Abstract:
The present invention relates to image acquisition technology. In particular, the present invention relates to tracking of an object of interest in ultrasound image information. For interventional procedures, an object of interest, e.g. the tool by which a procedure is performed, is depicted in image information acquired by a certain imaging modality. However, e.g. during transcatheter intracardiac procedures, tools are employed, which are rather narrow objects that may experience substantive passive motion due to heartbeat and blood flow as well as active motion due to the interventionist steering the tool. The present invention to allows steering two-dimensional ultrasound image acquisition, in particular steering of an imaging plane of an ultrasound image acquisition device, so that an object of interest remains visible in the acquired images, in particular without continuous readjustment of imaging parameters. Accordingly, a method for ultrasound image acquisition, in particular for tracking an object of interest in ultrasound image information, is provided comprising receiving ( 12 ) X-ray image information ( 40 ) and ultrasound image information ( 42 ), detecting ( 14 ) an object of interest ( 36 ) in the X-ray image information ( 40 ) and steering ( 16 ) two-dimensional ultrasound image acquisition such that the object of interest ( 36 ) is within a first ultrasound image plane ( 42 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to image acquisition technology. In particular, the present invention relates to tracking of an object of interest in ultrasound image information. More particularly, the present invention relates to a method for ultrasound image acquisition, a system for ultrasound image acquisition, a computer-readable medium as well as a program element for ultrasound image acquisition. 
       BACKGROUND OF THE INVENTION 
       [0002]    A current trend in interventional procedures is a tighter integration of ultrasound and X-ray modalities. In other words, different image modalities are not employed separately but rather conjointly during a specific procedure. Thereby, drawbacks of certain modalities may be compensated by advantages of other modalities. 
         [0003]    E.g., although modern interventional ultrasound solutions offer the possibility of 3D imaging, it is often relied upon two-dimensional ultrasound imaging for various reasons. Amongst those reasons may be a higher image quality, a higher frame rate and no need for cropping, volume orientation or rendering parameter tuning when employing two-dimensional ultrasound images versus 3D imaging. 
         [0004]    For interventional procedures, an object of interest, e.g. the tool by which the procedure is performed, is depicted in the image information acquired by the respective modalities. By depicting said object or overlaying image information of said object and the image information of a respective modality, a person performing the procedure readily obtains visual information about the location and/or orientation of the object of interest with regard to anatomic structures shown in the modality&#39;s image information. E.g., the path of a tool tip through tissue, may be tracked in the respective image information, i.e. visually followed by the person performing the procedure. Such tracking thus allows to reconfirm the correctness of a procedure, i.e. whether the procedure is correctly conducted. 
         [0005]    However, during transcatheter intracardiac procedures, such as mitral clip or arterial fibrillation ablation, transcatheter tools being employed, e.g. a mitral clip or ablation catheter, are rather narrow objects that may experience substantive passive motion due to breathing, heartbeat and blood flow as well as active motion due to the interventionist steering the tool. 
         [0006]    In such procedures, ultrasound images are regularly acquired by employing a transesophageal echocardiogram or TEE. The TEE is performed by a specialized probe being inserted into a patient&#39;s esophagus, which contains an ultrasound transducer at its tip. The ultrasound transducer is adapted for image acquisition and Doppler evaluation of ultrasound images. TEEs regularly provide clearer images, especially with regard to tissue structures that are difficult to view transthoracically, i.e. through the chest wall by employing an external ultrasound transducer. 
         [0007]    Since the TEE itself lies in the esophagus, it experiences very low levels of motion. In particular for transcatheter procedures, the motion of the TEE probe regularly is below the motion of transcatheter tools previously described. In other words, the spatial relationship and/or the distance between the transcatheter tool and the TEE transducer is non-constant and consequently, when considering 2D image acquisition of a particular ultrasound plane, the tool tip periodically goes out of an ultrasound plane currently under observation. 
         [0008]    To make the tool tip appear again in the imaging plane, readjustment of the imaging plane is required. This in turn requires a tedious coordination between the interventionist operating the transcatheter tool and the echographer operating in particular the ultrasound image acquisition device. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the invention to allow steering of two-dimensional ultrasound image acquisition, in particular steering of an imaging plane of an ultrasound image acquisition device, so that an object of interest remains visible, in particular without continuous readjustment of imaging parameters. 
         [0010]    Accordingly, a method for ultrasound image acquisition, in particular for tracking an object of interest in ultrasound image information, a system for ultrasound image acquisition, a computer-readable medium as well as a program element for ultrasound image acquisition according to the independent claims are provided. Preferred embodiments may be taken from the dependent claims. 
         [0011]    These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiments described hereinafter. Exemplary embodiments of the present invention will be described below with reference to the following drawings. 
         [0012]    The illustration in the drawings is schematic. In different drawings, similar or identical elements are provided with similar or identical reference numerals. 
         [0013]    The figures are not drawn to scale, however may depict qualitative proportions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows an exemplary embodiment of the relationship between ultrasound image information and X-ray image information according to the present invention; 
           [0015]      FIG. 2  shows an exemplary embodiment of a system for ultrasound image acquisition according to the present invention; and 
           [0016]      FIG. 3  shows an exemplary embodiment of a method for ultrasound image acquisition according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0017]    One aspect of the present invention pertains to steering and/or aligning an imaging plane of an ultrasound image so that an object of interest remains visible, i.e. within the imaging plane, without the requirement of, in particular constant, readjustment of imaging parameters, in particular ultrasound plane position and orientation. 
         [0018]    During a certain interventional procedure, both an X-ray imaging modality e.g. by using a C-arc, as well as an ultrasound imaging modality like TEE may be employed, with each modality generating in particular two-dimensional image information. 
         [0019]    Regularly, the ultrasound transducer comprises a transducer array for obtaining image information of a particular, two-dimensional ultrasound image plane, which can be adjusted in 3D by varying the ultrasound emissions of the transducer array. 
         [0020]    One aspect of the present invention is that the orientation of an ultrasound plane is steered such that the ultrasound plane comprises the X-ray source. Further, in case the ultrasound imaging plane also contains the projection of an object of interest, e.g. a tool tip of a transcatheter tool, on the X-ray detector, said spatial information is sufficient to establish an ultrasound imaging plane, in which the tool tip of a transcatheter tool may move without being able to leave the ultrasound plane. In other words, in case the ultrasound plane is steered to (always) contain the X-ray source as well as the projection of the tool tip visible in the X-ray image information, which projection however is non-static on the X-ray detector and may thus move, thereby requiring adjustment of the imaging plane, the tool tip of the transcatheter tool will remain visible within the ultrasound plane. Thereby, the ultrasound plane is automatically steered such that the object of interest lies within the image. No additional use of three-dimensional images is required, which in this case would only be used for tracking the tool tip while degrading the frame rate of the ultrasound image acquisition. 
         [0021]    The steering of the ultrasound plane according to the present invention allows to present a moving object of interest inside the ultrasound plane, thereby avoiding a repetitive appearance and disappearance of the object due to out of plane movement, either due to active motion of the object of interest or passive motion e.g. due to the cardiac beats. 
         [0022]    To allow the implementation of the method for ultrasound image acquisition according to the present invention, it is first assumed that the ultrasound image information and X-ray image information are registered. In other words, the spatial relationship between the ultrasound image information and C-arm imaging information, in particular comprising further information such as the relative position of an X-ray source, is known. E.g., when considering a two-dimensional X-ray image as well as a two-dimensional ultrasound image, the angle and position of both images with respect to each other is assumed to be known in case said data is registered. Also, a common coordinate system or reference coordinate system may be established and subsequently employed, thereby spatially linking the C-arm imaging information and the ultrasound image information. Further, particular image information visible in either two-dimensional image is also assumed to be known. One example of registering ultrasound image information and X-ray image information is described in international patent application WO 2011/070477 of the instant applicant. 
         [0023]    Further, it is assumed that the imaging plane of the ultrasound image information may be steered electronically, e.g. by employing a suitable ultrasound transducer array as known in the art. 
         [0024]    Also, it is assumed that the object of interest can be tracked, i.e. is visual and detectable within the X-ray image information. In other words, the X-ray image shall contain a projection of the object of interest, e.g. the tool tip of the transcatheter tool. 
         [0025]    Subsequently, the ultrasound plane is steered such that both the X-ray source as well as the tracked/projected object of interest in the X-ray image is contained in the ultrasound plane. In other words, the X-ray source as well as the tracked object of interest in the X-ray image describe two independent and precisely defined points of the ultrasound imaging plane. Since the ultrasound imaging plane also passes through the ultrasound probe or the ultrasound transducer, which therefore provides a third defined point, the ultrasound plane is completely and precisely determined. Since the ultrasound plane is thus readjusted continuously so that the X-ray source as well as the tracked object of interest in the X-ray image remains within the ultrasound plane, also the object of interest itself remains within the ultrasound imaging plane, since the object of interest naturally is situated on the line between the X-ray source and the projection of the object of interest. Indeed, the ultrasound plane is determined by the object of interest itself as well as the X-ray source, however with the object of interest being visualized by its projection on the X-ray detector. Thereby, it is ensured that the object of interest is also present in the ultrasound imaging plane. Inaccuracies in the imaging system, e.g. due to X-ray/ultrasound data registration errors or inaccuracies in the object of interest localization in the X-ray image, may require to tune the ultrasound slice thickness accordingly. Regularly, a slice thickness of 1, 2 or 3 mm is employed. 
         [0026]    According to the present invention, a first ultrasound image plane is thus determinable by employing three points in space, i.e. the ultrasound transducer tip, the X-ray source as well as the projection of the object of interest on the X-ray detector. Using said information, a further, second ultrasound image plane may be set up, which contains the vector or line between the ultrasound transducer and the object of interest itself in 3D space within the reference coordinate system, which position itself is known from the first ultrasound image information under particular consideration of the registration between the ultrasound image data and the X-ray image data. Consequently, a second two-dimensional ultrasound image may be acquired containing said vector between the transducer tip and the object of interest, leaving a further degree of freedom, which can be arbitrarily adjusted. In other words, a second two-dimensional ultrasound plane may be turned about the line or vector between the transducer tip and the object of interest located within patient tissue. Often, such a second two-dimensional ultrasound plane can be placed orthogonally to the first ultrasound image plane, thereby acquiring at least some three-dimensional information, while still only using (two) two-dimensional images. 
         [0027]    Put another way, assuming that the object of interest is also tracked in the first ultrasound plane, the second ultrasound image plane may be placed so that it passes through the object of interest. This leaves an extra degree of freedom for the interventionist to choose the orientation of the second ultrasound image plane freely, while still showing the object of interest. The tracking in the first ultrasound plane is greatly helped by the fact that the object of interest position is already known in the X-ray image and therefore the object of interest is constrained to lie on the corresponding epipolar line in the ultrasound image. 
         [0028]    Now referring to  FIG. 1 , an exemplary embodiment of the relationship between ultrasound image information and X-ray image information according to the present invention is depicted. 
         [0029]    In  FIG. 1 , a schematic of an imaging system employing both X-ray image acquisition as well as ultrasound image acquisition is depicted. An X-ray source  30  generates X-radiation  31 , which is exemplarily embodied as a cone-beam directed towards an X-ray detector  32 . X-ray detector  32  is exemplarity embodied as a two-dimensional X-ray detector, comprising a plurality of individual detector pixel elements arranged in a pixel array, which however is not depicted in detail in  FIG. 1 . In the path of X-radiation  31 , an object of interest  36 , the tool tip of a transcatheter tool, is depicted. The object of interest  36  generates a projection  38  on the two-dimensional X-ray detector  32 . Accordingly, a vector or line  48  between the X-ray source  30  and the projection  38  of the object of interest  36  on X-ray detector  32  is established, on which line  48  also the object of interest  36  is required to be arranged. 
         [0030]    Further, an ultrasound imaging apparatus embodied exemplarily as an ultrasound transducer  34  is shown in  FIG. 1 . Ultrasound transducer  34  is generating an ultrasound transmission  35 , here directed towards the object of interest  36 . A first line or vector  42   a  from the ultrasound transducer  34  to the X-ray source  30  as well as a second vector or line  42   b  from the ultrasound transducer  34  to the projection  38  of the object of interest  36  is depicted in  FIG. 1 . The first ultrasound plane  42  is thus established by the points in space of the ultrasound transducer  34 , the X-ray source  30  and the projection  38  of the object of interest  36 . Put another way, the ultrasound plane is established by paths or vectors  42   a,b,  and  48 . 
         [0031]    Establishing the first ultrasound plane  42  requires a known spatial relationship between the X-ray image information  40  and the ultrasound image information  42 , which may be performed by a registration operation of the C-arm acquisition geometry information and the ultrasound image information for obtaining a spatial relationship between said two image information. Known image processing techniques or further means like electromagnetic tracking unsing EM tracker elements may be employed for said registering operation. With said first ultrasound plane  42 , also a location in space of the object of interest  36  is known. Subsequently, for a second ultrasound plane  44 , this known location of the object of interest  36  together with the location of the ultrasound transducer  34  establishes a line or vector  50  between the ultrasound transducer  34  and the object of interest  36 . The second ultrasound plane  44  thus comprises always the object of interest  36  by assuming that said second ultrasound plane  44  always comprises line  50 , thereby allowing a degree of freedom  46 , which can be chosen arbitrarily, e.g. by an operator of the imaging system  54 . In other words, a further ultrasound image plane  44  may be turned about line  50  between the ultrasound transducer  34  and the object of interest  36 . One preferred alignment of the first ultrasound plane  42  and the second ultrasound plane  44  is arranging both planes perpendicular to each another to establish some three-dimensional reference frame for an interventionist. 
         [0032]    Now referring to  FIG. 2 , an exemplary embodiment of a system for ultrasound image acquisition according to the present invention is depicted. 
         [0033]    An exemplarily application scenario employs a C-arc  33  comprising an X-ray source  30  as well as an X-ray detector  32  with X-radiation  31  being generated by the X-ray source  30  and being directed towards the X-ray detector  32 . An object to be examined  52  is arranged in the path of X-radiation  31 , which is subject to a transcatheter procedure with an object of interest  36 , e.g. a transcatheter tool being inserted appropriately into object  52 . An ultrasound transducer  34  is arranged in the vicinity of the object to be examined  52 , in particular in the esophagus, embodied as a TEE ultrasound source. Said particular embodiment however is not depicted in  FIG. 2 , which is rather referring to a general ultrasound transducer  34 . 
         [0034]    A first ultrasound plane  42  is established by the X-ray source  30 , the projection  38  of the object of interest  36  and the ultrasound transducer  34  in three-dimensional space. 
         [0035]    Imaging system  54  comprises a processing element  56  with control elements  58 , exemplarily embodied as keyboard and manual input devices, as well as a display unit  60  for displaying at least some image information of the X-ray image information and the first and second ultrasound image information. The operator of the imaging system  54  may provide a suitable image information on display  60   a  to the interventionist operating to assist in the procedure. By determining the three-dimensional position in space of the object of interest  36 , a line  50  between the transducer  34  and the object of interest  36  may be established, allowing a second ultrasound plane  44 , not depicted in  FIG. 2  to be established, which may be turned about line  50  at the discretion of one of the operator and the interventionist. 
         [0036]    Now referring to  FIG. 3 , an exemplary embodiment of a method for ultrasound image acquisition according to the present invention is depicted. 
         [0037]      FIG. 3  describes a method  10  for ultrasound image acquisition, in particular for tracking an object of interest in ultrasound image information comprising receiving  12  X-ray image information and ultrasound image information, detecting  14  an object of interest in the X-ray image information and steering  16  two-dimensional ultrasound image acquisition such that the object of interest is within the first ultrasound image plane. A registering procedure  18  aids in obtaining a spatial relation between the X-ray image information and the ultrasound image information. The determined first ultrasound image plane may be visualized  20 . A second ultrasound image information corresponding to a second ultrasound image plane may be obtainable  22 , which may also be visualized separately or conjointly with at least one of the first ultrasound image plane and the X-ray image information. An operator may adjust  24  the thickness of at least one of the first ultrasound image plane and the second ultrasound image plane to aid in having the object of interest within at least one of the respective image planes. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           10  method for ultrasound image acquisition 
           12  receiving X-ray image information and ultrasound image information 
           14  detecting an object of interest in the X-ray image information 
           16  steering two-dimensional ultrasound image acquisition 
           18  registering X-ray image information and ultrasound image information 
           20  visualizing 
           22  obtaining second ultrasound image information 
           24  adjusting slice thickness of ultrasound image plane 
           30  X-ray source 
           31  X-radiation 
           32  X-ray detector 
           33  C-arc 
           34  ultrasound transducer 
           35  ultrasound transmission 
           36  object of interest 
           38  projection 
           40  X-ray image information 
           42  first ultrasound image plane 
           44  second ultrasound image plane 
           46  degree of freedom 
           48  line X-ray source projection object of interest 
           50  line ultrasound transducer object of interest 
           52  object to be examined 
           54  imaging system 
           56  processing element 
           58  control element 
           60  display unit