Abstract:
A method and apparatus for locating an elongated object in a three dimensional data array are disclosed. A slice of data generally lengthways of the elongated object is selected. Points on the object in the selected slice are identified. Data including the points are transposed parallel to the slice and transversely to the elongated object to align the points in a direction parallel to the slice and transverse to the elongated object. A current slice is selected that is rotated around the length direction of the object relative to the previously selected slice. The identifying and transposing are repeated to align points on the object in a direction parallel to the current slice and transverse to the elongated object.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/682,971, filed May 20, 2005, which is incorporated herein by reference in its entirety. 
     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. 11/437,525 entitled LOCATION OF FOCAL PLANE; 
     Ser. No. 11/437,524 entitled LOCATION OF CURVED STRUCTURE; and 
     Ser. No. 11/437,526 entitled PANORAMIC VIEW GENERATOR. 
    
    
     BACKGROUND 
     The invention relates to locating an elongated object in a three-dimensional array of data, and especially, but not exclusively, to locating an elongated structure in a tomographic imaging dataset. The invention has particular application to locating the nerve canal in x-ray tomographic dental imaging of the mandible or lower jaw of a human or other mammal. 
     In certain forms of dental surgery, such as implant surgery, it is necessary to drill into the mandible. For secure implantation of, for example, a prosthetic tooth, a deep drilled hole is desirable. However, within the mandible there is a nerve canal containing the neurovascular bundle to the teeth and parts of the face. If the surgeon drills too deeply, and damages the nerve, permanent numbness of portions of the face may result, to the potentially considerable detriment of the patient. 
     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. 
     From the three-dimensional data, a section in a desired plane may be generated. In planning for dental implant work, the surgeon may display tomograms in planes along and across the mandible, and attempt to identify the nerve canal in the tomograms. However, the contrast of x-ray tomograms is often poor, and the interior of the mandible is formed from bone of comparatively low density. In addition, the nerve canal is not smoothly curved along its length, but undulates irregularly. The nerve may therefore appear in an unexpected place in a transverse section, and may be difficult to follow in a longitudinal section. The nerve may also disappear out of the plane of a longitudinal section regardless of how the section plane is aligned. Consequently, it is not always easy for the surgeon to recognize the mandibular nerve and plan surgery correctly. 
     There is therefore a hitherto unfulfilled need for a system by which the mandibular nerve can be accurately identified and marked on tomograms of the mandible, enabling the surgeon reliably to avoid the nerve when drilling for implantation, and to assess whether sufficient depth of sound bone is available for a good implant. 
     SUMMARY 
     According to one embodiment of the invention, there is provided a method and system for locating an elongated object in a three dimensional data array. A slice of data generally lengthways of the elongated object is selected. Points on the object in the selected slice are identified. Data including the points are transposed parallel to the slice and transversely to the elongated object to align the points in a direction parallel to the slice and transverse to the elongated object. A current slice is selected that is rotated around the length direction of the object relative to the previously selected slice. The identifying and transposing are repeated to align points on the object in a direction parallel to the current slice and transverse to the elongated object. 
     According to a preferred embodiment of the invention, the data array is a tomographic dataset of a human mandible, and the elongated object is the mandibular nerve or mandibular nerve canal. 
     By locating the whole, or a substantial part, of the mandibular nerve in the three-dimensional tomographic dataset, and exploiting the fact that the nerve is continuous along its length, the mandibular nerve can be identified with far greater confidence than is attainable when inspecting a single x-ray image or tomogram. 
     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. 
     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 
       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. 
       In the drawings: 
         FIG. 1  is a schematic view of apparatus for generating a tomographic image. 
         FIG. 2  is a flow chart. 
         FIG. 3  is a somewhat schematic panoramic view of a mandible, showing the nerve canals. 
         FIG. 4  is a somewhat schematic top view of a mandible, showing the nerve canals. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     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 . 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. 
     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. 
     In step  106 , an initial slice  202  through the tomographic dataset is selected. As shown in  FIG. 3 , the initial slice  202  is a “curved slice” similar to a synthesized panoramic view of the mandible. That is to say, the initial slice is formed from vertical (relative to the normal standing or sitting position of a human patient) columns  210  of voxels passing through a line that follows the curve of the mandible  204 . (In the interests of clarity, only some of the columns  210  are marked in  FIG. 3 .) 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  if the computer is programmed to conduct pattern recognition on the tomographic dataset and to locate the mandible. Great precision in choosing the initial slice is not required, since the mandibular nerve is not straight, and will cross any reasonable curved slice. However, precise knowledge of what initial slice has been chosen may be helpful in the next step. 
     In  FIG. 3 , the initial slice  202  is shown “developed” or unrolled into a flat surface, like a panoramic x-ray image. This is a useful arrangement where the slice  202  is displayed on the display  40  for a human operator. The initial slice  202  is selected so that the nerve canal  206  is visible along its length. The initial slice  202  may be a thick slice, several voxels in extent in a direction perpendicular to the plane of the image in  FIG. 3 , and positioned relative to the mandible  204  so that at least most of the nerve canal  206  is in the slice. Alternatively, the initial slice  202  may be a thin slice, possibly only a single voxel thick, positioned so that the nerve canal  206  repeatedly crosses the initial slice. 
     In the interests of linguistic simplicity, the description of  FIGS. 2 to 4  describes tracing the mandibular nerve  206  as if the mandibular nerve appears in the dataset as a nerve line that occupies one voxel in any slice transverse to the length of the nerve, and thus occupies a single pixel in any transverse section image. In a practical embodiment, the nerve and nerve canal will appear as a “bull&#39;s eye” pattern several voxels across, and may be recognized by pattern recognition of a transverse slice of the dataset centered on the voxel where the notional nerve line intersects the transverse slice. 
     In step  108 , a number of reference points  208  of the nerve canal  206  are identified in the initial slice  202 . Because the position of the mandibular nerve is known in general terms, if a suitable initial slice is chosen some assumptions can be made as to where the mandibular nerve will be found in the initial slice. The points  208  may be points where the mandibular nerve  206  crosses the initial slice  202 , or may be distinctive points of the nerve within the slice  202 . The operator may identify the points  208  visually on the display  40 , and may identify the points to the computer  24  by positioning a cursor or pointer on the display  40 . Alternatively, the initial points may be identified by pattern recognition. The effectiveness of computer pattern recognition may be limited since at times the nerve canal may be barely visible even to the human eye. Therefore, where a skilled human is available, he or she may be preferred. However, the quality of computer methods is constantly improving, and may become better than the human eye. The points  208  are chosen because they are identifiable with a high degree of certainty, and are spaced out along the length of the nerve canal, so that the distance between any two adjacent points  208  is not too long. The minimum number of points needed, or the maximum spacing between adjacent points, very much depends upon the extents of the canal. If the canal does not undulate much, just a few points—maybe 4 or 6 per side—may be sufficient. On the other hand, if the canal undulates a lot, many more points may be needed. Where a suitable set of initial reference points  208  cannot be identified with sufficient confidence, the computer may loop back to step  106  and select a different initial slice, in which the initial points may be more easily recognized. 
     In step  110 , the columns  210  of voxels containing the points  208  are transposed or shifted up or down so that the points  208  are in a straight, horizontal line in the view shown in  FIG. 3 . The columns  210  that are between points  208  may be transposed by a linear interpolation on each section, or by a more sophisticated interpolation, for example, a cubic spline or the like. Alternatively, sub-sections of the nerve canal may be detected algorithmically and shifted accordingly. The points  208  are not aligned perpendicular to the plane of  FIG. 3 . Where the slice  202  is thick enough to contain the entire nerve canal  206 , the shifting of each column  210  is applied to a row of columns that extends perpendicular to the plane of the view in  FIG. 3  for the thickness of the slice. Where the slice  202  is thinner, the columns of voxels in front and behind the slice are shifted correspondingly, so that the whole nerve line is aligned. 
     In step  112 , the plane of view is rotated, for instance, 90° about the line of points  208 , so that the nerve canal  206  is presented in a top view slice  212 . Because the top view slice  212  is based on the transposed dataset generated in step  110 , the voxels of the nerve  206  are concentrated in a top view slice that may be only one or a few voxels thick, in a vertical position determined by the shifting process. Consequently, the top view slice  212  may be a thinner slice than the frontal slice  208  and still contain the entire nerve canal  206 . However, the shifting in step  110  was perpendicular to the top view slice  212  and thus did not affect the positions of the nerve voxels along or across the plane of the top view slice. As shown in  FIG. 4 , the mandible with the nerve canal is presented in its natural curved shape, but it may instead be presented straightened out, corresponding to the “developed” panoramic image in  FIG. 3 . 
     The process then returns to step  108  to identify a set of reference points  208  in the newly presented image. The first time the plan view of  FIG. 4  is viewed, the same points  208  as in  FIG. 3  may be used, or new points that are easily identified in the plan view may be used. The process then proceeds to step  110  to move the newly identified reference points  208  into a straight line. In this iteration of step  110 , the columns  210  of voxels that are shifted to move the points  208  are radial columns in the plane of  FIG. 4 , together with enough corresponding radial columns just above and below the plane of  FIG. 4  to ensure the entire nerve canal  206  is being shifted. 
     Steps  112 ,  108 , and  110  are then repeated as often as necessary. At each iteration, the line of the nerve canal  206  in the images becomes more nearly straight, and easier to identify, so the reference points  208  can be found closer and closer together, until it is determined in step  114  that the reference points  208  are closer than the scale of the undulations in the nerve canal  206 , so that a straight alignment of the points  208  reliably indicates a straight alignment of the whole nerve canal. It is then determined in step  116  whether the nerve canal  206  is in fact substantially straight both in the frontal view plane  202  of  FIG. 3  and in the top view plane  212  of  FIG. 4 , and/or optionally in other arbitrary rotation views. 
     In step  120  the computer  24  verifies that the mandibular nerve  206  has been correctly traced. This may be done by displaying to a human operator the final forms of the frontal slice  202  and the top view slice  212 , which should have the entire mandibular nerve  206  displayed in a straight line, or in a smooth curve in  FIG. 4 , along the slice plane. Alternatively, a pattern recognition routine within the computer  24  may review the transposed mandibular nerve and nerve canal, centered on the straightened nerve  206 . Verifying by pattern recognition the presence of the straightened nerve in the transposed dataset is a computationally comparatively simple process. 
     In step  122 , the computer  24  re-creates the original path of the mandibular nerve by working back from the final transposed path of the nerve  206  and a record of the amount and direction of transposition of each nerve voxel. In step  124 , the re-created path of the nerve is then superimposed on the original tomographic dataset, and displayed to a human user. The re-created path may be displayed by highlighting appropriate pixels in the display of the tomographic dataset. For example, the nerve may be shown by brightly colored pixels marking the nerve line. Depending on the size and resolution of the image, for example, a single pixel or a cluster of pixels around the nerve line may be highlighted. The re-created path may be recorded by editing a copy of the tomographic dataset to highlight the appropriate voxels. Alternatively, the re-created path may be stored in a separate file, and combined with the tomographic dataset when a display image is generated. 
     Where the computer  24  in step  108  fails to identify a sufficient set of reference points  208  for the next iteration, various additional actions may be taken to recover. For example, the computer  24  may use a more computationally intensive process to recognize reference points, or may use a lowered threshold of confidence that a valid reference point  208  has been identified. For example, the computer may refer to a human operator to make additional selections. 
     If the recovery is successful, the computer proceeds to step  110 , and continues the iterative nerve straightening process. If the recovery is not successful, the computer may still proceed to step  120  and generate a validation image to be reviewed by a human operator, to decide whether the imperfectly straightened nerve path actually provides sufficient information to be useful for a specific surgical or other operation. 
     Various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. 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. 
     For example,  FIG. 1  shows that the computer  24  on which the process of  FIG. 2  is running is connected to the scanner  22 . A single computer  24  may both control the scanner  22  and run the process of  FIG. 2 . Alternatively, part or all of the process of  FIG. 2  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.