# Patent

## Title
Methods and apparatus for ultrasound imaging

## Abstract

Some embodiments include acquisition of color Doppler data associated with a pulse repetition frequency (fPRF) and a baseline shift, detection of a first area of the color Doppler data representing zero flow velocity, detection of a second area of the color Doppler data, detection of a first transition between one or more velocities of the second area and one or more velocities of an adjacent third area of the color Doppler data, detection of a second transition between the third area and an adjacent fourth area of the color Doppler data representing non-zero flow velocities in the first direction, subtraction of a velocity corresponding to fPRF from each of the flow velocities of the third area if the second direction is positive, and addition of the velocity corresponding to fPRF to each of the flow velocities of the third area if the second direction is negative.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/102,060, filed on Oct. 2, 2008 and entitled “Method and Apparatus for Ultrasound Imaging”, the contents of which are incorporated herein by reference for all purposes.


BACKGROUND
Systems and methods described herein generally relate to the field of ultrasound imaging. More specifically, embodiments described below relate to methods and systems for color flow imaging.
Ultrasound is used to image various internal structures, including but not limited to the heart, the liver, a fetus, and blood vessels. For diagnosis of cardiovascular diseases, color Doppler (or color flow) imaging is usually used to visualize blood flow in the heart or blood vessels. Abnormal conditions often increase blood flow velocity in comparison to that under normal conditions. The increased velocity may result in aliasing within a corresponding color Doppler image. Color Doppler uses a pulse ultrasound technology for its spatial sampling capability, which limits the maximum frequency which can be detected without experiencing aliasing. The pulse repetition frequency (PRF), which is also the sampling frequency, sets the maximum frequency limitation. This limitation, in turn, limits the maximum blood flow velocity which can be measured without exhibiting aliasing. This limitation may be particularly problematic in cardiac cases. For example, the PRF cannot be set high enough to measure abnormally high blood velocities that occur at substantial imaging depths such as, for example, regurgitation jets across heart valves. Therefore, under abnormal cardiac conditions, color Doppler often exhibits aliasing, thereby reducing the reliability of any diagnosis based on the blood flow image. Thus, there exists a need to address this aliasing problem.
SUMMARY
Some embodiments relate to methods, systems and program code stored on a tangible medium to correct aliasing in color Doppler data. Embodiments may include acquisition of color Doppler data associated with a pulse repetition frequency (fPRF) and a baseline shift, detection of a first area of the color Doppler data representing zero flow velocity, detection of a second area of the color Doppler data adjacent to the first area and representing non-zero flow velocities in a first direction, detection of a first transition between one or more velocities of the second area corresponding to Doppler shift frequencies greater than




  
    f
    PRF
  
  2




minus a first positive preset value minus the baseline shift if the first direction is positive or minus the baseline shift if the first direction is negative and one or more velocities of an adjacent third area of the color Doppler data in a second direction substantially opposite to the first direction and corresponding to Doppler shift frequencies greater than





  
    f
    PRF
  
  2




minus a second positive preset value plus the baseline shift if the second direction is negative or minus the baseline shift if the second direction is positive, detection of a second transition between the third data and an adjacent fourth area of the color Doppler data representing non-zero flow velocities in the first direction, subtraction of a velocity corresponding to fPRF from each of the flow velocities of the third area if the second direction is positive, and addition of the velocity corresponding to fPRF to each of the flow velocities of the third area if the second direction is negative.




BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A: Blood velocity distribution in a vessel and transmit ultrasound beam direction.
FIG. 1B: Non-aliased color Doppler image corresponding to the velocity distribution in FIG. 1A.
FIG. 2A: Blood velocity distribution in a vessel and transmit ultrasound beam direction.
FIG. 2B: Aliased color Doppler image corresponding to the velocity distribution in FIG. 2A.
FIG. 3: A diagram of an ultrasound diagnostic imaging system (prior art).
FIG. 4: A diagram of an ultrasound diagnostic imaging system according to some embodiments and using line data.
FIG. 5: A diagram of an ultrasound diagnostic imaging system according to some embodiments and using scan-converted images.
FIG. 6A: A representation of color flow lines.
FIG. 6B: A representation of a scan-converted color flow image.
FIG. 7A: A representation of B-mode lines.
FIG. 7B: A representation of a scan-converted B-mode image.
FIG. 8A: Aliasing detection in a vertical direction using color flow lines.
FIG. 8B: Aliasing detection in horizontal direction using color flow lines.
FIG. 8C: Aliasing detection in a diagonal direction using color flow lines.
FIG. 9A: Aliasing detection in vertical direction using a color flow image.
FIG. 9B: Aliasing detection in horizontal direction using a color flow image.
FIG. 9C: Aliasing detection in a diagonal direction using a color flow image.
FIG. 10: A flow diagram of a process to address aliasing according to some embodiments.
FIG. 11A: Color-coded Doppler shift frequency (velocity) scale with no baseline shift.
FIG. 11B: Color-coded Doppler shift frequency (velocity) scale with a baseline shift of




  
    
      f
      PRF
    
    4
  
  .




FIG. 11C: Color-coded Doppler shift frequency (velocity) scale with a baseline shift of




  -
  
    
      
        f
        PRF
      
      4
    
    .
  







DETAILED DESCRIPTION
Embodiments will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Before embodiments of the invention are explained in detail, it is to be understood that embodiments are not limited in their application to the details of the examples set forth in the following description or illustrated in the figures. Other embodiments may be practiced or carried out in a variety of applications and in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “mounted,” “connected,” and “coupled,” are used broadly and encompass both direct and indirect mounting, connecting, and coupling. Further, “connected,” and “coupled” are not restricted to physical or mechanical connections or couplings.
It should be noted that embodiments are not limited to any particular software language described or that is implied in the figures. One of ordinary skill in the art will understand that a variety of alternative software languages may be used for implementation of some embodiments. It should also be understood that some of the components and items are illustrated and described as if they were hardware elements, as is common practice within the art. However, one of ordinary skill in the art, and based on a reading of this detailed description, would understand that, in at least one embodiment, components in the method and system may be implemented in software or hardware.
Ultrasound is transmitted by an ultrasound transducer into a human body to image various internal structures, including but not limited to blood vessels, a fetus, and the heart. Scatterers in tissue scatter the ultrasound and the scattered ultrasound is returned to the transducer. A receive beamformer creates ultrasound beams and a post-processor creates an image of tissues from the amplitude of the returned ultrasound signal as a B-mode image.
Blood vessels or the heart are often imaged, since they indicate cardiovascular conditions of patients. Blood flow information is usually acquired using color Doppler and spectral Doppler techniques.
Color Doppler is a two-dimensional imaging technique commonly used for imaging blood flow by sending ultrasonic waves into the blood flow and detecting the scattered ultrasound from the moving red cells. It consists of many beams similar to a B-mode image. A description of a color Doppler technique now follows; embodiments are not limited to the specific details therein.
In order to detect flow velocity, color Doppler transmits ultrasound signals several times per position to detect motion. To create a two-dimensional flow image, the transmit position is shifted by sub-millimeters, or about the order of an ultrasound wavelength. The transmit position shifting is repeated about 100 times to cover several centimeters and to create a two-dimensional flow image in linear and convex formats. For a phased array transducer or a sector image format, the transmit direction is changed a small angle, for example, about 0.5-1.0 degrees. This is repeated approximately 100 times to cover about 90 degrees of a sector image. For each transmit position or direction, ultrasound is transmitted several times. Received beamformed RF ultrasound signals undergo quadrature demodulation resulting in complex, Doppler I-Q signals.
In a color Doppler technique, the ultrasound is transmitted at a pulse repetition frequency (PRF) and the blood flow velocity is detected as the shift in frequency (Doppler shift frequency) in the received ultrasound signal. The received ultrasound is mixed with in-phase (0 degrees) and quadrature (90 degrees) reference signals of the same frequency as the transmit ultrasound frequency. After low-pass filtering high frequency components (e.g., second harmonics), only the baseband signals are obtained. Wall filtering (i.e., high-pass filtering) is applied to the baseband signals to remove strong clutter noise from tissue and slowly moving tissues such as blood vessel walls, resulting in complex I-Q Doppler signals. The wall filtering is performed because the Doppler I-Q signals may contain blood flow signal components as well as stationary tissue signal components. The stationary components are typically 30-40 dB greater than the blood flow components. Therefore, it is desirable to reduce or eliminate the stationary signal components in order to detect blood flow accurately.
Generally, the wall-filtered complex I-Q signal is used to derive the Doppler shift frequency because the Doppler shift frequency and the blood velocity have the following relationship




  
    
      
        
          
            Δ
            ⁢
            
              
            
            ⁢
            f
          
          =
          
            
              2
              ⁢
              
                f
                t
              
              ⁢
              v
              ⁢
              
                
              
              ⁢
              cos
              ⁢
              
                
              
              ⁢
              θ
            
            c
          
        
        ,
      
    
    
      
        (
        1
        )
      
    
  




where Δf is the Doppler shift frequency, ft is the transmitted frequency, v is the blood velocity, θ is the angle between the ultrasound beam direction and the velocity vector and c is the speed of sound.
In the case of color Doppler, the number of the sampled signals is limited to only about 10. Therefore, an auto-correlation technique is usually used to determine the phase differences between the wall-filtered I-Q signal and then to determine the Doppler shift frequency and the blood flow velocity as follows. The color Doppler's I-Q signals z(n)=x(n)+jy(n) are used to calculate “auto-correlation” R as shown in the following equation, where z(n) is the wall-filtered complex I-Q Doppler signal, x(n) is the in-phase (real) signal, y(n) is the quadrature phase (imaginary) signal, n indicates the signal number, j is the imaginary unit and * indicates the complex conjugate.

R=Σz(n)·z*(n−1)  (2)

The real (Real(R)) and imaginary (Imag (R)) parts of R are used to obtain the phase φ as shown in the following equation.




  
    
      
        φ
        =
        
          
            tan
            
              -
              1
            
          
          ⁢
          
            
              
                
                  Im
                  ⁢
                  ag
                
                (
                R
                )
              
              ⁢
              
                
              
            
            
              
                Re
                ⁢
                al
              
              ⁡
              
                (
                R
                )
              
            
          
        
      
    
    
      
        (
        3
        )
      
    
  




Since tan−1 usually provides only −0.5π to 0.5π, the position of complex value R in the complex coordinate may be also used to derive φ in the range of −π to π. The phase φ is then related to the Doppler shift frequency as shown in the following equation.




  
    
      
        
          Δ
          ⁢
          
            
          
          ⁢
          f
        
        =
        
          
            φ
            ⁢
            
              
            
            ⁢
            
              f
              PRF
            
          
          
            2
            ⁢
            π
          
        
      
    
    
      
        (
        4
        )
      
    
  




Other techniques can be used to obtain the phase and the Doppler shift frequency and the blood flow velocity. The Doppler shift frequency indicates the blood flow velocity. Additionally, the power of the high-pass filtered Doppler I-Q signals indicates the existence of blood flow and the variance of the data indicates turbulence.
Because the color Doppler signals are obtained by the pulsed ultrasound (and also sampling) technique, sampling theory dictates a maximum frequency limit. The maximum frequency is generally half of the pulse repetition frequency (PRF) or fPRF. Since the autocorrelation is performed on the complex I-Q Doppler signals, blood flow velocity in a negative direction appears in the negative frequency domain. Therefore, the color Doppler frequency includes negative frequencies that correspond to negative velocities. For example, the Doppler shift frequency usually has a range of




  
    
      -
      
        
          f
          PRF
        
        2
      
    
    ⁢
    
      
    
    ⁢
    to
    ⁢
    
      
    
    ⁢
    
      
        f
        PRF
      
      2
    
  
  ,




which in turn corresponds to a range of negative and positive maximum velocities.

Some embodiments employ other Doppler shift frequency ranges. For example, the range may incorporate a “baseline shift” in which the center frequency of the range is not equal to zero. In some embodiments, the baseline shift may be selected from a range of frequencies between




  
    -
    
      
        f
        PRF
      
      2
    
  
  ⁢
  
    
  
  ⁢
  and
  ⁢
  
    
  
  ⁢
  
    
      
        f
        PRF
      
      2
    
    .
  




In a particular example as shown in FIG. 11C, a Doppler shift frequency range of





  
    -
    
      
        f
        PRF
      
      4
    
  
  ⁢
  
    
  
  ⁢
  to
  ⁢
  
    
  
  ⁢
  
    
      3
      ⁢
      
        f
        PRF
      
    
    4
  




reflects a baseline shift of





  -
  
    
      
        f
        PRF
      
      4
    
    .
  




This Doppler frequency range is associated with a negative maximum velocity corresponding to





  -
  
    
      f
      PRF
    
    4
  




and a positive maximum velocity corresponding to





  
    
      3
      ⁢
      
        f
        PRF
      
    
    4
  
  .




Similarly, a Doppler shift frequency range of





  
    -
    
      
        3
        ⁢
        
          f
          PRF
        
      
      4
    
  
  ⁢
  
    
  
  ⁢
  to
  ⁢
  
    
  
  ⁢
  
    
      f
      PRF
    
    4
  




reflects a baseline shift of





  
    f
    PRF
  
  4




as shown in FIG. 11B. This Doppler frequency range is associated with a negative maximum velocity corresponding to





  -
  
    
      3
      ⁢
      
        f
        PRF
      
    
    4
  




and a positive maximum velocity corresponding to





  
    
      f
      PRF
    
    4
  
  .




FIG. 11A corresponds to the Doppler shift frequency ranges of FIG. 1B and FIG. 2B, in which the baseline (i.e., 0 Hz) is in the center of the Doppler shift frequency (velocity) scale. When the baseline is shifted, e.g. by




  
    f
    PRF
  
  4




as shown in FIG. 11B, the positive maximum frequency becomes





  
    f
    PRF
  
  4




while the negative maximum frequency becomes





  -
  
    
      
        3
        ⁢
        
          f
          PRF
        
      
      4
    
    .
  




If the baseline shift is





  
    -
    
      
        f
        PRF
      
      4
    
  
  ,




the positive maximum frequency becomes





  
    3
    ⁢
    
      f
      PRF
    
  
  4




while the negative maximum frequency decreases to





  -
  
    
      f
      PRF
    
    4
  




as shown in FIG. 11C. In other words, the positive maximum frequency is decreased by the baseline shift while the absolute magnitude of the negative maximum frequency is increased by the baseline shift.

Often in cardiovascular applications, as well as in other applications, blood velocities may exceed these maximum velocities, resulting in aliasing. Color Doppler imaging uses color coding methods to display blood velocities (or corresponding Doppler shift frequencies) in colors. With respect to FIG. 1B, the positive velocities may be displayed in shades of red 140, with higher positive velocities represented by lighter red 142 and lower positive velocities represented by darker red 144, while the negative velocities may be displayed in shades of blue 150, with higher negative velocities represented by lighter blue 152 and lower negative velocities represented by darker blue 154. Other color coding methods can be used to represent blood flow velocities.
FIG. 1B depicts a Doppler shift frequency range of




  
    -
    
      
        f
        PRF
      
      2
    
  
  ⁢
  
    
  
  ⁢
  to
  ⁢
  
    
  
  ⁢
  
    
      
        f
        PRF
      
      2
    
    .
  




Therefore, the Doppler data of FIG. 1B is associated with a baseline shift of 0 Hz (i.e., no baseline shift). As mentioned above, embodiments are not limited thereto.

When aliasing occurs, the color flow image may “wrap around” at velocities corresponding to the positive maximum frequency 160, with velocities corresponding to frequencies which exceed the positive maximum frequency 160 represented by colors associated with negative velocities (e.g., shades of blue). Conversely, aliasing may cause velocities corresponding to frequencies which exceed (in absolute value) the negative maximum frequency 162 to be represented by colors associated with positive velocities (e.g., shades of red). Aliasing therefore complicates the blood velocity image and makes any diagnosis based thereon difficult.
FIG. 1A shows a typical blood velocity distribution 102 within a vessel 100. At the vessel walls 104, 106, blood flow velocities are generally zero due to non-slip conditions and according to the laws of fluid mechanics. Blood velocities increase with position away from the walls. Generally, the velocity is the highest near the center of vessel 108. A good example is a parabolic velocity distribution which is defined as




  
    
      
        
          V
          =
          
            
              V
              MAX
            
            ⁢
            
              {
              
                1
                -
                
                  
                    (
                    
                      r
                      
                        R
                        a
                      
                    
                    )
                  
                  2
                
              
              }
            
          
        
        ,
      
    
    
      
        (
        5
        )
      
    
  




where V is velocity 114 at radial position r 110, Ra is radius 112 and VMAX is the maximum velocity 116 in the vessel center. The velocity distribution is smooth as the velocity changes gradually across the lumen radius 112. Assuming the maximum velocity 116 corresponds to a Doppler shift frequency that is less than the




  
    
      f
      PRF
    
    2
  
  ,




color Doppler technique may produce a color flow image 120 as represented in FIG. 1B, although colors cannot be reproduced in this black and white document using the color coding 122 methods described previously. The transmit ultrasound beam 118 is pointing towards the blood flow as shown in FIG. 1A. Color flow image area 124 at the vessel center is light red while color flow image areas 126, 128 close to the vessel walls 130, 132 are dark red. This color flow image 120 is not aliased, and therefore does not exhibit any “wrap around” transitions from light red to light blue or light blue to light red as described previously.

On the other hand, FIG. 2A shows a case in which some velocities of blood flow velocity distribution 210 correspond to a frequency greater than the positive or negative maximum frequencies




  
    (
    
      
        in
        ⁢
        
          
        
        ⁢
        this
        ⁢
        
          
        
        ⁢
        example
      
      ,
      
        
          -
          
            
              f
              PRF
            
            2
          
        
        ⁢
        
          
        
        ⁢
        to
        ⁢
        
          
        
        ⁢
        
          
            f
            PRF
          
          2
        
      
    
    )
  
  .




These blood flow velocities may therefore be aliased as depicted in FIG. 2B. Specifically, the corresponding color flow image 230 of FIG. 2B exhibits transitions 232, 234 from light red to light blue. From the near wall side 240, the flow image 230 shows dark red 242, light red 244, light blue 246, darker blue 248, light blue 250, light red 252 and dark red 254 in colors. Color flow image 230 is therefore an inaccurate portrayal of velocity distribution 210 due to aliasing.

FIG. 10 is a flow diagram of a process 1000 to address aliasing according to some embodiments. Process 1000 may be embodied in any combination of hardware and/or software. In one example, process 1000 is embodied in processor-executable process steps stored on a tangible medium. Process 1000 may be executed by a correction unit as will be described below with respect to FIGS. 4 and 5.
It should be understood that process 1000 describes the processing of one contiguous aliased image area. Accordingly, a system according to some embodiments may process non-aliased image areas differently. A system according to some embodiments may also or alternatively process multiple aliased image areas by executing process 1000 more than once.
Initially, at 1010, color Doppler data associated with a pulse repetition frequency is acquired. As used herein, “color Doppler data” may refer to color flow line data or color Doppler image data (e.g., scan-converted image data) generated from such color flow line data. Accordingly, process 1000 may be performed on either type of Doppler data.
The color Doppler data acquired at 1010 is also associated with a baseline shift. For simplicity, the foregoing example assumes a baseline shift of 0 Hz, which corresponds to a Doppler shift frequency range of




  
    -
    
      
        f
        PRF
      
      2
    
  
  ⁢
  
    
  
  ⁢
  to
  ⁢
  
    
  
  ⁢
  
    
      
        f
        PRF
      
      2
    
    .
  




As described above, the acquired color Doppler data may be generated based on sampled data as shown in equation 2. The frequency of these samples is equal to the pulse repetition frequency mentioned at 1010.
Next, at 1020, a first area of the color Doppler data is detected. The first area represents a zero flow velocity. Turning to the example of FIG. 2B, near wall area 240 of the color Doppler data appears black 241, representing zero flow velocity. A second area adjacent to the first area is detected at 1030, where the second area represents non-zero flow velocities in a first direction. With respect to FIG. 2B, the detected second area may include dark red area 242 and light red area 244.
A first transition is detected at 1040. The first transition is a transition between one or more flow velocities of the second area corresponding to Doppler shift frequencies having magnitudes greater than half the pulse repetition frequency




  (
  
    
      f
      PRF
    
    2
  
  )




minus a first preset positive value and minus any baseline shift and one or more flow velocities of a third area of the color Doppler data. The flow velocities of the third area are in a second direction substantially opposite to the first direction and correspond to Doppler shift frequencies having magnitudes greater than half the pulse repetition frequency





  (
  
    
      f
      PRF
    
    2
  
  )




minus a second preset positive value and plus the baseline shift. The first preset positive value may be equal to or different from the second preset positive value.

The above description of 1040 assumes that the flow velocities of the second area are in a direction toward the ultrasound transducer used to obtain the sampled data (i.e., the first direction is “positive”). If the flow velocities of the second area are in a direction away from ultrasound transducer (i.e., the first direction is negative), the first transition is detected between one or more flow velocities of the second area corresponding to Doppler shift frequencies having magnitudes greater than half the pulse repetition frequency




  (
  
    
      f
      PRF
    
    2
  
  )




minus a first preset positive value and plus any baseline shift and one or more flow velocities of the third area in a second direction substantially opposite to the first direction and corresponding to Doppler shift frequencies having magnitudes greater than half the pulse repetition frequency





  (
  
    
      f
      PRF
    
    2
  
  )




minus a second preset positive value and minus the baseline shift. Again, the first preset positive value may be equal to or different from the second preset positive value.

Transition 232 of color flow image 230 may be detected at 1040 according to some embodiments. More specifically, transition 232 is a transition between one or more flow velocities (244) corresponding to Doppler shift frequencies having magnitudes greater than half the pulse repetition frequency




  (
  
    
      f
      PRF
    
    2
  
  )




minus a first preset positive value (and minus a 0 Hz baseline shift since the first direction is positive in the present example) and one or more flow velocities (246) of a third area in a second direction substantially opposite to the first direction and corresponding to Doppler shift frequencies having magnitudes greater than half the pulse repetition frequency





  (
  
    
      f
      PRF
    
    2
  
  )




minus a second preset positive value (and plus the 0 Hz baseline shift since the second direction is negative in the present example). Unlike the velocities (244, 246) immediately adjacent to the first transition, the magnitude of some velocities (e.g., 242) of the second area and some velocities (e.g., 248) of the third area might not exceed the above-described thresholds.

A second transition is detected at 1050, between the third area and an adjacent fourth area of the color Doppler data. The fourth area represents non-zero flow velocities in the first direction. Transition 234 of image 230 is an example of such a second transition according to some embodiments.
By virtue of the foregoing, it is determined that the third area between the first and second transitions is aliased. To correct the aliasing, it is determined at 1060 whether the second direction is positive (i.e., toward the ultrasound transducer used to obtain the sampled data). If so, the third area is corrected at 1070 by subtracting a velocity value corresponding to the pulse repetition frequency from each velocity of the third area. If the second direction is not positive, the third area is corrected at 1080 by adding the velocity value corresponding to the pulse repetition frequency to each velocity of the third area.
As mentioned above, process 1000 may be applied to color flow line data or color Doppler image data according to some embodiments. FIG. 6A shows color flow lines 600 before scan-conversion, although only 11 lines are shown as an example. A color flow line consists of many color flow data samples along the line. Color flow lines are created from color beam data and may not show correct spatial dimensions. Scan-conversion is a technique to convert the color flow lines to a raster video image by interpolating the color flow lines. In a scan-converted image (e.g., sector scan) shown FIG. 6B, the color flow image consists of color flow image pixels of the orthogonal (x-y) coordinate with the correct length relationship (vertical vs. horizontal dimensions) in contrast to the color flow lines shown in FIG. 6A. B-mode imaging also uses the scan-conversion technique to convert B-mode lines as shown in FIG. 7A to a B-mode image as shown in FIG. 7B by interpolating B-mode line data.
Process 1000 may begin at a near or far field in some embodiments. Alternately, process 1000 may be performed from both near and far fields.
Embodiments of process 1000 may operate upon color flow lines 800 by one line at a time as shown in FIG. 8A, where color flow data samples are represented by dots 820. More specifically, process 1000 may proceed from one end of a color flow line to another end and then onto the next color flow line until the last color flow line is processed. A color flow line 800 represents many color flow data samples 820 in a color flow beam.
In some embodiments, process 1000 is performed horizontally across color flow lines 840 as shown in FIG. 8B, using color flow line data samples 830. Process 1000 may be performed diagonally across color flow lines 860 as shown in FIG. 8C, using the color flow line data samples 850.
Process 1000, in some embodiments, is performed on a scan-converted color flow image 620 pixel by pixel. The color flow image 620 is obtained by scan-converting the color flow lines 600. In some embodiments, as shown in FIG. 9A using color flow data samples 920, process 1000 is performed vertically from top to bottom in a first color flow image column and moves onto the next column until the last column is reached. Alternately, as shown in FIG. 9B using color flow data samples 940, process 1000 may be performed horizontally. As shown in FIG. 9C, using color flow data samples 970, process 1000 may be performed diagonally across a color flow image 980. The path traversed by a process according to some embodiments may be a straight line or a curve.
To find the vessel or cardiac boundary (i.e., a first area representing zero flow velocity), a scan-converted B-mode image 720 may be used. Alternately, B-mode line data 700 may be used. In an alternate embodiment, the B-mode image 720 may not be used to find the vessel or cardiac boundary. Instead, color flow data 600 or 620 itself is used to find the boundary. On the boundary, the blood flow velocity is zero and thus is assigned the color black (or no color). The color flow velocity may be also used to find the boundary. The power of color flow data before and/or after the wall-filtering may also be used to find the boundary. The power of color flow data before the wall-filtering may be similar to the B-mode signal while the power of color flow data after the wall-filtering indicates flow.
FIG. 3 shows a diagram of a conventional ultrasound diagnostic imaging system with B-mode imaging, Doppler spectrum and color Doppler imaging (e.g., see U.S. Pat. No. 4,573,477, U.S. Pat. No. 4,622,977, U.S. Pat. No. 4,641,668, U.S. Pat. No. 4,651,742, U.S. Pat. No. 4,651,745, U.S. Pat. No. 4,759,375, U.S. Pat. No. 4,766,905, U.S. Pat. No. 4,768,515, U.S. Pat. No. 4,771,789, U.S. Pat. No. 4,780,837, U.S. Pat. No. 4,799,490, and U.S. Pat. No. 4,961,427). The system may include other imaging modes, e.g. elasticity imaging, 3D imaging, real-time 3D imaging, tissue Doppler imaging, tissue harmonic imaging, contrast imaging and others. An ultrasound signal is transmitted from an ultrasound probe 330 driven by a transmitter 310 through a transmit/receive switch 320. The probe 320 may consist of an array of transducer elements which are separately driven by the transmitter with different time-delays so that a transmit ultrasound beam is focused and steered. A beamformer 340 receives the received ultrasound signal(s) from the probe 330 through the switch 320 and processes the signal(s) 325. The beamformer applies delays and/or phases to the signals and the resultant signals are summed for focusing and steering a receive ultrasound beam. The beamformer may also apply apodization, amplification and filtering.
The processed signal 345 is coupled to a Doppler spectrum processor 350, a color flow processor 360, and a B-mode image processor 370. The Doppler spectrum processor 350 includes a Doppler signal processor and a spectrum analyzer, and processes Doppler flow velocity signals and calculates and outputs a Doppler spectrum 355. The color flow processor 360 processes the received signal 345 and calculates and outputs velocity, power and variance signals 365. The B-mode image processor 370 processes the received signal 345 and calculates and outputs a B-mode image 375 or the amplitude of the signal by an amplitude detection.
The Doppler spectrum signals 355, color flow processor signals (velocity, power, and variance) 365 and B-mode processor signals 375 are coupled to a scan converter 380 that converts the signals to scan-converted signals. The output of scan converter 380 is coupled to a display monitor 390 for displaying ultrasound images.
FIG. 4 shows a diagram of an ultrasound imaging system including a color Doppler aliasing correction processor 464 according to some embodiments. The aliasing correction processor 464 may perform process 1000 with respect to color flow line data as described above. The aliasing correction processor 464 receives output 465 from the color Doppler processor 460 as well as the scan-converted B-mode image from the scan converter 480. The scan-converted B-mode image may be used to find the boundary (walls) of the blood flow structure, e.g. the heart or vessels. Output 465 comprises color flow line data rather than the scan-converted color Doppler image. The aliasing correction processor 464 receives the scan-converted B-mode image from scan converter 480 and finds color Doppler image pixels corresponding to the boundary. Alternately, the B-mode image processor 470's output may be directly input to the aliasing correction processor to indicate the boundary. FIG. 5 shows a diagram of embodiments in which the testing and correction of color Doppler aliasing is performed in the scan-converted image domain rather than the line data domain which was discussed previously. The B-mode image and color Doppler image are scan-converted before the aliasing correction processor 564 performs processing thereon.
One or more embodiments have been described. Nevertheless, various modifications will be apparent to those in the art.

## Claims

1. A method comprising:
acquiring color Doppler data associated with a pulse repetition frequency (fPRF) and a baseline shift;
detecting a first area of the color Doppler data representing zero flow velocity;
detecting a second area of the color Doppler data adjacent to the first area and representing non-zero flow velocities in a first direction;
detecting a first transition between one or more velocities of the second area corresponding to Doppler shift frequencies greater than





  
    f
    PRF
  
  2




minus a first positive preset value minus the baseline shift if the first direction is positive or plus the baseline shift if the first direction is negative and one or more velocities of an adjacent third area of the color Doppler data in a second direction substantially opposite to the first direction and corresponding to Doppler shift frequencies greater than




  
    f
    PRF
  
  2




minus a second positive preset value plus the baseline shift if the second direction is negative or minus the baseline shift if the second direction is positive;
detecting a second transition between the third data and an adjacent fourth area of the color Doppler data representing non-zero flow velocities in the first direction;
subtracting a velocity corresponding to fPRF from each of the flow velocities of the third area if the second direction is positive; and
adding the velocity corresponding to fPRF to each of the flow velocities of the third area if the second direction is negative.

2. A method according to claim 1, wherein the color Doppler data comprises color flow line data.

3. A method according to claim 1, wherein the color Doppler data comprises scan-converted color flow image data.

4. A method according to claim 1, wherein detecting the first area of the color Doppler data representing zero flow velocity comprises:
detecting the first area of the color Doppler data based on B-mode data or the color Doppler data.

5. A method according to claim 1, wherein detecting the second area, the first transition and the second transition may proceed along a color flow line, a line across color flow lines, a vertical or horizontal line in a scan-converted color flow image, or a line across said scan-converted image columns.

6. A method according to claim 1, wherein the baseline shift is greater than or equal to




  -
  
    
      f
      PRF
    
    2
  




and less than or equal to




  
    
      f
      PRF
    
    2
  
  .

7. A system comprising:
an aliasing correction processor to:
acquire color Doppler data associated with a pulse repetition frequency (fPRF) and a baseline shift;
detect a first area of the color Doppler data representing zero flow velocity;
detect a second area of the color Doppler data adjacent to the first area and representing non-zero flow velocities in a first direction;
detect a first transition between one or more velocities of the second area corresponding to Doppler shift frequencies greater than





  
    f
    PRF
  
  2




minus a first positive preset value minus the baseline shift if the first direction is positive or plus the baseline shift if the first direction is negative and one or more velocities of an adjacent third area of the color Doppler data in a second direction substantially opposite to the first direction and corresponding to Doppler shift frequencies greater than




  
    f
    PRF
  
  2




minus a second positive preset value plus the baseline shift if the second direction is negative or minus the baseline shift if the second direction is positive;
detect a second transition between the third area and an adjacent fourth area of the color Doppler data representing non-zero flow velocities in the first direction;
subtract a velocity corresponding to fPRF from each of the flow velocities of the third area if the second direction is positive; and
add the velocity corresponding to fPRF to each of the flow velocities of the third area if the second direction is negative.

8. A system according to claim 7, further comprising a color Doppler processor to provide the color Doppler data to the aliasing correction processor,
wherein the color Doppler data comprises color flow line data.

9. A system according to claim 7, further comprising a scan converter to provide the color Doppler data to the aliasing correction processor,
wherein the color Doppler data comprises scan-converted color flow image data.

10. A system according to claim 7, wherein detecting the first area of the color Doppler data representing zero flow velocity comprises:
detecting the first area of the color Doppler data based on B-mode data or the color Doppler data.

11. A system according to claim 7, wherein detecting the second area, the first transition and the second transition may proceed along a color flow line, a line across color flow lines, a vertical or horizontal line in a scan-converted color flow image, or a line across said scan-converted image columns.

12. A system according to claim 7, wherein the baseline shift is greater than or equal to




  -
  
    
      f
      PRF
    
    2
  




and less than or equal to




  
    
      f
      PRF
    
    2
  
  .

13. A tangible non-transitory computer-readable medium storing processor-executable program code, the program code comprising:
code to acquire color Doppler data associated with a pulse repetition frequency (fPRF) and a baseline shift;
code to detect a first area of the color Doppler data representing zero flow velocity;
code to detect a second area of the color Doppler data adjacent to the first area and representing non-zero flow velocities in a first direction;
code to detect a first transition between one or more velocities of the second area corresponding to Doppler shift frequencies greater than





  
    f
    PRF
  
  2




minus a first positive preset value minus the baseline shift if the first direction is positive or plus the baseline shift if the first direction is negative and one or more velocities of an adjacent third area of the color Doppler data in a second direction substantially opposite to the first direction and corresponding to Doppler shift frequencies greater than




  
    f
    PRF
  
  2




minus a second positive preset value plus the baseline shift if the second direction is negative or minus the baseline shift if the second direction is negative;
code to detect a second transition between the third area and an adjacent fourth area of the color Doppler data representing non-zero flow velocities in the first direction;
code to subtract a velocity corresponding to fPRF from each of the flow velocities of the third area if the second direction is positive; and
code to add the velocity corresponding to fPRF to each of the flow velocities of the third area if the second direction is not positive.

14. A medium according to claim 13, wherein the color Doppler data comprises color flow line data.

15. A medium according to claim 13, wherein the color Doppler data comprises scan-converted color flow image data.

16. A medium according to claim 13, wherein the code to detect the first image area of the color Doppler data representing zero flow velocity comprises:
code to detect the first image area of the color Doppler data based on B-mode data or the color Doppler data.

17. A medium according to claim 13, wherein detection of the second image area, the first transition and the second transition may proceed along a color flow line, a line across color flow lines, a vertical or horizontal line in a scan-converted color flow image, or a line across said scan-converted image columns.

18. A medium according to claim 13, wherein the baseline shift is greater than or equal to




  -
  
    
      f
      PRF
    
    2
  




and less than or equal to




  
    
      f
      PRF
    
    2
  
  .