Abstract:
For dental and facial imaging, a source of x-rays ( 14 ) or other penetrating radiation and a detector ( 20 ) are mounted opposite one another on a rotatable gantry ( 28 ), so that the head of the patient can be positioned between the source ( 14 ) and the detector ( 20 ), with the axis of rotation ( 36 ) of the gantry passing through the patient&#39;s head. The detector or the source are mounted so they can translate and/or pivot horizontally or vertically. The gantry is angulated so that the source or the detector may not be at the same height relative to the patient&#39;s head. The gantry can telescope, moving the source and the detector closer together or further apart. The collimator changes dynamically with the motion of the gantry and/or the source and detector to scan a smaller portion of the scan field.

Description:
RELATED APPLICATIONS AND PRIORITY CLAIM 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/030,222, filed on Feb. 20, 2008, the contents of which are incorporated herein by reference. 
       STATEMENT OF INCORPORATION BY REFERENCE 
       [0002]    This application incorporates by reference PCT application Ser. No. PCT/US08/51922 filed on Jan. 24, 2008 that claims priority to the U.S. provisional application 60/897,421 filed Jan. 24, 2007. This application also incorporates by reference the same US provisional application 60/897,421 filed Jan. 24, 2007. 
     
    
     TECHNICAL FIELD 
       [0003]    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 
       [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 in CT imaging with high resolution is most of the extent of the skull in the horizontal plane, 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. 
         [0007]    To improve the situation just described, the device described and illustrated in PCT application Ser. No. PCT/US08/51922 (which was filed on Jan. 24, 2008 claiming priority to U.S. Provisional Application 60/897,421 filed Jan. 24, 2007) was developed. Both of the application mentioned in the previous sentence are assigned to the owner of the current application, and incorporated by reference herein. Those applications disclose a device that rotates the detector (also known as a receptor) from a landscape to a portrait orientation, and vice versa. This rotation provides increased field of view when used in certain procedures. However, even with the rotation from landscape to portrait and vice versa, there are other field of view issues that can be addressed to provide better x-ray tomographic images and processes. 
       SUMMARY 
       [0008]    For dental and facial imaging, a source of x-rays or other penetrating radiation and a detector are mounted opposite one another on a rotatable gantry, so that the head of the patient can be positioned between the source and the detector, with the axis of rotation of the gantry passing through the patient&#39;s head. The detector or the source are mounted so they can translate and/or pivot horizontally or vertically. The gantry is angulated so that the source or the detector may not be at the same height relative to the patient&#39;s head. The gantry can telescope, moving the source and the detector closer together or further apart. The collimator changes dynamically with the motion of the gantry and/or the source and detector to scan a smaller portion of the scan field. 
         [0009]    The invention also provides data produced by the methods and systems of the invention 
         [0010]    In one embodiment, the invention provides an apparatus for dental and facial imaging. The apparatus includes a controller; a rotatable gantry; and a source of penetrating radiation mounted on the gantry. The source of penetrating radiation includes a beam limiter with a plurality of doors. The apparatus also includes a detector of penetrating radiation mounted opposite the source on the gantry, so that the head of a patient can be positioned 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 translatably on the gantry between a first position and a second position and includes a first stop positioned at the first position and a second stop positioned at the second position. Movement of the detector between the first position and the second position is controlled so that the movement of the detector is slowed in a first direction before the detector reaches the first stop and movement of the detector is slowed in a second direction before the detector reaches the second stop. The controller is configured to coordinate movement of the rotatable gantry, the plurality of doors of the beam limiter, and movement of the detector. 
         [0011]    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 
         [0012]    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: 
           [0013]      FIG. 1  is a schematic view of apparatus for generating a tomographic image. 
           [0014]      FIG. 2  is a perspective view of part of a gantry of the apparatus of  FIG. 1 , with a cover removed to show a mechanism to rotate the detector panel. 
           [0015]      FIG. 3  is a different perspective view of the same part of a gantry as in  FIG. 2 . 
           [0016]      FIG. 4A  is an elevational view of the outside of the detector end of  FIG. 2  with the cover and a cable guard removed, where the detector is in a landscape orientation. 
           [0017]      FIG. 4B  is an elevational view of the outside of the detector end of  FIG. 2  with the cover and a cable guard removed, where the detector is in a portrait orientation. 
           [0018]      FIG. 5A  is an elevational view of the interior side of the detector end of  FIG. 2  in a landscape orientation. 
           [0019]      FIG. 5B  is an elevational view of the interior side of the detector end of  FIG. 2  in a portrait orientation. 
           [0020]      FIG. 6  is a detail view of a portion of the embodiment of  FIG. 2 . 
           [0021]      FIG. 7  is an embodiment, having a detector that rotates about an axis as shown and provides for tilt of the detector around its u axis. 
           [0022]      FIG. 8  is an embodiment having a source and a detector on the gantry in an angled manner, and at different heights relative to the object being scanned. 
           [0023]      FIG. 9  illustrates u, v, and w axes used to describe motion of a detector panel and source in a rotatable gantry imaging system. 
           [0024]      FIG. 10  illustrates an embodiment in which the gantry telescopes. 
           [0025]      FIG. 11  illustrates an embodiment in which the detector array translates. 
           [0026]      FIG. 12  illustrates components of the detector which are used to translate the detector array. 
           [0027]      FIG. 13  is an exploded view of the detector illustrating components that are used to translate the detector array. 
           [0028]      FIG. 14  is a partially exploded view of an x-ray source illustrating components of a beam limiter (or components that are used to collimate x-rays). 
           [0029]      FIG. 15  is a perspective view of a housing of the x-ray source shown in  FIG. 14  illustrating a panel that separates a chamber in which an x-ray source is located and a beam limiter or collimator is located. 
           [0030]      FIG. 16  is a partially exploded view of a collimator illustrating a number of leaves used in a door of the collimator. 
           [0031]      FIG. 17  is a perspective view of the components used to collimate x-rays. 
           [0032]      FIG. 18  is a partially cut-away view of doors in the collimator of the prior figures. 
           [0033]      FIG. 19A  is an exploded view of a portion of the beam limiter in  FIG. 14 . 
           [0034]      FIG. 19B  is another exploded view of the portion of the beam limiter in  FIG. 19A . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
         [0036]    Referring to the drawings,  FIG. 1  shows a tomographic apparatus  10  according to an embodiment of the invention, which includes a scanner  12  and a computer  13  controlled by a console  16  with a display  18 . The scanner  12  includes a source of x-rays  14 , an x-ray detector  20  including a movable housing  21  (e.g., a plastic housing) supporting a rectangular sensor array  22 . The scanner  12  also includes 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 or chair 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  includes a panel having a rectangular array of x-ray sensitive elements. The x-ray detector  20  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. 
         [0037]    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. 
         [0038]    A first embodiment of the invention is now described referring to  FIGS. 1-6 . The x-ray detector  20  is supported by a cylindrical roller bearing  30  on a mounting panel  32  (shown in  FIG. 2  and discussed in documents incorporated by reference) attached to the gantry  28 . Not shown are electrical cables and a guard that protects the cables from the moving parts. The x-ray source  14  is arranged to emit a beam of x-rays along an axis (the “x-ray axis”  33 ) 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 . 
         [0039]    The detector axis  38  ( FIGS. 3 ,  5 A, and  5 B) is offset horizontally from the x-ray axis  33 , 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 short edge  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 sensor array  22 . 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 a bottom short edge  42  in portrait orientation is on the same line as the bottom long edge  40  in landscape orientation. 
         [0040]    An additional embodiment also contemplated, but not shown, is as follows. The detector axis  38  is offset horizontally from the x-ray axis  33 , and is positioned equal distances from the top edge  44 , bottom edge  40 , and short edge  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. 
         [0041]    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 of the x-ray detector  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. 
         [0042]    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. 
         [0043]    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  55  that travel along two fixed rods  56  that resist the rotational forces of the motor. The grab block  52  is clamped to a belt  54 . 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  54  moves clockwise and counterclockwise to rotate the journal  58  and, thereby, the x-ray detector  20 . 
         [0044]    Attached to the journal  58  is an arm  60  carrying a head  62 . As the journal  58  rotates, the head  62  moves along 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 . 
         [0045]    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 protrusions or lobes  78   a  and  78   b  that act as actuators 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  for adjusting the speed of the motor. The grab block  52  continues to move at a speed dependent on the position of the journal  58  and comes to a stop when the contact  70  in the head  62  comes into contact with the contact  68 . A spring  82  is pivotally connected to the mounting panel  32  and the journal  58  to assist the motor and help prevent stalling of the motor during operation thereof. 
         [0046]    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 lobes  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 . 
         [0047]    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 may be different for different embodiments of the invention. The computer  13  may be 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. 
         [0048]    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. 
         [0049]    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. 
         [0050]    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. 
         [0051]    In addition to changing the orientation of the sensor array  22  from a landscape orientation ( FIG. 5A ) to a portrait orientation ( FIG. 5B ) it is possible to change orientation of the sensor array  22  with respect to the x-ray axis such that the x-ray axis intersects the front surface of the sensor array at a non-right angle. As noted above, the x-ray axis  33  intersects the gantry axis  36  perpendicularly, and normally, the sensor array is aligned so that x-ray axis  33  intersects the front surface of the sensor array  22  perpendicularly. In the embodiment shown in  FIG. 7 , the sensor array  22  is pivotally mounted to the x-ray detector  20  so that it can be moved along an arc  91  such that the x-ray axis  33  intersects the sensor array  22  at an angle  92  that is less than 90°. Changing the orientation of the sensor array  22  changes the focus of the x-rays and, therefore, the particular location of the subject that is imaged by the apparatus  10 .  FIG. 10  illustrates an embodiment where the gantry  28  has a telescoping arm  100 . The telescoping arm  100  is movable along a linear path LP between a first position P 1  and a second position P 2 . By changing the position of the arm  100  along the path LP the focus of the x-rays emitted from the source  14  may be changed. As the gantry expands or contracts in length, the relative positions of the source, the object, and the detector are varied. This results in variations in scan quality, field of view, and resolution. 
         [0052]    Specifically, when the distance between the detector and patient/object is increased, there is greater magnification, and greater resolution. However, there is a narrower field of view and the resolution gain will at some point be offset by blurring in the edges of the images as a result of focal spot effects, which are caused by the fact that the x-ray source is not a point source, but rather has a finite diameter such as 1 millimeter. Decreasing the distance between the detector and patient/object increases field of view, and reduces focal spot effects thereby improving the sharpness of edges. However, these features come at the expense of reduced magnification, and reduced resolution. A telescoping feature of the gantry permits selection of a desired combination of magnification, field of view, and control of focal spot effects for a given application. 
         [0053]    An additional embodiment is shown in  FIG. 8 , in which the source of x-rays  14  and the x-ray detector  20  are pivotable such that the angle at which the x-ray-axis  33  intersects a patient or subject is variable. The embodiment shown in  FIG. 8  permits imaging of a subject from different angles without requiring the subject to move. This variability may lead to better or enhanced images of particular locations of interest on the subject. For example, certain defects or abnormalities might be visible or detectable only when imaged from a particular orientation or point of view. The enhanced images may facilitate diagnosis and treatment. 
         [0054]    Other additional alternative embodiments for movement of the detector panel or source in a rotatable gantry CT imaging system, that may substitute or complement the rotational movement described above are discussed below. In the description provided, reference is made to three directional axes. These axes apply to both of the source and the detector. They are the u, v, and w axes and are shown in the  FIG. 9 . X, y, and z axes labels are intentionally not used, because they may be confused with the x, y, and z directions of the scan field volume data. 
         [0055]    Initially, some discussion of the factors involved in scanning is required. The “ray point” is defined as the location on the detector panel at which a perpendicular ray leaving the point source hits the detector panel. The location of this ray point to the u axis of the detector is a strong parameter, that is, the quality of reconstruction is a strong function of the relative positioning of the ray point on the u axis. To the v axis, the ray point location is an important parameter but is less strong of a parameter as the location to the u axis. These two ray point location parameters are tied to the registration and calibration of the translating detector. Each of the embodiments presented herein provides for registration and calibration of the detector in a manner that can improve the parameters affecting image quality. 
         [0056]    In a first alternative embodiment, detailed in  FIGS. 11 through 18  herein, the detector translates (moves) along the u axis as illustrated in  FIG. 9 . This linear shift occurs without any movement to the source and center of rotation. This introduces a large amount of shift (in the u direction) to the vertical rays incident on the detector, and typically would be used when the machine operates in half-beam mode. The result is a larger field of view without increasing detector size. A similar result may be obtained by rotating the detector from a portrait to a landscape mode as described above and in PCT application Ser. No. PCT/US08/51922 filed on Jan. 24, 2008. The difference with the present embodiment is that it shifts the panel rather than rotates it. The same registration is required as the case of the rotating detector since there needs to be precise relationship between the location of the detector and the gantry. 
         [0057]    In the illustrated embodiment, the x-ray detector  20  includes detector panel or housing  110  and a detector support (or guide plate)  112  having three guide wheels  114 ,  116 , and  118  that run along two guide rails  120  and  122 . The detector support  112  has a drive connector  124  connecting it to a drive shaft or a drive screw  126  and a stepper motor  128 . Powering the motor in one direction or the other translates the detector support  112  and thereby the sensor array  22 . It is important that the sensor array be precisely registered. There are rigid stops  130  and  131  ( FIG. 14 ) with hardened contacts mounted on the on the housing  110 . On the detector support  112  there are also rigid stops  133  and  135  with hardened contact surfaces. In alternate constructions, the stops may be adjusted by threading a backing screw into and out of the rigid stops and then locking them in place. To maintain precision it is important that the contacts do not wear or deform. To this end, the housing has two limit switches  137  and  139  and the detector support  112  has two trigger surfaces  142  and  143  for those limit switches. When the detector support contacts a limit switch, a set number of steps is sent to the motor to drive rigid stops  133  and  135  to a position near or adjacent to rigid stops  130  and  131 . However, in alternate constructions is may be desirable to drive the motor at variable speeds. For example, it may be desirable that the detector translates quickly such that most of the translation may be done quickly, but a slowed and lower power speed is instituted to the motor prior to contact between the rigid stops to lessen their impact. 
         [0058]    In a second alternative embodiment, the detector translates (moves) along the v axis. This may be accomplished by configuring the detector panel translating components, described previously, in positions rotated approximately 90 degrees. While translating the detector panel vertically may not increase field of view, it allows shaping of volume coverage. For example, the top of the scan area naturally exhibits a cone shape simply because of the direction of the cones. By shifting the detector in the v-direction this shape may be modified to increase/reduce the amount of cone or in some cases invert the cone. 
         [0059]    Although the figures show an embodiment in which the detector translates on the detector pod, while the connection between the horizontal beam member of the gantry and the detector pod remains fixed, it would also be possible to make the pod movable relative to the base in translation and/or rotation. The following discussion relates to other such embodiments. 
         [0060]    In a third and fourth alternative embodiments implementing principles of the present invention, the detector may be rotated about its u or v axis respectively. Rotation about the w axis is generally disclosed in the above-referenced patent application. Rotation about the u or v axes may increase the effective resolution and/or achieve a higher resolved volumetric scan. 
         [0061]    In a fifth and sixth alternative embodiment, translation along axis u or v may be done to the source, as was done in alternate embodiments 1 and 2, respectively, for the detector. 
         [0062]    In a seventh and eighth alternative embodiment, the same rotation about axis u or v may be done to the source, as was done in alternate embodiments 3 and 4, respectively, for the detector. Rotation of the source about the w axis (the axis through which x-ray is delivered from the source) is unlikely to have great effect on the image for the reason that the source is (nearly) a point source. 
         [0063]    In a ninth alternative embodiment, the gantry may be angulated (refer to  FIG. 8  as an example) so that the source and detector may be rotated along its u axis. An example of an application of this concept is to place the x-ray source  14  in a relatively low position, preferably below the shoulders of the subject, and point the source  14  upward. This may be accomplished in a number of ways including mounting the source on tracks or guides such that the source  14  is moveable along a vertical axis with respect to the end of the gantry  28 . When the source  14  is lower, the detector is, in relative terms, higher up. An angulated orientation may be static, or may be selected dynamically for a particular scan operation, using fixtures in the gantry that orients it in a controllable manner. As noted above, the embodiment shown in  FIG. 8  permits imaging of a subject from different angles without requiring the subject to move. This variability may lead to better or enhanced images of particular locations of interest. 
         [0064]    In a tenth alternative embodiment, dynamic beam limitation (sometimes referred to as dynamic collimation) of the x-ray to targets only one specific region of the scan field. In this embodiment, the machine rotates around the scan field in the same manner as if the whole field of view is considered. However, a beam limiter is used to trim the x-ray beam dynamically, so that the x-ray field covers only that volume that is of interest. 
         [0065]    In this tenth alternative embodiment, in some cases, the beam limiter may need to create a moving field in order to maintain illumination of the volume of interest as the gantry rotates. This requires the use of a dynamic beam limiter, programmably controlled to change the illuminated area as the machine rotates around the scan field. 
         [0066]    One embodiment of the beam limiter is illustrated in the attached  FIGS. 14 through 19 . A beam limiter  200  is shown in  FIG. 14 . The beam limiter  200  is part of the x-ray source  14  and is attached to a plate  202  located within a housing  204 . The embodiment of the beam limiter  200  shown includes a scatter shield structure  206  having scatter shield support plate  208  and a lead scatter shield  210 , and two shutter structures  212  and  214 . In the illustrated construction, shutter structures  212  and  214  include the same components, thus only one will be described for brevity. For example, shutter structure  212  includes a mounting plate  216  supporting two doors or shutters  218  and  220 , each door  218  and  220  being coupled to a corresponding carrier plate  222  and  224  for movement with respect to one another. The carrier plates  222  and  224  are each operated by motors  226  and  228 , each connected to a corresponding optical sensor  230  and  232 . The optical sensors  230  and  232  generate signals related to the position of the carrier plates  222  and  224  with respect to the sensors  230  and  232  to determine the relative position of the doors  218  and  220  and to move the doors  218  and  220  with respect to one another. 
         [0067]    The doors  218  and  220  of both shutter structures  212  and  214  are used to create a window at a controllable location for x-ray to be emitted. In the illustrated construction, the doors  218  and  220  of shutter structure  214  move horizontally in the u direction, and the doors  218  and  220  of shutter structure  212  move vertically in the v direction. Either set of doors can meet to block the source from illuminating the object, or be opened at a desired location. By having one set of doors open and the other partially closed a vertical or horizontal slit-like aperture is created. If the second set of doors is also brought to a partially closed position the length of the slit can be decreased. In this way the slit can be made into a rectangular or square aperture. By controlling the relative positions of the four doors, the rectangle or square can be dynamically moved to a number of different positions in front of the x-ray source  14 . By coordinating the movements of the four doors ( 218  and  220  of both shutter structures  212  and  214 ) with the movements of the gantry  28  and/or detector  20 , a specific part of the object being scanned can be targeted. For example, one tooth area of a jaw. Among reasons for doing this are to limit the x-ray dose to the patient, to limit the amount of x-ray scatter that produces image problems such as a halo, and to decrease the overall amount of data reaching a detector so that processing may be performed more quickly. 
         [0068]    The embodiment of the beam limiter  200  described above uses a scatter shield and doors resulting in rectangular of square apertures. However, other geometries are contemplated that could result in apertures of other shapes such as a circular shape. 
         [0069]    In other embodiments, the rotation of the detector  20  from portrait to landscape as described by PCT application Ser. No. PCT/US08/51922 filed on Jan. 24, 2008, may be combined with the translations, rotations, and angulations described around the u, v, and w axes of this application. One advantage of such combinations would be to improve speed by reading out only part of the panel. By changing whether the panel is in portrait or landscape mode while it acquires its data, the data can be captured by the portion of the panel that is more easily and thus more quickly read. This can speed up the process which is a desirable feature in all CT scanning applications. 
         [0070]    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 the 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. 
         [0071]    Various combinations, 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 combinations, modifications and variations of the embodiments described provided they come within the scope of the appended claims and their equivalents. 
         [0072]    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. 
         [0073]    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 an 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. 
         [0074]      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.