Abstract:
A phased array coil system is presented for use with a magnetic resonance system. The phased array coil system includes a first coil, a second coil, and an interface subsystem. The first coil defines a first region and the second coil defines a second region, with the first coil partially overlapping the second coil to define an overlap region formed by the intersection of the first and second region. Operably connected with the first and second coils, the interface subsystem includes (i) a power splitter for splitting radio frequency (RF) power for delivery to the first and second coils and (ii) a phase compensator for adjusting the phase relationship of the RF power delivered to the first and second coils so that a magnetic field produced thereby in the overlap region is approximately equal to that produced near the center of each of the first and second regions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application for patent is a continuation of U.S. application Ser. No. 10/151,491, filed May 20, 2002 now U.S. Pat. No. 6,714,013, which is a continuation of U.S. application Ser. No. 09/776,132, filed Feb. 2, 2001, now U.S. Pat. No. 6,396,273, which is a continuation of U.S. application Ser. No. 09/512,093, filed Feb. 24, 2000, now abandoned, which is a divisional of U.S. application Ser. No. 08/979,842, filed Nov. 26, 1997, now U.S. Pat. No. 6,040,697, the contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to the field of magnetic resonance imaging (MRI) systems and, more particularly, to coils for use in such systems. 
   It is well known in the field of MRI systems to provide radio frequency signals in the form of circularly polarized or rotating magnetic fields having an axis of rotation aligned with a main magnetic field. It is also well known to use receiving coils to intercept a radio frequency magnetic field generated by a human subject or an object in the presence of the main magnetic field in order to provide an image of the human subject or the object. 
   Receiving coils of this type were formed as volume coils in order to enclose a volume for receiving a body part such as a leg, arm or hand and intercept the radio magnetic field. See, for example, U.S. Pat. No. 4,411,270 issued to Damadian and U.S. Pat. No. 4,923,459 issued to Nambu. Additionally, surface receiving coils were developed for this purpose. The surface receiving coils were placed adjacent a region of interest. For a surface receiving coil, see U.S. Pat. No. 4,793,356 to Misic et al., for example. 
   Advances in the field of MRI systems have resulted in modifications to both volume receiving coils and surface receiving coils in order to improve their signal to noise ratios. This was achieved by modifying the receiving coils to receive perpendicular components of the radio frequency magnetic field. These improved coils are known as quadrature coils. Quadrature coils provided a significant signal to noise ratio improvement over non-quadrature coils. See, for example, U.S. Pat. No. 4,467,282 issued to Siebold and U.S. Pat. No. 4,707,664 issued to Fehn. 
   In U.S. Pat. No. 5,258,717, issued to Misic, a quadrature receiving coil system was provided, along with a data acquisition system. The data acquisition system taught by Misic included multiple image processing channels for processing a plurality of MRI signals and combining the processed signals to produce an image. The receiving coil system of Misic was formed of multiple quadrature receiving coils, the receiving coils being adapted to intercept both of the quadrature components of the magnetic resonance signals in a spatially dependent manner. Such quadrature coil systems provided coverage of a portion of a total target sensitive volume along an axis parallel to the main magnetic field. Consequently, each receiving coil of the system had a sensitive volume smaller than that which would otherwise be necessary. Thus, each receiving coil provided an improved signal to noise ratio for the region within its sensitive volume. Two leads were connected to each receiving coil and each lead was connected to a separate processing channel of the data acquisition system. The outputs of the processing channels were combined and a final data set from the entire target sensitive volume was calculated. The calculated data set had a better signal to noise ratio greater than that which could be achieved with a single receiving coil. 
   However, the various receiving coils of the prior art described had a number of artifact problems. For example, an image provided using the prior art receiving coils could have artifacts due to aliasing caused when the phase of a signal from a part of the anatomy within the field of the coil duplicates that of a location elsewhere. This occurs because a phase location of 370 degrees appears to the system as a phase location of 10 degrees. Thus, a signal from anatomy at a phase location of −350 or 370 degrees manifests itself in the image at a phase location of 10 degrees within the field of view. Elimination of phase wrap essentially halves the actual phase field of view, shifting from −90 to +90 degrees rather than from −180 to +180 degrees. However, this merely moves the alias location to more than +/−1.5 the field of view rather that eliminating it. 
   Another form of artifact, sometimes referred to as an annafact, can occur in either the frequency direction or the phase direction within prior art MRI systems. In this type of artifact, an area of anatomy that is at least partially within the excitation field of the body coil has a local Larmour frequency identical to a pixel within the imaging field of view. If there is any excitation and subsequent pickup of this material, it appears within the field of view superimposed upon the desired image, regardless of whether the artifact comes in from the frequency direction or the phase direction. The problems associated with this type of artifact are worsened by the use of higher speed gradients that are shorter in physical size and lower field uniformity. 
   It is therefore an objective of the invention to provide a coil system and/or method that eliminates soft tissue artifacts and aliasing artifacts typical of prior art systems and/or methods for imaging various regions of interest. 
   Another objective of the present invention is to provide improved signal to noise performance, for example, by permitting the use of smaller fields of view and thinner slices when performing imaging. 
   Another objective of the present invention is to provide greater image uniformity than provided in the prior art. 
   Another objective of the invention is to facilitate complete imaging of the regions of interest during a magnetic resonance imaging (MRI) procedure. 
   SUMMARY OF THE INVENTION 
   In a preferred embodiment, the invention provides a phased array coil system for use with a magnetic resonance imaging (MRI) system. The phased array coil system includes a first coil, a second coil, and an interface subsystem. The first coil defines a first region and the second coil defines a second region, with the first coil partially overlapping the second coil to define an overlap region formed by the intersection of the first and second regions. Overably connected with the first and second coils, the interface subsystem includes (i) a power splitter for splitting radio frequency (RF) power for delivery to the first and second coils and (ii) a phase compensator for adjusting the phase relationship of the RF power delivered to the first and second coils so that a magnetic field produced thereby in the overlap region is approximately equal to that produced near the center of each of the first and second regions. 
   In a related embodiment, the invention provides a phased array coil system for use with a magnetic resonance imaging (MRI) system. The phased array coil system includes a first coil, a second coil, and an interface subsystem. The first coil defines a first region and the second coil defines a second region, with the first coil partially overlapping the second coil to define an overlap region formed by the intersection of the first and second regions. Operably connected with the first and second coils, the interface subsystem includes (i) a power splitter for splitting radio frequency (RF) power for delivery to the first and second coils and (ii) a phase compensator for adjusting the phase relationship of the RF power delivered to the first and second coils to cause partial destructive/constructive interference thereof in the overlay region so that a magnetic field produced thereby in the overlay region is approximately equal to that produced near the center of each of the first and second regions. 
   In a further related embodiment, the invention provides a transmit/receive (T/R) phased array coil system for use with a magnetic resonance imaging (MRI) system. The T/R phased array coil system includes a first birdcage coil, a second birdcage coil, and an interface subsystem. The first birdcage coil encompasses a first region, the second birdcage coil encompasses a second region, with the first and second birdcage coils defining an overlap region in which one of the birdcage coils is partially overlapped by the other of the birdcage coils to form a phased array coil subsystem. Connected to the phased array coil subsystem, the interface subsystem includes a power splitter, an attenuator, a phase compensator, and a plurality of switches for enabling the interface subsystem to be switched between a transmit state and a receive state. In the transmit state, the power splitter allocates radio frequence (RF) power received from the MRI system between the first and second birdcage coils with the attenuator reducing the RF power directed to at least one of the first and second birdcage coils so that (A) a first magnetic field is applied through the first birdcage coil to the first region encompassed thereby and (B) a second magnetic field is applied through the second birdcage coil to the second region encompassed thereby with the phase compensator affecting a phase relationship between the first and second magnetic fields so that a resulting magnetic field produced thereby in the overlay region is approximately equal to the first and second magnetic fields produced near the center of the first and second regions, respectively. In the receive state, the interface subsystem receives from the phased array coil subsystem a response of an anatomical structure placed therein and conveys the response to the MRI system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a prior art multiple quadrature receiving coil system wherein each coil of the quadrature coil system is a birdcage coil. 
       FIG. 2  is a perspective view of a single quadrature birdcage coil forming part of the prior art multiple quadrature receiving coil system of FIG.  1 . 
       FIG. 3  is a schematic diagram illustrating possible electrical connections of the prior art multiple quadrature receiving coil system of FIG.  1 . 
       FIG. 4  is an exploded perspective view of a first preferred embodiment of the multiple quadrature receiver/transmitter coil system of the present invention. 
       FIG. 5  is a schematic diagram of a second preferred embodiment of the present invention illustrating one possible way in which the transmit/receive phased array coil system of the invention can be electrically connected to a magnetic resonance imaging (MRI) system. 
       FIG. 6  is a side view of a preferred embodiment of the multiple quadrature receiver/transmitter coil system of the present invention similar to that shown in FIG.  4 . 
       FIG. 7  is a side view of a further preferred embodiment of the multiple quadrature receiver/transmitter coil system according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIGS. 1 and 2 , there is shown a prior art multiple quadrature receiving coil system  10 . This prior art multiple quadrature receiving coil system  10  was formed of quadrature receiving coils  12  and  14 , which were designed for a variety of anatomical regions of the body, such as the knee, leg, arm or head. Quadrature receiving coils  12 ,  14  of quadrature receiving coil system  10  are thus volume coils. Coils  12 ,  14  are disposed around a hollow cylindrical drum support member  16 . Support rods  18  extending the length of cylindrical drum  16  can be provided to stabilize the cylindrical drum support member  16 . 
   Quadrature receiving coils  12 ,  14  are of a type referred to as birdcage coils, as well known in the art. They are formed of circular conductive loops  20 ,  22  connected to each other and spaced apart from each other by conductive connection members  24 . There may be eight electrically conductive connection members  24  or rods  24  joining circular conductive loops  20 ,  22 . Each receiving coil  12 ,  14  of coil system  10  formed in this manner can function as a separate quadrature receiving coil within coil system  10 . 
   Magnetic interaction between quadrature receiving coils  12 ,  14  is eliminated by positioning quadrature coils  12 ,  14  about drum support member  16  to provide radial symmetry about the axis parallel to the main magnetic field, which is the Z-axis shown in FIG.  1 . Moreover, quadrature coil  12  is slightly larger in diameter than quadrature coil  14  so that coil  12  can overlap a portion of coil  14 . The amount of overlap of coils  12 ,  14  is adjusted so that the net flux from one coil, as seen as the vector sum of the flux from the overlap region, exactly cancels the flux from the return through the balance of the coil. In this case the field vector of the overlap region can be substantially equal to the field vectors of the other two regions. This causes the net shared flux of zero and thus a net mutual inductance of zero between quadrature receiving coils  12 ,  14 . Coils  12 ,  14  maintain their isolation regardless of the relative rotational position about their common axis. 
   Quadrature coils  12 ,  14  of receiving coil system  10  have symmetry about two planes parallel to the Z-axis. The two planes of symmetry are at right angles to each other. In addition, quadrature coils  12 ,  14  are arranged so that the rotating magnetic vector of each receiving coil  12 ,  14  is in the X-Y plane. However, the net rotating magnetic vectors of coils  12 ,  14  are spatially displaced from each other along the Z-axis. In this regard, each receiving coil  12 ,  14  intercepts the quadrature components of the magnetic resonance signal within its own sensitive volume. 
   Referring now to  FIG. 3 , there is shown a schematic representation of electrical connections that can be made to quadrature receiving coils  12 ,  14  of prior art quadrature receiving coil system  10 . In this system, a plurality of electrical leads  30   a, b  are connected to quadrature coil  14  and a plurality of electrical leads  32   a, b  are connected to quadrature coil  12 . The electrical leads  30   a, b  and  32   a, b  are preferably coaxial leads. Each coaxial lead  30   a, b  and  32   a, b  thus includes a center conductor and a shield, as is well known in the art. Each coaxial lead  30   a, b  and  32   a, b  coupled to quadrature coils  12 ,  14  is connected to a respective port  1 - 4  of data acquisition system  34 . Data acquisition system  34  is described in an article by Roemer et al., entitled “The NMR Phased Array,” Magnetic Resonance in Medicine, vol. 16 (1990), pp. 192-225. System  34  is provided with multiple data processing channels  36 - 42 . Each data processing channel  36 - 42  includes an individual amplifier, filter, and A/D converter for processing the image signals received by a corresponding coaxial lead  30   a, b  or  32   a, b.  The outputs of data processing channels  36 - 42  are multiplexed by multiplexer  44  and combined by microprocessor  46  according to a weighting algorithm. The weighting algorithm is adapted to select the outputs of processing channels  36 - 42  and combine them to produce an overall image signal. For example, a combined image signal having a maximum signal to noise ratio can be provided by the weighting algorithm performed by microprocessor  46 . 
   Coaxial leads  30   a, b  are connected to quadrature coil  14  at points that are at ninety degrees relative to each other and to ports  1 ,  2  of data acquisition system  34 . Likewise, coaxial leads  32   a, b  are connected to quadrature coil  12  at points that are disposed at ninety degrees relative to each other. Coaxial leads  32   a, b  are also connected to ports  3 ,  4  of data acquisition system  34 . An electrical network (not shown) can be interconnected between quadrature receiving coils  12 ,  14  and coaxial leads  30   a, b  and  32   a, b  to appropriately connect the cables to quadrature coils  12 ,  14 . Such electrical networks are well known in the art. Furthermore, quadrature coils  12 ,  14  can be rotated (i.e., can accommodate different angular orientations) relative to one another without destroying the signal to noise improvements achieved in accordance with this prior art method. This is taught in U.S. Pat. No. 5,258,717 to Misic and incorporated by reference herein. 
   Referring now to  FIG. 4 , there is shown an embodiment of the magnetic receiver/transmitter coil array system  60  of the present invention. Transmit/receive coil array system  60  can be formed of two quadrature birdcage coils: outer quadrature coil  68  and inner quadrature coil  64 . Inner quadrature coil  64  can be disposed partially within the volume defined by outer quadrature coil  68 . The quadrature coils  64 ,  68  can thus be provided with a critical overlap to reduce the mutual inductance therebetween and to therefore reduce the signal coupling between the coils  64 ,  68  to a negligible level, in a manner substantially as described for coil array system  10 . In one possible preferred embodiment of coil array system  60 , inner quadrature coil  64  can have a diameter of approximately 19.4 centimeters and outer quadrature coil  68  can have a diameter of approximately 22.5 centimeters. While coil array system  60  is shown in an exploded view for purposes of illustration, it will be understood that inner quadrature coil  64  is disposed within outer quadrature coil  68  during normal operation of coil array system  60 . 
   In the preferred embodiment of coil array system  60  (as discussed in more detail below with respect to FIG.  6 ), inner coil  64  can be formed with eight electrically conductive rods  66  fixed to electrically conductive end rings  70   a,b.  Outer quadrature coil  68  can be formed of two sections: receive/transmit coil section  74  and auxiliary transmit coil section  72 . Receive/transmit section  74  can be provided with eight electrically conductive rods  82  fixed to electrically conductive end rings  80   a,b  which act cooperatively to define an array volume. Auxiliary transmit coil section  72  can be provided with eight electrically conductive rods  84  connecting electrically conductive end rings  80   b,c.  Rods  66 ,  82  and  84  as well as end rings  70   a,b  and  80   a,b,c  can be any kind of electrical conductors such as, for example, conductive tubing, etched copper or copper tape or any other material suitable for inducing and detecting a magnetic field. 
   Furthermore, each conductive rod  84  of auxiliary transmit coil section  72  can be provided with a PIN diode  88 . When PIN diodes  88  of auxiliary coil section  72  are forward biased, conductive rods  84  and ring  80   c  are electrically coupled to receive/transmit coil section  74 . Thus, PIN diodes  88  can be used to couple and decouple portions of rods  84  and thereby substantially couple and decouple transmit coil section  72  and transmit/receive coil section  74 . When conductive rods  84  or rod extensions  84  are switched in by PIN diodes  88  in this manner, the effective length of receive/transmit coil section  74  can be extended to thereby define a further array volume. PIN diodes  88  are forward biased and the length of coil section  74  is thereby extended in this manner when electrical energy is applied to outer coil  68  for the purpose of creating a magnetic field to form a magnetic resonance image. Although PIN diodes  88  can be used in the preferred embodiment for extending the length of outer coil  68 , any kind of coupling and decoupling circuit known to those skilled in the art can be used. 
   It will be understood that coil array system  60  can be used for left or right imaging of the musculoskeletal system of human subjects. For example, coil array system  60  can be used for imaging a knee, a foot, an ankle, a wrist or a hand. The anatomical structures that can be imaged and evaluated using coil array system  60  can include ligaments, tendons, cartilage, osseous structures, fluid filled bursa, adipose tissue, muscle and potential pathological lesions. Furthermore, coil array system  60  is adapted to permit easy placement of the anatomy of interest within the array volume defined by coils  64 ,  68  and adapted to be disposed on a base to permit positioning left and right of isocenter. 
   Referring now to  FIG. 5 , there is shown a second preferred embodiment of the present invention including a coil interface subsystem  90  coupled to coils  64 ,  68 . The subsystem  90  applies energy to extended transmit/receive coils  64 ,  68  and receives the output of coils  64 ,  68  to form images of selected regions of interest. Within subsystem  90 , electrical energy is received from a conventional transmitter port by splitter  98  for the purpose of applying a magnetic field to the region of interest by coil  64 , coil  68  or both. 
   Energy from splitter  98  is applied to phase compensator  108  to obtain the correct phase relationship between the fields of coils  64 ,  68 . The output of phase compensator  108  is applied to a ninety degree element  104 . The outputs of the ninety degree element  104  are applied to isolated contact points  91 , displaced ninety degrees from each other, by way of leads  92  and socket  96 . Disposing contact points  91  at ninety degrees with respect to each other causes the orthogonal components of the rotating magnetic field signal to be completely received within coil system  64 ,  68 . 
   Because the energy required by larger coil  68  may be more than the energy required by smaller coil  64 , the energy supply path of smaller coil  64  is provided with attenuator  102  at the output of splitter  98 . Additionally, a ninety degree element  104  is provided as previously described. The four outputs of the ninety degree elements  104  thus determine relative amplitudes and phases for driving the inputs of coil system  60  with the appropriate power levels and signal phases to provide the most uniform transmit field possible. For coil  64 , energy from the outputs of the ninety degree element  104  is applied to coil  64  at contact points  93  by way of socket  96  and leads  94 . Contact points  93  are disposed ninety degrees from each other as described with respect to contact points  91 . In this manner, coil system  60  is provided with two quadrature pairs separated spatially along the Z-axis. Additionally, the voltage level and the phase applied to coils  64 ,  68  can be adjusted to provide a uniform field. Coils  64  and  68  can be crossed saddle quadrature coils or Helmholtz pairs. 
   It will be understood that alternative arrangements of attenuation and phase compensation can be used to obtain the required results. For example, both the attenuation and the phase compensation can be applied to one of the coils  64 ,  68  without any additional attenuation or phase compensation being applied to the other coil  64 ,  68 . For example, the attenuation and phase compensation can be applied to the path of inner coil  64  only. Furthermore, if inner coil  64  serves as a receive only coil without serving as a transmit coil, then transmit power is applied only to outer coil  68 . In this case, the transmit power can be applied to contact points  91  by way of a ninety degree element without necessarily using any additional attenuation or phase compensation. Further in this case, inner coil  64  does not require transmit decoupling. 
   Referring now to  FIG. 6 , there is shown a side view of magnetic receiver/transmitter coil array system  150  of the present invention. Magnetic receiver/transmitter coil array system  150  is a preferred embodiment of the system of the present invention. Outer quadrature coil  154  and inner quadrature coil  162  are provided within magnetic receiver/transmitter coil array system  150  for performing substantially similar operations as those described with respect to coil array system  60 . 
   For example, outer quadrature coil  154  is formed of coil sections  156 ,  160  wherein conductor rods  168  of coil section  156  are provided with PIN diodes  164 . When PIN diodes  164  of coil section  156  are forward biased during transmission, the effective length of outer quadrature coil  154  is extended to be equal to the combined lengths of coil section  156  and coil section  160 . 
   Each of the conductive rods  66 ,  82  and  84  of coil array system  60  can be provided with an adjustable tuning capacitor located at its mechanical center. The use of tuning capacitors in this manner is well known in the art and is not shown in order to simplify the drawings. The value of the tuning capacitors can be selected to allow each conductive rod  66 ,  82  and  84  to resonate at 63.87 MHz. A variable capacitor can be provided between the conductive rods containing the output contacts  91 ,  93 . The additional variable capacitor can be used to optimize the isolation of the quadrature outputs. 
   A network for impedance matching the real part of the coil impedance to 50Ω through the two lattice baluns can be provided for each of the four conductive rods  66 ,  82  having contacts  91 ,  93 . This can be accomplished using a series capacitive divider and an impedance transformation in the baluns. Additionally, each output port  122  can be followed by a balancing network including two series connected lattice baluns that are resonant at 63.87 MHz. 
   As shown in  FIG. 5 , output lines  112  apply signals from coils  64 ,  68  to output ports  122  that can be coupled to a conventional four receiver (i.e., data acquisition) system. The physical length of output lines  112  is approximately 37 inches from its junction  120  to the receiver, corresponding electrically to ¼ wavelength. In the preferred embodiment, each output port  122  has a PIN diode  118  coupled to an output line  112 . PIN diodes  118  act as switches to connect coils  64 ,  68  to the receiver system during data acquisition and to disconnect the receiver system from the transmit port during the transmit stage. 
   Referring now to  FIG. 7 , there is shown magnetic receiver/transmitter coil array system  180 . Magnetic receiver/transmitter coil array system  180  is a preferred embodiment of the system of the present invention that includes outer coil element  184  and inner coil elements  186 ,  188 . In the embodiment of  FIG. 7 , inner coil elements  186 ,  188  function as both receive elements and transmit elements. Excitation can be applied to inner coil elements  186 ,  188  by means of inductive coupling from external loops added to the coil. For example, four such loops can be used to excite a quadrature field in each of inner coil elements  186 ,  188 . 
   As previously described with respect to coil system  60 , splitters  104  can be used to provide four outputs of a selectively determined relative amplitude and phase to drive the four loops added to the current coil design with the appropriate power levels and signal phases to provide the most uniform transmit field possible. PIN diode networks  192  can be used to isolate the coil elements from the coil during transmission. 
   Thus, quadrature receiving coil system  60  and its alternate embodiments provide an improvement over previous receiving coils when multiple means for processing image signals are available. The sensitive volume of the coil system is expanded allowing for the interception of both quadrature components of MR signals in a spatially dependent manner, with each coil providing coverage of a portion of the desired sensitive volume along the axis parallel to the main magnetic field. Consequently, each coil had a sensitive volume smaller than that which would otherwise be necessary and each such coil provided improved signal to noise ratio from the region within its sensitive volume. 
   The above description is intended by way of example only and is not intended to limit the present invention in any way, except as set forth in the following claims. For example, it is to be understood that the present invention is not limited to two coil systems. Rather, the present invention may be embodied as an N-quadrature coil system, where N is an integer, and where N processing means are available for each coil system. Furthermore, the present invention can include any method and system for adding transmit capability to a quadrature phased array coil element by extending one part of the coil in transmit only, transmitting with both coils with proper amplitude and phase, and using external local transmit coil elements.