Ultrasound imaging system for high-resolution broadband harmonic imaging

Abstract

An ultrasound imaging system sequentially transmits two beams in the same beam direction. The waveform of the second beam is a phase-reversed version of the first waveform, and each waveform contains first and second principal frequency components. Echoes from along the beam direction are received in response to each beam transmission, and the echoes are processed by combining the two echo sequences additively and subtractively on a common depth basis. The echo components produced by this processing range from high frequencies for good near-field resolution to low frequencies for good depth penetration, and include fundamental, harmonic, and sum / difference frequency components. Linear and nonlinear echo signal components may be frequencies combined for speckle reduction.

Description

Technical Field

[0001] The present invention relates to medical ultrasound imaging systems, and in particular to ultrasound systems that perform high-resolution broadband harmonic imaging. Background Art

[0002] Harmonic imaging currently finds two main applications in ultrasound imaging. One is the imaging of ultrasound contrast agents. The microbubbles of the contrast agent can be designed to oscillate or rupture when penetrated by ultrasound. This oscillation or destruction will make the echoes returned from the microbubbles rich in harmonic frequency components. The received echoes are filtered to separate the harmonic components from the fundamental frequency components. The preferred separation technique is known as pulse inversion, as described in U.S. Pat. No. 5,951,478 (Hwang et al.). The images produced using these echoes can sharply segment the blood flow and vascular system containing the contrast agent.

[0003] A second significant application of harmonic imaging is tissue harmonic imaging, in which a generally sinusoidal transmit waveform is allowed to undergo natural distortion as it passes through the body. The distortion results in the development of harmonic frequency components, with the most significant harmonic frequency components typically being at the second harmonic of the fundamental transmit frequency. The echoes are received and processed in a similar manner to the tissue harmonic signals to separate the harmonic components returning from the body. Tissue harmonic imaging is described in U.S. Pat. No. 5,879,303 (Averkiou et al.). Images generated from harmonic components are desirable for their low clutter levels caused by multipath scattering.

[0004] U.S. Pat. No. 6,440,075 (Averkiou) describes another ultrasound imaging technique that enhances the generation of nonlinear signal components. This is accomplished by transmitting a waveform having two primary frequencies. When the waveform passes through tissue or encounters microbubbles, the harmonic components of each transmit frequency will develop as described above. In addition, the two transmit frequency components will modulate each other, thereby developing sum and difference frequency components. These two types of nonlinear signals are received and used to form an image enhanced by using two nonlinear mechanisms. The patent gives examples of several ways in which sum and difference frequencies can be formed and positioned, such as by using the side of the transducer passband for the primary transmit frequency and using the center for the difference frequency and harmonic frequency components.

[0005] Tissue harmonic imaging has two significant limitations, its ability to image in the very near field and its ability to image in the far field. Since the harmonic components of tissue harmonic imaging begin to develop only after the transmit wave passes through the tissue, little or no energy is present in the very near field for the return of the harmonic echo. For imaging at greater depths in the body, which is often necessary for deep abdominal imaging (such as imaging the liver), lower frequencies are required to counter the effects of depth-related frequency attenuation. Since the second harmonic frequency is by definition twice the fundamental transmit frequency, it is high frequency by nature and is therefore more affected by depth-related attenuation. This attenuation can reduce the signal-to-noise characteristics of the received echo and therefore the diagnostic quality of the image. In addition, the harmonic components of the echo signal contain only a small portion of the energy reflected in the echo, also reducing its signal-to-noise characteristics. It is therefore desirable to be able to perform harmonic imaging in a manner that will improve the resolution of the image in both the near and far fields of the image. Summary of the invention

[0006] According to the principles of the present invention, an ultrasound system for performing high-resolution harmonic imaging by using a wide bandwidth of an echo signal is described. The transmit waveform is designed to contain two main frequency components. The transmit waveform is transmitted twice, each time with a different transmit modulation. The received echoes from the two transmissions are combined in two ways by addition and subtraction to extract the full spectrum of image information. This separates the fundamental, harmonic and intermodulated components of the two main frequency components. These different components include high-frequency intermodulation and / or harmonic components that provide good near-field resolution, and strong low-frequency fundamental components that provide good resolution in the far field. The content of these different frequency components can be emphasized by using a tracking filter after signal component separation. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In the attached picture:

[0008] Figure 1 An ultrasonic diagnostic imaging system constructed in accordance with the principles of the present invention is illustrated in block diagram form.

[0009] Figure 2 The combination of waveforms of different frequencies to produce a multi-frequency transmit waveform and its inverse is illustrated.

[0010] Figure 3a and 3b Two ways of generating multi-frequency transmit waveforms in an ultrasound system are illustrated.

[0011] Figure 4 The diagram illustrates different frequency components that can be extracted by adding and subtracting complementary echo signals in accordance with the principles of the present invention. DETAILED DESCRIPTION

[0012] First reference Figure 1 , shows an ultrasonic diagnostic imaging system constructed according to the principles of the present invention. Figure 1 The ultrasound system of the invention utilizes a transmitter 16 that transmits a multi-frequency beam for the generation of harmonic frequencies within the object being imaged as well as sum and difference frequency signals. The transmitter is coupled to the elements of the array transducer 12 of the ultrasound probe 10 through a transmit / receive (T / R) switch 14, which protects the beamformer from high voltage transmit pulses. The transmitter responds to a number of control parameters that determine the characteristics of the transmit beam, as shown in the accompanying drawings. The two main frequencies f1 and f2 of the multi-frequency transmit pulses are controlled, which determine the frequencies at which the sum (f1+f2) and difference (f1-f2) frequency components will fall. The amplitudes or intensities a and b of the two transmitted frequency components are also controlled, which causes the transmit pulse to have the form (asin(2πf1t)+bsin(2πf2t)). However, because the sum and difference signals are produced by the nonlinear effect of mutual modulation, the received sum and difference signal components will have an amplitude c that is not a linear product of the intensities of a and b. Harmonics of the two transmit frequencies are also produced by nonlinear effects.

[0013] exist Figure 1 In the embodiment of the present invention, the transducer array 12 receives echoes from the body containing a full spectrum of different frequencies (including fundamental frequencies f1 and f2, harmonics of these two frequencies, and sum and difference frequency components within the transducer passband). These echo signals are coupled to the beamformer 18 through the T / R switch 14, and the beamformer 18 appropriately delays the echo signals from different elements and then combines them to form a sequence of echo signals from shallow to deeper depths along the received echo direction. Preferably, the beamformer is a digital beamformer that operates on digitized echo signals to produce a sequence of discrete coherent digital echo signals from near to far depths of the field. The beamformer can be a multi-line beamformer that generates two or more sequences of echo signals along multiple spatially different receive scan lines in response to a single transmit pulse. The coherent beamformed echoes from along the beam direction are stored in the line A buffer 20a.

[0014] After this first reception of the line of echo signals, the second beam is transmitted in the same beam direction as the first beam. The transmitter 16 has another variable transmission parameter for the phase or polarity of the transmit pulse, as shown in the accompanying drawings. The ultrasound system transmits two or more beams of different transmit polarities, amplitudes and / or phases in the same beam direction. For the dual pulse embodiment, the echo received in response to the second transmit pulse is stored in the line B buffer 20b. The scan line echoes received in response to the two transmit pulses are then combined on a spatial (depth) basis in two ways, by addition and addition. Alternatively, the second beam of the received echo can be directly combined with the echo of the stored first beam without buffering.

[0015] When the echoes of the two beams are combined by the summer or adder 22, a process known as pulse inversion separation is performed. Pulse inversion separates nonlinear signal components, including the sum, difference and second harmonic components of the two frequencies of the transmitted pulse, as described more fully below. As a result of the different phases or polarities of the transmitted pulses, the out-of-phase basic (linear) echo components will cancel, and the in-phase nonlinear components will combine to reinforce each other, thereby producing enhanced and isolated nonlinear harmonics, sums and difference frequency signals. Nonlinear signal separation can alternatively be performed by differential power modulation, as described in U.S. Pat. No. 5,577,505 (Brock-Fisher et al.). According to the principles of the present invention, the received echoes are also combined in a subtractive manner on a spatial basis by a subtractor 24. The subtractive combination cancels the nonlinear components of the two echo signals and strengthens the basic linear frequency components from the two echo signals. The results of this subtractive combination process are also described more fully below. The signal components separated by addition and subtraction combinations are then combined into a single echo signal at each depth location by a combiner including a summer or adder 26 .

[0016] The echo signals produced by the summer or adder 26 are filtered by a filter 30 to remove undesirable signals, such as out-of-band noise. Preferably, the filter 30 is a tracking filter that changes its passband from high to low frequencies as the echo signals are received from increasing depths of the field. The tracking filter can be a bandpass filter having a passband that moves from a high frequency band to a low frequency band with increasing depth, or it can be a bandpass filter having a high frequency cutoff that is moved lower in frequency as the echoes are received from increasing depths. The depth-dependent tracking filter improves resolution by excluding the high frequencies from further processing when the high frequency band has no useful or uncontaminated signal information due to depth-dependent attenuation.

[0017] The filtered echo signal is detected by detector 32, which may be an amplitude or phase detector. Figure 1 In the illustrated embodiment, when the echo is to be processed to form a B-mode (grayscale) image, the detector is an amplitude detector that detects the envelope of the echo signal. When the echo signal is to be Doppler processed, phase detection is used. The amplitude-detected echo signal is processed by a B-mode processor 34 that performs operations such as scan conversion to form a grayscale image of the tissue in the image field, and then is also processed by an image processor 36 for display on an ultrasound image display 38.

[0018] Figure 2Conceptually illustrated is how a multi-frequency beam transmitted under the control of the transmitter 16 is formed. A waveform 80 of a first frequency is combined with a waveform 82 of a second frequency. The multi-frequency combination of these two waveforms is shown as a composite waveform 84. A multi-frequency waveform such as waveform 84 is used for a first beam transmitted by the transducer array 12. Waveform 80 is inverted (180° phase change) to produce a second waveform 86, a phase-inverted copy of the first beam. Waveform 86 is used for a second beam transmitted by the transducer array in the same beam direction as the first beam.

[0019] Figure 3a and 3b Two embodiments of the transmitter 16 suitable for use in the ultrasound system of the present invention are illustrated in detail. Figure 3a In an embodiment, the waveforms of the different frequency components f1 and f2 of the transmit pulse are separately formed in a digital operation and then combined to form a composite multi-frequency transmit signal applied to the transducer elements. The f1 generator 42 generates the f1 transmit signal component, and the f2 generator 44 generates the f2 transmit signal component. The generators are capable of, for example, algorithmically generating the signal components. The generators generate their corresponding transmit waveforms in response to input control parameters (such as f1Sel. and f2Sel., which select the f1 and f2 frequency components for the transmit beam as shown in the accompanying drawings). Other variable input parameters (not shown) may be intensity or toe-cut parameters a and b, and phase or polarity parameters for inverting the transmit signal. Alternatively, the output waveforms generated by the generators 42 and 44 may be varied in amplitude and phase or polarity before or after being combined by the summer 46 into a composite transmit waveform containing multiple transmit frequency components. In Figure 3a , the waveform produced by the generator is weighted by digital weighting processor circuits 43 and 45, which apply amplitude or apodization weighting factors a and b to the generated waveform. The weighting circuit can take the form of a digital multiplier, and the sign (+1, -1) of the weighting factor can be used to control the polarity of the output waveform. The composite transmit waveform is applied to a D / A converter 48 for conversion to an analog signal, which can be further amplified and filtered as needed and used to drive the transducer elements 12' of the transducer array.

[0020] Figure 3bA second transmitter embodiment is illustrated in which a composite multi-frequency transmit signal is generated in advance and then stored in a waveform library 50 which may take the form of a digital memory. When a particular multi-frequency transmit waveform is desired, it is selected from the library 50 by a selection signal Sel., weighted by a weighting circuit 51, and stored in a transmit register 52. When the transmitter is triggered to transmit a beam, the composite multi-frequency waveform is shifted out of the transmit register 52 by a clock signal CLK, converted to an analog signal by a D / A converter 48, and applied to the transducer element 12'. The amplitude of the transmit pulse may be varied by a digital multiplier before the A / D converter (such as that used in the weighting circuit) or by an analog amplifier after the A / D converter, and may be filtered in the analog or digital domain as desired. The individual frequency components may not be individually adjusted in amplitude after the transmit register 52, since the waveform is already a composite waveform at that point in this embodiment.

[0021] Figure 4 Illustration of how the ultrasound system uses Figure 2The transmit waveform for dual pulse transmission collects the broad spectrum of echo signal frequencies. In the top portion 4a of the accompanying drawings, the ultrasound system combines the echo signals 84e and 86e that have been collected and stored in the two line buffers 20a and 20b in an additive manner. The additive combination performed by the summer 22 separates the nonlinear difference, the second harmonic, and the sum signal components f2-f1, 2f1, 2f2, and f2+f1, respectively. The summer 22 will also produce a higher frequency second harmonic 2f2 of another transmitted frequency component that is not shown in the accompanying drawings. For the f1 and f2 components selected for transmission in this example, the nonlinear components fall in the transducer passband, as shown in the spectrum to the right of the summer 22. For the sake of clarity of the illustration, only the center frequency of the nonlinear component band is shown in the accompanying drawings, but it will be appreciated that each component is actually a band of frequencies around the center frequency. It should be seen that the spectrum includes high frequencies of the sum signal component f2+f1 around 5.8MHz, and intermodulation at high frequencies that produce good near-field resolution. These components are generated by the mutual modulation of the transmitted signal components f1 and f2 in the field shortly after the transmission. The middle part 4b of the accompanying drawings shows that the received echoes 84e and 86e are combined in a subtractive manner by a subtractor 24, thereby realizing the echo signals of the linear basic frequency components f1 and f2, as shown on the right side of the subtractor. Frequency f1 is a low frequency that can be returned at a detectable amplitude in the echo from the deeper image depth, where the low frequency resists the effect of depth-related attenuation. This signal is also the basic frequency and therefore contains most of the energy reflected from the tissue at a greater depth. This signal is also a combination of echoes received in response to two transmissions, and therefore, due to the subtractive combination, the components from the two echoes reinforce each other to produce a strong received echo from a greater depth. The basic frequency echo component f2 is similarly strengthened from the two receptions, but is more susceptible to the effect of depth-related attenuation. In this example, the f1 and f2 frequencies are carefully chosen so that f1 is the lowest frequency received and enables acceptable returns from greater depths, while the f2 frequency falls between the non-linear component 2f1 and the additive combination of f2+f1.

[0022] After the addition and subtraction combinations are combined by summer 26, the entire echo spectrum appears as shown in the bottom portion 4c of the drawing. In this example, the spectrum extends from a low frequency of less than 2 MHz to a high frequency of almost 6 MHz. Therefore, these echo signals will produce good high frequency resolution in the very near field due to the intermodulated sum frequency component f2+f1, and good resolution in the far field due to the strong low frequency fundamental signal component f1. The tracking filter 30 (such as FIG. 4A ) moves from higher frequencies to lower frequencies when processing echoes from near to far fields. Figure 1 ) will enhance the signal-to-noise performance even more and emphasize the benefits of the wide echo signal spectrum.

[0023] Figure 5 A second embodiment of the ultrasound system of the present invention is shown in block diagram form. Figure 5 The system is based on Figure 1 The system is constructed in the same manner as the system of FIG. 2 by addition and subtraction combiners 22 and 24. The output of each of these combiners is coupled to two time-varying filters 30a and 30b. These filters present a passband characteristic that changes as the echo signal is received from the increased depth of the field, and provide an emphasis on the frequency components that provide the best imaging signal at the corresponding depth of the image. The filtered linear and nonlinear signals are coupled to detectors 62a and 62b, which perform amplitude (envelope) detection of the corresponding signal components. The detected signals are then weighted by time-varying weighting circuits 64a and 64b, which can be constructed as multiplier circuits that multiply the applied detected signals by weights that change with time (depth). The weighted signals are then combined by a summing circuit 66. Since the linear signal from the subtractor 24 and the nonlinear signal from the adder 22 produce different speckle patterns in the image, the artifacts of the coherent signal imaging system (the combination of the two signal paths through the summing circuit 66) will achieve a reduction in the combined speckle pattern through a process known as frequency compounding. For a description of the process and effects of frequency compounding for speckle reduction, see U.S. Patent Re. 35148 (Lizzi et al.). The combined signal of the summing circuit 66 is then coupled to the B-mode processor 34 for image formation and as Figure 1 Subsequent image processing and display in ultrasound systems.

[0024] Figure 6a and 6bExamples of time-varying passband characteristics of filters 30b and 30a are illustrated, respectively. At shallow depths, the passband 70 of filter 30b is shaped to pass higher frequency nonlinear components generated by the heating combiner 22, which are intermodulated sum signal components f1+f2 and second harmonic components 2f1. At shallow depths, these higher frequency components have not been significantly affected by depth-related attenuation, and since higher frequencies produce good resolution, they are preferred for shallow (near field) depths of images. When receiving echo signals from increased depths (e.g., medium ranges), the filter characteristic 70 transitions to a passband that includes lower difference frequency components f2-f1, because these lower frequency nonlinear components are less affected by increased depth-related attenuation than higher frequency nonlinear components. Nonlinear echo signal components are emphasized in the medium range of images. When receiving echo signals from greater depths of field (depth), the passband 70 continues to transition to a lower passband that passes lower nonlinear signal components f2-f1 and 2f1 to relatively exclude the highest frequency nonlinear sum signal components. Thus, at each depth, the passband 70 is tailored to those signals whose passage will produce the best image while taking into account the good resolution of high frequency signals and the effect of depth-dependent attenuation of high frequencies.

[0025] Figure 6b An example of a time-varying filter characteristic 72 of a filter 30a that can be used for a linear signal component is provided. At shallow depths, the passband of the filter characteristic 72 is larger at higher frequencies to emphasize the higher fundamental frequencies of the f2 band. This provides higher frequencies with good resolution for the near-field region of the image, and the transmitted f2 frequencies will return echo signals from the near field where nonlinear signals have not yet fully formed, further improving the near-field image quality. In the mid-range, the filter characteristic 72 becomes more balanced for the passage of both fundamental frequency bands f1 and f2. At deeper depths, the response of the filter characteristic 72 shifts to a lower band to emphasize echoes from the low fundamental frequency signal f1 that is least susceptible to the effects of depth-related attenuation. Thus, a filter response characteristic (such as Figure 6b The characteristics shown in FIG. 4 will utilize the contribution from the f2 frequency echo to enhance the image at shallow depths, and will utilize the contribution from the echo in the f1 frequency band to enhance the image with good penetration at deeper depths.

[0026] Figure 7aAn example of a change in the weight applied to the nonlinear echo signal component by the weighting circuit 64b is provided. Curve 74 shows that the weight is relatively small at the shallowest depths and then gradually increases to a larger weighted value in the medium range. This provides a significant contribution from the nonlinear echo signal in the medium range after the nonlinear signal component has had time to develop in the sound field and before the effects of depth-related attenuation have become overly significant. At greater depths, curve 74 shows that the weight decreases as the signal-to-noise ratio of the nonlinear echo signal becomes more greatly affected by depth-related attenuation.

[0027] Figure 7b An example of a change in the weight applied to the linear echo signal components by the weighting circuit 64b is illustrated. Curve 76 shows a greater weight applied in the very near field (shallow depth), increasing the contribution of the fundamental frequency echo signal to the image at the shallowest depth, where the nonlinear signal has not yet developed significantly. Curve 76 decreases (less weighted) as echoes are received from the mid-range to provide a relatively greater contribution from the nonlinear echo signal with good resolution. At greater depths, the weight increases again, as indicated by the rising curve 76, which increases the contribution of the lower fundamental frequency echo with better penetration, because the effect of depth-related attenuation affects the higher frequency nonlinear echo signal more.

[0028] The values ​​of the weights of the weighting circuits preferably also take into account two other factors. One is the need to equalize the relative amplitudes of the signals in the linear and nonlinear signal paths for better frequency recombination. When the signals of the two signal paths are properly balanced, better reduction of speckle pattern artifacts is achieved. In this regard, see U.S. Pat. No. 5,957,852 (Hossack et al.). Another is logarithmic compression of image data. Grayscale values ​​for B-mode images are typically converted to logarithmically altered values ​​because it has been found that logarithmic conversion results in a range of grayscale values ​​that is more diagnostically useful. Although logarithmic conversion can be performed at a variety of points in the image signal path, it is efficient to incorporate the conversion function into the selection of weights used in the weighting circuits 64a and 64b. Thus, the weighting circuits 64a and 64b are capable of performing three functions: selection of the best combination of linear and nonlinear signal components at different image depths; equalization of nominal signal amplitudes for effective frequency recombination and speckle reduction; and logarithmic conversion of grayscale image values. Alternatively, the logarithmic conversion can be achieved by using a separate logarithmic conversion memory lookup table before or after each weighting circuit.

[0029] From the above content, it can be seen that the concept of the present invention is Figure 5 The implementation of the system not only enhances the image resolution and penetration over the entire depth of the image by selectively combining both linear and nonlinear echo signal components, Figure 1The same is true for systems that additionally provide images enhanced by speckle reduction.

[0030] It should be noted that ultrasound systems suitable for use in embodiments of the present invention and in particular Figure 1 , 3a The component structure of the ultrasound system of 3b can be implemented with hardware, software or a combination thereof. Various embodiments and / or components of the ultrasound system (e.g., a transmitter, a B-mode processor, an image processor, a weighting circuit, and a waveform generator, or components and controllers therein) can also be implemented as part of one or more computers or microprocessors. A computer or processor may include a computing device, an input device, a display unit, and an interface, for example, for accessing the Internet. A computer or processor may include a microprocessor. The microprocessor may be connected to a communication bus, for example, to access a PACS system or a data network for inputting training images. A computer or processor may also include a memory. Memory devices (such as waveform libraries and line buffers) may include random access memory (RAM) and read-only memory (ROM). A computer or processor may also include a storage device, which may be a hard drive or a removable storage drive, such as a floppy disk drive, an optical drive, a solid-state thumb drive, etc. A storage device may also be other similar modules for loading a computer program or other instructions into a computer or processor.

[0031] As used herein, the term "computer" or "module" or "processor" or "workstation" may include any processor-based or microprocessor-based system, including systems using microcontrollers, reduced instruction set computers (RISC), ASICs, logic circuits, and any other circuits or processors capable of performing the functions described herein. The above examples are exemplary only, and thus are not intended to limit the definition and / or meaning of these terms in any way.

[0032] A computer or processor executes a set of instructions stored in one or more storage elements in order to process input data. Storage elements may also store data or other information as needed or desired. Storage elements may take the form of an information source or a physical memory element within a processing machine.

[0033] The instruction set of the ultrasound system as described above (including those instructions that control the acquisition, processing and display of ultrasound images) may include various commands that instruct a computer or processor as a processing machine to perform specific operations (such as the methods and processes of various embodiments of the present invention, and specifically, the addition and subtraction of received echo signals as described above). The instruction set may take the form of a software program. The software may take various forms such as system software or application software, and it may be implemented as a tangible and non-transient computer-readable medium. In addition, the software may take the form of a collection of separate programs or modules, such as a beamformer control module, a program module within a larger program, or a portion of a program module. The software may also include modular programming in the form of object-oriented programming. The processing of input data by the processing machine may be in response to an operator command, or in response to the results of a previous process, or in response to a request made by another processing machine.

[0034] Furthermore, the following claim limitations are not written in a means-plus-function format and are not intended to be interpreted based on 35 U.S.C. 112, sixth paragraph, unless and until such claim limitations expressly use the phrase "means for..." followed by a description of functionality without further structure.

Claims

1. An ultrasound system for producing high-resolution broadband harmonic images, comprising: an ultrasound probe including an array transducer adapted to transmit an ultrasound beam and receive echo signals from directions along the transmitted ultrasound beam; a transmitter coupled to the ultrasound probe and adapted to control the probe to transmit first and second differently modulated transmit waveforms in a common beam direction, each waveform comprising at least two primary frequency components; a buffer adapted to store echo signals received in response to transmission of at least said first transmit waveform; a summer coupled to the buffer and adapted to additively combine, on a spatial basis, echo signals received in response to transmissions of the first transmit waveform and the second transmit waveform; a subtractor coupled to the buffer and adapted to subtractively combine echo signals received in response to transmissions of the first transmit waveform and the second transmit waveform on a spatial basis; wherein the summer is further adapted to generate a nonlinear echo signal component at the output, and the subtractor is further adapted to generate a linear echo signal component at the output; And the ultrasound system also includes: a first time varying filter coupled to the output of the summer and adapted to change from a high pass filter characteristic to a lower band filter characteristic when receiving non-linear echo signals from increasing depths; and a second time varying filter coupled to the output of the subtractor and adapted to change from a high pass filter characteristic to a lower band filter characteristic when receiving linear echo signals from increasing depths; and An image processor adapted to generate an ultrasound image and characterized in that: The summer is further adapted to generate echo signals of harmonic frequency components, sum frequency components and difference frequency components, and the subtractor is further adapted to generate an echo signal of a fundamental frequency component, wherein all frequency components of the fundamental frequency component, the harmonic frequency components, the sum frequency component and the difference frequency component are lower than the main frequency of the second harmonic component of the highest frequency transmission waveform.

2. The ultrasound system according to claim 1, wherein: The buffer is further adapted to store echo signals received in response to transmission of the second transmit waveform.

3. The ultrasound system according to claim 1, wherein: The transmitter is further adapted to control the probe to transmit a first waveform and a second waveform, the second waveform being an inverted version of the first waveform.

4. The ultrasound system according to claim 2, wherein: The transmitter is further adapted to control the probe to transmit a first waveform and a second waveform, the second waveform being in a 180° phase relationship with the first waveform.

5. The ultrasound system of claim 1, wherein: The array transducer is further adapted to receive echo signals at a fundamental frequency, harmonic frequencies, and sum or difference frequencies of the two component frequencies.

6. The ultrasound system of claim 1, further comprising a combiner coupled to the summer and the subtractor and adapted to generate echo signals of a fundamental frequency component, a harmonic frequency component, and a sum frequency component or a difference frequency component.

7. The ultrasound system of claim 6, further comprising a tracking filter coupled to the combiner and the image processor.

8. The ultrasound system of claim 1, wherein: The second time varying filter is further adapted to emphasize the contribution of high frequency linear echo signals to the image at shallow depths and to emphasize the contribution of low frequency linear echo signals to the image at greater depths.

9. The ultrasound system of claim 8, wherein: The first time varying filter is further adapted to emphasize the contribution of non-linear echo signals to the image in a mid-range.

10. The ultrasound system of claim 1, further comprising: a first detector coupled to an output of the first time-varying filter; a second detector coupled to an output of the second time varying filter; as well as a summing circuit coupled to the outputs of the first detector and the second detector, Wherein, the summing circuit is adapted to generate an echo signal with reduced speckle.

11. The ultrasound system of claim 10, further comprising: a first weighting circuit coupled between the first detector and the summing circuit and adapted to apply a weight that varies according to depth to the nonlinear echo signal; as well as A second weighting circuit is coupled between the second detector and the summing circuit and is adapted to apply a weight that varies according to depth to the linear echo signal.

12. The ultrasound system of claim 11, wherein: The first weighting circuit is further adapted to apply maximum weight to echo signals at a mid-depth of field; and Therein, the second weighting circuit is further adapted to apply maximum weights to echo signals at shallow depths and deeper depths of the field.

Images