Magnetic resonance imaging system and radio frequency transmission system for a magnetic resonance imaging system

Abstract

The present disclosure relates to the technical field of medical equipment, including a magnetic resonance imaging system and a radio frequency transmitting system for the magnetic resonance imaging system. By sequentially arranging a RF amplifier, a first signal transmission line, a signal processing unit and a second signal transmission line along a radio frequency transmitting direction in each radio frequency transmitting channel of the RF transmitting system, the signal processing unit comprising an impedance matching circuit and a signal transmission circuit, since the first signal transmission line, the second signal transmission line and the signal transmission circuit can adjust the phase delay of the RF pulse signal, and the impedance matching circuit can provide different impedance adjustment capabilities, the input impedance of the output end of the RF amplifier to the RF transmitting unit can be dynamically adjusted when scanning scanning objects of different weights, the impedance matching degree of the RF transmitting system can be improved without using an isolator, and the imaging effect of the MRI system can be improved.

Description

Technical Field

[0001] This disclosure relates to the field of medical device technology, and more particularly to radio frequency (RF) emission systems for magnetic resonance imaging (MRI) systems. Background Technology

[0002] Magnetic Resonance Imaging (MRI) is a medical imaging technique based on the phenomenon of nuclear magnetic resonance, widely used in the field of medical diagnostics. Generally, an MRI system includes a main magnet, a gradient amplifier, and an RF transmission system. The RF transmission system comprises an RF output unit, an RF amplifier, a signal processing unit, and an RF transmission unit. During MRI operation, the RF output unit generates RF pulse signals, which are amplified by the RF amplifier and then transmitted to the signal processing unit. Subsequently, in transmission mode, the signal processing unit transmits the amplified RF pulse signals to the RF transmission unit, which then transmits the RF pulses to the scanned object (such as a patient).

[0003] The RF transmitting unit includes an RF transmitting coil (e.g., a body coil), the load impedance of which changes with the weight of the object being scanned. This change in load impedance alters the RF amplifier's output characteristics, thereby affecting the performance of the RF transmitting system.

[0004] To avoid the impact of changes in the weight of the scanned object on the performance of the RF transmission system, an isolator is typically added between the RF amplifier and the RF transmitting coil. For example: [reference needed] Figure 1 The isolator shown can be connected to an RF transmitting coil at one end and an RF amplifier at the other end. In this case, the isolator can shield the RF amplifier from the influence of the signal reflected by the RF transmitting coil, thereby offsetting the influence of the load impedance on the output characteristics of the RF amplifier. Therefore, the performance of the RF amplifier is stable when the weight of the scanned object changes.

[0005] An isolator is a magnetic device that includes ferrite. During operation, a constant external bias magnetic field is applied to control the rotational characteristics of the ferrite, causing the polarization direction of the electromagnetic wave propagating in the ferrite to rotate due to the gyromagnetic effect. This allows the signal to be transmitted unidirectionally along the transmission direction, thereby shielding the signal reflected by the RF transmitting coil.

[0006] However, when the main magnetic field (i.e., the B0 magnetic field) of the MRI system is higher than the bias magnetic field of the ferrite, the gyromagnetic effect disappears. At this time, the isolator's isolation effect disappears, and the performance of the RF transmission system will still be affected by the weight change of the scanned object, resulting in the instability of the RF transmission system performance and the decline in the imaging quality of the MRI system. Summary of the Invention

[0007] According to one aspect of this disclosure, an RF transmission system for a magnetic resonance imaging system is provided, the RF transmission system comprising:

[0008] The RF output unit is used to generate and output RF pulse signals.

[0009] At least one radio frequency (RF) transmission channel is connected to the RF output unit, wherein each RF transmission channel includes:

[0010] An RF amplifier is used to amplify the RF pulse signal;

[0011] The signal processing unit includes an impedance matching circuit and a signal transmission circuit;

[0012] A first signal transmission line is connected between the RF amplifier and the impedance matching circuit, and between the RF amplifier and the signal transmission circuit. The impedance matching circuit includes at least two matching states, which provide different impedance adjustment capabilities.

[0013] A second signal transmission line is connected between the signal transmission circuit and the RF transmitting unit, wherein the RF transmitting unit transmits RF pulse signals.

[0014] The second signal transmission line and the RF transmitting unit together cause a phase delay of the RF pulse signal at a first angle; the signal transmission circuit and the first signal transmission line together cause a phase delay of the RF pulse signal at a second angle.

[0015] In one possible implementation, the at least two matching states include a first state and a second state; wherein the first state is the state in which the impedance matching circuit is turned on, and the second state is the state in which the impedance matching circuit is not turned on.

[0016] In one possible implementation, each impedance matching circuit includes at least two matching branches, and correspondingly, the first state includes at least two sub-states, with at least one matching branch corresponding to one sub-state of the impedance matching circuit when it is in operation.

[0017] In one possible implementation, the impedance matching circuit is in the first state when the mismatch index is greater than a preset threshold.

[0018] When the mismatch index is less than or equal to a preset threshold, the impedance matching circuit is in the second state.

[0019] The mismatch index is related to the forward feedback signal transmitted from the signal processing unit to the RF transmitting unit and the reverse feedback signal fed back from the RF transmitting unit to the signal processing unit and output by the signal processing unit.

[0020] In one possible implementation, the mismatch metric includes the voltage standing wave ratio (VSWR), and correspondingly, the preset threshold is 2.

[0021] In one possible implementation, the impedance matching circuit includes: two impedance matching branches, a first DC power supply device, a resistor, and a control port, wherein,

[0022] One of the two impedance matching branches has its first end connected to the signal transmission circuit and the first end of the second DC power supply device, respectively, with the second end of the second DC power supply device grounded; the first end of the other impedance matching branch is grounded; each impedance matching branch includes an inductor and a diode connected in series.

[0023] The first terminal of the first DC power supply device is connected to the second terminal of the two impedance matching branches, and the second terminal is connected to the first terminal of the resistor.

[0024] The control port is connected to the second end of the resistor and is used to receive a status control signal to control the impedance matching circuit to be in the first state or the second state.

[0025] In one possible implementation, each impedance matching branch includes: a first inductor, a second inductor, a diode, and a third inductor.

[0026] The first end of the first inductor serves as the first end of the impedance matching branch, the second end of the first inductor is connected to the first end of the second inductor, the second end of the second inductor is connected to the anode of the diode, the cathode of the diode is connected to the first end of the third inductor, and the second end of the third inductor serves as the second end of the impedance matching branch.

[0027] In one possible implementation, the signal transmission circuit includes a third transmission line, a first end of which is connected to the first signal transmission line and a second end of which is connected to the second signal transmission line, and a DC blocker is provided on the third transmission line.

[0028] In one possible implementation, the RF output unit is connected to the signal processing unit via a mode conversion unit, so that the RF output unit sends a status control signal to the signal processing unit via the mode conversion unit, causing the impedance matching circuit to be in one of the at least two matching states.

[0029] In one possible implementation, the first angle is (2×n+1)90°; where n is a natural number.

[0030] According to another aspect of this disclosure, an MRI system is provided, the MRI system comprising:

[0031] RF transmitting unit; and

[0032] An RF transmitting system connected to the RF transmitting unit, the RF transmitting system including the RF transmitting system described above.

[0033] By setting a first signal transmission line, a second signal transmission line, and a signal transmission circuit between the RF amplifier and the RF transmitting unit in the RF transmitting system, the phase delay of the RF pulse signal can be adjusted. At the same time, an impedance matching circuit is also set in the signal processing unit. This impedance matching circuit can provide different impedance adjustment capabilities, thereby dynamically adjusting the input impedance observed by the output terminal of the RF amplifier to the RF transmitting unit when scanning subjects of different weights. This makes the input impedance closer to the center point of the Smith circle, which can improve the impedance matching degree of the RF transmitting system and thus improve the imaging effect of the MRI system.

[0034] Other features and aspects of this disclosure will become clear from the following detailed description of exemplary embodiments with reference to the accompanying drawings. Attached Figure Description

[0035] The accompanying drawings, which are included in and form part of this specification, illustrate exemplary embodiments, features, and aspects of this disclosure together with the specification and serve to explain the principles of this disclosure.

[0036] Figure 1 A schematic diagram of an RF transmission system with an isolator according to an embodiment of the present disclosure is shown;

[0037] Figure 2 A schematic diagram of a Smith circle according to an embodiment of the present disclosure is shown;

[0038] Figure 3 A schematic diagram showing the VSWR and output power variation curves corresponding to different body weights during scanning according to an embodiment of the present disclosure;

[0039] Figure 4 A schematic diagram of an RF transmission system according to an embodiment of the present disclosure is shown;

[0040] Figure 5 A schematic diagram of an RF transmission system having a 1 / 8 wavelength transmission line according to an embodiment of the present disclosure is shown.

[0041] Figure 6This diagram illustrates the variation of impedance points in the Smith circle for scanning objects of different weights before and after the introduction of a 1 / 8 wavelength transmission line according to an embodiment of the present disclosure.

[0042] Figure 7 A schematic diagram showing the change curve of VSWR before and after introducing a 1 / 8 wavelength transmission line according to an embodiment of the present disclosure;

[0043] Figure 8 A schematic diagram showing the output power Pout of an RF amplifier before and after introducing a 1 / 8 wavelength transmission line according to an embodiment of the present disclosure as a function of the initial voltage standing wave ratio VSWR_Initial;

[0044] Figure 9 A schematic diagram of an RF transmitting system according to another embodiment of the present disclosure is shown;

[0045] Figure 10 This diagram illustrates the change in input impedance from the output of the signal transmission circuit corresponding to the scanning object of different weights to the RF transmitting coil as a function of resonant frequency, without the second signal transmission line and mode switching switch provided, according to an embodiment of the present disclosure.

[0046] Figure 11 A schematic diagram showing the desired impedance distribution of a signal processing unit according to an embodiment of the present disclosure is provided.

[0047] Figure 12 A schematic diagram showing an impedance matching circuit in a second state according to an embodiment of the present disclosure is illustrated.

[0048] Figure 13 A schematic diagram showing the impedance distribution of the input impedance observed from the input terminal of the signal transmission circuit to the RF transmitting unit according to an embodiment of the present disclosure;

[0049] Figure 14 A schematic diagram showing the impedance distribution in a second state of the impedance matching circuit, as indicated by a second desired requirement according to an embodiment of the present disclosure.

[0050] Figure 15 A schematic diagram of an impedance matching circuit according to an embodiment of the present disclosure in a first state is shown;

[0051] Figure 16 A schematic diagram showing the impedance distribution in a first state of the impedance matching circuit, as indicated by a second desired requirement according to an embodiment of the present disclosure.

[0052] Figure 17 A schematic diagram of an impedance matching circuit according to another embodiment of the present disclosure is shown;

[0053] Figure 18A schematic diagram of an impedance matching circuit according to yet another embodiment of the present disclosure is shown;

[0054] Figure 19 A schematic diagram of an RF transmission system having a mode conversion unit according to an embodiment of the present disclosure is shown;

[0055] Figure 20 A schematic diagram showing the RF pulse signal corresponding to each matching state according to an embodiment of the present disclosure;

[0056] Figure 21 A schematic diagram of an RF output unit according to another embodiment of the present disclosure is shown;

[0057] Figure 22 A schematic diagram of an MRI system according to an embodiment of the present disclosure is shown. Detailed Implementation

[0058] Various exemplary embodiments, features, and aspects of this disclosure will now be described in detail with reference to the accompanying drawings. The same reference numerals in the drawings denote elements that have the same or similar functions. Although various aspects of the embodiments are shown in the drawings, they are not necessarily drawn to scale unless specifically indicated otherwise.

[0059] As used herein, the terms “comprising,” “including,” “having,” or variations thereof are open-ended and include one or more of the stated features, integrals, elements, steps, components, or functions, but do not exclude the presence or addition of one or more other features, integrals, elements, steps, components, functions, or groups thereof.

[0060] When an element is referred to as “connected,” “coupled,” “responding,” or a variation thereof relative to another element, it may be directly connected, coupled, or responding to another element, or there may be an intermediate element present.

[0061] Although the terms first, second, third, etc., may be used herein to describe various elements / operations, these elements / operations should not be limited by these terms. These terms are only used to distinguish one element / operation from another. Therefore, without departing from the teachings of the inventive concept, a first element / operation in some embodiments may be referred to as a second element / operation in other embodiments.

[0062] The term “exemplary” as used herein means “serving as an example, embodiment, or illustration.” Any embodiment illustrated herein as “exemplary” is not necessarily to be construed as superior to or better than other embodiments.

[0063] Furthermore, to better illustrate this disclosure, numerous specific details are set forth in the following detailed description. Those skilled in the art will understand that this disclosure can be practiced without certain specific details. In some instances, methods, means, components, and circuits well known to those skilled in the art have not been described in detail in order to highlight the main points of this disclosure.

[0064] First, let me introduce some of the terms used in this application.

[0065] Impedance matching is used to ensure that all microwave signals are transmitted to the load with minimal signal reflection back to the source. Impedance (or complex impedance) consists of a real resistive part and an imaginary reactive part, and can be expressed as Z = R + jX. Impedance matching ensures that the load impedance equals the signal source impedance (equal magnitude and phase, or equal magnitude and phase).

[0066] Indicators used to indicate the degree of impedance matching include the Voltage Standing Wave Ratio (VSWR), which can be determined by the following formula:

[0067]

[0068] Where Γ represents the reflection coefficient, V 反射 Represents the reflected wave voltage, V 入射 Z represents the incident wave voltage. L Z represents the load impedance, which is the equivalent impedance connected to the end of the transmission line. In an MRI system, the load impedance is usually determined by the body coil and the patient tissue. Z0 represents the characteristic impedance of the transmission line.

[0069] Smith circle: Used to graphically solve transmission line impedance matching problems, it is a polar coordinate graphical representation of normalized impedance; that is, impedance can be transformed into normalized impedance and represented on the Smith circle. (Reference) Figure 2 The Smith circle shown. The center point of the Smith circle ( Figure 2 The normalized impedance of the circle (point 200 at the center) is 1, because the normalized impedance Therefore, in impedance matching (Z L When Z = Z0), the load impedance is located at the center point, and according to the above VSWR calculation formula, VSWR = 1 when impedance is matched. The VSWR of all impedance points on the same circle 201 centered on the center point is equal. Rotating around the same circle centered on the center point (hereinafter referred to as the equal VSWR circle) once represents half the wavelength of the transmission line length. In other words, the impedance will repeat every time the transmission line length changes by half a wavelength. Rotating half a circle represents a quarter wavelength.

[0070] The main axis in the middle of the Smith circle ( Figure 2 The thick horizontal line passing through the center point (shown in the image) represents the pure resistance axis (also known as the real axis). The rightmost point of the central axis represents infinite impedance (open circuit), and the leftmost point represents a short circuit. The arcs extending from the points representing open circuits (equal reactance arcs) (X) represent constant reactance. Figure 2 In the Smith chart, the points within the Smith circle that are not located at the center (i.e., non-center points, hereinafter referred to as impedance points) represent the distribution of load impedance at different resonant frequencies when the MRI system scans subjects of different weights. Each impedance point represents the load impedance of the body coil corresponding to each scanned subject at a resonant frequency of 0.127 GHz or 0.1282 GHz. Impedance points to the left of the center point represent the load impedance corresponding to heavy-weight scanned subjects, and impedance points to the right of the center point represent the load impedance corresponding to light-weight scanned subjects. In this application, "heavy-weight" and "light-weight" are relative to standard weight; that is, weight greater than standard weight is considered heavy-weight, and weight less than standard weight is considered light-weight. Standard weight refers to the weight at which impedance matching can be achieved. That is, the load impedance generated by a scanned subject of standard weight is basically located at the center point on the Smith circle, where "based on being located at the center point" includes being located at the center point or deviating from the center point within a preset allowable error range. For example, the standard weight is 60 kg, the large weight is a weight greater than 60 kg, and the small weight is a weight less than 60 kg. Figure 2 Circles 202, 203, and 204 passing through the impedance points represent equal-power circles. In MRI, the output power of the RF amplifiers corresponding to impedance points located on the same equal-power circle is equal. The farther the impedance point is from the center point, the worse the output power performance and the worse the impedance matching.

[0071] Combination Figure 2 It is known that the load impedance of the MRI system differs when scanning subjects of different weights, and this difference in load impedance leads to varying output power performance of the RF amplifier in the MRI system. To avoid RF amplifier instability, the traditional approach uses isolators to shield the RF amplifier from signals reflected from the RF transmitting coil. However, isolators can fail in MRI systems with high main magnetic fields. In such cases, the MRI system needs to increase the rated power of the RF amplifier to ensure image quality. For example: [reference needed] Figure 3Curve 310 represents the output power variation curve of a conventional RF amplifier, i.e., when using an MRI system with an isolator and the isolator's isolation function is effective, when scanning subjects of different weights. Curve 320 represents the output power variation curve of an RF amplifier without impedance control, i.e., when using an MRI system without an isolator and increasing the rated power (e.g., increasing the rated power to 18KW), when scanning subjects of different weights. Curve 330 represents the VSWR variation curve corresponding to scanning subjects of different weights. The lowest point of curve 330 is the impedance matching point. Correspondingly, the weight of the MRI system's scanning subjects is standard weight, at which point the VSWR corresponding to the standard weight scanning subjects is 1.

[0072] according to Figure 3 It can be seen that even without an isolator, increasing the rated power of the RF amplifier only results in a higher output power than curve 310 at the peak of curve 320. That is, without an isolator, even increasing the rated power of the RF amplifier only improves its performance within a certain weight range. Figure 3 Within the range of the dashed line, the RF amplifier performance is guaranteed, but when scanning subjects outside this weight range, the RF amplifier performance is still poor, and correspondingly, the imaging effect of the MRI system is poor.

[0073] Based on this, this application provides an RF transmission system for an MRI system, which can improve the impedance matching degree when scanning subjects of various weights without using an isolator, and avoid the problem of unstable performance of the RF transmission system.

[0074] The RF transmission system for an MRI system proposed in this application will now be described. Figure 4 A schematic diagram of an RF transmitting system according to an embodiment of the present disclosure is shown. Figure 4 As shown, the RF transmission system includes:

[0075] RF output unit 410 is used to generate and output RF pulse signals;

[0076] At least one (i.e., one or at least two) radio frequency transmission channels are connected to the RF output unit 410. Figure 4 This example uses two RF transmission channels. In actual implementation, the number of RF transmission channels can be more or less; this embodiment does not limit the number of RF transmission channels. Each RF transmission channel includes:

[0077] RF amplifier 420 is used to amplify RF pulse signals;

[0078] The signal processing unit 430 includes an impedance matching circuit 432 and a signal transmission circuit 431;

[0079] The first signal transmission line 440 is connected between the RF amplifier 420 and the impedance matching circuit 432, and between the RF amplifier 420 and the signal transmission circuit 431. The impedance matching circuit 432 includes at least two matching states, which provide different impedance adjustment capabilities.

[0080] The second signal transmission line 450 is connected between the signal transmission circuit 431 and the RF transmitting unit, which transmits RF pulse signals.

[0081] The second signal transmission line 450 and the RF transmitting unit together cause a first-angle phase delay in the RF pulse signal, so that the input impedance observed from the output of the signal transmission circuit 431 to the RF transmitting unit meets the first desired requirement; the signal transmission circuit 431 and the first signal transmission line 440 together cause a second-angle phase delay in the RF pulse signal, and the second-angle phase delay and the matching state of the impedance matching circuit 432 make the input impedance observed from the output of the RF amplifier 420 to the RF transmitting unit meet the second desired requirement.

[0082] In this application, by setting a first signal transmission line, a second signal transmission line, and a signal transmission circuit between the RF amplifier and the RF transmitting unit, the phase delay of the RF pulse signal is adjusted, thereby ensuring that the input impedance observed from the output of the RF amplifier to the RF transmitting unit meets the second desired requirement. Furthermore, in this embodiment, an impedance matching circuit is also provided in the signal processing unit. This impedance matching circuit can provide different impedance adjustment capabilities, thereby dynamically adjusting the input impedance observed from the output of the RF amplifier to the RF transmitting unit when scanning objects of different weights, making the input impedance closer to the center point of the Smith circle, further improving the impedance matching degree.

[0083] The following is a detailed explanation of each component in the RF transmission system.

[0084] refer to Figure 9 The RF output unit 410 includes a controller 411, an RF generator 412 and a feedback signal receiver 413 respectively connected to the controller 411.

[0085] The controller 411 is used to perform overall control of the RF transmission system. The control content includes, but is not limited to, controlling the RF generator 412 to generate RF pulse signals and receiving feedback signals received from the feedback signal receiver 413. The controller 411 can be implemented as a field-programmable gate array (FPGA). In other embodiments, the controller 411 can also be implemented as other devices with processing capabilities. This embodiment does not limit the implementation method of the controller 411.

[0086] Each RF transmission channel corresponds to a set of RF generators 412 and feedback signal receivers 413. That is, there are at least two RF generators 412 and at least two feedback signal receivers 413, and each corresponds to at least two RF transmission channels.

[0087] Each RF generator 412 is connected to a controller 411 to generate RF pulse signals under the control of the controller 411. During an MRI scan, the RF pulse signals generated by the RF generator 412 are applied to the RF transmitting unit. These RF pulse signals are used, for example, to excite the target site of the scanned object (e.g., a patient) or for other clinical applications. The RF generator 412 includes, but is not limited to, a digital-to-analog converter 4121 and an RF front-end circuit 4122. The digital-to-analog converter is used to generate the RF pulse signals. This digital-to-analog converter can be a direct digital synthesis digital-to-analog converter (DDSDAC). In other embodiments, the analog-to-digital converter can also be implemented in other forms. This embodiment does not limit the implementation of the digital-to-analog converter. The RF front-end circuit 4122 adjusts (e.g., attenuates) the RF pulse signals and outputs them to the RF amplifier 420.

[0088] The RF amplifier 420 in each RF transmission channel is used to amplify the received RF pulse signal and output the amplified RF pulse signal to the signal processing unit 430.

[0089] The signal processing unit 430 transmits the amplified RF pulse signal to the RF transmitting unit and outputs a feedback signal to the RF output unit 410. Correspondingly, the feedback signal receiver 413 in the RF output unit 410 receives the feedback signal output by the signal processing unit 430.

[0090] The feedback signals include forward feedback signals and reverse feedback signals. The forward feedback signal is the signal that the amplified RF pulse signal is processed by the signal processing unit 430 and transmitted to the RF transmitting unit; the reverse feedback signal is the signal that the forward feedback signal returns from the RF transmitting unit after reaching the RF transmitting unit, and is then processed by the signal processing unit 430 and output.

[0091] In this application, the RF transmitting system includes a transmitting mode and a receiving mode. The transmitting mode refers to the mode of transmitting RF pulse signals to the scanned object, and the receiving mode refers to the mode of receiving magnetic resonance signals generated by the scanned object after the transmission of RF pulses has stopped. Accordingly, the RF transmitting unit includes a mode switching switch and an RF transmitting coil. The mode switching switch is used to switch between the transmitting mode and the receiving mode. Exemplarily, the mode switching switch can be implemented as a differential transmit / receive switch (dTRSW). In other embodiments, the mode switching switch can also be implemented as any other switch that can achieve mode switching. This embodiment does not limit the implementation of the mode switching switch. The RF transmitting coil (or body coil) is used to provide a uniform RF field (i.e., B1 field) and to transmit or receive RF signals to the scanned object. The RF transmitting coil can be implemented as a birdcage body coil. In other embodiments, the RF transmitting coil can also be implemented in other forms. This embodiment does not limit the implementation of the RF transmitting coil.

[0092] In this application, the output terminal of the signal processing unit 430 (i.e., the end that outputs the amplified RF pulse signal) transmits the amplified RF pulse signal to the RF transmitting unit through the second signal transmission line 450. The second signal transmission line 450 can adjust the phase delay of the amplified RF pulse signal, so that the input impedance observed by the output terminal of the signal transmission circuit 431 to the RF transmitting unit meets the first expected requirement.

[0093] For example, the first desired requirement is determined based on the impedance distribution that the impedance matching circuit 432 can handle. For instance: Reference Figure 10 The diagram shows the change in input impedance from the output of the signal transmission circuit 431 corresponding to scanning objects of different weights to the RF transmitting coil as a function of resonant frequency, without the second signal transmission line 450 and the mode switching switch. Figure 10 The diagram below illustrates the variation of the resonant frequency freq from 127.1MHz to 128.4MHz. Figure 10 In the middle, the input impedance distribution corresponding to the large-weight scanning object is at the center point ( Figure 10The black dot at the center of the circle is to the left of the center point, and each impedance distribution curve is short with sparse intervals between adjacent impedance distribution curves; the input impedance distribution of the scanned object with small weight is to the right of the center point, and each impedance distribution curve is long with dense intervals between adjacent impedance distribution curves.

[0094] If the first desired requirement indicates that the impedance matching circuit 432 can handle the impedance distribution as follows: Figure 10 Based on the impedance distribution shown, rotate 180° clockwise around the center point. As mentioned above, rotating 180° clockwise on the VSWR circle at the impedance point results in a 90° phase delay for the signal transmitted on the transmission line. Therefore, to meet the first desired requirement, it is necessary to set up a second signal transmission line and an RF transmitting unit to jointly cause a (2×n+1)×90° (first angle) phase delay in the RF pulse signal generation. The resulting impedance distribution is as follows. Figure 11 As shown. Where n is a natural number. The value of n can be determined based on the distance between the output terminal of the signal transmission circuit 431 and the RF transmitting unit in actual implementation. The first angle refers to the angle of phase delay caused between the excitation point (i.e., the physical location where radio frequency energy is injected) of the RF transmitting coil (generally a birdcage coil) and the output terminal of the signal processing unit 430. The first angle is generally an odd multiple of 90°.

[0095] according to Figure 11 It can be seen that, with Figure 10 Compared to the impedance distribution shown, after setting the second signal transmission line 450 and the mode switching switch, the input impedance rotates 180° clockwise around the center as the resonant frequency changes, which meets the first expected requirement.

[0096] For example, the phase delay caused by the second signal transmission line 450 can be expressed by the following formula:

[0097] θ p1 =φ1-θ p2 ;

[0098] Where, θ p1 θ represents the phase delay caused by the second signal transmission line 450. p2 This represents the phase delay caused by the RF transmitting unit (e.g., including a mode switching switch and an RF transmitting coil). φ1 is the first angle. Figure 11 The diagram is illustrated using an example where the first angle is (2×n+1)90°.

[0099] After determining the phase delay caused by the second signal transmission line 450, the length of the second signal transmission line 450 can be designed based on this phase delay. Specifically, the length l of the second signal transmission line 450 can be obtained by dividing the phase delay caused by the second signal transmission line 450 by the phase constant β. λ represents the wavelength of the electromagnetic wave in the second signal transmission line.

[0100] In this application, the signal processing unit 430 has the capability to delay the phase of an RF pulse signal and adjust its impedance. Specifically, the signal processing unit 430 implements the phase delay of the RF pulse signal through the signal transmission circuit 431, and provides at least two impedance adjustment capabilities through the impedance matching circuit 432, which includes at least two matching states.

[0101] In one example, at least two matching states include a first state and a second state. The first state refers to the state where the impedance matching circuit 432 is turned on (or enabled), and the second state refers to the state where the impedance matching circuit 432 is not turned on (or disabled). In this case, if the mismatch index is greater than or equal to a preset threshold, the impedance matching circuit 432 is in the first state; if the mismatch index is less than the preset threshold, the impedance matching circuit 432 is in the second state.

[0102] The mismatch index is used to indicate the degree of input impedance mismatch observed at the output of the signal transmission circuit 431 to the RF transmitting unit. For example, the mismatch index is related to the forward feedback signal transmitted from the signal processing unit 430 to the RF transmitting unit and the reverse feedback signal fed back from the RF transmitting unit to the signal processing unit 430 and output by the signal processing unit 430.

[0103] Taking the mismatch index including VSWR as an example, the mismatch index is positively correlated with the ratio of the voltage value of the reverse feedback signal to the voltage value of the forward feedback signal. Accordingly, the RF pulse signal output by the RF output unit 410 is amplified by the RF amplifier 420 and processed by the signal processing unit 430. The forward feedback signal output by the signal processing unit 430 is sent to the RF transmitting unit and reaches the RF transmitting coil. Then, the RF transmitting coil returns a signal to the signal processing unit 430 based on the forward feedback signal. After processing by the signal processing unit 430, a reverse feedback signal is output. Both the forward and reverse feedback signals are returned to the RF output unit 410 through the feedback signal receiver 413. The RF output unit 410 (specifically, the controller 411) determines the ratio of the voltage value of the reverse feedback signal to the voltage value of the forward feedback signal to obtain the reflection coefficient, and determines the VSWR based on this reflection coefficient. The specific calculation process is described in the VSWR calculation formula above, and will not be repeated here in this embodiment.

[0104] Optionally, when the mismatch indicator includes VSWR, the preset threshold can be 2. In other embodiments, the preset threshold can also be other values ​​greater than 1, such as 1.5, 2.5, etc. This embodiment does not limit the value of the preset threshold. In addition, this embodiment uses VSWR as an example for explanation. In other embodiments, the mismatch indicator can also be represented by other parameters, such as: by the reflection coefficient, or other values ​​obtained by converting the reflection coefficient. Accordingly, the preset thresholds corresponding to different types of mismatch indicators can be different. This embodiment does not limit the value of the mismatch indicator and the preset threshold corresponding to the mismatch indicator.

[0105] The following section uses VSWR as an example to introduce different matching states (second state and first state).

[0106] First, if VSWR is less than or equal to 2, the weight of the scanned object is small, the impedance matching circuit 432 is in the second state, the impedance matching circuit 432 in the signal processing unit 430 does not perform impedance adjustment, but performs phase delay on the amplified RF pulse signal through the signal transmission circuit 431.

[0107] For example, refer to Figure 12 The diagram shows the impedance matching circuit 432 in its second state. The signal transmission circuit 431 includes a third transmission line ( Figure 12 (Indicated by the bold black line), one end of the third transmission line is connected to the first signal transmission line 440, and the other end is connected to the second signal transmission line 450. At this time, the third transmission line performs phase delay on the amplified RF pulse signal. According to Figure 12 It can be seen that the impedance matching circuit 432 is open (indicated by a cross), therefore, the impedance matching circuit 432 is in the second state. If the impedance distribution of the input impedance observed from the output terminal of the signal transmission circuit 431 to the RF transmitting unit varies with frequency as follows... Figure 11 As shown, after adding the third transmission line and causing a phase delay of 213° (i.e., 180° + 33°), the input impedance observed from the input terminal of the signal transmission circuit 431 to the RF transmitting unit is as follows. Figure 13 As shown, according to Figure 13 It can be seen that the impedance distribution in the Smith circle is relative to Figure 11 Rotate 360°+66° around the center point.

[0108] When the impedance matching circuit 432 is in its second state, the signal transmission circuit 431 and the first signal transmission line 440 jointly cause a second-angle phase delay in the RF pulse signal, so that the input impedance observed from the output of the RF amplifier to the RF transmitting unit meets the second desired requirement. Accordingly, the second desired requirement is determined based on the change of the RF amplifier's amplification performance with the second-angle phase delay. For example, parameters indicating the RF amplifier's amplification performance include, but are not limited to, output power, power dissipation, and the channel current of the RF transmitting channel. Combining the changes in the RF amplifier's output power with the second angle, power dissipation with the second angle, and channel current with the second angle, a second angle is determined that allows the output power, power dissipation, and channel current to all be within the desired range. At this time, the phase delay of this second angle ensures that the input impedance observed from the output of the RF amplifier to the RF transmitting unit meets the second desired requirement.

[0109] For example: Reference Figure 14 The second desired requirement shown indicates the impedance distribution when the impedance matching circuit 432 is in the second state. Figure 14 A schematic diagram of the impedance distribution range shown in Figure 1401. Compared to Figure 11 The impedance distribution shown is as follows: Figure 14 The impedance distribution shown has a phase delay of (m×180+115)°, meaning the impedance distribution is rotated 230° around the center point of the Smith circle. Here, m is a natural number. The value of m can be determined based on the distance between the output of the RF amplifier and the input of the signal transmission circuit in actual implementation.

[0110] Based on this, the phase delay caused by the first signal transmission line 440 can be expressed by the following formula:

[0111] θ p3 =φ2-θ p4 ;

[0112] Where, θ p3 θ represents the phase delay caused by the first signal transmission line 440. p4 This indicates the phase delay caused by the signal transmission circuit 431 in the signal processing unit 430. φ2 is the second angle. Figure 14 The example shown is based on the second angle of (m×180+115)°. In actual implementation, the value of the second angle can be (m×180+φ)°, and the value of φ is in the range of [0,180). The value of φ is determined according to the amplification performance parameters of the RF amplifier. This embodiment does not limit the value of φ.

[0113] After determining the phase delay caused by the first signal transmission line 440, the length of the first signal transmission line 440 can be designed based on this phase delay. Specifically, the length l of the first signal transmission line 440 can be obtained by dividing the phase delay caused by the first signal transmission line 440 by the phase constant β.

[0114] For example, in this application, a DC blocker is provided on the third transmission line. For instance... Figure 12 and Figure 15 As shown, the DC blocker includes a first DC blocker DC_Block1 and a second DC blocker DC_Block2. The first DC blocker DC_Block1 is disposed on a portion of the third transmission line between the first signal transmission line 440 and the impedance matching circuit, and the second DC blocker DC_Block2 is disposed on another portion of the third transmission line between the impedance matching circuit and the second signal transmission line 450. The first DC blocker DC_Block1 prevents DC current from flowing into the RF amplifier during the operation of the impedance matching circuit, thus preventing damage to the RF amplifier components. The second DC blocker DC_Block2 prevents DC current from flowing into the RF transmitting unit during the operation of the impedance matching circuit, thus preventing damage to the mode switching device in the RF transmitting unit.

[0115] In this embodiment, the phase delay of the RF pulse signal changes the impedance matching degree between the load impedance and the characteristic impedance of the system. Specifically, refer to... Figure 5 Assuming that in the RF transmission system used in an MRI system, the transmission line 51 (including the signal transmission line and / or other components that can cause a phase delay in the RF pulse signal, such as the first signal transmission line, the second signal transmission line, and the signal transmission circuit mentioned above) connected between the output of the RF amplifier and the RF transmission coil is a transmission line with a length of 1 / 8 wavelength (λ), then the phase delay... Before adding transmission line 51, if the load impedance Z of the RF transmitting coil is high when scanning a heavy object A, A The ratio between the characteristic impedance Z0 of the system and the characteristic impedance Z0 is p, and p > 1, that is:

[0116]

[0117] Accordingly, before adding transmission line 51, VSWR A =p.

[0118] After adding transmission line 51, the input impedance Z observed at the output of the RF amplifier to the RF transmitting coil is... in =Z B At this time, Z in It can be represented as:

[0119]

[0120] Wherein, the input impedance Z in The real part is:

[0121]

[0122] At this point, after adding transmission line 51

[0123] When scanning a small, lightweight object A', the load impedance Z of the RF transmitting coil is... A’ and input impedance Z B’ The representation process is the same as that for a large-weight scanned object A; only the VSWR needs to be changed. B′ Change to That's all.

[0124] refer to Figure 6 As shown, before adding transmission line 51, the load impedance Z corresponding to the heavy scanning object is... A The corresponding impedance point A in the Smith circle; after adding transmission line 51, the input impedance Z corresponding to the heavy scanning object. B This corresponds to impedance point B in the Smith circle. Because transmission line 51 produces, for example, a phase delay of 1 / 8 wavelength (λ / 8), impedance point A rotates clockwise on circle 601 centered at its center point. Impedance point B is obtained at 90°. The introduction of transmission line 51 causes the equal power circle 602 where impedance point A is located to become the equal power circle 603 where impedance point B is located. At this time, impedance point B is closer to the center of the Smith circle than impedance point A, and the power dissipation is smaller.

[0125] Similarly, before adding transmission line 51, the load impedance Z corresponding to the small-weight scanning object is... A’ The corresponding impedance point A' in the Smith circle; after adding transmission line 51, the input impedance Z corresponding to the small-weight scanning object. B’ The impedance point A' corresponds to point B' in the Smith circle. On circle 601 centered at the center point, the impedance point A' rotates clockwise. Impedance point B' is obtained. The introduction of transmission line 51 causes the equal power circle 604 where impedance point A' is located to become the equal power circle 605 where impedance point B' is located. At this time, impedance point B' is closer to the center point of the Smith circle than impedance point A', and the power dissipation is smaller.

[0126] The VSWR corresponding to impedance point B and impedance point B' in the Smith circle are as follows: Figure 6 The circle 600 represents, according to Figure 6 It can be seen that the radius of circle 600 is smaller than that of circle 601, which indicates that VSWR decreases, that is, the degree of impedance mismatch is reduced.

[0127] refer to Figure 7 The graph shown illustrates the change in VSWR before and after the introduction of transmission line 51. Figure 7 It can be seen that before introducing transmission line 51, if the initial voltage standing wave ratio (VSWR_Initial) = 2, then the voltage standing wave ratio (VSWR_Opt) after introducing transmission line 51 is 1.25; if VSWR_Initial = 3, then the voltage standing wave ratio (VSWR_Opt) after introducing transmission line 51 is 1.67; and if VSWR_Initial = 4, then the voltage standing wave ratio (VSWR_Opt) after introducing transmission line 51 is 2.13. Clearly, the VSWR_Opt after introducing transmission line 51 is closer to 1 than the VSWR_Initial before introducing transmission line 51, indicating a reduced impedance mismatch; in other words, a better impedance matching.

[0128] refer to Figure 8 The graph shows the change in the output power Pout of the RF amplifier with respect to the initial voltage standing wave ratio VSWR_Initial before and after the introduction of transmission line 51. Figure 8 In the diagram, straight line 810 represents the rated power of the RF amplifier, for example, a rated power of 18kW. Curve 830 shows the change in output power Pout_Initial of the RF amplifier as a function of the initial voltage standing wave ratio VSWR_Initial before the introduction of transmission line 51. Curve 820 shows the change in output power Pout_Opt of the RF amplifier as a function of the initial voltage standing wave ratio VSWR_Initial after the introduction of transmission line 51. According to... Figure 8 It can be seen that when VSWR_Initial = 2, the output power before introducing transmission line 51 is Pout_Initial = 18 / VSWR_Initial = 9kW, while the output power after introducing transmission line 51 is Pout_Opt = 18 / VSWR_Opt = 14.4kW. When VSWR_Initial = 3, the output power before introducing transmission line 51 is Pout_Initial = 18 / VSWR_Initial = 6kW, while the output power after introducing transmission line 51 is Pout_Opt = 18 / VSWR_Opt = 10.8kW. When VSWR_Initial = 4, the output power before introducing transmission line 51 is Pout_Initial = 18 / VSWR_Initial = 4.5kW, while the output power after introducing transmission line 51 is Pout_Opt = 18 / VSWR_Opt = 8.47kW. Clearly, the output power Pout_Opt after introducing transmission line 51 is improved compared to the output power Pout_Initial before its introduction, and the power dissipation is smaller.

[0129] In this embodiment, by setting a first signal transmission line, a second signal transmission line, and a signal transmission circuit between the RF amplifier and the RF transmitting unit, the phase delay of the RF pulse signal can be adjusted, thereby increasing the output power of the RF amplifier, reducing the power dissipation of the RF amplifier, and improving the impedance matching degree of the input impedance observed from the output terminal of the RF amplifier to the RF transmitting unit.

[0130] Second, if VSWR is greater than 2, the weight of the scanned object is large, the impedance matching circuit 432 is in the first state, the signal processing unit adjusts the impedance through the impedance matching circuit, and at the same time delays the phase of the amplified RF pulse signal through the signal transmission circuit.

[0131] For example, refer to Figure 15 The diagram shows the impedance matching circuit in its first state. According to... Figure 15 It can be seen that the impedance matching circuit is conducting at this time. The impedance matching circuit includes: two impedance matching branches, a first DC feed device DC_Feed1, a resistor R, and a control port. Among them, the first end of one of the two impedance matching branches is connected to the signal transmission circuit and the first end of the second DC feed device DC_Feed2, and the second end of the second DC feed device DC_Feed2 is grounded; the first end of the other impedance matching branch is grounded; each impedance matching branch includes an inductor and a diode connected in series.

[0132] The first terminal of the first DC feed device DC_Feed1 is connected to the second terminal of the two impedance matching branches, and the second terminal of DC_Feed1 is connected to the first terminal of the resistor R.

[0133] The control port is connected to the second end of resistor R1 and is used to receive status control signals to control the impedance matching circuit to be in the first or second state.

[0134] For example, the control port can be a power access point, such as disabling the impedance matching circuit when the control port receives high voltage (status control signal) and enabling the impedance matching circuit when it receives -1A DC (status control signal).

[0135] In this example, by setting up two impedance matching branches, the problem of overheating or even device damage in the diodes of the impedance matching branch can be prevented when only one impedance matching branch is set up.

[0136] Optionally, each impedance matching branch includes: a first inductor, a second inductor, a diode, and a third inductor. For example: Reference Figure 15Of the two impedance matching branches shown, one impedance matching branch includes the first inductor L1, the second inductor L2, the diode PIN1, and the third inductor L3; the other impedance matching branch includes the first inductor L4, the second inductor L5, the diode PIN2, and the third inductor L6.

[0137] In each impedance matching branch, the first end of the first inductor serves as the first end of the impedance matching branch, the second end of the first inductor is connected to the first end of the second inductor, the second end of the second inductor is connected to the anode of the diode, the cathode of the diode is connected to the first end of the third inductor, and the second end of the third inductor serves as the second end of the impedance matching branch.

[0138] The inductance values ​​of the first, second, and third inductors are determined based on the second desired requirement. This embodiment does not limit the values ​​of the first, second, and third inductors. Furthermore, in other embodiments, the number of inductors in each impedance matching branch may be less than three or more than three. This embodiment does not limit the number of inductors in each impedance matching branch.

[0139] The first DC feeder, DC_Feed1, allows DC current to pass through and prevents RF pulse signals from entering the control port. The second DC feeder, DC_Feed2, allows DC current to pass through and provides a stable zero-potential reference point.

[0140] When the impedance matching circuit is in the first state, the signal transmission circuit 431 and the first signal transmission line 440 together cause the RF pulse signal to have a second-angle phase delay. At the same time, the impedance matching circuit adjusts the input impedance observed from the output of the RF amplifier to the RF transmitting unit.

[0141] For example: Reference Figure 16 The second desired requirement shown indicates the impedance distribution when the impedance matching circuit 432 is in the first state (e.g.) Figure 16 A schematic diagram of the impedance distribution range shown in Figure 1601. Compared to... Figure 11 The impedance distribution shown is obtained by adding signal transmission circuit 431 and first signal transmission line 440. Figure 16 The impedance distribution shown achieves a phase delay of (m×180+115)°, that is, the impedance distribution rotates 230° around the center point of the Smith circle. At the same time, impedance adjustment is performed through impedance matching circuit 432, and each impedance point is closer to the center point of the Smith circle. At this time, if the VSWR before impedance adjustment is as shown in circle 1602, then the VSWR after impedance adjustment is as shown in circle 1603. The VSWR after impedance adjustment is closer to the center point, and the impedance matching degree is better.

[0142] In the above embodiments, the impedance matching circuit based on inductor is used as an example for explanation. In other embodiments, the impedance matching circuit can also be implemented in other ways that can reduce the VSWR of the input impedance observed from the output of the RF amplifier to the RF transmitting unit for a large scanning object, such as: constructing an impedance matching circuit based on inductor and capacitor.

[0143] Furthermore, in the above embodiments, the impedance matching circuit is described using the example of a first state and a second state as the matching states. In other embodiments, the first state of the impedance matching circuit may further include at least two sub-states, in which case the at least two matching states include at least two sub-states; or, the at least two matching states include at least two sub-states and a second state. Accordingly, each impedance matching circuit may include multiple matching branches, and at least one matching branch corresponds to one sub-state of the impedance matching circuit when it is working. In other words, one matching branch can correspond to one sub-state of the impedance matching circuit when it is working, and / or, one matching branch can also correspond to one sub-state of the impedance matching circuit when it is working simultaneously. The multiple sub-states have different impedance adjustment capabilities, and one end of each matching branch is connected to a signal transmission circuit and the other end is connected to a control port. Different matching branches are provided with different control ports. In this way, by inputting different state control signals to the control port of each matching branch, the impedance matching circuit can be switched to the desired sub-state, thereby performing dynamic impedance matching.

[0144] For example: Reference Figure 18 The impedance matching circuits for the two RF transmission channels are shown. Each impedance matching circuit includes two matching branches. One of the RF transmission channels ( Figure 18 The impedance matching circuit for the upper-middle RF transmission channel includes matching branch 1801 and matching branch 1802. Matching branch 1801 is connected to control port 1803, and matching branch 1802 is connected to control port 1804. The other RF transmission channel ( Figure 18The impedance matching circuit of the lower-middle RF transmission channel includes matching branch 1805 and matching branch 1806. Matching branch 1805 is connected to control port 1807, and matching branch 1806 is connected to control port 1808. At this time, by enabling matching branch 1801 through control port 1803 and disabling matching branch 1802 through control port 1804, impedance adjustment can be achieved through matching branch 1801. Similarly, by enabling matching branch 1801 through control port 1803 and enabling matching branch 1802 through control port 1804, impedance adjustment can be achieved through both matching branches 1801 and 1802. The switching method for matching branches 1805 and 1806 is similar. This allows for the use of different matching branches for impedance adjustment, further improving the dynamic impedance matching capability of the impedance matching circuit.

[0145] For example, the circuit structure of each matching branch may include: two impedance matching branches, a first DC feed device, a resistor, and a control port; the connection relationship of the two impedance matching branches, the first DC feed device, the resistor, and the control port is referenced. Figure 15 As shown, the impedance matching branches and resistors in different matching branches have different device parameters, thus providing different impedance adjustment capabilities.

[0146] In other implementations, the circuit structures of different matching branches can also be different. For example, one matching branch is based on an inductor, and another matching branch is based on an inductor and a capacitor. This embodiment does not limit the implementation method of the matching branch.

[0147] For example: Reference Figure 17 The diagram illustrates another implementation of the impedance matching circuit. It takes an RF transmission system with two RF transmission channels, where the impedance matching circuits are identical in both channels, as an example. Figure 17 In the middle, each RF transmission channel (because the impedance matching circuits in the two RF transmission channels are the same, Figure 17 The impedance matching circuit shown in the diagram (illustrated for one of the RF transmission channels) includes a DC power supply device 1730 and two impedance matching branches 1710 and 1720.

[0148] The first end of the impedance matching branch 1710 is connected to both the signal transmission circuit and the first end of the DC power supply device 1730, and the second end of the DC power supply device 1730 is grounded. The second end of the impedance matching branch 1710 is connected to the control port 1711.

[0149] Impedance matching branch 1710 includes an inductor 1714, a diode 1717, a capacitor 1715, a transmission line 1716, and a DC power supply device 1718 connected in sequence. The end of inductor 1714 not connected to diode 1717 forms the first end of impedance matching branch 1710, and the DC power supply device 1718 is grounded at the end connected to transmission line 1716. Impedance matching branch 1710 also includes a DC power supply device 1713 and a resistor 1712 connected in sequence. The end of DC power supply device 1713 not connected to resistor 1712 is connected between diode 1717 and capacitor 1715, and the end of resistor 1712 not connected to DC power supply device 1713 forms the second end of impedance matching branch 1710.

[0150] The first end of impedance matching branch 1720 is connected to impedance matching branch 1710, specifically between capacitor 1715 and transmission line 1716. The second end of impedance matching branch 1720 is connected to control port 1721.

[0151] Impedance matching branch 1720 includes an inductor 1724, a diode 1727, a transmission line 1726, a DC power supply device 1723, and a resistor 1722 connected in sequence. The end of inductor 1724 not connected to diode 1727 forms the first end of impedance matching branch 1720, and the end of resistor 1722 not connected to DC power supply device 1723 forms the second end of impedance matching branch 1720. Impedance matching branch 1720 also includes capacitors 1725 and 1728. The first end of capacitor 1725 is connected between diode 1727 and transmission line 1726, and the second end of capacitor 1725 is grounded. The first end of capacitor 1728 is connected between transmission line 1716 and DC power supply device 1718, and the second end of capacitor 1728 is connected between transmission line 1726 and DC power supply device 1723.

[0152] At this point, the first state of the impedance matching circuit includes two seed states, as follows:

[0153] Type 1: Sub-state when impedance matching branch 1710 and impedance matching branch 1720 are enabled. In this case, the impedance matching circuit performs impedance adjustment together through the impedance provided by inductor 1714, diode 1717, capacitor 1715, transmission line 1716, and DC power supply device 1718, as well as the impedance provided by inductor 1724, diode 1727, transmission line 1726, DC power supply device 1723, resistor 1722, and capacitors 1725 and 1728.

[0154] The second type: the sub-state when impedance matching branch 1710 is enabled and impedance matching branch 1720 is disabled. In this case, the impedance matching circuit adjusts the impedance through the impedance provided by inductor 1714, diode 1717, capacitor 1715, transmission line 1716, and DC power supply device 1718.

[0155] Figure 17 The impedance matching branches 1710 and 1720 shown are merely illustrative. In actual implementation, the impedance matching branches can be implemented in other ways, such as replacing transmission lines 1716 and 1726 with resistors and inductors, etc. This embodiment will not list them all here.

[0156] Optionally, when the first state of the impedance matching circuit includes at least two sub-states, multiple sub-thresholds can be used to determine the sub-state matched by the mismatch index, and all sub-thresholds are greater than the preset threshold mentioned above. The multiple sub-thresholds divide the numerical range from the preset threshold (excluding the preset threshold itself) to positive infinity into at least two numerical segments, with each of the at least two sub-states corresponding to one of the at least two numerical segments. In other words, if the mismatch index belongs to a certain numerical segment, then the sub-state corresponding to that numerical segment is used for impedance matching. For example, if there are two sub-states, then there is one sub-threshold, which divides the numerical range from the preset threshold (excluding the preset threshold itself) to positive infinity into at least two numerical segments, namely numerical segment 1 and numerical segment 2. Numerical segment 1 corresponds to sub-state 1, and numerical segment 2 corresponds to sub-state 2. In this case, if the mismatch index belongs to numerical segment 1, then impedance matching is performed in sub-state 1; if the mismatch index belongs to numerical segment 2, then impedance matching is performed in sub-state 2.

[0157] Optionally, the status control signal of the control port in the impedance matching circuit is sent through a mode conversion unit connected to the RF output unit. Accordingly, reference... Figure 19 The RF output unit 410 is connected to the signal processing unit 430 through the mode conversion unit 1900, so that the RF output unit 410 sends a status control signal to the signal processing unit 430 through the mode conversion unit 1900, so that the impedance matching circuit is in one of at least two matching states.

[0158] For each RF transmit channel, the process by which the RF output unit 410 sends a status control signal to the signal processing unit 430 via the mode conversion unit 1900 includes the following steps:

[0159] 1. During the pre-scan phase, the RF output unit 410 generates and sequentially outputs RF pulse signals corresponding to each matching state to the RF amplifier 420, and sends state control signals to the impedance matching circuit in the signal processing unit 430 through the mode conversion unit 1900.

[0160] During MRI, RF pulse signals can excite protons within the scanned object to resonate. When the precession frequency of the protons matches the center frequency of the RF pulse signal, energy exchange occurs, resulting in magnetic resonance. Therefore, an inaccurate center frequency of the RF pulse signal directly leads to misalignment in the obtained magnetic resonance image. Thus, center frequency correction is typically performed during the pre-scanning stage to determine the optimal resonance frequency (center frequency). Center frequency correction includes center frequency search (CFL) to ensure that the center frequency matches the vibration frequency of the protons within the scanned object. In this embodiment, reference... Figure 20 After completing the center frequency search (CFL), the system outputs the RF pulse signal and status control signal corresponding to each matching state during the automatic pre-scan phase to select the matching state that matches the current scan object.

[0161] Taking the matching states, including the first and second states of the impedance matching circuit, as an example, refer to... Figure 20 The diagram shows the RF pulse signals corresponding to each matching state. The RF pulse signal corresponding to the first state VVmode is Pulse1, and the corresponding state control signal is used to put the impedance matching circuit in the first state. The RF pulse signal corresponding to the second state IImode is Pulse2, and the corresponding state control signal is used to put the impedance matching circuit in the second state. Pulse1 and Pulse2 are generated based on the center frequency determined after CFL. Figure 20 This example uses a pulse width of 0.5ms and a transmission interval of 1ms for Pulse1 and Pulse2. In other embodiments, the pulse width and transmission interval of Pulse1 and Pulse2 can be other values, and this embodiment does not limit them.

[0162] 2. The RF amplifier 420 amplifies the RF pulse signals received in sequence and outputs them to the signal processing unit 430.

[0163] 3. The signal processing unit 430 adjusts the matching state according to the state control signal corresponding to each RF pulse signal, and transmits each amplified RF pulse signal to the RF transmitting unit according to the current matching state, and feeds back the feedback signal corresponding to the amplified RF pulse signal to the RF output unit 410; the feedback signal includes the forward feedback signal transmitted by the signal processing unit 430 to the RF output unit 410, and the reverse feedback signal fed back by the RF output unit 410 to the signal processing unit 430 and output by the signal processing unit 430.

[0164] 4. The RF output unit 410 receives the feedback signal corresponding to each RF pulse signal; determines the ratio of the voltage value of the reverse feedback signal to the voltage value of the positive feedback signal in the feedback signal to obtain the reflection coefficient; determines the VSWR corresponding to each RF pulse signal based on the reflection coefficient; determines the VSWR closest to 1 from the VSWRs corresponding to each RF pulse signal to obtain the target RF pulse signal corresponding to the VSWR closest to 1; outputs the target RF pulse signal to the RF amplifier 420 to determine whether the matching state corresponding to the target RF pulse signal is a suitable matching state, and sends the state control signal corresponding to the target RF pulse signal to the impedance matching circuit in the signal processing unit 430 through the mode conversion unit 1900.

[0165] For example: in Figure 20 The target RF pulse signal Pulse3 is used to confirm whether the RF output unit 410 has selected the appropriate matching state.

[0166] 5. The RF output unit 410 amplifies the target RF pulse signal and outputs it to the signal processing unit 430. The signal processing unit 430 transmits the target RF pulse signal to the RF transmitting unit and feeds back the feedback signal corresponding to the amplified target RF pulse signal to the RF output unit 410.

[0167] 6. After receiving the feedback signal corresponding to the target RF pulse signal, the RF output unit 410 determines the ratio of the voltage value of the reverse feedback signal to the voltage value of the forward feedback signal in the feedback signal to obtain the reflection coefficient; based on the reflection coefficient, it determines the VSWR corresponding to the target RF pulse signal; if the VSWR is equal to the VSWR closest to 1 in step 4, it is confirmed that the RF output unit 410 has selected the appropriate matching state for the impedance matching circuit, and the following step 7 is executed; if they are not equal, step 1 is executed again.

[0168] 7. After the RF output unit 410 determines the matching state, it generates a training pulse and outputs the training pulse to the RF amplifier 420.

[0169] For example: training pulses, such as Figure 20 The pulse signal 2001 is shown.

[0170] 8. After amplifying the training pulse, the RF amplifier 420 outputs it to the signal processing unit 430, which is in the matching state. The signal processing unit 430 outputs the amplified training pulse to the RF transmitting unit and feeds back the feedback signal corresponding to the training pulse to the RF output unit 410.

[0171] 9. The RF output unit 410 generates a lookup table for the RF amplifier 420 under the current scan object based on the feedback signal, and stores the lookup table in the memory.

[0172] Optionally, in this embodiment, reference is made to... Figure 21 The RF output unit 410 also includes a memory 414 connected to the controller 411, which stores a lookup table. The lookup table stores the correspondence between signal characteristic values ​​(such as voltage, current, or power) of the feedback signal and linear compensation values ​​(such as gain linear compensation values ​​and / or phase linear compensation values). For example, the lookup table includes the correspondence between the signal characteristic values ​​of the forward feedback signal and at least one of the gain linear compensation value and the phase linear compensation value.

[0173] For example, the RF output unit 410 determines the lookup table of the RF amplifier 420 under the current scanning object based on the feedback signal, including: the RF output unit 410 calculates the open-loop characteristics of the RF amplifier 420 under the current scanning object based on the forward feedback signal in the feedback signal; and generates a lookup table based on the open-loop characteristics to achieve the desired gain curve corresponding to the compensated RF pulse signal after compensation based on the corresponding linear compensation value. For example, the gain curve is kept in a high and flat state, that is, the RF amplifier 420 has a large gain and good linearity. Since the feedback signal can generally correspond to the signal characteristic value, the one-to-one correspondence between the signal characteristic value and the linear compensation value can also be considered as the correspondence between the feedback signal and the linear compensation value. The open-loop characteristic can be voltage, current, or power, etc., and this embodiment does not limit the type of parameter describing the open-loop characteristic.

[0174] Optionally, a preset lookup table is stored in the memory, and training pulses can be generated based on this preset lookup table. In this embodiment, by generating a new lookup table based on the current scan object, the RF output unit 410 can perform more accurate linear compensation, thereby improving the performance of the RF amplifier 420.

[0175] 10. The RF output unit 410 compensates the output RF pulse signal according to the lookup table stored in the memory, and outputs the compensated RF pulse signal to the RF amplifier 420.

[0176] Optionally, in this embodiment, the impedance matching circuits in different RF transmission channels may be in the same or different matching states. For example, the impedance matching circuits in two RF transmission channels may both be in the first state, or both in the second state, or one in the first state and the other in the second state. That is, the matching states of the impedance matching circuits in different RF transmission channels can be controlled individually.

[0177] Because the impedance matching circuits in different RF transmission channels may be in different matching states, the lookup tables may also be different. Therefore, the memory stores a lookup table corresponding to each RF transmission channel (e.g., ...). Figure 21 The table includes lookup table 1 corresponding to RF transmission channel 1 and lookup table 1 corresponding to RF transmission channel 2. Each RF transmission channel uses the corresponding lookup table to compensate for the RF pulse signal, further improving the flexibility and accuracy of linear compensation, thereby further improving the performance of RF amplifier 420.

[0178] This application also provides an MRI system. Figure 22 A schematic diagram of an MRI system according to an embodiment of the present disclosure is shown. Figure 22 As shown, the MRI system includes: an RF transmitting unit; and an RF transmitting system 2220 connected to the RF transmitting unit, the RF transmitting system 2220 including the RF transmitting system of the above embodiment.

[0179] Specifically, the RF transmitting unit is connected to the second signal transmission line of the RF transmitting system. For a detailed description of the RF transmitting system, please refer to the above embodiment. This embodiment will not repeat the description here.

[0180] As can be seen from the above embodiments, the RF transmitting unit includes an RF transmitting coil 2211 and a mode switching switch 2212.

[0181] The MRI system also includes other components required for the MRI process. For example, the MRI system also includes a main magnet assembly, gradient coil assembly 2240, gradient coil driver 2250, scanning table 2260, control assembly 2270, etc.

[0182] The scanning bed 2260 is used to place the object to be scanned.

[0183] The main magnet assembly typically includes a superconducting magnet 2230 with a main magnet coil arranged circumferentially around it. The superconducting magnet is mounted within an annular vacuum container and defines a cylindrical imaging space surrounding the object being scanned on a scanning bed 2260. A constant static magnetic field, such as a static magnetic field B0, is generated along the Z-direction of this imaging space. The MRI system uses the generated static magnetic field B0 to transmit a magnetic flux density pulse signal to the object being scanned, thereby ordering the precession of protons within the object and generating a longitudinal magnetization vector.

[0184] The RF transmitting coil 2211 in the RF transmitting unit is typically arranged along the inner ring of the main magnet and is used to respond to the RF pulse signal emitted from the RF transmitting system 2220 to emit a radio frequency field B1 orthogonal to the static magnetic field B0 to the scanned object, thereby exciting the atomic nuclei within the object and transforming the longitudinal magnetization vector into a transverse magnetization vector. After the RF excitation pulse ends, a magnetic resonance signal that can be acquired is generated.

[0185] The gradient coil assembly 2240 typically includes three sets of gradient coils arranged along the X, Y, and Z axes. These coils receive power drive signals generated by the gradient coil driver 2250 to generate a three-dimensional gradient magnetic field in the imaging space. This allows for three-dimensional encoding of the magnetic resonance signal, providing three-dimensional positional information of the magnetic resonance signal. Based on the three-dimensionally encoded magnetic resonance signal, a medical image of the scanned area of ​​the object can be reconstructed. The gradient coil driver 2250 generates the power drive signal under the control of the control assembly 2270.

[0186] The RF transmitting system is also used to receive the magnetic resonance signal collected by the RF transmitting coil 2211 in receive mode and send the magnetic resonance signal to the control component 2270.

[0187] The control component 2270 can perform preprocessing, reconstruction, and other operations on the received digitized magnetic resonance signals to obtain the desired images or image data. The control component 2270 may include a computer and a storage medium on which a predetermined data processing program to be executed by the computer and a control program for the MRI system are recorded. The storage medium may be a ROM, floppy disk, hard disk, optical disk, magneto-optical disk, CD-ROM, or non-volatile memory card, etc. This embodiment does not limit the implementation of the storage medium.

[0188] Optionally, the control component 2270 may include the controller 411 and memory 414 in the RF transmission system 2220, and may also include other controllers and memories that are set independently of the controller 411 and memory 414. This embodiment does not limit the implementation of the control component 2270.

[0189] The MRI system may also include external devices 2280 connected to the control component 2270, such as a keyboard and mouse, through which the operator can input operation signals to the control component 2270. For example, the user can send pre-set scanning parameters to the control component 2270 through the external device, which may include the determined scanning location of the object to be scanned and other attribute characteristics.

[0190] The MRI system 1000 may also include a display unit 2290 connected to the control component 2270. The display unit 2290 can display the user interface and images generated by the control component 2270.

[0191] In the RF transmission system of the MRI system provided in this application, a first signal transmission line, a second signal transmission line, and a signal transmission circuit are provided between the RF amplifier and the RF transmission unit to adjust the phase delay of the RF pulse signal. Simultaneously, an impedance matching circuit is provided in the signal processing unit. This impedance matching circuit can provide different impedance adjustment capabilities, thereby dynamically adjusting the input impedance observed from the output of the RF amplifier to the RF transmission unit when scanning subjects of different weights. This makes the input impedance closer to the center point of the Smith circle, ultimately ensuring that the input impedance observed from the output of the RF amplifier to the RF transmission unit meets the second desired requirement, improving the impedance matching degree of the RF transmission system, and thus improving the imaging effect of the MRI system.

[0192] The flowcharts and block diagrams in the accompanying drawings illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present disclosure. In this regard, each block in a flowchart or block diagram may represent a module, segment, or portion of an instruction containing one or more executable instructions for implementing a specified logical function. In some alternative implementations, the functions marked in the blocks may occur in a different order than those shown in the drawings. For example, two consecutive blocks may actually be executed substantially in parallel, and they may sometimes be executed in reverse order, depending on the functions involved. It should also be noted that each block in the block diagrams and / or flowcharts, and combinations of blocks in the block diagrams and / or flowcharts, may be implemented using a dedicated hardware-based system that performs the specified function or action, or using a combination of dedicated hardware and computer instructions.

[0193] The various embodiments of this disclosure have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A radio frequency transmission system for a magnetic resonance imaging system, characterized in that, The radio frequency transmission system includes: The RF output unit is used to generate and output RF pulse signals. At least one radio frequency (RF) transmission channel is connected to the RF output unit, wherein each RF transmission channel includes: An RF amplifier is used to amplify the RF pulse signal; The signal processing unit includes an impedance matching circuit and a signal transmission circuit; A first signal transmission line is connected between the RF amplifier and the impedance matching circuit, and between the RF amplifier and the signal transmission circuit. The impedance matching circuit includes at least two matching states, which provide different impedance adjustment capabilities. A second signal transmission line is connected between the signal transmission circuit and the RF transmitting unit, wherein the RF transmitting unit transmits RF pulse signals. The second signal transmission line and the RF transmitting unit together cause a phase delay of the RF pulse signal at a first angle; the signal transmission circuit and the first signal transmission line together cause a phase delay of the RF pulse signal at a second angle.

2. The radio frequency transmitting system according to claim 1, characterized in that, The at least two matching states include a first state and a second state; wherein, the first state is the state in which the impedance matching circuit is turned on, and the second state is the state in which the impedance matching circuit is not turned on.

3. The radio frequency transmitting system according to claim 2, characterized in that, Each impedance matching circuit includes at least two matching branches. Accordingly, the first state includes at least two sub-states, with at least one matching branch operating in one of the sub-states of the impedance matching circuit.

4. The radio frequency transmitting system according to claim 2, characterized in that, When the mismatch index is greater than a preset threshold, the impedance matching circuit is in the first state; When the mismatch index is less than or equal to a preset threshold, the impedance matching circuit is in the second state. The mismatch index is determined based on the forward feedback signal transmitted from the signal processing unit to the RF transmitting unit and the reverse feedback signal fed back from the RF transmitting unit to the signal processing unit and output by the signal processing unit.

5. The radio frequency transmitting system according to claim 2, characterized in that, The mismatch indicators include voltage standing wave ratio (VSWR).

6. The radio frequency transmitting system according to claim 2, characterized in that, The impedance matching circuit includes: two impedance matching branches, a first DC power supply device, a resistor, and a control port, wherein... One of the two impedance matching branches has its first end connected to the signal transmission circuit and the first end of the second DC power supply device, respectively, with the second end of the second DC power supply device grounded; the first end of the other impedance matching branch is grounded; each impedance matching branch includes an inductor and a diode connected in series. The first terminal of the first DC power supply device is connected to the second terminal of the two impedance matching branches, and the second terminal of the first DC power supply device is connected to the first terminal of the resistor. The control port is connected to the second end of the resistor and is used to receive a status control signal to control the impedance matching circuit to be in the first state or the second state.

7. The radio frequency transmitting system according to claim 6, characterized in that, Each impedance matching branch includes: a first inductor, a second inductor, a diode, and a third inductor. The first end of the first inductor serves as the first end of the impedance matching branch, the second end of the first inductor is connected to the first end of the second inductor, the second end of the second inductor is connected to the anode of the diode, the cathode of the diode is connected to the first end of the third inductor, and the second end of the third inductor serves as the second end of the impedance matching branch.

8. The radio frequency transmitting system according to claim 1, characterized in that, The signal transmission circuit includes a third transmission line, with a first end connected to the first signal transmission line and a second end connected to the second signal transmission line. A DC blocker is provided on the third transmission line.

9. The radio frequency transmitting system according to claim 1, characterized in that, The RF output unit is connected to the signal processing unit through the mode conversion unit, so that the RF output unit sends a status control signal to the signal processing unit through the mode conversion unit, so that the impedance matching circuit is in one of the at least two matching states.

10. The radio frequency transmitting system according to any one of claims 1 to 7, characterized in that, The first angle is (2×n+1)×90°; where n is a natural number.

11. A magnetic resonance imaging system, characterized in that, The magnetic resonance imaging system includes: RF transmitting unit; and A radio frequency transmission system connected to the RF transmitting unit, the radio frequency transmission system comprising the radio frequency transmission system according to any one of claims 1 to 10.

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