Abstract:
An examination region (12) is defined along a z-axis offset from a geometric center (30) of a gradient coil assembly (20). Cylinders of a non-conductive, non-magnetic material support x, y, and z-gradient coils (24, 26, 22) for causing orthogonal magnetic field gradients through the offset examination region. The z-gradient coil (FIG. 2 ) includes a plurality of distributed loop arrays with a winding pattern selected to cause a region of linear magnetic field gradients in the z-direction in a region offset toward a cylinder first end (32). The x and y-gradient coils each include two pairs of oppositely disposed windings (FIG. 3 ) which include a pair of inner spirals (96, 98) offset towards the first end of the cylinder and an outer spiral (90) extending therearound. The outer spiral bows in (92) toward the cylinder first end and fans out (94) toward the second end. In this manner, the x, y, and z-gradient coils create linear magnetic field gradients along x, y, and z-directions which have a region of primary linearity offset from their geometric center.

Description:
This application is a continuation of U.S. application Ser. No. 07/805,016, filed Dec. 11, 1991, and now abandoned which is a continuation-in-part of U.S. application Ser. No. 07/719,334, filed Jun. 21, 1991, now U.S. Pat. No. 5,177,441, which in turn is a continuation-in-part of U.S. application Ser. No. 07/368,047, filed Jun. 16,1989, now U.S. Pat. No. 5,036,282. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the art of magnetic field gradient generation. It finds particular application in conjunction with establishing gradient magnetic fields in magnetic resonance imaging techniques and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in spectroscopy and other processes and apparatus in which accurate and predictable magnetic field gradients are established or maintained. 
     In magnetic resonance imaging, a uniform magnetic field is created through an examination region in which a subject to be examined is disposed. A series of radio frequency pulses and magnetic field gradients are applied to the examination region. Gradient fields are conventionally applied as a series of gradient pulses with preselected profiles. These radio frequency and gradient pulses excite magnetic resonance, phase and frequency encode the resonance, and cause phase and frequency encoded magnetic resonance signals to be emitted. 
     More specifically, the gradient magnetic pulses are applied to select and encode the magnetic resonance. In some embodiments, the magnetic field gradients are applied to select one or more planes or slices to be imaged. Gradient field pulses are also applied for selectively modifying the uniform magnetic field to encode frequency and phase into the magnetization, hence the resonance signals, in order to identify a spatial location. 
     The magnetic resonance signals are then processed to generate two or three dimensional image representations of a portion of the subject in the examination region. The accuracy of the resultant image representation is dependent upon the accuracy, among other factors, with which the actually applied magnetic field gradient pulses conform to selected gradient pulse profiles. 
     Linear magnetic field gradients are commonly produced by cylindrical gradient field coils. Discrete coils are wound in a bunched or distributed fashion on a large diameter hollow right cylinder tube, commonly 65 centimeters in diameter or larger. Conventional bunched geometries include Maxwell or modified-Maxwell pairs for z-gradient production and single or multi-arc Golay saddle coils for x and y gradient production. The coils are normally wound in a series arrangement and positioned to give a magnetic field profile with the desired linearity over a predefined volume. The distributive windings on the cylinders are generally wound and in pairs and driven anti-symmetrically. The coils are driven in an anti-symmetric manner such that only odd derivatives are non-zero at the coil origin. The first derivative is the field gradient while the third and higher order derivatives represent distortion. If the diameter of the cylinder and coil placement are chosen properly, the third derivative is canceled at the origin making the relatively weak fifth derivative component the first distortion term. 
     The conventional gradient coils are constructed of copper rods or multi-strand wires that are wound on a fiberglass reinforced plastic tube. The inductance, which is related to the stored magnetic energy, is critical in gradient coil design. The inductance determines to how quickly the coil can switch the gradient field on and off with a given supply of voltage. Larger inductances, as are typical in wound cylindrical coils, slow the switching speed of the gradient magnetic fields. 
     For maximum efficiency, it would be advantageous to reduce the diameter of the gradient coil cylinders to be as close as possible to the subject, provided gradient linearity can be maintained. The required energy for field gradient production varies roughly as a fifth power of a gradient coil cylinder diameter in free space. In an actual magnetic resonance imager, the gradient coils interact with other adjoining structures, such as radiation shields of superconducting magnets, making the relationship somewhat greater than the fifth power. Thus, reducing the coil size has a dramatic effect on power consumption. 
     Typically, the sweet spot or most linear, useable region at the coils is within the geometric center. A common way for improving the linearity or increasing the linear region in the axial direction is to lengthen the coil. Conversely, shortening the axial length of the coil tends to reduce the size of the linear region. 
     When imaging the head, particularly lower portions of the head and upper portions of the neck, the coils must be large enough to receive the patient&#39;s shoulders. When the coil is reduced in diameter to improve the switching speed and power consumption such that it just fits the head, the shoulders limit how far the patient&#39;s head can be inserted into the coil. Commonly, the linear portion or sweet spot of the coil can only be brought down to a central portion of the patient&#39;s head before the leading edge of the coil abuts the patient&#39;s shoulders. To move the imaging region down to the jaw or the top of the spine, for example, whole body size coils or surface coils are typically utilized. The whole body coils, being often twice the diameter or more of a head coil, suffers from slower switching times and higher power requirements. 
     U.S. Pat. No. 4,689,591, issued Aug. 25, 1987 to McDougall illustrates a whole body size coil in which the homogeneous region is offset from the geometric center of the coil. 
     The present invention provides a new and improved gradient coil configuration which facilitates shifting the linear volume of a coil axially, even on smaller diameter head coils. 
     SUMMARY OF THE INVENTION 
     In accordance With one aspect of the present invention, a gradient coil is provided which has its linear region offset from its axial center. 
     More specifically, three gradient coils are provided, each for generating linear magnetic field gradients along one of three mutually orthogonal axes. All three gradient coils defines its linear region along a common central axis asymmetrically from the center of the coil such that the linear region of all three coils are coincident. 
     In accordance with a more limited aspect of the present invention, the gradient coils include a first pair of identical coils positioned around the central axis of the imaging volume diametrically opposite to one another and a second pair of identical coils positioned around the imaging volume diametrically opposite to one another and overlapping the first pair of coils. 
     In accordance with another aspect of the present invention, each gradient coil pair for generating a gradient in a direction transverse to the central axis includes a double spiral. 
     In accordance with a more limited aspect of the present invention, each double spiral includes a larger ring of coils disposed symmetrically relative to the central axis, a smaller ring of coils axially offset from the first ring of coils, and a third ring of coils surrounding the first and second rings. 
     In accordance with another aspect of the present invention, an axial gradient coil is provided which generates magnetic field gradients along the central axis of the imaging volume displaced axially from a center thereof. 
     In accordance with a further aspect of the present invention, the axial gradient coil includes a first, denser group of coils adjacent one edge of the coil, a second, less dense group of windings adjacent the first, a third, more dense group of windings adjacent the second, and a fourth, less dense group of windings adjacent the third. 
     In accordance with a still more limited aspect of the present invention, the first and third coil winding groups include windings with opposite current flows. 
     In accordance with another aspect of the present invention, the coil sets are mounted on a generally elliptical cylinder sized to fit a patient&#39;s head, but too small to receive a patient&#39;s shoulders. 
     One advantage of the present invention is that it permits imaging of regions of the lower head and upper spinal cord. 
     Another advantage of the present invention is that it allows better patient access to an imaging region. 
     Another advantage of the present invention is that it does not require extended radial space. Radial return path gradients are not required. 
     Another advantage of the present invention is that it provides rapid switching of gradient fields. 
     Yet another advantage of the present invention is that it minimizes coil volume and improves coil efficiency. 
     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 apparatus with a magnetic field gradient coil in accordance with the present invention; 
     FIG. 2 illustrates a preferred z-gradient coil assembly winding pattern in accordance with the present invention; 
     FIG. 3 illustrates a planar projection of one side of an x or y-gradient coil assembly winding pattern in accordance with the present invention; 
     FIG. 4 illustrates an alternate embodiment of the gradient coil assembly pattern in perspective; 
     FIG. 5 is a diagram of a coil arrangement according to the invention incorporating a &#34;bucking&#34; coil arrangement; and 
     FIG. 6 illustrates another alternate embodiment of the gradient coil assembly in which the coil assembly is generally elliptical. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a main magnetic field means 10 generates a substantially uniform magnetic field longitudinally through an examination region 12 having a center 14. The main magnetic field means includes a plurality of main field magnets 16 which are operated under control of a magnetic field control means and power supply 18. preferably, the main magnetic field is a strong, uniform field that is aligned with a z or longitudinal axis. However, permanent and iron core magnets, e.g. a C-shaped magnet, with the uniform field along other than the z axis are also contemplated. 
     A gradient magnetic field means 20 selectively creates gradient magnetic fields across the examination region 12 which is not Centrally located along the z or longitudinal axis with respect to the gradient coil. The gradient field means includes a z-gradient coil assembly 22 which applies or creates magnetic field gradients along the z-axis. An x-gradient coil assembly 24 selectively applies or creates magnetic field gradients in an x direction transverse to the z-axis. A y-gradient coil assembly 26 selectively applies or causes magnetic field gradients in a y direction, transverse to the x and z axes. Each of the x, y, and z gradient coils has a geometric center. In the preferred embodiment, the coils have a common central plane 28 through this common geometric center point 30 on the z axis. Each of the x, y, and z-gradient coil assemblies has a linear gradient field region or sweet spot in which the magnetic fields which it generates are linear. These linear regions are coincident with each other and the examination region 12 but are not coincident with the geometric center. The examination region is offset along the z or longitudinal axis toward a first end 32 of the gradient coils from the common geometric center. 
     A current supply means 34 selectively supplies electrical current to each loop of the x, y, and z-gradient coils. The current is supplied to the current loops such that the current flows through some of the loops of each coil assembly in a first or clockwise direction and through other loops of the same assembly in an opposite or counterclockwise direction. A gradient field control means 36 controls the current supply means 34 to cause it to apply appropriate current pulses to the gradient field windings to cause selected gradient pulses. 
     A resonance excitation means includes a radio frequency transmitter 42 for generating radio frequency pulses of the appropriate frequency spectrum for inducing resonance in selected dipoles disposed in the examination region. The radio frequency transmitter is connected to a radio frequency antenna 44 disposed adjacent the examination region for transmitting radio frequency pulses into a region of interest of the patient or other subject in the examination region. Although the radio frequency antenna is illustrated as being disposed peripherally around the gradient coil assemblies, it is to be appreciated that such antenna may also be disposed within the gradient coil assemblies. For example, a surface coil may be positioned contiguous to an examined patient or subject for controllably inducing magnetic resonance in a selected contiguous region of the patient. 
     A magnetic resonance receiving means 50 includes the radio frequency coil 44 which receives, as well as transmits, radio frequency signals in the illustrated embodiment. For other studies, separate transmit and receive coils are used. For example, receive only surface coils may be disposed contiguous to a selected region of the patient to receive resonance signals induced by the radio frequency coil 44. A radio frequency receiver 52 receives the radio frequency signals from the antenna 44. The received radio frequency signals are demodulated and reconstructed into an image representation. More specifically, a Fourier transform means 54 performs an inverse two-dimensional fast Fourier transform on the magnetic resonance signals to transform them into an image representation for storage in an image memory means 56. As is conventional in the art, the image may represent a planar slice through the patient, an array of parallel planar slices, a three dimensional volume, or the like. A display means 58, such as a video monitor, provides a man-readable display of the resultant image. Other conventional processing equipment, which is conventional in the art, is omitted from the illustration for simplicity. 
     The gradient magnetic field means 20, in the preferred embodiment, includes two cylindrical formers 60, 62 made of a non-magnetic and non-conducting material. The two formers or sleeves are configured such that one is telescopically received or nests snugly within the other. Preferably, the x-gradient coil 24 is formed on one of the formers and the y-gradient coil 26 on the other. This enables the formers to be mechanically rotated to adjust the orthogonality of the x and y axes. The z-gradient coil 22 is preferably formed on the opposite surface of one of the formers from the x or y-gradient coil. 
     With reference to FIG. 2, the z-gradient coil 22 includes a plurality of distributed windings encircling the z-axis. The distributed windings include a first loop array 70 adjacent the first end 32. The first loop array is a relatively dense winding assembly in which most of the windings convey current in a first (counterclockwise in the illustrated embodiment) direction. However, at least one of the windings 72 disposed closely adjacent an edge of the z-gradient coil conducts current in an opposite direction. A second group of windings 74 is disposed adjacent an interior edge of the first loop array. The second loop array is relatively sparse compared to the first loop array. A third loop array 76 is disposed adjacent an interior edge of the second loop array. The third loop array is divided between loops 78 carrying current in the first and loops 80 carrying current in a second, opposite direction (clockwise in the illustrated embodiment). 
     A fourth, relatively sparse loop array region 82 and a fifth loop array region 84 are disposed adjacent the third loop array. Currents in the fourth loop array region and the fifth loop array are directed in the second, clockwise direction. A sixth loop array 86 is disposed adjacent the fifth loop array at an opposite end of the z-gradient coil. Currents through the sixth loop array are primarily in the second direction. 
     In the illustrated embodiment, the current control means 34 supplies current in the above noted direction and with the same magnitude in each loop. Of course, like current flux densities can also be achieved by adjusting the current in more uniformly distributed loops. 
     In this manner, the z-gradient coil generates magnetic field gradients which are linear centered in the examination region disposed asymmetrically along the z-axis relative to a geometric center 30&#39; of the z-gradient coil. 
     With reference to FIG. 3, the x and y-gradient coils each include two windings disposed symmetrically on opposite sides of a plane along the z-axis. Each winding of the x and y-gradient coil includes a first, peripheral array of loops 90 through which current flows in a first (counterclockwise in the illustrated embodiment) direction. The first loop array bellies-in 92 adjacent the first end 32, i.e. adjacent the examination region, and fans out 94 away from the examination region 12. Each loop array further defines a second or major interior loop 96 in which currents flow in the first direction and a third loop array 98 in which the currents also flow in the first or counterclockwise direction. The two interior loop arrays 96, 98 are disposed generally symmetrically about a central axis of the coil with the major interior loop array disposed offset toward the first end 32 and the minor interior loop array 98 disposed between the major loop and the first end of the gradient coil. 
     In the preferred embodiment, each gradient coil winding is defined by covering about a half of the exterior surface of one of the formers with copper foil and etching or otherwise removing strips of copper such that the winding pattern of FIG. 3 remains. The illustrated winding pattern is ideal due to its minimal number of overlapping regions. The overlapping strip 100 extending from the minor loop is, preferably, formed from a separate piece of copper foil separated from the underlying layers by a thin insulator. 
     The current supply means 34 applies a like current through each winding of FIGS. 2 and 3 in the preferred embodiment. Optionally, substantially the same current density can be achieved by changing the number of conductors and making a corresponding adjustment in the current through each conductor. 
     In the embodiment of FIG. 4, a pair of z-gradient circular, Maxwell coils 102a, 102b selectively generate linear gradients along the z-axis in the imaging region 12&#39;. 
     Gradients in the x-direction are generated by four x-gradient saddle coils 104a, 104b, 104c, 104d. Saddle coils 104a and 104b are folded back over the other pair of saddle coils 104c and 104d. With this arrangement, the imaging region 12, of satisfactory gradient linearity is positioned towards one end of the coils 104. Analogously, linear gradients in the y-direction are generated with four y-gradient saddle coils 106a, 106b, 106c, 106d. One set of the saddle coils 106a, 106b is folded over the other pair of saddle coils 106c, 106d. This arrangement again places the region of satisfactory gradient linearity toward the one end of the coils. The position of the center of the region of satisfactory gradient linearity 12&#39; in the axial direction is central between the Maxwell coils 102a, 102b which are positioned at a distance apart equal to about 3 1/2   times the radius of the gradient coils. Thus, the center of the examination region 12&#39;  is about 0.866 times the radius from the nearer end of the coil arrays 104, 106, as compared with about 1.3 times the radius in a more conventional arrangement. 
     In FIG. 4, coils 104 and 106 are shown for clarity as not overlapping in the circumferential direction. In practice, the coils typically do so, preferably subtending an angle of about 120° . 
     Current flows in one direction through coils 104a, 104c and in an opposite direction through coils 104b, 104d. Preferably, current flows in one direction through coils 106a, 106c and in an opposite direction through coils 106b, 106d. 
     It will be seen from FIG. 4, that the four coils 104 now effectively comprise a single pair of coils 104a, 104c, and 104b, 104d correspondingly positioned at opposite sides of the imaging volume 12&#39;. The conductors of each coils 104a, 104c and 104b, 104d are spaced axially at each end, more particularly in two groups. It will be appreciated in this connection that the spacing of the conductors at the right-hand end of the coils in FIG. 4 controls gradient linearity at the imaging region 12&#39;. It also arises from the desirability of making all the coils 104 identical. 
     With the particular arrangement shown in FIG. 4, it is found that an improvement in gradient linearity can be effected by arranging the coils 104c, 104d to produce a larger number of ampere turns than coils 104a, 104b. In one particular arrangement of coils of an internal radius of 32 cm. and arranged carry identical currents, the coils 104a and 104b each have 15 turns and the coils 104c and 104d each have 12 turns. 
     It will be seen from FlG. 4, that the four coils 106 now effectively comprise a single pair of coils 106a, 106c, and 106b, 106d correspondingly positioned at opposite sides of the imaging volume 12, The conductors of each coils 106a, 106c and 106b, 106d are spaced axially at each end, more particularly in two groups. It will be appreciated in this connection that the spacing of the conductors at the right-hand end of the coils in FIG. 4 controls gradient linearity at the imaging region 12, It also arises from the desirability of making all the coils 106 identical. 
     With the particular arrangement shown in FIG. 4, it is found that an improvement in gradient linearity can be effected by arranging the coils 106c, 106d to produce a larger number of ampere turns than coils 106a, 106b. In one particular arrangement of coils of an internal radius of 32 cm. and arranged carry identical currents, the coils 106 a and 106b each have 15 turns and the coils 106c and 106d each have 12 turns. 
     One difficulty that arises with this coil is that the magnetic field produced when it is energized reacts with the static magnetic field to produce a net torque such as to tend to rotate the coils and hence the former on which the coil set is mounted. This is due to the non-symmetrical form of the coil set, and consequently this difficulty does not arise with symmetrical prior art coil sets, where the forces produced cancel one another out. 
     In order to overcome this problem, so-called bucking coils are used. As shown in FIG. 5, these coils 110, 112 are positioned beyond the ends of the coils 104, 106 remote from the volume 12, and are arranged so as to produce, when energized, forces equal and opposite to those produced by the coils 104, 106 and thereby counteract such forces. To this end, one of a pair of identical diametrically oppositely positioned coils 110a, 110b, 112a, 112b are suitably used for each gradient coil set 104, 106. Each pair of bucking coils 110a, 110b, 112a, 112b, is axially aligned with the coils of the corresponding gradient coil set 104, 106 and mounted on the former therewith. Optionally, the bucking coils may be mounted on other structures connected to other portions of the former and mounted inside of the main magnetic field. 
     The bucking coils 110, 112 are conveniently connected for energization in series with the gradient coils 104, 106. However, they may be energized separately if desired, for example, to avoid increase of the inductance presented to the gradient coil energizing currents due to the bucking coils 110, 112 and thereby avoid increase of gradient current pulse rise and fall times due to the bucking coils 110, 112. 
     The coils are preferably wound with a conductor of rectangular cross-section with the turns lying alongside of one another in a direction parallel to the axis of the cylindrical volume which they surround. It will be appreciated that while referring to the embodiment of FIG. 4, the cylindrical volume is of circular cross-section, this is not necessary. In other embodiments of the invention, the volume may have other cross-sections, particularly an elliptical cross-section. 
     With reference to FIG. 6, the formers 60, 62 are shaped with an ellipse or ovoid cross sectional shape which better matches the cross-section of the patient&#39;s head or other body region that is to be imaged. The x-gradient coil windings become elongated in the y-direction, but are otherwise unchanged. Analogously, the y-coils become compressed in the x-direction, if the minor axis of the ellipse is smaller than the radius of the circle. 
     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.