Abstract:
A phased array coil providing spatial discrimination for simultaneous spin acquisition maximizes transverse and longitudinal acceleration through the use of loops having boundaries tipped to the longitudinal axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to Magnetic Resonance Imaging (MRI) and in particular to a local coil for use with SENSE, SMASH, and other imaging techniques in which the local coil helps distinguish between simultaneously acquired, but separate groups of spins.  
           [0002]    In MRI, a uniform magnetic field, B 0 , is applied to an imaged object along a longitudinal axis or z-axis of a Cartesian coordinate system. The effect of the magnetic field is to align the objects nuclear spins along the z-axis. A radio frequency (RF) excitation signal of the proper frequency oriented in the transverse or x-y plane is then applied to cause the nuclear spins to precess. During a detection stage, this precession is captured as a nuclear magnetic resonance (NMR) signal. Water, because of its relative abundance in biological tissue and the property of its nuclei, is the principal source of such NMR signals in medical imaging.  
           [0003]    An image may be generated from the NMR signals by distinguishing among the locations of the source nuclear spins. In a conventional “slice select” imaging sequence, this is be done by limiting the RF excitation to a single slice in the x-y plane. Magnetic gradient fields are then applied along the transverse plane to modify the frequency and phase of the precession of the nuclear spins as a function of their location. A series of RF excitations with different x and y-axis gradient fields provides the data necessary to identify the contribution from nuclear spins in different locations to the NMR signal. The mapping of signal contribution to spin location provides the basis for an MRI image.  
           [0004]    The NMR data obtained after each RF excitation provides one line of data in “k-space”. Repetition of the RF excitation with different gradients provides different lines until an area is covered. The k-space area may then be converted to an image. A significant drawback to this other sequential acquisition of lines of k-space data is that the speed of generating in the image is severally limited.  
           [0005]    Several techniques have been developed which allow simultaneous acquisition of multiple lines of k-space data from spatially separated regions of the patient. These techniques generally use the spatial information that can be derived from multiple receiving loops placed on each patient which has a different reception pattern for receiving the NMR signal. This additional spatial information allows NMR signals from different locations to be distinguished even though the NMR signals may have the same phase and frequency. Such techniques include Simultaneous Acquisition of Spatial Harmonics (SMASH) and Sensitivity Encoding Technique (SENSE) imaging techniques known to those of skill in the art. These and other techniques that allow simultaneous acquisition of multiple lines of k-space data will henceforth be termed “parallel acquisition techniques”.  
           [0006]    Referring now to FIG. 1, a prior art coil  10  suitable for use with parallel acquisition techniques provides a generally cylindrical form  12  aligned with the longitudinal or z-axis. The outer circumference of the cylindrical form supports four, phased array loops  14   a  to  14   d.  The loops  14   a  to  14   d  may be generally rectangular and in pairwise opposition along the x-axis with each loop  14   a  to  14   d  extending approximately 180 degrees about the circumference of the cylindrical form  12 . Thus, the first pair of loops  14   a  and  14   b  are in opposition at one longitudinal end of the cylindrical form  12  and a second pair of loops  14   c  and  14   d  are in opposition about a second end of the form. Each of the loops  14   a  to  14   d  provides a separate signal transmission lead  16   a  to  16   d  so that NMR signals may be independently obtained from each coil and compared.  
           [0007]    Referring now to FIG. 2, the reception patterns  18   a  to  18   d,  respectively, of loops  14   a  to  14   d  differ along the longitudinal axis (principally in being displaced with respect to one another either transversely or longitudinally) causing each loop  14   a  to  14   d  to have different sensitivities to the NMR signal from a given spin, for example, spin  20   a.  This difference in reception patterns  18  allows parallel acquisition of NMR signals from longitudinally displaced spins  20   a  and  20   b  experiencing the same RF excitation and gradient fields and allows for parallel acquisition of spins in up to four separate regions. Such a coil  10  is said to have an acceleration of two in the transverse direction and an acceleration of two in the longitudinal direction.  
           [0008]    Considering now only loops  14   a  and  14   c  and two longitudinally displaced spins  20   a  and  20   b  within the reception patterns  18   a  and  18   c,  generally each of leads  16   a  and  16   c  will provide an NMR signal that is a combination of signals from spins  20   a  and  20   b.  For this simple case, the contributions of each spin  20   a  and  20   b  may be separated solving two equations relating the unknowns of contributions to known sensitivities of the two loops  14   a  and  14   c  (from their reception patterns  18   a  and  18   b ) and known signals from lines  16   a  and  16   b.  This process may be expanded for multiple spins and multiple coils using well-known algebraic techniques.  
           [0009]    The coil  10 , as shown in FIG. 1, may be further accelerated in the transverse plane by shortening the arc length of the loops  14  to of ninety degrees along the circumference of the cylindrical form  12  to provide four rather than two separate transverse regions of sensitivity. Unfortunately, limitations in the number of inputs in current MRI equipment for receiving separate leads  16  prevent the practical use of more than eight loops  14 . Thus, in this case, acceleration along the z-axis must be eliminated if a transverse acceleration of four is desired.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    The present invention provides a local coil using loops that have reception patterns that are locally sensitive to both longitudinal and transverse displacement of spins. In one embodiment, these loops are triangular. By using loops that can provide either longitudinal or transverse acceleration, a more versatile coil is created. Thus, for example, the present invention can produce in theory a coil providing six transverse acceleration or two longitudinal acceleration requiring only six loops and six leads to the MRI machine.  
           [0011]    Specifically, the present invention provides an MRI coil for use in a polarizing longitudinal magnetic field and having at least two loops transversely adjacent across an interface extending in part along the longitudinal axis. The interface is angled with respect to the longitudinal axis. Signal leads attached to the loops separately conduct signals received from each loop to processing circuitry.  
           [0012]    Thus, it is one object of the invention to provide a coil with loops that can function for both transverse and longitudinal acceleration.  
           [0013]    The two loops may be triangles, for example, right triangle or isosceles triangles.  
           [0014]    Thus, it is another object of the invention to provide a loop shape that may be simply fabricated and designed.  
           [0015]    The triangles may tile to fill a rectangular area.  
           [0016]    Thus, it is another object of the invention to provide a loop shape readily adaptable to the rectangular surfaces of unwrapped cylinders or planes commonly used in local coils.  
           [0017]    The loops may conform generally to the surface of a cylinder having an axis of radial symmetry parallel to the longitudinal axis or may conform to the surface of a plane parallel to the longitudinal axis or may conform to opposed surfaces of a rectangular prism extending along the longitudinal axis.  
           [0018]    Thus, it is another object of the invention to provide a design adaptable to a wide variety of common coil topologies.  
           [0019]    These particular objects and advantages may apply to only some embodiments falling within the claims, and thus do not define the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0020]    [0020]FIG. 1 is a perspective view of a simplified, prior art, multi-loop phased array coil useful for parallel acquisition techniques;  
         [0021]    [0021]FIG. 2 is a cross-sectional view of the coil of FIG. 1 taken along lines  2 - 2  showing the different reception patterns of the loops of the coil of FIG. 1 as aids in parallel acquisition of NMR data;  
         [0022]    [0022]FIG. 3 is a figure similar to that of FIG. 1 showing the coil design of the present invention using right triangular loops;  
         [0023]    [0023]FIG. 4 is a view of the surface of the cylindrical coil of FIG. 3 unwrapped, showing the angle interface between the loops such as provides for z-axis sensitivity;  
         [0024]    [0024]FIG. 5 is a figure similar to that of FIG. 4 showing an alternative design using loops that are isosceles triangles;  
         [0025]    [0025]FIG. 6 is a figure similar to that of FIG. 4 showing a six-channel coil where alternate loop pairs have the opposite orientation of their neighbors;  
         [0026]    [0026]FIG. 7 is an alternative embodiment in which the loops are parallelograms;  
         [0027]    [0027]FIG. 8 is a perspective view of a coil constructed of opposed planar elements using the present invention design; and  
         [0028]    [0028]FIG. 9 is a planar view of an alternative loop design using truncated triangles. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]    Referring now to FIG. 3, in a first embodiment of the present invention a cylindrical coil form  30  suitable for head or torso imaging may provide for six loops  14   a  to  14   f  positioned on the circumference of the cylindrical coil form  30 . Each of the loops  14   a  to  14   f  is a right triangle having one base aligned with a first or second end of the cylindrical coil form  30  (in alternating fashion), and a second base aligned with the longitudinal axis or z-axis  11 .  
         [0030]    The hypotenuse of each loop  14  is opposed to the hypotenuse of an adjacent loop  14  so that the loops  14   a  to  14   f  are paired. Thus, in this example, loops  14   a  and  14   f  are paired together to cover a rectangular area. Similarly, loops  14   b  and  14   c , and loops  14   d  and  14   e  are paired and cover rectangular areas all of which tile smoothly around the circumference of the cylindrical coil form  30 . The terms triangle and rectangle, as used herein are both triangles and rectangles on a plane and those on a curved surface such as the surface of a cylinder.  
         [0031]    The loops  14   a  to  14   f  may be, for example, copper foil attached to an insulating cylindrical coil form  30  of fiberglass or the like. Cutouts  37  may be provided for access to the patient. The cylindrical coil form  30  may be rigid or flexible and may be covered with a housing or fabric or other material as is understood in the art. Tuning capacitors  32  may be placed in series with the conductors of the loops  14   a  to  14   f  to tune the loops  14   a  to  14   f  into resonance at the resonant frequency of the MRI with which the coil  10  will be used. Leads  16   a  to  16   f  may be attached to the loops  14   a  to  14   f  to provide six independent channels of MRI data.  
         [0032]    Referring now to FIG. 4, an interface  34  between the opposed hypotenuses of the pairs of loop  14   a  to  14   f,  for example, between loops  14   a  and  14   f  or between loops  14   c  and  14   b  or between loops  14   e  and  14   d,  is angled with respect to the longitudinal or z-axis  11 . For coils  10  with some overlap between the loops  14   a  to  14   f  or where the opposing conductors of the  14   a  to  14   f  are not parallel, the interface  34  is a line defined by the locus of points midway between the adjacent loop walls.  
         [0033]    The triangular shape of the loops  14   a  to  14   f  in coil  10  results in a fundamental change in the area of the loops  14   a  to  14   f  as one moves along the z-axis  11  changing the sensitivity of the loops  14   a  to  14   f  to nearby spins as a function of their location along the z-axis  11 . Specifically, as a spin moves in the z-axis  11  downward in FIG. 4, the signal received by loop  14   a  will decrease while the signal received by coil  14   f  will increase. One feature of this embodiment is that for each loop  14  exhibiting a decreasing sensitivity with movement of spins along the z-axis in a first direction, there is a corresponding loop  14  that provides an increased sensitivity to those spins for the same axial movement.  
         [0034]    Referring now to FIG. 5, a variation on the coils of FIGS. 3 and 4 uses isosceles triangles as loops  14   a  to  14   f.  Here bases of the isosceles triangles for alternate loops  14   a  to  14   b  are on alternate edges  36  of the cylindrical coil form  30  and the equal length sides abut to define the interfaces  34 . Unlike the previous example, where the loops  14   a  to  14   f  will tile to cover a rectangular area, loops  14   a  to  14   f  of FIG. 5 will tile only to fill a cylindrical ring. Note however, that unlike the coils of FIGS. 3 and 4, no interfaces  34  are parallel to the z-axis  11 .  
         [0035]    Referring now to FIG. 6 in yet a further embodiment of the coil  10 , the loops  14  may be eight right triangular elements providing for loops  14   a  to  14   h.  In this case, adjacent coil pairs exhibit mirror symmetric with respect to their neighbors. The interfaces  34  of adjacent pairs of loops are nonparallel. This coil requires an MRI machine that can accept eight inputs or the use of a multiplexer.  
         [0036]    In yet a further embodiment shown in FIG. 7, the loops  14  are parallelograms providing tipped interfaces  34  between transversally adjacent coils  14 . Unlike the previous embodiments, this embodiment produces an ambiguous zone near the center of the coil  10  where z-axis location of simultaneously excited spins cannot be distinguished.  
         [0037]    Referring to FIG. 8, the present invention may be extended to coils  10  having a single planar or opposed planar loops. In the embodiment of FIG. 8, four loops  14   a  to  14   d  are arranged pairwise in two planes with their hypotenuses adjacent to define the interface  34 . Pairs of planes are offset as if on the surface of opposed faces of a rectangular prism.  
         [0038]    Referring to FIG. 9, the present invention may be extended to coils  10  having loops  14   a  and  14   b  with a perimeter that conforms to a quadrilateral polygon in which opposite sides converge and in which opposed parallel bases are of substantially different lengths so as to create in this case, but not be limited to, a truncated triangle. Pairs of loops  14   a  and  14   b  of this embodiment may be used in the above-described coils.  
         [0039]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.