阅读说明：本技术 一种基于扩散效应的磁共振系统梯度场测量方法 (Magnetic resonance system gradient field measurement method based on diffusion effect ) 是由 吴子岳 罗海 陈潇 叶洋 于 2020-03-17 设计创作，主要内容包括：本发明公开一种基于扩散效应的磁共振系统梯度场测量方法,由非均匀场磁体,核磁共振谱仪,射频功放、射频线圈和已知ADC系数、T2驰豫时间常数的标准定量水模等组成。通过具有不同扩散敏感梯度持续时间或者是具有不同回波间隔的磁共振序列采集多组信号,从多组信号中拟合出梯度场大小。该方法不需要额外的专用磁场检测设备,测量时间短,便于集成到磁共振系统中,使得在装机现场完成梯度场测量非常便捷,提升装机和服务效率。(The invention discloses a magnetic resonance system gradient field measuring method based on diffusion effect, which comprises a non-uniform field magnet, a nuclear magnetic resonance spectrometer, a radio frequency power amplifier, a radio frequency coil, a standard quantitative water model with known ADC coefficient and T2 relaxation time constant and the like. Multiple sets of signals are acquired by magnetic resonance sequences with different diffusion sensitive gradient durations or with different echo spacings, and the gradient field magnitude is fitted from the multiple sets of signals. The method does not need additional special magnetic field detection equipment, has short measurement time, is convenient to integrate into a magnetic resonance system, ensures that the gradient field measurement can be conveniently completed on the installation site, and improves the installation and service efficiency.)

1. A magnetic resonance system gradient field measurement method based on diffusion effect is characterized by comprising the following steps:

s100, collecting M groups of echo signals in a non-uniform field nuclear magnetic resonance system, wherein the echo signals are four-dimensional arrays S (M, n, a, p),

wherein the first dimension is an echo spacing vector τ, length M,

the second dimension is the echo train length, which is N,

the third dimension is the average degree a,

the fourth dimension is the number of sampling points of single-time read data, and the number is P;

s200, preprocessing data, converting the signal S (m, n, a, p) into a one-dimensional or two-dimensional array S':

s210, carrying out Fourier transform on the fourth dimension of the signal S to obtain frequency domain data, reserving a low-frequency part,

s220, averaging the data,

s230, taking the logarithm of all the data,

s240, calculating a time sequence t (m);

s300, fitting out an equivalent coefficient a with a fitting function of

S‘＝f(a，T2，t(m))

Wherein T2 is the transverse relaxation time constant of a known standard quantitative phantom;

s400, calculating a gradient field with a calculation function of

Where γ is the magnetic rotation ratio, D is the ADC coefficient of a known standard quantitative phantom, c is the constant coefficient, and G is the measured gradient field.

2. The measurement method according to claim 1, characterized in that: in step S100, in the inhomogeneous field nuclear magnetic resonance system, an excitation pulse, a refocusing pulse and a constant gradient field are applied,

the flip angle of the excitation pulse is theta, the excitation pulse is followed by a plurality of refocusing pulses, and the flip angle of the refocusing pulses is 2 theta;

the phase difference between the excitation pulse and the first refocusing pulse is 90 degrees, the time interval between the excitation pulse and the first refocusing pulse is tau/2, and the time interval from the first refocusing pulse to the first sampling window is tau/2;

the time interval between the echo pulses is echo interval, N echo signals are acquired by one-time excitation, the echo interval is tau, the echo signals are acquired for multiple times, and the average value is calculated;

and changing the echo interval tau, carrying out M times of measurement, and collecting M groups of echo signals.

3. The measurement method according to claim 2, characterized in that: in step S200, the signal S (m, n, a, q) is converted to S' (m, n),

step S220 is to average the third dimension,

in step S240, the time sequence t (m) [ τ (m) ] ^2, where τ (m) is the mth element in the echo spacing vector τ.

4. A measuring method according to claim 3, characterized in that: the step S300 includes the steps of,

s310, estimation of an equivalent coefficient a:

fitting S' (m, n) as follows, estimating the coefficient a,

wherein C is₁Are unknown constants.

5. The measurement method according to claim 4, characterized in that: in step S400, the constant coefficients of the gradient field calculation function

6. The measurement method according to claim 1, characterized in that: in step S100, in the inhomogeneous field nuclear magnetic resonance system, an excitation pulse, a refocusing pulse and a constant gradient field are applied,

the excitation pulses comprise first excitation pulses, the excitation pulse rotation angles are all theta,

the refocusing pulse comprises a first refocusing pulse, a second refocusing pulse … … nth refocusing pulse, the flip angles of the refocusing pulses are both 2 theta,

the phase difference between the first excitation pulse and the first refocusing pulse is 90 degrees, and the phase difference between the first refocusing pulse and the subsequent refocusing pulse is 0 degree;

the time interval between the first excitation pulse and the first refocusing pulse is T, referred to as the diffusion sensitive gradient duration;

the time interval between the first refocusing pulse and the first acquisition window is T;

the time interval between the first acquisition window and the second refocusing pulse is tau/2; the time interval between subsequent refocusing pulses is tau, and the time interval between subsequent acquisition windows is tau;

acquiring N echo signals by one-time excitation, acquiring the echo signals for multiple times, and calculating an average value;

and changing the diffusion sensitive gradient duration time T, performing M times of measurement, and collecting M groups of echo signals.

7. The measurement method according to claim 6, characterized in that: in step S200, the signal S (m, n, a, q) is converted to S' (m),

step S220 is averaging the third dimension and the second dimension,

in step S240, the time series T (m) [ T (m) ] ^2, where T (m) is the mth element in the diffusion sensitive gradient duration series T.

8. The measurement method according to claim 7, characterized in that: in step S300, a coefficient a is fitted, and the fitting function is

Wherein C is₂Are unknown constants.

9. The measurement method according to claim 8, characterized in that: in step S400, the constant coefficients of the gradient field calculation function

10. The method of measuring according to any one of claims 1 to 9, wherein: the inhomogeneous field nuclear magnetic resonance system comprises a console, a nuclear magnetic resonance spectrometer, a magnet, a radio frequency system and a standard quantitative water model,

the console is connected with the nuclear magnetic resonance spectrometer, sends instructions to control parameter selection and ROI positioning of a measurement sequence, receives magnetic resonance signals collected by the spectrometer and completes real-time data processing;

the magnet is designed as a permanent magnet;

the radio frequency system mainly comprises a radio frequency power amplifier, a preamplifier, a receiving and transmitting change-over switch and a radio frequency coil, wherein the radio frequency coil can transmit an excitation signal and also can receive a magnetic resonance signal through the receiving and transmitting change-over switch;

the standard quantitative water model is a solution with known standard ADC coefficients and T2 values contained in a glass container.

Technical Field

The invention relates to the technical field of nuclear magnetic resonance, in particular to a diffusion effect-based gradient field measurement method of a magnetic resonance system.

Background

The nuclear magnetic resonance technique is a technique for imaging or detecting the composition and structure of a substance by utilizing the nuclear magnetic resonance phenomenon of hydrogen protons. Nuclei in the human body containing a single proton, such as hydrogen nuclei, have a spin motion. The spin motion of the charged nuclei is physically similar to that of individual small magnets whose directional distribution is random without the influence of external conditions. When a human body is placed in an external magnetic field, the small magnets will realign with the lines of the external magnetic field. At this time, the nuclear magnetic resonance phenomenon is a phenomenon in which nuclei are excited by a radio frequency pulse of a specific frequency to deflect spins (small magnets) of the nuclei to generate resonance. After the emission of the radio frequency pulse is stopped, the excited atomic nuclei (small resonant magnets) are gradually restored to the state before excitation, electromagnetic wave signals are released in the restoration process, and magnetic resonance images or composition and structure information of substances can be obtained after the nuclear magnetic resonance signals are received and processed by special equipment.

Magnets are one of the most central components in nuclear magnetic resonance systems. Conventional nmr system magnets all require a highly uniform magnetic field, and therefore the design, production, maintenance, and cost of the magnets are very high. In recent years, inhomogeneous field nmr systems have been developed, in which the homogeneity of the magnet is low, for example, using a single-sided permanent magnet. The magnet of the nuclear magnetic resonance system is very small, simple and convenient in design and production and low in cost. The nuclear magnetic resonance system can be used for oil product detection, food detection, geological exploration and even medical detection, and has wide application prospect.

FIG. 1 is a schematic diagram of a single-sided magnet field distribution for a NMR system. As shown, at the underside of the magnet, the B0 field shifts more slowly, with a region closer to the magnet being the expected ROI region. Within the ROI, the magnetic field is relatively uniform. Outside the ROI and on the upper side of the magnet, the B0 field decays rapidly.

In nuclear magnetic resonance applications, the exact value of the gradient field is a very important necessary parameter, and therefore the gradient field needs to be measured in advance. In single-side magnet nmr systems, accurate measurement of the gradient field in the excitation region is a difficult task. This is because the single-sided magnet nmr system has the following characteristics: 1. the gradient field is very large; 2. the gradient fields are different at almost every location; 3. the actual excitation region is irregular and complex in geometric shape, and has a large difference from the ideal ROI region. FIG. 2 is a schematic diagram of the excitation region of a magnetic resonance system with a single-sided magnet.

Disclosure of Invention

The invention aims to provide a method for measuring a gradient field of a magnetic resonance system based on a diffusion effect, which does not need additional special magnetic field detection equipment, has short measurement time, is convenient to integrate into the magnetic resonance system, ensures that the gradient field measurement can be conveniently completed on the installation site, and improves the installation and service efficiency.

In order to achieve the purpose, the invention is realized by adopting the following technical scheme:

the invention discloses a magnetic resonance system gradient field measuring method based on diffusion effect, which comprises the following steps:

s100, collecting M groups of echo signals in a non-uniform field nuclear magnetic resonance system, wherein the echo signals are four-dimensional arrays S (M, n, a, p),

wherein the first dimension is an echo spacing vector τ, length M,

the second dimension is the echo train length, which is N,

the third dimension is the average degree a,

the fourth dimension is the number of sampling points of single-time read data, and the number is P;

s200, preprocessing data, converting the signal S (m, n, a, p) into a one-dimensional or two-dimensional array S':

s210, carrying out Fourier transform on the fourth dimension of the signal S to obtain frequency domain data, reserving a low-frequency part,

s220, averaging the data,

s230, taking the logarithm of all the data,

s240, calculating a time sequence t (m);

s300, fitting a coefficient a, wherein the fitting function is S' ═ f (a, T2, T (m))

Wherein T2 is the transverse relaxation time constant of a known standard quantitative phantom;

s400, calculating a gradient field with a calculation function of

Where γ is the magnetic rotation ratio, D is the known standard quantitative phantom ADC coefficient, G is the measured gradient field, and c is a constant coefficient.

Preferably, in step S100, in the inhomogeneous field nuclear magnetic resonance system, an excitation pulse, a refocusing pulse, a constant gradient field are applied,

the flip angle of the excitation pulse is theta, the excitation pulse is followed by a plurality of refocusing pulses, and the flip angle of the refocusing pulses is 2 theta;

the phase difference between the excitation pulse and the first refocusing pulse is 90 degrees, the time interval between the excitation pulse and the first refocusing pulse is tau/2, and the time interval from the first refocusing pulse to the first sampling window is tau/2;

the time interval between the echo pulses is echo interval, N echo signals are acquired by one-time excitation, the echo interval is tau, the echo signals are acquired for multiple times, and the average value is calculated;

and changing the echo interval tau, carrying out M times of measurement, and collecting M groups of echo signals.

Preferably, in step S200, the signal S (m, n, a, p) is converted into S' (m, n),

step S220 is to average the third dimension,

in step S240, the time sequence t (m) [ τ (m) ] ^2, where τ (m) is the mth element in the echo spacing vector τ.

Preferably, step S300 includes the steps of,

s310, estimation of an equivalent coefficient a:

fitting S' (m, n) as follows

Wherein C is₁Is a constant.

Preferably, in step S400, the coefficients of the gradient field calculation function are

Preferably, in step S100, in the inhomogeneous field nuclear magnetic resonance system, an excitation pulse, a refocusing pulse, a constant gradient field are applied,

the excitation pulses comprise first excitation pulses, the excitation pulse rotation angles are all theta,

the refocusing pulse comprises a first refocusing pulse, a second refocusing pulse … … nth refocusing pulse, the flip angles of the refocusing pulses are both 2 theta,

the phase difference between the first excitation pulse and the first refocusing pulse is 90 degrees, and the phase difference between the first refocusing pulse and the subsequent refocusing pulse is 0 degree;

the time interval between the first excitation pulse and the first refocusing pulse is T, referred to as the diffusion sensitive gradient duration;

the time interval between the first refocusing pulse and the first acquisition window is T;

the time interval between the first acquisition window and the second refocusing pulse is tau/2; the time interval between subsequent refocusing pulses is tau, and the time interval between subsequent acquisition windows is tau;

acquiring N echo signals by one-time excitation, acquiring the echo signals for multiple times, and calculating an average value;

and changing the diffusion sensitive gradient duration time T, performing M times of measurement, and collecting M groups of echo signals.

Preferably, in step S200, the signal S (m, n, a, p) is converted to S' (m),

step S220 is averaging the third dimension and the second dimension,

in step S240, the time series T (m) [ T (m) ] ^2, where T (m) is the mth element in the diffusion sensitive gradient duration series T.

Preferably, step S300 is to fit the coefficient a, and the fitting function is

Wherein C is₂Are unknown constants.

Preferably, in step S400, the constant coefficient in the gradient field calculation function is

Preferably, the inhomogeneous field nuclear magnetic resonance system comprises a console, a nuclear magnetic resonance spectrometer, a magnet, a radio frequency system and a standard quantitative water model,

the console is connected with the nuclear magnetic resonance spectrometer, sends instructions to control parameter selection and ROI positioning of a measurement sequence, receives magnetic resonance signals collected by the spectrometer and completes real-time data processing;

the magnet is designed as a permanent magnet;

the radio frequency system mainly comprises a radio frequency power amplifier, a preamplifier, a receiving and transmitting change-over switch and a radio frequency coil, wherein the radio frequency coil can transmit an excitation signal and also can receive a magnetic resonance signal through the receiving and transmitting change-over switch;

the standard quantitative water model is a solution with known standard ADC coefficients and T2 values contained in a glass container.

The invention has the beneficial effects that:

1. the invention is composed of nuclear magnetic resonance equipment and a standard A quantitative water model, the system adopts a magnetic resonance diffusion weighting sequence to collect signals, the equivalent gradient field size of an excitation area can be measured, and the measuring method does not need a special magnetic field measuring tool and is convenient and quick.

2. The invention has short measuring time.

3. The gradient field measurement system is convenient to integrate into a magnetic resonance system, so that the gradient field measurement can be conveniently completed on the installation site, and the installation and service efficiency is improved.

Drawings

FIG. 1 is a schematic diagram of a single-sided magnet field distribution for NMR;

FIG. 2 is a schematic representation of an excitation region of a single-sided magnet NMR system;

figure 3 is a schematic diagram of a magnetic resonance system gradient field measurement system based on the diffusion effect;

FIG. 4 is a gradient field measurement sequence of the first embodiment, belonging to the SE-CPMG sequence;

fig. 5 is a gradient field measurement sequence of the second embodiment, belonging to the CPMG sequence.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings.

In the present application:

NMR: nuclear Magnetic Resonance technology

MRI: magnetic Resonance Imaging

K-space: k-space, the frequency domain space of the magnetic resonance signal

DWI: diffusion Weighted Imaging, Diffusion Weighted Imaging or Diffusion Weighted Imaging

T1: time constant for growth of longitudinal magnetization after RF-pulse, the longitudinal magnetization vector recovery Time constant, i.e. the longitudinal relaxation Time constant

T2: time constant for transverse magnetization after RF-pulse, transverse magnetization vector decay Time constant, i.e. transverse relaxation Time constant

TR: repetition Time, Repetition Time or Repetition period

ADC: apparent diffusion coefficient of Apparent diffusion coefficient

EPI: echo planar imaging, planar Echo imaging technique

CPMG: a NMR pulse sequence and by magnetic resonance diagnostics (Carr, Purcell, Meiboom, Gill), nuclear magnetic resonance sequences named by Carr, Purcell, Meiboom, Gill, et al

SE-EPI: spin echo-echo planar imaging, Spin echo-planar echo sequence

As shown in fig. 3, the gradient field measurement system of the magnetic resonance system based on the diffusion effect is mainly composed of five parts: console, nuclear magnetic resonance spectrometer, magnet, radio frequency system, standard quantitative water model.

A block diagram of the system is shown in fig. 3.

The console is connected with the spectrometer, sends instructions to control parameter selection and ROI positioning of a measurement sequence, receives magnetic resonance signals acquired by the spectrometer, and completes real-time data processing. The magnets are typically permanent magnet designs, such as single-sided permanent magnets, with highly inhomogeneous magnetic fields within the ROI.

The radio frequency system mainly comprises a radio frequency power amplifier, a preamplifier, a transmitting-receiving conversion switch and a radio frequency coil. The radio frequency coil can transmit excitation signals and receive magnetic resonance signals through the receiving and transmitting conversion switch.

The standard quantitative water model is a container made of non-metallic materials and non-conductors and filled with a specific solution, and the standard ADC coefficient and the T2 value of the solution are known. For example, a glass container filled with pure water or a glass container filled with a solution containing 2% copper sulfate.