CN100346171C - Receiving method of nuclear magnetic resonance imaging signal - Google Patents

Abstract

The present invention relates to a nuclear magnetic resonance imaging method and a nuclear magnetic resonance imaging instrument class, more specifically a receiving method of a nuclear magnetic resonance imaging signal. In the receiving method, a counter which is used for measuring time of errors each time is arranged on a digital receiver. The counter transfers the error time obtained through measurement to a host computer. The linear phase of a spectrum line of the nuclear magnetic resonance imaging signal is corrected in the spectrum width of the receiver. The present invention has the advantages that under the premise that hardware is changed to the minimal extent, the phase jitter of the nuclear magnetic resonance receiver, which is caused by the digital receiver is fundamentally eliminated. The method is very compact and is easily enforced. Simultaneously, because rapid Fourier conversion and inverse Fourier conversion are used, the calculation burden of the host computer is greatly relieved.

Description

A kind of method of reseptance of Magnetic resonance imaging signal

Technical field

The present invention relates to magnetic resonance imaging method employing and NMR imaging instrument device class, more particularly relate to a kind of method of reseptance of Magnetic resonance imaging signal.

Background technology

Along with the development of magnetic resonance digital quadrature demodulation method, the application of digital receiver in imaging system more and more widely.Fig. 7 is typical nuclear magnetic resonance digital receiver system synoptic diagram.By the NMR signal that collects of probe (1), after low-noise preamplifier (2) amplifies, enter frequency mixer (3) and carry out detection, carry out digitized sampling through entering analog to digital converter ADC (5) after intermediate-freuqncy signal conditioner (4) amplification and the filtering high fdrequency component.According to being Qwest's sampling thheorem, the sample frequency fs of ADC is usually than the resonant frequency f of magnetic resonance signal _FidAt least the above undistorted collection that could realize of high twice to magnetic resonance signal.If in order further to reduce the quantizing noise of ADC, will realize the over-sampling to NMR signal, the sample frequency of ADC is often high tens to tens times than intermediate frequency.For now, from the difficult real-time processing of the high-speed digital signal of ADC output, there is the people once to solve this problem with the little processing concurrent working of a plurality of high-performance with microprocessor realization digital signal.Obviously, this can increase the complexity and the cost of system to a great extent.More easy way is to adopt special IC (ASIC) to realize.As Fig. 7, ADC sampling gained data are given digital down converter (6), wherein (7A), (7B) be digital quadrature mixing device with 90 degree fixed skew, magnetic resonance signal is down-converted near the zero-frequency by intermediate frequency through it, enter cascade comb filter (CIC) then (8A), (8B) carry out the first order of signal is extracted and filtering, obtain the slower code stream of speed, finite impulse response decimation filter (DEC1 able to programme, DEC2) (9A), (9B) then magnetic resonance signal is carried out some grades of filtering and extraction again, (I Q) deposits main frame (10) in the complex signal that obtains the nuclear magnetic resonance base band at last.

Use the magnetic resonance spectrogram that digital receiver can obtain not having the zero-frequency peak, the mirror image peak disturbs, owing to used digital filter in digital receiver, linear phase shift and out-of-band noise that it also has in the extraordinary passband suppress ability in addition.As can be seen from Figure 7, the digital signal (bandwidth is generally tens megahertzes) of high-speed ADC sampling output, after handling by digital receiver, at a slow speed the narrow-band digital signal (bandwidth is generally tens KHz) often of output.Fig. 2 is the sequential chart of digital receiver output signal, and wherein, A is the nuclear magnetic resonance complex digital signal of digital receiver output.A ' is the digital receiver data frame signal, and each signal has been represented the data sampling point one time.B is the sampling enabling signal, and it is risen along beginning sampling thereon by host computer control.C is actual nuclear magnetic resonance simulating signal.D represents N-1 sampled point of digital receiver and the position on actual NMR signal thereof.D ' then represents N sampled point of digital receiver and the position on actual NMR signal thereof.It is E that sampling is engraved in position corresponding on the actual NMR signal when starting.As shown in Figure 2, E and digital received receiver output sampled point D has t _pTime error.And following relation is arranged:

0≤t _p＜t _s

t _s＝1/SW

Wherein, t _sIt is the time interval of each sampling; SW is a receiver sampling spectrum width.

This time error shows as the randomized jitter of NMR signal phase place on the unitary sampling of NMR signal, thereby has influence on adding up or offsetting of NMR signal.Then showing as the pseudo-shadow that produces on the phase-encoding direction on the Magnetic resonance imaging.

In order to reduce the randomized jitter of this phase place, a direct way is the bandwidth that increases receiver, makes SW increase, and reduces t _pTime error.But, can make main frame that the signal of receiver output is carried out filtering extraction once more to obtain the suitable visual field (FOV) like this, export the increase of spectrum width in addition along with receiver, need transmit data with computer data bus faster, with bigger memory headroom store data, this meeting is soft to system, and hardware proposes very high requirement.

Summary of the invention

The objective of the invention is provides a kind of method of reseptance of Magnetic resonance imaging signal according to above-mentioned the deficiencies in the prior art part, and this method is determined the deviate of phase place by setting up counter, signal is proofreaied and correct eliminated phase jitter again.

The object of the invention realizes being finished by following technical scheme:

A kind of method of reseptance of Magnetic resonance imaging signal, it is characterized in that the nuclear magnetic resonance digital receiver is provided with the counter that is used to measure each error time, in each sampling, this counter is measuring after the error time value sends to main frame, spectral line to NMR signal carries out the linear phase correction in the receiver spectrum width again, thereby eliminates the phase jitter of NMR signal.

Before carrying out the linear phase correction, need the NMR signal that collects is done fast Fourier transform (FFT), NMR signal is transformed from the time domain to frequency domain.

Spectral line is done linear phase in the described receiver spectrum width and proofread and correct, refer in frequency domain, be multiplied by phase correction factor α for the data point of each Frequency point correspondence of spectrum width scope.

Proofread and correct the nuclear magnetic resonance spectral line through linear phase and need carry out quick inverse-Fourier transform, with NMR signal from the frequency domain transform to the time domain, thereby obtain NMR signal through overcorrection.

Advantage of the present invention is, and compares by increasing the method that the receiver spectrum width reduces NMR signal phase jitter, fundamentally eliminated the NMR signal phase jitter that is caused by digital receiver; And under the prerequisite that minimal hardware is changed, eliminated the phase jitter of nuclear magnetic resonance, very succinct, the easy row of its implementation; Simultaneously, alleviated the computational burden of main frame greatly owing to used Fourier transform and inverse-Fourier transform fast to realize correction to NMR signal phase jitter.

Summary of drawings

Accompanying drawing 1 is digital receiver structured flowchart among the present invention;

Accompanying drawing 2 is NMR signal sampling synoptic diagram among the present invention;

Accompanying drawing 3 is NMR signal phase correction synoptic diagram among the present invention;

Accompanying drawing 4 is self-rotary echo-pulse series synoptic diagram among the present invention;

Accompanying drawing 5A is original nmr echo signal time domain synoptic diagram;

Accompanying drawing 5B is a nmr echo signal frequency domain synoptic diagram;

Accompanying drawing 5C is the echoed signal time domain synoptic diagram behind phase correction;

Accompanying drawing 6A is SE sequence (not adding a phase encoding) NMR signal raw data;

Accompanying drawing 6B is the data of the SE sequence (not adding phase encoding) behind phase correction;

Accompanying drawing 7 is a digital receiver structured flowchart in the prior art;

Concrete technical scheme

Feature of the present invention and other correlated characteristic are described in further detail by embodiment below in conjunction with accompanying drawing, so that technician's of the same trade understanding:

Shown in Fig. 1-7, label 1-11 represents respectively: probe (1), low-noise preamplifier (2), frequency mixer (3), intermediate-freuqncy signal conditioner (4), ADC (5), digital down converter (6), digital quadrature mixing device (7A) (7B), cascade comb filter (CIC) (8A) (8B), decimation filter (9A) (9B), main frame (10), counter (11), register (12).

On the nuclear magnetic resonance digital receiver, connect the counter (11) that is used to measure each error time, in each sampling, this counter (11) is measuring after the error time value sends to main frame (10), spectral line to NMR signal carries out the linear phase correction in the receiver spectrum width again, thereby eliminates the phase jitter of NMR signal.

Specific embodiments is general divided for three steps carried out:

The first step is done fast Fourier transform (FFT) to the NMR signal that collects, and NMR signal is transformed from the time domain to frequency domain.

Second step, in frequency domain, spectral line is done the one-level phase correction in the receiver spectrum width, promptly the data point for each Frequency point correspondence of spectrum width scope is multiplied by phase correction factor α.

The 3rd step, the nuclear magnetic resonance spectral line is carried out quick inverse-Fourier transform, obtain NMR signal through overcorrection.

From Fig. 2 as seen, between the data point D of the data point E of sampling startup moment correspondence and digital receiver output t is arranged _pTime error.If the frequency of NMR signal is f, then Shi Ji NMR signal can be expressed as:

A(t)＝A(0)e ^i2πf×t (1)

If digital receiver sampling spectrum width is SW, the NMR signal that then obtains at last is

$A^{\&prime;}\left(t\right)=A\left(t\right)e^{i2\mathrm{\&pi;f}\&times;t_p}(0<t_p<1/\mathrm{SW})---\left(2\right)$

Relatively 1 formula and 2 formulas can obtain, and for each signal sampling, the error of phase jitter is

0＜Δφ＜2πf/SW (3)

For each sampling, if error time t _pValue can measure, we just can do linear phase to NMR signal and correct by introducing the phase place modifying factor, the expression formula of phase place modifying factor α is as follows:

$\mathrm{\&alpha;}=e^{-\mathrm{<figure-callout\; id="2"\; label="i"\; filenames="C20041005315400091.png,C20041005315400101.png"\; state="\{\{state\}\}">i</figure-callout>}2\mathrm{\&pi;f}\&times;t_p---\left(4\right)}$

2 formulas be multiply by 4 formulas, can get

A′(t)×α＝A(t) (5)

This shows, on the basis of raw data, multiply by phase correction factor and just can eliminate the phase error that causes by digital receiver.

Fig. 3 is a NMR signal phase correction synoptic diagram among the present invention.The counting clock welding system clock (frequency is generally tens megahertzes) of counter (11) wherein.The sampling enabling signal is used the position of rest of data frame signal as counter as the count enable signal of counter.When sampling started, at the rising edge of sampling enabling signal, counter began counting, stops counting when running into frame signal for high level, and the value of counter is deposited in register (12).Among Fig. 3, starting to counter from counter, to stop the used time be t _c, the count frequency of establishing counter is f _Clk, then the count value N of counter and error time t _pFollowing relation is arranged:

t _s-N/f _clk＝t _p (6)

T wherein _sIn the time interval at expression digital receiver consecutive number strong point,, can measure t by reading the count value N of counter _pValue.

In order to eliminate error time t _pTo the influence that NMR signal causes, also can utilize phase correction factor to carry out that in time domain NMR signal is carried out convolution algorithm and eliminate phase jitter.

With spin echo (Spin Echo) sequence of Magnetic resonance imaging for for example shown in Figure 4, Gs wherein, Gp, Gr are gradient fields, their effect is that sample is carried out space encoding.90 ° and 180 ° is that radio-frequency pulse and sample magnetization vector are pulled the angle down, and its effect is the nuclear spin system of excited sample, produces NMR signal.After sampling started, digital receiver was gathered NMR signal.For the imaging sequence of Fig. 4, the frequency of its radio-frequency pulse is 10.7MHz, and the spectrum width SW of digital receiver is 20KHz, and the bandwidth of NMR signal is 2KHz, and sampling number TD is 256 points.Then according to 3 formulas, when receiving data, digital receiver produces the maximum phase error and is 36 ° of 2 π * 2KHz/20KHz=0.2 π.Among Fig. 4, be t=5ms the interval time of 90 ° and 180 ° radio-frequency pulses, and according to the nuclear magnetic resonance theory, the time t ' on the echo summit that its signal produces also should be 5ms.Fig. 5 A is the time domain parent magnetic resonance signal that digital receiver collects.Because the generation randomized jitter of NMR signal phase place, thus from the figure echo summit moment of 5ms not as can be seen.This shows that if NMR signal is not carried out phase correction, then can introduce serious phase place randomized jitter at the phase-encoding direction of nuclear magnetic resonance data and disturb, this must cause very serious image artifacts.For NMR signal is carried out phase correction, adopt following steps:

The first step is carried out obtaining behind the fast Fourier transform (FFT) nuclear magnetic resoance spectrum such as Fig. 5 B of frequency domain to the time domain NMR signal.

Second step, in frequency domain, nuclear magnetic resoance spectrum is carried out phase correction, its method is as follows:

Because the sampling bandwidth of receiver is 20KHz, sampling number is 256 points, and according to the character of Fourier transform, the frequency values f that obtains n data point on the spectrogram is

f _n＝n×78.125Hz(0≤n＜256) (7)

Then the phase correction factor of each data point can be expressed as in the spectrum width:

${\mathrm{\&alpha;}}_n=e^{i2\mathrm{\&pi;}\&times;n\&times;f_n\&times;\mathrm{tp}}---\left(8\right)$

For this embodiment, phase correction factor can be expressed as:

α _n＝e ^{i2π×n×78.125×tp} (9)

Wherein, t _pIt is the time error value of each sampling.For this embodiment, the spectrum width of establishing digital receiver is 20KHz, then t _pMaximal value be 50us.When the system counts clock was 50MHz, count value N was 50us*50MHz=2500 to the maximum.For this sampling, the count value of establishing counter is 1000, then according to 6 formulas, obtains time error value t _p=50us-20us=30us.Again according to 9 formulas, each data point in the original spectrogram that digital receiver is obtained multiply by phase correction factor α _n

${\mathrm{\&alpha;}}_n=e^{i2\mathrm{\&pi;}\&times;n\&times;78.125\&times;30\&times;{10}^{-6}}(0\&le;n<256)$

Calculate α _nAfter the factor, begin from 0Hz in the nmr spectrum of gained that the data of each Frequency point are multiplied by the respective phase correction factor, the nuclear magnetic resoance spectrum after so just obtaining proofreading and correct in the SW spectrum width scope.

The 3rd step, the nuclear magnetic resoance spectrum through overcorrect is done inverse-Fourier transform, obtain not have the time domain NMR signal of phase jitter, as Fig. 5 C, echo summit A has eliminated the phase jitter that each sampling causes through after the phase correction.

Fig. 6 A is a parent magnetic resonance K spatial data, each line data in the K space is corresponding to the sampling of a NMR signal, its signal amplitude is represented with gray scale, the high expression of brightness signal amplitude is big, the low expression of brightness signal amplitude is little, can see clearly because the each sampling of digital receiver causes the phase jitter of NMR signal from the K space.Fig. 6 B is the NMR signal K spatial data that obtains after this method for correcting phase of process of present embodiment is handled, can it is evident that,, eliminated to a large extent after the NMR signal correction by the method because the signal phase shake of each sampling that digital receiver causes.

Claims (1)

1, a kind of method of reseptance of Magnetic resonance imaging signal is characterized in that the nuclear magnetic resonance digital receiver is provided with timer conter, in each sampling, and the each sampling time error value of this counter measures; By the sampling time error value that usage counter obtains, in the receiver spectrum width, the nuclear magnetic resonance spectral line is carried out linear phase and proofread and correct; The time error value of utilizing counter to obtain calculates phase correction factor α; Before carrying out the linear phase correction, need the NMR signal that collects is done fast fourier transform, NMR signal is transformed from the time domain to frequency domain; In the receiver spectrum width, spectral line is done linear phase and proofread and correct, promptly in frequency domain, be multiplied by phase correction factor α for the data point of each Frequency point correspondence of spectrum width scope; The nuclear magnetic resonance spectral line of proofreading and correct through linear phase need carry out quick inverse-Fourier transform, with NMR signal from the frequency domain transform to the time domain.

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