Coil assembly for single knee and double knee joint magnetic resonance imaging

Abstract

The application relates to the technical field of magnetic resonance imaging, in particular to a coil assembly for single-knee and double-knee joint magnetic resonance imaging, which comprises two cavities arranged along the left-right direction of a subject and used for receiving the left knee and the right knee of the subject respectively, two birdcage coils arranged around the two cavities respectively and used for providing radio frequency excitation signals to the left knee and the right knee respectively, two groups of receiving coils arranged at the two cavities respectively and used for receiving magnetic resonance signals from the left knee and the right knee respectively, and a power divider, two radio frequency power output ports of which are connected to the two birdcage coils via transmission blocking switching circuits to provide radio frequency excitation signals to the birdcage coils; the two birdcage coils are separated from each other, the receiving coils are located on the inner circumferential side of the birdcage coils, and third resistors are connected to the birdcage coils.

Description

Technical Field

[0001] This application relates to the field of magnetic resonance imaging technology, and more particularly to a coil assembly for magnetic resonance imaging of single and double knee joints. Background Technology

[0002] Magnetic resonance imaging (MRI) is widely used in clinical practice. It involves no radiation and offers advantages over X-rays and CT scans, including higher soft tissue resolution, multi-planar imaging, and multi-sequence imaging, significantly improving disease detection and assessment rates. However, the long scan time and numerous contraindications of MRI limit its clinical application.

[0003] Coils are an essential component of MRI; they are necessary to complete an MRI scan. The function of a coil is to be placed at the site to be examined to receive signals from specific tissues for MRI scanning and signal acquisition. Specific areas require dedicated coils; for example, there are dedicated coils for the head, abdomen, and joints. Current knee joint scans are generally unilateral, allowing only one knee to be scanned at a time, taking 15-20 minutes. For bilateral symmetrical joint scans (such as bilateral knees or ankles), the coil must be placed on the other side after scanning one side for repositioning; completing bilateral joint scans takes approximately 30-40 minutes. Current bilateral symmetrical joint scanning methods are time-consuming and limit clinical application. Clinically, there are many cases requiring bilateral symmetrical joint examinations, creating an urgent need for joint coils capable of simultaneously scanning both sides and shortening the scan time.

[0004] The coil assembly disclosed in CN120446835A can be used for bilateral and unilateral knee MRI. However, the coil unit configured within it has the following limitation: when performing unilateral knee MRI, due to its small FOV (Field of View), the signal from the other knee may be folded in during image reconstruction, resulting in image artifacts. Summary of the Invention

[0005] To address at least one of the aforementioned technical problems, this application provides a coil assembly for magnetic resonance imaging of single and double knee joints.

[0006] The coil assembly for unilateral and bilateral knee joint magnetic resonance imaging proposed in this application includes: Two cavities are arranged along the left-right direction of the subject, and are used to receive the subject's left knee and right knee respectively; Two birdcage coils are arranged around the two cavities respectively, and are used to provide radio frequency excitation signals to the left knee and the right knee respectively; Two sets of receiving coils are respectively disposed in the two cavities, and are used to receive magnetic resonance signals from the left knee and the right knee respectively; The power divider has two radio frequency power output ports connected to the two birdcage coils via transmit blocking switching circuits to provide radio frequency excitation signals to the birdcage coils. The two birdcage coils are spaced apart from each other, the receiving coil is located on the inner circumference of the birdcage coil, and a third resistor is connected to the birdcage coil.

[0007] Preferably, each of the birdcage coils comprises: Two end rings are arranged at intervals along the length of the subject; Multiple legs are arranged spaced apart along the length of the body, and each leg is connected to the two end rings. Each leg includes a third capacitor and a third resistor connected in parallel.

[0008] Preferably, it includes: The power divider has two radio frequency power output ports connected to the two birdcage coils via transmit blocking switching circuits to provide radio frequency excitation signals to the birdcage coils. Each of the aforementioned transmit blocking switching circuits includes: A first diode is connected between the RF power output port and the birdcage coil; A second capacitor, a first inductor, a first capacitor, and a first resistor are connected in series. The other end of the second capacitor is connected between the RF power output port and the first diode. The other end of the first resistor is grounded. The resistance of the first resistor is 50 ohms. A second inductor, a second diode, and a third diode are connected in series. The other end of the second inductor is connected between the second capacitor and the first diode, and the other end of the third diode is connected between the first capacitor and the first resistor. The circuit node between the second capacitor and the first inductor is connected to the circuit node between the second diode and the third diode. A bias control circuit is used to control the first diode, the second diode, and the third diode to be selectively in a forward-biased state or a reverse-biased state. When the bias control circuit controls the first diode, the second diode, and the third diode to be in a forward bias state, the second capacitor and the second inductor form a parallel resonant circuit; when the bias control circuit controls the first diode, the second diode, and the third diode to be in a reverse bias state, the second capacitor, the first inductor, and the first capacitor form a series resonant circuit.

[0009] Preferably, the two bias control circuits corresponding to the two birdcage coils can independently control the forward bias and reverse bias states of the first diode, the second diode, and the third diode in the corresponding transmit blocking switching circuit.

[0010] Preferably, the anode of the first diode is connected to the radio frequency power output port, and the cathode is connected to the birdcage coil; The anode of the second diode is connected to the second inductor, and the cathode is connected to the anode of the third diode; The bias control unit includes a bias voltage input node connected between the second capacitor and the second inductor.

[0011] Preferably, the cathode of the first diode is connected to the birdcage coil via a 90-degree power divider phase shifter.

[0012] Preferably, the two output ports of the 90-degree power divider phase shifter are connected to the birdcage coil, one input port of the 90-degree power divider phase shifter is connected to the cathode of the first diode, and the other input port is grounded via a 50-ohm resistor.

[0013] Preferably, it includes: A first housing having two first recesses recessed from the outer surface of the first housing, the two first recesses being adapted to receive the left knee and the right knee respectively from the front side of the subject, a portion of the birdcage coil being disposed within the first housing; A second housing detachably connected to the first housing has two second recesses recessed from the outer surface of the second housing, the two second recesses being adapted to receive the left knee and the right knee respectively from the rear side of the subject; When the second housing is connected to the first housing, the first recess and the second recess define the cavity.

[0014] Preferably, the two cavities are separated from each other and are not connected.

[0015] Preferably, the receiving coil is a toroidal coil.

[0016] According to the coil assembly provided in this application, on the one hand, a self-transmitting and self-receiving coil design is adopted—two birdcage coils are separated from each other, and the receiving coil is located on the inner periphery of the birdcage coil; on the other hand, resistors are set on the legs of the birdcage coils, thereby reducing the Q value of the birdcage coils, which can significantly reduce the coupling between the two birdcage coils. Although the scheme of setting parallel resistors on the birdcage coils requires increasing the transmission power, the transmission power of the magnetic resonance system has a large margin for local excitation, and increasing the transmission power is not a problem at all. Therefore, the coil assembly of this embodiment achieves no folding artifacts in single-sided imaging while simultaneously meeting the requirements of bi-sided imaging. In addition, the coil assembly provided in this application can operate in both single-knee scanning mode and bi-knee scanning mode, and in both single-knee and bi-knee modes, it is in a good impedance matching state. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings of the embodiments will be briefly described below. Obviously, the drawings described below only relate to some embodiments of this application, and are not intended to limit this application.

[0018] Figure 1 This is a front view of the coil assembly provided in the embodiments of this application.

[0019] Figure 2 This is a schematic diagram of the coil assembly after the first housing has been opened.

[0020] Figure 3 This is a schematic diagram showing the arrangement of the birdcage coil and the receiving coil in the coil assembly.

[0021] Figure 4 It is observed along the length of the examinee's body. Figure 3 A schematic diagram.

[0022] Figure 5 This is a circuit diagram showing the birdcage coil after it has been unfolded.

[0023] Figure 6 This is a schematic diagram of the radio frequency transmission blocking switching circuit of the birdcage coil.

[0024] Figure 7 yes Figure 6 A schematic diagram of a portion of the transmit blocking switching circuit.

[0025] Figure 8 This is when a reverse bias voltage is provided at the bias voltage input node. Figure 7 The equivalent circuit.

[0026] Figure 9 This is when a positive bias voltage is provided at the bias voltage input node. Figure 7The equivalent circuit.

[0027] Figure 10 This is when one bias voltage input node provides a reverse bias voltage and the other bias voltage input node provides a forward bias voltage. Figure 6 The equivalent circuit.

[0028] in: 1-First shell, 1a-First recess; 2-Second shell, 2a-Second recess; 3-cavity; 4-Birdcage coil; 5-End ring; 6-legs; 7-Power Amplifier; 8-Power divider; 9-Toroidal coil; 10-90 degree power divider phase shifter; 11 - Bias voltage input node; R1 - First resistor, R2 - First resistor, R3 - Third resistor, C1 - First capacitor, C2 - Second capacitor, C3 - Third capacitor, C4 - Fourth capacitor, L1 - First inductor, L2 - Second inductor, D1 - First diode, D2 - Second diode, D3 - Third diode. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the described embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. It is understood that, without conflict, some technical means of the various embodiments described herein can be substituted for or combined with each other.

[0030] In the description of this application, the terms "first" and "second" are used only to distinguish the objects being described and have no sequential or technical meaning.

[0031] Figures 1 to 10 A coil assembly according to an embodiment of this application is shown. The coil assembly can be used for magnetic resonance imaging of both knee joints or for magnetic resonance imaging of a single knee joint (left or right knee). It includes a first housing 1 and a second housing 2 detachably connected to the first housing 1.

[0032] The first housing 1 is made almost entirely of plastic and has two first recesses 1a recessed from the outer surface of the first housing 1. The second housing 2 is also made almost entirely of plastic and has two second recesses 2a recessed from the outer surface of the second housing 2.

[0033] The two first recesses 1a are designed to receive the subject's left and right knees from the front, respectively, and the two second recesses 2a are designed to receive the subject's left and right knees from the rear, respectively. When the second housing 2 is connected to the first housing 1, the two first recesses 1a and the two second recesses 2a define two cavities 3, which respectively receive the subject's left and right knees. The two cavities 3 are spaced apart from each other and do not communicate with each other.

[0034] The coil assembly also includes two birdcage coils 4 and two sets of receiving coils.

[0035] When the first housing 1 and the second housing 2 are connected to each other to define the aforementioned two cavities 3, two birdcage coils 4 are arranged around the two cavities 3 respectively, and are used to provide radio frequency excitation signals to the left knee and the right knee respectively. That is, both birdcage coils 4 are transmitting coils, one birdcage coil 4 is used to transmit radio frequency excitation signals to the left knee, and the other birdcage coil 4 is used to transmit radio frequency excitation signals to the right knee.

[0036] When the first housing 1 and the second housing 2 are connected to define the aforementioned two cavities 3, two sets of receiving coils are respectively disposed in the two cavities 3, and are used to receive magnetic resonance signals from the left knee and the right knee, respectively. That is, one set of receiving coils is used to receive the magnetic resonance signal from the left knee, and the other set of receiving coils is used to receive the magnetic resonance signal from the right knee. Furthermore, the receiving coils are located on the inner periphery of the birdcage coils 4, and the two birdcage coils 4 are spaced apart from each other.

[0037] For the dual-birdcage excitation dual-knee imaging coil provided in this embodiment, if only one knee joint needs to be imaged, the corresponding birdcage coil 4 and receiving coil are selected to work; if both knees need to be imaged simultaneously, the birdcage coil 4 and receiving coil on both sides are selected to work simultaneously.

[0038] In this embodiment, the receiving coil is a ring coil 9, and multiple ring coils 9 are arranged around the cavity 3 to form a coil array, which becomes the aforementioned set of receiving coils.

[0039] In this embodiment, a portion of the birdcage coil 4 is disposed within the first housing 1, and another portion is disposed within the second housing 2; a portion of the coils in each set of receiving coils is disposed within the first housing 1, and the other portion is disposed within the second housing 2. When the first housing 1 and the second housing 2 are connected to each other, the two portions of the birdcage coil 4 are connected to each other, thereby forming a complete birdcage coil 4; the two portions of each set of receiving coils cooperate with each other to form a coil array surrounding the cavity 3. In this embodiment, a first electrical connector and a second electrical connector are respectively provided on opposite sides of the first housing 1 and the second housing 2. The first electrical connector and the second electrical connector respectively connect a portion and another portion of the birdcage coil 4. When the first housing 1 and the second housing 2 are connected to each other, the first electrical connector and the second electrical connector are plugged into each other to form a complete birdcage coil 4.

[0040] The birdcage coil 4 includes two end rings 5 ​​and multiple legs 6. The two end rings 5 ​​are spaced apart along the length of the subject's body, and the multiple legs 6 are spaced apart around the length of the subject's body. Each leg 6 is connected to two end rings 5, and each leg 6 includes a third capacitor C3 and a third resistor R3 connected in parallel. Multiple fourth capacitors C4 are connected in series on the end rings 5 ​​and arranged sequentially around the length of the subject's body. Each fourth capacitor C4 is positioned between two different adjacent legs 6.

[0041] When performing MRI on a single knee joint (left or right) of a subject, the small FOV (Field of View) can cause signal artifacts from the other knee joint to be folded in during image reconstruction. Furthermore, the close proximity of the two birdcage coils results in significant coupling between them, affecting emission uniformity and consequently impacting imaging quality. Advantageously, in this embodiment, the use of a self-transmitting and self-receiving coil design—with the outer birdcage coil 4 serving as the transmitting coil and the inner ring coil array serving as the receiving coil, and the birdcage coils 4 on both sides being spaced apart—helps to eliminate folding artifacts in unilateral imaging. Furthermore, this embodiment incorporates a first resistor R1 connected in parallel with the first capacitor C1 on the support leg 6 of the birdcage coil 4, thereby reducing the Q value of the birdcage coil 4 and significantly reducing the coupling between the two birdcage coils 4. Although this parallel resistor on the birdcage coil 4 requires increased transmission power, the transmission power of the magnetic resonance system has a large margin for local excitation, making the increase in transmission power not a problem. Therefore, the coil assembly in this embodiment achieves unilateral imaging without folding artifacts while simultaneously meeting the requirements for bilateral imaging.

[0042] The coil assembly also includes a power divider 8 (RF power divider 8), whose input port is connected to a power amplifier 7 to receive RF power signals from the power amplifier 7. The power divider 8 has two RF power output ports, which are connected to two birdcage coils 4 respectively via a transmit blocking switching circuit to provide RF excitation signals to the birdcage coils 4. The transmit blocking switching circuit can operate in a blocking state, thereby not providing RF excitation signals to the birdcage coils 4.

[0043] Each transmit blocking switching circuit includes a first diode D1, a second diode D2, a third diode D3, a first inductor L1, a second inductor L2, a first capacitor C1, a second capacitor C2, a first resistor R1, and a bias control circuit.

[0044] The anode of the first diode D1 is connected to the RF power output port of the power divider 8, and the cathode is connected to the birdcage coil 4 via the 90-degree power divider phase shifter 10. The 90-degree power divider phase shifter 10 is a four-port power divider phase shifter with two input ports and two output ports.

[0045] The second capacitor C2, the first inductor L1, the first capacitor C1, and the first resistor R1 are connected in series. The other end of the second capacitor C2 is connected between the RF power output port and the first diode D1, and the other end of the first resistor R1 is grounded. The resistance of the first resistor R1 is 50 ohms.

[0046] The second inductor L2, the second diode D2, and the third diode D3 are connected in series. The other end of the second inductor L2 is connected between the second capacitor C2 and the first diode D1, and the other end of the third diode D3 is connected between the first capacitor C1 and the first resistor R1. The circuit node between the second capacitor C2 and the first inductor L1 is connected to the circuit node between the second diode D2 and the third diode D3—one end of a wire (not shown in the figure) is connected between the second capacitor C2 and the first inductor L1, and the other end is connected between the second diode D2 and the third diode D3.

[0047] The bias control circuit is used to selectively control the first diode D1, the second diode D2, and the third diode D3 to be in a forward-biased state or a reverse-biased state. In this embodiment, the bias control circuit includes a bias voltage input node 11 connected between the second capacitor C2 and the second inductor L2. It is understood that when the bias voltage input node 11 provides a forward bias voltage, the first diode D1, the second diode D2, and the third diode D3 are in a forward-biased state and conduct; when the bias voltage input node 11 provides a reverse bias voltage, the first diode D1, the second diode D2, and the third diode D3 are in a reverse-biased state and are cut off.

[0048] When the bias control circuit controls the first diode D1, the second diode D2, and the third diode D3 to be in the reverse-biased cutoff state (for ease of description, referred to as the first state), the ground portion of the emitter blocking switching circuit is equivalent to... Figure 9 The circuit shown consists of a second capacitor C2, a first inductor L1, and a first capacitor C1 forming a series resonant circuit. The part to ground is in a series resonant conducting state. Therefore, the RF excitation signal output from the RF power output port of the power divider 8 will be transmitted to the 50-ohm first resistor R1. The transmit path part of the transmit blocking switching circuit (the part where the first diode D1 is located) is in an open state. Therefore, the RF excitation signal output from the RF power output port of the power divider 8 cannot be transmitted to the corresponding birdcage coil 4, so that the birdcage coil 4 is in a non-transmitting state.

[0049] When the bias control circuit controls the first diode D1, the second diode D2, and the third diode D3 to be in the forward bias conducting state (for ease of description, referred to as the second state), the ground portion of the emitter blocking switching circuit is equivalent to... Figure 8 The parallel resonant circuit shown, consisting of the second capacitor C2 and the second inductor L2, has its ground portion in an open-circuit state. Therefore, the RF excitation signal provided from the RF power output port of the power divider 8 is not transmitted to the 50-ohm first resistor R1. Furthermore, the transmit path portion of the transmit blocking switching circuit is turned on, so the RF excitation signal provided from the RF power output port of the power divider 8 is transmitted to the corresponding birdcage coil 4, putting the birdcage coil 4 into a transmit state.

[0050] To ensure the quality of magnetic resonance imaging (MRI) scans, the MRI system must operate in an impedance-matched state. When a birdcage coil 4 is in the transmitting state, the impedance at the operating ports of all connected components (including power amplifier 7, power divider 8, and 90-degree power divider phase shifter 10) must be its characteristic impedance—50 ohms. For the power divider 8, the direct connection impedance of both of its RF power output ports must be 50 ohms to ensure that both RF power output ports operate in an impedance-matched state. That is, the connection impedance of one RF power output port will affect the connection impedance of the other RF power output port. Therefore, even if the other birdcage coil 4 is in a non-transmitting state while one birdcage coil 4 is in the transmitting state, the direct connection impedance of the RF power output port corresponding to the non-transmitting state must still be 50 ohms. Advantageously, in this embodiment, even if the circuit corresponding to one birdcage coil 4 is in the aforementioned first state and the circuit corresponding to the other birdcage coil 4 is in the aforementioned second state (single-knee scan mode), the impedance matching requirements of the system can still be guaranteed. The specific analysis is as follows: On the path corresponding to the first state, the RF excitation signal provided by the corresponding RF power output port of the power divider 8 is transmitted to the first resistor R1 of 50 ohms and disconnected from the 90-degree power divider phase shifter 10. Therefore, the direct access impedance at the RF power output port is 50 ohms. In the path corresponding to the second state, the ground portion of the transmit blocking switching circuit is in an open-circuit state. The RF excitation signal provided by the other RF power output port of the power divider 8 is distributed to the two ports of the birdcage coil 4 via one input port of the 90-degree power divider phase shifter 10 and then to the two output ports of the 90-degree power divider phase shifter 10. The two ports of the birdcage coil 4 are set to have an impedance of 50 ohms, thus making the direct impedance of the two output ports of the 90-degree power divider phase shifter 10 50 ohms. Furthermore, one input port of the 90-degree power divider phase shifter 10 is grounded via a 50-ohm second resistor R2, also with a direct impedance of 50 ohms. Therefore, the direct impedance of all three ports of the 90-degree power divider phase shifter 10 is 50 ohms. This also makes the input port of the 90-degree power divider phase shifter 10 connected to the power divider 8 have an impedance of 50 ohms, and the corresponding RF power output port's input impedance is 50 ohms. Therefore, both RF power output ports of the power divider 8 have an input impedance of 50 ohms, thus operating in an impedance-matched state.

[0051] The RF excitation signals output from the two output ports of the 90-degree power divider phase shifter 10 are 90° out of phase, which can ensure the circular polarization of the birdcage coil 4.

[0052] The coil assembly also includes a locking mechanism (not shown) capable of switching between a locked state and a released state. In the locked state, the locking mechanism prevents the second housing 2 from separating from the first housing 1. In the released state, the locking mechanism allows the second housing 2 to separate from the first housing 1. The locking mechanism may include a locking member and a locked member. The locked member is disposed on the first housing 1 and includes a latch extending towards the front of the subject, while the locking member is disposed on the second housing 2 and includes an unlocking button and an elastic member that applies a biasing force to the unlocking button.

Claims

1. A coil assembly for magnetic resonance imaging of a single and double knee joint, characterized in that, include: Two cavities are arranged along the left-right direction of the subject, and are used to receive the subject's left knee and right knee respectively; Two birdcage coils are arranged around the two cavities respectively, and are used to provide radio frequency excitation signals to the left knee and the right knee respectively; Two sets of receiving coils are respectively disposed in the two cavities, and are used to receive magnetic resonance signals from the left knee and the right knee respectively; The power divider has two radio frequency power output ports connected to the two birdcage coils via transmit blocking switching circuits to provide radio frequency excitation signals to the birdcage coils. The two birdcage coils are spaced apart from each other, the receiving coil is located on the inner circumference of the birdcage coil, and a third resistor is connected to the birdcage coil.

2. The coil assembly according to claim 1, characterized in that, Each of the birdcage coils includes: Two end rings are arranged at intervals along the length of the subject; Multiple legs are arranged spaced apart along the length of the body, and each leg is connected to the two end rings. Each leg includes a third capacitor and a third resistor connected in parallel.

3. The coil assembly according to claim 1, characterized in that, include: The power divider has two radio frequency power output ports connected to the two birdcage coils via transmit blocking switching circuits to provide radio frequency excitation signals to the birdcage coils. Each of the aforementioned transmit blocking switching circuits includes: A first diode is connected between the RF power output port and the birdcage coil; A second capacitor, a first inductor, a first capacitor, and a first resistor are connected in series. The other end of the second capacitor is connected between the RF power output port and the first diode. The other end of the first resistor is grounded. The resistance of the first resistor is 50 ohms. A second inductor, a second diode, and a third diode are connected in series. The other end of the second inductor is connected between the second capacitor and the first diode, and the other end of the third diode is connected between the first capacitor and the first resistor. The circuit node between the second capacitor and the first inductor is connected to the circuit node between the second diode and the third diode. A bias control circuit is used to control the first diode, the second diode, and the third diode to be selectively in a forward-biased state or a reverse-biased state. When the bias control circuit controls the first diode, the second diode, and the third diode to be in a forward bias state, the second capacitor and the second inductor form a parallel resonant circuit; when the bias control circuit controls the first diode, the second diode, and the third diode to be in a reverse bias state, the second capacitor, the first inductor, and the first capacitor form a series resonant circuit.

4. The coil assembly according to claim 3, characterized in that, The two bias control circuits corresponding to the two birdcage coils can independently control the forward bias and reverse bias states of the first diode, the second diode, and the third diode in the corresponding transmit blocking switching circuit.

5. The coil assembly according to claim 3, characterized in that, The anode of the first diode is connected to the radio frequency power output port, and the cathode is connected to the birdcage coil; The anode of the second diode is connected to the second inductor, and the cathode is connected to the anode of the third diode; The bias control unit includes a bias voltage input node connected between the second capacitor and the second inductor.

6. The coil assembly according to claim 5, characterized in that, The cathode of the first diode is connected to the birdcage coil via a 90-degree power divider phase shifter.

7. The coil assembly according to claim 6, characterized in that, The two output ports of the 90-degree power divider phase shifter are connected to the birdcage coil, one input port of the 90-degree power divider phase shifter is connected to the cathode of the first diode, and the other input port is grounded via a 50-ohm resistor.

8. The coil assembly according to claim 1, characterized in that, include: A first housing having two first recesses recessed from the outer surface of the first housing, the two first recesses being adapted to receive the left knee and the right knee respectively from the front side of the subject, a portion of the birdcage coil being disposed within the first housing; A second housing detachably connected to the first housing has two second recesses recessed from the outer surface of the second housing, the two second recesses being adapted to receive the left knee and the right knee respectively from the rear side of the subject; When the second housing is connected to the first housing, the first recess and the second recess define the cavity.

9. The coil assembly according to claim 8, characterized in that, The two cavities are separated from each other and are not connected.

10. The coil assembly according to claim 1, characterized in that, The receiving coil is a toroidal coil.

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