Patent Publication Number: US-2010113926-A1

Title: System and method for clutter filter processing for improved adaptive beamforming

Description:
BACKGROUND 
     Embodiments of the invention relate generally to imaging systems and more specifically to a method and system for improving image resolution and contrast in an ultrasound imaging system. 
     Typically, ultrasound systems comprise an array of transducer elements used for transmitting a set of waveforms into an imaging subject and for receiving a set of reflected ultrasound signals. Each waveform is emitted with a relative time delay chosen to focus the net transmitted waveform in a desired direction and depth and with a desired shape. Similarly each received signal is individually delayed to maximize the response of the system to reflected energy for a desired direction and depth and with a desired shape. The delayed receive signals are summed and processed to create and display an image of the imaging subject. 
     The transmit and receive time delays, known collectively as beamforming time delays, are typically calculated based on the assumption that sound propagates through the body with a known, constant speed. However, it is observed that different tissue types (e.g., muscle, fat, cartilage, etc.) vary significantly in their corresponding sound speed. For example, cardiac imaging is often performed by placing the transducer between two of the subject&#39;s ribs. This acoustic window contains complicated layers of fat and intercostal muscles, so that the assumption of uniform sound speed propagation may not be valid. When this assumption fails, the transmit and receive focusing is degraded and there will be a resulting loss of image resolution and contrast. 
     Conventional methods of estimating time delay corrections for ultrasound imaging are unlikely to produce reliable time delay estimates when imaging the heart. These methods either directly or indirectly estimate time delay differences between individual transducer element signals, or between transducer element signals and the sum of these element signals, or between various sub-groups transducer element signals. Nearly all time-delay estimation methods explicitly or implicitly weight the contributions of strongly reflecting scatterers more heavily than weakly reflecting scatterers. However, scattering of ultrasound from the blood pool in the interior of the heart is much weaker than scattering from the heart wall. Thus, when imaging the interior of the heart, it is quite likely that the reflected ultrasound signal may be dominated by strongly reflecting heart wall tissue in the sidelobes of the ultrasound beam and not by the weakly reflecting blood in the mainlobe of the ultrasound beam. Hence, current time-delay estimation algorithms are likely to produce time-delay corrections which erroneously steer the corrected beams toward the strongly reflecting heart wall tissue. 
     A similar problem arises when imaging any weakly scattering object which is surrounded by strong scatterers, such as blood vessels and the gall bladder. Rescaling the amplitude of the signals that are compared does not solve this problem. For example, consider the limit in which only the sign of the signals is used in the time-delay estimation. When both blood and tissue contribute to a particular sample, the sign of that sample will still tend to be dominated by the sign of the much larger tissue contribution. Rescaling the amplitude helps only when some of the samples used in the time-delay estimation are dominated by blood. 
     The display of blood flow information in the ultrasound image, commonly known as color flow or color power, is a widely used medical diagnostic tool. At deeper imaging depths, however, it is often the case that conventional color flow processing is unable to extract a usable blood velocity or power signal, especially when imaging subjects in which large beamforming aberrations are present. Similarly, since the attenuation of ultrasound increases with frequency, at a given depth a lower transmit frequency must be used to produce a useable blood signal, and this reduces the resolution of the displayed blood velocity or power information. 
     It is generally accepted that the use of point-like reflectors greatly improves the accuracy and reliability of time-delay estimation. Unfortunately point-like reflectors are generally not available in ultrasound imaging of living tissue. A method of increasing the contribution of the blood component in signals would allow small blood vessels in certain tissue structures to be used as approximate point-like reflectors for time-delay estimation. 
     Therefore there is a need for an ultrasound imaging system with improved time-delay correction capabilities. 
     BRIEF DESCRIPTION 
     Briefly in accordance with one aspect of the technique a method is provided. The method provides for extracting a blood component from an element signal produced by an element to obtain a filtered element signal, and extracting a blood component from a beamsum signal to obtain a filtered beamsum signal, calculating a time delay estimate between the filtered element signal and the filtered beamsum signal, or between the filtered element signal and the beamsum signal, or between the element signal and the filtered beamsum signal and applying the time delay estimate to correct transmit and receive beamforming time delays for the element. Systems and apparatus that afford functionality of the type defined by this method may be provided by the present technique. 
     In accordance with a further aspect of the present technique an ultrasound system is provided. The ultrasound system includes a transducer array comprising a set of array elements, each of the elements being separately operable to produce a pulse of ultrasound energy during a transmission mode and to produce an echo signal in response to energy reflected from an imaging object during receive mode, a transmitter coupled to the transducer array and being operable during the transmission mode to apply a separate transmit signal pulse with a respective time delay to each of the array elements such that a directed transmit beam is produced; a receiver coupled to the transducer array and being operable during the receive mode to sample the echo signal produced by each of the array elements as the vibratory energy impinges the imaging object and to impose a separate respective receiver time delay on each said echo signal sample to generate a corresponding plurality of receive signals, a clutter filter processor for filtering element signals to obtain a corresponding one or more filtered element signals and for filtering beamsum signals to obtain one or more filtered beamsum signals. The system further comprises a beamformer processor comprising a correlator processor for comparing the one or more filtered element signals with the one or more filtered beamsum signals, or the one or more filtered element signals with the beamsum signals or the element signals with the one or more filtered beamsum signals and a correlation sum processor for further processing to produce the beamforming time delays. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a block diagram of an ultrasound system implemented according to one aspect of the present technique; 
         FIG. 2  is a graphical representation of the signals in the clutter filtering according to one aspect of the present technique; 
         FIG. 3  is a block diagram of a clutter filter according to one aspect of the present technique; 
         FIG. 4  is a block diagram of clutter filter processing of the beamsum signal according to one aspect of the technique; 
         FIG. 5  is a block diagram of an example for calculating time delay estimates according to one aspect of the technique; 
         FIG. 6  is a schematic representation of an exemplary image display frame as obtained from one aspect of the technique; 
         FIG. 7  is a schematic representation of an exemplary image display frame as obtained according to another aspect of the technique; and 
         FIG. 8  is flow chart illustrating one method by which relative time delay estimates can be calculated in the ultrasound system. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are generally directed to improving image resolution and contrast in an ultrasound system by calculating relative time delays of blood components in ultrasound signals. The embodiments of the present invention are generally directed towards cardiac imaging but may be used for imaging other areas such as, but not limited to, the abdomen. Referring now to  FIG. 1 , a schematic diagram of an exemplary ultrasound system  10  is illustrated in accordance with aspects of the present technique. The ultrasound system  10  includes an acquisition subsystem  12  and a processing subsystem  14 . The acquisition subsystem  12  transmits ultrasound signals into a subject  16  and receives backscattered ultrasound signals from the subject  16 . The acquired ultrasound signals are then processed by the processing subsystem  14  to generate an image of the subject  16 . 
     The acquisition subsystem  12  includes a transducer assembly  18 , typically an acoustic transducer assembly, which is in contact with a patient or subject  16  during an imaging procedure. The transducer assembly  18  may comprise a plurality of transducer array elements fabricated from materials, such as, but not limited to, lead zirconate titanate (PZT), polyvinylidene difluoride (PVDF) and composite PZT. It should be noted that the transducer assembly  18  is a two-way transducer and is configured to transmit ultrasound waves into and receive reflected ultrasound waves from the subject  16 . In transmission mode, the transducer array elements convert the electrical energy into ultrasound waves and transmit it into the subject  16 . In reception mode, the transducer array elements convert the ultrasound energy received from the subject (backscattered waves) into electrical signals. 
     In the illustrated embodiment, the acquisition subsystem  12  further includes transmit/receive switching circuitry  20 , a transmitter  22 , a receiver  24 , and a beamformer  26 . Although not illustrated, in one embodiment, a clutter filter processor may form an integral part of the beamformer  26 . In another embodiment, the clutter filter processor may exist as a separate entity outside the beamformer  26 . In such an embodiment, the clutter filter processor may be in operative association with the beamformer  26 . The transmit/receive (T/R) switching circuitry  20  is coupled to the transducer array  18  for switching the transducer array  18  into transmission or reception mode. To generate ultrasound waves for transmission into the subject  16 , the processing subsystem  14  sends transmit command data to the beamformer  26 . In response to receiving the transmit command data, the beamformer  26  generates transmit parameters to create a beam of a desired shape originating from a certain point at the surface of the transducer array  18  at a desired steering angle. The beamformer  26  then sends the transmit parameters to the transmitter  22 . The transmitter  22  uses the transmit parameters to encode transmit signals to be sent to the transducer array  18  through the T/R switching circuitry  20 . The transmit signals are set at certain levels and phases with respect to each other and are provided to individual transducer elements of the transducer assembly  18 . The transmit signals excite the transducer elements to emit ultrasound waves with the same phase and level relationships. As a result, a beam of ultrasound energy is formed in a subject  16  within a scan plane along a scan line when the transducer assembly  18  is acoustically coupled to the subject  16 . This process is typically known as electronic scanning. 
     The transmitted ultrasound waves are then backscattered off the tissue and blood samples within the subject  16 . The transducer array elements receive the backscattered waves at different times depending on the distance into the tissue they return from and the angle with respect to the surface of the transducer assembly  18  at which they return. As mentioned above, the transducer array elements receive the backscattered ultrasound signals from the subject  16  and convert these backscattered signals into electrical signals. Subsequently, the electrical signals are routed through the T/R switching circuitry  20  to the receiver  24 . The receiver  24  amplifies and digitizes the received signals and may provide other functions such as gain compensation. The digitized received signals corresponding to the backscattered ultrasound waves received by each transducer element at various times preserve the amplitude and phase information of the backscattered waves. The digitized signals are then sent to the processing subsystem  14  through beamformer  26 . The processing subsystem  14  sends receive command data to beamformer  26 . The beamformer  26  uses the receive command data to form a receive beam originating from a point on the surface of the transducer assembly  18  at a steering angle typically corresponding to the point and steering angle of the previous ultrasound beam transmitted along a scan line. The beamformer  26  operates on the appropriate received signals by performing time delaying and focusing, according to the instructions of the command data from the control processor  28 , to create received beam signals corresponding to sample volumes along a scan line in the scan plane within the subject  16 . The phase, amplitude, and timing information of the received signals from the various transducer elements are used to create the received beam signals. 
     The processing subsystem  14  includes a control processor  28 , a demodulator  30 , an imaging mode processor  32 , a scan converter  34  and a display processor  36 . The control processor  28  interfaces with the imaging mode processor  32 , the scan converter  34  and the display processor  36 . Additionally the control processor  28  is responsible for sending transmit and receive command data to the beamformer  26 . The demodulator  30  demodulates the received beam signals to create pairs of I and Q demodulated data values corresponding to sample volumes within the scan plane. In one embodiment, demodulation may be accomplished by comparing the phase and amplitude of the received beam signals to a reference frequency. The I and Q demodulated data values preserve the phase and amplitude information of the received signals. 
     The demodulated data is transferred to the imaging mode processor  32 . The imaging mode processor  32  uses parameter estimation techniques to generate imaging parameter values from the demodulated data in scan sequence format. The imaging parameters may include parameters corresponding to various possible imaging modes such as B-mode, color velocity mode, spectral Doppler mode, and tissue velocity imaging mode, for example. The imaging parameter values are passed to the scan converter  34 . The scan converter  34  processes the parameter data by performing a translation from scan sequence format to display format. The translation includes performing interpolation operations on the parameter data to create display pixel data in the display format. 
     The scan converted pixel data is sent to the display processor  36  to perform any additional spatial or temporal filtering of the scan converted pixel data, to apply grayscale or color to the scan converted pixel data, and to convert the digital pixel data to analog data for display on the monitor  38 . The user interface  40  is coupled to the control processor  28  to allow a user to interface with the ultrasound system  10  based on the data displayed on the monitor  38 . 
     The display processor  36  is further coupled to a display monitor  38  for displaying images. User interface  40  interacts with the control processor  28  and the display monitor  38 . The control processor  28  may also be coupled to a remote connectivity subsystem  42  including a web server  44  and a remote connectivity interface  46 . The processing subsystem  14  may be further coupled to a data repository  48  configured to receive ultrasound image data. The data repository  48  interacts with image workstation  50 . 
     The components described in association with  FIG. 1  may be dedicated hardware elements such as circuit boards with digital signal processors or may be software running on a general-purpose computer or processor such as a commercial, off-the-shelf personal computer, or specialized workstation. The various components may be combined or separated according to various embodiments of the invention. Thus, those skilled in the art will appreciate that the ultrasound system  10  described above is provided by way of example, and the present techniques are in no way limited by the specific system configuration. 
     In certain embodiments, color flow processing may be utilized to enhance the fast moving components of an ultrasound signal and suppress the slow moving components in such a signal. The fast moving components in the ultrasound signal may be representative of the blood component and the slow moving components in the signal may be representative of the tissue component. In these embodiments, a set of transmit firings may be made in a common imaging direction and the received element signals or the beamsum signals for each firing may then be stored in a memory. The set of firings are known as a set of “clutter firings.” The memory locations may be organized as a two-dimensional memory that stores a range sample number and a firing number. The element signals or the beamsum signals may be filtered in the firing number dimension, which is often referred to as filtering in “slow time.” The element signals or the beamsum signals may be filtered in the range dimension, which is often referred to as filtering in “fast time”. As will be appreciated, the high pass filtering in “slow time” reduces the tissue contribution at a range while preserving the blood component for that range. Further, filtering in “fast time” may modify the spectral content of a signal to improve its signal-to-noise ratio or to minimize aliasing artifacts prior to decimating that signal. 
       FIG. 2  is a graphical representation  52  illustrating fast time and slow time. The horizontal axis  54  represents the “fast time” or range dimension and the vertical axis  56  represents the “slow time” or firing number dimension. Signals  68 ,  70 ,  72  and  74  represent a set of received signals for M clutter firings  58 ,  60 ,  62  and  64  for the case in which there is a large fast moving component. The filled circles highlighted by dashed line  66  represent the clutter firing samples in “slow time” corresponding to a particular range. As will be appreciated, high pass filtering these samples in “slow time” will tend to suppress the slow moving component of the clutter firing signals at that range while tending to preserve the fast moving component. Repeating this high pass filtering in “slow time” at every range results in a set of M clutter-filtered signals. 
     In the implementation described above, a set of M filtered signals is obtained for each element in the transducer for each transmit firing direction. A corresponding set of M filtered beamsum signals may be obtained by applying the same clutter filtering to the sum of unfiltered element signals. Alternatively, in another embodiment, the corresponding set of M filtered beamsum signals may be obtained by summing the filtered element signals. A relative time delay may then be calculated between the filtered element signals and the corresponding filtered beamsum signals for each element and each of the transmit firing directions. Alternatively, a relative time delay may be calculated between the filtered element signals and the beamsum signals or the element signals and the filtered beamsum signals. The resulting time delay estimates for a given element and transmit firing direction may be combined, for example by averaging the time delays or calculating the median of the time delays, to produce a time delay estimate for that particular element and the corresponding firing direction. The variation in the estimates for a given element and firing direction may be used as an estimate of the reliability of the time delay estimate, so that only reliable time delay estimates may be used to modify the beamforming time delays. Additionally, as described in U.S. Patent Application US 2007/0167802(A1) the estimates for all elements for a given firing direction may be processed collectively to improve their reliability. 
     In a conventional color flow processing, as many as 16 or more clutter firings may be used to achieve a desired “slow time” filter response. The amount of memory required to store firing information typically increases with the number of clutter firings. As described in detail with regard to  FIG. 3 , the amount of memory required may be reduced by decimating the input signals before they are stored in memory and by calculating only one of the set of M clutter-filtered signals. Decimation can considerably reduce the cost and size of the hardware required. The reduction in memory may be particularly useful in the clutter processing of element signals since 256 or more element signals may be required to adequately sample the spatial variation of time delays due to aberrations in a human body. Therefore, a small clutter filter implementation makes it economically feasible to include the clutter processing in the same integrated circuit that implements beamforming time delay processing. Further, for a fixed number of memory locations, the accuracy and robustness of the time delay estimation may be improved by decimating the samples in range before storing them in the memory. The decimation filter can also be chosen to improve the signal-to-noise ratio of the signal when the sampling rate is much higher than twice the signal bandwidth. For example, in a cardiac ultrasound system, the typical signal bandwidth of interest may extend from zero frequency to about 5 MHz, while the sampling rate may be about 50 MHz. The weights may be chosen according to the well-known techniques for the design of finite impulse response (FIR) filters. According to the present embodiment, integer value weights may be chosen to reduce the size and cost of hardware. 
       FIG. 3  is a schematic diagram of a clutter filter  76  according to another implementation of the present technique. In the illustrated embodiment, the clutter filter  76  includes a decimation block  78  and a clutter filter accumulator  86  coupled together. The decimation block  78  may further include a memory such as a register  82  and a decimator  84 . The clutter filter accumulator may further include a memory  88 . In the illustrated embodiment, the clutter filter processing may include filtering in range or “fast time” and decimation. As illustrated, a set of input signals S i  (where i=1, 2, 3 . . . M) is multiplied by a set of weights in a multiplier  80 , and a contiguous group, commonly known as a “block”, of the resulting samples are summed in the register  82 . Thereafter, the decimator  84  sends the weighted sum of input signals in the block to the clutter filter accumulator  86 . This process repeats for each block of samples in the input signal, thereby producing an output signal with fewer samples than the input signal. As illustrated, the clutter filter accumulator  86  comprises a memory  88  where the decimated signals may be accumulated and stored. The size of the memory  88  in the clutter filter accumulator  86  may be selected to accommodate the maximum desired number of decimated samples chosen by the operator or automatically by the system to contribute to time delay estimation. On the first clutter firing, the set of decimated samples from which a time delay estimate is to be produced may be written into the memory  88 . On subsequent clutter firings, each decimated sample may be added via an adder  87  to the corresponding range sample stored in the memory  88 . After the decimated samples from all the clutter firings have been added, the memory contains a filtered signal (S_f) with an enhanced blood component. This filtered element signal S_f may then be employed to calculate the relative time delays as described with regard to  FIG. 5 . 
     In one embodiment, the process as illustrated in  FIG. 3  may be performed using a single multiplier  80 , with the proper choice of weights, for the operations of filtering in “fast time” and filtering in “slow time”, thus reducing the size and cost of the implementation in hardware or the computational complexity of the implementation in software. In one example, three clutter firings with clutter weights [w 1 , w 2 , w 3 ] and decimation by a factor of four using decimation filter weights [u 1 , u 2 , u 3 , u 4 ] may be implemented. On the first clutter firing the weights [w 1 ×u 1 , w 1 ×u 2 , w 1 ×u 3 , w 1 ×u 4 ] would be applied to each block of four samples. On the second clutter firing, the weights [w 2 ×u 1 , w 2 ×u 2 , w 2 ×u 3 , w 2 ×u 4 ] would be used and the third clutter firing would use [w 3 ×u 1 , w 3 ×u 2 , w 3 ×u 3 , w 3 ×u 4 ]. 
     As will be appreciated, a beamsum signal is the weighted sum of two or more element signals. In the discussion that follows, the weights are assumed to be unity for simplicity. A filtered beamsum signal may be obtained by summing the filtered element signals as obtained in  FIG. 3 . Alternately, a filtered beamsum signal may be obtained by summing unfiltered element signals and then filtering the resulting sum. As described in U.S. Patent Application US2006/0004287(A1), one method of estimating beamforming time delay corrections converts the beamsum signal to a complex signal. This method is chosen as an illustration only. Other methods of estimating the time delay between signals known to those skilled in the art, such as methods which utilize two real signals or methods which utilize two complex signals, are equally applicable to the invention.  FIG. 4  illustrates the process  90  by which the real beamsum signal BS is converted into a complex, clutter-filtered signal BS_f. As illustrated, an input real beamsum signal BS is converted to a bandpass, analytic signal form by applying a pair of filters  92  and  94 . The real and imaginary parts of the analytic signal form, and the combined decimation- and clutter-filter weights may then be sent to a pair of clutter filters  96  and  98 . The outputs from the pair of clutter filters  96  and  98  are the real and imaginary parts of the filtered beamsum signal BS_f. In the illustrated embodiment, the real and imaginary parts of the filtered beamsum signal are interleaved in block  100 , as a single complex signal BS_f. This complex signal is the clutter filtered beamsum signal BS_f in which the blood component has been enhanced and tissue component has been suppressed. 
       FIG. 5  illustrates a process  102  to produce beamforming time-delay corrections. In the illustrated embodiment, each of the clutter filtered element signals, s_f 1 , s_f 2 , . . . , s_f N , as obtained in  FIG. 3  are compared with a reference beamsum signal BS_f as obtained in  FIG. 4  in correlator processors  106 ,  108  and  110 . The outputs of the correlator processors  106 ,  108  and  110  are the complex numbers CS 1 , CS 2 , . . . , CS N , where the phase of each of the complex numbers is proportional to a time delay between the filtered element signal and the filtered beamsum signal. These complex numbers CS 1 , CS 2 , . . . , CS N  are further processed in the correlation sum processor  112 , by means that includes but is not limited to smoothing, filtering and masking, to produce the beamforming time-delay corrections, Δτ 1 , Δτ 2 , . . . Δτ N . The output of a color flow processor  104 , which is available on many ultrasound imaging systems, operating on the beamsum data, may be an input to the correlation sum processor  112 . The time delay corrections Δτ 1 , Δτ 2 , . . . Δτ N  may then be used to modify the transmit and receive beamforming time delays. 
     It will be appreciated that the illustrated embodiment represents just one illustrative example of the invention. Suitable modifications may be made by those skilled in the art. In one example, the element signals in the imaging system may be converted to a complex analytic signal or to a complex baseband form. In another example, the clutter filtering operations may be performed using a software processor rather than integrated circuits. Various methods of estimating the relative time delay between two signals are known to those skilled in the art and are equally applicable to the invention. 
     In one embodiment, a color flow processor  104  produces an estimate of the velocity and magnitude, or power, of a blood signal. In some embodiments, the estimate of the magnitude of the blood signal may be used by the ultrasound system to overlay blood flow velocity or power information on the image display for those pixels for which the power estimate is above a threshold value. Where the power estimate is below the threshold value, the blood flow velocity or power information may not be displayed. Further, the power flow estimate from the color flow processor  104  may be used as an additional input to the time delay estimation and may be used to identify regions of the image that contain significant blood flow. This additional input may be used by the system to automatically choose regions containing a significant blood component for processing by the time-delay estimation algorithm. In general, these regions may be irregular and non-contiguous in range and in beam direction. For example, the region chosen need not correspond to the same set of ranges for all beam directions. As another example, the region need not include all the beam directions. In regions for which the color flow processor output indicates that there is little blood component, time-delay estimation can be performed in the conventional manner, without using clutter filtering, so that time delays are estimated using the tissue component of the signal. In this way, reliable time-delay estimates can be obtained for regions both with and without significant blood components. 
     In one embodiment a contrast agent may be used to increase the size of the blood component in the beamsum signal, or the element signal, or both the beamsum signal and the element signal. The injection of contrast agents in the bloodstream may be used to increase the reliability and robustness of the time delay estimates by increasing the amplitude of the blood component with respect to the tissue component. 
     In certain embodiments, the displayed output from the color flow processor may be used by an operator to choose regions on the image display for time delay estimation. 
       FIG. 6  illustrates one example of using the output of a color flow processor  104  to improve the accuracy and reliability of time-delay estimation using clutter filtering. As illustrated, significant blood flow is detected by the color flow processor in region  118 , which lies in region of interest  116  on image display  114 . For beam direction  120 , the set of samples  124  have been identified as a contiguous set for which a substantial fraction of the samples in the contiguous set contain a significant blood component. These samples will be input to the clutter filter time delay estimation blocks. Similarly, the set of samples  126  for beam direction  122  have been identified. 
       FIG. 7  illustrates the image display frame  128  a short time after the image display frame  114 , illustrated in  FIG. 6 . The figure illustrates a new set of range samples  130  for time delay estimation that is calculated for beam direction  120  using the new output from the color flow processor  104 . As illustrated, beam direction  122  does not have significant blood flow for this frame, therefore the clutter-filter time delay estimation for this beam direction is not used. 
     In  FIG. 6  and  FIG. 7 , a contiguous set of samples are chosen for each beam, but in other embodiments non-contiguous sets can be used. More generally, the output of the color flow processor  104  can be used to weight the contribution of each sample in the image to the clutter filter time-delay estimation. 
       FIG. 8 . is a flow chart illustrating a control scheme  132  for time delay estimation to correct transmit and receive beamforming time delays in ultrasound signals according to one embodiment of the present technique. In the illustrated embodiment, the control scheme  132 , includes the steps of extracting a blood component from an element signal to obtain a filtered element signal at step  134 , extracting a blood component from a beamsum signal to obtain a filtered beamsum signal at step  136 , calculating a time delay estimate between the filtered element signal and the filtered beamsum signal at step  138 . Further, the control scheme  132  includes the step of applying the time delay estimate to correct the transmit and receive beamforming time delays for the element at step  140 . 
     The above discussed techniques of enhancing ultrasound image resolution and contrast have many advantages including improved cardiac image resolution and improved color flow sensitivity. Moreover, the technique also provides for improved image resolution for difficult subjects such as abdominal imaging of heavy patients as well as for breast imaging. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.