Magnetic particle rotation frequency measuring method based on wavelet transformation

Abstract

The invention discloses a magnetic particle rotation frequency measurement method based on wavelet transformation, and belongs to the technical field of signal frequency measurement. The magnetic particle rotation frequency determination method based on wavelet transformation comprises seven steps of setting a working state of an SERF atom magnetometer, carrying out empty collection on an environment magnetic field signal and collecting a magnetic particle signal, carrying out detrending processing, carrying out adaptive filtering, carrying out continuous wavelet transformation, extracting a frequency component and carrying out time domain positioning, and outputting data. The method comprises the following steps: acquiring magnetic field signals of magnetic particles by using an ultrahigh-sensitivity SERF atom magnetometer, performing de-trending and adaptive filtering noise suppression, performing continuous wavelet transform on the magnetic field signals to obtain a time-frequency spectrogram of the signals, and obtaining frequency and time information corresponding to a maximum intensity point on the time-frequency spectrogram, namely, the rotation frequency of the magnetic particles and time domain positioning of the signals; and non-visual measurement of the rotation frequency of the magnetic particles is realized.

Description

Technical Field

[0001] The invention relates to the technical field of signal frequency measurement, and in particular to a method for measuring the rotational frequency of magnetic particles based on wavelet transformation. Background Art

[0002] In recent years, magnetic particles have shown great potential for application in medical technology fields such as angiography and cancer treatment. For deep tissues in the human body that are difficult to observe directly, non-invasive measurements can be performed by injecting magnetic particles. For example, magnetic particles can be used as contrast agents to measure cerebral blood volume and assist in the clinical diagnosis of brain damage. At the same time, the introduction of magnetic particles has also opened up a new way for the indirect measurement of blood flow velocity. Based on the correlation between the rotation frequency of magnetic particles in the fluid and the flow velocity, cardiovascular parameters can be measured, providing a novel technical means for the assessment of cardiovascular health status and the study of hemodynamics.

[0003] When visual measurement conditions cannot be met, contactless measurement using magnetic field signals has the significant advantage of high signal-to-noise ratio. Since the magnetic permeabilities of different tissues in the human body are similar and they are almost transparent to magnetic fields, magnetic field signals can penetrate tissues almost without loss, making human biomagnetic field detection technology (such as magnetocardiography and magnetoencephalography) have the characteristics of high signal-to-noise ratio. The use of a highly sensitive spin-exchange relaxation-free (SERF) atomic magnetometer to detect magnetic particles in flow can effectively capture the changes in magnetic field signals caused by particle rotation, and then accurately extract the rotation frequency of the particles from the signal to achieve accurate measurement of flow velocity.

[0004] Time-frequency domain analysis is an effective processing method for extracting the frequency components of signals. For time-varying magnetic field particle signals, compared with Fourier transform, wavelet transform is not only more advantageous in processing non-stationary signals, but also can provide time positioning and give local frequency information. In addition, since the particle signal waveform and the wavelet waveform have a high similarity, wavelet transform can efficiently process particle signals, accurately extract the particle rotation frequency, and accurately locate the signal moment. Summary of the invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the present invention provides a method for measuring the rotational frequency of magnetic particles based on wavelet transform. Based on an ultra-high sensitivity SERF atomic magnetometer, the magnetic field signal of the magnetic particles is collected, and after detrending and adaptive filtering to suppress noise, a continuous wavelet transform is performed on it to obtain a time-frequency spectrum of the signal, and the frequency and time information corresponding to the maximum intensity point on the time-frequency spectrum is obtained, that is, the time domain positioning of the rotational frequency of the magnetic particles and the signal, thereby realizing the non-visual measurement of the rotational frequency of the magnetic particles.

[0006] The technical solution of the present invention is as follows:

[0007] A method for measuring the rotational frequency of magnetic particles based on wavelet transform, characterized in that it comprises the following steps:

[0008] Step 1, setting the working state of the SERF atomic magnetometer measurement system;

[0009] Step 2, using a SERF atomic magnetometer to collect environmental magnetic field signals and magnetic field signals of magnetic particles in the fluid respectively;

[0010] Step 3, detrending the magnetic field signal of the magnetic particles to remove the baseline drift in the signal, and using the environmental magnetic field signal as a reference signal;

[0011] Step 4, the adaptive filter outputs a filtered signal according to the input detrended magnetic particle magnetic field signal and the reference signal;

[0012] Step 5, performing continuous wavelet transform on the filtered signal to obtain a time-frequency spectrum of the wavelet transform;

[0013] Step 6, obtaining the frequency and time at the maximum intensity on the time spectrum, which are used as the rotation frequency ω of the magnetic particles and the occurrence time t of the magnetic field signal of the magnetic particles, respectively.

[0014] Step 1 includes setting the gas chamber heating temperature of the SERF atomic magnetometer, performing residual magnetic field compensation, and setting the liquid flow rate for transporting magnetic particles.

[0015] The environmental magnetic field signal in step 2 is also called an air-sampled environmental magnetic field signal. Under the condition that the SERF atomic magnetometer does not respond to magnetic particles, the response of the SERF atomic magnetometer to the remaining magnetic field changes is collected.

[0016] Collecting the magnetic field signal of the magnetic particles in the fluid in step 2 includes the magnetic particles being transported by the fluid through the detection area of ​​the SERF atomic magnetometer. The magnetic particles are spherical with a diameter in the micrometer range. The magnetic field signal of the magnetic particles decays with the spatial distance. The detection area refers to the spatial range in which the magnetic particles can cause the atomic magnetometer to produce an output response change. At this time, the collected signal is the superposition of the magnetic field signal of the magnetic particles and the environmental noise.

[0017] Step 3 includes fitting the collected signal to a sixth-order polynomial, and using the sixth-order polynomial fitting result to detrend the collected signal to remove the baseline offset of the signal to obtain a detrended signal.

[0018] Step 5 includes using Morse wavelet to perform continuous wavelet transform on the filtered signal to obtain a time-frequency spectrum of the wavelet transform; the time-frequency spectrum of the wavelet transform is a two-dimensional image, the horizontal axis represents time, the vertical axis represents frequency, the color depth of each point in the image represents the intensity of the signal at the corresponding time and frequency, and the time-frequency spectrum intuitively reflects the distribution of the signal at different times and frequencies.

[0019] Step 6 includes searching for the coordinate point with the maximum intensity on the time spectrum, which represents the most significant or highest energy frequency component in the signal, and obtaining the frequency and time information corresponding to the point. The frequency is the rotation frequency of the magnetic particles, and the time is the time domain positioning of the magnetic field signal of the magnetic particles.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The present invention discloses a method for measuring the rotational frequency of magnetic particles based on wavelet transform, which is based on the reference signal and particle signal collected by an ultra-high sensitivity SERF atomic magnetometer, and uses adaptive filtering to effectively suppress the power frequency noise, magnetic heart noise, etc. in the particle signal.

[0022] 2. The present invention discloses a method for measuring the rotation frequency of magnetic particles based on wavelet transform, which uses wavelet transform to perform time-frequency domain analysis, avoiding the shortcomings of Fourier transform and extracting the particle rotation frequency and the occurrence time of the signal at the same time.

[0023] 3. The present invention discloses a method for measuring the rotational frequency of magnetic particles based on wavelet transform, which utilizes the similarity between the waveform of magnetic field particle signals and wavelets to obtain a significant frequency spectrum distribution, which is beneficial to improving the accuracy of frequency recognition and also makes the method more robust. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic flow chart of a method for measuring the rotational frequency of magnetic particles based on wavelet transform according to the present invention. Figure 1 The method comprises the steps of: 1, setting the working state of the SERF atomic magnetometer measurement system; 2, using the SERF atomic magnetometer to respectively collect the environmental magnetic field signal and the magnetic field signal of the magnetic particles in the fluid; 3, performing detrending processing on the collected magnetic particle magnetic field signal to remove the baseline drift in the signal; and taking the air-sampled environmental magnetic field signal as the reference signal; 4, the adaptive filter outputs the filtered signal according to the input detrended magnetic particle magnetic field signal and the reference signal; 5, performing continuous wavelet transform on the filtered signal to obtain a time-frequency spectrum of the wavelet transform; and 6, obtaining the frequency and time at the maximum intensity on the time-frequency spectrum, which are respectively used as the magnetic particle rotation frequency ω and the occurrence time t of the magnetic particle magnetic field signal.

[0025] Figure 2 include Figure 2 (a) Figure 2 (b) Figure 2 (c) and Figure 2 (d). Figure 2 (a) is a schematic diagram of the output signal curve of the magnetometer after detrending the magnetic particle signal obtained by implementing a method for measuring the rotational frequency of magnetic particles based on wavelet transform of the present invention. Figure 2 The horizontal axis of (a) is time (s, scale value is 0, 1, 2, 3), and the vertical axis is the magnetic field magnitude (pT, scale value is -10, -5, 0, 5, 10). Figure 2 The curve in (a) represents the detrended signal. Figure 2 (b) is a schematic diagram of the air-sampled environmental magnetic field signal curve of the SERF atomic magnetometer obtained by implementing a magnetic particle rotation frequency measurement method based on wavelet transform of the present invention. Figure 2 The horizontal axis of (b) is time (s, scale value is 0, 1, 2, 3), and the vertical axis is the magnetic field magnitude (pT, scale value is -5, 0, 5). Figure 2 The curve in (b) represents the empty sampling signal. Figure 2 (c) Yes Figure 2 Schematic diagram of the filtered signal for the detrended signal in (a). Figure 2 The curve in (c) represents the adaptive filtering output signal. Figure 2 (d) is for Figure 2 (c) is the time-frequency spectrum of the adaptive filtering output signal after wavelet transform. Figure 2 The horizontal axis of (d) is time (s, scale values ​​are 0.5, 1, 1.5, 2, 2.5, 3), and the vertical axis is frequency (Hz, scale values ​​are 10, 20, 30). Figure 2 The bright area in the middle of (d) indicates the maximum signal intensity. DETAILED DESCRIPTION

[0026] Below is the attached figure ( Figure 1-Figure 2 ) and Examples illustrate the present invention.

[0027] Figure 1 It is a schematic flow chart of a method for measuring the rotational frequency of magnetic particles based on wavelet transform according to the present invention. Figure 2 (a) is a schematic diagram of the output signal curve of the magnetometer after detrending the magnetic particle signal obtained by implementing a method for measuring the rotational frequency of magnetic particles based on wavelet transform of the present invention. Figure 2 (b) is a schematic diagram of the air-sampled environmental magnetic field signal curve of the SERF atomic magnetometer obtained by implementing a magnetic particle rotation frequency measurement method based on wavelet transform of the present invention. Figure 2 (c) Yes Figure 2 Schematic diagram of the filtered signal for the detrended signal in (a). Figure 2(d) is for Figure 2 (c) The time-frequency spectrum of the adaptive filtering output signal after wavelet transformation. Figure 1 to Figure 2 As shown, a method for determining the rotation frequency of magnetic particles based on wavelet transform comprises the following steps: step 1, setting the working state of the SERF atomic magnetometer measurement system; step 2, using the SERF atomic magnetometer to respectively collect the environmental magnetic field signal and the magnetic field signal of the magnetic particles in the fluid; step 3, detrending the magnetic field signal of the magnetic particles to remove the baseline drift in the signal, and using the environmental magnetic field signal as a reference signal; step 4, an adaptive filter outputs a filtered signal according to the input detrended magnetic particle magnetic field signal and the reference signal; step 5, performing continuous wavelet transform on the filtered signal to obtain a time-frequency spectrum of the wavelet transform; step 6, obtaining the frequency and time at the maximum intensity on the time-frequency spectrum, which are respectively used as the rotation frequency ω of the magnetic particles and the occurrence time t of the magnetic particle magnetic field signal.

[0028] Step 1 includes setting the air chamber heating temperature of the SERF atomic magnetometer, performing residual magnetic field compensation, and setting the liquid flow rate for transporting magnetic particles. The environmental magnetic field signal in step 2 is also called the air-collected environmental magnetic field signal. Under the condition that the SERF atomic magnetometer does not respond to magnetic particles, the response of the SERF atomic magnetometer to the remaining magnetic field changes is collected. The magnetic field signal of the magnetic particles in the fluid collected in step 2 includes the magnetic particles being transported by the fluid through the detection area of ​​the SERF atomic magnetometer. The magnetic particles are spherical bodies with a diameter in the micrometer range. The magnetic field signal of the magnetic particles decays with the spatial distance. The detection area refers to the spatial range in which the magnetic particles can cause the atomic magnetometer to produce an output response change. At this time, the collected signal is the superposition of the magnetic field signal of the magnetic particles and the environmental noise.

[0029] Step 3 includes fitting the collected signal to a sixth-order polynomial, and using the sixth-order polynomial fitting result to detrend the collected signal to remove the baseline offset of the signal and obtain a detrended signal. Step 5 includes using Morse wavelet to perform continuous wavelet transform on the filtered signal to obtain a time-frequency spectrum of wavelet transform; the time-frequency spectrum of wavelet transform is a two-dimensional image, the horizontal axis represents time, the vertical axis represents frequency, the color depth of each point in the image represents the intensity of the signal at the corresponding time and frequency, and the time-frequency spectrum intuitively reflects the distribution of the signal at different times and frequencies. Step 6 includes searching for the coordinate point with the maximum intensity on the time-frequency spectrum, which represents the most significant or highest energy frequency component in the signal, and obtaining the frequency and time information corresponding to the point, the frequency is the rotation frequency of the magnetic particles, and the time is the time domain positioning of the magnetic field signal of the magnetic particles.

[0030] The present invention discloses a method for measuring the rotational frequency of magnetic particles based on wavelet transform, and belongs to the technical field of signal frequency measurement. The method for measuring the rotational frequency of magnetic particles based on wavelet transform includes seven steps: setting the working state of a SERF atomic magnetometer, sampling environmental magnetic field signals and collecting magnetic particle signals, detrending, adaptive filtering, continuous wavelet transform, extracting frequency components and time domain positioning, and outputting data. The magnetic field signal of the magnetic particles is collected by an ultra-high-sensitivity SERF atomic magnetometer, and after detrending and adaptive filtering to suppress noise, a continuous wavelet transform is performed on it to obtain a time-frequency spectrum of the signal, and the frequency and time information corresponding to the maximum intensity point on the time-frequency spectrum are obtained, that is, the rotational frequency of the magnetic particles and the time domain positioning of the signal, thereby realizing the non-visual measurement of the rotational frequency of the magnetic particles.

[0031] refer to Figure 1 , a method for determining the rotational frequency of magnetic particles based on wavelet transform, comprising the following steps:

[0032] Step 1, setting the working state of a spin-exchange relaxation-free (SERF) atomic magnetometer measurement system;

[0033] Step 2, using a SERF atomic magnetometer to collect the environmental magnetic field signal and the magnetic field signal of the magnetic particles in the fluid respectively;

[0034] Step 3, detrending the collected magnetic particle signal to remove the baseline drift in the signal;

[0035] Step 4, using the air-sampled environmental magnetic field signal of the SERF atomic magnetometer as a reference signal of the adaptive filter, using the detrended magnetic particle signal as an input signal, and the adaptive filter outputs a filtered signal;

[0036] Step 5, performing continuous wavelet transform on the filtered signal to obtain a time-frequency spectrum of the wavelet transform;

[0037] Step 6, obtaining the frequency and time at the maximum intensity on the time spectrum, which are used as the rotation frequency of the magnetic particles and the occurrence time of the particle signal, respectively;

[0038] Step 7, output data.

[0039] The setting of the working state of the SERF atomic magnetometer measurement system in step 1 includes the gas chamber heating temperature, residual magnetic field compensation, and liquid flow rate for transporting magnetic particles of the SERF atomic magnetometer.

[0040] The SERF atomic magnetometer in step 2 samples the environmental magnetic field signal in vain, and collects the response of the SERF magnetometer to the 50 Hz power frequency signal, the heart magnetic field, and the laboratory magnetic field fluctuation under the condition that the SERF magnetometer does not respond to magnetic particles.

[0041] The SERF atomic magnetometer in step 2 collects magnetic particle signals in the fluid. The magnetic particles are transported by the fluid through the detection area of ​​the SERF atomic magnetometer. Since the magnetic field signal of the magnetic particles decays with spatial distance, the detection area refers to the spatial range in which the magnetic particles can cause the SERF atomic magnetometer to produce an output response change. This signal is a superposition of the magnetic particle signal and the ambient magnetic field noise.

[0042] In step 3, the collected magnetic particle signal is detrended; the collected signal is fitted to a sixth-order polynomial, and the sixth-order polynomial fitting result is used to detrend the collected signal to remove the baseline offset of the signal, and the following is obtained: Figure 2 Detrending signal shown in (a).

[0043] Step 4 will be as follows Figure 2 The ambient magnetic field signal of the air sample shown in (b) is used as the reference signal of the adaptive filter, and the particle signal after trend removal is used as the input signal of the adaptive filter. The adaptive filter tracks the reference signal and filters it out from the particle signal, and the output is as follows: Figure 2 (c) The filtered signal is shown; it can suppress power frequency signals, magnetic-cardiographic noise, laboratory magnetic field fluctuations and other noises.

[0044] In step 5, the filter signal is subjected to continuous wavelet transform using Morse wavelet to obtain a time-frequency spectrum diagram of the wavelet transform; the time-frequency spectrum diagram of the wavelet transform is a two-dimensional image, such as Figure 2 As shown in (d), the horizontal axis represents time and the vertical axis represents frequency. The color depth of each point in the image represents the strength of the signal at the corresponding time and frequency. The time-frequency spectrum diagram intuitively reflects the distribution of the signal at different times and frequencies.

[0045] The step 6 searches for the coordinate point with the maximum intensity on the time-frequency spectrum, which represents the most significant or highest energy frequency component in the signal, and obtains the frequency and time information corresponding to the point. The frequency component is the rotation frequency of the magnetic particle, and the moment is the time domain positioning of the magnetic particle signal.

[0046] The step 7 outputs data including the rotation frequency ω and the signal time t.

[0047] The magnetic particles are spherical with a diameter in the micrometer range.

[0048] The contents not described in detail in the specification of the present invention belong to the prior art known to the professional and technical personnel in the field. It is pointed out here that the above description helps those skilled in the art to understand the invention, but does not limit the protection scope of the invention. Any equivalent replacement, modification and / or simplification of the above description without departing from the essence of the invention falls within the protection scope of the invention.

Claims

1. A method for determining the rotational frequency of magnetic particles based on wavelet transform, characterized in that: The following steps are involved: Step 1, setting the working state of the SERF atomic magnetometer measurement system; Step 2, using a SERF atomic magnetometer to collect environmental magnetic field signals and magnetic field signals of magnetic particles in the fluid respectively; Step 3, detrending the magnetic field signal of the magnetic particles to remove the baseline drift in the signal, and using the environmental magnetic field signal as a reference signal; Step 4, the adaptive filter outputs a filtered signal according to the input detrended magnetic particle magnetic field signal and the reference signal; Step 5, performing continuous wavelet transform on the filtered signal to obtain a time-frequency spectrum of the wavelet transform; Step 6, obtaining the frequency and time at the maximum intensity on the time spectrum, which are used as the rotation frequency ω of the magnetic particles and the occurrence time t of the magnetic field signal of the magnetic particles, respectively.

2. The method for determining the rotational frequency of magnetic particles based on wavelet transform according to claim 1, characterized in that: Step 1 includes setting the gas chamber heating temperature of the SERF atomic magnetometer, performing residual magnetic field compensation, and setting the liquid flow rate for transporting magnetic particles.

3. The method for determining the rotational frequency of magnetic particles based on wavelet transform according to claim 1, characterized in that: The environmental magnetic field signal in step 2 is also called an air-sampled environmental magnetic field signal. Under the condition that the SERF atomic magnetometer does not respond to magnetic particles, the response of the SERF atomic magnetometer to the remaining magnetic field changes is collected.

4. The method for determining the rotational frequency of magnetic particles based on wavelet transform according to claim 1, characterized in that: Collecting the magnetic field signal of the magnetic particles in the fluid in step 2 includes the magnetic particles being transported by the fluid through the detection area of ​​the SERF atomic magnetometer. The magnetic particles are spherical with a diameter in the micrometer range. The magnetic field signal of the magnetic particles decays with the spatial distance. The detection area refers to the spatial range in which the magnetic particles can cause the atomic magnetometer to produce an output response change. At this time, the collected signal is the superposition of the magnetic field signal of the magnetic particles and the environmental noise.

5. The method for determining the rotational frequency of magnetic particles based on wavelet transform according to claim 1, characterized in that: Step 3 includes fitting the collected signal to a sixth-order polynomial, and using the sixth-order polynomial fitting result to detrend the collected signal to remove the baseline offset of the signal to obtain a detrended signal.

6. The method for determining the rotational frequency of magnetic particles based on wavelet transform according to claim 1, characterized in that: Step 5 includes using Morse wavelet to perform continuous wavelet transform on the filtered signal to obtain a time-frequency spectrum of the wavelet transform; the time-frequency spectrum of the wavelet transform is a two-dimensional image, the horizontal axis represents time, the vertical axis represents frequency, the color depth of each point in the image represents the intensity of the signal at the corresponding time and frequency, and the time-frequency spectrum intuitively reflects the distribution of the signal at different times and frequencies.

7. The method for determining the rotational frequency of magnetic particles based on wavelet transform according to claim 1, characterized in that: Step 6 includes searching for the coordinate point with the maximum intensity on the time spectrum, which represents the most significant or highest energy frequency component in the signal, and obtaining the frequency and time information corresponding to the point. The frequency is the rotation frequency of the magnetic particles, and the time is the time domain positioning of the magnetic field signal of the magnetic particles.

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