Abstract:
A jig for calibrating an image-guided radiotherapy apparatus is disclosed. The jig includes a ball bearing and a three-axis positioner. Once the ball bearing has been moved to the calculated radiation isocenter of the apparatus, other calibration procedures can be performed by directing light onto the jig.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to radiotherapy, and more particularly relates to image-guided radiotherapy (“IGRT”). In its most immediate sense, the invention relates to a quality-control jig for use with IGRT apparatus. 
         [0002]    In IGRT apparatus, two instruments are mounted upon a rotatable gantry. One of these is a linear accelerator (a “linac”). The linac produces a beam of high-energy radiation (the “treatment beam”) used to destroy tumor tissue inside the patient&#39;s body. The other is an imager (which is usually but not necessarily a cone-beam computed-tomography imager). This produces a lower-energy beam of radiation (the “imaging beam”) used to create a three-dimensional image of the patient&#39;s body region in which the patient&#39;s tumor is located. The linac&#39;s treatment beam and the imager&#39;s imaging beam are at right angles to each other. 
         [0003]    Before a patient is imaged and subjected to radiation therapy, it is necessary to make sure that the IGRT apparatus is properly calibrated. This is because IGRT apparatus operates neither with theoretical perfection nor with absolute repeatability. 
         [0004]    For these reasons, radiation technicians routinely carry out quality-control procedures on IGRT apparatus before patients are imaged and subjected to radiation therapy. This enables the technicians to monitor the actual performance of the IGRT apparatus and to make sure that the apparatus is properly calibrated. However, such procedures are time-consuming and complicated. 
         [0005]    It would be advantageous to provide a jig that would make it easier to carry out quality-control procedures on IGRT apparatus, and to make it possible to carry out those procedures more quickly. 
         [0006]    Objects of the present invention are to provide a jig that facilitates and speeds the performance of quality-control procedures on IGRT apparatus. 
         [0007]    The invention proceeds from the realization that a ball bearing that is used to carry out a common quality-control procedure can be incorporated as a detachable part of a jig that can be used to check other apparatus parameters by shining light on the jig. 
         [0008]    In accordance with the invention, a jig has an elongated stylus with a proximal end and a distal end. The proximal end of the stylus is secured to a three-axis positioner. 
         [0009]    A ball bearing is provided, as is a ball bearing cap that is secured to the ball bearing and adapted to fit over the distal end of the stylus to detachably mount the ball bearing thereto. A pointer having a distal tip is provided, as is a pointer cap that is secured to the pointer and adapted to fit over the distal end of the stylus to detachably mount the pointer thereto. The ball bearing, the ball bearing cap, the pointer, and the pointer cap are all dimensioned such when the pointer cap is mounted to the distal end of the stylus, the distal tip of the pointer has the same location as does the center of the ball bearing when the ball bearing cap is mounted to the distal end of the stylus. 
         [0010]    A flat plate is provided, as are means for fixing the plate to the stylus in such a manner that the pointer will cast a shadow on the plate when a light is directed onto the pointer from a direction normal to the plate. 
         [0011]    This jig makes it easy to carry out many commonly-performed quality-control quickly and efficiently. Once the positioner has been used to place the ball bearing at the calculated radiation isocenter of the IGRT apparatus, measurements of other apparatus parameters can be carried out relative to the known position of the radiation isocenter by directing light onto the jig. 
         [0012]    Advantageously, the fixing means is adapted to fix the plate to the stylus at a 0° orientation, a 90° orientation, a 180° orientation, and a 270° orientation. This allows the jig to be conveniently reconfigured for use at four gantry positions. Likewise advantageously, an axially-elongated hollow phantom is provided. The phantom is detachably securable to the positioner in a manner that the stylus extends along the axis of the phantom and has surface markings indicating locations that are axially aligned with the center of the ball bearing and that are also rotationally aligned with gantry orientations of 0°, 90°, and 270°. This makes it possible to align the lasers used to position the patient. Additionally, four infrared-reflecting markers are mounted on the anterior surface of the phantom. The locations of these markers are known precisely, making it possible to calibrate and quality-control optical tracking equipment such as is conventionally used with IGRT apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention will be better understood with reference to the following illustrative and non-limiting drawings, in which: 
           [0014]      FIG. 1  shows an image-guided radiation therapy apparatus; 
           [0015]      FIG. 2  shows a preferred embodiment of the invention, configured to determine the radiation isocenter of an image-guided radiation therapy apparatus with which it is used; 
           [0016]      FIG. 3  shows a portion of the preferred embodiment of the invention, configured to determine the mechanical isocenter of an image-guided radiation therapy apparatus with which it is used; 
           [0017]      FIG. 4  shows a portion of the preferred embodiment of the invention, configured to check the calibration of an optical distance indicator in the linac of an image-guided radiation therapy apparatus with which it is used; 
           [0018]      FIG. 5  is a perspective view of the preferred embodiment of the invention with a phantom mounted on it; and 
           [0019]      FIG. 6  is a side view of the preferred embodiment of the invention with the phantom mounted on it. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0020]    The Figures herein are not necessarily to scale; various components may be enlarged or reduced for clarity of illustration. The same element is always indicated by the same reference numeral in all the Figures. 
         [0021]      FIG. 1  shows a conventional image-guided radiation therapy (“IGRT”) apparatus generally indicated by reference numeral  100 . A toroidal gantry  2  is supported to rotate in a vertical plane. A linac  4  is attached to the gantry  2 , as is an imager  6 . (In this example, the imager  6  is a cone-beam computed tomography imager, but this is not required. Another imaging device, such as an X-ray apparatus, can be used instead.) 
         [0022]    The IGRT apparatus  100  has a patient table  8 ; in use, a patient (not shown) is supported on the table  8 . Initially, the table  8  and the imager  6  are adjusted so that the imager  6  is aimed at the body region where a tumor (likewise not shown) is located. In an initial imaging phase, the gantry  2  is rotated while the imager  6  is operated to acquire image data from the patient. Once image data of the patient have been acquired through 360° of rotation of the gantry  2 , a three-dimensional image is reconstructed and registered. In this image, the location of the tumor within the body is accurately identified. Then, in a subsequent treatment phase, the gantry  2  is rotated through 360 degrees while the linac  4  is operated in accordance with the registered image information. Radiation from the linac  4  necrotizes the tumor. 
         [0023]    Before carrying these steps out on a patient, it is necessary to carry out quality-control procedures to make sure that the IGRT apparatus  100  is properly calibrated. This is because the IGRT apparatus  100  is not mechanically perfect and does not operate with absolute repeatability. For example, the parts of the gantry  2 , the linac  4 , and the imager  6  are not absolutely rigid and all mechanical parts are subject to wear. As a result, the various parts of the IGRT apparatus  100  flex during rotation of the gantry  2 . This flexure makes points in the patient&#39;s image appear to move with rotation of the gantry  2 . To minimize flexure-caused distortion of the image reconstructed from data acquired by the imager  6 , the flexure is conventionally measured at various orientations of the gantry  2  and taken into account during image reconstruction. 
         [0024]    To do this, in one universally-practiced quality control procedure, a ball bearing is moved to the geometric center of the radiation field of the linac  4 . The ball bearing is then imaged by the linac  4  at four orientations of the linac  4  (i.e. at gantry orientations 0°, 90°, 180°, and 270°). This localizes the ball bearing within the radiation field of the linac  4 , and the ball bearing can then be moved to the computed radiation isocenter of the IGRT apparatus  100 . After this has been done, the position of the ball bearing is measured in the coordinate system of the imager  6  throughout the range of orientations of the gantry  2 , thereby creating a so-called “Flex Map” that is used when an image is reconstructed from data acquired using the imager  6 . 
         [0025]    This is not the only quality-control procedure applicable to calibration of IGRT apparatus; others are used as well. These will be explained below in connection with the following description of a preferred embodiment of the invention. 
         [0026]    Referring now to  FIG. 2 , a ball bearing  10  is mounted to a ball bearing cap  12 . The ball bearing cap  12  is sized to fit over the distal end of a stylus  14 , and the proximal end of the stylus  14  is secured to a three-axis positioner  16 . The positioner  16  is mounted on a base  18 . 
         [0027]    Initially, the ball bearing cap  12  is fit over the distal end of the stylus  14 , the base  18  is placed upon the table  8 , and the ball bearing  10  is imaged by the linac  4  at the 0°, 90°, 180°, and 270° orientations of the gantry  2 . The radiation isocenter of the IGRT apparatus  100  is then computed, and the ball bearing  10  is then moved to that computed position by adjustment of the positioner  16  along directions parallel to the table  8 . 
         [0028]    Once the ball bearing  10  has been moved to the computed radiation isocenter of the IGRT apparatus  100 , it is possible to check whether the height of the table  8  is proper. This can be done by measuring the distance between the ball bearing  10  and the table  8  and comparing that measured distance with the distance that is expected to be present. 
         [0029]    The preferred embodiment of the invention makes it possible to check the relationship between the radiation isocenter of the IGRT  100  and the mechanical isocenter of the IGRT apparatus  100 . To do this, the ball bearing cap  12  is detached from the distal end of the stylus  14  and replaced with an assembly made up of a pointer  20  and a pointer cap  22  that is secured to the pointer  20  ( FIG. 3 ). The pointer cap  22 , like the ball bearing cap  12 , fits over the distal end of the stylus  14 . And, importantly, the dimensions of the ball bearing  10 , the ball bearing cap  12 , the pointer  20 , and the pointer cap  22  are chosen so that when the pointer cap is mounted to the distal end of the stylus  14 , the distal tip of the pointer  20  is located where the center of the ball bearing  10  is located when the ball bearing cap  12  is mounted to the distal end of the stylus  14 . 
         [0030]    Advantageously although not necessarily, two different ball bearings  10  are provided, each being mounted on a cap  12  so as to be mountable on the stylus  14 . The ball bearings  10  and their associated caps  12  are dimensionally identical, but the two ball bearings  10  are of different densities; one has a higher density than the other. The higher density ball bearing  10  is used as stated above to compute the radiation isocenter of the IGRT apparatus  100 . In a subsequent step, the higher density ball bearing  10  and its attached cap  12  can be removed from the stylus  14  and replaced by the lower density ball bearing  10  and its attached cap  12 . Then, an image of the lower density ball bearing  10  can be acquired using the imager  6  (which is typically rotated through 360° in order to acquire the image). The displacement between the radiation isocenter and the isocenter of the imager  6  is then measured to determine whether this displacement is within specifications (it should typically be less than 2 mm). Since the radiation isocenter is sometimes referred to as the “MV isocenter” and the isocenter of the imager  6  is sometimes referred to as the “KV isocenter”, this quality control measure is referred to as a “KV and MV isocenter coincidence check”. 
         [0031]    Each of the linac  4  and the imager  6  has a light (not specifically shown) at its radially inward face. And, a flat plate  24  is mounted to the stylus  14 . As can be seen in  FIG. 3 , the flat plate  24  is so located that the pointer  20  casts a shadow on a ruled side  26  of the plate  24  when light is directed upon the pointer  20 . (The ruled side  26  bears a Cartesian coordinate system calibrated in millimeters so that the precise position of the shadow cast by the pointer  20  can be noted. The ruled side  26  can be permanently ruled or a ruled piece of paper can be detachably secured to it.) 
         [0032]    Thus, when a light from either the linac  4  or the imager  6  is directed upon the pointer  20 , the pointer  20  casts a shadow onto the ruled side  26  of the plate  24  and the position of that shadow indicates the actual position of the linac  4  or imager  6  (as the case may be). This provides a way to track the mechanical isocenter of the IGRT apparatus  100 ; at various positions of the gantry  2 , the position of the tip of the shadow on the ruled side  26  of the flat plate  24  is noted. Advantageously, the plate  24  is so mounted to the stylus  14  that the plate can be rotated to, and fixed in, positions corresponding to the 0°, 90°, 180°, and 270° orientations of the gantry  2 . Thus, if the gantry  2  is set to the 0° orientation (i.e. with the linac  4  facing directly downwardly), the plate  24  will be rotated so that the ruled side  26  faces up; if the gantry  2  is set to the 270° orientation (i.e. with the linac  4  facing left) the plate  24  will be rotated so that the ruled side  26  faces right. A review of the variation in the position of the shadow tip at these gantry orientations makes it possible to determine whether the mechanical performance of the gantry  2  is within applicable specifications. 
         [0033]    The linac  4  will typically have an optical distance indicator (“ODI”, not shown) that measures distance from the linac  4  to the patient to be treated. The operation of the linac  4  can be checked by rotating the plate  24  so that its non-ruled side  28  faces the linac  4 , aiming the linac  4  at the non-ruled side  28  so that the ODI directs light thereon, and checking to see if the distance measured by the ODI is within the tolerance required. 
         [0034]    Conventionally, a room in which an IGRT apparatus is installed has a laser system that requires alignment. This is schematically illustrated in  FIG. 1 , which shows laser beams  50 ,  52 , and  54  that are projected toward the radiation isocenter of the IGRT apparatus  100  from lasers (not shown) that are mounted in the room in which the IGRT apparatus  100  is located. In order to make sure this laser system is properly aligned, an axially-elongated cylindrical acrylic phantom  30  ( FIGS. 5 and 6 ) is threaded onto the positioner  16  in such a manner that the stylus  14  extends along the axis of the phantom  30 . 
         [0035]    The phantom  30  has surface markings indicating locations that are axially aligned with the center of the ball bearing  10  and that are also rotationally aligned with gantry orientations of 0°, 90°, and 270°. The circular line  32  is axially aligned with the center of the ball bearing  10 . Each of surface lines  34 ,  36 , and  40  is parallel to the stylus  14 . The line  34  is on the top of the phantom  30 , at a position corresponding to a 0° orientation of the gantry  2 , the line  36  is on the side of the phantom  30 , at a position corresponding to a 90° orientation of the gantry  2 , and the line  40  is on the other side of the phantom  30 , at a position corresponding to a 270° orientation of the gantry  2 . Thus, the surface of the phantom  30  has three points of intersection (one being at the intersection of lines  32  and  34 , another being at the intersection of lines  32  and  36 , and the third being at the intersection of lines  32  and  40 ). 
         [0036]    Once the ball bearing  10  has been moved to the computed radiation isocenter, the phantom  30  can be mounted to the positioner  16  (as by being threaded onto it). The room lasers can then be calibrated or checked by turning them on and seeing how closely the beams  50 ,  52 , and  54  are projected to these three points of intersection. 
         [0037]    Four infrared-reflecting spherical markers  42  are mounted on the anterior top surface of the phantom  30 . Each marker  42  is 1.2 cm in diameter. The four markers  42  form a 5 cm by 5 cm square centered on the center of the ball bearing  10 . The coordinates of the four markers  42  are precisely known, which enables the calibration and quality control of optical tracking equipment such as is conventionally used with IGRT apparatus. 
         [0038]    Although a preferred embodiment has been described above, the scope of the invention is determined only by the following claims: