CN106226715A - Resonance digital receives system - Google Patents

Abstract

A magnetic resonance digital receiving system relates to magnetic resonance digital signal processing. It is equipped with magnetic resonance receiving coil, preamplifier, analog-to-digital converter, decimation filter, mixer, frequency synthesizer, CIC filter, FIR filter, compensation filter, FPGA, memory, host computer; magnetic resonance receiving The coil is connected to the preamplifier, the analog-to-digital converter, and the decimation filter in turn, the output terminals of the decimation filter are respectively connected to the input terminals of the first and second mixers, and the output terminals of the frequency synthesizer are respectively connected to the first and second mixers. The input terminals of the two mixers are connected, the output terminals of the first mixer are connected with the input terminals of the first CIC filter, the first FIR filter and the first compensation filter successively, and the output terminals of the second mixer are successively connected. It is connected with the input terminals of the second CIC filter, the second FIR filter and the second compensation filter, and the output terminals of the first and second compensation filters are respectively connected with FPGA.

Description

Magnetic resonance digital receiving system

technical field

The invention relates to magnetic resonance digital signal processing, in particular to a magnetic resonance digital receiving system.

Background technique

With the development and maturity of software radio technology, software radio technology is being applied in the field of magnetic resonance digital signal processing. Magnetic resonance digital receivers often adopt a digital intermediate frequency receiver structure based on the principle of software radio intermediate frequency digitization. The FID (free induction decay) radio frequency signal is received by the magnetic resonance receiving coil, converted into a digital signal after resampling by an analog-to-digital converter, and then digitalized. Orthogonal demodulation, and after decimation and filtering, the FID digital signal at a lower rate is finally obtained.

CIC (Cascaded Integral Comb) filter has the characteristics of simple structure and suitable for high-magnification decimation, so it is widely used in the field of decimation filtering of magnetic resonance receivers. However, in order to apply resampling technology more effectively to achieve the purpose of improving the signal-to-noise ratio of the receiver, especially in applications with high static magnetic field strength, the CIC filter will work at a higher rate (up to several Hundreds of megahertz), which puts forward higher requirements on the operation speed and stability of the device; at the same time, because the integrator of the CIC filter is located before the structure of the decimation, such a high-speed working speed will make the integrator's computing burden increases, the power consumption of the receiver increases accordingly. In addition, the traditional receiving system usually places the mixer for digital down-conversion after the analog-to-digital converter, and the application of resampling technology in magnetic resonance digital reception will make the mixer work at a higher frequency. The hardware stability of the mixer creates pressure, which puts forward higher requirements on the performance of the mixer.

Chinese patent CN103135079A discloses a method for receiving magnetic resonance signals. The coil units in the receiving coil array are divided into different coil unit groups; for each coil unit, the carrier frequency and the signal received by each coil unit in the coil unit group are established Correspondence between them; according to the correspondence, low-noise amplification, filtering and frequency mixing are performed on the signals received by all the coil units in the coil unit group to obtain intermediate frequency signals carried on respective corresponding carrier frequencies of the same channel; After the intermediate frequency signal is amplified and filtered, it is output to an analog-to-digital conversion unit for digital sampling to obtain a digital domain signal.

Contents of the invention

In order to improve the shortcomings of the application of CIC filters in the processing of magnetic resonance digital signals, the purpose of the present invention is to provide a magnetic resonance digital receiving system.

The invention is provided with a magnetic resonance receiving coil, a preamplifier, an analog-to-digital converter, a decimation filter improved by polyphase decomposition, a first mixer, a second mixer, a direct digital frequency synthesizer, and a first CIC filter device, the second CIC filter, the first FIR (finite-length unit impulse response) filter, the second FIR filter, the first compensation filter, the second compensation filter, FPGA (field programmable gate array), memory, upper computer;

The output end of the magnetic resonance receiving coil is connected to the input end of the preamplifier, the output end of the preamplifier is connected to the input end of the analog-to-digital converter, and the output end of the analog-to-digital converter is connected to the multiphase decomposition improved decimation filter The input end of the polyphase decomposition improved decimation filter is connected with the input end of the first mixer and the input end of the second mixer respectively, and the output end of the direct digital frequency synthesizer is connected with the first mixer respectively. The input terminal of the mixer is connected with the input terminal of the second mixer, the output terminal of the first mixer is connected with the input terminal of the first CIC filter, and the output terminal of the first CIC filter is connected with the first FIR filter The input end of the first FIR filter is connected to the input end of the first compensation filter, the output end of the second mixer is connected to the input end of the second CIC filter, and the output of the second CIC filter End is connected with the input end of the second FIR filter, the output end of the second FIR filter is connected with the input end of the second compensation filter, the output end of the first compensation filter and the output end of the second compensation filter are respectively connected with The input terminals of the FPGA are connected, and the FPGA is bidirectionally connected with the upper computer and the memory respectively.

A band-pass filter may be provided between the output end of the preamplifier and the input end of the analog-to-digital converter.

The invention adopts the decimation filter improved by polyphase decomposition, and reduces the working speed of the whole decimation filter. In order to improve the disadvantages such as low polyphase decomposition filter decimation rate and inflexible filter coefficient modification, and give full play to the advantages of CIC filter with simple structure and high extraction rate, the present invention adopts the decimation filter improved in polyphase decomposition A structure combined with a CIC filter. In order to improve the shortcomings of the traditional method of placing the mixer for digital down-conversion after the analog-to-digital converter, the invention adopts the method of placing the mixer after the decimation filter improved by polyphase decomposition, so that the mixer is far away from the Analog-to-digital converters, thereby relieving the mixer pressure and reducing the mixer operating frequency. The invention greatly reduces the working frequency of some key modules of the magnetic resonance digital receiving system, improves the real-time processing capability, and satisfies the high static magnetic field environment and high-magnification resampling technology more effectively.

Description of drawings

FIG. 1 is a composition block diagram of an embodiment of the present invention.

Fig. 2 is the functional block diagram of the decimation filter improved by polyphase decomposition.

detailed description

Referring to Fig. 1, the present invention is provided with magnetic resonance receiving coil 1, preamplifier 2, analog-to-digital converter 4, polyphase decomposition improved decimation filter 5, first mixer 6, second mixer 7, direct Digital frequency synthesizer 8, the first CIC filter 9, the second CIC filter 10, the first FIR (finite length unit impulse response) filter 11, the second FIR filter 12, the first compensation filter 13, the first Two compensation filters 14, FPGA (Field Programmable Gate Array) 15, memory 16, upper computer 17;

The magnetic resonance receiving coil 1 receives the magnetic resonance radio frequency signal input, the output end of the magnetic resonance receiving coil 1 is connected to the input end of the preamplifier 2, and the output end of the preamplifier 2 is connected to the input end of the analog-to-digital converter 4, The output end of the analog-to-digital converter 4 is connected with the input end of the decimation filter 5 improved by polyphase decomposition, and the output end of the decimation filter 5 improved by polyphase decomposition is connected with the input end of the first mixer 6 and the second mixer respectively. The input terminal of the frequency converter 7 is connected, the output terminal of the direct digital frequency synthesizer 8 is connected with the input terminal of the first mixer 6 and the input terminal of the second mixer 7 respectively, and the output terminal of the first mixer 6 Be connected with the input end of the first CIC filter 9, the output end of the first CIC filter 9 is connected with the input end of the first FIR filter 11, the output end of the first FIR filter 11 is connected with the first compensation filter 13 The input end is connected, and the output end of the second mixer 7 is connected with the input end of the second CIC filter 10, and the output end of the second CIC filter 10 is connected with the input end of the second FIR filter 12, and the second FIR filter The output terminal of the device 12 is connected with the input terminal of the second compensation filter 14, the output terminal of the first compensation filter 13 and the output terminal of the second compensation filter 14 are connected with the input terminal of the FPGA 15 respectively, and the FPGA 15 is respectively connected with the upper Machine 16 and memory 17 are bidirectionally connected.

A band-pass filter 3 may be provided between the output end of the preamplifier 2 and the input end of the analog-to-digital converter 4 .

In Fig. 1, the mark s _sin (n) is the digital local oscillator signal of the first mixer 6, and the mark s _cos (n) is the digital local oscillator signal of the second mixer 7; I (n) is in-phase FID digital baseband signal, Q(n) is an orthogonal FID digital baseband signal.

The invention proposes a magnetic resonance digital receiving system. A decimation filter improved by polyphase decomposition is designed by using N-order CIC filter. The traditional N-order CIC filter transfer function is:

H(z)=[(1-z- ^R )/(1-z ^-1 )] ^N ,

Where R is the extraction factor. suppose

R=2 ^K M,

Where K and M are positive integers, the transfer function H(z) can be decomposed into non-recursive parts

${\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; \&lsqb;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; z</span>}}^{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; K</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; z</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&rsqb;</span>}^{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; \&lsqb;</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; z</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; \&rsqb;</span>}^{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; \&Pi;</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; z</span>}}^{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; i</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; ,</span>$

and the recursive part

${\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; z</span>}}^{{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; K</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; \&lsqb;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; z</span>}}^{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; K</span>}}\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; z</span>}}^{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; K</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&rsqb;</span>}^{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; .</span>$

It can be easily seen from the expression that H ₁ (z) is a filter with a decimation factor of 2 ^K , is a filter with a decimation factor of M. recursive part first Do the identity transformation, its structure is equivalent to the decimator whose decimation factor is M on the H ₂ (z) cascade, where

H ₂ (z)=[(1-z- ^M )/(1-z ^-1 )] ^N .

Then do multiphase decomposition on the non-recursive part H ₁ (z) to get

${\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; z</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; i</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; z</span>}}^{{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; K</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span>$

Where E _i (z) is the polyphase component of the i (i=0,1,...,2 ^K -1) polyphase branch, the expression is

E _i (z)=(a _i0 +a _i1 z ^-1 +...+a _i(N-1) z ^1-N ),

The coefficients a _ij (j=0,...,N-1) are jointly determined by N and K. Thus, 2 ^K multiphase decomposition branches are obtained, and the i-th multiphase branch is cascaded with a delay unit z ^-i multiphase components A decimator with a decimation factor of ^2K is then cascaded. Then perform Nobel identity transformation on each polyphase branch, and the structure of the i-th polyphase branch is obtained by cascading an decimator with a decimation factor of 2 ^K on a delay unit z ⁱ and then cascading the polyphase component E _i (z); and wherein the delay unit z ⁱ can adopt the structure of the unit delay unit z- ¹ cascaded on the basis of the delay of the i-1 branch, so the delay unit z ⁱ can be formed by the unit The delay unit z ^-1 is equivalently replaced. The result of each polyphase branch is accumulated through an accumulator, and the result is equivalent to the non-recursive part H ₁ (z) after polyphase decomposition. Therefore, the non-recursive part after polyphase decomposition is cascaded to H ₂ (z) and then cascaded to a decimator with a decimation factor of M to obtain an improved decimation filter structure for polyphase decomposition, as shown in Figure 2 .

The complete workflow of the receiving system is described below:

The FID radio frequency signal is received by the magnetic resonance receiving coil to obtain the FID analog intermediate frequency signal, which is amplified by the preamplifier to an amplitude suitable for sampling by the analog-to-digital converter. Afterwards, the out-of-band noise is filtered out by a band-pass filter, and then the analog-to-digital converter is used to resample the FID analog IF signal by L times (usually L is greater than 2) to obtain the FID digital IF signal, assuming its sampling rate F _s . After that, the FID digital intermediate frequency signal is sent to the decimation filter improved by polyphase decomposition, and the decimation factor is

R=2 ^K M,

Where K and M are positive integers, 2 ^K times extraction is completed by the non-recursive part after multiphase decomposition, and M times extraction is completed by the recursive part.

Because this magnetic resonance digital receiving system adopts digital quadrature demodulation technology, it is necessary to obtain two FID digital baseband signals of in-phase and quadrature. The FID digital intermediate frequency signal extracted by the decimation filter improved by polyphase decomposition is divided into two channels, which are respectively input to the first mixer and the second mixer, wherein the first mixer is used for digital down-conversion to obtain in-phase FID digital For the baseband signal, the second mixer is used for digital down-conversion to obtain an orthogonal FID digital baseband signal. Both the digital local oscillator signals of the first mixer and the second mixer are generated by a direct digital frequency synthesizer. In principle, the digital local oscillator signal s _sin (n) input to the first mixer should be the same frequency and phase as the digital local oscillator signal s _t (n) of the magnetic resonance digital transmission system, while the digital local oscillator signal input to the second mixer The local oscillator signal s _cos (n) should be at the same frequency as s _sin (n), but out of phase by 90 degrees. In this receiving system, since the first mixer and the second mixer are located after the polyphase decomposition improved decimation filter, it is assumed here that the frequency of the analog carrier signal required for a certain transmission is f _c , then the direct digital The digital local oscillator signal generated by the type frequency synthesizer is

s _t (n)=sin(2πf _c n),

Therefore, the digital local oscillator signal input to the first mixer by the direct digital frequency synthesizer can be obtained as

s _sin (n) = sin(2πf _c 2 ^K Mn/F _s ),

The digital local oscillator signal input from the direct digital frequency synthesizer to the second mixer is

s _cos (n)=cos(2πf _c 2 ^K Mn/F _s ).

The first mixer outputs the in-phase FID digital baseband signal I ₁ (n) with a rate of F _s /(2 ^K M), and the second mixer outputs a quadrature FID digital baseband with a rate of F _s /(2 ^K M) Signal Q ₁ (n).

Input I ₁ (n) to the first CIC filter for second-stage decimation, the decimation factor is D, and the extracted signal is sent to the first FIR filter to eliminate redundant frequency components, and then input to the first compensation filter , to correct the amplitude distortion caused by the uneven passband of the first two stages of decimation filters; finally output the in-phase FID digital baseband signal I(n), and its rate is F _s /(2 ^K MD). Input Q ₁ (n) to the second CIC filter for second-stage decimation, the decimation factor is D, and the extracted signal is sent to the second FIR filter to eliminate redundant frequency components, and then input to the second compensation filter , to correct the amplitude distortion caused by the uneven passband of the first two stages of decimation filters; the final output is the quadrature FID digital baseband signal Q(n), and its rate is F _s /(2 ^K MD). The I(n) and Q(n) signals are input to the FPGA in parallel, and the FPGA performs a series of signal processing such as the third decimation filter, K space reconstruction, compression coding, and transmission of FID data to the upper computer. At the same time, the FPGA stores the compressed and encoded in-phase FID digital baseband signal and the quadrature FID digital baseband signal in the memory, so that the operator can repeatedly read the FID data information many times later.

The invention adopts the polyphase decomposition improved decimation filter to cooperate with the structure of the CIC filter to move the position of decimation forward, thereby alleviating the pressure of the integrator in the CIC filter and reducing the speed performance requirement of the decimation hardware. At the same time, the mixer is placed after the decimation filter improved by multi-directional decomposition, which reduces the working frequency of the mixer. The invention greatly reduces the working frequency of some key modules of the magnetic resonance digital receiving system, improves the real-time processing capability, and satisfies the high static magnetic field environment and high-magnification resampling technology more effectively.

Claims (2)

1. The magnetic resonance digital receiving system is characterized in that it is provided with a magnetic resonance receiving coil, a preamplifier, an analog-to-digital converter, a polyphase decomposition improved decimation filter, a first mixer, a second mixer, a direct digital Type frequency synthesizer, first CIC filter, second CIC filter, first FIR filter, second FIR filter, first compensation filter, second compensation filter, FPGA, memory, host computer; The output end of the magnetic resonance receiving coil is connected to the input end of the preamplifier, the output end of the preamplifier is connected to the input end of the analog-to-digital converter, and the output end of the analog-to-digital converter is connected to the multiphase decomposition improved decimation filter The input end of the polyphase decomposition improved decimation filter is connected with the input end of the first mixer and the input end of the second mixer respectively, and the output end of the direct digital frequency synthesizer is connected with the first mixer respectively. The input terminal of the mixer is connected with the input terminal of the second mixer, the output terminal of the first mixer is connected with the input terminal of the first CIC filter, and the output terminal of the first CIC filter is connected with the first FIR filter The input end of the first FIR filter is connected to the input end of the first compensation filter, the output end of the second mixer is connected to the input end of the second CIC filter, and the output of the second CIC filter End is connected with the input end of the second FIR filter, the output end of the second FIR filter is connected with the input end of the second compensation filter, the output end of the first compensation filter and the output end of the second compensation filter are respectively connected with The input terminals of the FPGA are connected, and the FPGA is bidirectionally connected with the upper computer and the memory respectively. 2. The magnetic resonance digital receiving system according to claim 1, characterized in that a band-pass filter is provided between the output end of the preamplifier and the input end of the analog-to-digital converter.