Abstract:
A method for controlling an ultrasound imaging system includes defining a sample volume gate on a two-dimensional (2D) ultrasound image, the sample volume gate defining a location at which flow is to be estimated, automatically calculating a SNR for an initial transmit and receive steering position (aperture location) and aperture size, automatically calculating a SNR for a different second transmit and receive steering position (aperture location) and aperture size, automatically comparing the SNR for the first set of apertures to the SNR for the second set of apertures, and automatically adjusting the steering angle and an aperture size of an ultrasound probe&#39;s transmit and receive events based on the comparison.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates generally to ultrasound imaging systems, and more particularly, to a method and apparatus for improving an aperture selection of an ultrasound probe. 
         [0002]    Ultrasound imaging systems may be utilized to measure the velocity of blood flow using spectral Doppler techniques. In operation, an ultrasound probe may transmit pulsed wave (PW) or continuous wave (CW) Doppler waveforms into an object and receive backscattered and reflected ultrasonic echoes. To measure the blood flow characteristics, returning ultrasound waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers, such as for example, blood cells. The frequency shift translates into the velocity of the blood flow. 
         [0003]    PW or CW Doppler waveforms may be computed and displayed in real-time as a spectrum or spectral image of the Doppler frequency (or velocity) versus time with the gray-scale intensity (or color) modulated by the spectral power. Each spectral line represents an instantaneous measurement of blood flow within a sampling gate. To identify a specific location to acquire the spectral Doppler information, a user typically places an indicator, also referred to as a sampling gate, on the B-mode image that indicates a position at which the user desires to acquire the blood flow velocity. A user may then manually steer the ultrasound transmit beam to any desired transmit angle. 
         [0004]    In medical ultrasound imaging it is desirable to optimize the signal-to-noise ratio (SNR). SNR is the ratio of the amplitude of the sound waves to the undesired system and acoustic noise. In spectral Doppler imaging, the SNR is related to the angle at which the ultrasound waves intercept the blood cells. In operation, an optimal Doppler angle is achieved when the sound waves are parallel to the flow of blood in the blood vessel. As the angle of the sound waves intercepting the blood flow increases from the optimal parallel angle, the received signal decreases. For example, if the ultrasound probe is positioned approximately perpendicular to the flow of blood cells, the sound waves intercept the blood cells at approximately 90 degrees resulting in no Doppler frequency shift and therefore no useful signal. Optionally, if the ultrasound probe is positioned approximately parallel to the flow of blood cells, the Doppler shift and therefore returning signal is relatively high. Thus, for maximum signal it is desirable to position the ultrasound probe such that the sound waves emit in a direction that is parallel to the flow of blood cells. 
         [0005]    However, many blood vessels are not perpendicular to a surface of the patient&#39;s skin. Therefore, in many cases, the ultrasound probe can not be positioned in such a manner that the sound waves emit in a direction that is parallel to the flow of blood in the blood vessel to achieve the maximum signal. To improve the SNR, at least some known ultrasound imaging systems enable the operator to manually change the steering angle of the transmitted beam. However, increasing the steering angle of the sound waves decreases the acoustic efficiency of the imaging system. More specifically, when the sound waves are emitted in a substantially perpendicular direction from the element face, the acoustic elements in the ultrasound probe have the highest efficiency in translating electrical energy to the acoustic energy and translating acoustic energy back to electrical energy. However, as the steering angle of the sound waves is moved in a direction to a greater angle, the efficiency of the imaging system decreases. 
         [0006]    Accordingly, in operation the user balances two competing interests: positioning the ultrasound probe to be as parallel to the blood flow as possible; and positioning the ultrasound probe to emit sound waves in a substantially perpendicular direction to the face of the ultrasound probe in order to maximize SNR. As a result, it may be difficult or time consuming for an operator to position the ultrasound probe, and manually adjust the steering angle of the sound waves emitted from the ultrasound probe, in such a manner that both competing interests are taken into account and the optimal SNR is arrived at. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    In one embodiment, a method for controlling an ultrasound imaging system is provided. The method includes defining a sample volume gate on a two-dimensional (2D) ultrasound image, the sample volume gate defining a location at which flow is to be estimated, automatically calculating a SNR with an initial aperture size and position, automatically calculating a SNR with a different aperture size and or position, automatically comparing the SNR with the first aperture to the SNR with the second steering aperture, and automatically adjusting the steering angle (aperture location) and aperture size for the transmit and receive beams of an ultrasound probe based on the comparison. 
         [0008]    In another embodiment, an ultrasound imaging system is provided. The ultrasound imaging system includes an ultrasound probe having a transducer emitting ultrasound beams into a patient, the ultrasound probe acquiring a volume of ultrasound data including a blood vessel, a user interface for defining a sample volume within the blood vessel, and a processor. The processor is configured to automatically calculate a SNR with an initial aperture size and location, automatically calculate a SNR with a different second aperture size and location, automatically compare the SNR with the first aperture to the SNR with the second aperture, and automatically adjust the steering angle (aperture location) and an aperture size for the transmit and receive beams of the ultrasound probe based on the comparison. 
         [0009]    In a further embodiment, a non-transitory computer readable medium is provided. The non-transitory computer readable medium is programmed to instruct a computer to automatically calculate a SNR for an initial aperture size and location, automatically calculate a SNR for a second aperture size and location, automatically compare the SNR at the first steering position to the SNR at the second steering position, and automatically adjust the steering angle (aperture location) and an aperture size of the ultrasound probe based on the comparison. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram of a medical imaging system formed in accordance with various embodiments. 
           [0011]      FIG. 2  is flowchart of an exemplary method of improving the signal-to-noise (SNR) of acquired spectral data on an ultrasound imaging system. 
           [0012]      FIG. 3  is diagram of an ultrasound scan that may be performed in accordance with various embodiments. 
           [0013]      FIG. 4  is a B-mode image that may be generated in accordance with various embodiments. 
           [0014]      FIG. 5  is a spectrogram that may be generated in accordance with various embodiments. 
           [0015]      FIG. 6  is flowchart of a portion of the method shown in  FIG. 1  in accordance with various embodiments. 
           [0016]      FIG. 7  is a diagram of an ultrasound scan that may be performed in accordance with various embodiments. 
           [0017]      FIG. 8  is another spectrogram that may be generated in accordance with various embodiments. 
           [0018]      FIG. 9  is a block diagram illustrating a portion of the imaging system shown in  FIG. 1  in accordance with various embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The foregoing summary, as well as the following detailed description of certain embodiments, will be better understood when read in conjunction with the appended drawings. To the extent that the figures illustrate diagrams of the functional blocks of various embodiments, the functional blocks are not necessarily indicative of the division between hardware circuitry. Thus, for example, one or more of the functional blocks (e.g., processors, controllers or memories) may be implemented in a single piece of hardware (e.g., a general purpose signal processor or random access memory, hard disk, or the like) or multiple pieces of hardware. Similarly, the programs may be stand alone programs, may be incorporated as subroutines in an operating system, may be functions in an installed software package, and the like. It should be understood that the various embodiments are not limited to the arrangements and instrumentality shown in the drawings. 
         [0020]    As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. 
         [0021]    Also as used herein, the phrase “reconstructing an image” is not intended to exclude embodiments of the present invention in which data representing an image is generated, but a viewable image is not. Therefore, as used herein the term “image” broadly refers to both viewable images and data representing a viewable image. However, many embodiments generate, or are configured to generate, at least one viewable image. 
         [0022]    Described herein are various embodiments for automatically adjusting a size and a location of an ultrasound probe active aperture to improve the signal-to-noise (SNR) for a particular patient, exam, and probe location, being examined. In operation, the system of various embodiments automatically identifies a plurality of aperture sizes and locations and automatically selects the aperture size and location for both the transmit and receive apertures that provides the highest SNR capabilities. At least one technical effect is to automatically identify a beam steering angle that results in the optimal SNR. 
         [0023]    Various embodiments described herein may be implemented in an ultrasound system as shown in  FIG. 1 . More specifically,  FIG. 1  is a block diagram of an exemplary ultrasound imaging system  10  that is constructed in accordance with various embodiments. The ultrasound system  10  is capable of electrical or mechanical steering of a soundbeam (such as in 3D space) and is configurable to acquire information (e.g., image slices) corresponding to a plurality of 2D representations or images of a sample volume location (SVL) in a subject or patient, which may be defined or adjusted as described in more detail herein. The ultrasound system 10 is configurable to acquire 2D images in one or more planes of orientation. The ultrasound system  10  may be embodied in a small-sized system, such as laptop computer, a portable imaging system, a pocket sized system as well as in a larger console-type system. 
         [0024]    The ultrasound system  10  includes a transmitter  12  that, under the guidance of a beamformer  14 , drives an array of elements  16  (e.g., piezoelectric elements) within a probe  18  to emit pulsed or continuous ultrasonic signals, i.e. sound waves, into a body. A variety of geometries may be used. The sound waves are back-scattered from structures in the body, like blood cells flowing through a blood vessel, to produce echoes that return to the elements  16 . The echoes are received by a receiver  20 . The received echoes are processed by the beamformer  14 , which performs receive beamforming and outputs an RF signal. The RF signal then passes through an RF processor  22 . Optionally, the RF processor  22  may include a complex demodulator (not shown) that demodulates the RF signal to form IQ data pairs representative of the echo signals. The RF or IQ signal data may then be routed directly to a buffer  24  for storage. 
         [0025]    In the above-described embodiment, the beamformer  14  operates as a transmit and receive beamformer. Optionally, the probe  18  includes a 2D array with sub-aperture receive beamforming inside the probe  18 . The beamformer  14  may delay, apodize and/or sum each electrical signal with other electrical signals received from the probe  18 . The summed signals represent spatially focused receive echoes. The summed signals are output from the beamformer  14  to the RF processor  22 . The RF processor  22  may generate different data types, e.g. B-mode, color Doppler (velocity/power/variance), tissue Doppler (velocity), and Doppler energy, for multiple scan planes or different scanning patterns. For example, the RF processor  22  may generate blood flow Doppler data for multi-scan planes. The RF processor  22  gathers the information (e.g. I/Q, B-mode, color Doppler, tissue Doppler, and Doppler energy information) related to multiple data slices/receive events and stores the data information, which may include time stamp and orientation/rotation information, in the buffer  24 . 
         [0026]    The ultrasound system  10  also includes a processor  26  to process the acquired ultrasound information (e.g., RF signal data or IQ data pairs) and prepare frames and or volumes of ultrasound information for display on a display  28 . The processor  26  is adapted to perform one or more processing operations according to a plurality of selectable ultrasound modalities on the acquired ultrasound data. Acquired ultrasound data may be processed and displayed in real-time during a scanning session as the echo signals are received. Additionally or alternatively, the ultrasound data may be stored temporarily in the buffer  24  during a scanning session and then processed and displayed in an off-line operation. 
         [0027]    The processor  26  is connected to a user interface  30  that may control operation of the processor  26  as explained below in more detail. The display  28  may include one or more monitors that present patient information, including diagnostic ultrasound images to the user for diagnosis and analysis. The buffer  24  and/or a memory  32  may store two-dimensional (2D) or three-dimensional (3D) data sets of the ultrasound data, where such 2D and 3D data sets are accessed to present 2D (and/or 3D images). The images may be modified and the display settings of the display  28  may also be manually adjusted using the user interface  30 . 
         [0028]    In various embodiments, the ultrasound system  10  also includes an automatic aperture selection module  50 . The automatic aperture selection module  50  may be programmed to identify a size and a location of an aperture based on inputs received from the probe  18 . The aperture selection module  50  may be software running on the processor  26  or hardware provided as part of the processor  26 . More specifically, the aperture selection module  50  may be embodied as a set of instructions or program that is executed by the processor  26 . The program instructions may be written in any suitable computer language, e.g., Matlab. The processor  26  may therefore be any one or a combination of suitably appropriate processing systems, such as, for example, a microprocessor, a digital signal processor, and a field programmable logic array, among others. The processing system may be embodied as any suitable computing device, e.g., a computer, personal digital assistant (PDA), laptop computer, notebook computer, a hard-drive based device, or any device that can receive, send, and store data. 
         [0029]      FIG. 2  is a flowchart of an exemplary method  100  that may be performed by the imaging system  10  shown in  FIG. 1 . In various embodiments, the method  100  may be implemented using the aperture selection module  50  also shown in  FIG. 1 . More specifically, the method  100  may be provided as a non-transitory computer-readable medium or media having instructions recorded thereon for directing the processor  26  to perform one or more embodiments of the methods described herein. The medium or media may be any type of CD-ROM, DVD, floppy disk, hard disk, optical disk, flash RAM drive, or other type of computer-readable medium or a combination thereof. 
         [0030]    In operation, in various embodiments, the method  100  enables the ultrasound system  10  to automatically generate either a stronger Doppler signal or a Doppler signal with less noise more quickly than could be manually obtained by the user. Moreover, the method  100  may search for the strong signal using more options than the user currently has available. 
         [0031]    At  102 , a volumetric scan of a patient is performed. The volumetric scan may be performed by operating the probe  18  to emit, B-mode waveforms and generate image (not spectral) data. In operation, the user may position the probe  18  on an area of the patient to be imaged. For example, as shown in  FIG. 3 , the probe  18  may be used to acquire a volume  150  having blood vessel  152  therein. The volume  150  is defined by a plurality of sector shaped cross-sections with radial borders  154  and  156  diverging from one another at an angle  158 . The probe  18  (shown in  FIG. 1 ) electronically focuses and directs ultrasound firings longitudinally/in-plane/laterally to scan along adjacent scan lines in each scan plane and electronically or mechanically focuses and directs ultrasound firings elevationally to scan adjacent scan planes. The scan planes obtained by the probe  18  are stored in the memory  24  or  32  and are scan converted from spherical or cylindrical coordinates to Cartesian coordinates by the volume scan converter processor  26 . A volume including the scan planes is output from the processor as a rendering region  160 . The rendering region  160  is formed from multiple adjacent scan planes. 
         [0032]    Referring again to  FIG. 2 , at  104 , an image based on the volumetric scan is generated and displayed, such as on the display  28  shown in  FIG. 1 . In various embodiments, the image may be a B-mode image with or without a color Doppler image, or any other type of image that shows the vessel  152 . Moreover, the image may be a two-dimensional (2D) image or a three-dimensional (3D) image. For example,  FIG. 4  illustrates an exemplary 2D B-mode image  180  that may be generated based on information acquired from the 2D probe  18  and may be displayed on the display  28 . The user may adjust the area to be imaged by manually viewing a cursor (not shown) that is displayed on the display  28 . The user may then manually reposition the probe  18  to the desired area while concurrently viewing the location of the cursor on the B-mode image  180 . Thus, the operator may ascertain the location of the probe  18  while viewing the location of the cursor on the display  28 . 
         [0033]    Referring again to  FIG. 2 , at  106  a user defines the sample volume location (SVL) on the image generated at  104 . For example, referring again to  FIG. 4 , the user may define a SVL  186  using, for example, the user interface  30 . A sample volume gate  188  may be placed and moved within the SVL  186  to a desired location where the user wishes to evaluate flow.  FIG. 4  also illustrates an exemplary steered beam  189  that denotes the current steering angle of the ultrasound beams. In various embodiments, the SVL  186  and/or the sample volume gate  188  may be placed at the edges of the B-mode image  180 . In various embodiments, the user may also change a size and a shape of the sample volume gate  188  within the displayed B-mode image  180 . The sample volume gate  188  is illustrated on the B-mode image  180 , using a pair of lines, however it should be realized that the sample volume gate  188  may be a single point. More specifically, the user may select a single point in the blood vessel  152  to acquire flow information. Moreover, in various embodiments, the user does not need to define the SVL  186 . Rather, as described above, the user may simply point to a single area in the blood vessel  152  using the user interface  30  to define the sample volume gate  188 . 
         [0034]    Referring again to  FIG. 2 , at  108  a spectrogram is generated based on the sample volume gate  188  defined at  106 . More specifically, the processed PW or CW Doppler received echoes are computed and displayed in real-time as a spectrogram or spectral image of Doppler frequency (or velocity) versus time with the gray-scale intensity (or color) modulated by the spectral power. For example,  FIG. 5  illustrates an exemplary spectrogram  200  that may be generated at  108 . As shown in  FIG. 5 , the spectrogram  200  is generated using a plurality of spectral lines  202  wherein each spectral line  202  represents an instantaneous measurement of blood flow within the sampling gate  188 . All of the spectral lines  202  taken together therefore form the spectrogram or spectrum  200 . Accordingly, each vertical line or spectral line  202  in the spectrogram  200  corresponds to a Doppler frequency spectrum at a given time instant. Positive Doppler frequencies correspond to flow towards the probe  18 , and negative frequencies correspond to flow away from the probe  18 , as referenced by a baseline  204  at frequency equal to zero. 
         [0035]    Referring again to  FIG. 2 , at  110 , the steering angle of transmit and or receive beam (aperture location) as well as the aperture size is adjusted, or automatically optimized, such that the SNR of PW or CW Doppler waveforms received at the probe  18  is increased or maximized. In various embodiments, optimization of the steering angle may be manually initiated by the operator. For example, the display  28  may include an icon  52  (shown in  FIG. 1 ). In operation, the user may operate the user interface  30  to activate the optimization of the steering angle by selecting the icon  52 . Once the user has selected the icon  52 , the aperture selection module  50  automatically selects an aperture size and an aperture location that increases or maximizes the SNR of the PW or CW Doppler received waveforms. More specifically, in one embodiment, the aperture selection module  50  is programmed to automatically select an optimal steering angle that increases or maximizes the SNR. 
         [0036]      FIG. 6  is a flowchart illustrating the method step  110  shown in  FIG. 2 . In the exemplary embodiment, at  250  the automatic aperture selection module  50  is configured to direct one or more of the elements  16  of the probe  18  to transmit and/or receive. In this way both a transmit and receive aperture contain one or more elements is defined. The transmit and receive apertures can be defined with a center position as well as size (in units of length or elements). For example,  FIG. 7  is a diagram of an ultrasound scan that may be performed in accordance with various embodiments at step  110 . The ultrasound beam may be, for example, a non-steered beam  270  (e.g. a beam that is perpendicular to the face of the probe  18 ) or a steered beam  272  (e.g. a beam that is not perpendicular to the face of the probe  18 ). The non-steered beam  270  may be an ultrasound beam transmitted in a direction generally along the major axis of the probe  18 . The steered beam  272  may be an ultrasound beam transmitted in a direction other than that of the non-steered beam  270 . For example, the steered beam  272  may have a propagation path that is  10  degrees from the propagation path of a non-steered beam  270 . In various embodiments, the methods described herein, when selecting apertures, generates a beam which intersects with the defined center location of the SVG. The beam steering, defined either by the user or by the SNR optimization selects steered beams that intersect at the SVG. 
         [0037]    At  252 , ultrasound information acquired from either the non-steered beam  270  or the steered beam, if selected, is analyzed to determine one or more pixels selected as ‘signal’ in the computed spectrum. For example, referring again to  FIG. 5 , assume that a pixel within the line  274  represents a pixel selected as a ‘signal’ in the non-steered beam  270 . A pixel within the line  274  may be identified as the ‘signal’ for example, by identifying the pixel intensity values for each of the pixels within the line  274  and then selecting the pixel having the highest pixel intensity value. The remaining pixels or a subset of the remaining pixels in the line  274  may then be classified as ‘noise’. The signal-to-noise-ratio (SNR) of the calculated spectral line, e.g. line  274  is then calculated. In various embodiments, the SNR of the spectral line  274  may be calculated by dividing the average or maximum intensity value of the pixels labeled ‘signal’ within spectral line  274  by an average or maximum of the pixel intensity values of the pixels labeled ‘noise’ in the spectral line  274 . 
         [0038]    At  254 , the SNR of calculated spectral line  274  and the transmit and receive apertures and angles at which the ultrasound information was acquired is recorded. 
         [0039]    At  256 , the automatic aperture selection module  50  is configured to direct one or more of the elements  16  of the probe  18  to transmit and/or receive to define the transmit and receive apertures with a second size and location. In this way both a transmit and receive aperture contain one or more elements is defined with a second size at a second location. The transmit and receive apertures can be defined with a center position as well as size (in units of length or elements. For example, referring again to  FIG. 7 , the ultrasound beam may be, for example, a second ultrasound beam  276  that is 10 degrees from the propagation path of the non-steered beam  270 . Optionally, a second beam  278  may be, for example, a second ultrasound beam  278  that is 10 degrees from the propagation path of the steered beam  272 . Thus, in the exemplary embodiment, the scan step size between the first and second scan positions is approximately 10 degrees. However, other steering angles may be used. 
         [0040]    At  258 , ultrasound information acquired from either the non-steered beam  270  or the steered beam, if selected, is analyzed to determine one or more pixels selected as ‘signal’ in the computed spectrum. Ultrasound information acquired from either the non-steered beam  276  or the steered beam  278 , if selected is analyzed to determine the brightest pixel  280 . It should be realized that the spectrogram shown in  FIG. 5  is continuously updating, therefore the brightest pixel  280 , at the second scan position, may not be shown concurrently with the brightest pixel  274  at the first scan position. It should be further realized that in the exemplary embodiment, the methods described at step  108  are performed by the automatic aperture selection module  50  without user input, and therefore in various embodiments, spectrograms showing the brightest pixels, or any other information, may not be generated and displayed at  108 . Therefore, the pixels  274  and  280  are only shown in  FIG. 5  to more clearly describe the various embodiments described herein. 
         [0041]    In operation, the brightest pixel  280 , at the second scan position, may be identified for example, by identifying the pixel intensity values for each of the pixels within the sampling gate  188 , at the second scan position, and then selecting the pixel having the highest pixel intensity value. The remaining pixels in the sampling gate  188 , at the second scan position, may then be classified as noise. The SNR of the brightest pixel, e.g. the pixel  280 , at the second scan position is then calculated. In various embodiments, the SNR of the pixel  280  may be calculated by dividing the intensity value of the pixel  280  by an average of the pixel intensity values of the remaining pixels in the sampling gate  188  at the second scan position. 
         [0042]    At  260 , the SNR of the brightest pixel, e.g. the pixel  280  and the angle at which the ultrasound information at which the pixel  280  was acquired is recorded. 
         [0043]    At  262 , the SNR of the pixel  274  is compared to the SNR of the pixel  280  to generate a third or revised scan position. For example, assume that the SNR of the pixel  280  is greater than the SNR of the pixel  274 . Accordingly, the automatic aperture selection module  50  may determine that the SNR of the ultrasound beams may be increased, or improved, by steering the ultrasound beam “left” from the original or starting scan position. More specifically, the automatic aperture selection module  50  may determine that the SNR of ultrasound beams is increased when the ultrasound beam are steered at the angle as compared to the ultrasound beams  270 , as described in more detail below. 
         [0044]    At  264 , the automatic aperture selection module  50  is configured to continuously scan at various angles to identify the aperture angle and size that results in ultrasound beams having the highest SNR. More specifically, for any given aperture size and location (angle), a SNR may be calculated. Moreover, the probe  18  defines a 2D space that indicates every possible aperture angle and every possible aperture location possible. Accordingly, in operation the automatic aperture selection module  50  is configured to search this space for the best SNR. For example, at  262 , the automatic aperture selection module  50  determined that the SNR of the ultrasound beams, indicated by the line  276 , were greater than the SNR of the ultrasound beams indicated by the line  270 . Accordingly, based on a priori information, e.g. that the SNR is improved by steering the ultrasound beams away from the first scan position, denoted by the line  270  toward the second scan position denoted by the line  276 , the automatic aperture selection module  50  may perform at scan at a third position that is between the first and second scan positions, e.g. 5 degrees from the first and second scan positions. The automatic aperture selection module  50  then identifies the brightest pixel, at the third scan position and compares the SNR of the brightest pixel at the third scan position to the SNR of the brightest pixels at the first and second scan positions. In this manner, the aperture selection module  50  iteratively scans the blood vessel  152  to steer the probe  18  to various scan angles, acquire the SNR information at the various scan angles, and identify which scan angle has the highest SNR. 
         [0045]    Accordingly, in various embodiments, the aperture selection module  50  is configured to continuously scan at various angles based on a step size, for example, the aperture selection module  50  may scan the blood vessel  152  using five degrees step sizes, 10 degree step sizes, etc. Moreover, once the aperture selection module  50  has identified the steering angle that results in an increased SNR using the scan steps, the aperture selection module  50  may focus in on the area by scanning intermediate areas using smaller step sizes until the aperture selection module  50  identifies the beam steering angle that results in the highest SNR. In various embodiments, moving the beam steering angle using various angles steps reduces the overall time required to identify the steering angle at which the ultrasound beams have the highest SNR. 
         [0046]    Optionally, the aperture selection module  50  may be configured to scan the entire 2D scan plane  254  using some predetermined step size. For example, the aperture selection module  50  may be configured to perform the 3D sweep in one degree increments. The aperture selection module  50  may then identify the brightest pixel at each one degree increment, determine the SNR for the brightest pixel at each one degree increment, and then compare the SNR for each pixel at each one degree increment to identify the beam steering angle that produces the highest SNR signals. Accordingly, in various embodiments, the scan steps of the search area may have a very fine resolution, or a coarse resolution to reduce the scan time. Thus, at  264 , the aperture selection module  50  is configured to identify the steering angle and aperture size that produces ultrasound beams having the highest SNR. 
         [0047]    In various embodiments, additional transmit beams may be interleaved temporally with the transmit beams utilized to determine the steering angle and aperture size having the highest SNR as described above. For example, because a certain position in the image is scanned, using the sample volume gate  158 , the depth of the scanned position is known. Moreover, in various embodiments, the ultrasound beams may be emitted in relatively short bursts having relatively short time duration. Accordingly, there is a time duration between bursts of ultrasound beams wherein the ultrasound system is inactive, such that the ultrasound system  10  is not transmitting ultrasound beams to the target. 
         [0048]    In operation, the distance between one transmit firing and a second transmit firing is referred to as a pulse repetition time (PRT). 1/PRT=a pulse repetition frequency (PRF). The PRF may be modified by the operator by changing the sampling frequency. For example, according to the Nyquist law, if a certain frequency is desired to be sampled or to identify a given frequency, sampling must be performed at twice the desired frequency or twice the frequency to be identified. Therefore, the PRF may be driven higher to observe higher velocities in the body. However, if the PRF is relatively low, the resultant images may have aliasing artifacts. Accordingly, in various embodiments, the operator may select to increase the PRT and thus reduce potential aliasing artifacts and increase the resolution of a generated image. 
         [0049]    Referring again to  FIG. 2 , at  112 , a spectrogram is generated based on the transmit and receive steering angle (aperture location) and aperture size identified at  110 . More specifically, the PW or CW Doppler waveform is computed and displayed in real-time as a spectrum or spectral image of Doppler frequency (or velocity) versus time with the gray-scale intensity (or color) modulated by the spectral power using the aperture angle and size defined at  188 . 
         [0050]    For example,  FIG. 8  illustrates an exemplary spectrogram  300  that may be generated at  112 . As shown in  FIG. 8 , each spectral line  302  represents an instantaneous measurement of blood flow within the sampling gate  188 . All of the spectral lines  302  taken together therefore form the spectrogram or spectrum  300 . Accordingly, each vertical line or spectral line  302  in the spectrogram  300  corresponds to a Doppler frequency spectrum at a given time instant at the revised, or adjusted, scanning position determined at  264 . Positive Doppler frequencies correspond to flow towards the probe  18 , and negative frequencies correspond to flow away from the probe  18 , as referenced by a baseline  304  at frequency equal to zero. In various embodiments, the spectrogram  300  may be continuously updating as the aperture selection module  50  is continuously scanning at various angles to identify the aperture size and location that produces higher or the highest SNR. Thus, various embodiments provide a method and system that enables an operator to automatically change the aperture size and location of the ultrasound probe to increase SNR and therefore automatically improve the image quality. 
         [0051]    The various components of the ultrasound system  10  may have different configurations. For example,  FIG. 9  illustrates an exemplary block diagram of an ultrasound processor module  350 , which may be embodied as a portion of the processor  26  shown in  FIG. 1 . The ultrasound processor module  350  is illustrated conceptually as a collection of sub-modules, but may be implemented utilizing any combination of dedicated hardware boards, DSPs, processors, etc. Alternatively, the sub-modules of  FIG. 9  may be implemented utilizing an off-the-shelf PC with a single processor or multiple processors, with the functional operations distributed between the processors. As a further option, the sub-modules of  FIG. 9  may be implemented utilizing a hybrid configuration in which certain modular functions are performed utilizing dedicated hardware, while the remaining modular functions are performed utilizing an off-the shelf PC and the like. The sub-modules also may be implemented as software modules within a processing unit. 
         [0052]    The operations of the sub-modules illustrated in  FIG. 9  may be controlled by a local ultrasound controller  352  or by the processor  26 . The sub-modules  354 - 366  perform mid-processor operations. The ultrasound processor module  350  may receive ultrasound data  370  in one of several forms. In the embodiment of  FIG. 9 , the received ultrasound data  370  constitutes I,Q data pairs representing the real and imaginary components associated with each data sample. The I,Q data pairs are provided to one or more of a color-flow sub-module  354 , a power Doppler sub-module  356 , a B-mode sub-module  358 , aspectral Doppler sub-module  360  and an M-mode sub-module  362 . Optionally, other sub-modules may be included such as an Acoustic Radiation Force Impulse (ARFI) sub-module  364  and a Tissue Doppler (TDE) sub-module  366 , among others. 
         [0053]    Each of sub-modules  354 - 366  are configured to process the I,Q data pairs in a corresponding manner to generate color-flow data  372 , power Doppler data  374 , B-mode data  376 , spectral Doppler data  378 , M-mode data  380 , ARFI data  382 , and tissue Doppler data  384 , all of which may be stored in a memory  390  (or memory  24  or memory  32  shown in  FIG. 1 ) temporarily before subsequent processing. For example, the B-mode sub-module  358  may generate B-mode data  376  including a plurality of B-mode image planes, such as the image  180  shown in  FIG. 4 . 
         [0054]    The data  372 - 484  may be stored, for example, as sets of vector data values, where each set defines an individual ultrasound image frame. The vector data values are generally organized based on the polar coordinate system. 
         [0055]    A scan converter sub-module  392  accesses and obtains from the memory  390  the vector data values associated with an image frame and converts the set of vector data values to Cartesian coordinates to generate an ultrasound image frame  393  formatted for display. The ultrasound image frames  393  generated by the scan converter module  392  may be provided back to the memory  390  for subsequent processing or may be provided to the memory  24  or the memory  32 . 
         [0056]    Once the scan converter sub-module  392  generates the ultrasound image frames  393  associated with, for example, B-mode image data, and the like, the image frames  393  may be restored in the memory  390  or communicated over a bus  396  to a database (not shown), the memory  24 , and the memory  32  and/or to other processors. 
         [0057]    The scan converted data may be converted into an X,Y format for video display to produce ultrasound image frames. The scan converted ultrasound image frames are provided to a display controller (not shown) that may include a video processor that maps the video to a grey-scale mapping for video display. The grey-scale map may represent a transfer function of the raw image data to displayed grey levels. Once the video data is mapped to the grey-scale values, the display controller controls the display  28  (shown in  FIG. 1 ), which may include one or more monitors or windows of the display, to display the image frame. The image displayed in the display  28  is produced from image frames of data in which each datum indicates the intensity or brightness of a respective pixel in the display. 
         [0058]    Referring again to  FIG. 9 , a 2D video processor sub-module  394  combines one or more of the frames generated from the different types of ultrasound information. For example, the 2D video processor sub-module  394  may combine a different image frames by mapping one type of data to a grey map and mapping the other type of data to a color map for video display. In the final displayed image, color pixel data may be superimposed on the grey scale pixel data to form a single multi-mode image frame  398  (e.g., functional image) that is again re-stored in the memory  390  or communicated over the bus  396 . Successive frames of images may be stored as a cine loop in the memory  390  or memory  390 . The cine loop represents a first in, first out circular image buffer to capture image data that is displayed to the user. The user may freeze the cine loop by entering a freeze command at the user interface  30 . The user interface  30  may include, for example, a keyboard and mouse and all other input controls associated with inputting information into the ultrasound system  10  (shown in  FIG. 1 ). 
         [0059]    A 3D processor sub-module  400  is also controlled by the user interface  30  and accesses the memory  390  to obtain 3D ultrasound image data and to generate three dimensional images, such as through volume rendering or surface rendering algorithms as are known. The three dimensional images may be generated utilizing various imaging techniques, such as ray-casting, maximum intensity pixel projection and the like. 
         [0060]    The various embodiments and/or components, for example, the modules, or components and controllers therein, such as of the imaging system  10 , also may be implemented as part of one or more computers or processors. The computer or processor may include a computing device, an input device, a display unit and an interface, for example, for accessing the Internet. The computer or processor may include a microprocessor. The microprocessor may be connected to a communication bus. The computer or processor may also include a memory. The memory may include Random Access Memory (RAM) and Read Only Memory (ROM). The computer or processor further may include a storage device, which may be a hard disk drive or a removable storage drive such as an optical disk drive, solid state disk drive (e.g., flash RAM), and the like. The storage device may also be other similar means for loading computer programs or other instructions into the computer or processor. 
         [0061]    As used herein, the term “computer” or “module” may include any processor-based or microprocessor-based system including systems using microcontrollers, reduced instruction set computers (RISC), application specific integrated circuits (ASICs), logic circuits, and any other circuit or processor capable of executing the functions described herein. The above examples are exemplary only, and are thus not intended to limit in any way the definition and/or meaning of the term “computer”. 
         [0062]    The computer or processor executes a set of instructions that are stored in one or more storage elements, in order to process input data. The storage elements may also store data or other information as desired or needed. The storage element may be in the form of an information source or a physical memory element within a processing machine. 
         [0063]    The set of instructions may include various commands that instruct the computer or processor as a processing machine to perform specific operations such as the methods and processes of the various embodiments of the invention. The set of instructions may be in the form of a software program, which may form part of a tangible non-transitory computer readable medium or media. The software may be in various forms such as system software or application software. Further, the software may be in the form of a collection of separate programs or modules, a program module within a larger program or a portion of a program module. The software also may include modular programming in the form of object-oriented programming. The processing of input data by the processing machine may be in response to operator commands, or in response to results of previous processing, or in response to a request made by another processing machine. 
         [0064]    As used herein, the terms “software” and “firmware” may include any computer program stored in memory for execution by a computer, including RAM memory, ROM memory, EPROM memory, EEPROM memory, and non-volatile RAM (NVRAM) memory. The above memory types are exemplary only, and are thus not limiting as to the types of memory usable for storage of a computer program. 
         [0065]    It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the various embodiments without departing from their scope. While the dimensions and types of materials described herein are intended to define the parameters of the various embodiments, they are by no means limiting and are merely exemplary. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the various embodiments should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in 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 statement of function void of further structure. 
         [0066]    This written description uses examples to disclose the various embodiments, including the best mode, and also to enable any person skilled in the art to practice the various embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the various embodiments is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if the examples have structural elements that do not differ from the literal language of the claims, or the examples include equivalent structural elements with insubstantial differences from the literal languages of the claims.