RADIO FREQUENCY COIL UNIT FOR MAGNETIC RESONANCE IMAGING AND RADIO FREQUENCY COIL

Abstract

The invention discloses an RF coil element and an RF coil for magnetic resonance imaging, wherein the RF coil element is connected with an active loss circuit capable of actively dissipating and absorbing RF power in the RF coil element to decrease the Q value of the coil element. The active loss circuit is connected to the coil element to absorb the RF power in the coil element to decrease the Q value of the coil element, so that the coupling degree (correlation coefficient) between every two elements of an array coil formed by the coil elements is decreased, thus improving the parallel transmission (pTX) performance and the uniformity of a magnetic resonance RF transmission field.

Claims

1. An RF coil element for magnetic resonance imaging, being connected with an active loss circuit which is able to actively dissipate and absorb RF power in the RF coil element to decrease a Q value of the coil element.

2. The RF coil element for magnetic resonance imaging according to claim 1, wherein the active loss circuit is a resistor in series or parallel connection with a circuit component in the RF coil element.

3. The RF coil element for magnetic resonance imaging according to claim 1, wherein the active loss circuit is a low-Q-value component in series or parallel connection with a circuit component in the RF coil element.

4. The RF coil element for magnetic resonance imaging according to claim 1, wherein the active loss circuit is a conductor, with a conductivity smaller than that of copper, in series connection with a circuit component in the RF coil element.

5. The RF coil element for magnetic resonance imaging according to claim 1, wherein the active loss circuit is an equivalent resistor module in series or parallel connection with a circuit component in the RF coil element.

6. The RF coil element for magnetic resonance imaging according to claim 1, wherein a loss circuit on-off element used to turn on/off the active loss circuit is connected to the coil element.

7. The RF coil element for magnetic resonance imaging according to claim 6, wherein the coil element is also connected with: a frequency compensation circuit, an impedance compensation circuit, a frequency compensation circuit on-off element used to turn on/off the frequency compensation circuit, and an impedance compensation circuit on-off element used to turn on/off the impedance compensation circuit.

8. The RF coil element for magnetic resonance imaging according to claim 7, wherein the coil element comprises a resonance circuit and a matching network connected with the resonance circuit, wherein the active loss circuit is in series or parallel connection with a circuit component in the resonance circuit or the matching network, the frequency compensation circuit is in series or parallel connection with a circuit component in the resonance circuit, and the impedance compensation circuit is in series or parallel connection with a circuit component in the matching network.

9. The RF coil element for magnetic resonance imaging according to claim 8, wherein the resonance circuit is a closed circuit formed by series connection of one or more conductors and one or more capacitors, and the matching network comprises a capacitor or an inductor.

10. The RF coil element for magnetic resonance imaging according to claim 9, wherein the resonance circuit comprises at least two capacitors which are connected in series, the active loss circuit is connected in series with a first diode and is then connected in parallel with one said capacitor in the resonance circuit, a first inductor is connected in series with a second diode and is then connected in parallel with the other capacitor in the resonance circuit, the first diode constitutes the loss circuit on-off element, and the second diode constitutes the frequency compensation circuit on-off element.

11. The RF coil element for magnetic resonance imaging according to claim 9, wherein the active loss circuit is connected in series with a second inductor and a third diode and is then connected in parallel with one said capacitor in the resonance circuit, the second inductor constitutes the frequency compensation circuit, and the third diode constitutes the frequency compensation circuit on-off element and the loss circuit on-off element.

12. The RF coil element for magnetic resonance imaging according to claim 11, wherein two terminals of the active loss circuit and the second inductor are connected in parallel with a first capacitor, and the second inductor and the first capacitor constitute the frequency compensation circuit jointly.

13. The RF coil element for magnetic resonance imaging according to claim 9, wherein a second capacitor is connected in series with a fourth diode and is then connected in parallel with the capacitor or inductor in the matching network, the second capacitor constitutes the impedance compensation circuit, and the fourth diode constitutes the impedance compensation circuit on-off element.

14. An RF coil for magnetic resonance imaging, being an array coil, wherein the RF coil comprises at least one RF coil element according to claim 1.

15. The RF coil for magnetic resonance imaging according to claim 14, wherein the RF coil is a transceiver-only RF array coil, a receiver-only RF array coil, or a transceiver RF array coil.

16. An RF coil for magnetic resonance imaging, being a birdcage coil, wherein an active loss circuit is connected to the RF coil to actively dissipate and absorb RF power in the RF coil to decrease the Q value of the coil.

17. The RF coil for magnetic resonance imaging according to claim 16, wherein the active loss circuit is connected in series or parallel with a capacitor on a leg or terminal ring in the RF coil.

18. The RF coil element for magnetic resonance imaging according to claim 8, the active loss circuit is arranged at a position away from the resonance circuit and is connected to a position away from the resonance circuit.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0067] FIG. 1 is a principle block diagram of a traditional RF coil;

[0068] FIG. 2 is a schematic circuit diagram of the traditional RF coil;

[0069] FIG. 3 is an equivalent circuit diagram of the traditional RF coil;

[0070] FIG. 4 is a schematic circuit diagram of a traditional RF receiver coil;

[0071] FIG. 5 is a schematic circuit diagram of a traditional ultrahigh-field RF transceiver array coil;

[0072] FIG. 6 is a coupling diagram of two identical coil elements;

[0073] FIG. 7 is a schematic diagram of the magnetic flux of overlap inductive decoupling of the two coil elements;

[0074] FIG. 8 is a schematic diagram of capacitive decoupling between the two coil elements;

[0075] FIG. 9 is an impedance analysis diagram of a resonance circuit of the right coil element in FIG. 8;

[0076] FIG. 10 is a schematic circuit diagram of an RF coil element in Embodiment 1 of the invention;

[0077] FIG. 11 is a schematic circuit diagram of an RF coil element in Embodiment 2 of the invention;

[0078] FIG. 12 is a schematic circuit diagram of an RF coil element in Embodiment 3 of the invention;

[0079] FIG. 13 is a schematic circuit diagram of an RF coil element in Embodiment 4 of the invention;

[0080] FIG. 14 is a schematic circuit diagram of an RF coil element in Embodiment 5 of the invention;

[0081] FIG. 15 is an equivalent circuit diagram of the RF coil element in a reception state in Embodiment 5 of the invention;

[0082] FIG. 16 is an equivalent circuit diagram of the RF coil element in a transmission state in Embodiment 5 of the invention;

[0083] FIG. 17 is a schematic circuit diagram of a transmitter-only coil element in Embodiment 6 of the invention;

[0084] FIG. 18 is a schematic circuit diagram of a transceiver coil element in Embodiment 7 of the invention;

[0085] FIG. 19 is a schematic circuit diagram of an RF coil element in Embodiment 8 of the invention;

[0086] FIG. 20 is a schematic circuit diagram of a traditional birdcage coil;

[0087] FIG. 21 is a schematic circuit diagram of a birdcage coil added with a loss circuit in Embodiment 9 of the invention;

[0088] FIG. 22 is a schematic circuit diagram of an 8-channel transceiver RF array coil in Embodiment 10 of the invention;

[0089] FIG. 23 is a diagram of an RF transmission field B1 of the array coil in Embodiment 10 of the invention;

[0090] FIG. 24 is a diagram of an RF transmission field B1 of a traditional solution.

DETAILED DESCRIPTION OF THE INVENTION

[0091] This application is further expounded below in combination with the embodiments and accompanying drawings. The invention can be implemented in various forms, and is not limited to the implementations described in the following embodiments. The following embodiments are provided for the purpose of a clearer and more comprehensive understanding of the contents of this application.

[0092] However, those skilled in the art would appreciate that one or more specific details in the following description can be omitted, or other methods, components, or materials can be adopted. In certain embodiments, some implementations are not described or not described in detail.

[0093] In addition, the technical characteristics and technical solutions in this description can be appropriately combined at random in one or more embodiments. It is appreciable for those skilled in the art that the sequence of steps or operations relating to the embodiments provided in this description can be changed. Thus, any sequences in the accompanying drawings and embodiments are only for the purpose of explanation, and do not, unless otherwise specifically stated, indicate that the steps or operations must be performed in certain sequences.

[0094] The serial numbers of components such as first and second in this description are only used for distinguishing the objects referred to, and do not have any sequential or technical indications.

Embodiment 1

[0095] FIG. 10 shows the first embodiment of the RF coil element for magnetic resonance imaging of the invention (hereinafter referred to as coil element). Identical with traditional RF coil elements, the coil element of the invention also comprises a resonance circuit and a matching network connected with the resonance circuit. Wherein, the resonance circuit is a closed circuit which is formed by series connection of a plurality of (n) capacitors (FIG. 10 specifically shows five capacitors C.sub.P, C.sub.H, C.sub.F2, C.sub.Fn-1, and C.sub.Fn constituting the resonance circuit) through a conductor (the conductor is typically a copper wire), and the matching network consists of a capacitor C.sub.S.

[0096] The key improvement of this embodiment lies in that active loss circuits are additionally arranged in the RF coil element to actively dissipate and absorb the RF power in the RF coil element (namely to dissipate transmission energy of the coil element and to weaken a signal during reception of the coil) to decrease the Q value of the RF coil element (namely to reduce the sensitivity of the coil element). That is to say, the efficiency of the RF coil element during transmission is significantly reduced.

[0097] Particularly, two active loss circuits are arranged in the RF coil element, as shown in FIG. 10, wherein one active loss circuit R.sub.LOSS1 is connected to the RF resonance circuit and is particularly connected in parallel with the capacitor C.sub.F2 in the resonance circuit, and the other active loss circuit R.sub.LOSS2 is connected to the matching network.

[0098] It should be noted that in FIG. 10, the active loss circuit R.sub.LOSS1 is connected in parallel to two terminals of the capacitor C.sub.F2, but the connection mode of the active loss circuit R.sub.LOSS1 to the radio-frequency circuit is not limited to the such mode, for example, the active loss circuit R.sub.LOSS1 may be connected in series with one capacitor in the resonance circuit optionally. The connection mode of the active loss circuit R.sub.LOSS2 to the matching network is not limited to the one shown in FIG. 10 either.

[0099] Clearly, it is also feasible to configure only one active loss circuit, and if this is the case, the active loss circuit is selectively connected to the resonance circuit or the matching network. Generally speaking, in the case where only one active loss circuit is configured, the active loss circuit is typically connected to the resonance circuit, that is, the active loss circuit is connected in series or in parallel with a circuit component in the resonance circuit.

[0100] It should be noted that in the case where the resonance circuit and the matching network of the coil element are not strictly marked off or even the matching network essentially belongs to the resonance circuit, it is impossible to definitely point out whether the active loss circuit is connected to the resonance circuit or the matching network. In another case where the impedance across the two terminals of the resonance circuit of some special coil elements is a characteristic impedance (such as 50), it is unnecessary to configure a matching network, which means that such coil elements do not have a matching network. In these two cases, the active loss circuit can be connected to any feasible position of the coil element as long as the active loss circuit is able to actively dissipate and absorb the RF power in the RF coil element to decrease the Q value of the RF coil element.

[0101] When the RF coil element in the first embodiment is used to fabricate an RF coil for magnetic resonance imaging, particularly an array coil, the active loss circuits R.sub.LOSS1 and R.sub.LOSS2 additionally configured in the RF coil element are able to actively dissipate and absorb the RF power in the RF coil element to decrease the Q value of the RF coil element, that is, the efficiency of the RF coil element during transmission is reduced, so that the coupling degree between coil elements is decreased, thus improving the performance of the array coil used for transmission and particularly significantly improving the uniformity of the transmission field B1.

[0102] The active loss circuit R.sub.LOSS1 and R.sub.LOSS2 in FIG. 10 can be any structure forms capable of actively dissipating and absorbing the RF power in the RF coil element to decrease the Q value of the RF coil element, and all such circuit modules can be used as the active loss circuits to be applied to the coil element to improve the transmission performance of the coil and to improve the uniformity of the transmission field B1.

[0103] Particularly, in this embodiment, the active loss circuit R.sub.LOSS1 and the active loss circuit R.sub.LOSS2 shown in FIG. 10 are both resistors.

[0104] There are at least the following four types of common active loss circuits: 1, resistors in series or parallel connection with circuit components in the RF coil element; 2, low-Q-value components in series or parallel connection with circuit components in the RF coil element; 3, conductors, with a conductivity smaller than that of copper, in series or parallel connection with circuit components in the RF coil element; 4, equivalent resistor modules in series or parallel connection with circuit components in the RF coil element. Clearly, the active loss circuits may also be combinations of the resistors, low-Q-value components, low-conductivity conductors and equivalent resistor modules.

Embodiment 2

[0105] FIG. 11 shows a second embodiment of the RF coil element for magnetic resonance imaging of the invention. In this embodiment, the RF coil element for magnetic resonance imaging also comprises a resonance circuit and a matching network connected with the resonance circuit. Wherein, the resonance circuit is a closed circuit formed by series connection of a plurality of capacitors (FIG. 11 specifically shows five capacitors C.sub.P, C.sub.F1, C.sub.F2, C.sub.Fn-1, and C.sub.Fn constituting the resonance circuit) through a conductor (the conductor is typically a copper wire), and the matching network consists of a capacitor C.sub.S.

[0106] Identical with the first embodiment, an active loss circuit R.sub.LOSS is particularly arranged in the RF coil element to dissipate and absorb the RF power in the RF coil element to decrease the Q value of the RF coil element.

[0107] Different from the first embodiment, one active loss circuit is arranged in the RF coil element in this embodiment, and the active loss circuit is arranged at a position away from the resonance circuit and is connected to a position away from the resonance circuit instead of being directly connected to the resonance circuit like the first embodiment.

[0108] Similarly, the active loss circuit R.sub.LOSS in the second embodiment is able to actively dissipate and absorb the RF power in the RF coil element to decrease the Q value of the RF coil element, that is, the efficiency of the RF coil element during transmission is reduced. Thus, when the RF coil element in the second embodiment is used to fabricate an RF coil for magnetic resonance imaging, particularly an array coil, the coupling degree between coil elements in the array coil can be reduced, thus improving the performance of the array coil used for transmission and particularly significantly improving the uniformity of the transmission field B1.

Embodiment 3

[0109] FIG. 12 shows a third embodiment of the RF coil element for magnetic resonance imaging of the invention, and the RF coil element in this embodiment also comprises a resonance circuit and a matching network connected with the resonance circuit. Wherein, the resonance circuit is a closed circuit formed by series connection of n capacitors (FIG. 12 specifically shows five capacitors C.sub.P, C.sub.H, C.sub.F2, C.sub.Fn-1, and C.sub.Fn constituting the resonance circuit) through a conductor (the conductor is typically a copper wire), and the matching network consists of a capacitor C.sub.S.

[0110] Identical with the second embodiment, an active loss circuit R.sub.LOSS is particularly arranged in the RF coil element to actively dissipate and absorb the RF power in the RF coil element to decrease the Q value of the RF coil element, and the active loss circuit R.sub.LOSS is arranged at a position away from the resonance circuit and is connected to a position away from the resonance circuit.

[0111] Different from the second embodiment, the active loss circuit R.sub.LOSS in this embodiment is a secondary resonance circuit (the secondary resonance circuit is equivalent to a resistor connected in parallel to two terminals of C.sub.Fn-1, thus being referred to as an equivalent resistor module or a resistance generation circuit) arranged at a position away from the resonance circuit instead of a simple resistor element. Obviously, the secondary resonance circuit in FIG. 12 is able to actively dissipate and absorb the RF power in the RF coil element to decrease the Q value of the RF coil element.

[0112] Similarly, the active loss circuit R.sub.LOSS in the third embodiment is able to actively dissipate and absorb the RF power in the RF coil element to decease the Q value of the RF coil element, that is, the active loss circuit R.sub.LOSS is able to reduce the efficiency of the RF coil element during transmission. Thus, when the RF coil element in the third embodiment is used to fabricate an RF coil for magnetic resonance imaging, particularly an array coil, the coupling degree between coil elements in the array coil can be decreased, thus, improving the performance of the array coil used for transmission and particularly significantly improving the uniformity of the transmission field B1.

Embodiment 4

[0113] FIG. 13 shows a fourth embodiment of the RF coil element for magnetic resonance imaging of the invention, and the RF coil element in this embodiment also comprises a resonance circuit and a matching network connected with the resonance circuit. Wherein, the resonance circuit is a closed circuit formed by series connection of a plurality of capacitors (FIG. 13 specifically shows five capacitors C.sub.P, C.sub.F1, C.sub.F2, C.sub.Fn-1, and C.sub.Fn constituting the resonance circuit) through a conductor (the conductor is typically a copper wire), and the matching network consists of a capacitor C.sub.S.

[0114] In this embodiment, an active loss circuit is particularly arranged in the RF coil element to actively dissipate and absorb the RF power in the RF coil element to decrease the Q value of the RF coil element.

[0115] Different from the first embodiment, the second embodiment and the third embodiment, the conductor used for series connection of the capacitors (including C.sub.P, C.sub.F2, C.sub.Fn-1, and C.sub.Fn) is a conductor with a conductivity lower than that of copper instead of a traditional copper wire. In this embodiment, the conductor is specifically an aluminum wire.

[0116] Obviously, the replacement of a traditional copper wire with the aluminum wire with a lower conductivity is equivalent to series connection of a small-resistance resistor to the resonance circuit, so that the RF power in the RF coil element can be actively dissipated and absorbed to decrease the Q value of the RF coil element.

[0117] Similarly, the active loss circuit in the fourth embodiment is able to actively dissipate and absorb the RF power in the RF coil element to decease the Q value of the RF coil element, that is, the active loss circuit R.sub.LOSS is able to reduce the efficiency of the RF coil element during transmission. Thus, when the RF coil element in the fourth embodiment is used to fabricate an RF coil for magnetic resonance imaging, particularly an array coil, the coupling degree between coil elements in the array coil can be decreased, thus, improving the performance of the array coil used for transmission and particularly significantly improving the uniformity of the transmission field B1.

Embodiment 5

[0118] FIG. 14 shows a fifth embodiment of the RF coil element for magnetic resonance imaging of the invention, and the RF coil element in this embodiment also comprises a resonance circuit and a matching network connected with the resonance circuit. Wherein, the resonance circuit is a closed circuit formed by series connection of a plurality of capacitors (FIG. 14 specifically shows five capacitors C.sub.P, C.sub.F1, C.sub.F2, C.sub.Fn-1, and C.sub.Fn constituting the resonance circuit) through a conductor (the conductor is typically a copper wire), and the matching network consists of a capacitor C.sub.S.

[0119] In this embodiment, an active loss circuit R.sub.LOSS is particularly arranged in the RF coil element to actively dissipate and absorb the RF power in the RF coil element to decrease the Q value of the RF coil element.

[0120] As can be known from the above description, the active loss circuit connected to the RF coil element in the first, second, third and fourth embodiments is able to actively dissipate and absorb the RF power in the RF coil element to decrease the Q value of the RF coil element, that is, the active loss circuit is able to reduce the efficiency of the RF coil element during transmission. Thus, when the RF coil element is used to fabricate an RF coil for magnetic resonance imaging, particularly an array coil, the coupling degree between coil elements in the array coil can be decreased, thus, improving the performance of the array coil used for transmission and particularly significantly improving the uniformity of the transmission field B1.

[0121] However, in the aforesaid five embodiments, the active loss circuit added to the RF coil element is only able to improve the performance of the RF coil element used for transmission (to reduce the coupling degree). However, when the RF coil element is used for reception, the active loss circuit still absorbs the RF power in the RF coil element to decrease the Q value of the RF coil element, which in turn reduces the efficiency of the RF coil element used for reception (the reception efficiency is drastically reduced), and this is undesired. The reception efficiency (signal to noise ratio of reception) is the first factor that should be taken into consideration during reception, and the coupling degree can be decreased by configuration of a pre-amplifier. So, the active loss circuit added to the RF coil element may reduce the most important reception performance, namely the signal to noise ratio of reception, of the coil. It is completely fine to apply the RF coil element to an RF transmitter array coil which does not involve reception because there is no problem about the reduction of the reception efficiency in this case. However, if the RF coil element is applied to an RF transceiver array coil, the reception efficiency of the coil will be drastically reduced inevitably during reception, which in turn results in blurred magnetic resonance images.

[0122] In order to solve this problem, an ingenuous solution is provided in the fifth embodiment: referring to FIG. 14, a diode D1 in series connection with the active loss circuit R.sub.LOSS is configured; when the coil element is used for transmission, the diode D1 is turned on, the active loss circuit R.sub.LOSS is connected to the coil element (the active loss circuit R.sub.LOSS is turned on), and in this case, the transmission uniformity focused by users is improved. When the coil element is used for reception, the diode D1 is turned off, the active loss circuit R.sub.LOSS is turned off accordingly (the active loss circuit R.sub.LOSS is not connected to the coil element), so that the reception efficiency focused by users will not be reduced in spite of the presence of the active loss circuit R.sub.LOSS.

[0123] Clearly, the diode D1 can be replaced with other components which are able to turn on the active loss circuit R.sub.LOSS when the coil is used for transmission and to turn off active loss circuit R.sub.LOSS when the coil is used for reception, and such components (such as the diode D1 in FIG. 14) are referred to as loss circuit on-off elements.

[0124] Because the active loss circuit R.sub.LOSS is turned on when the coil is used for transmission and is turned off when the coil is used for reception, the resonance circuit of the coil element have different frequencies and impedances during transmission and reception, while the structure of the matching network is constant during transmission and reception, and thus, magnetic resonance images cannot be acquired easily. In view of this, the structure of the coil element is further improved in the fifth embodiment particularly as follows:

[0125] In the fifth embodiment, a frequency compensation circuit, an impedance compensation circuit, a frequency compensation circuit on-off element used to turn on/off the frequency compensation circuit, and an impedance compensation circuit on-off element used to turn on/off the impedance compensation circuit are also configured in the RF coil element, wherein the frequency compensation circuit is specifically connected to the resonance circuit of the coil element, and the impedance compensation circuit is specifically connected to the matching network.

[0126] Generally speaking, when the coil element is used for transmission, the loss circuit on-off element, the frequency compensation circuit on-off element and the impedance compensation circuit on-off element are all turned on to allow the active loss circuit, the frequency compensation circuit and the impedance compensation circuit to be connected to the coil element; and when the coil element is used for reception, the loss circuit on-off element, the frequency compensation circuit on-off element and the impedance compensation circuit on-off element are all turned off to disconnect the active loss circuit, the frequency compensation circuit and the impedance compensation circuit from the coil element. In this way, the resonance frequency and impedance (characteristic impedance, generally 50) are kept consistent when the coil element is used for reception and transmission, and clear magnetic resonance images are obtained.

[0127] More particularly, as shown in FIG. 14, the active loss circuit R.sub.LOSS and the diode D1 which are in series connection are further connected in series with an inductor L.sub.F. Two terminals of the active loss circuit R.sub.LOSS, the diode D.sub.1 and the inductor L.sub.F which are connected in series are connected in parallel with the capacitor C.sub.H, and two terminals of the active loss circuit R.sub.LOSS and the diode D.sub.1 are connected in parallel with the capacitor C.sub.F. Herein, the inductor L.sub.F and the capacitor C.sub.F constitute the frequency compensation circuit, the diode D.sub.1 constitutes the frequency compensation circuit on-off element and the loss circuit on-off element. In addition, a capacitor C.sub.S2 and a diode D.sub.2 are additionally configured in the matching network, wherein after the capacitor C.sub.S2 and the diode D.sub.2 are connected in series, two terminals (namely two terminals of the capacitor C.sub.S2 and the diode D.sub.2) are connected in parallel with the capacitor C.sub.S in the matching network. Herein, the capacitor C.sub.S2 constitutes the impedance compensation circuit, and the diode D.sub.2 constitutes the impedance compensation circuit on-off element.

[0128] When the coil element is used for transmission, the diode D.sub.1 and the diode D.sub.2 are turned on to allow the active loss circuit R.sub.LOSS and the frequency compensation circuit (the inductor L.sub.F and the capacitor C.sub.F) and the impedance compensation circuit (the capacitor C.sub.S2) to be connected to the coil element, and at this moment, the whole equivalent circuit of the coil element is shown in FIG. 16. In this case, the capacitor C.sub.S2 is connected to the matching network to participate in impedance matching and is regarded as a constituent part of the matching network; and the active loss circuit R.sub.LOSS is connected to the resonance circuit to participate in resonance and is regarded as a constituent part of the resonance circuit.

[0129] When the coil element is used for reception, the diode D.sub.1 and the diode D.sub.2 are turned off to disconnect the active loss circuit R.sub.LOSS and the frequency compensation circuit (the inductor L.sub.F and the capacitor C.sub.F) and the impedance compensation circuit (the capacitor C.sub.S2) from the coil element, and as shown in FIG. 15, the whole equivalent circuit of the coil element at this moment is equivalent to an original (traditional) coil element. During transmission, the resonance frequency of the resonance circuit will be changed due to the presence of the active loss circuit R.sub.LOSS, while the inductor L.sub.F and the capacitor C.sub.F can compensate for a deviation of the resonance frequency. In addition, although the impedance of the coil turns into Z.sub.Coil, the capacitor C.sub.S used for reception is replaced with the capacitor C.sub.S and the capacitor C.sub.S2 which are connected in parallel in the matching network, so that Z.sub.Coil still matches the characteristic impedance 50. In this case, the capacitor C.sub.S2 is not connected to the matching network and does not participate in impedance matching, and the active loss circuit R.sub.LOSS is not connected to the resonance circuit and does not participate in resonance.

[0130] That is to say, as long as the corresponding relationship among the active loss circuit R.sub.LOSS, the inductor L.sub.F and the capacitor C.sub.F is properly designed, the resonance frequency and characteristic impedance can be kept consistent (matching) both in the reception stage and in the transmission stage of the coil element.

[0131] It should be noted that the frequency compensation circuit and the impedance compensation circuit are not limited to the specific structure forms shown in FIG. 14, any circuits (various circuit components in the coil element) that are able to keep the resonance frequency matching the characteristic impendence during transmission and reception can be used as the frequency compensation circuit and the impedance compensation circuit. For example, in FIG. 14, the capacitor C.sub.F connected in parallel to the two terminals of the active loss circuit R.sub.LOSS and the capacitor C.sub.F can be removed, and in this case, the frequency compensation circuit is formed by the inductor L.sub.F only. The capacitor C.sub.F is connected in parallel to the two terminals of the active loss circuit R.sub.LOSS and the capacitor C.sub.F in the fifth embodiment for the purpose of easier control during adjustment for frequency compensation.

[0132] It should be noted that the matching network may be of various structures. In certain embodiments, the matching network further includes an inductor, and in this case, the impedance compensation circuit can be selectively connected in parallel to the two terminals of the inductor of the matching network.

Embodiment 6

[0133] When the coil element shown in FIG. 14 is used for transmission only (for example, the coil element is applied to a transmitter-only array coil) that does not involve state switching, and the diode D.sub.1, the diode D.sub.2, the inductor L.sub.F, the capacitor C.sub.F and the impedance compensation circuit (capacitor C.sub.S2) can be removed. On the basis of the RF coil element shown in FIG. 14, the transceiver-only coil element shown in FIG. 17 can be obtained by necessary RF-Trap (Balun) and RF amplifier power feed.

Embodiment 7

[0134] On the basis of the coil element shown in FIG. 14, an RF transceiver array coil element can be obtained by the addition of a high-power RF switch, necessary Balun and a pre-amplifier used for reception. The circuit structure of the RF transceiver array coil element is shown in FIG. 18.

[0135] The working principle of the coil element shown in FIG. 18 is as follows:

[0136] When a magnetic resonance system is in an RF transmission state, the RF switch is switched to a transmission link, two RF diodes (D.sub.1 and D.sub.2) are turned on, at this moment, the capacitor C.sub.S and the capacitor C.sub.S2 which are connected in parallel in the matching network are turned on to regulate the impedance Z.sub.coil generated by the resonance circuit to the characteristic impedance 50, and the RF amplifier and the coil element are in a good power matching condition.

[0137] When the magnetic resonance circuit is in an RF reception state, the RF switch is switched to a reception link, the two RF diodes (D.sub.1 and D.sub.2) are turned off, at this moment, the capacitor C.sub.S in the matching network regulates the impedance Z.sub.Coil generated by the resonance circuit to the characteristic impedance 50, and the pre-amplifier and the coil element are in a good noise matching condition.

[0138] In conclusion, no matter whether the coil element is in a transmission state or a reception state, the coil element is in a good power matching or noise matching condition. In the transmission state, the sensitivity of the coil element is drastically reduced due to the presence of the active loss circuit R.sub.LOSS, so that the coupling between coil elements can be reduced during transmission.

Embodiment 8

[0139] FIG. 19 shows another embodiment of the RF coil element for magnetic resonance imaging of the invention. In this embodiment, the RF coil element for magnetic resonance imaging also comprises a resonance circuit and a matching network connected with the resonance circuit, wherein the resonance circuit is a closed circuit formed by series connection of a plurality of capacitors (FIG. 19 specifically shows five capacitors C.sub.P, C.sub.F1, C.sub.F2, C.sub.Fn-1, and C.sub.Fn constituting the resonance circuit) through a conductor (the conductor is typically a copper wire), and the matching network consists of a capacitor C.sub.S.

[0140] In this embodiment, an active loss circuit R.sub.LOSS is particularly arranged in the RF coil element to actively dissipate and absorb RF power in the RF coil element to decrease the Q value of the RF coil element. The active loss circuit R.sub.LOSS is connected in parallel to two terminals of the capacitor C.sub.F2 in the resonance circuit.

[0141] On the basis of the same consideration as the fifth embodiment, a loss circuit on-off element used to control the active loss circuit R.sub.LOSS to be turned on/off, a frequency compensation circuit, an impedance compensation circuit, a frequency compensation circuit on-off element used to turn on/off the frequency compensation circuit, and an impedance compensation circuit on-off element used to turn on/off the impedance compensation circuit are arranged in the RF coil element, wherein the frequency compensation circuit is specifically connected to the resonance circuit of the coil element, and the impedance compensation circuit is specifically connected to the matching network.

[0142] The structural forms of the loss circuit on-off element, the frequency compensation circuit, the impedance compensation circuit, the frequency compensation circuit on-off element and the impedance compensation circuit on-off element in this embodiment are completely different from those in the fifth embodiment. Particularly, in this embodiment, the active loss circuit R.sub.LOSS is connected in series with a diode D.sub.1 and is then connected in parallel with the capacitor C.sub.F2 in the resonance circuit, an inductor L.sub.F is connected in series with another diode D.sub.2 and is then connected in parallel with the capacitor C.sub.F1 in the resonance circuit, and a capacitor C.sub.S2 is connected in series with another diode D3 and is then connected in parallel with the capacitor C.sub.S in the matching network. Appreciably, the inductor L.sub.F in parallel connection with the capacitor C.sub.F2 constitutes the frequency compensation circuit, the capacitor C.sub.S2 in parallel connection with the capacitor C.sub.S constitutes the impedance compensation circuit, the diode D.sub.1 in series connection with the active loss circuit R.sub.LOSS constitutes the loss circuit on-off element, the diode D.sub.2 in series connection with the inductor L.sub.F constitutes the frequency compensation circuit on-off element, and the diode D3 in series connection with the capacitor C.sub.S2 constitutes the impedance compensation circuit on-off element.

Embodiment 9

[0143] Different from array coils, birdcage coils have no distinct element concept or distribution, and have a corresponding port concept. The principle of the invention is also applicable to the birdcage coils (not matter how many ports are configured).

[0144] The circuit principle of a traditional birdcage coil (one structural form of RF coils) is shown in FIG. 20, wherein CR represents capacitors at terminal rings, and CL represents capacitors on the legs.

[0145] FIG. 21 shows a birdcage coil improved by the inventor of this application. As shown in FIG. 21, a corresponding active loss circuit is connected in parallel to two terminals of the capacitors on the ring legs of the birdcage coils, R.sub.1 is connected in parallel to the two terminals of C.sub.L1, R.sub.K is connected in parallel to the two terminals of CLK, and R.sub.n is connected in parallel to the two terminals of C.sub.Ln. The active loss circuits can also be arranged on a terminal ring circuit.

[0146] The active loss circuits R.sub.1, R.sub.K, and R.sub.n are able to actively dissipate and absorb RF power in the birdcage coil to decrease the Q value of the birdcage coil, that is, the active loss circuits R.sub.1, R.sub.K, and R.sub.n significantly reduce the efficiency of the birdcage coil during transmission. Similarly, the coupling between the ports can be effectively reduced, thus effectively improving the transmission performance of the birdcage coil.

Embodiment 10

[0147] Referring to FIG. 22, the technical solution of the invention is introduced in detail below with an 8-channel transceiver RF array coil as an example.

[0148] The 8-channel transceiver RF array coil in this embodiment adopts 8 coil elements mentioned in the seventh embodiment (FIG. 18), and every two adjacent coil elements overlap partially. It should be noted that the coil in this embodiment is a cylindrical coil, and the 8 coil elements are adjacently arrayed end-to-end around a cylinder to form the array coil, that is, the element 1 and the element 8 also overlap partially.

[0149] In order to verify the validity of the patent, this embodiment is subjected to a comparative test in a Siemens Verio 3.0T system. FIG. 23 shows specific results of this embodiment, and FIG. 24 shows test results of a traditional 8-channel transceiver coil. The quantity and shape (symmetry) of black stripes in the figures reflect the uniformity of the RF transmission field. As can be seen from the test results, the uniformity of the transmission field B1 in this embodiment is significantly improved.

[0150] Compared with the existing common array coil shown in FIG. 5, the transceiver RF array coil in this embodiment has the following advantages and disadvantages:

[0151] 1. Coupling between the elements during transmission: when the coil is in a transmission state, the Q value and the sensitivity of the coil element are drastically reduced due to the present of the active loss circuit R.sub.LOSS, so that the coupling between elements is greatly reduced.

[0152] 2. Transmission efficiency of the coil: because the Q value of the resonance circuit and the sensitivity of the coil element are drastically reduced, the transmission efficiency of the coil is also drastically reduced; however, the coil in this patent is generally used for multi-channel transmission during which multiple RF power amplifiers work synchronously, so that the requirement for the output power of each RF power amplifier is low, and common commercial RF power amplifiers can meet the requirement.

[0153] 3. The uniformity of the transmission field: in the transmission state, the sensitivity of each element is reduced due to the presence of the active loss circuit R.sub.LOSS, and the coupling between the coil elements is greatly reduced, thus guaranteeing that the matching and sensitivity of the elements are highly consistent and remarkably improving the uniformity of the transmission field.

[0154] 4. The stability of the transmission field: in the traditional design, the coil element with a high sensitivity is very sensitive to a load during transmission, and the transmission field may severely fluctuate due to load fluctuations. However, in this patent, the sensitivity of each element is reduced by the active loss circuit R.sub.LOSS, the fluctuation caused by load fluctuations is reduced accordingly, and thus, the stability and consistency of the transmission field under different load conditions are improved.

[0155] 5. Parallel transmission (pTX) performance: because the pTX performance is highly related to the coupling between the elements, the pTX performance will be improved accordingly by reduction of the coupling between the elements.

[0156] 6. Coupling during reception: when the coil is in the reception state, the active loss circuit RLoss is disconnected from the resonance circuit, the Q value of the resonance circuit and the sensitivity of the coil element are increased to the existing common coil level, and the coupling degree is increased accordingly. In the reception state, the coupling is generally acceptable under the effect of pre-amp decoupling.

[0157] 7. Signal to noise ratio during reception: because of the pre-amp decoupling function, the signal to noise ratio during reception is not affected in this embodiment.

[0158] 8. Penetration capacity during reception: as shown in FIG. 5, in order to reduce the coupling between elements during transmission, the area of the element is much smaller than the area of the element in this embodiment, and thus, the penetration capacity and depth of the coil in this embodiment are significantly improved.

[0159] The invention further has various other embodiments. All technical solutions obtained by means of equivalent substitutions or transformations should also fall within the protection scope of the invention.