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 . 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 . 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 . 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 . 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 . 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 knee cbct imaging , a capability not available with gantry or other conventional solutions . the perspective and top views of fig2 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 . this defines a partial circular sector , bounded by this arc and radii at start and end - of - scan . 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 fig3 , 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 fig2 - 4 due to the detector transport 34 housing , in position near subject 20 for obtaining the first projection image in sequence . 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 ( fig2 ) and the non - imaged paired leg within the space defined by circumferential gap 38 . circumferential gap 38 extends approximately 180 degrees plus the fan angle , which is determined by source - detector geometry and distance . the top views of fig5 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 . the top views of fig6 continue the operational sequence begun in fig5 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 fig6 . 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 fig6 , is also possible , with the corresponding changes in initial and terminal scan positions . 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 fig7 . 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 fig5 . 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 . fig8 a and 8b show perspective views of knee imaging in a horizontal position , with the patient seated and the leg outwardly extended . full 360 degree rotation about an 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 fig9 . 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 fig1 and 11 . in these embodiments , there is a circumferential gap 38 only in the source orbit . referring back to the schematic diagrams of fig6 , 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 . 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 fig1 a and enlarged in fig1 b , 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 fig1 , 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 . it should be emphasized 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 fig7 - 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 . fig1 a 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 . fig1 b 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 fig6 . the top views of fig1 , 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 fig1 . imaging begins at this position and continues as rotating member 68 revolves source and detector components about axis a . for the example of fig1 and 16 , rotating member 68 moves in a clockwise direction . counter - clockwise rotation would also be possible . rotating member 68 can also be used with an imaging configuration for upper extremities , as shown in fig1 . 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 axis q . imaging of 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 . the top view of fig1 shows , for example , that the angular range for cbct scanning of the foot , for a standing patient , is about 50 degrees less than that for knee imaging , for example . a range of optional devices can also be provided to facilitate the imaging process . for example , a horizontal or vertical foot support can be provided for support of the patient &# 39 ; s foot . optionally , the foot support can be adjustable to some oblique angle between horizontal and vertical , such as at a 33 degree or 45 degree angle for example . 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 .
an apparatus for cone beam computed tomography of an extremity has a digital radiation detector and a first device to move the detector along a circular detector path extending so that the detector moves both at least partially around a first extremity of the patient and between the first extremity and a second , adjacent extremity . the detector path has radius r1 sufficient to position the extremity approximately centered in the detector path . there is a radiation source with a second device to move the source along a concentric circular source path having a radius r2 greater than radius r1 , radius r2 sufficiently long to allow adequate radiation exposure of the first extremity for an image capture by the detector . a first circumferential gap in the source path allows the second extremity to be positioned in the first circumferential gap during image capture .