Magnetic resonance CEST imaging pre-saturation method and device based on amplitude and off-resonance frequency modulation

Abstract

Disclosed in the present invention are a magnetic resonance CEST imaging pre-saturation method and device based on amplitude and off-resonance frequency modulation, belonging to the field of magnetic resonance imaging. The method uses a Bloch-McConnell mathematical simulation model. The amplitude and off-resonance frequency parameters of CEST pre-saturation sub-pulses are used as adjustable parameters for nonlinear optimization, so as to minimize the variation of the CEST effect values under radiofrequency field-strength inhomogeneity, thereby reducing the effect of radiofrequency field inhomogeneity on CEST imaging. The present disclosure does not rely on additional hardware or radiofrequency field shimming processes, does not require the acquisition of additional data for post-processing correction, and ensures easy implementation without increasing additional acquisition time.

Description

A pre-saturation method and equipment for magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation Technical Field

[0001] This invention belongs to the field of magnetic resonance imaging, specifically relating to the optimization of presaturation pulses for magnetic resonance chemical exchange saturation transfer imaging. Background Technology

[0002] Chemical exchange saturation transfer (CEST) imaging is a novel molecular imaging technique that utilizes the chemical exchange process between hydrogen protons in trace amounts of large protein molecules in the human body and hydrogen protons in free water to indirectly detect information about these proteins and thus probe the body's metabolic processes.

[0003] To achieve higher spectral resolution and signal-to-noise ratio, CEST imaging has been increasingly extended to higher fields, and its applications in these fields have been explored. As the field strength increases, the wavelength of the radio frequency pulse shortens, leading to the formation of standing waves within the tissue. This results in constructive interference near the center of the tissue, while destructive interference occurs at a distance of one-quarter of a wavelength from the center, manifesting as spatial inhomogeneity of the radio frequency field (B1). Since both the pre-saturation and imaging modules of CEST imaging require radio frequency excitation, the quality of CEST imaging is significantly affected by B1 field inhomogeneity.

[0004] Over the past few decades, various methods have been developed to mitigate the impact of B1 field inhomogeneities on magnetic resonance imaging (MRI). In the 1980s, with the development of spectrometers and radio frequency pulse technology, amplitude- and frequency-modulated pulses, known as adiabatic pulses, were proposed, which can achieve uniform excitation flip angles under inhomogeneous B1 fields. However, the adiabatic pulse theory requires neglecting the relaxation effect, and its feasibility in scenarios requiring longer pulses, such as CEST presaturation, has not yet been explored. Parallel excitation is another method proposed in recent years, mitigating the effects of B1 field inhomogeneities through modulation between multiple excitation coils. Parallel excitation achieves a uniform B1 field intensity in a local space by modulating the amplitude and phase between each excitation channel. However, this method requires additional hardware and a more complex system calibration process, and therefore has not yet been widely used clinically. Besides improvements at the data acquisition end, numerous data correction algorithms have been invented to correct the effects of B1 field inhomogeneity at the post-processing end. These include methods using the steady-state relationship between the CEST signal and the B1 field, methods using a polynomial fitting model to fit the correction curve of the B1 field inhomogeneity, and correction methods using Omega plots. However, these algorithms all rely on acquiring data from multiple pre-saturated powers for B1 field inhomogeneity correction, thus significantly increasing the acquisition time. Summary of the Invention

[0005] The purpose of this invention is to provide a magnetic resonance imaging (MRI) CEST pre-saturation method and device that does not rely on additional hardware or require additional data acquisition, and has high clinical and commercial value, in order to solve the problem of CEST imaging being affected by the non-uniformity of radio frequency field intensity.

[0006] The specific technical solution adopted in this invention is as follows:

[0007] In a first aspect, the present invention provides a method for optimizing presaturation parameters of magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation, which includes the following steps:

[0008] S1. For the target object to be subjected to magnetic resonance CEST imaging, a multi-group Bloch-McConnell mathematical simulation model is used to simulate the target CEST pre-saturation power. During the simulation, the radio frequency field intensity non-uniformity factor of the CEST pre-saturation sub-pulse is adjusted to simulate the change in radio frequency field intensity caused by radio frequency field non-uniformity during the pulsed CEST pre-saturation process. The z-spectrum under different CEST pre-saturation sub-pulse modulation parameters is obtained, and the CEST effect value at the frequency of interest is calculated based on the z-spectrum. The CEST pre-saturation sub-pulse modulation parameters are composed of the amplitude and off-resonance frequency of each sub-pulse in the CEST pre-saturation module.

[0009] S2. Using the CEST pre-saturation sub-pulse modulation parameters and corresponding CEST effect values ​​obtained from simulation calculations as sample data, and taking the minimum variation of CEST effect values ​​under changes in RF field intensity during CEST pre-saturation as the optimization objective, nonlinear optimization is performed on the amplitude and off-resonance frequency of each sub-pulse in the CEST pre-saturation module to obtain the optimal modulation parameters of the CEST pre-saturation sub-pulse that are least affected by changes in RF field intensity under the target CEST pre-saturation power.

[0010] As a preferred embodiment of the first aspect above, the multi-group-based Bloch-McConnell mathematical simulation model is represented by the following formula:

[0011] Where expm represents an exponential function with base e, and Δt represents the increment at time t. Let these represent the magnetization vectors at times t+Δt and t, respectively. and matrix n is the number of solute groups, n≥1; These represent the magnetization vector components of the free water group in the x, y, and z directions, respectively. These represent the magnetization vector components of n solute groups, where the magnetization vector components of the i-th solute group in the x, y, and z directions are respectively... This represents the magnetization vector component of the semi-solid magnetization transfer group in the z-direction, while the magnetization vector components of the semi-solid magnetization transfer group in the x and y directions are negligible; coefficient matrix in:

[0012] Where Δω w Δω represents the difference between the partial resonant frequency ω of the applied CEST presaturated sub-pulse and the resonant frequency of the free water group. i The difference between the partial resonance frequency ω of the applied CEST presaturated subpulse and the resonance frequency of the i-th solute group; These represent the longitudinal relaxation rate and the transverse relaxation rate of the free water group, respectively. These represent the longitudinal and transverse relaxation rates of the i-th solute group, respectively. and These represent the longitudinal and transverse relaxation rates of the semi-solid magnetization transfer group, respectively. These represent the steady-state magnetization vectors of the free water group, the i-th solute group, and the semi-solid magnetization transfer group, respectively. These represent the hydrogen proton exchange rates from the free water group to the i-th solute group and from the i-th solute group to the free water group, respectively. The exchange rates are related to the steady-state magnetization vector as follows: k w_MT k MT_w These represent the hydrogen proton exchange rates from the free water group to the semi-solid magnetization transfer group, and from the semi-solid magnetization transfer group to the free water group, respectively. The exchange rates are related to the steady-state magnetization vector as follows: ω1 represents the angular frequency of the applied CEST pre-saturation sub-pulse, determined by the gyromagnetic ratio γ and the sub-pulse amplitude B1. ω1 = γ·B1, and the CEST pre-saturation sub-pulse amplitude B1 = B 1_weight ·B 1_modulation ·B 1_desired B 1_desired The desired applied CEST presaturation sub-pulse amplitude is determined by the target CEST presaturation power corresponding to the current simulation; B 1_weight A radio frequency field intensity non-uniformity factor within the range [1-ε, 1+ε] is used to simulate the radio frequency field intensity variation caused by radio frequency field non-uniformity, where ε is a preset variation amplitude; B 1_modulation The value of the CEST pre-saturated sub-pulse amplitude adjustment factor is to be optimized.

[0013] As a preferred embodiment of the first aspect mentioned above, during the simulation process, for all sub-pulses in the CEST pre-saturation module, it is necessary to determine the B value that satisfies the target CEST pre-saturation power corresponding to the current simulation. 1_desired Meanwhile from B 1_weight Different values ​​are sampled within the variation range and assigned to all sub-pulses to simulate the change in radio frequency field intensity caused by radio frequency field inhomogeneity. Then, a different B value is assigned to each sub-pulse. 1_modulation The partial resonant frequency ω is used to form different CEST pre-saturated sub-pulse modulation parameters. Finally, the Bloch-McConnell mathematical simulation model based on multi-group is used to simulate the excitation process of each sub-pulse in the CEST pre-saturation module in the time domain under different CEST pre-saturated sub-pulse modulation parameters, so as to obtain CEST imaging data and calculate the z-spectrum.

[0014] As a preferred embodiment of the first aspect mentioned above, during the simulation process, the radio frequency field intensity non-uniformity factor B... 1_weight The variation range is 0.8 to 1.2, B 1_weight The step size of the change is 0.05.

[0015] As a preferred embodiment of the first aspect, in the CEST pre-saturated sub-pulse modulation parameters, the amplitude of the sub-pulse needs to be set within an optimized upper and lower bound range to ensure that the SAR value of the sequence does not exceed the system limit, the off-resonance frequency of the sub-pulse needs to be superimposed with an additional modulation frequency on the basis of the original CEST polarization frequency, and the modulation frequency needs to be set within an optimized upper and lower bound range to ensure the frequency selectivity of CEST imaging.

[0016] As a preferred embodiment of the first aspect mentioned above, when performing nonlinear optimization on the amplitude and partial resonant frequency of each sub-pulse in the CEST pre-saturation module, a nonlinear optimization process based on sequence quadratic programming is adopted, and the objective function used is as follows:

[0017] Among them: coef pulsei This represents the CEST pre-saturation sub-pulse modulation parameters, including the amplitude B1 and the partial resonant frequency ω of each sub-pulse in the CEST pre-saturation module; C(CEST(B1, ω) tatget )) is the loss function, used to calculate when B 1_weight The variation in [1-ε, 1+ε] at the frequency ω of interest target The degree of variation of the CEST effect value at point C(CEST(B1, ω) target Defined as: ΔCEST=CEST(B1,ω target )-CEST(B 1_desired ω target )

[0018] Where: CEST(B1, ω) target This indicates that when the amplitude of the CEST pre-saturated sub-pulse is B1, at the frequency ω of interest... target CEST effect value at point, CEST(B) 1_desired ω target This indicates that when the amplitude of the CEST pre-saturation sub-pulse is B... 1_desired At the frequency of interest ω target CEST effect value at the location;

[0019] Penalty items Defined as:

[0020] In the formula: This indicates that in Bloch-McConnell simulations using only the free water group (i.e., Bloch-McConnell mathematical simulation models that do not consider solute groups and semi-solid magnetization transfer groups), when the amplitude of the CEST pre-saturated sub-pulse is B1 at the frequency ω of interest... targetThe CEST effect value at the position is used to constrain the distortion and center shift of the z-spectrum caused by the use of partial resonant frequency modulation, thereby minimizing the additional CEST effect value introduced by the partial resonant frequency modulation; α is a weighting parameter used to balance the penalty term and the loss function.

[0021] Secondly, the present invention provides a pre-saturation method for magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation, comprising:

[0022] Obtain a pre-constructed parameter dictionary; the parameter dictionary is constructed as follows: for different CEST pre-saturation powers, according to the magnetic resonance CEST imaging pre-saturation parameter optimization method described in any of the first aspects above, obtain the optimal modulation parameters of the CEST pre-saturation sub-pulse that are least affected by the change in radio frequency field intensity under each CEST pre-saturation power, and then associate and store the CEST pre-saturation power and the optimal modulation parameters of the CEST pre-saturation sub-pulse in the parameter dictionary.

[0023] Before performing CEST imaging presaturation, the optimal modulation parameters of the corresponding CEST presaturation sub-pulse are found from the parameter dictionary according to the specified CEST presaturation power. After setting the amplitude and off-resonance frequency of each sub-pulse in the CEST presaturation module, the CEST presaturation module is executed to acquire the CEST image that is least affected by the change in radio frequency field intensity.

[0024] Thirdly, the present invention provides a computer-readable storage medium, characterized in that the storage medium stores a computer program, which, when executed by a processor, implements the magnetic resonance CEST imaging presaturation parameter optimization method based on amplitude and off-resonance frequency modulation as described in any of the first aspects above, or implements the magnetic resonance CEST imaging presaturation method based on amplitude and off-resonance frequency modulation as described in the second aspect above.

[0025] Fourthly, the present invention provides a computer electronic device, characterized in that it includes a memory and a processor;

[0026] The memory is used to store a computer program; the processor is used to, when executing the computer program, implement the magnetic resonance CEST imaging presaturation parameter optimization method based on amplitude and off-resonance frequency modulation as described in any of the first aspects above, or implement the magnetic resonance CEST imaging presaturation method based on amplitude and off-resonance frequency modulation as described in the second aspect above.

[0027] Fifthly, the present invention provides a magnetic resonance imaging device, characterized in that it includes a magnetic resonance scanner and a control unit, wherein the control unit stores a computer program, which, when executed, controls the magnetic resonance scanner to acquire CEST images that are least affected by changes in radio frequency field intensity according to the magnetic resonance CEST imaging pre-saturation method based on amplitude and off-resonance frequency modulation as described in the second aspect above.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] (1) The method provided by the present invention is easy to implement, does not require the use of additional multi-channel coils and does not require a complex system correction process. It only requires adjusting the amplitude and off-resonance frequency parameters of the pre-saturation pulse according to the desired pre-saturation pulse power.

[0030] (2) The method provided by this invention relies on the prior knowledge provided by simulation, but does not rely on any post-processing correction algorithm. Therefore, it does not require additional data collection of multiple different pre-saturation powers. Compared with existing post-processing correction methods, it greatly improves the acquisition efficiency and reduces the acquisition time. Attached Figure Description

[0031] Figure 1 shows a comparison of the differences between the traditional CEST presaturation method and the CEST presaturation method proposed in this invention in the time domain and frequency domain.

[0032] Figure 2 shows the scanning results of the phantom embodiment. (A): CEST images of the phantom under different radio frequency field intensities; (B): Quantitative scanning results of the radio frequency field intensity coefficient, which ranged from 0.8 to 1.2 in the phantom experiment; (C): MTR calculated using two different presaturation methods in two regions of interest with the maximum and minimum 10% of the radio frequency field intensity. asym Signal distribution diagram.

[0033] Figure 3 shows the average z-spectrum (first and second rows) and average MTR calculated in the phantom experiment within two regions of interest where the radio frequency field intensity is at its maximum and minimum 10%. asym Musical score (third and fourth lines).

[0034] Figure 4 shows the scanning results of an example of a healthy volunteer. (A): CEST images of the healthy volunteer under different radio frequency field intensities; (B): CEST source image at 15 ppm applied to white matter segmentation; (C): Quantitative scanning results of the radio frequency field intensity coefficient, which varied from 0.85 to 1.15 in the healthy volunteer experiment; (D): MTR calculated using two different presaturation methods in two regions of interest with the highest and lowest radio frequency field intensities within the 10% of the region where the white matter volume fraction is greater than 90% in the healthy volunteer.asym Signal distribution diagram.

[0035] Figure 5 shows the average z-spectrum (first and second rows) and average MTR calculated in two regions of interest (the 10% regions with the highest and lowest radiofrequency field intensity in the white matter volume fraction of healthy volunteers greater than 90%). asym Musical score (third and fourth lines). Detailed Implementation

[0036] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. Technical features in various embodiments of the present invention can be combined accordingly without mutual conflict.

[0037] This invention provides a method for optimizing presaturation parameters in magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation. This method requires simulation using the Bloch-McConnell equation. The conventional water-solute-semi-solid magnetization transfer (MT) three-group exchange model can be expressed by the following formula, where, due to the ultrashort T2 time of the MT group, the transverse magnetization vector of the MT group is assumed to reach a steady state instantaneously, and therefore the change in the transverse magnetization vector of the MT group is ignored:

[0038] Where Δω w Δω represents the difference between the externally applied partial resonant frequency pulse and the resonant frequency of the free water group. s Δω represents the difference between the externally applied partial resonant frequency pulse and the resonant frequency of the solute group. MT This represents the difference between the externally applied partial resonant frequency pulse and the resonant frequency of the MT group; These represent the longitudinal relaxation rate and the transverse relaxation rate of the free water group, respectively. R represents the longitudinal relaxation rate and the transverse relaxation rate of the solute group, respectively. rfMT This represents the attenuation rate of the longitudinal magnetization vector of the MT group under the influence of the saturation pulse. This represents the lateral relaxation rate of the MT Group; These represent the steady-state magnetization vectors of the free water group, solute group, and MT group, respectively; k w_s k represents the rate of hydrogen proton exchange from the free water group to the solute group. s_wThis represents the hydrogen proton exchange rate from the solute group to the free water group. The exchange rate is related to the steady-state magnetization vector as follows: k w_MT k represents the hydrogen proton exchange rate from the free water group to the MT group. MT_w This represents the hydrogen proton exchange rate from the MT group to the free water group, and the two exchange rates are related to the steady-state magnetization vector as follows: ω1 represents the angular frequency of the applied radio frequency pulse, determined by the gyromagnetic ratio γ and the radio frequency pulse amplitude B1, where ω1 = γ·B1. The above Bloch-McConnell equation simulation can co-simulate multiple different solute groups and be rewritten in the following matrix form to represent the multi-group-based Bloch-McConnell mathematical simulation model, used for iterative solution:

[0039] Where expm represents an exponential function with the natural constant e as the base, and Δt represents the increment at time t. Let these represent the magnetization vectors at times t+Δt and t, respectively. in matrix n is the number of solute groups, n≥1; These represent the magnetization vector components of the free water group in the x, y, and z directions, respectively. These represent the magnetization vector components of n solute groups, where the magnetization vector components of the i-th solute group in the x, y, and z directions are respectively... This represents the magnetization vector component in the z-direction of the semi-solid magnetization transfer group (MT), while the magnetization vector components in the x and y directions are negligible due to their extremely short T2 time. Coefficient matrix. in:

[0040] Where Δω i The difference between the partial resonance frequency ω of the applied partial resonance frequency pulse and the resonance frequency of the i-th solute group; These represent the longitudinal relaxation rate and the transverse relaxation rate of the i-th solute group, respectively. These represent the T1 relaxation time and T2 relaxation time of the i-th solute group, respectively. The steady-state magnetization vector represents the i-th solute group; The hydrogen proton exchange rate represents the rate from the free water group to the i-th solute group. Let represent the hydrogen proton exchange rate from the i-th solute group to the free water group, and the two exchange rates are related to the steady-state magnetization vector as follows: ω1 represents the angular frequency of the applied CEST pre-saturation sub-pulse, determined by the gyromagnetic ratio γ and the amplitude B1 of the CEST pre-saturation sub-pulse. ω1 = γ·B1, and the amplitude B1 of the CEST pre-saturation sub-pulse is B1 = B 1_weight ·B 1_modulation ·B 1_desired B 1_desired The amplitude of the desired CEST pre-saturation sub-pulse is determined by the target CEST pre-saturation power in the current simulation. In practical applications, the amplitude B of a single sub-pulse is calculated based on the target CEST pre-saturation power and a preset pulse waveform (such as a Gaussian pulse). 1_desired B 1_weight For a radio frequency field intensity non-uniformity factor in the range [1-ε, 1+ε], during the simulation, it can be obtained through B. 1_weight The variation of ε is used to simulate the change in radio frequency field intensity caused by radio frequency field non-uniformity. ε is a preset variation amplitude. This invention only considers the spatial non-uniformity of radio frequency field intensity and ignores the temporal fluctuation of radio frequency field intensity. Therefore, the same B is used between each sub-pulse. 1_weight By B 1_weight ·B 1_desired The sub-pulse amplitude variation caused by spatial non-uniformity of the radio frequency field was calculated; B 1_modulation B is the CEST pre-saturated sub-pulse amplitude adjustment factor to be optimized. 1_weight and B 1_modulation All are dimensionless proportional values ​​and can be superimposed on B. 1_desired This changes the actual amplitude of the CEST pre-saturated sub-pulse. And B... 1_modulation This is the actively adjustable parameter in this invention, namely, in the CEST pre-saturation sub-pulse modulation parameters, the amplitude of each sub-pulse in the CEST pre-saturation module is actually determined by B. 1_modulation It is controlled by [the relevant authority].

[0041] Therefore, in the above multi-group Bloch-McConnell mathematical simulation model, the CEST pre-saturated sub-pulse modulation parameters are determined by the amplitude B1 of each sub-pulse in the CEST pre-saturation module (actually determined by B). 1_modulation (To control) and the partial resonant frequency ω.

[0042] Therefore, in the embodiments of the present invention, based on the above-mentioned multi-group Bloch-McConnell mathematical simulation model, a method for optimizing presaturation parameters of magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation is provided, which includes the following steps:

[0043] S1. For the target object to be subjected to magnetic resonance CEST imaging, a multi-group Bloch-McConnell mathematical simulation model is used to simulate the target CEST pre-saturation power. During the simulation, the radio frequency field intensity inhomogeneity factor of the CEST pre-saturation sub-pulse is adjusted to simulate the change in radio frequency field intensity caused by radio frequency field inhomogeneity during the pulsed CEST pre-saturation process. The z-spectrum under different CEST pre-saturation sub-pulse modulation parameters is obtained, and the CEST effect value at the frequency of interest is calculated based on the z-spectrum. The CEST pre-saturation sub-pulse modulation parameters are composed of the amplitude and off-resonance frequency of each sub-pulse in the CEST pre-saturation module.

[0044] S2. Using the CEST pre-saturation sub-pulse modulation parameters and corresponding CEST effect values ​​obtained from simulation calculations as sample data, and taking the minimum variation of CEST effect values ​​under changes in RF field intensity during CEST pre-saturation as the optimization objective, nonlinear optimization is performed on the amplitude and off-resonance frequency of each sub-pulse in the CEST pre-saturation module to obtain the optimal modulation parameters of the CEST pre-saturation sub-pulse that are least affected by changes in RF field intensity under the target CEST pre-saturation power.

[0045] During the simulation, since the sub-pulses in the CEST pre-saturation module are applied continuously, time-domain simulation calculations are required according to the specific excitation time of each sub-pulse. For all sub-pulses in the CEST pre-saturation module, it is necessary to determine the B value that satisfies the target CEST pre-saturation power corresponding to the current simulation. 1_desired Meanwhile from B 1_weight Different values ​​are sampled within the variation range and assigned to all sub-pulses to simulate the changes in radio frequency field intensity caused by radio frequency field inhomogeneity. Then, a different B value is assigned to each sub-pulse. 1_modulation The partial resonant frequency ω is used to form different CEST pre-saturation sub-pulse modulation parameters. Finally, a multi-group Bloch-McConnell mathematical simulation model is used to simulate the excitation process of each sub-pulse in the CEST pre-saturation module in the time domain under different CEST pre-saturation sub-pulse modulation parameters. CEST imaging data is obtained through the readout module, and the z-spectrum is calculated. The signal value of each voxel in the CEST imaging data is the aforementioned parameter. The method of calculating the z-spectrum from CEST imaging data is existing technology and will not be elaborated further.

[0046] Additionally, it should be noted that the frequency of interest ω in this invention... target The CEST effect value at a given location is calculated based on the z-spectrum. The specific frequency of interest and CEST effect value can be selected according to the actual situation. In the embodiment of the present invention, the selected CEST effect value is ω. targetThe APT signal of -3.5 ppm was obtained by subtracting the signal value of +3.5 ppm from the signal value of -3.5 ppm on the z-spectrum.

[0047] In embodiments of the present invention, in order to mitigate the impact of radio frequency field inhomogeneity on CEST images, simulation of B... 1_weight The variability of the CEST effect value was measured when the value varied between 0.8 and 1.2 (with a step size of 0.05), and the amplitude of the pre-saturated sub-pulse and the off-resonance frequency were used as adjustable parameters for nonlinear optimization. The optimization process of this invention uses the frequency of interest ω... target The degree of variation in the CEST effect value at point B is used as a loss function to calculate the loss when B... 1_weight The variation in [1-ε, 1+ε] at the frequency ω of interest target The degree of variation of the CEST effect value at point C(CEST(B1, ω) target Defined as: ΔCEST=CEST(B1,ω target )-CEST(B 1_desired ω target )

[0048] Where: CEST(B1, ω) target This indicates that when the amplitude of the CEST pre-saturated sub-pulse is B1, at the frequency ω of interest... target CEST effect value at point, CEST(B) 1_desired ω target This indicates that when the amplitude of the CEST pre-saturation sub-pulse is B... 1_desired At the frequency of interest ω target The CEST effect value at the location. In this embodiment, ε is taken as 0.2, and B1 is sampled in steps of 0.05 within the range of 0.8 to 1.2, and then C(CEST(B1, ω)) is calculated. target Actually, it was B 1_weight Nine (ΔCEST) values ​​at 0.80, 0.85, 0.90, 0.95, 1.00, 1.05, 1.10, 1.15, and 1.20. 2 The sum is obtained.

[0049] Because this invention uses off-resonance frequency modulation, this modulation method causes a certain degree of center shift and distortion in the z-spectrum, resulting in abnormal signal peaks during z-spectrum asymmetry analysis. Therefore, an additional penalty term needs to be added during the optimization process. Defined as:

[0050] In the formula: This indicates that in Bloch-McConnell simulations using only the free water group (i.e., Bloch-McConnell mathematical simulation models that do not consider solute groups and semi-solid magnetization transfer groups), when the amplitude of the CEST pre-saturated sub-pulse is B1 at the frequency ω of interest... target The CEST effect value at a certain point is used to constrain the distortion and center shift of the z-spectrum caused by polarization frequency modulation. This penalty term simulates the response of the optimized pulse in the presence of only direct water saturation, thus making... The minimum value is achieved at each off-resonance frequency to suppress the z-spectral center drift and distortion caused by off-resonance frequency modulation, thereby minimizing the additional CEST effect value introduced by off-resonance frequency modulation.

[0051] Based on the penalty term P and the loss function C, the two can be summed using a weight parameter α, resulting in the following objective function:

[0052] Among them: coef pulsei This represents the CEST pre-saturated sub-pulse modulation parameters, including the amplitude B1 and the off-resonance frequency ω of each sub-pulse in the CEST pre-saturation module.

[0053] In an embodiment of the present invention, when performing nonlinear optimization on the amplitude and partial resonant frequency of each sub-pulse in the CEST pre-saturation module, a nonlinear optimization process based on sequence quadratic programming is used to perform CEST optimization. pulsei The optimal solution.

[0054] Furthermore, when optimizing the modulation parameters of the CEST pre-saturated sub-pulses, necessary constraints need to be set for the amplitude and off-resonance frequency of each sub-pulse in the CEST pre-saturation module. The off-resonance frequency of the sub-pulses in this invention can be considered as an additional modulation frequency superimposed on the original CEST polarization frequency (determined by the original design of the CEST pre-saturation module). To ensure that the SAR value of the invented amplitude and off-resonance frequency modulation method does not exceed the system limit, and to ensure the frequency selectivity of the CEST imaging signal, upper and lower limits need to be set for the amplitude modulation and off-resonance frequency modulation of the sub-pulses.

[0055] Furthermore, the present invention can also provide a pre-saturation method for magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation, which includes the following steps:

[0056] Obtain a pre-constructed parameter dictionary; the parameter dictionary is constructed as follows: for different CEST presaturation powers, according to the magnetic resonance CEST imaging presaturation parameter optimization method, obtain the optimal modulation parameters of the CEST presaturation subpulse that are least affected by the change in radio frequency field intensity under each CEST presaturation power, and then associate and store the CEST presaturation power and the optimal modulation parameters of the CEST presaturation subpulse in the parameter dictionary.

[0057] Before performing CEST imaging presaturation, the optimal modulation parameters of the corresponding CEST presaturation sub-pulse are found from the parameter dictionary according to the specified CEST presaturation power. After setting the amplitude and off-resonance frequency of each sub-pulse in the CEST presaturation module, the CEST presaturation module is executed to acquire the CEST image that is least affected by the change in radio frequency field intensity.

[0058] Figure 1 shows a comparison between the amplitude and off-resonance frequency modulation-based pre-saturation method for magnetic resonance CEST imaging of the present invention and the traditional pre-saturation method. In the traditional pre-saturation module, all sub-pulses have the same amplitude and corresponding off-resonance frequency. However, in the pre-saturation method of the present invention, after optimization, the amplitude and off-resonance frequency of each sub-pulse will differ.

[0059] It should be noted that the form of the parameter dictionary is not limited, as long as it can establish a mapping relationship between the corresponding CEST pre-saturation power and the optimal modulation parameters of the CEST pre-saturation sub-pulse. Since the CEST pre-saturation power determined by simulation is generally discrete, in the CEST imaging experiment, based on the actual expected CEST pre-saturation power, the optimal modulation parameters of the CEST pre-saturation sub-pulse corresponding to the closest CEST pre-saturation power are found in the parameter dictionary, and the CEST image with the least influence from changes in RF field strength is acquired accordingly.

[0060] The following example will further demonstrate the aforementioned method for optimizing pre-saturation parameters of magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation, as well as the technical effects that the magnetic resonance CEST imaging pre-saturation method can achieve, so that those skilled in the art can better understand the essence of the present invention.

[0061] Example

[0062] In this embodiment, the CEST scan parameters are set as follows:

[0063] This invention utilizes a Siemens 3T scanner (MAGNETOM Prisma, Siemens Healthcare, Erlangen, Germany) with a 20-channel receiver head coil and a CEST imaging sequence based on Fast Spin Echo Readout (TSE). The CEST pre-saturation pulses use B375 Gaussian pulses provided by Siemens Pulse Tool, with a single pulse duration of 100 ms, for a total of 10 saturation pulses and a total saturation time of 1 s. The pulse interval is 10 ms, during which a destructive gradient is applied to remove the transverse magnetization vector, ensuring a smooth z-spectrum. Readout uses a single-layer TSE sequence with a slice thickness of 5 mm and a field of view (FOV) of 212*185.5 mm. 2 The acquisition matrix size is 96*84, and the in-plane voxel size is 2.2*2.2mm. 2 TR = 3000ms, TE = 7.2ms, echo train length = 48, and fat signal suppression was performed using Fat Sat. In this example, a total of 41 polarization frequencies (Inf, ±6, ±5, ±4.5, ±4, ±3.75, ±3.5, ±3.25, ±3, ±2.5, ±2, ±1.5, ±1, ±0.75, ±0.5, ±0.25, 0, 10, 15, 20, 30, 40, 50, 60, 70, 80ppm) were applied to the CEST saturation pulse, with a total scan time of 4 minutes and 11 seconds. The water saturation offset reference method (WASSR) was used to correct for inhomogeneity of the main magnetic field. The WASSR sequence acquired 26 frequencies; except for Inf, the remaining 25 frequencies were evenly distributed between -1.5ppm and +1.5ppm, and the sequence acquisition time was 55 seconds. In addition, a quantitative RF field intensity coefficient sequence provided by the manufacturer was scanned, with an acquisition time of 10 seconds. This quantitative RF field intensity coefficient sequence maintained the same spatial positioning, FOV, acquisition matrix, phase encoding direction, and shimming mode as the WASSR sequence and the CEST imaging sequence. All scan sequences underwent channel merging and normalization using coil sensitivity maps to avoid erroneous conclusions due to inhomogeneities in the receiving coil.

[0064] 1. MRI phantom experiment:

[0065] The phantom experiment used a 100 mM / L nicotinamide phantom for CEST testing, and manganese chloride and agar were used to adjust the T1 and T2 values ​​of the phantom. The concentration of manganese chloride used in the phantom was 0.1653 mM / L, the concentration of agar was 2.55 w / v, and a cylindrical sealed container with a waist circumference of 470 mm and a total volume of 5 L was used. The final scan T1 of this phantom was 1150 ms, and the T2 was 98 ms, which is consistent with the T1 and T2 value range of normal human cranial tissue.

[0066] The phantom experiment was simulated using the Bloch-McConnell equation with 3 groups (number of solute groups n=2) to obtain the optimal pulse parameters. Specific simulation parameters were: free water (T1=1150ms, T2=98ms, M0=74.59), amide protons (T1=1000ms, T2=10ms, M0=0.0745, k s_w =50Hz, Δω=3.5ppm), amine protons (T1=1000ms, T2=10ms, M0=0.03, k s_w =2000Hz, Δω=3.0ppm).

[0067] The phantom experiment scanned four different CEST presaturation pulse powers (1.5uT, 2.0uT, 2.5uT, 3.0uT), so it is necessary to optimize the optimal pulse parameters for each of these four different presaturation pulse powers.

[0068] 2. MRI experiment on healthy volunteers:

[0069] All experiments involving healthy volunteers in the examples were approved by the local ethics review committee. A total of four volunteers participated in the experiments (two men and two women, aged 22-24).

[0070] The phantom experiment was simulated using the Bloch-McConnell equation with 7 solute groups (n=6) to obtain optimal pulse parameters. Specific simulation parameters were: free water (T1=1000ms, T2=100ms, M0=74.59), amide protons (T1=1000ms, T2=10ms, M0=0.0745, k...). s_w =50Hz, Δω=3.5ppm), amine protons (T1=1000ms, T2=10ms, M0=0.03, k s_w =2000Hz, Δω=3.0ppm), semi-solid-state magnetized proton transfer (T1=1000ms, T2=0.01ms, M0=6.0, k s_w =45Hz, Δω=0ppm), nuclear Oferhauser effect protons (T1=1000ms, T2=1ms, M0=0.5, k s_w =16Hz, Δω=-3.5ppm), creatine protons (T1=1000ms, T2=10ms, M0=0.04, k s_w =500Hz, Δω=1.8ppm), hydroxyl protons (T1=1000ms, T2=55ms, M0=0.1, k s_w =2000Hz, Δω =0.9ppm).

[0071] The experiment with healthy volunteers scanned five different CEST presaturation pulse powers (1.0uT, 1.5uT, 2.0uT, 2.5uT, 3.0uT), and the optimal pulse parameters need to be optimized for each of these five different presaturation pulse powers.

[0072] The data processing and analysis methods for the example are as follows:

[0073] In the phantom experiment, based on the quantitative map of the acquired radio frequency field intensity coefficient, two regions of interest (ROIs) were extracted within the phantom, representing the 10% of the maximum and minimum radio frequency field intensity, respectively. The average z-spectrum and MTR within the two ROIs were then calculated. asym Spectrum. In the healthy volunteer experiment, a CEST source image with a pre-saturation frequency of 15 ppm was first selected and segmented. Voxels with a white matter volume fraction greater than 90% were extracted from the segmented white matter regions for analysis. Based on the acquired radio frequency field intensity coefficient quantitative map, two Regions of Interest (ROIs) with the highest and lowest radio frequency field intensities (within the 10% of the selected white matter region) were selected. The average CEST z-spectrum and MTR within the two ROIs were calculated respectively. asym The spectrum was analyzed, and the signal distribution within the two ROIs was examined.

[0074] The results of this embodiment are analyzed as follows:

[0075] Figure 2A shows the experimental results on a phantom using a conventional pre-saturation pulse (first row) and the proposed pulse based on amplitude and off-resonance frequency modulation (second row). Experiments were conducted with pre-saturation powers of 1.5 µT, 2.0 µT, 2.5 µT, and 3.0 µT. Figure 2B is a quantitative diagram of the RF field strength coefficient of the phantom, consistent with the pattern of higher RF field strength at the center and lower RF field strength at the edges under circular polarization excitation mode. Opposite CEST contrast inhomogeneities were observed in the results for 1.5 µT and 3.0 µT. 1.5 µT exhibits a relatively high CEST signal in the region of high RF field strength, while 3.0 µT exhibits a relatively low CEST signal in the same region. The second row of Figure 2A shows the results of the modulation-based pre-saturation pulse proposed in this invention, which, compared to the conventional saturation method (first row of Figure 2A), can mitigate the influence of RF field strength inhomogeneity and obtain a more uniform CEST image. This result can be further corroborated by the box plot in Figure 2C. When the presaturation power is 1.5uT, 2.5uT and 3.0uT, there are significant differences in signal distribution between the two ROI regions when using the traditional saturation method, while the proposed saturation method can greatly reduce the differences in signal distribution between the two ROIs.

[0076] Figure 3 shows the z-spectrum (first and second rows) and MTR of the average values ​​within the ROI at the maximum and minimum 10% of the radio frequency field intensity within the phantom.asym Spectrum (third and fourth rows). The results show that the z-spectrum did not change significantly before and after pre-saturated pulse optimization, while the average MTR in the two ROI regions after optimization... asym The spectral lines are closer to 3.5 ppm than before optimization.

[0077] Figure 4 shows the experimental results of using the conventional saturation method and the invented pre-saturation method based on amplitude and off-resonance frequency modulation on a healthy volunteer. As shown in Figure 4A, in the region of the prefrontal cortex with low radio frequency intensity, the conventional saturation method (first row of Figure 4A) shows an abnormal region of low CEST signal in the prefrontal cortex (indicated by the red arrow in Figure 4A), while the invented saturation method (second row of Figure 4A) can mitigate the influence of radio frequency field intensity changes at the corresponding location and does not show a low CEST signal. The box plot in Figure 4D shows that in the healthy volunteer experiment, the conventional saturation method showed a significant difference in signal distribution between the two ROI regions with different radio frequency field intensities, while the invented saturation method can reduce the difference in signal distribution between the two ROIs.

[0078] Figure 5 shows the average z-spectrum (first and second rows of Figure 5) and MTR within two regions of interest (ROIs) where the radiofrequency field intensity is at its maximum and minimum 10% in areas with a white matter volume fraction greater than 90% in healthy volunteers. asym The spectrum (Figure 5, rows 3 and 4) shows that the z-spectrum did not change significantly before and after optimization of the pre-saturated pulse, while the optimized MTR... asym The spectral lines are closer together in the two ROI regions around 3.5 ppm.

Claims

1. A method for optimizing presaturation parameters in magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation, characterized in that, Includes the following steps: S1. For the target object to be subjected to magnetic resonance CEST imaging, a multi-group Bloch-McConnell mathematical simulation model is used to simulate the target CEST pre-saturation power. During the simulation, the radio frequency field intensity non-uniformity factor of the CEST pre-saturation sub-pulse is adjusted to simulate the change in radio frequency field intensity caused by radio frequency field non-uniformity during the pulsed CEST pre-saturation process. The z-spectrum under different CEST pre-saturation sub-pulse modulation parameters is obtained, and the CEST effect value at the frequency of interest is calculated based on the z-spectrum. The CEST pre-saturation sub-pulse modulation parameters are composed of the amplitude and off-resonance frequency of each sub-pulse in the CEST pre-saturation module. S2. Using the CEST pre-saturation sub-pulse modulation parameters and corresponding CEST effect values ​​obtained from simulation calculations as sample data, and taking the minimum variation of CEST effect values ​​under changes in RF field intensity during CEST pre-saturation as the optimization objective, nonlinear optimization is performed on the amplitude and off-resonance frequency of each sub-pulse in the CEST pre-saturation module to obtain the optimal modulation parameters of the CEST pre-saturation sub-pulse that are least affected by changes in RF field intensity under the target CEST pre-saturation power.

2. The method for optimizing pre-saturation parameters of magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation according to claim 1, characterized in that, The multi-group-based Bloch-McConnell mathematical simulation model is represented by the following formula: Where expm represents an exponential function with base e, and Δt represents the increment at time t. Let these represent the magnetization vectors at times t+Δt and t, respectively. and matrix n is the number of solute groups, n≥1; These represent the magnetization vector components of the free water group in the x, y, and z directions, respectively. These represent the magnetization vector components of n solute groups, where the magnetization vector components of the i-th solute group in the x, y, and z directions are respectively... This represents the magnetization vector component of the semi-solid magnetization transfer group in the z-direction, while the magnetization vector components of the semi-solid magnetization transfer group in the x and y directions are negligible; coefficient matrix in: Where Δω w Δω represents the difference between the partial resonant frequency ω of the applied CEST pre-saturated sub-pulse and the resonant frequency of the free water group. i The difference between the partial resonance frequency ω of the applied CEST presaturated subpulse and the resonance frequency of the i-th solute group; These represent the longitudinal relaxation rate and the transverse relaxation rate of the free water group, respectively. These represent the longitudinal and transverse relaxation rates of the i-th solute group, respectively. These represent the longitudinal and transverse relaxation rates of the semi-solid magnetization transfer group, respectively. These represent the steady-state magnetization vectors of the free water group, the i-th solute group, and the semi-solid magnetization transfer group, respectively. These represent the hydrogen proton exchange rates from the free water group to the i-th solute group and from the i-th solute group to the free water group, respectively; ω1 represents the angular frequency of the applied CEST pre-saturation sub-pulse, determined by the gyromagnetic ratio γ and the sub-pulse amplitude B1: ω1 = γ·B1, where B1 = B 1_weight ·B 1_modulation ·B 1_desired B 1_desired The desired applied CEST presaturation sub-pulse amplitude is determined by the target CEST presaturation power corresponding to the current simulation; B 1_weight A radio frequency field intensity non-uniformity factor within the range [1-ε, 1+ε] is used to simulate the radio frequency field intensity variation caused by radio frequency field non-uniformity, where ε is a preset variation amplitude; B 1_modulation The value of the CEST pre-saturated sub-pulse amplitude adjustment factor is to be optimized.

3. The method for optimizing pre-saturation parameters of magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation according to claim 2, characterized in that, During the simulation, for all sub-pulses in the CEST pre-saturation module, it is necessary to determine the B value that satisfies the target CEST pre-saturation power corresponding to the current simulation. 1_desired Meanwhile from B 1_weight Different values ​​are sampled within the variation range and assigned to all sub-pulses to simulate the change in radio frequency field intensity caused by radio frequency field inhomogeneity. Then, a different B value is assigned to each sub-pulse. 1_modulation The partial resonant frequency ω is used to form different CEST pre-saturated sub-pulse modulation parameters. Finally, the Bloch-McConnell mathematical simulation model based on multi-group is used to simulate the excitation process of each sub-pulse in the CEST pre-saturation module in the time domain under different CEST pre-saturated sub-pulse modulation parameters, so as to obtain CEST imaging data and calculate the z-spectrum.

4. The method for optimizing pre-saturation parameters of magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation according to claim 2, characterized in that, During the simulation process, the radio frequency field intensity non-uniformity factor B 1_weight The variation range is 0.8 to 1.2, and the variation step is 0.

05.

5. The method for optimizing pre-saturation parameters of magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation according to claim 1, characterized in that, In the CEST pre-saturated sub-pulse modulation parameters, the amplitude of the sub-pulse needs to be set with optimized upper and lower bounds to ensure that the SAR value of the sequence does not exceed the system limit. The off-resonance frequency of the sub-pulse needs to be superimposed with a modulation frequency on the basis of the original CEST polarization frequency. The modulation frequency needs to be set with optimized upper and lower bounds to ensure the frequency selectivity of CEST imaging.

6. The method for optimizing pre-saturation parameters of magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation according to claim 1, characterized in that, When performing nonlinear optimization on the amplitude and partial resonant frequency of each sub-pulse in the CEST presaturation module, a nonlinear optimization process based on sequential quadratic programming is adopted, and the objective function used is as follows: Among them: coef pulsei This represents the CEST pre-saturation sub-pulse modulation parameters, including the amplitude B1 and the partial resonant frequency ω of each sub-pulse in the CEST pre-saturation module; C(CEST(B1, ω) target )) is the loss function, used to calculate when B 1_weight The variation in [1-ε, 1+ε] at the frequency ω of interest target The degree of variation of the CEST effect value at point C(CEST(B1, ω) target Defined as: ΔCEST=CEST(B1,ω target )-CEST(B 1_desired ,ω target ) Where: CEST(B1, ω) target This indicates that when the amplitude of the CEST pre-saturated sub-pulse is B1, at the frequency ω of interest... target CEST effect value at point, CEST(B) 1_desired ω target This indicates that when the amplitude of the CEST pre-saturation sub-pulse is B... 1_desired At the frequency of interest ω target CEST effect value at the location; Penalty items Defined as: In the formula: This indicates that, using only the Bloch-McConnell simulation performed by the Freewater Group, the amplitude of the CEST pre-saturated sub-pulse at the frequency ω of interest is B1. target The CEST effect value at the location; α is the weighting parameter used to balance the penalty term and the loss function.

7. A pre-saturation method for magnetic resonance CEST imaging based on amplitude and off-resonance frequency modulation, characterized in that, include: Obtain a pre-constructed parameter dictionary; the parameter dictionary is constructed as follows: for different CEST pre-saturation powers, according to the magnetic resonance CEST imaging pre-saturation parameter optimization method described in any one of claims 1 to 6, obtain the optimal modulation parameter of the CEST pre-saturation subpulse that is least affected by the change in radio frequency field intensity under each CEST pre-saturation power, and then associate and store the CEST pre-saturation power and the optimal modulation parameter of the CEST pre-saturation subpulse in the parameter dictionary; Before performing CEST imaging presaturation, the optimal modulation parameters of the corresponding CEST presaturation sub-pulse are found from the parameter dictionary according to the specified CEST presaturation power. After setting the amplitude and off-resonance frequency of each sub-pulse in the CEST presaturation module, the CEST presaturation module is executed to acquire the CEST image that is least affected by the change in radio frequency field intensity.

8. A computer-readable storage medium, characterized in that, The storage medium stores a computer program, which, when executed by a processor, implements the magnetic resonance CEST imaging presaturation parameter optimization method based on amplitude and off-resonance frequency modulation as described in any one of claims 1 to 6, or implements the magnetic resonance CEST imaging presaturation method based on amplitude and off-resonance frequency modulation as described in claim 7.

9. A computer electronic device, characterized in that, Including memory and processor; The memory is used to store a computer program; the processor is used to implement, when executing the computer program, the magnetic resonance CEST imaging presaturation parameter optimization method based on amplitude and off-resonance frequency modulation as described in any one of claims 1 to 6, or the magnetic resonance CEST imaging presaturation method based on amplitude and off-resonance frequency modulation as described in claim 7.

10. A magnetic resonance imaging device, characterized in that, It includes a magnetic resonance scanner and a control unit, wherein the control unit stores a computer program that, when executed, controls the magnetic resonance scanner to acquire CEST images that are least affected by changes in radio frequency field intensity, according to the magnetic resonance CEST imaging pre-saturation method based on amplitude and off-resonance frequency modulation as described in claim 7.

Images