HK40054287A - A method for determining a speed of sound in a medium, an ultrasound imaging system implementing said method - Google Patents

Description

Method for determining the speed of sound in a medium, ultrasound imaging system implementing said method

Technical Field

More specifically, for the present invention, the media includes an outer surface, an intermediate region below the outer surface, and a target region below the intermediate region.

The method is directed to determining the speed of sound within the target region by using an ultrasound system having a probe in contact with an outer surface of the medium.

Background

The method of the invention may be used to determine the speed of sound within the liver of a mammalian body to identify and quantify hepatic steatosis in the liver in one example.

Hepatic steatosis is the accumulation of fat in the liver. Which is typically determined by Magnetic Resonance Imaging (MRI). Magnetic resonance imaging allows the measurement of the amount of fat media, but the use of MRI is costly and implies the use of scarce resources. Another diagnostic method consists in invasive liver biopsy followed by histology.

Another way to determine fat accumulation in the liver is by assessing the speed of sound in the liver. In fact, the speed of sound within tissue varies with the amount of fat present in the tissue. The speed of sound in fat is lower than in healthy liver. Thus, the sound velocity of a steatosis liver is slightly lower than that of a healthy liver. For example, the speed of sound in a steatosis liver can typically be reduced from 1580m/s to 1460m/s compared to that of a healthy liver. Proportionally, this reduction represents only 5% to 10%. Therefore, it may be difficult to identify the velocity characterizing fatty liver of a person with marginal measurement errors.

Therefore, in order to detect actual hepatic steatosis, the speed of sound should be measured as accurately as possible.

Since the liver is located in the body in a region below the surface mid-region including the skin, some fat and eventually the intercostal muscles, the presence of the mid-surface region may compromise the measurement.

Literature Robust sound velocity estimation for ultrasound-based liver steatosis assessment, Marion immune et al, Physics in medicine & Biology, 2017 discloses a method of measuring the overall sound velocity in a medium by correcting aberrations occurring during propagation of ultrasound waves in the medium. The method then uses virtual point source generation and an iterative focusing algorithm within the medium to ensure that the virtual point sources are well focused. The overall speed of sound is then estimated. After this first step, the speed of sound in the liver can then be calculated by assuming that the thickness and speed of sound in fat and muscle are well known. These may be determined using reference (average) values obtained from literature (e.g. of the speed of sound) and/or may be determined using manual estimation (e.g. for thickness). The disadvantage of manual estimation is operator dependence.

However, this technique requires several iterations to handle the aberration correction, and the thickness and speed of sound in fat and muscle must be known. Finally, this method does not take the inhomogeneity of the medium in the upper region into account correctly, so that the additional calculations for correcting the measured values and determining the speed of sound in the liver are mostly arbitrary.

Accordingly, there is a need for a more accurate way to determine the speed of sound in the inner region of a mammalian site.

Disclosure of Invention

It is an object of the present invention to provide a method of determining the speed of sound in a target region of a medium (i.e. a region below the middle region) without predetermined information of the middle region.

The method is implemented by using an ultrasound imaging system comprising at least a probe adapted to sense backscattered waves and to provide sensed signals corresponding to the backscattered waves to a processing unit (16) of the ultrasound system.

The method comprises the following steps:

-determining the position of at least one interface in the medium on a morphological image, the interface dividing the medium in depth direction into an intermediate region of the medium and the target region,

-determining a first speed of sound of the middle region based on at least some of the sensed signals, an

-determining the target speed of sound within the target region based on at least some of the sensed signals and taking into account the position of the interface and the first speed of sound.

With these features, the method constructs a heterogeneous model of the medium having an interface, a first speed of sound, and a target speed of sound. Such a method allows obtaining an accurate estimate of the target sound speed.

This accurate determination of the target sound speed can then be used to detect small changes from the reference healthy sound speed value for the same medium than the sound speed within the target region. This accurate method allows for the use of ultrasound techniques for the relevant detection of diseases such as hepatic steatosis.

In various embodiments of the method, one and/or other of the following features may optionally be incorporated.

In one aspect of the method, the probe is adapted to be in functional contact with an outer surface of the medium, the probe being adapted to transmit an excitation wave into the medium in a depth direction towards the target region, the excitation wave being backscattered in the medium towards the probe.

In one aspect of the method:

-the first sound speed is determined by taking into account a first reference sound speed and one of a plurality of first assumed sound speeds, and

-the target sound speed is determined by considering: a position of the interface, a target reference sound speed applied to the target region, the first sound speed applied to the intermediate region, and one of a plurality of assumed target sound speeds of the target region.

In one aspect of the method, the method further comprises the steps of: prior to determining the first sound speed of the middle region, calculating a plurality of first image data associated with a first representative region of the middle region, the first sound speed being based on the plurality of first image data, each of the first image data being determined based on a beamforming algorithm applied to at least the sensed signals corresponding to the first representative region, and the beamforming algorithm having as a parameter a first reference sound speed and one of a plurality of first assumed sound speeds.

In one aspect of the method, the method further comprises the steps of: prior to determining the target sound speed within the target region, calculating a plurality of target image data associated with a target representative region of the target region, the target sound speed being based on the plurality of target image data in the representative region of the target region, each of the target image data being determined based on a beamforming algorithm applied to at least the sensed signals corresponding to the representative region, and the beamforming algorithm having as parameters: a position of the interface, the target reference sound speed applied to the target region, the first sound speed applied to the intermediate region, and one of the plurality of assumed target sound speeds of the target region.

In one aspect of the method, at least one of:

-the plurality of first assumed sound speeds and the plurality of assumed target sound speeds have the same value, an

-the first assumed sound speed is a known sound speed of the middle region, and the assumed target sound speed is a known sound speed of the target region.

In one aspect of the method, the determination of the location of the interface is based on at least some of the sensed signals.

In one aspect of the method, the determination of the location of the interface is based on automatic image processing of the morphological image.

In one aspect of the method, the position of the interface is determined based on a change in magnitude of image data of the medium between the intermediate region and the target region in the depth direction, the image data being determined based on the sensed signals and a beamforming algorithm having the reference sound speed as a parameter.

In one aspect of the method, the first sound speed and/or the target sound speed are each calculated using respective first and/or target focus criteria, a plurality of respective first and/or target focus values being obtained by applying the respective first and/or target focus criteria to the plurality of first image data of the first representative region and/or the plurality of target image data of the target representative region, respectively, the first sound speed being a selected one of the plurality of respective first focus values and/or the target sound speed being a selected one of the plurality of respective target focus values.

In one aspect of the method, the first speed of sound is a maximum of the plurality of respective first focus values and/or the target speed of sound is a maximum of the plurality of respective target focus values.

In one aspect of the method, the focus criteria is a coherence criteria.

In one aspect of the method, the method further comprises the steps of:

-determining the position of a sub-interface in the intermediate region, which sub-interface divides the intermediate region in the depth direction into a second region of the intermediate region close to the outer surface and a first region of the intermediate region close to the interface,

-determining a second sound velocity of the second region based on at least some of the sensed signals and taking a second reference sound velocity and one of a plurality of second assumed sound velocities as parameters,

wherein

The determination of the first speed of sound is based on at least some of the sensed signals and takes into account: a position of the sub-interface, the first reference sound speed applied to the first region, the second reference sound speed applied to the second region, and one of the plurality of first assumed sound speeds of the first region.

In one aspect of the method, the determination of the position of the sub-interface is based on at least some of the sensed signals.

In one aspect of the method, the second region of the intermediate region contains the second representative region and the first region of the intermediate region contains the first representative region.

In one aspect of the method, the method further comprises the steps of: prior to determining the second sound velocity of the second region based on the plurality of second image data, calculating a plurality of second image data associated with a second representative region of the second region, each of the second image data being determined based on a beamforming algorithm applied to at least the sensed signal corresponding to the second representative region, and the beamforming algorithm having as parameters a second reference sound velocity and one of a plurality of second assumed sound velocities.

In one aspect of the method, the medium is a mammalian body and the outer surface is the skin of the mammal, and the target region is the liver of the mammal, the intermediate region being a region of the medium comprised between the liver and the skin in the depth direction.

It is another object of the invention to provide an ultrasound imaging system for determining a target speed of sound within a target region of a medium, the ultrasound imaging system comprising:

-a probe adapted to sense backscattered waves and provide a corresponding sensed signal to the ultrasound system, an

-a processing unit configured to:

-determining a position of at least one interface in the medium on a morphological image, the interface dividing the medium in the depth direction into an intermediate region of the medium and the target region,

-determining a first speed of sound of the middle region based on at least some of the sensed signals, and

-determining the target speed of sound within the target region based on at least some of the sensed signals and taking into account the position of the interface and the first speed of sound.

In various embodiments of the system, one and/or other of the following features may optionally be incorporated.

In one aspect of the system, the probe is adapted to be in functional contact with an outer surface of the medium, the probe being adapted to transmit an excitation wave into the medium in a depth direction towards the target region, the excitation wave being backscattered in the medium towards the probe.

In one aspect of the system:

-the first sound speed is determined by taking into account a first reference sound speed and one of a plurality of first assumed sound speeds, and

-the target sound speed is determined by considering: a position of the interface, a target reference sound speed applied to the target region, the first sound speed applied to the intermediate region, and one of a plurality of assumed target sound speeds of the target region.

In one aspect of the system, prior to determining the first sound speed of the middle region, the system further calculates a plurality of first image data associated with a first representative region of the middle region, the first sound speed being based on the plurality of first image data, each of the first image data being determined based on a beamforming algorithm applied to at least the sensed signal corresponding to the first representative region, and the beamforming algorithm having as parameters a first reference sound speed and one of a plurality of first assumed sound speeds.

In one aspect of the system, prior to determining the target sound speed within the target region, the system further calculates a plurality of target image data associated with a target representative region of the target region, the target sound speed being based on the plurality of target image data in the representative region of the target region, each of the target image data being determined based on a beamforming algorithm applied to at least the sensed signal corresponding to the representative region, and the beamforming algorithm having as parameters: a position of the interface, the target reference sound speed applied to the target region, the first sound speed applied to the intermediate region, and one of the plurality of assumed target sound speeds of the target region.

In one aspect of the system, at least one of:

-the plurality of first assumed sound speeds and the plurality of assumed target sound speeds have the same value, an

-the first assumed sound speed is a known sound speed of the middle region, and the assumed target sound speed is a known sound speed of the target region.

In one aspect of the system, the determination of the location of the interface is based on at least some of the sensed signals.

In one aspect of the system, the determination of the location of the interface is based on automatic image processing of the morphological image.

In one aspect of the system, the position of the interface is determined based on a change in magnitude of image data of the medium between the intermediate region and the target region in the depth direction, the image data being determined based on the sensed signals and a beamforming algorithm having the reference sound speed as a parameter.

In one aspect of the system, the first sound speed and/or the target sound speed are each calculated using respective first and/or target focus criteria, a plurality of respective first and/or target focus values being obtained by applying the respective first and/or target focus criteria to the plurality of first image data of the first representative region and/or the plurality of target image data of the target representative region, respectively, the first sound speed being a selected one of the plurality of respective first focus values and/or the target sound speed being a selected one of the plurality of respective target focus values.

In one aspect of the system, the first speed of sound is a maximum of the plurality of respective first focus values and/or the target speed of sound is a maximum of the plurality of respective target focus values.

In one aspect of the system, the focus criteria is a coherence criteria.

In one aspect of the system, the system further:

-determining the position of a sub-interface in the intermediate region, which sub-interface divides the intermediate region in the depth direction into a second region of the intermediate region close to the outer surface and a first region of the intermediate region close to the interface,

-determining a second sound velocity of the second region based on at least some of the sensed signals and taking a second reference sound velocity and one of a plurality of second assumed sound velocities as parameters,

wherein

The determination of the first speed of sound is based on at least some of the sensed signals and takes into account: a position of the sub-interface, the first reference sound speed applied to the first region, the second reference sound speed applied to the second region, and one of the plurality of first assumed sound speeds of the first region.

In one aspect of the system, the determination of the position of the sub-interface is based on at least some of the sensed signals.

In one aspect of the system, the second region of the intermediate region contains the second representative region and the first region of the intermediate region contains the first representative region.

In one aspect of the system, prior to determining the second acoustic velocity of the second region based on the plurality of second image data, the system further calculates a plurality of second image data associated with a second representative region of the second region, each of the second image data is determined based on a beamforming algorithm applied to at least the sensed signal corresponding to the second representative region, and the beamforming algorithm takes as a parameter a second reference acoustic velocity and one of a plurality of second assumed acoustic velocities.

In one aspect of the system, the medium is a mammalian body and the outer surface is the skin of the mammal, and the target region is the liver of the mammal, the intermediate region being a region of the medium comprised between the liver and the skin in the depth direction.

Drawings

Further features and advantages of the invention will become apparent from the following detailed description of embodiments of the invention, given by way of non-limiting example, with reference to the accompanying drawings. In the drawings:

FIG. 1 is a schematic diagram of a system for determining the speed of sound in a target region of a medium, showing a schematic diagram of the medium divided into two regions;

FIG. 2 is an example of a visual image 25 of a medium obtained using the system of FIG. 1, revealing an intermediate region and a target region;

FIG. 3 is a flow chart of a method for determining the speed of sound in a target region of a medium;

FIG. 4 is an example of a graph of coherence values versus speed of sound in the middle region of the medium;

FIG. 5 is an example of a graph of coherence values versus speed of sound in a target region of the medium;

FIG. 6 is a schematic view of the medium of FIG. 1 shown as being divided into three regions; and is

FIG. 7 is an example of a spatial autocorrelation calculated to estimate the resolution of a region in image data.

Detailed Description

FIG. 1 shows a schematic view of aIs a schematic illustration of an ultrasound imaging system 10 for determining a medium 14 (referred to herein as the target region 12), referred to herein as the target sound speed cT. The schematic diagram of fig. 1 is not drawn to scale and is merely provided as a presentation system 10 and demonstrates the functionality thereof.

The medium 14 may be part of any part of the mammalian body, such as a mouse or a human. In one embodiment, the medium 14 is an abdominal portion of a person containing a liver. In another embodiment, the medium 14 is a portion of a human breast. In another embodiment, the medium 14 is a portion of a human brain.

Target region 12 is any interior region of media 14 that is below the surface of media 14, i.e., away from the surface of media 14. In one embodiment, the target region 12 is a portion of the liver contained in the abdominal portion.

The system 10 may be used to determine various characteristics of the medium 14, and in particular to determine the speed of sound in the target region 12.

The system 10 comprises a processing unit 16, which in this example is shown embedded within a computer system 18, and a probe 20 for sending and receiving excitation waves 21 to and from the medium 14 (only a few schematic excitation waves 21 are shown in fig. 1). The processing unit 16 is functionally associated with the probe 20, controlling the probe 20, and also receiving information collected by the probe 20.

Computer system 18 may include an interface, such as a keyboard 22, for inputting instructions to processing unit 16. Such as the shape and intensity of the excitation wave 21. It is contemplated that the keyboard 22 may be replaced by a touch screen keyboard and may be wirelessly associated with the processing unit 16. Computer system 18 may also include a display 24 for visualizing an image 25 (referred to herein as "visual image 25") of medium 14, the image corresponding to the data collected by probe 20. An example of such a visual image 25 is shown in fig. 2. The display 24 may be wirelessly connected to the computer system 18. It is contemplated that the keypad 22, display 24 and/or probe 20 may be physically remote from each other and from the processing unit 16. They may be located, for example, in different rooms or locations, allowing a person remote from the probe 20 to analyze or not analyze the data collected by the probe 20 in real time.

Probe 20 is an element of system 10 that is capable of sending and receiving excitation waves in the form of ultrasound waves to and from medium 14 so that processing unit 16 can at least process them into a visual image 25 representative of medium 14. This type of imaging is commonly referred to as echography. As will be described below, the processing unit 16 may further utilize data corresponding to the waves received from the medium 14 to determine the speed of sound c in the target region 12.

In operation, as shown in FIG. 1, the probe 20 is functionally in contact with the outer surface 26 of the medium 14 (i.e., directly or indirectly through the ultrasound gel). If the medium 14 is an abdominal portion of a human body, the outer surface 26 is, for example, skin.

The probe 20 comprises a plurality of transducers 28 adjacent to each other which transmit each excitation wave 21 (acoustic or ultrasonic) into the medium 14 in the depth direction D towards the target region 12. Typically, the depth direction D is substantially perpendicular to the emitting surface 32 of the probe 20. The emitting surface 32 extends in a surface direction D1 that substantially corresponds to the spatial distribution of the transducers 28. The transducers 28 may be aligned to form a row of transducers in the surface direction D1 or may form a planar 2D surface of transducers. Target region 12 may be deep within media 14 such that acoustic waves 21 first pass through outer surface 26 and intermediate region 30 of media 14 before reaching target region 12. For example, if the medium 14 is an abdominal portion of a human body and the target region 12 is a liver, the intermediate region 30 may be composed of muscle, connective tissue, and/or some fat. The intermediate region 30 affects the propagation of waves in the medium 14 and, therefore, any calculation of the speed of sound based on the sensed signals from these waves.

The transducer 28 is adapted to transmit any type of wave into the medium 14, i.e., focused or unfocused, such as convergent, divergent or plane or quasi-plane waves. In one embodiment, transducer 28 emits a series of plane waves having a penetration angle α with respect to surface direction D1. To generate plane waves at the penetration angle α, the transducers 28 are actuated with a time lag relative to each other.

The transducer 28 is also adapted to receive waves transmitted by the medium 14. These waves are referred to as backscattered waves 29. When the excitation wave 21 is transmitted into the medium 14, a portion of the backscattered wave will be emitted back from the medium 14 towards the probe 20 when encountering a reflective element in the medium. The reflective elements may be particles that are heterogeneous as a medium. For example, the particles may be collagen particles contained in any mammalian tissue. These reflective elements produce points in the visual image 25 that are designated as "speckles". The intensity of these backscattered waves 29 may thus be used as a way to distinguish various tissues within medium 14 to form visual image 25 of medium 14.

Reference is now made more specifically toFIG. 2An example of a visual image 25 determined from data collected by probe 20 (i.e., backscattered waves 29) is provided. Visual image 25 shows media 14 having one or more non-uniformities. The medium 14 may be divided as follows: a first quasi-homogeneous region of the medium 14 (the liver in this example) corresponding to the target region 12; and a second heterogeneous portion of the target region 12 that is in turn different from that corresponding to the intermediate region 30. An interface 34 may be identified between the target zone 12 and the intermediate zone 30. The interface 34 corresponds to a transition between tissues of different characteristics, such as composition or density. As will be discussed below, the position of the interface 34 will be considered in the calculation of the sound speed c in the target region 12 (hereinafter referred to as "target sound speed cT"). Since the excitation wave 21 reaches the target region 12 (e.g. the liver) only after passing through the intermediate region 30 (e.g. connective and muscular tissue), the calculation of the speed of sound in the target region 12 may be altered by the presence of the intermediate region 30.

Interface 34 divides medium 14 into target zone 12 and intermediate zone 30. The target area 12 and the intermediate area 30 may have different sizes. For example, the intermediate region 30 may be shallow compared to the target region 30.

Reference is now made toFIG. 3A method 40 for determining the target sound speed cT in the target region 12 according to the first embodiment will be described. The method 40 uses the ultrasound imaging system 10. The probe 20 is in contact with an outer surface 26 of the medium 14 (e.g., skin) so as to be directed toward the target area in the depth direction DThe field 12 (e.g. the liver) transmits an excitation wave 21 into the medium 14. The excitation wave 21 is backscattered in the medium 14 towards the probe 20. The probe 20 senses the backscattered waves 29 and provides a sensed signal corresponding to the backscattered waves 29 to the processing unit 16 of the ultrasound system 10. The sensed signal is a time-dependent signal commonly referred to as a "raw signal" or "RF signal". These signals are not beamformed signals. The signal is processed by the processing unit 16 to obtain information about the medium 14.

The image data is a set of values obtained from the sensed signals that provide an indication of the resistance to backscatter excitation waves 21 for each spatial position of the region of the medium 14 covered by the transducer 26. In one example, the image data is used to form a visual image 25 of the medium 14. The image data is an image having preferably more than one pixel value and for example more than ten pixel values. The image data may be a matrix of pixel values if the image data corresponds to a physical rectangular area in the medium. The matrix may have a size of 3 x 3 pixels or more.

The sensed signals are processed into image data (i.e., a set of values) by a beamforming algorithm. There are various known beamforming algorithms. Although only one is described herein, the method may be adapted to other beamforming algorithms.

In the beamforming algorithm, the time lag between the transmission of the excitation wave 21 by each transducer 26 is compensated. When the excitation wave 21 is transmitted in a medium which is considered to be homogeneous, the speed of sound in said medium is usually taken into account. Typically, a known, non-measured standard sound speed is selected as the parameter. Thus, in a non-homogeneous medium, the known non-measured standard sound speed is only an approximation of the true sound speed in the medium.

The method described herein proposes to compensate for the inhomogeneity of the medium in order to more accurately approximate the speed of sound of the target region 12. It is indeed desirable to estimate the speed of sound accurately, especially in the liver example, because the difference between normal and diseased liver speeds of sound is very small.

An example of beamforming calculations in the case of a plane wave to be transmitted into a homogeneous medium is given below. To transmit a plane wave in a medium, the following delays will be applied to the transducer in the probe:

d(i)＝i.p

wherein

i is the index of the transducers in the probe, ranging from 1 to N, N is the number of transducers, and

p is a predetermined length of time in seconds.

Such delays apply to the transmission of ultrasonic pulses in a medium by a transducer.

Then, the plane wave is inclined at a penetration angle α with respect to the surface direction D1, and:

sin(α)＝p.c0/Δx，

wherein

c0 is a constant reference speed of sound in the medium,

Δ x is the spatial separation of the transducers in the probe (linear probe array).

Thus, the penetration angle α can be recalculated for any speed of sound.

The time required for the emitted excitation wave 21 to reach a location in the medium 14 is then referred to as the "forward delay D_forward"and is defined as follows:

D_forward＝1/c0.[x.sin(α(c0))+z.cos(α(c0))](equation 1)

Wherein:

α is a penetration angle α with respect to the surface direction D1 of the plane excitation wave 21

x, z are the truncations of the position in the direction D1 and the depth direction D, and

c0 is a constant reference speed of sound in the medium, and

sin (), cos () are sine (sine) and cosine (cosine) trigonometric functions.

The time required for the backscattered wave 29 to travel back to a given transducer at position xp is referred to as the "backward delay D_backward"and is defined as follows:

D_backward＝1/c0.sqrt((x-xp)²+z²) (equation 2)

Wherein:

sqrt () is a square root mathematical function.

The total travel time D of the guided plane wave to reach the location and travel back is therefore:

D＝D_forward+D_backward(equation 3)

A simplified version of the beamforming algorithm is to first sum the sensed signals from all transducers indexed by i and phase control by the total travel time:

wherein

b (i) is a transducer weighting function of the sensed signals, and

S_i(t) is the sensed signal received by the transducer indexed i corresponding to position xp.

Thereafter, the beamforming algorithm calculates the coherent signal S for a time window_coherentTo derive a pixel value having coordinates (x, z) in the image data.

The method 40 for determining the target sound speed cT includes the step of beamforming the image data on each of the intermediate region 30 and the target region 12 to obtain image data for calculating the local sound speeds in the intermediate region 30 and the target region 12. The beamforming algorithm for the target region 12 uses the speed of sound previously determined using the beamforming algorithm for the middle region 30.

The method 40 includes at least the following steps:

-at step S1And realizing virtual division. The position of an interface 34 between the intermediate region 30 of the medium 14 (e.g., muscle and connective tissue) and the target region 12 (e.g., liver) in the depth direction D is determined on the morphological image of the medium. This determination will be used to apply different sound velocities in the beamforming algorithm to different identified regions.

The determination of the location of interface 34 may be based on a morphological image of medium 14. The morphological image may be:

some sensed signal received by the probe 20, or

Image data obtained by processing the sensed signals by a beamforming algorithm, such as a B-mode image, or

Recorded images previously stored in the ultrasound imaging system, such as ultrasound images previously recorded by the system or from another system, or Magnetic Resonance Images (MRI) previously recorded from the magnetic resonance imaging system, or

Any other image corresponding to the medium 14.

The determination of the location of the interface 34 may be based on some of the sensed images. A beamforming algorithm may be used to generally obtain image data of medium 14. These image data may then be processed to obtain the visual image 25 shown in fig. 2. The image data is used to determine the location of the interface 34. An example of determining the interface 34 using image data will be provided below.

At step S2A plurality of first image data associated with the first representative region 42 of the intermediate region 30 is calculated, and the first sound speed c1 of the intermediate region 30 is determined based on the plurality of first image data. In one embodiment, the proximity interface 34 selects the first representation area 42. The first representative region 42 is typically a sub-region of the intermediate region 30. Taking the first representative region 42 as a representative of the middle region 30, as for the sound speed determined using the plurality of first image data, it is considered as the sound speed of the middle region 30, referred to herein as a first sound speed c 1. Each of the first image data is determined based on a beamforming algorithm applied to at least the sensed signals corresponding to the first representative region 42, and the beamforming algorithm has as parameters a first reference sound speed c _ ref _1 and one of a plurality of first assumed sound speeds c _ supp _1.

The first reference sound speed c _ ref _1 is, for example, a known sound speed of the middle region 30.

For the calculation to be performed, the determined value of the first speed of sound c1 will be used when the speed of sound in the middle region 30 is used as a parameter.

-In step S3 position(s)A plurality of target image data associated with the target representative region 44 of the target region 12 are calculated, and a target sound speed cT within the target region 12 is determined based on the plurality of target image data. The target representative region 44 may be selected in a manner similar to that described above for the first representative region 42. Each of the target image data is determined based on a beamforming algorithm (which may be the same as the beamforming algorithm used above or another beamforming algorithm) applied at least to the sensed signals corresponding to the target representation area 44, and which has as parameters: the position of the interface 34, the second reference sound speed c _ ref _2 applied to the target region 12, the first sound speed c1 applied to the middle region 30, and one of the second assumed sound speeds c _ supp _2 of the target region 12.

Means for determining the position of the interface

In one embodiment, the location of the interface 34 is found by analyzing the gradient of the image data in the depth direction D. Since the density of the intermediate region 30 is substantially different from the density of one of the target regions 12 (e.g. adipose tissue versus liver), by analyzing the change in image data values in the depth direction D (and optionally in the surface direction D1), it may be possible to preferably automatically determine the transition from the intermediate region 30 to the target region 12.

Thus, in one embodiment, the location of interface 34 is determined based on a change in the magnitude of the image data of medium 14 along depth direction D between intermediate region 30 and target region 12. For example, the variation of the amplitude of the image data of the medium 14 in the depth direction D may be fitted by any optimization algorithm to a parametric curve defining a known evolution in such a medium. The parametric curve includes several parameters, including, for example, the location of the interface 34 and other parameters.

The image data may be in the form of a 2D matrix and is determined based on the sensed signals and a beamforming algorithm that has the reference speed of sound c0 as a parameter. The reference sound speed c0 may correspond to a typical value of the sound speed of the medium 14, i.e., the overall reference sound speed c _ ref _ 0.

In one embodiment, the location of the interface 34 is determined by automatic image processing of the morphological image or any 2D image data corresponding to the medium.

The automatic image processing may use shape recognition algorithms to determine the location of the interface 34.

Automatic image processing may use conventional contour detection filters to determine the location of the interface 34. Examples of such filters are Canny or Gabor filters.

Other ways for determining the location of the interface 34 are contemplated. For example, image data is processed by a neural network trained with a plurality of classified image data of media-like, indexed by physical characteristics of the media, and containing the location of interfaces in each of the classified image data. The above-described process may be a deep learning method based on hundreds or thousands of classified image data.

In another embodiment, the location of the interface 34 may be arbitrarily selected. Any selection of the location of the interface 34 may be made for a known value of the depth of the intermediate region 30. For example, it is standard that a layer of skin and fat cells is 2cm thick in a human body. In this case, the position of the interface 34 will be chosen to be at a distance of 2cm in the depth direction D relative to the outer surface 26.

In yet another embodiment, the location of the interface is selected such that the ballistic echo (ballistic echo) between the two regions separated by the interface 34 is strong. In other words, at the time t when the sensed signal or the phased sensed signal (beamformed signal) is maximum_iThe location of the interface 34 is determined.

The position of the interface may be only the coordinates in the depth direction D, or the coordinates in the depth direction D and the surface direction D1.

According to the above method, the position of the interface 34 is preferably automatically determined in at least one of the image data. Finally, several image data (image data acquired at various times) are used, and the optimal position is determined using a plurality of determined positions, for example, an average of the successively determined positions through the interface 34.

Selection of a first representation area

More specifically, referring to step S2, the first representative area 42 is automatically selected by the processing unit 16 following a representative area selection algorithm. The first representative region 42 may be selected as a fixed portion of the intermediate region 30. For example, the first representative region 42 may be selected as a rectangle that is 1cm wide in the depth direction D and 2cm long in the surface direction D1.

Considering that the abdominal part of the human body has muscle and connective tissue, which is typically between 1cm and 3cm thick in the depth direction D, it may be advantageous to program the processing unit 16 to treat the first representation area 42 as a rectangle of the visual image 25 positioned, for example, 2cm in the depth direction and halfway the image in the surface direction D1. In other embodiments, the representative region 42 may be selected for the image data collectively. For example, a quasi-homogeneous region of the intermediate region 30 may be selected.

Advantageously, the automatic selection allows the same result to be obtained for the same data input. Additionally, once the patient data is collected, the above steps may be performed at a distance and/or time from the data collection without requiring his or her presence to perform additional scans.

Obtaining first image data

The determination of the first image data is based on at least some of the sensed signals and a beamforming algorithm (such as the beamforming algorithm described above), and the algorithm has the first reference speed of sound c _ ref _1 as a parameter. The first reference sound speed c _ ref _1, which may be derived from a memory of the processing unit 16 and may correspond to a classical value of the sound speed in the medium 14, is a constant parameter. This value is independent of the sensed signal.

For example, the beamforming calculation in the case of a plane wave propagating into a homogeneous medium in the first representation area 42 in the middle area 30 may be calculated by the following formula:

D_forward＝(1/c1).[x.sin(α(c1))+z.cos(α(c1))](equation 5)

D_backward＝(1/c1).sqrt((x-xp)²+z²) (equation 6)

D＝D_forward+D_backward(equation 7)

Wherein:

D_forwardin the general case a forward delay as explained above,

D_backwardin the general case, a backward delay as explained above, and

c1 is the speed of sound to be determined in the medium of the first representative zone 42 in the middle area 30,

these equations are corrected by the following equations:

z-h (c _ ref _1), c1/c _ ref _1 (equation 8)

α (c1) ═ asin (c1/c _ ref _1 α (c _ ref _1)) (equation 9)

Wherein:

asin () is an arcsine (arcsinus) trigonometric function, and

h (c _ ref _1) is a depth reference of the first representative region 42.

These corrections are to compensate for the change in the speed of sound among the first assumed speed of sound c _ supp _1 so that the first representative zone 42 does not move in the intermediate region 30 and so that the excitation wave as a plane wave has the same penetration angle α with respect to the surface direction D1. Due to these corrections, the generated first image data can be compared with each other and processed as explained to calculate the first sound speed c 1.

The beamforming algorithm then applies the sensed signals S from all the transducers indexed i_iSumming, and phasing by the total travel time:

and calculating a coherent signal S for a time window_coherentTo derive a first number of imagesFrom which the pixel value with coordinates (x, z).

The first reference speed of sound c _ ref _1 may correspond to a known value of the speed of sound in the entire medium 14. The first reference speed of sound may be equal to the reference speed of sound c 0. The first reference sound speed c _ ref _1 may correspond to a known value of the sound speed only in the middle region 30. For example, the speed of sound in adipose tissue is known to be about 1480 m/s. The first reference sound speed c _ ref _1 may alternatively correspond to a known value of the sound speed only in the region of interest 12. It is known that the speed of sound in the liver may be, for example, about 1540 m/s.

The first plurality of assumed speeds of sound c _ supp _1, which form a range of possible reference speeds of sound, may be various known values of speed of sound in the medium 14 (e.g., the entire medium 14 or the intermediate region 30 or the target region 12). Such asFIG. 4The possible reference sound speed range may be a range between 1300m/s and 1700m/s, shown on the abscissa of (a). The first reference sound speed c _ ref _1 may be one of a plurality of first assumed sound speeds c _ supp _1.

Calculation of the first speed of sound

The first speed of sound c1 is calculated using a first focusing criterion. The plurality of first focus values is obtained by applying a first focus criterion to each of the plurality of first image data of the first representative region 42. The first speed of sound c1 is then the selected first focus value (i.e., the speed of sound corresponding to the selected first focus value) of the plurality of respective first focus values. In one embodiment, the first speed of sound c1 is the maximum of a plurality of corresponding first focus values.

If the first focusing criterion contains a coherence criterion, the first sound speed c1 will be selected as the maximum of a plurality of values obtained using the first coherence criterion applied to the plurality of first image data, i.e., the first sound speed c _ supp _1 for each hypothesis. Fig. 4 shows an example of a coherence value obtained using a plurality of first sound speeds c _ supp _1 (the coherence value is each point of the graph, a value on the ordinate axis and a corresponding assumed sound speed for obtaining the coherence value on the abscissa axis). The first sound speed c1 is selected as the sound speed corresponding to the maximum value among the calculated coherence values. In the example of fig. 4, the maximum value is the highest point of the bell-shaped graph, which is about 2.47 on the vertical axis, and which corresponds to the speed of sound 1460 m/s.

In another embodiment, the focus criteria comprises a resolution criteria such as maximum lateral resolution. The resolution of the ultrasound image is improved when the beamforming focusing delay accurately compensates for the flight propagation time. Therefore, the estimation using the resolution in the ultrasound image (image data) can be used as a focusing criterion for estimating the sound speed in the medium.

In a first example, a fourier transform is used on image data corresponding to vertical or lateral lines (i.e., lines with a constant z-coordinate) of the first image data. The first focusing criterion may then be a frequency bandwidth of frequency values obtained by fourier transformation. Similarly, the first sound speed c1 will be selected as the maximum value among a plurality of values obtained using the first focusing criterion applied to the plurality of first image data, i.e., the first sound speed c _ supp _1 for each hypothesis. In other words, the first sound speed c1 is selected as a sound speed that provides a larger frequency bandwidth to the frequency value obtained by fourier transform. This corresponds to having the best (maximum) resolution among the plurality of first image data.

In a second example, the resolution criterion is estimated by calculating a spatial autocorrelation of the image data and by estimating a magnitude of the spatial autocorrelation.

Fig. 7 shows an example of spatial autocorrelation of two-dimensional (2D) image data. This spatial autocorrelation is also a 2D surface with a peak in the center (i.e., coordinates (0,0) in the axial and lateral lags). In this figure, the spatial autocorrelation is represented as a plurality of horizontal iso-curves.

The peak width can then be derived in each of the directions, inferentially in each of the axial and transverse directions. The width may be defined as the width of the peak-shaped curve (i.e., the width at-6 dB) with a height of half the peak maximum. The magnitude of the spatial autocorrelation is then a function of the width in each direction, and may be, for example, the maximum or average of the width, or determined by any other modulo function.

The focus criteria may comprise more than one criteria. For example, the focus criteria may include a coherence criterion and a maximum resolution criterion.

Other models of the focus criteria are contemplated.

Selection of target representation area

More specifically, referring to step S3, the processing unit 16 automatically selects the target representative area 44 in a manner similar to that used for the first representative area, but applied to the target area 12. The target representative area 44 may be selected as a fixed portion of the target area 12. For example, the target representing region 44 may be selected as a rectangle that is 1cm wide in the depth direction D and 2cm long in the surface direction D1. The size and shape of the target representative area 44 may be different from or the same as the size and shape of the first representative area 42.

The target representation area 44 may also be selected to be at a fixed location of the visual image 25. Considering that the abdominal part of the human body has muscle and connective tissue, which is typically between 1cm and 3cm thick in the depth direction D, it may be advantageous for the human being to program the processing unit 16 to treat the target representation area 44 as a rectangle of the visual image 25 positioned, for example, at 7cm in the depth direction and halfway the image in the surface direction D1. In other embodiments, the target representative region 44 may be selected collectively for the image data. For example, a quasi-homogeneous region of the target region 12 may be considered.

Obtaining target image data

The determination of target image data associated with the target representation area 44 is based on at least some of the sensed signals and a beamforming algorithm as described above, and having as parameters: a target reference sound speed c _ ref _ tar, the position of the interface 34, a first sound speed c1, and a plurality of assumed target sound speeds c _ supp _ tar.

The target reference sound speed c _ ref _ tar is, for example, a known sound speed for the target region 12.

The target reference sound speed c _ ref _ tar, which may be derived from the memory of the processing unit 16 and which may correspond to a classical value of the sound speed in the medium 14, is a constant parameter. This value is independent of the sensed signal.

For example, the beamforming calculation in the case of a plane wave propagating into a homogeneous medium in the target representation area 44 in the target region 12 may be calculated by the following formula:

D_forward＝(1/c1).[h1/cos(α(c1))]+

(1/c) [ (x-h1.tan (α (c1)). sin (α (c1))) + (z-h1). cos (α (c)) ] (equation 11)

D_backward＝min_xi[(1/c1).sqrt((xi-xp)2+h2)+

(1/c2).sqrt((x-xi)²+(z-h1)²)](equation 12)

D＝D_forward+D_backward(equation 13)

Wherein:

D_forwardis the forward delay of the time that the delay is,

D_backwardit is the backward delay that is the delay of the frame,

h1 is the thickness of the intermediate region 30 (i.e., the location of the interface 34),

c1 is the speed of sound previously determined in the middle region 30,

c is the speed of sound to be determined in the medium of the target representative zone 44 in the target zone 12,

these equations are corrected by the following equations:

z _ ref.c/c _ ref _2+ h1.(1-c/c _ ref _2) (equation 14)

Wherein

z _ ref is a depth reference for the target zone 44.

This correction is intended to compensate for a change in the sound speed in the assumed target sound speed c _ supp _ tar so that the target representative zone 44 does not move in the target region 12. Due to this correction, the generated target image data can be compared with each other and processed as explained above to calculate the target sound speed c.

The beamforming algorithm then applies the sensed signals S from all the transducers indexed i_iSumming, and phasing by the total travel time:

and calculating a coherent signal S for a time window_coherentTo derive a pixel value having coordinates (x, z) in the target image data.

The target reference speed of sound c _ ref _ tar may correspond to a known value of the average speed of sound in the entire medium 14 considered to be homogeneous. The target reference sound speed may be set equal to the reference sound speed c0 and/or the first reference sound speed c _ ref _1. The target reference sound speed c _ ref _ tar may correspond to a known value of the sound speed only in the middle region 30. For example, the speed of sound in adipose tissue is known to be about 1480 m/s. The target reference sound speed c _ ref _ tar may alternatively correspond to a known value of the sound speed only in the region of interest 12. For example, the speed of sound in the liver is known to be about 1540 m/s.

A plurality of assumed target sound speeds c _ ref _ tar, which form a possible reference sound speed range, are various known values of sound speeds (e.g., the entire medium 14, the middle region 30, or the target region 12). The plurality of assumed target sound speeds c _ supp _ tar may be the same as the plurality of first assumed sound speeds c _ supp _1. The target reference sound speed c _ ref _ tar may be one of a plurality of assumed target sound speeds c _ supp _ tar.

Calculation of target sound velocity

Similarly to the first sound speed c1, the target sound speed cT is calculated using the focusing criterion. As such, the detailed description of this step is very similar to that of the first step. The plurality of second focus values are obtained by applying a target focus criterion to each of the plurality of target image data of the target representative region 44. The target focus criteria may be the same as or different from the first focus criteria. As described above, the focus criteria may include a coherence criterion and/or a maximum lateral resolution criterion.

Reference toFIG. 5An example of a map of coherence values obtained for different assumed speeds of sound using target image data is provided.

Each point of the graph corresponds to a coherence value (ordinate axis) obtained using one of a plurality of assumed sound speeds (abscissa axis) in the beamforming algorithm to obtain first image data. The target sound speed cT is selected as the sound speed corresponding to the maximum value among the calculated plurality of coherence values. In the example of fig. 5, the maximum value is the highest point of the bell-shaped diagram, which maximum value is about 0.55 on the ordinate axis and which maximum value corresponds to the speed of sound 1585 m/s.

Aberration-adding constant correction

Can be controlled by taking into account the aberration delay D_aberrationTo improve the beamforming algorithm. These aberration delays D_aberrationIs any time of flight that is not due to wave propagation in the medium and is not taken into account by the propagation model, and therefore, if a homogeneous model is selected, it cannot be compensated for by adjusting parameters of the model such as the overall speed of sound. These aberration delays can compensate for delays in each physical transducer, delays in electronics for signal amplification, and delays in material layers in the probe, which may be slightly different from each other.

The formula for the total travel time D for beamforming may then be modified to:

D＝D_forward+D_backward+D_aberration，

this may improve the phasing of the sensed signals in the beamforming summation of equations 4, 10 and 15.

Examples of such improvements are given in literature "Robust sound velocity estimation for ultrasound-based assessment of hepatic steatosis" (Robust sound velocity estimation for ultrasound-based hepatic steatosis assessment "), Marion impedance et al. However, such aberration correction cannot correctly and accurately compensate for the propagation of the speed of sound in the inhomogeneous medium, i.e. change the speed of sound during wave propagation in the medium.

In contrast, the method disclosed herein determines at least a first speed of sound c1 and a target speed of sound cT. Thus, the method is constructed with_aberrationA plurality of sound speeds (at least two) of the medium whose amplitude is minimized, andand thus results in a more accurate estimate of the target sound speed cT, which may be very important in medical applications such as assessing hepatic steatosis or other diseases.

Case of three regions

Reference is now made toFIG. 6The above may be summarized as 3 or more stacked regions. The division in the finer region of the middle region 30 will improve the accuracy of the value calculated for the target sound speed cT.

In one embodiment, the middle region 30 may be divided into two regions. The sub-interface 48 divides the intermediate region 30 in the depth direction D into a second region 50 of the intermediate region 30 proximate the outer surface 26 and containing a second representative region 54, and a first region 52 of the intermediate region 30 proximate the interface 34 and containing the first representative region 42. The sub-interface 48 may, for example, correspond to an interface between a skin layer and a muscle layer.

Then, the target sound speed cT may be determined based on the first sound speed c1 as previously described, but the first sound speed c1 will be determined based on the second sound speed c2 of the second region 50. Therefore, the target sound speed cT in the medium divided into the plurality of stack regions is calculated by continuous determination of the adjacent stack regions from the region farthest from the target region 12 to the region nearest to the target region 12.

A method of determining the target sound speed cT using the division into three regions may be as follows:

-a step 10: the position of the sub-interface 48 in the intermediate region 30 and the position of the interface in the medium 14 are determined, the position of the sub-interface 48 and the position of the interface 34 being determined based on at least some of the sensed signals. The determination of the position of the interface 34 may be delayed until after step 11 and before step 12.

-a step 11: a plurality of second image data associated with the second representation area 54 of the second area 50 is calculated and a second sound velocity c2 of the second area 50 is determined based on the plurality of second image data. The second representative region 54 may be determined in a similar manner as the first representative region 42. Each of the second image data is determined based on a beamforming algorithm applied to at least the sensed signals corresponding to the second representative region 54, and the beamforming algorithm has as parameters the second reference sound speed c _ ref _2 and one of the plurality of second assumed sound speeds c _ supp _ 2. The second reference speed of sound c _ ref _2 may be a reference speed of a certain type of tissue found in the second region 50. For example, the second reference sound speed may be a standard sound speed in a skin type of tissue. The second assumed speed of sound c _ supp _2 may be the conventional expected speed of sound for that type of tissue.

-a step 12: a plurality of first image data associated with the first representative region 42 of the intermediate region 30 is calculated, and a first speed of sound c1 for the intermediate region 30 is determined based on the plurality of first image data. In one embodiment, the first representation area 42 is proximate to the interface 34 in the second region 52. Each of the first image data is determined based on a beamforming algorithm applied to at least the sensed signals corresponding to the first representation area 42, and the beamforming algorithm takes as parameters: the position of the sub-boundary 48, the second sound speed c2, the first reference sound speed c _ ref _1, and one of the first assumed sound speeds c _ supp _1. As previously used, the first reference speed of sound c _ ref _1 may be a reference speed of tissue of the type found throughout the first region 52 or the middle region 30. For example, the first reference sound speed may be a standard sound speed in a muscle. The first assumed speed of sound c _ supp _1 may be the conventional expected speed of sound for this type of tissue. The first reference sound speed c _ ref _1 and/or the first assumed sound speed c _ supp _1 may or may not be equal to the second reference sound speed c _ ref _2 and/or the second assumed sound speed c _ supp _2, respectively.

-step 13: a plurality of target image data associated with the target representative region 44 of the target region 12 are calculated, and a target sound speed cT within the target region 12 is determined based on the plurality of target image data in the representative region 44 of the target region 12. Each of the target image data is determined based on a beamforming algorithm applied to at least the sensed signals corresponding to the representative zone 44, and the beamforming algorithm takes as parameters: the position of the interface 34, the target reference sound speed c _ ref _ tar applied to the target region 12, the first sound speed c1 applied to the middle region 30, and one of the plurality of assumed target sound speeds c _ supp _ tar of the target region 12.

Will now be described in accordance withSecond embodimentFor determining a target sound speed cT in the target region 12. This embodiment does not use calculation of the image data in the representative region. Instead, the original sensed signal S is compared_iDirectly for a predetermined location in the medium in each representative zone. This embodiment can advantageously reduce the calculation for obtaining the first sound speed and the target sound speed.

Similar to the first embodiment of the method 40, the medium 14 is divided into the intermediate area 30 and the target area 12 using one of the approaches described above.

After segmenting the medium 14, in a first step, the first speed of sound c1 for the middle region 30 is determined directly based on at least some of the sensed signals. The determination of the first sound speed c1 takes into account the first reference sound speed c _ ref _1 and one of the plurality of first assumed sound speeds c _ supp _1.

The first focus criterion may be calculated at the following ratio C:

wherein

Wherein:

S_iis the sensed signal for the transducer of index i,

n is the number of transducers,

τ_iis the total travel time D for the wave to reach the predetermined focal position and travel back to the transducer, as defined above, and

brackets indicate time window averages.

Thus, the ratio C is the ratio between the coherent intensity and the total incoherent intensity, and it satisfies the following inequality:

0≤C≤1

this ratio C thus represents the quality of the focusing function applied to calculate the total travel time, and it can be used as a first focusing criterion. The ratio C is a maximum value (as represented in fig. 4) if the first assumed sound speed C _ supp _1 is related to the best focus function, i.e. if the first assumed sound speed C _ supp _1 is close to (ideally equal to) the first sound speed C1.

In a second step, the target sound speed cT is also determined directly in the target zone 12 based on at least some of the sensed signals. The determination of the target sound speed cT takes into account the following: the position of the interface 34, the target reference sound speed c _ ref _ tar applied to the target region 12, the first sound speed c1 applied to the middle region 30, and one of the plurality of assumed target sound speeds c _ supp _ tar of the target region 12.

The target focus criterion is calculated using the ratio C. This ratio C is now at the parameter τ_iThe interface 34 and the first speed of sound c1, the parameter τ_iIs the total travel time D for the wave to reach the predetermined focal position and travel back to the transducer. The ratio C is used as a target focusing criterion, and has a maximum value (as represented in fig. 5) if it is assumed that the target sound speed is close to (ideally equal to) the target sound speed cT.

A variation of the above-described second embodiment is now explained. In a second embodiment, the ratio C takes into account the sum of the sensed signal and the sensed signal time-averaged by a bracket < > mathematical function, and is calculated for a predetermined focus position (x, z) in the medium.

In this variant, the time average is replaced by an average over a plurality of focus positions. Then, the first focusing criterion is calculated by the following spatial ratio Cs:

wherein

S_iIs the sensed signal for the transducer of index i，

N is the number of transducers,

τ_i,Mjis the total travel time D for the wave to reach the focal position Mj and travel back to the transducer i, as defined above, and

mj is a position in the media among a plurality of focus positions.

Will now be described in accordance withThird embodimentFor determining a target sound speed cT in the target region 12. This embodiment does not use calculation of the image data in the representative region. Instead, the original sensed signal Si is directly applied to a predetermined focus position in the medium in each representative zone. This embodiment can also reduce the calculation for obtaining the first sound speed and the target sound speed.

Similar to the first embodiment of the method 40, the medium 14 is divided into the intermediate area 30 and the target area 12 using one of the approaches described above.

After segmenting the medium 14, in a first step, the first speed of sound c1 for the middle region 30 is determined directly based on at least some of the sensed signals. The determination of the first sound speed c1 takes into account the first reference sound speed c _ ref _1 and one of the plurality of first assumed sound speeds c _ supp _1.

The following spatial covariance R can be calculated by means of the following formula:

wherein

S_i(t) is the sensed signal for transducer i,

n is the index distance from transducer i,

n is the number of transducers,

t_fis the time of focus at the predetermined focus position,

τ_iis the total travel time D for the wave to reach the predetermined focal position and travel back to the transducer, as defined above, and

t is the length of the time window used for analysis.

For n-0, such a transducer spatial covariance is equal to 1 for the transducer distance n and decreases with increasing distance n. Thus, if the focus at the predetermined focus position is perfect: that is, if the delay time law or the estimation of the forward and backward flight times is correct, its decreasing curve is ideally a triangle with a maximum at n-0. This is the case when the speed of sound used in these times of flight is correct. Conversely, if an incorrect sound speed is used, the transducer spatial covariance decreases even more and its curve is located more and more below the ideal triangular shape.

Thus, the first focusing criterion may be estimated by an integration of the spatial covariance R over the distance index n and this integration is a maximum if the first assumed sound speed c _ supp _1 is related to the best focusing function, i.e. if the first assumed sound speed c _ supp _1 is close to (ideally equal to) the first sound speed c 1.

In a second step, the target sound speed cT is also determined directly in the target zone 12 based on at least some of the sensed signals. The determination of the target sound speed cT takes into account the following: the position of the interface 34, the target reference sound speed c _ ref _ tar applied to the target region 12, the first sound speed c1 applied to the middle region 30, and one of the plurality of assumed target sound speeds c _ supp _ tar of the target region 12.

The target focus criterion is calculated using the integral of the above-mentioned spatial covariance R over the distance index n. This calculation now takes into account the interface 34 and the first speed of sound c1 in the total travel time D for the parameter wave to reach the predetermined focal position and travel back to the transducer. This calculation is used as a target focusing criterion, and has a maximum value if it is assumed that the target sound speed is close to (ideally equal to) the target sound speed cT.

Thus, in all embodiments, the above-described method 40 for determining a target speed of sound within a target region of a medium uses an ultrasound imaging system. The method comprises the following steps: determining a location of an interface in the medium; determining a first speed of sound for a middle region above the interface and determining a target speed of sound within a target region below the interface based on at least some of the sensed signals and taking into account the position of the interface and the first speed of sound.

As described in all of the detailed description above, this method and the ultrasound system implementing the method allow for the construction of a heterogeneous model of the medium and accurate estimation of the target sound speed cT.

Claims (17)

1. A method (40) for determining a target speed of sound (cT) within a target region (12) of a medium (14) using an ultrasound imaging system (10), the ultrasound imaging system (10) comprising at least a probe (20), the probe (20) being adapted to sense backscattered waves and to provide sensed signals corresponding to the backscattered waves to a processing unit (16) of the ultrasound system (10), the method (40) comprising:

-determining the position of at least one interface (42) in the medium (14) on a morphological image, the interface (34) dividing the medium (14) in a depth direction (D) into a middle region (30) of the medium (14) and the target region (12),

-determining a first speed of sound (c1) of the middle region (30) based on at least some of the sensed signals, and

-determining the target sound speed (cT) within the target region (12) based on at least some of the sensed signals and taking into account the position of the interface (34) and the first sound speed (c 1).

2. The method according to claim 1, wherein the probe (20) is adapted to be in functional contact with an outer surface (32) of the medium (14), the probe (20) being adapted to transmit an excitation wave into the medium (14) in a depth direction (D) towards the target region (12), the excitation wave being backscattered in the medium (14) towards the probe (20), and

wherein the first sound speed (c1) is determined by taking into account a first reference sound speed (c _ ref _1) and one of a plurality of first assumed sound speeds (c _ supp _1), an

Wherein the target sound speed (cT) is determined by considering: a position of the interface (34), a target reference sound speed (c _ ref _ tar) applied to the target region (12), the first sound speed (c1) applied to the intermediate region (30), and one of a plurality of assumed target sound speeds (c _ supp _ tar) of the target region (12).

3. The method of claim 2, further comprising, prior to determining the first sound speed (c1) of the middle region (30), calculating a plurality of first image data associated with a first representative region (42) of the middle region (30), the first sound speed being based on the plurality of first image data, each of the first image data being determined based on a beamforming algorithm applied to at least the sensed signals corresponding to the first representative region (42), and the beamforming algorithm having as parameters a first reference sound speed (c _ ref _1) and one of a plurality of first assumed sound speeds (c _ supp _1).

4. The method of any of claims 2 or 3, further comprising, prior to determining the target sound speed (cT) within the target region (12), calculating a plurality of target image data associated with a target representative zone (44) of the target region (12), the target sound speed (cT) being based on the plurality of target image data in the representative zone (44) of the target region (12), each of the target image data being determined based on a beamforming algorithm applied to at least the sensed signals corresponding to the representative zone (44), and the beamforming algorithm having as parameters: a position of the interface (34), the target reference sound speed (c _ ref _ tar) applied to the target region (12), the first sound speed (c1) applied to the intermediate region (30), and one of the plurality of assumed target sound speeds (c _ supp _ tar) of the target region (12).

5. The method according to any of claims 1 to 4, characterized by at least one of the following:

-the plurality of first assumed sound speeds (c _ supp _1) and the plurality of assumed target sound speeds (c _ supp _ tar) have the same value, and

-the first assumed sound speed (c _ supp _1) is a known sound speed of the middle region (30) and the assumed target sound speed (c _ supp _ tar) is a known sound speed of the target region (12).

6. The method of any one of claims 1 to 5, wherein the determination of the position of the interface (34) is based on at least some of the sensed signals.

7. The method according to any one of claims 1 to 5, characterized in that the determination of the position of the interface (34) is based on automatic image processing of the morphological image.

8. The method according to any one of claims 1 to 6, characterized in that the position of the interface (34) is determined based on a variation in the magnitude of image data of the medium (14) in the depth direction (D) between the intermediate region (30) and the target region (12), the image data being determined based on the sensed signals and a beamforming algorithm having the reference sound speed (c0) as a parameter.

9. The method of any of claims 1 to 8, wherein the first sound speed (c1) and/or the target sound speed (cT) are each calculated using a respective first and/or target focusing criterion, a plurality of respective first and/or target focus values being obtained by applying the respective first and/or target focusing criterion to the plurality of first image data of the first representative region (42) and/or the plurality of target image data of the target representative region (44), respectively, the first sound speed (c1) being a selected one of the plurality of respective first focus values and/or the target sound speed (cT) being a selected one of the plurality of respective target focus values.

10. The method of claim 9, wherein the first sound speed (c1) is a maximum of the plurality of respective first focus values and/or the target sound speed (cT) is a maximum of the plurality of respective target focus values.

11. The method of claim 9 or 10, wherein the focusing criterion is a coherence criterion.

12. The method of any one of claims 1-11, further comprising:

-determining the position of a sub-interface in the intermediate region (30), which sub-interface divides the intermediate region (30) in the depth direction into a second region of the intermediate region close to the outer surface and a first region of the intermediate region close to the interface (34),

-determining a second sound velocity (c2) of the second region based on at least some of the sensed signals and having as parameters a second reference sound velocity (c _ ref _2) and one of a plurality of second assumed sound velocities (c _ supp _2),

wherein the content of the first and second substances,

the determination of the first speed of sound (c1) is based on at least some of the sensed signals and takes into account: a position of the sub-interface, the first reference sound speed (c _ ref _1) applied to the first region, the second reference sound speed (c _ ref _2) applied to the second region, and one of the plurality of first assumed sound speeds (c _ supp _1) of the first region.

13. The method of claim 12, wherein the determination of the location of the sub-interface is based on at least some of the sensed signals.

14. The method according to any one of claims 1 to 13, wherein the second region of the intermediate region contains the second representative region and the first region of the intermediate region contains the first representative region.

15. The method of any of claims 1 to 14, further comprising, prior to determining the second sound velocity (c2) of the second region based on the plurality of second image data, calculating a plurality of second image data associated with a second representative region of the second region, each of the second image data being determined based on a beamforming algorithm applied to at least the sensed signal corresponding to the second representative region, and the beamforming algorithm having a second reference sound velocity (c _ ref _2) and one of a plurality of second assumed sound velocities (c _ supp _2) as sound velocity parameters.

16. The method of any one of claims 1 to 15, wherein the medium (14) is a mammalian body and the outer surface (26) is the skin of the mammal, and

wherein the target region (12) is a liver of the mammal and the intermediate region (30) is a region of the medium (14) comprised between the liver and the skin in the depth direction (D).

17. An ultrasound imaging system (10) for determining a target speed of sound (cT) within a target region (12) of a medium (14), the ultrasound imaging system (10) comprising:

-a probe (20) adapted to be in functional contact with an outer surface (26) of the medium (14), the probe (20) being adapted to transmit an excitation wave (21) into the medium (14) in a depth direction (D) towards the target region (12), the excitation wave (21) being backscattered in the medium (14) towards the probe (20), the probe (20) being adapted to sense the backscattered wave (29) and to provide a corresponding sensed signal to the ultrasound system (10), and

-a processing unit (16) implementing the method according to any one of claims 1 to 16.