Abstract:
A system and method are presented that facilitate imaging of the joints of the upper and lower extremities including, for example, the hip and shoulder as well as sections of the spine, among others. One embodiment of the invention includes a short bore cylindrical magnet with an imaging volume smaller than that of a comparable whole body system, an articulated table that allows placement of the joint to be imaged in the center of the magnet homogeneous volume while maintaining a high degree of patient comfort and openness. The gradient and RF coils may be positioned above and below the patient instead of 360 degrees surrounding the patient. A smaller RF and gradient coil is made feasible because of the reduced imaging volume.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to International Patent Application No. PCT/US2008/006028, filed May 12, 2008, which in turn claims priority to U.S. Provisional Patent Application Ser. No. 60/928,622, filed May 10, 2007. Each of the aforementioned patent applications is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and system for generating medical images. Particularly, the present invention is directed to a method and system for generating magnetic resonance (“MR”) images of a patient. 
     2. Description of Related Art 
     A variety of systems and related methods are known in the art for generating magnetic resonance images of a patient. Currently, whole body MRI systems utilize, among other things, cylindrical superconducting magnets to facilitate imaging of the entire human body using linear moving patient tables and magnets with large homogeneous volumes. The inner diameter typically accessible to the patient is from about 600 mm to about 700 mm. The magnets are typically 1.75 to 2.25 meters long. Some systems have magnets as short as 1.25 meters. The typical imaging volume is 350 mm to 450 mm in each of three dimensions with the shorter systems having smaller homogeneous volumes. As known in the art, references herein to such homogeneous volumes refer to regions of sufficient magnetic field uniformity to facilitate creation of useful MRI images. 
     Some extreme engineering design measures can be taken whereby the amount of superconducting wire in the main magnet is greatly increased in attempts to improve the size of the homogeneous volume at shorter magnet lengths. However, this can only help to a limited extent. The physics and engineering limits require a reduction in homogeneous volume as the magnet is made shorter. 
     The hip and shoulder joints are off patient center in left/right directions during imaging. The preferred imaging field-of-view for imaging these joints is about 160 mm or less for an imaging volume centered about the joint. However, the magnet&#39;s useful imaging volume must be larger to place the desired joint in a good portion of the imaging volume even though the necessary imaging field-of-view is substantially less than the overall imaging volume of the system. An off center imaging sequence is prescribed resulting in an image acquisition centered about the joint. 
     Along with a large imaging volume in such devices, there is also a requirement to create an RF transmit magnetic field and gradient magnetic field that that spans the entire imaging volume. These coils are generally cylindrical occupying the volume between the magnet and patient bore for a full 360 degrees surrounding the patient. 
     Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, such systems are necessarily large. The larger the system, the larger the main magnet needs to be to surround the patient volume, gradient and RF coils. These larger gradient coils, in turn, require more power. Extra space in the facility along with a larger Radio Frequency shielded room is also required. The main magnet is mainly made using low temperature superconductors (“LTS”) such as niobium-titanium and the like. These materials are expensive. All of these factors increase the system costs. As such, there is a continuing need in the art for more suitably designed imaging systems that minimize these and other drawbacks. The present invention provides a solution for these and other problems. 
     SUMMARY OF THE INVENTION 
     The purpose and advantages of the present invention will be set forth in and become apparent from the description that follows. Additional advantages of the invention will be realized and attained by the methods and systems particularly pointed out in the written description hereof, as well as from the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied herein, a new and useful system is presented that facilitates imaging of the joints of the upper and lower extremities including, for example, the hip and shoulder as well as sections of the spine. One embodiment of the invention includes a short bore cylindrical magnet with an imaging volume smaller than that of a comparable whole body system, an articulated table that allows placement of the joint to be imaged in the center of the magnet homogeneous volume while maintaining a high degree of patient comfort and openness. 
     In accordance with a further aspect of the invention, the gradient and RF coils may be positioned above and below the patient instead of 360 degrees surrounding the patient, as in prior art systems. This aspect may allow for a smaller magnet to be used with the same or improved left/right patient aperture. A smaller RF and gradient coil is made feasible because of the reduced imaging volume. The smaller RF coil requires less power for the same performance and reduce RF power deposition (SAR or Specific Absorption Rate) that can cause heating and less dB/dt (time rate of field) that can cause peripheral nerve stimulation. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention disclosed herein. 
     The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a representative embodiment of an imaging device made in accordance with the present invention, illustrating positioning of a patent in a straight orientation with respect to the device. 
         FIG. 2  is a schematic view of the embodiment of  FIG. 1  illustrating positioning of a patient in the device to facilitate imaging a shoulder of the patient. 
         FIG. 3  is a schematic view of the embodiment of  FIG. 1  illustrating positioning of a patient in the device to facilitate imaging an elbow of the patient. 
         FIG. 4  is a schematic view of the embodiment of  FIG. 1  illustrating positioning of a patient in the device to facilitate imaging a hand of the patient. 
         FIG. 5  is a schematic view of the embodiment of  FIG. 1  illustrating positioning of a patient in the device to facilitate imaging a hip of the patient. 
         FIG. 6  is a schematic view of the embodiment of  FIG. 1  illustrating positioning of a patient in the device to facilitate imaging a knee of the patient. 
         FIG. 7  is a schematic view of the embodiment of  FIG. 1  illustrating positioning of a patient in the device to facilitate imaging a foot of the patient. 
         FIG. 8  is a schematic end view of the embodiment of  FIG. 1  illustrating placement of the main magnet of the device and gradient and radiofrequency coils. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The method and corresponding steps of the invention will be described in conjunction with the detailed description of the system. 
     The devices and methods presented herein may be used for generating medical images. Particularly, the present invention is directed to a method and system for generating magnetic resonance (“MR”) images of a patient. 
     Embodiments of the invention depicted herein include a combination of magnet and articulated patient table that allow the hip, shoulder, foot, ankle, knee, hand, wrist and elbow joints to be positioned and imaged in a smaller cylindrical MRI magnet. Particular embodiments described herein allow all of the joints of interest of the human body to be centered in the magnet, yet permit use of a magnet with an associated homogenous volume that is substantially smaller than typical systems known in the art. This results in a smaller, lighter, more compact and less costly system that is conveniently more open for the patient. 
     For purpose of explanation and illustration, and not limitation, views of an exemplary embodiment of an imaging system made in accordance with the invention are shown in  FIGS. 1-8  and is designated generally by reference character  100 . 
     As depicted, in  FIGS. 1-7 , system  100  includes a table  110  operably coupled to a magnet assembly  150  defining a bore  152  therethrough. The magnet assembly  150  can be a conventional design with a cylindrical gradient coil and RF body coil or may be open on the sides as permitted by the teachings of the embodiment illustrated and described herein in further detail below with reference to  FIG. 8 . 
     As depicted in  FIGS. 1-7 , patient table  110  includes a stationary base portion  112  upon which is mounted an articulated portion  120  upon which a patient  300  rests. Articulated portion  120  of table  110  includes a first linear displaceable segment  122 , a second generally round pivoting segment  124  and a third angularly displaceable segment  126 . The three components of articulated portion  120  of table  110  may be displaced along an axial direction “Z” of the device  100  along a track  114  formed on base portion  112  through bore  152 . 
     Angularly displaceable segment  126  may be angularly displaced, for example, about a pivot point  128  defined in the center of second segment  124 . If desired, second segment  124  and third segment  126  may rotate together about pivot point  128 . 
     As depicted, second and third segments  124 ,  126  are preferably upholstered for the comfort of patient  300 . In addition, table  110  is further provided with pads  130  that may be used for supporting various portions of a patient&#39;s anatomy while being imaged. Similarly, as depicted, a displaceable RF coil  132  for imaging the elbow or hand is also provided that is adapted and configured to slide along axis “Z” in a track  134 . 
     For purposes of illustration, the device  100  may be used to examine the shoulder joint. The shoulder joint is furthest from the center of the body in the left/right (“X”) direction and is the most difficult joint to place in the center in the magnet. In accordance with certain embodiments of the invention, it is desired to place the shoulder of a patient at or near the center of a significantly smaller imaging volume than is typical of a conventional whole body magnet. This allows for a much smaller and lower cost magnet. 
     Using conventional technology as a starting point, if one starts with a conventional whole body magnet design of 2.5 meters long and 0.9 m inside diameter (not including the gradient and RF body coil) the resultant useful imaging volume is about a 450 mm diameter sphere. If the magnet is reduced in length to &lt;1 meter, the resultant volume is reduced to about 200 mm in diameter. Further shortening the magnet will reduce the useful volume further. 
     Such a shorter magnet is depicted in the embodiments of  FIGS. 1-7 .  FIG. 1  depicts a top view of a 1 meter long (along the axis Z) magnet with a 200 mm diameter imaging volume  200  and a 700 mm wide patient bore  152 . It will be appreciated that the length of the magnet may be modified somewhat without departing from the scope or spirit of the invention. Preferably, the length of the magnet is between about 0.75 meters and about 1.25 meters. More preferably, the magnet is about one meter long. In accordance with another embodiment, the magnet may be less than one meter long. 
     The patient is a male with height of 172 cm (68″) (average is 175 cm) and distance of 154 mm from body center to the center of the shoulder joint. As clearly depicted in  FIG. 1 , off center imaging of the shoulder is no longer possible because the magnet is too small. Even with a patient bore as large as 700 mm, and an imaging volume of 200 mm diameter, it is not possible to place the shoulder of a person of average size in the magnet center while still laying flat and parallel to the bore axis (“Z” direction). 
     However, by angling portion  126  of the patient table  110  as shown in  FIG. 2 , the shoulder of patient  300  can now be placed into the center of the imaging volume  200 . Angling the patient table is actually facilitated by virtue of the shorter magnet assembly  150 . Shorter magnet assembly  150  in turn provides a reduced imaging volume  200 . The angled patient table  110  and a relatively short magnet assembly  150  work together to allow the shoulder to be imaged in a smaller, less expensive magnet. 
     Angling the patient table  110  also improves imaging the elbow. As depicted in  FIG. 3 , the elbow of patient  300  is positioned in the center of the imaging volume  200  and the center of Radio Frequency Coil  132 . The patient&#39;s arm is not completely extended above the head, which would cause patient discomfort and motion, and the torso is not adjacent to the imaging volume  200 , which would otherwise possibly interfere with the image obtained as in the case of a conventional whole body MRI system. Angling the head slightly allows further rotation of portion  126 , thereby lowering arm extension resulting in increased patient comfort. 
     For imaging the hand of patient  300 , the angling of the table  110  is possible with greater patient comfort. As shown in  FIG. 4 , the hand of patient  300  is in the center of the imaging volume  200 , resulting in the highest possible image quality. Similarly, the hip is easily centered in the magnet  150  in a manner that would be similar to a whole body system in  FIG. 5 . Likewise, the knee joint is easily centered in the magnet  150  in a manner that would be similar to a whole body system in  FIG. 6 . Furthermore, the ankle joint is easily centered in the magnet assembly  150  in a manner that would be similar to a whole body system in  FIG. 7 . 
     The same RF coil  132  has been depicted for imaging the hand, foot, elbow and knee for illustrative purposes only, and not limitation. As known in the art, in practice, different size coils optimized for each anatomical position may be used. The invention described herein is intended to encompass all such embodiments. 
       FIG. 8  presents a schematic end view of the magnet assembly  150  including a main superconducting magnet,  154 . As depicted, the imaging volume  200  is about 200 mm in diameter, while the patient bore  152  is about 500 mm in height and about 700 mm in width. 
     As will be appreciated by those of skill in the art, the size of the imaging volume  200  can be varied in accordance with the size of the magnet assembly  150 . For example, the size of the imaging volume can range anywhere from about 50 mm in diameter to about 500 mm in diameter, more preferably from about 100 mm in diameter to about 300 mm in diameter, and most preferably about 200 mm in diameter. 
     Similarly, the dimensions of the bore can be varied in accordance with the size of the magnet assembly. For example, the width of the bore can vary from about 500 mm to about 1000 mm or larger, more preferably from about 600 mm to about 800, 850 or 900 mm, and most preferably about 700 mm. By way of further example, the height of the bore can vary from about 300 mm to about 1000 mm or larger, more preferably from about 400 mm to about 700 mm, and most preferably about 500 mm. 
     As depicted in  FIG. 8 , it is possible to use a smaller RF coil  160  and gradient coil  170  placed above and below the patient instead of a larger set of cylindrical coils surrounding the patient as in a conventional whole body system. This is feasible because of the reduced imaging volume  200  that is needed to perform imaging. Any suitable gradient coil  170  design may be used, as known in the art. The RF coils may include one or more transmit elements that are adapted and configured to transmit signals to a region of interest such as in the imaging volume and receive MR signals from tissue in the region of interest. 
     The embodiments disclosed herein thus present certain advantages that are heretofore not present in the art. For example, the patient opening can be provided with a larger width by virtue of placement of the RF and gradient coils above and below the patient, instead of surrounding the patient. This, for example, allows for improved access for the shoulder. Moreover, with the RF and gradient coil above and below the patient, the RF transmit uniformity and gradient linearity is feasible for a smaller volume. The gradient amplifier power and RF power required is reduced by virtue of the smaller imaging volume. This allows reduced SAR and dB/dt, which are highly desirable. Perhaps more importantly, the magnet inside diameter can be reduced lowering the cost of the magnet and/or improving homogeneity of the magnetic field while still maintaining access for the shoulder of a patient. A reduced inside diameter of the magnet also permits use a shorter magnet for the same homogeneous volume. 
     Generally, as the length L of magnet assembly  150  is reduced in length, the homogeneous volume of the magnetic field is necessarily made smaller. The physics of magnetic field generation dictate that the size of the homogeneous volume is reduced in all dimensions even if the magnet is made shorter in just one dimension. However, some limited asymmetric shaping of the homogeneous volume can take place. One example is the oblate spheroid where the equatorial dimension is larger than the polar dimension. In other words, instead of a spherical imaging volume  200 , the imaging volume can resemble an ellipsoid that has been compressed along the “Z” dimension in the embodiments depicted in  FIGS. 1-7 . Such an imaging volume would appear oval from a top view as depicted in  FIGS. 1-7 , but round in  FIG. 8  with the height dimension of the imaging volume in  FIGS. 1-7  along the “Z” axis being less than the transverse dimension along the “X” axis and “Y” axis. 
     It should also be apparent that imaging the head, sections of the spine or any other portion of a patient&#39;s anatomy that may be placed in the imaging volume  200  is also possible and straightforward. For example, the spine is near the center of the body which is readily imaged. It will be further appreciated that, while dimensions of magnets and the like are depicted herein, these dimensions are intended to be exemplary and not limiting. It will be further appreciated that system may be operated at any suitable background field produced by main magnet  154 . For example, main magnet may adapted to produce a field at 1.0 T, 1.5 T, 2.0 T, 2.5 T, 3.0 T, 4.0 T, 5.0 T, 6.0 T, 7.0 T and the like, as desired. The other portions of system  100  (e.g., coils  160 ,  170 ) are accordingly adjusted to accommodate the difference in main field strength. 
     As can be seen, the methods and systems of the present invention, as described above and shown in the drawings, provide for an imaging system with superior qualities as compared to prior art systems. It will be apparent to those skilled in the art that various modifications and variations can be made in the device and method of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the subject disclosure and equivalents.