Method and system for detecting magnetic flux density of a magnetic resonance device

Abstract

A method for detecting the magnetic flux density of a magnetic resonance imaging (MRI) device within a patient channel of the MRI device includes: determining a first spatial direction and / or a second spatial direction, wherein the magnetic flux density of the B0 magnetic field of the B0 main magnet of the MRI device in the first spatial direction and / or the second spatial direction is below a preset value at at least one location within the patient channel; placing at least one magnetic field strength sensor at at least one location within the patient channel, wherein the magnetic field strength sensor is used to detect the magnetic flux density at least in the first spatial direction and / or the second spatial direction; and detecting the magnetic flux density in the first spatial direction and / or the second spatial direction by the at least one magnetic field strength sensor. Furthermore, the invention also relates to an application of at least one magnetic flux density measured according to the method, a system for detecting the magnetic flux density of an MRI device within a patient channel of the MRI device, and computer program elements.

Description

Technical Field

[0001] The present invention relates to a method and system for detecting the magnetic flux density of a magnetic resonance device, particularly a gradient coil, a magnetic resonance device, a corresponding computer program element, and an application of the measured magnetic flux density. Background Technology

[0002] Magnetic resonance imaging (MRT) is an imaging device that uses a strong external magnetic field to align the nuclear spins of the object being examined, and then excites the spins to precess around this alignment direction using an alternating magnetic field, thereby creating an image of the object. When the spins return from this excited state to a lower energy state, an alternating magnetic field is generated, which can be received by an antenna. Using a gradient magnetic field provided by gradient coils, the signal can be positionally encoded, thus assigning the received signal to volume elements. The received signal can then be evaluated to provide a three-dimensional image of the object being examined. This process is also known as image reconstruction. For image reconstruction, the gradient magnetic field actually provided during the examination is particularly important. The magnetic field used to generate the magnetic resonance image, in particular, requires quite high strength.

[0003] In this regard, a method and system are needed to detect the magnetic flux density of a magnetic resonance imaging (MRI) device, especially the magnetic flux density of the gradient coils of the MRI device, and the changes in magnetic flux density caused by the activation of the gradient coils.

[0004] Document US 10 641 858 B2 describes a magnetic resonance imaging system that uses magnetic field sensors to measure the lateral components of the magnetic field along the x, y, and z axes. The signals from these sensors are used to determine the magnetic field gradient along the longitudinal axis. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to provide a method and system for detecting the magnetic flux density of a magnetic resonance device, especially the gradient coil of the magnetic resonance device, and / or its changes.

[0006] The aforementioned technical problems, as well as other technical problems mentioned in the following description or that can be identified by a person skilled in the art, are solved by the technical solutions described in this invention.

[0007] According to the present invention, a method for detecting the magnetic flux density of a magnetic resonance imaging (MRI) device within the patient channel of the MRI device is disclosed, the method comprising:

[0008] A first spatial direction and / or a second spatial direction are determined, wherein the magnetic flux density of the B0 main magnet of the magnetic resonance device in the first spatial direction and / or the second spatial direction is below a preset value at at least one location within the patient channel;

[0009] At least one magnetic field strength sensor is disposed at at least one location within the patient passage, wherein the magnetic field strength sensor is disposed and configured to detect magnetic flux density at least in a first spatial direction and / or a second spatial direction; and

[0010] The magnetic flux density is detected in the first spatial direction and / or the second spatial direction by the at least one magnetic field strength sensor.

[0011] According to one aspect of the present invention, a method for detecting the magnetic flux density of a gradient coil in a magnetic resonance imaging (MRI) device within a patient channel of the MRI device is disclosed, the method comprising:

[0012] Determine at least one position within the patient channel such that the magnetic flux density of the B0 main magnet of the magnetic resonance device in the first spatial direction and the second spatial direction is below a preset value;

[0013] At least one magnetic field strength sensor is arranged at the at least one location, wherein the magnetic field strength sensor is arranged and configured to detect magnetic flux density at least in a first spatial direction and a second spatial direction; and

[0014] The magnetic flux density is detected in the first spatial direction and the second spatial direction by the at least one magnetic field strength sensor.

[0015] According to one aspect of the present invention, a method for detecting the magnetic flux density of a magnetic resonance imaging (MRI) device within a patient channel of an MRI device is disclosed, the method comprising:

[0016] A first spatial direction is determined, and the magnetic flux density of the B0 magnetic field of the B0 main magnet of the magnetic resonance device in the first spatial direction is below a preset value at at least one position within the patient channel;

[0017] At least one magnetic field strength sensor is disposed at at least one location within the patient passage, wherein the magnetic field strength sensor is disposed and configured to detect magnetic flux density at least in a first spatial direction; and

[0018] The magnetic flux density is detected in the first spatial direction by the at least one magnetic field strength sensor.

[0019] Magnetic resonance imaging (MRI) devices utilize high magnetic flux densities; for example, the magnetic flux density of the B0 main magnet in a known MRI device is 3 T. Therefore, magnetic field strength sensors, especially three-dimensional Hall sensors, are used to detect or measure the magnetic flux density of gradient coils within the patient channel of the MRI device. The problem is that such magnetic field strength sensors typically have relative measurement errors within a percentage range. A magnetic field strength sensor with a measurement range of 3 T and a relative error of 1% has an absolute error of 30 mT. Therefore, such a magnetic field strength sensor cannot be used to detect the magnetic flux density or changes in magnetic flux density of gradient coils with sufficiently high accuracy, such as detecting fluctuations or defects in the gradient field within the microtesla range. If a more accurate magnetic field strength sensor, i.e., a sensor with a linear measurement range within the microtesla range, is used, this magnetic field strength sensor enters a saturation region when the magnetic flux density is 3 T, thus failing to provide reliable measurements. In this invention, the concept of magnetic flux density should be understood in a broad sense. For example, magnetic flux density can also represent magnetic field strength. Magnetic field strength can also be measured simultaneously during the measurement of magnetic flux density.

[0020] This invention is based on the understanding that the B0 magnetic field of the B0 main magnet of a magnetic resonance imaging (MRI) device has a high magnetic field strength component in only one spatial direction within a uniform region, namely, the spatial direction parallel to the axis of symmetry of the B0 magnetic field of the B0 main magnet. Typically, detailed descriptions of position encoding and spatial orientation are based on the XYZ coordinate system. The Z-axis is generally defined as the axis of symmetry of the B0 main magnet of the MRI device, extending along the preferred direction of the B0 field through the patient channel of the B0 main magnet, and this invention is no exception. In the conventional installation of an MRI device, the Z-axis is horizontally oriented and passes through the center of the coil opening of the B0 main magnet. The subject is typically placed parallel to the Z-axis on a movable patient platform and pushed into the patient channel. The Z-axis, together with the X and Y axes, constitutes a space, wherein the axes are preferably orthogonal, and the X-axis is preferably horizontal and the Y-axis is preferably vertical.

[0021] Based on this XYZ coordinate system, within the uniform region of the B0 main magnet, only the Z-axis contains the relevant field strength component of the B0 magnetic field, which is also called the Z-field strength component (B0.z). The field strength components in the X and Y axes are usually called the X-field strength component (B0.x) and the Y-field strength component (B0.y), respectively.

[0022] Furthermore, this invention is based on the understanding that the B0 magnetic field of the B0 main magnet has multiple locations where the X-field strength component (B0.x) and the Y-field strength component (B0.y) are essentially zero; in other words, the magnetic flux density in these two spatial directions is essentially zero. To accommodate the magnetic field strength sensor, it is necessary to determine locations where the magnetic flux density is below a preset value, preferably essentially zero. In a preferred design, the (measuring) axis of the magnetic field strength sensor is collinearly oriented with the X-field strength component (B0.x) and the Y-field strength component (B0.y). For example, if a three-dimensional magnetic field strength sensor, preferably a three-dimensional Hall sensor, is used, and the measuring axis is aligned with the axis of the XYZ coordinate system, minute changes in magnetic flux density or in the X and Y directions can be detected, regardless of whether there is a high magnetic flux density in the Z direction or whether the magnetic field strength sensor for the Z-field strength component (B0.z) has reached saturation. Therefore, a high-precision magnetic field strength sensor can be used to detect the magnetic flux density in the X and Y directions with correspondingly high precision.

[0023] In other words, according to at least one aspect, this invention proposes first determining locations where the X-field component (B0.X) and / or Y-field component (B0.Y) of the main magnet within the patient channel are below a given value, preferably substantially zero, and arranging one or more magnetic field strength sensors at these locations to detect / measure the X-field component (B0.X) and Y-field component (B0.Y) at those locations. These locations can be determined in various ways, for example, by measuring the magnetic field, or by calculation or simulation. These locations can be calculated based on the geometry of the main magnet (B0). Magnetic resonance imaging (MRI) devices typically also provide a distribution map of the B0 magnetic field, from which suitable locations can be derived.

[0024] The proposed method allows for the recording of magnetic flux density in a gradient field and direct measurement of its changes. The recorded magnetic flux density can then be used for various applications. For example, it can be used to continuously record and monitor gradient fields or their variations. The acquired magnetic flux density can be used for image reconstruction, such as in the presence of undesirable vortex effects. Furthermore, it allows for monitoring of the magnetic flux density and, if it exceeds a predefined range, an emergency shutdown. Additionally, it allows for comparison of target and actual values ​​of the magnetic flux density, such as for evaluating the quality and / or operating status of gradient coils. Finally, the Z-field component can be determined based on the X and Y field strength components.

[0025] Here, the definition of patient access should be understood in a broad sense. It specifically refers to the interior of the typical "tube" of most MRI devices, also known as the "hole," but it can also refer to the sensing area suitable for imaging in MRI devices of any shape.

[0026] As described above, the axis of the magnetic field strength sensor is preferably collinear with at least two spatial directions of the B0 magnetic field (the X-component (B0.x) and the Y-component (B0.y)) or with at least one of the two spatial directions in which the magnetic flux density is detected. However, this disclosure is not limited thereto. Depending on the specific design, the axis of the magnetic field strength sensor may not be collinear, or it may be arranged at an angle to the two spatial directions.

[0027] The preset value of magnetic flux density is preferably between -0.02T and 0.02T, more preferably between -0.01T and 0.01T, and most preferably between -5µT and 5µT.

[0028] As previously described, according to one variation, the magnetic field strength sensor is a three-dimensional magnetic field strength sensor used to detect the field strength of the three components of the B0 magnetic field in three spatial directions, and the axis of the magnetic field strength sensor is preferably collinear with the three spatial directions of the B0 magnetic field. However, the present invention is not limited to using a three-dimensional magnetic field strength sensor. Additionally and / or alternatively, a two-dimensional magnetic field strength sensor may also be used, which is preferably collinear with two spatial directions of the B0 magnetic field (the X field strength component (B0.x) and the Y field strength component (B0.y)).

[0029] The magnetic field strength sensor is preferably a Hall sensor, anisotropic magnetoresistive sensor, magnetic tunneling resistance sensor, or fluxgate magnetometer. In one embodiment, different magnetic field strength sensors may also be used. Preferably, at least one magnetic field strength sensor has a linear measurement range between 0.01T and -0.01T.

[0030] Preferably, at different locations within the patient access channel, the magnetic flux density of the B0 main magnet of the MRI machine is lower than a preset value in the first and second spatial directions. Magnetic field strength sensors are arranged at different locations within the patient access channel, wherein the magnetic flux density of the B0 magnetic field in the first and second spatial directions is lower than the preset value. The magnetic field strength sensors are respectively arranged and configured to detect the magnetic flux density at least in the first and second spatial directions. Preferably, one or more "sensor arrays" consisting of multiple magnetic field sensors are used; these sensors can be arranged not only side-by-side but also stacked.

[0031] The method further includes the following steps:

[0032] Based on the detected magnetic flux density in the first and second spatial directions, the magnetic flux density in the third spatial direction is determined. In a preferred design, the X and Y field components are detected as described above. Then, since the magnetic flux density is passive, the Z field component can be calculated or derived because there are only closed field lines in the magnetic field. Applying the second Maxwell's equations, also known as Gauss's law of magnetic fields, yields the following result:

[0033]

[0034] This indicates that the magnetic field is source-free. The divergence of the vector field is:

[0035] B = (B x B y B z )

[0036] Defined as

[0037]

[0038] according to Solving for:

[0039]

[0040] By installing multiple magnetic field strength sensors, not only can the magnetic flux density, i.e. the local X and Y field strength components, be directly measured, but the corresponding derivatives in the two spatial directions can also be determined, thereby calculating the magnetic flux density in the Z direction.

[0041] According to one embodiment, the at least one magnetic field strength sensor comprises a plurality of magnetic field strength sensors disposed at multiple locations within the patient channel, particularly at least four different locations, preferably at least ten locations, and particularly preferably at least twenty locations. The magnetic field strength sensors are arranged and configured to detect magnetic flux density in a first spatial direction. The magnetic flux density is detected in the first spatial direction by the plurality of magnetic field strength sensors. Based on the detected magnetic flux density in the first spatial direction, the magnetic flux density and / or the local derivative of the magnetic flux density in a second and / or third spatial direction are determined, particularly calculated. Specifically, the position-dependent magnetic flux density within a defined volume in at least two, preferably three, spatial directions is determined. The magnetic field strength sensors can be particularly designed and arranged to detect magnetic flux density only in the first spatial direction. In particular, all magnetic field strength sensors can be designed and arranged to detect magnetic flux density only in the first spatial direction. In this context, the first spatial direction can be a spatial direction perpendicular to the B0 field and / or the (longitudinal) axis of symmetry of the B0 magnet. For example, the first spatial direction can be the x-direction or y-direction of the system, or a linear combination of the x-direction and y-direction. In this invention, it has been found that detecting magnetic flux density in only a single spatial direction is sufficient to determine magnetic flux density in other spatial directions. In particular, it can be specified to determine magnetic flux density curves in all three spatial directions within the patient passage and / or defined volume, especially within the defined volume of the patient passage. The magnetic field strength sensor can be positioned, in particular, at the edge region of the defined volume. It can also be specified that signals from the magnetic field strength sensor are acquired substantially simultaneously, and corresponding derivatives and / or magnetic flux densities are determined for these simultaneously acquired signals. Measurements of magnetic flux density can be repeated over time in the first spatial direction, particularly multiple times. For example, measurements can be repeated at the same time intervals or almost continuously. The magnetic field strength sensor that measures magnetic flux density only in the first spatial direction is inexpensive and simple to manufacture. The first spatial direction is orthogonal to the second and / or third spatial directions.

[0042] According to one embodiment, magnetic flux density in a second spatial direction and / or a third spatial direction is determined based on calculations using spatial harmonic functions, particularly conventional spatial harmonic functions. It is shown here that spatial harmonic functions are particularly suitable for performing the calculations in this invention due to geometric conditions. In particular, the local derivatives of the other two spatial directions can be directly determined using only one spatial direction. It is conceivable that a first field gradient model is determined in a defined volume within the patient passage and / or patient passage based on the determined magnetic flux density in the first spatial direction. The field gradient model can also be referred to as an incomplete field model. For example, a first field gradient model can be obtained by fitting a particularly complete first field model, especially a full-field model, to measurements from a magnetic field strength sensor and using spatial harmonic functions. For this purpose, the corresponding coefficients of the first field model can be adjusted to match the measured magnetic flux density. Based on the first field gradient model in the first spatial direction, a second field gradient model in the second or third spatial direction can be calculated. Based on the first field gradient model in the first spatial direction or based on the already calculated second field gradient model, a third field gradient model in the remaining spatial directions, such as the third spatial direction, can be calculated. Furthermore, the first-order model coefficients of the field model, especially the first-order model coefficients of the second-field model, and / or the first-order model coefficients of the third-field model can be calculated. To calculate the model coefficients, additional measurements can be used, particularly measurements of magnetic flux density, or estimations can be made by assuming that the magnetic field decays over greater distances.

[0043] According to one embodiment, at least two field detectors are further disposed within the patient passage. These field detectors are designed to detect local absolute values ​​of magnetic field strength. Specifically, at least two field detectors detect the absolute values ​​of magnetic field strength in each passage. Based on the detected magnetic flux density in a first spatial direction and the absolute values ​​of the magnetic field strength detected by the at least two field detectors, magnetic flux densities in a second and / or third spatial direction are determined. Using at least two field detectors is a particularly simple feasibility, allowing for the determination of not only the local derivatives of the magnetic flux density in two other spatial directions but also the actual magnetic flux density. It can be specified that the signals from the field detectors are acquired substantially simultaneously with those from the magnetic field strength sensor, and that the magnetic flux density is determined separately for these simultaneously acquired signals. For the aforementioned second and / or third field models, first-order model coefficients, particularly the first-order model coefficients of the second field model and / or the third field model, can be calculated using the local measurements from the field detectors.

[0044] Based on the first field model, the second field model, and / or the third field model, the position-dependent magnetic flux density within the aforementioned defined volume can typically be calculated. The calculation of the position-dependent magnetic flux density can be particularly targeted at different time points, especially based on the time point at which the magnetic flux density is measured in the first spatial direction.

[0045] The following section introduces computational and physical fundamentals and provides examples to illustrate how to calculate the magnetic flux density in the second and third spatial directions, as well as the position-dependent magnetic flux density in multiple spatial directions, based on the magnetic flux density measured in the first spatial direction.

[0046] Spherical harmonic functions (German: Kugelflächenfunktion) are defined as solutions to the spherical harmonic differential equation, which is given as the angular components of the Laplace equation in spherical coordinates (r=r0=constant). Since spherical functions form a complete set of orthogonal functions with an orthogonal basis on the sphere (r=r0=constant), any function defined on the sphere can be expressed as a weighted sum of these spherical functions. Spatial harmonic functions, also known as solid-state harmonic functions, are well-defined solutions to the Laplace equation defined inside the sphere (r>r0). Spatial harmonic functions can be divided into regular spatial harmonic functions that are well-defined at the origin and unconventional spatial harmonic functions that have a singularity at the origin. The following examples are based on regular spatial harmonic functions in Cartesian coordinates. Since spatial harmonic functions form a complete set of orthogonal functions with an orthogonal basis in three-dimensional space, any function defined inside the sphere can be expressed as a weighted sum of these spatial harmonic functions.

[0047] Typically, a three-dimensional scalar function f(r) or a three-dimensional vector function F(r) applicable to Δ · f(r) = 0 or Δ · F(r) = 0 (Laplace's equations and the Laplace operator Δ) can be called a harmonic function or harmonic field vector. Spatial harmonic functions (also called solid-state harmonic functions) are orthogonal solutions to these Laplace equations. In principle, any harmonic function or harmonic field can be represented by a weighted sum with appropriate calibration. In a space without current, the vector field of magnetic flux density B is a harmonic field vector, whose Cartesian component B... x B y and B z It is a harmonic function. According to Maxwell's equations, ∇×B and ∇·B are both zero, meaning that B is a field without eddies and without sources.

[0048] The target field is linearly decomposed into a weighted sum of N spatial harmonic (SH) functions, with the addition of N scalar coefficients C. i (i=1…N), can represent a three-dimensional field distribution. For the three spatial directions x, y, z, this can be represented as:

[0049]

[0050] Where index i represents the l-th order spatial harmonic function, its order is .

[0051] m = -l, 0, l

[0052] Table 1 summarizes the 16 conventional space harmonic functions for the first three levels. These terms serve as examples of field models. The completeness of these field models implies that, for a sufficient number of N terms, the development can converge to an accurate result. Scalar coefficients C i The set of coefficients allows us to calculate the Cartesian components of the magnetic field or magnetic flux density at any position r(x,y,z) within the volume. Such a set of coefficients is called a full-field model (FFM).

[0053] Table 1:

[0054]

[0055] Based on the field model, the field gradient model can be derived. For this, the first three partial derivatives of the Cartesian components can be used. For example, B x The field gradient model has three parts, which can be completely represented by N-1 model coefficients CX. i Describe using (t), i=2…N:

[0056]

[0057] because

[0058]

[0059] (See Table 1) Therefore, in this example, the field gradient model actually has only N-1 terms, which are determined by the corresponding (final) N-1 model coefficients of the initial field model, especially the FFM. The reduced N-1 model coefficients (e.g., CX2(t)…CX) N (t) can be called an incomplete field model (IFM) or a field gradient model. Similarly, it can also be the Cartesian component of the magnetic field and the magnetic flux density B. y and B z The Y-gradient and Z-gradient are given corresponding expressions. Their main difference lies in B. y Model coefficients CY i (t) and B z Model coefficients CZ i (t). Therefore, there are a total of 9 mathematical expressions that can be used to calculate all 9 Cartesian field gradients at any position r(x,y,z) in the defined volume.

[0060] In particular, the same set of model coefficients can be used simultaneously for the corresponding field model and the corresponding field gradient model, i.e., the corresponding spatial direction or Cartesian component B of the magnetic field or magnetic flux density. x B y Or B z .

[0061] Due to the vortex-free nature of the magnetic vector field or magnetic flux density ( ) and passive ( The following equation can be derived.

[0062]

[0063] The following equation can be derived.

[0064]

[0065] as well as

[0066]

[0067] It can be concluded that

[0068]

[0069] Therefore, the field gradients of the Cartesian components are not completely independent, but can be calculated from each other. For example, all field gradients B z Both can be based on the known field gradient B x and / or B y The calculation yielded the result.

[0070] To compute the N coefficients CX of the Full-Field Model i The field values ​​can be obtained from M different measurement points, where M ≥ N. The more measurement points M there are, the higher the measurement accuracy. Here, 10 to 50 measurement points, preferably 20 to 30, are particularly advantageous. The number of measurement points is the same as the number of magnetic field strength sensors. By formulating field model equations for all known field values, a linear system can be established, containing M ≥ N equations and N unknowns, i.e., model coefficients CX. i :

[0071]

[0072] This linear system can be solved using known arithmetic methods or numerical algorithms to find the complete field model (B in this case). x The N coefficients of ) can be calculated using the complete field model, at any position r. k (x k , y k , z k The field value or magnetic flux density Bx ,(r k , t j The value of ), especially at locations other than the initial measurement point.

[0073] Once the model coefficients of the first field model are determined, B can be determined using the first partial derivative of the complete field model. x Field gradient model:

[0074]

[0075] Using these equations, B at each location r within the defined volume can be calculated. x The three magnetic field gradients. In this invention, the complete field model and the field gradient model can use the same model coefficients. Furthermore, these equations also apply to the Cartesian components B in other spatial directions. y and B z Simply change the model coefficients CX i Replace them respectively with those for B y CY i and targeting B z CZ i That's all.

[0076] Using the equations and fundamental relationships of the first field gradient model described above

[0077]

[0078] It is possible to calculate r at (at least) M ≥ N-1 positions. k Where the value of the second field gradient at k=1…M is used, and...

[0079]

[0080] Perform the calculation. This allows us to calculate the value of B for each point r within the defined volume. y The gradient of X. This generates M equations and N-1 unknowns CY. i Linear systems of i=2…N:

[0081]

[0082] This system of linear equations can be solved using arithmetic methods or numerical algorithms to determine the CY of the second field gradient model. i The model coefficients are i=2…N.

[0083] Using the equations and fundamental relationships of the first field gradient model described above

[0084]

[0085] It is also possible to calculate (at least) M ≥ N-1 positions r k The gradient value of the third field, where k=1…M, is used

[0086]

[0087] Perform the calculation. This allows us to calculate the value of B for each point r within the defined volume. z The gradient of X. This generates a equation with M equations and N-1 unknowns, CZ. i Linear systems of i=2…N:

[0088]

[0089] This system of linear equations can be solved using arithmetic methods or numerical algorithms to determine the CZ of the second field gradient model. i The model coefficients are i=2…N.

[0090] The calculation method shown here is only a possible example. Other variations can be used to determine the field gradient model. In particular, there are many methods for determining the field gradient model using spatial harmonic functions. For example, one can first calculate the third field gradient model from the first gradient model, and then determine the second field gradient model from either the first or third field gradient model.

[0091] Finally, an example is provided to illustrate how to determine the first-order model coefficients based on measurements from a field detector. The field detector provides measurements from two different locations r. a and r b The absolute values ​​of the magnetic field or magnetic flux density. These absolute values ​​can be expressed using the field model described above as follows:

[0092]

[0093] Only two unknowns remain in the equation: the second field model CY1 (used for component B). y ) and the third field model CZ1 (for component B) z The model coefficients are obtained by defining the two positions of the field detector (r = r). a and r = r b Formulating this equation yields a system of quadratic linear equations with two unknowns. This system can be solved using arithmetic methods or numerical algorithms to find the missing first-order model coefficients CY1 and CZ1.

[0094] Therefore, all model coefficients for the three complete field models have been determined. Thus, using the equations of the field models, the Cartesian component B can be determined at any position r(x,y,z) within the defined volume. x B y B zTherefore, the local value B of the magnetic field vector or magnetic flux density B is:

[0095]

[0096] Local field values ​​can be used as input for field calibration or field mapping, such as for offline field cameras, or to correct image artifacts and image distortion, for example, for line field cameras.

[0097] According to one embodiment, a magnetic field strength sensor is arranged at a position substantially located on the surface of an imaginary sphere, wherein the magnetic field strength sensor is arranged equidistantly on the surface of the imaginary sphere. Alternatively or additionally, the magnetic field strength sensor is at least partially arranged on a local coil. The positioning of the spherical surface can be performed with reference to the description in document EP 4 407 334 A1. This positioning of the spherical surface can correspond to offline field calibration or an offline field camera. The offline field camera maps the distribution of the magnetic field and uses the stored map to correct data acquired during clinical scanning. The positioning of the local coil can be performed, for example, with reference to the description in document EP US 11 815 575 B2. This is equivalent to an online field camera.

[0098] Furthermore, the present invention also relates to a system for detecting the magnetic flux density of a magnetic resonance imaging (MRI) device within a patient channel of the MRI device, particularly for detecting the magnetic flux density of a gradient coil (18) within a patient channel (14) of the MRI device (1), the system comprising:

[0099] Processing loop;

[0100] Storage media; and

[0101] Data port;

[0102] The storage medium includes a computer program with multiple instructions that, when executed, cause the processing loop to perform the method for detecting the magnetic flux density of the gradient coil of a magnetic resonance imaging (MRI) device within the patient channel of the MRI device. The data port is configured to receive magnetic flux density detected by at least one magnetic field strength sensor in a first spatial direction and a second spatial direction. The above-described embodiments of the disclosed method also apply to this system.

[0103] In one example, the system includes a magnetic resonance imaging (MRI) device. The system also includes at least one gradient coil and at least one magnetic field strength sensor, preferably at least four magnetic field strength sensors or four "sensor arrays," which are arranged in different regions of the gradient coil and / or the system, preferably generally arranged in mutually opposite regions of the gradient coil and / or generally arranged on the surface of an imaginary sphere. Optionally, at least one magnetic field strength sensor may be part of and / or fixedly connected to the gradient coil.

[0104] Preferably, the system includes at least one patient platform configured for movement into the patient channel of the magnetic resonance imaging (MRI) device. The patient platform includes at least one magnetic field strength sensor arranged on the patient platform such that it is positioned within the patient channel at a location where the magnetic flux density of the B0 main magnet of the MRI device in a first spatial direction and / or a second spatial direction is below a preset value. For example, this magnetic field strength sensor can be positioned on the patient platform so that it is moved into the patient channel together with the patient platform and is located as close as possible to the origin of the XYZ coordinate system.

[0105] Preferably, the system includes at least one local coil configured to be positioned next to or on the patient and moved into the patient channel of the MRI machine. The local coil includes at least one magnetic field strength sensor, preferably at least four, arranged on the local coil such that at least one magnetic field strength sensor is located at a position within the patient channel where the magnetic flux density of the B0 main magnet of the MRI machine in a first spatial direction and / or a second spatial direction is below a preset value. This local coil can be a head coil or a chest coil.

[0106] To minimize interference with the measurements of the magnetic resonance imaging (MRI) device, it is recommended to place microcontrollers near one or more adjacent magnetic field strength sensors, thus providing a sensor controller unit. This sensor controller unit is preferably equipped with a high-frequency shielding material, such as a carbon copper braided mesh. This sensor controller unit can then transmit the measurement data to a computer unit via optical fiber.

[0107] Furthermore, the present invention also relates to a computer program element comprising instructions which, when run on a data processing device in a data processing environment, are configured to perform the steps of the method described above in the aforementioned system.

[0108] Finally, the present invention also relates to an application of at least one magnetic flux density measured according to the above method, wherein the magnetic flux density is used as an input value for:

[0109] - Comparison of nominal and actual magnetic flux density values;

[0110] - Methods for image reconstruction; and / or

[0111] - Monitoring of at least one magnetic flux density.

[0112] As described above, the measured magnetic flux density can be used in various applications. For example, it can be used to continuously detect and monitor gradient fields or changes in gradient fields. The measured magnetic flux density can be used for image reconstruction, for example, in the presence of undesirable eddy current effects. Furthermore, if the detected magnetic flux density exceeds a predefined range, it can be used for emergency shutdown. Additionally, target values ​​of magnetic flux density can be compared with actual values, for example, for quality and / or operational evaluation of gradient coils. Attached Figure Description

[0113] Unless otherwise expressly stated, all embodiments described herein can be combined with each other. Other features, advantages, and applications of the invention will become apparent from the following description, examples, and illustrations. In the drawings:

[0114] Figure 1 This shows a traditional magnetic resonance imaging (MRI) device;

[0115] Figure 2 A schematic diagram of the system according to the invention is shown;

[0116] Figure 3 A schematic diagram of the method according to the present invention is shown;

[0117] Figure 4 A graph showing the X and Y field strength components of the B0 magnetic field;

[0118] Figure 5 A first view of a gradient coil with multiple magnetic field strength sensors is shown;

[0119] Figure 6 Showing according to Figure 5 A second view of the gradient coil;

[0120] Figure 7 A schematic diagram showing the arrangement of multiple magnetic field strength sensors on the surface of a sphere surrounding a defined volume is provided.

[0121] Figure 8 A schematic diagram showing the arrangement of the magnetic field strength sensor on the head coil;

[0122] Figure 9A schematic diagram of a method according to the invention according to another embodiment is shown. Detailed Implementation

[0123] Figure 1 A conventional magnetic resonance imaging (MRI) device 1 is shown. The MRI device 1 includes a magnetic field generating unit 11 having a B0 main magnet or B0 magnet 12, which has one or more permanent magnets, electromagnets, or superconducting magnets for generating a strong and particularly uniform B0 main magnetic field or B0 magnetic field 13. Furthermore, the MRI device 1 includes a patient channel 14, also referred to as an aperture 14, for accommodating a patient 15. In the illustrated embodiment, the patient channel 14 is designed as a cylinder and is circumferentially surrounded by the B0 main magnet 12. In principle, different patient channel 14 structures are also conceivable. The patient channel 14 can substantially correspond to the image acquisition area of ​​the MRI device 1.

[0124] exist Figure 1 In the example shown, patient 15 can be positioned in patient channel 14 by patient positioning device 16 of magnetic resonance imaging device 1. For this purpose, patient positioning device 16 has a horizontally movable patient platform 17.

[0125] The magnetic field generating unit 11 has a gradient system with at least one gradient coil 18 for generating a gradient magnetic field used for position encoding during magnetic resonance imaging (MRI). The gradient coil 18 is controlled by a gradient control unit 19 of the MRI device 1. It is conceivable that the gradient system includes multiple gradient coils 18 for generating gradient magnetic fields along different, preferably mutually orthogonal, spatial directions.

[0126] The magnetic field generating unit 11 also includes a high-frequency system with a high-frequency coil, which in this embodiment is designed as a body coil 20 fixed in the magnetic resonance imaging (MRI) device 1. The body coil 20 is designed to excite nuclear spins in the main magnetic field 13 generated by the main magnet 12 (B0). The body coil 20 is controlled by the high-frequency control unit 21 of the MRI device 1 and transmits high-frequency excitation pulses to the image receiving area, which is mainly composed of the patient receiving area 14 of the MRI device 1. The body coil 20 can also be designed to receive magnetic resonance signals and constitute a receiving unit or part of a receiving unit in the MRI device 1.

[0127] To control the magnetic resonance imaging (MRI) device 1, particularly the gradient control unit 19 and the high-frequency control unit 21, the MRI device 1 has a control unit 22. The control unit 22 is specifically designed to coordinate the execution of imaging sequences, such as GRE (Gradient Echo) sequences, TSE (Turbine Spin Echo) sequences, or UTE (Ultra-Short Echo Time) sequences. Furthermore, the control unit 22 also includes a computing unit 28 for evaluating the MRI signals acquired via the imaging sequences during the MRI examination.

[0128] The magnetic resonance imaging (MRI) device 1 may include a user interface 23, which is signal-connected to a control unit 22. Control information, such as imaging parameters of the MRI examination, may be displayed on a display unit 24, for example, at least one monitor, which is part of the user interface 23. The display unit 24 may be specifically designed to provide a graphical user interface for displaying relevant body parts of the patient 15. Furthermore, the user interface 23 also has an input unit 25 through which the user can input or change parameters of the MRI measurement.

[0129] The magnetic resonance imaging (MRI) device 1 may include other components, such as a local coil 26. The local coil 26 may be placed at an appropriate location on a body part relevant to the diagnosis or treatment of the patient 15. The local coil 26 preferably has multiple antenna elements designed to detect MRI signals from the relevant body part of the patient 15 and transmit the signals to the computing unit 28 and / or the control unit 22. For this purpose, the local coil 26 may be connected to the high-frequency control unit 21 and the control unit 22 via an electrical connection 27 or other signal connection. Similar to the body coil 20, the local coil 26 may also be designed to excite nuclear spins in the jaw region 31 of the patient 15. For this purpose, the local coil 26 may be controlled by the high-frequency control unit 21.

[0130] Typically, the magnetic field generating unit 11 and the magnetic holding structure are surrounded by a housing 30. The housing 30 may be designed to protect the components of the magnetic resonance imaging device 1 from external influences and / or provide contact protection for the patient 15.

[0131] Figure 2 An embodiment of a system 50 according to the present invention is shown, the system being used to detect the magnetic flux density of the gradient coil 18 of a magnetic resonance imaging (MRI) device 1 within a patient channel 14 of the MRI device 1. The system includes: a processing circuit 60; a storage medium 70; and a data interface 80. The storage medium 70 includes a computer program having instructions that, when executed, cause the processing circuit 60 to perform a method for detecting the magnetic flux density of the gradient coil of the MRI device, particularly within the patient channel of the MRI device. The data interface 80 is configured to receive the detected magnetic flux density in a first spatial direction and / or a second spatial direction via at least one magnetic field strength sensor.

[0132] Figure 3 A schematic diagram of the process of detecting the magnetic flux density of the gradient coil 18 of the magnetic resonance device 1 within the patient channel 14 of the magnetic resonance device 1 is shown.

[0133] In the first step 40, at least one position is determined within the patient channel 14, at which the magnetic flux density of the B0 magnetic field 13 of the B0 main magnet 12 of the magnetic resonance device 1 is lower than a preset value in the first spatial direction, preferably the X direction, and / or in the second spatial direction, preferably the Y direction, which is preferably substantially 0.

[0134] Figure 4 The diagram shows the X (dashed line) and Y (solid line) field strength components (B0.x; B0y) of the magnetic field at position Z with a magnetic field of B0. The values ​​are expressed in tons (T). Figure 4 As shown, there are multiple locations where the X-field component (B0.x) and Y-field component (B0.y) are essentially zero. In other words, the magnetic flux density in these two spatial directions is essentially zero at these locations. These locations are the optimal placement positions for the magnetic field strength sensor, preferably collinear with the X-field component (B0.x) and Y-field component (B0.y).

[0135] For example, for a 12 x 60 cm B0 main magnet, the following coordinates / positions can be determined where the X-field component (B0.x) and Y-field component (B0.y) are essentially zero:

[0136] ·Z = 0cm; Y = + / -30cm; X = 0cm;

[0137] ·Z = + / -7.6cm; Y = + / -30cm; X = 0cm;

[0138] ·Z = + / -7.6cm; Y = 0cm; X = + / -30cm;

[0139] ·Z = + / -7.6cm; Y = + / -29cm; X = + / -7.6cm

[0140] In another step 41, one or more magnetic field strength sensors are arranged at one or more locations determined in the previous step 40, wherein the magnetic field strength sensors are arranged and configured to detect magnetic flux density at least in a first spatial direction and / or a second spatial direction.

[0141] Figure 5 and Figure 6 A schematic diagram of gradient coil 18 is shown, with multiple magnetic field strength sensors 35 or sensor array 35 installed at the positions determined in step 40. Figure 7 A schematic diagram is shown showing multiple magnetic field strength sensors 35 arranged on a sphere 90, which surrounds a defined volume. Figure 8 This shows a schematic diagram of the magnetic field strength sensor 35 arranged on the local coil, i.e., the head coil 91.

[0142] In a step 42, the magnetic field strength sensor 35 can now detect the magnetic flux density and its changes in a first spatial direction, preferably the X field strength component, and / or a second spatial direction, preferably the Y field strength component.

[0143] In another optional step 43, the magnetic flux density in the third spatial direction, preferably the Z-field component, can be calculated based on the two detected magnetic flux densities. In a preferred design, the X and Y field components are detected as described above. As mentioned above, since there are only closed field lines in the magnetic field, the Z-field component can be calculated or derived from the so-called passivity of the magnetic flux density. Applying the second Maxwell's equations, also known as Gauss's law of magnetic fields, the following results can be obtained:

[0144]

[0145] This indicates that the magnetic field has no source. The divergence of the vector field is a scalar field:

[0146] B = (B x B y B z )

[0147] Defined as

[0148]

[0149] according to Solving for:

[0150]

[0151] By arranging multiple magnetic field strength sensors, it is possible not only to measure the local magnetic flux density (X and Y field strength components), but also to determine the corresponding derivatives in the two spatial directions, preferably the X and Y directions, thereby determining the magnetic flux density in the Z direction.

[0152] Figure 9 The diagram illustrates the process of measuring magnetic flux density within the patient channel 14 of a magnetic resonance imaging (MRI) device 1. In the first step 101, a first spatial direction is determined in which the magnetic flux density of the B0 magnetic field 13 of the B0 main magnet 12 of the MRI device 1 is lower than a preset value at at least one location within the patient channel 14. This direction could be, for example, the X direction. Then, multiple magnetic field strength sensors 35 are arranged at multiple locations within the patient channel 14, particularly at least four different locations, and the magnetic flux density is measured according to a Cartesian coordinate system (e.g., B...). x The magnetic flux density 35 is detected in the first spatial direction. For this purpose, the magnetic field strength sensor can be particularly arranged around or on the surface of the defined volume, for example, as... Figure 7 As shown, arranged on the surface of a sphere, or as Figure 8 The arrangement is shown on the surface of a local coil. In another step 102, a first complete field model is fitted to the Cartesian component of the magnetic flux density in a first spatial direction using a conventional spatial harmonic function. In another step 103, a field gradient model for the first vector magnetic flux density, applicable to the Cartesian component in the first spatial direction, is calculated based on the first complete field model. In another step 104, a second field gradient model, applicable to the second spatial direction, such as B, is calculated based on the first field gradient model. y The Cartesian component of the second spatial direction is orthogonal to the first spatial direction. In another step 105, a third field gradient model is calculated based on the first or second field gradient model, which applies to the Cartesian component of the third spatial direction orthogonal to the first and second spatial directions. The third spatial direction can be oriented, in particular, along the axis of symmetry (z-direction) of the main magnet. In another step 106, the vector magnetic flux density at at least the locations of the two field detectors, in particular the NMR field detector, is determined using the absolute value of the magnetic flux density, based on measurement data from the two field detectors in the patient channel, thereby determining the first-order model coefficients of the second and third complete field models. Now, using these three field models, the magnetic flux density at any location, at least within a defined volume, can be calculated. The steps of this method can be repeated, in particular at equally spaced time points, to determine the time-dependent magnetic flux density.

[0153] This invention is not limited to the embodiments described above, as long as they fall within the scope of the following claims. Furthermore, it should be noted that the words "comprising" and "having" do not exclude other elements or steps, and the indefinite article "a" does not exclude multiple. Additionally, it should be noted that the features or steps described with reference to the above embodiments can also be used in combination with other features.

Claims

1. A method of detecting the magnetic flux density of a magnetic resonance apparatus (1) within a patient access (14) of the magnetic resonance apparatus (1), characterized in that, The method includes: A first spatial direction and / or a second spatial direction are determined, wherein the magnetic flux density of the B0 magnetic field (13) of the B0 main magnet (12) of the magnetic resonance device (1) in the first spatial direction and / or the second spatial direction is below a preset value at at least one location within the patient channel (14); At least one magnetic field strength sensor (35) is disposed at at least one location within the patient channel (14), wherein the magnetic field strength sensor (35) is disposed and configured to detect magnetic flux density at least in a first spatial direction and / or a second spatial direction; and The magnetic flux density is detected in the first spatial direction and / or the second spatial direction by the at least one magnetic field strength sensor (35).

2. The method according to claim 1, characterized in that Determine at least one location within the patient channel (14) such that the magnetic flux density of the B0 magnetic field (13) of the B0 main magnet (12) of the magnetic resonance device (1) in the first spatial direction and the second spatial direction is below a preset value at the at least one location. In particular, the at least one magnetic field strength sensor (35) is arranged and configured such that the axis of the magnetic field strength sensor (35) is collinear with two spatial directions of the B0 magnetic field (13), and the magnetic flux density of the B0 magnetic field is detected. The magnetic flux density is detected in the first spatial direction and the second spatial direction by the at least one magnetic field strength sensor (35). Optionally, the method may also include the following steps: determining (43) the magnetic flux density in the third spatial direction based on the detected magnetic flux density in the first spatial direction and the second spatial direction.

3. The method according to claim 1, characterized in that The at least one magnetic field strength sensor (35) includes a plurality of magnetic field strength sensors (35) disposed at multiple locations within the patient channel (14), particularly at least four different locations, preferably at least ten locations, and particularly preferably at least twenty locations. The magnetic field strength sensor (35) is arranged and configured to detect magnetic flux density in a first spatial direction. Among them, the magnetic flux density is detected in the first spatial direction by multiple magnetic field strength sensors (35). Specifically, based on the detected magnetic flux density in the first spatial direction, the magnetic flux density and / or the local derivative of the magnetic flux density in the second and / or third spatial directions are determined, and in particular, calculated. In particular, the position-dependent magnetic flux density within a defined volume is determined in at least two, preferably three, spatial directions.

4. The method according to claim 3, characterized in that At least two field detectors are also installed within the patient access channel. These field detectors are designed to detect the local absolute value of the magnetic field strength. Each channel uses at least two field detectors to detect the absolute value of the magnetic field strength. Specifically, the magnetic flux density in the second and / or third spatial directions is determined based on the detected magnetic flux density in the first spatial direction and the absolute values ​​of the magnetic field strength detected by at least two field detectors.

5. The method according to claim 3 or 4, characterized in that, Based on the calculations performed using the spatial harmonic function, the magnetic flux density in the second spatial direction and / or the third spatial direction is obtained.

6. The method according to any one of the preceding claims, characterized in that Multiple magnetic field strength sensors (35) are arranged such that they are generally arranged on the surface (90) of an imaginary sphere. These magnetic field strength sensors (35) are arranged, in particular, at equal intervals on the surface (90) of the imaginary sphere, and / or These magnetic field strength sensors (35) are at least partially arranged on local coils.

7. The method according to any one of the preceding claims, characterized in that The first spatial direction and / or the second spatial direction are orthogonally oriented relative to the third spatial direction, which is parallel to the axis of symmetry of the B0 main magnet (12). The first spatial direction and the second spatial direction are orthogonal to each other.

8. The method according to any one of the preceding claims, characterized in that The preset value of the magnetic flux density is between -0.02T and 0.02T, preferably between -0.01T and 0.01T, particularly preferably between -5μT and 5μT, and / or The at least one magnetic field strength sensor (35) has a linear measurement region between 0.01T and -0.01T.

9. The method according to one of the preceding claims, characterized in that, The at least one magnetic field strength sensor (35) is a Hall sensor, an anisotropic magnetoresistive sensor, a tunnel magnetoresistive sensor, or a fluxgate magnetometer.

10. System for detecting the magnetic flux density of a magnetic resonance device (1) within a patient access (14) of the magnetic resonance device (1), in particular for detecting the magnetic flux density of a gradient coil (18) within a patient access (14) of the magnetic resonance device (1), characterized in that The system includes: Processing loop (60); Storage medium (70); and Data port (80); The storage medium (70) includes a computer program having a plurality of instructions which, when the program is run, cause the processing loop (60) to perform the method according to any one of claims 1 to 9, wherein the data port (80) is configured to receive magnetic flux density detected by at least one magnetic field strength sensor (35) in a first spatial direction and a second spatial direction.

11. The system of claim 10, wherein, The system includes at least one gradient coil (18) and at least one magnetic field strength sensor (35), preferably at least four magnetic field strength sensors (35), which are arranged in different regions of the gradient coil (18) and / or the system, preferably in mutually opposite regions of the gradient coil (18) and / or generally arranged on the surface of an imaginary sphere.

12. The system of claim 10 or 11, wherein, The system includes at least one patient platform configured for movement into a patient channel (14) of a magnetic resonance imaging (MRI) device (1), wherein the patient platform includes at least one magnetic field strength sensor (35) arranged on the patient platform such that the magnetic field strength sensor (35) is located at a position within the patient channel (14) where the magnetic flux density of the B0 magnetic field (13) of the B0 main magnet (12) of the MRI device (1) in a first spatial direction and / or a second spatial direction is below a preset value.

13. The system according to one of claims 10 to 12, characterized in that The system includes at least one local coil, which is configured to be arranged next to or on the patient and moved into the patient channel (14) of the magnetic resonance device (1), wherein the local coil includes at least one magnetic field strength sensor (35), preferably at least four magnetic field strength sensors (35), and one or more magnetic field strength sensors are arranged on the local coil such that at least one magnetic field strength sensor (35) is located at a position within the patient channel (14) where the magnetic flux density of the B0 magnetic field (13) of the B0 main magnet (12) of the magnetic resonance device (1) is below a preset value in a first spatial direction and / or a second spatial direction.

14. A computer program element comprising a plurality of instructions, wherein the instructions are configured to perform the steps of the method according to any one of claims 1 to 9 within a system according to any one of claims 10 to 13 when a program device of a computer environment is executed.

15. An application of at least one magnetic flux density measured by the method according to any one of claims 1 to 9, wherein the magnetic flux density is used as an input value for: Comparison of nominal and actual magnetic flux density values; Image reconstruction methods; and / or Monitoring of at least one magnetic flux density.

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