direction as an example, assuming that the gradient strength in the direction is Gx, the magnetic field strength at the position of the coordinate x=x_1 is (B 0 +gx x_1). X=0 at the central position of the gradient magnetic field, and the magnetic field intensity at the central position is B no matter how the amplitude and the positive and negative polarities of the gradient intensity Gx change 0 . The present invention takes advantage of this feature to determine whether a gradient magnetic field focus exists in the imaging system and to determine the location of the gradient magnetic field focus.
Example 1
Referring to fig. 3, the method for detecting the focus of the gradient magnetic field in the magnetic resonance imaging system provided by the invention comprises the following specific steps:
step 1: carrying out three-dimensional imaging scanning by using a standard water model, confirming that the gradient linearity of an imaging area meets the standard requirement, confirming that gradients generated by three gradient systems are mutually perpendicular, and then taking out the standard water model;
step 2: placing a detection water ball at the center of an imaging area; the position is taken as an origin, and the origin is marked by x=0, y=0 and z=0;
step 3: measuring the central offset of the gradient magnetic field in the x direction, updating the x component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 3.1: measuring and detecting the resonance frequency of a sample in the water ball when positive and negative gradient magnetic fields with gradient strength Gx are applied respectively, and marking the resonance frequency as f_px and f_nx;
step 3.2: calculating the x-direction gradient magnetic field center offset delta_x=abs (f_px-f_nx)/2γgx;
step 3.3: updating the x component of the origin coordinate to x= -delta_x, and placing the detection water ball at the new origin x=0, y=0, z=0 position;
step 4: measuring the central offset of the gradient magnetic field in the y direction, updating the y component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 4.1: measuring the resonance frequency of the sample in the detection water ball when positive and negative gradient magnetic fields with gradient strength Gy are applied, and marking as f_py and f_ny;
step 4.2: calculating the y-direction gradient magnetic field center offset delta_y=abs (f_py-f_ny)/2γgy;
step 4.3: updating the y component of the origin coordinate to y= -delta_y, and placing the detection water ball at the new origin x=0, y=0, z=0 position;
step 5: measuring the central offset of the gradient magnetic field in the z direction, updating the z component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 5.1: measuring and detecting the resonance frequency of a sample in the water ball when positive and negative gradient magnetic fields with gradient strength Gz are applied, and marking the resonance frequency as f_pz and f_nz;
step 5.2: calculating the z-direction gradient magnetic field center offset delta_z=abs (f_pz-f_nz)/2γgz;
step 5.3: updating the z component of the origin coordinate to be z= -delta_z, and placing the detection water ball at a new origin x=0, y=0, z=0 position;
step 6: measuring the central offsets delta_x and delta_y of the gradient magnetic fields in the x direction and the y direction again; if delta_x and delta_y are both within the error allowable range, the focus of the three-way gradient magnetic field is at the new original point x=0, y=0 and z=0; if delta_x or delta_y exceeds the error allowable range, the three gradient magnetic fields have no common center, namely the imaging system has no gradient magnetic field focus;
wherein:
in the above formula, abs () represents a calculated absolute value, and γ represents a gyromagnetic ratio of protons.
The standard water model is used for checking the linearity of gradients and the water model with three paths of gradients mutually perpendicular in the standard configuration of the magnetic resonance imaging system. Referring to fig. 2, the water model is square with a designated side length (for example, 50 cm side length); the organic glass plate forms a peripheral frame of a cube; the space in the peripheral frame is uniformly divided into smaller cube spaces (for example, the side length is 1 cm) by the organic glass plate; the standard component ratio of the copper sulfate solution (the mass ratio of the copper sulfate to the sodium chloride to the water is 1:1:98) is positioned in the small separated cube spaces. In magnetic resonance imaging, the plexiglas does not generate a signal, and the copper sulfate solution generates a signal.
The gradient linearity meets the standard requirement, namely, the nominal value of the gradient linearity in the magnetic resonance imaging system is reached. For example, gradient linearity is better than 5% (i.e., gradient nonlinearity error less than 5%) over a sphere with a radius of 25 cm with the imaging spatial center point as the origin.
The detection water ball is filled with a copper sulfate solution (the mass ratio of copper sulfate, sodium chloride and water is 1:1:98) with standard component proportion, and the volume of the ball is determined according to the requirement of a user on precision. For example, the sphere volume is the volume corresponding to 1000 pixels at standard spatial resolution of the imaging system.
The error allowable range is determined according to the requirement of a user on precision. For example, the error does not exceed an offset corresponding to 10 pixels.
Example 2
Referring to fig. 4, the method for detecting the focus of the gradient magnetic field in the magnetic resonance imaging system provided by the invention comprises the following specific steps:
step 1: carrying out three-dimensional imaging scanning by using a standard water model, confirming that the gradient linearity of an imaging area meets the standard requirement, confirming that gradients generated by three gradient systems are mutually perpendicular, and then taking out the standard water model;
step 2: placing a detection water ball at the center of an imaging area; the position is taken as an origin, and the origin is marked by x=0, y=0 and z=0;
step 3: measuring the central offset of the gradient magnetic field in the z direction, updating the z component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 3.1: measuring and detecting the resonance frequency of a sample in the water ball when positive and negative gradient magnetic fields with gradient strength Gz are applied, and marking the resonance frequency as f_pz and f_nz;
step 3.2: calculating the z-direction gradient magnetic field center offset delta_z=abs (f_pz-f_nz)/2γgz;
step 3.3: updating the z component of the origin coordinate to be z= -delta_z, and placing the detection water ball at a new origin x=0, y=0, z=0 position;
step 4: measuring the central offset of the gradient magnetic field in the y direction, updating the y component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 4.1: measuring the resonance frequency of the sample in the detection water ball when positive and negative gradient magnetic fields with gradient strength Gy are applied, and marking as f_py and f_ny;
step 4.2: calculating the y-direction gradient magnetic field center offset delta_y=abs (f_py-f_ny)/2γgy;
step 4.3: updating the y component of the origin coordinate to y= -delta_y, and placing the detection water ball at the new origin x=0, y=0, z=0 position;
step 5: measuring the central offset of the gradient magnetic field in the x direction, updating the x component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 5.1: measuring and detecting the resonance frequency of a sample in the water ball when positive and negative gradient magnetic fields with gradient strength Gx are applied respectively, and marking the resonance frequency as f_px and f_nx;
step 5.2: calculating the x-direction gradient magnetic field center offset delta_x=abs (f_px-f_nx)/2γgx;
step 5.3: updating the x component of the origin coordinate to x= -delta_x, and placing the detection water ball at the new origin x=0, y=0, z=0 position;
step 6: measuring the central offsets delta_z and delta_y of the gradient magnetic fields in the z direction and the y direction again; if delta_z and delta_y are both within the error allowable range, the focus of the three-way gradient magnetic field is at the new original point x=0, y=0 and z=0; if delta_z or delta_y exceeds the error allowable range, the three gradient magnetic fields have no common center, namely the imaging system has no gradient magnetic field focus;
wherein:
in the above formula, abs () represents a calculated absolute value, and γ represents a gyromagnetic ratio of protons.
The standard water model is used for checking the linearity of gradients and the water model with three paths of gradients mutually perpendicular in the standard configuration of the magnetic resonance imaging system. Referring to fig. 2, the water model is square with a designated side length (for example, 40 cm side length); the organic glass plate forms a peripheral frame of a cube; the space in the peripheral frame is uniformly divided into smaller cube spaces (for example, the side length is 1 cm) by the organic glass plate; the standard component ratio of the copper sulfate solution (the mass ratio of the copper sulfate to the sodium chloride to the water is 3:1:96) is positioned in the small separated cube spaces. In magnetic resonance imaging, the plexiglas does not generate a signal, and the copper sulfate solution generates a signal.
The gradient linearity meets the standard requirement, namely, the nominal value of the gradient linearity in the magnetic resonance imaging system is reached. For example, gradient linearity is better than 3% (i.e., gradient nonlinearity error less than 3%) over a sphere of radius 20 cm with the imaging spatial center point as the origin.
The detection water ball is filled with a copper sulfate solution (the mass ratio of copper sulfate, sodium chloride and water is 1:1:98) with standard component proportion, and the volume of the ball is determined according to the requirement of a user on precision. For example, the sphere volume is the volume corresponding to 500 pixels at the standard spatial resolution of the imaging system.
The error allowable range is determined according to the requirement of a user on precision. For example, the error does not exceed an offset corresponding to 5 pixels.
Example 3
Referring to fig. 5, the method for detecting the focus of the gradient magnetic field in the magnetic resonance imaging system provided by the invention comprises the following specific steps:
step 1: carrying out three-dimensional imaging scanning by using a standard water model, confirming that the gradient linearity of an imaging area meets the standard requirement, confirming that gradients generated by three gradient systems are mutually perpendicular, and then taking out the standard water model;
step 2: placing a detection water ball at the center of an imaging area; the position is taken as an origin, and the origin is marked by x=0, y=0 and z=0;
step 3: measuring the central offset of the gradient magnetic field in the x direction, updating the x component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 3.1: measuring and detecting the resonance frequency of a sample in the water ball when positive and negative gradient magnetic fields with gradient strength Gx are applied respectively, and marking the resonance frequency as f_px and f_nx;
step 3.2: calculating the x-direction gradient magnetic field center offset delta_x=abs (f_px-f_nx)/2γgx;
step 3.3: updating the x component of the origin coordinate to x= -delta_x, and placing the detection water ball at the new origin x=0, y=0, z=0 position;
step 4: measuring the central offset of the gradient magnetic field in the z direction, updating the z component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 4.1: measuring and detecting the resonance frequency of a sample in the water ball when positive and negative gradient magnetic fields with gradient strength Gz are applied, and marking the resonance frequency as f_pz and f_nz;
step 4.2: calculating the z-direction gradient magnetic field center offset delta_z=abs (f_pz-f_nz)/2γgz;
step 4.3: updating the z component of the origin coordinate to be z= -delta_z, and placing the detection water ball at a new origin x=0, y=0, z=0 position;
step 5: measuring the central offset of the gradient magnetic field in the y direction, updating the y component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 5.1: measuring the resonance frequency of the sample in the detection water ball when positive and negative gradient magnetic fields with gradient strength Gy are applied, and marking as f_py and f_ny;
step 5.2: calculating the y-direction gradient magnetic field center offset delta_y=abs (f_py-f_ny)/2γgy;
step 5.3: updating the y component of the origin coordinate to y= -delta_y, and placing the detection water ball at the new origin x=0, y=0, z=0 position;
step 6: measuring the central offsets delta_x and delta_z of the gradient magnetic fields in the x direction and the z direction again; if delta_x and delta_z are both within the error allowable range, the focus of the three-way gradient magnetic field is at the new original point x=0, y=0 and z=0; if delta_x or delta_z exceeds the error allowable range, the three gradient magnetic fields have no common center, namely the imaging system has no gradient magnetic field focus;
wherein:
in the above formula, abs () represents a calculated absolute value, and γ represents a gyromagnetic ratio of protons.
The standard water model is used for checking the linearity of gradients and the water model with three paths of gradients mutually perpendicular in the standard configuration of the magnetic resonance imaging system. Referring to fig. 2, the water model is square with a designated side length (for example, 20 cm side length); the organic glass plate forms a peripheral frame of a cube; the space in the peripheral frame is uniformly divided into smaller cube spaces (for example, the side length is 0.5 cm) by the organic glass plate; the standard component ratio of the copper sulfate solution (the mass ratio of the copper sulfate to the sodium chloride to the water is 1:1:98) is positioned in the small separated cube spaces. In magnetic resonance imaging, the plexiglas does not generate a signal, and the copper sulfate solution generates a signal.
The gradient linearity meets the standard requirement, namely, the nominal value of the gradient linearity in the magnetic resonance imaging system is reached. For example, gradient linearity is better than 1% over a sphere of radius 10 cm with the imaging spatial center point as the origin (i.e., gradient nonlinearity error less than 1%).
The detection water ball is filled with a copper sulfate solution (the mass ratio of copper sulfate, sodium chloride and water is 5:1:94) with standard component proportion, and the volume of the ball is determined according to the requirement of a user on precision. For example, the sphere volume is the volume corresponding to 100 pixels at standard spatial resolution of the imaging system.
The error allowable range is determined according to the requirement of a user on precision. For example, the error does not exceed an offset corresponding to 1 pixel.
Claims (1)
1. A method for detecting a focus of a gradient magnetic field in a magnetic resonance imaging system, the method comprising the steps of:
step 1: carrying out three-dimensional imaging scanning by using a standard water model, confirming that the gradient linearity of an imaging area meets the standard requirement, confirming that gradients generated by three gradient systems are mutually perpendicular, and then taking out the standard water model;
step 2: placing a detection water ball at the center of an imaging area; the position is taken as an origin, and the origin is marked by x=0, y=0 and z=0;
step 3: measuring the central offset of the gradient magnetic field in the x direction, updating the x component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 3.1: measuring and detecting the resonance frequency of a sample in the water ball when positive and negative gradient magnetic fields with gradient strength Gx are applied respectively, and marking the resonance frequency as f_px and f_nx;
step 3.2: calculating the x-direction gradient magnetic field center offset delta_x=abs (f_px-f_nx)/2γgx;
step 3.3: updating the x component of the origin coordinate to x= -delta_x, and placing the detection water ball at the new origin x=0, y=0, z=0 position;
step 4: measuring the central offset of the gradient magnetic field in the y direction, updating the y component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 4.1: measuring the resonance frequency of the sample in the detection water ball when positive and negative gradient magnetic fields with gradient strength Gy are applied, and marking as f_py and f_ny;
step 4.2: calculating the y-direction gradient magnetic field center offset delta_y=abs (f_py-f_ny)/2γgy;
step 4.3: updating the y component of the origin coordinate to y= -delta_y, and placing the detection water ball at the new origin x=0, y=0, z=0 position;
step 5: measuring the central offset of the gradient magnetic field in the z direction, updating the z component of the origin coordinate, and placing the detection water ball at a new origin position, wherein the method specifically comprises the following steps:
step 5.1: measuring and detecting the resonance frequency of a sample in the water ball when positive and negative gradient magnetic fields with gradient strength Gz are applied, and marking the resonance frequency as f_pz and f_nz;
step 5.2: calculating the z-direction gradient magnetic field center offset delta_z=abs (f_pz-f_nz)/2γgz;
step 5.3: updating the z component of the origin coordinate to be z= -delta_z, and placing the detection water ball at a new origin x=0, y=0, z=0 position;
step 6: measuring the central offsets delta_x and delta_y of the gradient magnetic fields in the x direction and the y direction again; if delta_x and delta_y are both within the error allowable range, the focus of the three-way gradient magnetic field is at the new original point x=0, y=0 and z=0; if delta_x or delta_y exceeds the error allowable range, the three gradient magnetic fields have no common center, namely the imaging system has no gradient magnetic field focus;
wherein:
said abs () represents the calculated absolute value and γ represents the gyromagnetic ratio of the proton;
the standard water model is used for checking the linearity of gradients and the water model with three paths of gradients mutually perpendicular in the standard configuration of the magnetic resonance imaging system; the inside of the water mould is filled with copper sulfate solution with standard component proportion and is evenly divided into grids;
the gradient linearity meets the standard requirement, namely, reaches the nominal value of the gradient linearity in the magnetic resonance imaging system;
the detection water ball comprises: the copper sulfate solution with standard component proportion is filled in the ball body, and the volume of the ball body is determined according to the requirement of a user on precision;
the error allowance range: determining according to the requirement of a user on precision;
the sequence of the step 3, the step 4 and the step 5 is arbitrary; the two directions detected again in step 6 are the two directions detected first among the three detection directions.
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