Field of Invention

The group of inventions disclosed herein relates to the field of nondestructive methods of solid materials investigation and can be used for the monitoring of physical properties, internal texture and potential voids in solid and biological tissue.

Description of the Related Art

Ultrasonic testing, which is a method of investigation of inhomogeneities in an object by launching high-frequency ultrasonic beam onto the test object and detection of the waves reflected by the objects and its internal texture, allows visualizing the internal structure of the test object. This method affords detecting both surface and internal defects in metals, plastics, composites or biological tissue.

The new modality of ultrasonic NDT is laser ultrasonics. Pulsed laser radiation is explored for the excitation of high amplitude short ultrasonic pulse with sharp tempotce shape. The laser excited ultrasonic beam contains side lobes. Ultrawide band piezoelectric transducers is used for the detection of reflected and scattered ultrasonic signals. Due to separation of ultrasonics excitation and detection there is no "dead-zone" of testing. So, enhanced spatial resolution and sensitivity of testing is provided. (V.E. Gusev and A.A. Karabutov, Laser Optoacoustics, Moscow, Nauka, 1991, p. 304, ISBN 5-02- 14172-0.)

For studying objects having a complex texture it is more preferable to use high-power ultrasonic pulses generated by laser radiation and cross-correlation of the reflected signal with a reference signal using fast Fourier transformation. This allows obtaining a two-dimensional image of the internal structure of the object and, given preliminary calibration, provides an image of the internal structure of the test object, e.g., soft biological tissues, with a high resolution.

Example of this technical solution can be Ultrasonic Tomograph (RF Patent 2526424, published 20.08.2014, Bulletin #23) in which the working surface of planar electroacoustic transducers is arranged in a tangential plane to the circle inside which the tomographically studied organ is located, and orthogonal to the plane of said circle. Disadvantage of said technical solution is that planar electroacoustic transducers cannot generate high-power ultrasonic pulses and focus said high-power ultrasonic pulses on the required point of the test object.

Also known is the Quantitative Imaging System (WO2019195614 (Al) — 2019-10-10) comprising a laser that emits instant pulses, a fiberoptic bundle, one ultrasonic array of ultrasonic transducers configured to deliver ultrasonic pulses to the tissue region of interest and to detect ultrasonic signals reflected or transmitted via the tissue region, in which said optoacoustic array of ultrawide-band ultrasonic transducers and said ultrasonic array of ultrasonic transducers are coupled into a single arc-shaped array.

Said system allows one to obtain high-power ultrasonic pulses but does not allow focusing said pulses in the required point of the test object because the width of the emitter determines the width of the region studied. (A. Oraevsky and A. Karabutov, Limit Sensistivity of Optoacoustic Detection with Time Resolution, Educ.SPIE, 3916 (2000) 228-239. https:

//doi.org/10.1117/12.386326.). Disadvantage of said technical solution is that the suggested array design reduces the accuracy of the image obtained.

Known is the Laser Component, Laser Generating Apparatus and Optical Coherence Tomograph (CN107661088(A), priority date 06.02.2018.) in which the optoacoustic pulse from the emitter/receiver is delivered to a reflecting mirror and concentrated in one point. This method allows obtaining high- power ultrasonic pulses and easily adjust their parameters depending on the test object. Disadvantage of said technical solution is that the device for its implementation focuses ultrasonic pulses in the required region of the test object in the form of a band thus not allowing one to achieve sufficient accuracy, and the presence of an additional element, i.e., the mirror, reduces the efficiency of the signal and requires increasing the emitter power.

The prototype of the present invention is considered to be the Method of Relative Excitation for the Ultrasonic Tomography System (CN109655525 (A), priority date 19.04.2019.) comprising a plurality of sequentially excited piezoelectric ultrasonic sensors evenly distributed in the form of a circle on a side wall of a metallic tube, some of said sensors being emitters and the others receivers, wherein the number of said sensors is at least 16, further wherein the test object is placed in the center of said tube which is in turn filled with an immersion fluid. Due to the large number of emitters and receivers this method allows focusing optoacoustic pulses in one point and obtaining a reflected wave with minimal distortions.

Disadvantages of said technical solution are that the accuracy of test object study depends on the linear dimensions of the emitters and that optoacoustic pulses are focused in the center of the device used for the implementation of the method and therefore test object movement and positioning are complicated, especially for large test objects. Also complicated is control of operations performed in biological objects, e.g. visualization of acupuncture needles that are inserted into tissues to a preset depth. This technical solution is considered as the prototype.

Disclosure of the Invention

The technical result of the invention disclosed herein is increasing the accuracy of the real time visualization system and providing a device which uses the method disclosed herein for studying various objects of arbitrary shape, including biological objects. Said technical result is achieved by generating an optical pulse with a pulse-periodic laser, transmitting said pulse via an optical system and converting it into an acoustic signal with a distributed optoacoustic transducer having a curvilinear shape, emitting said acoustic signal to the test object placed into an immersion media and receiving the reflected acoustic signal with an array of piezoelectric elements, amplifying the received signal, converting the received signal from analog to digital form, and constructing a model of the internal structure of the test object, wherein the acoustic signal emitted to the test object is focused not on the surface of said test object but at some depth which is determined by the distance between the two focuses of Gaussian beams corresponding to waves in the XY planes which describe the position of said distributed acoustooptic transducer having a curvilinear shape and said array of piezoelectric elements, and which is in turn the working zone for the analysis of the acoustic signal reflected from the internal structure of said test object, furthermore the movement of said test object or said acoustooptic transducer having a curvilinear shape and their positioning are effected in an indexed manner after the completion of the acoustic signal emission / reflected acoustic signal receipt cycle, so that said working zone of said acoustic signal is permanently inside said test object, further wherein a 2D model of a discrete layer of said test object constructed using a back projection algorithm and an algorithm of accounting acoustic signal refraction on the test object surface.

Said technical result is further achieved by that the laser pulses and the clock signal generator that controls the analog to digital converter are synchronized by time-shifting the signal using Fourier phase filtering.

Said technical result is further achieved by that the focusing depth of the acoustic signal is determined by the dimensions of the test object and the minimum thickness of the scanned layer is 100 pm. It is suggested to use a device for the implementation of the technical result claimed in the method disclosed herein. Said device comprises a pulse modulated laser, an optical system, a distributed optoacoustic transducer, an array of piezoelectric elements forming a single unit having a curvilinear shape with said acoustooptic transducer, said unit being placed into a tank containing immersion fluid, a multichannel amplifier with a multichannel analog to digital converter, a computer with interface components, wherein said distributed optoacoustic transducer and said array of piezoelectric elements forming a single unit have a toroidal shape with an aperture of 90 to 180 arc deg, the number of receiving elements in said unit is equal to or greater than the number of emitting elements and is a factor of eight, and furthermore the positions of said acoustooptic transducer and said array of piezoelectric elements are coincident in the coordination system describing their shapes and dimensions.

Said technical result is further achieved by that said single unit having a curvilinear shape comprises alternating strips of emitters and receivers that are mutually parallel and do not contact with one another, and are in turn mounted on a rigid massive platform in a toroidal shape groove(cavity) which replicates the parameters that are preset for said acoustooptic transducer and said array of piezoelectric elements, wherein all said strips of emitters and said strips of receivers are interconnected via respective buses which are in turn connected to said multichannel amplifier and further to said multichannel analog to digital converter, further wherein the dimensions of said platform of said single unit and its curvature radius are determined on the basis of the sizes of test objects.

Brief Description of Figures

The method and device disclosed herein will be hereinafter illustrated with the Figures, wherein Figure 1 presents a general schematic of achieving the technical result claimed herein, Figure 2 presents a schematic of the formation of acoustic waves, Figure 3 presents an arbitrary cross-section of the test object and the single unit, Figure 4 shows an expanded view of the acoustooptic transducer and the array of piezoelectric elements (a single unit), and Figure 5 shows charts of experimental parameters for the sensitivity area and spatial resolution of the device disclosed herein.

General schematic of achieving the technical result claimed herein is shown in Figure 1. For this purpose, in accordance with the method disclosed herein, an optical pulse is generated by the pulse modulated laser 1 , said optical pulse is transmitted via the optical system 2, converted to an acoustic signal with the photoacoustic transducer 3 having a form of an array of piezoelectric elements, and the reflected scattered acoustic field is received with the wide band piezoelectric transducers 4 also having a form of an array. Both said transducers form a single unit the working surface of which facing the test object 5 has a toroidal shape. The trajectory of the acoustic signals 6 is arbitrarily shown in the schematic. The reflected acoustic signal 6 received by the wide-band piezoelectric transducers 4 is transmitted to a multiband analog amplifier and transducer 8 for amplification and further transmission to a data collection and processing system 9. Digitalized information is converted by the computer 10 with a back projection algorithm and an algorithm of accounting acoustic signal refraction on the surface of the test object 5 and visualized by a graphical processor 11. Furthermore the computer 10 controls the analog to digital amplifier and transducer 8 by time-shifting the received signal using Fourier phase filtering.

In accordance with the method disclosed herein all the movements of the test object 5 are preset in the coordinates that are preset relative to the single unit 12 and are only effected in an indexed manner after the completion of the acoustic signal emission / reflected acoustic signal receipt cycle. Initially one layer of the preset are of the test object 5 is studied, and after the completion of its 2D model, transfer is made to the subsequent layer the minimum thickness of which is 100 pm. Schematic of acoustic wave formation for the achievement of the technical result claimed herein is shown in Figure 2. For this purpose, in accordance with the method disclosed herein, the shape of the photoacoustic transducer 3 and the wide-band piezoelectric transducer 4 are completely similar and are part of a toroid with the curvature radius in the horizontal plane R and the curvature radius in the vertical plane r as well as their apertures F and Q. Components of the photoacoustic transducer 3 and the wide-band piezoelectric transducer 4 are in the form of strips 13 and 14 and alternate, the number of components always being a factor of eight. Acoustic signal is simultaneously excited in all the components of the photoacoustic transducer 3. The shape of the generated acoustic wave 6 has two nodes of Gaussian beams 15 and 16 the distance between which is determined by the difference between R and r. The center of the beam 15 is accepted as the origin of the coordination system 17 in which the test object 5 is positioned because the position of this point relative to the single unit of the photoacoustic transducer 3 and the wide band piezoelectric transducer 4 is known precisely. In accordance with the method disclosed herein both the test object 5 and the single unit 12 are movable. In this case the coordination system of the single unit 12 is tied to an additional coordination system, e.g. one connected with the tank containing the immersion fluid 7 in which the test object 5 is placed.

The distance between the single unit 12 and the test object 5 is preset so that the zone 18 of the wave 6 between the two nodes of the Gaussian beams 15 and 16 is completely within this distance. In Fig. 3 which shows an arbitrary cross-section of the test object 5 and the single unit 12 comprising the photoacoustic transducer 3 and the wide-band piezoelectric transducer 4, this distance is shown as x _n. The zone 18 in accordance with the method disclosed herein is the working zone for acoustic signal analysis having the highest resolution and the smallest distortion when the internal structure of the test object is imaged. Furthermore the test object 5 is moved so that the working zone 18 remains within the limits of the currently analyzed layer of the test object 5 which faces the device the limits of which are denoted in Figure 2 as 19. The test object 5 is moved along the Y axis also so that the working zone 18 remains within the limits of the currently analyzed layer of the test object 5 and is perpendicular to this axis. These limits are denoted in Figure 3 as 20.

Acoustic waves reflected from defects inside the test object 5 are detected by the wide-band piezoelectric transducer 4 and since the shapes of the receiver and the emitter and the distance between them and the test object 5 are completely similar, the received waves will undergo minimum distortion. Processing this type of waves with suitable algorithms, e.g. LU FBP and knowing the distance to the test object and the number of emitters one can exactly evaluate the scattering force(intensity) and the reflection coefficient at the media interface and obtain the most authentic image of the internal structure of the test object with the preset resolution.

The movement cycle of the test object 5 during the study can also be easily determined because the laser pulses generated by the pulse modulated laser 1 and the clock signal generator which controls the analog to digital converter 8 are synchronized by time-shifting the signal using Fourier phase filtering and can easily be achieved using the method disclosed herein because as noted above the emitted and received acoustic waves have similar shapes. Furthermore the distance of test object movement per cycle can be increased without compromise in accuracy due to the presence of the working zone, rather than a single point, with optimum resolution and minimum distortion.

Embodiments of the Invention

As an embodiment of said method is suggested a device comprising the pulse modulated laser 1, the optical system 2, the distributed optoacoustic transducer 3 having a curvilinear shape, the array 4 of piezoelectric elements having a curvilinear shape forming the single unit 12 with the optoacoustic transducer, placed in a tank with immersion fluid 7, as well as the multichannel amplifier 8 with the multichannel analog to digital converter 9, the computer 10 and the graphical processor 11.

The optoacoustic transducer 3 and the array 4 of piezoelectric elements forming the single unit 12 are shown in an expanded view in Figure 4. The single unit 12 has a toroidal shape with a receiving antenna aperture of 90 to 180 arc deg and the number of receiving elements equal to or greater than the number of emitting elements and being a factor of 8.

The single unit 12 of the device alternating strips of emitters 3 and receivers 4 that are mutually parallel and do not contact with one another, and are mounted on a rigid massive platform with a toroidal shape groove(cavity). The single unit 12 is placed in a tank with immersion fluid 7. The single unit 12 can only be moved with the platform (not shown in Figure 4).

The shape of the groove(cavity) in the platform corresponds to the designed shape of the single unit 12 and provides for mounting of the distributed optoacoustic transducer 3 and the array 4 of piezoelectric elements in the preset position. The distances between the elements of the distributed optoacoustic transducer 3 and the array 4 of piezoelectric elements are selected as the minimum ones precluding electric contact between said elements and are similar for all the elements of the single unit 12. The emitting elements of the optoacoustic transducer 3 are additional insulators for the elements of the array 4 of piezoelectric elements.

All the elements of the distributed optoacoustic transducer 3 and the array 4 of piezoelectric elements are interconnected via respective buses 22 and 23 which are in turn connected to the multichannel analog to digital transducer 9 via electric wiring 24.

The elements of the distributed optoacoustic transducer 3 and the array 4 of piezoelectric elements are in the form of elongated strips the width of which is substantially smaller than the length, by at least one order of magnitude. The aspect ratio of the elements of the single unit 12 is determined based on the parameters R and r and the dimensions of the platform (not shown in Fig. 4). For example the width of the elements of the single unit 12 is determined as the minimum one required for providing the necessary emitted power and the maximum length is determined based on the parameter r of the single unit 12.

The dimensions, curvature radii R and r and apertures F and Q of the single unit 12 are determined on the basis of the sizes of the test object 5. Therefore the device suggested for the implementation of the method disclosed herein may comprise several single units.

Exemplified embodiment of the method.

In an experimental device the array had the following parameters: r_f = 40 mm, R = 4 mm, F = 20 °, Q = 15 °. The model of the point acoustic diffuser was a 0.2 mm diameter needle clamped in a duraluminum holder. The needle moved along the X axis of the single unit 12 inside a water filled tank. The reflected signals were processed using the FBP algorithm. The obtained laser- ultrasonic field image was restored in real time mode with an up to 30 Hz frame rate in the XZ plane perpendicular to the needle. The image parameters were 1 mm x 1 mm and 400 x 400 pixels. The resultant experimental characteristics of the high sensitivity and spatial resolution area are shown in Figure 5 where 24 is the near-zero region of the X axis, 25 is the width of the sensitivity area in mm, 26 is the normalized sensitivity of the measurement unit, 27 is the axial resolution in mm and 28 is the lateral resolution.

The characteristics demonstrate that in the near-zero region 24 all the parameters describing needle position and sizes had minimum deviation from its actual position.

Thus the method disclosed herein and device for its implementation provide for the claimed technical result. All the aspects of the group of inventions described herein are required and sufficient for its implementation and are practically achievable.