Abstract:
A system suitable for reducing static or slow moving echoes from acoustic boundaries in a system such as pipe walls, blood vessels, tissue structures so that an image from flowing or time varying media such as water, oil, blood etc may be imaged more clearly, the system including an analog-to-digital converter for digitizing a received analog signal from a detector, means for digitally separating static or slow moving components of the digital signal, a digital-to-analog converter to provide an analog version of the separated static or slow moving components of the signal, and a subtractor to subtract the analog version of the static or the slow moving components of the signal from an analog signal received from a detector to produce an analog signal corresponding to the remaining flow components of the signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to flow imaging, in particular to suppression of echoes from stationary or slow moving reflections in an image. 
     Flow of media through a conduit, such as fluid through a pipe or blood through a blood vessel for example, may be imaged by transmitting waves such as ultrasound waves into the conduit and detecting echoes reflected from the flowing media and the surrounding structure. However, a problem with such imaging is that echoes from stationary or slow moving reflecting acoustic boundaries in the system such as pipe walls, blood vessels, tissue structures etc are far stronger than those from the flowing or time varying media such as water, oil, blood etc. The stationary or slow moving echoes may for example be 100 times larger than the echoes from the flowing media such that it is very difficult to precisely determine data for the flowing media. 
     In order to more precisely determine the data for the flowing media, the signal for the static or slow moving reflections is generally cancelled in the digital domain by digitally subtracting the static or slow moving element from the digitized incoming signal, as shown for example in EP 0 947 853. However, because of the very large difference between the magnitude of the signal from the static or slow moving reflections and the signal for the flowing media, for example 100:1, only a very small proportion of the resolution of the analogue-to-digital converter is used to digitize the portion of the incoming signal relating to the flowing media of interest, for example 1%. Furthermore, the extent to which the combined analogue signal can be amplified prior to analogue-to-digital conversion is limited by the need not to clip the static element, which has the flowing data of interest superimposed upon it, so that data is not lost. A high resolution analogue-to-digital converter is sometimes used but this can be very expensive often comprises the sampling rate possible. 
     It would be desirable to be able to reduce or remove the portion of a signal relating to echoes from static or slow moving reflecting acoustic boundaries in an image whilst overcoming or reducing one or more of the problems explained above. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a system suitable for reducing static or slow moving echoes from an image, the system comprising: an analogue-to-digital converter for digitizing a received analogue signal from a detector; means for digitally separating static or slow moving components of the digital signal; a digital-to-analogue converter to provide an analogue version of the separated static or slow moving components of the signal and a subtractor to subtract the analogue version of the static or slow moving components of the signal from an analogue signal received from a detector to produce an analogue signal corresponding to the remaining flow components of the signal. 
     Subtracting the static or slow moving component from the incoming measured signal in the analogue domain allows a resultant analogue difference signal to be produced corresponding to the flowing media of interest. This analogue signal corresponding to the flowing media of interest may then be amplified to a much greater degree without being clipped by the analogue-to-digital converter, producing a far better signal to noise ratio in the subsequently processed and displayed signal. 
     An amplifier with a controllable gain, in an embodiment, may be provided to receive the subtracted analogue signal corresponding to the flow components of the signal so that the extent to which the signal is amplified is dependent upon its gain setting. Controlling the gain of the amplifier enables the input range of the analogue-to-digital converter to be more fully utilised for more precise results. 
     An imaging system may be provided including a transducer array, a transmitter, a receiver, an output for a display and the system for reducing static and slow moving echoes from an image of the first aspect of the present invention. The system could be provided in the receiver or form part of a controller for the imaging system for example. 
     According to a second aspect of the present invention, there is provided a method for reducing static or slow moving echoes from an image, the method comprising digitizing an incoming analogue signal from a detector, separating the static or slow moving components of the digitized signal, converting the separated static or slow moving components into an analogue signal and subtracting the analogue version of the static or slow moving components from an analogue signal received from detector to produce an analogue signal corresponding to the flow components of the image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  illustrates flow of media through a conduit; 
         FIG. 2  shows an example of an imaging system which may be used with an example of the present invention; 
         FIG. 3  shows an example of a system illustrating our invention for reducing static or slow moving echoes from an image; 
         FIG. 4  shows a more detailed example of a system illustrating the present invention; 
         FIG. 5  shows an example a typical received signal from a prior art imaging system; 
         FIG. 6  shows an example of a received signal using an example of a system of the present invention; 
         FIG. 7  is a flow diagram of a method illustrating the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a conduit  1  which may, for example, be a tube or pipe such as a portion of a pipeline, a water pipe, a blood vessel or artery. The conduit  1  is arranged to convey a flowing or time varying media such as water, oil, gas or mixtures thereof or bodily fluids such as blood. Particles or bubbles  2  within the flowing media produce echoes when a transmitting/receiving transducer  3  of an imaging system is used to form an image of the flowing media. However, echoes from the walls of the conduit  1  produce time-static or slow moving (for example due to temperature variations in the conduit) echoes which are much larger than the echoes produced by the flowing media  2 . The echoes form the conduit  1  may be even larger due to multiple reflections within the conduit  1 . These static or slow moving echoes may be typically 100 times larger than echoes from the flowing media such that any imaging produced from the flowing media  2  is only seen at a very low resolution. 
       FIG. 2  shows an example of an imaging system  10  suitable for producing images of a flowing media  2 . In this example the imaging system  10  uses ultrasonic waves but any suitable signal may be used such as magnetic, optical, thermal, electrical impedance, sound waves or microwaves for example. The imaging system  10  of this example has a transducer array  11  comprising a plurality of transducer elements  12 , each of which may be separately driven and each of which produces a burst of ultrasonic energy when energised by a pulsed waveform produced by a transmitter  13 . The ultrasonic energy reflected back to the transducer array  11  from a flowing media such as shown in  FIG. 1  is converted to an electrical signal by a receiving transducer element  14  which may be in the same transmitting transducer array  11  or a separate receiving transducer. The electrical signals from the receiving transducer elements  14  are applied to a receiver  15  through a set of transmit/receive switches  16 . The transmit/receive switches  16  are typically semiconductors which protect the receive electronics from the high voltages generated by the transmit electronics. The transmit signal causes the semiconductor protection to limit the signal to the receiver  15 . The transmitter  13  and receiver  15  may be operated under the control of a synchronizing controller  17 . 
     A scan may be performed by acquiring a series of echoes in which the transmitter  13  is gated ON momentarily to energise each transmitting transducer element  12  and the subsequent echo signals produced by each receiving transducer element are applied to the receiver  15 . The receiver  15  combines the separate echo signals from each transducer element to produce a single echo signal which is used to produce a line in an image which may be provided on a display monitor  18 . 
       FIG. 3  shows an example of a system illustrating an embodiment of the invention suitable for reducing or substantially removing the static or slow moving echoes from an image. The system  20  includes an analogue-to-digital converter  21  for digitising a received analogue signal from a detector  11  via the node Rx. Means  22  is provided for digitally separating the static or slow moving component from the digitised signal from the analogue-to-digital converter  21 . Any suitable separator  22  may be used such as a low pass filter, a decimation filter or the use of an appropriate software function in a suitable processor for example to provide the static or slow moving components of the signal. A digital-to-analogue converter  23  is provided to receive the separated static or slow moving components of the signal and provide an analogue version of the separated static or slow moving components of the signal. A subtractor  24  is then used to subtract the analogue version of the static or slow moving components of the signal provided by digital-to-analogue convertor  23  from the analogue input provided by a transducer from node Rx to produce an analogue signal corresponding to the remaining flow components of the signal. The resultant signal represents echoes from the flowing media  2  and is digitised  21  and may be displayed  18 . As the static or slow moving component of the incoming measured signal is subtracted in the analogue domain, a resultant analogue difference signal corresponding to the flowing media of interest is produced which has a much higher signal to noise ratio. The signal with the much higher signal to noise ratio may then be processed to provide a far higher resolution measurement and image of the flowing media  2 . 
     One or more of the components of the system  21 ,  22 ,  23 ,  24 , such as the digital-to-analogue converter  23 , in an embodiment, is arranged to operate at an appropriate sample rate such that it seeks a sample value at appropriate time points or acquisition windows that leads to a net zero static or slow moving signal at the analogue-to-digital converter  21 . An algorithm may be used to separate the static or slow moving components of the signal. The algorithm may be adaptive and dependent upon the processing time of the other relevant components in the circuit, in particular the separator  22 , the digital-to-analogue convertor  23  and the subtractor  24  to overcome any latency offset issues or time-based echo drift. 
       FIG. 4  shows a more detailed example of the system of  FIG. 3  and includes an amplifier  25 . In this example the gain of the amplifier  25  may be controlled by an input labelled Rx Gain. The gain of the amplifier  25  may be controlled either manually by an operator or by a controller to more fully utilise the input range of the analogue-to-digital convertor  21  for more precise results even though the peak to peak size or magnitude of the analogue signal corresponding to the flow components may vary. This provides even further enhanced signal to noise ratios giving even more precise results. The function of the amplifier could be included in another component of the system, such as the analogue-to-digital converter  21 . 
       FIG. 5  illustrates an example of a signal that may be received by a conventional prior art imaging system. As can be seen, the signal is dominated by the static or slow moving elements produced by echoes from conduit walls. In  FIG. 5  the first static or slow moving sine wave  30  corresponds to echoes from the near wall  1  in  FIG. 1  and the later static or slow moving sine wave  31  corresponds to echoes from the far wall. The echoes from flowing or time-varying media within a conduit can be seen superimposed on the static or slow moving signal and account for only a very small proportion of the overall signal amplitude. 
     In contrast,  FIG. 6  shows an example of the analogue signal output by the subtractor  24  in the example of  FIG. 3  or  FIG. 4  of our description. As can clearly be seen, the removal of the slow moving or static elements  30  from  FIG. 5  produce a signal corresponding to the flowing or time-varying media within a conduit with a significantly enhanced signal to noise ratio which when displayed provides a far more precise image of the flowing component. 
       FIG. 7  shows a flow diagram of a method of reducing or removing static or slow moving echoes from a signal, in particular when the static or slow moving echoes are substantially larger than the portion of the signal which is to be displayed. At step  100  an incoming analogue signal from a detector is digitized. At step  200  the static or slow moving components of the digitized signal are separated. The separation may be provided in any suitable way, for example by using a low pass filter, a decimation filter or by a processor. At step  300  the separated static or slow moving components of the signal are converted into an analogue signal. At step  400  the analogue version of the static or slow moving components of the signal are subtracted from an analogue signal received from the detector to produce an analogue signal corresponding to the flow components of the signal. The resultant subtracted signal may then be amplified, digitized, possibly subjected to further processing and may then be displayed. The gain of any amplification may be varied depending upon the magnitude of the resultant subtracted signal to more fully utilize the input range of the analogue-to-digital converter. 
     The examples described above are able to provide a signal corresponding to the flowing or time-varying fluid within a conduit which is very clear and precise without having to use large and expensive components. 
     Many variations may be made to the examples described above whilst still falling within the present invention. For example, the components shown in  FIGS. 2, 3 and 4  may be provided by any suitable electronic hardware or by software provided on a processor or computer for example. Furthermore, the separator  22  may be provided by any appropriate component such as a low pass filter a decimation filter or by the use of an appropriate software function in a suitable processor or computer. 
     This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention 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 they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.