Patent Publication Number: US-2022233092-A1

Title: Treatment couch with localization array

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/875,966, filed Jan. 19, 2018, which is a divisional of U.S. patent application Ser. No. 12/214,771, filed Jun. 18, 2008, now U.S. Pat. No. 9,883,818, which claims the benefit of U.S. Provisional Patent Application No. 60/936,388, which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present invention relate to the field of radiation treatment, and in particular, to the positioning of an electromagnetic localization array for use in image guided radiation treatment utilizing wireless transponders. 
     BACKGROUND 
     Radiotherapy and radiosurgery are non-invasive radiation treatment options widely used to treat patients with a variety of tumors such as brain tumor, lung tumor, and liver tumor. Fiducial tracking is one among a variety of conventional tracking methods utilized in performing radiation treatments. 
     Electromagnetic tracking systems are used to track the positions of fiducials in near real time (e.g., 10 Hz). In one type of system (i.e., Calypso® 4D Localization System, available from Calypso, of Seattle, Wash.), an array of four transmitter coils spread out in space induces a resonance in a fiducial or transponder coil system. When the magnetic field is switched off, the transponder signal during relaxation is sensed by an array of receiver coils and used to establish the position of the transponder. In the Calypso 4D Localization System, the transmitter and receiver coils are embedded in an array wired to a common device, and the fiducials or transponders are wireless. 
     A tracking system may provide the positions of fiducials or transponders that are implanted in a patient before acquisition of a treatment planning computed tomography (CT) scan, for example for radiation therapy. The fiducial positions are known relative to the transmitter coils or electromagnetic localization array. If the transmitter coil or array position is known in the treatment room, the fiducial positions relative to the treatment room isocenter (treatment room coordinate system origin) can be obtained. The fiducial positions can then be used to guide treatment, for example, by directing a radiation beam at a target tracked by reference to the fiducial positions. 
     In such a tracking system, inappropriate placement of the localization array may cause problems such as susceptibility to interference in the array or physical obstruction of other elements of the treatment system. 
     Another problem with electromagnetic tracking systems is that the accuracy of the position and orientation information is affected by changes in the magnetic field other than those created by the transmitter coils. Perturbations in the magnetic field can be caused by the presence of metal (for example, in radiation therapy, a gantry or robotic manipulator and a linear accelerator) or other conductive material (for example, a treatment table top made out of conductive material). Thus the position information reported by an electromagnetic tracking system in practical use, for example, tracking a target region inside a patient during radiation therapy, can have a higher error than a system specification determined in a carefully controlled laboratory setting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. 
         FIG. 1  illustrates a treatment delivery system including localization array for tracking implanted fiducials according to one embodiment of the invention. 
         FIG. 2  illustrates a localization array integrated within a treatment couch according to one embodiment of the invention. 
         FIG. 3A  illustrates a localization array mounted underneath a treatment couch using a motorized attachment mechanism according to one embodiment of the invention. 
         FIG. 3B  illustrates a localization array mounted underneath a treatment couch using a motorized attachment mechanism according to one embodiment of the invention where the localization array is positioned beyond an imaging field of an imaging device. 
         FIG. 3C  illustrates a localization array mounted underneath a treatment couch using a motorized attachment mechanism according to one embodiment of the invention where the localization array is rotated beyond an imaging field of an imaging device. 
         FIG. 4  illustrates a localization array attached to a treatment couch by an attachment mechanism according to one embodiment of the invention. 
         FIG. 5  is a flow chart illustrating a process for using a localization array and treatment couch assembly according to one embodiment of the invention. 
         FIG. 6  illustrates one embodiment of a treatment delivery system with an electromagnetic tracking system capable of compensating for positional information error. 
         FIG. 7  illustrates one embodiment of a localization array with embedded fiducial markers. 
         FIG. 8  illustrates one embodiment of a treatment delivery system with an electromagnetic tracking system capable of compensating for positional information error. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein is a method and apparatus for positioning a localization array for use in image guided radiation treatment utilizing wireless transponders. The following description sets forth numerous specific details such as examples of specific systems, components, methods, and so forth, in order to provide a good understanding of several embodiments of the present invention. It will be apparent to one skilled in the art, however, that at least some embodiments of the present invention may be practiced without these specific details. In other instances, well-known components or methods are not described in detail or are presented in simple block diagram format in order to avoid unnecessarily obscuring the present invention. Thus, the specific details set forth are merely exemplary. Particular implementations may vary from these exemplary details and still be contemplated to be within the spirit and scope of the present invention. 
     According to one embodiment of the present invention, a localization array to be used for tracking fiducial markers may be coupled with a treatment couch. For example, the localization array may be contained within or attached underneath or above the treatment couch. Such an arrangement may ensure that the localization array maintains its position with respect to the treatment couch, even if the treatment couch moves. Thus, the position of the localization array in the treatment room can be tracked with reference to the treatment couch, rather than tracked independently. 
     Attachment of the localization array to the treatment couch may also eliminate the need for additional equipment or structures for supporting the array. The presence of such equipment or structures in the treatment room may block imaging equipment, such as an x-ray imager, or may physically obstruct moving equipment, such as a robotic arm for positioning a linear accelerator (LINAC). Accordingly, attachment of the localization array to the treatment couch may also reduce the likelihood of collisions between these pieces of equipment. 
     Many approaches may be used to couple a localization array to a treatment couch at a desired position. For example, in one embodiment, the localization array may be contained within the treatment couch. In other embodiments, the localization array may be mounted underneath or above the treatment couch. Alternatively, the localization array may also be laid on top of the treatment couch. In this case, a patient undergoing radiation treatment may then lie on top of the localization array. A pad or other covering may be used to separate the patient and the localization array. 
     The localization array may further be coupled to the treatment couch using an attachment mechanism that allows the localization array to be repositioned while remaining attached to the treatment couch. In other embodiments, the attachment mechanism may also allow the localization array to be detached from the treatment couch entirely and then reattached to the treatment couch at a different location. 
       FIG. 1  illustrates a radiation treatment delivery system including a localization array for tracking fiducial markers implanted in a patient&#39;s body, according to one embodiment of the invention. Treatment delivery system  100  includes robotic arm  101 , linear accelerator (LINAC)  102 , treatment couch  104  for supporting patient  103 , localization array  105  for detection position of implanted fiducial markers  106 , and processing unit  107 . 
     Robotic arm  101  carries a LINAC  102  that is capable of producing a beam suitable for use in radiation treatment. Robotic arm  101  is capable of maintaining the direction of the beam of LINAC  102  at a desired target, for example, a tumor within the body of patient  103 . Robotic arm  101  may also respond to signals from a target tracking system, such as processing unit  107 , to compensate for movement of the target during the treatment process. Treatment couch  104  may be used to support the body of patient  103  during the treatment process and may be a robotic treatment couch. For example, the treatment couch  104  may be a robotic couch capable of movement with at least five degrees of freedom. Alternatively, the treatment couch may be a robotic couch capable of movement with less than five degrees of freedom (e.g., four, three, or fewer degrees of freedom). 
     In an embodiment where the treatment couch can be repositioned using a robotic arm, the position of the treatment couch in the treatment room can be determined using mechanical encoders such as sensors attached to the robot arm. The sensors may determine the position of the treatment couch and the localization array attached to the couch based on the joint orientations of the robotic arm. 
     The treatment couch  104  may also respond to signals from a target tracking system to help compensate for movement of the target. Alternatively, treatment couch  104  may be another type of treatment couch, such as the Axum® treatment couch developed by Accuray, Inc. of California. Alternatively, other types of treatment tables may be used. 
     According to one embodiment of the invention, a localization array  105  may be attached to the treatment couch  104 . For example, localization array  105  may be attached to the underside of treatment couch  104  so that the detection field, a volume within which fiducial tracking is most effective, of localization array  105  is directed upwards through the treatment couch  104  and the body of patient  103 . The localization array  105  may be placed so that its detection field encompasses implanted fiducial markers  106 , which are implanted within the body of patient  103 . The localization array can then be used to track the movement of implanted fiducial markers  106 , and may include transmitter coils, receiver coils, or both transmitter and receiver coils for this purpose. Alternatively, transmitter coils may be removed from localization array  105 , so long as the signal from the transmitter coils can still reach the implanted fiducial markers  106 . Processing unit  107  may be used to collect tracking data from localization array  105  and direct robotic arm  101  to reposition the beam of LINAC  102  according to the most recently determined target position. The position of the target may, for example, be determined in reference to the implanted fiducials  106  if the target has a known location with respect to the implanted fiducials  106 . 
       FIG. 2  illustrates a treatment couch assembly including an integrated localization array according to one embodiment of the invention. Treatment couch assembly  200  includes localization array  201 , which is contained within treatment couch  202 . Localization array  201  contains electromagnetic coils  203  which may be used for tracking of fiducial markers located within detection field  204 . Electromagnetic coils  203  may include transmitter coils, receiver coils, or a combination of transmitter and receiver coils. 
     In  FIG. 2 , localization array  201  is exposed for clarity, although in some embodiments, localization array  201  may be concealed beneath the surface of treatment couch  202 . Alternatively, localization array  201  may also be laid on top of treatment couch  202  and may or may not be secured to treatment couch  202 . Whether laid on top or integrated within treatment couch  202 , localization array  201  may be covered. For example, localization array  201  may be covered to protect from physical damage or for aesthetic reasons. The covering material may be chosen to have minimal effect on the fiducial tracking capability of the localization array  201 . The covering material may also be chosen for the comfort of a patient  103  or to protect the patient  103  or the localization array  201 . For example, the covering may be a pad placed over the localization array  201 . Localization array  201  may also be centered with respect to treatment couch  202  along one or more axes of treatment couch  202 . For example, localization array  201  may be centered along the width of treatment couch  202 . 
     Localization array  201  may, in one embodiment, cover or underlie only a portion of the surface of treatment couch  202 . Alternatively, localization array  201  may cover or underlie the entire surface of treatment couch  202 . While in one embodiment, treatment couch  202  contains only one localization array  201 , other embodiments of treatment couch  202  may contain more than one localization array  201 . 
     During a treatment session, a patient  103  may lie on treatment couch  202  so that implanted fiducial markers  106  implanted in the body of patient  103  are within the detection field  204  of localization array  201 . Localization array  201  may then be used to track the locations of implanted fiducials  106 . 
       FIG. 3A  illustrates a treatment couch assembly that includes a localization array mounted underneath the surface of a treatment couch according to one embodiment of the invention. Treatment couch assembly  300  includes a localization array  301  mounted underneath a treatment couch  302 . Localization array  301  may be attached to a positioning mechanism  303  through an extension arm  304 . Positioning mechanism  303  may be mounted on a platform  305  attached to treatment couch  302 . 
     Localization array  301  may be positioned underneath the surface of treatment couch  302  so that the detection field of localization array  301  extends through and above the surface of treatment couch  302 . Localization array  301  may include transmitter coils, receiver coils, or both transmitter and receiver coils. Localization array  301  may also be attached to a positioning mechanism  303 . In one embodiment, positioning mechanism  303  may be mounted on platform  305 . Alternatively, positioning mechanism  303  may be mounted on the underside of treatment couch  302 . Platform  305  may be rigidly attached to treatment couch  302  so that positioning mechanism  303  does not move with respect to treatment couch  302 . Thus, repositioning of localization array  301  may occur solely through the operation of positioning mechanism  303 . In alternative embodiments, positioning mechanism  303  may be attached to treatment couch  302  in a manner that allows movement between treatment couch  302  and positioning mechanism  303  and localization array  301 . For example, treatment couch  302  may be mounted on slides that allow treatment couch  302  to move along the x-axis  310  with respect to positioning mechanism  303  and localization array  301 . Positioning mechanism  303  may operate to move localization array  301  with respect to the treatment couch  302 . For example, positioning mechanism  303  may move localization array  301  along the x-axis  310  or y-axis  311 . Thus, during a radiation treatment session, the localization array  301  may be moved to a desired position so that its detection field may encompass fiducial markers being tracked without moving the patient or the treatment couch  302 . Positioning mechanism  303  may also operate to move localization array  301  outside the imaging field of other imaging devices in the treatment room. For example, if an x-ray imager is in use, the localization array  301  may be moved out of the imaging field of the x-ray imager so that the localization array  301  is not captured in the x-ray image. Localization array  301  may also be moved for other reasons, such as to avoid obstructing moving equipment or personnel. Positioning mechanism  303  may also operate to move localization array  301  in the direction indicated by z-axis  312  in order to bring localization array  301  closer to or farther away from treatment couch  302 . 
     Positioning mechanism  303  may in one embodiment be implemented using an X-Y table. Alternatively, other mechanisms such as robots or non-motorized manual positioning mechanisms may be used. Positioning mechanism  303  may also accept inputs from a computer or other device in order to automate the movement of localization array  301  during a treatment session. Furthermore, positioning mechanism  303  may also send signals to another device to indicate the position of localization array  301 . In one embodiment, mechanical encoders are used to determine the position of the localization array in the treatment room. For example, positioning mechanism  303  may utilize servo motors capable of indicating the rotational positions of the motors, which can then be used to calculate the position of the localization array  301 . Alternatively, other methods for determining the location of localization array  310  may be used in conjunction with positioning mechanism  303 , such as positioning marks or optical sensors. 
     Localization array  301  may be attached to positioning mechanism  303  using an extension arm  304 . Extension arm  304  may be used so that localization array  301  is kept away from positioning mechanism  303 . For example, in cases where the operation or structure of positioning mechanism  303  may degrade the electromagnetic signals received by localization array  301 , extension arm  304  may enable localization array  301  to be sufficiently removed from positioning mechanism  303  so that the electromagnetic signals are unaffected by positioning mechanism  303 . Alternatively, localization array  301  may in other embodiments be attached to positioning mechanism  303  without using extension arm  304 . 
       FIG. 3B  illustrates a treatment couch assembly where the localization array is positioned beyond an imaging field of an imaging device according to one embodiment of the invention. Imaging device  306  is configured to capture images from within imaging field  307 . Imaging device  306  may be a device such as an x-ray imager. 
     Localization array  301  may be moved using positioning mechanism  303  so that localization array  301  is positioned beyond imaging field  307 . With the localization array  301  in this position, imaging device  306  may avoid capturing localization array  301  in an image. For example, imaging device  306  may be used to capture images of a patient&#39;s anatomy while the patient is lying on treatment couch  302 . In this case, allowing localization array  301  to remain in the imaging field may obscure the anatomical features which are the desired subject of the resulting image. 
       FIG. 3C  illustrates a treatment couch assembly having a localization array that is rotated beyond an imaging field of an imaging device according to one embodiment of the invention. A rotational movement may also be used to position localization array  301  outside the imaging field  307  of imaging device  306 . For example, positioning mechanism  303  may include a swivel connection to allow positioning mechanism  303  to swing localization array  301  out of the imaging field  307 . 
       FIG. 4  illustrates a treatment couch assembly including a localization array that is secured to the treatment couch using an attachment mechanism. Treatment couch assembly  400  includes treatment couch  401  having a number of attachment points such as attachment point  402 , clips  403  and  404 , a number of positioning marks such as positioning mark  405 , localization array  406 , and slides  407  and  408 . Clips  403  and  404  and slides  407  and  408  constitute an attachment mechanism for securing localization array  406  to treatment couch  401 . Localization array  406  has a detection field  409 . 
     Treatment couch  401  may have a number of attachment points such as attachment point  402 , where clips  403  and  404  may be attached. Localization array  406  may then be attached to clips  403  and  404 . The attachment points may be located at evenly spaced positions on both sides of treatment couch  401 . Each attachment point may also have a corresponding positioning mark such as positioning mark  405 . The positioning marks may also be labeled with numbers or other symbols used to identify the attachment points. Identifying attachment points where clips  403  and  404  are attached may further facilitate determining the location of the localization array  406  with respect to treatment couch  401 . This relative location may ultimately be used to locate fiducial markers such as implanted fiducials  106  tracked by the localization array  406  with respect to the beam of LINAC  102  during a radiation treatment session. In one embodiment, the position of the localization array  406  with respect to the treatment couch may be determined using mechanical encoders. For example, the attachment points or clips  403  and  404  may have sensors to indicate to which attachment points the localization array  406  is attached. Alternatively, if the position of the localization array  406  relative to the treatment couch is known, the position of the couch in the treatment room can be determined using mechanical encoders. For example, if the couch is movable on a track, sensors on the track can be used to identify the position of the couch on the sliders. The position of the localization array in the treatment room then can be determined by reference to the position of the treatment couch. 
     The clips  403  and  404  may be attached to the attachment points using fasteners. For example, hooks or screw-type fasteners may be used to attach clips  403  and  404  to the attachment points. Other types of fasteners may also be used. The fasteners may be selected based on their ability to support the weight of localization array  406  and clips  403  and  404 . In one embodiment, clips  403  and  404  may be separate from localization array  406  such that clips  403  and  404  may be attached to treatment couch  401  independently from localization array  406 . In other embodiments, clips  403  and  404  and localization array  406  may be a single assembly to be attached to treatment couch  401  in a single piece. 
     Localization array  406  may be repositioned along the x-axis  310  with respect to the treatment couch  401  by detaching clips  403  and  404 , and then reattaching clips  403  and  404  at different attachment points. Localization array  406  may also be repositioned along the z-axis  312  by exchanging clips  403  and  404  with a set of shorter or longer clips. Alternatively, clips  403  and  404  may be adjustable in length so that the same clips  403  and  404  may be used to adjust the position of localization array  406  along the z-axis  312 . Localization array  406  may be equipped with slides  407  and  408  to support localization array  406  on clips  403  and  404  while allowing localization array  406  to be repositioned along the y-axis  311  relative to the treatment couch  401 . Using these and other mechanisms known in the art, localization array  406  may be repositioned so that its detection field  409  is in an optimal position for tracking a set of fiducial markers. 
       FIG. 5  is a flow chart illustrating a process for setting up and using an electromagnetic localization array and treatment couch assembly according to one embodiment of the invention. Localization array setup process  500  begins with block  501 , where an optimal position for a localization array such as localization array  105  is determined. In one embodiment, the optimal position for localization array  105  may be a position where the detection field of localization array  105  encompasses implanted fiducial markers  106 . Alternatively, an optimal position for a localization array may be determined based on other factors or a combination of factors. For example, the optimal position may be determined based on a desired characteristic of a signal received by the localization array  105  from the implanted fiducials  106 . The optimal position may also be chosen to avoid obstruction by the localization array  105  of imaging equipment or moving equipment such as robotic arm  101 . 
     After the optimal position for the localization array is identified in block  501 , execution proceeds to block  502 , where positioning marks may be used to identify an attachment location for the localization array. For example, in one embodiment, positioning marks such as positioning mark  405  may identify one or more attachment points such as attachment point  402  where clips  403  and  404  may be attached so that localization array  406  is positioned at the optimal position. In other embodiments, positioning marks may not be required to identify an attachment location because the localization array may already be attached to the treatment couch. For example, the localization array may be integrated into the treatment couch, such as localization array  201 , or the localization array may be designed to remain attached to the treatment couch between treatment sessions. In these cases, localization array setup process  500  may not require block  502 . 
     Following block  502  is block  503 , where the localization array may be attached to the attachment location identified in block  502 . For example, clips  403  and  404  supporting localization array  406  may be attached to attachment points identified in block  502  so that the localization array  406  can be positioned at the optimal position identified in block  501 . In some embodiments, localization array setup process  500  may not require block  503  because the localization array may already be attached to the treatment couch. 
     From block  503 , execution may proceed to block  504 , where the localization array may be moved to the identified optimal position. At this stage, the localization array may already be attached to the treatment couch. Thus, moving the localization array to the optimal position identified in block  501  may entail using a positioning mechanism, such as positioning mechanism  303  or slides  407  and  408 , to move the localization array along one or more axes in relation to the treatment couch. 
     After the localization array has been moved to the optimal position, execution may proceed to block  505 , where the localization array is maintained at the optimal position during a treatment session. The position of the localization array may be maintained in accord with block  505  for only a portion of the entire treatment session. For example, if the fiducial markers move during the treatment session, the position of the localization array may be adjusted to compensate for the movement before the end of the treatment session. Furthermore, the localization array needs only to be sufficiently stationary to allow tracking of fiducial markers. For example, processing unit  107  or another part of the tracking system may tolerate or compensate for small movements of the localization array  105 . The position of the localization array may be maintained by physically immobilizing the localization array with respect to the treatment couch. For example, if the localization array is attached to the treatment couch through a positioning mechanism such as positioning mechanism  303 , the positioning mechanism may be turned off or locked in place. If the localization array is equipped with slides such as slides  407  and  408 , then the slides may be locked, clamped, or otherwise immobilized to prevent movement of the localization array. Alternatively, the localization array may maintain its position without further action. For example, locking or clamping of slides  407  and  408  may not be required if friction is sufficient to prevent undesired movement of localization array  406 . Also, if the localization array is integrated into the treatment couch, such as localization array  201 , the localization array may already be immobilized with respect to the treatment couch. 
     While the localization array is being maintained in a known position, the localization array may be used to track locations of fiducial markers relative to the localization array, as provided in block  506 . If the position of the localization array relative to the treatment couch and the position of the treatment couch relative to other equipment are also known, then the locations of the tracked fiducial markers relative to other equipment may be determined. For example, the locations of the fiducial markers may be determined relative to the location of a LINAC administering radiation treatment. Block  506  may also be repeated during the course of the treatment session so that the location of the fiducial markers can be periodically updated. Thus, maintaining the localization array in a stable position relative to the treatment couch may eliminate the need for recalculating or re-detecting the positional offset of the localization array from the treatment couch each time the locations of the fiducial markers are determined. Furthermore, if the localization array is attached to the treatment couch, the localization array may maintain its positional offset relative to the treatment couch even if the treatment couch is moved during the treatment session. Another advantage of maintaining the positional offset between the localization array and the treatment table is that the fiducial markers within the detection field of the localization array may remain within the detection field of the localization array even if the treatment couch is moved. For example, implanted fiducial markers  106  within patient  103  lying on treatment couch  104  may be encompassed by the detection field of localization array  105 . If localization array  105  is attached to treatment couch  104 , then patient  103 , implanted fiducials  106 , localization array  105 , and the detection field of localization array  105  may all move concurrently with treatment couch  104 . Thus, the detection field will continue to encompass the implanted markers  106  even if the treatment couch  104  is moved, facilitating reliable tracking of the implanted fiducials  106 . By tracking the locations of fiducial markers, a treatment delivery system such as treatment delivery system  100  may then use robotic arm  101  to position LINAC  102  to deliver radiation to a target that can be located with reference to the implanted fiducials  106 . 
       FIG. 6  illustrates one embodiment of a treatment delivery system with a localization array. Treatment delivery system  600  is configured to compensate for positional information error in an electromagnetic tracking system resulting from perturbations in the electromagnetic tracking system&#39;s magnetic field. Such distortions of the magnetic field tend to increase error in the determination of the position of implanted fiducials by the electromagnetic tracking system. The electromagnetic tracking system of treatment delivery system  600  may include a processing unit  107 , localization array  105 . 
     In one embodiment, the bias or offset error caused by perturbations in the magnetic field may be initially compensated by calibrating or characterizing the effect of the treatment delivery system (e.g., gantry system or robot-mounted LINAC  102 ), which could for example be a one-time procedure involving precalibration and storage of distortion offset for one or more possible positions of a gantry or robotic arm. In some embodiments, electromagnetic disturbances caused by equipment can be minimized by constructing the equipment from low conductivity or non-conductive materials. For example, the treatment couch  104  may be constructed from Kevlar. As compared to other light-weight materials, such as carbon-fiber, Kevlar is less conductive and thus tends to cause less distortion in the electromagnetic field. Nevertheless, additional sources of magnetic field distortion may not be accounted for through material selection for the treatment couch. A possible additional source of error is metal implants inside a patient (e.g., hip prosthesis), for which changing the material of the treatment couch or precalibrating the system may not be feasible. 
     In another embodiment, an independent fiducial localization method is used in order to compute the distortion offset. The positions of the implanted fiducial may obtained through use of an imaging system (e.g., X-ray, x-ray computed tomography, etc.) that can localize the implanted fiducials with very high accuracy (e.g., several tenths of a millimeter or better). The difference between the imaging acquired implanted fiducial positions and the fiducial positions determined using localization array  105  is, thus, the bias or offset of the electromagnetic tracking positions. An x-ray imager is discussed hereafter for ease of explanation purposes. It should be noted again that other imaging modalities and systems such as cone-beam CT (i.e., x-ray imaging system rotated about the patient to generate a cone-beam CT) and ultrasound may be used. 
     One embodiment of an X-ray imaging system includes both an X-ray source and an X-ray detector panel. Treatment delivery system  600  includes X-ray sources  610  and X-ray detector panels  611 . The X-ray sources  610  may be mounted angularly apart, for example, about 90 degrees apart, and aimed through the treatment target (e.g., tumor within the patient) toward the X-ray detector panels  611 . Alternatively, a single large detector may be used that would be illuminated by each of the X-ray sources  610 . In the single detector imaging system, the X-ray sources  610  may be positioned apart at an angle less than 90 degrees to keep both images on the single detector surface. 
     The detector panels  611  may be placed below the treatment target (e.g., on the floor), on the treatment couch  104 , or underneath the LINAC  102 , and the X-ray sources  610  may be positioned above the treatment target (e.g., the ceiling of the treatment room), to minimize magnification of the images and, therefore, the required size of the detector panels  611 . In an alternative embodiment, the positions of the X-ray sources  610  and the detector panels  611  may be reversed, e.g. the X-ray sources  610  below the treatment target and the detector panels  611  above the treatment target. In another embodiment, the detector panels  611  are arranged in a manner such that they move into position for imaging and are moved out of the way during positioning of the LINAC  102  or the treatment couch  104  or during delivery of the radiation beam from the LINAC  102 . 
     By using two X-ray imagers, which could be mounted approximately orthogonally, the 2D positions of the implanted fiducials  106  in the X-ray images can be back projected to obtain their 3D positions in the treatment room. The fiducial positions obtained using x-ray image localization can be known with very high accuracy (several tenths of a millimeter or better). The difference between the X-ray image-based fiducial positions and the fiducial positions determined using localization array  105  is thus the bias or offset of the electromagnetic tracking positions. In one embodiment, the electromagnetic bias or offset is calculated by processing unit  107 , which may be connected to localization array  105  and X-ray detector panels  611 . Thus, processing unit  107  may receive the locations of the implanted fiducials  106  as determined by the localization array  105  and as determined by X-ray imaging. Processing unit  107  may further use the calculated electromagnetic bias or offset in controlling robotic arm  101  to direct the beam of LINAC  102  at a treatment target. 
     The electromagnetic bias often changes slowly with spatial location. Thus, for small motions, the offset can be used to correct the real-time electromagnetic tracking positions, as determined using the localization array  105 . Since electromagnetic tracking with the localization array  105  does not involve radiation exposure to the patient, it may be used to track the positions of implanted fiducials  106  in near real time. The use of electromagnetic tracking to avoid the use of X-ray imaging is desirable because very frequent X-ray images of the patient may result in excessive radiation exposure to the patient. Any bias in the electromagnetic tracking positions can be corrected by taking occasional X-ray images to locate the implanted fiducials  106  with greater accuracy. The frequency of the X-ray images can be varied as necessary, for example, at every position of the treatment delivery system (e.g., gantry or LINAC  102 ). Alternatively, X-ray images may be captured periodically over time. New X-ray images could also be acquired whenever the electromagnetic tracking fiducials move beyond a certain threshold. Note that using an X-ray imaging system also requires that the treatment couch (e.g., in the form of a table or chair) be relatively radiolucent. 
     In order to determine the fiducial positions relative to the treatment room coordinate system, the position of localization array  105  in the treatment room needs to be known. The location of the localization array  105  can be obtained, for example, by tracking the array with an optical tracking system. However, this requires a line of sight between the array and the optical system. Placing the localization array  105  under the patient  103  is desirable for many reasons. This placement reduces potential collision issues with the treatment delivery system (gantry system or robotic arm  101  with LINAC  102 ). Placing the localization array  105  under the patient  103  could be accomplished by placing the array  105  on the treatment couch  104  and placing a pad over the array  105 . The localization array  105  could also be integrated into the treatment couch  104 . The localization array  105  may be movable to accommodate different patient sizes and setup positions and different treatment target positions. For example, localization array  105  could be placed in a platform or housing such that it can be easily moved along the long axis of the table and possibly also along the short axis of the table. Placing the array  105  under the patient  103 , whether on or integrated into the treatment couch  104 , may require a non-optical method for obtaining the position of the array  105  in the treatment room. One non-optical method for tracking the position of array  105  is to use one or more (e.g., two) X-ray imagers. 
       FIG. 7  illustrates one embodiment of a localization array  700  that can be tracked using X-ray imaging. Localization array  700  includes fiducial markers  701  that are located outside of patient  103 , and also includes electromagnetic coils  203 . Localization array  700  is integrated in treatment couch  202  so that detection field  204  encompasses an area above treatment couch  202 . 
     With external fiducial markers  701  embedded in array  700 , the position of array  700  can be tracked using a tracking device such as an X-ray imager. It should be noted that the term “external fiducial markers” is used herein to distinguish from the implanted fiducials that are internal to a patient. Accordingly, an external fiducial marker is one which is disposed outside of a patient&#39;s body. External fiducial markers  701  may be made from a radio-opaque material so that external fiducials  701  will appear in X-ray images. By using two X-ray imagers, which could be mounted approximately orthogonally, the 2D positions of the external fiducials  701  in the X-ray images can be back projected to obtain their 3D positions. The 3D positions are transformed to the treatment room coordinate system and, thus, the position of the localization array  700  in the treatment room coordinate system is determined. Such transformations are known in the art; accordingly, a detailed description is not provided herein. Alternatively, electromagnetic coils  203  in the array  700  could be used as fiducials to be tracked by a tracking device. If the tracking device is an X-ray imager, the coils may be uniquely distinguishable radiographically. For example, each of the electromagnetic coils  203  may have a unique shape so that the coil and its position can be uniquely identified in an X-ray image. The position of localization array  700  could be determined with other methods as well. In one embodiment, if the localization array  700  is integrated into the treatment couch  202  and is movable with respect to treatment couch  202 , a position tracking device may include mechanical encoders which can be used to obtain the position of the localization array  700  relative to the treatment couch  202 . Mechanical encoders may also be used to determine the position of treatment couch  202  in the treatment room. Treatment couch  202  could be a conventional treatment table, or could be a table top mounted on a robotic manipulator. 
     Thus, the position of localization array  700  relative to the treatment room coordinate system and the positions of implanted fiducial markers  106  in patient  103  relative to localization array  700 , as electromagnetically tracked by the localization array, can be used to determine the positions of implanted fiducials  106  relative to the treatment room coordinate system. In addition, bias or offset error in the electromagnetically tracked position of the implanted fiducials  106  caused by disturbances in the electromagnetic field can be corrected by using occasional X-ray images to more accurately locate the implanted fiducials  106 . This method for determining the location of the implanted fiducials  106  may be used not only with localization arrays that are integrated with treatment couch  104 , but also with localization arrays that are not coupled to the treatment couch  104 . 
       FIG. 8  illustrates a treatment delivery system capable of tracking the location of fiducial markers using a localization array external to the treatment couch, in conjunction with X-ray imaging according to one embodiment of the invention. Treatment delivery system  800  includes electromagnetic tracking system  810  connected to localization array  811 . Localization array  811  has a detection field  812  that extends downwards to encompass implanted fiducials  106 . Treatment delivery system  800  also includes X-ray imagers comprising X-ray sources  610  and X-ray detector panels  611 . 
     Localization array  811  may be similar to localization array  700 , having electromagnetic coils that can be used to determine the positions of implanted fiducials  106 . Localization array  811  is not attached to treatment couch  104 , and thus may be moved independently of treatment couch  104 . In one embodiment, localization array  811  is mechanically supported by electromagnetic tracking system  810 . For example, localization array  811  may be mounted on an adjustable arm  813  connected to electromagnetic tracking system  810 , which allows localization array  811  to be situated above patient  103 . Accordingly, localization array  811  may have a detection field  812  that extends downward to encompass implanted fiducials  106  so that localization array  811  can track the positions of implanted fiducials  106 . Electromagnetic tracking system  810  that is attached to localization array  811  may be an external unit containing electronics, such as drive circuitry and sensors used during the operation of localization array  811 . Electromagnetic tracking system  810  may receive fiducial positions from localization array  811 , and may further be connected to processing unit  107  so that the fiducial positions can be transmitted to processing unit  107 . At processing unit  107 , the fiducial positions can be used to control the movement of robotic arm  101 . 
     Localization array  811  may also be affected by disturbances in the electromagnetic field, such that error is increased in the electromagnetically tracked positions of the implanted fiducials  106 . X-ray imagers comprising X-ray sources  610  and X-ray detector panels  611  can be used to compensate for bias or offset caused by such electromagnetic disturbances. By using two X-ray imagers mounted approximately orthogonally, the 2D positions of the implanted fiducials  106  in the X-ray images can be back projected to obtain their 3D positions in the treatment room with high accuracy. The difference between the X-ray image-based fiducial positions and the fiducial positions determined using localization array  811  is thus the bias or offset of the electromagnetic tracking positions. This electromagnetic bias or offset may be calculated by processing unit  107 , which receives the positions of implanted fiducials  106  as determined by the localization array  105  and the X-ray imaging system. The offset can then be used to correct the real-time electromagnetically tracked positions, as determined using the localization array  811 . Any bias in the electromagnetically tracked positions can be corrected by taking occasional X-ray images to locate the implanted fiducials  106  with greater accuracy. The frequency of the X-ray images can be varied as necessary, for example, at every spatial position of the treatment delivery system (e.g., gantry or LINAC  102 ). New X-ray images could also be acquired whenever the electromagnetically tracked implanted fiducials  106  move beyond a certain positional threshold. 
     The resulting corrected positions of implanted fiducials  106  can be used by processing unit  107  to control robotic arm  101 . Using these positions, processing unit  107  may direct robotic arm to aim LINAC  102  so that the beam of LINAC  102  intersects a treatment target that can be located by reference to the implanted fiducials  106 . 
     Alternatively, treatment delivery system  100  may be a type of system other than a robotic arm-based system. For example, treatment delivery system  100  may be a gantry-based (isocentric) intensity modulated radiotherapy (IMRT) system. In a gantry based system, a radiation source (e.g., a LINAC) is mounted on the gantry in such a way that it rotates in a plane corresponding to an axial slice of the patient. Radiation is then delivered from several positions on the circular plane of rotation. In IMRT, the shape of the radiation beam is defined by a multi-leaf collimator that allows portions of the beam to be blocked, so that the remaining beam incident on the patient has a pre-defined shape. The resulting system generates arbitrarily shaped radiation beams that intersect each other at the isocenter to deliver a dose distribution to the target region. In IMRT planning, the optimization algorithm selects subsets of the main beam and determines the amount of time that the patient should be exposed to each subset, so that the prescribed dose constraints are best met. In one particular embodiment, the gantry-based system may have a gimbaled radiation source head assembly. 
     It should be noted that the methods and apparatus described herein are not limited to use only with medical diagnostic imaging and treatment. In alternative embodiments, the methods and apparatus herein may be used in applications outside of the medical technology field, such as industrial imaging and non-destructive testing of materials. In such applications, for example, “treatment” may refer generally to the effectuation of an operation controlled by the treatment planning system, such as the application of a beam (e.g., radiation, acoustic, etc.) and “target” may refer to a non-anatomical object or area. 
     Certain embodiments may be implemented as a computer program product that may include instructions stored on a computer-readable medium. These instructions may be used to program a general-purpose or special-purpose processor to perform the described operations. A computer-readable medium includes any mechanism for storing or transmitting information in a form (e.g., software, processing application) readable by a computer. The computer-readable medium may include, but is not limited to, magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto-optical storage medium; read-only memory (ROM); random-access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory; or another type of medium suitable for storing electronic instructions. 
     Additionally, some embodiments may be practiced in distributed computing environments where the computer-readable medium is stored on and/or executed by more than one computer system. In addition, the information transferred between computer systems may either be pulled or pushed across the communication medium connecting the computer systems. 
     Although the operations of the method(s) herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operation may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be in an intermittent and/or alternating manner. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.