Abstract:
Systems, methods and apparatus are provided through which in some embodiments a magnetic resonance imaging system includes at least two cryostats, each cryostat having a portion of a superconducting coil. Some embodiments provide force balancing between the sections. Some embodiments provide an ability to use more efficient superconducting coil geometry which would otherwise physically trap the gradient between the coils. Some embodiments provide an ability to install, remove or upgrade magnet without dismantling the imaging room.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to magnetic resonance imaging systems, and more particularly to a multi-section magnet for a magnetic resonance imaging system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Magnetic resonance imaging (MRI) is a technique in which an object is placed in an electromagnetic field and subjected to pulses of the electromagnetic field at a frequency. The pulses cause nuclear magnetic resonance in the object and the spectra obtained thereby is processed numerically to form cross-sectional images of the object. MRI imaging is especially useful for medical or veterinary applications because different living tissues emit different characteristics of resonance signals, thus enabling visualization of the different living tissues in the obtained image. An MRI apparatus thus operates in general by the application of a radio frequency (RF) electromagnetic field in the presence of other magnetic fields, and the subsequent sensing and analysis of the resulting nuclear magnetic resonances induced in the body. 
         [0003]    Conventional MRI systems include a main magnet which generates a strong static magnetic field of a high temporal stability and a high spatial homogeneity within a field-of-view (FOV) where the imaging takes place. Conventional MRI systems also include a gradient coil assembly located in the bore between the main magnet and an RF coil. The gradient coil assembly generates space-varying fields that cause the response frequency and phase of the nuclei of the patient body to depend upon position within the FOV thus providing a spatial encoding of the body-emitted signal. Conventional MRI systems further include RF coil/coils arranged within the bore which emit RF waves and receive resonance signal from the body. The main magnet may be a superconducting magnet that includes a plurality of concentric coils placed inside a cryostat which is designed to provide a low temperature operating environment for superconducting coils. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    In accordance with an embodiment, an apparatus includes a first section of a superconducting coil operable to generate a magnetic field. The first section of the superconducting coil is contained in a first cryostat. The apparatus also includes a second section of the superconducting coil. The second section of the superconducting coil contained in a second cryostat. 
         [0005]    In accordance with another embodiment, a system includes a plurality of cryostats. Each cryostat contains a section of a superconducting coil operable to generate a magnetic field. The system also includes a gradient coil positioned at a radius inward of the plurality of cryostats. 
         [0006]    In accordance with another embodiment, a magnetic resonance imaging system includes a gradient coil and a first cryostat having at least a portion positioned at a first end of the gradient coil. The first cryostat contains a first set of superconducting coils including a first bucking coil and a first section of a primary coil. The first section of the primary coil is operable to generate a magnetic field. The magnetic resonance imaging system also includes a second cryostat having at least a portion positioned at a second end of the gradient coil, the second cryostat containing a second set of superconducting coils including a second bucking coil and a second section of the primary coil. The second section of the primary coil is operable to generate a magnetic field. At least one of the first set of superconducting coils and the second set of superconducting coils is removable from the magnetic resonance imaging system. 
         [0007]    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 
         [0008]      FIG. 1  is a simplified cross section block diagram of a magnetic resonance imaging system having at least two cryostats, according to an embodiment; 
           [0009]      FIG. 2  is a simplified cross section block diagram of magnetic resonance imaging system having at least two cryostats, according to an embodiment; 
           [0010]      FIG. 3  is a simplified cross section block diagram of magnetic resonance imaging system having two sections of a superconducting magnet enclosed in separate cryostats, according to an embodiment; 
           [0011]      FIG. 4  is an isometric diagram of a conventional MRI transverse gradient magnetic field body coil that is compatible with the MRI system of  FIGS. 1-3 , according to an embodiment; 
           [0012]      FIG. 5  is a cross section diagram of a conventional transverse gradient coil that is compatible with the MRI system of  FIGS. 1-3 , according to an embodiment; 
           [0013]      FIG. 6  and  FIG. 7  are cross section diagrams of a transverse folded single layer continuous gradient coil that is compatible with the MRI system of  FIGS. 1-3 , according to an embodiment; and 
           [0014]      FIG. 8  is cross section diagram of a crescent gradient coil that is compatible with the MRI system of  FIGS. 1-3 , according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    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. 
         [0016]      FIG. 1  is a simplified cross section block diagram of a magnetic resonance imaging (MRI) system  100  having at least two cryostats, according to an embodiment. MRI system  100  includes a gradient coil  102 . MRI system  100  also includes a first cryostat  104  and a second cryostat  106 . The gradient coil  102 , the first cryostat  104  and the second cryostat  106  are enclosed in a housing  108  that surrounds a cylindrical patient volume or bore  110 . Various other elements of MRI system  100  such as an RF coil (or coils), suspension members, brackets, a patient table or support, etc. are omitted from  FIG. 1  for clarity. In  FIG. 1 , two cryostats  104  and  106  are shown, however, in other embodiments more than two cryostats may be used. 
         [0017]    The first cryostat  104  and the second cryostat  106  are separate components and do not exchange fluids such as liquid helium or other liquid coolant. The first cryostat  104  and the second cryostat  106  each house superconducting magnet components (e.g., superconducting coils) that are used to generate a magnetic field in the patient volume or bore  110 . The gradient coil  102  is positioned radially inward of the first cryostat  104  and the second cryostat  106  and is used to generate magnetic field gradient pulses used for spatially encoding acquired signals. At least one of the first cryostat  104  and the second  106  cryostat is removable from the housing as shown in  FIG. 2 .  FIG. 2  is a simplified cross section block diagram of a magnetic resonance imaging (MRI) system having at least two cryostats, according to an embodiment. In  FIG. 2 , the second cryostat  106  is shown being separated and unmounted from the housing  108 . In an alternative embodiment, both cryostats  104 ,  106  may be separable and removable from the housing  108 . 
         [0018]    Returning to  FIG. 1 , as mentioned, the first cryostat  104  and the second cryostat  106  are separate components. The first cryostat  104  and the second cryostat  106  are shown with an asymmetrical split so that the first cryostat  104  and the second cryostat  106  do not have the same dimensions. In alternative embodiments, the first cryostat  104  and the second cryostat  106  may have similar or the same dimensions. Preferably, at least one of the cryostats  104 ,  106  has dimensions that allow the cryostat to be removed from the housing  108 , as shown in  FIG. 2 . In an embodiment, at least one of the cryostats  104 ,  106  has dimensions that allow the cryostat to pass through a conventional-sized doorway, for example, a doorway of approximately 3.0 feet by 6.5 feet. Accordingly, transporting such cryostats to and from the site of the MRI system  100  does not necessarily require dismantling of the doorways and walls of the room in which the system is located. The dimensions of the separate cryostats  104 ,  106  provide easier maneuverability and transportation (e.g., less mechanical force may be required) when accessing and removing or replacing the cryostats. The separate cryostats  104 ,  106  also provide easier access to the gradient coil  102 . Accordingly, the maintenance of the MRI system  100 , gradient coil  102  and cryostats  104 ,  106  may be more convenient. The separate (or split) cryostat configuration provides access to portions of the MRI system such as the cryostats (and the magnet components contained therein) and the gradient coil  102  for maintenance, repair and upgrading. For example, the second cryostat  106  may be removed, as shown in  FIG. 2 ), to access the gradient coil  102  for fitting, removal or replacement. Alternatively, the cryostats  104 ,  106  may be removed for maintenance. 
         [0019]    As mentioned above, the first cryostat  104  and the second cryostat  106  each contain superconducting magnet components that are used to generate a magnetic field in the patient volume or bore  110 .  FIG. 3  is a simplified cross section block diagram of a magnetic resonance imaging (MRI) system having two sections of a superconducting coil enclosed in separate cryostats, according to an embodiment. The MRI system  300  includes a first cryostat  104  and a second cryostat  106 . The first cryostat  104  has a first length  310  and contains a first bucking (or shielding) coil  302  and a first section  304  of a primary superconducting coil. The terms primary coil and primary superconducting coil are used throughout to denote coils located on an inner radius of the cylindrical cryostats. The first section  304  of the primary superconducting coil is operable to generate a magnetic field. The second cryostat  106  has a second length  312  and contains a second bucking (or shielding) coil  306  and a second section  308  of the primary superconducting coil. The second section  306  of the primary superconducting coil is operable to generate a magnetic field. The primary superconducting coil for the MRI system  300  is composed in aggregate from the plurality of sections of the primary superconducting coil. For example, in  FIG. 3 , the primary superconducting coil is composed in aggregate of the first section  304  of the primary superconducting coil and the second section  308  of the primary superconducting coil. 
         [0020]    Similar to system  200  in  FIG. 2  above, in  FIG. 3 , at least one of the two cryostats  104 ,  106 , and thus one of the two section(s) of the superconducting coil are removable from the housing  108  and the MRI system  300 . In  FIG. 3 , the first length  310  of the first cryostat  104  and the second length  312  of the second cryostat  106  are different, illustrating an asymmetrical split of the superconducting coils. However, MRI system  300  is not limited by any particular length of the cryostats  104 ,  106  or sections of superconducting coil. In alternative embodiments, the cryostats  104 ,  106  may have similar or the same lengths. 
         [0021]    Preferably, the coils within each of the plurality of cryostats have a net balanced axial magnetic force so that there is no (or little) net attractive force between the plurality of cryostats. To provide a net balanced axial magnetic force, the bucking coil(s) in a particular cryostat have an axial magnetic force about equal and opposite to an axial magnetic force of the primary superconducting coil in that particular cryostat. Thus, the equal (or about equal) and opposite magnetic forces of the bucking coils and the primary superconducting coils yield a net balanced axial magnetic force within each cryostat, or a near balanced axial force within each cryostat. For example, in the first cryostat  104  the bucking coil  302  has an axial magnetic force about equal and opposite to an axial magnetic force of the first section  304  of the primary superconducting coil. In another example, in the second cryostat the bucking coil  306  has an axial magnetic force  314  about equal and opposite to an axial magnetic force  316 ,  318  of the second section  308  of the primary superconducting coil. The balanced axial magnetic force of the coils in each cryostat yields a net axial magnetic force of about zero. Accordingly, the cryostats  104  and  106  are not imparting significant magnetic force upon each other. 
         [0022]    In order to balance the axial force acting on a given section of the superconducting coil, a cryostat may include at least one coil in which the current flow is opposite to that in adjoining coils. For example, the coils in a cryostat inboard of end coils in the cryostat may be reversed coils to enable a net balanced axial magnetic force within the cryostat. In addition, such reversed flow in combination with a relatively large number of coils (e.g., more than 6 coils) enables the use of a short, yet homogeneous primary coil geometry. 
         [0023]    The separate cryostat configuration (as described above) using a plurality of cryostats is compatible with efficient primary superconducting coil geometries. For example, a short magnet system is obtained by turns of the primary superconducting coil that are arranged in a U-shaped geometry with the opening of the “U” facing a longitudinal axis  320  of the MRI system  300 . At least a portion of the first cryostat  104  is positioned at first end  322  of the gradient coil  102  and at least a portion of the second cryostat  106  is positioned at a second end  324  of the gradient coil  102 . As a result at least a portion of the gradient coil  102  is located between the two cryostats  104 ,  106 . In this configuration, the magnetic field generated by the primary coil has a very high degree of homogeneity and the size of the “U”-shaped geometry remains sufficiently large. The “U” configuration of the primary (inner) coils reduces the length of the primary coils and/or improves homogeneity while still providing sufficient space to house the gradient coil. In the “U” configuration, by using separable sections (e.g., sections  304 ,  308  shown in  FIG. 3 ) of the primary coil, the gradient coil  102  is not physically trapped by the primary coils, thus the gradient coil can be readily repaired or replaced. 
         [0024]    Where a “U”-shaped geometry for the primary superconducting coils is implemented, some embodiments include a gradient coil geometry with reduced axial extent such that the gradient coil can nest between the end primary coils.  FIGS. 4-8  describe exemplary shortened gradient coil configurations that are compatible with the MRI system of  FIGS. 1-3 .  FIG. 4  is an isometric diagram of a conventional transverse Golay gradient coil  400  that is compatible with the MRI system of  FIGS. 1-3 , according to an embodiment. The transverse gradient coil  400  has four quadrants each of which have a “fingerprint” winding pattern  402 ,  404 ,  406 ,  408 , similar to that shown in  FIG. 5 . Current flows according to, or opposite arrows  410 ,  412 ,  414  and  416 . Quadrants  402 ,  404 ,  406  and  408  are electrically connected in series with each other.  FIG. 4  also includes a subject  418  and a magnet means  420 . 
         [0025]      FIG. 5  is a cross section diagram of a conventional transverse Golay gradient coil  500  that is compatible with the MRI system of  FIGS. 1-3 , according to an embodiment. In each of the “fingerprint” coils of  FIG. 4 , a surface current is designed to pass through a region  502  from A to B to cause a magnetic field to be produced. The current path passing through a region  502  from A to B is designed to provide the desired magnetic field gradient. The region  504  from B to C is necessary to provide a current return path, completing the circuit. The return path in region  504 , however, increases the power dissipation and gradient coil length without providing a useful imaging gradient. However, in the example of the return path in region  504 , the return path region has more tightly packed turns in order to provide a slightly reduced gradient coil length without compromising the field linearity. 
         [0026]      FIG. 6  and  FIG. 7  are cross section diagrams of a transverse folded single layer continuous gradient coil  600  that is compatible with the MRI system of  FIGS. 1-3 , according to an embodiment. A transverse folded single layer continuous gradient coil geometry is more efficient and has reduced axial extent, which could be particularly advantageous when implemented in conjunction with a “U”-shaped primary coil geometry. Gradient coil  600  includes a first region  602  having a plurality of half-loops  604  for carrying a current, a second region  606  having a plurality of half-loops also for carrying a current, and a third region  608  having conductors which connect each half-loop  604  with a corresponding half-loop to create a single gradient coil  600 . Gradient coil  600  is intended to be folded or bent along the lines  610  and  612  to result in a shape which is disposed upon two cylinders of radii a.sub. 1 , and a.sub. 2 , as shown in  FIG. 7 . Section  602  is disposed upon a cylinder of radius a.sub. 1 , while section  606  is disposed upon a cylinder of radius a.sub. 2 . Section  608  is an intermediate used to link individual current paths of sections  602  and  606 , connecting each turn of section  602  at radius a.sub. 1  to each corresponding turn of section  606  at radius a.sub. 2 . This complication can be solved by appropriately soldering and supporting a connecting wire between each turn of coils at radius a.sub. 1  to those of radius a.sub. 2 . 
         [0027]      FIG. 8  is cross section diagram of a crescent gradient coil  800  that is compatible with the MRI system of  FIGS. 1-3 , according to an embodiment. The crescent gradient coil  800  includes a first radius  802  and a second radius  804 . In crescent gradient coil  800 , a return path for forward arcs is on the second radius  804 , reducing the axial extent of the gradient and thereby making the crescent geometry advantageous when implemented in conjunction with a “U”-shaped primary coil geometry. To further enhance performance, a crescent gradient coil may be combined with a more conventional Golay gradient coil, whereby some forwards arcs have a return path on the second radius and some forward arcs have a return path on the same radius. 
         [0028]    A separable superconducting coil magnetic resonance imaging (MRI) system is described. Although specific embodiments are 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. 
         [0029]    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 coil, different cryostats, and new MRI systems. 
         [0030]    The terminology used in this application is meant to include all MRI systems, cryostats and magnetic coils and alternate technologies which provide the same functionality as described herein