Abstract:
Quality assurance device for calibrating and testing the accuracy of movement-correlated computed tomography (“4D CT”) target-locating systems has a test unit sub-assembly adapted to be combined with a dynamic phantom system. The test unit sub-assembly has an axially and rotationally moveable test rod slideably disposed inside of a substantially hollow fixed housing. A matrix of markers, or “fiducials”, are located in the wall of the housing. A single fiducial is located near the distal end of the moveable test rod. The distal end portion of the moveable test rod is adapted to be connected to a motion actuator, which is programmed to oscillate the test rod in a predetermined pattern. When the test unit sub-assembly is inserted into a tissue equivalent phantom member, the combined sub-assembly and phantom member can then be subjected to four-dimensional imaging to generate a visual image. A visual comparison actual relative positions of the fiducials to the know positions of the fiducials in time indicates the accuracy of the 4D CT system.

Description:
RELATED APPLICATIONS 
       [0001]    The present patent document claims the benefit of the filing date under 35 U.S.C. Sec. 119(e) of Provisional U.S. Patent Application Ser. No. 61/000,789, filed Oct. 29, 2007, which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates generally to four-dimensional (4D) computed tomography (CT) quality assurance (QA) equipment. More particularly the present invention relates to devices used to calibrate, confirm and test the accuracy of movement-correlated 4D CT target-locating systems. 
         [0004]    2. Description of the Prior Art 
         [0005]    The technologies of intensity-modulated radiotherapy (IMRT) have undergone rapid change. Four-dimensional CT acquisition—that is, CT acquisition of a moving three-dimensional target object (for example, a tumor)—is commercially available, and can provide important information on the shape and trajectory of a tumor and normal tissues. The primary advantage of four-dimensional imaging over light breathing helical scans is the reduction of motion artifacts during scanning that can significantly alter tumor appearance. 
         [0006]    Precise knowledge and control of three-dimensional dose distribution in considered to be essential for a favorable therapeutic outcome. The ability to deliver highly conformal dose distributions through intensity-modulated radiotherapy has become common for sites such as head and neck and prostate. When the target moves due, for example, to respiration, precise delivery of dose becomes more challenging. 
         [0007]    Artifacts due to motion (known as “temporal aliasing artifacts”) during tomographic scans have been appreciated for many years. Three-dimensional CT images are typically obtained by taking a series of adjacent image slices (or, alternatively, a continuous helix of images) of a subject who/that is typically placed on a platform (e.g., bed) that moves relative to the scanner, and are then digitally stitching (via computer software) the various image slices together. If the subject moves relative to the bed, as for example during breathing, while adjacent image slices are being scanned, the movement can result in temporal aliasing artifacts. 
         [0008]    In order to minimize such temporal aliasing artifacts, motion-correlated CT systems have been proposed. Motion-correlated CT systems that acquire 3-dimensional image data are referred to herein as four-dimensional CT (“4D CT”) systems. 
         [0009]    Respiration-correlated CT uses a surrogate signal, tracking movement such as of the abdominal surface, or of respiratory air flow, or of internal anatomy to provide a signal that permits re-sorting of the reconstructed image data, resulting in multiple coherent spatiotemporal data sets at different respiratory phases. The scan time for 4DCT with multislice scanners is on the order of a few minutes. In general, in order to re-sort and correlate the image data, each image slice is time- and/or position-stamped and each surrogate signal is time- and/or position-stamped. Computer software is then used to re-sort and correlate the various image slices into the proper sequences as dictated by the time- and/or position-stamps of the respective surrogate and the image slices. The output of this process is typically 10 CT volumes, each with a temporal resolution of approximately 1/10 th  of the respiratory period. 
         [0010]    The ability of a 4D CT system to accurately re-sort and faithfully reconstruct three-dimensional data sets of a moving target volume within a subject depends heavily on how accurately and precisely the system can track the actual position in space of that target volume relative to the subject at all times during the scanning process. 
         [0011]    There is a need, then, for a means to confirm, measure and calibrate the accuracy and precision with which a 4D CT system tracks the actual position in space of a scanned target volume. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention is directed to a quality assurance method and apparatus, for use in conjunction with a 4D CT system, for determining the accuracy and precision to which the 4D CT system tracks the position in space of a target volume relative to the position of a scanned subject in which the target volume is located. 
         [0013]    It is an object of the present invention to provide a quality assurance device of the character described that can be used in conjunction with CT, PET, MRI or ultrasound imaging systems. 
         [0014]    Other features and advantages of the invention will be apparent from the following detailed description accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a cross-sectional view of a test rod and test cylinder sub-assembly in accordance with the present invention; 
           [0016]      FIG. 2  is an elevation view of the front of a test cylinder constructed in accordance with the present invention; 
           [0017]      FIG. 3  is an elevation view of the side of a test cylinder constructed in accordance with the present invention; 
           [0018]      FIG. 4  is an elevation view of the front of a test rod constructed in accordance with the present invention; 
           [0019]      FIG. 5  is an elevation view of the side of a test rod constructed in accordance with the present invention; 
           [0020]      FIG. 6  is a perspective view of a 4D CT QA device constructed in accordance with the present invention; 
           [0021]      FIG. 7  is a front elevation view of a tissue equivalent phantom used in a preferred embodiment of the invention; 
           [0022]      FIG. 8  is a cross-sectional view of the tissue equivalent phantom shown in  FIG. 7 ; and, 
           [0023]      FIG. 9  is a perspective view of tissue equivalent phantom partially cut away to show the test apparatus sub-assembly. 
       
    
    
     REFERENCE NUMERALS IN DRAWINGS 
       [0000]    
       
         D 1  outside diameter (of test rod distal shoulder  16   b ) 
         D 2  outside diameter (of test cylinder  14 ) 
           10  4D CT QA device, general 
           12  Test apparatus sub-assembly 
           14  Test cylinder
         14   a  inside surface (of test cylinder)     14   b  outside surface (of test cylinder)     
           15  Collar (of test cylinder) 
           16  Test rod
         16   a  shaft (of test rod)     16   b  distal shoulder (of test rod)     16   c  proximal shoulder (of test rod)     
           18  Fiducial (static) 
           20  Fiducial (moveable) 
           22  Hole (in distal shoulder of test rod) 
           24  Counterbored hole (in proximal shoulder) 
           28  Holes (in test cylinder  14 ) 
           30  Opening (in phantom body)
         30   a  Opening (in phantom body)     
           32  Phantom body 
           33  Phantom base member 
           34  Assembly base member 
           35  Controller support fixture 
           38  Motion actuator assembly 
           40  Motion controller assembly 
           42  Actuator rod 
       
     
       DETAILED DESCRIPTION 
       [0050]    A quality control device  10 , constructed in accordance with a preferred embodiment of the present invention, is shown in  FIG. 6 . As will be discussed in detail below, the device, hereafter referred to as a four-dimensional computed tomography quality assurance device (or “4D CT QA device”)  10 , may be used in a preferred embodiment to calibrate, confirm and/or test the accuracy of motion-correlated CT systems that acquire 3-dimensional image data. 
         [0051]    Referring to  FIGS. 1 and 6 : A four-dimensional computed tomography quality assurance (4D CT QA) device  10  comprises a test apparatus sub-assembly  12 , which comprises a test cylinder  14  and a test rod  16 , as illustrated in  FIG. 1 . Test rod  16  is disposed inside of test cylinder  14 . Preferably, test rod  16  is both axially and rotationally moveable within test cylinder  14 . 
         [0052]    The test apparatus sub-assembly  12  has at least two sets of CT markers (“fiducials”)  18  and  20 , which are located in the test cylinder  14  and the test rod  16 , respectively. In operation, since test rod  16  is moveable within test cylinder  14 , the two sets of fiducials  18  and  20  are moveable relative to each other. More specifically, fiducials  18  are operationally static (and are, therefore, referred to herein below as “static fiducials”  18 ); and fiducial  20  is operationally moveable (and is, therefore, referred to herein below as the “moveable fiducial”  20 ). 
         [0053]    Referring now to  FIGS. 2 and 3 : Test cylinder  14  is preferably constructed of a solid material such as acrylic. By way of example, in the preferred embodiment of the invention, the test cylinder  14  is 170 mm long, has outside diameter of 2.5 inches and an inside diameter of 1.75 inches. A cylinder collar  15 , approximately 1.75 inches square by 0.25 inches thick, is attached to one end of the test cylinder  14 . The inside surface  14   a  of the test cylinder  14  extends through the cylinder collar  15 . The outside diameter D 2  of test cylinder  14  is preferably sized so as to allow the outside (cylindrical) surface  14   b  of test cylinder  14  to slip fit inside of a corresponding opening  30  in a phantom body  32  (as will be described more fully herein below). 
         [0054]    A matrix of holes  28  is located intermediately along test cylinder  14 . In the preferred embodiment of the invention, each of the holes is radially oriented with respect to the longitudinal axis of test cylinder  14 , although the holes  28  may, alternatively, be aligned parallel to each other. In the preferred embodiment of the invention, seven radially spaced apart rows of seven holes  28  each are counterbored into the wall of test cylinder  14  as shown in  FIGS. 2 and 3 . By way of example, in the preferred embodiment of the invention each hole  28  is approximately 1 mm diameter by 8.5 mm deep, so as each to accommodate a 1 mm by 5 mm steel fiducial  18  and adhesive (not shown). The seven rows of holes  18  are preferably radially spaced 5.0 degrees apart (center to center); and adjacent holes within each row are preferably spaced apart 5.0 mm (center to center). 
         [0055]    Referring now to  FIGS. 4 and 5 : Test rod  16  is preferably constructed of a solid material such as ABS, polyethylene or acrylic. By way of example, in the preferred embodiment of the invention, test rod  16  is 175 mm long and has a 30 mm diameter shaft  16   a  extending between a test rod distal shoulder  16   b  and a test rod proximal shoulder  16   c . The test rod distal shoulder  16   b  is preferably 20 mm thick and has an outside diameter D 1  that is sized so as to slip-fit into the inside surface  14   a  of the test cylinder  14 . An approximately 1 mm diameter by approximately 5.5 mm deep hole  22  extends radially inwardly from the cylindrical surface of the test rod base  16   b  so as to accommodate a 1 mm diameter by 5 mm steel fiducial  20  and adhesive (not shown in  FIG. 5 ). It will be understood that, in the preferred embodiment of the invention, the spaced-apart proximal shoulder  16   c  and distal shoulder  16   b  facilitate maintenance of alignment of text rod  16  inside of test cylinder  14 . 
         [0056]    A counterbored hole  24  is preferably provided in the end of test rod proximal shoulder  16   c  for attachment of test rod  16  to a motion actuator assembly  38 . 
         [0057]    Referring now to  FIG. 6 : In the preferred embodiment of the invention, the test apparatus sub-assembly  12  is designed to be used in conjunction with a dynamic phantom system (such as the one described in U.S. Pat. No. 7,151,253, which is included herein by reference thereto), comprising a tissue equivalent phantom body  32 ; a motion actuator assembly  38 ; and a motion controller assembly  40 . 
         [0058]    The tissue equivalent phantom body  32  is preferably secured to a phantom base member  33 , which is preferably attached to an assembly base  34 . The motion actuator assembly  38  is supported from the assembly base  34  by actuator support fixture  35 . 
         [0059]    In the preferred embodiment of the invention, the tissue equivalent phantom body  32  is provided with a pair of through-holes  30  and  30   a  each of which is a diameter adapted to slideably receive the outside surface  14   b  of test cylinder  14 . As described in referenced U.S. Pat. No. 7,151,253, one of the through holes  30  in the tissue equivalent phantom member  32  preferably runs longitudinally through the phantom approximately parallel to the axis of the phantom; and the other through hole  30   a  is preferably oriented not parallel to the axis of the phantom. 
         [0060]    In the preferred embodiment of the present invention, motion actuator assembly  38  comprises a moveable actuator rod  42 . Actuator rod  42  is attached, (for example, by threaded engagement) to test rod proximal shoulder  16   c  at counterbored hole  24 . In operation, a motion controller assembly  40  electrically sends signals to motion actuator assembly  38 , which causes actuator rod  42  to oscillate axially and/or rotationally, which causes test rod  16  to slideably move inside of test cylinder  14 . As test rod  16  oscillates inside of test rod  16 , moveable fiducial  20  inside of test rod  16  moves relative to static fiducials  18  inside of test cylinder  14 . 
         [0061]    In the preferred embodiment of the invention, cylinder collar  15  is in a plane perpendicular to the axis of cylinder  14 , as shown in  FIG. 3 . The cylinder collar  15  provides a physical stop for proper insertion of the test apparatus sub-assembly  12  into opening  30  in phantom body  32 . In alternative embodiments of the invention, cylinder collar  15  can be omitted. 
       Basic Operation: 
       [0062]    An exemplary method of using the 4D CT QA device  10  to calibrate, confirm and/or test the accuracy of a motion-correlated CT system that acquires 3-dimensional image data is described. In response to predetermined signals from motion controller assembly  40 , test rod  16  (and, therefore, fiducial  20 ) moves periodically (so as to mimic, for example, breathing motion) a known distance (excursion, displacement) relative to the static fiducials  18  at any given direction, for example anterior-posterior (AP), left-right (LR) and/or inferior-superior (IS) or rotationally. 
         [0063]    The range of motion of test rod  16  is preferably set so that the location of moving fiducial  20  and the location of static fiducials  18  match at the maximum excursion of the test rod  16 . 
         [0064]    A motion-correlated 4D CT scanner may then acquire and sort images of the moving 4D CT QA device  10  at different motion phases. Zero percent and 50% phases should, preferably, each provide an image wherein the static and moving fiducials  18  and  20  are positioned next to each other. 
         [0065]    Any difference observed and measured between the positions of the static and moving fiducials  18  and  20 , on the static images generated by the 4D CT system, will be indicative of the accuracy (or lack thereof) of the 4D CT system&#39;s performance. Alternatively, if the range of motion of the test rod  16  is initially set up so that the positions of fiducials  18  and  20  do not match (i.e., are not in alignment with one another) at the maximum excursion, then the distance between the fiducials  18  and  20  can be measured in a static CT (without motion), and the generated images of the 4D CT phases should be evaluated against the known static position of the fiducials  18  and  20 . 
         [0066]    The above example describes a preferred embodiment of the invention that is particularly well suited for calibrating 4D CT systems. I should be understood, however, that modified embodiments of the invention are equally well suited for use in calibrating other medical imaging apparatus, including positron emission tomography (PET-CT), magnetic resonance imaging (MRI) and ultrasound imaging, Regardless of the nature of the imaging system that the present invention is to be used to calibrate, in each case the static fiducials  18  comprise discontinuities in the test cylinder  14 ; and the moveable fiducial  22  comprises a discontinuity in the test rod. Thus, for CT applications it is desirable that the fiducials  18  and  22 , have different mass densities and/or radiopacity than that of the material of construction of test cylinder  14  and test rod  16 ; and, for MRI applications it is desirable that the fiducials  18  and  22 , have different T 1  and T 2  values than that of the material of construction of test cylinder  14  and test rod  16 , so that the magnetic resonance “signature” of the fiducials is distinctive from the test rod and test cylinder. For MRI applications, for example, the material of construction of test cylinder  14  and test rod  16  may be acrylic, and the fiducials  18  and  22  may be a paramagnetic material such as ferrous oxides, nickel chloride or copper sulfate. 
         [0067]    While certain advantageous embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention. For example:
       The test apparatus sub-assembly  12  can be used in conjunction with a motion actuator assembly, but without insertion of the sub-assembly  12  into a tissue equivalent phantom member (such as phantom body  32 );   The tissue equivalent phantom body  32  can have one or more through holes ( 30 ,  30   a ) for receiving the test apparatus sub-assembly  12 ; and those holes may be oriented either parallel to or not parallel to a major axis of the phantom member;   The motion actuator assembly  38  may be designed to cause the actuator rod  42  (and, therefore, the test rod  16 ) to move axially or rotationally, or both;   In embodiments of the invention wherein the motion actuator assembly  38  doesn&#39;t cause the actuator rod  42  (and, therefore, the test rod  16 ) to move rotationally, the cross-sectional geometry of the test rod distal shoulder  16   b , and the cross-sectional geometry of the inside surface  14   a  of the test cylinder, can each be of a shape other than circular, provided the two are substantially the same shape;   The number, size, spacing and shape of the holes  28  in the test cylinder  28  can be other than those described for the preferred embodiment of the invention;   The “fiducials”  18  and  20  that are inserted into the holes  28  and  22  in the test cylinder  14  and test rod  16 , respectively, may be of any material having a density different from that of the test cylinder  14  and test rod  16 ;   The holes  28  and  22  in the test cylinder  14  and test rod  16 , respectively, may alternatively be left empty, so that the “fiducials” comprise only air, which, being of a different density than that of the material of the test cylinder and test rod, may render the holes, themselves, as visible markers on static CT images of the apparatus;   The matrix of holes  28  (as well as the fixed fiducials  18  inserted therein) may be aligned so that their respective axes are parallel to each other, or, alternatively, so that their respective axes are each oriented radially with respect to the longitudinal axis of the test cylinder  14 ;   Use of the device  10  is not limited to use in conjunction with computed tomography (CT) systems, but may be used in a substantially similar manner to provide quality assurance data in other medical imaging systems, including PET, MRI and ultrasound systems;   Although the matrix of holes  28  comprise blind (i.e., counterbored) holes into which fiducials  18  may be inserted, the holes may alternatively be through-holes that extend from the outside surface  14   b  to the inside surface  14   a  of the test cylinder  14 ;   Various common attachment means, other than by threaded engagement at counterbored hole  24 , may be used for connecting the proximal end of the test rod  16  to a motion actuator  38 ;   Means, such as treaded fasteners or pins extending, for example, through collar  15  may be provided in order to secure test cylinder  14  to phantom body  32 ;   The test rod  16  may be constructed without a distal shoulder  16   b  and proximal shoulder  16   a , provided that the test geometry of the test rod conforms with (and slip fits with) the geometry of the inside surface  14   b  of the test cylinder in the proximity of the moveable fiducial  20 ; and,   The material of construction of test rod  16  and test cylinder  14  may be other than ABS, polyethylene or acrylic, provided such material is substantially transparent to the imaging equipment (e.g., CT, PET, MRI, and ultrasound) that it is to be used to calibrate;   The fiducials  18 ,  20  may comprise a gas (i.e., air), a liquid, solid or gel material;   The fiducials  18 ,  20  may extend from the outside surfaces of the test cylinder  14  and test rod  16 , respectively, or they may be imbedded inside the test cylinder  14  and test rod  16 , respectively.   Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.