CN111650653A - Magnetic resonance signal denoising method based on noise correlation and wavelet threshold method - Google Patents
Magnetic resonance signal denoising method based on noise correlation and wavelet threshold method Download PDFInfo
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Abstract
The invention relates to a magnetic resonance signal denoising method based on noise correlation and wavelet threshold method, which comprises the steps of simultaneously recording data collected by a detection coil and a reference coil by using a nuclear magnetic resonance underground water detector, wherein the data comprises air-mining noise s (t) collected by the detection coil and a magnetic resonance signal s containing noisex(t) reference noise data r (t) and reference noise data r acquired synchronously by the reference coilx(t), t is a sampling time; data r acquired for a reference coil according to the principle of wavelet thresholdingx(t) processing to filter out the effective magnetic resonance signal r contained thereins(t) obtaining clean noise data; and calculating noise data in the detection coil according to the correlation of the space acquisition noise in the detection coil and the reference coil, and removing the noise data from the noise-containing signal to obtain a noise-removed magnetic resonance signal. The invention can effectively eliminate the magnetic resonance detection signal under the condition of not limiting the distance between the detection coil and the reference coilThe related noise in the signal is not lost, so that the noise elimination effect is improved, and manpower and material resources are saved.
Description
Technical Field
The invention belongs to the field of ground nuclear magnetic resonance data preprocessing, and particularly relates to a magnetic resonance signal denoising method based on noise correlation and a wavelet threshold method.
Background
The magnetic resonance underground water detection technology is the only existing geophysical method for directly detecting underground water in a non-invasive way, and is widely applied to underground water resource exploration and disaster water body detection and explanation in recent years. However, because the magnetic resonance signal is very weak, the magnetic resonance signal is often interfered by ambient noise in the field detection process, including spike noise, power frequency noise and random noise, wherein the power frequency harmonic interference caused by the power line is particularly serious. The power frequency noise in the magnetic resonance signal can be effectively filtered by utilizing the transfer characteristic of the noise and the wavelet transformation principle, and the accuracy of the later inversion interpretation of the underground water body can be effectively improved.
Patent CN102053280A discloses a nuclear magnetic resonance underground water detection system with reference coil and detection method, the invention synchronously collects the nuclear magnetic resonance signals in the transmitting/receiving coil and the full waveform data of the noise signals in the reference coil by a multi-path a/D collection unit, realizes the layout of the best position and number of the reference coil by calculating the maximum correlation between the noise signals collected by the reference coil and the nuclear magnetic resonance signals, adopts a variable step length self-adaptive algorithm under the condition that the statistical characteristics of the signals and the noise are unknown, and maximally cancels the noise in the transmitting/receiving coil to obtain the noise in the nuclear magnetic resonance signals, the method realizes the extraction of the nuclear magnetic resonance signals under the interference of multi-field source complex noise, but in the exploration process, in order to avoid receiving effective magnetic resonance signals in the reference coil, the layout distance between the reference coil and the detection coil is required to be larger than the side length of the detection coil, often limited by terrain in field surveys.
The patent CN 108459353B discloses "a method and a device for extracting weak magnetic resonance signals under electromagnetic noise background", in the invention, a magnetic resonance detection system is used to obtain a group of magnetic resonance signals s (t); establishing a chaos detection system based on Duffing arrays; decomposing the magnetic resonance signal according to time and the pre-estimated hydrological information characteristics, and segmenting into segmented signals; respectively carrying out m continuation on the formed segmented signals; respectively sending the extended signals into a chaotic detection system to obtain signal amplitudes; and sequentially combining the signal amplitude of the detected signal according to time and a pre-segmentation state to obtain a denoised magnetic resonance signal envelope image, and finishing the extraction of effective signals in the magnetic resonance signal S (t) in the detection data. The method can realize the extraction of the magnetic resonance signal under the condition of low signal-to-noise ratio, but the chaos detection system needs a large amount of data during the detection of weak signals, and an instrument is required to carry out high-frequency sampling during actual detection, so that the difficulty and the cost of instrument manufacture are increased, and the working efficiency of field detection is reduced.
Patent CN109782363A discloses a magnetic resonance signal denoising method based on time domain modeling and frequency domain symmetry, which first models noise according to the characteristic that power frequency noise has a long duration and is a series of sine waves fixed at integral multiples of the fundamental frequency of a power line, and uses a multi-channel instrument to collect nuclear magnetic resonance signals and noise data, converts the power frequency noise in a reference channel into power frequency noise in a main channel, and avoids signal distortion when eliminating the power frequency noise near larmor frequency. And then, the residual power frequency noise and the Gaussian white noise are further eliminated by utilizing the different symmetries of the nuclear magnetic resonance signal and the noise component in the frequency domain after Fourier transformation. The method reduces signal distortion generated when power frequency noise near the Larmor frequency is eliminated, but the fundamental frequency searching efficiency is low in the modeling process, and when small errors exist in the selection of the fundamental frequency, the noise elimination effect is obviously reduced.
Disclosure of Invention
The invention aims to provide a magnetic resonance signal denoising method based on noise correlation and a wavelet threshold method, which can effectively eliminate correlated noise in a magnetic resonance detection signal without losing signal components under the condition of not limiting the distance between a detection coil and a reference coil, thereby improving the denoising effect and saving manpower and material resources.
The present invention is achieved in such a way that,
a magnetic resonance signal denoising method based on noise correlation and wavelet threshold method comprises the following steps:
a. using nuclear magnetic resonance groundwater detectors recording acquisition by both detection coil and reference coilData including null noise s (t) and noisy magnetic resonance signal s acquired by the detection coilx(t) reference noise data r (t) and reference noise data r acquired synchronously by the reference coilx(t), t is a sampling time;
b. data r acquired for a reference coil according to the principle of wavelet thresholdingx(t) processing to filter out the effective magnetic resonance signal r contained thereins(t) obtaining clean noise data rN(t)＝rx(t)-rs(t)；
c. Calculating noise data s in the detection coil according to the correlation h of the space-time acquisition noise in the detection coil and the reference coilN(t)＝h·rN(t) removing the noise-containing signal to obtain a noise-removed magnetic resonance signal sc(t)，sc(t)＝sx(t)-sN(t)。
Further, the wavelet threshold method in step b specifically comprises the following steps:
1) data r acquired for a reference coilx(t) carrying out wavelet decomposition to extract detail coefficients d of each layer1,…,dkAnd the last layer approximation coefficient akK is the number of wavelet decomposition layers;
2) carrying out hard threshold processing on detail coefficients of each layer, setting the detail coefficients larger than the threshold to zero, and keeping the detail coefficients smaller than the threshold unchanged;
3) and reconstructing the effective magnetic resonance signal r acquired in the reference channel by using the last layer of wavelet approximation coefficient and each layer of detail coefficient after threshold processings(t)。
Further, the specific calculation step of the correlation h in the step c is as follows:
the correlation of the hollow sampling noise of the detection coil and the reference coil is
rs is the cross-correlation coefficient of r (t) and s (t), rr is the autocorrelation coefficient of r (t), h is the transfer coefficient, and the correlation relationship h is expressed as h ═ r (r) in a matrix formTr)-1·rTs, T are matrix transpose symbols.
Compared with the prior art, the invention has the beneficial effects that: according to the invention, the noise in the reference coil is preprocessed by using a wavelet threshold method, the magnetic resonance signal detected in the reference coil is removed, the noise in the detection coil is calculated according to the noise in the reference coil based on the noise correlation, the noise is effectively eliminated, the magnetic resonance signal component in the detection coil is ensured not to be lost, the accuracy of the inversion interpretation of the subsequent underground aquifer is greatly improved, the coil laying of the multi-channel nuclear magnetic resonance underground water detector is not limited by distance, and the flexibility of field detection is enhanced.
Drawings
Fig. 1 is a flow chart of a magnetic resonance signal denoising method based on noise correlation and wavelet threshold method.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
A magnetic resonance signal denoising method based on noise correlation and wavelet threshold method, the processing flow is shown in figure 1, and the method comprises the following steps:
a. simultaneously recording data collected by a detection coil and a reference coil by using a nuclear magnetic resonance underground water detector, wherein the data comprises air mining noise s (t) and a noise-containing magnetic resonance signal s collected by the detection coilx(t) reference noise data r (t) and reference noise data r acquired synchronously by the reference coilx(t), t is the sampling time, wherein the noise-containing magnetic resonance signal comprises the effective magnetic resonance signal and the noise, i.e. sx(t)＝sc(t)+sN(t) the noise acquired synchronously by the reference coil comprises noise and the detected weak magnetic resonance signal, i.e. rx(t)＝rN(t)+rs(t)；
b. Data r acquired for a reference coil according to the principle of wavelet thresholdingx(t) processing to filter out the effective magnetic resonance signal r contained thereins(t) obtaining the clean noise figureAccording to rN(t)；
c. Calculating noise data s in the detection coil according to the correlation h of the space-time acquisition noise in the detection coil and the reference coilN(t)＝h·rN(t) and removing the noise-containing signal to obtain a noise-removed magnetic resonance signal sc(t), i.e. sc(t)＝sx(t)-sN(t)。
The wavelet threshold method in the step b comprises the following specific steps:
1) data r acquired for a reference coilx(t) carrying out wavelet decomposition to extract detail coefficients d of each layer1,…,dkAnd the last layer approximation coefficient akK is the number of wavelet decomposition layers;
2) carrying out hard threshold processing on detail coefficients of each layer, namely setting the detail coefficients larger than the threshold to be zero, and keeping the detail coefficients smaller than the threshold unchanged;
3) and reconstructing the effective magnetic resonance signal r acquired in the reference channel by using the last layer of wavelet approximation coefficient and each layer of detail coefficient after threshold processings(t)；
The specific calculation step of the correlation h in the step c is as follows:
the correlation of the hollow sampling noise of the detection coil and the reference coil is
rs is the cross-correlation coefficient of r (t) and s (t), rr is the autocorrelation coefficient of r (t), h is the transfer coefficient, and the correlation relationship h can be expressed as h ═ r (r) through a matrix formTr)-1·rTs, T are matrix transpose symbols.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.
Claims (3)
1. A magnetic resonance signal denoising method based on noise correlation and wavelet threshold method is characterized by comprising the following steps:
a. simultaneously recording data collected by a detection coil and a reference coil by using a nuclear magnetic resonance underground water detector, wherein the data comprises air mining noise s (t) and a noise-containing magnetic resonance signal s collected by the detection coilx(t) reference noise data r (t) and reference noise data r acquired synchronously by the reference coilx(t), t is a sampling time;
b. data r acquired for a reference coil according to the principle of wavelet thresholdingx(t) processing to filter out the effective magnetic resonance signal r contained thereins(t) obtaining clean noise data rN(t)＝rx(t)-rs(t)；
c. Calculating noise data s in the detection coil according to the correlation h of the space-time acquisition noise in the detection coil and the reference coilN(t)＝h·rN(t) removing the noise-containing signal to obtain a noise-removed magnetic resonance signal sc(t)，sc(t)＝sx(t)-sN(t)。
2. The method according to claim 1, wherein the wavelet thresholding in step b comprises the following specific steps:
1) data r acquired for a reference coilx(t) carrying out wavelet decomposition to extract detail coefficients d of each layer1,…,dkAnd the last layer approximation coefficient akK is the number of wavelet decomposition layers;
2) carrying out hard threshold processing on detail coefficients of each layer, setting the detail coefficients larger than the threshold to zero, and keeping the detail coefficients smaller than the threshold unchanged;
3) and reconstructing the effective magnetic resonance signal r acquired in the reference channel by using the last layer of wavelet approximation coefficient and each layer of detail coefficient after threshold processings(t)。
3. The method according to claim 1, wherein the step of calculating the correlation h in step c specifically comprises:
the correlation of the hollow sampling noise of the detection coil and the reference coil is
rs is the cross-correlation coefficient of r (t) and s (t), rr is the autocorrelation coefficient of r (t), h is the transfer coefficient, and the correlation relationship h is expressed as h ═ r (r) in a matrix formTr)-1·rTs, T are matrix transpose symbols.
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| CN115808648A (en) * | 2022-11-18 | 2023-03-17 | 无锡鸣石峻致医疗科技有限公司 | Device and method for measuring ringing noise of magnetic resonance system |
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| CN115356774B (en) * | 2022-08-12 | 2023-05-23 | 中国科学院地质与地球物理研究所 | Semi-aviation electromagnetic detection device and method based on coaxial coplanar mutual reference coil set |
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