Inductive decoupling of an rf coil array

Description of equivalent WO 2007117604 A2

Inductive Decoupling of an RF Coil Array

Field of the Invention

[0001] 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

[0002] 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.

[0003] 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 < 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. [0004] 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-πoise 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.

[0005] 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

[0006] 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.

[0007] 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. [0008] 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

[0009] 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.

[00010] Figure 1 illustrates a circuit diagram according to an exemplary embodiment of the invention.

[00011] Figure 2A illustrates a circuit diagram according to an exemplary embodiment of the invention.

[00012] Figure 2B illustrates a circuit diagram according to an exemplary embodiment of the invention.

[00013] Figure 3 illustrates a circuit diagram according to an exemplary embodiment of the invention.

[00014] Figure 4 illustrates a circuit diagram according to an exemplary embodiment of the invention.

[00015] Figure 5 A shows individual fast low-angle shot (FLASH) images of coils in an exemplary four coil array. [00016] Figure 5B shows the sum of squares reconstruction for the images of Figure 5A.

[00017] Figures 6A, 6B, and 6C show images acquired with generalized autocalibrating partially parallel acquisition (GRAPPA) reconstruction at different acceleration factors for the exemplary four coil array.

Detailed Description

[00018] 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.

[00019] 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. [00020] 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.

[00021] 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.

[00022] The MRI system may be also used for magnetic resonance spectroscopy (MRS).

Other imaging embodiments may further be used with the invention.

[00023] Figure 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 Ll and L2. The coil array 8 may include additional coils (not shown). Each coil in the coil array is coupled to its own respective preamplifier. Coil Ll is coupled to preamplifier Pl, and coil L2 is coupled to preamplifier P2. The two coils Ll and L2 may have different inductances and may have a mutual coupling M.

[00024] The network circuit 10 for coil Ll includes a capacitor C12 connected in parallel to a transformer Tl. This parallel circuit is connected in series through a capacitor CI l to coil Ll. The transformer Tl couples coil Ll to its respective preamplifier Pl. The transformer Tl is coupled to coil Ll through its primary coil 12 and to the preamplifier Pl through its secondary coil 13. The secondary coil 13 of transformer Tl is connected in series through a capacitor C13 to the preamplifier Pl, which has an input impedance of ZpI. The primary and secondary coils 12, 13 of transformer Tl may have a turn ratio of n:l, where n is greater than 0, including fractional numbers. Through the capacitor C 13, the impedance of the secondary coil 13 of transformer Tl matches the input impedance ZpI of the preamplifier Pl. The transformer Tl has a coupling coefficient Kl . The network circuit 11 for coil L2 is similar to the network circuit 10 for coil Ll and couples coil L2 to the preamplifier P2 via the transformer T2. The impedance of the secondary coil 13 of the transformer T2 matches the impedance of the preamplifier P2. The ground GND3 of circuit 10 is electrically isolated from the ground GND2 of circuit 11 and from the ground GNDl of the preamplifier Pl . The preamplifiers Pl and P2 are connected to the ground GNDl.

[00025] The turn ratio of transformers Tl and T2 may be selected to minimize noise for the preamplifiers Pl and P2, 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 Ll and L2 in coil array 8 may be enhanced by using high input impedance for the preamplifiers Pl and P2, respectively. [00026] The transformers Tl and T2 are passive transformers. The transformer Tl, T2 may include helical windings, solenoidal windings, or stripline baluns. The transformer Tl₅ T2 may include an electrostatic shield comprised of a conducting material. The transformer Tl₅ T2 may utilize superconducting materials.

[00027] The signal that reaches the preamplifier Pl is coupled inductively to coil Ll through the primary coil of the transformer Tl. Because the primary and secondary coils of the transformer Tl are isolated, the preamplifier circuit (and the MRI scanner electronics connected thereto (not shown)) are electrically isolated from coil Ll. 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, as 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. [00028] Referring to the exemplary circuit diagram of Figure 1, it may be shown that the impedance of the circuit 10 as seen from the circuit 11, may be given as:

<img file="WO2007117604A2_D0001.tif" /> where ω is the NMR Larmor frequency, M is the inductive coupling between Ll and L2, L is the inductance of coil L2, Rl is the resistance of coil Ll, R2 is the resistance'of coil L2, Kl is the coupling coefficient for the transformer Tl₅ n is from the turn ratio n:l between the primary and secondary coils 12, 13 of the transformer Tl, and ZpI is the input impedance of the preamplifier Pl. In order to achieve high isolation between coils Ll and L2, Z should be made as close as possible to R2. Thus, the second term in equation (1) should be made as negligible as possible. This can be accomplished if both n and ZpI are chosen to be high.

[00029] The exemplary embodiment, described above with reference to Figure 1, was tested in a 7T/30 cm Bruker Avance MRI system. A four-coil array for imaging a rat brain was built using the circuit described with reference to Figure 1. A positive-intrinsic-negative (PIN) diode circuitry (not shown in the circuit of Figure 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, 1 Ω input impedance.

[00030] The measured isolation between the channels in the test was better than 45 dB.

Figure 5A shows individual FLASH axial images obtained with each coil, and Figure 5B shows a combined image obtained using a sum of squares reconstruction from the four images shown in Figure 5A. As can be seen by comparing the individual images in Figure 5A to the combined image in Figure 5B, no significant coupling is observed between the RF coils. Figures 6A, 6B, and 6C 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 Figure 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.

[00031] Figure 2A illustrates a circuit diagram according to an exemplary embodiment of the invention. The circuit diagram of Figure 2A depicts another technique to inductively decouple the coils Ll and L2, as discussed above for the exemplary embodiment of Figure 1. The circuit diagram in Figure 2A depicts network circuit 20, for example, a balun circuit, for coupling the coil Ll to the preamplifier Pl via the transformer Tl and depicts network circuit 21, for example, a balun circuit, for coupling the coil L2 to the preamplifier P2 via the transformer T2. The balun circuit 20 for coil Ll includes a capacitor C20, capacitors Ca and Cb, inductors La and Lb, a capacitor Cm21, and the transformer Tl. The balun circuit 20 converts the low impedance of the transformer Tl into a high impedance for the coil Ll. The capacitor Cm22 matches impedance of the transformer Tl to the input impedance ZpI of the preamplifier Pl via the secondary coil 22 of transformer Tl . The circuit 21 for coil L2 is similar to the circuit 20 for coil Ll and matches impedance of the coil L2 to the impedance of the preamplifier P2. The ground GND3 of circuit 20 is electrically isolated from the ground GND2 of circuit 21 and from the ground GNDl of the preamplifier Pl.

[00032] Referring to an exemplary circuit diagram of Figure 2A, it may be shown that the impedance of circuit 20 as seen from the resistive impedance of Ll is given by:

Z = n² . — . - (2)

Z _PI C

where n is the turn ratio between the secondary and the primary coils of the transformer Tl, ZpI is the impedance of the preamplifier Pl, 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 Tl are negligible and also that the coupling coefficient Kl for the transformer Tl is 1. For best decoupling of the coils Ll and L2, the impedance load in circuit 20 as seen from coil Ll should be high, i.e., ZpI should be made as large as possible. According to equation (2), the presence of the transformer Tl in circuit 20 enhances the impedance seen from coil Ll by a factor of n².

[00033] With reference to Figure 2B, this exemplary embodiment is similar to an exemplary embodiment of Figure 2A except that each balun circuit 20, 21 in Figure 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 Tl into a high impedance for the coil Ll. 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.

[00034] The capacitor Cm22 matches impedance of the transformer Tl to the input impedance ZpI of the preamplifier Pl via the secondary coil 22 of transformer Tl. The coaxial cable is connected at each end in series with the capacitors Cm20, Cm21. The capacitors Cm20, Cm21 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 Cm20, Cm21 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 Figure 2A.

[00035] Figure 3 illustrates a circuit diagram according to an exemplary embodiment of the invention. The circuit diagram of Figure 3 depicts another technique to inductively decouple the coils Ll and L2, as discussed above for the exemplary embodiment of Figure 1. The circuit diagram in Figure 3 depicts network circuit 30, which differs from network circuit 10 in Figure 1, for coupling the coil Ll to the preamplifier Pl via the transformers Tl₅ T3, and T4 and depicts network circuit 31, which differs from network circuit 11 in Figure 1, for coupling the coil L2 to the preamplifier P2 via the transformers T2, T5, and T6. In circuit 30, the detected signal in the coil Ll is distributed in a balanced configuration to transformers T3 and T4 at the same time. The outputs from the transformers T3 and T4 are amplified in preamplifiers P3 and P4, respectively, and combined in transformer Tl. Impedance of the transformer Tl is matched to impedance of the MRI receiver chain, e.g. the input impedance of the preamplifier Pl. Since noise signals coming from preamplifiers P3 and P4 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 L2 is similar to the circuit 30 for coil Ll . The ground GND3 of circuit 30 is electrically isolated from the ground GND2 of circuit 31 and from the ground GNDl of the preamplifier Pl. The uncoupling between coils Ll and L2 for this exemplary embodiment is twice as effective as the uncoupling between the coils Ll and L2 for the exemplary embodiment of Figure 1.

[00036] Figure 4 illustrates a circuit diagram according to an exemplary embodiment of the invention. The circuit diagram of Figure 4 depicts another technique to inductively decouple the coils Ll and L2, as discussed above for the exemplary embodiment of Figure 1. The circuit diagram in Figure 4 depicts network circuit 40, which differs from network circuit 10 in Figure 1, for coupling the coil Ll to the preamplifier Pl via the transformer Tl and 'depicts network circuit 41, which differs from network circuit 11 in Figure 1, for coupling the coil L2 to the preamplifier P2 via the transformer T2. The circuit 40 includes capacitors C41, C42 and C43, an inductor L43, and the transformer Tl. Through the capacitor Cm41, the impedance of the secondary coil 42 of transformer Tl matches the input impedance ZpI of the preamplifier Pl. The circuit 41 for coil L2 is similar to the circuit 40 for coil Ll . The ground GND3 of circuit 40 is electrically isolated from the ground GND2 of circuit 41 and from the ground GNDl of the preamplifier Pl. [00037] To achieve high levels of decoupling between the coils Ll and L2, the circuits 40 and 41 require low input impedance for the preamplifiers Pl and P2, respectively. The transformers Tl and T2 can be used to lower the equivalent input impedance of the preamplifiers Pl and P2, respectively. For circuit 40 (and similarly for circuit 41), it may be shown that the equivalent input impedance Zin of the preamplifier Pl measured on the primary coil 43 of transformer Tl is given by:

^ = % (3) n

where ZpI is the input impedance of the preamplifier Pl, and n is the turn ratio between the secondary and the primary coils 42, 43 of the transformer Tl . According to equation (3), the transformer Tl lowers the equivalent input impedance Zin of the preamplifier Pl by a factor of n².

[00038] 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.