Abstract:
Systems and methods for delivery of radiotherapy in conjunction with magnetic resonance imaging in which various conductors, shields and shims may be used to solve issues occurring when radiation therapy equipment is placed in the vicinity of an magnetic resonance imaging system.

Description:
TECHNICAL FIELD 
     The present disclosure relates to systems and methods for the delivery of radiotherapy in conjunction with magnetic resonance imaging. 
     BACKGROUND 
     It is desirable to combine radiation therapy with repetitive real-time imaging using a magnetic resonance imaging system (MRI) in order to better locate and treat therapy targets while sparing nearby healthy tissue. While MRIs and radiation treatment systems such as linear accelerators or radioisotope beams have operated separately from one another for quite some time, combining the two technologies presents many significant technological challenges. Examples of such challenges include the magnetic fields and eddy currents generated in ferromagnetic and conductive radiotherapy equipment through the MRI&#39;s main and gradient magnets, both of which can destroy an MRI&#39;s ability to provide quality images. 
     SUMMARY 
     Disclosed herein are systems and methods for combining radiotherapy with magnetic resonance imaging. 
     In one embodiment, a system may include a magnetic resonance imaging system, a gantry, one or more radiation therapy heads fixed to the gantry, one or more collimating devices associated with the one or more heads, including a plurality of leaves, one or more leaf drive motors for moving the leaves, the one or more leaf drive motors including one or more position sensors, and a magnetic shield substantially surrounding the one or more position sensors, one or more permanent magnets adjacent the one or more leaf drive motors, the one or more permanent magnets oriented so as to counteract the MRI&#39;s main magnetic field, and one or more additional conductive elements disposed around the gantry in a substantially symmetrical manner. The one the one or more additional conductive elements may have a shape, volume or material permeability designed to substantially match the shape, volume or material permeability of the collimating devices. In addition, the one or more additional conductive elements may be substantially similar to the outer housings of the collimating devices. In one embodiment, only one radiation therapy head is fixed to the gantry and only one collimating device is associated with the one radiation therapy head and the one or more additional conductive elements is a single conductive element extending substantially around the portion of the gantry not occupied by the one collimating device. Furthermore, the magnetic shield may be made of steel and may be cylindrical in shape. 
     In another embodiment, a system may include a magnetic resonance imaging system, a gantry, one or more radiation therapy heads fixed to the gantry, one or more collimating devices associated with the one or more heads and one or more additional conductive elements disposed around the gantry in a substantially symmetrical manner. The one or more additional conductive elements may be additional collimating devices or may be substantially similar to the outer housings of the collimating devices. In addition, the one or more additional conductive elements may have a shape, volume or material permeability designed to substantially match the shape, volume or material permeability of the collimating devices. In another embodiment, the one or more radiation therapy heads may be three radioisotopic sources, and the one or more additional conductive elements may be three conductors substantially similar to the outer housings of the collimating devices. In yet another embodiment, only one radiation therapy head is fixed to the gantry and only one collimating device is associated with the one radiation therapy head and the one or more additional conductive elements comprises a single conductive element extending substantially around the portion of the gantry not occupied by the one collimating device. 
     In still another embodiment, the system may include a magnetic resonance imaging system, a gantry, one or more radiation therapy heads fixed to the gantry, one or more collimating devices associated with the one or more heads, including a plurality of leaves, one or more leaf drive motors for moving the leaves, the one or more leaf drive motors including one or more position sensors and a magnetic shield substantially surrounding the one or more position sensors. The magnetic shield may have a cylindrical shape and may be made out of ferromagnetic material, such as steel. 
     In another embodiment, the system may include a magnetic resonance imaging system, a gantry, one or more radiation therapy heads fixed to the gantry, one or more collimating devices associated with the one or more heads, including a plurality of leaves, one or more leaf drive motors for moving the leaves and one or more permanent magnets adjacent the one or more leaf drive motors, the one or more permanent magnets oriented so as to counteract the MRI&#39;s main magnetic field. Permanent magnets may be placed on either side of the one or more leaf drive motors and may be made from Neodymium. 
     These and other features, aspects, and advantages of the present disclosure will become better understood with reference to the following description and claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Features, aspects, and implementations of the disclosure are described in conjunction with the attached drawings, in which: 
         FIG. 1  is a simplified diagram illustrating aspects of a radiation therapy device operating in conjunction with a magnetic resonance imaging system consistent with implementations of the current subject matter; 
         FIG. 2  is a simplified diagram illustrating an example of a treatment system consistent with implementations of the current subject matter; 
         FIG. 3  illustrates an exemplary collimating device, specifically a multi-leaf collimator; 
         FIG. 4A  illustrates one example of an arrangement of conductive elements consistent with implementations of the current subject matter; 
         FIG. 4B  illustrates an example of an arrangement of one collimating device and one conductive element consistent with implementations of the current subject matter; 
         FIG. 5A  is a simplified diagram illustrating a leaf motor combined with an example magnetic shield consistent with implementations of the current subject matter; 
         FIG. 5B  is a simplified representation of the effect of a magnetic shield on an MRI&#39;s main magnetic field; 
         FIG. 6  is a simplified illustration of permanent magnets being placed adjacent a bank of leaf motors consistent with implementations of the current subject matter; and, 
         FIG. 7  illustrates an exemplary magnetic orientation of permanent magnets with respect to an MRI&#39;s main magnetic field consistent with implementations of the current subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein are systems and methods for combining radiotherapy with magnetic resonance imaging.  FIG. 1  is a simplified schematic view of an implementation of a radiation therapy system including a radiation therapy head  104  mounted on a gantry  106 , which can rotate to different positions to enable radiation delivery from different angles. The exemplary system depicted in  FIG. 1  also includes an MRI  102 , which may be used for real-time imaging during radiation therapy and may be of the split or open type of MRI as shown. Radiation therapy head  104  can be used to direct a treatment beam at a target within patient  108  lying on couch  110 . 
       FIG. 2  depicts an example of a radiation therapy system that may be used with the disclosed systems and methods. The example depicted includes three radioisotopic sources  200 , such as Cobalt-60, mounted on gantry  106 , directing three radiotherapy beams at patient  108 . While this example utilizes radioisotopic beams, this disclosure contemplates and is applicable to other radiotherapy beam types such as linear accelerators, proton beams, etc. In addition, while the example of  FIG. 2  shows three radiotherapy heads spaced in an equidistant manner around the gantry, the systems and methods disclosed herein apply to any number of radiotherapy heads (i.e., one or more). 
       FIG. 2  also depicts collimating devices  202  attached to gantry  106  and associated with each of the sources  200 . Collimating devices  202  may, for example, be multi-leaf collimators (MLCs), as shown in further detail in  FIG. 3 . MLCs typically have two banks of opposing pairs of leaves  302 , which move independently and can open to form apertures of various shapes and sizes. The number of leaves  302  can vary. Leaves  302  may be made of tungsten or any suitable material or materials for blocking radiation. MLCs may also employ a tongue and groove arrangement on the long sides and front of the leaves  302  to limit interleaf radiation leakage and can be configured for inter-digitation of the leaves  302  in the closed position. 
     Each leaf  302  of each bank of leaves may be capable of independent motion and may be driven by leaf motors  304  through connecting rods  306 . An MLC control system can control the two opposing banks of leaves  302  to independently position the edge of each leaf  302  to a specified location in order to block a radiation beam and form a field size of a specific shape. 
     The MLC leaves  302 , motors  304 , and other components may be supported by housing  308  that then attaches to gantry  106 . Housing  308  may be, for example, made from aluminum. 
       FIG. 4A  depicts an exemplary gantry  106  layout for the example radiation therapy system depicted in  FIG. 2  having three radioisotopic sources  200  along with three corresponding collimating devices  202 . The embodiment of  FIG. 4A  further includes additional conductive elements  400 . Conductive elements  400  may be made of a single material or combine multiple materials. The conductive elements preferably have a shape, volume and material permeability (i.e., the degree of magnetization that a material obtains in response to an applied magnetic field) designed to substantially match those of collimating devices  202  in such respects as are pertinent to the generation of eddy currents during energizing of the MRI&#39;s gradient coils. For example, in the embodiment depicted in  FIG. 4A , conductive elements  400  could comprise identical multi-leaf collimators. 
     Because including additional sets of identical multi-leaf collimators would be expensive, and because eddy currents are primarily generated in the outer housings  308  of collimating devices  202 , conductive elements  400  may merely be substantially similar to the housings  308  of collimating devices  202 . For example, conductive elements  400  may be pairs of aluminum multi-leaf collimator housings. Alternatively, conductive elements  400  may be other shapes or other materials that substantially mimic the eddy current properties of the collimating devices  202  or housings  308 . 
     Conductive elements  400  are preferably placed in positions around gantry  106  to create a substantially symmetrical pattern, as depicted in  FIG. 4A . The positions and number of conductive elements  400  may vary, but the resulting arrangement is preferably symmetrical about the circumference of gantry  106 . As noted above, this disclosure contemplates and is applicable to any type of radiotherapy device. In the case where there is a single radiation therapy head, for example, a linear accelerator, the same principles for the additional conductive elements  400  described above apply. In one example, there may be five conductive elements  400  substantially equally spaced around gantry  106 , along with the single collimating device  202 . In another example, a single conductive element  400  may extend around substantially the entire portion of the circumference of gantry  106  not occupied by collimating device  202 , for example as shown in  FIG. 4B . While a number of examples have been given for the design and layout of conductive elements  400 , any design and layout meeting the spirit of this disclosure is contemplated. 
     Leaf motors  304  associated with collimating devices  202 , typically include one or more position sensors  500  capable of determining the position of associated leaves  302 , as shown in  FIG. 5A . For example, position sensor  500  may be a Hall effect encoder, which is capable of determining how many times a motor has rotated. A preferred embodiment includes magnetic shields  502  substantially surrounding sensors  500 . Magnetic shield  502  is preferably cylindrically shaped and made of a ferromagnetic material, in one example. One such ferromagnetic material may be steel, but other materials and shapes suitable for magnetic shielding can be used. In one embodiment, the wall thickness of a steel cylinder is in the range of 1.5-2.0 mm and reduces the magnetic field within the shield to below 140 Gauss. In one preferred embodiment, the wall thickness of the steel cylinders 1.6 mm.  FIG. 5B  demonstrates how magnetic shield  502  can decrease the magnetic field to which sensor  500  is objected from that of the main magnet magnetic field  504  to a reduced magnetic field  506  observed within the shield. 
     Position sensors  500  are preferably oriented within magnetic shield  502  so as to be in the location at which the magnetic field is weakest, as shown in  FIG. 5B  as the top and bottom of magnetic shield  502 . 
     In one embodiment of the disclosure, permanent magnets  600  may be placed adjacent to the bank of motors  304 , as shown in  FIG. 6 . Permanent magnets  600  may be held in place by housings  402  and, in one embodiment, may be made from Neodymium (NdFeB). The polar orientation of the permanent magnets  600  should be such that it counteracts the MRI&#39;s main magnetic field and the magnetic field induced in the motors and motor shields  304 .  FIG. 7  illustrates this orientation of permanent magnets  600  amidst the MRI&#39;s main magnetic field  700 , showing the north-south polarity of permanent magnets  702  opposing the main magnet&#39;s magnetic field  700 . 
     One embodiment of the disclosure contemplates one permanent magnet being placed on either side of the bank of motors  304  as shown in  FIG. 6 . However, any number and arrangement of properly oriented permanent magnets  600  may be used that results in substantial cancellation of the magnetic field induced in motor shields  304  and other ferromagnetic materials in the area (for example, the motor housings). As an alternative to permanent magnets  600 , active windings could be used as well. The canceling effect of the magnet design&#39;s strengths, field orientations, and locations can be determined utilizing modeling software such as FARADAY, available from Integrated Engineering Software, or any other appropriate software such as VectorField, for example, and with further analysis of results potentially being performed in a program such as MATLAB or any other appropriate software such as FORTRAN, for example. 
     In one embodiment of the systems and methods of the disclosure, the leaf motors  304  may also be oriented so that the permanent magnets stators within them either cancel out the group of motors&#39; magnetic field, or may alternatively be oriented in a manner that causes the group&#39;s magnetic field to be known or predictable. 
     While placing permanent magnets  600  adjacent the banks of leaf motors  304  substantially improves field homogeneity, the main magnetic field can be further improved, and the effect of radiation therapy equipment on MRI  102  further decreased. In addition to the leaf motors  304 , magnetic shields  502 , motor housings, stators within the motors and the like, the collimating devices  202  contain leaves  302  that may also have ferromagnetic properties. For example, leaves  302  may be made from a material such as tungsten, which has a relatively low permeability of approximately 1.03, but which, due to the large volume of the leaves, may also have a significant effect on the MRI&#39;s magnetic field. Magnetic field homogeneity may be in further improved, for example, by placing additional permanent magnet shims within the system. For example, in the embodiment depicted in  FIG. 4A  and discussed above, additional permanent magnets may be placed on the three conductive elements  400 . While such location has proven beneficial for this embodiment, there are numerous other locations that would prove beneficial to field homogeneity, as can be determined using modeling software such as FARADAY or any other appropriate software such as VectorField, for example. Other locations could include, for example, on the collimating devices  202  themselves. In addition, active windings could be used in place of, or in conjunction with, the permanent magnets. Finally, standard ferromagnetic shims may be used. 
     While various implementations in accordance with the disclosed principles have been described above, it should be understood that they have been presented by way of example only, and are not limiting. Thus, the breadth and scope of the invention(s) should not be limited by any of the above-described exemplary implementations, but should be defined only in accordance with the claims and their equivalents issuing from this disclosure. The present disclosure contemplates that the calculations disclosed in the implementations herein may be performed in a number of ways, applying the same concepts taught herein, and that such calculations are equivalent to the implementations disclosed. Furthermore, the above described advantages are not intended to limit the application of any issued claims to processes and structures accomplishing any or all of the advantages. 
     Additionally, section headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically, and by way of example, although the headings refer to a “Technical Field,” such claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, the description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the invention(s) set forth in issued claims. Furthermore, any reference to this disclosure in general or use of the word “invention” in the singular is not intended to imply any limitation on the scope of the claims set forth below. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby.