Double-layer magnetic resonance radio frequency receiving coil array

Abstract

The invention discloses a double-layer magnetic resonance radio frequency receiving coil array, and belongs to the technical field of magnetic resonance imaging. According to the coil array, the problem of signal attenuation caused by the fact that an existing single-layer receiving coil cannot be tightly attached to the human body is solved, and potential safety hazards caused by the fact that a wireless coil is inserted into the single-layer receiving coil are avoided. An inner-layer coil and outer-layer coil structure is adopted, the inner-layer coil can be tightly attached to subjects of different body types so as to directly receive signals, and the outer-layer coil efficiently receives signals from the inner layer through inductive coupling. According to the design, the safety in the radio frequency emission period is ensured through a detuning protection mechanism, and efficient transmission of magnetic resonance signals is achieved by optimizing the distance and the coupling state between the inner layer coil and the outer layer coil. By applying the coil array, the image signal-to-noise ratio and the image signal-to-noise ratio after parallel imaging acceleration can be remarkably improved under the condition that the number of receiving channels is the same or smaller, particularly, the imaging effect of a small-size subject is remarkably improved, and meanwhile good compatibility with an existing magnetic resonance system is kept.

Description

Technical Field

[0001] This invention relates to the field of magnetic resonance imaging technology, and more specifically to a dual-layer magnetic resonance radio frequency receiving coil array. Background Technology

[0002] Magnetic resonance imaging (MRI) medical diagnostic equipment non-invasively acquires images of any cross-section of the human body, providing clear soft tissue images and anatomical information. It is currently the most advanced clinical diagnostic equipment for detecting and diagnosing early-stage cancer and many other diseases. An MRI system typically has two fields: a main magnetic field B0 generated by a magnet and a radio frequency field B1 generated by a radio frequency transmitting coil. In the B0 field, atomic nuclei excited by the radio frequency field B1 undergo energy level transitions. When the radio frequency pulse is turned off, the nuclei gradually release energy and return to their initial equilibrium state; this process is called relaxation. The energy released by the nuclei during relaxation is detected by the receiving coil, forming a magnetic resonance signal.

[0003] The pursuit of high-quality MRI images relies on advancements in radio frequency (RF) receiving coil technology. Currently, MRI RF receiving coils utilize single-layer, multi-channel receiving coil arrays. Commercially available coils are categorized for different body parts, including head coils, body coils, spine coils, and ankle coils. Head coils typically have 24, 32, or 64 channels. The coil units in a multi-channel receiving coil array are arranged around the body, with a fixed distribution range and size. Because human bodies vary in size, commercially available coils cannot be placed in close proximity. The MRI signal received by the coil attenuates with distance, resulting in an inadequate signal-to-noise ratio. This is especially true for smaller subjects, where the distance between the receiving coil and the subject is greater, leading to weaker signals. Furthermore, the receiving coil requires a robust support structure for mounting a preamplifier and coaxial cable, making close placement within the body difficult.

[0004] Current designs employ a wireless coil placed close to the human body, then using inductive coupling to enhance the magnetic resonance signal acquired by the receiving coil, thereby improving the image signal-to-noise ratio and parallel imaging image quality. However, this technology has several drawbacks: while the wireless coil can be placed anywhere on the subject, it offers limited protection. The wireless coil relies on only one passive detuning circuit, which may not operate ideally, especially when multiple coils are simultaneously conducting and there is mutual coupling between them. Furthermore, if one of the bidirectional diodes in this circuit fails, the coil itself may not detune properly, leading to excessive coupling of transmitted energy into the coil. If radio frequency transmitted energy couples to the wireless coil, it could result in excessive local radio frequency radiation, exceeding the electromagnetic power absorption or consumption (SAR) limit per unit mass of human tissue, thus posing a safety risk. Additionally, the wireless coil design does not address inductive coupling with the receiving coil. Strong inductive coupling degrades the performance of both the wireless and receiving coils; only with high energy transfer efficiency can the image signal-to-noise ratio be improved. Summary of the Invention

[0005] To address the technical problems mentioned above, this invention applies active detuning, passive detuning, and a fuse simultaneously in two layers of coils, ensuring that each coil in the dual-layer receiving coil array can be fully detuned during radio frequency transmission, thus preventing the radio frequency transmission signal from coupling into the receiving coil.

[0006] This invention also incorporates the theory of magnetic resonance wireless power transfer, using the principle of inductive coupling for analysis. The transmission efficiency between the two coil layers directly affects the signal received by the receiving coil. The operating frequency is the first core factor affecting transmission efficiency; the highest transmission efficiency is achieved when both coil layers operate simultaneously at the magnetic resonance Larmor frequency. The second factor is the coupling coefficient, which is mainly determined by the placement of the inner and outer coil layers and the distance between them. The third core factor is the coil's own quality factor; the coil's material, structure, and size all affect the quality factor. A larger coupling coefficient and a higher quality factor result in higher transmission efficiency between the coils.

[0007] This invention aims to address the problem that currently available commercially available coils cannot be placed in close contact with the human body, resulting in attenuation of the received MRI signal with distance, thus failing to achieve the optimal signal-to-noise ratio. This is especially true for smaller subjects, where the distance between the receiving coil and the subject is greater, leading to weaker signals.

[0008] To achieve the above objectives, the present invention provides a dual-layer magnetic resonance radio frequency receiving coil array, comprising: The inner coil array, placed close to the subject, is used to acquire and couple magnetic resonance signals. It includes a coil array covering the sample area, consisting of several radio frequency coils with the same resonant frequency as the magnetic resonance frequency. The outer coil array is set at a preset distance from the inner coil array and is connected to the receiving coil and the preamplifier, so that the signal is amplified and enters the receiving channel of the system spectrometer through the coaxial line.

[0009] Preferably, each resonant circuit in the inner and outer coil arrays is provided with an active detuning circuit, a passive detuning circuit, and a fuse; The inner coil array and the outer coil array transmit magnetic resonance signals through inductive coupling. The outer coil array is a multi-channel receiving coil covering the magnetic resonance sample region and the inner coil, containing several receiving coils with the same frequency as the inner coil.

[0010] Preferably, the active detuning circuit includes a first capacitor, a first inductor, and a first diode; The first capacitor and the first inductor are connected in series to form a first LC resonant circuit, and the resonant frequency of the first LC resonant circuit is the same as the magnetic resonance operating frequency of the coil circuit in which it is located. The first diode is connected in parallel with the first LC resonant circuit and is turned on or off according to the DC bias voltage provided by the system. When the first diode is turned on under DC bias, the first LC resonant circuit causes the coil circuit in which it is located to detune at the magnetic resonance operating frequency. The passive detuning circuit includes a second capacitor, a second inductor, and a second diode; The second capacitor and the second inductor are connected in series to form a second LC resonant circuit; The second diode is connected in parallel with the second LC resonant circuit and is configured to be self-biased and turned on by the energy generated by the radio frequency transmission pulse, thereby detuning the coil circuit in which it is located. The fuse is connected in series in the coil circuit.

[0011] Preferably, it further includes a fixing bracket for fixing the inner coil array and the outer coil array to maintain the preset distance and the relative position of the two coil arrays.

[0012] Preferably, the inner coil array also includes a coil housing made of medical-grade polyurethane rubber and configured to provide a variety of sizes for subjects of different body types.

[0013] Preferably, the inner coil array receives the magnetic resonance signal from the subject and transmits the magnetic resonance signal to the outer coil array through inductive coupling; the outer coil array receives the magnetic resonance signal coupled through the inner coil array and the magnetic resonance signal directly from the subject, and transmits the combined signal to the preamplifier.

[0014] Preferably, the preset distance between the inner coil array and the outer coil array is adjusted so that the two coil arrays operate in the near-field strong coupling region, and the frequency response measured at the input end of the outer coil array is within the transition distance range from Lorentz resonance to coupling splitting, so as to maximize the signal transmission efficiency at the magnetic resonance frequency.

[0015] Preferably, the process for adjusting the preset distance includes: The outer coil array is gradually moved closer to the inner coil array from a position away from it. Monitor the frequency response at the input of the outer coil array; When the frequency response changes from a single Lorentz resonance peak to a split double peak, the preset distance is determined as the distance that keeps the frequency response in the transition state between the Lorentz resonance peak and the double peak.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: The dual-layer magnetic resonance radio frequency receiving coil array provided by this invention is suitable for subjects with smaller body sizes, significantly improving the signal-to-noise ratio of their magnetic resonance images. By receiving signals with the inner coil close to the body, the distance between the signal source and the receiving coil is effectively shortened, thereby enhancing the strength of the received signal. Simultaneously, this design ensures that the coils are fully detuned during the radio frequency transmission phase, preventing excessive radio frequency energy from coupling to the coils and ensuring the safety of the subject.

[0017] The coil array is highly adaptable, allowing for close fit to various body shapes by replacing inner coils of different sizes. Optimized spacing between the inner and outer coils ensures efficient coupling, enabling efficient transmission of magnetic resonance signals from the inner to the outer layer. While maintaining good compatibility with existing commercial MRI equipment, the entire system achieves an image signal-to-noise ratio comparable to or even better than that of multi-channel single-layer coil systems. It also provides a higher image signal-to-noise ratio for parallel imaging undersampled data reconstruction, improving the cost-effectiveness and clinical application flexibility of the equipment. Attached Figure Description

[0018] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of a double-layer receiving coil circuit according to an embodiment of the present invention; Figure 2 This is an equivalent circuit diagram of the magnetic resonance double-layer receiving coil according to an embodiment of the present invention; Figure 3 This is a schematic diagram of a double-layer receiving coil according to an embodiment of the present invention; Figure 4 This is a comparison of the signal-to-noise ratio of the head double-layer coil of this invention and a 32-channel commercial head coil; Figure 5 This is a comparison chart of image signal-to-noise ratios using the parallel imaging acceleration factor R in an embodiment of the present invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] Example 1: like Figure 1 The diagram shown is a schematic of the double-layer receiving coil circuit in this embodiment. Components C1, C2, C3, C4, C5, and C6 in the diagram are fixed capacitors in the outer receiving coil circuit. T For variable capacitors, F is a fuse. C1, L1, and D1 form an active detuning circuit. L1 is a hand-wound inductor, and D1 is a non-magnetic PIN diode. The DC bias from the system output is applied across D1. When a forward bias voltage is applied to D1, causing it to conduct, the resonant circuit formed by C1 and L1 has the same resonant frequency as the coil. Therefore, the resonant peak of the coil resonates and splits, detuning the coil at the magnetic resonance frequency. C2, L2, and D2 form a passive detuning circuit. L2 is a hand-wound inductor, and D2 are two non-magnetic diodes designed to protect magnetic resonance imaging (MRI) receivers from the high radio frequency (RF) energy fields present in most MRI devices, including long RF pulses and RF spikes. The diodes do not require a forward bias voltage to turn on; instead, they are self-biased by the pulse power of the RF transmitter. P1 and C P2 The capacitor, used as a coil tuning capacitor, is used to adjust the coil impedance to match the input impedance of the preamplifier.

[0023] C'1, C'2, C'3, C'4, C'5, and C'6 are fixed capacitors in the inner coil circuit. TF is a variable capacitor and F' is a fuse. C'1, L'1 and D'1 form an active detuning circuit, and C'2, L'2 and D'2 form a passive detuning circuit.

[0024] Figure 2 This is the equivalent circuit diagram of a magnetic resonance double-layer receiving coil. In wireless power transfer theory, electrical energy is transferred via induction through a pair of coupled coils. Figure 2 The magnetic resonance dual-layer receiving coil system employs magnetic coupling, and the coupling coefficient between the two layers of coils is... k It is directly proportional to the mutual inductance M: in, L 1 represents the inductance of the inner coil in a double-layer coupled coil. L 2 represents the inductance of the outer coil.

[0025] To improve power transmission capability and system efficiency, resonant circuits need to be constructed in both layers of coupled coils. For example... Figure 2 As shown, ω 0 represents the resonant frequency of the inner and outer coils. The outer receiving coil is connected to the preamplifier. Z R This indicates the input impedance of the preamplifier. R p This represents the equivalent impedance of the preamplifier. C R2 and L R2 Used to adjust Z R To meet the matching requirements of the preamplifier. Among them, V ( ω 0) indicates the Larmor frequency in magnetic resonance. ω At position 0, the signal voltage coupled from the inner coil to the outer coil is the effective signal that ultimately enters the preamplifier. R 1 and R 2 represents the equivalent series resistance of the inner and outer coils, reflecting the coil's inherent loss characteristics. Its value affects the system's quality factor and signal transmission efficiency. When the two coils are close together, strong resonant coupling can cause the resonance peak to... ω A split occurs at point 0, causing the preamplifier to receive... ω 0 frequency signal voltage V ( ω 0) Weakening. Therefore, the distance between the two coil layers needs to be selected within a certain range, that is, the transition range from the Lorentz resonance to the slight splitting of the resonance peak, so as to ensure high coupling efficiency while avoiding signal attenuation caused by excessive coupling.

[0026] Example 2: The following will describe in detail, with reference to this embodiment, how the present invention solves the technical problems in practical work.

[0027] The inner coil of this invention employs the same protection measures as the outer receiving coil. An active detuning circuit is added to the inner coil, consistent with that of the outer receiving coil, and is controlled by the system's DC bias signal. The diode is turned on when the system is transmitting radio frequency signals, and turned off when receiving magnetic resonance signals.

[0028] If the inner coil consists of multiple resonant coil loops, each loop has an active detuning circuit, a passive detuning circuit, and a fuse. When the DC bias provided by the system is applied across the diodes, the LC oscillation loop with the same resonant frequency as the coil begins to resonate, causing the coil to detune. During operation, if the first-layer active detuning circuit fails, the second-layer passive detuning circuit acts as a protective measure. When the RF transmission signal is coupled to the coil, the voltage causes the bidirectional diode to conduct, resulting in coil detuning. The third layer of protection is the fuse, which melts when the current is too high to protect the subject's safety.

[0029] The inner coil is placed directly close to the human body, and the DC bias connection line controlling the detuning of the inner coil is connected to the system via a socket. Suitable inner coils can be replaced according to different needs. Mutual coupling between inner coils affects the coil's quality factor, requiring an overlap decoupling method. The coil positions must remain relatively fixed, so a material with both good moldability and strength is chosen. The inner coil's retaining shell is made of medical-grade polyurethane rubber, available in small, medium, and large sizes to accommodate different body shapes.

[0030] The inductive coupling between the outer coil and the inner coil directly affects the signal transmission efficiency, and the outer coil needs to be matched with the inner coil.

[0031] The human magnetic resonance imaging (MRI) signal and the inner coil close to the body are used as the signal transmitters, while the outer receiving coil array and preamplifier are used as the signal receivers. The human magnetic resonance signal is received by the inner coil and then transmitted to the outer receiving coil, which simultaneously receives both the signal from the body and the signal transmitted from the inner coil. After amplification by the preamplifier, the signal is transmitted via a coaxial cable to the MRI system's spectrometer for signal processing. When the transmitter and receiver are in resonance, both coils resonate at the Larmor frequency of the magnetic resonance. The transmission efficiency depends on the distance between the inner and outer coils and their resonance matching.

[0032] Efficient transmission requires adjusting the distance to the near-field strong coupling region. Measuring the frequency response of the outer receiving coil at the preamplifier input reveals that when the outer coil is far away and the coupling is weak, the frequency response exhibits Lorentz resonance. As the outer coil moves closer to the inner coil, the coupling effect strengthens, and the depth of the Lorentz resonance in the frequency response increases. However, as the distance decreases further, coupling splitting occurs in the frequency responses of the outer and inner coils, resulting in very low efficiency of the received signal at the resonant frequency. Therefore, selecting a distance between the Lorentz resonance and the point of excessive coupling splitting maintains a relatively high frequency response at the resonant frequency, leading to higher transmission efficiency.

[0033] The design of a dual-layer receiving coil array requires consideration of the transmission efficiency between the inner and outer coil arrays. The distance between the inner and outer coil arrays is a crucial factor; therefore, the distance between the outer and inner coils is fixed, necessitating a fixed support to maintain their relative positions. The inner coils mentioned in this application, suitable for different human body sizes, are all equipped with a fixed coil housing and a human body support.

[0034] Example 3: To verify the superiority of this invention over existing technologies, this embodiment is provided as a comparative embodiment. For example... Figure 3 As shown, the outer layer is a 12-channel head coil, and the inner layer is an array of 20 resonant coupled coils. A DC bias signal line is led out from the arm and connected to the active detuning circuit of the 20 inner coils. This dual-layer coil was compared with a 32-channel commercial head coil. For subjects with a smaller head circumference (approximately 20 cm in diameter), the dual-layer coil showed a significantly higher image signal-to-noise ratio (SNR). Comparing the SNR of pixels within a circular area, the data showed a 127% improvement in SNR in the prefrontal cortex, a 46% improvement in the temporal lobe, and a 16% improvement in the center of the head. Results are as follows... Figure 4 As shown.

[0035] Figure 5 The image in the middle shows a comparison of signal-to-noise ratio (SNR) using the parallel imaging acceleration factor R, with images cropped from the subject's brain. AP indicates that the MRI data acquisition order is from front to back, and LR indicates that the MRI data acquisition order is from left to right. In the figure, the image SNR when the parallel imaging acceleration factor R=1 is defined as 1000. As R increases, the average SNR of the image decreases relatively. The numbers in the figure represent the average pixel SNR of the brain image. With the same acceleration factor, the image SNR obtained by the 32-channel commercial head coil is consistently higher than that of the dual-layer head coil. However, the dual-layer head coil uses 12 receiving channels, indicating that the inner coil provides more image SNR for the reconstruction of undersampled data from parallel imaging, contributing to the improved SNR after image acceleration.

[0036] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A dual-layer magnetic resonance radio frequency receiving coil array, characterized in that, include: The inner coil array, placed close to the subject, is used to acquire and couple magnetic resonance signals. It includes a coil array covering the sample area, consisting of several radio frequency coils with the same resonant frequency as the magnetic resonance frequency. The outer coil array is set at a preset distance from the inner coil array and is connected to the receiving coil and the preamplifier, so that the signal is amplified and enters the receiving channel of the system spectrometer through the coaxial line.

2. The dual-layer magnetic resonance radio frequency receiving coil array according to claim 1, characterized in that, Each resonant circuit in the inner and outer coil arrays is equipped with an active detuning circuit, a passive detuning circuit, and a fuse. The inner coil array and the outer coil array transmit magnetic resonance signals through inductive coupling. The outer coil array is a multi-channel receiving coil covering the magnetic resonance sample region and the inner coil, containing several receiving coils with the same frequency as the inner coil.

3. The dual-layer magnetic resonance radio frequency receiving coil array according to claim 1, characterized in that, The active detuning circuit includes a first capacitor, a first inductor, and a first diode; The first capacitor and the first inductor are connected in series to form a first LC resonant circuit, and the resonant frequency of the first LC resonant circuit is the same as the magnetic resonance operating frequency of the coil circuit in which it is located. The first diode is connected in parallel with the first LC resonant circuit and is turned on or off according to the DC bias voltage provided by the system. When the first diode is turned on under DC bias, the first LC resonant circuit causes the coil circuit in which it is located to detune at the magnetic resonance operating frequency. The passive detuning circuit includes a second capacitor, a second inductor, and a second diode; The second capacitor and the second inductor are connected in series to form a second LC resonant circuit; The second diode is connected in parallel with the second LC resonant circuit and is configured to be self-biased and turned on by the energy generated by the radio frequency transmission pulse, thereby detuning the coil circuit in which it is located. The fuse is connected in series in the coil circuit.

4. The dual-layer magnetic resonance radio frequency receiving coil array according to claim 1, characterized in that, It also includes a fixing bracket, which is used to fix the inner coil array and the outer coil array to maintain the preset distance and the relative position of the two coil arrays.

5. The dual-layer magnetic resonance radio frequency receiving coil array according to claim 4, characterized in that, The inner coil array also includes a coil housing made of medical-grade polyurethane rubber and configured to provide multiple sizes for subjects of different body types.

6. The dual-layer magnetic resonance radio frequency receiving coil array according to claim 1, characterized in that, The inner coil array receives magnetic resonance signals from the subject and transmits them to the outer coil array via inductive coupling. The outer coil array receives the magnetic resonance signals coupled through the inner coil array and the magnetic resonance signals directly from the subject, and transmits the combined signals to the preamplifier.

7. The dual-layer magnetic resonance radio frequency receiving coil array according to claim 1, characterized in that, Adjust the preset distance between the inner coil array and the outer coil array so that the two coil arrays operate in the near-field strong coupling region, and make the frequency response measured at the input end of the outer coil array within the transition distance range from Lorentz resonance to coupling splitting, so as to maximize the signal transmission efficiency at the magnetic resonance frequency.

8. The dual-layer magnetic resonance radio frequency receiving coil array according to claim 7, characterized in that, The process for adjusting the preset distance includes: The outer coil array is gradually moved closer to the inner coil array from a position away from it. Monitor the frequency response at the input of the outer coil array; When the frequency response changes from a single Lorentz resonance peak to a split double peak, the preset distance is determined as the distance that keeps the frequency response in the transition state between the Lorentz resonance peak and the double peak.

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