Abstract:
An ultrasound imaging method comprises the steps of: transmitting a prescanning ultrasonic beam toward a subject and measuring an ultrasonic echo from a bone structure inside a cranium in advance; determining a transfer function of an ultrasonic wave in the cranium based on the measured said ultrasonic echo; transmitting an imaging ultrasonic beam toward the subject; generating a cancelling ultrasonic beam having an opposite phase as an ultrasonic echo reflected by the inner surface of the cranium due to said imaging ultrasonic beam, based on said transfer function; receiving an ultrasonic echo from the subject while cancelling said ultrasonic echo from the bone structure inside the cranium by transmitting the cancelling ultrasonic beam in accordance with the ultrasonic echo from the bone structure inside the cranium due to said imaging ultrasonic beam; and generating an ultrasound image of the subject.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an ultrasound imaging method and apparatus which produce ultrasound images, and in particular, to an ultrasound imaging method and apparatus which produce intracranial ultrasound images. 
         [0002]    Ultrasound imaging which produces ultrasound images by transmitting an imaging ultrasonic beam from an ultrasound probe toward a subject and receiving the ultrasonic echo reflected by the subject is known. In recent years, ultrasound imaging has been applied to intracranial observation. For example, intracranial ultrasound images have been produced by transmitting an imaging ultrasonic beam toward a subject inside a cranium from a thin place on the cranium such as the temple. 
         [0003]    In this type of intracranial ultrasound imaging, multiple echoes occur, wherein part of the imaging ultrasonic beam transmitted inside the cranium is repeatedly reflected by the inner surface of the cranium and so forth. Multiple echoes of the imaging ultrasonic beam result in noise in the produced ultrasound image, and also have the risk of affecting brain tissue. 
         [0004]    Thus, in JP 2007-117165 A, for example, a technique is proposed whereby, in multiple echoes occurring between the surface of the ultrasound probe and inside the body, noise in the ultrasound image is reduced by cancelling out echoes reflected by the ultrasound probe. 
         [0005]    However, even if the echo reflected by the surface of the ultrasound probe is cancelled out, multiple echoes of the ultrasonic echo occurring on the inner surface of the cranium still exist, and there is risk that the remaining ultrasonic echoes affect brain tissue. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to provide an ultrasound imaging method and apparatus which resolve such problems of the past, and can suppress multiple echoes of ultrasonic echo inside the cranium. 
         [0007]    An ultrasound imaging method according to the present invention comprises the steps of: 
         [0008]    transmitting a prescanning ultrasonic beam toward a subject and measuring an ultrasonic echo from a bone structure inside a cranium in advance; 
         [0009]    determining a transfer function of an ultrasonic wave in the cranium based on the measured said ultrasonic echo; 
         [0010]    transmitting an imaging ultrasonic beam toward the subject; 
         [0011]    generating a cancelling ultrasonic beam having an opposite phase as an ultrasonic echo reflected by the inner surface of the cranium due to said imaging ultrasonic beam, based on said transfer function; 
         [0012]    receiving an ultrasonic echo from the subject while cancelling said ultrasonic echo from the bone structure inside the cranium by transmitting said cancelling ultrasonic beam in accordance with the ultrasonic echo from the bone structure inside the cranium due to said imaging ultrasonic beam; and 
         [0013]    generating an ultrasound image of the subject based on the ultrasonic echo received from the subject. 
         [0014]    An ultrasound imaging apparatus according to the present invention comprises: 
         [0015]    echo measurement means which transmits a prescanning ultrasonic beam toward a subject and measures an ultrasonic echo from a bone structure inside a cranium; 
         [0016]    transfer function calculation means which determines a transfer function of an ultrasonic wave in the cranium based on said ultrasonic echo measured by said echo measurement means; 
         [0017]    imaging means which transmits an imaging ultrasonic beam toward a subject and receives ultrasonic echoes from the subject to generate an ultrasound image of the subject; and 
         [0018]    cancelling ultrasonic beam transmission means which, based on said transfer function, generates a cancelling ultrasonic beam having an opposite phase as an ultrasonic echo reflected by the inner surface of the cranium due to said imaging ultrasonic beam, and transmits it in accordance with the ultrasonic echo from the bone structure inside the cranium due to said imaging ultrasonic beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a block diagram illustrating a configuration of a ultrasound imaging apparatus according to one embodiment of the present invention. 
           [0020]      FIG. 2  is a block diagram illustrating an arrangement position of probes used in the embodiment. 
           [0021]      FIG. 3  is a drawing illustrating a state of ultrasonic beam propagation in prescanning. 
           [0022]      FIG. 4  is a drawing illustrating a state where an ultrasonic echo from a cranium is cancelled out. 
           [0023]      FIG. 5  is a drawing illustrating a state of ultrasonic beam propagation in imaging. 
           [0024]      FIG. 6  is a drawing illustrating an arrangement position of probes in a modification of the embodiment. 
           [0025]      FIG. 7  is a drawing illustrating an arrangement position of probes in another modification of the embodiment. 
           [0026]      FIG. 8  is a drawing illustrating a configuration of a probe used in still another modification of the embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    The present invention is described in detail hereinafter based on a preferred embodiment shown in the accompanying drawings. 
         [0028]      FIG. 1  illustrates a configuration of an ultrasound imaging apparatus according to one embodiment of the present invention. The ultrasound imaging apparatus comprises an imaging probe  1 , a cancelling probe  2  and an apparatus body  3 . 
         [0029]    The imaging probe  1  and the cancelling probe  2  each have a plurality of ultrasound transducers lined up in an array. When imaging, the imaging probe  1  transmits an imaging ultrasonic beam toward a subject and receives ultrasonic echoes reflected by the subject, and the cancelling probe  2  transmits cancelling ultrasonic beams toward the inside of a cranium. When prescanning, the imaging probe  1  transmits a prescanning ultrasonic beam toward the inside of the cranium and receives ultrasonic echoes from a bone structure inside the cranium. 
         [0030]    The apparatus body  3  comprises a reception signal processor  4  connected to the imaging probe  1 , an imaging transmission signal generator  5  connected to the imaging probe  1 , and a cancelling transmission signal generator  6  connected to the cancelling probe  2 . 
         [0031]    Reception signals corresponding to the ultrasonic echoes received by the imaging probe  1  are input from the imaging probe  1  to the reception signal processor  4 . The imaging transmission signal generator  5  generates imaging transmission signals and prescanning transmission signals, and outputs them to the imaging probe  1 . The cancelling transmission signal generator  6  generates cancelling transmission signals corresponding to cancelling ultrasonic beams having the opposite phase as the ultrasonic echoes reflected by the inner surface of the cranium, and outputs them to the cancelling probe  2 . 
         [0032]    The reception signal processor  4 , the imaging transmission signal generator  5  and the cancelling transmission signal generator  6  are each connected to a controller  7 . The controller  7  controls input and output of signals to and from the parts in the apparatus body  3 . 
         [0033]    A transfer function calculator  8  and a display unit  9  are also each connected to the controller  7 . The transfer function calculator  8  determines a transfer function of ultrasonic waves that propagate through the cranium, based on the ultrasonic echoes from the bone structure inside the cranium obtained in advance in prescanning. Here, the transfer function includes a propagation delay quantity and a waveform deformation quantity (including amplitude, phase, etc.) of the ultrasonic wave arising due to a thickness of the cranium at respective positions on the cranium. 
         [0034]    The display unit  9  displays an ultrasound image produced based on the reception signals corresponding to the ultrasonic echoes received by the reception signal processor  4 . 
         [0035]    The echo measurement means of the present invention is constructed of the above-described imaging probe  1 , the reception signal processor  4  and the imaging transmission signal generator  5 , and the cancelling ultrasonic beam transmission means of the present invention is constructed of the cancelling probe  2  and the cancelling transmission signal generator  6 . 
         [0036]    Next, the operation of the ultrasound imaging apparatus illustrated in  FIG. 1  will be described. 
         [0037]    First, as shown in  FIG. 2 , the imaging probe  1  is arranged at a specified position on the head, and the cancelling probe  2  is arranged at a position adjacent to the imaging probe  1 . When the imaging probe  1  and the cancelling probe  2  have been arranged, the imaging transmission signal generator  5  generates a prescanning transmission signal, and outputs it to the imaging probe  1 . As a result, a prescanning ultrasonic beam is transmitted from the imaging probe  1  toward the inner surface of the cranium, and prescanning of the bone structure inside the cranium is performed. 
         [0038]    As shown in  FIG. 3 , the prescanning ultrasonic beam transmitted from the imaging probe  1  propagates through the cranium H and reaches point A on the inner surface of the cranium, and an ultrasonic echo Ea reflected at point A on the inner surface of the cranium and returned is received by the imaging probe  1 . Here, when the ultrasonic wave propagates through the cranium H, deformation of the waveform caused by the thickness of the cranium H occurs due to propagation delay and attenuation. 
         [0039]    On the other hand, a part of the prescanning ultrasonic beam that passed through point A on the cranium H reaches point B on the inner surface of the cranium on a side opposite the side where the imaging probe  1  is arranged, and an ultrasonic echo Eb reflected from its bone structure again reaches point A on the cranium H. The ultrasonic echo Eb that passed through point A on the cranium H is received by the imaging probe  1 . Also, a part of the ultrasonic echo Eb is additionally reflected at point A on the cranium H. 
         [0040]    Reception signals for the ultrasonic echoes Ea and Eb from the bone structure received by the imaging probe  1  are input into the reception signal processor  4 . When the reception signal processor  4  outputs reception signals corresponding to the input ultrasonic echoes Ea and Eb to the controller  7 , the controller  7  outputs those reception signals to the transfer function calculator  8 . 
         [0041]    Based on the reception signal of the ultrasonic echo Ea received from the bone structure, the transfer function calculator  8  determines the transfer function of the ultrasonic wave in the cranium H. That is, the transfer function calculator  8  determines the transfer function of the ultrasonic wave in the cranium H based on a time of transmission and a waveform of the prescanning ultrasonic beam transmitted from the imaging probe  1 , and on a time of reception and a waveform of the ultrasonic echo Ea received by the imaging probe  1 . Also, the transfer function calculator  8  determines a transit time required for the ultrasonic beam transmitted from the imaging probe  1  to be reflected at point B on the cranium H and reach point A on the cranium H, based on the times of reception of ultrasonic echoes Ea and Eb received by the imaging probe  1 . The propagation delay quantity, the waveform deformation quantity and the transit time determined in this way are output to the controller  7 , and from the controller  7  they are output to the cancelling transmission signal generator  6 . 
         [0042]    Next, imaging of the subject inside the cranium H will be described. As shown in  FIG. 4 , the imaging transmission signal generator  5  generates an imaging transmission signal and outputs it to the imaging probe  1 , and from the imaging probe  1 , an imaging ultrasonic beam St corresponding to the imaging transmission signal is transmitted at time T 0 . 
         [0043]    The imaging ultrasonic beam St propagates through the cranium H, passing through point A on the cranium H, and reaches the subject such as a blood vessel inside the cranium H. A ultrasonic echo Sr reflected by the subject again passes through point A on the cranium H and is received by the imaging probe  1 . On the other hand, the part of the imaging ultrasonic beam St that propagated through the subject reaches point B on the cranium H, and the first echo E 1  reflected from its bone structure again reaches point A on the cranium H at time T 1 . 
         [0044]    At this time, based on the propagation delay and transit time input from the transfer function calculator  8  via the controller  7 , the cancelling transmission signal generator  6  determines a time Δt for the ultrasonic wave to reach from the cancelling probe  2  to point A on the cranium H, and determines a time T 1  at which the first echo E 1  reaches point A on the cranium H. Then, the cancelling transmission signal generator  6  transmits a cancelling ultrasonic beam having the opposite phase as the first echo E 1 , from the cancelling probe  2  at time T 1   a , which is Δt earlier than time T 1  at which the first echo E 1  reaches point A of the cranium H. Here, based on the waveform deformation quantity of the ultrasonic wave at the cranium H determined by the transfer function calculator  8 , the cancelling transmission signal generator  6  transmits a cancelling ultrasonic beam C 1  having the opposite phase as the first echo E 1  and having an amplitude so as to leave the first echo E 1  to the degree required to create an image. Note that the degree required to create an image is the amount at which the position of the bone structure can be confirmed without being buried in noise when an image of the first echo E 1  is created. 
         [0045]    In this way, as shown in  FIG. 5 , the cancelling ultrasonic beam C 1  transmitted from the cancelling probe  2  propagates through the cranium H while being attenuated, and at point A on the cranium H, it interferes with the first echo E 1  by a waveform having the opposite phase as the first echo E 1  and having an amplitude so as to leave the first echo E 1  to the degree required to create an image. 
         [0046]    Also, the part of the first echo E 1  that was reflected without passing through point A on the cranium H again reaches point B on the cranium H, and a second echo E 2  reflected here reaches point A on the cranium H at time T 2 . Similarly, the cancelling transmission signal generator  6  transmits a cancelling ultrasonic beam C 2 , so as to completely cancel out the second echo E 2 , from the cancelling probe  2  at time T 2   a  which is Δt earlier than time T 2  at which the second echo E 2  reaches point A on the cranium H, thereby completely cancelling out the second echo E 2  which reaches point A on the cranium H. Furthermore, in the case where the second echo E 2  cannot be completely cancelled out and a third echo E 3 , a fourth echo E 4 , etc. are generated, they can be respectively cancelled out by it transmitting cancelling ultrasonic beams C 3 , C 4 , etc., so as to completely cancel out the third echo E 3 , fourth echo E 4 , etc., from the cancelling probe  2  at times T 3   a , T 4   a , etc. which are Δt earlier than times T 3 , T 4 , etc. at which the third echo E 3 , the fourth echo E 4 , etc. reach point A on the cranium H. 
         [0047]    In this way, by reducing the first echo E 1  and completely cancelling out echoes E 3 , E 4 , etc. beyond the second echo E 2 , it is possible to suppress multiple echoes of ultrasonic echo inside the cranium H. 
         [0048]    Thus, by transmitting cancelling ultrasonic beams, the majority of the ultrasonic echoes reflected and returned at point B on the cranium H are cancelled out, and, as shown in  FIG. 4 , a residual signal of these ultrasonic echoes is equivalent only to the difference between the first echo E 1  and the cancelling ultrasonic beam C 1 . This residual signal and the reception signal corresponding to the ultrasonic echo Sr of the subject are output from the imaging probe  1  to the reception signal processor  4 , and the reception signal processor  4  outputs them to the controller  7 . The controller  7  outputs the input reception signal corresponding to the ultrasonic echo Sr of the subject and the residual signal to the display unit  9 , and the display unit  9  generates and displays an ultrasound image based on the input signals. In this way, an image obtained by reducing the first echo E 1 , which is later than the ultrasonic echo from the deepest part of the image display region inside the cranium, and cancelling out the second echo E 2 , the third echo E 3 , the fourth echo E 4 , etc. is displayed on the display unit  9 . 
         [0049]    According to the ultrasound imaging apparatus of this embodiment, it is possible to suppress multiple echoes of ultrasonic echo inside the cranium H by reducing the first echo E 1  and completely cancelling out the echoes E 3 , E 4 , etc. beyond the second echo E 2 . 
         [0050]    Note that in this embodiment, the arrangement of the cancelling probe  2  on the head is not particularly limited, provided that the imaging ultrasonic beam can be transmitted from the imaging probe  1  and the ultrasonic echo from the bone structure inside the cranium H can be cancelled out by the cancelling ultrasonic beam. For example, as shown in  FIG. 6 , the cancelling probe  2  may be arranged at a position which opposes the imaging probe  1  arranged on the head. Also, as shown in  FIG. 7 , ultrasonic echo from the bone structure which diffuses and propagates inside the cranium H can be cancelled out with high precision by arranging two cancelling probes  2  adjacent to the imaging probe  1  arranged on the head. 
         [0051]    Furthermore, it may be configured such that the cancelling probe  2  is omitted, the imaging probe  1  and the cancelling transmission signal generator  6  are connected, and the cancelling transmission signal generator  6  transmits cancelling ultrasonic beams from some of a plurality of ultrasound transducers of the imaging probe  1 . For example, as shown in  FIG. 8 , an imaging probe  10  has a plurality of imaging transducers  11  arranged in the center, and a plurality of cancelling transducers  12  arranged on both sides sandwiching the plurality of imaging transducers  11 . By so doing, it is possible to suppress multiple echoes of ultrasonic echo inside the cranium H using only the imaging probe  10 . It is also possible to configure the imaging probe  10  such that a plurality of cancelling transducers  12  are adjacent to only one of the plurality of imaging transducers  11 . 
         [0052]    Also, in this embodiment, an imaging ultrasonic beam and a prescanning ultrasonic beam are transmitted from the imaging probe  1  and a cancelling ultrasonic beam is transmitted from the cancelling probe  2 , but the method of transmitting the ultrasonic beams is not particularly limited, provided that the first echo can be cancelled out by the cancelling ultrasonic beam. For example, the imaging ultrasonic beam may be transmitted from the imaging probe  1 , and the prescanning ultrasonic beam and cancelling ultrasonic beam may be transmitted from the cancelling probe  2 .