Abstract:
An apparatus for magnetic resonance imaging having a magnetic assembly with a main magnet, a shielding system positioned outside the main magnet, and a first and second space for positioning the imaged and non-imaged extremity. Various apparatus with distinct vessels and casings are described with a magnetic assembly and ferromagnetic shielding systems. In yet a further aspect, an apparatus for resonance imaging having additional shim elements for compensating effects from non-axisymmetrical shape of ferromagnetic shielding casing is disclosed. Additionally, an apparatus for magnetic resonance imaging is described having a dedicated space outside the imaging region and where the dedicated space has support element for resting non-image extremity as part of the structure of the imager.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to magnetic resonance imaging (MRI) apparatus. More particularly, this invention relates to an MRI apparatus having a region for a non-imaged extremity.  
       BACKGROUND OF THE INVENTION  
       [0002]     MRI scanners, which are used in various fields such as medical diagnostics, typically use a computer to create images based on the operation of a magnet, a gradient coil assembly, and a radiofrequency coil(s). The magnet creates a uniform main magnetic field, which makes nuclei, such as that of hydrogen atoms, responsive to radiofrequency excitation. The gradient coil assembly imposes a series of pulsed, spatially varying magnetic fields upon the main magnetic field, in order to give each point in the imaging volume a spatial identity corresponding to its unique magnetic field values during the imaging pulse sequence. The radiofrequency coil(s) generate(s) an excitation frequency pulse that causes a temporary oscillating transverse magnetization of the nuclei. The nuclei relaxation from that states results in an emitted signal which is detected by the radiofrequency coil and used by the computer to create the image.  
         [0003]     The typical MRI system is provided with shielding means designed to prevent exposure to static stray magnetic field to the operator, surrounding equipment and facilities. The typical stray magnetic field limit imposed by the U.S. Food and Drug Administration (FDA) with respect to external personnel exposure is 5 gauss; additional stray field limitations of higher values may be imposed in the design to prevent interference with electronics and other nearby equipment.  
         [0004]     The shielding means of superconductive magnets may include superconductive bucking coils (active shielding) and/or ferromagnetic (iron) shielding elements. The superconductive bucking coils carry electric currents of generally opposite direction to the electric current carried in the superconductive main coils. The superconductive bucking coils are positioned radially outward from the superconductive main coils to counterbalance magnetic moments created by the main coils. Likewise, the cylindrical iron shield is positioned radially outward from the superconductive main coils to prevent leakage outside the magnet of the magnetic field created by the main coils.  
         [0005]     Orthopedics is a medical specialty concerned with correction of deformities or functional impairments of the skeletal system, particularly, of the extremities and the spine, and associated structures, such as muscles and ligaments. For example, diagnosis and treatment of broken hand or leg bones is a common practice in orthopedics. Because many orthopedic health problems are subcutaneous, imaging anatomy under the skin is a very important capability in orthopedics. Magnetic resonance imaging (MRI) is one imaging technique implemented in orthopedic diagnosis.  
         [0006]     Use of conventional whole body (WB) MRI systems in orthopedic imaging has its intrinsic limitations. The distance between the front face and the field of view is hardly sufficient to allow the patient to extend their arm or leg into the centrally located field of view from the outside of the MRI system, therefore patients must egress into the center of the MRI even for orthopedic imaging of limbs. For claustrophobic patients, this can be a traumatic experience. In addition, the large size and large stray field footprint of conventional WB MRI systems require a large floor space in which to site the MRI system and associated increased facilities cost. Furthermore, conventional full body MRI systems have higher cost than the dedicated orthopedic system, as the large bore inevitably leads to much larger overall dimensions, forces and amount of superconductor, as well as complexity of structural and cryogenic designs.  
         [0007]     Thus, in the case of orthopedic application for MRI systems there are considerable advantages in terms over cost and sitability for small diameter bore magnet systems dedicated to extremity imaging such as human legs and arms. However, an orthopedic MRI system suffers from the limitation that the shielding increases the outside dimensions of the magnet system which limits the access for leg imaging due to the difficulty of positioning the subject&#39;s second leg outside the imaging region.  
         [0008]     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 orthopedic imaging that is not limited by the outside dimensions of the magnet system. There is also a need for improved magnetic resonance imaging of extremities, which does not compromise or affect the accuracy or operation of the MRI.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0009]     The above-mentioned shortcomings, disadvantages and problems are addressed herein, which will be understood by reading and studying the following specification.  
       SUMMARY OF THE INVENTION  
       [0010]     An apparatus for resonance imaging having a magnetic assembly positioned about the center of a vessel comprising main coils and/or other means to generate main magnetic field, e.g. ferromagnetic elements, having an inner diameter and an outer diameter for magnetic resonance imaging, and regions between the main magnetic assembly and a shielding system so as to position the imaging extremity in the center and non-imaging extremity in the region between the main magnetic assembly and the shield system.  
         [0011]     In yet another aspect, an apparatus for resonance imaging having a main magnetic assembly positioned about the center of an annulus vessel comprising a main coil assembly and/or other means to generate main magnetic field, e.g. ferromagnetic elements, having an inner diameter and an outer diameter, a gradient system adjacent to the magnetic assembly, shielding assembly, and regions between the gradient assembly and magnetic assembly so as to position the non-imaging extremities.  
         [0012]     An magnetic resonance imaging system is described having a magnetic assembly positioned in an annulus vessel, a ferromagnetic shielding casing positioned around the magnetic assembly and in partial contact with the magnetic assembly, a first space for placing the imaged extremity in the bore of said magnetic assembly, a second space adjacent to said portion of said ferromagnetic casing which is in contact with said magnetic assembly for positioning the non-imaged extremity, and additional shim elements for compensating effects from non-axisymmetrical shape of said ferromagnetic shielding casing.  
         [0013]     In a further aspect, an apparatus for resonance imaging having a magnetic assembly positioned about the center of a vessel comprising a main coil assembly and/or other means to generate main magnetic field, e.g. ferromagnetic elements, having an inner diameter and an outer diameter, a shielding assembly and shim elements for placement within the magnetic assembly so as to become magnetized and compensate magnetic field inhomogeneities created by non-axisymmetric shape of shielding assembly elements, and regions in the annulus vessel to position the non-imaging extremities.  
         [0014]     In yet a further aspect, an apparatus for resonance imaging having a magnetic assembly positioned about the center of an annulus vessel comprising a main coil assembly and/or other means to generate main magnetic field, e.g. ferromagnetic elements, having an inner diameter and an outer diameter, a shim elements for placement within the magnetic assembly so as to become magnetized and compensate magnetic field inhomogeneities created by non-axisymmetric shape of shielding assembly elements, and a substantially bulge shape region and a annular shape region between the shield and main magnetic assembly to position the non-imaging extremity.  
         [0015]     In still yet a further aspect, an apparatus for resonance imaging having a magnetic assembly positioned about the center of a warped annulus vessel resembling a clover leaf shape comprising a main coil assembly and/or other means to generate main magnetic field, e.g. ferromagnetic elements, having an inner diameter and an outer diameter. The lobes of the cloverleaf vessel is used to position the non-imaging extremities  
         [0016]     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  
       [0017]      FIG. 1  is an end view of a vessel according to an embodiment for positioning extremities for medical resonance imaging;  
         [0018]      FIG. 2  is an end view of a vessel according to an embodiment for positioning extremities for medical resonance imaging between a gradient system and magnetic system;  
         [0019]      FIG. 3  is a side view (vertical cross-section) of the MRI system according to an embodiment;  
         [0020]      FIG. 4  is an end view of an annulus vessel according to an embodiment for positioning extremities for medical resonance by the use of cut outs;  
         [0021]      FIG. 5  is a top view (horizontal cross-section) of the MRI system according to an embodiment;  
         [0022]      FIG. 6  is an end view of an annulus vessel according to an embodiment for positioning extremities for medical resonance by the use of bulges in the shielding system; and  
         [0023]      FIG. 7  is an end view of an annulus vessel according to an embodiment for positioning extremities for medical resonance imaging in a clover-type shape. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     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.  
         [0025]      FIG. 1  is an end view that provides a system level overview of an apparatus having an annulus outer perimeter to image a subject using magnetic resonance. It should be understood, however, that the underlying concepts are not depended upon a particular shape so it is conceivable that other shapes can be used without departing from the inventive concepts. System  100  solves the need in the art for more comfortable access to an MRI system by a patient during imaging of an extremity of the patient. System  100  also solves the need in the art for reduced floor space requirements of MRI systems.  
         [0026]     System  100  includes a casing  10  having an annular shaped outer perimeter while a leg or arm of the patient is placed in the system  100  for imaging. The imaged extremity is placed in the bore of the magnet  108  and the non-imaged extremity is placed in either space  102  or  104 . Thus, system  100  solves the need in the art for more comfortable access to a MRI system by a patient during imaging of an extremity&#39;s of the patient. In addition, the small overall outer diameter  208  of the casing  110  reduces floor space requirement of the MRI system. The smaller length does not require egress into the center of the entire system for orthopedic medical imaging, which more readily accommodates claustrophobic patients.  
         [0027]     System  100  also includes a first annular space  102  and second annular space  104  in the annulus casing  110 . The magnet for imaging has a magnetic assembly or plurality of magnetic coils (not shown) positioned in the casing  110  around the bore in close proximity to the first annular space  102  and the second annular space  104 . When imaging of an extremity is being performed the patient will insert it in the bore and use either space  102  or  104  to rest the non-imaging extremity. Further, positioned inside the outer edge of annulus vessel  110  is a shielding system  106 . The shielding system  106  can include active shielding such as superconductive bucking coil, or passive ferromagnetic shielding, or any known or future developed shielding. In the latter implementation, the shielding system  106  can be a room temperature shielding such as iron or like that surrounds the main magnetic assembly and substantially reduces leakage stray field so as to protect space outside of the annulus casing. When the MRI system is being used, for example for imaging legs, the non-imaged extremity can be placed in either annular space  102  or  104 . Otherwise, the non-imaged extremity would extend outside the annulus casing  110  causing discomfort to the patient and limiting the region that can be imaged.  
         [0028]      FIG. 2  is an end view that provides a system level overview of an apparatus having an annulus outer perimeter  212  to image a subject using magnetic resonance. System  200  solves the need in the art for more comfortable access to an MRI system by a patient during imaging of an extremity. System  200  also solves the need in the art for reduced floor space requirements of MRI systems by encasing the system the MRI in a casing with smaller outer diameter The smaller size does not require egress into the center of the entire system for orthopedic medical imaging, which more readily accommodates claustrophobic patients.  
         [0029]     System  200  also includes a first area  202  and a second area  210  within the annulus casing  212  for positioning a non-imaged extremity. A magnetic assembly creates a Field of View (FOV)  204  in the bore of casing  212  in close proximity to the first area  202  and the second area  210 . The imaged extremity is then placed in the bore to be imaged by the MRI system  200  while the non-imaged extremity can be comfortably placed in either area  202  or  210 . The coils or other magnetic field generated devices can be placed in space  206  along with the necessary shielding for the MRI system  200  which is positioned closer to the outer edge  212  of the annulus vessel. Additionally, a gradient structure  208  can be positioned adjacent to the FOV  204  or in the alternative the gradient structure  208  could be placed adjacent to the magnetic field generating devices in space  206 . In the later arrangement the first and second areas  202  and  210  are between the gradient system  208  and the top part of the bore. The shielding in space  206  can be active shielding such as superconductive bucking coil, passive ferromagnetic shielding, or any known or future developed shielding that can be placed inside or on outside an annulus vessel. When the MRI system is being used for imaging legs, the non-imaged leg can be placed in either area  202  or  210 . Otherwise, the non-imaged leg would extend outside the annulus casing  212  causing discomfort to the patient and limiting the region that can be imaged.  
         [0030]      FIG. 3  is a side view diagram that provides an apparatus having an annulus outer perimeter to image a subject using magnetic resonance. System  300  includes the entrance to the bore  306  for positioning the imaged extremity at the imaging region  204  so as to perform imaging of extremities, gradient structure, and entry points or first and second areas  202 , 210  for positioning the non-imaged extremity. Furthermore,  302  shows the diameter of the casing,  306  shows the dimension for the entrance to the bore, and  304  shows the diameter of the combined first and second areas and the entrance to the bore. Those in the art should understand that these dimensions are only for illustration purposes and that these dimensions could be changed or altered without departing from the spirit of the invention.  
         [0031]      FIG. 4  is an end view that provides a system level overview of an apparatus having an annulus outer perimeter to image a subject using magnetic resonance. MRI System  400  solves the need in the art for more comfortable access to an MRI system by a patient during imaging of an extremity of the patient. System  400  also solves the need in the art for reduced floor space requirements of MRI systems.  
         [0032]     The MRI System  400  includes a first space  410  and second space  412  that have been cutout from an otherwise circular casing  416 . The shielding system  416  is a ferromagnetic envelope. In the alternative, the casing  416  can be defined as a region that ends at spaces  410  and  412  and takes the shape as shown in  FIG. 4 . A magnetic assembly and shielding system (not shown) can be positioned in the casing  416  in region  404  in close proximity to bore  402  where a field of view (FOV) is created for imaging an extremity. When the MRI system is being used for imaging legs the non-imaged leg can be placed in either annular space  410  or  412  that have been cutout from annulus casing  416 . Otherwise, the non-imaged leg would extend outside the casing  416  causing discomfort to the patient and limiting the region that can be imaged. When annular spaces  410  or  412  are cutout from the annulus vessel  416  the possibility that the shielding in  404  is not substantial to isolate stray magnetic field from outside the vessel is increased. To circumvent non-uniform contribution to the imaging field caused by such leakage, shimming elements  406  can be placed substantially near the outer diameter (OD)  414  of bore  402 . These additional shimming elements compensate from non-axisymmetrical shape of said ferromagnetic shielding casing  
         [0033]     The shimming elements  406  are selected based on a process known as shimming to improve the homogeneity of the magnetic field produced by magnetic assembly to the necessary levels by making small adjustments to the overall magnetic field. In the case of passive shimming, the shim elements  406  are placed within the magnetic field so as to become magnetized and have an effect on the magnetic field. A non-magnetic holder receives the shim element such that the shim element is disposed at a fixed position with respect to the magnet. In the alternative, the shim elements could be placed in either a dedicated mounting or welded on the surface of the magnet. The shim element  406  may represent a plurality of shim elements of the same or varying sizes. The magnetic material may be a soft ferromagnetic material. radial terms can be attenuated by proper positioning accomplished through iterative measuring and calibration or magnetic analysis that can be used to determine the proper shim elements  406 .  
         [0034]      FIG. 5  is a horizontal cross-sectional view that provides a system level overview of an apparatus having an outer perimeter to image a subject using magnetic resonance. System  500  solves the need in the art for more comfortable access to an MRI system by a patient during imaging of an extremity of the patient. System  500  also solves the need in the art for reduced floor space requirements of MRI systems.  
         [0035]     System  500  shows a second space  412  and view of the inner part of the bore  402  where the imaging of extremities is conducted along path  506 . Further, part of the magnetic system such as field shaping coils  502  that with end coils  504  produce a magnetic field for imaging of extremities are shown at field of view  510 . The distance between imaged and non-imaged leg is represented by  508 .  
         [0036]      FIG. 6  is an end view that provides a system level overview of an apparatus having an annulus outer perimeter to image a subject using magnetic resonance. System  600  solves the need in the art for more comfortable access to an MRI system by a patient during imaging of an extremity of the patient. System  600  also solves the need in the art for reduced floor space requirements of MRI systems.  
         [0037]     System  600  includes a casing  614  capable of holding imaged and non-imaged extremities, magnetic assembly, and shielding system. System  600  also includes a first space  606  and second space  608  in the annulus casing  614 . These spaces are for positioning the non-imaged extremity during the imaging cycle. For example, if imaging at field of view (FOV)  616  the right leg or arm a patient (facing system  600 ) could rest the non-imaged extremity in space  608 . A magnetic assembly or pluralities of magnetic coils (not shown) are positioned in the casing  614  in close proximity to the first annular space  606  and the second annular space  608 . The MRI system  600  may use ferromagnetic shielding (not shown) which may be at room temperature (RT) and may form part of the outer casing at  612  for example, or be part of the imaging room. RT shielding on outside the magnet use less space then the actively-shielded magnet with bucking coils, and provides the opportunity for accommodating second, non-imaged leg or arm.  
         [0038]     To avoid effect of changing magnetization with ambient temperature, the temperature of the RT shield may be automatically controlled by maintaining it constant, and elevated over that of the room e.g. by using heaters. In embodiments where RT shielding is used, an asymmetric magnet exerts a non-zero axial force on the symmetric RT shielding thus affecting internal suspension system, thus the RT shield is formed asymmetric to achieve balanced zero axial force. System  600  shows additional shim elements  604  that are positioned at a predetermined distance from field of view  616  at the outer edge of the bore  610 . The additional shim elements become magnetized and have an effect of the projected magnetic field produced by the magnetic system.  
         [0039]      FIG. 7  is an end view that provides a system level overview of an apparatus to image a subject&#39;s extremities using magnetic resonance. In  FIG. 7 , system  700  solves the need in the art for more comfortable access to an MRI system by a patient during imaging of an extremity of the patient. System  700  also solves the need in the art for reduced floor space requirements of MRI systems. The casing resembles the shape of a cloverleaf that is anchor on floor  710 . As shown in  FIG. 7  first and second spaces  702 ,  704  are bulges protruding or distorting an otherwise ring like shape of the vessel.  
         [0040]     System  700  includes a casing  716  in a cloverleaf-type shaped system. The magnet  708  is built in the conventional manner and the distance  706  from the field of view (FOV) is the same as the previously described implementations. The casing has been stretched at different parts leading to the bulges and resembling a cloverleaf with a cutoff bottom for placing on floor  710 . System  700  shows non-axisymmetric shielding a space for positioning a non-imaged extremity. The smaller length of the dedicated orthopedic imager does not require egress into the center of the entire system for orthopedic medical imaging, which more readily accommodates claustrophobic patients. System  700  also includes a first space  702  and second space  704  in the casing  716 . The regions  702  and  704  are farther positioned from the magnetic assembly resulting in an air gap between the magnet and the non-imaged extremity. A magnetic assembly (not shown) is positioned in close proximity to the first annular space  702  and the second annular space  704  in the inner part of the casing such as in space  712 . The casing  716  acts as a shielding system when a non-axisymmetric ferromagnetic envelope is fashioned into part of the cloverleaf shape. One positive effect of this non-axisymmetric shielding is that magnetic field is mostly confined to the fourth order radial terms. When the MRI system  700  is being used for imaging legs the non-imaged leg can be placed in either space  702  or  704 . Otherwise, the non-imaged leg would extend outside the casing  716  causing discomfort to the patient and limiting the region that can be imaged. Further, within the casing  716  a space can be provided for service area such as electrical supplies, cryocooler, and storage for other needed supplies.  
       CONCLUSION  
       [0041]     A MRI system 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. For example, although described as using an annulus vessel, one of ordinary skill in the art will appreciate that implementations can be made in any shape or any size that provides the required function.