MRI devices utilize RF transmitting coils to transmit RF signals to an imaged portion of a body and utilize RF receiving coils to detect RF signals that are received from the imaged portion of the body during an MRI process. These RF coils are used in conjunction with matching circuits to tune the coils to a particular matched impedance, generally 50.OMEGA.. These impedance matching circuits include variable passive elements that can be tuned by human adjustment to the matched impedance value. These adjustments are required as a result of the various external factors that will affect the reactive component values in the RF coil and matching circuit.
FIG. 1 shows a basic impedance matching circuit. The RF coil is shown as an inductor on the left side of FIG. 1 in series with resistance R.sub.s that occurs due to losses from the patient and the RF coils themselves. Two variable capacitors, C.sub.p and C.sub.s, are added to bring the impedance of the circuit to 50.OMEGA.. The capacitors, C.sub.p and C.sub.s are variable to allow for a manual or automatic adjustment to bring the circuit impedance to 50.OMEGA..
Although the circuit is simple enough to be solved by a series of algebraic manipulations to find a real part of the circuit impedance to be 50.OMEGA. and the imaginary part of the circuit impedance to be 0.OMEGA., it is much easier to solve for the matching condition using a Smith chart (such as is shown in FIG. 1A).
FIG. 1A is a Smith chart configuration showing the impedance of the matching circuit of FIG. 1. The Smith chart in FIG. 1A illustrates the need for two variable capacitors, C.sub.p and C.sub.s, in FIG. 1 to match the impedance to the 50.OMEGA. requirement. As is well-known, the Smith chart illustrates normalized impedance (either to 50.OMEGA. or 75.OMEGA., although normally for RF and microwave frequencies a 50.OMEGA. system is employed) at the center of the circle (1.0)). The impedance of the RF coil before any matching is shown as Z.sub.rx. In the chart of FIG. 1A, the values L=0.6 .mu.H and Q=10 at 64 MHz were used. The effect of the parallel capacitor C.sub.p brings the impedance closer to the open circuit point (.infin.) of the Smith chart and the series capacitance C.sub.s brings the impedance shown on the Smith chart to the normalized impedance of 1.0.
As can be seen in FIG. 1A, bringing the impedance point from Z.sub.rx to the matched point (1.0) requires both series and parallel capacitance. That is, if only parallel capacitance is added to Z.sub.rx, the impedance point will rotate counterclockwise past the horizontal line (resistance axis) of FIG. 1A and, ultimately, reach to the short circuit point (0.0). Similarly, adding only series capacitance at Z.sub.rx will take the impedance point around (in a counter-clockwise direction) through approximately the 0.5 point then toward the open circuit point (.infin.). In neither case (using only series or parallel capacitance, but not both) will the impedance point intersect the resonance point (1.0). Thus, both variable parallel capacitance C.sub.p and variable series capacitance C.sub.s, are used in the circuit of FIG. 1.
Additional capacitors may also be present on the matched circuit. For high frequency operation, for example, such as is used in MRI procedures, additional capacitors connected in series with the receiver coil segments are advantageous in reducing frequency downshifting due to the contribution of the dielectric characteristics in the load. These additional capacitors are commonly called "distributed capacitors" and are shown in FIG. 2 as C.sub.d. In FIG. 2A, distributed capacitance C.sub.d is combined with the variable capacitor C.sub.p and the variable capacitor C.sub.s.
FIG. 2C is the corresponding Smith chart for the matching circuit of FIG. 2A. As can be seen in FIG. 2C, the distributed capacitor C.sub.d adds series capacitance to the matching circuit. The parallel and series capacitors are adjusted to achieve the impedance matching requirement shown in FIG. 2C.
As discussed above, in matching circuits of the type shown in FIGS. 1 and 2A, at least two capacitors (one in parallel and one in series) are present and are variable in order to bring the matching circuit to complex conjugate of the load impedance.
FIG. 2B is an alternative matching circuit, which is a variation of the circuit of FIG. 2A. In FIG. 2B, since the capacitors C.sub.d and C.sub.p of FIG. 2A provide the required parallel and series variable capacitances needed to tune the matching circuit, the series capacitor C.sub.s has been removed completely. The configuration in FIG. 2B is simpler and preferred over FIG. 2A. The corresponding Smith chart for the circuit of FIG. 2B is shown in FIG. 2D. Again, the characteristics of the illustration in the Smith chart of FIG. 2D is L=0.6 .mu.H and Q=10 at 64 MHz.
The circuits of FIGS. 1, 2A and 2B do not illustrate the effect of parasitic capacitance in parallel with C.sub.p that may be included in the coil or matching circuit. The effect of the parasitic capacitance can be significant. For example, if the parasitic capacitance value is too large, the impedance of the receiving coil and matching circuit can fall below the horizontal line on the Smith chart. When this happens, variable capacitors C.sub.d, C.sub.s or C.sub.p alone cannot tune the circuit to 50.OMEGA..
For example, if parasitic parallel capacitance is present in the circuit of FIG. 2B, the impedance value shown in FIG. 2D will reflect a greater total parallel capacitance value. That, of course, would bring the impedance value shown as resting at 1.0 in FIG. 2D below the horizontal line (moving in a clockwise direction along the same segment identified as "C.sub.p " in FIG. 2D). If the parasitic capacitance is small, it can be corrected by removing some of the parallel capacitance contributed by capacitor C.sub.p through its variable adjustment. But, as the parallel parasitic capacitance value exceeds the value of C.sub.p (FIG. 2D), the capacitance contribution from the parallel parasitic capacitance alone will cause the impedance value to fall below the horizontal line in FIG. 2D (through the matched impedance point 1.0). Under those conditions, no amount of reduction in the capacitance C.sub.p will bring the matched circuit back into 50.OMEGA. matching.
The following table summarizes typical capacitance values of the three different matching circuits in FIGS. 1, 2A and 2B for an RF receiving coil whose inductance is 0.6 .mu.H and Q is 10 operating at 64 MHz (for a 1.5 T MRI system).
TABLE 1 ______________________________________ CIRCUIT TOPOLOGY C.sub.d (pF) C.sub.p (pF) C.sub.s (pF) ______________________________________ FIG. 1 N/A 3.1 7.3 FIG. 2A 12.5 19.7 51.4 FIG. 2B N/A 51.3 11.5 ______________________________________
As shown in Table 1, the circuit in FIG. 1 requires very small capacitance values that are unsuitable for a circuit having larger parasitic capacitance. Larger capacitance values are more desirable for stable and repeatable matching operations. The circuits in FIGS. 2A and 2B have more reasonable variable capacitance values for situations where parasitic capacitance may be large.
The amount of parasitic capacitance present can depend on the type of circuit board that may be used in the tuning and matching circuit of the RF receiving coil. FR4-based printed circuit material (which has a dielectric constant over 4), for example, may add a large parasitic capacitance to tuning and matching circuits printed on them. On the other hand, circuits that are printed on a teflon/glass substance (such as is marketed under the name "Duroid"), which has a low dielectric constant (of, for example, approximately 2), will exhibit less parasitic capacitance over the same circuitry printed on the FR4-based printed circuit material. Unfortunately, however, the teflon/glass based printed circuit board material is more costly than the FR4-based printed circuit board material. Reducing the cost of the RF coils, while reducing the effect of parasitic capacitance is desirable.
FIG. 6, for example, shows a schematic representation of a RF receiving coil fabricated on a printed circuit board based on the circuit configuration of FIG. 2B. The RF coil shown in FIG. 6 is a representation of a lumbar spine coil for a high frequency MRI. The RF coil assembly 10 includes a RF coil trace board 12 and a matching circuit board 30. The RF coil trace board 12 may be either the FR4- or teflon/glass-type onto which coil 22 is imprinted. The RF coil trace 22 is broken in several places to include series capacitances. At least one of the series capacitance in the coil 22 must be adjustable as shown in FIG. 6. As discussed previously, these adjustable series capacitances allow the user to tune the RF coil matching circuit to the desired impedance.
Certain problems exist in accessing this variable series capacitor in the MRI system. To illustrate this problem, the RF coil assembly 10 shown in FIG. 6 is shown in its MRI application in FIG. 7. Specifically, in FIG. 7, the assembly 10 is included as part of a table 40 within the MRI structure 42. The RF coil assembly 10 includes a surface that is roughly contiguous with the surface of the table 40 such that the patient is provided with a smooth table to lie on during the MRI process. In FIG. 7, however, the surface of the assembly 10 is removed to illustrate the RF coil trace board 12 within a cavity 14.
The coil 22 which is on the upper surface of the RF coil trace board 12 can be seen in FIG. 7. The coil 22 includes an adjustable capacitor in series, as shown in FIG. 6. The adjustment for this capacitor is typically provided at the end of the RF coil assembly 10, and the adjustment is typically accessed by an adjustment hole 16 (FIG. 7).
As can be particularly seen in FIG. 7, once the human body is on the table 40, access to adjustment hole 16 is significantly limited. Nevertheless, as described with respect to the Smith charts above, the series capacitor (C.sub.s) must be adjusted to tune the RF coil to the desired frequency. Thus, even though difficult, access to the adjustments 16 is requisite.
In addition, the series capacitance C.sub.s is subjected to very high voltages induced on the RF receiving coil during the RF transmission cycle. Usually, there is not enough space to have a high voltage, non-magnetic variable capacitor on the coil trace board.
The prior matching circuits thus suffered from problems in accessing the variable series capacitance adjustments in the RF receiving coils and problems with arcing across the variable series capacitor.
The schematic diagram for the assembly 10 in FIGS. 6 and 7 is shown in FIG. 4. The RF coil 22 is shown as inductance L, together with the series capacitance C.sub.s and resistance R.sub.s caused by load loss. As described previously, with respect to FIGS. 1, 2A and 2B, the prior art also includes a parallel capacitance C.sub.p, as shown in FIG. 4, to adjust the resonance to a typically 50.OMEGA. matched impedance. The RF coil trace board 12 and matching board 30 provide an inherent parasitic capacitance C.sub.parasitic, as shown in FIG. 4. FIG. 4 also shows the RF output connection 60.
A ground breaker 50 is placed on matching board 30 to reduce the unwanted ground loop between the RF coil and the rest of the circuit shown in FIG. 4. A typical ground breaker is made up of a coaxial cable wound as a coil, together with a tuning capacitor connected across the outside conductor of the coaxial cable. This type of ground breaker 50 is described in U.S. Pat. No.4,682,125 to Harrison et al., which is incorporated herein by reference. The outside conductor of the coaxial cable and the parallel capacitor act as a parallel tuned circuit and exhibit a very high impedance at the resonant frequency. As a result, little RF ground current will flow along the outside of the coaxial cable.
In FIG. 4, for a 64 MHz operation, the inductance of the coaxial cable formed as a coil can be approximately 0.1 .mu.H and the electrical length is 26 degrees, which is about 1/14 wavelength. The value of C.sub.parasitic can be approximately on the order of 20 to 30 pF, depending on which type of printed circuit material is used and how the traces on the printed circuit board are laid out.
As is described above, the parasitic capacitance C.sub.parasitic can be relatively large that the variable capacitors C.sub.p and C.sub.s can no longer bring the RF coils into a matched impedance condition. In addition, obtaining access to the adjustment 16 to vary the capacitance C.sub.s to obtain a matched impedance condition can be difficult in particular applications.
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