An active noise reduction method for magnetic resonance equipment based on sequential electromagnetic interference sampling

Abstract

The present invention relates to an active noise reduction method for magnetic resonance equipment based on sequential electromagnetic interference sampling, and belongs to the field of nuclear magnetic resonance technology. The method adopts an additional electromagnetic interference sampling imaging sequence, that is, after collecting magnetic resonance signals during sequence operation, the remaining repetition time is used to collect electromagnetic interference signals to simultaneously obtain two sets of k-space data, namely the magnetic resonance signal k-space and the electromagnetic interference k-space. The correlation coefficient matrix is ​​calculated using the least squares method or a neural network. For fixed frequency interference or residual electromagnetic interference signals due to insufficient correlation, a transform domain subtraction method is used to filter them out, that is, the difference between the magnetic resonance signal k-space and the electromagnetic interference k-space after active noise reduction is made in the transform domain to eliminate electromagnetic interference of specific modes. The present invention can achieve effective filtering of electromagnetic interference signals in the magnetic resonance radio frequency receiving coil.

Description

Technical Field

[0001] The present invention belongs to the technical field of nuclear magnetic resonance and relates to an active noise reduction method for magnetic resonance equipment based on sequential electromagnetic interference sampling. Background Art

[0002] In low-field MRI applications, achieving good image quality often requires a large, enclosed RF shield to eliminate external electromagnetic interference signals during the scanning process. However, such shields not only require strict installation and occupy a large area, but are also difficult to use in bedside MRI systems (such as in intensive care units and operating rooms).

[0003] The effectiveness of existing active noise reduction methods for mobile low-field magnetic resonance imaging systems depends heavily on the correlation between the noise detection channel and the RF receiving coil. Specifically, when one or more noise detection channels are unable to detect or fit the synchronous electromagnetic interference signals in the receiving coil, the active noise reduction system fails to function properly. For fixed-frequency electromagnetic interference signals, while traditional least-squares fitting-based active noise reduction algorithms can eliminate the interference signals to a certain extent, image artifacts introduced by the fixed-frequency electromagnetic interference can still be observed in the reconstructed image. For electromagnetic interference signals of multiple frequencies, traditional active noise reduction algorithms calculate channel correlation coefficients using the edge k-space signals (containing a small amount of magnetic resonance signal) between the receiving coil and the noise detection coil. The k-space signals of the remaining noise detection coils are then used to eliminate the electromagnetic interference signals in the receiving coil using the correlation coefficients. However, using correlation coefficients derived from a small amount of data to represent the correlation between coils is not optimal, and traditional methods often fail when the correlation between coils is poor.

[0004] Therefore, there is an urgent need for a new active noise reduction method that can be used in mobile low-field magnetic resonance imaging systems to solve the above problems. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide an active noise reduction method for magnetic resonance equipment based on sequential electromagnetic interference sampling, which adopts a noise detection coil combined with additional electromagnetic interference sampling in the imaging sequence to achieve effective filtering of electromagnetic interference signals in the magnetic resonance radio frequency receiving coil.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A method for active noise reduction of a magnetic resonance device based on sequential electromagnetic interference sampling specifically comprises the following steps:

[0008] S1: Using an additional electromagnetic interference sampling imaging sequence, specifically: using a single RF receiving coil and a multi-channel noise detection coil, after acquiring magnetic resonance signals during the sequence, the remaining repetition time TR is used to acquire electromagnetic interference signals, so as to simultaneously obtain two sets of k-space data, namely magnetic resonance signal k-space data and electromagnetic interference k-space data, and calculate the correlation coefficient matrix of the two spatial data;

[0009] S2: combining the magnetic resonance signal k-space data and the electromagnetic interference k-space data detected by the noise detection coil with the correlation coefficient matrix to calculate the electromagnetic interference signal I and the electromagnetic interference signal II in the radio frequency coil;

[0010] S3: Subtract the magnetic resonance signal k-space data and electromagnetic interference k-space data collected by the RF receiving coil from the previously obtained electromagnetic interference signal I and electromagnetic interference signal II, respectively, to obtain preliminary noise-reduced k-space data and residual electromagnetic interference signal; and perform the difference in the transform domain, and then obtain the final noise-reduced k-space data through inverse transformation.

[0011] Furthermore, in step S1, the correlation coefficient matrix of the two spatial data is obtained by adopting the least square method or neural network training.

[0012] Furthermore, in step S1, the correlation coefficient is calculated using the least squares method in the frequency domain to obtain the relationship between the RF receiving and noise detection coils; assuming that the number of noise detection coils is C and the number of sub-bands is f, the transfer coefficient matrix between each noise channel and the RF receiving channel is This can be achieved by the least squares method in the complex field:

[0013]

[0014] in, are the electromagnetic interference k-space data (f-th sub-band) collected by the c-th channel noise detection coil and the RF receiving coil, respectively. N is the number of sampling points. The superscripts “H” and “-1” represent conjugate transpose and inverse matrix operations, respectively.

[0015] Further, step S3 specifically includes: obtaining the transfer coefficient of the system Afterwards, the electromagnetic interference in the magnetic resonance signal k-space can be obtained by the transfer coefficient matrix Suppression; frequency domain magnetic resonance signal after noise reduction It can be expressed as:

[0016]

[0017] in, The k-space data (f-th sub-band) of the magnetic resonance signal collected by the noise detection coil of the c-th channel and the radio frequency receiving coil are respectively; the time domain magnetic resonance signal is finally obtained by inverse Fourier transform;

[0018] The residual electromagnetic interference signal can also be calculated using the above method.

[0019] The beneficial effects of the present invention are:

[0020] (1) The present invention adopts an imaging sequence with additional electromagnetic interference sampling, which completes the sampling of magnetic resonance signal k-space and electromagnetic interference k-space by destroying the gradient after the magnetic resonance echo sampling is completed while keeping the total imaging time unchanged.

[0021] (2) The present invention utilizes electromagnetic interference filtering based on transform domain subtraction, and utilizes the similarity of electromagnetic interference characteristics in the magnetic resonance signal k-space and the electromagnetic interference k-space to perform a difference between the two sets of k-space signals in the transform domain to achieve the elimination of residual electromagnetic interference artifacts.

[0022] Other advantages, objects, and features of the present invention will be described in part in the following description and, in part, will be apparent to those skilled in the art upon examination of the following description or may be learned from practice of the present invention. The objects and other advantages of the present invention may be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings, in which:

[0024] Figure 1 This is a structural diagram of a mobile unshielded low-field magnetic resonance imaging system;

[0025] Figure 2 for imaging sequences with additional electromagnetic interference sampling;

[0026] Figure 3 This is a flow chart of the active noise reduction algorithm for magnetic resonance equipment based on sequential electromagnetic interference sampling of the present invention;

[0027] Figure 4 This is a comparison chart of active noise reduction algorithms. DETAILED DESCRIPTION

[0028] The following describes the embodiments of the present invention by means of specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0029] See also Figures 1 to 4 The present invention proposes a method for active noise reduction of magnetic resonance equipment based on sequential electromagnetic interference sampling. First, a sequential additional electromagnetic interference sampling scheme is adopted. That is, after collecting magnetic resonance signals during sequence operation, the electromagnetic interference signal is collected using the remaining repetition time (TR) to simultaneously obtain two sets of k-space data (magnetic resonance signal k-space and electromagnetic interference k-space). At this time, the correlation between the coils can be calculated using the complete electromagnetic interference k-space data collected by the receiving coil and the noise detection coil. Then, the correlation coefficient matrix is ​​calculated using the least squares method or a combination of neural networks. Furthermore, for fixed frequency interference or residual electromagnetic interference signals due to insufficient correlation, a transform domain subtraction method is used to filter them out. That is, the difference between the magnetic resonance signal k-space after active noise reduction and the electromagnetic interference k-space is taken in the transform domain to eliminate the electromagnetic interference of specific patterns.

[0030] The structure of the mobile unshielded low-field magnetic resonance imaging system used in the method of the present invention is as follows: Figure 1 As shown in the figure, the system mainly includes the main magnet, coils, peripheral equipment and mechanical support structure. The main magnet part mainly includes a bipolar magnet to generate a 50mT background main magnetic field and other auxiliary components (pole shoes, anti-eddy current plates, shim rings, etc.). The coil part includes RF transmitting and receiving coils to achieve the excitation and detection of magnetic resonance signals; gradient coils, which are used for spatial encoding of signals and shim field in imaging; noise detection coils ( Figure 1 Only one is shown in the figure) and is used to detect electromagnetic interference in space. Peripheral equipment includes an RF power amplifier, a gradient power amplifier, a magnetic resonance imaging spectrometer, and an RF receiver amplifier module (including coil matching circuits, preamplifier, and other circuits). The system is powered by 220V / 50Hz AC power. After the sequence parameters are set, the imaging spectrometer controls the RF and gradient power amplifiers to stimulate the hydrogen proton signal within the imaging area, generating a spatially encoded magnetic resonance signal. Simultaneously, the RF receiver and noise detection coils transmit the detection signals to the imaging spectrometer via the RF receiver module. Finally, signal processing and image reconstruction are completed in a computer.

[0031] The imaging sequence of additional electromagnetic interference sampling used in the embodiment of the present invention is as follows: Figure 2As shown in the figure, taking a 3D gradient echo sequence with a spoiled gradient as an example, after acquiring the echo signal, the RF receiving coil applies a spoiled gradient in advance through the gradient coil to disperse the remaining magnetic resonance signal. Then, an additional electromagnetic interference signal is sampled once during the remaining repetition time TR (excluding the magnetic resonance signal). For a single RF receiving coil or noise detection coil, two sets of k-space data are obtained after the sequence is completed: the magnetic resonance signal k-space and the electromagnetic interference k-space, as shown in the figure. Figure 2 The same electromagnetic interference sampling method can also be applied to imaging sequences with longer TR times, such as three-dimensional fast spin echo sequences.

[0032] After completing the acquisition of two sets of k-space data, the active noise reduction algorithm based on additional electromagnetic interference sampling proposed in the present invention is as follows: Figure 3 As shown in the figure, the algorithm first uses the electromagnetic interference k-space data collected by the multi-channel (1-N) noise detection coil and the RF receiving coil to obtain the correlation coefficient matrix through the least square method or neural network training. Then, the magnetic resonance signal k-space data detected by the noise detection coil and the electromagnetic interference k-space data are combined with the correlation coefficient matrix to calculate the electromagnetic interference signal 1 and electromagnetic interference signal 2 in the RF coil (as shown in the figure). Figure 3 ). Furthermore, the MRI k-space data and EMI k-space data collected by the RF receiving coil are subtracted from the previously obtained EMI signal 1 and EMI signal 2, respectively, to obtain the preliminary noise-reduced k-space data and the residual EMI signal. Finally, the two sets of signals are subtracted in the transform domain and then inversely transformed to obtain the final noise-reduced k-space data.

[0033] Taking the calculation of k-space data after preliminary noise reduction as an example, the least squares method in the frequency domain is used to calculate the correlation coefficient matrix TF to obtain the relationship between the RF receiving and noise detection coils. Without loss of generality, assuming that the number of noise detection coils is C and the number of sub-bands is f, the transfer coefficient matrix TF of each noise channel and RF receiving channel is c f This can be achieved by the least squares method in the complex field:

[0034]

[0035] in, are the electromagnetic interference k-space data (f-th sub-band) collected by the c-th channel noise detection coil and the RF receiving coil, respectively. N is the number of sampling points. The superscripts “H” and “-1” represent conjugate transpose and inverse matrix operations, respectively.

[0036] The transfer coefficient of the system is obtained Afterwards, the electromagnetic interference in the nuclear magnetic signal k-space can be Suppression. Frequency domain magnetic resonance signal after noise reduction It can be expressed as:

[0037]

[0038] in, These are the k-space data (fth sub-band) of the nuclear magnetic resonance signals collected by the noise detection coil and the RF receiving coil of the c-th channel, respectively. The time-domain magnetic resonance signals are finally obtained through an inverse Fourier transform. The residual electromagnetic interference signal can also be calculated using the above method.

[0039] Comparison algorithm effect Figure 4 As shown in the figure, compared with the traditional active noise reduction method, the algorithm of the present invention can more effectively remove the highlight artifacts in the image.

[0040] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for active noise reduction of magnetic resonance equipment based on sequential electromagnetic interference sampling, characterized in that: The method specifically comprises the following steps: S1: Using an additional electromagnetic interference sampling imaging sequence, specifically: using a single RF receiving coil and a multi-channel noise detection coil, after acquiring magnetic resonance signals during the sequence, the remaining repetition time TR is used to acquire electromagnetic interference signals, so as to simultaneously obtain two sets of k-space data, namely magnetic resonance signal k-space data and electromagnetic interference k-space data, and calculate the correlation coefficient matrix of the two spatial data; S2: combining the magnetic resonance signal k-space data and the electromagnetic interference k-space data detected by the noise detection coil with the correlation coefficient matrix to calculate the electromagnetic interference signal I and the electromagnetic interference signal II in the radio frequency coil; S3: Subtract the magnetic resonance signal k-space data and electromagnetic interference k-space data collected by the RF receiving coil from the previously obtained electromagnetic interference signal I and electromagnetic interference signal II, respectively, to obtain preliminary noise-reduced k-space data and residual electromagnetic interference signal; and perform the difference in the transform domain, and then obtain the final noise-reduced k-space data through inverse transformation.

2. The method for active noise reduction of magnetic resonance equipment based on sequential electromagnetic interference sampling according to claim 1, characterized in that: In step S1, the correlation coefficient matrix of the two spatial data is obtained by adopting the least square method or the neural network training method.

3. The method for active noise reduction of magnetic resonance equipment based on sequential electromagnetic interference sampling according to claim 2, characterized in that: In step S1, the correlation coefficient is calculated using the least square method in the frequency domain to obtain the relationship between the radio frequency receiving and noise detection coils; Assume that the number of noise detection coils is C , the number of sub-bands is f , then the transfer coefficient matrix between each noise channel and the RF receiving channel is , implemented by the least squares method in the complex field: in, , Respectively c Electromagnetic interference k-space data collected by the channel noise detection coil and the RF receiving coil. N is the number of sampling points, and the superscripts "H" and "-1" represent conjugate transpose and inverse matrix operations, respectively.

4. The method for active noise reduction of magnetic resonance equipment based on sequential electromagnetic interference sampling according to claim 3, characterized in that: Step S3 specifically includes: obtaining the transfer coefficient of the system Afterwards, the electromagnetic interference in the magnetic resonance signal k-space is transferred through the transfer coefficient matrix Suppression; frequency domain magnetic resonance signal after noise reduction Expressed as: in, , Respectively c The k-space data of the magnetic resonance signal collected by the channel noise detection coil and the radio frequency receiving coil; the time domain magnetic resonance signal is finally obtained by inverse Fourier transform; The residual electromagnetic interference signal is also calculated using the above method.

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