CN103630859A - Method for searching optimal shimming value of magnetic resonance - Google Patents

Abstract

The invention discloses a method for searching an optimal shimming value of magnetic resonance. The method mainly comprises that the maximum AC value is searched firstly, and then a corresponding current value is searched according to the maximum AC value correspondingly and transmitted to a coil. With application of the method, the optimal shimming value can be accurately searched so that the corresponding current value is detected and supplied to the coil correspondingly, and thus uniformity of a main magnetic field is great.

Description

A kind of method of finding the optimum shimming value of magnetic resonance

Technical field

The present invention relates to a kind of method of finding the optimum shimming value of magnetic resonance.

Background technology

Magnetic resonance imaging system requires to try one's best uniform main field, and actual main field is because the each side reasons such as manufacturing process, external interference can not be completely evenly.This unevenness can reduce by method for shimming.Shimming is divided into initiatively two kinds of shimmings and passive shimming, and wherein initiatively shimming is by obtain the process of more uniform main field by suitable electric current in shim coil.

Known according to physical principle, resonant frequency and magnetic field intensity are proportional, and the AC value that FID signal is done after Fourier transform is relevant with the homogeneity in magnetic field: magnetic field is more even, and this AC value is larger.So can judge by AC value the quality of shimming effect.

Traditional method is manually to find optimum shim current by operating personnel, alternately changes the shim current of three directions until find maximum AC value.Its shortcoming is that the optimum shim current of artificial searching expends time in, efficiency is low, and final effect is also difficult to assurance, and accuracy is not high.

Summary of the invention

For solving the problems of the technologies described above, the invention provides a kind of method of finding the optimum shimming value of magnetic resonance.Specifically comprise the steps:

(1) initial shimming value shimvalue[i], i=1 wherein, 2,3; Initial direction direction=[0,0,0 of searching]; Initial step length K_step=300; Gather FID signal AC1.

(2) carry out shimvalue[i]=shimvalue[i]+1000 after, i=1 wherein, 2,3; Gather FID signal AC2.

(3) whether judgement (AC1-AC2)/AC1 is less than 0.5, if be less than 0.5 execution step (4), if be not less than 0.5, performs step (4a).

(4a) carry out shimvalue[i]=shimvalue[i]-1000, i=1 wherein, 2,3; AC0=AC2; Execution step after completing (9).

(4) gather respectively direction for [6000,0,0], [6000,0,0], [0,6000,0], [0 ,-6000,0], the AC value of [0,0,0], is designated as AC[i], i=1...5 wherein, i is integer.

(5) the AC value judgement Max value and the min value that according to step (4), collect, Max=Max (AC[i]) wherein, min=min (AC[i]).

(6) judge whether Max is less than 50, if be less than 50, exits, and prompting " not finding signal ", if be not less than 50, perform step (7).

(7) whether judgement (Max-min)/min is less than 0.5, if be less than 0.5, exit, and prompting " inspection gradient amplifier ", if be not less than 0.5, perform step (8).

(8) search [K that Max is corresponding _x, K _y, K _z], wherein, K _xfor the shim current of x direction, K _yfor the shim current of y direction, K _zshim current for z direction; And assignment is to shimvalue[i], assignment AC0=Max.

(9) gather respectively shimvalue[i]+300 AC value, be designated as ACi=1,2,3.

(10) judge whether ACi is greater than AC0; If so, perform step (11), if not, perform step (10a).

(10a) gather shimvalue[i]-300 AC value, be designated as ACi.

(10b) whether the ACi in comparison step (10a) is greater than AC0, if so, perform step (11), if not, perform step (12).

(11) carry out direction[i]=ACi-AC0.

(12) judge whether direction equals [0,0,0], if so, perform step (18), if not, perform step (13).

(13), if direction is not equal to [0,0,0], AC1-AC0, carries out $\mathrm{direction}\left[i\right]=\frac{\mathrm{direction}\left[i\right]}{\sqrt{\mathrm{\Σdirection}{\left[i\right]}^2}}.$

(14) carry out shimvalue[i]=shimvalue[i]+K_step * direction[i], direction[i wherein] be the direction[i drawing in step (13)].

(15) gather shimvalue[i] AC value, be designated as AC2.

(16) judge whether AC2 is greater than AC1; If so, perform step (16a), if not, perform step (17).

(16a) assignment K_step=K_step * 2; AC0=AC1; AC1=AC2; Return to afterwards step (14).

(17) judge that new initial value is between AC0 and shimming value corresponding to AC2, shimming value corresponding to maximizing in this is interval, and assignment is shimvalue[i].

(18) judge that optimum shimming value is at shimvalue[i] in ± 300 scopes, find corresponding optimum shimming value.

The invention has the beneficial effects as follows: by the optimum shimming value of the said method fast automatic searching of energy, save operating personnel's time and efforts.Shimming result is accurate, the error that there will not be human factor to cause.

Accompanying drawing explanation

Fig. 1 is nuclear magnetic resonance equipment K _x, K _ythe distribution situation of AC value in two-dimensional space.

Fig. 2 is process flow diagram of the present invention.

Embodiment

Below in conjunction with the drawings and specific embodiments, the present invention is further elaborated.

For each group shim current K _x, K _y, K _za corresponding unique AC value, can be K _x, K _y, K _zas independent variable, AC value is as functional value.Shimming problem can be simplified to the optimization problem of maximizing in a three dimensions, and this three-dimensional coordinate axis represents x, y, and tri-direction shim current values of z, functional value represents AC value.

Fig. 1 has shown a nuclear magnetic resonance equipment K _x, K _ythe distribution situation of AC value in two-dimensional space.As can be observed from Figure: near the AC value of maximal value changes rapider, and curved surface tends towards stability and has other local maximums to occur when away from maximal value.

As shown in Figure 2, the method that the present invention finds the optimum shimming value of magnetic resonance comprises the steps:

(1) initial shimming value shimvalue[i], i=1 wherein, 2,3; Initial direction direction=[0,0,0 of searching]; Initial step length K_step=300; Gather FID signal AC1.

(2) carry out shimvalue[i]=shimvalue[i]+1000 after, i=1 wherein, 2,3; Gather FID signal AC2.

(3) whether judgement (AC1-AC2)/AC1 is less than 0.5, if be less than 0.5 execution step (4), if be not less than 0.5, performs step (4a).

(4a) carry out shimvalue[i]=shimvalue[i]-1000, i=1 wherein, 2,3; AC0=AC2; Execution step after completing (9).

(4) gather respectively direction for [6000,0,0], [6000,0,0], [0,6000,0], [0 ,-6000,0], the AC value of [0,0,0], is designated as AC[i], i=1...5 wherein, i is integer.

(5) the AC value judgement Max value and the min value that according to step (4), collect, Max=Max (AC[i]) wherein, min=min (AC[i]).

(6) judge whether Max is less than 50, if be less than 50, exits, and prompting " not finding signal ", if be not less than 50, perform step (7).

(7) whether judgement (Max-min)/min is less than 0.5, if be less than 0.5, exit, and prompting " inspection gradient amplifier ", if be not less than 0.5, perform step (8).

(8) search [K that Max is corresponding _x, K _y, K _z], wherein, K _xfor the shim current of x direction, K _yfor the shim current of y direction, K _zshim current for z direction; And assignment is to shimvalue[i], assignment AC0=Max.

(9) gather respectively shimvalue[i]+300 AC value, be designated as ACi=1,2,3.

(10) judge whether ACi is greater than AC0; If so, perform step (11), if not, perform step (10a).

(10a) gather shimvalue[i]-300 AC value, be designated as ACi.

(10b) whether the ACi in comparison step (10a) is greater than AC0, if so, perform step (11), if not, perform step (12).

(11) carry out direction[i]=ACi-AC0.

(12) judge whether direction equals [0,0,0], if so, perform step (18), if not, perform step (13).

(13), if direction is not equal to [0,0,0], AC1-AC0, carries out $\mathrm{direction}\left[i\right]=\frac{\mathrm{direction}\left[i\right]}{\sqrt{\mathrm{\Σdirection}{\left[i\right]}^2}}.$

(14) carry out shimvalue[i]=shimvalue[i]+K_step * direction[i], direction[i wherein] be the direction[i drawing in step (13)].

(15) gather shimvalue[i] AC value, be designated as AC2.

(16) judge whether AC2 is greater than AC1; If so, perform step (16a), if not, perform step (17).

(16a) assignment K_step=K_step * 2; AC0=AC1; AC1=AC2; Return to afterwards step (14).

(17) judge that new initial value is between AC0 and shimming value corresponding to AC2, shimming value corresponding to maximizing in this is interval, and assignment is shimvalue[i].

(18) judge that optimum shimming value is at shimvalue[i] in ± 300 scopes, find corresponding optimum shimming value.

The invention has the beneficial effects as follows: by the optimum shimming value of the said method fast automatic searching of energy, save operating personnel's time and efforts.Shimming result is accurate, the error that there will not be human factor to cause.

According to above-mentioned steps, can accurately search out optimum shimming value, thereby corresponding current value be detected, and be passed in coil according to the current value detecting, make the good uniformity of main field.

Claims (1)

1. find a method for the optimum shimming value of magnetic resonance, it is characterized in that comprising the steps:,

(1) initial shimming value shimvalue[i], i=1 wherein, 2,3; Initial direction direction=[0,0,0 of searching]; Initial step length K_step=300; Gather FID signal AC1;

(2) carry out shimvalue[i]=shimvalue[i]+1000 after, i=1 wherein, 2,3; Gather FID signal AC2;

(3) whether judgement (AC1-AC2)/AC1 is less than 0.5, if be less than 0.5 execution step (4), if be not less than 0.5, performs step (4a);

(4a) carry out shimvalue[i]=shimvalue[i]-1000, i=1 wherein, 2,3; AC0=AC2; Execution step after completing (9);

(4) gather respectively direction for [6000,0,0], [6000,0,0], [0,6000,0], [0 ,-6000,0], the AC value of [0,0,0], is designated as AC[i], i=1...5 wherein, i is integer;

(5) the AC value judgement Max value and the min value that according to step (4), collect, Max=Max (AC[i]) wherein, min=min (AC[i]);

(6) judge whether Max is less than 50, if be less than 50, exits, and prompting " not finding signal ", if be not less than 50, perform step (7);

(7) whether judgement (Max-min)/min is less than 0.5, if be less than 0.5, exit, and prompting " inspection gradient amplifier ", if be not less than 0.5, perform step (8);

(8) search [K that Max is corresponding _x, K _y, K _z], wherein, K _xfor the shim current of x direction, K _yfor the shim current of y direction, K _zshim current for z direction; And assignment is to shimvalue[i], assignment AC0=Max;

(9) gather respectively shimvalue[i]+300 AC value, be designated as ACi=1,2,3;

(10) judge whether ACi is greater than AC0; If so, perform step (11), if not, perform step (10a);

(10a) gather shimvalue[i]-300 AC value, be designated as ACi;

(10b) whether the ACi in comparison step (10a) is greater than AC0, if so, perform step (11), if not, perform step (12);

(11) carry out direction[i]=ACi-AC0;

(12) judge whether direction equals [0,0,0], if so, perform step (18), if not, perform step (13);

(13), if direction is not equal to [0,0,0], AC1-AC0, carries out $\mathrm{direction}\left[i\right]=\frac{\mathrm{direction}\left[i\right]}{\sqrt{\mathrm{\Σdirection}{\left[i\right]}^2}}.$

(14) carry out shimvalue[i]=shimvalue[i]+K_step * direction[i], direction[i wherein] be the direction[i drawing in step (13)];

(15) gather shimvalue[i] AC value, be designated as AC2;

(16) judge whether AC2 is greater than AC1; If so, perform step (16a), if not, perform step (17);

(16a) assignment K_step=K_step * 2; AC0=AC1; AC1=AC2; Return to afterwards step (14);

(17) judge that new initial value is between AC0 and shimming value corresponding to AC2, shimming value corresponding to maximizing in this is interval, and assignment is shimvalue[i];

(18) judge that optimum shimming value is at shimvalue[i] in ± 300 scopes, find corresponding optimum shimming value.

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