CN1006578B - Method of reducing artefacts in images formed by means of fourier zeugmatography - Google Patents
Method of reducing artefacts in images formed by means of fourier zeugmatographyInfo
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- CN1006578B CN1006578B CN 85101765 CN85101765A CN1006578B CN 1006578 B CN1006578 B CN 1006578B CN 85101765 CN85101765 CN 85101765 CN 85101765 A CN85101765 A CN 85101765A CN 1006578 B CN1006578 B CN 1006578B
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Abstract
The present invention relates to a nuclear magnetic resonance image generated by a Fourier yoke type graphic method, which comprises an agitation error which is mainly caused an inherent interference signal (For example, a reverse rotation pulse of 180 DEG is not ideal because of a constant value error.) According to the present invention, alternate exciting pulses with phase difference of 90 DEG are applied to contiguous measuring circulation. During a measuring period, a signal sampling value is obtained, and is stored in contiguous lines in an image frequency matrix. Sampling values of every other row in the matrix reverses a phase for one time, and consequently, an error component caused by the internal interference signal alters a symbol for one time every other row along the direction of lines. When Fourier conversion is carried out to lines, an error caused by the interference signal is moved to the edge of an image.
Description
The present invention relates to a kind of method, distribute in order to the nuclear magnetic resonance of determining a bodies position, this object be in magnetic direction be of first direction stable, uniformly among the magnetic field,
This method comprises the following steps:
A) produce a high-frequency electromagnetic pulse, its magnetic direction with stable, magnetic direction is vertical uniformly, so that the magnetization of the nucleon in object generation produces resonance signal like this with respect to the precession of first magnetic direction.
B) then, in the preparatory period, apply a gradient magnetic at least, its magnetic direction is consistent with first magnetic direction.
C) then, in measuring period, get one group of (n) signal sampling value.
D) then, through a latent period, repeat to comprise a), b) and measurement circulation c), to obtain many groups of (n ') data, at least to the intensity of a gradient magnetic in the preparatory period integrates, in different repetition time, different values is arranged, this is in order to obtain the n ' group sampled value of n signal, the form of numerical value with matrix stored away,, just can determine that the magnetized distribution of nucleon resembles through Fourier Tranform.
The invention still further relates to a kind of device, be used for determining that the nuclear magnetic resonance at a bodies position distributes, device comprises:
A) produce equipment stable, uniform magnetic field.
B) equipment of generation high frequency electromagnetic radiation.
C) equipment of at least two (first and second) gradient magnetics of generation.Its gradient direction mutually orthogonal.
D) sample devices.In measuring period, to a), b) resonance signal that produces of section institute define equipment samples.In preparatory period before sampling, resonance signal is by c) at least one gradient magnetic of producing of section define equipment regulates.
E) treatment facility.Be used for handling the obtained signal of sample devices.
F) opertaing device.Be used for controlling b at least) to e) the defined equipment of section, to produce, to regulate sampling and to handle a plurality of resonance signals, in the preparatory period, each resonance signal is regulated respectively, opertaing device is given c) equipment that defined of section provides control signal, with the intensity of adjusting at least one gradient magnetic and (or) duration, concerning the integration of intensity in the duration of at least one gradient magnetic, through being different behind each latent period.
Nuclear magnetic resonance distributes and can be regarded as the distribution of nuclear-magnetism field strength in this article.Velocity flow profile, relaxation time T
1, T
2Distribute or MR spectroscopy distribution (relation of MR spectroscopy and position), or the like.
Also have, in this article, resonance signal can be regarded as, with resonance excitation execute certainly the nucleon sum and thereby free induction decay (FID) signal that produces the two is relevant.
A kind of like this method (being also referred to as the fourier type graphical method) and equipment see German patent application DE-OS26.11.497.According to such method, checked object will bear stable, a uniform high-intensity magnetic field B
o, its magnetic direction, such as, consistent with the Z axle of rectangular coordinate system (X, Y, Z).This stabilizing magnetic field B
oCause the slight polarization of nuclear spin in the object, and nuclear spin can be produced magnetic field B
oThe precession of direction.Applying B
oAfter the magnetic field, preferably produce 90 ° of high frequency electromagnetic radiation pulses (angular frequency w=γ .B
o, γ is a gyromagnetic ratio here, B
oBe magnetic flux density), with the self-rotating nuclear direction of magnetization half-twist angle in the object.After 90 ° of end-of-pulsings, nuclear spin will begin relative magnetic direction B
oPrecession has so just encouraged resonance signal (free induction decay signal).Utilize and magnetic field B
oThe on all four gradient magnetic GX of direction, GY, GZ can produce a magnetic field B=B
o+ GX.X+GY.Y+GZ.Z, its magnetic flux density is relevant with the position, because each gradient magnetic GX, the intensity of GY and GZ is at X, Y and Z direction have gradient separately.
After 90 ° of pulses, at t
xAdd magnetic field G during this time
x, then, at t
yAdd magnetic field GY during this time, thereby the relevant mode of the precession opsition dependent of the nuclear spin that is energized changes.This preparatory period of process (is promptly passed through t
x+ t
y) afterwards, add magnetic field G
Z, at t
ZInterior during this time N
ZThe individual measurement free induction decay signal (being actually whole magnetization sums of nucleon) of constantly sampling.Above-mentioned measuring process then repeats 1 * m time, uses different t in each case
x(or) t
yValue.Like this, obtained (N
Z* m * 1) individual signal sampling value is wherein comprising X, Y, the magnetization distribution information at certain position of an object in the Z space.N the signal sampling value of tested group of this 1 * m deposited in the memory (at N
ZIn the memory cell of * m * 1), after, free induction decay (FID) signals sampling value is carried out three-dimensional Fourier Tranform, the distribution that just can obtain nuclear magnetic resonance resembles.Clearly, also can use the excitation of selecting, produce free induction decay (FID) signal by the nuclear spin (its initial point can be selected arbitrarily) in two-dimensional space only, like this, (for example), the FID signal only need produce m time, just can obtain m * N in selected two-dimensional space by two-dimentional Fourier Tranform
ZThe magnetization distribution of individual point resembles.
With institute's describing method can be in two dimension or three dimensions definite kernel spin magnetic flux distribution.With similar way,, be possible (this frequency spectrum is represented the chemical state of material in the corresponding picture dot zone) to the plain definite frequency spectrum of each picture dot in two dimension or the three dimensions.Accomplish this point, such as, in measuring period, do not apply gradient magnetic, but in the preparatory period, apply 1,2 or 3 gradient magnetic.With such method, signal sampling value can be formed 2 dimensions, 3 dimensions or 4 dimension matrixes, through 2 dimensions, 3 dimensions or 4 dimension Fourier Tranforms can be exchanged into the relevant frequency spectrum in position, its position correlation be 1,2 or 3 dimensions (X for example, perhaps X, Y, perhaps X, Y, Z).
Yet the nuclear magnetic resonance distribution plan with above-mentioned two dimension or three-dimensional NMR fourier type graphical method obtain comprises error component, and it influences the information of nuclear magnetic resonance (NMR) in resembling, even (partly) offsets this information.To what record, carry out 2 dimensions or 3 dimension Fourier Tranforms through the resonance signal of demodulation, can obtain nuclear magnetic resonance and resemble, be that the scope of f is 0≤f≤f and comprise frequency in the resonance signal of demodulation
MaxLow frequency signal.Resemble error and be exactly usually thus that undesirable signal causes in the frequency range.Error of first kind is clearly when picture dot is positioned at when resembling the center, and it has different intensity, is caused by factors such as bias voltages.Another kind of error resembles by second and is superimposed upon first and resembles and cause, and when adopting so-called spin echo technique, 180 ° of pulses of non-ideal use just produce this class error.This class error is considered to be caused by the relevant interference signal.Because these relevant interference signals, in fact two category informations have mixed, and it is very annoying that yes, also is that we are undesirable.An approach cancelling these errors is, each signal sampling value in the matrix is carried out twice measurement, measure for the first time with measurement for the second time during the resonance signal excitation phase opposite, these two signal sampling additions with compensating error, have promptly reduced error, yet, in this method, a measurement will take the time of two double-lengths, thereby is inconvenient.
The objective of the invention is for a kind of method and device are provided, be used for forming the nuclear magnetic resonance that not disturbed by error resemble (even, under the situation that adopts the spin echo nuclear magnetic resonance technique), its needed signal sampling time is long unlike the needed time of known method and apparatus, and to the latter, the appearance of error is inevitable.
In order to reach this purpose, the characteristics of the inventive method are, in one then the measurement according to gradient magnetic intensity integrated value order in the preparatory period circulates, encourage resonance signal like this, so that and introduce an additional phase angle △ φ between the resonance signal of the line correlation that continues connection, because each row in the matrix are carried out Fourier Tranform, and this additional phase angle has been cancelled.
The characteristics of first kind of scheme of the inventive method are: phase differential △ φ equals π radian per second or (n '-1). π/n ' radian per second (n ' be capable number).
The characteristics of the another kind of scheme of the inventive method are, when row n ' is even number, additional phase error equals the π radian per second, this be because, in measuring period, the row that continues is adopted alternative inversion excitation resonance signal, each row or even number line or odd-numbered line in the matrix are carried out Fourier Tranform by anti-phase present value.
Use the result advised to reach: the signal sampling value that obtains in measuring period repeatedly is stored in the memory that constitutes with matrix form, in the measuring period of respectively continuing, to gradient magnetic intensity integration in the preparatory period, the sequence of this integrated value has determined will be with the row position (line label) of the corresponding signal sampling (n) of behavior unit's storage.A relevant interference signal, for example, the deviation signal that detection and the desired electronic circuit of sampling resonance signal are produced occurs with same ability to see things in their true light each signal sampling.Yet the set of the resonance signal sampled value of alternative inversion excitation exists in the matrix reservoir, when sample of signal is when from then on extracting in every line in the matrix, after interlacing is anti-phase, to the influence of relevant interference signal from delegation to another capable opposite sign.Thereby process is to after each row carries out Fourier Tranform in the matrix, and this influence will only occur in the edge picture dot that resembles each row of matrix, because one change of the symbol delegation of undesired signal, so will include only the highest frequency that appears in the matrix.As the result who uses alternative excitation phase method, the influence of deviation voltage centering imago primitive element (0 frequently) will move to the edge of elephant.Promptly in fact reduced disturbing effect.Be noted that alternately the relevant interference signal is converted to known point spread function by Fourier Tranform, the maximum position of this function is determined by the number of numerical value in each row.When this number is an even number, the maximum value of point spread function will appear at a side of high column element.In the every other column element, the value of point spread function (P.S.f.) will be zero.When one row comprise the odd number value, the outermost element that is in these row both sides will be obtained very big share from P.S.f..It is far away apart from greatest member that adjacent element is obtained more and more littler share, and share is littler.(central element of row is zero).
When using the spin echo nuclear magnetic resonance technique,, apply 180 ° of driving pulses in order to produce spin echo signal.These 180 ° of pulses make 180 ° of spin phase shifts.Yet if it can not be desirable having used unfavorable 180 ° of pulses, spin flip conversion yet, some spin will be got and be given 180 ° of fixed phase places that rotatable phase is different.In addition, undesirable spin also will be added into 180 ° and give fixed rotatable phase, because 180 ° of driving pulses are relevant with 90 ° of driving pulses.Do not wish that the signal component that nuclear spin produced that encourages is superimposed on the desirable resonance signal,, cause in such resembling of producing undesirable information and desired information weave in through Fourier Tranform.
According to the present invention, carry out 90 ° of alternative inversion excitation resonance signals, sample of signal in matrix anti-phase in every line after, by the relevant interference signal that encourages nuclear spin to produce improperly, moved past multiple row by Fourier Tranform, move to the edge of the elephant of such calculating.Be noted that in order to reduce most effectively to resemble error that it is preferable adding ready gradient magnetic in the cycle between 90 ° of driving pulses and 180 ° of pulses by what unfavorable 180 ° of pulses caused.
A preferred plan according to the inventive method, to each measurement that continues circulation, the integrated value equivalent of gradient magnetic in the whole preparatory period increases, it is characterized in that: in the measuring period of continuing, resonance signal is encouraged by alternative inversion, the signal sampling value that so records is anti-phase every a value, so that the value in each row is carried out Fourier Tranform.
The present invention also can be used for carrying out three-dimensional fourier type diagram.As a kind of scheme of determining the nuclear magnetic resonance location mode of an object in 3D region, this object be in one that be energized, stable, uniformly among the magnetic field.This scheme comprises the following steps:
A) produce a high-frequency electromagnetic pulse,, produce resonance signal thus so that cause the magnetization precession of nucleon in the object.
B) then, in the preparatory period, apply first and second gradient magnetic at least, their gradient direction is mutually orthogonal.
C) then, in measuring period, resonance signal is got one group of (n) sample signal.
D) then, every through a latent period, repeat to comprise a), b) and c) measurement circulation, through repeatedly (m * n ' inferior) repetition, first integration comprises the integration of the first gradient magnetic intensity in the preparatory period, has the first variate number (m), and second integral comprises the integration of the second gradient magnetic intensity in the preparatory period, variate number (n ') is arranged, the plane is provided first number (m), in a three-dimensional matrice, there is line number second number (n ') of (n) individual sampled signal on each plane, after three-dimensional matrice is Fourier Tranform, can determine the nuclear magnetisation distribution plan of in the 3D region of an object, inducting.
It is characterized in that, in one then one measurement circulates, to the first gradient magnetic intensity in the preparatory period integrates, successive integration value according to correspondence, so encourage resonance signal: between said per twice continuous coverage round-robin resonance signal in the above, introduce an additional phase differential △ φ, (it equals the π radian per second or equals (m-1). π/m radian per second, wherein m is a number of planes), because the value on m plane is carried out Fourier Tranform, this additional phase differential has been cancelled.
For nuclear magnetic resonance in the 3D region of determining an object distributes, characteristics according to another scheme of the inventive method are, wherein number of planes m is an even number, additional phase error is that the π radian per second-the anti-phase resonance signal of alternative excitation obtains in the circulation by measuring one by one, numerical value every a plane will be by anti-phase, so that the signal sampling value in first number (m) carries out Fourier Tranform in the matrix plane to being illustrated in.Like this, the two-dimensional matrix group of stacker is full of signal sampling value.Every provisional capital of a two-dimensional matrix has been full of the signal value of adopting sample, and its each signal sampling value all is to obtain under identical excitation phase.Matrix all is full of by the signal sampling value of obtaining under the resonance signal excitation of interval phase place one by one.Through after the three-dimensional Fourier Tranform, error or be moved to: with the corresponding image plane of highest frequency (under the situation on even number plane) appears in the picture frequency matrix, perhaps be dispersed on the plane of two outermosts and between the plane of directly adjoining (under the situation on odd number plane) corresponding to known point spread function.
Filling with signal sampling value in the process of a matrix, when this signal sampling value is when taking from the resonance signal of anti-phase alternative excitation line by line, then pass through after the three-dimensional Fourier Tranform, error will be moved to the edge (all is under the situation of even number at number of planes and line number) on outermost plane, perhaps spread to the outermost row (at number of planes is even number, and line number is under the situation of odd number) of both sides, outermost plane.
Furthermore, also can only reduce to resemble (because for example the puppet of generation such as deviation voltage resembles and must avoid respectively or eliminate) by the puppet that unfavorable 180 ° of pulses produce.
According to a kind of scheme of the inventive method, this method be used for determining the nuclear magnetisation in the object area distribute (this object be in one stable, uniformly among the magnetic field, magnetic direction is a first direction).Comprise the following steps:
A) produce a high-frequency electromagnetic pulse,, thereby produce the excitation resonance signal so that the nuclear magnetisation in the object produces the precession with respect to first direction.
B) then, in the preparatory period, add a gradient magnetic at least.
C) then, produce 180 ° of high-frequency electromagnetic pulses, at least once, so that the counter-rotating of nuclear magnetisation direction has so just produced the nuclear spin echo signal, signal is sampled in the measuring period of and then 180 ° of pulses, obtains one group of (n) signal sampling value.For periodically adopt one group of the nuclear spin echo signal (n) sampled value, be split into sampling interval measuring period.
D) whenever just repeat step a) through behind the latent period, b) and c), obtain (n ') secondary data, to each repetition, the intensity integration of at least one gradient magnetic has different values.So that obtain gang's signal sampling value, it is carried out Fourier Tranform after, can determine that the nuclear magnetisation that produces distributes.
Characteristics are: in the cycle, 180 ° of electromagnetic pulses are pressed the corresponding order of gradient magnetic intensity at preparatory period inner product score value at a different measuring of following one, and alternative inversion ground produces one by one.
According to a kind of apparatus features of the present invention be, comprise programmable computer in the opertaing device, be used for providing control signal to the equipment that produces high frequency electromagnetic radiation, described control signal is suitable for giving according to one decides program and produces, measure in the circulation at one, 90 ° of driving pulses or first phase place is arranged perhaps have second phase place, relative first phase shift of second phase place 180 °.With such device, described method can realize under the situation that does not increase described error, and is such just as already described.
Hereinafter the object lesson in is with reference to the accompanying drawings described the present invention in detail.In the accompanying drawing:
Fig. 1 is the coil system configuration diagram of a kind of device of realizing the inventive method.
Fig. 2 is for realizing the calcspar of a kind of device of method among the present invention.
Fig. 3 shows a kind of simple form according to the prior art horizontal process.
Fig. 4 and Fig. 5 represent the corresponding optimum form according to the inventive method respectively.
Fig. 6 shows in order to realize the part according to a kind of device of the inventive method.
Fig. 1 has shown a coil system 10, it is device 15(Fig. 2) a part, be used for determining that the nuclear magnetic resonance of the spinning nucleon in the position of object 20 distributes, this position have thickness (such as), △ Z is positioned at the X-Y plane of the X-Y-Z coordinate system shown in the figure.Its Y-axis and drawing planar quadrature.Coil system 10 produces uniform steady magnetic field B
o, its magnetic direction is parallel with the Z axle, three gradient magnetic G
x, G
yAnd G
zMagnetic direction be parallel to the Z axle, gradient direction respectively with X, Y is parallel with the Z axle, coil system also produces a high frequency magnetic field.For this reason, coil system 10 comprises one group of main coil 1, is used for producing stabilizing magnetic field B
o, its density from 0.3 to 1.5 tesla.Main coil 1 can be placed on the surface of (for example) ball 2, ball 2 be centered close to rectangular coordinate system X, Y, the initial point of Z, the axis of main coil 1 is consistent with the Z axle.
Coil system 10 also comprise (such as), four coil 3a, 3b, they are placed in same sphere and produce gradient magnetic G
ZFor reaching this purpose, the exciting current of first group of 3a is opposite with the exciting current direction of second group of 3b, among the figure with symbol
Represent that with ⊙ ⊙ represents certain part that electric current enters coil 3 among the figure,
The expression electric current leaves certain part of coil.
Coil system 10 comprises, such as four square coil 5(only illustrate wherein two) or four other coils, for example " the sharp coil of dagger-axe " is used for producing gradient magnetic G
y, for producing gradient magnetic G
x, used four coils 7, they are identical with coil 5 shapes and rotated 90 ° of angles with respect to coil 5 around the Z axle.Fig. 1 also expresses coil 11, is used for producing and the detection electromagnetic field of high frequency.
Fig. 2 has shown a kind of device device 15, is used for realizing according to a kind of method of the present invention.Device 15 comprises coil 1,3,5,7,11, and they have contrasted Fig. 1 and have been illustrated; Current feedback circuit 17,19,21,23 are respectively applied for drive coil 1,3, and 5 and 7; With a high frequency signal generator 25 that is used for drive coil 11.This device 15 also comprises a high-frequency signal detecting device 27, a detuner 28, a sampling circuit 29, signal handling equipment, for example, an analog digital converter 31, a memory 33 and a computing circuit 35 that is used for carrying out Fourier Tranform, a control module 37 that is used for controlling sampling instant also has a display device 43 and central control equipment 45, and its effect and connection relation will describe in detail afterwards.
The method that the nuclear magnetic resonance of spinning nucleon distributes in the device 15 realization mensuration objects 20 is as mentioned below.The method comprises quick duplicate measurements circulation, is divided into several steps and measure circulation itself.Measure in the circulation at one, some spinning nucleons in the object are by resonance excitation, resonance excitation for spinning nucleon, central control equipment 45 is turn-on current generator 17 at first, thereby coil is energized and keep being energized state in the measurement that continues circulation, has so just produced a stable and magnetic field B uniformly
oThen, radio-frequency generator 25 is switched on an of short duration time, so that coil 11 produces an electromagnetic field of high frequency (with reference to magnetic field).Encourage in the magnetic field that certain spinning nucleon in the object 20 can be applied in, and the nuclear magnetisation of synthetic precession will be with respect to uniform magnetic field B
oGet certain angle, such as 90 ° (90 ° of reference pulses).The location at object position and spinning nucleon thereof, will be depended on: magnetic field B except other factors by the element that resonates and encouraged like this
oDensity, added any gradient magnetic, the angular frequency of electromagnetic field of high frequency
o, because must satisfy equation in order to resonate
o=γ .B
o(1), here γ be gyromagnetic ratio (to free proton, H for example
2The O proton, the π=42.576MH of γ/2.
Z/ T).Through an Energizing cycle, high frequency signal generator 25 is turn-offed by central control equipment 45.Resonance excitation is always measured at each and is carried out when round-robin begins.To some method of operating, in measuring circulation, reference pulse is also sensed goes into object.These reference pulses may be, for example 180 ° of reference pulses or the sequence formed by 180 ° of reference pulses, and these pulses are gone into object by induction periodically.The latter produces one and is called the response of " many spin echoes ".Spin echo technique is described in the article of I.L.Pykett to some extent, and this article is entitled as " application of nuclear magnetic resonance in medical treatment ", is published on " American of science " in May, 1985.
Next step is gathered useful signal sampling value.Used gradient magnetic for this purpose, each gradient magnetic is produced under the control of central control equipment 45 respectively by generator 19,21 and 23.Detecting resonance signal (being called free induction decay (or FID) signal) realizes by connecting high-frequency detector 27, detuner 28, sampling circuit 29, analog digital converter 31 and control module 37.The appearance of this FID signal is that nuclear magnetisation is with respect to magnetic direction B
oThe result of precession, precession is to be caused by the excitation that the parametric excitation pulse is produced.This nuclear magnetisation precession effect induces an induced voltage in magnetic test coil, its amplitude is to weigh the yardstick of nuclear magnetisation amount.
FID signal simulation, sampling that derives from sampling circuit 29 is transformed device 31 digitizings and exists in the memory 33.In the end sampled signal moment te adopted after, central control equipment 45 disconnects generators 19,21 and 23, sampling circuit 29, control module 37 and mould one number converter 31.
The free induction decay that has been sampled (FID) signal deposits in and remains in the memory 33.Then, carry out the next one and measure circulation, the FID signal is energized, is sampled and deposited in memory 33 again.(the FID signal number that will survey depends on when the FID of enough numbers signal is recorded, for example, desirable resolution), through 2 dimensions or 3 dimension Fourier Tranform imaging (this depends on the application of gradient magnetic, and the FID letter is produced and is sampled under its effect).
Fig. 3 has provided a measurement circulation example that is undertaken by state-of-the art, and its explanation can be with reference to device shown in Figure 2 15.Utilize radio-frequency coil 11, after connecting main coil 1, produce the pulse P of 90 ° of phase places
1, this pulse has produced the stable and uniform magnetic field B
oWhen utilizing spin echo technique, can make synthetic resonance signal (FID signal) F
1Decay.In the time interval
After, radio-frequency coil 11 produces the pulse P of 180 ° of phase places
2At t
V1Produce gradient fields G in the part-time at interval
x(use curve G
1Expression), its reason will be described hereinafter.Through with
Time equates
After time, by 180 ° of pulse P
2Echo resonance signal (the spin echo FID signal) F that produces
2To reach peak value.So-called spin echo technique (180 ° of pulse P
2) employing can prevent that phase angle error from appearring in the resonance signal that nuclear spin causes; This error is because stabilizing magnetic field B
oHeterogeneity cause.At curve G
2In the time that represented gradient magnetic Gx exists, press the sampling interval of tm the echo resonance signal is sampled.
As everyone knows, in gradient magnetic Gx, the phase angle of the magnetization at some x place is determined by following formula:
∫
tγ·G
x（τ）x·dτ
Like this, picture frequency rate K
xBe defined as:
k
x＝γ∫
tG
x（τ）·dτ
Therefore, each sampling period tm will obtain a corresponding signal sampling value later, thereby produce a different picture frequency rate K accordingly
xWill there be the picture frequency rate variance in adjacent picture frequency rate:
△Kx＝γ∫
tmG
x·（τ）·dτ
Obviously, when repeating above-mentioned measurement circulation time,, then obtain the signals sampling value before sampling if be to apply another gradient fields Gy, be accompanied by produce the picture frequency rate to (Kx, Ky).When no gradient fields Gy, resulting signal sampling value supervene the picture frequency rate (Kx, O).Can prove, supervene the picture frequency rate to (Kx when collecting one group of signal sampling value, Ky) matrix, promptly in this matrix the picture frequency rate be a Kx to+Kx and-Ky is in the scope of+Ky the time, then utilizes 2 dimension Fourier transforms to obtain the distribution of the magnetization on the XY plane from this group signal sampling value.Use pulse P when comprising
1After period of time T in the measurement of beginning circulates in finishes, has similar measurement pulse P
1The next one measure the circulation beginning, its objective is that obtaining row is accompanied by and produces the picture frequency rate to (Kx, new signal sampling value Ky), Ky are constants and are to predesignate, at pulse P
1' and P
2' between
Remove gradient fields G in time
1Do not draw among the gradient fields Gy(figure in addition) outward.According to the method that prior art adopted, the time T that adjacent secondary is measured between the round-robin starting point is 0.5 second to 1 second.Further this time interval of minimizing will make in next FID signal amplitude of measuring generation in the circulation and reduce, because the spinning nucleon that major part is energized, for so short time interval, a long relaxation time is arranged, and only be that those spin axiss have returned to main field B
oThe spinning nucleon of direction is just influential to next spin echo signal.
Fig. 4 has introduced a measurement circulation according to the inventive method.This measurement circulation has much in common with measurement circulation shown in Figure 3.For the sake of clarity, Fig. 4 does not draw gradient fields and only draws some relevant signals like this: driving pulse P
1, P
1', 180 ° of pulse P
2P
2' and resonance signal F
1F
1' F
2F
2'.In addition, in the measurement circulation that continues (cycle of each is T), different (predetermined) gradient magnetic G
1G
1' G
1" ... also be illustrated.There is following difference each other in the measurement circulation that continues: 90 ° of driving pulse P that continue
1P
1' P
1" ... waiting its phase place is that (this just means high-frequency impulse P to alternative inversion
1' with respect to high-frequency impulse P
1And P
1" 180 ° phase shift is arranged.And the gradient fields G that continues
1, G
1' G
1" ... intensity increase by step-by-step system.Because its gradient fields G in each measurement period T
1Be that stepping increases, therefore, from the spin echo signal F that continues and produce
2, F
2', F
2" in can provide (Kx, Ky) signal sampling values of adjacent two row in the picture frequency rate matrix.
Because the phase place of driving pulse is an alternative inversion, so initial FID signal F
1, F
1', F
1" ... phase place and the spin echo signal F of back
2, F
2', F
2" ... phase place also be alternative inversion.The alternative excitation pulse to the influence of spin echo signal owing to the anti-phase of sampled value in every line is compensated.Because drive characteristic does not influence the relevant interference signal, so because the anti-phase of sampled value in every line makes that the influence of these undesired signals also is that interlacing is anti-phase.With the misalignment signal is example, and its influence is actually in row from 0 frequency to the sampling maximum tolerance frequency that principle determined and increases.Therefore, according to Fourier transform, according to the point spread function of having introduced, it will be moved to an edge (or two edges) of conversion row.
Fig. 5 has provided an example according to the inventive method; The present invention is used in so-called multiple echo (multiple-echo) technology now.At 90 ° of driving pulse P
1After, by means of 180 ° of rp pulse P
21, P
22, P
23The compartment of terrain produces many NMR signal F
21, F
22, F
23In fact, high-frequency electromagnetic rp pulse P
21, P
22... be not desirable, therefore, resonance signal F
21, F
22The influence of nuclear spin will be subjected to.If 180 ° of pulses are desirable, then this influence does not just exist yet.To the resonance signal sampling, the 1st resonance signal F
21Sampled value, second resonance signal F
22Sampled value ... be stored in the 1st picture frequency rate matrix respectively, the 2nd picture frequency rate matrix, the 3rd picture frequency rate matrix ... delegation.With t
NAfter the measurement loop ends that constantly begins, the next measurement circulates in t
N+1Constantly begin, remove preparation gradient fields (for clarity sake being omitted) and 180 ° of high frequency inverse pulse P
21', P
22' ... in addition, this measurement circulation is identical with last measurement circulation.Its rp pulse P
21', P
22' ... with 180 ° of high frequency inverse pulses of last measurement round-robin P
21, P
22180 ° phase differential is arranged.The resonance signal F that is produced
21', F
22', F
23' ... also measure the round-robin method by last one and be sampled, sampled value also is stored in the matrix in same mode.By incremental step of each measurement ringing that continues, fill each row of each picture frequency rate matrix continuously to the preparation gradient fields.Because 180 ° of rp pulse P
21, P
22, P
21', P
22' ... phase place enter another and alternative inversion along with measuring circulation from one, the relevant interference signal that is caused by imperfect 180 ° of rp pulses is stored in the matrix, its symbol is the alternate along with the variation of row also.Therefore the picture frequency rate matrix through Fu Likuang be for conversion into resemble matrix after, the puppet that undesired signal causes resembles only at an edge (or as previously mentioned at two edges).
So far, in these methods that we narrated, the influence of undesirable undesired signal, in the picture frequency rate matrix of measuring, alternation consequently makes the puppet that resembles in the matrix resemble and is moved to the edge its symbol by row.But be not the excitation that all needs alternative inversion in all cases.In the picture frequency rate matrix, the additional phase error between the resonance signal relevant with continuous row (or plane) is:
△ψ＝（n′-1）· (π)/(n)
(under the best circumstances), wherein n ' is line number (or number of planes).For a correct understanding is arranged, existing situation with one dimension is done an explanation, and its signal is by the equidistant sampling in time of sampling interval tj.After Fourier transform, the bandwidth of sampled signal is
Fw＝ 1/(tj) ，
Resolution (frequency interval) is △ f=1/(NS * tj).Wherein NS is the number of sampling.Highest frequency in frequency spectrum is F
Max
When hits is even number: F
Max=NS (△ f)/2
When hits is odd number: F
Max=(NS-1) (△ f)/2
Differ (when this highest frequency) between two sampled values that continue is
△ψ＝2π·F
max/tj。
With 1/ (tj)=△ fNS and F
Max
Behind the formula substitution following formula, then differ into:
When NS is even number: △ φ=π
When NS is odd number: △ φ=(NS-1). π/NS.
In order to select and/or to adjust given pulse order and measuring the round-robin time interval, preferably use programmable computer.At device 15(Fig. 2) an embodiment in, central control unit 45 comprises that a programmable calculator that is used for Data Control 51 and a PPG programme pulse generator 53(who has input and output worktable 52 sees Fig. 6).The output 55 of PPG programme pulse generator 53 is seen Fig. 2 by bus 50() be connected to current feedback circuit 17,19,21,23 and 25, realize 53 couples of coil 3a of generator, 3b, 5,7 and 11 control.Obviously, output 55 also can directly be linked top said generator.Computing machine (Philips model P857) can be by the given programming of back appendix.According to program and control data by worktable 52 inputs, computing machine just can rely on this programmed control PPG programme pulse generator 53(Nicolet model 293 B).The order set that adopts in program (row of the 3rd in the program) is the instruction (instruction JSA exception, it makes program jump to start address) of PPG programme pulse generator 53.Each has all defined a time cycle at the 4th row, and generator 53 has signal output in this one-period.The 4th row of program are the states (fixed female S makes an exception) of the output 55 of the generator 53 represented with 16 ary codes.Address or memory address are shown in the 5th tabulation.The symbol I of the 6th row represents occur to interrupt, and the output code (part) in the output 55 of it and generator 53 is finished some additional functions together, as:
A replaces 90 ° of pulses with the new waveform generator 25(that " packs into " with 180 ° of pulses).The b driving pulse is anti-phase.Or c shows the beginning of a new pulse train.In appendix given program to 90 ° of driving pulses only use+or-the y pulse, 180 ° of pulses are only used+the X pulse.
Claims (11)
1, be used for measuring the method that the nuclear magnetic resonance at a certain position of object distributes, this object places among the stable uniform magnetic field that is produced, and this method comprises the following steps:
A) produce a high-frequency electromagnetic pulse, make of the direction generation motion of the nuclear magnetisation intensity of testee, thereby produce resonance signal around stabilizing magnetic field,
B) then, in the preparatory period, apply at least with a gradient magnetic,
C) then, in measuring period, get the sampled value of one group of n resonance signal,
D) then, repeat by step a) after through a latent period every, b) and c) the measurement circulation formed, it is inferior to repeat n ', the each repetition, the intensity at least one gradient magnetic in latent period is carried out integration, each gained result is different.Purpose is will obtain n ' row n signal sampling value arranged, and these numerical value are stored with the matrix structure form.Just can determine that the distribution of responding to nuclear magnetisation resembles;
The characteristics of this method are, in the circulation of one by one different measuring corresponding in the preparatory period, the gradient field intensity being carried out successive integration, integrated value sequentially encourages resonance signal like this, make it between adjacent row and row, introduce an additional phase differential △ φ, and, and this additional phase error is balanced out by sampled value of each row of matrix is carried out Fourier Tranform.
2, method according to claim 1 is characterized in that, phase differential △ φ equal the π radian per second or etc. give (n '-1).π/n ' radian per second, n ' is a line number here.
3, method according to claim 2, it is characterized in that, when if line number n ' is even number, additional phase error △ φ equals the π radian per second, this is by in measuring circulation, to the alternately anti-phase excitation resonance signal of adjacent row, then or with even number line or the signal value of odd-numbered line is anti-phase, again value of each row of matrix is carried out that Fourier Tranform realizes.
4, method according to claim 3, the integrated value equivalent increase of each consecutive measurement circulation relatively in preparatory period inside gradient magnetic field intensity, the characteristics of this method are in the measurement circulation that continues, resonance signal is encouraged by alternative inversion, the signal sampling value that records like this, have anti-time phase every one group, again the value in the rectangular array is carried out Fourier Tranform.
According to claim 3 or 4 described methods, it is characterized in that when carrying out Fourier Tranform that 5, the row to matrix carries out the one dimension Fourier Tranform earlier, and is after this anti-phase to transformation results in every line, then each row carried out Fourier Tranform.
6, be used for determining the method that the nuclear magnetic resonance of object in 3D region distributes that this object places in the stable and uniform magnetic field of a formation, this method comprises the following steps:
A) produce a high-frequency electromagnetic pulse, make that the nucleon magnetization produces motion around the direction of stabilizing magnetic field in the object, so just produced resonance signal,
B) then, add the first and second two gradient magnetics at least in the preparatory period, these two magnetic directions are orthogonal,
C) then, in measuring period, resonance signal is got one group of (n) sampled value,
D) then, after a latent period, repeat to comprise a), b) and c) measurement of step circulation, obtain many groups of (m * n ' group) data, first integration comprises the integration of first gradient magnetic in the preparatory period, its tool m different integrated value, m is first number, and second integrated value comprises the integration of second gradient magnetic in the preparatory period, and it has the individual different integrated value of n ', and n ' is second number, the first number m so then is provided so a plurality of planes, each plane has so multirow of the second number n ', and every row has n sampled data, constitutes the form of a three-dimensional data matrix, it is carried out Fourier Tranform can determine that an object produces the nuclear magnetisation distribution and resembles in 3D region
These method characteristics are, in measurement circulation one by one, corresponding to the order of the first gradient magnetic intensity in the preparatory period integrates, alternately anti-phase excitation resonance signal, make between the described in the above measurement round-robin resonance signal in succession, introduce an additional phase differential △ φ, and when the sampled value on m plane was carried out Fourier Tranform, this additional phase differential had been cancelled.
According to the described method of claim 6, it is characterized in that 7, this additional phase error △ φ equals the π radian per second or equals (m-1). π/m radian per second, m is the number on plane here.
8, determined according to claim 7 in order to determine object magnetic nuclear resonance method in 3D region, be characterized in, if number of planes m is an even number, then additional phase error is the π radian per second, it is by in measurement circulation in succession, alternately anti-phase excitation resonance signal obtains, and measured sampled data is had anti-time phase every a plane, then to carrying out Fourier Tranform with the signal sampling value in the represented matrix plane of the first number m.
9, according to the described method of claim 7, it is characterized in that, will have anti-time phase every an all sampled value in plane.
10, method according to claim 8, the individual integrated value of its described second number (n ') increases with equivalent, changing in the inferior process of n ', first integrated value all remains unchanged at every turn, and each two individual different numerical value of the desirable n ' of integrated value, these integrated values circle are between two extreme values, in the inferior measurement circulation of n ', it is increased to another extreme value like an extreme value ladder, these method characteristics are, inside an inferior measurement circulation of n ', alternately produce resonance signal, the pumping signal of the alternate phase place that its phase place is corresponding with row is anti-phase, this pumping signal in the inferior continuous coverage of n ' circulation when first integral changes, its phase place is once anti-phase, or with the even number line on the odd-numbered line on odd number plane and even number plane or with the sampling numerical value opposite sign in the odd-numbered line on the even number line on odd number plane or even number plane.
11, determine the method that nuclear magnetic resonance distributes in a certain zone of object, this object places a stable uniform magnetic field, and the direction in this magnetic field is called first magnetic direction, and this method comprises the following steps:
A) produce a high-frequency electromagnetic pulse,, thereby produce resonance signal so that the nuclear magnetisation intensity of object produces motion around first magnetic direction,
B) then, in the preparatory period, apply a gradient magnetic at least,
C) then, produce again one 180 ° the high-frequency electromagnetic pulse at least once, so that nuclear magnetisation direction counter-rotating, so just produced a nuclear spin echoed signal, in the measuring period of and then 180 ° of pulses, to signal sampling, obtain one group of (n) sampled signal, in order periodically to adopt to such an extent that n is one group a nuclear spin echo signals sampling value, will be divided into many sampling interval measuring period
D) then, every behind a latent period, promptly repeat step a), b) and c), obtain (n ') secondary data, the each repetition carried out integration to a gradient magnetic intensity at least in the preparatory period, the each repeating part of the integrated value of gained is all different, so just obtain gang's signal sampling value, it is carried out Fourier Tranform, can determine to respond to the distribution of nuclear magnetisation intensity
The characteristics of this method are, at one then in one the measurement circulation, and with corresponding to gradient magnetic intensity integrated value order in the preparatory period, alternately anti-its phase place of 180 ° electromagnetic pulse.
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| CN105785457B (en) * | 2016-05-13 | 2018-04-06 | 吉林大学 | The measuring method of ground nuclear magnetic resonance T2 based on bipolar pulse |
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