WO2021010836A1

WO2021010836A1 - Method and system for using wave analysis for speed of sound measurement

Info

Publication number: WO2021010836A1
Application number: PCT/NL2020/050470
Authority: WO
Inventors: Verya DAEICHIN; Nicolaas De Jong; Martin Daniël VERWEIJ
Original assignee: Technische Universiteit Delft
Priority date: 2019-07-18
Filing date: 2020-07-17
Publication date: 2021-01-21
Also published as: NL2023523B1

Abstract

Method and system for measuring speed of sound in an object 1, with a transducer assembly 12 arranged for providing sound waves to the object 1 and for detecting acoustic energy from the object 1. A processing unit 11 is connected to the transducer assembly 12 and arranged for controlling the transducer assembly 12 for insonifying the object 1 by the sound waves; detecting a plurality of Lamb waves induced in the object 1; and determining the speed of sound in the object 1 from the detected plurality of Lamb waves, by determining a plateau level in the phase velocity – frequency domain for symmetric modes of the detected plurality of Lamb waves, i.e. without a need to fit any individual Lamb wave modes to a model.

Description

Method and system for using wave analysis for speed of sound measurement Field of the invention

The present invention relates to a method for non-invasive, non-destructive measurement of speed of sound in an object, comprising providing sound waves to the object, wherein the object is insonified by the sound waves.

Background art

International patent publication WO01/71338 discloses a method of ultrasonic testing using insonification of a test piece. A transducer is used having multiple independent elements, allowing to‘steer’ the acoustic waves in the test piece.

International patent publication WO2014/147122 discloses an ultrasonic imaging system and method. A test object is subjected to insonifying ultrasonic signals form one of two sub-arrays. From the echo signals, images are produced of the test object.

American patent publication US 8,392,133 discloses a method for measuring shear wave speed in an isotropic plate. The method calculations include transformations of signals into propagation wavenumbers for given Lamb waves from peaks within the spectra, and applying a Newton-Raphson gradient method using the propagation wavenumbers of Raleigh-Lamb dispersion curve equations.

American patent publication US2005/004457 discloses a method capable of measuring the velocity of at least two ultrasonic waves propagating in a bone at the same time, where at least one of said waves is a guided wave according to Lamb wave theory. This object is achieved through a method and the device as defined in the accompanying claims. The invention makes use of the well-known Lamb wave theory for guided waves in solid elastic plates, demonstrating how these guided waves can be applied for the non-invasive assessment of bones.

Summary of the invention

The present invention seeks to provide a non-invasive and non-destructive method for measurement of both sound velocity and thickness of a (non-uniform) object.

According to the present invention, a method as defined above is provided, wherein the method further comprises detecting a plurality of Lamb waves induced in the object, and determining the speed of sound in the object from the detected plurality of Lamb waves, by determining a plateau level in the phase velocity - frequency domain for symmetric modes of the detected plurality of Lamb waves, i.e. without a need to fit any individual Lamb wave modes to a model.

Short description of drawings

The present invention will be discussed in more detail below, with reference to the attached drawings, in which Fig. 1 shows a simplified diagram of a system set-up in accordance with the present invention embodiments;

Fig. 2 shows a graph of Lamb wave dispersion curves in an object;

Fig. 3 shows a graph of Lamb wave modes occurring in an object 1 when insonified, in the velocity - frequency (v-f) domain;

Fig. 4 shows a graph of Lamb wave modes in the frequency - wavenumber (f-k) domain;

Fig. 5 shows an intensity plot of a superposition of Lamb waves in an insonified object, with a slope determined in the high intensity area; and

Fig. 6 shows the intensity plot of Fig. 5 after a thresholding operation.

Description of embodiments

The present invention in general terms relates to embodiments of a method and a system utilizing wave analysis for speed of sound measurement in an object. These embodiments have wide application possibilities, but are particularly suitable for determining speed of sound in the context of measurement to a uniform plate-like materials and/or a non-uniform object, such as the human skull or other bone material, in combination with ultrasound imaging applications. Further applications can relate to measurements in an industrial environment, e.g. non-destructive testing of materials, pipes, concretes, wafers, etc.

Fig. 1 shows a simplified diagram of a system set-up in accordance with the present invention embodiments. The system 10 comprises a transducer assembly 12 arranged for providing sound waves (e.g. ultrasound signals) to an object 1 and for detecting acoustic energy from the object 1 . A processing unit 1 1 is provided which is connected to the transducer assembly 12, and in further embodiments to an optional display unit 13.

The availability of a non-invasive express method for in-vivo measurement of both sound velocity and thickness of the human skull bone would be of great benefit to various transcranial ultrasonic imaging and treatment applications. The existing methods of measuring the speed of sound in human skull bone are based on the prior knowledge of its thickness. On the other hand, the methods of thickness measurement are based on the knowledge of the sound velocity. The ability to measure both parameters non-invasively would open the possibility for applying automatic corrections to the ultrasonic signals and for real-time adaptive focusing and beamforming through the skull. To date, several methods have been proposed for simultaneously measuring the sound speed and the sample thickness, which however require solving two independent equations and/or two independent measurements.

One existing method uses a combination of four transducers mounted on a multifaceted wedge. Two of these probes are oriented obliquely to the sample to launch and receive a longitudinal creeping wave along its surface. These probes provide the measurement of the sound velocity. The other pair of probes sends and receives the waves through the sample, providing data for measuring its thickness. Although this method works well in uniform objects, its application to the skull bone is hardly possible due to inhomogeneity and layered structure of the bone. The velocity of the surface wave propagating in the thin top layer of the cortical bone may not be directly related to the average longitudinal sound speed across the skull, as the latter has intermediate layers with slightly different sound speed values. These layers include the outer and inner tables composed of cortical bone with little or no porosity, and the middle table composed of porous trabecular bone (diploe). The diploe layer may be absent in certain cases including newborn skulls or thin temporal bones, but it is usually present in the thickest sections of an adult skull.

In another simultaneous measurement method swept frequency signals are used capable of exciting several standing wave resonances in the sample. One equation relates sound velocity to the frequency interval between spectral peaks corresponding to adjacent resonances. The second equation is derived from the time of flight measurement in the temporal domain, which is obtained by inverting the spectral data. While this method works well in industrial applications (e.g. pipeline monitoring), its adaptation to the highly attenuating skull may require increased power levels to excite standing waves. The necessary power level is too dangerous for a real biomedical application.

In the present invention embodiments, a compression wave velocity (speed of sound) is measured, benefiting from Rayleigh-Lamb Waves generated when the object is insonified with ultrasound. Whenever a plate is insonified, Lamb waves are generated. The higher the thickness of a plate and the higher the frequency, the more Lamb wave modes are generated. The amount of Lamb waves often limits a conventional approach when trying to fit one or a few Lamb waves modes to a baseline signal. Lamb waves are commonly used in non-destructive-testing. Generally, only one or very few Lamb wave modes are generated by tuning the excitation frequency to make the fitting possible. When insonifying human skull with a medical ultrasound frequency range (1 -3 MHz) many modes are being generated and the fitting becomes very difficult if not impossible. In this context, the term‘insonified’ is to be interpreted as‘to flood an area or an object with carefully- controlled sound waves’.

In a first aspect, the present invention relates to a method for non-invasive, non-destructive measurement of speed of sound in an object 1 , comprising providing sound waves to the object 1 , wherein the object 1 is insonified by the sound waves, detecting a plurality of Lamb waves induced in the object 1 , and determining the speed of sound in the object 1 from the detected plurality of Lamb waves. The same measurement data or a new set of pulse-echo measurements can be used for detecting a thickness of the object 1 by time measurement of reflection pulses originating from the sound waves, and using the determined speed of sound in the object 1 . This allows to obtain a combined determination of the speed of sound and a thickness measurement.

Further exemplary embodiments of the present invention relate to applications having an automatic correction to ultrasonic signals, real-time adaptive focusing and beamforming through a non-uniform object 1 , such as a (human or animal) bone, e.g. a human skull. The present invention embodiments have specific advantageous application when the object 1 is elongated (i.e. the thickness of the object 1 is its smallest dimension), and the speed of sound measured is a longitudinal wave velocity. In these embodiments, the sound waves are advantageously provided in a frequency range of e.g. between 1 and 3MHz which is a frequency range allowing insonification of a (human/animal) bone, and which is compatible with ultrasound analysis/imaging. For further clinical applications, the frequency range might depend on the specific application, for example for neonate babies with a thinner skull the frequency can be as high as 10 MHz. It is noted that the frequency range can be from KHz to GHz depending on the specific application. E.g. when making measurements in thin plates and (e.g. semiconductor wafers) the range is in the order of 100 MHz to 1 GHz, for measurements in pipes and concert the range will be in the order of several kHz, for geological applications it can even be lower. In a specific application the sound waves emitted are such that multiple lamb waves are being generated in the medium of interest.

As already indicated with reference to Fig. 1 above, the sound waves are provided and the plurality of induced Lamb waves are detected using a transducer assembly 12. Such a transducer assembly 12 allows to transform a control signal (from the processing unit 11) into mechanical vibrations in the object 1 and vice versa.

In a further embodiment, the method further comprises obtaining an ultrasound image (of the object 1 and/or of an area behind the object 1) using the determined speed of sound in (and thickness of) the object 1 . As the thickness of the object 1 and the speed of sound in the object 1 can be obtained simultaneously using the same measurement set-up, this provides for a very efficient and reliable (ultrasound) imaging method.

The Rayleigh-Lamb equations are described as follows:

wherein

(3)

and h: thickness of object 1

w: angular frequency

k: wave number

v_P: compression wave velocity or longitudinal velocity

v_s: shear wave velocity

Note that a positive exponent in equation (1) represents a symmetric wave, a negative component an asymmetric wave. At high frequencies the AO and SO modes approach the Rayleigh wave velocity, as shown in the Lamb wave dispersion curves in the graph of Fig. 2. In contrast, all higher modes approach the shear wave velocity of the object 1. At low frequencies, the SO dispersion curve approaches the quasi-longitudinal wave velocity. The same applies for all the other S-modes at a certain frequency, where they form a plateau in the velocity - frequency (v-f) domain, as deducible from the graphs in Fig. 2 and Fig. 3. Fig. 3 shows a graph of Lamb wave modes occurring in the object 1 when insonified, with the frequency on the horizontal axis and the phase velocity / shear wave velocity on the vertical axis.

The group velocity v_g is defined as the slope of the dispersion curve in the frequency - wave number (f-k) domain:

άw (4) V⁼ dk

Therefore, the quasi-longitudinal wave velocity which appears as a plateau in the graph of Fig, 3 will appear as constant slopes in the f-k domain as shown in the graph of Fig. 4.

The superimposition of the quasi-longitudinal wave velocity for different symmetric modes of the Lamb waves which appears as a plateau in the v-f domain (Fig. 2 and 3) and as a slope in the f-k domain (Fig. 4) can be exploited to derive the speed of sound in the material of object 1 .

In a first embodiment, determining the speed of sound in the object from the detected plurality of Lamb waves comprises determining a plateau level in the phase velocity - frequency domain for symmetric modes of the detected plurality of Lamb waves. At respective low frequency parts in the graphs, this results in a proper and reliable determination of the speed of sound in the object 1 .

In a further embodiment, determining the speed of sound in the object from the detected plurality of Lamb waves comprises determining the slope of high amplitude overlapping areas of symmetric modes of the detected plurality of Lamb waves in a frequency-wavenumber (f-k) domain [claim 3], as is exemplified in the graph of Fig. 4.

A further simple way to derive the speed of sound in the material of object 1 is to apply a radon transform on the f-k domain data to derive the slope of the high intensity area as shown in Fig. 5. Note that after multiplying the determined slope by 2p the speed of sound in the medium of the object 1 is directly obtained. Thus, in a further embodiment, the method further comprises applying a radon transform on the frequency-wavenumber domain amplitude data of the symmetric modes of the detected plurality of Lamb waves. A radon transform is the integral transform which takes a function f defined on the plane to a function Rf defined on the (two-dimensional) space of lines in the plane, whose value at a particular line is equal to the line integral of the function over that line.

An even further way is to threshold the f-k domain data such that only the high intensity area corresponding to the overlap of the longitudinal wave velocity of the different symmetric modes is above the threshold limit. Subsequently a linear line can be fit to the remaining data to get the slope as depicted in in the graph of Fig. 6. So in this further embodiment, the method further comprises determining a threshold area in the frequency-wavenumber domain having an amplitude above a predetermined threshold, and fitting a linear line through the determined threshold area.

For all these exemplary embodiments, further validation has been done, which showed that the determined slope based on the overlapping areas of the symmetric modes of the Lamb waves in the insonified object 1 is independent from the thickness and the shear wave velocity of the material of the object 1 . As mentioned above, the present invention can also be implemented as a system, more in particular a system for measuring speed of sound in an object 1 , comprising a transducer assembly 12 arranged for providing sound waves to the object 1 and for detecting acoustic energy from the object 1 , and a processing unit 1 1 connected to the transducer assembly 12. The processing unit

1 1 is arranged for controlling the transducer assembly 12 for insonifying the object 1 by the sound waves, detecting a plurality of Lamb waves induced in the object 1 , and determining the speed of sound in the object 1 from the detected plurality of Lamb waves. The processing unit 1 1 can then further be arranged to execute the method according to any one of the exemplary embodiments described herein.

The transducer assembly 12 is arranged to provide the sound waves, and to detect the plurality of induced Lamb waves in a further embodiment. Alternatively, the transducer assembly

12 may comprise separate transducers, e.g. one for providing and guiding the sound waves into the object 1 , and a second for detecting the plurality of induced Lamb waves.

The transducer assembly 12 has a predetermined focus range in a further embodiment. This is particularly advantageous when using the present invention embodiments wherein the speed of sound and the thickness of the object 1 is determined using the same measurement data. For practical implementations, the transducer assembly 12 may comprise one or more of: a piezo transducer, a piezo-ceramic transducer, a micro-machined transducer, a capacitive micro- machined ultrasonic transducer, a mechanical transducer, an optical transducer.

In even further embodiments, the present invention relates to an ultrasound imaging system comprising a system 10 according to any one of the embodiments described herein, and a display unit 13 connected to the processing unit 1 1 . The processing unit 1 1 and/or the display unit 13 may be further arranged to obtain an ultrasound image using the determined speed of sound in (and the thickness of) the object 1 . The ultrasound image can cover an area of/in the object 1 , but also an area behind the object 1 .

The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.

Claims

1. Method for non-invasive measurement of speed of sound in an object, comprising providing sound waves to the object, wherein the object is insonified by the sound waves;

detecting a plurality of Lamb waves induced in the object; and

determining the speed of sound in the object from the detected plurality of Lamb waves, by determining a plateau level in the phase velocity - frequency domain for symmetric modes of the detected plurality of Lamb waves.

2. Method according to claim 1 , wherein determining the speed of sound in the object from the detected plurality of Lamb waves comprises:

determining a slope of high amplitude overlapping areas of symmetric modes of the detected plurality of Lamb waves in a frequency-wavenumber domain.

3. Method according to claim 2, further comprising applying a radon transform on the frequency-wavenumber domain amplitude data of the symmetric modes of the detected plurality of Lamb waves.

4. Method according to claim 2, further comprising determining a threshold area in the frequency-wavenumber domain having an amplitude above a predetermined threshold, and fitting a linear line through the determined threshold area.

5. Method according to any one of claims 1-4, further comprising detecting a thickness of the object by time measurement of reflection pulses originating from the sound waves and using the determined speed of sound in the object.

6. Method according to any one of claims 1-5, wherein the object is elongated, and the speed of sound measured is a longitudinal wave velocity.

7. Method according to any one of claims 1-6, wherein the object is a bone.

8. Method according to any one of claims 1-7, wherein the sound waves are provided in a frequency range between 1 and 3MHz.

9. Method according to any one of claims 1-8, wherein the sound waves are provided and the plurality of induced Lamb waves are detected using a transducer assembly.

10. Method according to claim 9, wherein the transducer assembly has a predetermined focus range.

1 1 . Method according to claim 9 or 10, wherein the transducer assembly comprises one or more of: a piezo transducer, a piezo-ceramic transducer, a micro-machined transducer, a capacitive micro-machined ultrasonic transducer, a mechanical transducer, an optical transducer.

12. Method according to any one of claims 1 -1 1 , further comprising

obtaining an ultrasound image using the determined speed of sound in the object.

13. System for measuring speed of sound in an object (1), comprising

a transducer assembly (12) arranged for providing sound waves to the object (1) and for detecting acoustic energy from the object (1 ), and

a processing unit (1 1) connected to the transducer assembly (12) wherein the processing unit (1 1) is arranged for:

- controlling the transducer assembly (12) for insonifying the object (1) by the sound waves;

- detecting a plurality of Lamb waves induced in the object (1); and

- determining the speed of sound in the object (1) from the detected plurality of Lamb waves, by determining a plateau level in the phase velocity - frequency domain for symmetric modes of the detected plurality of Lamb waves.

14. System according to claim 13, wherein the processing unit (1 1) is further arranged to execute the method according to any one of claims 2-8.

15. System according to claim 13 or 14, wherein the transducer assembly (12) is arranged to provide the sound waves, and to detect the plurality of induced Lamb waves.

16. System according to any one of claims 13-15, wherein the transducer assembly (12) has a predetermined focus range.

17. System according to any one of claims 13-16, wherein the transducer assembly (12) comprises one or more of: a piezo transducer, a piezo-ceramic transducer, a mechanical transducer, an optical transducer.

18. Ultrasound imaging system comprising a system (10) according to any one of claims 13-17 and a display unit (13) connected to the processing unit (1 1).

19. Ultrasound imaging system according to claim 18, wherein the processing unit (1 1) and the display unit (13) are further arranged to obtain an ultrasound image using the determined speed of sound in the object (1).