Abstract:
The invention relates to a system and a method for visualizing a time-variant parameter at a plurality of positions in a biological structure, wherein a visualization display is configured to display a first representation showing a first visualization parameter at a plurality of time intervals, determined in a first volume between a first and a second boundary of the structure, and to display a second representation showing first and second visualization parameters determined in the first and in a second volume extending between the first and the second boundary of the structure. By providing the user with this combination of the two representations, a higher resolution of data may be processed and meaningfully visualized for intermediate volumes between the first and the second boundary. This is based on the insight that it is desirable to view data between boundaries of a structure as well as at different positions through the structure. However, without simple means of visualization, processing a higher resolution of data is not feasible. This is especially useful when performance differences may be present between the structure at the first and the second boundary. For example, perfusion measurements within the myocardium are different for the endocardial and epicardial layers. Therefore, the relative position of the measurements relative to these layers yields valuable data in the evaluation of perfusion. This increases the information which the healthcare professional can extract from imaging data without complicating the representations required to visualize it.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a system and method for visualizing a time-variant parameter in a biological structure. 
       BACKGROUND OF THE INVENTION 
       [0002]    As medical imaging and analysis techniques are continuously improving, the healthcare professional is presented with an ever increasing complexity of the data available for visualization. Starting with measurements of a parameter within a volume of interest, the parameter may be analyzed in different quantitative ways, for example to extract maximum intensity, mean intensity, minimum intensity, average slope values, maximum slope values, minimum slope values, for any desired piece of the volume of interest. The professional must then be able to visualize the data in a meaningful way. For example, visualization of time-intensity curves and associated quantitative analysis data derived from medical images, such as for images of a diseased heart acquired with first pass enhancement cardiac magnetic resonance imaging (MRI). 
         [0003]    At present, two-dimensional (2D) visualization techniques are typically used to visualize such time-intensity curves and the associated quantitative analysis data. 
         [0004]    For example, a visualization representation known as a perfusogram for a biological structure is known from US application 2005/0124861. Such a perfusogram representation  10  is depicted in FIG.  7 —it comprises a 2D array of pixels  15 , each pixel having a grey value or color value to represent a mean intensity level. A key  20  is also depicted indicating the mean intensity associated with each grey value or color value. The number of pixels in the horizontal direction  30  is determined by the number of time intervals for which the mean intensity values are determined. The number of pixels in the vertical direction  40  is determined by the number of segments selected for analysis. In other words, the vertical direction provides spatial information, and the horizontal direction provides temporal information. 
         [0005]    Such a perfusogram may be visualized in combination with a color overlay on an anatomical gray value image, so that a healthcare professional may relate the spatial information to the anatomy of the patient. It is also known in the art to overlay the anatomical gray value image with color-coded segments, such that the value of a parameter for that segment may be related to the color chosen to depict that segment. For example, as shown in  FIG. 6 , a color overlay representation  50  comprises an anatomical gray value image  60  of a human heart, overlaid in the region of the myocardium by at least one colored segment  70 . The color of the segment  70  is an indication of the mean segment intensity as a measure of perfusion, and a key  80  is also depicted indicating the mean intensity associated with each color. 
         [0006]      FIGS. 6 and 7  illustrate the difficulties in presenting complex multi-dimensional data to the user— FIG. 7  makes it difficult to relate the representation to the actual anatomy of the patient, and  FIG. 6  does not show changes in intensity observed over time. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the invention to provide a system for visualizing a time-variant parameter at a plurality of positions. 
         [0008]    The invention is defined by the independent claims. Advantageous embodiments are defined in the dependent claims. 
         [0009]    According to a first aspect of the invention, a system is provided for visualizing a time-variant parameter at a plurality of positions in a biological structure, the system comprising: 
         [0010]    a determination unit configured to determine a value of the time-variant parameter at a plurality of positions in the biological structure, the structure extending in mutually perpendicular X, Y and Z directions; determine the positions of a first and a second boundary extending in the Z direction, and the positions of a first volume extending in Z disposed between the first and the second boundary; determine the positions of a second volume generated by extending the first volume in the X-direction to both the first and the second XY boundary; determine a first visualization parameter for a plurality of time intervals from the time-variant parameters at a plurality of positions in the first volume; determine a second visualization parameter from the time-variant parameters at a plurality of positions in the second volume, and 
         [0011]    a visualization display configured to display a first representation showing the first visualization parameter at the plurality of time intervals, and display a second representation showing the first and second visualization parameters. 
         [0012]    By providing the user with this combination of the two representations, a higher resolution of data may be processed and meaningfully visualized for intermediate volumes between the first and the second boundary. This is based on the insight that it is desirable to view data between boundaries of a structure as well as at different positions through the structure. However, without simple means of visualization, processing a higher resolution of data is not feasible. This is especially useful when performance differences may be present between the structure at the first and the second boundary. For example, perfusion measurements within the myocardium are different for the endocardial and epicardial layers. Therefore, the relative position of the measurements relative to these layers yields valuable data in the evaluation of perfusion. This increases the information which the healthcare professional can extract from imaging data without complicating the representations required to visualize it. 
         [0013]    According to a further aspect of the invention, the visualization display is further configured to display a third representation comprising an anatomical gray-value image comprising an XY section through the biological structure, overlaid with the XY section of the first and the second boundary corresponding to the XY section through the structure, and the XY section through the first volume corresponding to the XY section through the structure. 
         [0014]    By overlaying, on an anatomical image, the contours representing some of the parameters used in the determination of the representations, the relationship between the representations and the patient&#39;s anatomy is further clarified. 
         [0015]    According to a further aspect of the invention, the system is further configured to display the first representation comprising a spatial indicator of one of the plurality of first volumes, and a temporal indicator of one of the plurality of time intervals. 
         [0016]    In addition or alternatively, the system is configured to display the second representation comprising a spatial indicator of one of the plurality of first volumes. 
         [0017]    By providing meaningful indicators to the user, the relationships between the different representations are clarified. This improves the intuitive feel of the displayed information, and makes it easier for the healthcare professional to relate the displayed information to a medical condition. 
         [0018]    According to an aspect of the invention, the system further comprises interactive means for a user to determine a parameter in the determination unit or visualization display from the group consisting of: the positions of the first boundary, the positions of the second boundary, the positions of the first volume, the extent of the first volume in XY, the extent of the biological structure in Z, the plurality of time intervals, the first visualization parameter, the second visualization parameter, the XY section through the biological structure and any combination thereof. 
         [0019]    By providing high resolution and meaningful data, the system can be made more advantageous and intuitive by allowing the user to directly change parameters used in the determination and visualization of the data. 
         [0020]    According to a still further aspect of the invention, the visualization display is further configured to display the second representation using volume rendering. 
         [0021]    By introducing the healthcare professional to the possibility of a higher resolution of data analysis for biological structures, a whole new way of data representation becomes possible. Volume rendering techniques, never considered previously for such intermediate volumes in a structure, may be employed to provide even more advantageous representations. 
         [0022]    According to an aspect of the invention, a method is provided for visualizing a time-variant parameter at a plurality of positions in a biological structure, comprising determining a value of the time-variant parameter at a plurality of positions in the biological structure, the structure extending in mutually perpendicular X, Y and Z directions; determining the positions of a first and a second boundary extending in the Z direction, and the positions of a first volume extending in Z disposed between the first and second boundaries; determining the positions of a second volume generated by extending the first volume in the X-direction to both the first and the second XY boundary; determining a first visualization parameter for a plurality of time intervals from the time-variant parameters at a plurality of positions in the first volume; determining a second visualization parameter from the time-variant parameters at a plurality of positions in the second volume, and displaying a first representation showing the first visualization parameter at the plurality of time intervals, and displaying a second representation showing the first and second visualization parameters. 
         [0023]    According to a further aspect of the invention, the method further comprises displaying a third representation comprising an anatomical gray-value image comprising an XY section through the biological structure, overlaid with the XY section of the first and the second boundary corresponding to the XY section through the structure, and the XY section through the first volume corresponding to the XY section through the structure. 
         [0024]    According to an aspect of the invention, a computer program product is provided for carrying out the method of the invention when loaded and run on a computer. 
         [0025]    It will be appreciated by those skilled in the art that two or more of the above-mentioned embodiments, implementations, and/or aspects of the invention may be combined in any way deemed useful. 
         [0026]    Modifications and variations of the image acquisition apparatus, of the workstation, of the system, and/or of the computer program product, which correspond to the described modifications and variations of the method, can be carried out by a person skilled in the art on the basis of the present description. 
         [0027]    A person skilled in the art will appreciate that the system may visualize any form of time-variant multidimensional image data, e.g., to 2-dimensional (2-D), 3-dimensional (3-D) or 4-dimensional (4-D) images, acquired by various acquisition modalities such as, but not limited to, standard X-ray Imaging, Computed Tomography (CT), Magnetic Resonance Imaging (MRI), Ultrasound (US), Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT), and Nuclear Medicine (NM). 
         [0028]    The embodiments may also be advantageously combined with the “System For Analyzing Images and Corresponding Method” disclosed in the co-pending application, applicant reference number PH-012697, filed by the same applicant and on the same day as this application. The co-pending application discloses establishing a gradient which is representative of the rate of change in data values between a first and a second border. This gradient may advantageously be used in the embodiments of the current application to determine one of the visualization parameters. Additionally, it may be advantageous to further configure the system of this application to also display the gradientogram disclosed in said co-pending application, or to display the gradientogram in place of the first representation. In particular, the visualization of perfusion in a myocardium may be enhanced by combining the disclosures of the co-pending application with those of the present application. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
           [0030]    In the drawings: 
           [0031]      FIG. 1  shows an example of a biological structure for which a representation may be determined, 
           [0032]      FIG. 2  depicts the biological structure of  FIG. 1 , viewed along the Z axis, so that the XY plane is closest to the viewer, 
           [0033]      FIG. 3  shows an example of a visualization representation, 
           [0034]      FIG. 4  depicts a second example of a biological structure for which a representation may be determined, 
           [0035]      FIG. 5  depicts the biological structure example of  FIG. 4 , viewed along the Z axis, so that the XY plane is closest to the viewer, 
           [0036]      FIG. 6  shows an example of a color overlay representation, 
           [0037]      FIG. 7  depicts an example of a perfusogram representation, 
           [0038]      FIG. 8  depicts the biological structure of  FIG. 1  and the different volumes which may be defined, 
           [0039]      FIG. 9  depicts examples of data representations, 
           [0040]      FIG. 10  depicts a biological structure which approximates a hollow cylinder volume, 
           [0041]      FIG. 11  depicts further examples of data representation, 
           [0042]      FIG. 12  depicts a further example of a data representation, 
           [0043]      FIG. 13  depicts displays that may be presented to the user, 
           [0044]      FIG. 14  shows a possible implementation of the representation combination of  FIG. 13B , 
           [0045]      FIG. 15  depicts an example of a method of visualizing a time-variant parameter in a biological structure, 
           [0046]      FIG. 16  shows a system for visualizing a time-variant parameter, and 
           [0047]      FIG. 17  depicts volumetric data representations. 
       
    
    
       [0048]    The Figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. Similar components in the Figures are denoted by the same reference numerals as much as possible. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0049]      FIG. 1  depicts a biological structure  100  for which a representation may be determined. The structure  100  extends in mutually perpendicular X, Y and Z directions  200 . These directions  200  are chosen arbitrarily—any other coordinate system or convention may be used. The structure  100  is defined by a first boundary  110  and a second boundary  120 , each boundary extending in YZ planes. The structure is also bounded by an XY plane  180 . 
         [0050]    The skilled person will realize that the structure  100  and its boundaries here described do not necessarily coincide with the anatomical extent of biological tissue. For example, the boundaries of the structure  100  may coincide with the walls of a blood vessel or tissue walls, such as the endocardial and epicardial layers of the myocardium, but they may also be selected to define a volume of interest for imaging purposes inside or outside such walls. Similarly, any boundaries in the Z direction may also be arbitrarily selected. Typically, however, the imaging data will have been acquired as slices along XY planes which are digitally assembled. In such a case, the extent in the Z direction may be equal to a selected number of such XY slices. 
         [0051]    Note that for the purposes of this invention, the XY slices are assumed to be made at the same time interval. 
         [0052]      FIG. 2  depicts the same structure  100 , viewed along the Z axis, so that the XY plane  180  is closest to the viewer. In this example, the first boundary  110  and the second boundary  120  are also depicted, defining the edges of this XY plane  180 . One or more pieces of the XY plane  180  are identified as segments  131 ,  132 ,  133 ,  134 , which may be of any convenient size and may even be different in size with respect to each other. 
         [0053]    Each segment  131 ,  132 ,  133 ,  134  is associated with a segment volume, bounded by the relevant piece of the XY plane  180  and the first boundary  110  and the second boundary  120 . For each segment  131 ,  132 ,  133 ,  134 , positions in the segment volume are determined, and a parameter associated with each position in the segment volume is analyzed to generate a visualization parameter. 
         [0054]    Typically, a series of medical scans are made of such a structure, because the healthcare professional wishes to visualize how the parameter changes in time. This means that a visualization parameter for each segment volume may be determined, and a series of visualization parameters may be determined for each segment volume. 
         [0055]    During a first-pass myocardial perfusion examination, the uptake of a contrast agent in the myocardium is monitored dynamically. For example, during a period of 20-40 seconds, 3-5 short axis slices are acquired every 1-2 heart beats, using ECG triggering. The time-intensity curves at individual locations in the myocardium contain important information about local myocardial blood perfusion. Note that for the purposes of this invention, the slices are assumed to be made at the same time interval. 
         [0056]    An example of a visualization representation  400  is depicted in  FIG. 3 . The representation  400  comprises a 2D array of pixels, each pixel having a grey value to represent a value of the visualization parameter, for example the higher the value, the whiter the pixel. A color value scheme may also be used. The 2D array has several rows  431 ,  432 ,  433 ,  434  arranged vertically above each other, and several columns  451 ,  452 ,  453 ,  454 ,  455 ,  456 ,  457 ,  458  arranged horizontally, the rows representing a spatial position and the columns representing a temporal position, in other words moments in time. An arrow  450  indicates the progression of the time interval, from left to right. Each row  431 ,  432 ,  433 ,  434  represents the change in the visualization parameter for each segment volume associated with the segments  131 ,  132 ,  133 ,  134 , respectively, over a number of time intervals  451 ,  452 ,  453 ,  454 ,  455 ,  456 ,  457 ,  458 . The time intervals  451 ,  452 ,  453 ,  454 ,  455 ,  456 ,  457 ,  458  represented may comprise all the acquired time series of data, or an arbitrary selection. 
         [0057]    For example, if the parameter associated with each position is an intensity measured during a medical scan, then the visualization parameter may be the mean segment intensity. If the intensity is a measure of, for example, perfusion, then the visualization parameters may be visually represented to the user as a perfusogram, which is a specific example of the representation  400  depicted in  FIG. 3 . An example of a perfusogram representation is illustrated in  FIG. 7 , as explained above. 
         [0058]    Producing such a representation  400  to visualize a time-variant parameter at a plurality of positions in a biological structure  100  may be performed by a suitable system. An example of such a system  1  is depicted in  FIG. 16 . The system comprises a determination unit  2  and a visualization display  3 . The determination unit  2  is configured to determine a value of the time-variant parameter at a plurality of positions in the biological structure  100 , and to determine the positions of a first boundary  110  and a second boundary  120  extending in the Z direction, and the positions of a segment volume, associated with a segment  131 ,  132 ,  133 ,  134 , extending in Z disposed between the first  110  and the second  120  boundary, and to determine the visualization parameter for a plurality of time intervals  451 ,  452 ,  453 ,  454 ,  455 ,  456 ,  457 ,  458  from the time-variant parameters at a plurality of positions in the segment volumes. The visualization display  3  is configured to display a representation  400  showing the visualization parameter at a plurality of time intervals  451 ,  452 ,  453 ,  454 ,  455 ,  456 ,  457 ,  458 . 
         [0059]    Typically, the user is provided with a means  7  to interact with the system, so that the user may influence what is displayed. A user may use a workstation  4  to perform these interactions, for example during image acquisition, image viewing, image analysis and image modification. The workstation  4  comprises the visualization display  3  for displaying one or more representations  400 , and typically for displaying anatomical gray value image  60 . The user interactions may be provided in one or more forms, such as icons, thumbnails, menus, and pull-down menus. The workstation  4  also comprises a means  7  for the user to interact with the workstation  4 , which may comprise a keyboard, mouse, trackball, pointer, drawing tablet. 
         [0060]    The biological sample  100  in  FIGS. 1 and 2  approximates a rectangular volume. However, the same representation may be used for any structure which approximates a 3D geometric figure with approximately parallel congruent bases having approximately the same orientation.  FIG. 4  depicts a biological structure  300  which approximates a hollow cylinder volume. The structure  300  extends in mutually perpendicular X, Y and Z directions  200 . Again, these directions  200  are chosen arbitrarily—any other coordinate system or convention may be used. In particular, a radial coordinate system may also be advantageous for such a structure. The structure  300  is defined by a first boundary  310  and a second boundary  320 , each boundary extending in the Z planes. The structure is also bounded by an XY plane  380 . 
         [0061]      FIG. 5  depicts the same structure  300 , viewed along the Z axis, so that the XY plane  380  is closest to the viewer. In this example, the first boundary  310  and the second boundary  320  are also depicted, defining the edges of this XY plane  380 . One or more pieces of the XY plane  380  are identified as segments  331 ,  332 ,  333 ,  334 . The number of segments chosen and their sizes is arbitrary, chosen depending on the biological structure  300  and the type of analysis desired. 
         [0062]    Each segment  331 ,  332 ,  333 ,  334  is associated with a segment volume, bounded by the relevant piece of the XY plane  380  and the first boundary  310  and second boundary  320 . For each segment  331 ,  332 ,  333 ,  334 , positions in the segment volume are determined, and a parameter associated with each position in the segment volume is analyzed to generate a visualization parameter. 
         [0063]    If a series of medical scans are made of such a structure, a visualization parameter for each segment volume may be determined, and a temporal, or time-variant, series of visualization parameters may be determined for each segment volume. This may also be visually displayed to the healthcare professional as the representation  400  depicted in  FIG. 3 . In this case, each row  431 ,  432 ,  433 ,  434  represents the change in the visualization parameter of the segment volume associated with segments  331 ,  332 ,  333 ,  334 , respectively, over a number of time intervals  451 ,  452 ,  453 ,  454 ,  455 ,  456 ,  457 ,  458 . 
         [0064]    It will be apparent to the skilled person that small sections of the cylindrical structure  300  may approximate a rectangular volume  100  as depicted in  FIG. 1 , enabling this technique to be used for many types and shapes of biological structure. 
         [0065]    For example, the structure may be one or more sections of a human myocardium for which the degree of perfusion is to be measured. The most frequently occurring heart disease is ischemia due to the (partial) occlusion of a coronary artery. First pass enhancement cardiac MRI may be used to assess the severity of perfusion deficits caused by coronary artery occlusions, by studying the uptake of contrast agent in the myocardial tissue after the first pass of a contrast bolus. This procedure is often performed on patients at rest and under stress (using pharmacological agents to increase the heart rate) as perfusion deficits are often stress induced. The myocardium may be conveniently divided into segments, such as those depicted in  FIGS. 2 and 5 , for performing the perfusion determination and for visualization of the results, using the representation of  FIG. 3 . 
         [0066]    It may be advantageous to provide the healthcare professional with representations of layers within a biological structure  100 ,  300 , instead of segment volumes. Using the segments  131 ,  132 ,  133 ,  134  in  FIG. 1  and the segments  331 ,  332 ,  333 ,  334  in  FIG. 5 , the user is limited to the selection of particular segments. This limits the resolution which can be visualized. However, if the determination is modified so as to provide the visualization of layers between the first boundary  110 ,  310  and the second boundary  120 ,  320 , the user is given additional freedom to examine the biological structure. 
         [0067]      FIG. 8A  depicts the biological structure  100  of  FIG. 1  for which a representation may be determined. It may be advantageous in some applications to provide an extra degree of freedom by determining the time-variant parameter within the biological structure  100  for an intermediate layer  150  situated between the first boundary  110  and the second boundary  120 . 
         [0068]    For example, if the biological structure is the myocardium, the first boundary  110  may represent the epicardial (or outer) layers and the second boundary  120  may represent the endocardial (or inner) layers. When determining perfusion, it may be advantageous to visualize layers at different positions between the endocardial and epicardial layers, because it is known, from physiology, that the endocardium shows a higher rest perfusion, but is more susceptible to ischaemia than the epicardium. As such, visualizations of perfusion in different layers of the myocardial wall provide for accurate diagnosis and staging of ischemic heart disease. 
         [0069]    Determining parameter values for such a layer  150  requires a high resolution of differentiation between the different tissues found between the inner boundary  120  and outer boundary  110  of the biological structure  100 . Correspondingly, increasing the spatial resolution of the determination increases the number of data points which must be visualized. This greatly complicates the interaction with the user because there are more choices available, and more information must be displayed to the user at the same time. 
         [0070]      FIG. 8B  depicts the same structure  100 , wherein a point  160  on the layer  150  has been selected, which the user wishes to see visualized. The point  160  may be considered a piece in the XY plane  180  which is smaller in the X direction than the distance between the first boundary  110  and the second boundary  120 . Any convenient size and shape of point  160  may be used, balancing the need between higher resolution and faster processing. Typically, the point  160  will have a minimum size of 1 voxel. The point  160  is associated with a first volume  170 , extending in the Z direction at a position between the inner boundary  110  and the outer boundary  120 , in other words a section of the intermediate layer  150  extending in both the Y and Z directions. For the first volume, positions in the volume are determined, and a parameter associated with each position in the volume is analyzed to generate a first visualization parameter for a plurality of time intervals. 
         [0071]      FIG. 8C  depicts the same structure  100 , wherein a second volume  190  is generated by extending the first volume until it intersects both the first boundary  110  and the second boundary  120 . The positions in the second volume are determined, and a parameter associated with each position in the second volume is analyzed to generate a second visualization parameter for one of the plurality of time intervals. 
         [0072]      FIG. 9A  depicts, in a representation  900 , one row  170  of the first visualization parameter at several periods in time  950 , namely at intervals  951 ,  952 ,  953 ,  954 ,  955 ,  956 ,  957 ,  958 . This Figure shows the user the change with respect to time of the visualization parameter in the first volume, disposed between the first boundary  110  and the second boundary  120 . 
         [0073]      FIG. 9B  is a representation  600  of the relationship at a selected time interval, being one of the plurality of time periods, between the first visualization parameter  630  determined at said selected time interval, and a second visualization parameter  610 , derived from the data analysis of the second volume. Optionally, a grid  601  may be provided to indicate the scale. For example, the second visualization parameter  610  may represent the maximum value of the first visualization parameter determined in the second volume  190 . This is particularly advantageous in cases where each parameter visualized can be referenced to the same scale, preferably having the same unit of measure. 
         [0074]    The second visualization parameter  610  may also represent the value of the first visualization parameter at a particular position in the second volume  190  between the first boundary  110  and the second boundary  120 . For example, if perfusion in the myocardium is measured, the intensity for the epicardial layer or the intensity for the endocardial layer may be selected as a suitable second visualization parameter  610 . 
         [0075]    It may also be advantageous to determine a plurality of visualization parameters from the positions in the second volume  190 , for example the value of the first visualization parameter at both the first boundary  110  and the second boundary  120  to visualize a second  610  and a third  620  visualisation parameter, respectively. For example, if perfusion in the myocardium is measured, the intensity for the epicardial layer may be used for the second visualization parameter  610  and the intensity for the endocardial layer may be used for the third visualization parameter  620 . 
         [0076]    Other possible quantitative analysis techniques that may be used to determine visualization parameters include upslope, deconvolution, and Patlak. Additionally, the skilled person will realize that such techniques may be combined with arithmetical and statistical operations, such as averaging or weighting, to provide a suitable and meaningful representation. 
         [0077]    As described above, a similar determination may be performed with any structure which approximates a 3D geometric figure with approximately parallel congruent bases having approximately the same orientation.  FIG. 10  depicts a biological structure  500  which approximates a hollow cylinder volume. The structure  200  is viewed along the Z axis, so that the XY plane  580  is closest to the viewer. In this example, the first boundary  510  and the second boundary  520  are also depicted, defining the edges of this XY plane  580 . 
         [0078]    On the layer  550  a point  560  has been selected which the user wishes to see visualized. The point  560  may be considered a piece in the XY plane  580 , which piece is smaller than the distance between the first boundary  510  and the second boundary  520 . Any convenient size and shape of point  560  may be used, balancing the need between higher resolution and faster processing. Typically, the point  560  will have a minimum size of 1 voxel. The point  560  is associated with a first volume, extending in the Z direction at a position between the inner boundary  510  and the outer boundary  520 . For the first volume, the positions in the volume are determined, and a parameter associated with each position in the volume is analyzed to generate a first visualization parameter for a plurality of time intervals. A second volume is generated by extending the first volume until it intersects both the first boundary  510  and the second boundary  520 . For the second volume, the positions in the second volume are determined, and a parameter associated with each position in the second volume is analyzed to generate a second visualization parameter for one of the plurality of time intervals. 
         [0079]      FIG. 11A  depicts a representation  900  similar to the one depicted in  FIG. 9A , with this difference that the representation  900  shown here comprises a plurality of rows  971  of the first visualization parameter at several periods in time  950 . Each row  971  displays the first visualization parameters for a particular spot position  560  along the intermediate layer  550 . This shows the user the change in the time-variant parameter in the first volume, disposed between the first boundary  510  and the second boundary  520 . 
         [0080]      FIG. 11B  depicts representation  901 , which is an alternative to representation  900 . Here the representation  901  comprises a plurality of circular segments  971  of the first visualization parameter at several periods in time  950 . Each segment  971  displays the first visualization parameters for a particular spot position  560  along the intermediate layer  550 . This shows the user the change in the time-variant parameter in the first volume, disposed between the first boundary  510  and the second boundary  520 . Such a circular representation may be advantageous if the biological structure  300  is also approximately circular in cross-section. 
         [0081]      FIG. 12  depicts a representation  600  similar to the one depicted in  FIG. 9B , except that the first  630 , second  610  and third  630  visualization parameters have been determined for a plurality of spot positions  560  around the intermediate layer  550 , and the grid has been made circular to create a polar plot representation  600 . The grid  601  may optionally be indicated, and where useful, may indicate a scale for the polar plot. The polar plot  600  also comprises an indicator  660 ,  690 , so that the radial position of a selected spot of interest  560  may be indicated to the user, such as a small rectangle  660  linked by a bar  690  to the centre of the polar plot  600 . This is particularly advantageous in cases where each parameter visualized can be referenced to the same scale, preferably having the same unit of measure. 
         [0082]    For example, if perfusion in the myocardium is measured, the intensity for the epicardial layer may be used for the second visualization parameter  610  and the intensity for the endocardial layer may be used for the third visualization parameter  620 . 
         [0083]      FIG. 13A  depicts a display that may be presented to the user. The display comprises the polar plot  600  depicted in  FIG. 12  and the 2D pixel array  900  of  FIG. 11A . The 2D pixel representation  900  is provided with a temporal indicator  962 , which indicates a selected time interval for which the second visualization parameter is determined. The 2D pixel representation  900  is also provided with a spatial indicator  961 , which indicates a selected spot position. The polar plot  600  comprises an indicator  660 ,  690 , which indicates the selected spot position  560 . 
         [0084]    Typically, the user is provided with a means to interact with the system, so that the user may influence what is displayed. The workstation  4  comprises input means  7  for the user to interact with the workstation  4 , such as a keyboard, mouse, trackball, pointer, or drawing tablet. It may be advantageous to allow the user to change one or more of the  settings associated with these indicators, using the input means  7 . 
         [0085]    If the user moves the temporal indicator  962  to select a different time interval, the 2D pixel array  900  remains the same. However, the first  630 , second  610  and third  630  visualization parameters are determined again for a plurality of spot positions  560  around the intermediate layer  550  for the selected time interval. Therefore, the user will see the shape of the polar plots of the first  630 , second  610  and third  630  visualization parameters change as the temporal indicator  962  is moved. 
         [0086]    If the user moves the spatial indicator  961  to select a different selected spot position, the 2D pixel array  900  remains the same. Also the polar plots of the first  630 , second  610  and third  630  visualization parameters remain the same. However, in the polar plot representation  600 , the selected spot indicator  660 ,  690 , is determined again. Therefore, the user will see the selected spot indicator  660 ,  690  move around the polar plot  600  as the spatial indicator  961  is moved. Conversely, if the user moves the selected spot indicator  660 ,  690  to select a different selected spot position, the user will see the spatial indicator  961  move to a different selected spot position. 
         [0087]    By combining and linking the 2D pixel array  900  with the polar plot  600 , the user is provided with an intuitive mean to visualize the visualization parameters, and to interpret the relationship between the time-series in the 2D pixel-array  900  and the associated quantitative analysis data in the polar plot  600 . Additionally, as the number of time-series are increased due to a higher number of spot positions  560  around the intermediate layer  550 , the user is only confronted with an increase in the density of the 2D-pixels  900 —the relationship with the quantitative analysis data in the polar plot  600  remains clear due to the linked spatial position indicator  961  and the selected spot indicator  660 ,  690 . 
         [0088]    The skilled person will appreciate that the changes realized by the user interactions with any of the indicators is dependent on the role of the dimension represented by the indicator in the generation of the representation. For example, if perfusion in the myocardium is measured, some measure of the intensity, such as the average or mean, over a plurality of time intervals may be determined for the epicardial layer and may be used for the second visualization parameter  610 . Similarly some measure of intensity for the endocardial layer over a plurality of time intervals may be used for the third visualisation parameter  620 . The first visualization parameter value  630  may also represent the same measure of intensity in the first volume  170 , associated with the selected spot position  560 , over a plurality of time intervals. In this case, if the user moves the temporal indicator  962  to select a different time interval, the 2D pixel array  900  remains the same, and the first  630 , second  610  and third  630  visualization parameters remain the same. Therefore, the user will see no change in either the 2D pixel array representation  900  or the polar plot representation  600 . 
         [0089]    In a typical application, the user workstation  4  is provided with further analytical possibilities, such as upslope, deconvolution, Patlak and arithmetic and statistic operations such as average, mean, maximum, minimum etc. These may be selected by the user to further analyze the parameter values in the first and second volume associated with the currently selected spot position. The results of this analysis may be displayed in an analysis window  5 , as shown in  FIG. 16 , as simple figures or any suitable graphical representation. 
         [0090]    It may be advantageous to add a further representation  500  to complement the 2D pixel array  500  and the polar plot  600 , as depicted in  FIG. 13B . The representation  500  comprises an anatomical gray value image  570  depicting an image slice in the XY plane  200 , overlaid with an indication of the boundaries of the structure  500  as viewed along the Z axis, so that the XY plane  580  is closest to the user. In particular, the first boundary  510 , the second boundary  520  and the intermediate layer  530  are overlaid over the associated positions of the biological structure  500 . This provides the user with an indication of the relationship between the structure  500  for which the parameters are determined and the anatomical structure. It may also be advantageous to indicate the position of the selected spot  560  on the intermediate layer. 
         [0091]    The example of the user interaction described in relation to  FIG. 13A  is slightly modified in  FIG. 13B . 
         [0092]    If the user moves the temporal indicator  962  to select a different time interval, the 2D pixel array  900  and the structure overlay  500  remain the same. The polar plot  600  changes as described in relation to  FIG. 13A . The anatomical gray value image  570  changes to display the acquired data for the selected time interval. 
         [0093]    If the user moves the spatial indicator  961  to select a different selected spot position, the 2D pixel array  900 , the structure overlay  500  and the polar plot  600  remain the same. However, in the polar plot representation  600 , the selected spot indicator  660 ,  690 , is determined again. Therefore, the user will see the selected spot indicator  660 ,  690  move around the polar plot  600  as the spatial indicator  961  is moved. Similarly, in the structure overlay the selected spot  560  will move around the intermediate layer  530  as the spatial indicator  961  is moved. Conversely, if the user moves the selected spot  560  or the selected spot indicator  660 ,  690  to select a different selected spot position, the user will see the spatial indicator  961  move to a different selected spot position. 
         [0094]    Interaction by the user with the workstation  5  may be improved by highlighting the corresponding angular position in the polar plot  600  when the user moves the mouse cursor in the 2D pixel array  900 . Alternatively, the corresponding row of the 2D pixel may be highlighted if the user moves the mouse cursor in the polar plot. 
         [0095]    If the user moves the intermediate layer  530  to a different position between the first boundary  510  and the second boundary  520 , the positions within the first volume will change, and the first visualization parameter will be re-determined. This means that the user will see a change in the values represented by the 2D pixel array  900  corresponding with the shift in position of the intermediate layer  530  as represented in the structure overlay  500 . As the skilled person will realize, the intermediate layer  530  may also be selected to coincide with inner boundary  510  and outer boundary  520 . 
         [0096]    It may be particularly intuitive for a user to provide a means of moving the intermediate layer  530  between the first boundary  510  and the second boundary  520 , using the scroll-wheel typically found in a mouse, as part of the user interfacing means  7 . 
         [0097]    As the imaging data may be comprised of a plurality of XY plane slices, it may also be particularly intuitive for the user to be provided with a means of moving through the image stack in effect in the Z direction by using the scroll wheel. 
         [0098]    These scroll wheel functions may be enabled when a cursor is positioned over a particular area, or a particular area of one of the representations, to increase the intuitive feeling of the interface. 
         [0099]    It is envisioned that the representation  500  may also comprise anatomical markers, such as the positions of blood vessels or arches in blood vessels. This may be particularly advantageous in helping the user associate the representations and the data with actual anatomical positions. For example, in analyzing perfusion data for the myocardium, the positions and orientations of the coronary arteries are important as the source of oxygenated blood, so proximity to the arteries is a factor in accurately interpreting the results. Also markers for the right ventricular inflection points may be considered advantageous. Such spatial markers may also be indicated in the 2D pixel array  400 ,  900  and/or the polar plot  600  at the appropriate places. 
         [0100]      FIG. 14  depicts a possible implementation of the representation combination of  FIG. 13B .  FIG. 14  shows a further representation  500  as depicted in  FIG. 13B . The representation  500  comprises an anatomical gray value image  570  depicting an image slice in the XY plane, overlaid with an indication of the boundaries of the structure  500  as viewed along the Z axis, so that the XY plane is closest to the user. In particular, the first boundary, the second boundary and the intermediate layer are overlaid over the associated positions of the biological structure, in this case the myocardium. The representation  500  is complemented by the associated 2D pixel array  900  and the polar plot  600 . The spatial indicator  961  and the temporal indicator  962  have been chosen to be lines overlaying the 2D pixel array  900 . Optionally, an analysis window  5  may be provided to provide details on the analysis performed during the visualization of the imaging data, or to provide access to and results of additional analysis possibilities. 
         [0101]    Additional indicators may be provided in any of the representations to indicate reference points currently selected or used in the analysis performed during the visualization of the imaging data, or selected or used during additional analysis. 
         [0102]    By combining and linking the 2D pixel array  900  with the polar plot  600  and the structure overlay representation  500 , the user is provided with an intuitive means to visualize the visualization parameters, which can be more easily related to the anatomical data. Additionally, as the number of time-series are increased due to a higher number of possible intermediate layer  550  positions, this increase is shielded from the user. The user is presented with far more analysis possibilities, without an increase in the complexity of the representations. 
         [0103]    The possibility to visualize such a high resolution both along the intermediate layer  550  and between the first boundary  510  and the second boundary  520  means that very thin sections of tissue may be analysed. The biological structure  500  may therefore be an organ, a part of an organ, a lobe of an organ, a skeletal bone, a part of a skeletal bone, a muscle, a part of a muscle, a lymph node, part of a lymph node, a vessel, and part of a vessel. In addition, the biological structure may also be a tumor, primary tumor, metastatic tumor, cyst, pseudocyst, neoplasm, lymph node, lymphoma, fibroid, or nevus. 
         [0104]    The ability to visualize a higher degree of data density means that new types of representation become possible and particularly advantageous.  FIG. 17  depicts alternative representations  701 ,  702 ,  703 ,  704  which may be described as volumetric representations. 
         [0105]    For example, conventional volume rendering techniques may be used to visualize a time-variant parameter at a selected time interval at positions throughout a biological structure  100 , such as that depicted in  FIGS. 1 and 8 , or structure  300  in  FIG. 4 . In the volumetric representation, the whole volume, or a selected portion of the whole volume, of a biological structure  100 ,  300  is visualized at the same time. By applying these conventional rendering techniques to a new type of volume, a powerful visualization tool has been provided. 
         [0106]    Some of the possible types of volume rendering for a selected time interval are provided as examples in  FIG. 17 , one of which relates to a measure of perfusion in the myocardium between the epicardial and endocardial layers: 
         [0107]    Representation  701  is a direct volume rendering, derived by mapping every sample value to opacity and a color. This is done with a “transfer function” which can be a simple ramp, a piecewise linear function or an arbitrary table. Once converted to an RGBA (red, green, blue, alpha) value, the composed RGBA result is projected on the correspondent pixel of the representation. 
         [0108]    Representation  702  is an iso-surface rendering, wherein a surface is derived upon which all positions have the same measure of perfusion. 
         [0109]    Representation  703  is a second direct volume rendering, displaying a different orientation and based on a different look-up table for colors and transparency.
       Representation  704  is a third direct volume rendering, displaying a still further different orientation and based on yet another different look-up table for colors and transparency.       
 
         [0111]    It may be advantageous to define various cut planes at various locations and/or orientations so that only a part of the volumetric representation  701 ,  702 ,  703 ,  704  is shown which corresponds to the intermediate layer  150  of  FIG. 8 . 
         [0112]      FIG. 15  depicts an example of a method  800  of visualizing a time-variant parameter in a biological structure. In particular, it is a method of visualizing perfusion in a human myocardium. The explanation of the method will focus on perfusion of the left ventricle. However, the method may also be used to visualize time-variant parameters in different layers of other parts of the heart (right ventricle and atria) or even other organs (kidney, prostate) that are imaged using first-pass enhancement MRI. Furthermore, the method is also applicable to time-variant parameters obtained from other imaging modalities (PET, SPECT, CT, etc.). The method  800  comprises the following parts: 
         [0113]    Acquiring  810  the imaging data to be visualized. This may be performed by the system depicted in  FIG. 16 , if the system is capable of image acquisition using a suitable modality, or it may have been acquired at some earlier point in time by an independent image acquisition apparatus. It is also envisioned that the system may be comprised in an image acquisition apparatus.
       Correcting  820  the imaging data for unwanted motion. For some imaging applications, it is important to remove certain time-dependent spatial variations, as these may affect the final visualization results, thereby creating artifacts. For example, when making images of the heart, a common problem is the patient&#39;s breathing motion that is present in the images. The first pass enhancement images may be registered to correct for the breathing motion that is present in the images. Appropriate registration techniques are known to the skilled person. Note that this correcting  820  is considered optional—such movement may also be prevented by asking the patients to hold their breath during image acquisition  810 .       
 
         [0115]    Delineating  830  the desired contours of the biological structure of interest. In other words, determining the positions corresponding to the extent of the biological structure. In this example, the left ventricular contours are delineated  830 , either manually, semi-automatically or fully automatically. 
         [0116]    Delineating  840  an intermediate layer within the biological structure. In other words, determining the positions corresponding to the intermediate layer. Delineating  830 ,  840  may be performed by any conventional means, such as interpolation or segmentation known in the art. 
         [0117]    Sampling  850  the time-variant parameters, for example in the form of time-intensity curves, from the image data at positions along as well as across the myocardium. Optionally, these time-variant parameters may be filtered. 
         [0118]    Analyzing  860  the time-variant parameters, using the desired and/or appropriate analysis technique. For example, upslope, deconvolution, and Patlak. 
         [0119]    Visualising  870  the time-variant data as a perfusogram representation  900 ,  901  in combination with a polar plot  600  that shows associated quantitative analysis data as a result of the analysis  860 . As the analysis of the time-variant data may be dependent on the position in the biological structure, the different plots  610 ,  620 ,  630  in the polar plot  600  relate to different myocardium layers. 
         [0120]    It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. 
         [0121]    For example, any convenient visual means may be employed as an indicator  560 ,  961 ,  962 ,  660 ,  690 , such as arrows, lines, blocks, dots, color coding etc. The indicators  560 ,  660 ,  690 ,  961 ,  961  may be arranged adjacent to the associated representation, or the indicators may be overlaid, on the associated representation, with visual aids such as highlighting, shading, boxes, lines and dashed lines. It is also envisioned that associated indicators and marks in the different representations will be given the same or a similar sort of indicator to illustrate the association, for example using the same colors. 
         [0122]    Alternatively, the display may contain a plurality of one or more of the representations to increase the amount of data being visualized. 
         [0123]    The skilled person, provided with the details of the methods disclosed, will be able to implement a computer program to carry out these methods when they are loaded and run on a computer. 
         [0124]    In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. 
         [0125]    In the system claim enumerating a determination unit and a visualization display, several of these means may be embodied by one and the same item of hardware. 
         [0126]    The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.