Abstract:
A segmentation method and system for medical images of a structure, including at least one substructure are described herein. Examples of such structure and substructures are a vertebra and its spinal canal or a sacrum with its two foramens. The method consists of first providing a plurality of digital images representing consecutive slices of the structure. The set of images is then divided into groups for which initial substructures center position are estimated. Using this information, the pixels corresponding to each substructure and common to all the images in the group are determined. Iterations are performed between these two last steps until the best candidates are obtained. The method consists then in estimating the substructures center position in each image of the group. After the substructures have been identified in each image, the images are then processed to remove noise in the substructures, in the rest of the structure and outside the structure.

Description:
This application claim benefit of provisional application No. 60/087,090 filed May 28, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to medical imaging. More specifically, the present invention is concerned with a segmentation method for medical images of structures such as, for example, bones. 
     BACKGROUND OF THE INVENTION 
     Most standard medical scanning devices, for example CT (Computerized Tomography) and MRI (Magnetic Resonance Imagery) scanners produce sets of two dimensional images that represent slices of the imaged anatomical structures. By successively examining these slices, a physician can build, in his mind, a three dimensional representation of the anatomical structure, from which a pathology can then be identified. 
     Recent advances in the field of medical imaging and computer graphics now allow the computation and the visualization of accurate three dimensional models of anatomical structures. In order to obtain these three dimensional models, every pixels that correspond to the desired structure must be identified in each two dimensional image, which is a long and tedious process given the large number of images that are generally required to obtain an accurate three dimensional representation of an anatomical structure. 
     The segmentation process, i.e. the identification of the desired pixels, is in general a complex process since some noise usually remains after applying standard image processing techniques, such as thresholding and noise reduction filtering. An additional challenge comes from the fact that some anatomical structures contain cavities or empty regions that must not be filled or merged with adjacent structures during the segmentation process. 
     According to a conventional segmentation process, a user reviews each image, one by one, using interactive image processing tools, such as thresholding, painting, and filling, in order to identify to which anatomical structure each pixel belongs. Accordingly, this process is long and tedious, and usually requires hours before the three dimensional model of the desired anatomical structure can be obtained. 
     A semi-automatic segmentation method is proposed in the international PCT application WO 98/39736, filed on Mar. 3, 1998 and naming HIBBARD as the inventor. This method includes the steps of:1) providing digital images corresponding to slices of a structure; 2) drawing by a user of a polygon within a ROI (Region Of Interest) or substructure to be segmented; 3) expanding the polygon by iteratively testing pixels of the images outside of, but adjacent to, the pixels that the polygon currently subtends. Pixels will be added to the polygon if the value of a decision rule function has a predetermined value. The perimeter of the polygon is considered the boundary of the substructure once it is found that none of the perimeter pixels satisfy the decision rule. The last step of the method is to compute the contour of the segmented substructure based on the boundary. 
     A drawback of Hibbard&#39;s method is that the user input involved in step 2 must be done for every slice (image) of the structure, therefore making the segmentation method time consuming. Moreover, to obtain an accurate contour, the user may have to perform step 2 a second time to provide new boundary segments for the polygon, depending of the complexity of the substructure. 
     More automated and reliable segmentation method and system are thus desirable. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is therefore to provide a segmentation method and system free of the above described drawbacks. 
     Another object of the invention is to provide a reliable segmentation method and system that need less user interactions than the conventional methods and systems of the prior art. 
     SUMMARY OF THE INVENTION 
     More specifically, in accordance with the present invention, there is provided a segmentation method, comprising: 
     receiving a plurality of digital images representing consecutive slices of a structure that includes a substructure; 
     thresholding the plurality of images; 
     dividing the plurality of images in at least one group of images; 
     for each of at least one group of images, 
     a) estimating a substructure center position corresponding to the group; 
     b) using the substructure center position to find pixels of the substructure common to all images in the group; 
     c) for each image in the group, 
     c1) using the group substructure center position and the substructure common pixels to estimate a substructure center position corresponding to the image; and 
     c2) using the image substructure center position to remove noise both outside and inside the substructure, thereby producing segmented images. 
     In accordance with an aspect of the present invention, there is also providing a segmentation system comprising: 
     a first storing device for receiving a plurality of digital images representing consecutive parallel slices of a structure that includes a substructure; 
     a computer for thresholding the plurality of images; 
     dividing the plurality of images in at least one group of images; 
     for each of said at least one group of images, 
     a) estimating a substructure center position corresponding to the group; 
     b) using said substructure center position to find pixels of the substructure common to all images in the group; 
     c) for each image in the group, 
     c1) using said group substructure center position and said substructure common pixels to estimate a substructure center position corresponding to the image; and 
     c2) using the image substructure center position to remove noise both outside and inside the substructure, thereby producing segmented images. 
     In accordance with an aspect of the present invention, there is also providing an article of manufacture comprising: 
     a computer usable medium having a computer readable code means embodied in the medium for segmentation of images, the computer readable program code in the article of manufacture comprising: 
     computer readable program code means for causing the computer to receive a plurality of digital images representing consecutive parallel slices of a structure; the structure including a substructure; 
     computer readable program code means for causing the computer to threshold the plurality of images; 
     computer readable program code means for causing the computer to divide the plurality of images in at least one group of images; 
     computer readable program code means for causing the computer to estimate a substructure center position corresponding to each of at least one group images; 
     computer readable program code means for causing the computer to use the substructure center position to find pixels of the substructure common to all images in each group of at least one group of images; 
     computer readable program code means for causing the computer to use the group substructure center position and the substructure common pixels to estimate a substructure center position corresponding to each image in at least one group of images; and 
     computer readable program code means for causing the computer to use the image substructure center position to remove noise both outside and inside the substructure, thereby producing segmented images. 
     It is to be noted that in the following the terms pixel and point will both be used to described the smallest distinguishable element of a digital image. 
     Other objects, advantages and features of the present invention will become more apparent upon reading of the following non restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the appended drawings: 
     FIG. 1 is a schematic bloc diagram of a segmentation system according to an embodiment of the present invention; 
     FIG. 2 is a flow chart of a method for segmentation of medical images according to an embodiment of the present invention; 
     FIG. 3 is a slice image of a lumbar vertebra after the thresholding step; 
     FIG. 4 is the image of FIG. 3 after the noise removing step; 
     FIG. 5 is the image of FIGS. 3 and 4 in the form of a binary map of the structure superimposed on the grey-level background; 
     FIG. 6 is a slice image of a sacrum after the thresholding step; 
     FIG. 7 is the image of FIG. 6 after the noise removing step; and 
     FIG. 8 is the image of FIGS. 6 and 7 in the form of a binary map of the structure superimposed on the grey-level background. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 of the appended drawings, a segmentation system  10 , according to an embodiment of the present invention, will be described. 
     The segmentation system  10  includes a computer  12 , a storing device  14 , an output device in the form of a display monitor  16 , and an input device  18 . The storing device  14 , the display monitor  16  and the input device  18  are all connected to the computer  12  via standard connection means, such as, for example, wires. 
     The computer  12  can be a conventional personal computer or any processing machine that includes a processor, a memory and input/output ports (not shown). The input/output ports may include network connectivity to transfer the images to and from the storing device  14 . 
     The storing device  14  can be, for example, a hard drive, a cd-rom drive or other well known storing means. It can be directly connected to the computer  12  or remotely via a computer network, such as, for example the Internet. According to this embodiment of the invention, the storing device  14  is used to store both the non-segmented medical images as well as the resulting segmented images as computer files. Those files can be stored in any format and resolution that can be read by the computer  12 . 
     The display monitor  16  is used to visualize the medical images both before and after the segmentation process. With the input device  18 , the display monitor  16  also allows the input of guidance points by the user as will be described hereinbelow. The display monitor  16  is finally used to display a user interface, to facilitate the interaction between the user and the computer  12 . It is believed within the reach of a person of ordinary skills in the art to provide another output device that allows for the visualization of the medical images. 
     The input device  18  can be a conventional mouse, a keyboard or any other well known input devices or combinations thereof. 
     Of course, the computer  12  runs a software that embodies the method of the present invention thereof. 
     Other aspects and characteristics of the system  10  will become more apparent upon reading of the following description of a segmentation method according to an embodiment of the present invention. 
     Referring now to FIG. 2 of the appended drawings, generally stated, the method of the present invention consists in performing the following steps in sequence: 
       100 —starting the system  10 ; 
       102 —receiving a set of images of a structure that comprises a substructure; 
       104 —thresholding the set of images; 
       106 —selecting the image range; 
       108 —selecting a predefined segmentation strategy (optional); 
       110 —selecting guidance points in the substructure according to the strategy; 
       112 —partitioning into groups the images in the range; 
     for all group of images ( 114 - 118 ): 
       114 —identifying the region of the substructure common to all images in the group; 
       116 —identifying the substructure in each image of the group; 
       118 —removing the noise in each image of the group; 
       120 —storing the segmented images on the storing device; and 
       122 —stopping the system  10 . 
     Before describing these general steps in greater details, it is to be noted that steps  110  to  116  are advantageously performed for every substructure to be identified in the images. 
     After the segmentation process  10  has been started (step  100 ), the step  102  consists in receiving a set of images representing cuts or slices of a structure to the computer  12 . An example of an image  150  to be segmented is shown in FIG. 3, where the structure is a lumbar vertebra  152 . The substructure that needs to be identified by the segmentation process is, in this example, the spinal canal  154 . One can see in FIG. 3, that the vertebra  152  and its spinal canal  154  are not perfectly defined in the images received. In other application, the substructure can be any part of a structure that can be visually isolated from the structure. 
     The image  150  is a two dimensional array of pixels that has been previously produced by an imaging system, such as, for example a CT scanner or a MRI scanner. It is to be noted that the set of images is provided sequentially in the order that they appear in the three-dimensional object. In other words, successive images come from adjacent slices of the three dimensional object. 
     In step  104 , a thresholding operation is performed on the set of images. The tresholding step  104  consists in selecting pixels on the image that have values between a minimal and a maximal values. Step  104  is performed by the user and can be viewed as a pre-segmentation step that facilitates the up-coming segmentation steps. 
     The image range is then defined in step  106  by selecting first and last images of the range. If, for example, the set of images comprises slices of the entire spinal cord, the user can select a range of images corresponding only to the lumbar section of the spine, if it is the only section for which the user wants to segment the images. This is simply done by selecting the first and last image of the range. 
     In step  108 , a predefined segmentation strategy can optionally be selected. If, for example, the system  10  is often used for the segmentation of lumbar vertebrae, a segmentation strategy, that takes into account the fact that such vertebrae contain a substructure consisting of a cavity (the spinal canal), can be selected. Of course, these segmentation strategies may be stored and kept for future uses. In the following description, we will assume that the segmentation strategy for lumbar vertebra has been selected. Consequently the present example relates to a structure including a single substructure, even though the invention can deal with more than one such substructure. 
     After the segmentation strategy has been selected, the user is asked to identify the substructure  154 , i.e. the cavity of the spinal canal. Step  110  consists in using the input device  18  to select a point (pixel) included in the representation of the substructure  154 , preferably in the first and in the last image in the range of images selected in step  106 . 
     The method according to the present invention can also work if other images in the range are used to select the points. However, if the images are too close in the set to each other, the segmentation process, that will be describe hereinbelow, would not be as efficient. Also, the method according to the present invention can also be implemented if only a single image is used to choose a point for each substructure. However, it has been found advantageous to select such pixels in the first and last image of the range to facilitate the estimation of the center position of the substructure, as will be describe hereinbelow. Also, it is preferable that the guidance points be selected near the center position of the substructure, to increase the speed of the segmentation process. 
     In step  112 , the images selected in the range of images are then divided into smaller groups. One reason to divide the images is to increase the speed of-the segmentation process by taking into account the common structural properties of the structure  152  in adjacent images. 
     The computer  12  then searches the region of the substructure  154  common to all the images in the group (step  114 ). The computer  12  achieves this, first by predicting the group substructure center position. It involves a linear extrapolation between the center of the substructure in the first image of the current group and the center of the previous group, and another extrapolation between the center of the previous group and the center in the last image of the group. The average of the two predicted centers is the predicted center of the substructure in the present group. 
     The computer  12  can then search all possible candidates that may represent the common region of the group. The prediction is based on the region properties such as the size of the region, the distance of its center from the predicted center and criteria of confinement. All those values and criteria have default values that may be adjusted during the possible candidate searching process. Using criteria of confinement consists in allowing more importance to pixels that are parts of a substructure in the images. 
     The computer  12  performs iterations of the searchings of the common region and the center point. In the case of the first image of the first group of images, the starting point of this iterative process is one of the guidance point selected in step  110 . 
     If the common region of a group can not be determined by the iterative process using the default values mentioned hereinabove, a dynamic adjustment that generally consists in reducing the number of images in the group and reducing the parameter associated to selection criteria. The adjustment is possible only if a minimal number of images in the group and the minimal size of the substructure to be identified are satisfied. 
     If the center of the substructure can not be determined with the dynamic adjustment of the values, the computer  12  tries to find the center with less strict values. If, after that, the detection still fails, the user can be asked to enter interactively the substructure position for a given image of the group. 
     When the identification of the group center position (step  114 ) is successful and the pixels of the substructure common to every images of the group have been identified, the identification of the position of the substructure  154  in each images of the group (step  116 ) can then be performed. 
     The center position of the substructure in each image is determined by taking into account the relative position of each image inside its group and with respect to the previous group. Again, an extrapolation between center position values from the group and the previous group is computed using a weight factor. The weight factor is adjusted with respect to the position of the studied image into the range. The closer to the end is the images, the more the weight factor gives importance to the current group center position. 
     Once the position of the substructure  154  in the structure  152  has been identified in each images  150 , it is possible to remove the noise in each image  150  (step  118 ). The computer first removes noise in the region that is outside a given radius from the center determined in step  116 . Afterward, it erases the noise and fills empty regions in the structure  166 , knowing the position and size of the substructure. Criteria for removal of noises are based on size, connectivity and relative positioning with respect to the substructure. An example of a processed image  160  is shown in FIG. 4 of the appended drawings. FIG. 4 corresponds to the image  150  of FIG. 3 after step  118  is executed. 
     After an image in the range has been processed and the substructure is well defined, the resulting images can be stored on the storing device  14  of the system  10  in the form of a contour map or as a filled structure (step  120 ). An example of a filled structure image  162  (or binary map) corresponding to the images  150  and  160  is shown in FIG. 5, where each pixel of the image  162  is either part of the substructure  154 , or of the rest of the structure (see region  166 , in white on FIG.  5 ). 
     Once all the images of the range have been segmented, a person skilled in the art can build a three dimensional model of the structure, comprising the substructure, by using the segmented images  162  and a conventional three-dimensional reconstruction system. 
     FIGS. 6,  7  and  8  of the appended drawings illustrate the use of the method according to this invention to segment medical images  170  of the sacrum  172  comprising the two foramens  174 . 
     In the case of such a structure, all the step from  100  to  120  are executed as described hereinabove, except for the following differences: 
     in step  108  another predefined segmentation strategy is selected to take into account the fact that there are now two additional cavities in the structures to identify in the images: the two foramens  174 ; and 
     in step  110 , the user must now select additional guidance points for each foramen  174 . 
     FIG. 7 shows the image of FIG. 6 after the noise removing step, while FIG. 8 shows a binary map superimposed on a grey-level background and corresponding to FIG.  7 . 
     Although the present embodiment has been described with bones as the structure, the structure represented on the images can be any physical object having at least one substructure that can be independently identified. 
     Although the present invention has been described hereinabove by way of preferred embodiments thereof, it can be modified, without departing from the spirit and nature of the subject invention as defined in the appended claims.