DESCRIPTION

HIGH FIELD HEAD COIL FOR DUAL-MODE OPERATION IN MAGNETIC RESONANCE IMAGING

Cross-Reference to Related Application

The present application claims the benefit of U.S. provisional patent application Serial No. 60/644,439, filed January 14, 2005, which is hereby incorporated by reference herein in its entirety, including any figures, tables, or drawings.

Background of Invention

In most head coil designs, a double coil structure is used. Typically, the outside coil is a volume coil for RF excitation and the inner coil is a surface coil array for signal reception.

In head imaging, two important design criteria for the coil design are homogeneity in RF excitation and sufficient sensitivity in signal reception. These requirements are usually achieved using the combination of a volume coil, such as a TEM or Birdcage coil, and a surface coil array with active decoupling circuits.

It has been shown that placing a shield on the side of a surface coil can offer good decoupling. This is equivalent to the use of a counter rotating current (CRC) coil, which is also efficient in coil decoupling. However, placing a shield on the side of a surface coil does not allow good control in the coil decoupling.

Brief Description of Drawings

Figure 1 shows an embodiment of a coil for use in magnetic resonance imaging in accordance with an embodiment of the subject invention.

Figure 2 shows a circuit diagram for decoupling between transmitting structure and receiving structure for the coil in Figure 1, in accordance with an embodiment of the subject invention.

Figure 3 shows a photograph of a coil that is schematically shown in Figure 9, in accordance with an embodiment of the subject invention.

Figure 4 shows a schematic of a portion of the transmit coil array shown in Figure 3, in accordance with an embodiment of the subject invention.

Figure 5 shows an embodiment utilizing a power splitter design in accordance with an embodiment of the subject invention.

Figure 6 shows a ground structure in accordance with an embodiment of the subject invention.

Figure 7 shows the receive structure for a coil in accordance with an embodiment of the subject invention.

Figure 8 shows a circuit diagram for decoupling between transmitting and receiving structure for the coil in Figure 7, in accordance with an embodiment of the subject invention.

Figure 9 shows a coil incorporating double-loop coil elements in accordance with an embodiment of the subject invention.

Figure 10 shows a schematic for utilizing the coil of Figure 9 in accordance with an embodiment of the subject invention.

Detailed Disclosure

Embodiments of the subject invention can utilize a single coil structure for dual-mode operation. Embodiments of the invention pertain to a method for utilizing a single coil structure for dual mode operation. In a specific embodiment, a single coil structure can be used as a volume coil in the transmitting phase and as a phase array in the receiving phase. In an embodiment, a transmit coil array in accordance with the subject invention can be utilized for one or more of the following: 1) parallel RF excitation on an MRI scanner equipped with a multiple-channel transmitter; 2) serial RF excitation with the use of a switch system with a single channel transmitter; and 3) generation of a homogeneous B ₁ field for regular MRI with the use of a power splitter on a standard scanner. In a specific embodiment, the use of a concentric double loop coil with different tunings of inner and outer loops can be implemented. In an embodiment, the coil elements can be decoupled using the CRC mode of a concentric double loop coil. In an embodiment, a surface coil transmit array can provide better compatibility with a receive array. In a specific embodiment, an over-coupled technique can be used to improve the phase control of the transmitting pulse.

In a specific embodiment, an array structure is constructed directly from the elements in the volume coil. In a further specific embodiment, each element of a TEM coil structure can be connected to a low input impedance preamplifier so as to receive signals simultaneously from all the elements. However, SNR may suffer from the weak coupling between a single element and the sample. To improve sensitivity coverage, larger coil loops can be formed by connecting two elements of the TEM coil. Figure 1 shows the coil structure of a specific embodiment of the subject invention. Figure 2 shows a circuit configuration that can be used in accordance with this embodiment of the subject invention.

Referring to Figure 1, a modified 8-leg TEM coil structure in accordance with the subject invention is shown. Although this embodiment incorporates 8 rungs on the inside and 8 rungs on the outside to make 8 legs, other numbers of legs can be used. Preferably an even number of legs can be used. Specific embodiments can use 16 or 32 legs, although the number of legs is not limited to powers of 2. The outer rungs 2 are grounded. The inner rungs 4 carry the currents for B ₁ field generation. In the embodiment shown, two washers 6 and 8, machined from PCB circuit board, are used to support the inner and outer rungs and provide a grounded shield. The end-ring structure for mode degeneration and the circuit structures for decoupling are shielded out of the ground layer.

Figure 2 shows a circuit diagram for decoupling between transmitting structure and receiving structure. Referring to Figure 2, the circular structure is unfolded on a flat plane. Only six of the inner rungs 4 in the TEM coil structure from Figure 1 are shown. Each coil loop is connected to a low input impedance preamplifier 12 (note that only 2 preamplifiers of the 4 are shown in Figure 2). AU the diodes are switched on during the transmitting phase and off during the receiving phase. When the diode packs 10 are switched on during the transmitting phase, they form a short allowing a direct connection between the inner rung 4 and the outer rung 2 for each leg. In this way current travels in the inner rungs 4 and produces transmit RF signals for MRI. When the diode packs are switched off during the receiving phase, current can be induced in the receive coils formed by the path from the preamplifier 12, along a portion of ring shield 8, down one of the inner rungs 4, along the other ring shield 6, back up another inner ring 4, and back over to the preamplifier 12 via a portion of ring shield 8. In the embodiment shown in Figure 2, the receive coils are formed by the paths utilizing the 1 ^st and 4 ^th inner rungs 4, and the 3 ^rd and 6 ^th inner rungs 4, and the 5 ^th

and 8 ^th (not shown) inner rungs 4, and the 7 ^th (not shown) and 2 ^nd inner rungs 4. Other permutations of inner rungs 4 can be utilized to make the receive coils, particularly in embodiments having 16, 32, or other number of legs. The mutual coupling between two adjacent loops are suppressed using a shared capacitor 14 and some overlap of two loops.

A concentric double loop surface coil can be used to produce two resonance modes: a co-rotating current mode and a counter-rotating current (CRC) mode. In the CRC mode, the magnetic field produced can be reduced due to the opposing currents of the inner and outer coil loops. The degree of this field reduction depends on the current ratio between the inner loop and the outer coil, which can be easily adjusted by tuning the two coil loops. In an embodiment, double loop surface coils can be used to make up a transmit array and tuned to the CRC mode, with the inductive coupling between coil elements controlled by tuning capacitors.

Figure 3 shows an eight-element transmit array, in accordance with the subject invention, utilizing tuning capacitors to decouple the double loop surface coils when the transmit array is tuned to the CRC mode. The schematic of the transmit coil array shown in Figure 3 is shown in Figure 4. Referring to Figure 3, eight coils were positioned on a clear acrylic tube of 27cm in diameter. Each element is a concentric double rectangular loop. The size of the inner loop is 8x13 cm and the size of the outer loop is 9.5x17 cm. The inner and outer loops can be tuned differently such that the coupling between next nearest neighbors can be suppressed to a satisfactory level. Between nearest neighbors, a shared inductor can be used for decoupling. Figure 4 shows inductive decoupling between adjacent inner loops. In an embodiment, an RF balun was used at the excitation port of every element to remove the coupling through cables. The array can be placed in an RF shield. In the embodiment shown in Figure 3, the array is placed in an RF shield with a distance of about 16 mm to the coils.

To provide the imaging capability on the MRI scanner equipped with only a one- channel transmitter, an eight-way power splitter was used to excite every coil unit with a phase delay of 45° between nearest neighbors. Because the coil matching is extremely sensitive to loading condition at this field, which dramatically affects the phase control in the RF transmission to every coil unit, the coils were over-coupled to a certain degree such that the phase of the current in every coil is insensitive to the load change. Figure 5 shows an embodiment implementing a power splitter design.

Embodiments of the invention can include a combination of volume coil and a receive coil array. Additional embodiments can include a transmit/receive coil array. The same resonator structure can be used in transmit and receive mode. In an embodiment having a combination of a volume coil and a receive coil array, active decoupling circuits (such as a diode network) can be used to switch the configuration of the coil between rf transmission and signal reception. In an embodiment having a transmit/receive coil array, the mode configuration can be switched using a power splitter and combiner or T/R (transmit/receive) switch.

The same ground structure shown in Figure 6 can be utilized with embodiments having a volume coil and a receive coil and in embodiments having a transmit/receive coil array. The ground structure 1 can include multiple rungs 2 connected to two washers, one washer at each of the two ends of the rung. In an embodiment, the inner diameter of each washer can be equal to the coil diameter. Even though 16 rungs are shown in Figure 6, other numbers of rungs can be also be used. Rungs 2 can be tubing, rods, foil or other shapes. The use of tubing or rods can allow a person to see out of the ground structure from inside the ground structure.

Figure 7 shows the coil structure in an embodiment of the invention having a volume coil and a receive coil. The coil structure is shown during receive. During transmit, for example as a TEM coil, the rungs 4 connect to ground. The structure can include two parts: 16 rungs (4 in Figure 7) and the top plate part. In the embodiment shown schematically in Figure 7, the 16 rungs 4 are placed equally spaced on a cylindrical surface. In an embodiment, the rungs 2 from the ground structure 1 shown in Figure 6 are positioned at the same angular position as the rungs 4, when the ground structure 1 is placed outside the coil structure.

The cross-over of elements 3, to form a triangular loop in Figure 7, can be used for decoupling a pair of coils from an adjacent pair of coils. The coil structure of Figure 7 has 4 pairs of coils used for receiving MR signals. The coil structure is used in MR with the static magnetic field parallel with rungs 4. The rungs 4 can also be positioned such that the cross- section shape of the rungs 4 position in a plane transverse to the static magnetic field is elliptical. Preferably, the ratio of the minor axis of such an ellipse to the major axis is 0.8 or greater. Referring to Figure 7, capacitors 16 help with decoupling. The 16 rungs 4 in Figure

7 are segmented into 4 pairs of coils. Each pair of coils utilizes 4 rungs 4 such that the first and third rungs are part of one coil, along with conductors along the top and bottom, and the second and fourth rungs are part of the other coil of the coil pair, along with conductors along the top and bottom. In this way, the coils in the coil pair overlap to reduce coupling.

Figure 8 shows the electrical connections of the structures, a portion of which are shown in Figure 7, in accordance with an embodiment of the invention. During the transmit mode, the diodes are on and ground most of the coil structure. During the transmit mode, the 16 inner rungs 4 are not grounded. Combining the structure shown in Figure 7 with the ground structure shown in Figure 6, during transmit the 16 pairs of inner 4 and outer 2 rungs form the basic TEM coil structure, which can be used to implement RP transmission. An example of such a basic TEM coil structure is shown in Figure 1, for 8 rungs. During the receive mode, the diodes are off and an eight-channel surface coil array is formed based on the inner rung 4 structure, as shown in Figure 7.

Referring to Figure 9, in another specific embodiment, eight double-loop coil elements are placed equally spaced on a cylindrical surface. In another embodiment, the cross-section can be elliptical rather than circular. Also, a different number of double-loop coils can be utilized, such as, for example, 4, 16 or 32. The counter rotating current (CRC) mode is used to generate the rf field inside the coil. A stronger rf current is generated in the inner loop than in the outer loop. The currents in the inner and outer loops are in opposite directions. The current ratio between the inner loop and outer loop is dependent on the requirements for the decoupling from next nearest neighbors in the coil array. This ratio can be adjusted by, for example, tuning the inner and outer coil. The decoupling between the nearest neighbors in the coil array can be implemented by sharing the inductance or capacitance. The balun circuit can be connected to the inner loop for rf transmission and signal reception. The coil can be capacitively overcoupled to increase the phase stability for different sample loading conditions. The ground structure shown in Figure 6 can be placed outside of this coil resonator.

The ground structure shown in Figure 6 can be positioned such that two of rungs 2 are symmetrically positioned outside of the double-loop coil. Other ground structures can be utilized, such as other ground structures having symmetry around the center axis of the cylindrical surface and rungs of tubular structure, rod structure, and/or foil structure. Figure

10 shows how the baluns, matching circuits, and power splitter/combiner can be utilized with the coil structure of Figure 9. Figure 5 shows an embodiment utilizing the coil structure of Figure 9 and the ground structure of Figure 6 to transmit (T) and receive (R), where transmit and receive are interleaved with each other in time. The inner loops 9 are shown inductively coupled to the adjacent inner loop 9 for decoupling purposes. The outer loops 11 are shown in Figure 10 as well.

All patents, patent applications, provisional applications, and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.