Abstract:
A magnet primarily for use in MRI applications comprises a pair of poles oriented about a plane of symmetry parallel to each therebetween defining an air gap region, magnetic field sources secured on the surfaces of the poles opposite the air gap that have yokes disposed on them, the yokes connected to each other by returns so that the entire magnet assembly can form a closed magnetic flux circuit to substantially confine the magnetic fields generated by the apparatus in the air gap where an imaging region is formed to place subjects for the purposes of examination. The main assembly being cylindrical in geometry has permanent magnets for magnetic field sources that are composed of two regions, a central disk-like portion magnetized substantially along the axial direction and an outer ring-like region magnetized substantially along the radial direction extending axially to form part of the pole together producing a very efficient and even flux distribution throughout the entire magnet assembly with minimal flux leakage. A further means of reducing flux leakage is incorporated in the yokes which have two sections, a disk-like region and an ring-like section to enclose the permanent magnets. The poles are made of multiple sections with a central disk-like region and an outer ring-like region that is a combination of permanent magnets and high permeability materials. This magnet assembly can achieve 1.0 Tesla or greater magnetic fields for whole-body scanning without saturating the magnet pole and other structures.

Description:
PRIORITY CLAIM 
     This application claims priority from U.S. Provisional Application 60/751,450, filed Dec. 19, 2005. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a new class of permanent magnet designs that can generate very strong and highly homogeneous fields for NMR, MRI and MRT use. In particular, the magnet is capable of generating field strengths of 1.0 Tesla or greater and still accommodate whole-body imaging without saturating the pole piece and surrounding magnet structures. 
     2. Description of the Related Art 
     In MRI the strength of the NMR signal is proportional to the magnetic field and so the greater the magnetic field the stronger the NMR signal which translates into improved image quality and faster acquisition of image data. For whole-body imaging field strengths greater than 0.5 Tesla were only achieved using superconducting coils typically in cylindrical configurations although recently there have been 1.0 Tesla open configurations as well. However, magnets based on superconducting coil technology require dual cryogen systems and periodic refilling making them expensive to buy and maintain. Consequently, a cheaper alternative is desirable. 
     As the overall MRI technology progresses demand for permanent magnet based MRI scanners has grown. Most whole-body permanent magnet based scanners are still below 0.5 Tesla and are limited in certain important applications that require greater field strengths. The limitations become severe when the required magnetic field is greater than 0.5 Tesla. First the available permanent magnets of which the strongest today is a Neodymium-Iron-Boron (NdFeB) compound can only produce a limited amount of magnetization—NdFeB compounds are capable of producing magnetic energy densities of up to 50 MGOe. Thus, there is a fundamental limit to how much magnetization these materials can produce and the magnets made from them if they are to be compact in size and weight. Second, the ferromagnetic structures and in particular the poles will saturate rendering the magnet inoperative. 
     A conventional magnet typically consists of permanent magnet blocks magnetized along the main magnet assembly axis which is cylindrical in configuration and are arranged to create a dipolar field with one section of the permanent magnets forming a North Pole and the other a South Pole. Each permanent magnet group has a yoke which are connected by returns and poles on the opposite sides that together concentrate the magnetic field in an air gap between the two poles where subjects are placed for the purposes of examination. In conjunction with actively shielded gradient coils and very high energy density permanent magnets such as NdFeB, with energy densities of 45 MGOe or greater, these systems can be optimized for energy efficiency to yield designs that can generate magnetic fields up to 0.55 T before becoming unwieldy in aspect ratio or size, weight and saturating the very important poles rendering the design inefficient, the field inhomogeneous, creating eddy current and residual magnetization problems and thermal drift issues. 
     The main source of the problem in conventional magnets is the poles. By design, these structures are used to create a constant potential surface across the pole-air gap interface whereby magnetic field lines emanating at this interface are perpendicular to the pole surface and create a highly homogeneous and energy efficient magnetic circuit. However, for typical whole-body systems, as the magnetic field strength in the air gap is required to be greater than about 0.4 T, the poles start to saturate making them ineffective or leading to designs that become impractical because they have to be made very thick. Physically, the poles introduce a boundary discontinuity and the magnetic field lines crossing this interface are forced to follow a path that is substantially curved away from the axial direction. At field strengths greater than 0.4 T, a significant part of the magnetic field energy gets concentrated in the poles saturating them and leading to an inefficient design. Moreover, the abrupt change of direction of magnetic field lines at the pole-air gap interface causes more magnetic energy leakage making the field in the air gap inhomogeneous along with many of the other problems already mentioned. 
     To overcome some of the problems posed by the poles more permanent magnets can be added to increase the magnetic field in the air gap. However, this solution leads to increased inefficiency because the yokes and returns have to be proportionally bigger in order to carry the additional fluxes generated that would otherwise lead to substantial leakage of energy through these structures as well. 
     In addition to the problems caused by the poles, the yoke and returns are further sources of magnetic energy leakage. In typical designs, there is a substantial flux path connecting the permanent magnets to the yoke and return. To minimize this leakage the returns can be placed further away from the air gap region. However, this increases the overall size, weight and footprint of the magnet. 
     Consequently, new methodologies are required to make stronger permanent magnet based designs. The canonical permanent magnet system to create a uniform dipolar field most efficiently is the spherical magnet with a vertically oriented uniform magnetization distribution on the surface of the sphere. However, this is a closed system and doesn&#39;t easily lend itself for MRI purposes because the subject under examination needs to be able to go in and out of the sphere. The next best solution is a cylindrical system that is infinite in length and has a vertically oriented uniform magnetization distribution on the surface of the cylinder. In practical implementations, the axial length and radial thickness of the cylinder is finite and instead of a continuous magnetization distribution an even and discrete distribution such as a Halbach array or a Magic cylinder is used. 
     Based on these efficient configurations, an open system can be formed by simply knocking out the middle portions of the magnetization blocks of the Halbach systems. Similar to conventional systems, rotating this system about the vertical axis will sweep out a circular geometry and make it very efficient. However, this system is obtained by breaking the original symmetry of the cylindrical and spherical systems. Therefore, one has to restore as much of the original symmetry to retain the full benefit of the canonical systems. In this invention, many features and variations on this theme have been used to compensate for the loss of symmetry of the Halbach type magnets and to design a system that is as efficient as possible. 
     Designs based on this approach far exceed current limits of conventional permanent magnet utility and have several novel features not found in previous MRI permanent magnet designs that simultaneously solve many of the drawbacks inherent in conventional designs. 
     SUMMARY OF THE INVENTION 
     Based on the above considerations, it is an object of the present invention to provide a magnet system that is suitable for generating very high magnetic fields, up to 1 T or greater, using permanent magnets for the purposes of whole-body MRI applications in an open configuration. 
     A main objective of this invention is that the magnet system be based on permanent magnets made up of two major sections: a disk portion and a ring portion that together generate a very strong and uniform magnetic field. The magnetization directions in the disk portion are axially oriented while in the ring portions they are substantially orthogonally oriented with respect to the disk portions. 
     Accordingly, such an arrangement of permanent magnet arrays has the effect of concentrating the flux generated in the center while minimizing the flux externally. A yoke system further concentrates the flux in the center and minimizes leakage while also being used to support the permanent magnets. 
     A further means of homogenizing the magnetic field in the center is provided by a pole that forms a constant potential surface to account for manufacturing Tolerance buildups and material inhomogenieties in the permanent magnet (PM) blocks. The pole system&#39;s effectiveness is enhanced by the drastically reduced flux generated in it due to the combination of the disk and ring portions of the PM blocks. 
     Another feature of this invention relates to the composition of the disk PMs that are made of multiple sections each exhibiting slightly different orientations so that flux saturation is minimized and field homogeneity in the imaging region is optimized. Moreover, a ferromagnetic portion is added midway in the disk PMs to reduce mechanical forces and increase the magnetic field in the center. 
     In one preferred embodiment of this invention the ring magnets are also multisectioned and their positions can be mechanically adjusted. The magnetization directions are substantially oriented in the radial direction with the lower section extending axially to form part of the shim portion of the poles. The ring magnets can be split further into two sections according to yet another embodiment of the present invention. In the upper portion the length, thickness and magnetization orientation adjusts flux distribution throughout the magnet and central field value. The lower portion adjusts saturation of the pole, particularly the Rose shims or shim rings, and field homogeneity in the air gap. Each section is separately and mechanically movable for fine tuning field strength and homogeneity. 
     In more particular embodiments of this invention the pole system is multisectioned. The central base section is primarily composed of a ferromagnetic material. Making the outer base section a PM material increases the central field value while at the same time reducing the overall saturation of the pole. The Rose shims can either be ferromagnetic or PM material or a combination of both. The size of the Rose shims is variable and is adjusted to optimize homogeneity and field strength. 
     In yet another preferred embodiment of this invention there is provided a T-section with the main yoke. In one preferred embodiment this ring section is made of a ferromagnetic material and plays the role of a field clamp significantly reducing fringe field generation. In another embodiment this ring section can be a permanent magnet material magnetized substantially in the axial direction and thereby extend the homogeneous field region in the air gap by up to 5 cm or more. The placement of the T-yoke, particularly the ring portion aids in a much more even flux distribution throughout the yoke and return yielding a more efficient yoke and return design. 
     In a further aspect of this invention the return portion can be placed all the way against the ring portion of the T-yoke and still maintain a highly efficient and practical design without impacting any performance parameters. Such a placement decreases the amount of flux flowing through the top portion of the T-yoke and further reduces its size and weight. 
     Other aspects of the invention will become clear from the drawings and detailed description to follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate the main aspects of the specification and together with the detailed description establish the advantages of the invention. 
         FIG. 1  A conventional C-shaped, double return, permanent magnet based MRI magnet. 
         FIG. 2  2D, axisymmetric half-plane, finite difference magnetic field model of the magnet in  FIG. 1 . 
         FIG. 3  Illustrates the magnetic field generated by a uniformly magnetized solid sphere. 
         FIG. 4  A uniformly magnetized solid cylinder obtained by cutting the uniformly magnetized sphere with a plane parallel to the magnetization direction and extruding it in opposite directions. 
         FIG. 5  Infinitely long uniformly magnetized hollow cylinder. 
         FIG. 6  An axial cross-sectional view of an Eight-element Halbach array. 
         FIG. 7  2D, axisymmetric half-plane, finite difference magnetic field model of the Eight-element Halbach array of  FIG. 6  and the midplane vertical field profile. 
         FIG. 8  The Halbach array of  FIG. 6  with the middle array elements eliminated. 
         FIG. 9  2D, axisymmetric half-plane, finite difference magnetic field model of  FIG. 8  and the midplane vertical field profile. 
         FIG. 10  The midplane vertical field profile of the model in  FIG. 8  as the angular magnetization orientation of the outer array elements is varied. 
         FIG. 11  A pictorial depiction of the magnet generated by rotationally sweeping the model of  FIG. 8  180° about the central vertical axis. 
         FIG. 12  The midplane vertical field profile of the model in  FIG. 11  as the angular magnetization orientation of the ring magnet is varied. 
         FIG. 13  A pictorial depiction of the magnet of  FIG. 11  with supporting yokes, flux return posts and magnet poles. 
         FIG. 14  Nomenclature for the new magnet circuit elements of the model in  FIG. 13 . 
         FIG. 15  A detailed cross-sectional view of the magnet in this invention with the many different configurations and embodiments. 
         FIG. 16  2D, axisymmetric half-plane, finite difference magnetic field model of the basic magnet in this invention. 
         FIG. 17  3D, octant model of the magnetic field produced by the disk and ring portions of the permanent magnets of this invention. 
         FIG. 18  2D, axisymmetric half-plane, finite difference magnetic field model of  FIG. 17  with yokes and poles added. 
         FIG. 19  2D, axisymmetric half-plane, finite difference magnetic field model of  FIG. 18  when the return posts are moved next to the ring section of the yoke in this invention. 
         FIG. 20  3D, octant model of the magnetic field produced by splitting the ring portion of the permanent magnets of this invention. 
         FIG. 21  2D, axisymmetric half-plane, finite difference magnetic field model of  FIG. 15  for the consideration of analyzing the many configurations and embodiments possible in this invention. 
         FIG. 22  Table showing the analysis of the different configurations and embodiments in  FIG. 21 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     System  10  of  FIG. 1  is a typical open MRI permanent magnet. It consists of permanent magnet (PM) blocks  2  and  4 . They are each magnetized in the same direction and are disk-like in configuration. The magnetizations are vertically or axially oriented as depicted by  2   a  and  4   a . The PM blocks are attached to ferromagnetic yokes  12  and  14  which are connected to each other by the ferromagnetic returns  16  and  18  to form a closed magnetic circuit. The air gap between the PM blocks  2  and  4  accommodates patients for the purposes of examination. To achieve the requisite homogeneity poles  6  and  8  are attached to the PM blocks  2  and  4  respectively. 
       FIG. 2  shows a 2D, axisymmetric half-plane, finite difference field model of system  10 . The dimensions have been optimized to yield a central field of 4500 Gauss (G) and homogeneity of 2,000 parts-per-million (ppm) on a 40 cm diameter-spherical-volume (DSV) centered on the coordinates (0,0) of the model. The DSV is shown as  13 . For the PM blocks  2  and  4  a 47 MGOe energy Neodymium-Iron-Boron (NdFeB) material was used. In the model the returns  16  and  18  are actually one piece because it is a 2D axisymmetric model. However, as those skilled in the art of magnet design will recognize, a 2D model of the type in  FIG. 2  is an integral part of 3D magnet design and the effects are negligible for the purposes of the description that follows. Although this is a very good performance magnet the poles  6  and  8  are saturated, particularly the shim ring portions  6   a  and  8   a . This can be seen from an increased concentration of flux lines in the shim rings  6   a  and  8   a  and the poles  6  and  8  where they join the base portion. The poles consist of typical high-grade low-carbon steel such as 1006 steel. These materials start to saturate at about 1.6 Tesla or 16,000 Gauss. The poles in this example have fields that exceed this limit and reach as high as 2.3 Tesla. 
     To increase the field more PMs can be added but the poles will progressively get more and more saturated. This leads to other problems. The inhomogeneity in  13  gets worse, the magnet becomes more sensitive to thermal drifts because the saturated poles  6   a  and  8   a  leak more flux. Eddy current and residual magnetization problems are also more severe because the materials that are used to suppress them are rendered inoperative when they are saturated. In addition, flux leakage from the yokes and returns is increased. 
     In this invention, a fundamentally different approach is taken to address these issues and produce a magnet that is free from these problems and yet be capable of producing central fields of one Tesla or greater in whole-body magnet geometries. Starting from first principles, a magnet system is developed that is highly efficient and also suitable for open MRI type magnet applications. 
     System  20  of  FIG. 3  is the canonical example of a magnetized object. It is comprised of sphere  20  with radius a,  22  and has a uniform, vertically oriented magnetization M 0 ,  24 . Being a uniformly magnetized sphere it is the simplest example because the sphere is the simplest and most efficient geometry. Moreover, this example has a well known closed form analytical solution which produces a uniform magnetic field inside the sphere. Therefore, it is a very important example to consider and is the starting point for many permanent magnet designs. 
     The solution proceeds by solving first for the magnetic scalar potential and then calculating the magnetic field from it. Since the magnetization is uniform the effective magnetic charge density inside the sphere contributes zero potential (ρ M =  ∇ ·  M =0) while on the surface it is σ M ={circumflex over (n)}·  M =M 0 δ(r−a)cos θ because the magnetization  26  and the unit normal to the surface  28  make an angle θ,  30  with respect to each other as shown in  FIG. 3 . The form of this expression implies azimuthal symmetry about the vertical axis and solving Laplace&#39;s equation for this charge distribution yields the very well known result 
     
       
         
           
             
               
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     The field can now be calculated using  H =−  ∇ φ M (  r ). Inside the sphere this gives a constant field  31  whereas externally the field is equivalent to that of a point dipole source as indicated by some of the flux lines  32 ,  33 ,  34  and  35 . In summary, the key feature that yielded this result is a uniform magnetization distribution that exhibited a sinusoidal variation on the surface of a closed sphere. 
     Based on this teaching, the next natural example to consider is that of a solid cylinder which is infinite in extent as depicted in  FIG. 4 . System  30  can be obtained from system  20  by cutting a plane  40  that is parallel to the magnetization axis across sphere  20  and then extruding it infinitely in the two axial directions  43   a  and  43   b . The solid cylinder  42  with radius a,  44  so obtained will again have a uniform magnetization M,  46  throughout that is vertically oriented making an angle φ,  47  to the unit normal  48  on the surface. 
     To calculate the field we proceed as before and first find the magnetic scalar potential. The only part that contributes is the surface magnetization distribution which is given by σ M ={circumflex over (n)}·  M =M 0  cos φ in cylindrical coordinates. The potential is then 
     
       
         
           
             
               
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     The internal field is once again constant since the potential is linear. Compared to the spherical example the infinite solid cylinder yields a similar result in that the critical feature was a sinusoidal magnetization distribution on the surface of the cylinder, just like the sphere, that yields a constant internal field and an external field that is dipolar in nature. 
     This naturally leads to the Halbach cylinder which is depicted in  FIG. 5  as system  40 . It is a hollow, solid cylinder of inner radius a,  7  and outer radius b,  9  that extends infinitely in the axial directions  11   a  and  11   b . In this example instead of a uniform magnetization a continuously changing magnetization,  10  that exhibits a sinusoidal variation is impressed in the material between the inner and outer radii  7  and  9 . It is given by
 
   M =M   0 [sin(2φ) î −cos(2φ) ĵ] 
 
in Cartesian coordinates,  1  and
 
   M =M   0 [sin φ{circumflex over (ρ)}−cos φ{circumflex over (φ)}]
 
in cylindrical coordinates,  3 . Such a distribution is motivated by the previous two solutions noting that the sinusoidal distribution on the surface of the material was the key feature. Additionally, as will be seen in the solution that follows a finite thickness surface adds the feature of making these systems extremely efficient because the entire field generated is internal with minimal fields externally.
 
     In system  40  the potential has contributions both from inside the material and the surfaces. The effective magnetic charge distribution in the solid portion of the cylinder is 
               ρ   M     =       ∇   ⇀     ⁢     =       ·     M   ⇀       =     2   ⁢     M   0     ⁢     1   ρ     ⁢   sin   ⁢           ⁢   φ                 
while the effective surface charge density is
 
     
       
         
           
             
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     The solutions for these distributions are well know and the potential from the volume portion contributes 
     
       
         
           
             
               
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     The potential from the surface portion is 
     
       
         
           
             
               
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     Combining these together, the total solution is 
     
       
         
           
             
               
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     Remarkably, this ideal solution predicts that for system  40  as described above and as depicted in  FIG. 5 , the internal field is uniform while the external field is zero. Consequently, this example implies that a continuously changing magnetization, one that is particularly sinusoidal in nature is the key to generating efficient uniform dipolar magnetic fields internally with minimal fields externally. 
     Although, the Halbach cylinder is an idealized example, a more practical implementation is the Halbach array which is depicted pictorially in  FIG. 6  as system  50 . Instead of a continuously changing magnetization these systems are typically made of eight or sixteen discrete elements. System  50  is an eight-element Halbach array and extends infinitely along the axis. It is comprised of array elements  51  through  58 . Each element has a magnetization direction given by orientations  61  through  68 . These orientations differ from each other by 90° between any two adjacent elements. As  FIG. 6  pictorially depicts a uniform dipolar field,  69  is generated internally with minimal fields externally. 
     A finite-difference model for system  50  is shown in  FIG. 7 . It is a quarter-model as shown in  FIG. 7   a  and the dimensions can be seen from the graphs. The internal opening is about 100 cm while the dimensions of the array elements  51 - 58  are about 50 cm on a side. Here only elements  51 - 53  are shown with only the symmetric half portions of  51  and  53  modeled. For the magnet a 47 MGOe energy NdFeB material was used.  FIG. 7   b  shows the field profile for the model with  FIG. 7   c  showing the vertical (By) component of the field plotted along the x-axis at y=0. The field is very constant with a value of about 8500 G and a spherical harmonic decomposition on a 40 cm DSV centered on (0,0) yields 1200 ppm on this volume. 
     Although the Halbach array is a crude approximation to the Halbach cylinder system  50  it still yields a very useful magnetic field performance and there are many practical applications for these systems including whole-body MRI scanners. However, system  50  is not an open configuration and by eliminating array elements  53  and  57  it can be opened. This is a key step in the invention disclosed herein. This new configuration is designated system  50   a  and is depicted in  FIG. 8 . It consists of array elements  71 - 76  with corresponding magnetization orientations  81 - 86  respectively. 
     In contrast to the conventional open magnets designated herein as system  10  of  FIG. 1  this system has many more magnetization orientations. Orientations  82  and  85  of the new system  50   a  are collinear and are oriented vertically similar to orientations  2   a  and  4   a  of system  10 . However, there are four more additional orientations,  81 ,  83 ,  84  and  86 . These orientations actually form a quadrupolar arrangement amongst themselves. Starting with orientation  81 , going around in the plane the successive orientations  83 ,  84  and  86  maintain an antiparallel orientation whereas opposite elements such as  81  and  84  and  83  and  86  maintain a parallel orientation with respect to each other. Orientations,  81 ,  83 ,  84  and  86  all maintain an orthogonal orientation with respect to orientations  82  and  85 . 
       FIG. 9  depicts a finite-difference model of system  50   a . It is a quarter-model showing only the symmetric half of element  72  and  73  with the corresponding orientations  82  and  83  respectively. While  FIG. 9   a  shows the layout  FIG. 9   b  is the actual field profile after solving the model. As can be clearly seen, there is significant field leakage where elements  53  and  57  of system  50  were eliminated. Moreover, there is considerable bending of the flux lines inside the array about position (0,0). This outward bulging of the field lines is clearly due to a severe breaking of the symmetric arrangement of the array elements of system  50 . Consequently, the central field has dropped from 8500 to 5600 G.  FIG. 9   c  shows a plot of the vertical (By) component of the field on the x-axis starting at (0,0) and ending at (50,0). It is obvious the homogeneity has deteriorated considerably and a spherical harmonic decomposition analysis yields a 140,000 ppm homogeneity on a 40 cm DSV. 
     Hereinafter, the objective is to develop an open magnet system based on system  50   a . Therefore, a main principle of this invention is to restore as much of the original symmetry of system  50  and thereby recover most of the loss in field strength and homogeneity of system  50   a . One approach that has proven fruitful is varying the angle of the orientations  81 ,  83 ,  84  and  86 . In the model of  FIG. 9  this is simply achieved by just varying the orientation of  83  and by symmetry the other orientations, namely  81 ,  84  and  86  will have the corresponding variations as well.  FIG. 10   a  shows a plot of the vertical (By) component of the field on the x-axis starting at (0,0) and ending at (50,0) after varying the angular orientation of  83  and it makes an angle of 30° with respect to the horizontal axis or makes an angle of 60° with respect to orientation  82 . At this angle the best homogeneity is achieved as shown in the plot G 2  of  FIG. 10   b  which is a plot of homogeneity as a function of angular variation of  83 . Compared to  FIG. 9   c  much of the original homogeneity has been recovered. However, the loss in field strength, as shown by plot G 1  of  FIG. 10   b  (plot of central field value as a function of angular variation of  83 ), has not improved because the missing elements  53  and  57  of system  50  are necessary to restore the field to the original value in this planar configuration. 
     Consequently, another approach is required to restore the field strength of system  50   a . The approach in this invention is to make system  50   a  cylindrically symmetric about the axis  89  of  FIG. 8  which recovers most of the loss of field strength. System  60  of  FIG. 11  depicts pictorially the resulting magnet if any plane perpendicular to the axial axis of system  50   a  is rotated about  89  as indicated by  70   a  and  70   b  in the upper half and  80   a  and  80   b  in the lower half. The configuration so swept out would look similar to system  60 . A finite-difference model of this new system is easily obtained by changing the x-axis of  FIG. 9   a  to a radial axis and the y-axis to the vertical or z-axis and making the z-axis an axisymmetric axis.  FIG. 12   a  shows a plot of the axial (Bz) component of the field on the radial or ρ-axis starting at (0,0) and ending at (50,0). 
     The central field is now 7800 G and is closer to the original value of 8500 G of system  50 . Moreover, compared to system  50   a  which had an inhomogeneity of 140,000 ppm on 40 cm DSV before varying the angular orientation of  83  this new system has an improved inhomogeneity of 90,000 ppm on 40 cm DSV. Therefore, this new system is a major enhancement over  50   a . Furthermore, varying the angular orientation of the radial magnetization improves the homogeneity to 4,700 ppm on 40 cm DSV when the angle with respect to the z-axis is 40° as shown by the plot of the axial (Bz) component of the field on the radial axis in  FIG. 12   b . Plots G 3  and G 4  of  FIG. 12   c  are the field and homogeneity variations as a function of angular variation of the radial magnetization orientation. 
     Based on this teaching system  60  of  FIG. 11  is a more practical implementation of these ideas and this invention which is a fundamentally new configuration in open MRI magnet geometries. Disks  92   a  and  92   b  are the central PM blocks with axial magnetization orientations  91   a  and  91   b  which are collinear and point in the same axial direction. PM blocks  94   a  and  94   b  are annular sections that have magnetization orientations that are substantially oriented in the radial directions where  93   a  is radially outward and  93   b  is pointing radially inward. The space between the upper and lower portions provides an opening to insert patients for the purposes of MRI examinations. It is important to note that a vertical plane that is collinear with the cylindrical axis,  95  cutting through system  60  would exhibit a sinusoidal magnetization orientation when viewing the orientations  91   a ,  93   a ,  91   b  and  93   b  going around in a circle on the plane. Therefore, the principle of a sinusoidally varying magnetization distribution is still maintained except the gap portion. 
     A further enhancement of system  60  is that of system  70  shown in  FIG. 13 . System  70  is just system  60  but with yokes  100   a  and  100   b  added and connected to each other with returns  102   a  and  102   b  to form a closed magnetic circuit and limit the amount of fringe fields produced. Adding these structures can add between 25-50% to the central field. Further fringing fields can be contained by adding field clamps  104   a  and  104   b  to the ring sections  94   a  and  94   b . Additionally, pole  106  has been added with a corresponding pole in the upper section not shown in  FIG. 13 . The poles provide a constant potential surface while the Rose shims correct the second order (dominant error) term, and together homogenize the central field. 
     When system  70  is viewed as a 2D axisymmetric system it has many new magnet circuit elements that are quite different from the conventional system  10  of  FIG. 2 . Accordingly, we use the following nomenclature for these new circuit elements as shown in  FIG. 14 . The yoke and field clamp together we designate a T-yoke. The disk and ring PM blocks together we designate as an L-magnet and the base plate portion of the poles with the shim rings together an L-pole. A more detailed consideration of system  70  and the interplay between all the pieces to fine tune this invention is considered by looking at the view cut through by  101  of  FIG. 13 . 
     System  90 , shown in  FIG. 15 , is a detailed view of system  70  with the many more configurations and embodiments of this invention that enhance the magnetic performance. In one preferred embodiment of this invention an L-magnet comprised of disk portion  120   a  with vertical orientation  121   a  and ring sections  122   a  and  122   b  with corresponding radial magnetizations  123   a  and  123   b  respectively are attached to ferromagnetic T-yoke  110   a  and  114   a . To close the magnetic circuit a ferromagnetic return,  112  connects a symmetric portion of the upper L-magnets and T-yokes on the bottom. An L-pole to homogenize the field in the air gap is formed of base plates  116   a , shim rings  111  and  118   a  all made of ferromagnetic material. A symmetric L-pole on the lower L-magnet exists as well. In the example considered, the L-magnets are all made of 47 MGOe energy NdFeB material and the T-yokes and returns are 1010 low-carbon steel whereas the L-poles are 1006 low-carbon steel. 
     A 2D axisymmetric finite-difference model of the magnetic field generated by this system is shown in  FIG. 16 . A central field of 7,500 G is obtained and a close inspection of the L-poles reveals much less bunching of field lines compared to system  10  in  FIG. 2  and hence no saturation. Moreover, a spherical harmonic decomposition of the field on a 40 cm DSV centered on the coordinates (0,0) reveals a homogeneity of 1257 ppm. The DSV is also depicted as  115  in  FIG. 15 . This performance is significantly better since in system  10  a central field of only 4,500 G was reached when the onset of saturation was initiated. Furthermore, the geometric dimensions of system  70  are not much more than system  10 . Overall, the volume and weight of system  70  are only 15% more than that of system  10  for a 40% increase in field. Usually, the scaling of system dimensions with central field is nonlinear and in view of this fact the performance of the new invention is recognized as being much more efficient than conventional designs. 
       FIGS. 17   a - 17   c  show one octant of a 3D model of the magnetic field produced by the L-magnets alone when using a 47 MGOe energy NdFeB magnet material.  FIG. 17   a  is the disk portion  110   a . The field profile in the plane S 1  is shown where the lower left corner is centered on (0,0,0) and the upper right corner is at (100,0,100). The disk has a radius of 52.5 cm and a height of 35 cm. It produces an average central field of 4,000 G but has homogeneity of only 170,000 ppm. The ring section  122   a  and  123   a  shown in  FIG. 17   b  has inner and outer radii 52.5 cm and 73.5 cm respectively with a height of 35 cm. In the same plane S 1  it produces an average central field of 2,000 G. A close inspection of the flux lines shows that below line L 1 , in region R 4  the fluxes are vertically oriented in the central region and are clockwise in the outer regions whereas above line L 1  the fluxes are all counterclockwise. Compared to the disk  110   a  where all the fluxes are clockwise the ring portions  122   a  and  123   a  add fluxes in a very efficient way. Above L 1  the fluxes from  110   a ,  122   a  and  123   a  oppose each other and cancel. Below L 1  the fluxes from  110   a ,  122   a  and  123   a  add.  FIG. 17   c  depicts this when the disk and ring are added together to form the L-magnet and clearly shows where the efficiency of this invention is derived from. In the external regions R 7 , that is where the T-yokes and returns are placed, the fluxes are very minimal and therefore the amount of ferromagnetic material required to carry these fluxes is also lowered. However, in region R 5  the fluxes add together to give an average central field of about 6,000 G with homogeneity of 14,000 ppm which is significantly better than the disk  110   a  of  FIG. 17   a . Moreover, region R 6  of  FIG. 17   c  shows a lower flux bunching compared to the same region, R 2  of  FIG. 17   a . That is, the poles will also carry less flux because of the manner in which the ring and the disk add fluxes in this region. 
     The gains in efficiency obtained by adding the disk portion  110   a  and the ring portions  122   a  and  123   a  as shown in  FIGS. 17   a - 17   c  are even more dramatic when the T-yokes and L-poles are included.  FIGS. 18   a - 18   c  depict a 2D axisymmetric finite difference model of the magnetic field generated with the T-yokes and L-poles.  FIG. 18   a  shows the magnetic field flux flows with just the disk portion Dk 1 . The flux lines flow in a clockwise direction through the disk, Dk 1 , the upper yoke, Ty 1 , the back section of the T-yoke, Ty 3  and into the returns to the lower symmetric section of the magnet system back to the air gap, Ag 1  which has the desired vertical orientation. The flux flows through the ring section, Ty 2  and the L-poles, Lp 1  are also in a clockwise direction. In contrast to  FIG. 18   a ,  FIG. 18   b  shows the flux flows with just the ring portion, Rg 1 . The flux flows are now in a counter clockwise direction through the upper yoke, Ty 1 , the region where the disk magnet would have been and into the L-poles, Lp 1 , the ring magnet, Rg 1 , and into the ring, Ty 2 , which when it reaches the upper yoke splits into two directions, one back towards Ty 1  and the other towards the back portion of the T-yokes, Ty 3  and into the returns to the lower symmetric section of the magnet system and closes the loop back through the air gap region, Ag 2  which is vertically oriented. When the disk, Dk 1  and ring, Rg 1  are present as in  FIG. 18   c  then the flux flows of  FIGS. 18   a  and  18   b  superimpose in the desired fashion of this invention resulting in the dramatic efficiencies of the magnet system. 
     The central field in the air gap, Ag 1  of the disk, Dk 1  in  FIG. 18   a  produced a value of about 4,600 G whereas the central field in the air gap, Ag 2  of the ring, Rg 1  in  FIG. 18   b  produced a value of about 3,100 G and the total central field in the air gap, Ag 3  of both Dk 1  and Rg 1  in  FIG. 18   c  produced a value of about 7,600 G. Remarkably, this system produced increased field values in the air gap region Ag 3  which was very desirable while at the same time decreasing the magnetic field values in all other regions. In  FIG. 18   c , the regions Ty 1 , Ty 2 , Rg 1 , Dk 1  and the very important L-poles, Lp 1  all had significantly reduced field values because the flux flows for these regions in  FIGS. 18   a  and  18   b  were in opposite senses. For example, field values in Dk 1  in  FIG. 18   a  range from a minimum value of about 9,300 G to a maximum of about 11,500 G whereas in  FIG. 18   c  the field value in Dk 1  was fairly constant throughout at about 7,700 G. Similarly for Rg 1  in  FIG. 18   b  the field values ranged throughout this region from about 6,600 G to about 11,000 G whereas in  FIG. 18   c  they ranged from 1,500 G to about 8,500 G with the exception of some corners approaching 10,000 G. The yoke region Ty 1  in  FIG. 18   a  was saturated with peak fields of about 20,000 G whereas in  FIG. 18   c  it is back down mostly under 10,000 G except the back sections towards Ty 3 . The very important L-poles, Lp 1  which were saturated with field values of up to 25,000 G throughout most of this region in  FIGS. 18   a  and  18   b  are now down to 12,000 G in  FIG. 18   c . This is one of the most important features of this invention. Moreover, the addition of the T-yokes boosts the central field by another 1,500 G and the L-poles improve the homogeneity to well below 2,000 ppm. Further varying the magnetization orientation of the ring magnets improves the homogeneity to well below 1,000 ppm. This is considered in other embodiments of this invention. 
     The system of  FIG. 18   c  generates such a low fringe field that in another embodiment of this invention the returns  112  can be placed next to the T-yoke as shown in  FIG. 19 . In this example, the returns have been moved in by more than 20 cm and doing so has minimal effects on the central field value and homogeneity. However, the same can not be done in system  10  because the leakage field is to detrimental to the homogeneity of the central field and it can&#39;t be corrected by passive shimming techniques as is well known by those skilled in the art. Moving in the returns against the T-yokes has the benefits of a much reduced weight and volume with an overall smaller aspect ratio compared to system  10 . This has the further advantage of a smaller footprint magnet that takes up less space in a hospital or clinical setting. 
     Moreover, some or the entire ring portion of the T-yokes can be made of PM blocks with magnetization substantially in the axial direction instead of a ferromagnetic section. With this change in the ring portion of the T-yokes the central field can be further extended radially outward enhancing the shimming and giving another degree-of-freedom in the shimming of the overall magnet. 
     In a further embodiment of this invention, the ring magnet can be split into two or more sections. Referring to the model of the ring magnet depicted in  FIG. 17   b  and splitting it, for example into two sections will have an effect as shown in  FIG. 20   a . The flux flow in region Sp 1  will be altered in a way that has both an effect on the strength of the overall magnetic field produced in the air gap region and the homogeneity of that field including the saturation of the L-poles. More particularly, as shown in  FIG. 20   b , when the splitting is a bit more pronounced the flux flows in the region Sp 1  reverse directions to the one shown in  FIG. 20   a  which has the effects mentioned. These considerations will be analyzed in further detail below. 
       FIG. 21  is a 2D, axisymmetric finite difference magnetic field model for the consideration of analysis of the many configurations and embodiments of  FIG. 15 . As shown in the table of  FIG. 22  the many embodiments of this invention are analyzed as case studies. In reference to  FIG. 21  the table lists the particular elements and their configuration for each study with the last two columns reporting the central field value and the homogeneity, respectively. As further shown in the Legend of  FIG. 22  F designates a ferromagnetic material, A an air or vacuum region and an angle represents a permanent magnet material, a 47 MGOe energy NdFeB magnet material with their angular orientations given with respect to the ρ-axis, P of  FIG. 21 . Case  1  is the basic magnet system of this invention with elements  111  and  118 , the edges of the base plate part of the L-poles and the Rose shims, respectively, when each are ferromagnetic materials and the various parts of the disk and ring portions of the L-magnets having their magnetizations simply vertical and orthogonal, respectively. This was already analyzed in  FIG. 16  producing a central field of about 7,450 G and homogeneity of about 1,400 ppm on a 40 cm DSV. 
     In Case  2 , the ring magnet portions are all exhibiting a tilted magnetization orientation of 15° with respect to the horizontal, ρ-axis of  FIG. 21 . This is a very beneficial aspect of the invention as already mentioned previously adding about 100 G to the central field value and improving the homogeneity to about 500 ppm. A further enhancement upon this performance is changing the middle portions of the disk permanent magnets,  117   a  and  117   b  to a ferromagnetic material. As Case  3  shows, doing so adds about 30 G and has minimal impact on the central field value. However small this change is though the benefit from this part of the invention is for the assembly part of the magnet because the magnetic forces are significantly reduced if the disk magnets are split into at least two halves before insertion into the magnetic circuit. Furthermore, the flux lines are pulled back towards the returns away from the central portion of the magnet by making elements  117   a  and  117   b  a ferromagnetic material which aids in reducing the saturation of the L-poles. 
     A similar insertion of a ferromagnetic material,  113 , in the middle of the ring magnets does not have a beneficial result as shown in Case  4 . However, splitting the rings and leaving an air region as in Case  5  reduces the central field by about 100 G without to much effect on the homogeneity. As discussed previously, this enables the mechanical adjustment of these two rings separately. The upper ring adjusts the central field value while the lower adjusts both the homogeneity and the saturation of the L-poles. 
     Cases  6 - 9  are studies on the effect of changing the magnetization orientation of the different parts of the disk magnets away from the vertical orientation of 90°. For these studies,  113  is again a permanent magnet as indicated in the table. In Case  6 , the lower, outer disk portion,  126 , has a magnetization orientation of 70° instead of 90°. As shown in the table the central field value is increased by about 110 G but comes at the cost of reducing the homogeneity and worsening the saturation of the L-poles. If the magnetization orientation of element  128  is 110° instead of 90° as in Case  7 , the central field value drops by about 50 G and the homogeneity deteriorates to 1000 ppm but the saturation of the L-poles is completely removed with an average magnetic field value throughout the base region of only about 12,000 G. This is a very significant gain for the overall performance of the magnet system. Case  8  is a check of tilting the magnetization of  128  to  70 ′ instead of 110°. The results are similar. As Case  9  shows there is not much change from Case  2  and this series of studies shows that adjusting the magnetization orientation of the lower portion of the disk magnets  126  and  128  has more significant benefits than the upper portions which only affect the flux flows in the T-yokes. 
     Case  10  is a check on the effect of just changing the magnetization orientation in the upper portion of the ring magnet  124 . The result clearly shows that the central field value is increased by about 30 G without any effect on the homogeneity. Therefore, adjustments of this portion of the ring magnet give fine control over the central field value. Case  11  shows the benefits of changing the edge portion of the L-poles,  111  to a permanent magnet. It adds about 250 G to the central field value and significantly reduces the overall saturation of the L-poles. In contrast changing the Rose shims,  118 , to permanent magnets, Case  12 , has a similar effect as well and varying the magnetization orientation adjusts saturation of the L-poles and the overall homogeneity. 
     In more practical implementations of this magnet system a polygonal shape to the L-magnets and the L-poles is easier in which case the sections have to be at least 8 and preferably 32 or more for a better circumferential approximation. Furthermore, having a slight gap between the disk and ring portions of the L-magnets allows room for mechanically adjusting the ring magnets. 
     In summary, this new magnet invention has substantial benefits over previous or conventional permanent magnet based magnet designs including but not limited to:
         1. minimal thermal drifts because the poles are not saturated;   2. eddy currents will be reduced because high resistivity materials can be used and will not be rendered inoperable by the poles being saturated as in conventional designs;   3. poles that operate in the linear region throughout the pole volume generate an inherently much higher homogeneous magnetic fields in the air gap as compared to conventional designs;   4. the magnet is easily shimmable because of point #3;   5. the very high efficiency of the design means less demanding materials can be used throughout the magnet;   6. the non-saturation of the poles allows the use of low saturation point, soft magnetic materials;   7. the poles operate in the theoretically desired regime of approaching an infinite permeability medium so the surfaces behaves as true constant potential;   8. higher magnetic field designs are possible.       

     This breakthrough in permanent magnet design can yield practical designs for whole-body MRI permanent magnet based systems of field strengths up to 10 T or more. In the examples considered throughout this patent the conventional designs in  FIG. 1  yielded fields in the air gap of about 0.45 T and operating points of the permanent magnets of about 0.7 T. The new designs, using the same dimensions of the conventional design of  FIG. 1  with the addition of the ring permanent magnets and ring yoke yielded fields of 0.75 T in the air gap and operating points of the permanent magnets of 0.7 T as depicted in  FIG. 16 . The two designs yield completely different performances. In particular, the new design is a much more efficient magnetic circuit and exhibits no saturation in the poles, yokes and returns. 
     The drawings and descriptions while demonstrating the main objects of the invention, together with the claims below are in on way meant to limit the scope and spirit of the invention. Changes in form and details of the invention will be understood not to depart from the current invention.