CN115656900B - Method and device for reducing influence of system error on magnetic resonance imaging - Google Patents
Method and device for reducing influence of system error on magnetic resonance imaging Download PDFInfo
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Abstract
The invention discloses a method and a device for reducing the influence of system errors on magnetic resonance imaging, comprising the following steps: determining a first pulse preset position and a second pulse preset position of different gradients of the spin echo; simultaneously applying amplitude values with equal magnitudes and opposite directions on the basis of applying a dispersion gradient at the first pulse preset position and the second pulse preset position; calculating vortex generated by any gradient according to the amplitude value, and determining that the vortex generated by the amplitude value is smaller according to the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value; counteracting remanence generated by the dispersion gradient through the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value and opposite in direction; based on the eddy current and residual magnetism, the influence of systematic errors caused by dispersion gradient on magnetic resonance imaging is reduced.
Description
Technical Field
The invention relates to the technical field of magnetic resonance imaging, in particular to a method and a device for reducing the influence of system errors on magnetic resonance imaging.
Background
Diffusion weighted imaging is a new magnetic resonance imaging technique, and hydrogen protons show different signal amplitudes in the gradient direction after applying a diffusion gradient by utilizing the motion diffusion characteristic of water molecules. Since the diffusion coefficients of different tissues are different, the different tissues can be distinguished by the signal intensity.
The fast spin echo in the diffusion weighted imaging has the characteristics of high scanning speed, low requirement on shimming conditions and small motion artifact, and can effectively reduce the deformation and artifact of plane echo images when being used in the diffusion weighted imaging. The fast spin echo has some defects, and because the system is difficult to reach a theoretical ideal state, a large number of phase-focusing pulses and gradients cause the difference of signal phases in an echo chain, and the signals show non-negligible artifacts in images.
Disclosure of Invention
In view of the foregoing, the present invention provides a method for reducing the impact of systematic errors on magnetic resonance imaging, comprising:
determining a first pulse preset position and a second pulse preset position of different gradients of the spin echo;
simultaneously applying amplitude values with equal magnitudes and opposite directions on the basis of applying a dispersion gradient at the first pulse preset position and the second pulse preset position;
calculating vortex generated by any gradient according to the amplitude value, and determining that the vortex generated by the amplitude value is smaller according to the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value; counteracting remanence generated by the dispersion gradient through the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value and opposite in direction; based on the eddy current and residual magnetism, the influence of systematic errors caused by dispersion gradient on magnetic resonance imaging is reduced.
Further, the method further comprises the following steps:
and (3) moving the stimulated echo out of an imaging space through amplitude values applied by the first pulse preset position and the second pulse preset position, so that the influence of systematic errors caused by dispersion gradients on magnetic resonance imaging is reduced.
Further, determining a first pulse preset position and a second pulse preset position of different gradients of the spin echo comprises:
the first pulse preset position is before the first diffusion gradient, and the second pulse preset position is after the last diffusion gradient.
Further, the magnitude value is smaller than the diffusion gradient value.
Further, the eddy current generated by any gradient is calculated, and a specific calculation formula is as follows:
wherein G is the vortex generated by the gradient, G is the gradient,
e (t) is a convolution symbol, and e (t) is a function for calculating eddy current response.
Further, according to the amplitude value applied to the preset position of the second pulse being smaller than the dispersion gradient value, it is determined that the eddy current generated by the amplitude value becomes smaller, including:
according to a calculation formula of the eddy current, the gradient of the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient, so that the eddy current generated by the amplitude value is determined to be smaller.
The invention also provides an apparatus for reducing the effect of systematic errors on magnetic resonance imaging, comprising:
a preset position determining unit for determining a first pulse preset position and a second pulse preset position of different gradients of the spin echo;
the amplitude value applying unit is used for simultaneously applying amplitude values with equal magnitudes and opposite directions on the basis of applying a dispersion gradient at the first pulse preset position and the second pulse preset position;
the influence reducing unit is used for calculating vortex generated by any gradient according to the amplitude value, and determining that the vortex generated by the amplitude value is smaller according to the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value; counteracting remanence generated by the dispersion gradient through the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value and opposite in direction; based on the eddy current and residual magnetism, the influence of systematic errors caused by dispersion gradient on magnetic resonance imaging is reduced.
Further, the method further comprises the following steps:
and the stimulated echo shifting-out unit is used for shifting the stimulated echo out of the imaging space through amplitude values applied by the first pulse preset position and the second pulse preset position, so that the influence of systematic errors caused by dispersion gradients on magnetic resonance imaging is reduced.
Further, the preset position determining unit includes:
the preset position determining subunit is configured to determine that the preset position of the first pulse is before the first diffusion gradient and the preset position of the second pulse is after the last diffusion gradient.
Further, the influence reducing unit includes:
and the eddy current diminishing determining subunit is used for determining that the eddy current generated by the amplitude value is diminishing by applying the amplitude value gradient at the preset position of the second pulse to be smaller than the dispersion gradient according to the calculation formula of the eddy current.
The invention provides a method and a device for reducing the influence of systematic errors on magnetic resonance imaging, wherein amplitude values with equal magnitudes and opposite directions are simultaneously applied to a first pulse preset position and a second pulse preset position on the basis of applying a dispersion gradient; the amplitude value is smaller than the dispersion gradient, dG/dt is reduced, and the eddy current influence caused by the rising edge and the falling edge of the gradient can be effectively reduced; the positive and negative gradient switching can offset the influence of residual magnetism on the image to a certain extent; because the applied diffusion gradients 1 and 2 are very small, in the image with b=0, the two gradients can be selectively not closed, and after the second 180-degree phase-focusing pulse, the stimulated echo signal is subjected to the dephasing action of the two gradients, and in the subsequent echo signals, the same as the diffusion gradients exist, the stimulated echo can not be generated in the image space, and the consistency of the signals is maintained.
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FIG. 1 is a flow chart of a method for reducing the effect of systematic errors on magnetic resonance imaging provided by the present invention;
FIG. 2 is a timing diagram of the eddy current effect generated by the dual phase, radio frequency pulse reduction system according to the present invention;
FIG. 3 is a diffusion gradient application station in accordance with the present invention;
fig. 4 is a comparison of the magnetic resonance image processing before and after the processing according to the present invention;
fig. 5 is a schematic structural diagram of an apparatus for reducing the influence of systematic errors on magnetic resonance imaging according to the present invention.
Detailed Description
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be embodied in many other forms than those herein described, and those skilled in the art will readily appreciate that the present invention may be similarly embodied without departing from the spirit or essential characteristics thereof, and therefore the present invention is not limited to the specific embodiments disclosed below.
In article Reduction of eddy-current-induced distortion in diffusion MRI using a twice-refocused spin echo it is proposed that the eddy current effect generated by a dual phase, poly-phase rf pulse reduction system can be exploited to improve image quality, as shown in fig. 2. This method can eliminate the influence of the eddy current to some extent, but it can be found from the formula (1) that the eddy current is related to only the gradient change,
wherein G is the vortex generated by the gradient, G is the gradient,
e (t) is a convolution symbol, and e (t) is a function for calculating eddy current response.
This approach has less effect on the rising and falling edges and has limited effect. Moreover, this mode gradient may cause stimulated echo when b=0, which affects the image signal value and ultimately affects the accuracy of the ADC.
In order to solve the above-mentioned problems, the present invention provides a method for reducing the influence of systematic errors on magnetic resonance imaging, as shown in fig. 1, comprising the steps of:
step S101, determining a first pulse preset position and a second pulse preset position of different gradients of the spin echo.
As shown in fig. 3, the first pulse preset position is before the first diffusion gradient, i.e., the position shown by arrow 1 in fig. 3, and the second pulse preset position is after the last diffusion gradient, i.e., the position shown by arrow 2 in fig. 3.
Step S102, on the basis of applying a dispersion gradient, amplitude values with equal magnitudes and opposite directions are simultaneously applied at the first pulse preset position and the second pulse preset position.
The dispersion gradients 1 and 2,1 and 2 are added at the position shown in fig. 3, have equal magnitudes and opposite polarities, and then magnitude values with equal magnitudes and opposite directions are simultaneously applied on the basis of applying the dispersion gradients, wherein the magnitude values are smaller than the dispersion gradient values. The amplitudes of 1 and 2 are smaller than the dispersion gradient, and the value of the dispersion gradient is generally in the range of 0.2-0.6.
Step S103, calculating vortex generated by any gradient according to the amplitude value, and determining that the vortex generated by the amplitude value is smaller according to the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value; counteracting remanence generated by the dispersion gradient through the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value and opposite in direction; based on the eddy current and residual magnetism, the influence of systematic errors caused by dispersion gradient on magnetic resonance imaging is reduced.
And (3) calculating the eddy current generated by any gradient according to the formula (1), and further determining that the gradient of the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient according to the formula (1), so as to determine that the eddy current generated by the amplitude value becomes smaller.
Counteracting remanence generated by the dispersion gradient through the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value and opposite in direction; based on the eddy current reduction and the remanence reduction, the stimulated echo is moved out of an imaging space through amplitude values applied by the first pulse preset position and the second pulse preset position, so that the influence of a systematic error caused by a dispersion gradient on magnetic resonance imaging is reduced.
By processing the magnetic resonance image in the above-described manner, fig. 4 (a) is an ADC image before improvement, and fig. 4 (b) is an ADC image after improvement. From the results, it can be seen that the apparent gradient deformation, the blurring artifact and the signal disappearance caused by the dephasing exist at the arrow in fig. 4 (a), and the blurring artifact can be slightly seen in fig. 4 (b), and other problems are obviously improved.
Based on the same inventive concept, the present invention provides an apparatus 500 for reducing the effect of systematic errors on magnetic resonance imaging, as shown in fig. 5, comprising:
a preset position determining unit 510 for determining a first pulse preset position and a second pulse preset position of different gradients of the spin echo;
an amplitude value applying unit 520, configured to apply amplitude values with equal magnitudes and opposite directions at the first pulse preset position and the second pulse preset position simultaneously on the basis of applying a dispersion gradient;
an influence reducing unit 530, configured to calculate an eddy current generated by any gradient according to the amplitude value, and determine that the eddy current generated by the amplitude value becomes smaller according to the amplitude value applied to the preset position of the second pulse being smaller than the dispersion gradient value; counteracting remanence generated by the dispersion gradient through the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value and opposite in direction; based on the eddy current and residual magnetism, the influence of systematic errors caused by dispersion gradient on magnetic resonance imaging is reduced.
Further, the method further comprises the following steps:
and the stimulated echo shifting-out unit is used for shifting the stimulated echo out of the imaging space through amplitude values applied by the first pulse preset position and the second pulse preset position, so that the influence of systematic errors caused by dispersion gradients on magnetic resonance imaging is reduced.
Further, the preset position determining unit includes:
the preset position determining subunit is configured to determine that the preset position of the first pulse is before the first diffusion gradient and the preset position of the second pulse is after the last diffusion gradient.
Further, the influence reducing unit includes:
and the eddy current diminishing determining subunit is used for determining that the eddy current generated by the amplitude value is diminishing by applying the amplitude value gradient at the preset position of the second pulse to be smaller than the dispersion gradient according to the calculation formula of the eddy current.
The invention provides a method and a device for reducing the influence of systematic errors on magnetic resonance imaging, wherein amplitude values with equal magnitudes and opposite directions are simultaneously applied to a first pulse preset position and a second pulse preset position on the basis of applying a dispersion gradient; the amplitude value is smaller than the dispersion gradient, dG/dt is reduced, and the eddy current influence caused by the rising edge and the falling edge of the gradient can be effectively reduced; the positive and negative gradient switching can offset the influence of residual magnetism on the image to a certain extent; because the applied diffusion gradients 1 and 2 are very small, in the image with b=0, the two gradients can be selectively not closed, and after the second 180-degree phase-focusing pulse, the stimulated echo signal is subjected to the dephasing action of the two gradients, and in the subsequent echo signals, the same as the diffusion gradients exist, the stimulated echo can not be generated in the image space, and the consistency of the signals is maintained.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that modifications and equivalents may be made to the specific embodiments of the present invention without departing from the spirit and scope of the present invention, and all modifications and equivalents are intended to be included in the scope of the claims of the present invention.
Claims (9)
1. A method for reducing the effects of systematic errors on magnetic resonance imaging, comprising:
determining a first pulse preset position and a second pulse preset position of different gradients of the spin echo;
simultaneously applying amplitude values which are equal in magnitude and opposite to the dispersion gradient direction on the basis of applying the dispersion gradient at the first pulse preset position and the second pulse preset position; the amplitude value is smaller than the dispersion gradient value;
calculating vortex generated by any gradient according to the amplitude value, and determining that the vortex generated by the amplitude value is smaller according to the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value; counteracting remanence generated by the dispersion gradient through the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value and opposite in direction; based on the eddy current and residual magnetism, the influence of systematic errors caused by dispersion gradient on magnetic resonance imaging is reduced.
2. The method as recited in claim 1, further comprising:
and (3) moving the stimulated echo out of an imaging space through amplitude values applied by the first pulse preset position and the second pulse preset position, so that the influence of systematic errors caused by dispersion gradients on magnetic resonance imaging is reduced.
3. The method of claim 1, wherein determining the first pulse preset position and the second pulse preset position for different gradients of spin echo comprises:
the first pulse preset position is before the first diffusion gradient, and the second pulse preset position is after the last diffusion gradient.
4. The method of claim 1, wherein the eddy current generated by any gradient is calculated by the following formula:
wherein G is the vortex generated by the gradient, G is the gradient,
e (t) is a convolution symbol, and e (t) is a function for calculating eddy current response.
5. The method of claim 1, wherein determining that the eddy current generated by the amplitude value becomes smaller based on the amplitude value applied to the preset position of the second pulse being smaller than the dispersion gradient value, comprises:
according to a calculation formula of the eddy current, the amplitude value applied to the preset position of the second pulse is smaller than the amplitude value of the dispersion gradient, so that the eddy current generated by the amplitude value is determined to be smaller.
6. An apparatus for reducing the effect of systematic errors on magnetic resonance imaging, comprising:
a preset position determining unit for determining a first pulse preset position and a second pulse preset position of different gradients of the spin echo;
the amplitude value applying unit is used for simultaneously applying amplitude values which are equal in size and opposite to the dispersion gradient direction on the basis of applying the dispersion gradient at the first pulse preset position and the second pulse preset position; the amplitude value is smaller than the dispersion gradient value;
the influence reducing unit is used for calculating vortex generated by any gradient according to the amplitude value, and determining that the vortex generated by the amplitude value is smaller according to the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value; counteracting remanence generated by the dispersion gradient through the fact that the amplitude value applied to the preset position of the second pulse is smaller than the dispersion gradient value and opposite in direction; based on the eddy current and residual magnetism, the influence of systematic errors caused by dispersion gradient on magnetic resonance imaging is reduced.
7. The apparatus as recited in claim 6, further comprising:
and the stimulated echo shifting-out unit is used for shifting the stimulated echo out of the imaging space through amplitude values applied by the first pulse preset position and the second pulse preset position, so that the influence of systematic errors caused by dispersion gradients on magnetic resonance imaging is reduced.
8. The apparatus according to claim 6, wherein the preset position determining unit includes:
the preset position determining subunit is configured to determine that the preset position of the first pulse is before the first diffusion gradient and the preset position of the second pulse is after the last diffusion gradient.
9. The apparatus according to claim 6, wherein the influence reducing unit includes:
and the eddy current reduction determining subunit is used for determining that the eddy current generated by the amplitude value is reduced according to the eddy current calculation formula, and the amplitude value applied to the preset position of the second pulse is smaller than the amplitude value of the dispersion gradient.
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| DE19834698C2 (en) * | 1997-08-19 | 2001-06-21 | Siemens Ag | Diffusion detection using magnetic resonance |
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| CN109242847B (en) * | 2018-09-20 | 2020-12-29 | 上海东软医疗科技有限公司 | Magnetic resonance diffusion weighted imaging method and device |
| CN113391251B (en) * | 2020-03-12 | 2023-05-26 | 上海联影医疗科技股份有限公司 | Magnetic resonance image reconstruction method, device and equipment |
| CN112881959B (en) * | 2020-06-10 | 2021-12-21 | 苏州纽迈分析仪器股份有限公司 | Gradient eddy current compensation method and system for magnetic resonance imaging |
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