A method for measuring the rotational frequency of magnetic particles based on wavelet transform

Abstract

The present invention discloses a method for measuring the rotational frequency of magnetic particles based on wavelet transform, which 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 ambient 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 signals of magnetic particles are collected using an ultra-high-sensitivity SERF atomic magnetometer. After detrending and adaptive filtering to suppress noise, continuous wavelet transform is performed on the signals to obtain a time-frequency spectrum diagram of the signals. The frequency and time information corresponding to the maximum intensity point on the time-frequency spectrum diagram, i.e., the rotational frequency of the magnetic particles and the time domain positioning of the signals, are obtained, thereby realizing non-visual measurement of the rotational frequency of the magnetic particles.

Description

Technical Field

[0001] The present 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 demonstrated remarkable potential in medical technologies such as angiography and cancer treatment. Injecting magnetic particles allows for non-invasive measurement of deep tissues in the human body that are difficult to observe directly. For example, using magnetic particles as contrast agents can be used to measure cerebral blood volume, assisting in the clinical diagnosis of brain injury. Furthermore, the introduction of magnetic particles has opened up new avenues for indirect measurement of blood flow velocity. The correlation between the rotational frequency of magnetic particles in a fluid and flow velocity allows for the measurement of cardiovascular parameters, providing a novel technical approach for the assessment of cardiovascular health and the study of hemodynamics.

[0003] When visual measurement conditions are unavailable, contactless measurement using magnetic field signals offers the significant advantage of a high signal-to-noise ratio. Because different tissues in the human body have similar magnetic permeabilities and are nearly transparent to magnetic fields, magnetic field signals can penetrate tissue with almost no loss, enabling human biomagnetic field detection technologies (such as magnetocardiography and magnetoencephalography) to possess a high signal-to-noise ratio. Using a highly sensitive spin-exchange relaxation-free (SERF) atomic magnetometer to detect flowing magnetic particles effectively captures changes in the magnetic field signal caused by particle rotation, and then accurately extracts the particle rotation frequency from the signal, enabling precise measurement of flow velocity.

[0004] Time-frequency domain analysis is an effective method for extracting the frequency components of a signal. For time-varying magnetic field particle signals, the wavelet transform, compared to the Fourier transform, not only offers advantages in processing non-stationary signals but also provides temporal localization, providing localized frequency information. Furthermore, due to the high similarity between the particle signal waveform and the wavelet waveform, the wavelet transform can efficiently process the particle signal, accurately extract the particle rotation frequency, and precisely 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. After detrending and adaptive filtering to suppress noise, a continuous wavelet transform is performed on the signal to obtain a time-frequency spectrum diagram. The frequency and time information corresponding to the maximum intensity point on the time-frequency spectrum diagram are obtained, namely the time-domain positioning of the magnetic particle rotation frequency and the signal, thereby realizing non-visual measurement of the rotational frequency of the magnetic particles.

[0006] The technical solutions of the present invention are as follows:

[0007] A method for measuring the rotational frequency of magnetic particles based on wavelet transform, characterized by comprising the following steps:

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

[0009] Step 2: Use SERF atomic magnetometer to collect the environmental magnetic field signal and the magnetic field signal of magnetic particles in the fluid respectively;

[0010] Step 3, performing detrending processing on the magnetic particle magnetic field signal to remove the baseline drift in the signal, and using the ambient 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: Obtain the frequency and time at the maximum intensity on the time-frequency spectrum, which are used as the rotation frequency ω of the magnetic particle and the generation time t of the magnetic particle magnetic field signal, respectively.

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

[0015] The ambient magnetic field signal in step 2 is also called an air-sampled ambient magnetic field signal. When 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] In step 2, collecting the magnetic field signal of the magnetic particles in the fluid 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 on the order of microns. 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 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, with the horizontal axis representing time and the vertical axis representing frequency. The color depth of each point in the image represents the intensity of the signal at the corresponding time and frequency. 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 particle, and the time is the time domain positioning of the magnetic field signal of the magnetic particle.

[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. Based on the reference signal and particle signal collected by an ultra-high-sensitivity SERF atomic magnetometer, adaptive filtering is used to effectively suppress power frequency noise, cardiac magnetic noise, etc. in the particle signal.

[0022] 2. The present invention discloses a method for measuring the rotational 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 spectral 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 includes 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 collected magnetic particle magnetic field signal to remove the baseline drift in the signal; and using the unsampled environmental magnetic field signal as a reference signal; step 4, the adaptive filter outputs a filtered signal based on the input detrended magnetic particle magnetic field signal and the reference signal; step 5, performing a continuous wavelet transform on the filtered signal to obtain a time-frequency spectrum of the wavelet transform; and step 6, obtaining the frequency and time of the maximum intensity on the time-frequency spectrum as the magnetic particle rotation frequency ω and the occurrence time t of the magnetic particle magnetic field signal, respectively.

[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 magnetic particle rotation frequency measurement method 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 ambient 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 magnitude of the magnetic field (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 magnetic particle rotation frequency measurement method based on wavelet transform of the present invention. Figure 2 (b) is a schematic diagram of the air-sampled ambient 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 spectrum of the adaptive filtering output signal after wavelet transform. Figures 1 to 2 As shown, a method for measuring the rotational frequency of magnetic particles based on wavelet transform includes 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 particle magnetic field signal 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 based on the input detrended magnetic particle magnetic field signal and the reference signal; step 5, performing a 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 magnetic particle rotation frequency ω and the occurrence time t of the magnetic particle magnetic field signal.

[0028] 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. The environmental magnetic field signal in step 2 is also called the 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. 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 with a diameter on the order of microns. 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 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 acquired signal to a sixth-order polynomial, and using the sixth-order polynomial fitting result to detrend the acquired 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 the wavelet transform; the time-frequency spectrum of the wavelet transform is a two-dimensional image, with the horizontal axis representing time and the vertical axis representing frequency. The color depth of each point in the image represents the intensity of the signal at the corresponding time and frequency. 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, which 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 ambient 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 signals of magnetic particles are collected using an ultra-high-sensitivity SERF atomic magnetometer. After detrending and adaptive filtering to suppress noise, continuous wavelet transform is performed on the signals to obtain a time-frequency spectrum diagram of the signals. The frequency and time information corresponding to the maximum intensity point on the time-frequency spectrum diagram, i.e., the rotational frequency of the magnetic particles and the time domain positioning of the signals, are obtained, thereby realizing non-visual measurement of the rotational frequency of the magnetic particles.

[0031] refer to Figure 1 A method for measuring the rotational frequency of magnetic particles based on wavelet transform comprises the following steps:

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

[0033] Step 2: Use 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 ambient magnetic field signal of the SERF atomic magnetometer as the reference signal of the adaptive filter, and using the detrended magnetic particle signal as the input signal, 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 generation 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 of the SERF atomic magnetometer, the residual magnetic field compensation, and the flow rate of the liquid for transporting magnetic particles.

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

[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. Because 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] The 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. The particle signal after detrending 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. The output is as follows: Figure 2 (c) The filtered signal is shown; it can suppress power frequency signals, magnetic field noise, laboratory magnetic field fluctuations and other noises.

[0044] In step 5, the filtered signal is subjected to continuous wavelet transform using Morse wavelet to obtain a time-frequency spectrum of the wavelet transform; the time-frequency spectrum 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 of each point in the image represents the signal strength at that time and frequency. The time-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 on the order of micrometers.

[0048] Any content not described in detail in this specification is prior art known to those skilled in the art. It should be noted that the above description is intended to help those skilled in the art understand the present invention, but does not limit the scope of protection of the present invention. Any equivalent substitution, modification, improvement, and / or simplification of the above description that does not depart from the essence of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A method for measuring 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: Use SERF atomic magnetometer to collect the environmental magnetic field signal and the magnetic field signal of magnetic particles in the fluid respectively; Step 3, performing detrending processing on the magnetic particle magnetic field signal to remove the baseline drift in the signal, and using the ambient magnetic field signal as a reference signal; Step 4: the adaptive filter outputs a filtered signal based on 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: Obtain the frequency and time at the maximum intensity on the time-frequency spectrum, which are used as the rotation frequency ω of the magnetic particle and the generation time t of the magnetic field signal of the magnetic particle, respectively.

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

3. The method for measuring the rotational frequency of magnetic particles based on wavelet transform according to claim 1, characterized in that: The ambient magnetic field signal in step 2 is also called an air-sampled ambient magnetic field signal. When 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 measuring the rotational frequency of magnetic particles based on wavelet transform according to claim 1, characterized in that: In step 2, collecting the magnetic field signal of the magnetic particles in the fluid 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 on the order of microns. 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 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 measuring 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 measuring 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, with the horizontal axis representing time and the vertical axis representing frequency. The color depth of each point in the image represents the intensity of the signal at the corresponding time and frequency. The time-frequency spectrum intuitively reflects the distribution of the signal at different times and frequencies.

7. The method for measuring 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 particle, and the time is the time domain positioning of the magnetic field signal of the magnetic particle.

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