Abstract:
An apparatus for cone beam computed tomography of lower leg portions of a patient has a radiation source and a source transport actuable to move the source along an arcuate source path within a housing, from one side of a circumferential gap to the other and has a radius R2 about a center. A housing is provided for placement of the patient&#39;s foot. A digital radiation detector has a detector transport actuable to move the detector along an arcuate detector path within the housing, the detector path having a radius R1 about the center and concentric with the source path, wherein R1 is less than R2, and wherein the detector path extends from one side of the pedestal indent to the other. A gap closure apparatus is movable to a position that continues the detector path across the circumferential gap and encloses the detector path.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Ser. No. 61/322,516, provisionally filed on Apr. 9, 2010, entitled “Cone Beam CT Apparatus for Medical Imaging”, to Yorkston et al. 
         [0002]    This application claims priority to U.S. Ser. No. 61/422,679, provisionally filed on Dec. 14, 2010, entitled “EXTREMITY IMAGING APPARATUS FOR CONE BEAM COMPUTED TOMOGRAPHY”, to Yorkston et al. 
         [0003]    This is a Continuation-in-Part of U.S. Ser. No. 12/771,250, filed on Apr. 30, 2010, entitled “EXTREMITY IMAGING APPARATUS FOR CONE BEAM COMPUTED TOMOGRAPHY” to Yorkston et al., which claims priority to U.S. Ser. No. 61/175,091 provisionally filed on May 4, 2009, entitled “Cone Beam Computed Tomography (CBCT) For Extremity Imaging”. 
         [0004]    These references are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0005]    The invention relates generally to diagnostic imaging and in particular to cone beam imaging systems used for obtaining volume images of extremities. 
       BACKGROUND OF THE INVENTION 
       [0006]    3-D volume imaging has been employed as a diagnostic tool that offers advantages over earlier 2-D radiographic imaging techniques for evaluating the condition of internal structures and organs. 3-D imaging of a patient or other subject has been made possible by a number of advancements, including the development of high-speed imaging detectors, such as digital radiography (DR) detectors that enable multiple images to be taken in rapid succession. 
         [0007]    Cone beam computed tomography (CBCT) or cone beam CT technology offers promise as one type of diagnostic tool for providing 3-D volume images. Cone beam CT systems capture volumetric data sets by using a high frame rate digital radiography (DR) detector and an x-ray source, typically affixed to a gantry that rotates about the object to be imaged, directing, from various points along its orbit around the subject, a divergent cone beam of x-rays toward the subject. The CBCT system captures projections throughout the rotation, for example, one 2-D projection image at every degree of rotation. The projections are then reconstructed into a 3D volume image using various techniques. Among the most common methods for reconstructing the 3-D volume image are filtered back projection approaches. 
         [0008]    Although 3-D images of diagnostic quality can be generated using CBCT systems and technology, a number of technical challenges remain. In some cases, for example, there can be a limited range of angular rotation of the x-ray source and detector with respect to the subject. CBCT Imaging of legs, arms, and other extremities can be hampered by physical obstruction from a paired extremity. This is an obstacle that is encountered in obtaining CBCT image projections for the human leg or knee, for example. Not all imaging positions around the knee are accessible; the patient&#39;s own anatomy prevents the radiation source and image detector from being positioned over a portion of the scan circumference. 
         [0009]    To illustrate the problem faced in CBCT imaging of the knee, the top view of  FIG. 1  shows the circular scan paths for a radiation source  22  and detector  24  when imaging the right knee R of a patient as a subject  20 . Various positions of radiation source  22  and detector are shown in dashed line form. Source  22 , placed at some distance from the knee, can be positioned at different points over an arc of about 200 degrees; with any larger arc, left knee L blocks the way. Detector  24 , smaller than source  22  and typically placed very near subject  20 , can be positioned between the patient&#39;s right and left knees and is thus capable of positioning over the full circular orbit. 
         [0010]    A full 360 degree orbit of the source and detector is not needed for conventional CBCT imaging; instead, sufficient information for image reconstruction can be obtained with an orbital scan range that just exceeds 180 degrees by the angle of the cone beam itself, for example. However, in some cases it can be difficult to obtain much more than about 180 degree revolution for imaging the knee or other joints and other applications. Moreover, there can be diagnostic situations in which obtaining projection images over a certain range of angles has advantages, but patient anatomy blocks the source, detector, or both from imaging over that range. 
         [0011]    For imaging the leg, one way around this problem is to arrange the patient in a pose such that the subject leg is extended into a CBCT scanning apparatus and the paired leg is supported in some other way or bent with respect to the subject leg, such as at a right angle. This is the approach used, for example, in the CT scanner device taught in U.S. Pat. No. 7,394,888 entitled “CT Scanner for Lower Extremities” to Sukovic et al. In the methods of the Sukovic et al. &#39;888 disclosure, the other leg must either be lifted out of place or spread at a distance, or is relaxed while the subject leg is lifted out of place and extended into the scanner equipment. This arrangement can be particularly disadvantageous for a number of reasons. It can be helpful, for example, to examine the condition of a knee or ankle joint under the normal weight load exerted on that joint by the patient. But, in requiring the patient to assume a position that is not usually encountered in typical movement, the Sukovic et al. &#39;888 apparatus may obtain an image when there is excessive strain, or insufficient strain, or poorly directed strain, on the joint. 
         [0012]    Another problem with conventional approaches relates to imaging of a load-bearing extremity such as the human leg. Because of the inability to image the leg under a normal load, as the patient is in a standing position, various artificial ways of mimicking load conditions have been attempted. Such approaches have used various types of braces, compression devices, and supports. As one example intended to remedy the shortcomings of conventional imaging techniques, the Sukovic et al. &#39;888 disclosure teaches simulating the normal loading of the leg by elevating the leg to a non-standing position, then applying an external force against the leg. However, it can be readily appreciated that while this type of simulation allows some approximation of load-bearing limb response, it can be inaccurate. The knee or ankle joint, under some artificially applied load and at an angle not taken when standing, may not behave exactly as it does when bearing the patient&#39;s weight in a standing position. 
         [0013]    Another difficulty with the Sukovic et al. &#39;888 apparatus and with other devices designed to address knee, lower leg, and foot imaging relates to poor image quality. For image quality, the CBCT sequence requires that the detector be up close to the subject and the source of the cone beam radiation be at a sufficient distance from the subject. This provides the best image and reduces image truncation and consequent lost data. Positioning the subject midway between the detector and the source, as Sukovic et al. &#39;888 apparatus and with other devices require, not only noticeably compromises image quality, but also places the patient too near the radiation source, so that radiation levels are considerably higher. 
         [0014]    One example of this strategy is shown in German patent publication DE 10146915. With the C-shaped gantry arrangement shown, centering the subject at the center of rotation of source and detector would apply considerably higher radiation amounts with each projection and severely compromise image quality. Any other positioning of the subject, such as closer to the detector, might reduce radiation levels over some part of the image capture sequence, but would result in unduly complex image reconstruction problems, since this would actually vary the distances between radiation source and subject and between subject and detector with each projection image obtained. Attempted imaging of the knee with such a system would require the patient to be supported in some way, balancing on the leg being imaged. It can be appreciated that this requirement is unreasonable or impossible for many situations in which an injured knee is being imaged. Thus, the C-shaped gantry shown would not be suitable for imaging only one knee of the patient. The solution described would irradiate both knees, increasing the amount of tissue exposed to radiation and reducing image quality. 
         [0015]    Imaging of the foot and ankle presents additional obstacles for CBCT projection image capture. The shape of the foot itself makes it difficult to provide a suitable path for the x-ray source and detector for obtaining a set of projection images that can be used for accurate 3-D reconstruction. Approaches such as that given in the Sukovic et al. &#39;888 disclosure, centering the foot between source and detector, suffer from the same problems of poorly positioned exposure and noticeably compromised image quality. 
         [0016]    A number of conventional imaging approaches for the lower leg function by imaging both legs at the same time. This type of conventional solution suffers from reduced image quality and increases the amount of patient anatomy that receives radiation. 
         [0017]    It can be seen that although a number of solutions have been proposed to address the problem of CBCT extremity imaging, conventional solutions fall short of what is needed with regard to both usability and performance. 
       SUMMARY OF THE INVENTION 
       [0018]    It is an object of the present invention to advance the art of diagnostic imaging of extremity body parts, particularly jointed or load-bearing, paired extremities such as knees, legs, ankles, fingers, hands, wrists, elbows, arms, and shoulders. 
         [0019]    It is a feature of the present invention that it provides an apparatus with different radii for orbital paths of sensor and radiation source components. 
         [0020]    It is an advantage of the present invention that it allows imaging of foot and ankle extremities for a patient who is standing or sitting. 
         [0021]    According to one aspect of the present invention there is provided an apparatus for cone beam computed tomography of a portion of a lower leg of a patient, the apparatus comprising: a radiation source; a radiation source transport actuable to move the source along at least a portion of an arcuate source path within a housing, wherein the source path extends from one side of a circumferential gap in the housing to the other side of the circumferential gap and has a radius R 2  about a center; a pedestal indent on the housing for placement of the patient&#39;s foot; a digital radiation detector; a detector transport actuable to move the detector along at least a portion of an arcuate detector path within the housing, the detector path having a radius R 1  about the center and concentric with the source path, wherein R 1  is less than R 2 , and wherein the detector path extends from one side of the pedestal indent to the other; and a gap closure apparatus that is movable to a position that continues the detector path across the circumferential gap and that encloses the detector path. 
         [0022]    These objects are given only by way of illustrative example, and such objects may be exemplary of one or more embodiments of the invention. Other desirable objectives and advantages inherently achieved by the disclosed invention may occur or become apparent to those skilled in the art. The invention is defined by the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the embodiments of the invention, as illustrated in the accompanying drawings. The elements of the drawings are not necessarily to scale relative to each other. 
           [0024]      FIG. 1  is a schematic view showing the geometry and limitations of CBCT scanning for portions of the lower leg. 
           [0025]      FIG. 2  shows a top and perspective view of the scanning pattern for an imaging apparatus according to an embodiment of the present invention. 
           [0026]      FIG. 3  is a perspective view showing patient access to an imaging apparatus with the access door open according to an embodiment of the present invention. 
           [0027]      FIG. 4  is a perspective view showing the patient in a scanning position with the access door closed. 
           [0028]      FIG. 5  is a series of top schematic views showing the sequence for patient access and system preparation for CBCT imaging. 
           [0029]      FIG. 6  is a series of top schematic views showing the sequence for obtaining CBCT projections at a number of angular positions. 
           [0030]      FIG. 7  is a perspective view showing optional height adjustment. 
           [0031]      FIGS. 8A and 8B  are perspective views that show extremity imaging for an extended leg in an alternate configuration. 
           [0032]      FIG. 9  is a perspective view that shows a configuration of the imaging apparatus for upper extremity imaging. 
           [0033]      FIG. 10  is a perspective view that shows imaging with the detector transport fully encircling the lower extremity. 
           [0034]      FIG. 11  is a perspective view that shows imaging with the detector transport fully encircling the upper extremity. 
           [0035]      FIG. 12A  shows perspective views of imaging apparatus with and without covers. 
           [0036]      FIG. 12B  is a perspective view of an imaging apparatus using a turntable for source and detector transport. 
           [0037]      FIG. 13  is a top view of the transport arrangement shown in  FIG. 12B . 
           [0038]      FIG. 14A  shows a top view of the imaging apparatus with the housing partially transparent. 
           [0039]      FIG. 14B  shows internal components in start and stop scan positions. 
           [0040]      FIG. 15  shows top views of the turntable transport arrangement for initial positioning of the extremity of the patient and beginning of scan. 
           [0041]      FIG. 16  shows a top view during the scan sequence. 
           [0042]      FIG. 17  shows perspective views of an embodiment for extremity imaging at a horizontal position. 
           [0043]      FIG. 18A  is a top view that compares angular considerations for foot and knee imaging. 
           [0044]      FIG. 18B  is a schematic view showing the shape of the foot relative to source and detector paths. 
           [0045]      FIG. 19A  is a schematic view showing patient positioning for CBCT imaging of the foot in a non-load-bearing position. 
           [0046]      FIG. 19B  is a schematic view showing patient positioning for CBCT imaging of the foot in a load-bearing position. 
           [0047]      FIG. 19C  is a schematic view showing patient positioning for CBCT imaging of the foot in a non-load-bearing position with the patient seated and the leg extended. 
           [0048]      FIG. 20  shows perspective views of an imaging apparatus for foot imaging that employs an insert for setting up the detector path. 
           [0049]      FIG. 21  is a top view of an imaging apparatus for foot imaging that employs an insert for setting up the detector path. 
           [0050]      FIG. 22  is a top view of an imaging apparatus for foot imaging that employs an insert for setting up the detector path and with translucent covers. 
           [0051]      FIGS. 23 ,  24 , and  25  are perspective views that show different positions of the imaging hardware for foot and ankle imaging. 
           [0052]      FIGS. 26 and 27  are perspective views showing a CBCT imaging apparatus rotated to different imaging positions. 
           [0053]      FIG. 28  is a perspective view of a CBCT imaging apparatus in an alternate embodiment that uses one or more doors to complete the detector path. 
           [0054]      FIG. 29  is a perspective view of a CBCT imaging apparatus in the embodiment that uses one or more doors to complete the detector path, with the doors in closed position. 
           [0055]      FIG. 30  is a top view of a CBCT imaging apparatus in an alternate embodiment that uses one or more doors to complete the detector path. 
           [0056]      FIG. 31  is a top view of a CBCT imaging apparatus in the embodiment that uses one or more doors to complete the detector path, with the doors in closed position. 
           [0057]      FIGS. 32 and 33  show door open and door closed positions, respectively, for a CBCT imaging apparatus in rotated position for extended leg imaging of the foot or ankle. 
           [0058]      FIG. 34  is a schematic diagram showing alternate radiation paths for foot and ankle imaging. 
           [0059]      FIG. 35A  is a schematic diagram that shows a housing tilted to an oblique angle for foot imaging according to one embodiment of the present invention. 
           [0060]      FIG. 35B  is a schematic diagram that shows a housing inverted and tilted to an oblique angle for foot imaging according to an alternate embodiment. 
           [0061]      FIG. 36A  is a schematic diagram that shows a housing without a foot insert tilted to an angle for foot and toe imaging according to an alternate embodiment of the present invention. 
           [0062]      FIG. 36B  is a schematic diagram that shows a housing with a foot insert tilted to an angle for foot and toe imaging according to an alternate embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0063]    The following is a detailed description of the preferred embodiments of the invention, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the several figures. 
         [0064]    In extremity imaging, particularly for imaging the lower paired extremities, improvements are needed, including the following:
       (i) improved placement of the radiation source and detector to provide acceptable radiation levels and image quality throughout the scanning sequence;   (ii) system flexibility for imaging at different heights with respect to the rotational axis of the source and detector, including the flexibility to allow imaging with the patient standing or seated comfortably, such as with a foot in an elevated position, for example;   (iii) improved patient accessibility, so that the patient does not need to contort, twist, or unduly stress limbs or joints that may have been injured in order to provide images of those body parts;   (iv) improved ergonomics for obtaining the CBCT image, allowing the patient to stand with normal posture, for example. This would also allow load-bearing extremities, such as legs, knees, and ankles, to be imaged under the normal load exerted by the patient&#39;s weight, rather than under simulated loading conditions as taught in the Sukovic et al. &#39;888 disclosure and elsewhere;   (v) capability for CBCT imaging of the foot or ankle. The inherent difficulty that is due to the shape of the foot is one factor; other problems with foot imaging include the need to be able to image the foot in a number of different positions, with and without weight-bearing.       
 
         [0070]    In the context of the present disclosure, the term “extremity” has its meaning as conventionally understood in diagnostic imaging parlance, referring to knees, legs, ankles, fingers, hands, wrists, elbows, arms, and shoulders and any other anatomical extremity. The term “subject” is used to describe the extremity of the patient that is imaged, such as the “subject leg”, for example. The term “paired extremity” is used in general to refer to any anatomical extremity wherein normally two or more are present on the same patient. In the context of the present invention, the paired extremity is not imaged; only the subject extremity is imaged. 
         [0071]    To describe the present invention in detail, the examples given herein for embodiments of the present invention focus on imaging of the load-bearing lower extremities of the human anatomy, such as the leg, the knee, the ankle, and the foot, for example. However, these examples are considered to be illustrative and non-limiting. 
         [0072]    In the context of the present disclosure, the term “arc” or, alternately, “circular arc”, has its conventional meaning as being a portion of a circle of less than 360 degrees or, considered alternately, of less than 2π radians for a given radius. 
         [0073]    Embodiments of the present invention address the difficulties of lower extremity imaging by providing an imaging apparatus that defines orbital source and detector paths, concentric about a center point, wherein components that provide the source and detector paths are configured to allow patient access prior to and following imaging and configured to allow the patient to stand with normal posture during the CBCT image capture series. In embodiments of the present invention, this capability is effected by using a detector transport device that has a circumferential access opening allowing positioning of the extremity, wherein the detector transport device is revolved about the positioned extremity once it is in place, enclosing the extremity as it is revolved through at least a portion of the scan. 
         [0074]    It is instructive to consider dimensional attributes of the human frame that can be considerations for design of CBCT equipment for scanning extremities. For example, an adult human patient of average height in a comfortable standing position has left and right knees generally anywhere from about 10 to about 35 cm apart. For an adult of average height, exceeding about 35-40 cm (14-15.7 inches) between the knees becomes increasing less comfortable and out of the range of normal standing posture. It is instructive to note that this constraint makes it impractical to use gantry solutions such as that shown in DE 10146915, described earlier, for knee imaging. Either the source or the detector must be able to pass between the legs of a standing patient for single-knee CBCT imaging, a capability not available with gantry or other conventional solutions. 
         [0075]    The perspective and top views of  FIG. 2  show how the scanning pattern is provided using various embodiments of a CBCT imaging apparatus  10  according to the present invention. A detector path  28  of a suitable radius R 1  from a central axis A is provided by a first device, a detector transport  34 . A source path  26  of a second, larger radius R 2  is provided by a second device, a source transport  32 . The extremity, subject  20 , is substantially centered along central axis A so that central axis A can be considered as a line through points in subject  20 . The limiting geometry for image capture is due to the arc of source transport  32 , blocked by patient anatomy, such as by a paired limb, to typically about 200 degrees, as noted previously. Another limitation relates to the desirable goal of not exposing the patient more than necessary. This defines a partial circular sector, bounded by this arc and radii at start and end-of-scan. 
         [0076]    Detector transport  34 , while capable of a fully circular orbit because it can be moved between the standing patient&#39;s legs, follows the necessary complementary arc to that of source transport  32 . Patient access before scanning is eased by providing a circumferential gap  38  in detector transport  34 . With detector transport  34  in the open position shown in  FIG. 3 , the patient can freely move in and out of position for imaging. When the patient is properly in position, detector transport  34  is revolved about axis A, substantially 180 degrees. This orbital movement confines the extremity more narrowly and places detector  24 , not visible in  FIGS. 2-4  due to the detector transport  34  housing, in position near subject  20  for obtaining the first projection image in sequence. 
         [0077]    Circumferential gap  38  not only allows access for positioning of the subject leg or other extremity, but also allows sufficient space for the patient to stand in normal posture during imaging, placing the subject leg for imaging in the central position of axis A ( FIG. 2 ) and the non-imaged paired leg within the space defined by circumferential gap  38 . Circumferential gap  38  extends approximately 360 degrees minus the sum of 180 degrees plus the fan angle, which is determined by source-detector geometry and distance. 
         [0078]    The top views of  FIG. 5  show the sequence for patient access for imaging apparatus  10 . In an open access position  40 , circumferential gap  38  permits access of the extremity so that it can be centered in position along central axis A. The outline of the foot corresponding to an open access position  42  indicates positioning of the patient and is shown for reference. In this example, the left leg is the subject imaged; the paired right leg would lie within or just outside circumferential gap  38 . Once the patient&#39;s leg or other extremity is in place, detector transport  34 , or a hooded cover or other member that defines this transport path, can be revolved into position, closing the detector portion of circumferential gap  38 , as shown in a revolving transport position  44 . A transport in place position  46  shows detector transport  34  in suitable position for executing the CBCT imaging sequence. 
         [0079]    The top views of  FIG. 6  continue the operational sequence begun in  FIG. 5  and show the sequence for obtaining CBCT projections at a number of angular positions when using imaging apparatus  10 . The relative positions of radiation source  22  and detector  24 , which may be concealed under a hood, as noted earlier, are shown in  FIG. 6 . The outline of the foot is a reference for indicating the relative position of the patient&#39;s leg; imaging of the foot itself is shown and described in more detail subsequently. The source and detector are diametrically opposite at each position during the CBCT scan and projection imaging. The sequence begins at a begin scan position  50 , with radiation source  22  and detector  24  at initial positions to obtain an image at a first angle. Then, both radiation source  22  and detector  24  revolve about axis A as represented in interim scan positions  52 ,  54 ,  56 , and  58 . Imaging terminates at an end scan position  60 . As this sequence shows, source  22  and detector  24  are in diametrically opposing positions relative to subject  20  at each imaging angle. Throughout the scanning cycle, detector  24  is within a short distance D 1  of subject  20 . Source  22  is positioned beyond a longer distance D 2  of subject  20 . The positioning of source and detector components can be carried out by separate actuators, one for each transport path, or by a single rotatable member, as described in more detail subsequently. It should be noted that scanning motion in the opposite direction, that is, clockwise with respect to the example shown in  FIG. 6 , is also possible, with the corresponding changes in initial and terminal scan positions. 
         [0080]    Other features of imaging apparatus  10  are provided by the capability to move both source and detector transports  32  and  34  along the axis direction as a unit, as shown in the perspective view of  FIG. 7 . A vertical support  70  provides vertical transport of the imaging apparatus, so that the source and detector can be translated upwards or downwards in the direction of the central axis in order to suit patients of different heights and to image different portions of the leg. The height adjustment can be made before or after the patient&#39;s subject leg to be imaged is enclosed by detector transport  34  using the setup sequence of  FIG. 5 . 
         [0081]    In one embodiment, vertical support  70  also allows rotation of the CBCT imaging apparatus  10  to allow imaging of an extremity that is disposed horizontally or is extended at some oblique angle other than vertical.  FIGS. 8A  and  8 B show perspective views of knee imaging in a horizontal position, with the patient seated and the leg outwardly extended. Full 360 degree rotation about a tilt axis Q is possible. It should be noted that, with this application, similar patient accessibility applies, with detector transport  34  revolved into position once the extremity is centered in place. Further height adjustment is also possible, such as for arm, elbow, or shoulder imaging, as shown in  FIG. 9 . 
         [0082]    Using revolving detector transport  34  simplifies patient access and provides sufficient imaging path for CBCT imaging, since the angular limitation of the orbital imaging path is due to source obstruction, rather than to the detector path. Thus, for example, detector transport  34  could fully encircle the limb, as shown in the examples of  FIGS. 10 and 11 . In these embodiments, there is a circumferential gap  38  only in the source orbit. 
         [0083]    Referring back to the schematic diagrams of  FIG. 6 , radiation source  22  and detector  24  each orbit the subject along an arc with radii R 2  and R 1 , respectively. Within source transport  32 , a source actuator could be used, cooperating with a separate, complementary detector actuator that is part of detector transport  34 . Thus, two independent actuator devices, one in each transport assembly, can be separately controlled and coordinated by an external logic controller to move source  22  and detector  24  along their respective arcs, in unison, about subject  20 . 
         [0084]    In an alternate embodiment, source and detector transport components are mechanically linked to a single revolving or rotating assembly. One such arrangement, shown at the right in  FIG. 12A  and enlarged in  FIG. 12B , provides source and detector transports  32  and  34  using a single mechanical assembly, a rotating member  68 , on a turntable  64  that revolves about central axis of rotation A and provides the needed radii for source  22  and detector  24 . As is best shown in the top view of  FIG. 13 , detector  24  rides along the surface of the C-shaped turntable  64 , orbiting the subject at radius R 1 . Source  22  is connected to turntable  64  along an arm  66  that provides the longer radius R 2 . Circumferential gap  38  extends across both source and detector paths. 
         [0085]    It is noted that the embodiments shown using rotating member  68  on turntable  64  can be encased in one or more housings, thereby providing similar appearance to imaging apparatus  10  shown in  FIGS. 7-11 , for example. This type of arrangement has advantages for isolating the patient from moving components and for alleviating at least some of the patient anxiety that might be caused by automatically moving components during imaging. 
         [0086]      FIG. 14A  shows sources and detector transports  32  and  34  and source and detector  22  and  24  components as they are fitted within covers  80  that protect moving mechanical parts and help to prevent patient contact with moving components.  FIG. 14B  shows the covered system with internal components in begin and end scan positions  50  and  60  respectively, when using the scan sequence described earlier with reference to  FIG. 6 . 
         [0087]    The top views of  FIGS. 13 ,  15 , and  16  show how patient access is provided using this mechanical arrangement. Once the patient is positioned, rotating member  68  is swung around the positioned extremity, to a start position  72 , as shown at the bottom in  FIG. 15 . Imaging begins at this position and continues as rotating member  68  revolves source and detector components about axis A. For the example of  FIGS. 15 and 16 , rotating member  68  moves in a clockwise direction. Counter-clockwise rotation would also be possible. 
         [0088]    Rotating member  68  can also be used with an imaging configuration for upper extremities, as shown in  FIG. 17 . Because none of the patient anatomy blocks the transport path, a full circular orbit is permitted for scanning with this configuration. Again, full 360 degree rotation of the components in the plane of rotating member  68  is possible, about tilt axis Q. 
         [0089]    In the context of the present disclosure, the term “lower leg” is considered to include any portion of the human leg from just above the knee outward, thus including the knee as well as tibia/fibula structures, ankle, and foot. Imaging of extreme portions of the lower leg, including the ankle and foot is also possible with CBCT imaging apparatus  10 . However, because the foot protrudes outward into the desired detector transport path, the allowable angular range for foot imaging is more constrained than the range for leg and knee imaging in general. The top view of  FIG. 18A  shows, for example, that the angular range for CBCT scanning of the foot, for a standing patient using CBCT imaging apparatus  10  described previously, is about 50 degrees less than that for knee imaging, for example. 
         [0090]    As shown schematically in  FIG. 18B , the general shape of the foot makes it difficult to image in a CBCT apparatus with the arrangement just described for CBCT imaging apparatus  10 . In this figure, the inner dashed line circle represents the possible arcuate detector path  28 ; the outer dashed circle represents the possible arcuate source path  26 . Paths  26  and  28  both lie in a transport plane T that can be tilted, as described subsequently. As shown previously, paths  26  and  28  are concentric and source and detector devices are arranged to be opposite each other along these respective paths. Where the foot is extended, source path  26  is not obstructed by patient anatomy. Because of limitations to detector path  28 , which must be in the form of a circular arc that substantially extends from one side of the foot to the other, different approaches are needed for obtaining suitable projection images of the foot and ankle. 
         [0091]    Schematic diagrams of  FIGS. 19A ,  19 B, and  19 C show patient positioning for obtaining images of the foot under various conditions and tilt angles when using a CBCT imaging apparatus that is configured according to embodiments of the present invention.  FIG. 19A  shows patient positioning for CBCT imaging of the foot in a non-load-bearing position. The patient places one foot into a CBCT imaging apparatus  100 , such as onto a pedestal indent  102  formed into a housing  80  of CBCT imaging apparatus  100  or other suitable support that maintains the foot at the proper height for imaging. Source  22  and detector  24  then orbit the foot over a range of angles for obtaining the needed image projections. Here, transport plane T is substantially horizontal. It should be noted that tilt axis Q, shown in  FIGS. 8A ,  8 B, and elsewhere, is substantially parallel to transport plane T, but may lie above or below plane T. 
         [0092]      FIG. 19B  shows patient positioning for CBCT imaging of the foot in a load-bearing position. With this arrangement, the patient is able to stand in a natural position and apply some weight to the foot during imaging. Here, transport plane T is also substantially horizontal. 
         [0093]      FIG. 19C  is a schematic view showing patient positioning for CBCT imaging of the foot in a non-load-bearing application with the patient seated and the leg extended. This position takes advantage of the rotatable housing of CBCT imaging apparatus  100 . Here, transport plane T is substantially vertical, or at other suitable upright angle. 
         [0094]    The perspective view of  FIG. 20  shows an embodiment of CBCT imaging apparatus  100  that uses, as a type of detector path or gap closure apparatus, providing gap closure across circumferential gap  38  and enclosing the detector path, a removable insert  110  that is added to the base CBCT imaging apparatus when foot or ankle imaging is required. Insert  110  acts as a cover over the travel path of the detector and defines the boundaries of the detector orbit for foot imaging. Here, insert  110  slips into place within outer housing  80  as needed. Insert  110  is mechanically keyed to fit into housing  80  only in the proper orientation and is designed with interlocks that sense when insert  110  is in place and provide a signal to controlling logic to modify the source-detector paths accordingly. With insert  110  in place, closure of gap  38  is provided, completing the portion of the detector path that lies across gap  38 . 
         [0095]    The top view of  FIG. 21  shows from an outside view, with cover and housing opaque, insert  110  in place within CBCT imaging apparatus  100  and shows the reference position of a foot F when placed within insert  110 . The top view of  FIG. 22  shows the covers of insert  110  and housing  80  of imaging apparatus  100  as transparent, so that the positions of source  22  and detector  24  can be seen. For foot imaging, the travel path changes so that detector  24  travel is around the sides and rear of the foot and not around the front of the foot. This pattern is shown in the sequence of perspective views of  FIGS. 23 ,  24 , and  25 .  FIGS. 23 and 25  show the extreme ends of the detector  24  travel path. Where clockwise orbit is used,  FIG. 23  shows the starting position of the detector  24  orbit and  FIG. 25  the ending position. Where counter-clockwise movement is used,  FIG. 25  shows the starting position for detector  24  and  FIG. 23  the ending position. 
         [0096]    Height and angle adjustment are useful for foot and ankle imaging.  FIGS. 26 and 27  show different rotation and height positions that can also be provided to allow more flexible patient positioning. For example, the arrangement shown in  FIG. 27  is suitable for imaging of the foot with the patient in an extended leg position, as was described earlier with reference to  FIG. 19C . 
         [0097]    In addition to rotation about a tilt axis Q as shown in  FIG. 27 , housing  80  is also at least partially rotatable about a perpendicular vertical axis A, shown in  FIG. 27  and elsewhere, in one embodiment. 
         [0098]    While insert  110  is useful as a detector path closure apparatus or gap closure apparatus for providing the continuation of the detector transport path around the back of the foot, the insert must be removed in order to allow imaging of the knee or other extremities. The alternate embodiment shown in  FIGS. 28 ,  29 ,  30 ,  31 ,  32 , and  33  uses an alternate approach to allow foot imaging or knee imaging using the same device. The perspective views of  FIGS. 28 and 29  show CBCT imaging apparatus  100  that has a set of doors  114  that act as a gap closure apparatus for detector path closure and that enclose that portion of the arcuate detector path. Doors  114  are opened to allow entry of the patient&#39;s foot and can be closed once the foot is positioned on pedestal indent  102 . Closing doors  114  completes detector path  28 , extending this path behind the patient so that detector  24  can pass behind the foot during the CBCT image capture sequence, as was shown earlier with reference to  FIGS. 23 ,  24 , and  25 .  FIGS. 30 and 31  show door-open and door-closed positions from a top view for this alternate embodiment. 
         [0099]      FIGS. 32 and 33  show door open and closed positions, respectively, with CBCT imaging apparatus  100  in rotated position for extended leg imaging of the foot or ankle. It should be noted that a single door could be used as an alternative to the double-door arrangement of  FIGS. 28-33 . A sensor  82  ( FIG. 28 ) detects when a foot is positioned in the pedestal and adjusts the imaging and transport patterns accordingly. 
         [0100]    The source-detector travel paths provided for the conventional patient positions shown in  FIGS. 19A ,  19 B, and  19 C can be used for accurate 3-D reconstruction of the ankle and upper areas of the foot, but can be of less accuracy for reconstructing a 3-D image of the foot itself. Referring to the schematic diagram of  FIG. 34 , it can be seen that the direct head-on radiation path P 1  from source  22  to detector  24  is through a number of bone structures. This is a problem encountered by a scanning pattern in which the source and detector orbits are in the plane of radiation path P 1 . 3-D reconstruction from such an image is complicated by the presence of these bones in different arrangements for or missing from different image projections. One solution is to tilt the foot; however, this may not be possible for the patient. Moreover, having the patient hold the foot in an awkward position may not provide what is most helpful for diagnostic analysis. The pedestal (not shown for clarity in  FIG. 34 ) seats the foot along a plane W that may or may not be horizontal. 
         [0101]    An alternative solution that is available with embodiments of CBCT imaging apparatus  100  is to tilt the imaging apparatus itself about tilt axis Q ( FIG. 28  and elsewhere), at an angle θ so that housing  80  is at an oblique angle by 1 or more degrees relative to either the horizontal or vertical, such as at 10, 20, or 40 degrees. 
         [0102]    With respect to the schematic diagram in  FIG. 34 , this arrangement allows the source and detector orbits in transport plane T to be shifted to any of a range of angular positions, shown as planes T 1  and T 2  corresponding to paths P 1  and P 2 , for example, allowing improved imaging of parts of the foot and ankle. As is shown in  FIG. 34 , this arrangement effectively tilts transport plane T to any of a range of angles relative to plane W of the foot, so that planes W and T are at different angles. The tilt angle of the tilt axis can thus be disposed to direct radiation to the foot at an angle that is offset from base of the foot by a variable range, from a few degrees to more than 10 or 20 degrees, and even allowing the tilt axis to vary over a full 90 degree range or more for this purpose. 
         [0103]    The schematic diagram of  FIG. 35A  shows housing  80  tilted to an oblique angle for foot imaging according to one embodiment. This orients the orbits of source  22  and detector  24  to an oblique angle with respect to horizontal and also expands the possible angular range of both source and detector travel paths. The schematic diagram of  FIG. 35B  shows an alternate arrangement, with housing  80  inverted and at an oblique angle, for improving the image coverage of parts of the foot. 
         [0104]    Because such an extended angular range is permitted, the problem of imaging for the front of the foot and imaging of the toes can also be addressed by embodiments of the present invention.  FIG. 36A  shows an embodiment without using foot insert  110 . In this embodiment, the extended foot can be positioned for imaging, but the shin of the leg prevents the foot from being fully extended into the imaging area. With foot insert  110 , as shown in  FIG. 36B , the foot can be extended a few more inches into the imaging area, since the shin of the patient has more room to be moved forward. 
         [0105]    It can be appreciated that the alternate embodiment shown in  FIGS. 28-33 , not using insert  110  but instead using some sort of sliding door arrangement, has advantages over embodiments that require the use of insert  110 . One advantage relates to more general use of CBCT imaging apparatus  100  for obtaining images of different extremities. For example, the same apparatus can be used for imaging a foot or ankle as well as for imaging a knee or other portion of the leg or other extremity. 
         [0106]    The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.