Abstract:
In a method to control the acquisition and/or evaluation procedure of image data in medical examinations, in a previously acquired planning image data set entirely or partially covering a target volume, spatial information of the target volume is determined automatically using a statistical model of the target volume based on data about real anatomy. The acquisition and/or evaluation operation is controlled using the spatial information. A statistical model of at least one greyscale value distribution in the region of the surface of the target volume is used to calculate the location information.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a method to control the acquisition and/or evaluation procedure of image data in medical examinations of the type wherein, in a previously acquired planning image data set entirely or partially covering a target volume, location (spatial) information of the target volume is determined automatically using a statistical model of the target volume based on data about real anatomy, and the acquisition and/or evaluation procedure is controlled using the location information. 
     2. Description of the Prior Art 
     In serial examinations of pathologies or examinations of a possible medicinal effect on these pathologies, for the examiner it is extremely important to discover the precise orientation and positioning of the pathological object as quickly as possible. Since the resolution capability in small animal measurements is, for example, in the range of 100 μm and is in a range of half a millimeter in humans, prefabricated parameter sets cannot simply be adopted. Rather, the position and orientation of the subject to be examined must be rediscovered in every new examination. For this purpose, a planning image data set is typically acquired. The more precisely that the orientation of the subject to be examined should be determined, the greater the resolution of the planning image data set must be. If this panning image is three-dimensional, in magnetic resonance tomography acquisitions it is the case that, for doubled resolution in the phase coding directions, a measurement time that is twice as long must be scheduled. For a three-dimensional data set, two phase coding gradients are required, meaning that a doubled resolution in these two directions corresponds to a quadrupling of the measurement time. In addition, after this time-consuming acquisition, the positioning of the subject must still also be precisely determined by an operator in order to establish the parameter sets for subsequent acquisitions. An exact positioning of the measurement slice is therefore necessary since (for example in flow measurements) the measurement slice must be situated perpendicular to the vessel through which medium flows, because otherwise it artifacts can occur in the exposures. Such position determinations can thereby easily occupy half of the available measurement time. It is therefore desirable to be able to automatically and exactly register the position and orientation of the subject to be examined with an optimally low-resolution planning image data set. 
     DE 10 2006 017 932 A1 describes a method in which a target volume in a planning image data set is automatically determined using a statistical model of the target volume that is based on data about real anatomy, and the acquisition and/or evaluation operation is then controlled with this information. The statistical model is acquired by means of measurements on multiple test subjects. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method of the above type that can be reproduced with less tendency toward error and that also requires less data for the planning image data set. 
     To solve this problem in a method of the aforementioned type, according to the invention a statistical model of at least one greyscale value distribution in the region of the surface of the target volume is used to calculate the spatial information. 
     In addition to the description of the target volume via a network of points, according to the invention the target volume is described by a network of profiles. Instead of one value, a series of values is thereby available for each point that should describe the target volume in a three-dimensional model. Spatial information thus can be obtained from the planning image data set entirely or partially covering the target volume in order to control the acquisition and/or evaluation operation of the target volume. The average model, thus the statistical model of the target volume that is based on data of real anatomy, is thereby adapted to the values of the planning image data set. The statistical average model is modified until the data of the planning image data set are reproduced with the highest probability. 
     A statistical model of the greyscale value distributions in the region of the surface of the target volume that is determined using model data sets of multiple people can be particularly advantageously used. Such a model has the advantage that it is based on real data, and that the statistical variance is reflected as close to reality as possible via suitable selection of people and, for example, MR method selection. It is also possible to divide the people into groups and thus to respectively achieve a model for children, women and men, for example, or to achieve a division based on age groups. The variance in the model data set can be reduced and the precision can thus be increased. 
     Correspondence points can advantageously be established on the surface of the target volume of each model data set. The volume of the data to be processed can be significantly reduced by this procedure. The more simply that the target volume is structured, the fewer correspondence points are required for description. For example, it is possible to register target volumes of relatively simple geometric structure (such as the liver or the kidneys) with relatively few correspondence points while more correspondence points are required for target volumes of more complex structure (such as the heart). 
     The greyscale values in the direction of the surface normals of each correspondence points of the target volume can advantageously be determined as profiles. The profiles are naturally dependent on their orientation relative to the surface. Optimally clear transitions from a target volume into regions outside the target value can be achieved by the selection of a preferred direction in the direction of the surface normals. A portion of the data points of the profile thereby lies within the target subject; some few lie in the transition regions, and multiple points additionally lie outside of the target subject. In the ideal case, the transition would ensue in stages, meaning that the transition from the target volume into the surrounding tissue would be recognizable as a jump in the greyscale values of the profile. In reality, however, for the most part one to two measurement points lie in this jump region, i.e. between the values that the target volume exhibits and the values that the surrounding tissue has. Conveniently, no exact point at which the surface of the target volume ends must be established via the use of the profile. Such an establishment is inherently always plagued with errors and can be omitted with the method according to the invention. After normalization of the profiles from the profiles of corresponding correspondence points for each correspondence point, an average profile representing the statistical model and a covariance matrix representing the statistical deviation can advantageously be determined. Statistical conclusions are possible only after measurement of at least one test subject; as already mentioned, these can also be divided into groups. In order to now obtain a statistical model from all of these target volumes of the model data sets, these must first be normalized, meaning that the value range of the variables must be transformed to a specific range. It is subsequently possible to determine an average profile and a covariance matrix for each correspondence point from the model data sets. This can also occur given group divisions, wherein only the model data sets associated with the group are then respectively incorporated into the generation of the average profile and the covariance matrix. 
     Alternatively, a reference image can be determined form the correspondence points of the model data sets via transformation of all test subject images to the average model. In this case, not only can the profiles be considered with regard to the surface normals of the correspondence points (thus this calculation of the reference image is computationally intensive); the statistical model outside of the measurements is created with test subjects, and this time factor is thus of less concern. The advantage lies in the fact that more data are available via the reference image. A normalized average image representing an additional statistical model and a normalized variance image representing the statistical deviation can then advantageously be determined from the reference image. By this procedure not only are profiles obtained with regard to the surface normals of the target volume, but also profiles of the greyscale value distribution in every direction relative to the surface of the target volume. A broader initial database is thus obtained. 
     Depending on the application field, various two-dimensional or three-dimensional data sets, or data sets from multiple two-dimensional images are considered as a planning image data set. It is possible for localizer exposures to be used as a planning image data set. The data acquired in such a manner are naturally subjected to one or more post-processing routines. For example, MR data are typically acquired in k-space and only converted into three-dimensional space via a Fourier transformation. In this post-processing the spatial resolution can then also be altered, for example by zero filling. Such reconstruction steps are necessary since the statistical model was naturally created in three-dimensional space from image data sets that have likewise previously been reconstructed. Since a statistical model based on real data is used (thus from the outset assumptions about the actual appearance of the target volume are plugged into the method), even low-resolution image data sets with a small coverage of the target area containing the target volume are sufficient, such that they can be used as a planning image data set. The term “coverage” is used herein to mean that, in the case of two-dimensional images, only thin sections are taken from the target volume, and thus a small slice thickness is realized. However, the spatial resolution of the actual image could be high. A smaller coverage is thus achieved with a small slice thickness and a low number of images. The method according to the invention here has a particularly advantageous field of application since these localizer exposures can already be produced before the additional data set acquisition. 
     Naturally it is also possible to use a previous diagnostic image data set as a planning image data set. Such a diagnostic image data set can have already been acquired for various reasons, or can be directly evaluated with the use of the method. For example, an organ in a diagnostic image data set can be localized in order to subsequently determine parameters (for example its volume). 
     Consequential spatial information of the target volume are obtained with the use of the statistical model. For this purpose, a start position for a generated model instance of the model can initially be established under consideration of the type of the target volume to determine the location information, for example using an ellipsoid model of the torso of a patient, and the model instance can be adapted to the image data in the planning image data set in an optimization process, wherein the location information is obtained from the adapted model instance after termination of the optimization process. 
     A start position must consequently initially be located. How this can best be established depends on the type of the target volume. For example, if this is an organ arranged in the torso of a patient, its rough position in the torso is relatively well known. To automatically locate the start position, for example, the torso can simply be considered as an ellipsoid. The relative position in the ellipsoid at which the corresponding organ is typically located is then assumed. In practice, the first and second moments from the planning image data set according to which the ellipsoidal torso can be defined are determined for this purpose, wherein the start position can then be selected. 
     In an embodiment, the optimization process include the steps of generating a sub-model that comprises those correspondence points of the statistical model for which corresponding image points are present in the planning image data set, calculating the current profiles of the correspondence points of the sub-model using the planning image data set, calculating a most probable displacement (shift) for each correspondence point of the sub-model using the current profiles and the respective statistical profiles, calculating that transformation that best reproduces the most probable displacement projecting the transformed sub-model to the statistical sub-model, and repeating the preceding steps until the model converges. 
     As noted above, the planning image data set should cover as little as possible for a fast implementation of the method. The spatial coverage of the model data sets will therefore exceed that of the planning order distribution system. Fewer data points are therefore located in the planning image data set than in the statistical model of the target volume. Therefore, a sub-model must be generated in which those correspondence points of the statistical model are located for which there are correspondences in the planning image data set. The corresponding profile of the greyscale value distribution can subsequently be respectively calculated for the current, existing correspondence points, and these profiles can then be compared with the profiles of the statistical sub-model. The transformation for the most probable displacement can be calculated from these data, and the statistical sub-model can be accordingly transformed. Upon convergence of the calculations, that statistical model or sub-model has been found that best reflects the measurement data. 
     As already mentioned, the method according to the invention can be used to control the further acquisition operation of image data. In particular, the acquisition of a second image data set (in particular of a magnetic resonance image data set) can thereby be controlled using the location information. This is particularly advantageous when the image acquisition device used to acquire the planning image data set is used to acquire the second image data set. The planning image data set can in turn be composed of localizer exposures. The patient advantageously remains unmoved between the acquisition of the second image data set and the acquisition of the planning image data set. Defined movements (in particular of the patient table) can be incorporated into the spatial information. 
     Alternatively, it is also possible to acquire an overview image data set, and the planning image data set is registered with the overview image data set and a second image data set is acquired with the patient unmoved in comparison to the overview image data set, wherein the control of the acquisition ensues using the registration and the spatial information. For example, such a scenario can occur when a diagnostic data set acquired in a past examination or even some other image data set is used. Then it is naturally also necessary to establish a correlation between the planning image data set and the coordinate system of the image acquisition device or, respectively, of the current bearing and position of the patient. This occurs with the aid of the overview image data set. For example, these can again be localizer exposures. 
     The acquisition of the second image data set can be controlled in many kinds of ways. The spatial information can thus be used to determine image acquisition parameters of slices to be acquired. This is primarily helpful in magnetic resonance acquisitions. The laborious manual marking of slices to be acquired is consequently discarded in a display of the planning image data set. Alternatively or additionally, the spatial information can be used for positioning of a navigator. With the use of the known position, orientation and shape of the target volume, by means of suitable algorithms it is easily possible to enable an idea positioning of a navigator (for example PACE) that is far less error-prone. 
     Furthermore, the spatial information can be used for positioning of the patient. For example, an ideal positioning of a patient bed in a magnetic resonance apparatus can be determined so that maximum homogeneity predominates in the target volume. 
     As a last example, it is ultimately advantageous to use the spatial information to adapt a measurement protocol in magnetic resonance acquisitions. For example, the repetition time can be optimized in what are known as link acquisitions. 
     Often it is also possible that a larger number of image data sets are acquired in the framework of a complete examination or an examination in multiple steps. For steady improvement and optimization of the spatial information, it can be useful to use the second image data set as a planning data set for a further implementation of the method. The establishment of the location information is therefore continuously improved iteratively from acquisition to acquisition. The results of the last adaptation of the statistical model can always be used as start values. 
     Since the same adapted model instance ultimately results with different exposures (thus in different image data sets) given the adaptation of the model to the corresponding patients, thus these ultimately differ only due to the positioning of the patient during the acquisition (thus in bearing and positioning and possibly due to deformation), the two mentioned image data sets can nevertheless be connected via comparison of marked points of these two model instances. Thus, first and second location information can be obtained from a first planning image data set acquired in a first examination and a second planning image data set serving for planning a follow-up examination, with both the first location information and the second location information being used for control. For example, in the first planning image data set a specific point can be located at which (for example) an irregularity exists that should be examined in more detail the scope of a follow-up examination. Since the model instances significantly correspond, the same point of interest can naturally also be located in the instance of the statistical model that is adapted to the second planning image data set. The items of information obtained from two planning image data sets are thus advantageously correlated with one another in order to localize the actual region of interest for the follow-up examination in the second planning image data set and to control the follow-up examination operation accordingly. In summary, given different position, orientation and/or shape of the target volume during the acquisition of the first planning image data set and during the acquisition of an additional image data set that is to be controlled, the first and the second items of spatial information serve to determine a spatial relationship (in particular a registration) based on which the control ensues. A simple registration is also consequently possible using the method. 
     In another variant of the method, evaluation information about the target volume can be determined under consideration of the spatial information in the framework of a control of the evaluation operation. For example, a start value for a segmentation process can be determined from the location information. Where the adaptation of the statistical model alone already represents a very good segmentation, as a start value for a finer segmentation algorithm it can be a starting point for even more precise and better results. 
     Alternatively or additionally, the spatial information is drawn upon to determine physiological parameters or data. For example, such parameters or data can be the volume of an organ or the number of lesions occurring therein or the volume proportion of the lesions in the organ. The perfusion of the target volume can also be determined. The spatial information is thereby primarily used to limit the region used to obtain the parameters or data. For example, from the image data set only the portion that also belongs to the target volume is considered, and then the physiological parameters or data are determined from this sub-image data set. 
     However, the method is also applicable when multiple target volumes are visible in a planning image data set. Spatial information of all target volumes can then be determined using a respective statistical model per target volume. In a subsequent step, for example, an image acquisition device can be controlled so that, for each of these target volumes, second image data set that show only these target volumes are generated. The method also turns out to be particularly advantageous given the association of conspicuities with specific target volumes or organs. The spatial information can be used to associate determined physiological parameters or data with target volumes. For example, the number of lesions can therefore be displayed for all organs of the lower abdomen. For this such lesions are initially determined in an entire planning image data set, including their position. If the position then lies within a target volume localized with the aid of the statistical model, thus within a specific organ, it is associated with this organ. This is naturally possible not only for lesions but also for other abnormalities (for example carcinomas) that are then associated with different target volumes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates production of a statistical model by means of the “offline strategy”. 
         FIG. 2  schematically illustrates production of a statistical model by means of the “on-the-fly strategy”. 
         FIG. 3  schematically illustrates a magnetic resonance system for implementation of the method according to the invention. 
         FIG. 4  is a flow chart of an embodiment of the method according to the invention. 
         FIG. 5  is a flow chart of Step S 4 . 
         FIG. 6-9  schematically illustrates the model adaptation. 
         FIG. 10  schematically illustrates the calculation of the most probable displacement. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 3  shows a magnetic resonance system  20  as an example for implementation of the method according to the invention. It is also possible to implement the method with radiation-based imaging modalities and even other imaging modalities, wherein the goal of the method would be less in time savings than in a reduction of the radiation exposure. 
     To create the statistical model, various patients  21  are moved into the magnetic resonance system  20  on a patient bed  22 . Respective image data sets (what are known as model data sets) are acquired to create the statistical model. Additional measurements as the acquisition of high-resolution model data sets are not provided with these patients  21 . The model data sets are then relayed to a computer  23  equipped with monitor  24  and keyboard  25 . Whether the same patient or different patients are respectively used for different target volumes (for example brain, liver, kidneys or also the aorta and the carotids) is incidental. It is only important that sufficient model data sets in order to be able to draw statistical conclusions are respectively available for modeling for a specific target volume. The statistical spread in the model data sets can thereby be reduced when sub-groups whose variance among one another is low can be located in the patients. For example, a division into children and adults or children, women and men, or generally in an age division (which can occur according to decades, for example) is possible. Depending on the subject to be examined, the most fitting statistical model can then be selected to implement the method according to the invention. 
     There are two strategies to get from the model data sets to the statistical models. 
       FIG. 1  shows the “offline strategy”. Correspondence points are thereby established on the surfaces of the target volumes of the model data sets. The profiles of the greyscale values are then determined in the direction of the surface normals for every correspondence point. One part of the data points of the profiles thereby lies within the target volume, one part outside the target volume and one part in the transition region. An exact separation of the target volume from the environment (characterized by a jump in the profile) is not possible in reality. 
     The curves  2   a ,  2   b  and  2   c  therefore also show not a jump but a steady transition. The curves  2   a ,  2   b  and  2   c  thereby show the profile of the grey values that are indicated by the surface normals  8   a ,  8   b  and  8   c  of the target volumes  1   a ,  1   b  and  1   c . The x-axes  3   a ,  3   b  and  3   c  respectively point in the direction of the surface normals  8   a ,  8   b  and  8   c  while the y-axes  4   a ,  4   b  and  4   c  indicate relative intensity profiles. A greyscale value profile is thus initially obtained for each model data set and every correspondence point. In the “offline strategy”, the statistical model is now formed in that an average value is formed from the greyscale value profiles of corresponding correspondence points, and a corresponding covariance matrix is calculated for this. The averaging thus respectively ensues across the model data sets, and as many greyscale value profiles and covariance matrixes are obtained as correspondence points that have been established. 
     The procedure in the “on-the-fly strategy” is something different, as is visible in  FIG. 2 . Here the target volumes  1   a ,  1   b  and  1   cc  are also provided with correspondence points; however, a reference image is then acquired from these via transformation. A normalized average image and a normalized variance image can then be calculated. Not only are greyscale value profiles obtained with regard to the surface normals, but also in all arbitrary directions for every correspondence point. In the normalized average image  10 , the surface normal  11  is drawn for illustration. Analogous considerations can be implemented at this point for every profile orientation. The corresponding greyscale value profile  12  is plotted over the x-axis  13 , which lies in the direction of the surface normal  11 . The y-axis  14  indicates the intensity distribution of the greyscale profile of the average image  10  at the point of the surface normal  11 . Corresponding specifications can also be presented for the variance. The x-axis  15  corresponds to the x-axis  13  but the variance is plotted on the y-axis  16 ; the corresponding curve  17  is likewise mapped in  FIG. 2 . 
       FIG. 5  shows how the average model is adapted to the values of the planning image data set. In a first step S 4 , a sub-model is generated. The statistical model was acquired with high resolution or, respectively, with full coverage in order to obtain optimally location information and size information of the target volume. By contrast, the planning image data set is for the most part acquired with a low resolution or coverage in order to save time. Therefore not every correspondence point in the statistical model has a correspondence in the planning image data set. Therefore, those correspondence points of the statistical model that have a correspondence in the planning image data set are selected in the sub-model. In addition to the specification of the correspondence points, the statistical model also contains a greyscale value profile for every correspondence point; corresponding greyscale value profiles then likewise exist in the sub-model for every correspondence point. In addition to these previously known and previously calculated greyscale value profiles, the current profiles are calculated in Step S 42 ; these are the profiles that can be obtained from the planning image data set. Following this, the most probable displacement is calculated for every correspondence point, via which the profile known from the statistical model can be transformed into the current profile. In Steps S 42  and S 43  it should be noted that, given the presence of a statistical model based on the “offline strategy”, only profiles with regard to a surface normal can be used, while profiles in arbitrary directions can be used given use of a statistical model based on the “on-the-fly strategy”. An additional difference for the two different strategies also arises in the calculation of the probable displacement. For example, in the “offline strategy” it ensues via the Mahalanobis distance of the profile while in the “on-the-fly strategy” the minimal Gaussian distance is used, for example. The definition of the Mahalanobis distance is:
 
 d   Mahalanobis =( p−  p   ) 1   *C   −1 ( p−  p   ).
 
wherein p thereby stands for the profile and C for the covariance. The Gaussian distance is defined by:
 
                 d   Gauss     =       ∑   i     ⁢         (       p   i     -       p   _     i       )     2       σ   i   2           ,         
wherein the p i  are the individual values of the profile and the σ i  are the variances of the points. A transformation of the sub-model that best reproduces the most probable displacements is subsequently calculated in Step S 44 . The sub-model transformed in such a manner is then projected onto the statistical sub-model in Step S 45  according to the formula
 
 b=P   sub   −1 *( x   sub −   x   sub   ).
 
     The values of b are thereby to be limited to a valid range. Steps S 41 -S 45  are then repeated until the model converges. The statistical average model was thus varied until the data of the planning image data set are reproduced with the greatest probability.  FIG. 6-9  show a graphical representation of the adaptation of the statistical model to the planning image data set. The planning image data set thereby consists of six localizer images, wherein three parallel slices  31   a ,  31   b  and  31   c  were respectively acquired as well as slices  32   a ,  32   b  and  32   c  perpendicular to the slices  31   a ,  31   b  and  31   c  and in turn parallel to one another. The statistical average model  30  with the correspondence points  35   a - 35   g  is then placed in these data. As can be seen, correspondences are found in the planning image data set for the correspondence points  35   a ,  35   b ,  35   c ,  35   d ,  35   f  and  35   g , however not for the correspondence point  35   e . This is therefore absent in the sub-model. The surface normal  33  is drawn as an example for the correspondence point  35   c , as well as a freely oriented vector  34 . These represent the greyscale value profiles to be obtained. Here the advantage of the “on-the-fly strategy” is clear. While only the data points that are indicated by the surface normal  33  are available in the “offline strategy”, the points indicated by the vector  34  are also available in the “on-the-fly strategy”; significantly more data are thus available for calculation of the displacement of the greyscale value profiles. 
     The most probable displacements  36   a ,  36   b ,  36   c ,  36   d ,  36   f  and  36   g  for the respective correspondence points are respectively plotted for the correspondence points of the sub-model. After implementing this displacement, a new sub-model  38  results (as shown in  FIG. 8 ). The correspondence points displaced in such a manner are then transformed to the most probable model instance  39  by a similarity transformation and projection onto the sub-model, as  FIG. 9  shows. A high resolution statistical model  39  of the target subject could thus be created by means of 6 quick localizers of the planning image data set, wherein both the measurement of the planning image data set and the calculation of the most probable model instances could take place quickly. 
       FIG. 7  schematically shows the discovery of the most probable displacement. Three possible displacements  42   a ,  42   b ,  42   c  are conceivable for the correspondence point  41  of the sub-model  40 . The current profile  44  is taken from the planning image data set while the possible profiles  43   a ,  43   b  and  43   c  based on the displacement are taken from the statistical model. As can be recognized, a maximum correlation results for the displacement  42   b , which therefore is selected for the correspondence point  41 . The method proceeds accordingly for all further correspondence points of the sub-model  40 . 
     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.