Abstract:
A patient positioning system for use with a radiation therapy system that monitors the location of fixed and movable components and pre-plans movement of the movable components so as to inhibit movement if a collision would be indicated. The positioning system can also coordinate movement of multiple movable components for reduced overall latency in registering a patient. The positioning system includes external measurement devices which measure the location and orientation of objects, including components of the radiation therapy system, in space and can also monitor for intrusion into the active area of the therapy system by personnel or foreign objects to improve operational safety of the radiation therapy system.

Description:
[0001]    RELATED APPLICATIONS 
         [0002]    This application is a continuation of U.S. patent application Ser. No. 10/917,220, filed Aug. 12, 2004, and claims the benefit of the U.S. Provisional Application No. 60/494,699 filed Aug. 12, 2003 and U.S. Provisional Application No. 60/579,095 filed Jun. 10, 2004 both entitled “Precision Patient Alignment and Beam Therapy System” and all of which are incorporated in their entireties herein by reference. 
     
    
     GOVERNMENT SUPPORT 
       [0003]    This invention was made with United States Government support under the DAMD17-99-1-9477 and DAMD17-02-1-0205 grants awarded by the Department of Defense. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    The invention relates to the field of radiation therapy systems. One embodiment includes an active path planning and collision avoidance system to facilitate movement of objects in a radiation therapy environment in an efficient manner and so as to proactively avoid possible collisions. 
         [0006]    2. Description of the Related Art 
         [0007]    Radiation therapy systems are known and used to provide treatment to patients suffering a wide variety of conditions. Radiation therapy is typically used to kill or inhibit the growth of undesired tissue, such as cancerous tissue. A determined quantity of high-energy electromagnetic radiation and/or high-energy particles are directed into the undesired tissue with the goal of damaging the undesired tissue while reducing unintentional damage to desired or healthy tissue through which the radiation passes on its path to the undesired tissue. 
         [0008]    Proton therapy has emerged as a particularly efficacious treatment for a variety of conditions. In proton therapy, positively charged proton subatomic particles are accelerated, collimated into a tightly focused beam, and directed towards a designated target region within the patient. Protons exhibit less lateral dispersion upon impact with patient tissue than electromagnetic radiation or low mass electron charged particles and can thus be more precisely aimed and delivered along a beam axis. Also, upon impact with patient tissue, protons exhibit a characteristic Bragg peak wherein a significant portion of the kinetic energy of the accelerated mass is deposited within a relatively narrow penetration depth within the patient. This offers the significant advantage of reducing delivery of energy from the accelerated proton particles to healthy tissue interposed between the target region and the delivery nozzle of a proton therapy machine as well as to “downrange” tissue lying beyond the designated target region. Depending on the indications for a particular patient and their condition, delivery of the therapeutic proton beam may preferably take place from a plurality of directions in multiple treatment fractions to maintain a total dose delivered to the target region while reducing collateral exposure of interposed desired/healthy tissue. 
         [0009]    Thus, a radiation therapy system, such as a proton beam therapy system, typically has provision for positioning a patient with respect to a proton beam in multiple orientations. In order to determine a preferred aiming point for the proton beam within the patient, the typical procedure has been to perform a computed tomography (CT) scan in an initial planning or prescription stage from which multiple digitally reconstructed radiographs (DRRs) can be determined. The DRRs synthetically represent the three dimensional data representative of the internal physiological structure of the patient obtained from the CT scan in two dimensional views considered from multiple orientations. A desired target isocenter corresponding to the tissue to which therapy is to be provided is designated. The spatial location of the target isocenter can be referenced with respect to physiological structure of the patient (monuments) as indicated in the DRRs. 
         [0010]    Upon subsequent setup for delivery of the radiation therapy, an x-ray imager is moved into an imaging position and a radiographic image is taken of the patient. This radiographic image is compared or registered with the DRRs with respect to the designated target isocenter. The patient&#39;s position is adjusted to, as closely as possible, align the target isocenter in a desired pose with respect to the radiation beam as indicated by the physician&#39;s prescription. The desired pose is frequently chosen as that of the initial planning or prescription scan. Depending on the particular application, either the patient and/or the beam nozzle will need to be moved. 
         [0011]    There is a desire that movement of components of the therapy system to achieve alignment be done in an accurate, rapid manner while maintaining overall system safety. In particular, a radiation therapy apparatus is an expensive piece of medical equipment to construct and maintain both because of the materials and equipment needed in construction and the indication for relatively highly trained personnel to operate and maintain the apparatus. In addition, radiation therapy, such as proton therapy, is increasing being found an effective treatment for a variety of patient conditions and thus it is desirable to increase patient throughput both to expand the availability of this beneficial treatment to more patients in need of the same as well as reducing the end costs to the patients or insurance companies paying for the treatment and increase the profitability for the therapy delivery providers. As the actual delivery of the radiation dose, once the patient is properly positioned, is relatively quick, any additional latency in patient ingress and egress from the therapy apparatus, imaging, and patient positioning and registration detracts from the overall patient throughput and thus the availability, costs, and profitability of the system. 
         [0012]    The movable components of a radiation therapy system also tend to be rather large and massive, thus indicating powered movement of the various components. As the components tend to have significant inertia during movement and are typically power driven, a safety system to inhibit damage and injury can be provided. Safety systems can include power interrupts based on contact switches. The contact switches are activated at motion stop range of motion limits to cut power to drive motors. Hard motion stops or limiters can also be provided to physically impede movement beyond a set range. However, contact switches and hard stops are activated when the corresponding component(s) reach the motion limit and thus impose a relatively abrupt motion stop which adds to wear on the machinery and can even lead to damage if engaged excessively. In addition, particularly in application involving multiple moving components, a motion stop arrangement of contact switches and/or hard limiters involves significant complexity to inhibit collision between the multiple components and can lead to inefficiencies in the overall system operation if the components are limited to moving one at a time to simplify the collision avoidance. 
         [0013]    From the foregoing it will be understood that there is a need for providing a collision avoidance system to maintain operating safety and damage control while positioning multiple movable components of a radiation therapy delivery system. There is also a desire to maintain the accuracy and speed of the patient registration process when implementing such a collision avoidance system. 
       SUMMARY OF THE INVENTION 
       [0014]    Embodiments of the invention provide a patient positioning system for a therapeutic radiation system having moving components. The patient positioning system pre-plans and analyzes movements to increase movement efficiency for decreased latency and to pro-actively avoid collisions. The patient positioning system includes multiple cameras that can both determine the location of fixed and movable components of the system as well as monitor for possible intrusion into a movement path of a foreign object or personnel. The system provides significant safety advantages over systems employing motion stops. 
         [0015]    One embodiment comprises a radiation therapy delivery system having fixed and movable components, the system comprising a gantry, a patient pod configured to secure a patient substantially immobile with respect to the patient pod, a patient positioner interconnected to the patient pod so as to position the patient pod along multiple translational and rotational axes within the gantry, a radiation therapy nozzle interconnected to the gantry and selectively delivering radiation therapy along a beam axis, a plurality of external measurement devices which obtain position measurements of at least the patient pod and nozzle, and a controller which receives the position measurements of at least the patient pod and nozzle and determines movement commands to position the patient in a desired pose with respect to the beam axis and corresponding movement trajectories of the patient pod with respect to other fixed and movable components of the therapy delivery system based upon the movement commands and determines whether a collision is indicated for the movement commands and inhibits movement if a collision would be indicated. 
         [0016]    Another embodiment comprises a path planning and collision avoidance system for a radiation therapy system having fixed and movable components and selectively delivering a radiation therapy beam along a beam axis, the positioning system comprising a plurality of external measurement devices arranged to obtain position measurements of the components so as to provide location information, a movable patient support configured to support a patient substantially fixed in position with respect to the patient support and controllably position the patient in multiple translational and rotational axes, and a controller receiving position information from the plurality of external measurement devices and providing movement commands to the movable patient support to automatically align the patient in a desired pose and determining a corresponding movement envelope wherein the controller evaluates the movement envelope and inhibits movement of the patient support if a collision is indicated else initiates the movement. 
         [0017]    A further embodiment comprises a method of registering and positioning a patient for delivery of therapy with a system having fixed and at least one movable components, the method comprising the steps of positioning a patient in an initial treatment pose with a controllable patient positioner, externally measuring the location of selected points of the fixed and at least one movable components, determining a difference vector between the observed initial patient pose and a desired patient pose, determining corresponding movement commands and a movement trajectory for the patient positioner to bring the patient to the desired patient pose, and comparing the movement trajectory with the measured locations of the selected points of the fixed and at least one movable components so as to inhibit movement of the patient positioner if a collision is indicated. 
         [0018]    Yet another embodiment comprises a system for delivering radiation therapy to a pre-selected location within a patient, the system comprising a plurality of movable components including a patient positioner and a nozzle, the system further comprising an external monitoring system that monitors the physical location of the plurality of movable components and provides signals indicative thereof and wherein the system further includes internal monitoring systems that also monitor the movement of the plurality of movable components and provides signals indicative thereof and wherein the system monitors the signals from the external and internal monitoring systems and inhibits movement of the plurality of components if the signals indicate that a collision of components is likely to occur. 
         [0019]    These and other objects and advantages of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0020]    A schematic diagram of one embodiment of a radiation therapy system with a patient positioning system in a first orientation is shown in  FIG. 1A  and in a second orientation in  FIG. 1B ; 
           [0021]      FIG. 2A  illustrates one embodiment of retractable imagers in an extended position and  FIG. 2B  illustrates the imagers in a retracted position; 
           [0022]      FIG. 3  illustrates one embodiment of a patient positioner to which a patient pod can be attached; 
           [0023]      FIGS. 4A-4E  illustrate various position error sources of one embodiment of a radiation therapy system; 
           [0024]      FIG. 5  is a flow chart of one embodiment of a method of determining the position and orientation of objects in a radiation therapy environment; 
           [0025]      FIG. 6  illustrates one embodiment of external measurement devices for a radiation therapy system; 
           [0026]      FIG. 7  illustrates further embodiments of external measurement devices for a radiation therapy system; 
           [0027]      FIG. 8  is a block diagram of one embodiment of a precision patient positioning system of a radiation therapy system; 
           [0028]      FIG. 9  is a block diagram of one embodiment of an external measurement and 6D coordination system of the patient positioning system; 
           [0029]      FIG. 10  is a block diagram of a patient registration module of the patient positioning system; 
           [0030]      FIG. 11  is a block diagram of a path planning module of a motion control module of the patient positioning system; 
           [0031]      FIG. 12  is a block diagram of an active collision avoidance module of the motion control module of the patient positioning system; 
           [0032]      FIG. 13  is a block diagram of one embodiment of the collision avoidance module and a motion sequence coordinator of a motion control module; and 
           [0033]      FIG. 14  is a flow chart of the operation of one embodiment of a method of positioning a patient and delivering radiation therapy. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0034]    Reference will now be made to the drawings wherein like reference designators refer to like parts throughout.  FIGS. 1A and 1B  illustrate schematically first and second orientations of one embodiment of a radiation therapy system  100 , such as based on the proton therapy system currently in use at Loma Linda University Medical Center in Loma Linda, Calif. and as described in U.S. Pat. No. 4,870,287 of Sep. 26, 1989 which is incorporated herein in its entirety by reference. The radiation therapy system  100  is designed to deliver therapeutic radiation doses to a target region within a patient for treatment of malignancies or other conditions from one or more angles or orientations with respect to the patient. The system  100  includes a gantry  102  which includes a generally hemispherical or frustoconical support frame for attachment and support of other components of the radiation therapy system  100 . Additional details on the structure and operation of embodiments of the gantry  102  may be found in U.S. Pat. No. 4,917,344 and U.S. Pat. No. 5,039,057, both of which are incorporated herein in their entirety by reference. 
         [0035]    The system  100  also comprises a nozzle  104  which is attached and supported by the gantry  102  such that the gantry  102  and nozzle  104  may revolve relatively precisely about a gantry isocenter  120 , but subject to corkscrew, sag, and other distortions from nominal. The system  100  also comprises a radiation source  106  delivering a radiation beam along a radiation beam axis  140 , such as a beam of accelerated protons. The radiation beam passes through and is shaped by an aperture  110  to define a therapeutic beam delivered along a delivery axis  142 . The aperture  110  is positioned on the distal end of the nozzle  104  and the aperture  110  may preferably be specifically configured for a patient&#39;s particular prescription of therapeutic radiation therapy. In certain applications, multiple apertures  110  are provided for different treatment fractions. 
         [0036]    The system  100  also comprises one or more imagers  112  which, in this embodiment, are retractable with respect to the gantry  102  between an extended position as illustrated in  FIG. 2A  and a retracted position as illustrated in  FIG. 2B . The imager  112  in one implementation comprises a commercially available solid-state amorphous silicon x-ray imager which can develop image information such as from incident x-ray radiation that has passed through a patient&#39;s body. The retractable aspect of the imager  112  provides the advantage of withdrawing the imager screen from the delivery axis  142  of the radiation source  106  when the imager  112  is not needed thereby providing additional clearance within the gantry  102  enclosure as well as placing the imager  112  out of the path of potentially harmful emissions from the radiation source  106  thereby reducing the need for shielding to be provided to the imager  112 . 
         [0037]    The system  100  also comprises corresponding one or more x-ray sources  130  which selectively emit appropriate x-ray radiation along one or more x-ray source axes  144  so as to pass through interposed patient tissue to generate a radiographic image of the interposed materials via the imager  112 . The particular energy, dose, duration, and other exposure parameters preferably employed by the x-ray source(s)  130  for imaging and the radiation source  106  for therapy will vary in different applications and will be readily understood and determined by one of ordinary skill in the art. 
         [0038]    In this embodiment, at least one of the x-ray sources  130  is positionable such that the x-ray source axis  144  can be positioned so as to be nominally coincident with the delivery axis  142 . This embodiment provides the advantage of developing a patient image for registration from a perspective which is nominally identical to a treatment perspective. This embodiment also includes the aspect that a first imager  112  and x-ray source  130  pair and a second imager  112  and x-ray source  130  pair are arranged substantially orthogonal to each other. This embodiment provides the advantage of being able to obtain patient images in two orthogonal perspectives to increase registration accuracy as will be described in greater detail below. The imaging system can be similar to the systems described in U.S. Pat. Nos. 5,825,845 and 5,117,829 which are hereby incorporated by reference. 
         [0039]    The system  100  also comprises a patient positioner  114  ( FIG. 3 ) and a patient pod  116  which is attached to a distal or working end of the patient positioner  114 . The patient positioner  114  is adapted to, upon receipt of appropriate movement commands, position the patient pod  116  in multiple translational and rotational axes and preferably is capable of positioning the patient pod  116  in three orthogonal translational axes as well as three orthogonal rotational axes so as to provide a full six degree freedom of motion to placement of the patient pod  116 . 
         [0040]    The patient pod  116  is configured to hold a patient securely in place in the patient pod  116  so to as substantially inhibit any relative movement of the patient with respect to the patient pod  116 . In various embodiments, the patient pod  116  comprises expandable foam, bite blocks, and/or fitted facemasks as immobilizing devices and/or materials. The patient pod  116  is also preferably configured to reduce difficulties encountered when a treatment fraction indicates delivery at an edge or transition region of the patient pod  116 . Additional details of preferred embodiments of the patient positioner  114  and patient pod  116  can be found in the commonly assigned application (serial number unknown, attorney docket number LOMARRL.132A) entitled “Modular Patient Support System” filed concurrently herewith and which is incorporated herein in its entirety by reference. 
         [0041]    As previously mentioned, in certain applications of the system  100 , accurate relative positioning and orientation of the therapeutic beam delivery axis  142  provided by the radiation source  106  with target tissue within the patient as supported by the patient pod  116  and patient positioner  114  is an important goal of the system  100 , such as when comprising a proton beam therapy system. However, as previously mentioned, the various components of the system  100 , such as the gantry  102 , the nozzle  104 , radiation source  106 , the imager(s)  112 , the patient positioner  114 , the patient pod  116 , and x-ray source(s)  130  are subject to certain amounts of structural flex and movement tolerances from a nominal position and orientation which can affect accurate delivery of the beam to that patient. 
         [0042]      FIGS. 1A and 1B  illustrate different arrangements of certain components of the system  100  and indicate by the broken arrows both translational and rotational deviations from nominal that can occur in the system  100 . For example, in the embodiment shown in  FIG. 1A , the nozzle  104  and first imager  112  extend substantially horizontally and are subject to bending due to gravity, particularly at their respective distal ends. The second imager  112  is arranged substantially vertically and is not subject to the horizontal bending of the first imager  112 .  FIG. 1B  illustrates the system  100  in a different arrangement rotated approximately 45° counterclockwise from the orientation of  FIG. 1A . In this orientation, both of the imagers  112  as well as the nozzle  104  are subject to bending under gravity, but to a different degree than in the orientation illustrated in  FIG. 1A . The movement of the gantry  102  between different orientations, such as is illustrated in  FIGS. 1A and 1B  also subjects components of the system  100  to mechanical tolerances at the moving surfaces. As these deviations from nominal are at least partially unpredictable, non-repeatable, and additive, correcting for the deviations on a predictive basis is extremely challenging and limits overall alignment accuracy. It will be appreciated that these deviations from the nominal orientation of the system are simply exemplary and that any of a number of sources of error can be addressed by the system disclosed herein without departing from the spirit of the present invention. 
         [0043]      FIGS. 4A-4E  illustrate in greater detail embodiments of potential uncertainties or errors which can present themselves upon procedures for alignment of, for example, the nozzle  104  and the target tissue of the patient at an isocenter  120 .  FIGS. 4A-4E  illustrate these sources of uncertainty or error with reference to certain distances and positions. It will be appreciated that the sources of error described are simply illustrative of the types of errors addressed by the system  100  of the illustrated embodiments and that the system  100  described is capable of addressing additional errors. In this embodiment, a distance SAD is defined as a source to axis distance from the radiation source  106  to the rotation axis of the gantry, which ideally passes through the isocenter  120 . For purposes of explanation and appreciation of relative scale and distances, in this embodiment, SAD is approximately equal to 2.3 meters. 
         [0044]      FIG. 4A  illustrates that one of the potential sources of error is a source error where the true location of the radiation source  106  is subject to offset from a presumed or nominal location. In this embodiment, the therapeutic radiation beam as provided by the radiation source  106  passes through two transmission ion chambers (TIC) which serve to center the beam. These are indicated as TIC  1  and TIC  3  and these are also affixed to the nozzle  104 . The source error can arise from numerous sources including movement of the beam as observed on TIC  1  and/or TIC  3 , error in the true gantry  102  rotational angle, and error due to “egging” or distortion from round of the gantry  102  as it rotates.  FIG. 4A  illustrates source error comprising an offset of the true position of the radiation source  106  from a presumed or nominal location and the propagation of the radiation beam across the SAD distance through the aperture  110  providing a corresponding error at isocenter  120 . 
         [0045]      FIG. 4B  illustrates possible error caused by TIC location error, where TIC  1 , the radiation source  106 , and TIC  3  are offset from an ideal beam axis passing through the nominal gantry isocenter  120 . As the errors illustrated by  FIG. 4A and 4B  are assumed random and uncorrelated, they can be combined in quadrature and projected through an assumed nominal center of the aperture  110  to establish a total error contribution due to radiation source  106  error projected to the isocenter  120 . In this embodiment, before corrective measures are taken (as described in greater detail below), the radiation source error can range from approximately ±0.6 mm to ±0.4 mm. 
         [0046]      FIG. 4C  illustrates error or uncertainty due to position of the aperture  110 . The location of the radiation source  106  is assumed nominal; however, error or uncertainty is introduced both by tolerance stack-up, skew, and flex of the nozzle  104  as well as manufacturing tolerances of the aperture  110  itself. Again, as projected from the radiation source  106  across the distance SAD to the nominal isocenter  120 , a beam delivery aiming point (BDAP) error is possible between a presumed nominal BDAP and an actual BDAP. In this embodiment, this BDAP error arising from error in the aperture  110  location ranges from approximately ±1.1 mm to ±1.5 mm. 
         [0047]    The system  100  is also subject to error due to positioning of the imager(s)  112  as well as the x-ray source(s)  130  as illustrated in  FIGS. 4D and 4E .  FIG. 4D  illustrates the error due to uncertainty in the imager(s)  112  position with the position of the corresponding x-ray source(s)  130  assumed nominal. As the emissions from the x-ray source  130  pass through the patient assumed located substantially at isocenter  120  and onward to the imager  112 , this distance may be different than the SAD distance and in this embodiment is approximately equal to 2.2 meters. Error or uncertainty in the true position of an imager  112  can arise from lateral shifts in the true position of the imager  112 , errors due to axial shifting of the imager  112  with respect to the corresponding x-ray source  130 , as well as errors in registration of images obtained by imager  112  to the DRRs. In this embodiment, before correction, the errors due to each imager  112  are approximately ±0.7 mm. 
         [0048]    Similarly,  FIG. 4E  illustrates errors due to uncertainty in positioning of the x-ray source(s)  130  with the position of the corresponding imager(s)  112  assumed nominal. Possible sources of error due to the x-ray source  130  include errors due to initial alignment of the x-ray source  130 , errors arising from movement of the x-ray source  130  into and out of the beam line, and errors due to interpretation of sags and relative distances of TIC  1  and TIC  3 . These errors are also assumed random and uncorrelated or independent and are thus added in quadrature resulting, in this embodiment, in error due to each x-ray source  130  of approximately ±0.7 mm. 
         [0049]    As these errors are random and independent and uncorrelated and thus potentially additive, in this embodiment the system  100  also comprises a plurality of external measurement devices  124  to evaluate and facilitate compensating for these errors. In one embodiment, the system  100  also comprises monuments, such as markers  122 , cooperating with the external measurement devices  124  as shown in  FIGS. 2A ,  2 B,  6  and  7 . The external measurement devices  124  each obtain measurement information about the three-dimensional position in space of one or more components of the system  100  as indicated by the monuments as well as one or more fixed landmarks  132  also referred to herein as the “world”  132 . 
         [0050]    In this embodiment, the external measurement devices  124  comprise commercially available cameras, such as CMOS digital cameras with megapixel resolution and frame rates of 200-1000 Hz, which independently obtain optical images of objects within a field of view  126 , which in this embodiment is approximately 85° horizontally and 70° vertically. The external measurement devices  124  comprising digital cameras are commercially available, for example as components of the Vicon Tracker system from Vicon Motion Systems Inc. of Lake Forrest, Calif. However, in other embodiments, the external measurement devices  124  can comprise laser measurement devices and/or radio location devices in addition to or as an alternative to the optical cameras of this embodiment. 
         [0051]    In this embodiment, the markers  122  comprise spherical, highly reflective landmarks which are fixed to various components of the system  100 . In this embodiment, at least three markers  122  are fixed to each component of the system  100  of interest and are preferably placed asymmetrically, e.g. not equidistant from a centerline nor evenly on corners, about the object. The external measurement devices  124  are arranged such that at least two external measurement devices  124  have a given component of the system  100  and the corresponding markers  122  in their field of view and in one embodiment a total of ten external measurement devices  124  are provided. This aspect provides the ability to provide binocular vision to the system  100  to enable the system  100  to more accurately determine the location and orientation of components of the system  100 . The markers  122  are provided to facilitate recognition and precise determination of the position and orientation of the objects to which the markers  122  are affixed, however in other embodiments, the system  100  employs the external measurement devices  124  to obtain position information based on monuments comprising characteristic outer contours of objects, such as edges or corners, comprising the system  100  without use of the external markers  122 . 
         [0052]      FIG. 5  illustrates one embodiment of determining the spatial position and angular orientation of a component of the system  100 . As the component(s) of interest can be the gantry  102 , nozzle  104 , aperture  110 , imager  112 , world  132  or other components, reference will be made to a generic “object”. It will be appreciated that the process described for the object can proceed in parallel or in a series manner for multiple objects. Following a start state, in state  150  the system  100  calibrates the multiple external measurement devices  124  with respect to each other and the world  132 . In the calibration state, the system  100  determines the spatial position and angular orientation of each external measurement device  124 . The system  100  also determines the location of the world  132  which can be defined by a dedicated L-frame and can define a spatial origin or frame-of-reference of the system  100 . The world  132  can, of course, comprise any component or structure that is substantially fixed within the field of view of the external measurement devices  124 . Hence, structures that are not likely to move or deflect as a result of the system  100  can comprise the world  132  or point of reference for the external measurement devices  124 . 
         [0053]    A wand, which can include one or more markers  122  is moved within the fields of view  126  of the external measurement devices  124 . As the external measurement devices  124  are arranged such that multiple external measurement devices  124  (in this embodiment at least two) have an object in the active area of the system  100  in their field of view  126  at any given time, the system  100  correlates the independently provided location and orientation information from each external measurement device  124  and determines corrective factors such that the multiple external measurement devices  124  provide independent location and orientation information that is in agreement following calibration. The particular mathematical steps to calibrate the external measurement devices  124  are dependent on their number, relative spacing, geometrical orientations to each other and the world  132 , as well as the coordinate system used and can vary among particular applications, however will be understood by one of ordinary skill in the art. It will also be appreciated that in certain applications, the calibration state  150  would need to be repeated if one or more of the external measurement devices  124  or world  132  is moved following calibration. 
         [0054]    Following the calibration state  150 , in state  152  multiple external measurement devices  124  obtain an image of the object(s) of interest. From the images obtained in state  152 , the system  100  determines a corresponding direction vector  155  to the object from each corresponding external measurement device  124  which images the object in state  154 . This is illustrated in  FIG. 6  as vectors  155   a - d  corresponding to the external measurement devices  124   a - d  which have the object in their respective fields of view  126 . Then, in state  156 , the system  100  calculates the point in space where the vectors  155  ( FIG. 6 ) determined in state  154  intersect. State  156  thus returns a three-dimensional location in space, with reference to the world  132 , for the object corresponding to multiple vectors intersecting at the location. As the object has been provided with three or more movements or markers  122 , the system  100  can also determine the three-dimensional angular orientation of the object by evaluating the relative locations of the individual markers  122  associated with the object. In this implementation, the external measurement devices  124  comprise cameras, however, any of a number of different devices can be used to image, e.g., determine the location, of the monuments without departing from the spirit of the present invention. In particular, devices that emit or receive electromagnetic or audio energy including visible and non-visible wavelength energy and ultra-sound can be used to image or determine the location of the monuments. 
         [0055]    The location and orientation information determined for the object is provided in state  160  for use in the system  100  as described in greater detail below. In one embodiment, the calibration state  150  can be performed within approximately one minute and allows the system  100  to determine the object&#39;s location in states  152 ,  154 ,  156 , and  160  to within 0.1 mm and orientation to within 0.15° with a latency of no more than 10 ms. As previously mentioned, in other embodiments, the external measurement devices  124  can comprise laser measurement devices, radio-location devices or other devices that can determine direction to or distance from the external measurement devices  124  in addition to or as an alternative to the external measurement devices  124  described above. Thus, in certain embodiments a single external measurement device  124  can determine both range and direction to the object to determine the object location and orientation. In other embodiments, the external measurement devices  124  provide only distance information to the object and the object&#39;s location in space is determined by determining the intersection of multiple virtual spheres centered on the corresponding external measurement devices  124 . 
         [0056]    In certain embodiments, the system  100  also comprises one or more local position feedback devices or resolvers  134  (See, e.g.,  FIG. 1 ). The local feedback devices or resolvers  134  are embodied within or in communication with one or more components of the system  100 , such as the gantry  102 , the nozzle  104 , the radiation source  106 , the aperture  110 , the imager(s)  112 , patient positioner  114 , patient pod  116 , and/or world  132 . The local feedback devices  134  provide independent position information relating to the associated component of the system  100 . In various embodiments, the local feedback devices  134  comprise rotary encoders, linear encoders, servos, or other position indicators that are commercially available and whose operation is well understood by one of ordinary skill in the art. The local feedback devices  134  provide independent position information that can be utilized by the system  100  in addition to the information provided by the external measurement devices  124  to more accurately position the patient. 
         [0057]    The system  100  also comprises, in this embodiment, a precision patient alignment system  200  which employs the location information provided in state  160  for the object(s). As illustrated in  FIG. 8 , the patient alignment system  200  comprises a command and control module  202  communicating with a 6D system  204 , a patient registration module  206 , data files  210 , a motion control module  212 , a safety module  214 , and a user interface  216 . The patient alignment system  200  employs location information provided by the 6D system  204  to more accurately register the patient and move the nozzle  104  and the patient positioner  114  to achieve a desired treatment pose as indicated by the prescription for the patient provided by the data files  210 . 
         [0058]    In this embodiment, the 6D system  204  receives position data from the external measurement devices  124  and from the resolvers  134  relating to the current location of the nozzle  104 , the aperture  110 , the imager  112 , the patient positioner  114 , and patient pod  116 , as well as the location of one or more fixed landmarks  132  indicated in  FIG. 9  as the world  132 . The fixed landmarks, or world,  132  provide a non-moving origin or frame of reference to facilitate determination of the position of the moving components of the radiation therapy system  100 . This location information is provided to a primary 6D position measurement system  220  which then uses the observed data from the external measurement devices  124  and resolvers  134  to calculate position and orientation coordinates of these five components and origin in a first reference frame. This position information is provided to a 6D coordination module  222  which comprises a coordinate transform module  224  and an arbitration module  226 . The coordinate transform module  224  communicates with other modules of the patient alignment system  200 , such as the command and control module  202  and the motion control with path planning and collision avoidance module  212 . 
         [0059]    Depending on the stage of the patient registration and therapy delivery process, other modules of the patient alignment system  200  can submit calls to the 6D system  204  for a position request of the current configuration of the radiation therapy system  100 . Other modules of the patient alignment system  200  can also provide calls to the 6D system  204  such as a coordinate transform request. Such a request typically will include submission of location data in a given reference frame, an indication of the reference frame in which the data is submitted and a desired frame of reference which the calling module wishes to have the position data transformed into. This coordinate transform request is submitted to the coordinate transform module  224  which performs the appropriate calculations upon the submitted data in the given reference frame and transforms the data into the desired frame of reference and returns this to the calling module of the patient alignment system  200 . 
         [0060]    For example, the radiation therapy system  100  may determine that movement of the patient positioner  114  is indicated to correctly register the patient. For example, a translation of plus 2 mm along an x-axis, minus 1.5 mm along a y-axis, no change along a z-axis, and a positive 1° rotation about a vertical axis is indicated. This data would be submitted to the coordinate transform module  224  which would then operate upon the data to return corresponding movement commands to the patient positioner  114 . The exact coordinate transformations will vary in specific implementations of the system  100  depending, for example, on the exact configuration and dimensions of the patient positioner  114  and the relative position of the patient positioner  114  with respect to other components of the system  100 . However, such coordinate transforms can be readily determined by one of ordinary skill in the art for a particular application. 
         [0061]    The arbitration module  226  assists in operation of the motion control module  212  by providing specific object position information upon receipt of a position request. A secondary position measurement system  230  provides an alternative or backup position measurement function for the various components of the radiation therapy system  100 . In one embodiment, the secondary position measurement system  230  comprises a conventional positioning functionality employing predicted position information based on an initial position and commanded moves. In one embodiment, the primary position measurement system  220  receives information from the external measurement devices  124  and the secondary position measurement system  230  receives independent position information from the resolvers  134 . It will generally be preferred that the 6D measurement system  220  operate as the primary positioning system for the previously described advantages of positioning accuracy and speed. 
         [0062]      FIG. 10  illustrates in greater detail the patient registration module  206  of the patient alignment system  200 . As previously described, the 6D system  204  obtains location measurements of various components of the radiation therapy system  100 , including the table or patient pod  116  and the nozzle  104  and determines position coordinates of these various components and presents them in a desired frame of reference. The data files  210  provide information relating to the patient&#39;s treatment prescription, including the treatment plan and CT data previously obtained at a planning or prescription session. This patient&#39;s data can be configured by a data converter  232  to present the data in a preferred format. The imager  112  also provides location information to the 6D system  204  as well as to an image capture module  236 . The image capture module  236  receives raw image data from the imager  112  and processes this data, such as with filtering, exposure correction, scaling, and cropping to provide corrected image data to a registration algorithm  241 . 
         [0063]    In this embodiment, the CT data undergoes an intermediate processing step via a transgraph creation module  234  to transform the CT data into transgraphs which are provided to the registration algorithm  241 . The transgraphs are an intermediate data representation and increase the speed of generation of DRRs. The registration algorithm  241  uses the transgraphs, the treatment plan, the current object position data provided by the 6D system  204  and the corrected image data from the imager(s)  112  to determine a registered pose which information is provided to the command and control module  202 . The registration algorithm  241  attempts to match either as closely as possible or to within a designated tolerance the corrected image data from the imager  112  with an appropriate DRR to establish a desired pose or to register the patient. The command and control module  202  can evaluate the current registered pose and provide commands or requests to induce movement of one or more of the components of the radiation therapy system  100  to achieve this desired pose. Additional details for a suitable registration algorithm may be found in the published doctoral dissertation of David A. LaRose of May 2001 submitted to Carnegie Mellon University entitled “Iterative X-ray/CT Registration Using Accelerated Volume Rendering” which is incorporated herein in its entirety by reference. 
         [0064]      FIGS. 11-13  illustrate embodiments with which the system  100  performs this movement.  FIG. 11  illustrates that the command and control module  202  has provided a call for movement of one or more of the components of the radiation therapy system  100 . In state  238 , the motion control module  212  retrieves a current position configuration from the 6D system  204  and provides this with the newly requested position configuration to a path planning module  240 . The path planning module  240  comprises a library of three-dimensional model data which represent position envelopes defined by possible movement of the various components of the radiation therapy system  100 . For example, as previously described, the imager  112  is retractable and a 3D model data module  242  indicates the envelope or volume in space through which the imager  112  can move depending on its present and end locations. 
         [0065]    The path planning module  240  also comprises an object movement simulator  244  which receives data from the 3D model data module  242  and can calculate movement simulations for the various components of the radiation therapy system  100  based upon this data. This object movement simulation module  244  preferably works in concert with a collision avoidance module  270  as illustrated in  FIG. 12 .  FIG. 12  again illustrates one embodiment of the operation of the 6D system  204  which in this embodiment obtains location measurements of the aperture  110 , imager  112 , nozzle  104 , patient positioner and patient pod  114  and  116  as well as the fixed landmarks or world  132 .  FIG. 12  also illustrates that, in this embodiment, local feedback is gathered from resolvers  134  corresponding to the patient positioner  114 , the nozzle  104 , the imager  112 , and the angle of the gantry  102 . 
         [0066]    This position information is provided to the collision avoidance module  270  which gathers the object information in an object position data library  272 . This object data is provided to a decision module  274  which evaluates whether the data is verifiable. In certain embodiments, the evaluation of the module  274  can investigate possible inconsistencies or conflicts with the object position data from the library  272  such as out-of-range data or data which indicates, for example, that multiple objects are occupying the same location. If a conflict or out-of-range condition is determined, e.g., the result of the termination module  274  is negative, a system halt is indicated in state  284  to inhibit further movement of components of the radiation therapy system  100  and further proceeds to a fault recovery state  286  where appropriate measures are taken to recover or correct the fault or faults. Upon completion of the fault recovery state  286 , a reset state  290  is performed followed by a return to the data retrieval of the object position data library in module  272 . 
         [0067]    If the evaluation of state  274  is affirmative, a state  276  follows where the collision avoidance module  270  calculates relative distances along current and projected trajectories and provides this calculated information to an evaluation state  280  which determines whether one or more of the objects or components of the radiation therapy system  100  are too close. If the evaluation of stage  280  is negative, e.g., that the current locations and projected trajectories do not present a collision hazard, a sleep or pause state  282  follows during which movement of the one or more components of the radiation therapy system  100  is allowed to continue as indicated and proceeds to a recursive sequence through modules  272 ,  274 ,  276 ,  280 , and  282  as indicated. 
         [0068]    However, if the results of the evaluation state  280  are affirmative, e.g., that either one or more of the objects are too close or that their projected trajectories would bring them into collision, the system halt of state  284  is implemented with the fault recovery and reset states  286  and  290 , following as previously described. Thus, the collision avoidance module  270  allows the radiation therapy system  100  to proactively evaluate both current and projected locations and movement trajectories of movable components of the system  100  to mitigate possible collisions before they occur or are even initiated. This is advantageous over systems employing motion stops triggered, for example, by contact switches which halt motion upon activation of stop or contact switches, which by themselves may be inadequate to prevent damage to the moving components which can be relatively large and massive having significant inertia, or to prevent injury to a user or patient of the system. 
         [0069]    Assuming that the object movement simulation module  244  as cooperating with the collision avoidance module  270  indicates that the indicated movements will not pose a collision risk, the actual movement commands are forwarded to a motion sequence coordinator module  246  which evaluates the indicated movement vectors of the one or more components of the radiation therapy system  100  and sequences these movements via, in this embodiment, five translation modules. In particular, the translation modules  250 ,  252 ,  254 ,  260 , and  262  translate indicated movement vectors from a provided reference frame to a command reference frame appropriate to the patient positioner  114 , the gantry  102 , the x-ray source  130 , the imager  112 , and the nozzle  104 , respectively. 
         [0070]    As previously mentioned, the various moveable components of the radiation therapy system  100  can assume different dimensions and be subject to different control parameters and the translation modules  250 ,  252 ,  254 ,  260 , and  262  interrelate or translate a motion vector in a first frame of reference into the appropriate reference frame for the corresponding component of the radiation therapy system  100 . For example, in this embodiment the gantry  102  is capable of clockwise and counterclockwise rotation about an axis whereas the patient positioner  114  is positionable in six degrees of translational and rotational movement freedom and thus operates under a different frame of reference for movement commands as compared to the gantry  102 . By having the availability of externally measured location information for the various components of the radiation therapy system  100 , the motion sequence coordinator module  246  can efficiently plan the movement of these components in a straightforward, efficient and safe manner. 
         [0071]      FIG. 14  illustrates a workflow or method  300  of one embodiment of operation of the radiation therapy system  100  as provided with the patient alignment system  200 . From a start state  302 , follows an identification state  304  wherein the particular patient and treatment portal to be provided is identified. This is followed by a treatment prescription retrieval state  306  and the identification and treatment prescription retrieval of states  304  and  306  can be performed via the user interface  216  and accessing the data files of module  210 . The patient is then moved to an imaging position in state  310  by entering into the patient pod  116  and actuation of the patient positioner  114  to position the patient pod  116  securing the patient in the approximate position for imaging. The gantry  102 , imager(s)  112 , and radiation source(s)  130  are also moved to an imaging position in state  312  and in state  314  the x-ray imaging axis parameters are determined as previously described via the 6D system  204  employing the external measurement devices  124 , cooperating markers  122 , and resolvers  134 . 
         [0072]    In state  316 , a radiographic image of the patient is captured by the imager  112  and corrections can be applied as needed as previously described by the module  236 . In this embodiment, two imagers  112  and corresponding x-ray sources  130  are arranged substantially perpendicularly to each other. Thus, two independent radiographic images are obtained from orthogonal perspectives. This aspect provides more complete radiographic image information than from a single perspective. It will also be appreciated that in certain embodiments, multiple imaging of states  316  can be performed for additional data. An evaluation is performed in state  320  to determine whether the radiographic image acquisition process is complete and the determination of this decision results either in the negative case with continuation of the movement of state  312 , the determination of state  314  and the capture of state  316  as indicated or, when affirmative, followed by state  322 . 
         [0073]    In state  322 , external measurements are performed by the 6D system  204  as previously described to determine the relative positions and orientations of the various components of the radiation therapy system  100  via the patient registration module  206  as previously described. In state  324 , motion computations are made as indicated to properly align the patient in the desired pose. 
         [0074]    While not necessarily required in each instance of treatment delivery, this embodiment illustrates that in state  326  some degree of gantry  102  movement is indicated to position the gantry  102  in a treatment position as well as movement of the patient, such as via the patient positioner  114  in state  330  to position the patient in the indicated pose. Following these movements, state  332  again employs the 6D system  204  to externally measure and in state  334  to compute and analyze the measured position to determine in state  336  whether the desired patient pose has been achieved within the desired tolerance. If adequately accurate registration and positioning of the patient has not yet been achieved, state  340  follows where a correction vector is computed and transformed into the appropriate frame of reference for further movement of the gantry  102  and/or patient positioner  114 . If the decision of state  336  is affirmative, e.g., that the patient has been satisfactorily positioned in the desired pose, the radiation therapy fraction is enabled in state  342  in accordance with the patient&#39;s prescription. For certain patient prescriptions, it will be understood that the treatment session may indicate multiple treatment fractions, such as treatment from a plurality of orientations and that appropriate portions of the method  300  may be iteratively repeated for multiple prescribed treatment fractions. However, for simplicity of illustration, a single iteration is illustrated in  FIG. 14 . Thus, following the treatment delivery of state  342 , a finished state  344  follows which may comprise the completion of treatment for that patient for the day or for a given series of treatments. 
         [0075]    Thus, the radiation therapy system  100  with the patient alignment system  200 , by directly measuring movable components of the system  100 , employs a measured feedback to more accurately determine and control the positioning of these various components. A particular advantage of the system  100  is that the patient can be more accurately registered at a treatment delivery session than is possible with known systems and without an iterative sequence of radiographic imaging, repositioning of the patient, and subsequent radiographic imaging and data analysis. This offers the significant advantage both of more accurately delivering the therapeutic radiation, significantly decreasing the latency of the registration, imaging and positioning processes and thus increasing the possible patient throughput as well as reducing the exposure of the patient to x-ray radiation during radiographic imaging by reducing the need for multiple x-ray exposures during a treatment session. 
         [0076]    Although the preferred embodiments of the present invention have shown, described and pointed out the fundamental novel features of the invention as applied to those embodiments, it will be understood that various omissions, substitutions and changes in the form of the detail of the device illustrated may be made by those skilled in the art without departing from the spirit of the present invention. Consequently, the scope of the invention should not be limited to the foregoing description but is to be defined by the appended claims.