Abstract:
In a method and magnetic resonance apparatus for segmenting a balloon-type volume having an inner surface and an outer surface in an image data record, that is provided to a computer, the image data record at least partially mapping a balloon-type volume, the computer is provided with a starting area and determines a first boundary surface as an inner surface of the balloon-type volume. The computer is provided with a starting surface in the balloon-type volume, and determines a second boundary surface as an outer surface of the balloon-type volume on the basis of the starting surface. The balloon-type volume is determined in the computer as a volume within the first boundary surface and the second boundary surface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    The invention concerns a method and a magnetic resonance system (apparatus) for segmenting a balloon-type volume having an inner surface and an outer surface in a three-dimensional image data record. 
         [0003]    Description of the Prior Art 
         [0004]    With image data records acquired using an imaging modality such as a computed tomography apparatus or a magnetic resonance apparatus, there is frequently a need for automated segmentation, i.e. detection of delimited image areas, the basis of which is formed in most cases by a different image content. 
         [0005]    Depending on whether the image is a two-dimensional image or a three-dimensional image, this may involve segmenting surfaces or segmenting volume elements. 
         [0006]    A large number of methods are known with which image data records can be automatically segmented. For instance, an image data record can be examined using pattern recognition methods. Segmentation can accordingly be performed by the sought pattern being differentiated from the rest of the image data record, and is thus segmented. 
         [0007]    With three-dimensional image data records, difficulties in terms of segmentation increase significantly because translational and rotational as well as deformation movements can take place in all directions. 
         [0008]    Balloon-type volumes are volumes that are predominantly curved and at least partially enclose a space. Balloon-type volumes are therefore neither spherical nor necessarily completely closed. They also do not need to be rigid, but can form the basis of a periodic or aperiodic movement. The left ventricle of the heart and the myocardium of the left ventricle of the heart are examples of balloon-type volumes. Whether the myocardium by itself, or together with adjacent tissues such as the epicardium and the endocardium, are among the volumes to be segmented depends on the diagnostic question to be answered, the type of imaging, and other factors. The type of imaging influences the question of the volume to be segmented because different types of imaging map tissue in different ways, and in particular the contrasts between different tissues can be different. If the image data record does not allow a difference to be identified between two adjacent tissues of organs such as the liver and stomach, a segmentation of the liver (for instance) cannot take place. Each volume that at least partially encloses a space thus can be regarded as a balloon-type volume. 
         [0009]    There is a particular interest in automatically segmenting the myocardial tissue in magnetic resonance image data records, in order to be able to automatically evaluate scar tissue, which can be displayed highlighted using late gadolinium enhancement (LGE). 
       SUMMARY OF THE INVENTION 
       [0010]    An object of the present invention is to provide a method of the type noted above wherein an automatic or semiautomatic segmentation of a balloon-type volume is better enabled. 
         [0011]    This object is achieved in accordance with the invention by a method for automatically or semi-automatically segmenting a balloon-type volume having an inner surface and an outer surface in an image data record, having the steps:
       a) providing an image data record to a computer, which at least partially maps a balloon-type volume,   b) providing a starting area within the balloon-type volume to the computer,   c) determining a first boundary surface in the computer as an inner surface of the balloon-type volume to the computer,   d) providing a starting surface in the balloon-type volume to the computer,   e) determining a second boundary surface in the computer as an outer surface of the balloon-type volume based on the starting surface, and   f) ascertaining the balloon-type volume as a volume within the first boundary surface and the second boundary surface, and making an electronic signal that represent the balloon-type volume available from the computer.       
 
         [0018]    The core of the invention is that the boundary surfaces of the balloon-type volume are defined and the position of the balloon-type volume is thus finally also known. This proceeds from the inside to the outside. 
         [0019]    Also applicable are the definition described in the introduction and the cited examples of a balloon-type volume. In addition, the right heart ventricle, the atria of the heart, and the liver are examples of other balloon-type volumes. 
         [0020]    The starting area may be a point or a volume. A pixel in a two-dimensional image data record and a voxel in a three-dimensional image data record are such a point. If the starting area is automatically predetermined, this involves an automatic method. If the starting area is alternatively predetermined by a user, this is referred to as a semiautomatic method. The starting area is in any case disposed within the balloon-type volume and therefore not in the sought volume itself. Since the balloon-type volume is predominantly curved, it encloses a volume. The starting area is disposed in this enclosed volume. “Within” is used below to refer to the enclosed volume, “in the balloon-type volume” is the volume occupied by the balloon-type volume itself. 
         [0021]    With these definitions, the starting surface is disposed in the balloon-type volume, i.e. it is disposed radially outside of the starting area and also of the first boundary surface. 
         [0022]    The balloon-type volume is defined by the definition of the first and second boundary surface. As described, this proceeds from the inside to the outside. In particular, the second boundary surface is disposed radially outside of the balloon-type volume and radially outside of the first boundary surface. In this case the structure needs to be neither spherical nor symmetrical, as already mentioned; radial is only defined by way of the distance from the starting area. 
         [0023]    The steps c) to f) are always performed in an evaluation processor. Only step b) may require the help of a user. 
         [0024]    An edge detection method can be used to determine the first boundary surface. Such methods isolate planar areas in an image by segmenting them along lines with the aid of different color or gray values. In particular an “active contour” method can be used. These methods are also referred to as “snakes”. Here the object contour is described by parametric curves and corrected by way of energies. Different models and functions can be used for these energies. 
         [0025]    An estimated boundary surface can preferably be calculated on the basis of the starting area and the first boundary surface can be calculated on the basis of the estimated boundary surface. In particular, an “active contour” method, as just described, can be used to ascertain the estimated boundary surface. I.e. the “active contour” method is not the very last step in determining the first boundary surface, but instead a step which follows the definition of the starting area. On the basis of the estimated boundary surface, at least one further step then follows for defining the first boundary surface. 
         [0026]    The estimated boundary surface or the entire three-dimensional image data record and the estimated boundary surfaces can be converted into polar coordinates for each slice and the calculation of the first boundary surface into polar coordinates can be performed. The use of polar coordinates has also proven to be advantageous in non-cylindrical balloon-type volumes. The center of gravity of the estimated boundary surface can be used as a center of the image in polar coordinates. 
         [0027]    The first boundary surface can be ascertained from the estimated boundary surface using a second edge detection method. The first edge detection method then only has the task of “approximately” determining the shape, while the second edge detection method is more sensitive to color differences. 
         [0028]    A Canny method can be used here as a second edge detection method. The Canny method is also known as “Canny algorithm” and “Canny edge detector”. By comparison with “active contour” methods, the Canny method is more insensitive to noise, as a result of which it is particularly advantageous in image data records with a low SNR. 
         [0029]    Preferably the starting surface can be ascertained as a function of the first boundary surface. If we assume an even thickness of the balloon-type volume, the starting surface is always parallel to the first boundary surface, i.e. the distance from one point of the first boundary surface to a corresponding point of the starting surface is always the same. The distance is typically determined on the basis of a tangent through the point of the first boundary surface. With a three-dimensional image data record, a tangential surface is similarly used as the basis. 
         [0030]    The thickness of the balloon-type volume is naturally never constant over the entire surface; a starting surface would otherwise not be required. However, the thickness is often only variable with short distances, so that the starting surface is a very good starting point. A fixed distance from the first boundary surface and from there outwards can preferably be predetermined, as described, in order to ascertain the starting surface. 
         [0031]    An edge detection method is preferably used to ascertain the second boundary surface. A Canny method is preferably used here again, as described. 
         [0032]    Advantageously the volume elements ascertained with the edge detection method, also known as voxels, can be smoothed by a polynomial regression. For example, the second boundary surface is smoothed. It has namely been shown that the second boundary surface is not as accurately defined by the edge detection method as the first boundary surface. Smoothing is therefore advantageous. 
         [0033]    Naturally the first boundary surface can also be smoothed with a polynomial regression. However, the use of a number of edge detection methods is preferred with the first boundary surface. 
         [0034]    The starting area can preferably be ascertained from a starting point, by a predetermined radius being used to determine a starting sphere as a starting area. The starting point can be manually predetermined or automatically determined. As noted above, a starting point in a three-dimensional image data record is a starting voxel. 
         [0035]    Advantageously, myocardial tissue and/or endocardial tissue and/or epicardial tissue can be used as a balloon-type volume, as a first boundary surface and/or as a second boundary surface respectively. The myocardial tissue is then disposed within the first and the second boundary surface, namely within the endocardial tissue and the epicardial tissue. 
         [0036]    The myocardial tissue of a left ventricle of the heart can preferably be determined as a balloon-type volume. The method described above is then a method for segmenting myocardial tissue of a left ventricle of the heart. This is only a preferred application, other myocardial tissue or entirely different tissue can also be segmented, as already described. 
         [0037]    In more general terms, the balloon-type volume may be an organ or an organ part, which is mapped in the three-dimensional image data record. 
         [0038]    A magnetic resonance image data record is preferably used as the image data record. A magnetic resonance image data record is naturally an image data record that was recorded using a magnetic resonance apparatus. 
         [0039]    An image data record that has been recorded following administration of a contrast agent to the examination object, which is mapped in the image data record, is preferably used as an image data record. In particular, an LGE magnetic resonance image data record can be used. 
         [0040]    Preferably an image data record that was recorded with parameters, in which the signal from the balloon-type volume is minimized, is used as the image data record. This can take place in addition or alternatively to the administration of a contrast agent. One example of the myocardial tissue may be a magnetic resonance image data record, in which an inversion pulse was used and the recording starts when the signal of the myocardial tissue has its zero crossing. This takes place by suitably selecting the inversion time. 
         [0041]    A three-dimensional image data record can be used particularly preferably as an image data record. The method can also be performed on a two-dimensional image data record, wherein there is no synchronization across a number of slices. Ultimately with a three-dimensional image data record, many of the described steps are performed on individual slices, with the slices being considered as two-dimensional image data records. With a three-dimensional image data record, a synchronization across a number of slices can still take place beyond the procedure on a two-dimensional image data record. 
         [0042]    The above object underlying the invention is also achieved with a magnetic resonance system having a control computer. The control computer is configured to perform the method as described above. 
         [0043]    Further advantageous embodiments of the inventive magnetic resonance apparatus correspond to the described embodiments of the inventive method. 
         [0044]    The aforementioned methods can be implemented in the control apparatus as software, or as (hard-wired) hardware. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]      FIG. 1  shows a magnetic resonance system in accordance with the invention. 
           [0046]      FIG. 2  shows a flowchart for recording a three-dimensional magnetic resonance data record in accordance with the invention. 
           [0047]      FIG. 3  shows a three-dimensional image data record. 
           [0048]      FIG. 4  shows a first image for explaining the invention. 
           [0049]      FIG. 5  shows a cross-section through a cardiac wall. 
           [0050]      FIG. 6  shows a second image for explaining the invention. 
           [0051]      FIG. 7  shows a third image for explaining the invention. 
           [0052]      FIG. 8  shows a fourth image for explaining the invention. 
           [0053]      FIG. 9  shows a fifth image for explaining the invention. 
           [0054]      FIG. 10  shows an extract from the fifth image. 
           [0055]      FIG. 11  shows a flowchart for segmenting the myocardium of a left ventricle of the heart in accordance with the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0056]      FIG. 1  shows a magnetic resonance apparatus  1 . In addition to a radio frequency coil  2  embodied as a body coil, this has a coil array  3  with coils  4 ,  5 ,  6  and  7  and a control computer  8 . A body coil such as the coil  2  is used to excite the magnetization of nuclear spins. It is therefore also called an excitation coil. 
         [0057]    The coil array  3  is used to read out the MR signal that results from the excitation. The coils  4 ,  5 ,  6  and  7  of the coil array  3  read out the measured signal simultaneously. This is parallel imaging if more than one coil is used to read out the measured signal. An individual coil can also be used as a detection coil instead of the coil array  3 . Particularly with high field devices having a basic magnetic field strength of greater than 10 T and a patient bore of 40 mm to 200 mm, it is common practice to use coils that are simultaneously excitation and detection coils. An image data record can also be recorded herewith, as described further below. 
         [0058]    Gradient coils  9 ,  10  and  11  are required for imaging purposes. The gradient coils  9 ,  10  and  11  generate gradient fields in three directions. These are overlaid in order to generate the gradients used in a recording sequence, these gradients being in the read, phase and slice direction. Depending on their position, the gradients used in a sequence are composed of the gradients individually or in any combination. 
         [0059]    The gradient coils  9 ,  10  and  11  or the fields generated therewith are, as is known, required for spatial encoding. A phase encoding is produced scanned by repeatedly varying at least one current feed value of one of the gradient coils  9 ,  10  and  11 . 
         [0060]      FIG. 2  shows a flowchart for recording a three-dimensional magnetic resonance image data record of a left ventricle of the heart. This can be segmented using the described method for instance. 
         [0061]    In step S 1  the patient is positioned in the magnetic resonance apparatus  1 , wherein the basic settings such as shimming or determining the axis positions are also performed with the aid of scout scans. 
         [0062]    A gadolinium-based contrast agent is administered in step S 2 . 
         [0063]    The three-dimensional image data record will be recorded at a predetermined time following administration of the gadolinium in step S 3 . This can be recorded segmented, i.e. with interruptions. It can however also be acquired “in one step”. A recording method of this type is known for instance from Shin et al., Rapid Single-Breath-Hold 3D Late Gadolinium Enhancement Cardiac MRI Using a Stack-of-Spirals Acquisition, JMRI 40: 1496-1502 (2013). The recording parameters are selected here such that the myocardial tissue provides no signal and thus has minimal or, disregarding the noise signal, no signal values in the image data record. This can be achieved for instance by suitably selecting an inversion time, see above. 
         [0064]    Finally, the measured signals thus recorded are processed in step S 4 ; inter alia a Fourier transform takes place so that a three-dimensional image data record  12  is obtained. 
         [0065]      FIG. 3  shows the three-dimensional image data record  12  as a structure made of voxels  13 ,  14  and  15 . The other voxels have no reference numerals. The three directions of the image data record  12  are shown using the arrows  16 ,  17  and  18 . Images in the form of individual slices  19 ,  20  or  21  can be easily extracted along these directions, which are the read, phase and slice directions. It is known to position the read, phase and slice direction such that the evaluation of the image data record  12  is simplified, for instance by a structure to be examined being intersected vertically by one of the read, phase or slice direction. It can be freely selected as to which of the arrows  16 ,  17  and  18  shows the read, phase and slice direction, respectively. 
         [0066]    Depending on the choice of the gradients and the position of the patient, the 2D images or slices  19 ,  20  and  21  are sagittal, coronal or axial sectional views or are at an angle hereto. 
         [0067]      FIG. 4  shows an image of the slice  19  as an example. The left ventricle of the heart  22  and the surrounding tissue  23  are shown schematically. The left ventricle of the heart  22  consists in this sectional view of a strip of tissue  24  within which the space  25  is disposed. 
         [0068]    The strip of tissue  24  is illustrated in detail in  FIG. 5 . The endocardium  26 , the myocardium  27  and the epicardium  28  follow from the space  25 . The pericardium  29 , which for its part is subdivided again, then also appears. However this subdivision has no further relevance to the present application. 
         [0069]    The starting area  30  and the estimated boundary surface  31  are also mapped in  FIG. 4 . On the basis of the starting area  30 , the estimated boundary surface  31  is obtained using an “active contour” method for instance. 
         [0070]    The first boundary surface  32  is ascertained on the basis of the estimated boundary surface  31 , by the starting image being transformed into polar coordinates and a further edge detection method such as the Canny method being applied to this polar image.  FIG. 6  shows the image  19  with the first boundary surface  32  which is transformed again into Cartesian coordinates. The first boundary surface  32  differs from the estimated boundary surface  31  in the section below to the right for instance, where the notch is omitted. 
         [0071]    Prior to the transformation into Cartesian coordinates, the shape of the first boundary surface is preferably smoothed by consideration of the slices parallel to the evaluated slice  19 . The first boundary surface  32  which is smoothed in this way is shown in  FIG. 7 . 
         [0072]    Here this first boundary surface  32  is the boundary between the endocardium  26  and the myocardium  27 , which is ascertained automatically. 
         [0073]    On this basis a starting surface  33  can be ascertained for determining the second boundary surface, by a fixed distance  34  radially outwards being predetermined on the basis of the first boundary surface  32 , as  FIG. 8  shows.  FIG. 9  shows an extract from  FIG. 8 . The distance  34  is defined at any point on the basis of the tangential surface. Here the tangential surface is a tangent  35  by virtue of the two-dimensional representation. 
         [0074]      FIG. 10  shows the second boundary surface  36 , which was ascertained by means of a Canny method on the basis of the starting surface  33 . The first boundary surface  32  is also drawn with dashed lines. The myocardium  27  is then the volume between the first boundary surface  32  and the second boundary surface  36 . 
         [0075]      FIG. 11  shows a flowchart for segmenting the myocardium  27  of a left ventricle of the heart  22 . 
         [0076]    Step S 5  with the substeps S 5 . 1  to S 5 . 3  shows the ascertaining of the estimated boundary surface  31 . 
         [0077]    In step S 5 . 1 , a three-dimensional image data record  12  is provided, which maps the left ventricle of the heart  22  and was recorded using an LGE method. In summary, LGE means that a gadolinium contrast agent administration took place before the recording and the signal of the myocardium  27  is suppressed as well as possible by the parameter selection of an inversion module. 
         [0078]    Subsequently in step S 5 . 2  a start voxel is predetermined by selecting a voxel in the space  25  within the myocardium  27  and a starting sphere is calculated therefrom as a starting area  30 , by a fixed radius being placed around the starting voxel. 
         [0079]    On the basis of the starting sphere, the estimated boundary surface  31  is ascertained using an “active contour” method as a first edge detection method in step S 5 . 3 . 
         [0080]    The determination of the first boundary surface  32  in the form of the endocardium  26  then follows on as step S 6  with substeps S 6 . 1  to S 6 . 4 . 
         [0081]    In step S 6 . 1  a volume around the estimated boundary surface  31  is selected, which is selected generously such that it reliably contains the entire left ventricle of the heart  22  but not the immaterial image areas further outside. This steps helps to save on computing time, but is not compulsory. 
         [0082]    In the following step S 6 . 2 , the estimated boundary surface  31  and the further voxels of the selected volume are converted into polar coordinates. The center of gravity of the estimated boundary surface  31  is preferably used as a center of the image in polar coordinates. 
         [0083]    In step S 6 . 3 , the first boundary surface  32  is ascertained by means of a Canny method as a second edge detection method from the estimated boundary surface  31 . A factor σ=2 can preferably be used here so that smaller corners are extracted. 
         [0084]    The first boundary surfaces  32  thus obtained are smoothed as step S 6 . 4  in the direction vertical to the direction of the slice  19 , as for instance in the direction of the arrow  17 , by considering the first boundary surfaces  32  found in the adjacent slice or slices. Individual inaccuracies in a slice can be compensated in this way. 
         [0085]    In step S 7 , the epicardial structure is determined. 
         [0086]    In step S 7 . 1 , a starting surface  33  is predetermined on the basis of the first boundary surface  32 , by a fixed distance being plotted radially outwards. The radius is selected such that the starting surface is reliably disposed in the myocardium  27 . 
         [0087]    In step S 7 . 2 , a Canny method is used again in order to define the second boundary surface  36 . This is the third time that an edge detection method is used. 
         [0088]    The voxels of the second boundary surface  36  thus found are smoothed in step S 7 . 3  by a polynomial regression. The second boundary surface  36  is the boundary surface between the myocardium  27  and the epicardium  28 . 
         [0089]    The steps S 7 . 1  and S 7 . 2  are also preferably performed in polar coordinates. A conversion into Cartesian coordinates is only appropriate for displaying individual images. 
         [0090]    With the described method, the myocardium can be segmented without a further image data record being required. The myocardium has hitherto typically been segmented in another image data record, for instance a Cine data record, and this structure placed in an LGE image data record. It is possible with the described method to segment the myocardium directly in a two-dimensional or three-dimensional LGE image data record. 
         [0091]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.