Abstract:
A method and apparatus for generating a panoramic image of part of a target, comprises providing a scanner having a source of penetrating radiation facing a detector having an array of sensors for the penetrating radiation. The target is positioned between the source of penetrating radiation and the detector, and scanned with relative rotation of the scanner and the target. Radiation received at a plurality of the sensors spaced apart in the circumferential direction of the relative rotation is separately recorded. A panoramic image is generated by combining for each pixel of the panoramic image outputs from different sensors at different times during the relative rotation, the combined outputs being selected to represent rays of radiation passing through a point on a defined curve. The generated panoramic image and a graphical representation of the defined curve are displayed. Instructions are received from a user to alter the defined curve. The generating and displaying of the panoramic image are repeated using the altered curve.

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.  
         [0002]     This application is related to the following United States Patent Applications which are filed on even date herewith and which are incorporated herein by reference:  
         [0003]     Ser. No. 11/______ (Attorney Docket No.: 45058-0005-00-US (225687) entitled LOCATION OF ELONGATED OBJECT;  
         [0004]     Ser. No. 11/______ (Attorney Docket No.: 45058-0008-00-US (226556) entitled LOCATION OF CURVED STRUCTURE; and  
         [0005]     Ser. No. 11/______ (Attorney Docket No.: 45058-0009-00-US (226711) entitled PANORAMIC VIEW GENERATOR. 
     
    
     BACKGROUND  
       [0006]     The invention relates to imaging a curved object in a panoramic array of data, and especially, but not exclusively, to panoramic imaging of the dental arch. The invention has particular application to imaging 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 panoramic x-ray image is conventionally made by placing the target between a source of x-rays and a detector of the x-rays and causing the source and the detector to rotate around the target. The source is collimated to produce a narrow fan of x-rays in a plane parallel to the axis of rotation, and the detector is arranged to expose a narrow strip of detecting surface aligned along the fan of x-rays. X-ray absorbing structures in the target cast an x-ray shadow on the detector strip. Any structure that is not exactly on the axis of rotation casts a shadow that moves across the strip as the source and detector rotate. In a conventional panoramic x-ray machine, the detector strip is part of an x-ray sensitive photographic film, exposed through a slit in a metal mask. By moving the film across the slit at a controlled speed, a part of the structure at a specific distance from the axis of rotation can be brought into sharp focus, because the shadow of the selected part of the structure follows the movement of the film exactly. More recent panoramic x-ray machines use a charge-coupled device (CCD) or a CMOS detector array, and a similar panoramic image can also be produced utilizing an Amorphous Silicon Flat Panel detector, which is also used in Cone Beam CT systems. By stepping the acquired charge across the detector array and integrating the image at a controlled speed, the focusing action of a moving film can be exactly imitated.  
         [0009]     The human jaw is not an exact arc of a circle, so a simple circular motion of the source and detector, with the source, detector, and film transport or CCD stepping moving at a constant speed, does not provide a well-focused image of the whole jaw. Panoramic x-ray devices have been developed in which the focus position is adjusted so as to follow the actual curve of the jaw, either by a complicated movement of the x-ray source and/or detector, or by varying the rate of CCD readout, or both. However, such devices can adjust the focus position only at the time of x-ray exposure, and only to a pre-programmed jaw contour. If the actual jaw does not match the pre-programmed contour, or if the jaw is not in exactly the expected position relative to the scanner, an incorrectly focused image may result, in which the plane of sharp focus diverges from the jaw for part or all of the dental arch.  
         [0010]     The depth of focus can be adjusted by adjusting the width of the detecting strip. A narrower detector strip gives a deeper area of sharp focus, at the expense of requiring either a more intense or a slower exposure to achieve the same level of saturation of the x-ray film or CCD pixels. However, with pre-existing panoramic x-ray devices the strip width also can be adjusted only at the time of exposure.  
         [0011]     There is therefore a hitherto unfulfilled need for a system by which the line of sharp focus and/or the depth of focus in a panoramic x-ray can be adjusted after the x-ray exposure.  
       SUMMARY  
       [0012]     According to one embodiment of the invention, there is provided a method and system for generating a panoramic image of part of a target, comprising providing a scanner having a source of penetrating radiation facing a detector having an array of sensors for the penetrating radiation, positioning the target between the source of penetrating radiation and the detector, scanning the target with relative rotation of the scanner and the target, separately recording radiation received at a plurality of sensor positions spaced apart in the circumferential direction of the relative rotation, generating a panoramic image by combining for each pixel of the panoramic image outputs from different sensor positions at different times during the relative rotation, the combined outputs being selected to represent rays of radiation passing through a point on a defined curve, displaying the generated panoramic image and a graphical representation of the defined curve, receiving from a user instructions to alter the defined curve, and repeating the generating and displaying of the panoramic image using the altered curve.  
         [0013]     According to a preferred embodiment of the invention, the data array is a dataset of a human head or part thereof, and the curved object is the upper or lower jaw, or both.  
         [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 x-ray image 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 view of a display of curves representing a contour of a jaw.  
         [0021]      FIG. 4  is a schematic view similar to  FIG. 3 .  
         [0022]      FIG. 5  is a schematic view similar to  FIG. 3 .  
         [0023]      FIG. 6  is a schematic plan view of part of the apparatus of  FIG. 1  in operation. 
     
    
     DETAILED DESCRIPTION  
       [0024]     Reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0025]     Referring to the drawings, and initially to  FIGS. 1, 2 , and  6 , 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, shown symbolically by the contour line  42  in  FIG. 6 . 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, as shown by the arrow  44  in  FIG. 6 .  
         [0026]     The x-ray source  28  is arranged to emit a fan-shaped beam of x-rays  48 , with the plane of the fan parallel to the axis of rotation  46  of the scanner  22 , vertical as seen in  FIG. 1  and perpendicular to the plane of  FIG. 6 . The x-ray detector  30  has an array of sensors in the form of pixels  38  aligned with the fan-shaped beam of x-rays. The array of sensors is long enough to receive the whole imaging part of the fan, and is several pixels wide. The scanner  22  may be a cone-beam computed tomographic scanner, with the x-ray beam collimated down to a narrow fan. Suitable apparatus is available commercially, for example, the i-CAT Cone Beam 3-D Dental Imaging System from Imaging Sciences International of Hatfield, Pa. The part of the x-ray detector  30  not within the fan may then be masked off, or may simply be inactive, so that any data received by sensors  38  outside the fan is discarded. Alternatively, the scanner  22  may be a dedicated panoramic scanner, in which the x-ray source  28  produces only a fan-shaped beam and the array of sensors  38  consists only of the strip facing the fan-shaped beam.  
         [0027]     The detector  30  is arranged to read out the contents of every sensor  38  separately, without integrating across rows. Non-integrated readout may be achieved by the design of the detector  30 , or may be achieved by operating the x-ray source  28  in pulses, and/or by reading out the detector between pulses. The scanner  22  may then be rotated by, for example, one or more pixel widths of the detector  30  between pulses.  
         [0028]     Referring now to  FIG. 2 , in step  102 , the x-ray detector  30  produces a stream of x-ray data from the sensors  38 . Each sensor output represents the total density of a line  48  through the patient&#39;s head at a known alignment. At each readout interval, each column of sensors  38  represents lines forming a vertical fan. The different columns of sensors represent lines forming different fans spread apart sideways. If the scanner follows a simple circular motion, the middle of the fan is typically on the axis  46 .  
         [0029]     The computer  24  receives the x-ray image data from the scanner  22 , and in step  104  calculates a panoramic image by summing data from sensors in the same row in different columns in different readout intervals, corresponding to different positions of the detector  30 . The summed data then represent intersecting lines  48  through the patient&#39;s head. By selecting specific readout intervals or sensor positions for specific columns, lines can be chosen that intersect at a specific point within the patient&#39;s head. Then, the density information for the specific point in all the lines sum additively, whereas the density data for other points in the lines, which are to a greater or lesser extent independent, tend to sum randomly, and partly cancel out. The specific point is thus brought into focus. Typically, the selected readout intervals will be in order, but not necessarily evenly spaced, from one side of the detector strip to the other.  
         [0030]     For example, in  FIG. 6 , the x-ray source  28  and the detector  30  are shown in three exemplary positions as  28 ,  28 ′,  28 ″ and  30 ,  30 ′,  30 ″, and three exemplary x-ray lines  48  are shown at each position from the x-ray source  28  to a leading sensor position  50 , a middle sensor position  52 , and a trailing sensor position  54  of the array of sensors  38 .  FIG. 6  shows seven points  46 ,  56  where three exemplary rays  48  meet, and a further six points  58  where two exemplary rays  48  meet. It may be seen how each of those points may be selected by a suitable choice of sensor  50 ,  52 ,  54  or none of those sensors at each of the positions  28 ,  28 ′,  28 ″. By using a larger number of positions  28 ,  28 ′,  28 ″, etc. of the x-ray source and positions  30 ,  30 ′,  30 ″, etc. of the detector, and by making suitable selections from among the sensors  38  at each position, any point within the region of the jaw  42  or other target can be selected to an accuracy comparable to the size of the detector sensors  38 .  
         [0031]     In a practical embodiment, fewer than all of the rays  48  suggested by  FIG. 6  may be used for a specific point  46 ,  56 ,  58 . In particular, some of the points  56 ,  58  are at the intersections of some rays between the source  28  and the axis  46 , and are at the intersections of other rays between the axis  46  and the detector  30 . Because the x-rays  48  are fanning out vertically more than they are fanning out circumferentially, gathering every possible ray  48  through a point  56 ,  58  above or below the center of the source  28  requires selecting sensors  38  in different rows of the detector  30 , as well as in different columns. Where the target is a contour  42  at approximately a constant distance from the axis  46 , and only the rays  48  passing through a chosen point on the contour  42  between the axis  46  and the detector  30  are selected, then a summation of detector outputs from a single horizontal row of detector sensors can be used with only slight vertical blurring.  
         [0032]     However, as may be seen from the example of the points  58  in  FIG. 6 , for a point further away from the center axis  46  than the width of the x-ray fan at the axis, not all positions of the detector  30  will produce a sensor position representing an x-ray line  48  through that point. In particular, the outer points  58  are typically seen only on x-ray lines close to a radial direction from the center axis  46  to the point in question, and not on x-ray lines at a high angle to that radial direction. Consequently, the voxel represented by an image pixel is typically not approximately cubic, but is elongated in the radial direction. As the scanner  22  rotates, an object feature that is radially inside or outside the selected point  58  advances across the sensors  38  at a different speed from the selected point  58 , and is summed into different image pixels at successive readout intervals. As a result, the object feature becomes in effect smeared out across different image pixels, rather than being focused in a single image pixel. The wider the fan, or the greater the distance between the first and last active detector sensor positions  50 ,  54 , the greater the number of image pixels over which any given object feature is smeared out, and the more pronounced the defocusing becomes. As a result, a narrow fan leads to a greater radial depth of focus. As will be explained below, this phenomenon can be used to advantage.  
         [0033]     In accordance with the present embodiment of the invention, the output from each detector sensor  38  at each readout interval, corresponding to successive positions of the detector  30 ,  30 ′,  30 ″, etc., is recorded separately. The sensors are not integrated across the detector array by indexing the CCD cell contents backward one sensor at a time as the detector  30  is moved forward. The summing of selected sensors from selected readout intervals is done in software after the scanning is completed. It is therefore possible to repeat the summing process as often as desired.  
         [0034]     By repeating the summing process selecting a different set of readout intervals, a different point in the patient&#39;s head is brought into focus. By selecting systematically staggered sets of readout intervals, a series of points forming a curve within the patient&#39;s head, for example, a series of points lying on an estimate of the contour  42  of the patient&#39;s dental arch, can be brought into focus. By repeating the same set of sums for each horizontal row of detector sensors, a vertical surface following the curve can be brought into focus, creating a panoramic image of the patient&#39;s dental arch.  
         [0035]     The depth of the region around the selected curve that is in effectively sharp focus is known as the “focal layer thickness” or “focal trough.” For dental work, where the panoramic view is essentially an elevation view of the dental arch, or a large part thereof, as seen from the inside or the outside, a view with fine detail in the vertical and circumferential directions, but considerable focal trough depth in the radial direction, perpendicular to the plane of the image, is frequently desirable. Data from object features outside the effective focal trough are “smeared” out over different image pixels to such an extent that they contribute little to the final image. The wider the strip of sensors  38  used to sum the image points is, the more rapidly the data will cease to be related and become smeared as the distance from the selected image point at the center of the focal trough increases, and thus the smaller is the depth of focus of the final image.  
         [0036]     In step  106 , the resulting panoramic image is displayed to a user on the monitor  40 .  
         [0037]     In describing step  104 , the system for choosing the curve  42  on which the selected points of focus lie was not explained. The initial curve  42  may be arbitrary. Typically, the initial curve is a conventional estimate of the contour of a patient&#39;s dental arch. However, different patients have dental arches of different shapes and sizes and, even using the headrest  36 , different patients may not be positioned in exactly the same position in the scanner  22 . When the panoramic image is inspected in step  106 , it may become apparent to the user that the curve  42  on which the panoramic image is based does not entirely agree with the actual contour of the patient&#39;s dental arch. If the disagreement is greater than half the thickness of the focal trough, the image does not bring the whole jaw into sharp focus.  
         [0038]     Referring now also to FIGS.  3  to  5 , in step  108  an image  202  of the shape of the initially selected curve  42  in plan view is displayed on the monitor  40 . The monitor  40  also displays a control curve  204  that can be moved by the user. In FIGS.  3  to  5 , the initial curve  202  is shown in a continuous line, and the control curve  204  is shown in a broken line. In a practical embodiment, the curves  202 ,  204  may be shown on the monitor  40  as, for example, lines of different colors or different weights.  
         [0039]     In step  110 , the user may be allowed to move the control curve  204  backwards and forwards relative to the general orientation of the jaw, as shown in  FIG. 3 , or sideways, as shown in  FIG. 4 . The user may be allowed to make the control curve  204  wider or narrower, as shown in  FIG. 5 . The user may be allowed to rotate the control curve  204 . The adjustments may be made by using cursor or other keys on a keyboard, or by dragging the control curve  204  on screen, or by dragging handles attached to the control curve on screen.  
         [0040]     Especially when dragging the control curve  204  on screen, more elaborate changes of position are possible. For example, the user may be permitted to lock down parts of the control curve  204  that are correctly focused, while moving parts of the control curve that are not yet correctly focused, or may be able to drag the curve into a different shape.  
         [0041]     The process then returns to step  104 , and computes a new panoramic image from the original recorded data from the detector  30 , using the control curve  204 , instead of the original curve  42 ,  202 , to select the focus points. Steps  104  through  110  may be repeated as often as necessary or desirable. For example, the user may continue to adjust the control curve  204  until the user decides that he or she has achieved an adequately sharp image, or the sharpest practical image, of the dental arch in the panoramic display in step  106 .  
         [0042]     Where the computer  24  has sufficient processing power, the panoramic image may be updated in real time, or substantially in real time, as the user adjusts the control curve  204 . The panoramic image and the control curve  204  may then be displayed side by side on the monitor  40 , allowing the user to see the effect of adjustments to the control curve as the user adjusts the control curve.  
         [0043]     The focal trough depth can be increased by shortening the row of sensors  38  from which the outputs are summed in step  104 . However, reducing the number of sensors summed typically reduces the overall quality of the image, and is usually advantageous only when a deep focal trough is desired in a special case to supplement and/or augment the diagnostic information.  
         [0044]     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.