CN112462311B

CN112462311B - Method and device for correcting and measuring transverse magnetization vector decay time constant, computer equipment and non-uniform field magnetic resonance system

Info

Publication number: CN112462311B
Application number: CN202011242388.8A
Authority: CN
Inventors: 潘子异; 张洁莹; 王伟谦; 吴子岳; 叶洋
Original assignee: Wuxi Marvel Stone Healthcare Co Ltd
Current assignee: Wuxi Marvel Stone Healthcare Co Ltd
Priority date: 2020-11-09
Filing date: 2020-11-09
Publication date: 2021-06-15
Anticipated expiration: 2040-11-09
Also published as: CN112462311A

Abstract

The invention relates to the technical field of nuclear magnetic resonance imaging, and discloses a method, a device, computer equipment and a non-uniform field magnetic resonance system for correcting and measuring a transverse magnetization vector decay time constant, wherein in a nuclear magnetic resonance system with a very non-uniform magnetic field or a very short echo time, a plurality of acquired echo signals acquired by SE-CPMG sequences applying different b values are firstly fitted to obtain an ADC-T2 two-dimensional map, and then the transverse magnetization vector decay time constant T2 is corrected, so that the problem of small T2 measurement value caused by diffusion can be avoided. In addition, a more accurate ADC-T2 two-dimensional map can be obtained based on the corrected T2 measured value, the T2 value measurement is not influenced by echo time and molecular diffusion, the requirement on the system is low, and the system cost can be reduced.

Description

Method and device for correcting and measuring transverse magnetization vector decay time constant, computer equipment and non-uniform field magnetic resonance system

Technical Field

The invention belongs to the technical field of nuclear magnetic resonance imaging, particularly relates to a magnetic resonance spectrometer technology, and particularly relates to a method and a device for correcting and measuring a transverse magnetization vector decay time constant, computer equipment and a non-uniform field 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 are obtained after the nuclear magnetic resonance signals are received and processed through special equipment.

The speed at which the nuclear transverse magnetization vector goes to zero depends on the strength of the spin-spin interaction, described by the transverse magnetization vector decay time constant T2 (also known as the spin-spin relaxation time T2). In the magnetic resonance technology, T2 is generally measured by using Spin Echo Sequence (SE), such as CPMG nuclear magnetic resonance sequence (nmr pulse sequence by magnetic resonance systems Carr, Purcell, Meiboom and Gill, i.e. nuclear magnetic resonance sequence named by Carr, Purcell, Meiboom and Gill, etc.), i.e. applying 90-degree radio frequency pulse on x-axis, then applying 180-degree radio frequency pulse on y-axis at T τ,3 τ,5 τ, …, (2n-1) τ, and then obtaining Echo signals at T τ,4 τ,6 τ, …,2n, as shown in fig. 1. The CPMG sequence is short in time, and error accumulation caused by inaccuracy of 180-degree radio frequency pulse (due to limited radio frequency field uniformity) can be avoided. In inhomogeneous field magnetic resonance systems, CPMG sequences are also commonly used to acquire signals. Generally, T2 in solids is much shorter than T2 in liquids. Typical biological tissue has a T2 in the range of 20-150ms, and free water T2 is much longer than bound water T2. Clinically, the observed prolongation of T2 at the lesion is interpreted as an increased proportion of free water.

Molecules in a substance all have some degree of diffusive motion, with random direction, called thermal or brownian motion of the molecules. Free diffusion is said to occur if the water molecules are not constrained in their diffusive motion. In the human body, diffusion movement of water molecules such as cerebrospinal fluid and urine is relatively less restricted and is considered to be freely diffused. In fact, the diffusion movement of water molecules in biological tissues is limited to different degrees due to the constraint of surrounding media, which is called limiting diffusion, and the diffusion movement of water molecules in general tissues belongs to limiting diffusion. The apparent diffusion coefficient is a physical quantity that describes the ability of water molecules to diffuse in tissue. After the magnetic resonance signal is excited, the diffusion movement of water molecules in the direction of the gradient magnetic field causes the attenuation of the magnetic resonance signal, and if the water molecules are more freely diffused in the direction of the gradient magnetic field, the larger the diffusion distance is during the application of the gradient magnetic field, the larger the magnetic field change is experienced, and the more obvious the attenuation of the tissue signal is. Therefore, the apparent diffusion coefficient of the object can be measured by the nuclear magnetic resonance technology, so that the microstructure characteristics and the change of the object are indirectly reflected.

In the magnetic resonance imaging technology, the apparent diffusion coefficient is widely used as an important clinical diagnosis index. The measurement is generally performed by Diffusion Weighted Imaging (DWI), such as Spin echo-echo planar Imaging (SE-EPI), i.e. Spin echo Sequence (SE) for Diffusion gradient encoding and planar echo sequence (EPI) for signal readout. In inhomogeneous magnetic field magnetic resonance systems (also shortly called inhomogeneous magnetic field magnetic resonance systems), similar diffusion weighted imaging techniques are introduced for measuring the apparent diffusion coefficient of a substance. Several typical pulse sequences for measuring apparent diffusion coefficients are shown in figure 2. Fig. 2 (a) shows a Spin echo-CPMG sequence (SE-CPMG), i.e., a diffusion gradient encoding is performed based on Spin echo, and then signal readout is performed using an ultra-fast CPMG sequence. Fig. 2 (b) shows a Dual spin echo-CPMG sequence (DSE-CPMG), i.e. diffusion gradient encoding is performed based on the Dual echo sequence, and signal readout is performed by using the ultra-fast CPMG sequence, which can reduce the influence of low-speed liquid flow. Fig. 2 (c) shows a Stimulated echo-CPMG sequence (STE-CPMG), that is, diffusion gradient encoding is performed based on the Stimulated echo sequence, which can reduce the influence of T1 recovery, and when T1/T2 of the detected object is small, measurement accuracy can be improved by using the sequence to measure an Apparent Diffusion Coefficient (ADC).

In the prior art, an ADC (Apparent Diffusion Coefficient) measurement pulse sequence is composed of a Diffusion gradient coding module and a signal readout module. Because the gradient Magnetic field is very large in the inhomogeneous field Magnetic Resonance system, the gradient Magnetic field is usually 2-3 orders of magnitude higher than that of a conventional Magnetic Resonance Imaging (MRI) system, and the change of the gradient Magnetic field in a signal readout stage cannot be controlled (the DWI technology in the MRI system can control the gradient field to be reduced in the signal readout stage, and in the inhomogeneous magnet, the static gradient Magnetic field G is usually very large and is a constant uncontrollable gradient field). Therefore, an ultra-fast signal readout module is required to reduce the influence of diffusion effect in the signal readout process. For example, an echo spacing of 40us is used. Therefore, the requirements on spectrometer equipment, a radio frequency power amplifier and a radio frequency coil of a nuclear magnetic resonance system are very high, otherwise, due to the existence of a gradient magnetic field, the molecular diffusion of substances can cause the loss of CPMG signals, so that the echo peak value cannot reach the due height, particularly, when tau is longer, the influence of the diffusion is increased, a larger error is generated on the measurement of T2, the finally measured T2 is smaller, the two-dimensional map distribution of T2 and ADC coefficients cannot be accurately measured, and the accurate identification of the substance components of the detected object is not facilitated.

Disclosure of Invention

In order to solve the problem that the two-dimensional map distribution of T2 and ADC coefficients cannot be accurately measured due to the fact that a transverse magnetization vector attenuation time constant T2 obtained by measurement is small in a nuclear magnetic resonance system with an extremely-uneven magnetic field or extremely-short echo time cannot be achieved, the invention aims to provide a method, a device, computer equipment, an inhomogeneous-field magnetic resonance system and a computer-readable storage medium for correcting and measuring the transverse magnetization vector attenuation time constant.

In a first aspect, the present invention provides a method for calibrating and measuring a transverse magnetization vector decay time constant, comprising:

acquiring multiple groups of echo signals acquired based on an SE-CPMG nuclear magnetic resonance sequence, wherein an SE part in the SE-CPMG nuclear magnetic resonance sequence is used for performing diffusion gradient coding and applying different diffusion sensitivity coefficient b values for each group of echo signals, and a CPMG part in the SE-CPMG nuclear magnetic resonance sequence is used for reading each group of echo signals;

fitting an ADC-T2 two-dimensional map according to the multiple groups of echo signals to obtain a fitting value of an apparent diffusion coefficient and a transverse magnetization vector attenuation time constant, wherein the ADC-T2 two-dimensional map is a two-dimensional map with variables including the apparent diffusion coefficient and the transverse magnetization vector attenuation time constant;

the correction value T of the transverse magnetization vector decay time constant is calculated according to the following formula_2-corr：

In the formula, T_2-fitFitting values representing the transverse magnetization vector decay time constant, D_fitRepresents the fitted value of the apparent diffusion coefficient, gamma represents the magnetic rotation ratio of the hydrogen nucleus, G₀Denotes the natural constant gradient field magnitude, t, of a magnet in a magnetic resonance system_EDAnd the time length from the central point moment of the first echo signal to the central point moment of the second echo signal in the single group of echo signal acquisition process is represented.

Based on the above disclosure, in a nuclear magnetic resonance system having a very non-uniform magnetic field or incapable of achieving a very short echo time, a two-dimensional map of ADC-T2 is fitted first by acquiring a plurality of echo signals acquired by SE-CPMG sequences applying different b values, and then the transverse magnetization vector decay time constant T2 is corrected, so that the problem of a small T2 measurement value caused by diffusion can be avoided. In addition, a more accurate ADC-T2 two-dimensional map can be obtained based on the corrected T2 measured value, the T2 value measurement is not influenced by echo time and molecular diffusion, the requirement on the system is low, and the system cost can be reduced. The error caused by molecular diffusion in the CPMG sequence measurement T2 can be corrected, the robustness of the T2 measurement is ensured, and the T2 value is not changed along with the change of the echo interval; for a magnetic field with extremely non-uniform intensity (including but not limited to a single-sided magnet) or a nuclear magnetic resonance system which cannot realize extremely short echo time, the method can still measure the ADC-T2 map to obtain a more accurate T2 measured value; the hardware requirement of the system can be reduced when the non-uniform field nuclear magnetic resonance system for measuring ADC-T2 is designed, so that the hardware cost is reduced; more accurate T2 measurement can be obtained, so that water and fat components in the detected object can be accurately distinguished, water and fat can be further accurately distinguished in an ADC-T2 two-dimensional map (including but not limited to human liver fat quantitative detection), tissue components can be identified (including but not limited to liver fibrosis detection), and the like.

In one possible design, the diffusion sensitivity coefficient b is a value of (t) and_EE)³is in direct proportion, wherein t_EEThe time length from the central point time of the first 90-degree excitation pulse to the central point time of the first echo signal in the single group echo signal acquisition process is shown.

In one possible design, fitting the ADC-T2 two-dimensional map according to the multiple sets of echo signals to obtain fitted values of the apparent diffusion coefficient and the transverse magnetization vector decay time constant includes:

fitting the following integral formula to the multiple groups of echo signals:

wherein, S' (b)_i,t_j) Representing the sum of the intermediate variable b in the sets of echo signals_iAnd t_jThe corresponding echo signal is then transmitted to the receiver,

t_j＝j·t_ED，i＝1,2,…,M，j＝1,2,…,N，t_EEithe time length from the central point moment of the first 90-degree excitation pulse to the central point moment of the first echo signal in the collection process of the ith group of echo signals is represented, M represents the total application number of the diffusion sensitivity coefficient b values, namely the total group number of a plurality of groups of echo signals, N represents the echo chain length of the single group of echo signals, D_maxRepresenting the boundaries of maxima of the apparent diffusion coefficient, D_minRepresenting the boundary of the minimum value of the apparent diffusion coefficient, T_2-maxMaximum value boundary, T, representing the decay time constant of the transverse magnetization vector_2-minThe boundary of the minimum value, S (D, T), representing the decay time constant of the transverse magnetization vector₂) Representing the ADC-T2 two-dimensional map to be solved, D representing the apparent diffusion coefficient, T₂Indicating transverse magnetisationVector decay time constant, e represents the base of the natural logarithm;

setting the dimension of the ADC-T2 two-dimensional map to be solved as p × q, and then converting the integral formula into the following matrix formula:

S′＝K₂SK₁ ^T,s′＝K·s

wherein S 'represents vect (S'), and S 'represents S' (b)_i,t_j) Vect (S ') denotes S' (b)_i,t_j) In the form of a vector of (a),

K₁the representation is composed of N × q elements

The arrangement is such that a matrix is formed,

K₁ ^Trepresentation matrix K₁Transposed matrix of, K₂The representation is composed of M × p elements

The arrangement is such that a matrix is formed,

T_2jrepresents the transverse magnetization vector decay time constant, T, corresponding to the j point in the two-dimensional map of ADC-T2_i＝i·t_ED，D_jRepresents the apparent diffusion coefficient corresponding to the j-th point in the ADC-T2 two-dimensional map, S is vect (S), S represents S (D, T₂) Vect (S) represents S (D, T)₂) The vector form of (1);

according to the matrix formula, solving S (D, T)₂) The problem of (2) is transformed into a solution optimization problem of the following formula:

in which F(s) denotes with respect to the variable sFunction, | | | purple₂Representing the two-norm of the vector, and λ represents the constraint term coefficient;

solving according to the optimization solving problem to obtain a minimum fitting vector s which is larger than 0;

calculating to obtain a fitting value D of the apparent diffusion coefficient according to the fitting vector s obtained by solving_fitFitting value T of transverse magnetization vector decay time constant_2-fit。

In one possible design, before the fitting of the integration formula to the multiple sets of echo signals, the method further includes preprocessing the multiple sets of echo signals in any one or any combination of the following manners (a) to (C):

(A) performing fast Fourier transform on the multiple groups of echo signals on the dimension of the number of sampling points of the single-time read data to obtain frequency domain data, and then reserving a low-frequency part lower than a preset frequency threshold value and performing equalization processing to obtain multiple groups of echo signals which are not different on the dimension of the number of sampling points;

(B) averaging the multiple groups of echo signals in an average frequency dimension to obtain multiple groups of echo signals which are not different in the average frequency dimension;

(C) and carrying out filtering processing based on singular value decomposition on the multiple groups of echo signals in the echo chain length dimension to obtain multiple groups of echo signals without noise influence in the echo chain length dimension.

In one possible design, the correction value T of the decay time constant of the transverse magnetization vector is calculated_2-corrThereafter, the method further comprises:

according to the correction value T_2-corrObtaining a corrected ADC-T2 two-dimensional map;

and carrying out interpolation processing on the corrected ADC-T2 two-dimensional map again to obtain a squared new ADC-T2 two-dimensional map.

In a second aspect, the invention provides a device for correcting and measuring a transverse magnetization vector decay time constant, which comprises a signal acquisition module, a map fitting module and a constant correction module, wherein the signal acquisition module, the map fitting module and the constant correction module are sequentially in communication connection;

the signal acquisition module is used for acquiring multiple groups of echo signals acquired based on a SE-CPMG nuclear magnetic resonance sequence, wherein a SE part in the SE-CPMG nuclear magnetic resonance sequence is used for performing diffusion gradient coding and applying different diffusion sensitivity coefficient b values to each group of echo signals, and a CPMG part in the SE-CPMG nuclear magnetic resonance sequence is used for reading each group of echo signals;

the map fitting module is used for fitting an ADC-T2 two-dimensional map according to the multiple groups of echo signals to obtain a fitting value of an apparent diffusion coefficient and a transverse magnetization vector attenuation time constant, wherein the ADC-T2 two-dimensional map is a two-dimensional map with variables containing the apparent diffusion coefficient and the transverse magnetization vector attenuation time constant;

the constant correction module is used for calculating and obtaining a correction value T of the transverse magnetization vector decay time constant according to the following formula_2-corr：

In a third aspect, the present invention provides a computer device, comprising a memory and a processor, wherein the memory is used for storing a computer program, and the processor is used for reading the computer program and executing the calibration measurement method according to the first aspect or any one of the possible designs of the first aspect.

In a fourth aspect, the present invention provides a non-uniform field magnetic resonance system, which comprises a console, a nuclear magnetic resonance spectrometer, a magnet and a radio frequency subsystem;

the console is communicatively connected to the nuclear magnetic resonance spectrometer, and is configured to send an instruction to the nuclear magnetic resonance spectrometer so as to control parameter selection and region-of-interest positioning of a measurement sequence, and receive a magnetic resonance signal acquired by the nuclear magnetic resonance spectrometer, and complete data processing, where the data processing includes performing the calibration measurement method according to any one of the first aspect and the possible design of the first aspect;

the nuclear magnetic resonance spectrometer is in communication connection with the radio frequency subsystem and is used for executing instructions from the console, transmitting radio frequency excitation signals of a measurement sequence through the radio frequency subsystem and receiving the magnetic resonance signals;

the magnet is used for being arranged right above the detected object and the signal transceiving component in the radio frequency subsystem;

and the radio frequency subsystem is used for transmitting a radio frequency excitation signal of a measurement sequence and receiving the magnetic resonance signal under the control of the nuclear magnetic resonance spectrometer.

In one possible design, the radio frequency subsystem comprises a radio frequency power amplifier, a preamplifier, a transmit-receive switch and a radio frequency coil;

the signal input end of the radio frequency power amplifier is electrically connected with the signal output end of the nuclear magnetic resonance spectrometer, and the signal output end of the radio frequency power amplifier is electrically connected with the first switching end of the transceiving switch;

the signal input end of the preamplifier is electrically connected with the second switching end of the transceiving switch, and the signal output end of the preamplifier is electrically connected with the signal input end of the nuclear magnetic resonance spectrometer;

the controlled end of the transceiving switch is in communication connection with the output end of the nuclear magnetic resonance spectrometer, and the switching public end of the transceiving switch is electrically connected with the radio frequency coil;

the radio frequency coil is used as a signal transceiving component of the radio frequency subsystem, and is used for transmitting the radio frequency excitation signal to the detected object and receiving the magnetic resonance signal from the detected object.

In a fifth aspect, the present invention provides a computer-readable storage medium having stored thereon instructions which, when executed on a computer, perform the calibration measurement method as described above in the first aspect or any one of the possible designs of the first aspect.

In a sixth aspect, the present invention provides a computer program product comprising instructions which, when run on a computer, cause the computer to perform the calibration measurement method as described above in the first aspect or any one of the possible designs of the first aspect.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

Fig. 1 is an exemplary diagram of a CPMG sequence-based measurement T2 in the prior art.

FIG. 2 is an exemplary diagram of a prior art ADC measurement pulse sequence employed in a non-uniform field magnetic resonance system, wherein (a) a SE-CPMG sequence is employed; (b) adopting a DSE-CPMG sequence; (c) the STE-CPMG sequence was used.

FIG. 3 is a schematic flow chart of a calibration measurement method provided by the present invention.

FIG. 4 is an exemplary diagram of an ADC-T2 measurement pulse sequence employed in a non-uniform field magnetic resonance system provided by the present invention.

FIG. 5 is an exemplary graph of different diffusion sensitivity coefficient b values adopted when the tested substances are 0.5mmol/L MnCl2 solution and peanut oil.

FIG. 6 is an exemplary graph of CPMG measurement data obtained for different values of diffusion susceptibility coefficient b when the substance to be tested is 0.5mmol/L MnCl2 solution and peanut oil.

FIG. 7 is a comparative example diagram of ADC-T2 two-dimensional maps before and after correction when the detected substance is 0.5mmol/L MnCl2 solution, wherein (a) is ADC-T2 two-dimensional map before correction; (b) is a corrected ADC-T2 two-dimensional map.

FIG. 8 is a two-dimensional map example of ADC-T2 corrected when the tested substances are 0.5mmol/L MnCl2 solution and peanut oil provided by the invention.

Fig. 9 is a schematic structural diagram of a calibration measurement device provided by the present invention.

Fig. 10 is a schematic structural diagram of a computer device provided by the present invention.

FIG. 11 is a schematic structural diagram of a non-uniform field magnetic resonance system provided by the present invention.

Detailed Description

The invention is further described with reference to the following figures and specific embodiments. It should be noted that the description of the embodiments is provided to help understanding of the present invention, but the present invention is not limited thereto. Specific structural and functional details disclosed herein are merely illustrative of example embodiments of the invention. This invention may, however, be embodied in many alternate forms and should not be construed as limited to the embodiments set forth herein.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments of the present invention.

It should be understood that, for the term "and/or" as may appear herein, it is merely an associative relationship that describes an associated object, meaning that three relationships may exist, e.g., a and/or B may mean: a exists alone, B exists alone, and A and B exist at the same time; for the term "/and" as may appear herein, which describes another associative object relationship, it means that two relationships may exist, e.g., a/and B, may mean: a exists independently, and A and B exist independently; in addition, for the character "/" that may appear herein, it generally means that the former and latter associated objects are in an "or" relationship.

It will be understood that when an element is referred to herein as being "connected," "connected," or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. Conversely, if a unit is referred to herein as being "directly connected" or "directly coupled" to another unit, it is intended that no intervening units are present. In addition, other words used to describe the relationship between elements should be interpreted in a similar manner (e.g., "between … …" versus "directly between … …", "adjacent" versus "directly adjacent", etc.).

It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes" and/or "including," when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, numbers, steps, operations, elements, components, and/or groups thereof.

It should also be noted that, in some alternative designs, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed substantially concurrently, or the figures may sometimes be executed in the reverse order, depending upon the functionality/acts involved.

It should be understood that specific details are provided in the following description to facilitate a thorough understanding of example embodiments. However, it will be understood by those of ordinary skill in the art that the example embodiments may be practiced without these specific details. For example, systems may be shown in block diagrams in order not to obscure the examples in unnecessary detail. In other instances, well-known processes, structures and techniques may be shown without unnecessary detail in order to avoid obscuring example embodiments.

As shown in fig. 3 to 8, the method for calibrating and measuring the transverse magnetization vector decay time constant provided in the first aspect of the present embodiment may be, but is not limited to, performed by a console (which is connected to a nuclear magnetic resonance spectrometer to control parameter selection and region of interest positioning of a measurement sequence, and receives magnetic resonance signals acquired by the nuclear magnetic resonance spectrometer to complete data processing) in a nuclear magnetic resonance system with a very inhomogeneous magnetic field or a very short echo time, so as to obtain a calibrated transverse magnetization vector decay time constant T2. The method for correcting and measuring the decay time constant of the transverse magnetization vector comprises but is not limited to the following steps S101 to S103.

S101, acquiring multiple groups of echo signals acquired based on an SE-CPMG nuclear magnetic resonance sequence, wherein an SE part in the SE-CPMG nuclear magnetic resonance sequence is used for performing diffusion gradient coding and applying different diffusion sensitivity coefficient b values to the groups of echo signals, and a CPMG part in the SE-CPMG nuclear magnetic resonance sequence is used for reading the groups of echo signals.

In step S101, as shown in fig. 4, the T2 measurement sequence includes two parts: (1) the former part is a spin echo SE part used for diffusion gradient encoding, and in order to correct diffusion influence, the b value of a diffusion sensitivity coefficient applied by a diffusion gradient needs to be changed in advance (in DWI technology, the parameter of an applied diffusion sensitivity gradient field is called as the b value or the diffusion sensitivity coefficient) so as to obtain a plurality of groups of different echo signals, and therefore the b value of the diffusion sensitivity coefficient is equal to (t) and (t)_EE)³Is in direct proportion, wherein t_EEThe time duration from the time of the center point of the first 90 ° excitation pulse to the time of the center point of the first echo signal during the single set of echo signal acquisition is shown in fig. 4. (2) The latter part is a CPMG part for reading out the echo signals of each group, i.e. a typical θ -2 θ -2 θ -2 θ … … radio frequency pulse sequence is adopted: the flip angle of the first excitation pulse is theta, and then a plurality of refocusing pulses follow the first excitation pulse, wherein the flip angle is 2 theta; the phase difference between the first excitation pulse and the first refocusing pulse is 90 degrees, the time interval between the first excitation pulse and the first refocusing pulse is tau/2, and the time interval between the first refocusing pulse and the first sampling window is tau/2; pulse of refocusingThe time intervals between are all tau and are called echo intervals. In a nuclear magnetic resonance system, the constant gradient field is the natural gradient field of the magnet and no control is required. In addition, although N echo signals (where N is a positive integer) can be acquired by one excitation, the excitation needs to be repeated multiple times to improve the signal-to-noise ratio by averaging the signals.

S102, fitting an ADC-T2 two-dimensional map according to the multiple groups of echo signals to obtain a fitting value of an apparent diffusion coefficient and a transverse magnetization vector attenuation time constant, wherein the ADC-T2 two-dimensional map is a two-dimensional map with variables including the apparent diffusion coefficient and the transverse magnetization vector attenuation time constant.

In step S102, the acquired multiple sets of echo signals may be represented by a 4-dimensional array S (M, n, a, q), where the first-dimensional array S (M) is based on different values of the diffusion sensitivity coefficient b and has a length of a positive integer M, i.e., t is corresponding to M different lengths_EE(ii) a The second dimension array S (N) is based on different echo chain lengths, and the length is a positive integer N; a third dimension array S (a) is based on different average times when repeated collection is carried out, and the length is a positive integer A; and the fourth dimension array S (Q) is based on different sampling points of single-time read data and has the length of a positive integer Q. Based on the four-dimensional array, the ADC-T2 two-dimensional map estimation may be performed first, and then the diffusion influence correction may be performed on the T2 measurement value based on the estimated map, and the specific estimation fitting process may include, but is not limited to, the following steps S201 to S205.

S201, fitting the multiple groups of echo signals by the following integral formula:

t_j＝j·t_ED，i＝1,2,…,M，j＝1,2,…,N，t_EEithe time length from the central point moment of the first 90-degree excitation pulse to the central point moment of the first echo signal in the collection process of the ith group of echo signals is represented, M represents the total application number of the diffusion sensitivity coefficient b values, namely the total group number of a plurality of groups of echo signals, N represents the echo chain length of the single group of echo signals, D_maxRepresenting the boundaries of maxima of the apparent diffusion coefficient, D_minRepresenting the boundary of the minimum value of the apparent diffusion coefficient, T_2-maxMaximum value boundary, T, representing the decay time constant of the transverse magnetization vector_2-minThe boundary of the minimum value, S (D, T), representing the decay time constant of the transverse magnetization vector₂) Representing the ADC-T2 two-dimensional map to be solved, D representing the apparent diffusion coefficient, T₂Represents the transverse magnetization vector decay time constant, and e represents the base of the natural logarithm.

In step S201, it is considered that the sets of echo signals are a 4-dimensional array S (m, n, a, q), which has too many dimensions and noise influence, and which may increase unnecessary integral computation amount and bring errors, so that it is necessary to perform a down-scaling and/or de-noising process, that is, before performing an integral equation fitting on the sets of echo signals, the pre-processing of the sets of echo signals in any one or any combination of the following manners (a) to (C) is included, but not limited: (A) performing fast Fourier transform on the multiple groups of echo signals on the dimension of the number of sampling points of the single-time read data to obtain frequency domain data, and then reserving a low-frequency part lower than a preset frequency threshold value and performing equalization processing to obtain multiple groups of echo signals which are not different on the dimension of the number of sampling points; (B) averaging the multiple groups of echo signals in an average frequency dimension to obtain multiple groups of echo signals which are not different in the average frequency dimension; (C) in the echo chain length dimension, the multiple groups of echo signals are subjected to filtering processing based on Singular Value decomposition (SVD is one of algorithms which are widely applied in the field of machine learning and is also one of cornerstones which can not be bypassed by learning machine learning algorithms). Therefore, through the processing in the manners (a) and (B), the four-dimensional array S (m, n, a, q) can be reduced into the two-dimensional array S (m, n), which is beneficial to reducing subsequent calculation.

S202, setting the dimension of the ADC-T2 two-dimensional map to be solved as p × q, and then converting the integral formula into the following matrix formula:

S′＝K₂SK₁ ^T,s′＝K·s

K₁the representation is composed of N × q elements

The arrangement is such that a matrix is formed,

The arrangement is such that a matrix is formed,

T_2jrepresents the transverse magnetization vector decay time constant, T, corresponding to the j point in the two-dimensional map of ADC-T2_i＝i·t_ED，D_jRepresents the apparent diffusion coefficient corresponding to the j-th point in the ADC-T2 two-dimensional map, S is vect (S), S represents S (D, T₂) Vect (S) represents S (D, T)₂) In the form of a vector.

S203, solving S (D, T) according to the matrix formula₂) The problem of (2) is transformed into a solution optimization problem of the following formula:

where F(s) represents a function with respect to variable s, | | | | | non-woven phosphor₂Representing the two-norm of the vector and λ representing the constraint term coefficients.

And S204, solving according to the optimization solving problem to obtain a minimum fitting vector s which is larger than 0.

In step S204, a specific process of solving the fitting vector S according to the optimization problem is an existing conventional solution.

S205, calculating to obtain a fitting value D of the apparent diffusion coefficient according to the fitting vector s obtained by solving_fitFitting value T of transverse magnetization vector decay time constant_2-fit。

In step S205, the fitting vector S ═ vect (S) ═ vect (S (D, T)₂) And therefore can be solved for two fit values in a conventional manner.

S103, calculating a correction value T of the transverse magnetization vector attenuation time constant according to the following formula_2-corr：

In step S103, the CPMG sequence may play a role of frequency coding and a role of diffusion coding at all times, taking into account a strong gradient field existing in the inhomogeneous field. That is, if the echo interval of the CPMG sequence is larger, the signal is lower, wherein the influence of the diffusion effect on the CPMG signal can be described by the following formula:

in the formula, γ represents a magnetic rotation ratio of a nucleus, D' represents an ADC coefficient of a substance, G represents a gradient magnetic field magnitude, and τ represents an echo interval of the CPMG portion. As can be seen from the formula, the CPMG signal is related to the ADC coefficient, the gradient field and the echo interval of the substance, so that the ADC coefficient can be estimated by collecting SE-CPMG signals with different b values, and the measured T2 value is corrected.

The decay time constant of the transverse magnetization vector after correction is larger than that before correction, namely, the problem that the T2 value measurement is smaller due to diffusion is solved. In addition, the correction value T of the attenuation time constant of the transverse magnetization vector is obtained in the calculation_2-corrThereafter, the method further comprises: according to the correction value T_2-corrObtaining a corrected ADC-T2 two-dimensional map; and carrying out interpolation processing on the corrected ADC-T2 two-dimensional map again to obtain a squared new ADC-T2 two-dimensional map. Through the further processing, after the T2 for correcting the diffusion influence is obtained, a more accurate ADC-T2 two-dimensional map can be obtained, and the material composition of the detected object can be accurately identified.

As shown in FIGS. 5-8, the following is to measure the concentration of 0.5mmol/L MnCl on a non-uniform field NMR system₂The test of the solution and peanut oil is taken as an example to specifically illustrate the experimental results of this example. The experimental parameters were mainly as follows: the Region of Interest (ROI) has a constant magnetic field of 0.07T, a gradient field of 110Gauss/cm, and a diffusion gradient encoded T of SE-CPMG sequence _EE16 groups (i.e., M is 16) of 0.4ms, 2.1ms, 2.6ms, 3.0ms, 3.3ms, 3.6ms, 3.8ms, 4.0ms, 5.0ms, 7.1ms, 8.4ms, 9.3ms, 10.1ms, 10.8ms, 11.4ms and 12.0ms respectively, the echo chain length of the CPMG sequence is 256 (i.e., N is 256), the average number of times is 64 (i.e., a is 64), and the number of echo sampling points is 64 (i.e., Q is 64).

As can be seen from fig. 5, the b-value is piecewise linear to accommodate molecular diffusion of different tissue components; see fig. 6 for an exampleThe attenuation degrees of the CPMG sequences with different b values are different; from FIG. 7, 0.5mmol/L MnCl can be seen₂The solution is a uniform peak on an ADC-T2 two-dimensional map, the ADC coefficient corresponding to the peak top is 1.36e-3mm2/s, the T2 is 38.9ms, the value is close to the theoretical value, and further, the T2 measured value corresponding to the corrected map is 46.1 ms; it can be seen that, after correcting for the diffusion effects, the value of T2 becomes larger, consistent with theory; as can be seen from FIG. 8, MnCl₂The peaks of the solution and the peanut oil on the spectra are obviously distinguished: MnCl₂The ADC of the solution is larger, whereas peanut oil has a smaller ADC.

Therefore, according to the calibration measurement scheme described in detail in the above steps S101 to S103, in a nuclear magnetic resonance system having a very non-uniform magnetic field or being incapable of achieving a very short echo time, a two-dimensional map of ADC-T2 is fitted to echo signals acquired by a plurality of SE-CPMG sequences applying different b values, and then the transverse magnetization vector decay time constant T2 is calibrated, so that the problem of a small T2 measurement value caused by diffusion can be avoided. In addition, a more accurate ADC-T2 two-dimensional map can be obtained based on the corrected T2 measured value, the T2 value measurement is not influenced by echo time and molecular diffusion, the requirement on the system is low, and the system cost can be reduced. The error caused by molecular diffusion in the CPMG sequence measurement T2 can be corrected, the robustness of the T2 measurement is ensured, and the T2 value is not changed along with the change of the echo interval; for a magnetic field with extremely non-uniform intensity (including but not limited to a single-sided magnet) or a nuclear magnetic resonance system which cannot realize extremely short echo time, the method can still measure the ADC-T2 map to obtain a more accurate T2 measured value; the hardware requirement of the system can be reduced when the non-uniform field nuclear magnetic resonance system for measuring ADC-T2 is designed, so that the hardware cost is reduced; more accurate T2 measurement can be obtained, so that water and fat components in the detected object can be accurately distinguished, water and fat can be further accurately distinguished in an ADC-T2 two-dimensional map (including but not limited to human liver fat quantitative detection), tissue components can be identified (including but not limited to liver fibrosis detection), and the like.

As shown in fig. 9, a second aspect of this embodiment provides a virtual device for implementing the calibration measurement method according to any one of the first aspect or the first aspect, including a signal obtaining module, a map fitting module, and a constant calibration module, which are sequentially connected in a communication manner;

For the working process, working details and technical effects of the foregoing apparatus provided in the second aspect of this embodiment, reference may be made to the correction measurement method described in the first aspect or any one of the possible designs in the first aspect, which is not described herein again.

As shown in fig. 10, a third aspect of the present embodiment provides a computer device for executing the calibration measurement method according to any one of the possible designs of the first aspect or the first aspect, and the computer device includes a memory and a processor, which are communicatively connected, where the memory is used to store a computer program, and the processor is used to read the computer program and execute the calibration measurement method according to any one of the possible designs of the first aspect or the first aspect. For example, the Memory may include, but is not limited to, a Random-Access Memory (RAM), a Read-Only Memory (ROM), a Flash Memory (Flash Memory), a First-in First-out (FIFO), and/or a First-in Last-out (FILO), and the like; the processor may not be limited to the microprocessor of the model number employing the STM32F105 family. In addition, the computer device may also include, but is not limited to, a power module, a display screen, and other necessary components.

For the working process, working details and technical effects of the foregoing computer device provided in the third aspect of this embodiment, reference may be made to the first aspect or any one of the possible design of the calibration measurement method in the first aspect, which is not described herein again.

As shown in fig. 11, a fourth aspect of the present embodiment provides a non-uniform field magnetic resonance system for performing the calibration measurement method according to the first aspect or any one of the possible designs of the first aspect, including a console, a nuclear magnetic resonance spectrometer, a magnet, and a radio frequency subsystem;

In the specific configuration of the non-uniform field magnetic resonance system, as shown in fig. 11, the magnet is typically designed as a permanent magnet, such as a single-sided permanent magnet, so that there is still a highly non-uniform magnetic field in the region of interest.

In one possible design, the radio frequency subsystem includes a radio frequency power amplifier, a preamplifier, a transmit-receive switch and a radio frequency coil;

As shown in fig. 11, in a specific structure of the radio frequency subsystem, the radio frequency power amplifier is used for amplifying the radio frequency excitation signal to be transmitted; the preamplifier is used for amplifying the received magnetic resonance signal. The receiving and transmitting change-over switch is used for enabling the radio frequency coil to transmit the radio frequency excitation signal and receive the magnetic resonance signal through switching control.

For the working process, the working details and the technical effects of the inhomogeneous field magnetic resonance system provided in the fourth aspect of this embodiment, reference may be made to the first aspect or any one of the possible design of the corrected measurement method in the first aspect, which is not described herein again.

A fifth aspect of the present embodiment provides a computer-readable storage medium storing instructions including the calibration measurement method according to any one of the possible designs of the first aspect or the first aspect, that is, the computer-readable storage medium has instructions stored thereon, and when the instructions are executed on a computer, the computer performs the calibration measurement method according to any one of the possible designs of the first aspect or the first aspect. The computer-readable storage medium refers to a carrier for storing data, and may include, but is not limited to, floppy disks, optical disks, hard disks, flash memories, flash disks and/or Memory sticks (Memory sticks), etc., and the computer may be a general-purpose computer, a special-purpose computer, a computer network, or other programmable devices.

For the working process, working details and technical effects of the foregoing computer-readable storage medium provided in the fifth aspect of this embodiment, reference may be made to the first aspect or any one of the possible designs of the calibration measurement method in the first aspect, and details are not described herein again.

A sixth aspect of the present embodiment provides a computer program product comprising instructions which, when run on a computer, cause the computer to perform the calibration measurement method according to the first aspect or any one of the possible designs of the first aspect. The computer may be a general purpose computer, a special purpose computer, a network of computers, or other programmable devices.

The embodiments described above are merely illustrative, and may or may not be physically separate, if referring to units illustrated as separate components; if reference is made to a component displayed as a unit, it may or may not be a physical unit, and may be located in one place or distributed over a plurality of network units. Some or all of the units can be selected according to actual needs to achieve the purpose of the solution of the embodiment. One of ordinary skill in the art can understand and implement it without inventive effort.

The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: modifications may be made to the embodiments described above, or equivalents may be substituted for some of the features described. And such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Finally, it should be noted that the present invention is not limited to the above alternative embodiments, and that various other forms of products can be obtained by anyone in light of the present invention. The above detailed description should not be taken as limiting the scope of the invention, which is defined in the claims, and which the description is intended to be interpreted accordingly.

Claims

1. A method for correcting and measuring a transverse magnetization vector decay time constant, comprising:

2. The calibration measurement method of claim 1 wherein the diffusion sensitivity coefficient b value is equal to (t)_EE)³Is in direct proportion, wherein t_EEThe time length from the central point time of the first 90-degree excitation pulse to the central point time of the first echo signal in the single group echo signal acquisition process is shown.

3. The calibration measurement method of claim 2, wherein fitting an ADC-T2 two-dimensional map to obtain fitted values of apparent diffusion coefficient and transverse magnetization vector decay time constant based on the plurality of sets of echo signals comprises:

fitting the following integral formula to the multiple groups of echo signals:

t_j＝j·t_ED，i＝1,2,…,M，j＝1,2,…,N，t_EEithe time length from the central point moment of the first 90-degree excitation pulse to the central point moment of the first echo signal in the collection process of the ith group of echo signals is represented, M represents the total application number of the diffusion sensitivity coefficient b values, namely the total number of the multiple groups of echo signalsN denotes the echo train length, D, of a single group of echo signals_maxRepresenting the boundaries of maxima of the apparent diffusion coefficient, D_minRepresenting the boundary of the minimum value of the apparent diffusion coefficient, T_2-maxMaximum value boundary, T, representing the decay time constant of the transverse magnetization vector_2-minThe boundary of the minimum value, S (D, T), representing the decay time constant of the transverse magnetization vector₂) Representing the ADC-T2 two-dimensional map to be solved, D representing the apparent diffusion coefficient, T₂Represents the transverse magnetization vector decay time constant, e represents the base of the natural logarithm;

S′＝K₂SK₁ ^T,s′＝K·s

K₁the representation is composed of N × q elements

The arrangement is such that a matrix is formed,

The arrangement is such that a matrix is formed,

T_2jrepresents the transverse magnetization vector decay time constant, T, corresponding to the j point in the two-dimensional map of ADC-T2_i＝i·t_ED，D_jRepresenting the apparent diffusion system corresponding to the j point in the two-dimensional graph of the ADC-T2S denotes S (D, T) vect (S)₂) Vect (S) represents S (D, T)₂) The vector form of (1);

where F(s) represents a function with respect to variable s, | | | | | non-woven phosphor₂Representing the two-norm of the vector, and λ represents the constraint term coefficient;

4. The calibration measurement method of claim 3, further comprising preprocessing the sets of echo signals in any one or any combination of the following manners (A) to (C) before fitting the integration formula to the sets of echo signals:

5. As claimed in claim 1The calibration measurement method is characterized in that the calibration value T of the transverse magnetization vector decay time constant is obtained by calculation_2-corrThereafter, the method further comprises:

6. A device for correcting and measuring a transverse magnetization vector decay time constant is characterized by comprising a signal acquisition module, a map fitting module and a constant correction module which are sequentially in communication connection;

In the formula, T_2-fitFitting values representing the transverse magnetization vector decay time constant, D_fitRepresents the fitted value of the apparent diffusion coefficient, gamma represents the magnetic rotation ratio of the hydrogen nucleus, G₀Representing the natural constant gradient field magnitude of a magnet in a magnetic resonance system，t_EDAnd the time length from the central point moment of the first echo signal to the central point moment of the second echo signal in the single group of echo signal acquisition process is represented.

7. A computer device comprising a memory and a processor communicatively connected, wherein the memory is configured to store a computer program and the processor is configured to read the computer program and execute the calibration measurement method according to any one of claims 1 to 5.

8. A non-uniform field magnetic resonance system is characterized by comprising a console, a nuclear magnetic resonance spectrometer, a magnet and a radio frequency subsystem;

the console is in communication connection with the nuclear magnetic resonance spectrometer and is used for sending instructions to the nuclear magnetic resonance spectrometer so as to control parameter selection and region-of-interest positioning of a measurement sequence and receiving magnetic resonance signals acquired by the nuclear magnetic resonance spectrometer to complete data processing, wherein the data processing comprises executing the correction measurement method according to any one of claims 1-5;

9. The system of claim 8, wherein the rf subsystem comprises an rf power amplifier, a preamplifier, a duplexer, and an rf coil;

10. A computer-readable storage medium having stored thereon instructions for performing, when executed on a computer, a calibration measurement method according to any one of claims 1 to 5.