NL1006229C2 - Imaging system with numerical residual focusing. - Google Patents

Description

Imaging system with numerical residual focusing.

The present invention relates to a device for creating an image of an object, comprising: 5 - transmission means for generating and transmitting a transmission beam with a determined transmission beam property to a medium; receiving means for receiving within a transmission beam with a certain reception beam property of echo signals caused by the medium; - converting means for converting the echo signals into electrical receivers; a processor connected to the converting means and adapted to receive the electrical receiving signals and process them into an image; Such devices are known in practice.

Ultrasonic imaging systems use transducers to generate and record acoustic waves. Sometimes such imaging systems have separate transmit transducers and receive transducers. The transmit transducers generate a high-frequency acoustic wave that illuminates the medium, which may be tissue in medical applications. In acoustic inhomogeneities, the acoustic wave is reflected. With the receive transducers, the reflected signal is detected and converted into electrical signals, which can be visualized. In many ultrasonic applications, the transmit and receive transducer form one unit and are referred to as "single transducer pulse echo measurements". In applications where the transducer consists of multiple elements, a transducer array is used. By moving the single transducer or controlling the individual elements of a transducer array differently, the medium is exposed differently with each measurement and an image is obtained by combining and visualizing all measured signals.

It is desirable in practice to create high-resolution images. High-resolution imaging means creating an image with high resolution, both radially and axially. High axial resolution means that inhomogeneities can be distinguished one behind the other, seen from the transducer. High radial resolution means that inhomogeneities close to each other viewed from the transducer can be displayed separately.

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High axial resolution is generally achieved by exposing the medium with a short acoustic pulse. Spectral this means that the transmit transducer must generate a broadband acoustic pulse.

High radial resolution is generally achieved by minimizing the width of the generated beams from the transmit and receive transducers in a focal zone. For example, a focusing beam with one fixed focus point is in the shape of an "hourglass" that is narrowest in the focal zone. In practice, the focus point does not have infinitely small dimensions. Therefore, point particles in the focus point get broadened. Particles outside the focal zone are smeared even more.

The above applies not only to ultrasonic imaging systems, but to all types of imaging systems using either sound or electromagnetic radiation. Therefore, the present invention relates to all such imaging systems.

Prior art systems provide the best imaging of those parts of the medium that are in the focal zone. In order to obtain the best possible images at different distances from the imaging system, it has been proposed in the prior art to vary the distance from the focal zone to the imaging system using electro-mechanical or electrical means (dynamic focusing) . However, this leads to complex systems.

For imaging systems without a focusing beam, the aforementioned drawbacks apply to an even greater degree.

It is an object of the present invention to provide an imaging system with maximum achievable resolution, both inside and outside the focal zone.

To achieve this objective, the present invention proposes an object imaging device, as defined above, and characterized in that the processor is further arranged to transform the transmit beam, respectively, the receive beam in a virtual transmit beam of a predetermined desired shape, or a virtual receive beam of a predetermined shape for making the image.

With such an arrangement, the dynamic focusing during the measuring process is avoided and the focusing in both transmitting and receiving takes place virtually after the measuring process, namely during the processing by the processor of the electrical receiving signals. This allows the physical imaging system to remain robust and simple. For example, a device can be used that has only one fixed focal zone during transmission and reception. It uses the great power and flexibility of today's processors. Thus, a high-resolution image can be obtained at all depth levels.

In a preferred embodiment, the virtual transmit beam and / or the virtual receive beam have a constant cross section. The cross section of the bundles can be circular or elliptical, or any other desired shape. The cross section of said virtual beams is preferably chosen as small as possible, the minimum size of which depends on the transducer (s) used and the frequency content of the transmission signal.

To transform the bundle (s) into said virtual bundle (s), the shape of the bundle (s) must be known to the processor. For example, the shape of the beam (s) can be derived analytically by the processor based on data about the type of the transducer (s) used. Alternatively, the shape of the beam (s) can be measured in one plane in the medium. Using the wave theory, the entire three-dimensional shape of the bundle (s) can then be unambiguously determined. Measuring the shape of the beam (s) has the advantage that deviations from the transducer (s) are automatically included in the measuring process.

Preferably, the processor is arranged to perform a lateral deconvolution process on the image, which deconvolution process depends on the distance in the medium from the imaging system, which deconvolution process produces a filtered image.

The operations performed by the processor preferably take place in the Fourier domain. This allows the lateral deconvolution process to be replaced by a simple multiplication process. Thus, in such an embodiment, the processor is arranged to perform the following processing steps on the image: a. Calculating the Fourier transform of the image; b. performing lateral deconvolution on the Fourier transformed image as a multiplication process in the Fourier-1006229 4 domain; c. performing an inverse Fourier transform on the result of step b.

In general, the transmission beam will have too long a pulse shape. In an embodiment, the processor is also arranged to perform a temporal deconvolution step such that the pulse shape in the filtered image is shortened in time. This leads to a higher axial resolution.

The present invention is not limited to a device. The invention also relates to a method of making an image of an object, comprising: generating and transmitting a transmission beam with a determined transmission beam property to a medium; receiving within a receiving beam with a certain receiving beam property of echo signals caused by the medium; converting the echo signals into electric receive signals; processing the electrical receive signals into an image; characterized by the step of transforming the transmit beam, the receive beam, respectively, into a virtual transmit beam of a predetermined desired shape, or a virtual receive beam of a predetermined shape for making the image.

Further embodiments of the method are evident from the dependent method claims.

The present invention will be elucidated with reference to some figures, which are intended to illustrate further and not to limit the scope of protection thereof.

Figure 1 shows a beam generated by a focusing transducer in a known per se arrangement; Figure 2 shows a schematic depiction of six fully mirror-point diffractors, as produced by a single fixed focus-point imaging system, as is also known per se; Figure 3 shows a diagram for further illustration of the invention. Ultrasonic imaging systems use transmit and receive transducers to generate and record acoustic waves. The essence of the invention will be explained below with reference to an acoustic pulse echo imaging system, which uses a "hourglass shape" transmit / receive beam with one focus point. However, the general applicability of the mathematical formulation is not limited to this, as stated above. The shape of the beam can be arbitrary, as long as it is known to the processor 4 (see figure 3) of the system.

Figure 1 shows a transmit transducer 1, which generates a beam B + 5 in a coordinate system in which the lateral positions with? and the axial positions are indicated by z. In the arrangement shown in Figure 1, the origin of the beam B + emitted by transducer 1 is at the position (? K, zQ). The transmit transducer 1 shown generates a focusing beam B +, the focus of which is on position (? K, zf).

The radial resolution is determined by the width of the transmit / receive transducer. Thus, a focusing beam B + with a fixed focus point is in the shape of an "hourglass", which is narrowest in the focus zone at a distance zf-zQ from the transmit transducer 1, as shown in Figure 1.

In order to obtain the greatest possible radial and lateral resolution with the prior art systems, electro-mechanical or electrical measures are taken to be able to move the focus of the beam in position during the measuring process.

However, this electro-mechanical or electrical solution proves unnecessary. It appears to be possible to apply a numerical focus to the received image with the aid of a processor after performing the measuring process without actually moving the focus of the transducer (s) during the measurement.

First, the general mathematical formulation of the invention will be given below. Next is an example of the general mathematical formulation for an application in two dimensions.

The process, the mathematical formulation

The beams of a transmit transducer at position (?., Z0) and a receive transducer at position (fj, zQ) can be presented in any plane (z = zm) by functions B + (?, Zm;? I, respectively). z0; w) and B "(? j, z0;?, zm; ti)). The effective beam of the transceiver combination is given by the product of both beams.

This means that in a single transducer system (pulse echo system) the effective beam at position (?, Z0) of one perfectly specular 35 point diffractor at the point (? K, zm) is presented by

Gk „(f, z0, ω) (ï, z0; rk, z„; <ji) B * (ïk, zm; r. Ζ0; ω) da) 1006229 6 or, for a medium without lateral variations,

Gkm {f-ïk, z0, u>) = B ~ (F, z0; rk, zm; <1>) B * lrk, zm; r, ζ0; ω) (1b>

Each image P at position (?, ZQ) can be presented as 5 summation of images Pm (?, ZQ) because of all point diffractions in all planes z: m P (r, z0, ω) = Σ Pm (f, z0, ω ) mk where R + (fk, zm, u) represents the reflectivity distribution for depth level zm, Sm (u) is equal to the pulse shape for depth level zm, Gm = GJi1e ~ 2 -, “t” 10 and tm = (z0-zm) / c.

All this is further elucidated in figure 2. Figure 2 shows a schematic representation of six fully specular point diffractors, ie R + = 1, which are located at lateral position in a display system with one fixed focus point. Note that in Figure 2 the vertical axis is now the time axis and that the beam has its most limited size at time t = tf, ie the time when the beam reaches focus.

Note that formula 2a represents a lateral convolution process: P (f, z0, ω) = 52 [Gm (F, zQ, ω) * R * (f, zm, ω)] (2b) m 20

In the focal zone, the function Gm has the smallest width, which depends on the size, shape and frequency range of the transducer (s). Outside the focal zone, Gm widens due to a wave-propagation dependent spread. This spread is not transducingly dependent and can be calculated from wave theory if Gm is known on only one z-plane.

Thus, a filter Fm can be determined, which for each reflection depth zm with vertical reflection time 2tm = 2 (z0-zm) / c compresses the function G ^ at any depth to a magnitude G ^ which has the function G 'in the 1006229 7 focus point: /, O)

Filtering process (3) means applying a depth-dependent lateral deconvolution process to the measured image: -5 P0 (F, 20, ω) = Y'FJÏ, ζ0, ω) + Pm (ï, z0, v) »(4a) m where (?, <*>) represents a narrow depth-independent beam, which can be called "pencil beam".

Optionally, the filter Fm could also shorten the time pulse (tem-10 pore deconvolution) and / or change the shape of the beam in the focus point, for example, virtually decreasing the intensity of side lobes and narrowing the main lobe width: P0 (r , z0, ω) -50 (ω> δ0 '(*. ω) R * (F, zm. ω) e_w "e * (4b) m 15 where S0 (a>) presents the resulting pulse after the optional temporal" pulse shaping process and G ^ (ï, a) presents the resulting beam after the optional lateral beam shaping process.

Result (4b) represents the result of an integrated hybrid imaging process that simulates a virtual imaging system 20 with a narrow depth-independent beam ("pencil beam"), simulating, as it were, a variable focus on transmission and reception using numerical methods .

Example

The invention is now further elucidated on the basis of an example with reference to figure 3. An analog electrical signal is generated by means of a physical imaging system 2, which is fed to an A / D converter 3. The digitized output from the A / D converter 3 is supplied to a digital processor 4. The physical imaging system 2 may be any known imaging system. Likewise, the A / D converter 3 and the digital processor 4 may be commercially available instruments 1006229.

In figure 3 it is indicated by means of an elliptical line 5, that the physical imaging system 2 gives a "rough" image. This image is an image that can be obtained with prior art systems. In another elliptical line 6, it is schematically indicated that the digital processor 4 is programmed to obtain a high-resolution image. For this, the digital processor is provided with a program that applies the theory given above.

The digital processor 4 includes residual focusing operators associated with the filtering process Fm (f, z0, o). These operators treat the raw image (lateral deconvolution) to a result with an unprecedentedly high resolution that results in a fully variable focus in transmission and reception.

15 The theory given above is now illustrated in two dimensions (i.e.? Turns into x) for the situation of a single transducer system with one fixed focus point, the position of which is given by the two-way travel time tf = 2 (zf-zQ) / c. If p (x, t) represents the "raw" image and P (kx, u) its two-dimensional Fourier transform, then the residual Fourier transformed focusing result for the time level t = mAt (m = 0.1 , 2, ..., M; At = predetermined length of time) are written as: = - ^ / ί; (* χ, ω) Ρ (* Α., ΩΜω (53) ω where Fm is the Fourier transformed focusing operator , which is given by (5b)

FmUx, (o) = Ame * 2 ^ '£ · - ^ with:

Am = amplitude factor (5c) = jkl-v> 2 / cz

The amplitude factor Am in the Fourier transformed focus operator Fm may optionally be used for the transducer-dependent "pulse shaping process", ie the aforementioned time pulse shortening (temporal deconvolution) and / or the " beam shaping "process, ie the aforementioned adjustment of the beam in the focus point. If A = 1, the residual focus operator is a wave field operator and depends only on the wave speed c and the location of the focus point zf.

The total process in this example therefore consists of a double Fourier transform: p (x, t) -> P (kx, e) and residual focusing process (5a) for m = 1,2, ..., M, followed by single inverse Fourier transform rm (kx, tm) -> r (x, tm) for m = 1,2, ..., M.

10 In case a time-dependent focusing process has already been applied in the receiving phase in the physical imaging system 2, then (5b) must be replaced by (5d) FJkx, v) - 15 In case there are no focusing transducers in the physical imaging system 2 have been used and thus the beam becomes wider immediately from position zQ as a result of a wave propagation-independent spread, then in (5b) tf = 0 must be taken.

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List of most important symbols ω = angular frequency c = wave speed (ƒ, z) = position expressed in lateral position? and axial position z with respect to an origin; B * (f, z; zk, ζ0; ω) = transmission beam at position (?, Z) as a result of transmission transducer at position (? K, z0) with angular frequency ω; ζ; ω) = receive beam at position (?, z) due to 10 receive transducer at position (fJfz0) with angular frequency ω;

Gm {S-ïk, ζ0, ω) = B ". B + = effective beam at position (Fk, zm) due to a single transducer system at position (Z, zQ); 15 P (r, ζ0, ω) = unedited image at position (F, z0);

Pm {f, ζ0, ω) = raw image of all individual point reflections in plane (z = zm); P0 (r, ζα, ω) = image after depth-dependent lateral deconvolution of Ρ (?, Ζ0, ω); 20 Sm [w) = pulse shape in plane (z = zm); S0 (<o) = pulse shape after temporal deconvolution; R * {r, zm, ω) = reflectivity distribution in plane (z = zm); βΐ (τ, ω) = narrow depth-independent beam (pencil beam) as a function of? and ω; 25 Fm (F, z0, a>) = filtering process to make depth-dependent beam G ^ (?, z0, u) into depth-independent beam Gg (?, <O).

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Claims (12)

Device for creating an image of an object, comprising: 5. transmission means (1) for generating and transmitting a transmission beam with a determined transmission beam property (B +) to a medium; receiving means (2) for receiving within a transmission beam with a determined reception beam property (B ~) echo signals caused by the medium; 10. converting means (2) for converting the echo signals into electric receiving signals; a processor (4) connected to the converting means (2) and arranged to receive the electrical receiving signals and process them into an image (P (?, z0, u); p (x, t), 15 P (kx, u>)); characterized in that the processor (4) is further adapted to transform the transmit beam, the receive beam, respectively, into a virtual transmit beam of a predetermined desired shape, or a virtual receive beam of a predetermined shape, for producing the image (Ρ (?, Ζ0, ω); p (x, t), P (kx, u)). The device of claim 1, wherein the virtual transmit beam and the virtual receive beam have a constant cross section. The device of claim 1 or 2, wherein the processor (4) is arranged to perform a lateral deconvolution process (Fm (kx, ω)) on the image (Ρ (Ρ, ζ0, ω); p (x, t), P (kx, <a)), which deconvolution process (Fm (kx, o)) depends on the distance in the medium to the imaging system, which deconvolution process (Fm (kx, ω)) has a filtered -30 yields (PQ; r (x, tm), rm (kx, tm)). The device of claim 3, wherein the processor is configured to perform the following processing steps in the image (p (x, t)): a. Calculating the Fourier transform (P (kx, u)) of the image (p (x, t)); b. performing lateral deconvolution (Fm (kx, u)) on the Fourier transformed image (P (kx, u>)) as a multiplication 1006229 process in the Fourier domain; c. performing an inverse Fourier transform on the result of step b. Device according to any one of the preceding claims, wherein the image (Ρ (ί, ζ0, ω); p (x, t), P (kx, a>)) has a predetermined pulse shape (Sm (o>)) depending on the distance in the medium to the imaging system (2) and the processor (4) is also arranged to perform a temporal deconvolution step such that the pulse shape in the filtered image (PQ; r (x, tm), rm (kx, tm)) is shortened in time. 6. Device according to any one of the preceding claims, wherein the processor (4) is also arranged to mathematically change the shape of the original transmit beam and the original receive beam in the focus point afterwards, for example by reducing the intensity of side lobes and narrowing the width of a main lobe. A method of creating an image of an object, comprising: generating and transmitting a transmission beam with a given transmission beam property (B +) to a medium; receiving echo signals caused by the medium within a receiving beam with a certain receiving beam property (B '); Converting the echo signals into electric receiving signals; processing the electrical receive signals into an image (P (f, z0, ω); p (x, t), P (kx, ü)); characterized by the step of transforming the transmit beam, respectively the receive beam into a virtual transmit beam of a predetermined desired shape, or a virtual receive beam of a predetermined shape, for creating the image (Ρ (?, ζ0, ω) ; p (x, t), P (kx, ω)). The method of claim 7, wherein the virtual transmit beam and the virtual receive beam have a constant cross section. The method of claim 7 or 8, wherein the processor (4) is arranged to perform a lateral deconvolution process 1006229 (Fm (kx, u)) on the image (Ρ (?, Ζ0, ω); p (x , t), P (kx, u)), which deconvolution process (Fm (kx, (i>))) depends on the distance in the medium to the imaging system, which deconvolution process (Fm (kx, Q) ) yields a filtered image (P0; r (x, tm), rm (kx, tm)) 5 The method of claim 9, wherein the processor is arranged to perform the following processing steps in the image (p (x, t)): a. Calculating the Fourier transform (P (k, ω)) of the mapping (p (x, t)); b. performing the lateral deconvolution (Fm (k, ω)) on the Fourier transformed image (P (kx, w)) as a multiplication process in the Fourier domain; c. performing an inverse Fourier transform on the result of step b. A method according to any one of the preceding claims 7 to 10, wherein the mapping (P (f, zQ, u); p (x, t), P (kx, t>)) has a predetermined pulse shape (Sm ( w)) which depends on the distance in the medium to the imaging system (2) and the processor (4) is also arranged to perform a temporal deconvolution step such that the pulse shape in the filtered image {PQ; r (x, tm), rm (kx, tm)) is shortened in time. A method according to any one of the preceding claims 7 to 11, wherein the processor (4) is also arranged to mathematically change the shape of the original transmit beam and the original receive beam in the focus point afterwards, for example by reducing the side lobe intensity and narrowing the width of a main lobe. ***** 1006229