Abstract:
An apparatus for dental and facial imaging including a rotatable gantry having an axis of rotation generally in the direction of local gravitational vertical; a source of penetrating radiation mounted on the gantry; an elongated detector for detecting radiation and producing image data, the detector mounted opposite the source on the gantry, and having a length, a long axis, and a short axis The gantry, source, and detector configured to receive the head of a patient between the source and the detector, with the axis of rotation of the gantry passing through the patient&#39;s head. The detector is mounted rotatably to the gantry and movable between a first position where the long axis of the detector is generally perpendicular to the axis of rotation of the gantry and a second position where the long axis of the detector is generally parallel to the axis of rotation of the gantry.

Description:
RELATED APPLICATIONS AND PRIORITY CLAIM 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/524,403, filed on Mar. 1, 2010, which is a U.S. 371 National Phase filing of PCT application Serial No. PCT/US08/51922 filed on Jan. 24, 2008, which claims priority to the U.S. provisional application No. 60/897,421 filed Jan. 24, 2007, the contents of all of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to the examination of objects with penetrating radiation, and especially to a scanner with a detector that can be positioned in different orientations to generate different images. 
       BACKGROUND 
       [0003]    The viewing of objects, including parts of the human anatomy, by the use of x-rays and other forms of penetrating radiation is known. In the case of x-rays, the radiation is directed at the object from one side, and the part of the radiation that penetrates the object is detected at the opposite side. An image may thus be obtained in which parts of the object that are more absorbent of x-rays, typically more dense parts of the object, appear as darker shadows, for example, by detecting the x-rays electronically and generating an image using a computer. 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, or may be developed hereafter. From the three-dimensional data, a tomogram, which is a section in a desired plane, may be generated. 
         [0004]    In a typical dental CT system, the patient sits upright, and the x-ray source and detector are mounted on opposite ends of a gantry that rotates about a vertical axis through the middle of the patient&#39;s head. In order to obtain sufficient data to cover the desired part of the head, which is most of the extent of the skull in the horizontal plane in CT imaging with high resolution, the detector must span a substantial distance in the circumferential direction perpendicular to the axis of rotation of the gantry, that is to say, the horizontal, direction. 
         [0005]    Electronic detectors currently available include flat panel arrays of charge-coupled device (CCD) or other detectors, each of which converts incoming x-rays over a defined pixel area in a defined time to an electric charge that can easily be converted to a digital intensity value for subsequent computation. One flat panel detector commercially available from Varian Medical Systems, Inc., of Salt Lake City, Utah that is suitable for use in dental CT units has a pixel size of 127 μm (micrometers) square, and has an overall panel size of approximately 25 cm×20 cm. In dental imaging with a cone beam, because of the divergence of the beam towards the panel, that panel provides an effective Field of View approximately 16 cm×13 cm. When mounted with the long axis horizontal, the 25 cm length of the panel thus allows a Field of View with a diameter of approximately 16 cm, which is large enough to permit sufficient coverage of the imaged structures in the axial (horizontal) direction with high resolution for typical dental uses. 
         [0006]    However, for most normal adults, the 20 cm height of the panel allows imaging only from the bottom of the lower jaw to about the bottoms of the orbits of the eyes (about 13 cm effective height at the level of the object being viewed). That is sufficient for most dental and oral surgery applications, but for some classes of orthodontic and orthognathic surgery applications an x-ray image up to the level of the glabella, roughly the level of the eyebrows, is essential. Such images, known as “full face” have in the past been produced by conducting two overlapping scans of 13 cm height at different levels and merging the images. Conducting two scans increases the radiation dose to the patient. Merging the images seamlessly is difficult, especially as the time taken to reposition the gantry, or the patient, between the two scans allows the patient to move. It would be possible to use a 25 cm square detector panel, which would have both the width to produce full coverage CT scans of the mouth diametrally, and the height to produce full-face scans in a single scan for about 98% of human adults, but the cost of detector panels increases disproportionately to the size of the panel, and could not easily be justified, when the full height is seldom required. 
       SUMMARY 
       [0007]    According to one embodiment, the invention provides an apparatus for dental and facial imaging. The apparatus includes a rotatable gantry having an axis of rotation generally in the direction of local gravitational vertical, the gantry configured to be rotated during an imaging procedure. The apparatus also include a source of penetrating radiation mounted on the gantry and an elongated detector for detecting radiation from the source of penetrating radiation and producing image data. The detector is mounted opposite the source on the gantry, and has a length, a long axis, and a short axis. The rotatable gantry, source, and detector are configured to receive the head of a patient between the source and the detector when the patient is in an upright position, with the axis of rotation of the gantry passing through the patient&#39;s head. The detector is mounted rotatably to the gantry and movable between a first position in which the long axis of the detector is generally perpendicular to the axis of rotation of the gantry and a second position in which the long axis of the detector is generally parallel to the axis of rotation of the gantry. 
         [0008]    According to another embodiment, the invention provides a method for dental and facial imaging using an apparatus comprising a computing device, a rotatable gantry having an axis of rotation generally in the direction of local gravitational vertical, a penetrating radiation source mounted on the gantry, and an elongated detector mounted opposite the source on the gantry and having a length, a long axis, and a short axis. The method includes rotating the detector between a first position in which the long axis of the detector is generally perpendicular to the axis of rotation of the gantry and a second position in which the long axis of the detector is generally parallel to the axis of rotation of the gantry. The method further includes receiving the patient&#39;s head between the source and the detector when the patient is in an upright position, with the axis of rotation of the gantry passing through the patient&#39;s head; rotating the gantry about the axis of rotation of the gantry; generating penetrating radiation by the radiation source during the step of rotating the gantry; and detecting the penetrating radiation by the elongated detector to produce image data. 
         [0009]    The “vertical” direction is defined relative to the orientation of the patient&#39;s head when the patient is standing or sitting upright. Although for ease of construction and operation this direction and the direction of the axis of rotation of the gantry are substantially aligned and substantially the same as local gravitational vertical, consideration of the geometry and mechanics of the system, and the motility and flexibility of human beings, shows that exact alignment is not necessary. In addition, some dental x-ray machines operate with the patient lying down, and the “vertical” direction of the patient&#39;s head is then more nearly horizontal relative to gravity. 
         [0010]    In one disclosed embodiment, the detector panel is aligned so that with the long axis perpendicular to the gantry rotation axis (landscape orientation) the center of the panel is on the center axis of the beam of radiation from the source, which center axis intersects the axis of rotation of the gantry. 
         [0011]    In a disclosed embodiment the detector panel is mounted so that one bottom corner is in the same position in both the landscape orientation and the orientation with the long axis parallel to the gantry rotation axis (portrait orientation). Using this feature, or in other embodiments, in the portrait orientation the panel may extend more above the center of the beam, and less to the other side of the beam, than in the landscape orientation. Where the landscape orientation is optimized for imaging the mouth, the portrait orientation can then be optimized for full-face imaging at the same relative position of the patient&#39;s head and the scanner gantry. Where the landscape orientation is optimized for high resolution, the portrait orientation, extending as far from the axis as the landscape orientation on one side but less far on the other side, can provide a good compromise to maintain acceptable resolution with the narrower panel. 
         [0012]    The invention also provides computer software arranged to generate 3-D tomographic and other datasets and tomographic and other images 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. 
         [0013]    The computer software may be responsive to a signal from a switch that detects the position of the sensor panel. 
         [0014]    The invention also provides data produced by the methods and systems of the invention. 
         [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 perspective view of part of a gantry of the apparatus of  FIG. 1 , with a cover removed. 
           [0020]      FIG. 3  is a different perspective view of the same part of a gantry as in  FIG. 2 . 
           [0021]      FIG. 4A  is an elevational view of the outside of the detector end with the cover and a cable guard removed, in landscape orientation. 
           [0022]      FIG. 4B  is an elevational view of the outside of the detector end with the cover and a cable guard removed, in portrait orientation. 
           [0023]      FIG. 5A  is an elevational view of the interior side of the detector end in landscape orientation. 
           [0024]      FIG. 5B  is an elevational view of the interior side of the detector end in portrait orientation 
           [0025]      FIG. 6  is a detail view of a portion of the previous embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
         [0027]    Referring to the drawings, and initially to  FIGS. 1 and 2 , one form of tomographic apparatus  10  according to an embodiment of the invention, comprises a scanner  12  and a computer  13  controlled by a console  16  with a display  18 . The scanner  12  comprises a source of x-rays  14 , an x-ray detector  20  including a rectangular sensor array  22 , and a support  24  for an object to be imaged. In an embodiment, the scanner  12  is arranged to image the head, or part of the head, of a human patient (not shown), especially the jaws and teeth. The support  24  may then be a seat with a rest or restrainer  26  for the head or face (not shown) of the patient. The x-ray source  14  and x-ray detector  20  are then mounted on a rotating carrier or gantry  28  so as to circle round the position of the patient&#39;s head, while remaining aligned with one another. The x-ray detector  20  then records a stream of x-ray shadowgrams of the patient&#39;s head from different angles. The computer  13  receives the x-ray image data from the scanner  12 , and calculates a 3-dimensional spatial distribution of x-ray density. 
         [0028]    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. 
         [0029]    Referring now also to  FIG. 2  and  FIG. 3 , the x-ray detector  20  comprises a panel having a rectangular array of x-ray sensitive elements, supported by a cylindrical roller bearing  30  on a mounting panel  32  attached to the gantry  28 . Not shown are electrical cables and a guard that protects the cables from the moving parts. The electrical cables are partially retained between a guide  34  and the mounting panel  32 . The x -ray source  14  is arranged to emit a beam of x-rays with an axis (the x-ray axis, not shown) that is aligned with the center of the x-ray source  14 , and intersects perpendicularly the gantry axis of rotation  36  of the gantry  28  relative to the frame of the scanner  12 . The axis  38  of the roller bearing  30  (“the detector axis”) may be parallel to but offset from the x-ray axis of source  14 . The panel of the x-ray detector  20  is generally perpendicular to the detector axis  38 , and the detector axis passes through the panel. In the embodiments shown, when the x-ray detector  20  is in the landscape orientation with the long axis of the rectangular sensor array  22  horizontal (perpendicular to the gantry axis  36 ), the x-ray axis is aligned with the center of the rectangular sensor array  22 . 
         [0030]    The detector axis  38  is offset horizontally from the x-ray axis, and is positioned equal distances from the bottom, and one end of the rectangular sensor array  22 . For an example using numbers, in one embodiment, the panel has a rectangular sensor array  22  with an operative area 20 cm by 25 cm, and the detector axis  38  is then 10 cm from each of the bottom edge  40  and one end  42  (the left end as seen in  FIG. 2  or the right end as seen in  FIGS. 3 ,  5 A, and  5 B) of the array. Then, when the x-ray detector  20  is rotated 90° about the detector axis  38  from the landscape orientation seen in  FIG. 5A  to a portrait orientation  FIG. 5B , the detector assumes a position where the bottom short edge  42  in portrait orientation is on the same line as the bottom long edge  40  in landscape orientation. 
         [0031]    An additional embodiment also contemplated, but not shown, is as follows: The detector axis  38  is offset horizontally from the x-ray axis, and is positioned equal distances from the top edge  44 , bottom edge  40 , and one end  42  of the rectangular sensor array  22 . For an example, using the same numbers as in the embodiment shown, the panel has a rectangular sensor array  22  with an operative area 20 cm by 25 cm, and the detector axis  38  is then 10 cm from each of the top, bottom, and one end (the left end as seen in  FIG. 2 ) of the array. Then, when the x-ray detector  20  is rotated 90° about the detector axis  38  from the landscape orientation a portrait orientation, the detector assumes a position (not shown) where the left-hand long edge in portrait orientation is on the same line as the left-hand short edge in landscape orientation, and the bottom long edge in portrait orientation is on the same line as the left-hand short edge in landscape orientation. 
         [0032]    The roller bearing  30  is a large-diameter bearing, for example, 5.5 cm diameter, with minimal play and backlash. A high-resolution imaging panel  20  may have a pixel size of, for example, 127 μm. The positioning of the x-ray detector  20  should be stable, both within a scan and between scans, to within a fraction of a pixel, say, 0.1 mm (100 μm), for high-quality imaging without extra computation. A very stable bearing  30  is therefore desirable. 
         [0033]    Apart from its mounting, the x-ray detector  20  may be a commercially available flat-panel x-ray detector, such as those supplied by Varian Medical Systems, Inc., of Salt Lake City, Utah. 
         [0034]    The position of the x-ray detector  20  is controlled by components visible in  FIGS. 2 ,  4 A, and  4 B. A stepper motor  46  having an armature  48  (not shown) with internal threads extends and retracts an externally threaded shaft  50 . The shaft is connected to a grab block  52  that has guides  54  that travel along two fixed rods  56  that resist the rotational forces of the motor. The grab block  52  is clamped to a belt  55 . The belt passes over two pulleys  57 , and around a journal  58  that is attached to the roller bearing  30 . As the motor  46  extends and retracts the shaft  50 , the belt moves clockwise and counterclockwise to rotate the journal  58  and thereby the x-ray detector  20 . 
         [0035]    Attached to the journal  58  is an arm  60  carrying a head  62 . As the journal  58  rotates, the head  62  describes an arc between two end stops  64  positioned so that the journal  58  and the arm  60  can rotate exactly ninety degrees between the end stops  64 . The end stops  64  are provided with pins  66  for precise adjustment of the positions at which the head  62  is stopped by the end stops  64 . The stops have hardened contact surfaces  68  and the head has hardened contact surfaces  70  ( FIG. 6 ). The hardened surfaces are located within counterbores  72  in the head so that both hardened surfaces are along a single plane extending from the detector axis  38 . The non-contact end of the pins  66  rest against an adjustment screw  74  (not shown) within the end stop  64 . The stop is split, so that tightening of a screw  76  clamps down on the pin  66 . 
         [0036]    To maintain precision it is important that the contacts do not wear or deform. A slow, low-power rotational speed prior to contact between the arms and the stops lessens impact. To this end, the journal  58  is provided with actuators  78   a,    78   b  for a pair of limit switches  80  that detect when the journal  58  is near one of its end positions. The limit switches signal the position to the computer  13 . At an appropriate time power to the motor is stopped, and gravity completes the movement of the arm and the detector. The grab block  52  comes to a stop when power to the motor is cut-off, but the arm and detector, driven by gravity, cause journal  58  to slip within the belt  54 , until the contact  70  in the head  62  comes into contact with the contact  68 . To further decrease the speed and force of rotation, a spring  82  is pivotally connected to the mounting panel  32  and the arm  60  to carry a portion of the weight. 
         [0037]    The control of the position described above is just one way contemplated for low-speed contact. Alternatively, the movement of the journal  58  may be tracked in another way. For example, the actuators  78   a,    78   b  and limit switches  80  may have secondary contacts arranged to signal to the computer  13  when the head  62  is a short distance from one of the end stops  64 . 
         [0038]    In operation, the pins  66  are set so that the end positions of the x-ray detector  20  are landscape and portrait positions with the rectangular array aligned with the gantry axis  36 . The alignment may be precise to within a fraction of the size of a pixel over the length of the array, although the amount of precision required maybe different for different embodiments of the invention. The computer  13  maybe programmed to rotate the journal  58 , and thus the x-ray detector  20 , at a moderate speed, taking several seconds for a ninety degree rotation. Then, just before the head  62  reaches the end stop  64  at the far end of the rotation, the motor  46  is braked, and the head  62  closes gently against the pin  66 , so that the x -ray detector  20  is accurately positioned without an impact that might damage any part of the system. In particular, it is desirable to avoid, as far as practical, deformation of the head  62  or the pins  66  that might result in the end position of the x-ray detector  20  drifting. 
         [0039]    In use, the scanner  12  may be used with the x-ray detector  20  in landscape orientation for a computed tomography scan of the mouth region. A rectangular sensor array  22  that is 25 cm wide allows the detection of x-rays sufficiently far from the axis to allow for computed tomography of a quality sufficient for almost all dental and oral surgery. A rectangular sensor array  22  that is 20 cm high allows most human heads to be imaged over a region extending from just below the lower jaw to about the bottom of the eye-socket. If only a region of lesser height needs to be imaged for a specific purpose, the height of the x-ray beam can be reduced by an adjustable collimator to reduce the x-ray dose to the patient. A collimator with four independently controllable jaws is suitable, and in some jurisdictions is required, in order to collimate the x-ray beam to the changes in the position of the detector panel. Such collimators are well-known in the art and, in the interests of conciseness, are not further described here. 
         [0040]    With the x-ray detector  20  in portrait orientation, a rectangular sensor array  22  that is 25 cm high allows most human heads to be imaged over a region extending from just below the lower jaw to about the level of the glabella, roughly eyebrow level. The extra height is required for certain types of orthodontic and orthognathic surgery, and a 25 cm high rectangular sensor array  22  is then sufficient for about 98% of human adults. The positioning of the detector axis  38  relative to the x-ray detector  20  results in the bottom of the rectangular sensor array  22  of the detector panel being at the same level in both portrait and landscape orientations, so that the positioning of the patient in the scanner  12  is identical in both portrait and landscape scanning, which reduces the risk that a scan has to be repeated because the patient was incorrectly positioned. 
         [0041]    With a 20 cm by 25 cm rectangular sensor array  22  in portrait orientation, there is some loss of image quality, because of the reduced width of the detector. However, the positioning of the detector axis  38  relative to the x-ray detector  20  may be chosen so that one side of the rectangular sensor array  22  is the full 12.5 cm from the x-ray axis, with the other side of the array only 7.5 cm from the x-ray axis. The 12.5 cm extent on one side of the axis gives better coverage diametrally in computed tomography imaging than a panel extending to 10 cm on both sides of the x-ray axis, by using a known reconstruction method commonly referred to as “half beam mode.” Although the image quality is less than with a 25 cm wide array, it has been found to be sufficient for most forms of orthodontic and orthognathic surgery. Where a high resolution image of the actual teeth is required, for example, for some forms of oral diagnosis or surgery, the 25 cm high full-face scan can be combined with a second scan, using the x-ray detector  20  in 25 cm wide landscape orientation, but with the x-ray beam collimated to only 6 or 8 cm high. Such a double scan can be conducted with less x-ray dosage to the patient than would result from a full-face scan using two overlapping scans from a scanner with a fixed detector panel 25 cm wide by 20 cm high. 
         [0042]    Certain commercially available detector panels suitable for use as the x-ray detector  20  are provided with built-in electronics for a high-resolution panoramic imaging mode, in which the x-ray beam is collimated to a narrow vertical slit, and only the detector pixels in the corresponding part of the detector array are read out. That mode greatly speeds the readout process, by reducing the number of pixels read. However, the available panels support that panoramic slit mode only in landscape orientation. High resolution full-face panoramic imaging is not usually needed, but the rotatable x-ray detector  20  of the present device allows switching between operating modes including a 25 cm high full-face scan and the panoramic slit mode where a fixed detector panel, in either orientation, could offer only one of those modes. 
         [0043]    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. 
         [0044]    As an example, a detector panel with an array of sensors 20 cm by 25 cm has been used. That is only an example, and detector panels of other sizes may be used. 
         [0045]    As an example, a x-ray detector  20  positioned with the axis of the bearing  30  equidistant from two edges of the panel, so that the bottom edge and one side edge are at the same positions relative to the gantry in the landscape and portrait modes, has been described. Also described as an example is a x-ray detector  20  positioned with the axis of the bearing  30  equidistant from three edges of the panel, so that the bottom edge and one side edge are in the same positions relative to the gantry in the landscape and portrait modes. Certain reasons for those arrangements have been identified. However, other positions of the x-ray detector  20  relative to the axis of the bearing  30  are possible, and may be desirable for certain purposes or in certain scanner configurations. 
         [0046]      FIG. 1  shows that the computer  13  on which the image data are processed and analyzed is connected to the scanner  12 . A single computer  13  may both control the scanner  12  and process the data. Alternatively, part or all of the processing may be carried out on a separate computer. The data from the scanner  12  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. The detailed control of the motor  46  and the movement of the x-ray detector  20  may be controlled by a dedicated logic controller in the scanner  12 , with the computer  13  or other external controller merely issuing a command to adopt a specified one of the portrait and landscape orientations, and receiving a signal confirming that the x-ray detector  20  is in a specific orientation.