Abstract:
An insertable coil (40) is inserted in a bore (12) of a magnetic resonance imaging apparatus. Primary field magnets (10) create a temporally constant magnetic field longitudinally through the insertable coil. A computer control (58) controls a radio frequency coil (46) and a gradient coil (42) to create magnetic resonance imaging sequences and process received magnetic resonance signals into image representations. The insertable gradient coil includes a cylindrical, dielectric former (44) of appropriate diameter to receive a patient&#39;s head. A pair of parabolic cutouts (62) are defined adjacent a patient receiving end of the dielectric former and are of an appropriate size to receive the patient&#39;s shoulders. In this manner, the patient&#39;s head can be centered in a longer head coil. Four thumbprint type x-gradient coil windings (72) are mounted symmetrically on the dielectric former with the parabolic cutouts (74) centrally on one side of the thumbprint coil windings. The windings of the thumbprint coils are contoured to follow the parabolic cutout. Four y-gradient coil windings (84) rotated 90° from the x-gradient coils are mounted on the dielectric former. The y-gradient thumbprint coil windings are mounted between the parabolic cutouts and windings along their corners (82) conform to the parabola.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the magnetic resonance imaging art. It finds particular application in conjunction with insertable gradient coils for high speed imaging techniques and will be described with particular reference thereto. 
     Magnetic resonance imagers commonly include a large diameter, whole body gradient coil which surrounds a patient receiving bore. Main field magnets, either superconducting or resistive, and radio frequency transmission/reception coils also surround the bore. Although the whole body gradient coils produce excellent linear magnetic field gradients, they have several drawbacks. With large diameter gradient coils, the slew rate is sufficiently slow that it is a limiting factor on the rate at which gradient magnetic fields can be induced and changed. Large diameter whole body gradient coils have relatively low gradient field per unit drive ampere for a given inductance, which limits their use for some of the highest speed magnetic resonance imaging techniques. The energy stored in gradient coils is generally proportional to greater than the fifth power of the radius. Hence, large diameter, whole body coils require large amounts of energy. Further, superconducting main magnets have cold shields disposed around the bore. The larger the diameter of the gradient coil, the closer it is to the cold shields and hence the more apt it is to produce eddy currents. More shielding is needed to prevent the whole body gradient coils from inducing eddy currents in the cold shields than would be necessary for smaller diameter coils. 
     Due to these and other limitations in whole body coils, numerous insertable coils have been developed which are small enough to fit within the bore with the patient. Typically, the insertable coils are customized to a specific region of the body, such as a head coil, or a cardiac coil. Traditionally, head coils have been a cylinder sized to accommodate the human head easily, e.g. 28 cm in diameter, while cardiac coils have been biplanar sized to accommodate the human torso. Most brain examinations center around the portion of the brain that is substantially in the same plane as the eye sockets. In a symmetric coil, the magnetic and physical isocenters are both configured to be disposed in a common plane with the patient&#39;s eyes or patient&#39;s heart. 
     As a general rule, the longer the cylindrical head coil, the larger the region over which the gradient is linear and the more linear the region is. However, the patient&#39;s shoulders are a limiting factor on the length of a symmetric head gradient coil. The shoulders limit the isocenter to about 20 cm from the patient end. Thus, symmetric head coils have heretofore been limited to about 40 cm in length. 
     In order to achieve the beneficial effects of a longer head gradient coil, head coils have been designed in which the magnetic isocenter is offset toward the patient from the physical, geometric center of the coil. See, for example, U.S. Pat. No. 5,278,504 of Patrick, et al. or U.S. Pat. No. 5,177,442 of Roemer, et al. Although asymmetric head coils have beneficial effects on the linearity and the size of the linear region, the improvement is not without an offsetting difficulty. Within the main magnetic field, the asymmetric gradient coil is subject to mechanical torques from the main and gradient magnetic field interactions. In order to counteract these torques, the asymmetric head coils are mounted with rigid mechanical constraints. Even with substantial mechanical structures anchored to the main field magnet assembly, the torque still tends to cause mechanical vibration and noise. 
     Although conventional head gradient coils include a Maxwell pair for the z-axis or Golay saddle coils for the x or y-axes on the surface of a cylinder, others have proposed coils in which all windings do not lie on the cylinder surface. &#34;Compact Magnet and Gradient System For Breast Imaging&#34;, S. Pissanetzky, et al., SMRM 12th Annual Meeting, p. 1304 (1993) illustrates a compact asymmetric cylinder coil bent up radially at a 90° angle at the field producing end of the coil. The coil is designed for breast imaging with the coil pressed up against the chest. &#34;High-Order, Multi-Dimensional Design of Distributed Surface Gradient Coil&#34;, Oh, et al., SMRM 12th Annual Meeting, p. 310 (1993) attempts to optimize a gradient surface coil using current flows in three dimensions. One of the problems with the Oh surface gradient coil is that it was difficult to control the linearity. Further, the coil was difficult to manufacture due to its complicated shape and high current densities. 
     The present invention provides a new and improved insertable gradient coil which overcomes the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an insertable gradient coil is provided. Gradient magnetic field inducing windings are disposed adjacent a common physical and magnetic isocenter of the coil. Return windings extend symmetrically to opposite ends of the coil. At the patient receiving end of the coil, sections are cut out to receive otherwise interfering body portions. The return windings are configured to conform to the cutout portions. 
     In accordance with a more limited aspect of the present invention, the coil is dimensioned to receive a human head and the cutout portions are configured to receive the patient&#39;s shoulders. 
     In accordance with a yet more limited aspect of the present invention, the cutout portions are parabolic. 
     In accordance with another aspect of the invention analogous cut out sections are provided symmetrically at an opposite service end. 
     One advantage of the present invention is that it achieves better linearity by allowing an increase in coil length. 
     Another advantage of the present invention is that it provides better access for positioning the patient inside the coil. 
     Another advantage of the present invention resides in freedom from torque and vibrational effects. 
     Another advantage of the present invention resides in its higher efficiency and short rise time. 
     Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a magnetic resonance imaging system including an insertable coil in accordance with the present invention; 
     FIG. 2 is a perspective view of a head gradient coil in accordance with the present invention including a generally cylindrical dielectric former and exemplary x and y-gradient coils; 
     FIG. 3 is a detailed illustration of one of four symmetric quadrants of the x-gradient coil in accordance with the present invention; 
     FIG. 4 is a detailed illustration of one of four symmetric quadrants of the y-gradient coil in accordance with the present invention; and, 
     FIG. 5 is a perspective view of an alternate embodiment of a head coil in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a plurality of primary magnet coils 10 generate a temporally constant magnetic field along a longitudinal or z-axis of a central bore 12. In a preferred superconducting embodiment, the primary magnet coils are supported by a former 14 and received in a toroidal helium vessel or can 16. The vessel is filled with liquid helium to maintain the primary magnet coils at superconducting temperatures. The can is surrounded by a series of cold shields 18, 20 which are supported in a vacuum dewar 22. 
     A whole body gradient coil assembly 30 includes x, y, and z-coils mounted along the bore 12. Preferably, the gradient coil assembly is a self-shielded gradient coil assembly that includes primary x, y, and z-coil assemblies potted in a dielectric former 32 and a secondary gradient coil assembly 34 that is supported on a bore defining cylinder of the vacuum dewar 22. A whole body RF coil 36 is mounted inside the gradient coil assembly 30. A whole body RF shield 38, e.g. copper mesh, is mounted between RF coil 36 and the gradient coil assembly 30. 
     An insertable head coil 40 is removably mounted in the center of the bore 12. The insertable coil assembly includes an insertable gradient coil assembly 42 supported by a dielectric former. An insertable RF coil 46 is mounted inside the dielectric former 44. An RF shield 48 is mounted between the insertable RF and gradient coils. 
     An operator interface and control station 50 includes a human-readable display such as a video monitor 52 and an operator input means including a keyboard 54 and a mouse 56. Computer racks 58 hold a magnetic resonance sequence, a controller reconstruction processor, and other computer hardware and software for controlling the radio frequency coils 36 and 46 and the gradient coils 30 and 42 to implement any of a multiplicity of conventional magnetic resonance imaging sequences, including echo-planar echo-volumar imaging sequences. Echo-planar and echo-volumar imaging sequences are characterized by short data acquisition times and high gradient strengths and slew rates. The racks 58 also hold a digital transmitter for providing RF excitation and resonance manipulation signals to the RF coil and a digital receiver for receiving and demodulating magnetic resonance signals. An array processor and associated software reconstruct the received magnetic resonance signals into an image representation which is stored in computer memory or on disk. A video processor selectively extracts portions of the stored reconstructed image representation and formats the data for display by the video monitor 52. 
     With reference to FIG. 2, the active gradient coil windings of the insertable gradient coil assembly 42 are mounted on the dielectric former 44 which, in the preferred embodiment, is circularly cylindrical. At a patient receiving end, the dielectric former has cutouts 62 shaped to conform to a patient&#39;s shoulders, parabolic in the preferred embodiment. Analogous parabolic regions 62&#39; are defined at the opposite end for symmetry. An isocenter 64 is disposed in the geometric center of the former. The cylindrical former has an interior dimension sized to receive the human head, preferably with a radius ρ a  equal to about 15 cm. Preferably, the RF screen and the RF head coil are disposed inside the radius ρ a  between the subject&#39;s head and the gradient coils. The dielectric former has a length L equal to 2z b  with the distance between the apices of the parabolas equal to 2z a . The radius ρ a  =a with the span of the parabolas at the edge of the cylinder equal to 2x b . More specifically, an azimuthal distance from the top of the parabolic section at φ=0 is denoted as x a  =0.0. The numerical eccentricity of the parabola is ε=1. The axial distance of the end point of the parabolic section to the geometric center of the coil is denoted by z b  and coincides with the half length of the coil. The azimuthal distance from the end point of the parabolic section to the φ=0 position is denoted by x b  =ρ a  ·φ b . The latus rectum parameter p of the parabolic section is defined for the parabolic x-gradient coil as: ##EQU1## 
     This geometric shape for the gradient coil is symmetric, hence the overall torque is equal to zero. The imaging volume of the coil, i.e. the region with the best linearity and uniformity covers the entire human head and is centered on the brain. Due to the symmetry of the current density of the coil, its stored magnetic energy is less than the corresponding stored energy in an asymmetric gradient coil with the same specifications. The extended return path length permits lower turns densities compared to symmetric coil designs of 40 cm length and comparable gradient field specifications. 
     A first step in designing the x-gradient coil is to define the configuration of a traditional finite-size transverse x-gradient coil with radius ρ a  and total length L. The design of this type of gradient coil generates a gradient field which is anti-symmetric in the x-direction around the isocenter of the coil and is symmetric in the y and z-directions. Due to the finite length of the coil, the current density is expanded in terms of the sine and cosine Fourier series. Due to the symmetric conditions along the axial z-direction, only cosine Fourier series expansion is required. For this step and due to the symmetry conditions, the current density is constrained to lie on the surface of the cylinder and the resulting current density is constructed as a vector addition of two components. One component lies along an axial direction J z  (φ,z) and the other component lies along the azimuthal direction J.sub.φ (φ,z). Using the continuity equation in order to relate both components of the current density and expressing the z-component of the magnetic field B z  and the stored magnetic energy W in terms of either of these two components of the current density, a functional E is constructed in terms of W and B z  as: ##EQU2## where λ j  are the Lagrange multipliers and B zSC  represent the constraint values of the z-component of the magnetic field at the specified N points. TABLE 1 displays the position of the constraint points n=1,2,3 and the value of the gradient field at each point. 
     
                       TABLE 1______________________________________n           ρi    z.sub.i   B.sub.zSC______________________________________1           0.001     0.000     0.000040002           0.145     0.000     0.005800003           0.001     0.100     0.00004060______________________________________ 
    
     There are two constraint points along the x-axis of the gradient field to ensure the on-axis linearity of the gradient field. A third constraint point is located at the borders of the imaging volume on a plane which is perpendicular to the imaging axis x and controls the off-axis uniformity of the gradient field. Minimizing E, a quadratic function of the current with respect to the current coefficients j n   a , one obtains a matrix equation which j n&#39;   a  must satisfy: ##EQU3## where a=ρ a ,ψ n  (k) are functions which contain information about the system. See for example, U.S. Pat. No. 5,296,810 of Morich. The evaluation of the Lagrange multipliers is done via the constraint equation. Inverting Equation (3), one obtains a solution for j n   a  and hence for the current density. Once these coefficients have been determined, the stored magnetic energy and the magnetic field at any point in space is calculated. 
     In order to discretize the above-calculated continuous function, consider the continuity equation for the current density: 
     
         ∇·J=0                                    (4). 
    
     Analogously with the magnetic field, where a vector potential is introduced, the current density is expressed as the curl of the function  S , called a &#34;stream function&#34;. Specifically: 
     
         J=∇×S                                       (5). 
    
     Because the current is restricted to flow on the surface of a cylinder with radius a=ρ a  and has only angular and axial dependence, the relation between the current density and the stream function in cylindrical coordinates is: ##EQU4## and S.sub.ρ  is found from: ##EQU5## The contour plots of the current density are determined by: ##EQU6## where N is the number of current contours, S min  is the minimum value of the current density, and S inc  represents the amount of the current between two contour lines. The determination of S inc  is: ##EQU7## with S max  representing the maximum value of the current density. The contours which are generated by this method follow the flow of the current and the distance between them corresponds to a current equal to an amount of S inc  in amps. In the manufacturing process, discrete wires are positioned to coincide with these contour lines. 
     This process, of course, generates a discrete current pattern which lies on a surface of a cylinder. The next step of the process is to produce a current distribution which accommodates the parabolic cutout, i.e., a parabolic x-gradient coil configuration. 
     Starting with the original cylindrical surface, coordinates of the starting point for the parabolic section are chosen. These coordinates are represented by a vector expression (ρ a , 0.0, z a ). Up to this point, every segment for the original current patterns remains unchanged. The only current segments that are constrained to follow the parabolic path are those which are included inside the rectangular area which is bounded by the vectors (0.0, z a ) as the lower limit and (x b  =ρ a  ·φ b , z b ) as the upper limit. Inside this rectangular area, any point of the current segments is constrained to follow a parabola which is defined by the equation: ##EQU8## where (x p  =ρ a  φ b , z p ) represent the coordinates for any point inside the rectangular section. In this manner, a discrete current pattern for the transverse x-gradient coil is generated which is confined to a two-dimensional surface with a parabolic aperture in the center region of the coil return paths. FIG. 3 illustrates one of four symmetric quadrants of the x-gradient coil. 
     In order to evaluate the magnetic field for the parabolic x-gradient coil for the discrete current distribution, one uses the Biot-Savart law: ##EQU9## The area of integration includes only the region with the cylindrical surface. In this case, the current is restricted to flow on a cylindrical surface ρ=ρ a . Thus, each current segment is only a function of the azimuthal φ and axial z-directions. Thus, the expressed magnetic field which resulted from the current pattern is: ##EQU10## with z 1 ,φ 1  representing the coordinates of the origin for each line segment at the discrete current distribution, and z 2 ,φ 2  corresponding to the coordinates of the end point for the same line segment. 
     With particular reference to FIGS. 2 and 3, a parabolic x-gradient coil 70 lies on a cylindrical surface of the former 44. The x-gradient coil includes four like thumbprint coil windings 72 each with a parabolic aperture 74 for the return paths of the current patterns. Each of the four coil windings is laminate to one of four symmetric quadrants on the cylindrical former. The radius of the cylinder in the preferred embodiment is ρ a  =0.1579 meters. The total length of the coil of the preferred embodiment is L=0.6 meters. The distance from the geometric center of the coil to the top of the parabola is preferably z a  =0.20 meters. The maximum azimuthal distance x b  is preferably 0.1 meters and the axial distance z b  is preferably 0.3 meters. A coil of this configuration generates a 37.8 mT/m gradient strength over a 25 cm diameter spherical volume at 220 amps and stores 2.005 Joules of energy. 
     With reference to FIGS. 2 and 4, a y-gradient coil 80 is again sized to wrap around the former 60 of length L and radius ρ a . However, the y-gradient coil is rotated 90° around the cylinder from the x-gradient coil 70, i.e., the azimuthal distance from the top of the parabolic section to the φ=π/2 position is denoted by y a  =ρ a  ·π/2. The azimuthal distance from the end point of the parabolic section to the φ=0 position is denoted by y b  =ρ a  ·φ b . The latus rectum p of the parabolic section for the y-gradient coil is defined as: ##EQU11## with z b  &lt;z a . 
     Initially, design of the y-gradient coil starts with the configuration of a traditional finite transverse y-gradient coil of radius ρ a  and total length L. The design of this type of gradient coil generates a gradient field which is anti-symmetric in the y-direction around the geometric center of this coil and is symmetric along the x and z-directions. Due to the finite length of this gradient coil, the current density is expanded in terms of the sine and cosine Fourier series. Due to the symmetry along the axial or z-direction, only cosine Fourier series expansion terms are needed. Due to this symmetry and because the current density is constrained to lie on the surface of the cylinder, the resulting current density is constructed as the vector addition of two components. One along the axial direction J z  (φ,z) and the other along the azimuthal direction J.sub.φ (φ,z). Using the continuity equation in order to relate both components of the current density and expressing the z-component of the magnetic field B z  and the stored magnetic energy W in terms of either one of the two components of the current density, the functional E in terms of W and B is again defined by Equation (2). TABLE 2 illustrates the constraint points and the value of the gradient field for the y-gradient coil. 
     
                       TABLE 2______________________________________n        ρ.sub.i           φ.sub.i                      Z.sub.i                           B.sub.zSC______________________________________1        0.001  π/2     0.000                           0.000040002        0.145  π/2     0.000                           0.005800003        0.001  π/2     0.100                           0.00004060______________________________________ 
    
     Specifically, there are two constraint points along the y-axis of the gradient field to ensure the on-axis linearity and a third constraint located at the borders of the imaging volume on a plane which is perpendicular to the y imaging axis to control the off-axis uniformity of the gradient field. Minimizing E, a quadratic function of the current with respect to the coefficients j n   a , one obtains the matrix equation for j n&#39;   a  which satisfies: ##EQU12## where a=ρ a , ψ n  (k) are functions which contain information about the geometry of the system. Again, see U.S. Pat. No. 5,296,810. The evaluation of the Lagrange multipliers is done via the constraint equation. Inverting this matrix equation, one obtains the solution for j n   a  and hence for the current density. Once these coefficients are determined, one can calculate the stored magnetic energy and magnetic field at any point in the volume. 
     In order to convert the continuous function solution to a discrete current pattern, one considers Equation (4), the continuity equation for the current density. In analogy with the magnetic field where a vector potential is introduced, the current density can again be expressed as a function  S , called a &#34;stream function&#34; as described in Equation (5). Because the current is restricted to flow on the surface of a cylinder of radius a=ρ a  and has only angular and axial dependence, the relation between the current density and the stream function in cylindrical coordinates is again given by Equations (6) and (7). The contour plots are again determined by Equation (8), where N is the number of contour curves, S min  is the minimum value of the current density, and S inc  represents the amount of current between the two contour lines. The determination of S inc  is again found in accordance with Equation (9). The contours which are generated by this method follow the flow of the current and the distance between them corresponds to a current equal to an amount of S inc  in amps. Discrete wires are positioned in such a way as to coincide with these contour lines. This, of course, generates a discrete current pattern which lies on the surface of a cylinder. The next process is to redistribute the current to accommodate the parabolic cutouts 62 at the corners 82 of each of four like thumbprint windings 84 of the y-gradient coil configuration. 
     Starting with the original cylindrical surface, the coordinates of the starting point of the parabolic section 82 are selected. These coordinates are represented by the vector expression (ρ a , ρ a  ·π/2, z a ). The only current segments that are constrained to follow the parabolic path are those which are included inside the rectangular area which is bounded by the vectors (y a  =ρ a  ·π/2, z a ) as the lower limit and (y b  =ρ a  ·φ b , z b ) as the upper limit. The other current segments remain the same. Inside the rectangular area at any point, the current segment is constrained to follow a parabola which is defined by the equation: ##EQU13## where (y p  =ρ a  ·φ p , z p ) represents the coordinates for any point inside the rectangular section. In this manner, a discrete pattern for the parabolic y-gradient coil is generated. The discrete pattern is confined to a two-dimensional surface with parabolic apertures at the corners of each coil quadrant 84 as illustrated in FIG. 4. 
     In order to evaluate the magnetic field for the parabolic gradient from the discrete current distribution, we use the Biot-Savart law as set forth in Equation (11). The area of integration includes only the region with the cylindrical surface, not the region with the parabolic shoulder accommodating regions. In this case, the current is restricted to flow on a cylindrical surface ρ=ρ a  and each current segment is only a function of the azimuthal direction φ in the axial direction z. In a preferred embodiment, the y-gradient coil has a radius of ρ a  =0.1591 meters and a length L=0.6 meters. The distance from the geometric center of the coil to the top of the parabola is z a  =0.2 meters and the azimuthal distance is y b  =0.13 meters and the corresponding axial distance is z b  =0.3 meters. A coil of this configuration generates a 39.5 mT/m gradient strength over a 25 cm. diameter spherical volume, at 220 amps and stores 2.07 Joules of energy. 
     Suitable z-gradient coils are illustrated in parent applications 08/269,393 and 08/213,099. The z-gradients of suitable linearity are achieved with a coil of these configurations and a length of 0.4 meters, the distance between the parabolic cutouts (also the length of many prior art head coils). 
     With reference to FIG. 5, the repositioned return windings adjacent the parabolic shoulder cutouts can be raised off the surface of the cylinder. The former has flared parabolic sections or extensions 90 mounted adjacent the shoulder cutout and extending over the patient&#39;s shoulders. Mounting the return windings on these flared portions enables the return windings to be physically spaced more distantly. 
     It is to be recognized by those skilled in the art that a more conventional distributed or bunched z-gradient coil design can be configured with the parabolic x and y-gradients to form a three axis set. The z-gradient is constrained to less than 40 cm in overall length to avoid impinging upon the shoulder access region and preferentially resides at the largest of the three coil diameters due to its inherently high efficiency. 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.