Abstract:
A method of fusion of a first digital radiographic image obtained as a result of scanning with a second digital radiographic image obtained by magnetic resonance imaging (MRI). A CT interval of gray levels is selected first in the scanner image. Each pixel of the scanner image having a gray level lying within the CT interval is then replaced by a pixel obtained by digital processing of the pixel of the same coordinates as the MRI image. The final image corresponds to the scanner image in which the pixels of gray levels lying within the CT interval have undergone the digital processing.

Description:
This application claims the benefit of a priority under 35 USC 119 and 35 USC 365 and 371 to PCT/IB00/00606 and French Patent Application No. 99 05438 filed Apr. 29, 1999, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The invention concerns the fusion of two digital images of an object, the first image of which favors a particular constituent of the object, while the second image favors another. 
   It has a particularly important application in the medical field, in which a first image of a body organ obtained by scanning is fused with a second image of the same organ obtained by magnetic resonance imaging (MRI). 
   In fact, an image obtained by means of a scanner particularly reveals the bony part. In such an image, the bony part is white and all the other parts, especially, the soft tissues, are of a homogeneous gray without contrast. On the other hand, an image obtained by means of MRI reveals the soft tissues in different shades of gray levels and the other parts like the bony parts and empty space are black. 
   In general, in the medical field a scanner image is fused with an MRI image by integrating the pixels of the bony parts of the scanner image in the MRI image. 
   The scanner images possess an absolute scale of gray levels, that is, all the scanner images are compatible with one another, in the sense that a given gray level always represents a particular organ. This absolute scale is the Hounsfield scale, composed of positive and negative numbers, in which the 0 level is the gray level of water. 
   An MRI image does not possess an absolute scale. The gray levels depend on the patient and on the image acquisition conditions. Therefore, from one MRI image to another, the muscle, for example, as soft tissue, is not represented by the same gray level. Thus, fusion of an MRI image with a scanner image results in a final image whose scale is not absolute. 
   In other words, the fusion of an image possessing an absolute scale with another image not possessing an absolutely scale results in a final image not possessing any absolute scale. 
   Furthermore, an image not possessing any absolute scale cannot be used by any of the current scanner image processing software. In fact, all of that software uses a standard gray level format, which is the Hounsfield scale. 
   Thus, a final image originating from the fusion of both scanner and MRI images is incompatible with any scanner image processing software. It is necessary to develop specific image processing software not calibrated on the Hounsfield scale, in order to be able to use the final image. 
   BRIEF SUMMARY OF THE INVENTION 
   An embodiment of the invention provides a solution to that problem by a scaling of the gray levels of the MRI image, in order to render the final image compatible with all scanner image processing software. In other words, the final image will be calibrated on the Hounsfield scale. 
   It is an advantage of an embodiment of the invention to reduce the cost of investment in the development of specific software, if it is desired to carry out digital processing on the final image. An embodiment the invention uses a final image of as an image source for standard radiotherapy software, which is not the case with the fused images in the present state of the art. 
   An embodiment of the invention proposes a method of fusion of a first digital radiographic image obtained as a result of scanning with a second digital radiographic image obtained by magnetic resonance imaging (MRI). 
   In an embodiment of the invention, a CT interval of gray levels is selected in the scanner image and each pixel of the scanner image having a gray level lying within the CT interval is replaced by a pixel obtained by digital processing of the pixel of the same coordinates as the MRI image. The final image therefore corresponds to the scanner image in which the pixels of gray levels lying within the CT interval are thus modified. 
   Furthermore, with a view to effective digital processing, a two-dimensional recentering of both MRI and scanner images may be carried out by means of at least one rotation and/or translation operation, so that a pixel of the scanner image of coordinates (x,y) and a pixel of the MRI image of the same coordinates (x,y) represent the same portion of the organ X-rayed. 
   In other words, the range of gray levels corresponding to the soft tissues is replaced by a new range of gray levels. The values of the gray levels of that new range are obtained from an algorithm introducing certain gray levels of the MRI image. For a given pixel of the CT interval in the scanner image, the algorithm calculates the gray level value of the new pixel from a pixel of the MRI image having the same coordinates as the pixel of the CT interval having to be replaced. 
   In an embodiment of the invention, the upper limit B CT  of the CT interval is fixed at a gray level value on the Hounsfield scale, the gray level corresponding to the highest value of the gray levels representing the soft tissues visualized on the scanner image. The lower limit A CT  of the CT interval is fixed at a gray level value on the Hounsfield scale, the gray level corresponding to the lowest value of the gray levels representing soft tissues visualized on the scanner image. 
   More precisely, two thresholds are fixed, defining the CT interval corresponding to the soft tissues in the scanner image. 
   In practice, B CT  is fixed as the highest value of the soft tissues in the scanner image and A CT  is fixed as the lowest value of the soft tissues in the scanner image. 
   The interval thus selected is an interval included in the Hounsfield scale, since the scanner image is calibrated on that scale. 
   In general, in an embodiment of the invention, one selects an MR interval of gray levels in the MRI image, whose upper limit B MR  corresponds to a gray level above which the pixels are white, and whose lower limit A MR  corresponds to a gray level below which the pixels are black. 
   In other words, that interval takes into account all of the variation of gray levels in the MRI image. This variation, this contrast, represents the useful information on the soft tissues. 
   There are then two intervals, a first CT interval in the scanner image included in the Hounsfield scale and a second MR interval in the MRI image not linked to the Hounsfield scale. These two intervals represent a framing of the soft tissues. 
   In an embodiment of the invention, the digital processing consists of a linear interpolation by means of an affine function integrating the value of the lower limit A CT  and upper limit B CT  of the CT interval in the scanner image and the value of the lower limit A MR  and upper limit B MR  of the MR interval in the MRI image. 
   Carrying out a linear interpolation makes it possible to respect the choice of contrast in the MRI image. 
   Preferably, for a scanner pixel having a gray level V CT  lying within the CT interval, the gray level V MR  of the pixel of the same coordinates in the MRI image is determined, and then a gray level in the CT interval is determined from the affine function and from the level V MR . The gray level V OUT  of each pixel of the final image can then be obtained by the following algorithm:
         if V CT &lt;A CT , then   1) V OUT =V CT ,   if V CT &gt;B CT , then   2) V OUT =V CT      if A CT &lt;V CT &lt;B CT , then   3) V OUT =A CT +(B CT −A CT ) (V MR −A MR /(B MR −A MR .       

   In other words, while maintaining the resolution of the MRI image, the MRI image is scaled so that the black level A MR  of the MRI image corresponds to the lowest value A CT  of the soft tissues in the scanner image. Likewise, the white level B MR  of the MRI image corresponds to the highest value B MR  of the soft tissues in the scanner image. 
   In fact:
         for V MR =B MR , highest gray level in the MR interval of the MRI image,   one obtains by 3) V OUT =B CT , highest gray level in the CT interval of the scanner image,   and for V MR =A MR , lowest gray level in the MR interval of the MRI image,   one obtains by 3) V OUT =A CT , lowest gray level in the CT interval of the scanner image.       

   Scaling causes the MR interval not calibrated on the Hounsfield scale of the MRI image to undergo a digital processing which makes it correspond to the CT interval lying within the Hounsfield interval. 
   Thus, all the gray level values V OUT  of the final image will be contained in the Hounsfield scale, which is the standard scale of scanner image processing. 
   The invention is also directed to a system of fusion of a first digital radiographic image obtained by scanning with a second digital radiographic image obtained by MRI, comprising:
         means for reading pixels of the scanner image, the gray levels of which lie within a predetermined CT interval;   means for reading pixels of the MRI image, the coordinates of which are identical to those of the pixels of the CT interval of the scanner image; and   means for calculation of a third image composed of the scanner image in which the pixels whose gray levels lie within the CT interval are replaced by pixels obtained by digital processing of the pixels of the same coordinates as the MRI image in order to obtain an image making possible visualization of the soft tissues and bony tissues.       

   The final image obtained is of the scanner type. It can therefore be processed by standard software such as Advantage Sim or even Advantage Windows 3D Viewer, which is not the case with the fusion images of the prior art. The methods of the prior art require the use of specific software in order to be able to process their fusion images. 
   Other advantages and characteristics of the invention will appear on examination of the detailed specification of a nonlimitative embodiment and of the attached drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flow chart of an embodiment of the method according to the invention; 
       FIG. 2  schematically illustrates two images obtained by a tomography system. 
       FIG. 3  is a radiographic image acquisition by a scanner; 
       FIG. 4  is a radiographic image acquisition by magnetic resonance imaging; and; 
       FIG. 5  is a final radiographic image based on the fusion of the images of  FIGS. 3 and 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring in particular to the three  FIGS. 1 ,  3  and  4 , first of all, the two digital images are acquired. Acquisition  1  makes it possible to obtain a scanner image  11  illustrated in  FIG. 3 . This image represents a view of a patient&#39;s head along a given plane. A part outside the head can be distinguished there, that is, the air  12  represented in black. The white zone  14  corresponds to the bony tissues and all the grayish zones  13  correspond to the soft tissues. The scanner image  11  is of particular interest because it favors visualization of the bony tissue. Its principal characteristic is therefore a perfect display of the bony tissues  14 . On the other hand, the grayish zones  13  have a poor resolution, so that it is impossible to distinguish the contrasts in the soft tissues. 
   Acquisition  2  makes it possible to obtain an image  15  by magnetic resonance imaging. It represents a view of a patient&#39;s head along the same plane as the scanner image  11 . A black part  16  can also be distinguished there, corresponding to the air all around the head. Inside the head, the black zones  17  correspond to the bony tissues and to any element other than the soft tissues, like air, for example. The grayish zones  18  represent the soft tissues. The principal characteristic of that image is the display of soft tissues. The resolution is sufficient to distinguish contrasts, elements of interest  19 . On the other hand, it is difficult to delimit the bony parts  17 , for they are merged with the air and every other element appearing in black on the image  15 . 
   These two images  11  and  15  originating from two different methods of acquisition  1  and  2  represent a view of the head along a given cutting plane. Thus, a two-dimensional recentering  3  is made in order to render the two images  11  and  15  superposable. For this purpose, rotation and/or translation operations are possibly carried out. There are tools known to the expert that make it possible to control the recentering operation  3 . One can mention, notably, the tool using a pointer, that is, a particular element is pointed to on the scanner image  11 , for example, and a cursor appears on the same particular element on the MRI image  15 . The same idea is exploited in the tool using a magnifier. 
   Once the two images  11  and  15  are recentered, one determines in stage  4  the lowest value A CT  of the soft tissues in the scanner image, for example, −130, which is a low value of soft tissues in the Hounsfield scale. One also determines the highest value B CT  of soft tissues in the scanner image, for example, 80, which is a high value of soft tissues in the Hounsfield scale. These two values are both CT numbers. A CT number is defined from the attenuation coefficient of the tissue considered and from the attenuation coefficient of water: 
         CT   ⁢           ⁢   Number     =           μ   0     -     μ   w         μ   w       ×   1000         
         with
           μw: attenuation coefficient of water   μ0: attenuation coefficient of the tissue considered.   
               

   The CT number is expressed in Hounsfield unit. 
   Table of CT Numbers 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               ELEMENTS OF THE HUMAN BODY 
               CT NUMBER 
             
             
                 
                 
             
           
           
             
                 
               BONE (CORTEX) 
               &gt;250 
             
             
                 
               BONE (MARROW) 
               130 ± 100 
             
             
                 
               COAGULATED BLOOD 
               80 ± 10 
             
             
                 
               THYROID GLAND 
               70 ± 10 
             
             
                 
               LIVER 
               50 ± 10 
             
             
                 
               MUSCLE 
               45 ± 5  
             
             
                 
               BLOOD 
               40 ± 10 
             
             
                 
               BRAIN (WHITE MATTER) 
               35 ± 5  
             
             
                 
               KIDNEY 
               30 ± 10 
             
             
                 
               BRAIN (GRAY MATTER) 
               25 ± 5  
             
             
                 
               FATTY TISSUE 
               −100 ± 10    
             
             
                 
                 
             
           
        
       
     
   
   Two values A MR  and B MR  on the MRI image are also determined in stage  5 . A MR  is a gray level such that the lower gray levels are considered black. B MR  is a gray level such that the higher gray levels are considered white. 
   One then proceeds with an algorithm  6  making possible scaling of the MR interval. According to a preferred embodiment of the invention, algorithm  6  is applied in accordance with  FIG. 2 . A target pixel of the scanner image of gray level equal to V CT  is taken in the course of stage  6   a . In the first place, it is going to be determined whether that value is included in the CT interval. For that purpose, both values of the lower limit A CT  and upper limit B CT  are introduced. First of all, the value V CT  is compared to value A CT  in the course of stage  6   b . If the gray level of the target pixel V CT  is less than A CT , then the target pixel is outside the CT interval and it can then correspond to the bony tissue  14  or to the black background  12  of the scanner image. In that case, the target pixel maintains its value V CT  on the final image  20 . 
   Otherwise, if the gray level V CT  is higher than A CT , it is compared in the course of stage  6   c  to value B CT . If the gray level V CT  is higher than the upper limit B CT  of the CT interval, then the target pixel maintains its value V CT  on the final image  20  in the course of stage  6   f , that is, V OUT , the gray level of the target pixel on the final image  20 , is equal to V CT . Thus, for a value V CT  lower than A CT  or higher than B CT , the level V CT  is maintained as gray level V OUT  of the final image  20 . 
   On the other hand, if V CT  is higher than A CT  and lower than B CT , the gray level V MR  of a pixel of the MRI image of the same coordinates as the target pixel of the scanner image is then determined in the course of stage  6   d . It is then made to undergo a linear inter-polation at that gray level V MR  by introducing levels A CT , B CT , A MR  and B MR . A new value V OUT  independent of V CT  is then obtained in the course of stage  6   e.    
   Algorithm  6  is presented in that case in the form:
         if V CT &lt;−130, then   1) V OUT =V CT ,   if V CT &gt;80, then   2) V OUT =V CT ,   if − 130&lt;V   CT &lt;80, then   3) V OUT=− 130+(80+130) (V MR −A MR )/(B MR −A MR ).       

   Whatever the values A MR  and B MR , the final image  20  presents a range of gray levels according to the Hounsfield scale.  FIG. 5  shows the final image  20  in which the bony tissues  14  as well as the soft tissues  18  are distinguished. The background of the image  12  remains black, as on the scanner image  11 . 
   However, for a scanner image obtained according to a view of the lungs, a pixel of gray level V CT  representing the lungs in the scanner image will have value V OUT  equal to V CT  in the final image, whatever the gray level V CT  included or not in the CT interval. In other words, if the gray levels of the lungs in the final image are the gray levels of the lungs in the scanner image, the linear interpolation is not applied on the gray levels of the lungs. This is due to the fact that the gray levels representing the lungs possess such dynamics that the scanner image has a better resolution than the MRI image. 
   The final image  20 , whose gray levels are contained in the Hounsfield scale, was thus determined. This final image is then safeguarded in stage  7  in the form of a scanner image. It can be printed in stage  10  or even displayed in stage  8  on a screen for possible study. But the main advantage of that methods resides in the fact that this image can be delivered on entry of a standard radiotherapy treatment system in stage  9 . 
   The final image originating from fusion of a scanner image with an MRI image reveals soft tissues as well as bony tissues and can be used a source for all standard scanner image processing software, such as Advantage Sim, Isis or even Advantage Windows Viewer. 
   Various modifications in structure and/or steps and/or function may be made by one skilled in the art without departing from the scope of the invention.