High-bandwidth laser homodyne interferometer for high-intensity focused ultrasound sound pressure measurement

Abstract

The invention discloses a high-bandwidth laser homodyne interferometer for high-intensity focused ultrasound sound pressure measurement. Comprising a single-frequency stabilized-frequency laser light source, a first polarization splitting prism, a first quarter-wave plate, a reflector, a second quarter-wave plate, a lens group, a second polarization splitting prism, a phase delay wave plate, a first high-speed photoelectric detector, a second high-speed photoelectric detector, a high-speed digital oscilloscope and a signal demodulation processing unit. According to the invention, the problem that the traditional laser homodyne interference method cannot measure the HIFU sound pressure can be solved, the requirement of a heterodyne method for the system bandwidth and the complexity of system implementation are effectively reduced, and the measurement signal-to-noise ratio and accuracy are improved, so that the uncertainty level of HIFU pressure measurement and the measurement upper limit of the HIFU sound pressure are improved.

Description

Technical Field

[0001] The present invention can be applied in the technical field of ultrasonic sound pressure measurement. Specifically, it relates to a high-bandwidth laser homodyne interferometer for measuring the sound pressure of high-intensity focused ultrasound. Background Art

[0002] High-intensity focused ultrasound (HIFU) has the characteristics of strong directivity, good penetrability, focusability, and no radiation. The HIFU treatment technology focuses high-intensity focused ultrasound on the lesion site, and uses the hyperthermal effect, cavitation effect, mechanical effect, and sono-chemical effect to cause coagulative necrosis of the target tumor tissue, providing a non-invasive or minimally invasive method for treating diseased tissues.

[0003] The key to HIFU treatment is to cause irreversible coagulative necrosis of the target tissue in an extremely short time without damaging other tissues. The damaged area generated in the target tissue is mainly determined by the shape and position of the acoustic focal region in the focused sound field, and its degree of damage is closely related to the magnitude of the HIFU peak sound pressure and the treatment time (the combined thermal effect). To ensure the accuracy of the HIFU treatment dose and the safety of the treatment, it is necessary to accurately measure the peak sound pressure and the spatial distribution of the sound pressure in the HIFU sound field, that is, to achieve quantitative measurement of the HIFU sound field distribution.

[0004] The existing quantitative measurement of the HIFU sound field distribution is usually obtained by using a high-frequency hydrophone through a three-dimensional scanning method. The sound pressure sensitivity of the high-frequency hydrophone, that is, the ratio of the output voltage to the received sound pressure, needs to be strictly calibrated to monitor the safety of the sound output. However, the calibration process of the current high-frequency hydrophones used for high-intensity sound field measurement is completed in a low sound pressure field of hundreds of kPa or even below, while the sound pressure in the HIFU treatment sound field can reach dozens of MPa or even nearly 100 MPa, and the linearity of the sound pressure sensitivity of the high-frequency hydrophone in the high sound pressure field cannot be guaranteed.

[0005] The hydrophone calibration method based on the principle of optical interference is the recommended method by the International Electrotechnical Commission (IEC), which can realize the traceability of the sound pressure value in the medical ultrasound field. This method measures the vibration of the thin film in the sound field based on laser interference technology to achieve the quantitative measurement of sound pressure. Currently, only the quantitative measurement of sound pressure in the order of maximum MPa has been realized, and the quantitative measurement of the high sound pressure HIFU sound field has not been realized. One of the key reasons is that the existing laser interference measurement system cannot meet the measurement requirements of high frequency and high intensity of HIFU sound pressure. According to different principles, the laser interference method is divided into laser heterodyne interference method and laser homodyne interference method. The laser heterodyne interference method uses means such as acousto-optic frequency shift devices to make the measurement beam and the reference beam have a fixed initial frequency difference, that is, the optical carrier frequency. The magnitude of this frequency is directly related to the measurement upper limit of HIFU sound pressure. Therefore, the higher the sound pressure, the larger the required optical carrier frequency, which results in the complexity of its technical implementation.

[0006] The homodyne laser interferometer based on the phase working point locking is applicable to the case where the sound pressure is small, that is, the vibration displacement caused by the sound pressure is less than 1 / 8 of the laser wavelength. Therefore, it is not applicable to the measurement of HIFU sound pressure. Moreover, the feedback phase-locked circuit must be used to make the system always work at the quadrature working point position. However, when the external disturbance changes, the feedback phase-locked circuit is prone to unlocking, resulting in the oscillation of the compensation system and unable to stably measure the sound pressure for a long time.

[0007] In view of the above problems, the present invention proposes a high-bandwidth laser homodyne interferometer for measuring the sound pressure of high-intensity focused ultrasound. Summary of the Invention

[0008] In view of the above problems, the present invention proposes a laser homodyne interference system for measuring HIFU sound pressure, which solves the problems that the existing laser homodyne interference method is not applicable to the measurement of HIFU sound pressure and the high-frequency optical carrier is difficult to obtain and the high bandwidth cost of the interference system in the measurement of HIFU sound pressure for the laser heterodyne interference method.

[0009] The present invention includes a single-frequency frequency-stabilized laser light source, a first polarization beam splitter prism, a first quarter-wave plate, a mirror, a second quarter-wave plate, a lens group, a second polarization beam splitter prism, a phase delay wave plate, a first high-speed photodetector, a second high-speed photodetector, a high-speed digital oscilloscope, and a signal demodulation and processing unit. The emission end of the single-frequency frequency-stabilized laser light source is connected to the first incident end of the first polarization beam splitter prism. The measurement beam sequentially passes through the lens group and the second quarter-wave plate and is connected to the second incident end of the first polarization beam splitter prism. The first emission end of the first polarization beam splitter prism forms a synthesized measurement beam and a reference beam after passing through the first quarter-wave plate and the mirror. The second emission end of the first polarization beam splitter prism is connected to the first incident end of the second polarization beam splitter prism placed at an inclination of 45°. The first emission end of the second polarization beam splitter prism is connected to the input end of the first high-speed photodetector. A phase delay wave plate is arranged at the input end of the first high-speed photodetector. The second emission end of the second polarization beam splitter prism is connected to the input end of the second high-speed photodetector. After the two interference signals are transmitted to the high-speed digital oscilloscope for signal acquisition, through the signal demodulation and processing unit, the measured HIFU sound pressure is obtained by parameter demodulation.

[0010] Further, the phase delay amount of the phase delay wave plate needs to compensate for the phase difference, and finally the phase difference between the two interference signals is made 90°.

[0011] Further, the calculation steps for parameter demodulation of the high-bandwidth laser homodyne interferometer include:

[0012] A. Using the high-speed digital oscilloscope to collect the two interference signals of the first high-speed photodetector and the second high-speed photodetector;

[0013] B. Performing a DC removal operation on the two interference signals to obtain two AC interference signals;

[0014] C. Obtaining the AC amplitude of the interference signal and performing amplitude scaling to make the amplitudes of the two interference signals the same;

[0015] D. Using the arctangent algorithm to calculate the displacement of the measured vibration;

[0016] E. Calculating the vibration velocity by performing a differential algorithm on the displacement;

[0017] F. Deducing the HIFU sound pressure based on the vibration velocity.

[0018] Further, the two interference signals I cos 、I sin can be expressed as:

[0019]

[0020] Among them, a and c are the DC components of the two interference signals respectively, b and d are the AC amplitudes of the two interference signals respectively, n is the refractive index, x is the vibration displacement, and λ is the laser vacuum wavelength.

[0021] Further, for the arctangent algorithm, the formula for calculating the displacement x of the measured vibration is:

[0022]

[0023] Since the displacement x is a digital signal, the vibration velocity v can be calculated through the differential algorithm:

[0024] v(i) = F s ·[x(i + 1) - x(i)] (5)

[0025] Among them, F s is the sampling rate.

[0026] A high-bandwidth laser homodyne interferometer system for high-intensity focused ultrasound (HIFU) sound pressure measurement uses the high-bandwidth laser homodyne interferometer to achieve its functions. The system includes an HIFU transducer, a water medium, an HIFU sound field, a thin film fixing ring, an optical window, a measurement beam, a high-bandwidth laser homodyne interferometer, a water tank, and a sound-transmitting and light-reflecting thin film. An optical window is provided on one side of the water tank. The HIFU transducer, the optical window, and the high-bandwidth laser homodyne interferometer are on the same axis. A sound-transmitting and light-reflecting thin film is provided between the HIFU transducer and the optical window. The HIFU transducer and the sound-transmitting and light-reflecting thin film are arranged in the water tank. The water tank is filled with a water medium. The optical path output end of the optical window is connected to the input end of the high-bandwidth laser homodyne interferometer.

[0027] Further, the acoustic axis of the HIFU transducer is perpendicular to the sound-transmitting and light-reflecting thin film and coincides with the optical axis of the measurement beam.

[0028] Further, the focus of the acoustic axis of the HIFU transducer coincides with the sound-transmitting and light-reflecting thin film.

[0029] Compared with the prior art, the present invention has the following technical effects:

[0030] 1) The present invention uses the laser homodyne interference method, which can effectively reduce the requirement of the HIFU sound pressure measurement for the system bandwidth compared with the heterodyne interference method, and effectively reduce the technical complexity and measurement result instability caused by generating a large frequency difference optical carrier.

[0031] 2) By using a high-bandwidth detector and a signal acquisition unit and combining with the signal processing method of the present invention, the laser homodyne interference method that could not be originally applied to HIFU sound pressure measurement realizes HIFU sound pressure measurement and has certain technical advantages.

[0032] 3) The present invention uses a phase retardation wave plate to compensate for the additional phase difference introduced by the polarization beam splitter prism. In combination with the signal processing method of the present invention, it can reduce the non-linear error of vibration displacement measurement, thereby improving the accuracy of HIFU sound pressure measurement. Description of the Drawings

[0033] Figure 1 Schematic diagram of the high-bandwidth laser homodyne interferometer for HIFU sound pressure measurement in the high-intensity focused ultrasound sound pressure measurement of the present invention

[0034] Figure 2 Schematic diagram of the structure of the high-bandwidth laser homodyne interferometer for HIFU sound pressure measurement of the present invention.

[0035] Figure 3 Schematic diagram of the process of the high-bandwidth laser homodyne interferometer for HIFU sound pressure measurement of the present invention. Detailed Embodiments

[0036] In view of the problem that the current laser homodyne interferometry method cannot achieve HIFU sound pressure measurement, the present invention proposes a high-bandwidth laser homodyne interferometer for high-intensity focused ultrasound sound pressure measurement.

[0037] The implementation method of the system is as Figure 1 and Figure 2 shown. The device includes a HIFU transducer 1, a water medium 2, a HIFU sound field 3, a thin film fixing ring 4, an optical window 5, a measurement beam 6, a high-bandwidth laser homodyne interferometer system 7, a water tank 8, and a sound-transmitting and light-reflecting thin film 9. Among them, both the HIFU transducer 1 and the sound-transmitting and light-reflecting thin film 9 are in the water medium 2; the acoustic axis of the HIFU transducer 1 is perpendicular to the sound-transmitting and light-reflecting thin film 9 and coincides with the optical axis of the measurement beam 6, and the focus of the HIFU sound field 3 coincides with the sound-transmitting and light-reflecting thin film 9; the water tank 8 can be an open container or a sealed pressurized container; the sound-transmitting and light-reflecting thin film 9 can also be horizontally placed at the interface between the water medium and the air, and in this case, the optical window 5 is not required.

[0038] The device includes a single-frequency frequency-stabilized laser light source 11, a first polarization beam splitter prism 12, a first quarter-wave plate 13, a mirror 14, a second quarter-wave plate 15, a lens group 16, and a measurement beam 6 (i.e., Figure 1The measuring beam 6), the second polarization beam splitter prism 18, the phase retardation wave plate 19, the first high-speed photodetector 20, the second high-speed photodetector 21, the high-speed digital oscilloscope 22, and the signal demodulation and processing unit 23. Among them, the single-frequency frequency-stabilized laser light source 11 emits a laser beam with a single and stable frequency, which is incident on the first polarization beam splitter prism 12; the first polarization beam splitter prism 12 outputs two laser beams, and the transmitted light among them is used as the measuring beam. After passing through the second quarter-wave plate 15 and the lens group 16 in sequence, a converging measuring beam 6 is formed; the measuring beam 6 is perpendicularly incident on the sound-transmitting reflective film 9 and returns along the original path, so as to measure the vibration velocity / displacement of the sound-transmitting reflective film 9. The beam returning along the original path passes through the lens group 16 and the second quarter-wave plate 15 again, and is reflected by the first polarization beam splitter prism 12; the reflected light output by the light source incident on the first polarization beam splitter prism 12 is used as the reference beam, passes through the first quarter-wave plate 13 and returns along the original path through the mirror 14, and is transmitted by the first polarization beam splitter prism 12 after passing through the first quarter-wave plate 13 again, and is combined with the measuring beam into a beam of light; the combined measuring beam and reference beam pass through the second polarization beam splitter prism 18 placed at an inclination of 45°. Among them, the reflected light is received by the first high-speed photodetector 20 after passing through the phase retardation wave plate 19, and the transmitted light is received by the second high-speed photodetector 21; the two interference signals generated by the first high-speed photodetector 20 and the second high-speed photodetector 21 are transmitted to the high-speed digital oscilloscope 22 for signal acquisition. The acquired interference signals are demodulated by the signal demodulation and processing unit 23 to obtain the measured HIFU sound pressure.

[0039] The phase retardation wave plate 19 can generate a certain optical phase retardation, so that the phase difference between the two interference signals is 90°. The magnitude of the phase retardation of the phase retardation wave plate 19 is not the same as that of the commonly used half-wave plate. The reason is that the parameters of the actual second polarization beam splitter prism 18 are not ideal, and an additional phase difference will be introduced into the two interference signals. The phase retardation amount of the phase retardation wave plate 19 needs to compensate for this additional phase difference, and finally the phase difference between the two interference signals is 90°.

[0040] The signal demodulation and processing unit 23 adopts off-line data processing, that is, the method of collecting first and then processing. The specific signal processing flow is as Figure 3 shown. The two interference signals I cos 、I sin acquired by the high-speed digital oscilloscope 22 can be expressed as:

[0041]

[0042] Among them, a and c are the DC components of the two interference signals respectively, b and d are the AC amplitudes of the two interference signals respectively, n is the refractive index, x is the vibration displacement, and λ is the laser vacuum wavelength. First, perform a DC removal operation on the two interference signals to obtain two AC interference signals:

[0043]

[0044] Furthermore, by obtaining the AC amplitude of the interference signal and performing amplitude scaling, the amplitudes of the two interference signals are made the same:

[0045]

[0046] Using the arctangent algorithm, the displacement x of the measured vibration is calculated:

[0047]

[0048] Since the displacement x is a digital signal, the vibration velocity v can be calculated by the difference algorithm:

[0049] v(i) = F s ·[x(i + 1) - x(i)] (5)

[0050] where F s is the sampling rate. Finally, the HIFU sound pressure p can be deduced:

[0051] p = ρcv (6)

[0052] where ρ is the density of the water medium and c is the sound velocity in the water medium.

[0053] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and all are covered by the protection scope of the present invention.

Claims

1. A high-bandwidth laser homodyne interferometer for high-intensity focused ultrasound sound pressure measurement, characterized in that: The high-bandwidth laser homodyne interferometer comprises a single-frequency stable laser light source, a first polarization beam splitter prism, a first quarter-wave plate, a reflector, a second quarter-wave plate, a lens group, a second polarization beam splitter prism, a phase delay wave plate, a first high-speed photodetector, a second high-speed photodetector, a high-speed digital oscilloscope and a signal demodulation processing unit. The emission end of the single-frequency stable laser light source is connected to the first incident end of the first polarization beam splitter prism, the measuring light beam is connected to the second incident end of the first polarization beam splitter prism through the lens group and the second quarter-wave plate in sequence, and the first emission end of the first polarization beam splitter prism is connected to the first quarter-wave plate through the first transmitting end of the first polarization beam splitter prism. A wave plate and a reflector are connected to form a synthesized measuring beam and a reference beam, the second emitting end of the first polarization beam splitter prism is connected to the first incident end of the second polarization beam splitter prism placed at an angle of 45 degrees, the first emitting end of the second polarization beam splitter prism is connected to the input end of the first high-speed photodetector, the input end of the first high-speed photodetector is provided with a phase delay wave plate, the second emitting end of the second polarization beam splitter prism is connected to the input end of the second high-speed photodetector, the two interference signals are transmitted to a high-speed digital oscilloscope for signal acquisition, and then passed through a signal demodulation processing unit, and the measured HIFU sound pressure is obtained by parameter demodulation.

2. A high-bandwidth laser homodyne interferometer for high-intensity focused ultrasound sound pressure measurement according to claim 1, characterized in that: The phase delay amount of the phase delay wave plate needs to compensate the phase difference, so that the phase difference between the two interference signals is finally 90°.

3. The high-bandwidth laser homodyne interferometer for high-intensity focused ultrasound sound pressure measurement according to claim 1, characterized in that: The calculation steps of parameter demodulation by the high-bandwidth laser homodyne interferometer include: A. using a high-speed digital oscilloscope to collect two interference signals of the first high-speed photodetector and the second high-speed photodetector; B. Perform DC removal operation on the two interference signals to obtain two AC interference signals; C. Obtain the AC amplitude of the interference signal and scale the amplitude so that the amplitudes of the two interference signals are the same; D. Use the inverse tangent algorithm to calculate the displacement of the measured vibration; E. The vibration velocity is obtained by calculating the displacement using the differential algorithm; F. HIFU sound pressure is calculated based on the vibration velocity.

4. The high-bandwidth laser homodyne interferometer for high-intensity focused ultrasound sound pressure measurement according to claim 1, characterized in that: Two-way interference signal I cos ,I sin It can be expressed as: Among them, a and c are the DC components of the two interference signals, b and d are the AC amplitudes of the two interference signals, n is the refractive index, x is the vibration displacement, and λ is the vacuum wavelength of the laser.

5. The high-bandwidth laser homodyne interferometer for measuring high-intensity focused ultrasound sound pressure according to claim 3, characterized in that: The formula for calculating the displacement x of the measured vibration using the inverse tangent algorithm is: Since the displacement x is a digital signal, the vibration velocity v can be calculated by the difference algorithm: v(i)=F s ·[x(i+1)-x(i)] (5) Among them, F s is the sampling rate.

6. A high-bandwidth laser homodyne interferometer system for high-intensity focused ultrasound sound pressure measurement, using the high-bandwidth laser homodyne interferometer as claimed in claim 1 to achieve HIFU sound pressure measurement, characterized in that: The system includes a HIFU transducer, a water medium, a HIFU sound field, a film fixing ring, an optical window, a measuring beam, a high-bandwidth laser homodyne interferometer, a water tank, and a sound-transmitting reflective film. An optical window is arranged on one side of the water tank. The HIFU transducer, the optical window and the high-bandwidth laser homodyne interferometer are located on the same axis. A sound-transmitting reflective film is arranged between the HIFU transducer and the optical window. The HIFU transducer and the sound-transmitting reflective film are arranged in the water tank. A water medium is arranged in the water tank. The light path output end of the optical window is connected to the input end of the high-bandwidth laser homodyne interferometer.

7. A high-bandwidth laser homodyne interferometer system for high-intensity focused ultrasound sound pressure measurement according to claim 6, characterized in that: The acoustic axis of the HIFU transducer is perpendicular to the acoustically transparent reflective film and coincides with the optical axis of the measuring light beam.

8. The high-bandwidth laser homodyne interferometer system for high-intensity focused ultrasound sound pressure measurement according to claim 6, characterized in that: The focus of the acoustic axis of the HIFU transducer coincides with the sound-transmissive reflective film.

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