Abstract:
An ultrasound imaging method and apparatus forms high-quality color ultrasound images by using variable power threshold levels depending on pixel position. The method comprises the steps of determining a power threshold level that is varied depending on a noise, transmitting ultrasound signals from at least one transducer array to a target object, receiving a first signal including a signal reflected from the target object mixed with a noise, and imaging an ultrasound image of the target object corresponding to the first signal when the power level of the first signal is greater than the variable power threshold level. The apparatus comprises one or more transducers for transmitting an ultrasound signal to an object and receiving a first signal including a reflected signal from the object mixed with a noise; and a digital signal processor for forming an ultrasound image of the object corresponding to the first signal when a power level of the first signal is greater than a variable power threshold level.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an ultrasound imaging method and apparatus, and more particularly, to an ultrasound imaging method and apparatus for forming color images by using a power threshold level that is varied depending on a pixel&#39;s position within a color image for isolating noises and color signals.  
         BACKGROUND OF THE INVENTION  
         [0002]    An ultrasound imaging system with a color mode imaging capability is known to be able to provide a color image in addition to a B (Brightness) mode image. Referring to FIG. 1, an exemplary display of a color image within a color box is shown, wherein reference numerals  1 ,  2  and  3  indicate a B mode image, a color box, and a color image, respectively.  
           [0003]    Conventionally, a power threshold level for isolating noises and color signals is determined as a constant regardless of pixel&#39;s position. Referring to FIG. 2, there is illustrated a graphical representation of a conventional power threshold level  301  and a noise power level  302  corresponding to the pixel&#39;s position along the direction perpendicular to a probe in an image showing a target object. In FIG. 2, the horizontal transverse axis represents depth in an axial direction and the vertical longitudinal axis represents power level. As shown in FIG. 2, the power threshold level  301  has a constant value. Therefore, in the prior art, a color display is determined by comparing the noise power level  302  with the power threshold level  301  regardless of the variation of the noise power level  302  along the depth. Specifically, if the level of an input power is higher than the power threshold level  301 , a color corresponding to the input power is displayed. Otherwise, the input power is regarded as a noise, and the corresponding color is not displayed. Use of the power threshold level  301  as noted above causes problems such as weak input powers being regarded as noise. Consequently, a color corresponding to the weak input power may not be displayed on an image.  
           [0004]    Further, due to various characteristics depending on the pixel&#39;s position, such as TGC (Time Gain Compensation), an apodization using a function can be varied depending on an axial direction, and the change of aperture size, the gain of a color signal may also be varied depending on the pixel&#39;s position. Specifically, the TGC compensates the ultrasound attenuation caused by the human body such that it makes a pixel in proximity to a transducer have small gain and a pixel out of proximity thereto have large gain. In general, the function varied depending on the axial direction of a scan line is used as an apodization profile and utilized in the apodization that applies different gains to each channel. The aperture size of a scan line in proximity to both edges of a transducer is relatively smaller than that of another scan line. Thus, the actual range of a noise may be varied depending on the above-mentioned characteristics.  
           [0005]    Therefore, if a power threshold level having a constant value is used to process a receiving signal, e.g. a color Doppler signal, noise is filtered improperly or a color signal is extremely removed from the receiving signal, thereby deteriorating the quality of images.  
         SUMMARY OF THE INVENTION  
         [0006]    It is, therefore, an object of the present invention to provide an ultrasound imaging method and apparatus for forming high-quality color ultrasound images by using a variable power threshold level that is varied depending on the pixel&#39;s position.  
           [0007]    It is another object of the present invention to provide an ultrasound imaging method and apparatus capable of obtaining a noise profile depending on the pixel&#39;s position.  
           [0008]    According to one aspect of the present invention, there is provided an ultrasound imaging method, comprising the steps of (a) determining a variable power threshold level based on a noise, (b) transmitting ultrasound signals from at least one transducer array to a target object, (c) receiving a first signal including a signal reflected from the target object mixed with a noise, and (d) imaging an ultrasound image of the target object corresponding to the first signal when the power level of the first signal is greater than the variable power threshold level.  
           [0009]    According to another aspect of the present invention, there is provided an ultrasound imaging apparatus, comprising, one or more transducer(s) for transmitting an ultrasound signal to an object and receiving a first signal including a reflected signal from the object mixed with a noise, and a digital signal processor for forming an ultrasound image of the object corresponding to the first signal when a power level of the first signal is greater than a variable power threshold level. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings.  
         [0011]    [0011]FIG. 1 shows a B (Brightness) mode image.  
         [0012]    [0012]FIG. 2 is a graphical representation of a conventional power threshold level and a noise power level.  
         [0013]    [0013]FIG. 3 is a graphical representation of the variable power threshold level and noise power level of the present invention.  
         [0014]    [0014]FIG. 4 is a schematic block diagram of an ultrasound imaging apparatus in accordance with the present invention.  
         [0015]    [0015]FIG. 5 represents a detailed block diagram of the digital signal processor shown in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0016]    [0016]FIG. 3 is a graphical representation of a variable power threshold level  401  and a noise power level  402  according to the present invention. In FIG. 3, the horizontal transverse axis represents depth in an axial direction and the vertical longitudinal axis represents power level.  
         [0017]    As shown in FIG. 3, the present invention isolates noise and color signals by using the variable power threshold level  401  that is varied depending on the pixel&#39;s position. Specifically, a color corresponding to an input power is displayed if the level of input power is higher than the variable power threshold level  401 ; otherwise, the corresponding color is not displayed as it is regarded as noise. Unlike conventional technology, a weak color signal can be displayed as the variable power threshold level  401  has a profile varied along the noise power level  402 .  
         [0018]    As described in detail with reference to the accompany drawings, the variable power threshold level  401  is determined after obtaining a noise profile, wherein the noise profile may be varied depending on the ultrasound imaging circumstances.  
         [0019]    [0019]FIG. 4 is a schematic block diagram of an ultrasound imaging apparatus in accordance with the present invention. The ultrasound imaging apparatus  100  with a color mode imaging capability comprises a transducer array  102 , a pre-amplifier  103 , a TGC (Time Gain Compensation) amplifier  104 , an ADC (Analog-to-Digital Converter)  105 , a beamformer  106 , a quadrature demodulator  107 , a digital signal processor  108 , a display deice  109 , and a controller  110 .  
         [0020]    The transducer array  102  transmits ultrasound signals to a target object, such as an erythrocyte in a human body, and receives reflected signals mixed with noise from the target object. The transducer array  102  connected to the pre-amplifier  103  sends the reflected signals thereto. The pre-amplifier  103  coupled to the TGC amplifier  104  amplifies the reflected signals from the transducer array  102  and forwards it to the TGC amplifier  104 . The TGC amplifier  104  is connected to the ADC  105 . With respect to the reflected signals, the TGC amplifier  104  changes time-variable gain in order to compensate the attenuation due to an ultrasound transmission distance within the human body and amplifies the reflected signals to output to the ADC  105 . The ADC  105  connected to the beamformer  106  converts the output of the TGC amplifier  104  into digital signals and sends them to the beamformer  106 .  
         [0021]    The beamformer  106  connected to the quadrature demodulator  107  performs a receive-focusing operation on the digital signals from the ADC  105  and sends them to the quadrature demodulator  107 . The quadrature demodulator  107  connected to the digital signal processor  108  demodulates the digital signals from the beamformer  106  by using the known quadrature demodulation technology, and generates I (in-phase) and Q (quadrature) signals and outputs them to the digital signal processor  108 . The digital signal processor  108  coupled to the display device  109  obtains the power, velocity and variance of the reflected signals by using the I and Q signals transmitted from the quadrature demodulator  107  and outputs them to the display device  109 . The display device  109  then displays colors corresponding to the obtained power, velocity and variance.  
         [0022]    The controller  110  controls the transducer array  102  and the digital signal processor  108  in order to determine the variable power threshold level. The digital signal processor  108  isolates noises and color signals by using the variable power threshold level that is varied depending on the pixel&#39;s position. The controller  110  controls the transducer array  102  and the digital signal processor  108  in order to determine the variable power threshold level by obtaining a noise profile. For the purpose of obtaining the noise profile, the controller  110  controls the transducer array  102  to receive signals reflected from a target object without transmitting an ultrasound signal to the target object. The transducer array  102  does not receive the reflected signals. In this case, a power obtained in the digital signal processor  108  becomes a noise power. The controller  110  then controls the digital signal processor  108  to determine the variable power threshold level with reference to that noise power. It is preferable to consider a signal transmission interval in order to eliminate the interference of a previously transmitted signal in obtaining the noise power.  
         [0023]    [0023]FIG. 5 represents a detailed block diagram of the digital signal processor  108  shown in FIG. 4. The digital signal processor  108  includes an input cornering unit  200 , a clutter filter  201 , an auto-correlator  202 , a color converter  203 , a color signal display determiner  204 , a memory  205 , a post-processor  206 , and a level determiner  207 .  
         [0024]    The I and Q signals are inputted from the quadrature demodulator  107  shown in FIG. 4 to the input cornering unit  200  of the digital signal processor  108 . In order to obtain data for a specific scan line, ensemble data obtained by repetitive receiving and transmitting ultrasound signals with respect to that scan line is generally used. Thus, each of the I and Q signals is considered as a function having an ensemble index and a depth index as a variable, wherein the ensemble index is changed relatively slower than the depth index. The input cornering unit  200  re-arranges the inputted I and Q signals bearing data of the same depth in the order of the ensemble index and transmits the re-arranged I and Q signals to the clutter filter  201 . As will be disclosed in the following description, the clutter filter  201  and the auto-correlator  202  filter a series of data having the same depth to obtain an autocorrelation value.  
         [0025]    The re-arranged I and Q signals are inputted to the clutter filter  201 . The clutter filter  201  isolates signals of the re-arranged I and Q signals (referred to as “clutters”) reflected from a target object, such as tissue rather than blood, which moves at a relatively slow velocity. The reflected signals from the target object are primarily composed of low frequency components, so that the clutter filter  201  uses a high-pass filter. The clutter filer  201  filters the re-arranged I and Q signals inputted from the input cornering unit  200  to transmit same to the auto-correlator  202 .  
         [0026]    The auto-correlator  202  calculates auto-correlation values with respect to the filtered I and Q signals to obtain the power, velocity and variance thereof. The autocorrelation values are denoted as R(0) and R(1) in FIG. 5. An autocorrelation value R(n) is expressed as follow:  
               R        (   n   )       =       ∑     l   =     -   ∞       ∞            x        (   i   )                x   *          (     i   -   n     )       .                 (     Eq   .              1     )                               
 
         [0027]    The auto-correlator  202  is connected to the color converter  203 , the color signal display determiner  204 , and the level determiner  207 . The auto-correlator  202  transmits R(0) and R(1) to the color converter  203 , while it sends R(0) only to the color signal display determiner  204  and the level determiner  207 .  
         [0028]    According to one embodiment of the present invention, the level determiner  207  obtains a noise profile under control of the controller  110  shown in FIG. 4, by regarding a power R(0) provided from the auto-correlator  202  as a noise power when the transducer array  102  does not transmit an ultrasound signal to the target object. A variable power threshold level is determined depending on the noise profile to be stored in the memory  205 . For example, the noise profile may be changed by every scan line and/or depth. The noise profile within an ultrasound image being displayed may be changed depending on the pixel&#39;s position.  
         [0029]    The level determiner  207  determines whether the ultrasound imaging apparatus  100  normally operates based on the noise profile obtained to notify the determination results to an operator (not shown). For instance, when the noise profile is deviating from a normal value (i.e. the level of the noise power is too high), the digital signal processor  108  transmits to the operator an alarm message or notice that the ultrasound imaging apparatus  100  is abnormal, via a local area network (LAN) or the Internet (not shown).  
         [0030]    The memory  205  stores the variable power threshold level obtained in the level determiner  207  and provides it to the color signal display determiner  204 . The memory  205  stores a plurality of variable power threshold levels that is varied depending on depths and/or scan lines and provides to the color signal display determiner  204 , one of the stored variable power threshold levels corresponding to a pixel currently being processed.  
         [0031]    The color signal display determiner  204  compares R(0) inputted from the auto-correlator  202  with the power threshold level provided by the memory  205 . If R(0) is higher than the power threshold level, the color signal display determiner  204  transmits a display enable signal to the color converter  203 ; otherwise, it provides a display disenable signal thereto.  
         [0032]    In response to the display enable signal, the color converter  203  calculates the velocity, power and variance of I and Q signals based on R(0) and R(1) inputted from the auto-correlator  202  and transmits the calculated velocity, power and variance as a color signal to the post-processor  206 . The post-processor  206  then processes the color signal and transmits it to the display device  109  shown in FIG. 4. In response, the display device  109  displays a color corresponding to the color signal.  
         [0033]    In response to the display disable signal, the color converter  203  transmits the color signal having power=velocity=variance=0 to the post-processor  206  regardless of R(0) and R(1).  
         [0034]    For improving an image quality, the post-processor  206  performs smoothing filtering, frame averaging and frame interpolation, which are well known in the art, on the color signal inputted from the color converter  203 , and provides them to the display device  109 .  
         [0035]    Hereinbelow, an operation for displaying a color image by using the variable power threshold level according to the present invention will be described in detail.  
         [0036]    A noise profile depending on the pixel&#39;s position in the color image is measured and obtained when an ultrasound imaging apparatus is designed or manufactured, and the noise profile obtained can be used in subsequent procedures. The noise profile can be changed due to various reasons and it is possible to continuously measure the noise profile during the operation of the ultrasound imaging apparatus and determine the variable power threshold level.  
         [0037]    As described above, the level determiner  207  is a unit for obtaining the variable power threshold level according to the noise profile that was obtained corresponding to each scan line within one frame of the color box  2  shown in FIG. 1. As illustrated in FIG. 3, the level determiner  207  determines the variable power threshold level  401  varied depending on the noise power level  402 . Since a noise is isolated by using the variable power threshold level  401 , the ultrasound imaging apparatus can effectively display even weak color signals.  
         [0038]    In one embodiment according to the present invention, the controller  110  of FIG. 4 controls the transducer array  102  to determine the noise profile by performing a receiving operation without transmitting the ultrasound signal thereto during a time interval corresponding to one frame of the color box  2 . Thus, there is no reflected signal so that a power obtained in the auto-correlator  202  of the digital signal processor  108  is a noise power. The controller  110  controls the level determiner  207  to obtain the noise profile corresponding to each scan line along the direction of the depth by using the noise power obtained to determine the variable power threshold level based on the noise profile. Here, the noise profile is a 2-D (two-dimensional) profile and is stored in the memory  205 .  
         [0039]    As described above, when the noise profile corresponding to each scan line is obtained, a time interval corresponding to one frame is required in order to obtain the noise profile corresponding to all scan lines within said one frame of the color box  2 . If a color image is not obtained during that time interval, the frame rate of the color image is decreased.  
         [0040]    In another embodiment according to the present invention, it is assumed that a noise profile corresponding to one scan line within the color image is identical to that of the remaining scan lines in order to reduce time required to obtain all noise profiles corresponding to the scan lines. Thus, it is possible to use the obtained noise profile corresponding to the one scan line for the remaining scan lines as their noise profile. In order to achieve this, the transducer array  102 , under the control of the controller  110 , does not transmit an ultrasound signal to the target object during a time interval corresponding to one scan line and performs a receiving operation during that time interval so that the level determiner  207  of the digital signal processor  108  obtains a noise profile corresponding to one scan line. The noise profile is a 1-D (one-dimensional) profile and is stored in the memory  205 .  
         [0041]    In yet another embodiment according to the present invention, the controller  110  controls the transducer array  102  and the level determiner  207  to obtain a noise profile corresponding to one scan line, wherein the scan lines have the same aperture opening state to each other.  
         [0042]    In still another embodiment of the present invention, the controller  110  controls the transducer array  102  and the level determiner  207  to obtain a noise profile corresponding to one scan line within the color image during one frame. In this case, noise profiles corresponding to all scan lines within the color image can be obtained after passing a number of frames having the same number of all scan lines. Although the updating speed of the noise profile becomes slower, this is advantageous unless the noise profile itself is dramatically varied over passage of time.  
         [0043]    Although the methods for updating the noise profile with a predetermined rate have been described in the aforementioned embodiments, the present invention is by no means limited thereto. For instance, a user can update the noise profile with a variable rate.  
         [0044]    In still another embodiment of the present invention, the controller  110  controls the transducer array  102  and the level determiner  207  to obtain a noise profile corresponding to one scan line within the color image by using a number of ensembles less than the number of ensembles being used for obtaining an actual color Doppler image. To prevent increasing an estimation error, the noise profile can be obtained by averaging a newly obtained noise profile and a previously obtained noise profile.  
         [0045]    In one embodiment according to the present invention, R(0) applied from the auto-correlator  202  to the level determiner  207  is used as the noise profile representing a noise power when an ultrasound signal is not transmitted to the target object. In another embodiment of the present invention, R(1) outputted from the auto-correlator  202  is applied to the level determiner  207 , thereby using the absolute value of R(1) as the noise power.  
         [0046]    While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.