Patent Publication Number: US-2006017536-A1

Title: Systems, methods and apparatus for hybrid shielding in a magnetic resonance imaging system

Description:
FIELD OF THE INVENTION  
      This invention relates generally to magnetic resonance imaging systems, and more particularly to shielding in magnetic resonance imaging systems.  
     BACKGROUND OF THE INVENTION  
      Conventional magnetic resonance imaging (MRI) systems with a passive shielded magnet have iron shielding around a cryostat, such as a helium vessel. The helium vessel contains superconducting magnets. The iron shielding retains the stray field within certain prescribed limits and boundaries. Conventional magnetic resonance imaging systems with an active shielded magnet have two sets of superconducting coils, the first set of superconducting coils, referred to as the “main” coils, are positioned in the helium vessel relatively close to where a subject to be imaged is positioned during imaging. The second set of superconducting coils, referred to as the bucking coils, are positioned in the helium vessel on the outside from the main coils. A magnetic field of the bucking coil reduces the magnetic field of main coils outside of the magnet and retains the stray field within-certain prescribed limits and boundaries.  
      In regards to passive shielded magnets, the iron shielding is outside of the helium vessel and the iron shielding operates at room temperatures, approximately 21° C. The iron shielding is generally applied only to an MRI magnet system that has a low field (e.g. &lt;=0.5 Teslas), because an MRI magnet with a higher field requires a very heavy iron shield. For an active shielded magnet, the position of bucking coils outside the main coils results in a rather high amount of magnetic coupling between the main coils and the bucking coils. The magnetic field of the bucking coils interferes with the magnetic field of the main coils in an imaging region and reduces the magnetic field generated by the main coils, which in turn requires main coils with a much larger size to produce a magnetic field with sufficient strength to induce sufficient resonance in a subject in order to image the subject. The larger main coils require additional expense to manufacture, additional expense to operate, and a larger magnet size. The larger magnet size is particularly inappropriate for small medical facilities that lack generous amounts of floor space in the facility. The larger size is also particularly inappropriate for imaging procedures of a small portion of a subject, such as in orthopedic imaging procedures, in which a large magnetic resonance imaging system is not needed.  
      For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a magnetic resonance imaging system that has smaller main coils that are less expensive to manufacture. There is also a need for a magnetic resonance imaging system that has a smaller size that is more appropriate for smaller medical facilities and orthopedic imaging procedures.  
     BRIEF DESCRIPTION OF THE INVENTION  
      The above-mentioned shortcomings, disadvantages and problems are addressed herein, which will be understood by reading and studying the following specification.  
      In one aspect, an apparatus to image a subject comprises main coils that are operable to create a magnetic field of view (FOV) of the subject, bucking coils that are operable to retain the magnetic FOV within a predefined range, and a ferromagnetic shield positioned between the main coils and the bucking coils. The ferromagnetic shield separates the magnetic flux between the main coils and the bucking coils, which in turn reduces the magnetic coupling between the main coils and the bucking coils. The reduced magnetic coupling requires smaller main coils and smaller bucking coils to generate a magnetic field of sufficient strength to image a subject while retaining the stray field within certain prescribed limits and boundaries. The smaller main coils and bucking coils of the apparatus are less expensive to manufacture because of the reduced material cost of the smaller main coils and bucking coils. In one embodiment, the ferromagnetic shield is an iron shield.  
      In another aspect, the main coil, ferromagnetic shield and bucking coils are enclosed in a cryostat, such as a liquid helium vessel.  
      In yet another aspect, the apparatus comprises further ferromagnetic shields positioned outside of the cryostat.  
      In still another aspect, the cryostat and further ferromagnetic shields are enclosed a vacuum vessel.  
      In a further aspect, the apparatus has a size and shape that is particularly well-suited to orthopedic medical imaging, such as an outside diameter of about 64 centimeters, an inside diameter of about 32.2 centimeters, and a length along a longitudinal axis of about 55 centimeters.  
      Apparatus, systems, and methods of varying scope are described herein. In addition to the aspects and advantages described in this summary, further aspects and advantages will become apparent by reference to the drawings and by reading the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram that provides a system level overview of a magnetic resonance imaging (MRI) system with a ferromagnetic shield between main coils and bucking coils,  
       FIG. 2  is a block diagram of a MRI apparatus having an iron shield between main coils and bucking coils,  
       FIG. 3  is a block diagram of a MRI apparatus having a ferromagnetic shield in a helium vessel between main coils and bucking coils,  
       FIG. 4  is a block diagram of a MRI apparatus having an iron shield in a helium vessel between main coils and bucking coils, and  
       FIG. 5  is a flowchart of a method  500  for assembling a MRI system.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken in a limiting sense.  
      The detailed description is divided into five sections. In the first section, a system level overview is described. In the second section, apparatus of embodiments are described. In the third section, methods of embodiments are described. Finally, in the fourth section, a conclusion of the detailed description is provided.  
     System Level Overview  
       FIG. 1  is a block diagram that provides a system level overview of a magnetic resonance imaging (MRI) system with a ferromagnetic shield between main coils and bucking coils. System  100  solves the need in the art a MRI that has a smaller size that is more appropriate for smaller medical facilities and for use in orthopedic imaging procedures and that has smaller main coils and bucking coils that are less expensive to manufacture.  
      System  100  includes main coils  102  and bucking coils  104 . The main coils  102  are operable to generate a magnetic field of view (FOV)  106  of the subject (not shown) such as a human or a portion of a human. The bucking coils  104  are operable to retain the magnetic FOV within a predefined range. The bucking coils are also known as shielding coils.  
      System  100  also includes a ferromagnetic shield  108 . The ferromagnetic shield  108  is positioned between the main coils  102  and the bucking coils  104 . The ferromagnetic shield  108  separates the magnetic flux between the main coils  102  and the bucking coils  104 , which in turn reduces the magnetic coupling between the main coils  102  and the bucking coils  104 . The reduced magnetic coupling requires smaller main coils  102  and bucking coil  104  to generate the magnetic FOV  106  of sufficient strength to image a subject. The smaller main coils  102  and bucking coil  104  of system  100  are less expensive to manufacture because the material cost of the smaller main coils  102  and smaller bucking coils  104  is less. Thus, the ferromagnetic shield  108  of system  100  fulfills the need in the art for a MRI system that has smaller main coils  102  and bucking coils  104  that are less expensive to manufacture.  
      The smaller main coils  102  and smaller bucking coils of system  100  also provide an MRI system that has an overall smaller size. Thus, the ferromagnetic shield  108  of system  100  fulfills the need in the art for a MRI system that has a smaller size that is more appropriate for smaller medical facilities and orthopedic imaging procedures.  
      The system level overview of the operation of an embodiment has been described in this section of the detailed description. While the system  100  is not limited to any particular main coils  102 , bucking coils  104 , FOV  106  and ferromagnetic shield  108 , for sake of clarity a simplified main coils  102 , bucking coils  104 , FOV  106  and ferromagnetic shield  108  have been described.  
     Apparatus of an Embodiment  
      In the previous section, a system level overview of the operation of an embodiment was described. In this section, the particular apparatus of such an embodiment are described by reference to a series of diagrams.  
       FIG. 2  is a block diagram of a magnetic resonance imaging (MRI) apparatus  200  having an iron shield between main coils and bucking coils. Apparatus  200  fulfills the need in the art for a MRI that has a smaller size that is more appropriate for smaller medical facilities and that is more appropriate for use in orthopedic imaging procedures, and that has smaller main coils and bucking coils that are less expensive to manufacture.  
      Apparatus  200  includes an iron shield  202  positioned between main coils  102  and bucking coils  104 . The thickness of iron shield  202  depends on field strength and the geometry of main coils and bucking coils. In some embodiments, the thickness of the iron shield  202  is 3-5 centimeters.  
      The iron shield  202  separates the magnetic flux between the main coils  102  and the bucking coils  104 , which in turn reduces the magnetic coupling between the main coils  102  and the bucking coils  104 . The reduced magnetic coupling requires smaller main coils  102  and bucking coils  104  to generate the magnetic FOV  106  of sufficient strength to image a subject. The smaller main coils  102  and smaller bucking coils  104  of apparatus  200  are less expensive to manufacture because the material cost of the smaller main coils  102  and bucking coils is less. Thus, the iron shield  202  fulfills the need in the art for a MRI system that has smaller main coils  102  and bucking coils that are less expensive to manufacture. The smaller main coils  102 - and smaller bucking coils of apparatus  200  provide an MRI system that has an overall smaller size. Thus, the iron shield  202  of apparatus  200  fulfills the need in the art for a MRI system that has a smaller size that is more appropriate for smaller medical facilities and orthopedic imaging procedures.  
      An iron shield embodiment has been described in this section of the detailed description. While the apparatus  200  is not limited to any particular iron shield  202 , for sake of clarity a simplified iron shield  202  has been described.  
       FIG. 3  is a block diagram of a MRI apparatus  300  having a ferromagnetic shield in a helium vessel between main coils and bucking coils. Apparatus  300  fulfills the need in the art for a MRI that has a smaller size that is more appropriate for smaller medical facilities and that is more appropriate for use in orthopedic imaging procedures, and that has smaller main coils and bucking coils that are less expensive to manufacture.  
      Apparatus  300  includes a ferromagnetic shield  302  positioned in a helium vessel  304  between main coils  102  and bucking coils  104 . The ferromagnetic shield  302  is cooled by liquid helium (not shown) in the helium vessel  304 , and therefore is described as a “cold ferromagnetic shield.” Thus, the cooled ferromagnetic shield  302  operates as a passive shield between the main coils  102  and the bucking coils  104 , which in turn reduces the magnetic coupling between the main coils  102  and the bucking coils  104 .  
      The reduced magnetic coupling between the main coils  102  and the bucking coils  104  requires smaller main coils  102  and smaller bucking coils  104  to generate the magnetic FOV  106  of sufficient strength to image a subject. The smaller main coils  102  and smaller bucking coils  104  of apparatus  300  are less expensive to manufacture because the material cost of the smaller main coils  102  and bucking coils  104  is reduced. Thus, the ferromagnetic shield  302  fulfills the need in the art for a MRI system that has smaller main coils  102  and smaller bucking coils  104  that are less expensive to manufacture. The smaller main coils  102  and smaller bucking coils  104  of apparatus  300  provide an MRI system that has an overall smaller size. Thus, the ferromagnetic shield  302  of apparatus  300  fulfills the need in the art for a MRI system that has a smaller size that is more appropriate for smaller medical facilities and orthopedic imaging procedures.  
      In some embodiments, apparatus  300  also includes ferromagnetic shielding  306  and  308  outside of the helium vessel  304 . Ferromagnetic shielding  306  and  308  operate at the ambient temperature, such as “room temperature” approximately 21° C. and therefore are described as a “warm shield.” Thus, apparatus  300  includes three portions of ferromagnetic shielding,  302 ,  306  and  308 . The three portions and the bucking coils outside of the cooled shield in helium vessel comprise a hybrid shield.  
      An embodiment having a ferromagnetic shielding  302  in a helium vessel  304  and ferromagnetic shielding  306  and  308  outside of the helium vessel has been described in this section of the detailed description. While the apparatus  300  is not limited to any particular ferromagnetic shield  302 ,  306  and  308  or helium vessel  304 , for sake of clarity, simplified ferromagnetic shielding  302 ,  306  and  308  and or helium vessel  304  have been described.  
       FIG. 4  is a block diagram of a MRI apparatus  400  having an iron shield in a helium vessel between main coils and bucking coils. Apparatus  400  satisfies the need in the art for a MRI apparatus that has a smaller size that is more appropriate for smaller medical facilities and orthopedic imaging procedures, and that has smaller main coils and smaller bucking coils that are less expensive to manufacture and operate.  
      Apparatus  400  includes an iron shield  402  positioned in a helium vessel  304  between main coils  102  and bucking coils  104 . The iron shield  402  is cooled by liquid helium (not shown) in the helium vessel  304 , and therefore operates as an active shield between the main coils  102  and the bucking coils  104 , which in turn reduces the magnetic coupling between the main coils  102  and the bucking coils  104 .  
      The reduced magnetic coupling between the main coils  102  and the bucking coils  104  requires smaller main coils  102  and smaller bucking coils  104  to generate a magnetic FOV  106  of a sufficient strength to image a subject. Thus, the less expensive smaller main coils  102  and smaller bucking coils of apparatus  400  fulfills the need in the art for a MRI apparatus that has smaller main coils  102  and bucking coils that are less expensive to manufacture. The smaller main coils  102  and smaller bucking coils  104  of apparatus  300  provide an MRI system that has an overall smaller size.  
      In some embodiments, apparatus  400  also includes iron shielding  404  and  406  outside of the helium vessel  304 . Iron shielding  404  and  406  operate at the ambient temperature, such as “room temperature” approximately 21° C. and therefore are described as a “warm shield.” Thus, apparatus  400  includes three portions of iron shielding,  402 ,  404  and  406 .  
      In orthopedic embodiments of apparatus  400  and apparatus  200 ,  300  and  400 , an outside diameter  408  is about 64 centimeters. In addition, an inside diameter  410  is about 32.2 centimeters and a longitudinal axis  412  is about 55 centimeters.  
      In some embodiments of apparatus  400 , the center magnetic field is 3 Teslas, the homogeneity of the FOV  106  is 7.5 ppm at 16 DSV, the radial and axial dimensions of the 5 Gauss line is 1.5 meters X 2.0 meters, the current to the main coils is  780 A. In an environment of a high critical temperature (Tc) for superconducting of apparatus  400 , the mail coils  102  and the bucking coils  104  are made of high Tc (HTc) superconductors, the cold vessel  304  comprises a single cryostat containing gaseous molecular nitrogen (N 2 ) for rapid cooling of magnet (main coils  102  and bucking coils  104 ), the magnet is operated at a temperature slightly above the temperature of liquid N 2  to unify the magnet temperature and/or apparatus  400  does not include a thermal shield.  
      An embodiment having iron shielding  402  in a helium vessel  304  and iron shielding  404  and  406  outside of the helium vessel has been described in this section of the detailed description. While the apparatus  400  is not limited to any particular iron shield  402 ,  404  and  406  or helium vessel  304 , for sake of clarity, simplified iron shielding  402 ,  404  and  406  and or helium vessel  304  have been described.  
     Methods of an Embodiment  
      In the previous section, apparatus of the operation of an embodiment was described. In this section, the particular methods performed in a manufacturing process of such an embodiment are described by reference to a series of flowcharts.  
       FIG. 5  is a flowchart of a method  500  for assembling a MRI system according to an embodiment. Method  500  allows a system and/or apparatus to be manufactured that satisfies the need in the art for a MRI that has a smaller size that is more appropriate for smaller medical facilities and orthopedic imaging procedures, and that has smaller main coils and smaller bucking coils that are less expensive to manufacture and operate.  
      Method  500  includes assembling  502  a first ferromagnetic shield with main magnetic coils and bucking magnetic coils. Examples of the first ferromagnetic shield include ferromagnetic shield  108  in  FIG. 1  and ferromagnetic shield  302  in  FIG. 3 . Examples of the main coils and the bucking coils are main coils  102  and bucking coils  104  in  FIGS. 1, 2 ,  3  and  4 .  
      Thereafter, the assembled iron shield, main magnetic coils and bucking magnetic coils are assembled  504  into a helium vessel. In some embodiments, method  500  further includes assembling  506  one or more additional ferromagnetic shields outside the helium vessel. Examples of the additional ferromagnetic shields include ferromagnetic shielding  306  and  308  in  FIG. 3  and iron shielding  404  and  406  in  FIG. 4 .  
     CONCLUSION  
      A magnetic resonance imaging system (MRI) with a ferromagnetic shield positioned between the main coils and the bucking coils has been described. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations.  
      In particular, one of skill in the art will readily appreciate that the names of the methods and apparatus are not intended to limit embodiments. Furthermore, additional methods and apparatus can be added to the components, functions can be rearranged among the components, and new components to correspond to future enhancements and physical devices used in embodiments can be introduced without departing from the scope of embodiments. One of skill in the art will readily recognize that embodiments are applicable to future MRI devices, different main coils, and new bucking coils.  
      The terminology used in this application with respect to MRI is meant to include all medical and industrial environments and alternate technologies which provide the same functionality as described herein