CN113281689A

CN113281689A - Magnetic resonance distributed spectrometer

Info

Publication number: CN113281689A
Application number: CN202110555688.XA
Authority: CN
Inventors: 张双; 吴林; 何甜; 康承犇; 张涛; 余洁; 刘杭
Original assignee: University of Electronic Science and Technology of China
Current assignee: University of Electronic Science and Technology of China
Priority date: 2021-05-21
Filing date: 2021-05-21
Publication date: 2021-08-20

Abstract

The invention discloses a magnetic resonance distributed spectrometer, which is applied to the technical field of electronic information and aims at solving the problem of low signal-to-noise ratio of the existing spectrometer, the invention adopts a near-field receiving technology, and can effectively reduce the attenuation and interference of magnetic resonance signals in the transmission process; a clock management module with low phase noise is configured on the near-field receiver, so that the clock signal output by the receiver clock management module is homologous with the spectrometer master control equipment unit, and meanwhile, the jitter of the clock signal output by the receiver clock management module is not influenced by the quality deterioration of the clock sent to the receiver by the spectrometer master control equipment unit; the spectrometer adopts a link correction mode, so that all spectrometer hardware terminals are based on the same starting point.

Description

Magnetic resonance distributed spectrometer

Technical Field

The invention belongs to the technical field of electronic information, and particularly relates to a spectrometer architecture.

Background

The spectrometer is the most critical and core device of the magnetic resonance imaging system and is responsible for sequence operation, radio frequency signal generation, spatial localization gradient signal generation, radio frequency signal reception, acquired data reconstruction and the like. In the magnetic resonance imaging process, a spectrometer receiver generates and stores a uV-level magnetic resonance signal obtained by induction of a radio frequency receiving coil as k-space data for magnetic resonance image reconstruction after the uV-level magnetic resonance signal is subjected to filtering, amplification, sampling, analog-to-digital conversion, digital filtering and the like. It follows that in the case of main magnetic field determination, the architecture and performance of the spectrometer have a decisive influence on the signal-to-noise ratio of the magnetic resonance imaging system. The implementation of spectrometers is based on electronics and communication technology, among other things. Since this century, a large number of advanced electronic communication chips and signal sampling processing techniques have been widely used in emerging fields such as 5G. However, in the field of magnetic resonance imaging techniques, these latest techniques have not been well utilized for reasons of cost and the like. Therefore, the project provides the latest software and hardware technology and theory in the fields of electronics and communication, and innovatively develops the whole architecture and part of key technologies of the spectrometer, so as to greatly improve the signal-to-noise ratio of the magnetic resonance imaging system.

Firstly, in the signal acquisition link, the dynamic range of the signal in the magnetic resonance K space is large, and the signal in the whole dynamic range needs to be accurately sampled to acquire all the useful signals without distortion, and particularly, the magnetic resonance receiver needs to be capable of effectively acquiring the signal close to the thermal noise level in high-resolution three-dimensional imaging. The early spectrometer adopts an analog receiver, the signal-to-noise ratio of the early spectrometer is not enough, the magnetic resonance signal close to the thermal noise level cannot be effectively acquired, and the signal-to-noise ratio is limited. In recent years, analog receivers are gradually replaced by digital receivers, the digital receivers have higher signal sensitivity, and the circuit noise of the digital receivers is controlled at a lower level, so that the signal-to-noise ratio of a spectrometer and the spatial resolution of magnetic resonance are improved.

In terms of structural design, the existing spectrometer generally adopts an integrated structure, a receiver is placed in a spectrometer cabinet between devices, the structural design is convenient for the spectrometer receiver to be synchronous with the time sequence of other board cards of the spectrometer, but magnetic resonance signals induced by a radio frequency receiving coil need to be transmitted to the spectrometer between the devices from a magnet to a long distance, and spatial interference components between the magnets can crosstalk into useful received signals, so that the signal-to-noise ratio of the received signals can be reduced. Therefore, the spectrometer architecture in a centralized layout is also a large factor limiting the signal-to-noise ratio of the spectrometer.

The performance index of the analog-to-digital converter of the spectrometer receiver is also a key factor influencing the signal-to-noise ratio of the spectrometer. The signal-to-noise ratio of the sampled signal can be improved by increasing the number of bits of the analog-to-digital converter of the receiver, but the spectrometer hardware needs to be improved and designed by increasing the number of bits of the analog-to-digital converter, and meanwhile, the implementation cost of the scheme is high. In addition to the increase of the sampling signal-to-noise ratio by increasing the number of bits of the analog-to-digital converter, the sampling signal-to-noise ratio can also be increased by increasing the sampling rate of the analog-to-digital converter, but the sampling rate is limited by key indexes such as the sampling rate of the analog-to-digital converter and the conversion bit width on the market, the sampling rate of the analog-to-digital converter of the conventional commercialized magnetic resonance spectrometer receiver is generally designed to be below 100MHz, and the conversion bit width is generally 14 bits or 16 bits. In recent years, the performance of an analog-digital converter chip is rapidly improved, an analog-digital converter with the sampling rate of more than 1GHz and the conversion bit width of 16 bits appears, the price of the new analog-digital converter is only increased by 50% compared with the price of the analog-digital converter on the existing commercialized spectrometer receiver, and meanwhile, the material cost of the analog-digital converter of the receiver only accounts for a very small part of the whole MRI machine, so that the opportunity of applying the high-sampling-rate analog-digital converter to the magnetic resonance receiver is mature. In addition, noise scrambling techniques have been reported in the literature to improve the signal-to-noise ratio of sampled signals. In fact, noise scrambling technology has been applied in the communication industry in recent years, and has achieved good effect on improving the signal-to-noise ratio of the sampled signal, but the technology is not used in the magnetic resonance spectrometer at present.

Under the condition of not changing a spectrometer circuit, the signal-to-noise ratio of an imaging image can be improved by dynamically adjusting the receiving gain parameter of the spectrometer. The magnetic resonance imaging K space data needs to collect echo signals of all phase encoding lines, the amplitude of the echo signal of each phase encoding line is obviously different, and the amplitude of the echo signal of a larger phase encoding line is smaller than that of the echo signal of a smaller phase encoding line. Therefore, in the gain setting process, a fixed receiving gain parameter is set on the premise of ensuring that the point with the maximum echo amplitude of all phase encoding lines in the whole K space does not overflow, so that the whole echo signal does not reach the maximum signal-to-noise ratio. Mark A et al and C.H.OH et al found that optimal setting of receive gain parameters in different regions of K-space can increase the signal-to-noise ratio of magnetic resonance images by about two times, but the magnetic resonance imaging spectrometer platform they developed experiments does not support dynamic adjustment of receive gain parameters during sequence operation, so sequence scanning needs to be performed once under each receive gain parameter, and finally complete K-space data is obtained by splicing, and for this, even if the time for manually updating and setting the receive gain parameters is deducted, the total time of whole sequence scanning is 6 to 7 times of that of normal scanning one-pass sequence, which is obviously unacceptable in clinical or actual scientific research. Obviously, to complete all the above tasks in one sequence scan, the receive gain parameters must be dynamically updated during phase encoding line switching, and for this reason, the existing spectrometer architecture needs to be reconstructed to support the dynamic update of the scan parameters.

Disclosure of Invention

In order to solve the technical problems, the invention provides a magnetic resonance distributed spectrometer, which is based on a near-field receiving and distributed full-digitalization overall spectrometer architecture design scheme, and aims at synchronizing and coordinating all parts of the spectrometer under a distributed architecture, so that the signal-to-noise ratio of the spectrometer can be effectively improved.

The technical scheme adopted by the invention is as follows: a magnetic resonance distributed spectrometer comprising: a spectrometer body portion and a near field receiver portion, the spectrometer body portion comprising: the spectrometer comprises a magnet, a sequence host, a spectrometer Main Control equipment Unit (MCU), a gradient signal generating board, a radio frequency signal generating board and a serial server, wherein the sequence host, the gradient signal generating board, the radio frequency signal generating board and the serial server are respectively connected with the spectrometer Main Control equipment Unit; a plurality of near field receivers of the near field receiver part are arranged on the side surface of the magnet; a plurality of near-field receivers of the near-field receiver part are connected with the spectrometer main control equipment unit through a wireless network; the method also comprises the step of configuring a clock source with a low temperature drift coefficient in the spectrometer main control equipment unit, wherein a clock signal generated by the clock source is transmitted to the near-field receiver through a coaxial cable.

The method comprises the steps that a clock management module with low phase noise is configured on a near-field receiver, a first input port of the clock management module is connected with a clock signal sent to the near-field receiver by a spectrometer main control equipment unit, a second input port of the clock management module is connected with the output of a voltage-controlled crystal oscillator, and the output of the clock management module is connected with an analog-to-digital conversion module of the near-field receiver.

The clock jitter of the voltage controlled crystal oscillator is less than 200 fs.

In the running process of the magnetic resonance spectrometer, the near-field receiver, the radio frequency signal generating board and the gradient signal generating board actively track the states of the spectrometer software system and the spectrometer main control equipment unit.

When the radio frequency signal generating board transmits a signal, the receiver is disconnected from the receiving coil, and when the receiver receives a magnetic resonance signal, the radio frequency signal generating board stops transmitting the signal.

The communication among the sequence host, the spectrometer main control equipment unit, the gradient signal generating board, the radio frequency signal generating board, the serial server and the near field receiver adopts a link correction method.

The invention has the beneficial effects that: according to the invention, the receiver is separated from the spectrometer and distributed to the side face of the magnet, so that direct radio frequency sampling, extraction and digitization of a weak magnetic resonance signal induced by a radio frequency receiving coil are realized among the magnets, and then the weak magnetic resonance signal is transmitted to a reconstruction host in a digital coding mode, thereby avoiding interference caused by long-distance cable transmission analog signals and improving the signal-to-noise ratio of the radio frequency receiving signal; then configuring a clock source with a low temperature drift coefficient on the spectrometer MCU, transmitting the clock signal to the near-field receiver through a coaxial cable, synchronizing the time sequence of the near-field receiver and the radio frequency pulse signal generating board in a cooperative working mode of the main body part of the spectrometer and the near-field receiver of the spectrometer, and solving the problem that the quality of a sampling clock of a near-field receiving signal is influenced; the communication between the spectrometer components will use a method of link correction so that the spectrometer components start working based on the same time starting point.

Drawings

FIG. 1 is a spectrometer configuration of the present invention;

FIG. 2 is a block diagram of a receiver clock management module;

FIG. 3 is a schematic diagram of the implementation of timing synchronization between spectrometer components;

fig. 4 is a schematic diagram of an implementation of a single physical receive channel of a near field receiver.

Detailed Description

In order to facilitate the understanding of the technical contents of the present invention by those skilled in the art, the present invention will be further explained with reference to the accompanying drawings.

The spectrometer, as the most central and critical component in a magnetic resonance imaging system, is crucial to the signal-to-noise ratio and performance of the magnetic resonance imaging system.

The overall structure of the spectrometer provided by the invention is shown in FIG. 1, and the spectrometer provided by the invention is structurally divided into two parts: the spectrometer comprises a spectrometer Main body part and a near field receiver, wherein the spectrometer Main body part comprises a sequence host, a spectrometer Main Control Unit (MCU), a gradient signal generating board, a radio frequency signal generating board, a serial server and the like. The spectrometer components are introduced as follows:

(i) sequence host: the spectrometer hardware is responsible for interpreting time sequences and parameters input by a user into hardware parameter sequences, and in the sequence scanning process, the spectrometer hardware is used for timing according to a system synchronous clock so as to send a trigger signal to a sequence host at a fixed beat. And the sequence host expands the sequence parameters according to the beat, generates a hardware register control command and transmits the hardware register control command to the MCU through the high-speed optical fiber transmission path, and then the MCU forwards the register to the spectrometer hardware terminal board card.

(ii) And (3) a spectrometer MCU: the star-based topological structure is connected with each hardware terminal board card of the spectrometer (each hardware comprises a spectrometer main body part and a near field receiver part) through a high-speed optical fiber physical link, and each hardware terminal is managed, so that all spectrometer hardware equipment is formed into an organic whole, and in addition, each hardware terminal of the spectrometer exchanges information through an MCU (microprogrammed control unit), and synchronization is realized.

(iii) Gradient signal generation board: under the control of a sequence host, the analysis of a gradient event is realized, the generation of a three-axis high-precision gradient waveform is completed, and the compensation of pre-emphasis of a gradient waveform signal is synchronously completed while the gradient waveform is generated.

(iv) Radio frequency signal generation board: the radio frequency pulse signal generator is used for generating radio frequency pulse signals which can be expanded to 8 physical channels and outputting the radio frequency pulse signals to a radio frequency power amplifier. The radio frequency signal generating board communicates with a spectrometer MCU through a high-speed optical fiber link, a digital up-conversion algorithm is realized based on an FPGA, a radio frequency oversampling mode is adopted in a digital-to-analog conversion link to work, the sampling rate is 100Msps, the conversion data bit width is 16 bits, a digital-to-analog converter outputs an intermediate frequency analog signal with the frequency of 17.5MHz @ +/-500 KHz, the intermediate frequency analog signal is input to a mixer which is responsible for the up-conversion function, amplitude modulation is carried out on a high-precision local oscillator signal with the frequency of 110MHz generated by a phase-locked loop, the result of the amplitude modulation is a 3T magnetic resonance radio frequency pulse transmitting signal with the central frequency of 127.5MHz, and the signal is finally transmitted to a radio frequency power amplifier after being processed.

(v) A serial server: the system is used for managing the states of a large number of serial devices in the magnetic resonance system, such as a radio frequency power amplifier, a gradient power amplifier, a sickbed control unit, a water cooling unit and the like.

(vi) A near-field receiver: and receiving a weak magnetic resonance signal from a radio frequency receiving coil, and carrying out low-noise amplification, sampling quantization, orthogonal demodulation, extraction filtering and other processing on the signal. The coil induction signals are sent to a receiver, the signals are subjected to amplitude amplification through a low noise amplifier, then sent to an analog-to-digital converter for direct radio frequency sampling and quantization, and then transmitted to a spectrometer MCU through FPGA digital down-conversion processing. The MCU transmits the acquired data to the reconstruction host through the high-speed communication link, the reconstruction host is responsible for reconstructing the acquired data, and the image data output by the reconstruction host is uploaded to the scanning operation console.

The near-field receiving technology is adopted in the invention, so that the attenuation and interference of the magnetic resonance signal in the transmission process can be effectively reduced, and the signal-to-noise ratio of the spectrometer can be improved. But such a distributed architecture will cause problems of timing asynchrony, phase incoherence, etc. between the receiver and the spectrometer body part. In order to solve the problems, the invention is further improved as follows:

implementation of a high quality sampling clock for near field receivers:

and configuring a constant-temperature crystal oscillator with a temperature drift coefficient as low as ppb level in the spectrometer main control equipment unit to serve as a high-stability clock source of the near-field receiver and other board cards of the spectrometer, and transmitting the clock signal to the near-field receiver and other board cards of the spectrometer through a coaxial cable.

And a clock management module with low phase noise is configured on the near-field receiver, and the clock management module not only ensures that a sampling clock supplied to the analog-to-digital converter has clock jitter of a level as low as femtosecond, but also ensures that the frequency of the sampling clock is homologous with a system reference clock of the spectrometer main control equipment unit. To ensure that the receiver ADC achieves a better signal-to-noise ratio of the sampled signal, it is necessary to ensure that the sampling clock has a small jitter, and the clock management module is responsible for reducing the clock jitter. A phase-locked loop chip with good jitter performance is commonly used for processing clock jitter, and the voltage-controlled crystal oscillator works together with the phase-locked loop chip.

An implementation of the receiver clock management module is shown in fig. 2. The input port of the receiver clock management module is connected with a clock signal which is sent to the near-field receiver by the spectrometer main control equipment unit, and the input port is in synchronous reference clock connection, so that the receiver clock management module outputs a clock which is homologous with the clock of the spectrometer main control equipment unit. The clock jitter of the vcxo is generally controlled below 200fs, and the clock jitter of the vcxo determines the jitter of the output clock of the receiver clock management module.

The advantages of this clock generation approach are: the clock sent to the receiver by the spectrometer master control equipment unit is influenced by space and a transmission cable, and the clock jitter is generally large and generally in us magnitude; the clock jitter of the voltage controlled crystal oscillator is low, and can generally reach below 200 fs. The receiver clock generation method provided by the invention enables the output clock signal of the receiver clock management module to be homologous with the spectrometer main control equipment unit, meanwhile, the jitter of the output clock signal of the receiver clock management module is not influenced by the quality deterioration of the clock transmitted to the receiver by the spectrometer main control equipment unit, and the jitter of the output clock signal of the receiver clock management module is determined by the voltage-controlled crystal oscillator.

The cooperative working mode of the main body part of the spectrometer and the near-field receiver of the spectrometer is as follows:

in the running process of the magnetic resonance spectrometer, the spectrometer main control equipment unit is used as a hardware management pivot of a spectrometer hardware platform, each component of the spectrometer can actively track the states of a spectrometer software system and the spectrometer main control equipment unit, meanwhile, the spectrometer near-field receiver can actively report the working state and the abnormal state of the spectrometer main control equipment unit, and the spectrometer main control equipment unit can also issue control words related to a sequence host and a receiver to the near-field receiver in real time.

In order to protect the receiver from receiving high-intensity signals, the radio-frequency signal generating board and the radio-frequency receiving board cannot work simultaneously, when the radio-frequency signal generating board transmits signals, the receiver is disconnected from the receiving coil, when the receiver receives magnetic resonance signals, the radio-frequency signal generating board stops transmitting signals, and the coordination work is completed by the spectrometer MCU.

The design of the spectrometer near-field receiver and the radio frequency pulse signal generation board time sequence synchronization is as follows:

FIG. 3 is an implementation of timing synchronization between spectrometer components. The communication between the spectrometer components adopts a link correction method, so that the spectrometer components start to work based on the same time starting point, and the length of a cable of a transmission link is irrelevant. In a spectrometer ready state, the spectrometer main control equipment unit sends a link length measurement instruction to all hardware terminals through a real-time data channel, a calculator is started at the same time, the hardware terminals return confirmation messages of receiving the link length measurement instruction after receiving the instruction, and the spectrometer main control equipment unit stops counting after receiving the confirmation messages, so that the length of each path is saved by each counting register, and the length of each path is obtained by dividing the length by 2.

The transmitter in fig. 3 is one of the hardware terminals of the spectrometer and is responsible for the generation of the radio frequency pulse signal of the spectrometer.

In this embodiment, each spectrometer hardware terminal has a count register, which is used to correct the length of the spectrometer master control device unit and each spectrometer hardware terminal.

The developed spectrometer adopts a link correction mode, so that all spectrometer hardware terminals are based on the same starting point, and the starting point is irrelevant to the length of a link; and the spectrometer master control equipment unit simultaneously transmits the spectrometer reference clock to the spectrometer hardware terminal, so that the gradient signal generation, the radio frequency signal generation and the radio frequency signal receiving events synchronously work under the unified clock beat of the spectrometer. Through the measures, the problem of time sequence synchronization among spectrometer components can be solved.

Fig. 4 is a schematic diagram of an implementation of a single physical receive channel of a near-field receiver (receiver for short). Each receiver integrates a 16-channel radio frequency signal conditioning circuit, and fig. 4 is a signal flow chart of a single channel of the near-field receiver. The coil induction signals are sent to a receiver, the signals are subjected to amplitude amplification through a low noise amplifier, then sent to an analog-to-digital converter (ADC) for direct radio frequency sampling and quantization, and then transmitted to a spectrometer main control equipment unit MCU through FPGA digital down conversion processing. The MCU transmits the acquired data to the reconstruction host through the high-speed communication link, the reconstruction host is responsible for reconstructing the acquired data, and the image data output by the reconstruction host is uploaded to the scanning operation console.

It will be appreciated by those of ordinary skill in the art that the embodiments described herein are intended to assist the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited embodiments and examples. Various modifications and alterations to this invention will become apparent to those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A magnetic resonance distributed spectrometer, comprising: a spectrometer body portion and a near field receiver portion, the spectrometer body portion comprising: the spectrometer comprises a magnet, a sequence host, a spectrometer Main Control equipment Unit (MCU), a gradient signal generating board, a radio frequency signal generating board and a serial server, wherein the sequence host, the gradient signal generating board, the radio frequency signal generating board and the serial server are respectively connected with the spectrometer Main Control equipment Unit; a plurality of near field receivers of the near field receiver part are arranged on the side surface of the magnet; a plurality of near-field receivers of the near-field receiver part are connected with the spectrometer main control equipment unit through a wireless network; the method also comprises the step of configuring a clock source with a low temperature drift coefficient in the spectrometer main control equipment unit, wherein a clock signal generated by the clock source is transmitted to the near-field receiver through a coaxial cable.

2. The magnetic resonance distributed spectrometer according to claim 1, wherein a clock management module with low phase noise is configured on the near-field receiver, a first input port of the clock management module is connected with a clock signal sent to the near-field receiver by the spectrometer master control equipment unit, a second input port of the clock management module is connected with an output of the voltage controlled crystal oscillator, and an output of the clock management module is connected with an analog-to-digital conversion module of the near-field receiver.

3. A magnetic resonance distributed spectrometer according to claim 2, characterised in that the voltage controlled crystal oscillator has a clock jitter of less than 200 fs.

4. A magnetic resonance distributed spectrometer according to claim 3, characterised in that the near field receiver, the radio frequency signal generation board, the gradient signal generation board actively track the state of the spectrometer software system and the spectrometer master control device unit during operation of the magnetic resonance spectrometer.

5. A magnetic resonance distributed spectrometer according to claim 4 characterised in that the receiver is disconnected from the receive coil when the radio frequency signal generating plate is transmitting and the radio frequency signal generating plate ceases transmitting when the receiver is receiving a magnetic resonance signal.

6. The magnetic resonance distributed spectrometer according to claim 5, wherein the communication between the sequence host, the spectrometer master control equipment unit, the gradient signal generation board, the radio frequency signal generation board, the serial server and the near field receiver adopts a link correction method.