Abstract:
In a method and apparatus for locating in a three dimensional data array an arcuate object having an axial extent, slices of data generally transverse to the axial extent of the object are selected. Rays generally radially of the arcuate object are selected within the slices. Crossing points where the rays cross the boundaries of the arcuate object are located. The position of the arcuate object is determined from the positions of the located points.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/682,971, filed May 20, 2005, which is imported herein by reference in its entirety.  
         [0002]     This application is related to the following U.S. patent applications which are filed on even date herewith and which are incorporated herein by reference: 
        Ser. No. 11/XXX,XXX (Attorney Docket No.: 45058-0005-00-US (225687) entitled LOCATION OF ELONGATED OBJECT;     Ser. No. 11/XXX,XXX (Attorney Docket No.: 45058-0007-00-US (226421) entitled LOCATION OF FOCAL PLANE; and     Ser. No. 11/XXX,XXX (Attorney Docket No.: 45058-0009-00-US (226711) entitled PANORAMIC VIEW GENERATOR.       
 
     
    
     BACKGROUND  
       [0006]     The invention relates to locating a curved object in a three-dimensional array of data, and especially, but not exclusively, to locating an unevenly curved structure in a tomographic imaging dataset. The invention has particular application to locating the maxilla, the mandible, or both in a dataset of part of the head of a human or other mammal.  
         [0007]     In certain forms of dental medicine and surgery, a “panoramic” image of the jaw is used to examine the jaw, for example, for monitoring of dental health and condition, diagnosis, and planning of prosthetic and other surgical procedures. The panoramic image of the jaw, like a panoramic photograph, depicts the jaw as if it were imaged onto an imaginary approximately cylindrical sheet with the axis of the sheet upright, and the sheet were then unrolled into a flat form. However, the human jaw is not a perfect circular arc, so the “cylindrical” shape of the imaginary sheet is not exactly circular.  
         [0008]     A set of three-dimensional data relating to a property of an object that varies over space within the object may be obtained in various ways. For example, an x-ray image of a target may be obtained by placing the target between a source of x-rays and a detector of the x-rays. In a computed tomography (CT) system, a series of x-ray images of a target are taken with the direction from the source to the detector differently oriented relative to the target. From these images, a three-dimensional representation of the density of x-ray absorbing material in the target may be reconstructed. Other methods of generating a three-dimensional dataset are known, including magnetic resonance imaging, or may be developed hereafter.  
         [0009]     From the three-dimensional data, a desired section or “slice” may be generated, including a curved slice. For example, a slice curving along the jaw, corresponding to a panoramic view of the jaw may be generated, provided that the position of the jaw within the three-dimensional dataset is known. It has previously been proposed to display on a visual display unit (VDU) a horizontal section through the mandible or maxilla, and for a human user to identify, for example by controlling a cursor on the VDU or by using a stylus on a touch-sensitive VDU, enough points on the mandible or maxilla for the curve of the jaw to be interpolated. However, many dentists and oral surgeons are not skilled computer operators.  
         [0010]     There is therefore a hitherto unfulfilled need for a system by which the arc of the jaw can be accurately identified in a tomographic dataset without relying on the skill of a human operator, so that panoramic and similar images can be reliably automatically generated.  
       SUMMARY  
       [0011]     According to one embodiment of the invention, there is provided a method and system for locating in a three dimensional data array an arcuate object having an axial extent. Slices of data generally transverse to the axial extent of the object are selected. Rays generally radially of the arcuate object are selected within the slices. Crossing points where the rays cross the boundaries of the arcuate object are located. The position of the arcuate object is determined from the positions of the located points.  
         [0012]     According to a preferred embodiment of the invention, the data array is a tomographic dataset of a human head or part thereof, and the curved object is the upper or lower jaw.  
         [0013]     By locating the mandible or maxilla in the three-dimensional tomographic dataset, and exploiting the fact that the jaw is continuous along its length and has relatively sharp boundaries, the curve of the jaw can be identified with greater reliability than is attainable by most human operators without special skills or great effort.  
         [0014]     The invention also provides computer software arranged to generate an image in accordance with the method of the invention, and computer-readable media containing such software. The software may be written to run on an otherwise conventional computer processing tomographic data.  
         [0015]     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0017]     In the drawings:  
         [0018]      FIG. 1  is a schematic view of apparatus for generating a tomographic image.  
         [0019]      FIG. 2  is a flow chart of one embodiment of a method according to the invention.  
         [0020]      FIG. 3  is a schematic plan view of a jaw.  
         [0021]      FIG. 4  is a schematic side view of a head, with a curve of probability distribution.  
         [0022]      FIG. 5  is a flow chart of another embodiment of a method according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0023]     Reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0024]     Referring to the drawings, and initially to  FIGS. 1 and 2 , one form of tomographic apparatus according to an embodiment of the invention, indicated generally by the reference numeral  20 , comprises a scanner  22  and a computer  24  controlled by a console  26  with a display  40 . The scanner  22  comprises a source of x-rays  28 , an x-ray detector  30 , and a support  32  for an object to be imaged. In an embodiment, the scanner  22  is arranged to image the head, or part of the head, of a human patient (not shown), especially the jaws and teeth. The support  32  may then be a seat with a rest or restrainer  36  for the head or face (not shown) of the patient. The x-ray source  28  and detector  30  are then mounted on a rotating carrier  34  so as to circle round the position of the patient&#39;s head, while remaining aligned with one another. In step  102 , the x-ray detector  30  then records a stream of x-ray shadowgrams of the patient&#39;s head from different angles. The computer  24  receives the x-ray image data from the scanner  22 , and in step  104  calculates a 3-dimensional spatial distribution of x-ray density.  
         [0025]     The imaging of the patient&#39;s head and calculation of the spatial distribution may be carried out by methods and apparatus already known in the art and, in the interests of conciseness, are not further described here. Suitable apparatus is available commercially, for example, the i-CAT Cone Beam 3-D Dental Imaging System from Imaging Sciences International of Hatfield, Pa.  
         [0026]     In step  106 , an initial slice  202  through the tomographic dataset is selected. As shown in  FIG. 4 , the initial slice  202  is a horizontal slice, relative to the normal standing or sitting position of a human patient. For other species, the operator may select an appropriate orientation, either by positioning the patient for scanning, or by manipulation of the tomographic data. The slice may be selected by a human operator on the display  40  of the console  26 , or may be selected automatically by the computer  26 .  
         [0027]     In step  108 , a reference point  204  is selected. The reference point  204  is preferably on, or close to, the centerplane of the head, and approximately at the center of the dental arch. Because the dental arch is not an arc of a circle, and may not be symmetrical from side to side, very precise centering of the reference point  204  is not required, and may not be meaningfully possible. The same reference point  204  will be used in subsequent processing of other slices. Therefore, the reference point  204  may be treated as the intersection of a vertical reference line with the slice  202 , and either the reference line/point  204  or the slice  202  may be selected first, or they may be selected in parallel. For subsequent slices  202 , the reference point  204  is automatically selected as a point on the reference line through the reference point in the first slice.  
         [0028]     In step  110 , a probability count is set to zero.  
         [0029]     In step  112 , a ray  206  in the slice  202  extending radially away from the center point  204  through the jaw  200  is selected.  
         [0030]     In step  114 , the ray  206  is inspected to identify the points  208 ,  210  where the ray crosses the inner face  212  and the outer face  214  of the jaw  200 . These faces  212 ,  214  are typically recognized as ramps in the density profile along the ray, where the density rises rapidly towards the jaw  200 . The bone of the jaws typically has a markedly higher density than the surrounding soft tissue. The crossing points  208 ,  210  may be taken as points where the ramp crosses a selected density threshold. If the ramps are not clearly defined, for example, if the steepness or height of either or both ramp is less than a preset threshold, the ray  206  may be discarded.  
         [0031]     If crossing points  208 ,  210  are identified, in step  116  the crossing points may be subjected to a quality test. For example, the ray may be discarded if the identified crossing points  208 ,  210  are not approximately where the jaw  200  is expected to be, are too close together or too far apart, or are out of line with the crossing points  208 ,  210  in neighboring rays.  
         [0032]     If the ray passes the quality test, in step  117  the probability count is increased by one, and the position where the ray  206  crosses the jaw is recorded. In one embodiment, only the center of the jaw, which may be taken as a point  216  midway between the crossing points  208 ,  210 , is recorded. In another embodiment, the crossing points  208 ,  210  are recorded instead, or in addition. If the ray  206  is discarded either at step  114  because adequate ramps are not found or at step  116  because the quality test is failed, then step  117  is bypassed.  
         [0033]     The process then returns to step  112  to select another ray  206  in the same slice. A selected number of rays  206  evenly spaced around the arc of the jaw are selected. When it is determined in step  118  that all of the rays  206  for the slice  202  have been processed, the probability count for the slice  202  is recorded, and the process returns to step  106  to select another slice  202 .  
         [0034]     The process then repeats for each of a stack of slices  202  over a part of the tomographic dataset where, based on the normal positioning of the headrest  36  relative to the x-ray source  28  and detector  30 , the jaw  200  is likely to be present. It is not usually necessary to process every voxel slice of the original dataset. Preferably, a reasonable number of slices, which may be evenly spaced or may be more closely spaced in areas of expected high probability, are processed. The initial slice  202  may be the top or bottom slice of the stack of slices, with the process working systematically down or up the stack. However, that is not necessary, and the initial slice  202  may be an arbitrary slice.  
         [0035]     Various methods may be used to define the upper and lower bounds of the stack of slices  202  used in steps  106  to  120 . A bound may be set as the upper and/or lower edge of the field of the original scan of part of the patient&#39;s head. A bound may be set manually using a vertical display of the dataset. The upper or lower edge of the mandible, or the lower edge of the maxilla, may be easily found by detecting the point where high density tissue (usually teeth or bone) abruptly disappears. Other suitable methods may also be used.  
         [0036]     When it is determined in step  120  that all of the slices  202  have been processed, in step  122  the recorded probability counts for the different slices  202  are inspected. As shown by the curve  220  in  FIG. 4 , the probability counts typically vary from slice to slice, with the highest probability counts forming a peak where the jaw is most well defined and the probability value decreasing above and below the peak. Alternatively, a bimodal distribution with distinct peaks for the mandible and maxilla may be found. A high probability count indicates that few of the rays  206  were discarded, and that many of the rays have yielded points  216  that are believed correctly to indicate the position of the jaw.  
         [0037]     In step  124 , the slice  202  with the highest probability count is selected, and the center points  216  of the selected slice are selected, if the center points were calculated and recorded in step  117 , or are calculated, if only the crossing points  108 ,  110  were recorded in step  117 . If the peak of the probability curve  220  has a “plateau,” where more than one neighboring slice has the identical highest probability count, any one of those slices can be chosen arbitrarily. All of those slices are likely to lie in a “good” region of the jaw. A contour line  222  representing the dental arch is then constructed through the points  216 . Where a bimodal distribution is found, separate contour lines  222  may be constructed for the maxilla and the mandible.  
         [0038]     In step  126 , a synthesized panoramic slice formed from columns of voxels along the contour line  222  is then generated and presented to the user. The columns of voxels may be perpendicular to the plane of the slices  202 , or may be at an angle. Where separate mandibular and maxillary contour lines  222  have been constructed, the columns of voxels may be slanted or curved to pass through both contour lines  222 , giving a hybrid panoramic view that shows both the maxilla and the mandible. The synthesized panoramic slice may be one or more voxels thick. For example, the thickness of the panoramic slice may correspond to a typical value for the actual thickness of the mandible or maxilla perpendicular to the panoramic slice plane.  
         [0039]     The thickness of the human mandible and maxilla vary, both from person to person and from point to point within the jaw. Consequently, the synthesized panoramic slice displayed in step  126  may be less than optimal because it may include too much or too little of the thickness of the jaw. If the panoramic slice is too thick, bony structure may overlay or “shade” detail in the interior of the mandible or maxilla, such as the alveolar nerve, or blur details of the root of a tooth. If the panoramic slice is too thin, bony outgrowths, protruding calcification, or displaced teeth may be missed.  
         [0040]     Referring now to  FIG. 5 , in an alternative embodiment of a process according to the invention the crossing points  208 ,  210  are recorded in step  117 . Then, after step  120 , the process proceeds to step  302  and calculates inner and outer envelopes of the jaw  200 . The inner envelope may follow the points  208  from all the accepted rays  206  in all the slices  202 . The outer envelope may follow the points  210  from all the accepted rays  206  in all the slices  202 . In step  304 , the process then generates a panoramic slice containing the entire jawbone, except for projections too small to be reflected in the array of crossing points  208 ,  210 .  
         [0041]     Alternatively, if it is desired to view the internal structure of the mandible or maxilla, part of the cortical bone may be virtually “machined” away to generate a slice in which less bone is present to obscure the view. In step  306 , the process may select, or get from a user, a figure for the amount of machining, for example, a “percentage of erosion.” The panoramic slice is then generated in step  308 , omitting a corresponding part of the cortical bone. For example, where the panoramic slice in step  304  extends from −50% to +50% of the thickness of the jaw, centered on the contour  222  at 0%, a panoramic slice with 30% erosion may extend from −35% to +35%. Other patterns of erosion or machining may be used, for example, removal of a constant thickness of bone.  
         [0042]     In step  310 , the tomographic data generated as described above with reference to steps  102 ,  104  in  FIG. 2  may be subjected to a segmentation process known in other fields to discriminate between the targeted mandible or maxilla and other tissues. The segmentation process typically comprises of defining an initially arbitrary contour through the dataset. The contour may be a surface in the three dimensional dataset or a line in a slice  202 . In an iterative process, the contour is then adjusted and assessed by some criterion for its fit to the surface of the mandible or maxilla. The criterion may test, for example, how much of the contour is in areas of high density gradient. The iterative process may lock into place parts of the contour that are on local maxima of the density gradient, while continuing to adjust other parts of the contour. Segmentation processes and suitable iterative algorithms are well known in other fields of image processing, and in the interests of conciseness the segmentation is not further discussed here.  
         [0043]     Where the segmentation process is carried out using a surface as the contour, the final result may be equivalent to the envelope generated in step  302 , and may be passed directly to steps  304  and  306 . Where the segmentation process is carried out on individual slices  202 , the slice contours may be stacked and an envelope interpolated similarly to step  202 . Where both an envelope surface from step  302  and a segmentation surface from step  310  are available, the two surfaces may be compared in step  312  to corroborate the identification of the actual jaw surface. Alternatively, portions of the jaw may be located by the process of steps  106  to  120 , and portions by segmentation, and combined in step  312  to give a full surface. For example, a three dimensional segmentation process may be better suited to parts of the head where the bone surface is not close to vertical relative to the axis  204 .  
         [0044]     Also, the segmentation process can identify and distinguish separate bony structures in the same dataset. In  FIG. 2 , only one good set of points  216  is needed, so if some rays  206  inadvertently detect edges on structures other than the targeted mandible or maxilla, those rays, and if necessary entire slices  202 , can be discarded as aberrant. However in  FIG. 5 , it is desirable to have as many correctly identified points  208 ,  210  as possible, or as nearly a continuous surface found by segmentation as possible, so using the segmentation method to separate out other bony structures and reduce the amount of data discarded as ambiguous or unclear can be beneficial. Step  114  may then accept multiple crossing points  208 ,  210 , and a later step, for example, step  116 , may use segmentation or other discrimination process to select the crossing point  208  or  210  that belongs to the targeted mandible or maxilla.  
         [0045]     Various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. For example, the order of the steps may be changed. In  FIG. 2 , more than one, or all, of the rays  206  may be processed to identify the crossing points  208 ,  210  before the first rays  206  are subjected to the quality test in step  116 . In particular, the test for out-of-line points cannot usually be completed until points from neighboring rays  206  are available for comparison. Alternatively, or in addition, rays in neighboring slices  202  may be used for comparison.  
         [0046]     In a further alternative, in step  124  two or more slices  202  with high probability counts may be combined. An average of the positions of the contour lines  222 , which may be weighted according to the probability counts of the respective slices  202 , may be used, or a consensus curve may be determined by combining only contour lines  222  that agree within a narrow tolerance.  
         [0047]     As described with reference to  FIG. 2 , steps  106  through  120  repeat for each of a stack of slices  202  over the height of the jaw  200 . Alternatively, the process may start with a slice  202  near the expected location of the probability peak of curve  220 . The process may then proceed both up and down, with the probability count being reviewed as each slice is completed. Step  120  may then terminate the process in each direction when the probability count drops below a threshold, which may be either absolute or relative to the highest detected count, so that the process is confident that the peak is within the part of the stack of slices  202  that has been processed.  
         [0048]     Where steps  114  and  116  find only one clear crossing point  208  or  210 , step  302  may use that one point, although in step  124 , a single crossing point  208  or  210  may be unhelpful for constructing the center point  216 .  
         [0049]     For example,  FIG. 1  shows that the computer  24  on which the processes of  FIGS. 2 and 5  is running is connected to the scanner  22 . A single computer  24  may both control the scanner  22  and run the processes of  FIGS. 2 and 5 . Alternatively, part or all of the process of  FIG. 2  and/or  FIG. 5  may be carried out on a separate computer. The data from the scanner  22  may be transferred from computer to computer in a convenient format, for example the DICOM format, at a convenient stage of the process. The data may, for example, be transferred directly from computer to computer or may, for example, be uploaded to and downloaded from a storage server.  
         [0050]     For example, in the processes described above, it is assumed that the number of rays is equal in all slices, and the rays are uniformly spaced. The number of rays may vary from slice to slice, provided that an appropriate allowance is made at steps  122  and  124 . The rays may be more closely spaced at parts of the jaw where a small radius of curvature and/or a rapid change in curvature is expected. In the process of  FIG. 2 , for example, the selected slice may have a few discarded rays, and the ray spacing is desirably sufficiently close that a correct contour  222  can be interpolated with a desired degree of accuracy across at least one missing point  216 .  
         [0051]     For example, it is not necessary for the reference points  204  of the different slices  202  all to lie on a straight line. However, using consistent reference points  204  typically gives more consistent, and thus more comparable results, and may give a more accurate decision in step  124 . In step  302 , using consistent reference points  204  gives a more consistent relationship between the arrays of crossing points  208  and  210  and may make the generation of the envelopes computationally simpler. In addition, using a vertical line of points  204  is usually the simplest approach.  
         [0052]     Although distinct embodiments have been described, features of different embodiments may be combined. For example, the contour  222  shown in  FIG. 3  may be obtained as a middle line between inner and outer contours obtained by the segmentation process of step  310 .  
         [0053]     Thus, it is intended that the present invention cover modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.