Abstract:
A birdcage-like coil with a pair of electrically conductive ring elements separated in a longitudinal direction and interconnected by three longitudinally extending electrically conductive elongated strips, two of which are diametrically oppositely disposed and the third is azimuthally at 90E from both of them, can create an RF magnetic field gradient when driven in a certain resonance mode. A similarly structured birdcage-like coil with a fourth strip to have two diametrically oppositely disposed strips can create two switchable orthogonal magnetic field gradient by switching off a selected one of the strips and driving the coil in a certain mode. A coil for generating alternative a homogeneous field and selectably one of two orthogonal gradient fields is formed by sandwiching a prior art birdcage long-pass coil with a pair of such coils and by switching on and off suitable ones of the switches in the strips.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to high-resolution nuclear magnetic resonance (NMR) spectroscopy and imaging and in particular to so-called X-Y B1 gradient coils for producing two quickly switchable orthogonal magnetic field gradients in a plane which is perpendicular to the direction of a homogeneous static magnetic RF field. 
     It has been known to take advantage of a magnetic field with a gradient in NMR spectroscopy. U.S. Pat. No. 5,323,113 issued Jun. 21, 1994 to Cory et al, for example, disclosed an NMR probe for generating both a homogeneous RF field over a sample volume and a radial field comprising two orthogonal gradient fields in a plane transverse to the homogeneous field. The structure with two coils connected in parallel to a signal generator tends to give rise to interference problems. No commercially available coils of this kind exist within the knowledge of the inventors herein. 
     It has also been known to structure a radio-frequency coil like a birdcage in order to obtain a highly homogeneous magnetic field as described, for example, in U.S. Pat. No. 4,694,255 issued Sep. 15, 1987 to C. Hayes and “Experimental Design and Fabrication of Birdcage Resonators for Magnetic Resonance Imaging” (T. Vullo, et al., Magnetic Resonance in Medicine, 24, 243 (1992)). Birdcage coils are so called because of their general structure having a pair of loop-shaped conductive elements (the “rings”) separated in a longitudinal direction and a plurality of conductive segments (the “strips”) evenly spaced about the circumference of and interconnecting these two loop-shaped conductive elements. Capacitors are inserted either in the strips for a low-pass coil, or in the rings for a high-pass coil, as illustrated in FIGS. 1A and 1B, respectively. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a birdcage-like coil for generating an improved RF magnetic field gradient. 
     It is another object of this invention to provide a birdcage-like X-Y B1 gradient coil for generating two quickly switchable orthogonal magnetic field gradients, say, in the X-direction and the Y-direction. 
     It is still another object of this invention to provide an improved X-Y B1 gradient coil for generating both a homogeneous RF field over a sample volume and a radial field comprising two orthogonal gradient fields without giving rise to unwanted interference between the so-called transmitter and receiver coils. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
     FIGS. 1A and 1B are sketches showing birdcage coils respectively with low-pass and high-pass coil configurations; 
     FIG. 2A is a schematic diagonal view of a birdcage-like coil embodying this invention for generating an improved RF magnetic field gradient and FIG. 2B is a schematic sectional view taken along line  2 B— 2 B in FIG. 2A, FIGS. 2A and 2B being hereinbelow together referred to as FIG. 2; 
     FIG. 3 is a schematic diagonal view of another birdcage-like coil embodying this invention for generating two quickly switchable orthogonal magnetic field gradients; 
     FIG. 4 is a schematic diagonal view of still another birdcage-like coil embodying this invention for generating both a homogeneous RF field and a radial field comprising two orthogonal gradient fields; 
     FIG. 5 is a circuit diagram of the coil of FIG. 4; and 
     FIGS. 6A and 6B are each a schematic diagonal view of a birdcage-like coil with twisted strips embodying this invention. 
    
    
     Those components which are similar, although components of different coils, may be indicated by the same symbols for the sake of convenience without repetitious explanations. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows schematically the structure of a birdcage-like coil  10  for generating an improved magnetic field gradient, characterized as comprising a pair of electrically conductive rings  12  which are mutually separated in a longitudinal direction parallel to the central axis of the birdcage-like shape of the coil  10 . Unlike a real birdcage intended to keep a bird inside such that it cannot escape, the coil  10 , according to a representative embodiment of the invention, has only three strips which are elongated members extending longitudinally (say, along the Z-direction) between the rings  12 , interconnecting them at both ends and are not evenly spaced circumferentially along each of the rings  12 . For the convenience of description, one of these three strips will be hereinafter referred to as the center strip  14  and the other two as the flanking strips  15 , the flanking strips  15  being disposed diametrically opposite to each other (say, along a diameter in the X-direction) with respect to the rings  12  and the center strip  14  being at equidistance from them, that is, at a mid-way position therebetween separated azimuthally from both of the flanking strips  15  by 90E around the rings  12  (or on the diameter in the Y-direction). The invention does not impose any particular limitation on the physical structure of the rings  12  and the strips  14  and  15  except, as schematically shown in FIG. 2, that each of the strips  14  and  15  is capacitively coupled with the rings  12  at both ends. Methods of joining such a strip to a ring so as to form a capacitance therebetween have been known and will not be discussed herein. 
     Although only schematically shown in FIG. 2, the coil  10  is connected to a driving means  18 , or an RF generator for operating the coil  10  in a resonance mode wherein as a current with intensity I flows through the center strip  14  in one longitudinal direction, a current of intensity I/2 will flow through each of the pair of flanking strips  15  in the opposite longitudinal direction. The coil  10  has another resonance mode wherein no current flows through the center strip  14  and as a current with intensity I flows through one of the flanking strips  15  in one longitudinal direction, another current of the same intensity I flows through the other of the flanking strips  15  in the opposite longitudinal direction. Since it is well understood by persons skilled in the art how to operate the driving means  18  to activate the coil  10  in a desired mode, operation of the driving means  18  will not be described in any detail. 
     With the coil  10  thus being operated, a magnetic field with a uniform gradient in the Y-direction is generated inside the birdcage-like structure. 
     The invention does not limit the number of the strips to be three. More strips may be provided, flanking the center strip  14 . By controlling the current distribution among these strips, a more uniform gradient can be generated over a larger portion of the space inside the birdcage-like coil structure but the control of the driving means  18  will be accordingly more difficult. 
     FIG. 3 shows schematically the structure of another birdcage-like coil  20  for generating two quickly switchable orthogonal magnetic field gradients, characterized as comprising, like the coil  10  explained above with reference to FIG. 2, a pair of electrically conductive rings  22  which are mutually separated in a longitudinal direction parallel to the central axis of the birdcage-like shape of the coil  20  and again referred to as the Z-direction. Unlike the coil  10  shown in FIG. 2, however, the coil  20  has four strips disposed equally spaced circumferentially around the rings  22  to form a birdcage-like structure, or a two pairs of mutually diametrically disposed strips with respect to the rings  22 . For the convenience of description, the strips of one of the pairs (the “first pair”) disposed on the diameter defining the X-direction will be indicated by  24   a  and  24   b,  and those of the other of the pairs (the “second pair”) on the perpendicular diameter defining the Y-direction will be indicated by  25   a  and  25   b.  Each of the strips  24   a,    24   b,    25   a  and  25   b  is capacitively coupled to the rings  22 . 
     Symbols  28   x  and  28   y  in FIG. 3 schematically indicate driving means for the coil  20 , each connected to one of the rings  22  through a coupling capacitor and a switch  27 . As shown also schematically in FIG. 3, one each of the strips of each of the pairs ( 24   b  and  25   b ) includes a switch  29 . These switches may each comprise a pin diode. When a gradient in the Y-direction is desired, the switch  29  in the strip  25   b  and the switch  27  related to one of the driving means  28   y  are opened. The other driving means  28   x,  connected to the ring  22  operates the coil  20  in a mode wherein as a current with intensity I is caused to pass through the other of the strips of the second pair ( 25   a ) in one longitudinal direction, a current with intensity equal to I/2 will flow through each of the strips  24   a  and  24   b  of the first pair in the opposite longitudinal direction. It should be noted that the current configuration in this situation is identical to that described above with reference to FIG. 2, the strip  25   a  serving as the center strip and the strips  24   a  and  24   b  serving as the flanking strip of FIG.  2 . Thus, there results inside the birdcage-like structure of FIG. 3 a magnetic field gradient in the Y-direction, the B 1  field strength being large near the strip  25   a  and dropping to zero at the position of the strip  25   b.    
     Similarly, in a second mode of operation, the switch  29  in the strip  24   b  and the switch  27  related to the other of the driving means  28   x  are opened. The connected one of the driving means ( 28   y ) operates the coil  20  in this case such that as a current with intensity I is caused to pass through the connected one of the strips of the first pair ( 24   a ) in one longitudinal direction, a currents with intensity equal to I/2 will flow through each of the strips  25   a  and  25   b  of the second pair in the opposite longitudinal direction. In this case, the strip  24   a  serves as the center strip of FIG.  2  and the strips  25   a  and  25   b  serve as the flanking strip. Thus, there results inside the birdcage-like structure of FIG. 3 a magnetic field gradient in the X-direction, the B 1  field strength being large near the strip  24   a  and dropping to zero at the position of the strip  24   b.    
     FIG. 4 shows schematically the structure of still another birdcage-like coil  30  for generating both a homogeneous RF field and a radial field comprising two orthogonal gradient fields. Described briefly, this coil  30  may be said to be a combination of a center coil  31 , which is structured essentially like a prior art high-pass birdcage coil, and a pair of X-Y B1 gradient coils  41  on its both longitudinal ends so as to sandwich it in between. Thus, the center coil  31  is structured as schematically illustrated in FIG. 1B, having a pair of rings  32  which are separated in a longitudinal direction (the “Z-direction), and a plural number of elongated members (the “strips”)  34  extending in the Z-direction and interconnecting the rings  32  at junctions which are equally spaced circumferentially around the rings  32 . Each of these rings  32  are circumferentially divided into the same plural number of segments  33  each having a corresponding one of these junctions thereon and each mutually adjacent pair of these segments  33  are mutually capacitively coupled so as to form a high-pass coil as illustrated in FIG.  1 B. 
     Each of the X-Y B1 gradient coils  41  is structured essentially as shown in FIG. 3, that is, as a birdcage-like structure with two rings and four longitudinally extending strips  44   a,    44   b,    45   a  and  45   b  interconnecting them. Each of the gradient coils  41  makes use of a corresponding one of the rings  32  of the center coil  31  as one of its own rings. The other ring will be hereinafter referred to as the end ring  42 . The four strips  44   a,    44   b,    45   a  and  45   b  are equally spaced circumferentially around the coils  32  and  42  and are each capacitively coupled with the end coil  42 . The strips  44   a  and  44   b  make a diametrically disposed pair (the “first pair”) of strips and the strips  45   a  and  45   b  make another diametrically disposed pair (the “second pair”) of strips, the two pairs being disposed on two mutually perpendicular diameters (extending in the X-direction and the Y-direction, respectively) of the rings  32  and  42 , as explained above with reference to FIG.  3 . The four strips  44   a,    44   b,    45   a  and  45   b  each include a switch  46  which may be a Pin diode. The plural number of the strips  34 , and hence also the number of segments  33  of the ring  32 , is a multiple of four (FIG. 4 showing this number to be eight). The strips  44   a,    44   b,    45   a  and  45   b  of the end coils  41  are each connected to one of the segments  33  of the ring  32 . Thus, the circuit diagram of the coil  30  may be as shown in FIG.  5 . 
     When the coil  30  is used to create a field gradient in the Y-direction, the switches  46  in the first pair of strips  44   a  and  44   b  and in one of the second pair of strips (say,  45   a ) of each end coil  41  are all closed and the one in the other of the second pair of strips ( 45   b ) is opened. This means that the two strips  45   a  of the two end coils  41  are electrically connected through one of the strips  34  of the center coil  31  and the corresponding ones of the segments  33  forming the rings  32 . The same is true with each of the strips  44   a  and  44   b  of the two end coils  41 . Thus, the coil  30  is now structured like the coil  10  shown in FIG.  2 . So, when a driving means (not shown) establishes a current distribution by causing a current with intensity I to pass through the strips  45   a  and the corresponding one of the strips  34  of the center coil  31  in one longitudinal direction while a current with intensity I/2 passes through each of the second pair of the strips  44   a  and  44   b  of the end coils  41  in the opposite longitudinal direction. Those of the strips  46  of the center coil  31  not corresponding to the closed strips  45   a,    45   b  and  46   b  carry no current. As a result, as explained with reference to FIG. 2, a magnetic field gradient is created in the Y-direction. 
     Similarly, a magnetic field gradient in X-direction is created by closing the switches  46  in the second pair of strips  45   a  and  45   b  and one of the first pair of strips (say,  44   a ) while opening the switch in the other of the first pair of strips  44   b  of the each end coil  41  and operating the driving means (not shown) to establish a current distribution such that when a current with intensity I passes through the strips  44   a  through a corresponding one of the strips  34  of the center coil  31  in one longitudinal direction, a current with intensity I/2 will pass through each of the first pair of the strips  45   a  and  45   b  of the end coils  41  in the opposite longitudinal direction. Similarly as explained above, this current distribution creates a magnetic field gradient in the X-direction. 
     When the coil  30  is used as a detection coil, the switches  46  in the end coils  41  are all opened such that the center coil  31  functions as a detection birdcage coil as shown in FIG. 1B, activated by the driving means. Since the same current paths are used both for creating a gradient (either in the X-direction or in the Y-direction) and for detection, the coil  30  according to this invention eliminates the prior art problem of interaction between a transmitter coil and a receiver coil. 
     FIG. 6A shows still another coil  50  according to this invention for creating a B1 gradient in the Z-direction, characterized as being structured similarly to a birdcage B1 gradient coil  20  shown in FIG.  3 . Thus, the components which are similar between the two figures are indicated by the same numerals and will not be explained repetitiously. Unlike the coil  20  shown in FIG. 3, the coil  50  has its strips (of the first pair  54   a  and  54   b  and the second pair  55   a  and  55   b ) twisted, not extending in the longitudinal direction. In other words, the coil  50  may be formed by rotating one of the rings  22  of the coil  20  by 90E with respect to the other ring  22 . Thus, the first pair of strips  54   a  and  54   b,  which is on the diameter of one of the rings (the “first ring  52   a ”) in the X-direction, is on the diameter of the other of the rings (the “second ring  52   b ”) in the Y-direction. Similarly, the second pair of strips  55   a  and  55   b  is on the diameter of the first ring  52   a  in the Y-direction and on the diameter of the second ring  52   b  in the X-direction. 
     When a current distribution through the strips  54   a,    54   b,    55   a  and  55   b  is established, as explained above with reference to FIG. 3 for obtaining a gradient in X-direction, a gradient in X-direction will be established at the position of the first ring  52   a  (at the zero coordinate in the Z-direction, or at Z=0) but the gradient established at the position of the second ring  52   b  (at Z=h where h is the separation between the two rings  52   a  and  52   b ) will be in the Y-direction, rotated by 90E. At an intermediate position with Z-coordinate such that 0&lt;Z&lt;h, the direction of the magnetic field gradient (measured by the azimuthal angle θ from the X-direction will be such that 0E&lt;&lt;90E, varying as a function of Z. This functional relationship is dependent upon how the twisting is effected, or the relationship between the azimuthal angular position of (any of) the strips and the longitudinal position Z. The X-component of the gradient at a position Z=Z is B1 cos θ where B1 is the gradient at Z=0. Thus, by twisting the strips appropriately, it is possible to make a uniform gradient in the Z-direction. 
     FIG. 6B shows a coil  60  which may be considered to be a variation of the coil  50  shown in FIG.  6 A and hence similar components are indicated by the same numerals for convenience. The coil  60  in FIG. 6B is different only in that the twisting of the strips  54   a,    54   b,    55   a  and  55   b  between the two rings  52   a  and  52   b  is by 180E, not by 90E. Thus, as one moves in the longitudinal direction from Z=0 to Z=h, the direction of gradient changes, say, from the positive X-direction to the negative X-direction. Thus, the X-component of the gradient changes monotonically from the position of the first ring  52   a  (Z=0) to that of the second ring  52   b  (Z=h). The manner in which it changes again depends on the functional relationship between the azimuthal angular position of any of the strips and longitudinal position Z.