Abstract:
The invention relates to a method and device for navigating on a vision plane ( 1 ) in a multidimensional image data set ( 8 ), wherein the intersection angle and degree of freedom ( 3, 6, 7 ) of the vision plane ( 1 ) displacement correspond to the degree of freedom of a sample ( 2 ) displacement, in particular an ultrasound transducer ( 2 ) during an interactive image producing examination. Said invention also relates to a method and device for carrying out measurements in dynamic image data, wherein said method consists in stopping, for a predefined time interval, a sequence reproduction when a frame (Fp) is interesting, thereby enabling a user to carry out measurements.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a method and device for navigation and measurement in a multidimensional image data set of an object which has been obtained/acquired in particular by a medical imaging method. 
     BRIEF DISCUSSION OF RELATED ART 
     In many medical technology imaging methods, such as ultrasound, magnetic resonance tomography or optical coherence tomography (OCT), it is possible to obtain three-dimensional (3D) or four-dimensional (4D) image data sets of the object to be examined. A four-dimensional data set is a time sequence of plural three-dimensional images. However, orientation in such a 3D or 4D image data set of an organ is relatively difficult and requires considerable experience on the part of the user, generally of the cardiologist/radiologist or consultant. 
     To examine and carry out measurements in the image data, these are mostly observed on two-dimensional section images. In order to navigate the section planes through the image data set, in known methods the user can move the section plane in all 6 directions of space through the three-dimensional image data set. Often in this case 3 section images which are perpendicular to one another can be represented, which the user can then slide and rotate. In this type of navigation, the user must get used to and train navigating in the 3D/4D image data. 
     It is also known to represent a structure of interest by “surface rendering” or “volume rendering”. In this case, a 2D view of the object having shaded surfaces is generated from the 3D data volume. The structure of interest is however often covered by other structures of the object, which then have to be cut away by means of what are known as “scalpel” tools. A corresponding method for this is disclosed in DE 103 39 979 A1. In this case, on a section image, a vector is set up, which spans the structure of interest. A plane is set up through the starting point and/or the end point of the vector, which is at right-angles to the vector, and the image data on the other side of the planes are blanked out, so that only those image data are represented which lie between the planes. This method has the disadvantage that, due to the geometry of the planes, in each case only a slice-like area can be cut clear, which often does not correspond to the shape of the structure of interest. 
     In examining dynamic image data, further problems arise if the user wants to carry out a measurement, e.g. measure a certain distance in the structure, or to set markers at specified anatomical landmarks. In the prior art, this is only possible on a static image, so that the dynamics of the structure are not taken into account during measurement. In ultrasound examinations of the heart, for example, the examining doctor is used to seeing the heart in motion. Many diseases of the heart can be recognised only upon observing the beating heart. Therefore, precise and anatomically correct measurement is often only possible if the dynamics of the structure are being observed. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention makes available options for navigating and carrying out measurements in multidimensional, in particular 3D and 4D, image data sets which do not have the above-mentioned disadvantages. 
     This is achieved in the invention according to a first aspect with a method which comprises a step for navigation in a multidimensional image data set, in which the user moves a plane of view through the image data set by means of an input device, during which a section image corresponding to this plane of view is represented. The method is characterised in that in that the plane of view intersects the image data set at an angle which corresponds to an angle of view contained during an interactive imaging examination, and in that the degrees of freedom of the movement of the planes of view corresponds to the degrees of freedom of the movement of a “probe” or of an acquisition device during an interactive imaging examination. In the navigation according to the invention, therefore, the accustomed movements and ways of viewing of the user are simulated. The degrees of freedom of movement are consciously limited to the degrees of freedom which are available during the interactive imaging examination e.g. during an ultrasound examination. Thus navigation by the doctor is intuitive and can be quickly learnt. Furthermore, the doctor only sees section images which correspond to the usual perspectives and section images, so that there is no risk that he will lose his orientation in a 3D or 4D image data set. 
     Particularly preferably, the plane of view corresponds to the sound field of an ultrasound transducer, which may have e.g. a two-dimensional fan-shape. Accordingly, the movement of the plane of view corresponds preferably to the movement of the ultrasound transducer, i.e. the plane of view can be pivoted, rotated or slidably displaced on a virtual recording surface. The degrees of freedom of movement are preferably controlled via a single input device such as e.g. a computer mouse, the pivoting motion, rotation and sliding being controlled e.g. by the left-hand or right-hand mouse button, or by a scrolling wheel on the mouse. Thus the eye can be kept on the image data throughout the navigation. 
     If the image data set represents a moving heart, for example, the user usually searches first of all the 4-chamber view with the plane of view. What is known as the 2-chamber view and the longitudinal axis section are respectively rotated through approx. 60° relative to the 4-chamber view. Thus, according to a preferred embodiment of the invention, respectively 1 or 2 further section images are shown, which are rotated through a fixed angle relative to the plane of view. 
     Preferably, the method comprises a further step in the method of cutting the structure of interest clear, in which case a vector is set up on a section image and spans the structure of interest. A straight or curved plane or a free-form surface is set up through the image data set through the end point and/or the starting point of the vector, the plane extending perpendicularly on the vector. In this case, the image data on one side of the plane are blanked out, so that e.g. the structure lying between the planes is cut clear. Unlike the prior art, for the plane, a free-form or curved surface, in particular a spherical surface with variable radius, can be used, so that it can be adapted to the structure of interest. 
     According to a further aspect, the invention relates to a method which comprises a step of carrying out measurements and/or setting landmarks in a multidimensional dynamic image data set of the object, which contains a sequence composed of consecutively acquired 2D or 3D frames. The method is characterised in that a frame of interest is chosen, in which the measurement is to take place, and accordingly the sequence is played through by consecutive display of the individual frames, this display stopping at the frame of interest for a predetermined time, during which the user can set a marker or take a measurement. Thus the user can observe the object dynamically, but at the same time obtains for a time span of e.g. 1 to 2 seconds the option of studying a certain frame more closely and of setting a measuring point or landmark or measuring out a distance e.g. with the mouse. 
     Particularly preferably, the display loop is repeated more than once, in which case the display stops at the frame of interest for a predetermined time span in each case. According to another embodiment, the display loop is likewise run through more than once, but the display stops in each display loop on a different frame. This type of display can be repeated for various planes of view of a 4D data set. 
     From various measurements in various planes of section and/or at various points in time or time intervals, optionally measurements can be linked both in place and/or time. From these links, preferably further measurements are derived, such as for example, the change in a measurement over time and/or in space. This may involve for example areas, angles, volumes and their change over time, thus for example the change in volume of the heart during beating over time or the change in diameter of the mitral valve ring over time. 
     To represent such measurement results, according to a preferred embodiment, a two-dimensional rendering of the structure of interest is generated by surface rendering or volume rendering. In this rendering, at least one voxel is highlighted in colour according to the measuring results or the derived measurement results. 
     The invention relates also to a device which is suitable for carrying out the method according to the first and second aspect. 
     Finally, the invention also relates to a computer program product which contains software code stored on a computer-readable medium and which enables a computer to carry out the method described above when the computer program product is installed on the computer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained more fully with the aid of an embodiment with reference to the attached drawings, which show: 
         FIG. 1  a flow diagram showing the sequence of the method of examination according to one embodiment of the invention; 
         FIG. 2  a schematic representation of the degrees of freedom in moving a plane of view through an image data set; 
         FIG. 3  schematic representations of a section image with a structure of interest, which has been cut clear by an optional step in the method; 
         FIG. 4  a schematic representation of the dynamic display and measurement according to the second aspect of the method according to the invention; 
         FIG. 5  a rendering generated by volume rendering of a heart with areas of interest coloured in; 
         FIG. 6  a schematic representation of a device according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  gives an overview of an example of a complete examination method of a three-dimensional dynamic (i.e. four-dimensional) image data set  8 . At least the steps  14 ,  18  and  20  are in this case optional. 
     According to step  12 , the user navigates through the image data set in order to find a section image on which the structure of interest is shown advantageously. Optionally, he can segment the image data set in a step  14 , i.e. cut clear the structure of interest. In step  16 , the actual measurement is then carried out, and e.g. a landmark is set or a distance or an area on a section image is measured. These 3 steps are if necessary repeated in order to carry out further measurements. In step  18 , the measurement results are then linked in place and/or in time and are displayed in step  20 . In this case, there is an option of highlighting the measurement results in colour in the image data set. 
     The step  12  in the method of navigation will now be described more fully with the aid of  FIGS. 2 and 6 .  FIG. 2  shows a three-dimensional image volume  8 , which has been obtained/acquired by a three-dimensional ultrasound measurement. In this case, an ultrasound transducer has been pivoted on the surface of the patient&#39;s body in order thus to obtain image data from a conical data volume. Navigation of the plane of view through this image volume is then simulated by movement of the ultrasound head. The figure therefore shows a virtual probe  2 , which is firmly connected to a virtual sound plane  1 , the plane of view. In this case, further planes of view can also be navigated simultaneously, e.g. in capturing the heart, two longitudinal axis section and one short axis section. Pivoting of the virtual probe  2  according to the reference  3  in  FIG. 2  takes place about a base point  4 . This pivoting motion may be limited to e.g. ±80° in order thus to simulate the limitations in movement of an interactive examination. 
     With reference to  FIG. 6 , a section image  32  corresponding to the plane of view  1  is shown on a screen  28 . The screen is connected to a computer  30 , to which in turn the input device e.g. in the form of a computer mouse  22  is connected. The mouse has a left-hand mouse button  24 , a right-hand mouse button  26  and a scrolling wheel  25 . In addition to the section image  32  corresponding to the plane of view  1 , in the example shown on the screen  28 , two further section images  33 ,  34  are shown, which correspond to the same angle of incidence of the virtual probe  2 , but are rotated through the angle α or β relative to the plane of view  1 . This is shown on a further window  35  which shows a cross-section through the image data set  8 , on which the position of the section images  32 ,  33 ,  34  are shown as broken lines. This type of division of the screen  28  is however only one preferred example. 
     Returning to  FIG. 2 , the base point  4  of the virtual probe  2  can only be moved along a virtual recording surface  5 . The virtual recording surface is a simulation of the surface on which the probe or acquisition device rests during an interactive measurement and on which it can be moved. In ultrasound images, this is often the skin surface of the patient, but may also be a surface inside the body such as e.g. the windpipe if the ultrasound transducer is inserted through the same, or a bone surface or a surface outside the body determined by the type of image to be taken. The virtual recording surface  5  may be either planar or have a certain curvature, which for example simulates the object being scanned. 
     Rotation about the longitudinal axis of the probe  2  takes place according to the arrow  6 , e.g. with the scrolling wheel of the mouse and is advantageously limited to e.g. ±90°. Translation along the virtual plane of the skin  5  according to the direction of the arrow  7  is effected e.g. with the right-hand mouse button. 
     The virtual plane  5  of the skin can also be realised as a spherical surface, in which case the sphere preferably barely encloses the image volume  8 . Thus, in principle, any section planes are possible, which may however impede orientation in the data volume. 
       FIG. 3  shows the optional step  16  in the method in which the structure  38  of interest, in this case e.g. the head of a foetus, is cut clear. The left-hand diagram shows a section image through the image data set, the head  38  being covered by the structures  39  shown shaded, e.g. the womb. In order to cut these structures away, the user fixes a vector shown as an arrow  40  on the section image. A plane  43  extends through the starting point of the vector, and a spherical surface  42  through the end point. As can be seen in the right-hand diagram, all the image data lying outside the two surfaces  43 ,  42  are cut away. With the spherical shape of the surface  42 , the segmented volume of interest contains almost exclusively the head  38 . If then according to the middle diagram a surface view of the head is generated by volume rendering, the face is no longer hidden by the wall of the womb  39 . 
     If the user has thus found the image areas in which he wishes to carry out a measurement and has cut them clear, these are shown dynamically. The step  10  for carrying out measurement in a dynamic image data set will now be explained more fully with the aid of  FIG. 4 . In this, first of all a frame of interest Fp is defined or computed from the time sequence of 2D or 3D images (frames). Then the dynamic sequence of images is displayed with a normal playback speed. In the case of an image of the heart, e.g. 20 to 30 frames are played back which have been acquired during one heartbeat. A normal playback speed is in this case 20 frames per second. 
     If the frame of interest Fp is reached, the display stops for a predetermined time span.  FIG. 4  shows this frame referenced “prolonged frame Fp”. During this time, the user can then set the markers LM 1  and LM 2  and e.g. measure the distance between these landmarks so set in the image Fp which is displayed longer. The time span during which the display is halted can either be a predefined time of e.g. 0.5 to 5 seconds and particularly preferably 1 to 2 seconds, or according to another embodiment, the time span may last as long as the measurement takes. After the predetermined time duration and/or after the measurement has been completed, the display loop plays on. Thus the user has available both the dynamic impression of the sequence and also a static image from the sequence in order to be able to draw in the measurement exactly. This process can be repeated more than once in various image planes. After setting of the points, this method can also be applied for adjustment of the points set. 
     According to a further embodiment, the display loop is run through more than once, and the display stops each time at a different frame, e.g. at F 1 , F 2 , . . . Fn. Thus for example the same structure can be measured in each phase of movement. 
     From the measurements thus taken in various section planes and/or at various moments in time, measurements can be linked both in place and in time. From these links, further readings can be derived, such as for example the change in a measurement over time (step  18 ). In an optional step  20 , these measuring results are e.g. highlighted in colour in a two-dimensional view obtained by surface rendering.  FIG. 5  shows e.g. such a view of a heart, in which a shaded area  45  has been coloured e.g. red and a dotted region  46  e.g. in turquoise. The colour coding corresponds e.g. the moment of the contraction of the correspondingly coloured heart chamber wall. 
       FIG. 6  shows an embodiment of the device according to the invention schematically. This comprises a screen  28 , an input device  22  and a computing unit  30 , which may be a computer for example. The invention can also be incorporated in a computer program product, which contains software code sections which are installed on the computer  30  so that this carries out the re-written method. 
     The invention therefore makes available a method of simple navigation in multidimensional image data, an optional step for segmenting structures, a method of carrying out measurements on dynamic data, and a step for linking four-dimensional measurements and a step  20  for representing the dynamic measurement results in the four-dimensional data set.