Abstract:
An apparatus for imaging includes: a radio frequency (RF) coil array having a first RF coil and at least one additional RF coil, where the RF coil array is adapted to generate an image signal; a preamplifier having an input impedance, where the preamplifier is adapted to receive the image signal from the first RF coil; and a transformer to couple the first RF coil to the preamplifier, where impedance of the transformer is adapted to match the input impedance of the preamplifier.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to radio frequency (RF) coil arrays used in magnetic resonance imaging (MRI). Specifically, the invention relates to decoupling the coils in a RF coil array for MRI. 
     BACKGROUND 
     The elimination of inductive coupling is an important step for the use of RF coils arrays for MRI, such as in parallel imaging for MRI. For example, in nuclear magnetic resonance (NMR) imaging, if some effective mutual inductance remains among the coils in the coil array, the NMR signal obtained from one coil may disturb the flux in another coil, which may make it difficult to match and tune each circuit with a coil to the input impedance of the respective preamplifier circuit. 
     A common method to isolate the coils in the coil arrays avoids the build up of significant currents from the NMR signal among each of the coils in the coil array, in such way that the effects from the mutual inductances may be neglected. This may be conventionally achieved by connecting each coil to a circuit that should behave as an open circuit from its input port. A common method employed for this circuit implementation usually involves the use of a network transformer (or an impedance transformer) and preamplifiers with very low input impedance (typically &lt;2Ω) and decoupling networks with lumped elements. However, the construction of low noise figure preamplifiers with extremely low input impedance is not easy to accomplish. In addition, the use of preamplifiers with low input impedance imposes technical restrictions on coil design, especially when considering geometries that require overlapping loops where there is a significant amount of mutual inductance between the RF coils. 
     In addition, it is known that the coupling of coils to the matching network is usually a hard task to accomplish and is highly dependent on effects caused by sample loading. One such effect is produced by voltage differences in different parts of the coil that generate an electrostatic field around the coil. This electrostatic field may couple to the sample, causing dielectric losses and consequently a reduction of the received signal. Another undesirable effect is caused by standing waves that may be present in the cables that connect the coils to the preamplifier, which may feed back to the pickup coil or may also represent a radiation loss for the NMR signal. Conventionally, these disturbances in the signal-to-noise ratio may be improved by reducing the dielectric losses and also by reducing the currents in the ground loops that may provide resistive and radiation losses. 
     Another technique employed to decouple RF coils in an array attempts to cancel the flux between any two coils in the array. However, canceling flux is not an efficient method for isolating non-adjacent neighboring coils. Further, canceling flux tends to degrade the NMR signal by inevitable insertion of losses on coils and, moreover, may not work for arbitrary coil geometries. 
     SUMMARY 
     One embodiment of the invention includes an apparatus for imaging including: a RF coil array having a first RF coil and at least one additional RF coil, the RF coil array adapted to generate an image signal; a preamplifier having an input impedance, the preamplifier adapted to receive the image signal from the first RF coil; and a first transformer to couple the first RF coil to the preamplifier, impedance of the first transformer to match the input impedance of the preamplifier. 
     One embodiment of the invention includes a method for imaging, including: obtaining an image with a RF coil array including a first RF coil and at least one additional RF coil, the first RF coil providing an image signal; passing the image signal from the first RF coil through a first transformer to obtain a transformed signal; matching impedance of the first transformer to an input impedance of a preamplifier; and amplifying the transformed signal with the preamplifier. 
     One embodiment of the invention includes an apparatus for imaging, including: a RF coil array including a plurality of RF coils, the RF coil array adapted to generate an image signal, the RF coils being inductively decoupled from each other; and a plurality of preamplifiers, each preamplifier adapted to receive the image signal from a respective RF coil, a ground for each preamplifier being electrically isolated from a ground for each respective RF coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of various embodiments of the invention will be apparent from the following, more particular description of such embodiments of the invention, as illustrated in the accompanying drawings, wherein like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The left-most digit in the corresponding reference number indicates the drawing in which an element first appears. 
         FIG. 1  illustrates a circuit diagram according to an exemplary embodiment of the invention. 
         FIG. 2A  illustrates a circuit diagram according to an exemplary embodiment of the invention. 
         FIG. 2B  illustrates a circuit diagram according to an exemplary embodiment of the invention. 
         FIG. 3  illustrates a circuit diagram according to an exemplary embodiment of the invention. 
         FIG. 4  illustrates a circuit diagram according to an exemplary embodiment of the invention. 
         FIG. 5A  shows individual fast low-angle shot (FLASH) images of coils in an exemplary four coil array. 
         FIG. 5B  shows the sum of squares reconstruction for the images of  FIG. 5A . 
         FIGS. 6A ,  6 B, and  6 C show images acquired with generalized autocalibrating partially parallel acquisition (GRAPPA) reconstruction at different acceleration factors for the exemplary four coil array. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the invention are discussed in detail below. While specific exemplary embodiments are discussed, it should be understood that this is done for illustration purposes only. In describing and illustrating the exemplary embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the invention. It is to be understood that each specific element includes all technical equivalents that operate in a similar manner to accomplish a similar purpose. Each reference cited herein is incorporated by reference. The examples and embodiments described herein are non-limiting examples. 
     According to the exemplary embodiments of the invention, the RF coils in a RF coil array of a MRI system are inductively decoupled from each other. Each RF coil is coupled to its respective preamplifier with at least one transformer. A nearly perfect balanced signal is transferred to the preamplifier from the RF coil, and at the same time, the RF coil becomes electrically isolated from the preamplifier and its associated electronics. The electrical isolation may reduce some of the undesired effects of ground loops and parasite signals that typically appear in capacitively-coupled networks. The relationship between the turn ratio for the transformer and the input impedance of the preamplifier determines the decoupling intensity between the RF coils and also the signal-to-noise ratio for the amplified signal output from the preamplifier. 
     With the invention, the RF coils may be positioned according to different geometrical configurations, such as in a parallel arrangement, an overlapped mode, separated by gaps, or even stacked on top of each other. 
     With the invention, each preamplifier is connected to a separate receive channel of the MRI system, and the channels may be accessed independently and simultaneously, making it possible to perform parallel imaging acquisitions. 
     The MRI system may be also used for magnetic resonance spectroscopy (MRS). Other imaging embodiments may further be used with the invention. 
       FIG. 1  illustrates a circuit diagram according to an exemplary embodiment of the invention. The MRI imaging system includes, for example, an RF coil array having at least two RF coils L 1  and L 2 . The coil array  8  may include additional coils (not shown). Each coil in the coil array is coupled to its own respective preamplifier. Coil L 1  is coupled to preamplifier P 1 , and coil L 2  is coupled to preamplifier P 2 . The two coils L 1  and L 2  may have different inductances and may have a mutual coupling M. 
     The network circuit  10  for coil L 1  includes a capacitor C 12  connected in parallel to a transformer T 1 . This parallel circuit is connected in series through a capacitor C 11  to coil L 1 . The transformer T 1  couples coil L 1  to its respective preamplifier P 1 . The transformer T 1  is coupled to coil L 1  through its primary coil  12  and to the preamplifier P 1  through its secondary coil  13 . The secondary coil  13  of transformer T 1  is connected in series through a capacitor C 13  to the preamplifier P 1 , which has an input impedance of Zp 1 . The primary and secondary coils  12 ,  13  of transformer T 1  may have a turn ratio of n:1, where n is greater than 0, including fractional numbers. Through the capacitor C 13 , the impedance of the secondary coil  13  of transformer T 1  matches the input impedance Zp 1  of the preamplifier P 1 . The transformer T 1  has a coupling coefficient K 1 . The network circuit  11  for coil L 2  is similar to the network circuit  10  for coil L 1  and couples coil  12  to the preamplifier P 2  via the transformer T 2 . The impedance of the secondary coil  13  of the transformer T 2  matches the impedance of the preamplifier P 2 . The ground GND 3  of circuit  10  is electrically isolated from the ground GND 2  of circuit  11  and from the ground GND 1  of the preamplifier P 1 . The preamplifiers P 1  and P 2  are connected to the ground GND 1 . 
     The turn ratio of transformers T 1  and T 2  may be selected to minimize noise for the preamplifiers P 1  and P 2 , respectively. A number of winding in the primary coil of the transformer may be equal to, greater than, or fewer than a number of windings in the ns secondary coil of the transformer. Further, the isolation limits for coils L 1  and L 2  in coil array  8  may be enhanced by using high input impedance for the preamplifiers P 1  and P 2 , respectively. 
     The transformers T 1  and T 2  are passive transformers. The transformer T 1 , T 2  may include helical windings, solenoidal windings, or stripline baluns. The transformer T 1 , T 2  may include an electrostatic shield comprised of a conducting material. The transformer T 1 , T 2  may utilize superconducting materials. 
     The signal that reaches the preamplifier P 1  is coupled inductively to coil L 1  through the primary coil of the transformer T 1 . Because the primary and secondary coils of the transformer T 1  are isolated, the preamplifier circuit (and the MRI scanner electronics connected thereto (not shown)) are electrically isolated from coil L 1 . This arrangement provides an electrical balance and isolation between the channels of the coil array. With the Invention, traps and baluns in the circuits  10  and  11 , a used conventionally, may be unnecessary. Further, the inductively coupled inventive technique employs a transformer to match each coil in the coil array to its respective preamplifier. The inventive technique provides a balanced structure that may reduce dielectric losses and may isolate the lossy currents in ground loops. 
     Referring to the exemplary circuit diagram of  FIG. 1 , it may be shown that the impedance of the circuit  10  as seen from the circuit  11 , may be given as: 
                   Z   =       R   2     +           ω   2     ·     M   2     ·     L   2           R   1     +         n   2     ·     Z     p   ⁢           ⁢   1           K   1   2           .               (   1   )               
where ω is the NMR Larmor frequency, M is the inductive coupling between L 1  and L 2 , L is the inductance of coil L 2 , R 1  is the resistance of coil L 1 , R 2  is the resistance of coil L 2 , K 1  is the coupling coefficient for the transformer T 1 , n is from the turn ratio n:1 between the primary and secondary coils  12 ,  13  of the transformer T 1 , and Zp 1  is the input impedance of the preamplifier P 1 . In order to achieve high isolation between coils L 1  and L 2 , Z should be made as close as possible to R 2 . Thus, the second term in equation (1) should be made as negligible as possible. This can be accomplished if both n and Zp 1  are chosen to be high.
 
     The exemplary embodiment, described above with reference to  FIG. 1 , was tested in a 7 T/30 cm Bruker Avance MRI system. A four-coil array for imaging a rat brain was built using the circuit described with reference to  FIG. 1 . A positive-intrinsic-negative (PIN) diode circuitry (not shown in the circuit of  FIG. 1 ) was incorporated to allow decoupling of the coil array from the transmit coil. The coil array was connected to regular 50 Ω input impedance preamplifiers. The transformers were made very small with a 7:1 turn ratio between the primary coil and the secondary coil. No trap or baluns were employed in the circuit array. The isolation level for this configuration is described by equation (1), and this is equivalent to the isolation achieved when using low input impedance preamplifiers, with, for example, 1Ω input impedance. 
     The measured isolation between the channels in the test was better than 45 dB.  FIG. 5A  shows individual FLASH axial images obtained with each coil, and  FIG. 5B  shows a combined image obtained using a sum of squares reconstruction from the four images shown in  FIG. 5A . As can be seen by comparing the individual images in  FIG. 5A  to the combined image in  FIG. 5B , no significant coupling is observed between the RF coils.  FIGS. 6A ,  6 B, and  6 C show FLASH images acquired with the GRAPPA acquisition scheme at three different acceleration factors (AF), 1, 2, and 2.91, respectively. Reconstruction artifacts are only noticeable for AF=2.91 in  FIG. 6C  in the form of aliasing banding along the phase encoding directions. The inductive decoupling between the different channels shows good performance with excellent isolation of all four channels and immunity to standing waves or other parasitic signals. 
       FIG. 2A  illustrates a circuit diagram according to an exemplary embodiment of the invention. The circuit diagram of  FIG. 2A  depicts another technique to inductively decouple the coils L 1  and L 2 , as discussed above for the exemplary embodiment of  FIG. 1 . The circuit diagram in  FIG. 2A  depicts network circuit  20 , for example, a balun circuit, for coupling the coil L 1  to the preamplifier P 1  via the transformer T 1  and depicts network circuit  21 , for example, a balun circuit, for coupling the coil L 2  to the preamplifier P 2  via the transformer T 2 . The balun circuit  20  for coil L 1  includes a capacitor C 20 , capacitors Ca and Cb, inductors La and Lb, a capacitor Cm 21 , and the transformer T 1 . The balun circuit  20  converts the low impedance of the transformer T 1  into a high impedance for the coil L 1 . The capacitor Cm 22  matches impedance of the transformer T 1  to the input impedance Zp 1  of the preamplifier P 1  via the secondary coil  22  of transformer T 1 . The circuit  21  for coil L 2  is similar to the circuit  20  for coil L 1  and matches impedance of the coil L 2  to the impedance of the preamplifier P 2 . The ground GND 3  of circuit  20  is electrically isolated from the ground GND 2  of circuit  21  and from the ground GND 1  of the preamplifier P 1 . 
     Referring to an exemplary circuit diagram of  FIG. 2A , it may be shown that the impedance of circuit  20  as seen from the resistive impedance of L 1  is given by: 
                   Z   =       n   2     ·     1     Z     p   ⁢           ⁢   1         ·     L   C               (   2   )               
where n is the turn ratio between the secondary and the primary coils of the transformer T 1 , Zp 1  is the impedance of the preamplifier P 1 , L is the value for the inductances La or Lb (La=Lb=L), C is the value for the capacitor Ca or Cb (Ca=Cb=C). Equation (2) assumes that the primary and secondary resistances of the transformer T 1  are negligible and also that the coupling coefficient K 1  for the transformer T 1  is 1. For best decoupling of the coils L 1  and L 2 , the impedance load in circuit  20  as seen from coil L 1  should be high, i.e., Zp 1  should be made as large as possible. According to equation (2), the presence of the transformer T 1  in circuit  20  enhances the impedance seen from coil L 1  by a factor of n 2 .
 
     With reference to  FIG. 2B , this exemplary embodiment is similar to an exemplary embodiment of  FIG. 2A  except that each balun circuit  20 ,  21  in  FIG. 2A  is replaced by a coaxial cable  24 ,  25  with a length substantially equal to one fourth of a wavelength at the frequency of operation. The coaxial cable  24  converts the low impedance of the transformer T 1  into a high impedance for the coil L 1 . Of course, it is contemplated that the coaxial cable  24 ,  25  may be replaced with an equivalent circuit representing an equivalent of one fourth of a wavelength at the frequency of operation. 
     The capacitor Cm 22  matches impedance of the transformer T 1  to the input impedance Zp 1  of the preamplifier P 1  via the secondary coil  22  of transformer T 1 . The coaxial cable is connected at each end in series with the capacitors Cm 20 , Cm 21 . The capacitors Cm 20 , Cm 21  may be length compensation capacitors that are configured to cancel at least some of the phase shift in the coaxial cable. The values of the capacitors Cm 20 , Cm 21  may be selected based upon the length of the coaxial cable and the desired operating characteristics. The coaxial cable has shown better effective signal transmission than the balun circuit of an exemplary embodiment of  FIG. 2A . 
       FIG. 3  illustrates a circuit diagram according to an exemplary embodiment of the invention. The circuit diagram of  FIG. 3  depicts another technique to inductively decouple the coils L 1  and L 2 , as discussed above for the exemplary embodiment of  FIG. 1 . The circuit diagram in  FIG. 3  depicts network circuit  30 , which differs from network circuit  10  in  FIG. 1 , for coupling the coil L 1  to the preamplifier P 1  via the transformers T 1 , T 3 , and T 4  and depicts network circuit  31 , which differs from network circuit  11  in  FIG. 1 , for coupling the coil L 2  to the preamplifier P 2  via the transformers T 2 , T 5 , and T 6 . In circuit  30 , the detected signal in the coil L 1  is distributed in a balanced configuration to transformers T 3  and T 4  at the same time. The outputs from the transformers T 3  and T 4  are amplified in preamplifiers P 3  and P 4 , respectively, and combined in transformer T 1 . Impedance of the transformer T 1  is matched to impedance of the MRI receiver chain, e.g. the input impedance of the preamplifier P 1 . Since noise signals coming from preamplifiers P 3  and P 4  are not correlated, the combination of signals may provide better signal-to-noise ratio as compared to the circuit that uses just one preamplifier as shown in  FIG. 1 . The circuit  31  for coil L 2  is similar to the circuit  30  for coil L 1 . The ground GND 3  of circuit  30  is electrically isolated from the ground GND 2  of circuit  31  and from the ground GND 1  of the preamplifier P 1 . The uncoupling between coils L 1  and L 2  for this exemplary embodiment is twice as effective as the uncoupling between the coils L 1  and L 2  for the exemplary embodiment of  FIG. 1 . 
       FIG. 4  illustrates a circuit diagram according to an exemplary embodiment of the invention. The circuit diagram of  FIG. 4  depicts another technique to inductively decouple the coils L 1  and L 2 , as discussed above for the exemplary embodiment of  FIG. 1 . The circuit diagram in  FIG. 4  depicts network circuit  40 , which differs from network circuit  10  in  FIG. 1 , for coupling the coil L 1  to the preamplifier P 1  via the transformer T 1  and depicts network circuit  41 , which differs from network circuit  11  in  FIG. 1 , for coupling the coil L 2  to the preamplifier P 2  via the transformer  12 . The circuit  40  includes capacitors C 41 , C 42  and C 43 , an inductor L 43 , and the transformer T 1 . Through the capacitor Cm 41 , the impedance of the secondary coil  42  of transformer T 1  matches the input impedance Zp 1  of the preamplifier P 1 . The circuit  41  for coil L 2  is similar to the circuit  40  for coil L 1 . The ground GND 3  of circuit  40  is electrically isolated from the ground GND 2  of circuit  41  and from the ground GND 1  of the preamplifier P 1 . 
     To achieve high levels of decoupling between the coils L 1  and L 2 , the circuits  40  and  41  require low input impedance for the preamplifiers P 1  and P 2 , respectively. The transformers T 1  and T 2  can be used to lower the equivalent input impedance of the preamplifiers P 1  and P 2 , respectively. For circuit  40  (and similarly for circuit  41 ), it may be shown that the equivalent input impedance Zin of the preamplifier P 1  measured on the primary coil  43  of transformer T 1  is given by: 
                     Z   in     =       Z     p   ⁢           ⁢   1         n   2               (   3   )               
where Zp 1  is the input impedance of the preamplifier P 1 , and n is the turn ratio between the secondary and the primary coils  42 ,  43  of the transformer T 1 . According to equation (3), the transformer T 1  lowers the equivalent input impedance Zin of the preamplifier P 1  by a factor of n 2 .
 
     The invention is described in detail with respect to exemplary embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and the invention, therefore, as defined in the claims is intended to cover all such changes and modifications as fall within the true spirit of the invention.